Sediment waves on the Conrad Rise, Southern Indian Ocean: Implications for the migration history of the Antarctic Circumpolar Current

Marine Geology 348 (2014) 27–36

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Sediment waves on the Conrad Rise, Southern Indian Ocean: Implications for the migration history of the Antarctic Circumpolar Current Hisashi Oiwane a,⁎, Minoru Ikehara b, Yusuke Suganuma a, Hideki Miura a, Yasuyuki Nakamura c,d, Taichi Sato e, Yoshifumi Nogi a, Masako Yamane d,f, Yusuke Yokoyama d a

National Institute of Polar Research, 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan Center for Advanced Marine Core Research, Kochi University, B200 Monobe, Nankoku, Kochi 783-8502, Japan Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan d Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan e Geological Survey of Japan, Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan f Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan b c

a r t i c l e

i n f o

Article history: Received 30 May 2013 Received in revised form 9 October 2013 Accepted 17 October 2013 Available online 26 October 2013 Communicated by D.J.W. Piper Keywords: Conrad Rise Southern Ocean Antarctic Circumpolar Current sediment wave multi-channel seismic reflection

a b s t r a c t The Antarctic Circumpolar Current (ACC) is the worlds longest and largest current system; therefore, it plays a prominent role in the global distribution of heat, nutrients and greenhouse gasses. While past changes in the ACC have been reconstructed by a number of studies using sedimentary records in the Southern Ocean, a detailed understanding of the relationship between its temporal and spatial variability and the changes in the Earths climatic system remains unclear. In this study, we conducted multibeam bathymetry, multi-channel seismic reflection, and sediment coring surveys on the Conrad Rise, located in the southern Indian Ocean sector of the ACC. These data reveal large scale sediment wave structures with continuous and parallel reflectors that have low to moderate reflection amplitudes in the upper part of the seismic section (Unit A). These phenomena are most likely formed by bottom current interactions with the sea-floor and sediment transport under the ACC. The basal age of Unit A is estimated to be younger than Pliocene/Pleistocene boundary based on the extrapolation of sedimentation rates from a shallow sedimentary core. The lower part of the section (Unit B) is characterized by moderate to high amplitude sub-horizontal to horizontal reflectors that are interpreted as pelagic sedimentation with stronger current influence to the upper part of the unit. Based on the correlation with ODP sites in the Southern Ocean, we estimate that the upper part of Unit B mainly comprises calcareous ooze. Though the age of the change in the sedimentary environment from seismic Units B to A is not specified, it is thought to be caused by a northward shift of the ACC. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The Southern Ocean has a profound influence on the world's ocean and climate (e.g., Carter et al., 2009). Based on its salinity and temperature structure, the Southern Ocean has several latitudinal zones (Pollard et al., 2002). The Subtropical Front (STF), Subantarctic Front (SAF), Polar Front (PF), and the southern boundary of the ACC (SB) separate the Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ), and Antarctic Zone (AAZ), respectively (Fig. 1). The ACC is enclosed between the SAF and SB (Fig. 1), and the mean position of the ACC “axis” is given by the locus of the PF (Barker and Thomas, 2004). The ACC is also the most prominent current in the Southern Ocean, and it flows through three major ocean basins. The eastward flow of the net current of the ACC extends from the surface to the bottom of the ocean, and its path is guided by the seafloor topography (Orsi et al., 1995). The ACC is also

⁎ Corresponding author. Tel.: +81 42 512 0762; fax: +81 42 528 3479. E-mail address: [email protected] (H. Oiwane). 0025-3227/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.margeo.2013.10.008

responsible for the inter-basin exchange of heat, salinity, nutrients, and gasses, and it contributes to the thermohaline circulation (e.g., Rintoul, 2009). Furthermore, the ACC inhibits meridional transport of water, which causes thermal isolation of the Antarctica from the warm waters distributed to the north of the current (e.g., Rintoul, 2009). Therefore, the past variability of the ACC has a large impact on environmental change in the Southern Ocean and Earth's climatic system. Thus the establishment of ocean gateways and fluctuation of the ACC have been discussed in relation with the cooling of the Antarctica for example at the Eocene/Oligocene boundary (e.g. Exon et al., 2000; Barker and Thomas, 2004; Bijl et al., 2013), but these are also disputed (e.g. DeConto and Pollard, 2003; Huber and Nof, 2006). The ACC plays an important role in the zonation of the biogeochemical features and processes in the Southern Ocean (e.g., Pollard et al., 2002). Production of diatoms is greatly subordinate to other phytoplankton north of the PF due to the limitation of temperature and nutrients (e.g., Burckle and Cirlli, 1987; Zielinski and Gersonde, 1997; Nelson et al., 2001), resulting in a predominance of carbonate sediments (Hutchins et al., 2001). In contrast, diatoms are a dominant primary

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Fig. 1. Bathymetric and oceanographic map of the Indian and Atlantic sector of the Southern Ocean. Topographical features are labeled. SWIR: Southwestern Indian Ridge, DCR: Del Caño Rise, CR: Conrad Rise, KP: Kerguelen Plateau, MR: Maud Rise. Oceanic fronts are shown in white lines. STF: Subtropical Front, SAF: Subantarctic Front, PF: Polar Front, SB: Southern Boundary of ACC. Oceanic zones are also indicated. SAZ: Subantarctic Zone, PFZ: Polar Front Zone, AAZ: Antarctic Zone. Positions of the oceanic fronts and zones are based on Sokolov and Rintoul (2009a, b). ODP Site numbers are leveled.

producer south of the PF and account for up to two-thirds of the total global silica input to the deep oceans (Tréguer et al., 1995; DeMaster, 2002; Cortese et al., 2004). These characteristics make it possible to reconstruct past fluctuations of the ACC associated fronts, and water masses using geological, geophysical, and geochemical analyses of the Southern Ocean sedimentary archives and to evaluate their response to changing climate (e.g., Howard and Prell, 1992; Barron, 1996; Bohaty and Harwood, 1998; Becquey and Gersonde, 2002; Diekmann and Kuhn, 2002; Whitehead and McMinn, 2002; Gersonde et al., 2005; Flores and Sierro, 2007; Marino et al., 2009; Kemp et al., 2010; Williams et al., 2010; Etourneau et al., 2012). The meridional migration of zonal westerly winds and Southern Ocean fronts in response to global climate events, such as the late Pliocene global cooling, mid-Pleistocene transition, and mid-Brunhes event, has been proposed by several studies (e.g., Bard and Rickaby, 2009; Kemp et al., 2010; McKay et al., 2012). McKay et al. (2012) suggest that major ice expansion on Antarctica and sea-surface ocean cooling began at approximately 3.3 Ma, followed by a stepwise expansion of sea ice between 3.3 and 2.5 Ma. Based on the AND-1B sediment record from the Ross Sea and deep ocean drill core records from the global ocean, McKay et al. (2012) hypothesize that the northward migration of westerly winds and ocean fronts potentially reduced the Atlantic Meridional Overturning Circulation (AMOC) by restricting the surface water connectivity between the ocean basins. Kemp et al. (2010) highlighted a stepwise northward migration of the locus of the PF in the early to mid-Pleistocene based on the change of diatom ooze distribution. A key feature of these hypotheses is the northward migration of the ACC that restricts interconnectivity of subtropical gyres between ocean basins and, specifically, heat transport from the Indian Ocean to the Atlantic Ocean, which could potentially weaken the global ocean circulation. However, these paleoceanographic inferences are made based on a limited number of records. In this study, we provide direct evidence of migration of the ACC and associated oceanic fronts in the Indian Ocean. The depositional units we describe hold a record of the relationship of change in the ACC to global cooling events that is accessible through scientific drilling.

The Conrad Rise is an intra-plate aseismic topographic high located between 50°S and 55°S in the Indian sector of the Southern Ocean, and it is presently situated in the AAZ near the PF (Fig. 1). The ACC is unrestricted in the region and bifurcates at the western side of the Conrad Rise, forming jets along the 3500 m isobaths to the north and south, and it converges again on its eastern side (Ansorge et al., 2008). This oceanographic setting is suitable for addressing the past fluctuations of the ACC and its associating oceanic fronts. However, the sedimentary history of the Conrad Rise remains poorly understood because of very limited geological and geophysical data from the region. In this study, we interpret a northward shift of the paleo-ACC based on seismic reflection configuration and facies, swath bathymetry, and sediment coring from the Conrad Rise. This first report of the long-term sedimentary history from the Conrad Rise is correlated with ODP sites in the Kerguelen Plateau and southeastern Atlantic, and provides a direct record of Late Neogene ACC variability and new insights into the evolution of the Southern Ocean. 2. Methods We conducted swath bathymetric surveys, seismic reflection, and sediment coring in the southwestern slope of the Conrad Rise. Surveys were undertaken during two cruises in 2008 and in 2010-2011 using the Research Vessel (R/V) Hakuho-Maru from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Swath bathymetric data were obtained using a SeaBeam 2120 echosounder system along the ship's tracks (Fig. 2). 150 beams and a swath width of 120° were used for data collection. The raw data were edited to remove outlying depth values. The sea-floor gradient was calculated from the grid data spacing and the differences in elevation. We collected a sedimentary core (COR-1bPC) at 54° 16′ S, 39° 46′ E on the southwestern slope of the Conrad Rise, where the water depth was 2828 m (Fig. 2). The length of the core was 10.5 m. Color reflectance was measured at 1 cm resolution on the surface of split cores (Fig. 3). The Minolta CM-2002 photospectrometer was used to measure the hue and chroma attributes of the sediments, as well as the reflected

H. Oiwane et al. / Marine Geology 348 (2014) 27–36

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Fig. 2. Bathymetry and gradient map of the survey area. Locations of seismic profiles, the coring site, and erosional truncations are also shown. The bathymetric contour interval is 100 m. The coring site is the same as that of Katsuki et al. (2012). The thick red lines indicate the seismic profiles in Fig. 4.

visible light in 31 10-nm-wide bands ranging from 400 to 700 nm. The colors of the sediments were expressed in the L*, a*, and b* color space indexes. The lightness (variable L*) ranged from 0% (black) to 100% (white). The bulk density was analyzed using a sensor for gamma ray attenuation (GRA) on the Geotek multi-sensor core logger (MSCL) system at Kochi University. Data were sampled at 1-cm intervals. The GRA densitometer used a 10 mCi 137Cs capsule as the gamma ray source (with the principal energy peak at 0.662 MeV) and a scintillation detector. Because the attenuation coefficient was similar for most common minerals and for aluminum, the bulk density was obtained by directly calibrating the densitometer using aluminum rods of different diameters that were mounted in a core liner that was filled with distilled water. The bulk density reflects the combined effect of variations in porosity and grain density (the dominant mineralogy). Radiocarbon age is provided by accelerator mass spectrometry (AMS) 14 C dating of the planktonic foraminifer Neogloboquadrina pachyderma (sinistral) (Table 1). Four radiocarbon samples were treated according to the protocol described by Yokoyama et al. (2007, 2010) with target

graphites measured at two AMS facilities at the University of Tokyo with different terminal voltages (5 MeV and 250 keV). All dates are corrected for the reservoir age (890 yr) of the Southern Ocean (Bard, 1988) and converted to calendar years (cal yr BP) using the calibration program CALIB 6.1.0 (Stuiver and Reimer, 1993). The linear sediment accumulation rate calculated between age control points is 26.6 cm/ka. A multi-channel seismic reflection survey was also conducted during the two cruises. The sound source was a Sercel GI-gun with a total chamber volume of 355 in3 (250 in3 for the generator and 105 in3 for the injector). The GI-gun was fired at 125 bars (1800 psi) of air pressure. A 48-channel solid streamer with a group spacing of 25 m was used. The CDP (Common-Depth Point) intervals were 12.5 m, and the averaged fold number was 14. The record length was 10 s with a 1 ms sampling interval. The total line-length of the survey line was approximately 220 nautical miles comprising 12 survey lines. Data were edited and sorted, and then a normal move-out correction was applied based on velocity analysis results. After stacking, the data were migrated to remove diffraction interference.

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EPICA

COR-1bPC

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3

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1354

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35

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40 11 Depth (m)

→ white

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Age (kyr)

Fig. 3. Lithostratigraphy and physical properties of the COR-1bPC core and its correlation to the EPICA Dome C ice core. a) Lithology and photograph, b) bulk density, c) L* (pale blue) and 5-points moving average of L* (blue), d) calibrated age (kyr) of the COR-1bPC core, and e) temperature (ΔD) profile from the EPICA Dome C ice core (pale red) (Jouzel et al., 2007) and 5-points moving average of ΔD (red). An age tie point between the COR-1bPC core (4.7 m) and the EPICA Dome C ice core (17.4 ka) is shown by the dotted line.

areas A and B in Fig. 2, without a systematic downslope decrease of height and/or wavelength.

3. Results 3.1. Bathymetry

3.2. Sedimentary cores The swath bathymetric map of the southwestern slope of the Conrad Rise from 2400 to 3400 m is shown in Fig. 2. The bathymetry deepens in the southwestward direction, and a small rise with steep edges was observed in the middle-western part of the survey area. Two depressions are observed next to this small rise. Wavy geomorphic features are observed in the majority of the survey area and are generally parallel to the contour lines with a wavelength of 1–2 km, a height of 10–100 m, and a lateral continuity of 5–40 km. The larger waves are only observed in

The COR-1bPC core is composed of diatom ooze (Fig. 3a). The bulk density values are 1.2–1.3 g/cm3 from the bottom of the core (10.5 m) to 5.0 m, and shifts to 1.1 g/cm3 between 5.0 and 2.5 m. From 2.5 m to the seafloor (the top of the core), the bulk density is much smaller (1.0–1.1 g/cm3) (Fig. 3b). The lightness (L*) values are darker from the base of the core to 4.7 m, after which they change to lighter values. From 3.8 m to the seafloor, lighter values continue (Fig. 3c).

Table 1 Calibrated ages (cal yr BP) of planktonic foraminifera Neogloboquadrina pachyderma (sinistral) from core COR-1bPC. MTC: Micro Analysis Laboratory, Tandem accelerator, University of Tokyo. YAUT: Yokoyama Laboratory, Atmosphere and Ocean Research Institute, University of Tokyo. Depth (cm)

Lab code

14

30.2 211.5 526.9 739.3

MTC-15632 MTC-15638 MTC-16635 YAUT-000709

2315 10,055 16,285 24,120

C age (yr BP) ± ± ± ±

45 60 85 110

Reservoir age (Bard, 1988)

Reservoir age corrected 14C age (yr)

Calibrated age (1 sigma) (yr BP)

Mid calibrated age (yr BP)

890 890 890 890

1425 9165 15,395 23,230

1302 10,359 18,560 27,869

1406 10,513 18,706 28,250

1354 10,436 18,633 28,060

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3.3. Seismic stratigraphy The sedimentary succession on the Conrad Rise was divided into four seismic units based on unconformities and seismic facies: A, A′, B, and C in descending order. Four seismic profiles are shown in Fig. 4, which represent the characteristics of these units in this region. The thickness of Unit A is 0.1–0.5 s in two-way travel time (twt), which corresponds to a maximum thickness of approximately 387.5 m, assuming that the p-wave velocity of 1550 m/s reported at the top of the ODP Site 1093 core is the same of that of the Conrad Rise due to similar sediment composition (Gersonde et al., 1999). Unit A displays low to moderate amplitude, continuous and parallel reflectors with large scale sediment wave structures in the middle part of the slope (CDP 300–600 in Fig. 4a; CDP 250–350 in Fig. 4c). The upper boundary of this unit displays a wavy configuration. Internal reflectors are continuous and transparent to low reflection amplitude, and they also display a wavy configuration that is generally parallel to the upper boundary (cf. Mitchum et al., 1977; Normark et al., 2002; Wynn and Stow, 2002). Crests of sediment waves show a slight upslope migration. Erosional truncation of the underlying strata occurs locally (CDP 750–850 in Fig. 4a). Based on these characteristics, Unit A is subdivided into 6 subunits: A-1 to A-6, in ascending order. Faults, channels, and levees are not observed in this unit. Units A and B can be traced to the mounded deposit at the side of the small rise (Figs. 2 and 4c) via crossing seismic profiles 07-01 (Fig. 4c). Unit A′ is characterized by contorted lenticular configuration and only observed above regions of erosional truncation at the upper boundary of the Unit B (CDP 650–1050 in Fig. 4a). Internal reflectors of this unit are not clear. The configuration of the upper boundary of Unit B at less than 4.25 s (twt) is different from that of depths deeper than 4.25 s (twt). The shallower side (CDP 650–1300 in Fig. 4a) has undulating surface, and an erosional truncation of the underlying unit occurs. In the deeper part of the section towards the SW, the unit boundary is marked by a smoother relationship between the parallel and sub-parallel reflectors in both the overlying and underlying units (CDP 0–650 in Fig. 4a and CDP 300–600 in Fig. 4b). The internal reflectors of Unit B are semicontinuous, parallel to sub-parallel and of moderate to high amplitude. The reflectors are disturbed by faults with small displacements. A fault that cuts the upper boundary of the unit indicates a termination of the active faulting before the sedimentation of Unit A (Fig. 4b). Unit B is subdivided into 7 subunits, B-1 to B-7 in ascending order, which display progressively higher-amplitude reflections. The lower subunits (B-1 to B-3) display a sheet drape external form. Internal reflectors of the subunits have low reflection amplitudes and onlap underlying Unit C. In contrast, the upper subunits (B-4 to B-7) have a wedge-shaped external form thickening to the NE (Fig. 4a). The upper boundary of Unit C displays high amplitude reflections with an irregular discontinuous morphology (Fig. 4a, c and d). For the most part, this unit displays chaotic facies, whereas topographic lows tend to show a few stratified reflectors (CDP 300–500 and 600–800 in Fig. 4a). The stratified reflectors onlap the chaotic facies beneath the topographic highs (CDP 250–300 in Fig. 4d). 4. Discussion 4.1. Formation process of Unit A 4.1.1. Formation of sediment waves Seismic reflectors that are interpreted as sediment waves are almost parallel to the seafloor topography throughout Unit A, and they display transparent to low amplitudes (Fig. 4a), indicating that sedimentation is stable and lithologically homogeneous. The COR-1bPC core on the Conrad Rise and an earlier COR-1PC core reported by Katsuki et al. (2012) for the same region are comprised mainly of diatom ooze. The bulk density values of the COR-1bPC core (1.1–1.4 g/cm3) are similar to those of the diatom ooze from the ODP Site 1093 located in a similar

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oceanographic setting (Gersonde et al., 1999), indicating that Unit A, which forms sediment waves on the Conrad Rise, is most likely composed of diatom-dominated sediments. Sediment waves most likely form from turbidite or bottom current deposition (Faugéres et al., 1999; Wynn and Stow, 2002). Based on Wynn and Stow's (2002) criteria, sediment waves originating from turbidites typically display a basinward decrease in height and wavelength (Wynn and Stow, 2002). However, sediment waves on the Conrad Rise are not attenuated in height or wavelength (A and B in Fig. 2), implying an origin that is not associated with downslope depositional processes. In addition, the moderate amplitude continuous reflectors that follow the gross geometry of the unit, and the sediment wave structures are consistent with contourite deposition based on the definition by Nielsen et al. (2008), supporting our interpretation that this unit is not of turbidite origin. Furthermore, sediments do not bury two depressions at the middle-western part of the survey area, indicating that it is unlikely that the mounded deposit between the depressions originated from gravity flow sediments (Figs. 2 and 4c). Therefore, we concluded that bottom currents are the most likely process in the formation of sediment waves on the Conrad Rise. The ACC is widely regarded as a deep reaching or barotropic current that interacts with the ocean-floor around the periphery of the Antarctic continental margin (e.g., Orsi et al., 1995; Gordon, 2001). The position of the ACC flow is partly controlled by the sea floor topography (Gordon, 2001). Durgadoo et al. (2008) show that the ACC flow to the north and south is divided by the topographic high of the Conrad Rise. The estimated current speed is approximately 2–6 cm/s at 2500–3000 m in depth based on numerical ocean modeling (Durgadoo et al., 2008). This current speed enables the transportation of fine-grained sediments (Kidd and Hill, 1987). A number of studies, which were conducted in the Argentine Basin during Project MUDWAVES (Manley and Flood, 1993), indicate that fine-grained sediment waves can be formed beneath bottom-currents under these low current speeds (e.g., Manley and Flood, 1993; Weatherly, 1993). Oceanic fronts in the ACC cause an upwelling of nutrients to the surface (Rintoul, 2009), resulting in high diatom productivity to the south of the PF (e.g., Werfer and Berger, 2001; Diekmann, 2007). This is consistent with the sedimentation of diatom ooze on the Conrad Rise. Therefore, we concluded that the sediment waves on the Conrad Rise that constitute Unit A were formed by pelagic biosiliceous deposition under the influence of the ACC. 4.1.2. Estimated age of Unit A Sedimentation rate of Unit A is estimated by the correlation between L* and temperature (ΔD) record in the EPICA Dome C ice core (Fig. 3c and e). The drop in the L* color reflectance curve of the COR-1bPC from 4.7 m is most likely corresponding to the temperature decrease of the EPICA Dome C ice core, which represents the onset of deglaciation (4.7 m in Fig. 3c and 17.4 ka in Fig. 3e). This feature is also reported in the color reflectance record of the ODP Site 1094 from Southern Atlantic over the past 1.1 Ma (Kemp et al., 2010; Sigman et al., 2010). Accordingly, the sedimentation rate of COR-1bPC core can be estimated as 27.0 cm/kyr based on the onset of the deglaciation expressed as a significant drop of the L* curve. This sedimentation rate is obviously supported by 14C dating in planktonic foraminifer (Table 1 and Fig. 3d). The basal age of Unit A (387.5 m below seafloor) can be estimated as 1.43 Ma by extrapolation of the sedimentation rate (27.0 cm/kyr), however, slower sedimentation rate during glacial period should be considered. For example, Kanfoush et al. (2002) reported that the sedimentation rates during the glacial maximum periods are almost 50% of that in interglacial periods at the ODP Site 1094. Assuming the sedimentation rate in the glacial periods as half of that in interglacial periods and equal duration of glacial/interglacial periods in Unit A, average sedimentation rate is 20.25 cm/kyr. The basal age becomes 1.91 Ma by applying the average sedimentation rate. Thus, we conclude that the basal age of Unit A is not older than the Pliocene/Pleistocene boundary and most likely during the Gelasian.

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H. Oiwane et al. / Marine Geology 348 (2014) 27–36

CDP

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fault cuts the upper boundary

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Fig. 4. Seismic reflection profiles and their interpretations. Red lines show crests of sediment waves, blue lines show faults. Locations of the seismic lines are shown in Fig. 2. CDP: common depth point. a) Line 11. Inset box shows colors of subunits. b) Line 35. c) Line 34. d) Line 33.

H. Oiwane et al. / Marine Geology 348 (2014) 27–36

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The distribution of Unit A′ is limited and its internal structure is different from that of Unit A (CDP 600–1000 in Fig. 4a). However, the boundary between Units A and A′ shows wavy configuration parallel to overlying sediment waves, whereas the base of Unit A′ is marked by local erosional truncation of the underlying Unit B. This implies that the lower boundary is a regional unconformity, and we interpret Unit A′ as a localized drift sediment and/or a possible contourite sediment during the development of an invigorated ACC in the region, which subsequently stabilized during the deposition of Unit A.

constrains biogeographical zonation, similar sedimentary sequence between the Conrad Rise and these ODP sites is expected. As stated earlier, we observed diatom ooze and hiatus at Unit A/B boundary. Accordingly, we estimate that Unit B of the Conrad Rise mainly comprises of calcareous ooze in correlation with these ODP sites. These data indicate a fundamental change in the primary productivity of the Southern Ocean, which is recorded in the sedimentary environment as a transition from calcareous ooze to siliceous ooze with a clear hiatus in this area. Unit A, marked by sediment wave and diatom ooze is an indication of the ACC, and calcareous ooze is dominant in the north of the ACC (e.g. Hutchins et al., 2001). Accordingly, we associate the sedimentary change to the northward expansion of the ACC and intensification of abyssal flow at Conrad Rise with a period of erosion and subsequent biosiliceous deposition as sediment waves. The age of the oceanographic change and hiatus is not constrained by our data. However, if they are correlated to the ODP sites in the Kerguelen Plateau, they are estimated around Miocene/Pliocene boundary. They also can be correlated with paleoceanographic change reported in the Antarctic Peninsula that took place some time between start of Middle Miocene to the end of the Miocene (Hernández-Molina et al., 2006).

4.3. Formation of Unit B

4.4. Formation of Unit C

At the upper boundary of Unit B, erosional truncation features are observed only in the shallower NE side of the survey area (Fig. 4a). The erosional truncation does not continue downslope, or exhibit channel and levee structures at the unconformity. This suggests that the erosion is caused by a contour current rather than by a gravity-controlled current. Faults are observed only in Unit B, indicating that the sedimentation of Units A and B is temporally distinct. The generally parallel, continuous low amplitude internal reflectors of the lower subunits of Unit B (subunits B-1 to B-3) that downlap or onlap to the underlying unit (CDP 0–600 in Fig. 4a; CDP 250–350 in Fig. 4d) indicate a gravity-controlled sediment supply (e.g. Mitchum et al., 1977). However, the thicknesses of the upper subunits (B-4 to B-7) increase towards the shallower NE side of the area, which is more consistent with sheeted drift sediments originating from contour currents (Mitchum et al., 1977). The reflection amplitudes of the unit gradually increase from the lower to the upper subunits, suggesting a change from uniform sediments to more variable ones through the unit. Based on these interpretations, we interpret a gradual change in sedimentary environment during deposition of Unit B, from turbidite to contourite sedimentation. This implies gradual change of bottom current and/or sediment through the Unit. The reflection amplitude of the upper subunits of Unit B is clearly higher than that of the overlying unit, which indicates interlayered sediments with contrasting P-wave velocity and density. Similarly, depositional configuration of upper subunits of Unit B is different from that of Unit A. These indicate different patterns of sediment supply between Units A and B. Since Unit A is most likely deposited below the ACC, Unit A likely deposited without the influence of the ACC. This suggests that sediment composition of the upper Unit B is different from that of Unit A. Since Unit A is estimated as diatom ooze, upper Unit B likely comprises of other pelagic sediment and/or IRDs. Turbidites may not be suitable because of its depositional configuration. In addition to the discussion above, correlation with ODP sites in the Southern Ocean enables us to estimate that upper Unit B mainly comprise of calcareous ooze. In the Kerguelen Plateau, ODP sites 748, 749, 750, 751, and 1137 are located in the similar oceanographic setting to the Conrad Rise, ~500 km south of the Polar Front (Fig. 1). They commonly exhibit siliceous ooze (with foraminifera and IRDs) of Plio– Pleistocene age at the seafloor, calcareous ooze of Cretaceous to the late Miocene age below the siliceous ooze, and hiatus which includes the Miocene/Pliocene boundary observed between them (Barron et al., 1987; Schlich et al., 1988; Coffin et al., 2000; O'Brien et al., 2000). Diatom ooze and unconformity are common in the Conrad Rise. Since the ACC

The boundary between Units B and C shows irregular configuration in some places, but no clear erosional truncation is observed. Topographic highs tend to show chaotic facies (CDP 100–200, 350–450 in Fig. 4c; CDP 300–400 in Fig. 4d), and topographic lows tend to show weak stratified reflectors (CDP 300–500 and 600–800 in Fig. 4a). Stratified reflectors show an onlap to some chaotic areas (CDP 250–300 in Fig. 4d). These reflection configurations are similar to those in the volcanic regions (e.g., Tsuji et al., 2012; Crutchley et al., 2013). Because the Conrad Rise is regarded as one of the Large Igneous Provinces (e.g., Coffin et al., 2006), the chaotic facies in topographic highs and the stratified reflectors in topographic lows are interpreted as volcanic bodies and volcaniclastic sediments, respectively.

The sedimentation rate at the Conrad Rise is extremely high for normal pelagic sediments. This is possibly due to the high diatom productivity caused by a vigorous upwelling of nutrients in the ACC (Werfer and Berger, 2001), and a drifting of sediments due to the meandering of the ACC at the west end of the Conrad Rise (Durgadoo et al., 2008). Therefore, deposition of Unit A, marked by a high accumulation rate of diatom ooze with sediment wave structures, is interpreted as an indication of the geographic position of the ACC. 4.2. Formation of Unit A′

4.5. Oceanographic reconstructions based on the sedimentary sequence of the Conrad Rise and ODP sites nearby in the Southern Ocean Sedimentary history and the oceanographic setting over the Conrad Rise are summarized as follows (Fig. 5). The basement of the Conrad Rise (Unit C) is composed of the volcanic bodies and volcaniclastic sediments. Unit B, which is inferred to consist of calcareous ooze, shows a change in the sedimentary environment from a homogeneous gravitycontrolled environment to contour current depositional conditions (Fig. 5a). Because the calcareous ooze deposits to the north of the PF at present (e.g., Burckle and Cirilli, 1987), the PF during sedimentation of Unit B was probably located to the south of the Conrad Rise (Fig. 5b). The hiatus in the Conrad Rise region and the other areas in the Southern Ocean likely corresponds to a major oceanographic reorganization possibly at the Miocene–Pliocene boundary. After the hiatus, the PF is thought to have shifted northward, and associated colder waters produced diatom ooze to form the sediment waves with extremely high sedimentation rates from the Gelasian to present (Fig. 5c and d). Because the mean position of the ACC “axis” is given by the locus of the PF (Barker and Thomas, 2004), this PF shift is thought to correspond to the shift of the entire ACC. The northward shift of the ACC is probably related to a major oceanographic change in the Southern Ocean and the Earth's climatic system (e.g. Biastoch et al., 2008; Bard and Rickaby, 2009; McKay et al., 2012). Bard and Rickaby (2009) report the northward migration of the STF during glacials of the late Pleistocene, restricting flow of the Agulhas Current that controls the heat and salt from the Indo-Pacific Ocean to the Atlantic Ocean, creating consequences in the severity of the glacial periods at the Marine Isotope Stages (MISs) 10 and 12. McKay et al. (2012) proposed that ice sheet expansion in West Antarctica and cooling in the Southern Ocean led to an increased seasonal persistence of sea ice

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a

b

c

d

Fig. 5. Schematic sedimentation model and estimated path of the ACC around the Conrad Rise. a) Sedimentation of Unit B and b) ACC located in the south of the Conrad Rise, c) sedimentation of Unit A with d) ACC over the Conrad Rise. SWIR: Southwestern Indian Ridge.

between 3.3 and 2.6 Ma, which is thought to have affected the northward expansion of westerly winds and the northward migration of ocean fronts in the Southern Ocean. They suggest that this situation contributed to a similar restriction of subtropical inflow to the south Atlantic (e.g. Bard and Rickaby, 2009), ultimately leading to a slowdown of the interhemispheric AMOC beginning after 2.6 Ma, and it would have further intensified the cooling into the Pleistocene. Our result suggests that a major northward shift of the ACC in the Indian Ocean has occurred, and covered the Conrad Rise probably after Pliocene/ Pleistocene boundary. Additionally, our result suggests an increase of productivity over the Conrad Rise, which implies enhanced zonal circulation or migration of the southern hemisphere westerlies altering nutrient availability or oceanic temperature. While our results provide some new insights into the Late Neogene evolution of the ACC that has potentially large impacts on environmental change in the Southern Ocean, it is still possible that the hiatus separating Unit A from B may be older e.g. the Late Miocene if it is correlated with other ODP sites in the Southern Ocean (Barron et al., 1987; Schlich et al., 1988; Coffin et al., 2000; O'Brien et al., 2000). The uncertainty arises from the lack of deep sediment drill cores that enables the determination of a long-term sedimentation rate at this site. Thus, a well dated transect of high-resolution sediment records using IODPtype drilling is urgently needed to understand the role of the ACC variability on global climate change such as oceanic ventilation, heat transport, and productivity.

5. Conclusion Seismic reflection, multibeam bathymetry and sediment coring were conducted on the Conrad Rise, located in the Indian sector of the Southern Ocean. Based on these data, the seismic reflection profiles on the Conrad Rise were divided into four seismic units in descending order: A, A′, B, and C. Unit A is approximately 378.5 m thick and consists of siliceous ooze with sediment wave structures. Based on the extrapolation of the sedimentation rate in the uppermost 4.7 m of a piston core correlated with EPICA Dome C records, the basal age of Unit A is estimated to be younger than Pliocene/Pleistocene boundary. In contrast, upper subunits of Unit B possibly mainly comprise of calcareous ooze with different seismic characteristics than those of Unit A. The change in the sedimentary environment most likely reflects a transition of the oceanographic setting on the Conrad Rise, relating to a northward shift of the ACC. Unit C is interpreted as volcanic bodies and volcaniclastic sediments. Further research on sediments on the Conrad Rise, for example, an IODPtype drilling project, is urgently needed to better understand the fluctuation of the ACC and its relationship to change in the Earth's climatic system. Acknowledgments We thank R. McKay, R. Larter (Reviewers), and T. Naish for the comprehensive discussions about the oceanographic and climatic change in

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the Southern Ocean. We also thank J.V. Durgadoo and S. Aoki for the helpful discussions concerning oceanic current around the Conrad Rise. We also thank the officers, crews, and technicians who provided essential help with data acquisition during the R/V Hakuho-maru KH07-4 Leg 3 and KH-10-7 Leg 4 cruises. We also thank technical staff T. Matsuzaki, S. Yanagimoto, S. Sakaguchi, and C. Nishimori for nondestructive measurements of sediment cores in Kochi University. This work was supported by JSPS KAKENHI (19340156, 21253001, 90335919, and 23244102), NIPR through Advanced Project (KP-1), Kochi University research project GEEDS, and JAMSTEC feasibility study for IODP proposal. The GMT software (Wessel and Smith, 1998) was used for mapping bathymetric and gradient data. The production of this paper was supported by an NIPR publication subsidy. 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