Erosional and depositional processes on the submarine flanks of Ontong Java and Nukumanu atolls, western equatorial Pacific Ocean

Accepted Manuscript Erosional and depositional processes on the submarine flanks of Ontong Java and Nukumanu atolls, western equatorial Pacific Ocean

Sally J. Watson, Joanne M. Whittaker, Vanessa Lucieer, Millard F. Coffin, Geoffroy Lamarche PII: DOI: Reference:

S0025-3227(17)30058-0 doi: 10.1016/j.margeo.2017.08.006 MARGO 5667

To appear in:

Marine Geology

Received date: Revised date: Accepted date:

3 March 2017 3 August 2017 8 August 2017

Please cite this article as: Sally J. Watson, Joanne M. Whittaker, Vanessa Lucieer, Millard F. Coffin, Geoffroy Lamarche , Erosional and depositional processes on the submarine flanks of Ontong Java and Nukumanu atolls, western equatorial Pacific Ocean, Marine Geology (2017), doi: 10.1016/j.margeo.2017.08.006

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Erosional and depositional processes on the submarine flanks of Ontong Java and Nukumanu atolls, western equatorial Pacific Ocean 1

1

1

1,2,3

4, 5

, and Geoffroy Lamarche

PT

Sally J. Watson , Joanne M. Whittaker , Vanessa Lucieer , Millard F. Coffin

Institute for Marine and Antarctic Studies (IMAS), University of Tasmania, Hobart, Australia

2

School of Earth and Climate Science, University of Maine, Orono, Maine, USA

3

Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA

4

National Institute of Water and Atmospheric Research (NIWA), Wellington, New Zealand

5

School of Environment, University of Auckland, Auckland, New Zealand

MA

Corresponding Author: [email protected]

NU

SC

RI

1

D

Abstract

PT E

Ontong Java and Nukumanu atolls sit atop Earth’s largest oceanic plateau– the Ontong Java Plateau – in the western equatorial Pacific Ocean. In 2014, scientists aboard the Schmidt Ocean Institute’s RV Falkor mapped the seafloor surrounding Ontong Java Atoll (Solomon Islands) and Nukumanu Atoll (Papua New

CE

Guinea) for the first time using multibeam acoustic systems. These and pre-existing data help reveal the evolution of the atolls, which has involved volcanism, erosion, deposition, and carbonate growth. We

AC

use the new multibeam bathymetry and backscatter data, together with legacy seismic reflection data, to: (i) qualitatively characterise the submarine flanks of the atolls; (ii) identify broad and fine scale geomorphological features and classify the seafloor environment; (iii) investigate how submarine processes have influenced the morphological evolution of the atolls over geological timescales; and (iv) provide evidence that these atolls have fed sediment downslope to Kroenke Canyon that incises the NE flank of the Ontong Java Plateau. Submarine erosion is widespread on both atolls, and submarine landslides control, at least in part, the dynamic geomorphology and hence development of both atolls. Channels and gullies are abundant on

ACCEPTED MANUSCRIPT

the flanks of both atolls. Scarps are common on the upper flanks of both atolls, whereas undulating bedforms and crescent shaped bedforms are more common at the foot of the atolls’ slopes. The Ontong Java Atoll coastline has widespread bights that appear to be linked to submarine debris deposits downslope. The overall shape of Ontong Java Atoll could result from one or more extensive edifice failure(s), or multiple smaller cones amalgamating to form one island. Active erosion of Nukumanu Atoll

PT

is manifested by crescent shaped bedforms, although compared to Ontong Java, Nukumanu has a much 3

less sinuous coastline and more subdued mass wasting. We estimate ~1,500 km of volcaniclastic

RI

sediment has been eroded and transported from Ontong Java and Nukumanu atolls, some of which has

SC

likely fed Kroenke Canyon downslope.

NU

Keywords:

Multibeam bathymetry, geomorphology, seafloor classification, erosion, Kroenke Canyon, Ontong Java

MA

Plateau, Large Igneous Province.

D

1. Introduction

PT E

Ontong Java and Nukumanu atolls lie atop the Ontong Java Plateau (OJP) in the western equatorial Pacific Ocean (Fig 1a & b). A long-standing interpretation suggests that atolls start as volcanic islands, and subsequently erode to sea level and grow carbonate caps to attain their present configurations

CE

(Darwin, 1842). Volcanic islands and atolls are prone to erosion, mass wasting in particular, and edifice failures, due to both endogenetic factors - those relating to the internal architecture of the volcanic

AC

edifice including fault and dyke distribution, and exogenetic factors - sources of failures external to the volcano, including sea-level fluctuations, climate changes, extreme weather, and tectonism. Endogenetic sources of failure are more common during the active growth stage of the volcanic edifice, whereas exogenetic processes can occur at any stage of the volcanic island to atoll lifecycle (Keating & McGuire, 2000; Terry & Goff, 2013).

Ontong Java and Nukumanu atolls are subject to earthquakes and tsunamis related to the circum-Pacific subduction zone, ocean currents such as the South Equatorial Current, storms, and extreme weather

ACCEPTED MANUSCRIPT

(Hoeke et al., 2013; Keating & McGuire, 2000; Smithers & Hoeke, 2014; Fig. 1a). Storm waves can transport large volumes of sediment offshore and induce mass wasting, density flows, and shoreline regression, or cause the reactivation of sites prone to slope failure (Masson et al., 2006; Orpin et al., 2015; Pope at al., 2017; Prior et al., 1989; Puig et al., 2004). Mass wasting has the potential to cause inundation and substantial coastal erosion of Ontong Java and Nukumanu atolls, potentially reducing

PT

their already limited land area. An absence of modern geophysical data around these atolls has precluded investigating the role of mass wasting in their evolution. In particular, bathymetric data are

RI

important for understanding the mechanisms and extents of submarine erosion and deposition, and are

SC

essential in identifying regions prone to failure (Gee et al., 2001). In October 2014, scientists aboard 2

Schmidt Ocean Institute’s RV Falkor (FK141015) mapped ~1,234 km of seafloor surrounding Ontong

NU

Java Atoll and Nukumanu Atoll for the first time (Fig. 1c; Coffin et al., 2015a)

MA

In this paper, we present a terrain classification using multibeam bathymetry and backscatter data to divide the seafloor into geomorphic zones that help us identify regions where erosion has occurred.

D

Classification assists in the identification and distribution of morphological and geological seafloor

PT E

features that may otherwise be overlooked or difficult to discern within their broader context. We analyse multibeam acoustic bathymetry and backscatter data, combined with legacy seismic reflection data, around Ontong Java and Nukumanu atolls to (i) qualitatively characterise the submarine flanks of

CE

the atolls; (ii) identify broad and fine scale geomorphological features used to classify the seafloor environment; (iii) understand how submarine processes have influenced the morphological evolution of

AC

the atolls over geological timescales; and (iv) provide evidence that these relatively small atolls have actively fed sediment downslope to Kroenke Canyon.

2. Geomorphologic setting of Ontong Java and Nukumanu atolls Ontong Java and Nukumanu atolls are two of several atolls, islands, and reefs surmounting the OJP in the western equatorial Pacific (Inoue et al., 2008; Fig. 1a). Others include Nuguria, Malum, Tulun, Tauu, Roncador, and Sikaiana (Fig. 1b; Neal et al., 1997). The OJP is bordered by the North Solomon Trench (the convergence zone between the Pacific and the Indo-Australian Plates) to the south, and by the East

ACCEPTED MANUSCRIPT

Caroline Basin, East Mariana Basin, Nauru Basin, Stewart Basin and Ellice Basin to the west, north, NE, east, and SE, respectively (Fig. 1a & b; Kroenke et al., 2004; Mann & Taira, 2004). Subduction and accretion of the OJP has contributed to increasing congestion of the North Solomon Trench, manifesting as extensive deformation, seismicity, crustal thickening, uplift, obduction, and “collisional orogenesis”

The OJP (~1.86

6

PT

(Cloos, 1993; Mann & Taira, 2004; Petterson et al., 1997; Phinney et al., 2004; Wessel & Kroenke, 2008).

2

10 km ) was emplaced in a submarine environment during Early Cretaceous time

RI

(~122 Ma; Mahoney et al. 1993), with a later rift-related phase of volcanism at ~90 Ma (Gladczenko et

SC

al., 1997; Mahoney et al., 1993; Neal et al., 1997). The OJP consists of an up to ~42 km-thick crust (Furumoto et al., 1976; Gladczenko et al., 1997; Korenaga, 2011; Richardson et al., 2000; Tharimena et

NU

al., 2016) overlain by <1.5 km of sediments, largely comprising foraminifer chalk and limestone (Andrews et al., 1975; Kroenke et al., 1991; Mahoney et al., 2001). Samples retrieved from Deep Sea

MA

Drilling Program (DSDP) sites 289/586 (Legs 30/89) and Ocean Drilling Program (ODP) Site 1183 (Leg 192) (Fig. 1b; Andrews et al., 1975; Mahoney et al. 2001; Moberly et al., 1986) near the crest of the OJP,

D

suggest deposition in a low energy regime, high in the carbonate lysocline, and beneath the equatorial

PT E

divergence, resulting in low levels of carbonate dissolution and elevated pelagic sedimentation (Berger & Stax, 1994). Stratigraphic correlation and seismic reflection data indicate that the sedimentary layers

al., 1999).

CE

are laterally continuous over significant distances and form relatively uniform parallel strata (Phinney et

AC

The ages of the original volcanoes that are now Ontong Java and Nukumanu atolls are unknown. However, the most commonly accepted explanation is that these and the other atolls, islands, and reefs originated by volcanism post-dating Cretaceous emplacement of the main OJP (Kroenke et al., 2004). Tauu Atoll, ~240 km west of Ontong Java and Nukumanu atolls, is interpreted to have originated as a volcanic island that formed in the Tertiary (mid-Eocene to Oligocene/Miocene; Coffin et al., 2008; Inoue et al., 2008). DSDP Site 288, ~275 km east of the atolls, recovered ‘ash-rich’ foram-nannofossil ooze of Late Miocene to Pliocene age (Fig 1b; Andrews et al., 1975). Tertiary volcanism may thus be responsible for the atolls, islands, and reefs along the southern OJP, including Ontong Java and Nukumanu atolls.

ACCEPTED MANUSCRIPT

Ontong Java Atoll consists of several islands with a maximum elevation of ~13 m, distributed around the 2

perimeter (~219 km) of a large lagoon that covers ~1,480 km (Fig. 1c), similar in size to the Hawaiian Island of Kauai. Subaerially, Ontong Java Atoll has a distinctive boomerang shape, with its apex pointing to the SW and the two long axes oriented NNE and ESE. The morphology of the islands varies around the

PT

atoll. On the eastern portion of Ontong Java Atoll the islands are elongated narrow islands with submerged platforms (~1.5 km width), mostly continuous around the rim of the lagoon with the

RI

exception of a few channels on the SE rim (Fig. 1c). More than 50 islands border the western rim of the

SC

atoll. Here, the atoll rim is punctuated by numerous channels that allow transfer of ocean water into and out of the lagoon. Onshore, bight-like structures (Terry & Goff, 2013) are common in the atoll rim,

NU

suggesting that edifice failures and mass wasting have played roles in the geomorphological

MA

development of the atoll (Fig. 1c; Fairbridge, 1950; Stoddart, 1965; Terry & Goff, 2013).

Nukumanu Atoll (~46 km north of Ontong Java Atoll) is substantially smaller than Ontong Java Atoll, with 2

D

a perimeter of ~65 km (Fig. 1c), and a combined lagoon, reef and land area ~273 km similar to the

PT E

subaerial size of Molokai, Hawaii. The morphology of the atoll’s islands varies around its rim, with continuous linear narrow islands on the NE and SE perimeter and at least four discrete smaller islands on the west facing rim (max. elevation Nukumanu ~2 m; Fig. 1c). Nukumanu Atoll has a triangular rim

CE

with axes oriented NE, south, and west. It has a substantially less sinuous coastline compared to Ontong

AC

Java Atoll (Fig. 1c).

Kroenke Canyon is located on the OJP NE of the atolls (Fig. 1b & c); other than the Ontong Java and Nukumanu atolls (and their antecedent volcanic islands), no subaerial sources of sediment are apparent. Globally, canyons commonly incise continental margins, and regions with high subaerial sediment output from fluvial or glacial sources generally have more shelf-incising canyons than those without a substantial subaerial sediment source, although canyons can also form by non-deposition within the canyon (Druckman et al., 1995). Furthermore, ‘blind’ or ‘headless’ canyons lacking a subaerial sediment source and typically attributed to fluid flux are common on continental margins (Harris & Whiteway,

ACCEPTED MANUSCRIPT

2011); these can form by upslope regression from the mid-slope (Pratson & Coakley, 1996). Geologic samples suggest that the OJP was emplaced entirely in a submarine environment (Andrews et al., 1975; Kroenke et al., 1991; Mahoney et al., 2001). According to DSDP/ODP sites that penetrated igneous basement across the plateau (Sites: 289, 803, 807, 1183, 1185-1187; Fig. 1b) as well as sampling in the vicinity of the atolls (STA26-27 RD28, 235, PC009, 101G; Fig 1b, Appendix 1), sedimentary rock and

PT

sediment overlying OJP basement is also consistent with submarine deposition, comprising limestones, chalk, chert, and ooze (Andrews et al., 1975; Kroenke et al., 1991; Mahoney et al., 2001). Whether or

RI

not Ontong Java and Nukumanu atolls were subaerial sources of sediment contributing to the

3. Data and methods 3.1. Bathymetric data for seafloor characterisation

NU

SC

development of Kroenke Canyon has not been tested.

MA

Multibeam bathymetry data (Fig. 1c) were acquired aboard RV Falkor in 2014 (FK141015; Coffin et al., 2015a) using a 26.5-33.6 kHz Kongsberg EM302 (1°x1°) multibeam echosounder. The entire FK141015 2

D

multibeam survey encompassed 6,391 km around and between both atolls, and the region NE of the 2

PT E

atolls (Fig. 1b). The average water depth for the 1,234 km region surrounding the atolls and the upper reaches of Kroenke Canyon was ~1,800 m and ranged from 86 – 2,500 m (Fig. 1c), optimal water depths

CE

for the EM302 multibeam echosounder system.

Lamont-Doherty Earth Observatory undertook acoustic processing (Coffin et al., 2015a) and final project

AC

delivery including full processing and photo-mosaicking of the sonar data. Bathymetry and backscatter sensor files were exported from CARIS Hips & Sips 8.1.7 as grid files to ArcGIS v 10.3 at resolutions of 10 and 25 m. Surface characteristics were calculated in ArcGIS v 10.3 using DEM Surface Tools (Jenness, 2013) and the System for Automated Geoscientific Analysis (SAGA) 2.3.0 (Conrad et al., 2015). These results include the following bathymetric derivatives: hillshade, slope, curvature, rugosity, and bathymetric position index (BPI; Fig. 2). Hillshade is a raster generated from elevation data that provides a shaded surface depending on the angle and azimuth of a hypothetical light source. Hillshade raster is typically displayed underneath the transparent bathymetry raster to enhance visualisation of

ACCEPTED MANUSCRIPT

elevation data. Slope is defined as the maximum rate of change in a cell value (elevation) relative to neighbouring cells (3x3 neighbourhood). Also known as the first derivative of elevation (bathymetry in this context), the resulting raster shows the steepest gradient in degrees ranging from 0° (horizontal) to 90° (vertical; Jenness, 2013). Curvature is the second derivative of elevation, calculated in two directions also using a 3x3 neighbourhood (Jenness, 2013). The resulting raster reveals the local minima

PT

and maxima within the dataset. Positive values represent convexity, negative values represent concavity, and values close or equal to zero represent flat or uniform slope. Profile curvature is the

RI

second derivative of elevation, measured in the direction of maximum slope. Rugosity (also known as

SC

Terrain Ruggedness) measures the small-scale variation, complexity, and roughness within a neighbourhood defined by the user (a smaller neighbourhood measuring more localised variation;

NU

Wright et al., 2005). The rugosity tool provides a raster that measures the surface area of a neighbourhood compared to the planar area as a ratio (where values nearer to zero have less terrain

MA

variation than values closer to one). The BPI analyses the elevation of each cell relative to the average elevation of the neighbouring cells in a user-defined neighbourhood. Positive BPI values indicate

D

bathymetric/topographic highs (e.g., mounds, hills, mountains); likewise, negative BPI values indicate

PT E

bathymetric /topographic lows (e.g., gullies, troughs and valleys). Two BPIs were developed to identify both fine scale features (50 m scale factor) and broad scale features/zones (1,000 m scale factor). The broad BPI was adopted to generate zones within the study area (relative bathymetric highs and flat

CE

regions), whereas the fine-scale BPI was primarily used to identify small-scale physiographic features within the broad scale zones (scarps, ridges, gullies, and ravines). BPI values are standardised to account

AC

for spatial autocorrelation, and broad and fine BPI values can be classified and compared at any scale. Standardisation converts the BPI values such that zero represents flat terrain or consistent slope and ±100 represents 1 standard deviation from the mean and indicates crests (positive BPI) or depressions (negative BPI;(Lundblad et al., 2006; Verfaillie et al., 2007).

Seafloor backscatter data were acquired concurrently with the bathymetric data (Fig 2). The backscatter echo is the component of the transmitted sound signal that is reflected back from the seafloor and recorded by the sonar receiving arrays (Lurton, 2010). Backscatter intensity relates to the seafloor

ACCEPTED MANUSCRIPT

micro-roughness, and heterogeneities in the sediment volume, at the scale of the sonar wavelength (~0.05 m). Because backscatter strength is related to sediment grain size and micro-topography, it has been successfully used as a proxy for seafloor substrate (Jackson & Briggs, 1992; Lucieer & Lamarche, 2011; McGonigle & Collier, 2014). In this study, we only use the backscatter data qualitatively to provide a useful proxy for the nature of the substrate. As the backscatter data have not been calibrated, we are

PT

not able to use them quantitatively.

RI

We analysed 10 m and 25 m bathymetry, backscatter, and bathymetric derivatives to characterise and

SC

classify the submarine flanks of Ontong Java and Nukumanu atolls (Fig. 2; Wright et al., 2012). We employed ArcGIS 10.3 to identify and digitise seafloor features, and classify the submarine flanks of the

NU

atolls into physiographic zones based on spatial distribution of features and the characteristics of

MA

rasterised datasets (e.g., relative backscatter intensity and homogeneity).

3.2 Seismic reflection data for seafloor and sub-seafloor characterisation

D

Seismic reflection data in the vicinity of Ontong Java and Nukumanu atolls, both inside and outside the

PT E

area of RV Falkor multibeam echosounder coverage, were acquired on board RV Mahi in 1968. The acquisition system consisted of a 5000 joule EG&G sparker, a 30 m analogue streamer, seismic amplifier filters, and wet paper recorders. The analogue data were filtered in low (50-100 Hz or 65-130 Hz) and

CE

high (100-200 Hz or 125-250 Hz) bands to optimise penetration and resolution, respectively, and algebraically summed for display as single mixed-frequency paper records. Scans of these records are

AC

available from the U.S. National Oceanic and Atmospheric Administration (NOAA; Fig 1b).

We analysed and interpreted these data using seismic/sequence (e.g., Galloway, 1989; Mitchum et al., 1977) stratigraphic principles to study sediment relationships within a time-stratigraphic framework of repetitive, genetically related contemporaneous strata bounded by surfaces of erosion or nondeposition, or their correlative conformities. Seafloor unconformities were readily identifiable, and using the established seismic stratigraphic framework for OJP (Inoue et al., 2008; Mosher et al., 1993; Phinney et al., 1999), we identified prominent conformities and their intervening sequences.

ACCEPTED MANUSCRIPT

3.3 Sediment, sedimentary rock, and igneous rock samples No sampling was undertaken from RV Falkor. To ground-truth the multibeam echosounder and seismic reflection data and infer the geological nature of the seafloor and sub-seafloor, we used all available geological sample data collected in the Ontong Java and Nukumanu atolls region (Fig. 1b; Appendix 1).

PT

The results from eight piston and gravity cores proximal to the atolls are available from NOAA and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Relevant scientific ocean drilling

RI

samples were acquired by scientists aboard DSDP Legs 7 (Winterer & Reidel, 1971), 30 (Andrews et al.,

SC

1975), and 89 (Kennett et al., 1986; Moberly et al., 1986), and ODP Legs 130 (Kroenke et al., 1991) and

NU

192 (Mahoney et al., 2001a); the most proximal drill site is DSDP Site 288 (Fig. 1b).

4. Results

MA

4.1 Submarine geomorphology of Ontong Java and Nukumanu atolls

4.1.1 Ontong Java Atoll

D

The most distinctive feature on Ontong Java Atoll is its NE flank, a 39 km-wide “C”-shaped embayment,

PT E

with numerous smaller bights ranging from 3 to 10 km in length (Fig. 3a). Offshore, steep scarps characterise the submarine flanks on the upper slope. The NE embayment has a smooth chute-like

CE

morphology, typical of landslide valleys (Masson et al., 2002). In the NE embayment, the slope is ≤40° from ~100 – ~400 m water depth; the gradient sharply increases to >40° for ~300 m (~400 – 700 m

AC

water depth) forming a near continuous headscarp along the entire NE embayment. From ~700 m water depth, the slope then exponentially decreases with depth and distance from the atoll rim (Fig. 3b – Profile A-A’). In general, the slope is concave in profile (Fig 3b), and in some locations on the upper flank slope exceeds 60°. The chute has lower backscatter intensity and is bound to the east and north by rugged blocky debris associated with patches of higher backscatter intensity (slope failures C & D Fig 3e & f; Fig. 4).

ACCEPTED MANUSCRIPT

The SW flank of Ontong Java Atoll, the apex of the boomerang, is characterised by a highly irregular coastline with numerous bights. Downslope of bights, debris deposits are common. The SW corner of Ontong Java Atoll has a distinctive sinuous slope profile, contrasting with the concave NE flank (Fig. 3a & b). Here, the slope is steep in shallow water (~30° in <500 m water depth), then flattens to less than 15° before the gradient abruptly increases to >40° at the base of the atoll slope at ~1,400 m (Fig. 3b – Profile

PT

B-B’). The sinuous slope profile occurs only on the SW and southern flanks of Ontong Java Atoll, and shows high backscatter intensity (Fig. 4). The fringing bulge that forms the sinuous slope profile is

RI

incised by numerous gullies and channels occurs consistently between 1,100 and 1,400 m water depths

SC

(Fig. 3a).

NU

The irregular rim and shape of Ontong Java Atoll suggest extensive submarine erosion. Submarine slides are manifested as convex bathymetric contour lines directly downslope from concave segments or

MA

bights along the atoll rim (Fairbridge, 1950). The correspondence of bights in the emergent rim with abundant scarps, debris aprons, and mass wasting deposits offshore suggests physical relationships

D

between the onshore rim and the offshore slope of Ontong Java Atoll. We present four examples of

PT E

slope failures on the southern, eastern, and NE flanks of Ontong Java Atoll, all of which have debris deposits preserved at the foot of the atoll slope (slope failures A, B, C, and D; Fig. 3a, c-f).

CE

Slope failure A is located on the southern flank of Ontong Java Atoll, in an arcuate embayment ~11 km across (Fig. 3c). The debris deposit extends ~6 km in a southward direction from the steep amphitheatre

AC

in the atoll rim to ~1,400 m depth and is ~4 km wide. Slope failure A comprises multiple lobes, likely a result of multiple slides from a smooth landslide chute (Fig. 3c). The toe of the landslide is ~300 m shallower than surrounding seafloor and is associated with high backscatter (light grey-white; Fig. 4). One lobe extends 1,000 m farther south than the rest of the other debris lobes making up slope failure A. Directly downslope from slope failure A, a sediment lobe extends ≥9 km beyond the slope failure A complex in ≥1,700 m water depth. Although incompletely mapped, the lobe appears to laterally disperse with depth and distance from the atoll rim from 2.5 km to ≥6 km. Seafloor across the lobe is ~100-150 m shallower than adjacent ambient seafloor.

ACCEPTED MANUSCRIPT

Slope failure B is located on the eastern flank of Ontong Java Atoll (Fig. 3d). Although bights characterise the atoll rim in this location, they are less well defined compared to slope failure A and the NE flank embayment. Slope failure B is laterally dispersed south-eastwards from ~700 m wide on the atoll flank, to a series of bulbous lobes extending laterally over ~3,170 m at the foot of the atoll slope (Fig. 3d).

PT

Slope failure B has a thick debris deposit at the base of the slope with highly heterogeneous backscatter intensities, consisting of low backscatter intensity patches on its top and medium to high intensity areas

RI

around its boundary (Fig. 4). Similar to slope failure A, a sedimentary apron extends ~3.6 km from the

NU

or a single major debris flow arising from slope failure B.

SC

atoll rim beyond the main slope failure deposit, possibly evidence for numerous debris flows at this site,

Slope failures C and D border the NE flank embayment and are both characterised by blocky irregular

MA

terrain at the base of the slope (Fig. 3e & f). A patch of undulating seafloor lying ~40 m above adjacent ambient seafloor likely reveals the outer edges of a sedimentary apron that extends ~24 km downslope

D

from the NE flank embayment (Fig. 3e). At both slope failures C and D, individual blocks border the NE

PT E

embayment, display high backscatter, and appear randomly distributed at the foot of the slope (Fig. 4). Along the base of slope failure C on the eastern side of the large embayment, four or possibly five elongate bathymetric depressions ≤200 m deep align sub-parallel to the 1,500 m bathymetric contour, a

CE

distinction from other failure deposits in the region (Fig. 3e). The perimeters of these depressions show strong backscatter intensity at their bases (Fig. 4). The high backscatter intensity on the perimeters of

AC

the depressions may represent a harder, more durable substrate, possibly carbonate or volcanic.

Slope failure D is situated on the northern limb of the NE embayment. Relatively small, subdued bights have carved the atoll rim compared to slope failures A-C. From the atoll rim, the slope is between 10-25° for ~1-1.6 km, until a steep slope >40° at ca. 850 m water depth (Fig. 3f) associated with low backscatter intensity. At the foot of the atoll slope lie 10-12 blocks (asymmetric mounds up to 1.4 km across) with high backscatter intensity. Numerous channels radiate downslope to sediment lobes at the atoll base; these are probably related to the downslope movement and channelling of sediment (Fig. 3f).

ACCEPTED MANUSCRIPT

4.1.2 Nukumanu Atoll The eastern side of Nukumanu Atoll descends steeply into the upper reaches of Kroenke Canyon (Fig. 5a). Approximately 36 km NE of the atoll's rim, the seafloor deepens rapidly towards a valley at 2,240 m

PT

depth, which we interpret to be the canyon thalweg based on regional bathymetry (Fig. 1c). Seismic reflection data near Nukumanu Atoll illuminate the stratigraphy in regions both inside and outside the

RI

multibeam sonar coverage (Fig. 6a). The west-east profile (Fig 6b – Profile I-II) intersects the southern

SC

edge of Nukumanu Atoll, revealing stark differences between the atoll’s acoustic basement and the relatively uniform, layer-cake stratigraphy typical of the OJP on either side of the atoll (Mosher et al.,

NU

1993; Phinney et al., 1999; Inoue et al., 2008). Seismically opaque wedges extend from the flanks of Nukumanu Atoll. The wedge is thicker on the atoll’s eastern flank and extends ~19 km from the atoll. On

MA

the western flank the wedge is thinner and extends ~32 km from the atoll. To the east of Nukumanu Atoll, the adjacent seafloor dips steeply eastward to form a channel-like depression (Fig 6b). This depression occurs in a gap in the multibeam coverage, but correlates with the head of the Kroenke

PT E

D

Canyon as inferred from the multibeam and satellite-derived bathymetry.

Terraces NE of Nukumanu Atoll are delimited by long, continuous, and sinuous scarps (>160 m high and

CE

with slopes consistently >20°, with some locations >50°) conspicuous in the relatively flat region (gradient 0-10°; Figs. 5a, b). The scarps display high backscatter juxtaposed with low backscatter,

AC

surrounded by the terraces, which display relatively homogenous medium reflectivity (Fig. 4). The flat terrace and scarps are well expressed on profiles as bathymetric steps with gouges forming troughs at their bases (Fig. 5b). Hummocks at the bases of the scarps possibly originate from an accumulation of mass debris derived from upslope.

Offshore, headscarps around the atoll flanks are much more subdued than on Ontong Java Atoll. The western flank is a relatively straight 15 km stretch of coastline with no major bights in the atoll rim (Fig. 5c). Here, headscarps ≤1 km across, occur on the upper atoll slopes, and irregular undulating bedforms

ACCEPTED MANUSCRIPT

and gullies characterise the base of the slope. To the south, we interpret a distinctive blocky, irregular surface with a fan-like morphology at the atoll base to be a debris apron between ~1,100 – 1,600 m water depth (Fig. 5d). Upslope of the debris apron the flank has a markedly lower gradient than the adjacent flank. The debris apron is narrow on the upper slope before dispersing at ~870 m to form a triangular cone shape towards the base of the atoll. The debris extends ~8 km from the atoll rim (Fig.

PT

5d).

RI

Immediately north of Nukumanu Atoll lies a knoll at ~700 m water depth (Fig. 5a). The knoll is conical,

SC

with an irregular base and a distinctive mound at the summit, and displays high backscatter return (Fig. 4). The sea knoll is by far the largest positive relief feature in the mapped vicinity of the atolls, rising

NU

just <1 km above the surrounding seafloor. Considering the overall geology of the region and the high backscatter return, we interpret this knoll to be a volcano. Near the knoll in depths between 1,300 –

MA

1,700 m, numerous blocky features with similar high backscatter return are scattered randomly. These blocks could also be smaller volcanoes, or debris from mass wasting. Farther downslope from

D

Nukumanu Atoll, both seismic reflection profile III-IV and multibeam bathymetry reveal irregular

PT E

bathymetry (Fig. 6c). The multibeam and seismic reflection data reveal that these channels are trending downslope towards the thalweg of Kroenke Canyon, and are possibly acting as transport conduits for

CE

sediment eroded from the nearby Nukumanu Atoll.

Prominent linear gullies observed in the multibeam data are common around the bases of both Ontong

AC

Java and Nukumanu atolls; we suggest these are active transport conduits for unconsolidated sediment eroded from the upper atoll flanks to the bases. The gullies are characterised by high backscatter intensity, and radiate perpendicular to the atoll rims from their upper flanks to their bases (Fig. 5c). The gullies occur on all flanks of the atolls and vary substantially in size, from the resolution limit of the multibeam data to 400 m wide and up to 5 km long (Fig. 7a, Profile A-A’). Within some of these gullies (more common on the flanks of Nukumanu Atoll) are regularly spaced crescent-shaped bedforms (CSBs), sub-parallel to bathymetric contours, which are well identified in the profile curvature raster and in profile (Fig. 7b, Profile B-B’). Profiles within the gullies show that the CSBs are asymmetric sediment

ACCEPTED MANUSCRIPT

waves ≤10 m high and 200 m wide, and lie in water depths ≥~1,550 m (Fig. 7 - Profile B-B’). Such features have been identified in submarine canyons globally, and indicate sediment transport during high-energy gravity flows (Normandeau et al., 2014; Paull et al., 2011). The presence of CSBs on seafloor with slopes of 0°-10° and at depths of ~1,300 - ~1,700 m may represent a change from an erosional to a

PT

depositional regime on the atoll flanks (Fig. 7 – Profile B-B’).

4.2 Qualitative classification

RI

We defined five geomorphic zones and five physiographic features based on bathymetry, slope,

SC

curvature, terrain ruggedness, BPI, and backscatter (Fig. 2). These zones include the OJP, the atollplateau transition (or atoll base), the lower slope, the upper slope, and volcanic features. Within each

NU

zone we identified physiographic features: gullies, undulating bedforms, scarps, channels, blocks, and CSBs (Fig. 8a-c). Gullies are identified as small scale (10-100 m wide and typically <10 m deep) channels

MA

with steep sides, which incise slopes and provide conduits for downslope movement of sediment (Micallef & Mountjoy, 2011; Surpless et al., 2009). Undulating bedforms comprise irregular to sinuous

D

accumulations of debris forming piles or lobes with a stoss slope >5°, typically at or near a slope base.

PT E

Undulating bedforms also indicate downslope movement of sediment (Normandeau et al., 2014). Scarps are linear to arcuate features with relatively steep slopes (>40°) compared to surrounding areas. Channels are v-shaped linear incisions and are located on relatively flat seafloor (<5°). Blocks are

CE

discrete bathymetric mounds >100 m wide and long, and commonly display high backscatter. CSBs are asymmetric trough and crest undulations, and are distinguished from undulating bedforms by their

AC

rhythmic nature and crescent shape, concave downslope form. CSBs are up to tens of meters high and typically <200 m wide. CSBs can be confined within a channel (Fig 7a, profile A-A’) or occur on a flank slope (Normandeau et al., 2014).

All scarps occur on the atoll slope (upper and lower slope zones), and both atolls have a similar distribution of scarps at ~1 per km of atoll flank (Table 1). We observed 706 gullies around Ontong Java Atoll; 75% occur within the upper and lower slope zones. Around Nukumanu Atoll, we identified 75% of the gullies within the upper and lower slope zones (water depths <1,300 m). The Nukumanu Atoll

ACCEPTED MANUSCRIPT

environs contain 72% of all CSBs identified in this study. Undulating bedforms are more common within the atoll base zone than in any other zone around both atolls.

Table 1 – Distribution and density of features around Ontong Java and Nukumanu atolls Ontong Java Atoll

(Perimeter at -1,300 m water depth

(Perimeter at -1,300 m water depth =

= ~99 km)

~295 km)

Count

147

1.5

1.1

178

0.6

0.5

528

1.8

4.9

185

0.6

3.8

1035

3.5

293

3.0

779

2.6

80

0.8

75

0.3

0

N.A.

5

0.02

MA

Gullies in water 50

AC

Undulating

379

CE

of atolls

PT E

482

bedforms in water

D

<1,300 m deep

bedforms in vicinity

kilometre of atoll flank

320

>1,300 m deep

Total undulating

SC

1.2

NU

114

Gullies in water

CSBs

Frequency per

Count

kilometre of atoll flank Scarp

RI

Frequency per Feature

PT

Nukumanu Atoll

>1,300 m deep

Blocks in water >1,300 m deep

Blocks in water <1,300 m deep

ACCEPTED MANUSCRIPT

4.2.1 Ontong Java Plateau (OJP) zone The OJP zone is characterised by gently sloping seafloor (0°- 5°) with the exception of terrace scarps NE of Nukumanu Atoll. In general, the OJP zone is characterised by a gently undulating morphology, low

PT

broad BPI, homogeneous intermediate backscatter intensity, and water depths >1,800 m (Fig. 8c). The intermediate backscatter intensity (dark grey) suggests smooth and homogeneous sediments, possibly

RI

unconsolidated (Fig. 4), which we interpret as biogenic ooze at the top of sedimentary section overlying

SC

OJP’s igneous basement (as sampled DSDP 289/586, FFC022, FCC018 – Appendix 1 and Fig. 1b). Undulating bedforms and CSBs occur locally in the OJP zone, typically adjacent to the atoll base zone

NU

(Fig. 8a & b). Dendritic channels are predominantly concentrated to the east of Nukumanu Atoll proximal to where Kroenke Canyon may start (Fig. 8b).

MA

4.2.2 Atoll Base zones

The atoll base zones mark the transition from OJP to the atolls and is defined by an increase in slope to

D

>5°, high broad BPI, depths ranging from ~1,800 – 1,300 m, and a transition from predominantly

PT E

homogeneous intermediate intensity backscatter (OJP), to a highly variable range of backscatter intensities on the atoll flanks (Fig. 4). The atoll base zones have moderate to low roughness and

CE

undulating profile curvature (convex and concave). In the atoll base zones undulating bedforms, gullies, and blocks between 300 -1,000 m wide are common (Table 1), and CSBs become prevalent, particularly

AC

on Nukumanu Atoll (Table 1; Fig. 8a & b). The CSBs indicate downslope sediment transport, deposition, and redeposition at this location. The backscatter data display radial linear stripes downslope over the entire atolls’ flanks (Fig. 4).

4.2.3 Upper and lower slope zones The atolls’ flanks are characterised by a sharp increase in slope to >15° shoreward. We divided the flanks into two sections: upper and lower slopes (Fig. 8c). The lower slope occurs at ~1,300 – 1,000 m depth in regions where the gradient is consistently >15°. The lower slope consists of some regions (particularly

ACCEPTED MANUSCRIPT

on the NE flank of Ontong Java Atoll) that have lower intensity (darker) backscatter intensities than the OJP zone, possibly representing unconsolidated sediment (Fig. 4). In general, the atoll slopes (both lower and upper) exhibit abundant gullies and scarps, and few undulating bedforms (Table 1; Fig. 8a & b), probably reflecting dominant erosion on the atoll slopes. Shoreward, the upper slope is defined by increases in flank gradient to >30°, water depths <1,000 m, and consistently strong (light) backscatter

PT

intensity (Fig. 4 and 8c). Gradients >30° in this zone indicate consolidated sediment or sedimentary rock, as the angle of repose for unconsolidated sediments/talus is typically ~30° (Lee et al., 1994; Mitchell et

RI

al., 2000; Selby et al., 1982). Scarps are common in the upper portion of both atolls’ slopes (Table 1),

SC

commonly associated with undulating bedforms and in some cases debris deposits downslope (Fig. 8a & b). The NE flank of Ontong Java Atoll is consistently concave in profile, with a near continuous headscarp

NU

between 400 - 700 m water depth facing NE towards Kroenke Canyon (Fig. 3b & 8a). The ruggedness of

MA

the upper slope is highly variable, but the backscatter is consistently high intensity.

4.2.4 Volcanic zone

The zone classified as “volcanic features” encompasses bathymetric elements distinct from the atolls

PT E

D

themselves (including the sea knoll and smaller bathymetric highs to the north of Nukumanu Atoll; Fig. 8b). Although no geologic evidence is available to confirm these features as volcanic, their conical shapes, high backscatter intensities (distinct from surrounding OJP), and the surrounding geology

AC

5. Discussion

CE

together suggest these are composed of hard, probably volcanic rock.

Multibeam bathymetry (with associated spatial derivatives) and backscatter data form the basis of our geomorphological interpretations. The multibeam data reveal evidence for extensive submarine erosion and deposition of the flanks of Nukumanu and Ontong Java atolls, although we believe the dominant mechanisms (triggering, transport, emplacement) vary between the two atolls. We use geomorphological classification to calculate the present-day volume of the atolls and estimate the volume of the original volcanic edifices. Our analyses using multibeam bathymetry and seismic

ACCEPTED MANUSCRIPT

reflection data suggest that the atolls have contributed sediment to the nearby Kroenke Canyon, and probably continue to do so today.

5.1 Morphological evolution of Ontong Java and Nukumanu atolls Erosion appears to prevail over the upper and lower atolls slopes, which results in active shedding of

PT

sediments downslope. The distribution of fine-scale features across the surveyed area reveals that steep scarps trending parallel to bathymetric contours are common. The prevalence of scarps across the upper

RI

and lower slopes zones (Table 1) may be a result of continual erosion and evidence for a slope

SC

susceptible to mass wasting (Van Westen et al., 2003).

NU

The overall slope geometry relates to the underlying geology, and to the dominant sediment transport mechanisms (Adams & Schlager, 2000; Adams et al., 1998; Kenter, 1990). Exponential slopes (concave)

MA

are common in carbonate settings, as carbonate slopes are more prone to building steeper slopes than siliclastic settings (Adams & Kenter, 2014). The steep submarine slopes and high backscatter intensities

D

observed on both Ontong Java and Nukumanu atolls indicate that the upper flanks are predominantly

PT E

composed of hard substrate, limestone and/or volcanic rock. The angle of repose for unconsolidated sediment/talus is typically <40° (Lee et al., 1994; Mitchell et al., 2000), therefore suggesting that the upper flanks of the atolls are sediment starved and probably composed of consolidated rock rather than

CE

accumulated debris (Keating, 1998; Lee et al., 1994; Mitchell, 2003). Concave slopes such as the NE embayment reflects the decay in energy transport with distance from the source (Adams & Schlager,

AC

2000). As the slope decreases towards the atoll base at ~1,600 m water depth, unconsolidated sediment is increasingly deposited (Fig. 8c). The decrease in slope indicates a transition from an erosive regime in the upper flanks to a depositional regime towards the atoll base (Fig 7, Profile B-B’). Deviations from an exponential trend have been attributed to a change in the depositional regime or slumping (Adams et al., 1998). In the case of the NE embayment profile, the slight deviation from an exponential trend in the upper 400 m may be attributed to the influence of shallow water processes on the uppermost slopes or the presence of a lithological boundary.

ACCEPTED MANUSCRIPT

On the southern and SW flanks of Ontong Java Atoll, a fringing bulge lies consistently in depths between 1,100 – 1,400 m. The fringing bulge forms a sinuous slope profile, starkly different to the NE embayment concave profile (Fig. 3b). This unique slope profile is unlike both concave and Gaussian slope profiles described by Adams & Schlager (2000), as the steepest slope occurs close to the base of the atoll (Fig. 3b). In Hawaii, lava terraces form as lavas flow from the subaerial to the submarine environment. As hot

PT

lava enters water, it piles up and thickens, creating a steeper submarine slope due to the cooling effects of the ocean, the influence of the waves, and increased buoyancy (Mark & Moore, 1987). The convex

RI

morphology of the fringing bulge on the southern and SW flank of the Ontong Java Atoll is consistent

SC

with a model similar to that of the lava terraces around Hawaii that formed at sea level (Coulbourn et al., 1974). Subsided terraces around Hawaii are observed in water depths ≤4 km and have been

NU

interpreted to form by lava flows at sea level during active volcanism on the island (Moore, 1987). As these features form at sea level, their presence below sea level is an indication of subsidence. This depth

MA

may also represent the carbonate-volcanic transition on these atolls (Winterer & Sager, 1995). Subsidence estimates based on subsided terraces are broadly consistent with estimated total

D

subsidence values of the OJP since formation, 1,500 ± 400 m (Roberge et al.,2005). It is important to

PT E

note that total subsidence does not account for shorter wavelength subsidence or uplift (for example passing over a hotspot, or plate collision). If the fringing bulge on the southern and SW flank of Ontong Java Atoll represents subsided terraces like those in Hawaii, this suggests that the volcano antecedent to

CE

Ontong Java Atoll has subsided between 1,100 – 1,400 m.

AC

The seafloor immediately surrounding the atolls (the atoll base) is characterised by hummocky irregular seafloor, gullies, and ridges, suggesting the atoll base is prone to sediment deposition, whether by ongoing erosion, catastrophic mass wasting, or both. Sample sites FFC022 and FFC018 (Appendix 1, Fig. 1b) situated ~85 km to the west and east of the atolls, respectively, contain terrigenous mud, which may be sourced from the atolls and delivered to the OJP by downslope sediment transport.

The OJP zone is characterised by a sub-horizontal seafloor with subdued landforms, with the exception of terrace formations NE of the atolls. In general, the Cretaceous tholeiitic basalts that form the upper

ACCEPTED MANUSCRIPT

igneous crust of the OJP are overlain with calcareous biogenic sedimentary rock, sediment, and ooze (Andrews et al., 1975; Kennett et al., 1986; Kroenke et al., 1991; J. Mahoney et al., 2001; Moberly et al., 1986; Winterer & Reidel, 1971). Terraces NE of Nukumanu Atoll (Fig. 5b), may be stratigraphically controlled by the relatively uniform sedimentary layers overlying the volcanic basement of the OJP (Fig. 6b & c; Phinney et al., 1999). The relatively low backscatter intensity that characterises the OJP zone

PT

indicates low reflectivity material at the seafloor. DSDP Site 288 (Appendix 1, Fig. 1b), lying ~275 km east of the atolls, is the nearest deep stratigraphic control, bottoming in Aptian sedimentary rock 988.5 m

RI

below the seafloor (Packham & Andrews, 1975). Siliceous ooze is present at greater water depths

SC

(below the Calcite Compensation Depth, CCD), as well as terrigenous mud, pumice, and corals (Appendix 1, Fig. 1b). These data indicate that surface sediment atop the OJP (the “OJP zone”) is predominantly

NU

composed of calcareous ooze.

MA

5.1.1 Inter-atoll variations in erosion and deposition

Multibeam bathymetry, backscatter, and seismic reflection data reveal large and small-scale features

D

that indicate erosion, downslope movement of eroded sediment, and deposition and redeposition of

PT E

sediment. Both atolls have undergone substantial morphological change due to erosion and continue to contribute sediment to the OJP zone and likely to Kroenke Canyon. Both flanks (upper and lower slope zones) of Ontong Java and Nukumanu atolls are rife with gullies and scarps, with widespread undulating

CE

bedforms in water depths >1,300 m (Table 1). The size of debris deposits, the apparent mechanism of failure, and the extent to which erosion has and is contributing to the evolution of the atolls both

AC

onshore and offshore differ between the atolls.

Ontong Java Atoll has multiple large (km-scale) debris deposits in the atoll base zone, which connect to bights in the coastline (detailed in Section 4.1.1, Fig. 3a-f). Bights have long been recognised as evidence for mass wasting and major edifice failures (Fairbridge, 1950; Stoddart, 1965; Terry & Goff, 2013). The NE embayment of Ontong Java Atoll gives the atoll its distinctive boomerang shape. We attribute this embayment to either a result of an extensive flank collapse related to endogenetic factors during the

ACCEPTED MANUSCRIPT

shield-building phase of the volcano, or control by the antecedent morphology of the original volcanic cone(s) (discussed further in Section 5.2).

Slope failures C and D on Ontong Java Atoll have distinctive blocky deposits at the base of the slope, strongly suggestive of a debris avalanche in which remnant blocks have remained at the base of the

PT

slope whilst finer grained material was transported beyond the base (Fig. 3e & f). Slope failure C is also associated with a regressed atoll rim, which we again attribute to one or multiple slope failures. Rock

RI

falls and debris avalanches can occur due to detachment of a slab of rock/debris from the flank bound

SC

by either bedding planes or fractures (Hungr et al., 2014; Varnes, 1978). We cannot discern whether these slope failures are stratigraphically controlled in the context of Ontong Java Atoll; however, the

NU

near continuous headscarp along the NE flank of Ontong Java Atoll could reflect a lithological boundary

MA

lying between 400 and 700 m water depth (Section 4.1.1).

The bathymetric depressions at slope failure C (Section 4.1.1; Fig. 3e) may result from variations in the

D

hardness of neighbouring substrate or the continual downslope erosion of the atoll flanks, analogous to

PT E

the formation of plunge pools (Lee et al., 2002). Such troughs at the base of slopes may be attributed to excavation of rock less resistant to erosion at the base and/or focussing of current energy at the slope base. Similar features are observed at the base of the US continental slope; around the atolls the

CE

continual high momentum erosion from the upper slope either from recurring edifice failure and/or mass wasting may have contributed to the formation of these pothole-like depressions downslope.

AC

Downslope transport of sediment excavates the base of the slope, forming a trough, and deposition of sediment downslope forms in a rampart similar to those observed at the base of slope failure C (Lee et al., 2002).

Mass wasting of Nukumanu Atoll is more subdued than of Ontong Java Atoll, and it has not made a strong impact on Nukumanu’s rim. CSBs are common in water depths >1,300 m around Nukumanu Atoll (Table 1) indicating that submarine landslides are a major driver of its dynamic geomorphology. Farther downslope, from Nukumanu Atoll towards Kroenke Canyon, we observe terraces and step-like features

ACCEPTED MANUSCRIPT

(Fig. 5b), which we interpret as cyclic steps, similar to those described in submarine canyons and across continental slopes globally (Fildani et al., 2006; Kostic, 2011; Paull et al., 2013; Zhong et al., 2015). Cyclic steps are formed by turbidity currents that can be induced by a wide range of natural processes, including earthquakes, tsunamis, surface waves and storms, wind-driven currents, internal waves and tides, and mass wasting. Turbidity currents can transport sediment over distances >500 km (Heezen &

PT

Ewing, 1952; Piper, Cochonat, & Morrison, 1999).

RI

5.1.2 Multiple generations of erosion and deposition

SC

Although bights and regressions are associated with mass wasting, the cumulative volume of debris deposits observed in the multibeam bathymetry around Ontong Java Atoll does not suffice to account

NU

for such major regressions in the coastline. The accumulation of multiple mass wasting deposits at the base of the flanks of Ontong Java Atoll indicates that certain regions are more prone to mass wasting

MA

than others (e.g., slope failure A on the southern flank; Fig. 3c). The bight in the atoll rim upslope from slope failure A is ~11.6 km across. Slope failure A comprises a debris deposit at the foot of the slope

D

made up of multiple lobes ~4 km across with a maximum height of ~300 m above surrounding ambient

PT E

seafloor at the toe (Fig. 3c). Such sizeable bights are likely to have formed by mass wasting, analogous to formation of bight-like structures (Terry & Goff, 2013), although the bight in the atoll rim upslope from slope failure A could reflect the original morphology of the antecedent volcanic island(s). The amount of

CE

failed material identifiable on the seafloor at the foot of slope failure A’s bight is far less voluminous than the volume of the bight. Therefore, if the primary cause of the bight were slope failure, one or

AC

several much larger slope failures than those we observe in the multibeam bathymetry would have been required to generate the ~11.6 km embayment. Therefore, we would infer that the observed slope failure A deposit comprises only the most recent episode of mass wasting, with additional older deposits extending farther south beyond of the surveyed area (Fig. 3c), removed by currents, and/or overlain by posterior episodes. Alternatively, if the bight reflects antecedent volcanic island(s) morphology, the observed slope failure A deposit could account for the bulk of the volume of sediment eroded from the bight.

ACCEPTED MANUSCRIPT

At slope failure B, multiple lobes spreading laterally at the foot of the slope have formed fan-like debris deposits, also suggesting recurring mass wasting (Fig. 3d). Multiple debris deposits at the base of a large bight in the atoll rim suggest an older episode of mass wasting, and reflect a site prone to repetitive failures and slope destabilisations, similar to the Matakaoa debris flow along the New Zealand margin (Joanne et al., 2010). For slope failures A and B on Ontong Java Atoll, we believe that the arcuate

PT

headscarp, i.e., the bight in the rim of the atoll, is related to older episodes of mass wasting (Hungr et

RI

al., 2014; Terry & Goff, 2013; Varnes, 1978).

SC

The shape of the atoll perimeters combined with multibeam bathymetry reveal that multiple generations of slope failures have likely occurred. We propose that the major bights and regressions in

NU

the atoll rim formed during the shield-building phase of the volcano due to endogenetic factors, whereas slope failures and debris deposits identified in the multibeam bathymetry are more recent and

MA

likely smaller failures, which may be attributed to exogenetic factors. Evidence suggests that the onshore morphology of Ontong Java Atoll has been influenced by exogenetic factors such as

D

storms/hurricanes (Bayliss-Smith, 1988). We suggest such storms also exert a major influence on the

PT E

offshore slope geomorphology by inducing erosion. For Ontong Java and Nukumanu atolls, exogenetic sources of failure including seismicity, weather, vertical tectonics, sea level fluctuation, and instabilities associated with the substantial carbonate cap generated in shallow water (Puig et al., 2014), are likely to

CE

be the current drivers of erosion, as they can occur at any time (Keating & McGuire, 2000). Endogenetic sources of failure typically dominate during the shield-building phase of the volcano and are therefore

AC

not likely to be currently driving slope failure on either atoll. In fine, we propose that the bights and regression in the atoll rim of Ontong Java Atoll are sites of ancient of slope failures that formed due to endogenetic sources of failure and continued to evolve into the Present time under exogenetic natural pressures.

5.1.3 Evidence for recent erosion Although Nukumanu Atoll has a much less sinuous coastline and the influence of mass wasting on the atoll rim is more subdued compared to Ontong Java Atoll, CSBs provide evidence for recent erosion.

ACCEPTED MANUSCRIPT

Two debris deposits lie on the southern flank of Nukumanu Atoll, with no notable relationship to the atoll rim (Fig. 5d). The multibeam bathymetry reveals ~72% of all CSBs identified in this study are on the submarine flanks of the Nukumanu Atoll, and are present, although to a much lesser degree, on the submarine flanks of Ontong Java Atoll (Table 1; Fig. 8a & b). CSBs are present within channels and are particularly well represented on the NE flank of Nukumanu Atoll (Fig. 7a & b). Formed by multiple

PT

phases of gravity flows (Normandeau et al., 2014; Paull et al., 2010, 2011), CSBs are evidence for active erosion and sediment transport in canyons. Globally, CSBs are typically found in submarine canyons, are

RI

highly dynamic, and correlate with high sediment transport activity (~1 incidence of gravity flow per

SC

year; Clarke, 2014; Normandeau et al., 2014; Paull et al., 2010). The sediment transport mechanisms that form CSBs include slope failures and gravity flows by storm induced currents (Normandeau et al.,

NU

2014; Paull et al., 2010, 2011). Ontong Java and Nukumanu atolls lie in the path of the vigorous South Equatorial Current, and are also affected by high latitude storms and extreme weather causing high

MA

waves and contributing to coastal erosion and inundation (Hoeke et al., 2013; Smithers & Hoeke, 2014). Land-based and near-shore observations of Ontong Java Atoll have shown that hurricanes and storms in

D

the past have strongly influenced sediment distribution (Bayliss-Smith, 1988). Waves generated by high

PT E

latitude storms can cause shoreline erosion and flooding on low latitude islands (Smithers & Hoeke, 2014), and possibly induce the formation of CSBs offshore. Widespread CSBs on the submarine flanks of Nukumanu Atoll suggests that sediment transport mechanisms are actively funnelling sediment from

CE

the atolls downslope, and that frequent gravity flows sustain them (Fig. 7a & b).

AC

5.2 Original volcanic edifice and estimated eroded volume Ontong Java and Nukumanu atolls are the most likely sources of sediment to erode Kroenke Canyon. To approximate the volume of volcaniclastic material that has emanated from Ontong Java and Nukumanu atolls, we estimate the volume of their original volcanic foundations. Globally, seamounts have truncated cone morphology and are sub-circular/elliptical in plan view (Kim & Wessel, 2011; Wessel, 2001). As submarine volcanoes grow to rise above sea level and become volcanic islands, flank slopes decrease in gradient, e.g., Hawaii (Moore, 1964). Although volcanic islands typically have a much more complex structure than seamounts (Mitchell, 2001), to estimate the volume of the antecedent volcanic

ACCEPTED MANUSCRIPT

islands and therefore volcaniclastic material lost from the atolls we use the simplest geometry of a truncated cone. To calculate the volume of a truncated cone, we estimate the original volcanic cone basal radius using the 1,300 m bathymetric contour, as atoll flank slope changes markedly at this depth. We determine the top radius and the height of the truncated cone relative to the basal radius according to the generalised truncated cone formula (Kim & Wessel, 2011; Wessel, 2001). From these parameters,

PT

we calculate the volume of the original cone (Table 2; Fig. 9a). The present-day volume for both atolls was determined with the same basal depth (1,300 m) using the surface volume tool in ArcGIS. The basal

RI

radius of the volcanic edifice for Nukumanu Atoll is ~12 km, which would have formed a cone ~3 km

SC

above the basal depth of 1,300 m, or ~1.7 km above current sea level (Table 2; Fig. 9b). The present-day 3

thickness of the carbonate cap is unknown.

NU

volume of Nukumanu Atoll is ~433 km ; however, this likely overestimates the volcanic volume as the

MA

Estimating the original size of Ontong Java Atoll volcanic edifice is more complex than for Nukumanu Atoll due to the former’s irregular shape. We calculated the total eroded volume of Ontong Java Atoll

D

using two scenarios to account for the boomerang shape: 1) the El Hierro (Canary Islands) scenario (Fig.

PT E

9c), where we assume a single large cone with one or many edifice failure(s), and 2) the Isabela Island (Galapagos) scenario (Fig. 9d), where we assume three smaller volcanic edifices amalgamated to form

CE

one island.

El Hierro in the Canary Islands, has a distinct boomerang morphology that resembles Ontong Java Atoll

AC

(Fig. 10a & b), and is attributed to substantial endogenetically driven flank collapses and three rift zones that extend from the centre of the volcanic island (Fig. 10a; Gee et al., 2001; Krastel et al., 2001; Masson 3

et al., 2002). A major debris avalanche (El Golfo, volume 150-180 km and extent 65 km) on the NW flank has had the most influence on the onshore morphology of the El Hierro, creating an arcuate embayment and giving the island a shape analogous to Ontong Java Atoll (Fig. 10b). If similar processes formed the Ontong Java Atoll NE embayment, we would expect mass wasting greater than twice the size of El Golfo, given the size of the embayment (11 km compared with 39 km) and the relative size of the 2

Ontong Java Atoll compared to El Hierro (area of Ontong Java Atoll including the lagoon is ~1,480 km vs.

ACCEPTED MANUSCRIPT

2

278 km for El Hierro). Similar to El Hierro, such an extensive edifice failure may have been guided by endogenetic factors such as faults and rift-related activity, causing instabilities on the flanks of the original volcanic island (Masson et al., 2006). In the El Hierro scenario, the reconstructed basal radius of Ontong Java Atoll is ~33 km forming a truncated cone that would have risen >7 km above ambient

PT

seafloor, or ~6 km above current sea level (Fig. 9c; Table 2).

Alternatively, Ontong Java Atoll’s boomerang shape may be attributed to the antecedent morphology of

RI

the ancient volcanic islands. In this case, the boomerang shape may be related to the formation of one

SC

island by the amalgamation of multiple volcanic edifices. An analogue is Isabela Island in the Galapagos, where the unusual coastline results from six individual shield volcanoes (Fig. 10c). If the shape of Ontong

NU

Java Atoll is attributed to the amalgamation of three volcanic cones, it is still likely to have experienced edifice failure(s) to form such a sinuous coastline with widespread bight-like structures, although to a

MA

smaller degree. Three distinct vertical gravity gradient anomalies over Ontong Java Atoll (Kim & Wessel, 2011; Fig 10b; anomalies: KW-18768, KW-18771, KW-18770) could be three volcanic cones that are now

D

buried beneath a carbonate cap and central lagoon. In the Isabela scenario, the location of each cone is

PT E

based on the distribution of Ontong Java Atoll gravity anomalies (Kim & Wessel, 2011; Fig 10b). The reconstructed basal radius for each cone is similar to the size of the estimated original Nukumanu volcanic edifice (Table 2). These individual cones would have risen ≤4 km above ambient seafloor, or ≤3

CE

km above current sea level (Fig. 9d).

AC

Table 2 – estimated height, radii, and volumes for the original volcanic edifices (rounded to nearest whole number), calculated present day volumes, and estimated minimum eroded material for Ontong Java and Nukumanu atolls Nukumanu

Ontong Java Atoll

Ontong Java Atoll

Atoll

(El Hierro scenario)

(Isabela Island scenario)

12

33

12, 17, and 17

Original basal radius of volcanic

ACCEPTED MANUSCRIPT

edifice (km) Original truncated cone

3

7

571

11706

3, 4, and 4

height (km) Original volume

625, 1,510, and 1,564

3

Total = 3,699

PT

(km )

3

433

2331

138

9,384

24

MA

Present day

2331

Estimated eroded volume of

1,378

NU

volcaniclastics

SC

RI

volume (km )

3

(km ) Minimum eroded material

80

PT E

D

(%)

37

In both scenarios, the estimated total volume of eroded material from Ontong Java Atoll is an order of magnitude larger than that from Nukumanu Atoll. The El Hierro scenario implies that less than 20% of

CE

the original Ontong Java volcanic edifice remains today. Although it is possible that Ontong Java Atoll

AC

has lost 80% of the original volcano, if this is the case, we suggest the cause of the edifice failure was endogenetic due to the vast difference in eroded volume between Nukumanu Atoll and Ontong Java Atoll. The Isabela Island scenario has an erosion ratio more comparable to Nukumanu Atoll. The estimated size of the original volcanic cone, the calculated eroded volume for each scenario, and the distribution of gravity anomalies across Ontong Java Atoll lead us to believe the Isabela Island scenario is more plausible for Ontong Java Atoll. Therefore, the total volcaniclastic material eroded from the flanks of both atolls and shed in all directions, some of which was likely transported downslope feeding 3

Kroenke Canyon, is estimated to be ~1,500 km .

ACCEPTED MANUSCRIPT

5.3 Atoll erosion and canyon activity Submarine canyon activity may rely on a proximal source of sediments and adequate sediment transport mechanisms (e.g., ocean currents or mass-wasting; Normandeau et al., 2014; Paull et al., 2011; Puig et al., 2014). Alternatively, ‘blind’ or ‘headless’ canyons form by fluid escape independent of

PT

downslope erosive flows (Orange & Breen, 1992; Sultan et al., 2007). Multibeam coverage of the uppermost reaches of Kroenke Canyon is sparse (Fig. 11a); however, one swath of multibeam NE of

RI

Nukumanu Atoll indicates continuation of the canyon to the east of the atolls (Fig. 11a & b), which is

SC

corroborated by satellite-derived bathymetry (Becker et al., 2009; Sandwell et al., 2014). Seismic reflection data also document Kroenke Canyon to the east of Nukumanu Atoll (Fig. 6a & b). Kroenke

NU

Canyon appears to originate SE of Nukumanu Atoll and NE of Ontong Java Atoll; these relatively small atolls constitute the only proximal sediment sources. The close proximity of the atolls implies that

MA

downslope erosive flows have played a role in the development of Kroenke Canyon.

D

Our bathymetric analysis and seismic reflection interpretation shows that the atolls have been, and

PT E

probably still are actively shedding sediments downslope to Kroenke Canyon. We interpret the series of linear gullies east of Nukumanu Atoll, as well as those farther downslope observed in seismic reflection profile III-IV (Fig. 6c) to be drainage channels funnelling sediment from the atoll toward the canyon

CE

(Fig. 11b). Drainage channels such as these can form by bottom-up erosion and mass wasting, due to the aggradation and flux of sediment from upslope or by hydrodynamically driven density currents (Canals

AC

et al., 2006; Micallef & Mountjoy, 2011; Mountjoy & Micallef, 2012). The drainage channels to the east and NE of Nukumanu Atoll are v-shaped in cross-section, typical of gullies that feed submarine canyons (Fig 6c; Lonergan et al., 2013). In addition, we observe CSBs produced by recent mass wasting on the flank of Nukumanu Atoll facing Kroenke Canyon (Fig. 7a & b). That Kroenke Canyon formed so far from a major subaerial sediment source raises significant questions about its evolution. However, our data and analyses suggest a proximal source of sediment, albeit the relatively small atolls. Mass wasting around the atolls implies mechanisms driving significant downslope sediment transport.

ACCEPTED MANUSCRIPT

6. Conclusions Recently-collected, multibeam bathymetry and backscatter data (FK141015), combined with legacy seismic reflection data, have revealed for the first time the submarine landscape surrounding Ontong Java and Nukumanu atolls, located atop the OJP. We present broad and fine scale classification of the mapped region surrounding the atolls using manual qualitative classification in ArcGIS. Seafloor

PT

classification shows that submarine erosion and deposition is widespread (albeit quite different) on Ontong Java and Nukumanu atolls. The sinuous coastline of Ontong Java Atoll has widespread bights

RI

that appear to be linked to submarine debris deposits, whereas for Nukumanu Atoll the link between

SC

onshore and offshore geomorphology is less obvious. Classification has facilitated the identification of regions with debris deposits preserved at the base of the atoll slope, which suggest certain regions have

NU

been more prone to slope failure. The overall boomerang coastline of Ontong Java Atoll could result from one or more extensive edifice failure(s), or from multiple smaller cones amalgamating to form one

MA

island. The evidence presented in this study favours the latter scenario, analogous to Isabela Island in the Galapagos. Future geologic sampling and seismic reflection data acquisition of the atolls themselves

D

and downslope of Ontong Java Atoll could verify or refute the existence of mass wasting deposits

PT E

further associated with the proposed edifice failures and provide geochemical constraints for volcanism. The multibeam and seismic reflection data show that the submarine environment surrounding Nukumanu Atoll comprises a range of features including crescent shaped bedforms, scarps, channels,

CE

terraces, a sea knoll of likely volcanic origin, and the apparent uppermost reaches of Kroenke Canyon. Although the head of Kroenke Canyon remains to be surveyed, a combination of multibeam bathymetry,

AC

seismic reflection, and satellite-derived bathymetry data presented in this study provides evidence that sediments originating from Nukumanu Atoll are actively feeding Kroenke Canyon.

Acknowledgements We thank Master Philipp Guenther and his able crew for their vital contributions to the successful outcome of R/V Falkor voyage FK141015. We are especially grateful to Cruise Coordinator/Lead Marine Technician Colleen Peters and Marine Technician Paul ‘Jimbo’ Duncan for keeping the multibeam

ACCEPTED MANUSCRIPT

systems humming and educating the science party. We acknowledge the efforts of the science party in acquiring and initially processing the shipboard data: M.F. Coffin, N. Adams, M. Heckman, T. Ketter, J. Neale, A. Reyes, and A. Travers. We are grateful to L.W. Kroenke for providing original access to the R/V Mahi seismic reflection data, and to P. Wessel and B. Taylor for relevant technical advice. We thank M. Rebesco, N.C. Mitchell, and an anonymous reviewer for suggestions and comments that greatly

PT

improved this manuscript. We thank the Schmidt Ocean Institute for supporting the voyage. S.J.W. was supported by an Australian Government Research Training Program Scholarship. J.M.W. was supported

SC

RI

by ARC grant DE140100376 and DP150102887.

References

NU

Adams, E. W., & Kenter, J. A. (2014). So different, yet so similar: comparing and contrasting siliciclastic and carbonate slopes. Deposits, Architecture and Controls of Carbonate Margin, Slope and

MA

Basinal Settings. SEPM Special Publication, 105, 14–25.

Adams, E. W., & Schlager, W. (2000). Basic types of submarine slope curvature. Journal of Sedimentary

D

Research, 70(4), 814–828.

PT E

Adams, E. W., Schlager, W., & Wattel, E. (1998). Submarine slopes with an exponential curvature. Sedimentary Geology, 117(3), 135–141. Andrews, J. E., Packham, G., Eade, J. V., Holdsworth, B. K., Jones, D., deVries Klein, G., … van der Lingen,

CE

G. J. (1975). Site 289 (Principle Results). Deep Sea Drilling Project. Retrieved from http://www.deepseadrilling.org/30/volume/dsdp30_07.pdf

AC

Bayliss-Smith, T. P. (1988). The role of hurricanes in the development of reef islands, Ontong Java Atoll, Solomon Islands. Geographical Journal, 377–391. Becker, J. J., Sandwell, D. T., Smith, W. H. F., Braud, J., Binder, B., Depner, J., … others. (2009). Global bathymetry and elevation data at 30 arc seconds resolution: SRTM30_PLUS. Marine Geodesy, 32(4), 355–371. Berger, W. H., Kroenke, L. W., Mayer, L. A., & Shipboard Scientific Party. (1991). Ontong Java plateau, Leg 130: synopsis of major drilling results. (Proc. ODP, Init. Repts.,130: College Station, TX

ACCEPTED MANUSCRIPT

(Ocean Drilling Program), 497–537). Retrieved from http://wwwodp.tamu.edu/publications/130_IR/VOLUME/CHAPTERS/ir130_10.pdf Berger, W. H., & Stax, R. (1994). Neogene carbonate stratigraphy of Ontong Java Plateau (western equatorial Pacific): Three unexpected findings. Terra Nova, 6(5), 520–534. Canals, M., Puig, P., de Madron, X. D., Heussner, S., Palanques, A., & Fabres, J. (2006). Flushing

PT

submarine canyons. Nature, 444(7117), 354–357. Clarke, J. E. H., Marques, C. R. V., & Pratomo, D. (2014). Imaging active mass-wasting and sediment flows

SC

Consequences (pp. 249–260). Springer. Retrieved from

RI

on a fjord delta, Squamish, British Columbia. In Submarine Mass Movements and Their

http://link.springer.com/chapter/10.1007/978-3-319-00972-8_22

NU

Cloos, M. (1993). Lithospheric buoyancy and collisional orogenesis: Subduction of oceanic plateaus, continental margins, island arcs, spreading ridges, and seamounts. Geological Society of

MA

America Bulletin, 105(6), 715–737.

Coffin, M. F., Adams, N., Heckman, M., Ketter, T., Lucieer, V., Neale, J., … Whittaker, J. (2015a).

D

Deciphering Ontong Java Atoll, Nukumanu Atoll, and Kroenke Canyon, Western Equatorial

PT E

Pacific Final Project Report for RV Falkor FK141015 15 October–3 November 2014. Retrieved from http://schmidtocean.org/wpcontent/uploads/fk141015_ontong_java_final_project_report.pdf

CE

Coffin, M. F., Inoue, H., Mochizuki, K., Nakamura, Y., & Kroenke, L. (2008). Tertiary Magmatism on the Early Cretaceous Ontong Java Plateau. In AGU Fall Meeting Abstracts. Retrieved from

AC

http://adsabs.harvard.edu/abs/2008AGUFM.V23H..06C Conrad, O., Bechtel, B., Bock, M., Dietrich, H., Fischer, E., Gerlitz, L., … Böhner, J. (2015). System for Automated Geoscientific Analyses (SAGA) v. 2.1.4. Geosci. Model Dev., 8(7), 1991–2007. https://doi.org/10.5194/gmd-8-1991-2015 Coulbourn, W. T., Campbell, J. F., & Moberly, R. (1974). Hawaiian submarine terraces, canyons, and Quaternary history evaluated by seismic-reflection profiling. Marine Geology, 17(4), 215–234.

ACCEPTED MANUSCRIPT

Darwin, C. (1842). On the Structure and Distribution of Coral Reefs: Being the First Part of the Geology of the Voyage of the Beagle Under the Command of Captain Fitzroy, RN During the Years 1832 to 1836. Smith, Elder. Druckman, Y., Buchbinder, B., Martinotti, G. M., Tov, R. S., & Aharon, P. (1995). The buried Afiq Canyon (eastern Mediterranean, Israel): a case study of a Tertiary submarine canyon exposed in Late

PT

Messinian times. Marine Geology, 123(3–4), 167–185. Fairbridge, R. W. (1950). Landslide patterns on oceanic volcanoes and atolls. Geographical Journal, 84–

RI

88.

SC

Fildani, A., Normark, W. R., Kostic, S., & Parker, G. (2006). Channel formation by flow stripping: Large-

Sedimentology, 53(6), 1265–1287.

NU

scale scour features along the Monterey East Channel and their relation to sediment waves.

Fitton, J. G., Mahoney, J. J., Wallace, P. J., & Saunders, A. D. (2004). Origin and evolution of the Ontong

MA

Java Plateau: introduction. Geological Society, London, Special Publications, 229(1), 1–8. Furumoto, A. S., Webb, J. P., Odegard, M. E., & Hussong, D. M. (1976). Seismic studies on the Ontong

D

Java plateau, 1970. Tectonophysics, 34(1–2), 71–90.

PT E

Galloway, W. E. (1989). Genetic stratigraphic sequences in basin analysis I: architecture and genesis of flooding-surface bounded depositional units. AAPG Bulletin, 73(2), 125–142. Gee, M. J., Watts, A. B., Masson, D. G., & Mitchell, N. C. (2001). Landslides and the evolution of El Hierro

CE

in the Canary Islands. Marine Geology, 177(3), 271–293. Gladczenko, T. P., Coffin, M. F., & Eldholm, O. (1997). Crustal structure of the Ontong Java Plateau:

AC

modeling of new gravity and existing seismic data. Journal of Geophysical Research: Solid Earth, 102(B10), 22711–22729. Harris, P. T., & Whiteway, T. (2011). Global distribution of large submarine canyons: Geomorphic differences between active and passive continental margins. Marine Geology, 285(1), 69–86. Heezen, B. C., & Ewing, M. (1952). Turbidity currents and submarine slumps, and the 1929 Grand Banks earthquake. American Journal of Science, 250(12), 849–873.

ACCEPTED MANUSCRIPT

Hoeke, R. K., McInnes, K. L., Kruger, J. C., McNaught, R. J., Hunter, J. R., & Smithers, S. G. (2013). Widespread inundation of Pacific islands triggered by distant-source wind-waves. Global and Planetary Change, 108, 128–138. Hungr, O., Leroueil, S., & Picarelli, L. (2014). The Varnes classification of landslide types, an update. Landslides, 11(2), 167–194.

PT

Inoue, H., Coffin, M. F., Nakamura, Y., Mochizuki, K., & Kroenke, L. W. (2008). Intrabasement reflections of the Ontong Java Plateau: Implications for plateau construction. Geochemistry, Geophysics,

RI

Geosystems, 9(4). Retrieved from

SC

http://onlinelibrary.wiley.com/doi/10.1029/2007GC001780/full

Jackson, D. R., & Briggs, K. B. (1992). High-frequency bottom backscattering: Roughness versus sediment

NU

volume scattering. The Journal of the Acoustical Society of America, 92(2), 962–977. Jenness, J. (2013). DEM Surface Tools for ArcGIS (Version 2.1.399). Jenness Enterprises. Retrieved from

MA

http://www.jennessent.com/arcgis/surface_area.htm

Joanne, C., Collot, J.-Y., Lamarche, G., & Migeon, S. (2010). Continental slope reconstruction after a giant

D

mass failure, the example of the Matakaoa Margin, New Zealand. Marine Geology, 268(1), 67–

PT E

84.

Keating, B. H. (1998). Side-scan sonar images of submarine landslides on the flanks of atolls and guyots. Marine Geodesy, 21(2), 129–145.

CE

Keating, B. H., & McGuire, W. J. (2000). Island edifice failures and associated tsunami hazards. Pure and Applied Geophysics, 157(6–8), 899–955.

AC

Kennett, J. P., von der Borch, C. C., Baker, P. A., Barton, C. E., Boersma, A., Caulet, J. P., … Takeuchi, A. (1986). Initial Reports of Deep Sea Drilling Project (DSDP) Leg 90 (Initial Report No. DSDP Leg 90). Washington (U.S. Govt. Printing Office). Retrieved from http://www.deepseadrilling.org/90/volume/dsdp90.pdf Kenter, J. A. (1990). Carbonate platform flanks: slope angle and sediment fabric. Sedimentology, 37(5), 777–794. Kim, S.-S., & Wessel, P. (2011). New global seamount census from altimetry-derived gravity data. Geophysical Journal International, 186(2), 615–631.

ACCEPTED MANUSCRIPT

Korenaga, J. (2005). Why did not the Ontong Java Plateau form subaerially? Earth and Planetary Science Letters, 234(3), 385–399. Korenaga, J. (2011). Velocity–depth ambiguity and the seismic structure of large igneous provinces: a case study from the Ontong Java Plateau. Geophysical Journal International, 185(2), 1022– 1036.

PT

Kostic, S. (2011). Modeling of submarine cyclic steps: Controls on their formation, migration, and architecture. Geosphere, 7(2), 294–304.

RI

Krastel, S., Schmincke, H.-U., Jacobs, C. L., Rihm, R., Le Bas, T. P., & Alibés, B. (2001). Submarine

SC

landslides around the Canary Islands. Journal of Geophysical Research: Solid Earth (1978–2012), 106(B3), 3977–3997.

NU

Kroenke, L. W., Berger, W. H., Janecek, T. R., & Shipboard Scientific Party. (1991). Proc. ODP, Init. Repts., 130 (No. 130). College Station, TX (Ocean Drilling Program). Retrieved from http://www-

MA

odp.tamu.edu/publications/citations/cite130.html

Kroenke, L. W., Wessel, P., & Sterling, A. (2004). Motion of the Ontong Java Plateau in the hot-spot

D

frame of reference: 122 Ma-present. Geological Society, London, Special Publications, 229(1),

PT E

9–20.

Lee, H. J., Torresan, M. E., & McArthur, W. (1994). Stability of submerged slopes on the flanks of the Hawaiian Islands, a simplified approach. Geological Survey, Menlo Park, CA (United States).

CE

Retrieved from http://www.osti.gov/scitech/biblio/90387 Lee, S. E., Talling, P. J., Ernst, G. G., & Hogg, A. J. (2002). Occurrence and origin of submarine plunge

AC

pools at the base of the US continental slope. Marine Geology, 185(3), 363–377. Lonergan, L., Jamin, N. H., Jackson, C. A.-L., & Johnson, H. D. (2013). U-shaped slope gully systems and sediment waves on the passive margin of Gabon (West Africa). Marine Geology, 337, 80–97. Lucieer, V., & Lamarche, G. (2011). Unsupervised fuzzy classification and object-based image analysis of multibeam data to map deep water substrates, Cook Strait, New Zealand. Continental Shelf Research, 31(11), 1236–1247. Lundblad, E. R., Wright, D. J., Miller, J., Larkin, E. M., Rinehart, R., Naar, D. F., … Battista, T. (2006). A benthic terrain classification scheme for American Samoa. Marine Geodesy, 29(2), 89–111.

ACCEPTED MANUSCRIPT

Lurton, X. (2010). An introduction to underwater acoustics: principles and applications. Berlin: Springer,[2010]. Retrieved from http://library.canterbury.ac.nz/webapps/public/newtitles.php?type=subject&days=14&subject s_out=Physics&order=location Mahoney, J. J., Fitton, J. G., Wallace, P. J., & Shipboard Scientific Party. (2001a). Leg 192 Initial Report

http://www-odp.tamu.edu/publications/192_IR/192TOC.HTM

PT

(Proc. ODP, Init. Repts 192). College Station, TX (Ocean Drilling Program). Retrieved from

RI

Mahoney, J. J., Fitton, J. G., Wallace, P. J., & Shipboard Scientific Party. (2001b). Site 1183 (Proceedings

SC

of the Ocean Drilling Program). College Station, TX (Ocean Drilling Program). Retrieved from http://www-odp.tamu.edu/publications/192_IR/VOLUME/CHAPTERS/IR192_03.PDF

NU

Mahoney, J. J., Storey, M., Duncan, R. A., Spencer, K. J., & Pringle, M. (1993). Geochemistry and age of the Ontong Java Plateau. The Mesozoic Pacific: Geology, Tectonics, and Volcanism, 233–261.

MA

Mahoney, J., Wallace, P. J., & Bridges, B. (2001). Basement Drilling of the Ontong Java Plateau: Covering Leg 192 of the Cruises of the Drilling Vessel JOIDES Resolution, Apra Harbor, Guam, to Apra

D

Harbor, Guam, Sites 1183-1187, 8 September-7 November 2000. Texas A & M University.

PT E

Mann, P., & Taira, A. (2004). Global tectonic significance of the Solomon Islands and Ontong Java Plateau convergent zone. Tectonophysics, 389(3), 137–190. Mark, R. K., & Moore, J. G. (1987). Slopes of the Hawaiian ridge. US Geol. Surv. Prof. Pap, 1350, 101–107.

CE

Masson, D. G., Harbitz, C. B., Wynn, R. B., Pedersen, G., & Løvholt, F. (2006). Submarine landslides: processes, triggers and hazard prediction. Philosophical Transactions of the Royal Society of

AC

London A: Mathematical, Physical and Engineering Sciences, 364(1845), 2009–2039. Masson, D. G., Watts, A. B., Gee, M. J. R., Urgeles, R., Mitchell, N. C., Le Bas, T. P., & Canals, M. (2002). Slope failures on the flanks of the western Canary Islands. Earth-Science Reviews, 57(1), 1–35. Masson, D. G., Wynn, R. B., & Talling, P. J. (2010). Large landslides on passive continental margins: processes, hypotheses and outstanding questions. In Submarine mass movements and their consequences (pp. 153–165). Springer. Retrieved from http://link.springer.com/10.1007/97890-481-3071-9_13

ACCEPTED MANUSCRIPT

McGonigle, C., & Collier, J. S. (2014). Interlinking backscatter, grain size and benthic community structure. Estuarine, Coastal and Shelf Science, 147, 123–136. Micallef, A., & Mountjoy, J. J. (2011). A topographic signature of a hydrodynamic origin for submarine gullies. Geology, 39(2), 115–118. Mitchell, N. C. (2001). Transition from circular to stellate forms of submarine volcanoes. Journal of

PT

Geophysical Research: Solid Earth, 106(B2), 1987–2003. Mitchell, N. C. (2003). Susceptibility of mid-ocean ridge volcanic islands and seamounts to large-scale

RI

landsliding. Journal of Geophysical Research: Solid Earth (1978–2012), 108(B8). Retrieved from

SC

http://onlinelibrary.wiley.com.ezproxy1.library.usyd.edu.au/doi/10.1029/2002JB001997/pdf Mitchell, N. C., Tivey, M. A., & Gente, P. (2000). Seafloor slopes at mid-ocean ridges from submersible

NU

observations and implications for interpreting geology from seafloor topography. Earth and Planetary Science Letters, 183(3), 543–555.

MA

Mitchum Jr, R. M., Vail, P. R., & Thompson III, S. (1977). Seismic stratigraphy and global changes of sea level: Part 2. The depositional sequence as a basic unit for stratigraphic analysis: Section 2.

D

Application of seismic reflection configuration to stratigraphic interpretation. Retrieved from

PT E

http://archives.datapages.com/data/specpubs/seismic1/data/a165/a165/0001/0050/0053.htm Moberly, R., Schlanger, S. O., & Shipboard Scientific Party. (1986). Site 586 (Principle Results). Deep Sea Drilling Project. Retrieved from http://www.deepseadrilling.org/89/volume/dsdp89_04.pdf

CE

Moore, J. G. (1964). Giant submarine landslides on the Hawaiian Ridge. US Geological Survey Professional Paper, 501, 95–98.

AC

Moore, J. G. (1987). Subsidence of the Hawaiian ridge. US Geol. Surv. Prof. Pap, 1350(1), 85–100. Mosher, D., Mayer, L. A., Shipley, T. H., Winterer, E. L., Hagen, R. A., Marsters, J. C., … Lyle, M. (1993). Seismic stratigraphy of the Ontong Java Plateau. Retrieved from http://scholars.unh.edu/ccom_affil/23/?utm_source=scholars.unh.edu%2Fccom_affil%2F23&u tm_medium=PDF&utm_campaign=PDFCoverPages Mountjoy, J. J., & Micallef, A. (2012). Polyphase emplacement of a 30 km3 blocky debris avalanche and its role in slope-gully development. In Submarine Mass Movements and Their Consequences

ACCEPTED MANUSCRIPT

(pp. 213–222). Springer. Retrieved from http://link.springer.com/chapter/10.1007/978-94-0072162-3_19 Mulder, T., Zaragosi, S., Garlan, T., Mavel, J., Cremer, M., Sottolichio, A., … Schmidt, S. (2012). Present deep-submarine canyons activity in the Bay of Biscay (NE Atlantic). Marine Geology, 295, 113– 127.

PT

Neal, C. R., Mahoney, J. J., Kroenke, L. W., Duncan, R. A., & Petterson, M. G. (1997). The Ontong Java Plateau. Large Igneous Provinces: Continental, Oceanic, and Planetary Flood Volcanism, 183–

RI

216.

SC

Normandeau, A., Lajeunesse, P., St-Onge, G., Bourgault, D., Drouin, S. S.-O., Senneville, S., & Bélanger, S. (2014). Morphodynamics in sediment-starved inner-shelf submarine canyons (Lower St.

NU

Lawrence Estuary, Eastern Canada). Marine Geology, 357, 243–255. Orange, D. L., & Breen, N. A. (1992). The effects of fluid escape on accretionary wedges 2. Seepage

Solid Earth, 97(B6), 9277–9295.

MA

force, slope failure, headless submarine canyons, and vents. Journal of Geophysical Research:

D

Orpin, A. R., Rickard, G. J., Gerring, P. K., & Lamarche, G. (2015). Tsunami hazard potential for the

PT E

equatorial southwestern Pacific atolls of Tokelau from scenario-based simulations. Natural Hazards and Earth System Sciences Discussions, 3, 4391–4433. Packham, G., & Andrews, J. E. (1975). Principal Results: Leg 30 Deep Sea Drilling Project (No. 30).

CE

Retrieved from http://www.deepseadrilling.org/30/dsdp_toc.htm Paull, C. K., Caress, D. W., Lundsten, E., Gwiazda, R., Anderson, K., McGann, M., … Sumner, E. J. (2013).

AC

Anatomy of the La Jolla submarine canyon system; offshore Southern California. Marine Geology, 335, 16–34. Paull, C. K., Caress, D. W., Ussler, W., Lundsten, E., & Meiner-Johnson, M. (2011). High-resolution bathymetry of the axial channels within Monterey and Soquel submarine canyons, offshore central California. Geosphere, 7(5), 1077–1101. Paull, C. K., Ussler III, W., Caress, D. W., Lundsten, E., Covault, J. A., Maier, K. L., … Augenstein, S. (2010). Origins of large crescent-shaped bedforms within the axial channel of Monterey Canyon, offshore California. Geosphere, 6(6), 755–774.

ACCEPTED MANUSCRIPT

Petterson, M. G., Neal, C. R., Mahoney, J. J., Kroenke, L. W., Saunders, A. D., Babbs, T. L., … McGrail, B. (1997). Structure and deformation of north and central Malaita, Solomon Islands: tectonic implications for the Ontong Java Plateau-Solomon arc collision, and for the fate of oceanic plateaus. Tectonophysics, 283(1), 1–33. Phinney, E. J., Mann, P., Coffin, M. F., & Shipley, T. H. (1999). Sequence stratigraphy, structure, and

PT

tectonic history of the southwestern Ontong Java Plateau adjacent to the North Solomon Trench and Solomon Islands arc. Journal of Geophysical Research: Solid Earth, 104(B9), 20449–

RI

20466.

SC

Phinney, E. J., Mann, P., Coffin, M. F., & Shipley, T. H. (2004). Sequence stratigraphy, structural style, and age of deformation of the Malaita accretionary prism (Solomon arc–Ontong Java Plateau

NU

convergent zone). Tectonophysics, 389(3), 221–246.

Piper, D. J., Cochonat, P., & Morrison, M. L. (1999). The sequence of events around the epicentre of the

MA

1929 Grand Banks earthquake: initiation of debris flows and turbidity current inferred from sidescan sonar. Sedimentology, 46(1), 79–97.

D

Pope, E. L., Talling, P. J., Carter, L., Clare, M. A., & Hunt, J. E. (2017). Damaging sediment density flows

PT E

triggered by tropical cyclones. Earth and Planetary Science Letters, 458, 161–169. Pratson, L. F., & Coakley, B. J. (1996). A model for the headward erosion of submarine canyons induced by downslope-eroding sediment flows. Geological Society of America Bulletin, 108(2), 225–234.

CE

Prior, D. B., Suhayda, J. N., Lu, N.-Z., Bornhold, B. D., Keller, G. H., Wiseman, W. J., … Yang, Z.-S. (1989). Storm wave reactivation of a submarine landslide. Nature, 341(6237), 47–50.

AC

Puig, P., Ogston, A. S., Mullenbach, B. L., Nittrouer, C. A., Parsons, J. D., & Sternberg, R. W. (2004). Storm-induced sediment gravity flows at the head of the Eel submarine canyon, northern California margin. Journal of Geophysical Research: Oceans, 109(C3). Retrieved from http://onlinelibrary.wiley.com/doi/10.1029/2003JC001918/full Puig, P., Palanques, A., & Martín, J. (2014). Contemporary sediment-transport processes in submarine canyons. Annual Review of Marine Science, 6, 53–77. Richardson, W. P., Okal, E. A., & Van der Lee, S. (2000). Rayleigh–wave tomography of the Ontong–Java Plateau. Physics of the Earth and Planetary Interiors, 118(1), 29–51.

ACCEPTED MANUSCRIPT

Roberge, J., Wallace, P. J., White, R. V., & Coffin, M. F. (2005). Anomalous uplift and subsidence of the Ontong Java Plateau inferred from CO2 contents of submarine basaltic glasses. Geology, 33(6), 501–504. Sandwell, D. T., Müller, R. D., Smith, W. H. F., Garcia, E., & Francis, R. (2014). New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure. Science, 346(6205), 65–67.

PT

https://doi.org/10.1126/science.1258213 Selby, M. J., & others. (1982). Hillslope materials and processes. Hillslope Materials and Processes.

RI

Retrieved from https://www.cabdirect.org/cabdirect/abstract/19841986659

SC

Smithers, S. G., & Hoeke, R. K. (2014). Geomorphological impacts of high-latitude storm waves on lowlatitude reef islands—Observations of the December 2008 event on Nukutoa, Takuu, Papua

NU

New Guinea. Geomorphology, 222, 106–121.

Stoddart, D. R. (1965). The shape of atolls. Marine Geology, 3(5), 369–383.

MA

Sultan, N., Gaudin, M., Berne, S., Canals, M., Urgeles, R., & Lafuerza, S. (2007). Analysis of slope failures in submarine canyon heads: an example from the Gulf of Lions. Journal of Geophysical

D

Research: Earth Surface, 112(F1). Retrieved from

PT E

http://onlinelibrary.wiley.com/doi/10.1029/2005JF000408/full Surpless, K. D., Ward, R. B., & Graham, S. A. (2009). Evolution and stratigraphic architecture of marine slope gully complexes: Monterey Formation (Miocene), Gaviota Beach, California. Marine and

CE

Petroleum Geology, 26(2), 269–288. Terry, J. P., & Goff, J. (2013). One hundred and thirty years since Darwin:‘Reshaping’the theory of atoll

AC

formation. The Holocene, 23(4), 615–619. Tharimena, S., Rychert, C. A., & Harmon, N. (2016). Seismic imaging of a mid-lithospheric discontinuity beneath Ontong Java Plateau. Earth and Planetary Science Letters, 450, 62–70. Van Westen, C. J., Rengers, N., & Soeters, R. (2003). Use of geomorphological information in indirect landslide susceptibility assessment. Natural Hazards, 30(3), 399–419. Varnes, D. J. (1978). Slope movement types and processes. Special Report, 176, 11–33. Verfaillie, E., Doornenbal, P., Mitchell, A. J., White, J., & Van Lancker, V. (2007). The bathymetric position index (BPI) as a support tool for habitat mapping. Retrieved November, 20, 2011.

ACCEPTED MANUSCRIPT

Wessel, P. (2001). Global distribution of seamounts inferred from gridded Geosat/ERS-1 altimetry. Journal of Geophysical Research: Solid Earth, 106(B9), 19431–19441. Wessel, P., & Kroenke, L. W. (2008). Pacific absolute plate motion since 145 Ma: An assessment of the fixed hot spot hypothesis. Journal of Geophysical Research: Solid Earth, 113(B6). Retrieved from http://onlinelibrary.wiley.com/doi/10.1029/2007JB005499/full

Scripps Institution of Oceanography. Retrieved from

RI

http://www.deepseadrilling.org/07/volume/dsdp07pt1_01.pdf

PT

Winterer, E. L., & Reidel, W. R. (1971). DSDP Volume VII - Introduction (DSDP Shipboard Site Report).

SC

Winterer, E. L., & Sager, W. W. (1995). 31. SYNTHESIS OF DRILLING RESULTS FROM THE MID-PACIFIC MOUNTAINS: REGIONAL CONTEXT AND IMPLICATIONS1. Retrieved from

NU

https://www.researchgate.net/profile/William_Sager/publication/279613133_Synthesis_of_Dr illing_Results_from_the_Mid-

MA

Pacific_Mountains_Regional_Context_and_Implications/links/5667360108aea62726ee6fd2.pdf Wright, D. J., Lundblad, E. R., Larkin, E. M., Rinehart, R. W., Murphy, J., Cary-Kothera, L., & Draganov, K.

D

(2005). ArcGIS benthic terrain modeler. Oregon State University, Corvallis, OR, USA.

PT E

Wright, D. J., Pendleton, M., Boulware, J., Walbridge, S., Gerlt, B., Eslinger, D., … Huntley, E. (2012). ArcGIS Benthic Terrain Modeler (BTM) (Version 3.0). Massachusetts Office of Coastal Zone Management: Environmental Systems Research Institute, NOAA Coastal Services Center,.

CE

Retrieved from http://esriurl.com/5754. Zhong, G., Cartigny, M. J., Kuang, Z., & Wang, L. (2015). Cyclic steps along the South Taiwan Shoal and

AC

West Penghu submarine canyons on the northeastern continental slope of the South China Sea. Geological Society of America Bulletin, 127(5–6), 804–824.

CE

Figure Captions

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 1 - Regional context of the Ontong Java Plateau, Ontong Java and Nukumanu atolls, and Kroenke

AC

Canyon. a) Satellite-derived bathymetry from Sandwell et al. (2014); The western equatorial Pacific Ocean and location of the Ontong Java Plateau (outlined by solid white line); land (grey); Papua-New Guinea (PNG). North Solomon Trench (NST) is indicated with solid black line with subduction “teeth” on present day overriding plate. Dashed rectangle indicates the location of Fig. 1b. b) The Ontong Java Plateau is outlined by solid white line. ODP/DSDP drill sites (black crosses), and previous sampling in the vicinity of the atolls (black circles - for details refer to Appendix 1), Santa Isabel (SI) and Malaita (M) islands, part of the Solomon Islands. Atolls, islands and reefs referred to in the text are labelled. FK141015 multibeam coverage indicates data collected in 2014 (EM302: 25 m grid cell size) on board RV

ACCEPTED MANUSCRIPT

Falkor. Dashed rectangle indicates the location of Fig. 1c. c) Multibeam bathymetry of the data used in this study (25 m) with grey scale satellite bathymetry underneath, encompassing the region surrounding Ontong Java and Nukumanu atolls and the uppermost reaches of Kroenke Canyon. Seismic reflection lines in the vicinity of the Ontong Java and Nukumanu atolls (RV Mahi, 1968) are solid black lines. Canyon thalweg (Adams, 2015) (solid white line) and hypothesized continuation of thalweg (dashed

PT

white line) are indicated. Black filled in polygons are atoll islands identified using Landsat satellite

RI

imagery. Dashed rectangle indicates the location of Fig. 2.

SC

Fig. 2 – Transparent bathymetry raster images with grey scale hillshade behind; see Fig. 1c for location. Examples of all bathymetric derivatives and acoustic backscatter data used to classify and interpret

NU

geomorphic zones and features. Broad (1,000 m scale factor) and Fine (50 m scale factor) Bathymetric

atolls identified using Landsat imagery.

MA

Positions Index (BPI) Solid black polygons in this and Figs. 3-5, 7, 9, and 11 are islands constituting the

D

Fig. 3 a) EM302 multibeam bathymetry surrounding Ontong Java Atoll gridded using a 10 m grid cell

PT E

size. Locations of slope failures A-D (black rectangles). Features discussed in the text are labelled. Solid black lines are locations of slope profiles presented in Fig. 3b. b) Profiles of the SW flank of Ontong Java Atoll showing a sinuous slope profile (black) and of the NE embayment showing a concave slope profile

CE

(blue). For profile locations see Fig. 3a. c) Slope failure A (southern Ontong Java Atoll), dotted white lines outline the extent of debris deposits located downslope of the bight in the atoll rim, dotted black lines

AC

outline an older slope failure deposit, much larger and extending much farther than the lobes at the base of the atoll. For Figs. 3c-f, solid black lines are 100 m bathymetric contours; features described in the text are labelled. d) Slope failure B (eastern Ontong Java Atoll), dotted white lines outline the lobes making up slope failure B dispersing laterally as they reach the foot of the slope. Dashed black line outlines a sedimentary apron downslope. e) Slope failure C (eastern limb of Ontong Java Atoll), east of the NE embayment. Dashed white lines indicate bathymetric depressions found only at slope failure C. The multibeam data also capture one or more sedimentary aprons farther downslope between the 1,700 and 1,800 m bathymetric contours, ~24 km from the atoll rim, outlined by dashed black lines. f)

ACCEPTED MANUSCRIPT

Slope failure D (northern limb of Ontong Java Atoll), north of the NE embayment. Downslope of bights, dashed white lines show the axes of channels. Randomly distributed blocks and sedimentary aprons are labelled. A break in slope is indicated by a dashed white line; above the break is a scarp, possibly stratigraphically controlled.

PT

Fig. 4 - EM302 25 m grid cell size backscatter mosaic. Features described in the text are labelled. Red

RI

rectangles outline locations of slope failures A-D (see Fig. 3).

SC

Fig. 5 a) EM302 multibeam bathymetry surrounding Nukumanu Atoll, 10 m grid cell size. Solid black lines are 100 m bathymetric contours. The direction to Kroenke Canyon, the sea knoll, and the terraces are

NU

labelled. Dashed white lines indicate steep scarps that delimit the upslope or downslope extents of terraces. Profile across terrace and bathymetric step is shown in Fig. 5b. Locations of Figs. 5c and 7 are

MA

outlined by black rectangles. b) Profile across terrace and steep scarp, located to the north of Nukumanu Atoll (location A-A’ shown in Fig. 5a). The trough at the foot of the steep slope is well

D

expressed in profile. These scarps have steep slopes, in places >50°. c) Bathymetry (top) and backscatter

PT E

(bottom) on the western flank of Nukumanu Atoll showing gullies (white and red dashed lines) and scarps on the upper flanks and undulating bedforms towards the base of the atoll. The upper flank of the atoll has much higher intensity backscatter compared to the moderate intensity atoll base. High

CE

intensity radiating lineations, common on the upper flanks of both atolls, are labelled. d) 3D image of Nukumanu Atoll from its SW corner, facing NNE, with 6:1 vertical exaggeration. Triangular debris

AC

accumulations are outlined with dashed white lines.

Fig. 6 a) Satellite-derived bathymetry (Sandwell et al., 2014) with FK141015 multibeam data overlain and navigation for 1968 R/V Mahi seismic reflection data, b) Seismic profile I-II intersecting the southern flank of Nukumanu Atoll, traversing the unmapped upper reaches of Kroenke Canyon. Interpreted seismic profile shows transparent purple polygons are wedges of seismically opaque material, possibly volcanic and/or carbonate debris accumulations, c) Seismic profile III-IV extending NE from the east

ACCEPTED MANUSCRIPT

flank of Nukumanu Atoll downslope reveals irregular bathymetry at the base of the atoll and uniform stratigraphy typical of OJP. VE is vertical exaggeration. Profile locations on Fig. 6a.

Fig. 7 a) EM302 bathymetry of the NE flank of Nukumanu Atoll, facing Kroenke Canyon, showing Crescent Shaped Bedforms (CSBs) and gullies (solid black lines). See Fig. 5a for location. Dashed black

PT

lines show the locations of profiles A-A ‘and B-B’. Profile A-A’ shows a cross-slope profile of the gullies and ridges. Profile B-B’ shows a downslope profile within one of the gullies. The change from steep

RI

smooth upper slope to undulating lower slope is shown clearly in profile B-B’, which we suggest is a

SC

change from an erosional to a depositional regime. Crescent shaped bedforms are well expressed and labelled in Profile B-B’. b) Profile curvature of the same region. Dark grey to black represents concave

NU

regions; light grey to white represents convex regions. CSBs are well defined within the gullies as

MA

rhythmic alternating dark to light banding (concave-convex alterations). Ship track artefacts are labelled.

D

Fig 8. Qualitative terrain classification generated using bathymetry, bathymetric derivatives, and

PT E

backscatter. Zones are transparent with hillshade underneath a) Ontong Java Atoll with location of profile used for Fig. 8c shown as solid grey line (A-A’); b) Nukumanu Atoll; c) Generalised zone classification of atoll flank based on slope, depth, curvature, ruggedness, and backscatter (profile

CE

location in Fig. 8a).

AC

Fig. 9 a) Schematic of the Ideal Truncated Cone formula (Kim & Wessel, 2011; Wessel, 2001); b) Reconstruction of the original volcanic edifice of Nukumanu Atoll. Solid blue line is 1,300 m bathymetric contour and dashed black line is the generalised basal circumference, used to estimate volume. Estimated original volume (bold) uses truncated cone formula. b) Reconstruction of the original volcanic edifice of Nukumanu Atoll with volume estimation in bold. c) Reconstruction of the original volcanic edifice of Ontong Java Atoll: Hierro Scenario, single volcanic edifice, and d) Isabela Island Scenario, three volcanic edifices. Volume estimations in bold.

ACCEPTED MANUSCRIPT

Fig. 10 a) Landsat image of El Hierro, Canary Islands. Dashed white lines indicate headscarps of major landslides. Solid black lines show rift zones thought to be a cause of flank collapse on El Hierro b) Landsat image of Ontong Java Atoll. Red dots indicate locations of seamounts according to the Global Seamount Database (Kim & Wessel, 2011; see text for details). c) Landsat image of Isabela Island, Galapagos, dashed white lines outline individual shield volcanoes that have amalgamated to form one

PT

island.

RI

Fig. 11 a) Satellite-derived bathymetry (Sandwell et al., 2014) overlain by FK141015 multibeam

SC

bathymetry in the vicinity of Ontong Java and Nukumanu atolls and the uppermost reaches of Kroenke Canyon. Dashed black rectangle is the location of Fig. 10b. Canyon thalweg (Adams, 2015) (solid black

NU

line) and hypothesized continuation of thalweg (dashed black line) are indicated. b) Channels identified

AC

CE

PT E

D

sediment transport to Kroenke Canyon.

MA

in the multibeam bathymetric data (solid black arrows) are likely acting as conduits for downslope

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 1.

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 2.

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

Fig. 3.

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 4

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 5

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 6

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 7

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

Fig. 8

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 9

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

Fig. 10

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 11

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Graphical abstract

ACCEPTED MANUSCRIPT

Highlights: 

First ever high resolution bathymetry around atolls on the Ontong Java Plateau



Extensive submarine mass wasting of Ontong Java and Nukumanu atolls



Base-of-atoll debris deposits indicate where atoll flanks have failed



~1,500 km of volcaniclastic material shed from the original edifices



Volcaniclastic and carbonate erosional products of atolls have fed Kroenke Canyon

AC

CE

PT E

D

MA

NU

SC

RI

PT

3