Sediment waves on the Conrad Rise, Southern Indian Ocean: Implications for the migration history of the Antarctic Circumpolar Current
Graphical abstract
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 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 Cirilli, 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 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.
Section snippets
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 echo-sounder system along the ship's tracks (Fig. 2). 150 beams and a swath width of 120° were used for data collection. The raw
Bathymetry
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
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
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
Acknowledgments
We thank R. McKay, R. Larter (Reviewers), and T. Naish for the comprehensive discussions about the oceanographic and climatic change in 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 KH-07-4 Leg 3 and KH-10-7 Leg 4 cruises. We also thank technical staff T. Matsuzaki, S.
References (64)
- et al.
Origin, signature and palaeoclimatic influence of the Antarctic Circumpolar Current
Earth-Science Reviews
(2004) Diatom constraints on the position of the Antarctic Polar Front in the middle part of the Pliocene
Marine Micropaleontology
(1996)- et al.
Past hydrodynamic and climatic changes in the Subantarctic Zone of the South Atlantic — the Pleistocene record from ODP Site 1090
Palaeogeography, Palaeoclimatology, Palaeoecology
(2002) - et al.
Southern Ocean Pliocene paleotemperature variation from high-resolution silicoflagellate biostratigraphy
Marine Micropaleontology
(1998) - et al.
Opal sedimentation in the world ocean over the last 15 Myr
Earth and Planetary Science Letters
(2004) - et al.
Insights into the emplacement dynamics of volcanic landslides from high-resolution 3D seismic data acquired offshore Montserrat, Lesser Antilles
Marine Geology
(2013) The accumulation and cycling of biogenic silica in the Southern Ocean: revisiting the marine silica budget
Deep-Sea Research II
(2002)Sedimentary pattern in the late Quaternary Southern Ocean
Deep-Sea Research II
(2007)- et al.
Sedimentary record of the mid-Pleistocene climate transition in the Southern Atlantic
Palaeogeography, Palaeoclimatology, Palaeoecology
(2002) - et al.
Seismic features diagnostic of contourite drift
Marine Geology
(1999)
Pronounced mid-Pleistocene southward shift of the Polar Front in the Atlantic sector of the Southern Ocean
Deep-Sea Research II
Sea-surface temperature and sea ice distribution of the Southern Ocean at the EPILOG Last Glacial Maximum — a circum-Antarctic view based on siliceous microfossil records
Quaternary Science Reviews
Current system in the Southern Ocean
The contourite depositional system of the Gulf of Cádiz: A sedimentary model related to the bottom current activity of the Mediterranean outflow water and its interaction with the continental margin
Deep-Sea Research II
The ocean circulation in the southern hemisphere and its climatic impacts in the Eocene
Palaeogeography, Palaeoclimatology, Palaeoecology
Comparison of ice-rafted debris and physical properties in ODP Site 1094 (South Atlantic) with the Vostok ice core over the last four climatic cycles
Palaeogeography, Palaeoclimatology, Palaeoecology
Migration of the Antarctic Polar Front through the mid-Pleistocene transition: evidence and climatic implications
Quaternary Science Reviews
Project MUDWAVES
Deep-Sea Research II
Response of calcareous nannofossil assemblages to paleoenvironmental changes through the mid-Pleistocene evolution at Site 1090 (Southern Ocean)
Palaeogeography, Palaeoclimatology, Palaeoecology
A seasonal progression of Si limitation in the Pacific sector of the Southern Ocean
Deep-Sea Research II
Variability in form and growth of sediment waves on turbidite channel levees
Marine Geology
On the meridional extent and fronts of the Antarctic Circumpolar Current
Deep-Sea Research I
Physical controls on biogeochemical zonation in the Southern Ocean
Deep-Sea Research II
Antarctic Circumpolar Current
Hydrothermal fluid flow system around the Iheya North Knoll in the mid-Okinawa trough based on seismic reflection data
Journal of Volcanology and Geothermal Research
On deep-current and hydrographic observations from a mudwave region and elsewhere in the Argentine Basin
Deep-Sea Research II
Kerguelen Plateau Quaternary–late Pliocene palaeoenvironments: from diatom, silicoflagellate and sedimentological data
Palaeogeography, Palaeoclimatology, Palaeoecology
Evidence for iceberg armadas from East Antarctica in the Southern Ocean during the late Miocene and early Pliocene
Earth and Planetary Science Letters
Classification and characterisation of deep-water sediment waves
Marine Geology
Relation between acid dissolution time in the vacuum test tube and time required for graphitization for AMS target preparation
Nuclear Instruments and Methods in Physics Research Section B
Diatom distribution in Southern Ocean surface sediments (Atlantic sector): implications for paleoenvironmental reconstructions
Palaeogeography, Palaeoclimatology, Palaeoecology
The first oceanographic survey of the Conrad Rise
South African Journal of Science
Cited by (9)
Diatom-based reconstruction of the Subantarctic Front migrations during the late Miocene and Pliocene
2020, Marine MicropaleontologyCitation Excerpt :One of the important achievements of this study includes the detailed reconstruction of paleo-SAF locations. Although many researchers have attempted to clarify paleo-ACC changes, the Miocene to Pliocene history of ACC-fronts remains poorly understood (e.g., Barron, 1996; Ballegeer et al., 2012; Oiwane et al., 2014; Taylor-Silva and Riesselman, 2018). This study presents the first reliable evidence for repeated migrations of SAF between 9 and 5 Ma and a SAF position further north after 5 Ma based on abundance patterns of subtropical diatoms at Site 513.
Antarctic sea-ice and palaeoproductivity variation over the last 156,000 years in the Indian sector of Southern Ocean
2020, Marine MicropaleontologyCitation Excerpt :We however note that early-mid Holocene SST in core DCR1-PC were 8–10 °C (Crosta et al., 2020; Fig. 4e), slightly higher than modern SST at the core site, which suggests that the northern APF branch did not reach the core site at ~46°S and was probably located south of its present location at ~50°S (Durgadoo et al., 2008). We therefore propose that the lower-than-modern SST reconstructed during the early Holocene in cores SK 200/22a and SK 200/27 result from the northward transport of SIZ and POOZ diatoms to the core sites, probably by bottom currents that are deflected by the presence of the Conrad Rise (Oiwane et al., 2014). As such, we believe that the new results from core SK 200/33, obtained from a more stable region, represent a better image of past SST and sea-ice dynamics than the ones inferred from the SK 200/22a and SK 200/27 core.
Late Quaternary geomorphology, seabed evolution, and terrigenous sediment delivery to the Pandora and Moresby Troughs, Gulf of Papua
2016, Marine GeologyCitation Excerpt :Undulating topographies in the deep-sea have long ignited debate about their origin. Some researchers believe that such features are generated by slope failure, plastic flow or MTD's (Lee et al., 1981; Field and Barber, 1993; Booth et al., 1993; Gardner et al., 1999; Correggiari et al., 2001; Canals et al., 2004; Francis et al., 2008; Urgeles and Camerlenghi, 2013), while others favor long-term sedimentation processes (Damuth, 1979; Normark, 1980; Carter et al., 1990; Lee et al., 2002; Mosher and Thomson, 2002; Oiwane et al., 2015). Francis et al. (2008) first studied this seabed region, and suggested that the undulating seafloor topography in Moresby Trough is a confined fan-shaped MTD transported via a major canyon.