Modern sediment dispersal and accumulation on the outer Poverty continental margin

Marine Geology 270 (2010) 213–226

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Marine Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r g e o

Modern sediment dispersal and accumulation on the outer Poverty continental margin C.R. Alexander a,⁎, J.P. Walsh b, A.R. Orpin c a b c

Skidaway Institute of Oceanography, Savannah, GA 31411, United States Department of Geological Sciences, East Carolina University, Greenville, NC 27858, United States National Institute of Water and Atmospheric Research, Wellington 6021, New Zealand

a r t i c l e

i n f o

Available online 3 November 2009 Keywords: continental margin Poverty Bay Waipaoa River New Zealand 210 Pb 137 Cs sediment budget

a b s t r a c t The Poverty margin was sampled in 2005 and 2006 as part of an international initiative to examine the terrestrial and marine sedimentary response to natural and human impacts on dispersal systems at muddominated coasts: the NSF MARGINS Source-to-Sink Initiative. 210Pb accumulation rates generally decrease from ~ 1 g/cm2 y in the mid-shelf depocenter on the outer shelf to ~ 0.1 g/cm2 y on the mid-slope plateau (range 0.04–2.53 g/cm2 y). Higher accumulation rates are observed all along the outer Poverty shelf, extending over the shelf break onto the upper Poverty slope in canyons and gullies. Rates are fast in gullies that incise into the shelf edge (0.75–1 g/cm2 y), particularly in the area between Waipaoa Canyon and Poverty Canyon, and in the axis of Poverty Canyon (1.29–1.89 g/cm2 y). Below ~ 1200 m water depth, rates in the axis of Poverty Canyon are no more rapid than those found at open slope cores in similar water depths (0.12–0.15 g/cm2 y). Excess 210Pb profiles generally exhibit steady state characteristics, except in the axis of Poverty Canyon, where non-steady-state 210Pb profiles are observed. 137Cs was not found above minimum detection limits or above statistical background levels in any cores from the Poverty margin. 7Be was found distributed widely along and across the margin during both summer and winter periods, observed to depths of up to 4 cm. Widespread presence of 7Be on the margin in both winter and summer suggests that terrestrially derived sediment is reaching the outer shelf and upper slope throughout the year. A sediment budget based on 210Pb accumulation rate data shows that approximately 13–18% of the Waipaoa annual discharge is accumulating on the outer Poverty margin as a whole, and approximately 11–15% of the annual discharge is accumulating on the continental slope alone. Notably, approximately 28% of the sediment annually accumulating in the outer margin is being sequestered in slope gullies and canyons, although they represent only 6% of the area of the outer margin. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Small mountainous rivers at active continental margins are particularly sensitive to environmental change, driving variations in the sediment flux to the ocean over short time scales (e.g. Syvitski et al., 2005; Dadson et al., 2004; Gomez et al., 2004). Unlike large rivers that drain passive continental margins through extensive muddy deltas, these high-yield rivers typically discharge directly to the ocean (e.g. Milliman and Syvitski, 1992). Sediment sources and sinks are closely linked, and rapid inter-basin transport of erosion products is amplified by the severity of floods, short river courses, and narrow continental shelves (e.g. Sommerfield and Nittrouer, 1999; Dadson et al., 2005; Gomez et al., 2007). At the watershed of the Waipaoa River, northeastern New Zealand, colonisation brought about destruction of 97.5% of the old-growth native forests for pasture which, combined with ⁎ Corresponding author. Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411, United States. E-mail address: [email protected] (C.R. Alexander). 0025-3227/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2009.10.015

the vigorous maritime climate, caused accelerated erosion of the mudstone- and sandstone-dominated hinterland (e.g. Gomez et al., 1999; Marutani et al., 1999; Hicks et al., 2004). Simultaneously, these surficial erosion processes occur against a background of rapid tectonic uplift and margin instability driven by oblique subduction (e.g. Lewis et al., 1998; Mazengarb and Speden, 2000). These factors contributed to the Waipaoa Sedimentary System (WSS) being selected as part of an international initiative to examine the terrestrial and marine sedimentary response to natural and human impacts on dispersal systems at mud-dominated coasts: the NSF MARGINS Source-to-Sink Initiative. Mud depocenters occur on the adjacent Poverty continental shelf (Foster and Carter, 1997; Orpin et al., 2006; Gerber et al., 2010-this issue) and as localised infilling of upper slope topography (Walsh et al., 2007). High modern sedimentation rates indicate that down-slope sediment dispersal could be initiated at focal points, despite the high annual sediment supply and wide spatial extent of shelf depocenters. Orpin et al. (2006) speculated that current riverine sediment supply far exceeds millennial-scale margin sedimentation inferred from Holocene tephra stratigraphy. The corollary of this assertion is that under modern

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conditions a larger proportion of the Waipaoa sediment dispersal system extends onto the slope and beyond. Slope sediment accumulation is a critical element of the modern sediment budget (e.g. Kuehl et al., 2006) and a mechanistic understanding of the causal linkages between the land-shelf-slope system through time is a goal of this larger program. But quantification of off-shelf sediment escape is currently limited by a paucity of high-resolution cores and geochronological measurements. This paper presents new geochronological data to quantify the distribution of sediment accumulation rates seaward of the Poverty mid-shelf depocenters, and along the Poverty outer shelf and upper slope—the distal component of the Waipaoa Sedimentary System. We identify active canyons and escape paths for off-shelf sedimentation and compare these with modern hemipelagic accumulation in intra-canyon areas. The balance of hemipelagic versus canyon-focused sedimentation was shown to be critical to sediment dispersal at the Eel River margin (Alexander and Simoneau, 1999; Puig et al., 2003)—a sedimentary dispersal system of similar magnitude, behaviour, and active tectonic setting. Additionally, using short-lived radioisotopes we report the first evidence of seasonal variations in outer shelf–upper slope sedimentation for the WSS, indicating that dispersal across the margin is both rapid and frequent. Finally, we construct a sediment budget for the Poverty slope that reinforces the wide spatial extent of the WSS. 2. The Waipaoa catchment sediment source The WSS (Fig. 1) is located on the tectonically active and geologically complex northern Hikurangi margin of New Zealand, where oceanic crust of the Pacific Plate is being subducted obliquely beneath the Raukumara Peninsula along its eastern margin (Mazengarb and Speden, 2000). Subduction-related underplating is elevating the North Island axial ranges, including the catchment of the Waipaoa River, at an estimated rate of 3 mm y− 1 (Reyners and McGinty, 1999). The strongly jointed and clay-rich lithology results in highly unstable landforms, manifest as slumps, landslides, and extensive gully erosion (e.g. Berryman et al., 2000; DeRose et al., 1998), which leads to the very high sediment yields for the Waipaoa River catchment. The alluvium base along the Waipaoa River has an exponential form to within ~25 km of the coast, where regional neotectonism causes a transition from uplift to subsidence (Berryman et al., 2000). Despite a small catchment of 2205 km2, the Waipaoa River delivers 15 Mt y− 1 of suspended sediment and 0.15 Mt y− 1 of bedload sediment to coastal Poverty Bay (Hicks et al., 2000), with sediment yields in its upper catchment among the highest recorded on Earth (Walling and Webb, 1996). Large floods (>1900 m3 s− 1) are frequent and can occur throughout the year (Reid, 1999), although they are most common in the autumn and early winter months, from April to June. Hicks et al. (2004) suggest that flood discharges from the Waipaoa River approach or exceed the critical suspended sediment concentration (40,000 mg L− 1; Mulder and Syvitski, 1995) for hyperpycnal plume generation at the river mouth every 40 years. Analysis of field observations from the 2006 flood/storm season and historical data indicate that high discharges of the Waipaoa River occur during times of energetic waves (Harris et al., 2006), promoting seaward sediment dispersal. 3. The Waipaoa sedimentary sink: the Poverty shelf and slope Data from recent MARGINS field programs have identified three modern sediment depocenters on the shelf (Miller and Kuehl, 2010-this issue) which are broadly compatible with postglacial sediment thicknesses (Foster and Carter, 1997; Orpin et al., 2006; Kuehl et al., 2006; Gerber et al., 2010-this issue). In addition, seismic and stratigraphic studies have documented significant sediment accumulation locally on the slope (Kuehl et al., 2006; Walsh et al., 2007). The Holocene mud blanket extends to the shelf edge, except in the vicinity of emergent anticlinal ridges where relict gravel and sand surround

exposures of Neogene sedimentary rocks (Lewis, 1973; Foster and Carter, 1997). Contrary to classical margin sedimentation models, high modern accumulation rates (>0.5 cm y− 1) occur on the outer shelf and in canyon heads (Alexander et al., 2006; Walsh et al., 2007). Over millennial time scales, seismic stratigraphy on the outer shelf shows that deposition was sympathetic with fault activity on the lastglacial erosion surface and the creation of accommodation space, implying that sedimentation was not supply limited (Orpin et al., 2006; Walsh et al., 2007). These data indicate slower but steady sedimentation since the last transgression, and that significant across-shelf sediment transport pre-dates the increase in sedimentation resulting from colonisation and deforestation. Hence the efficiency with which riverine sediments are dispersed across the Poverty margin is longstanding and suggests strong oceanic drivers. Further offshore on the Poverty continental slope, accretionary tectonics and plate convergence have produced imbricate-thrust faults and folded Neogene slope sediments as a deforming backstop, with only a narrow accretionary prism locally forming in places at the toe of the slope (Lewis and Pettinga, 1993; Collot et al., 1996). The 1500 km2 Poverty indentation is a major continental margin depression extending from a re-entrant in the deformation front at the Hikurangi Trough to the continental shelf (Collot et al., 1996). The bathymetry of the Poverty Re-entrant is complex and comprises six basic morphologic components, in order of increasing water depth: (i) a heavily gullied upper slope; (ii) a gently sloping mid-slope trough with hemipelagic sediment cover (Paritu Trough); (iii) the beheaded Poverty Canyon system; (iv) margin-parallel lower slope ridges (North and South Paritu Ridges); (v) a V-shaped structural re-entrant in the deformation front at the canyon mouth; and, (vi) the largely flat expanse of the Hikurangi Trough, seamounts, and Hikurangi Channel seaward of the deformation front. On the slope, Orpin (2004) estimated from tephrostratigraphy that the average Holocene sediment mass accumulation rate in the Paritu Trough is ~0.05g cm− 2 y− 1 (0.04–0.07 cm y− 1), and by applying this rate over a 450 km2 area of the slope, identified zones of hemipelagic sedimentation from echo character and acoustic imagery, and speculated that the total mass accumulation on the slope is about 0.2 Mt y− 1. Some fraction of the hemipelagic drape could conceivably be sourced from the rivers further north on the Raukumara Peninsula (e.g. Uawa/ Hikuwai and Waiapu Rivers). Modern rates on the slope from radiochemical profiles are poorly constrained spatially but infer a slight increase in accumulation on the upper slope, presumably in response to increase rates of sediment supply post-deforestation (Orpin et al., 2006). 4. Oceanography of the Poverty margin Along eastern North Island, seaward of the northward-flowing Wairarapa Coastal Countercurrent (WCC) is the counter flowing East Cape Current (ECC), forming part of the Subtropical inflow to New Zealand (Heath, 1985; Chiswell and Roemmich, 1998; Chiswell and Booth, 1999). With a variable volume transport of 1–2 × 107 m3 s− 1, the ECC is nominally positioned seaward of the 1000 m isobath, although the geostrophic cross-section of Stanton et al. (1997) and current meter data south of East Cape (Chiswell and Roemmich, 1998) indicate that the ECC is topographically controlled along the 200-m isobath but extends seaward to at least 1300 m depth. The current's passage across the Poverty margin is likely modified by the topography of the reentrant. The effect of the ECC is to transport suspended sediment southward. A third component of the coastal circulation is large eddies formed by complex interactions between the southward-flowing ECC, the Wairarapa Eddy, and the East Cape Eddy (Chiswell and Roemmich, 1998; Roemmich and Sutton, 1998a,b; Chiswell, 2000). A dynamic situation exists along northeastern New Zealand (Chiswell, 2005), with 2–3 eddies moving southwest along the outer margin each year, stalling or merging with older features, and at times these eddies may be responsible for moderate currents at >2000 m depth (P. Sutton, NIWA unpublished data). The Poverty upper slope probably lies at the junction

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Fig. 1. Map showing the physiography of the study region on the eastern coast of the North Island of New Zealand. Waipaoa River drainage basin shown in upper left quadrant of figure; Poverty continental shelf and upper slope with canyons and gullies in center of image; mid-slope plateau and Hikurangi trough in lower right quadrant. Note the heavily dissected nature of the shelf/slope transition and upper slope.

of the ECC and WCC motions. Satellite images show the offshore dispersal of the surface plume (Foster and Carter, 1997). On the middle shelf, surface transport is strongly dependent upon wind stress (Foster and Carter, 1997; Chiswell, 2005), but little is known about mid-watercolumn and near-bed currents or sediment transport. Local tides are weak; at the entrance to Poverty Bay, peak tidal current speeds are 2– 4 cm s−1 at 10-m depth (Healy et al., 1998). On the open continental shelf, long period (8–12 s) southerly swell is persistent, typically 1–2 m in height (Foster and Carter, 1997). Numerical wave modelling and recent field measurements from within Poverty Bay indicate that 4-m swells occur a few times per year (pers. comm. A. Bever and J. McNinch, Virginia Institute of Marine Science, 2008). Modelling results suggest that during a typical

storm season, waves are capable of resuspending sediment more than half the time on the middle shelf, and for at least 10% of the time on the outer shelf (pers. comm., A. Bever, Virginia Institute of Marine Science, 2008). 5. Anthropogenic impact on landscape erosion Using fires, Polynesian settlers began to clear the Raukumara Ranges of thick temperate rain-forest 500–700 y BP (e.g. McGlone et al., 1994; McGlone and Wilmshurst, 1999), and forest clearing accelerated significantly with European colonisation in the mid-eighteenth century (Pullar, 1962). By 1880, most of the hinterland had been cleared, and by 1920 all but a few percent of the land had been converted to pasture.

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Following deforestation, an intense phase of landscape erosion was initiated in the upper reaches of the Waipaoa catchment around the beginning of the 20th century (Allsop, 1973). Since then, the river has aggraded rapidly in response to increased sediment yield (Gage and Black, 1979; Gomez et al., 1999) which was the major factor leading to a 3–4 fold increase in vertical sediment accumulation (Gomez et al., 2004, 2007; Orpin et al., 2006). Early estimates of Holocene sedimentation rates are approximately 0.15 cm y− 1 (Gomez et al., 2004), or approximately 0.1 g cm− 2 y− 1. 6. Methods 6.1. Field methods Cores in this study were collected during a summer cruise (February 2005; KM0503) and a winter cruise (September 2006; TAN0613; Fig. 2). On KM0503, 42 cores were collected with a single spade box corer (20 cm × 30 cm × 60 cm) and 12 cores were collected with a gravity corer (3-m long); on TAN0613, 41 cores were collected with a multicorer (9.4 cm dia. × 80 cm L), with the longest multicore from each corer deployment selected for sampling. Cores were sliced into 1-cm thick intervals in the upper 10-cm of each core and into 2-cm thick intervals below 10 cm. Twenty-one of the sites sampled on TAN0613 were reoccupations of core sites sampled on KM0503 to enable winter–summer comparisons in short-lived radionuclides and sedimentary processes. Slabs for X-radiography were removed from

each box core and multicore in Plexiglas trays and imaged on board ship with a digital X-ray unit. 6.2. Laboratory methods Sediment samples were analyzed for 210Pb (half-life= 22.3 y), 137Cs (half-life= 30.3 y), 7Be (half-life= 50 d) and 234Th (half-life= 24 d) activities to examine the rates of sediment processes on a variety of timescales. To determine sediment accumulation rates on 100-year timescales, samples were analyzed for 210Pb and 137Cs. 210Pb is naturally produced in the environment through the decay of its grandparent and parent, the long-lived 226Ra, and the gas 222Rn, respectively. 137Cs is a transient tracer (produced from atmospheric nuclear tests), which was first introduced into the environment in significant amounts in 1954 and reached a peak in 1963. Box and multicores were analyzed for 210Pb and 137Cs activities using gamma spectroscopy, following the techniques described in Alexander et al. (1993). Sediments were weighed, oven-dried (60 °C), then ground to a powder, sealed in 30 ml polypropylene jars and equilibrated for 20 days. Total 210Pb activities in sediment samples were directly determined by measuring the 46.5 keV gamma peak (Cutshall et al., 1983) and supported levels of 210 Pb were determined from the activities of 214Pb and 214Bi (at 295, 352 and 609 keV). 137Cs activities were determined by measurement of its 661.6 keV gamma peak (Kuehl et al., 1986). Gravity cores were analyzed for 210Pb using alpha spectroscopy techniques (e.g., Flynn, 1968), assuming equilibrium between 210Pb and 210Po. Dried sediment

Fig. 2. Station locations for all cores collected during this project and for which data are reported in Table 1. Most cores are located on the outer shelf and upper slope to constrain the mechanisms and quantity of off-shelf transport to the slope. Cores directly discussed in the text are labelled.

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samples were spiked with a known quantity of 209Po and successively leached in concentrated HNO3 and 6 N HCl. Polonium isotopes were plated onto a silver planchet suspended in a weak HCl leachate. The alpha activities of 209Po and 210Po were subsequently measured using silicon barrier detectors. For the gravity cores, supported 210Pb activities were estimated by averaging values deep in cores where uniform levels of activity were reached. Details of the gravity core analysis are given in Sumner (2007). In all cases, accumulation rates were estimated from a linear regression of the decay of excess 210Pb activity below the mixed surface layer. 7 Be is closely associated with terrestrially derived particles in the ocean, being dominantly delivered on flood-derived material eroded from land, thus providing a tracer for fluvial material that has been delivered to a sampling site on monthly timescales (e.g., Sommerfield et al., 1999). 7Be was quantified by measuring its 477-keV gamma peak (Larsen and Cutshall, 1981). 234 Th was used to determine relative seasonal differences in shortterm sedimentary processes and the intensity of biological mixing of the seabed (Aller and Cochran, 1976; DeMaster et al., 1985). 234Th, formed in the water column by decay of its parent 238U, is scavenged by sinking particles and emplaces an excess 234Th signal into the seabed, that is rapidly mixed downward by macrobenthos, where present. Because of thorium's short half-life, samples for mixing rate determinations were quickly returned to the laboratory, dried, and ground as

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described above. Samples were counted for 24 h each on one of three germanium detectors, allowing as many samples as possible to be counted before excess 234Th activity had decayed to levels difficult to detect (~2.5 half-lives). Total 234Th was quantified by observation of its 63.3-keV gamma peak (Buesseler et al., 1992) soon after sample collection. Samples were recounted after 4 months to quantify supported levels of 234Th and, by difference, to allow determination of excess 234Th. Biological mixing rates for the upper sediment column were calculated from profiles of excess 234Th with depth in the seabed by solving a 1-D advection–diffusion equation (Aller and Cochran, 1976; Nittrouer et al., 1983/84; DeMaster et al., 1985). 7. Results and discussion 7.1.

210

Pb and

137

Cs accumulation rates

Representative 210Pb profiles are shown in Fig. 3 and all data are reported in Table 1. Rates generally decrease from ~1 g cm− 2 y− 1 in the mid-shelf depocenter on the outer shelf to ~0.1 g/cm2 y on the mid-slope plateau (range 0.04–2.53 g cm− 2 y− 1; Table 1). Higher accumulation rates are observed all along the outer shelf and extending over the shelf break onto the upper slope in canyons, gullies and depressions on the sea floor. Rates are fast in gullies that incise into the shelf edge (0.75–1 g cm− 2 y− 1), particularly in the area between Waipaoa Canyon and Poverty

Fig. 3. Representative 210Pb profiles from the outer Poverty margin. Steady-state profiles are observed on the (A) shelf, (B) upper open slope and (C) mid-slope plateau, indicating nepheloid layer transport and relatively steady supply of sediment. (D) Cores for upper slope gullies and canyon axes exhibit non-steady-state 210Pb profiles, indicating episodic sediment supply by sediment gravity flows.

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Canyon, and in the axis of Poverty Canyon (1.29–1.89 g cm− 2 y− 1). In contrast, several cores from the axis of an unnamed canyon southwest of Poverty Canyon do not exhibit enhanced accumulation rates (Fig. 4). Below ~1200 m water depth, rates in the axis of Poverty Canyon are no more rapid than those found at open slope sites in similar water depths (Fig. 5). In contrast, rates at the slope gully debouchments (0.22– 0.27 g cm− 2 y− 1) are more rapid than rates for open slope cores in similar water depths (0.12–0.15 g/cm2 y). Excess 210Pb profiles (Fig. 3) generally exhibit steady state characteristics, except in the axis of Poverty Canyon, where non-steady-state 210Pb profiles are observed. 137 Cs was not found above detector statistical background levels in any cores from the Poverty margin. This lack of a strong 137Cs signature in southern latitudes has been documented previously by researchers in other southern, high-latitude settings such as the Antarctic (e.g., Picciotto and Wilgain 1963; Ledford-Hoffman et al., 1986). The distribution of accumulation rates illustrates that, on 100-y timescales, sediment is preferentially accumulating on the outer Poverty shelf, in the outer shelf depocenter, and is then transported into the heads of Poverty and Waipaoa Canyons, and into shelf-indenting slope gullies. These sediments are sequestered on the upper slope in these regions. However, higher rates are observed in slope gullies and canyons between ~200 m and 1200 m water depth than are documented from open slope sites, illustrating that these gullies and canyons must be important pathways for sediment transport from the shelf to the deeper margin. Addington et al. (2007) show that fluvial material is transiting the shelf on the Waiapu shelf just north of our study area, and sediment is reaching the slope rapidly in that region. Although frequent hyperpycnal flows are documented from the area and provide a mechanism for cross-shelf transport, favorable conditions for hyperpycnal flows are present on the Poverty margin as well (Hicks et al., 2004). 7.2. 7Be and

234

Th seasonal data

Both 7Be and 234Th are short-lived radionuclides that permit assessments of short-term sedimentary processes in coastal and continental-margin environments. 7Be is closely associated with terrestrially derived particles in the ocean, being dominantly delivered on flood-derived material eroded from land. Excess 234Th, produced by steady state decay of 238U in seawater, is scavenged from the water column by particles, which subsequently carry that signal to the seafloor. 7.2.1. 7Be 7 Be was found distributed widely along and across the margin during both summer and winter periods, penetrating to depths of up to 4 cm (Fig. 6A). In the summer, surface activities ranged from 0 to 1.6 dpm g− 1 and integrated seabed inventories of 7Be ranged from 0 to 1.4 dpm cm− 2 (Table 1). In winter, surface activities were higher and ranged from 0 to 4.5 dpm g− 1 and integrated seabed inventories of 7Be ranged from 0 to 3.9 dpm cm− 2. Average 7Be inventories were examined in two depth range groups at sites resampled seasonally on the two cruises: above 250 m water depth, which includes the shelf and upper canyon/gully reaches, where sediments are accumulating most rapidly (see discussion above) and below 250 m water depth, including the lower canyon/gully reaches and the mid-slope plateau, where sediments accumulate more slowly. Average seasonal 7Be inventories are not significantly different in water depths <250 m (winter: 0.7+/ −0.9 dpm cm− 2; summer: 0.5+/−0.4 dpm cm− 2) or in water depths > 250 m (winter: 0.4+/−0.9 dpm cm− 2; summer: 0.2+/−0.3 dpm cm− 2), nor are the inventories significantly different when the shallow and deep water values are compared. The widespread presence of 7Be on the margin in both winter and summer suggests that terrestrially derived sediment is reaching the outer shelf and upper slope throughout the year (Table 1). The distribution of the 7Be-tagged material documents preferential accu-

mulation on the outer shelf and in the upper reaches of shelf-indenting canyons and upper-slope gullies. These are the same sites where elevated accumulation rates point to offshore transport. 7.2.2. Excess 234Th activity and inventory Seasonally, the activity of average excess 234Th in surface sediments was similar in water depths <250 m (winter: 22.9+/−19.6 dpm g− 1; summer: 17.0+/−15.6 dpm g− 1) but differed markedly in water depths >250 m (winter: 25.5+/−16.8 dpm g− 1; summer: 9.6+/−9.6 dpm g− 1) (Fig. 6B). Similarly, average excess 234Th seabed inventories were similar at reoccupied sites in water depths <250 m (summer: 16.1+/ −9.7 dpm cm− 2; winter 15.0+/−18.8 dpm cm− 2; Table 1). In depths >250 m, inventories were dissimilar, as was observed with surficial activities, with wintertime inventories (13.4+/−10.2 dpm cm− 2) being greater than summertime inventories (7.1+/−5.5 dpm cm− 2). The theoretical seabed inventory that would be present, assuming all the available excess 234Th was stripped from the overlying water column, can be calculated knowing the salinity and concentration of 238U dissolved in sea water (Ku et al., 1977). This calculation was made for the water depths of each of our revisited sites to determine the ratio of the observed to the predicted seabed inventory present (Fig. 7). The fraction of predicted water column inventory in the seabed is highest in shallow waters and decreases rapidly offshore to water depths of 400–600 m. Below 600 m, inventory ratios are uniformly low and little of the potential water column inventory is in the seabed. An exponential curve fit to the winter and summer data yields an r2 of 0.68 and 0.80, respectively (not shown). Biological mixing rates calculated from excess 234Th profiles range from 1.0 to 58.0 cm2 y− 1 (Fig. 6B, Table 1). These rates demonstrate that biological mixing can be vigorous in the study area. These values are similar to those observed in other continental margin study sites (Carpenter et al., 1982; Aller, 1992; Alexander and Venherm, 2003; Wheatcroft et al., 2007). Depths of mixing range from 1 to 4 cm, and are shallower than mixing depths observed in many of the margin settings mentioned above. Mixing rates averaged over all water depths were greater in the summer (20.0+/–15.0 cm2 y− 1) than in winter (13.4+/ −12.5 cm2 y− 1). However, caution must be used when comparing these values because all but one of the mixing rates from the summer cruise were from cores from water depths between 1210 and 1612 m, whereas those from the winter cruise were from cores in depths between 42 and 1255 m. Where these two datasets overlap at ~1200 m, there is good agreement between summer and winter cruise values. If the datasets truly are comparable and can be combined, then biological mixing rates on the Outer Poverty margin are relatively constant (spanning a range of 2–20 cm2 y− 1, Table 1) over a wide range of depths down to about 1400 m. In deeper water, between 1400 and 1600 m, the biological mixing rates double to approximately 45 cm2 y− 1 (relationship not shown). 7.2.3. Seasonal variability of mixing rates Seasonal variability on the Outer Poverty margin is pronounced, particularly in shallower water depths (to 250 m). Cores from these water depths exhibit a seasonal decrease in mixing rate in winter. This summer to winter decrease in mixing rate is observed routinely in marine and freshwater systems and can be driven by decreased metabolic rates as temperatures decrease in the winter (White et al., 1987), or increased activity is summer associated with enhanced food availability from phytoplankton blooms (Wheatcroft, 2006). Both mechanisms may be operating in the WSS. This winter-time decrease in mixing rate occurs in concert with a winter-time increase in 234Th inventory in the seabed in water depths below 250 m. An increase in inventory indicates that the flux of sediment through the water column, which scavenges 234Th as it sinks, increases in the winter, an observation supported by sediment trap measurements in many study areas (e.g., Walsh and Nittrouer, 1999). This increase in sediment flux to the slope during the winter months is driven by the energetic physical

Table 1 Radionuclide data for all cores collected during summer (KM0503) and winter (TAN0613) cruises in 2005 and 2006. Latitude

Water depth (m)

Vertical accretion rate (cm y− 1)

Mass flux accumulation rate (g cm− 2y− 1)

178.2147 178.2945 178.3007 178.3767 178.3884 178.4602 178.4372 178.4158 178.3884 178.3042 178.2515 178.3247 178.3959 178.3627 178.3724 178.3508 178.2633 178.2412 178.2116 178.1790 178.2565 178.2310 178.2243 178.1679 178.3099 178.3198 178.3486 178.3069 178.6532 178.5787 178.4844 178.4884 178.4025 178.3651 178.1796 178.1990 178.2146 178.2240 178.2191 178.2113 178.1989 178.2685 178.3792 178.3482 178.2563 178.2256 178.2258 178.2143 178.2780 178.2912

108 129 125 146 480 656 539 372 144 1527 1413 1424 1190 1140 1222 1210 348 144 134 359 1235 240 238 210 1185 1263 1476 1412 1612 1494 1405 1183 1162 881 360 135 190 223 172 172 114 204 1160 1478 1230 891 891 190 1257 1243

1.22 0.31 0.54 0.24 0.80 0.17 0.17 0.48 0.30 0.20 0.22 0.16 0.27 0.43 0.24 0.08 0.77 0.63 0.79 2.62 0.47 0.87 0.66 2.54 0.31 0.18 0.21 0.18 0.06 0.10 0.11 0.11 0.46 0.50 1.69 0.37 0.61 0.54 – 0.32 0.59 – – – 1.00 2.09 – 0.90 0.38 0.34

0.99 0.25 0.44 0.20 0.61 0.12 0.13 0.37 0.26 0.13 0.16 0.09 0.17 0.30 0.17 0.06 0.55 0.49 0.62 1.89 0.33 0.64 0.50 1.71 0.22 0.12 0.15 0.12 0.04 0.06 0.07 0.10 0.32 0.39 1.29 0.29 – – – 0.24 – – – – 1.05 1.65 – 0.71 0.27 0.24

7

Be inventory (dpm cm− 2)

Observed excess 234 Th seabed inventory (dpm cm− 2)

Predicted 234Th water column inventory (dpm cm− 2)

Ratio observed to predicted 234 Th inventories

1.2 1.0 0.2 0.6 0.0 – 0.9 1.4 – 0.0 0.0 0.0 0.0 0.0 0.3 0.1 0.2 0.6 0.0 0.3 0.6 0.5 0.5 1.0 0.0 0.0 0.3 0.0 0.0 0.2 0.0 0.0 – 0.3 – – – – – – – – – – – – 0.0 0.0 0.1 0.0

27.3 29.1 1.5 18.5 8.2 – 2.7 9.4 – 5.5 4.6 2.1 6.5 4.4 2.2 3.6 9.1 9.0 4.9 24.9 6.5 23.6 26.9 15.2 7.1 3.6 6.5 4.7 8.1 2.6 1.4 6.9 – 6.4 – – – – – – – – – – – – 21.5 11.3 5.1 7.0

27 32 31 37 120 – 135 93 – 382 353 356 298 285 306 303 87 36 34 90 309 60 60 53 296 316 369 353 403 374 351 296 – 220 – – – – – – – – – – – – 223 48 314 311

1.01 0.90 0.05 0.51 0.07 – 0.02 0.10 – 0.01 0.01 0.01 0.02 0.02 0.01 0.01 0.10 0.25 0.15 0.28 0.02 0.39 0.45 0.29 0.02 0.01 0.02 0.01 0.02 0.01 0.00 0.02 – 0.03 – – – – – – – – – – – – 0.10 0.24 0.02 0.02

234

Th mixing rate (cm2 y− 1)

– – – – – – – – – 45.3 20.0 28.2 – – 5.2 1.1 – – – – 4.1 – – – – 17.4 27.2 21.4 41.6 39.0 3.3 – – – – – – – – – – – – – – – – – 20.2 5.8

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(continued on next page)

C.R. Alexander et al. / Marine Geology 270 (2010) 213–226

KM0503 station ID 1 − 38.9014 2 − 38.8365 3 − 38.8235 4 − 38.7835 5 − 38.7949 6 − 38.8445 7 − 38.7760 8 −38.7666 9 − 38.7688 10 − 39.1202 11 − 39.1202 12 − 39.0636 14 − 38.8873 15 − 38.8770 16 −38.9142 17 − 38.9391 18 − 38.8891 19 −38.9079 20 − 38.9433 21 − 38.9823 22 − 39.0262 23 − 38.9365 24 − 38.9541 25 − 38.9763 26 − 38.9531 27 − 38.9744 28 − 39.0203 29 − 39.0222 30 − 38.9905 31 − 39.0060 32 − 38.9850 33 − 38.9081 34 − 38.8476 35 − 38.8241 36 − 38.9821 37 − 38.9581 38 − 38.9637 39 −38.9540 40 − 38.9514 41 −38.9432 42 − 38.9383 43 − 38.9080 44 − 38.9001 45 − 39.0202 46 − 39.0260 47 − 38.9907 48 − 38.9904 49 − 38.9635 50 − 38.9932 51 − 38.9732

Longitude

220

Table 1 (continued) Be inventory (dpm cm− 2)

Observed excess 234 Th seabed inventory (dpm cm− 2)

Predicted 234Th water column inventory (dpm cm− 2)

Ratio observed to predicted 234 Th inventories

0.93 0.54 0.32

– 0.2 0.0

– 13.3 9.3

– 226 34

– 0.06 0.27

– – –

– – – – 0.12 – – – 2.53 0.73 – 0.93 – – – – 0.28 0.58 0.44 0.31 1.01 – – – – – – 0.88 – – 0.54 1.16 0.62 – – – 0.20 0.47 0.51 ND

0.4 0.0 0.7 0.0 0.0 0.0 0.0 0.0 0.1 0.7 0.0 0.2 0.9 1.4 0.0 0.0 0.0 3.9 0.5 0.5 0.4 0.2 0.0 0.0 3.8 0.0 0.6 0.7 0.2 0.9 0.0 0.6 0.5 – 1.2 0.0 0.1 1.6 0.7 0.4

11.7 10.8 14.6 5.2 5.8 2.7 10.9 0.6 11.4 15.1 13.4 11.4 5.9 24.1 4.8 2.0 39.1 86.8 25.2 11.0 11.0 2.7 1.5 11.2 30.3 4.3 25.6 20.8 6.1 18.3 12.4 14.0 17.5 – 17.7 7.5 4.8 15.0 12.4 4.6

47 33 27 34 94 284 219 32 77 70 224 87 59 65 253 301 131 55 45 38 38 220 314 308 90 48 64 70 51 84 27 36 81 36 36 133 299 22 14 11

0.25 0.32 0.54 0.15 0.06 0.01 0.05 0.02 0.15 0.22 0.06 0.13 0.10 0.37 0.02 0.01 0.30 1.59 0.57 0.29 0.29 0.01 0.00 0.04 0.34 0.09 0.40 0.30 0.12 0.22 0.46 0.39 0.22 – 0.50 0.06 0.02 0.68 0.87 0.43

– – – – 6.6 – – – 10.3 9.8 – 13.4 – – – – 7.2 5.7 17.1 2.1 58.0 18.1 16.7 – – – – 15.1 – – 8.9 26.3 8.4 – – – 1.0 5.7 17.5 7.3

Water depth (m)

Vertical accretion rate (cm y− 1)

Mass flux accumulation rate (g cm− 2y− 1)

KM0503 station ID 52 − 38.9627 53 − 38.8972 54 − 38.8545

178.2669 178.3060 178.2869

1044 903 136

1.18 0.76 0.38

TAN0613 (KM) 1 (49) 2 (20) 3 (1) 4 (54) 5 6 (15) 7 (35) 8 (2) 9 10 11 (53) 12 13 (23) 14 (24) 15(52) 16 (51) 17 18 19 20 21 22 (48) 23 (50) 24 (46) 26 (36) 27 (25) 28 29 30 (19) 31 (18) 32 33 34 35 (4) 36 (9) 37 (7) 38 (17) 39 40 41

178.2143 178.2117 178.2150 178.2873 178.3232 178.3625 178.3652 178.2956 178.3167 178.2682 178.3063 178.2492 178.2310 178.2243 178.2670 178.2903 178.2507 178.2212 178.2037 178.1870 178.1800 178.2252 178.2780 178.2570 178.1795 178.1695 178.1677 178.2253 178.2682 178.2637 178.2545 178.3155 178.3292 178.3772 178.3887 178.4373 178.3480 178.1863 178.1827 178.1122

187 133 108 135 377 1137 877 129 309 280 894 347 234 259 1013 1205 522 218 178 153 152 878 1255 1231 359 190 254 278 205 335 109 145 325 145 143 533 1194 89 57 42

– – – – 0.16 – – – 4.02 1.05 – 1.31 – – – – 0.37 0.77 0.55 0.39 1.34 – – – – – – 1.22 – – 0.63 1.49 0.89 – – – 0.29 0.54 0.45 ND

Station ID − 38.9637 − 38.9433 − 38.9013 − 38.8545 − 38.8917 − 38.8777 − 38.8240 − 38.8366 −38.8505 − 38.8757 − 38.8972 − 38.9208 − 38.9365 − 38.9540 − 38.9627 − 38.9725 − 38.9655 − 38.9632 − 38.9650 − 38.9672 − 38.9677 − 38.9900 −38.9940 −39.0275 − 38.9820 − 38.9755 − 38.9857 − 38.9622 − 38.9080 − 38.8892 − 38.8617 − 38.8210 − 38.8120 − 38.7835 − 38.7688 − 38.7760 − 38.9313 − 38.9238 − 38.8240 − 38.7875

7

Note: When two core site numbers are present, the site was occupied on both cruises; site number in parentheses is from KM0503.

234

Th mixing rate (cm2 y− 1)

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Longitude

Latitude

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221

Fig. 4. Distribution of sediment accumulation rates on the outer Poverty margin. Note that rates are more rapid on the outer shelf and in upper slope gullies and canyons.

oceanography and shelf environment (Foster and Carter, 1997; Chiswell, 2005). It seems clear that both direct input of sediment during floods of the river (Miller and Kuehl, 2010-this issue) and input

of flood material that is resuspended later in the winter because of the energetic physical environment are responsible for the supply of sediment to the slope.

Fig. 5. Sediment accumulation rate versus depth for the study area. Rates are rapid in the upper reaches of canyons and slope gullies when compared to open slope sites at similar water depths. Sediments are preferentially accumulating in canyons and gullies in waters shallower than 1200 m. Rates in open slope and canyon sites are similar below this water depth, suggesting that hemipelagic sedimentation is more important deeper than 1200 m water depth (Poverty canyon rates highlighted by way of example).

Fig. 6. Representative profiles of (A) 7Be, documenting the presence of terrestrially derived material on the outer Poverty margin, and (B) excess 234Th, documenting the depth and intensity of biological mixing. Mixing rates range from 1 to 50 cm2 y, indicating a weak to moderate level of biological modification of the seabed.

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8. Grain size and sedimentary structure

Fig. 7. Water depth versus the ratio of measured excess 234Th seabed inventory to that which would be present if all excess 234Th in the overlying water column was rapidly removed to the bottom by particles (see text). Data are only from stations that were reoccupied during both the summer and winter cruises. On the shelf and upper slope, ratios are typically greater than 0.4, suggesting greater amounts of suspended material is present to scavenge excess 234Th activity. Inventory ratios are small in water depths deeper than 600 m, suggesting low suspended sediment concentrations below these depths. Ratios are similar in winter and summer, indicating a vigorous resuspension environment throughout the year.

Unimodal sediments fine with distance from the mid-shelf to the mid-slope plateau from ~6.5 to ~9 phi (0.01 to 0.002 mm), with all but one sample falling within a group that consists of 0.5–20% sand, 35–55% silt and 35–65% clay (Figs. 8 and 9A–C). The coarse fraction of Poverty margin samples is typically fine quartz sand, tephra (volcanic ash) and/or rock fragments. The anomalous sandy sample on the ternary diagram is a single surface sample from the energetic mid-shelf, between two emergent anticlines, which does not reflect outer shelf and slope conditions. Representative histograms of outer shelf and slope surface sediments illustrate the unimodal nature of the surface sediments, as well as the shift in modal diameter with distance offshore. Texture in both outer shelf and open slope cores exhibit little fine-scale variability downcore, although a subtle coarsening with depth, of approximately 0.5 phi units, is evident in the data (Fig. 10). Although this coarsening with depth potentially signals a fining in material delivered to the shelf and slope over the past few centuries, there are also biological factors that may be creating this coarsening with depth. Many species of infauna act to retain finer grained particles at the sediment surface by their feeding strategies (e.g., Rhodes and Young, 1970). In contrast to the outer shelf and open slope cores, Poverty Canyon and slope gully cores (KM0503 cores 10 and 53) show greater downcore variability in grain size, probably related to the more episodic nature of sediment delivery to those types of sites on the margin. As illustrated by the non-steady-state canyon 210Pb profile (Fig. 4) and sedimentary structures discussed immediately

Fig. 8. Mean grain size on the outer Poverty margin. Sediments fine seaward from sandy silts to silty clays across the outer shelf to the mid-slope plateau. Most fining occurs between the outer shelf and the base of the upper slope.

C.R. Alexander et al. / Marine Geology 270 (2010) 213–226

223

Fig. 10. Downcore profiles of selected cores from this study. Note the relatively constant profiles from cores on the outer shelf and mid-slope plateau, which coarsen slightly with depth. Cores from canyons and gullies (10, 53) display more variability downcore, documenting the more episodic nature of sedimentation in those environments.

sediment gravity flows that transport shelf material rapidly into deep water. Based on recent analysis of four cores from the Poverty Canyon, Sumners (2007) estimates that these events occur approximately every 8.9 y. In contrast, given the angularity of the mudstone fragments, these mudstone layers represent an autochthonous material, sourced from within the slope itself and derived from mass wasting of gully walls, and do not represent transport of material over great distances. 9. Sediment budget for the outer Poverty margin

Fig. 9. Representative grain size histograms illustrating the fining from (A) upper slope to (B) mid-slope plateau and the shift in unimodal character along that gradient. (C) Ternary diagram of surface samples from all cores collected in this study. Note the dominance of silt and clay in all samples and that a few mid-shelf samples stand out here by reason of their sand content.

below, down canyon flows and mass wasting provide a more variable supply of material to these depositional environments. X-radiographs of outer shelf and slope cores are typically mottled to homogenous. Cores from the Poverty Canyon axis exhibit laminated zones, on the order of 5–10 cm thick, separated by whereas some cores from slope gullies exhibit isolated layers of semiconsolidated mudstone (Fig. 11A,B). These 5–10 cm thick laminated zones represent periodic

Rates of sediment accumulation documented in this study can be used to create a sediment budget that quantifies the current retention of Waipaoa fluvial material on the Poverty outer shelf, upper slope and mid-slope plateau. Regions of similar accumulation rate and sedimentary character were identified by integrating the sedimentological and radiochemical results reported in this paper. Subsequently, ESRI ARCGIS 9.2 was used to quantify the area of each of these regions (Table 2). These results show that, on a 100-y timescale, approximately 13–18% of the Waipaoa annual discharge (15 × 106 T y− 1) is accumulating on the outer Poverty margin as a whole, and approximately 11–15% of the annual discharge is accumulating on the upper continental slope and mid-slope plateau alone (see next paragraph for discussion of error terms). Notably, approximately 28% of the sediment annually accumulating in the outer margin is being sequestered in slope gullies and canyons, although they represent only 6% of the area of the outer margin. In addition, the budget shows that 3× more sediment is accumulating in the outer shelf depocenter and along the shelf edge

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each region is well-constrained, given the geomorphological boundaries upon which each region is based. Each region, except for the mid-slope plateau, is well-characterized in terms of a representative mean value of accumulation rate derived from widely distributed cores within each region. However, the comparatively few rates produced in this study for the mid-slope plateau are in agreement with rates for the same region provided in Orpin (2004), suggesting that the average rate used in the budget for this region is reasonable. The error which potentially most significantly affects the budget and which cannot be well-constrained at this time is the amount of material that may be transported to the midslope plateau from other parts of the margin, given the large size of the region when compared to others in the budget. Currents in the region could act to supply sediment from outside the system (Orpin, 2004). Assuming that only half of the material accumulating on the plateau is from the Waipaoa River, we arrive at the lower estimate for input to the margin given in the paragraph above. In the worst-case, assuming that all the material on the mid-slope plateau is derived from outside the Waipaoa system, the outer shelf and upper slope regions account for 1.5 × 106 T y− 1 (10% of the annual discharge), and the upper slope accounts for 1.1 × 106 T y− 1 (7% of the annual discharge) alone. To better constrain this error term, additional work on the geological, sedimentological and physical oceanographic setting in the area is required. 10. Comparison to other margins

Fig. 11. X-radiograph negatives showing the character of preserved stratification in (A) canyon and (B) slope gully settings. (A) Canyon stratification, representing down canyon gravity flows, is typically finely laminated, in beds 5–10 cm thick, and bounded by bioturbated units. (B) Gully stratification, although uncommon, typically consists of locally derived angular fragments of mudrock contained within a bioturbated matrix.

between Waipaoa and Poverty Canyons than there is north of Waipaoa Canyon. If we add this to the sediment accumulating on the shelf (18–30% of the Waipaoa discharge; Miller and Kuehl, 2010-this issue), we can account for 29–45% of the sediment discharged by the Waipaoa River annually, leaving 55–71% of the discharge unaccounted for on the shelf, upper slope and mid-slope plateau. Errors in the sediment budget could arise from: 1) error in the determination of accumulation rates as a whole (approximately 10% based on the standard error of the regressions calculated from 210Pb profiles); 2) error in the definition of the areal extent of each region; and 3) error in the average accumulation rate used in each region, which is directly related to the number of accumulation rates available to characterize each region (Fig. 3); and 4) error arising from input of sediment from sources other than the Waipaoa River. The areal extent of

Table 2 Outer Poverty margin sediment budget. Region

Area (m2)

Outer Shelf (N) 0.4 × 108 Outer Shelf (S) 0.5 × 108 Proximal canyons 0.75 × 108 Proximal intercanyons 0.75 × 108 Base of upper slope 1.2 × 108 Mid-slope plateau 8.6 × 108 Total 12.2 × 108 Percent of Waipaoa discharge – Including Outer Shelf – Excluding Outer Shelf

Mass accumulation (T y− 1) 0.1 × 106 0.3 × 106 0.7 × 106 0.1 × 106 0.3 × 106 1.0 × 106 2.5 × 106 2.0–2.5 × 106 1.6–2.0 × 106

Percent of total 4 12 28 4 12 40 100 13–18 11–15

These results show similarities to other active margins which have been documented in the past decade. The Eel margin, California, an active margin similar to the Poverty margin, has been extensively documented in the STRATAFORM Program, and may have the bestconstrained sediment budget for an active margin presently receiving sediment. The Eel River discharges a similar amount of sediment (~15× 106 T/y) to a narrow, energetic shelf. In this setting, approximately 20% of the annual discharge has been documented to accumulate on both the shelf and slope, leaving approximately 60% of the annual discharge to accumulate either in the nearby Eel Canyon, and/or to be redistributed widely along the margin (Alexander and Simoneau, 1999; Sommerfield and Nittrouer, 1999). Similarly, the energetic Santa Clara, California, shelf is trapping approximately 19–30% of the annual fluvial discharge from this river, leaving 70–81% to accumulate in offshore areas (Alexander and Lee, 2009). Integrating over the wider region, recent work has documented that the San Pedro and Santa Monica margins, California, trap approximately 25% of the region's annual input on the shelf, with the remainder accumulating on the slope and in deep offshore basins (Alexander and Lee, 2009). The southern Barcelona continental margin, Spain, (Sanchez-Cabeza et al., 1999) may also provide a good analogue to the Poverty margin, although the details of sediment accumulation on the margin are not as well constrained. However, they document that the upper and mid-canyon regions are major depocenters for sediment accumulation, based on 210Pb inventories and showing that little material is escaping farther down the canyon. In contrast, some active margins, particularly those with steep drainage basins and very narrow shelves, discharge directly into the sea, (i.e., Sepik in Papua New Guinea, Kuehl et al., 2004, and the Langyang in Taiwan, Hu et al., 2006) and retain very small amounts of sediment in the shelf. The Gulf of Lions (DeGeest et al., 2008), although having neither of the characteristics of the two systems just mentioned, shows only 6–8% retention of annual discharge on the shelf because of energetic conditions and a long sediment transport pathway along-shelf prior to being intercepted by a canyon, and little remains in the canyon system (~1%). Preferential accumulation of sediment in the southern part of the outer shelf and transfer of material from the outer shelf depocenter to the head of Poverty Canyon has been proposed as a major sediment transport pathway for escape of shelf sediment to the slope (Kuehl et al., 2006; Orpin et al., 2006; Walsh et al., 2007) and results reported here support that contention. The details of transport mechanisms that

C.R. Alexander et al. / Marine Geology 270 (2010) 213–226

deliver fluvial sediment to the outer shelf are not yet know, although the southward flowing East Cape Current is a likely candidate to impose a general southward transport on material transiting the shelf to the slope. Interestingly, none of this sediment that is presently escaping the shelf to deeper water via the canyon is transiting the length of the canyon to the deep sea. The similarity in accumulation rates both inside and outside the canyon below about 1200 m water depth suggests that below this water depth, sediment gravity flows are not a major source of sediment to the slope and nepheloid and/or hemipelagic sedimentation are the dominant mechanisms for sediment delivery. 11. Summary Short- and long-lived radionuclides document that sedimentary material is reaching the Poverty outer shelf and slope seaward of the Waipaoa River. The presence of short-lived 7Be documents that transfer of sediment from the land, across the shelf to the upper slope and midslope plateau is an on-going process that takes place on monthly to seasonal timescales, where it accumulates preferentially in the heads of shelf-indenting canyons and in upper slope gullies at rates approaching 3 cm y− 1, demonstrating a strong linkage between terrestrial source and marine sink. In addition, the sediment budget presented here documents that sediment is escaping the shelf in significant quantities on centennial timescales. Both surficial activities and excess 234Th inventories in the seabed document greater material in the water column during the winter months. These observations are similar to those from other active margins, where between 20 and 30% of annual fluvial discharges are accumulating on shelves, with the remainder accumulating in the slope and deep sea. However, large amounts of sediment (55–71%) are not found within the local shelf and slope regions, suggesting a widespread distribution for sediment within these energetic, tectonically active settings. Acknowledgements The authors appreciate the responsive efforts put forth by the crews of the US RV Kilo Moana and NZ RV Tangaroa during the cruises. Claudia Venherm, Lisa Northcote, Reide Corbett, Mike Robinson, Linda Meneghini, numerous US and NZ graduate students and several US teachers all contributed significantly to the core collection effort. Reviews by Jeff Borgeld, Don Gorsline and an anonymous reviewer provided comments that improved this manuscript. This work was supported by grants OCE0405726 and OCE-0646771 from the National Science Foundation within its MARGINS Source-to-Sink Initiative. AO was supported by the Foundation for Research Science and Technology NIWA contract C01X0203 and CO1X0702.

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