Research ArticleLate Oligocene-Miocene proto-Antarctic Circumpolar Current dynamics off the Wilkes Land margin, East Antarctica
Introduction
The Antarctic Circumpolar Current (ACC) is the Earth's strongest ocean current (137–162 sverdrup (Sv)) flowing eastward along a 20,000 km pathway around Antarctica (Rintoul et al., 2001; Sokolov and Rintoul, 2009). Owing to the absence of land barriers, the ACC is the only ocean current connecting the Pacific, the Atlantic and the Indian oceans, and consequently influences the entire global ocean circulation (Rintoul, 2018). The ACC pathway is constrained by ocean gateways (i.e. Drake Passage) and the bathymetry of the Southern Ocean. Its strength is mainly controlled by the seafloor topography (Olbers et al., 2004), and the position and intensity of the Southern Westerly Winds (SWW) (Thompson and Solomon, 2002; Aoki et al., 2005; Toggweiler and Russell, 2008; Rignot et al., 2019). At present, the vigorous zonal flow of the ACC prevents the intrusion of warm waters from lower latitudes to penetrate the Antarctic margin and, together with sea-ice presence, contributes to maintain the cold and arid glacial state of Antarctica (e.g., Olbers et al., 2004; Ferrari et al., 2014). The deep layers of the ACC are occupied by a relatively warm and saline water mass, the Circumpolar Deep Water (CDW) (Orsi et al., 1995). Recently, an increasing incursion of CDW into the continental margins has been shown to favor melting and thinning of the Antarctic ice shelves through basal melting (Pritchard et al., 2012; Liu et al., 2015; Nakayama et al., 2018; Rignot et al., 2019). Despite its importance for the Antarctic and the global climate, little is known about the onset and past dynamics of the ACC, as well as its linkages with the Antarctic Ice sheet (AIS) dynamics. This knowledge is especially relevant from past times when climatic conditions were close to the modern and future ones in terms of warmth and atmospheric CO2 concentration.
One of these times was the late Oligocene (i.e., ~24.5 Ma), when the reconstructed atmospheric CO2 concentrations dropped below 600 ppm (600–400 ppm) (Zhang et al., 2013). These values are similar to the modern and projected atmospheric CO2 concentrations within this century (IPCC, 2013; Meredith et al., 2019). Foster and Rohling (2013) argued that the global ice volume is supposedly less sensitive to CO2 fluctuations between 600 and 400 ppm. In contrast, benthic foraminiferal oxygen isotope records (e.g., Liebrand et al., 2017) suggest highly fluctuating ice volumes at this time. The drop in CO2 concentration in the late Oligocene (~24.5 Ma) likely led to climate cooling and ice sheet advance across the Antarctic continental shelves, connecting large areas of marine-based ice with the ocean (Pekar and Christie-Blick, 2008; Levy et al., 2019). Ice-proximal geological records (Barrett, 1975; Naish et al., 2008; Kulhanek et al., 2019; Levy et al., 2019) and seismic data (Anderson and Bartek, 1992; Sorlien et al., 2007) provide direct evidence for a major expansion of marine ice sheets across the Ross Sea continental shelf between 24.5 and 24 Ma. However, the oceanographic and climatic conditions leading to the maximum growth of the ice sheet remain poorly known.
The global deep-sea benthic δ18Ο records maximum expansion of the AIS between 23.2 and 23 Ma (Zachos et al., 2001; Beddow et al., 2016; Liebrand et al., 2017). However, deep-sea benthic δ18Ο records reflect a combination of ice-volume and bottom water temperature and their location (low to mid-latitude versus Antarctic proximal records) determines the different water masses influencing the record and thus masking information from the Antarctic glaciation (e.g., Pekar et al., 2006). Thus, most of the ice volume estimates based on deep-sea benthic δ18Ο records should be taken with caution. Variations in the Southern Ocean circulation and ocean heat transport across the Antarctic continental margin driven by obliquity forcing have been suggested to play a significant role on ice sheet sensitivity during the late Oligocene and Miocene (Salabarnada et al., 2018; Sangiorgi et al., 2018; Levy et al., 2019). This is especially true in times when ice sheets extended into the marine environments (e.g., Jovane et al., 2019; Levy et al., 2019). Sedimentary archives strategically located along latitudinal transects across the main ACC pathway and at the vicinity of the Antarctic ice sheet are however needed to provide direct links between changes in the ocean circulation and ice sheet dynamics (Escutia et al., 2019).
Sedimentary records across the Tasman Gateway (Pfuhl and McCave, 2005) and from the South Pacific (Lyle et al., 2007) document a shift to higher velocity bottom currents between 25 and 23 Ma. This shift has been interpreted to result from the onset of a strong, deep-reaching ACC during the late Oligocene. However, recent comparisons between the dinocysts preserved in sediments from the Integrated Ocean Drilling Program (IODP) Site U1356 off the East Antarctic Wilkes Land margin and strata from Tasmania and south of New Zealand indicate a weaker than present day ACC, at least until the middle Miocene (Bijl et al., 2018b). Oligocene and Miocene paleoceanographic reconstructions off the Wilkes Land margin based on sedimentological data (Salabarnada et al., 2018), dinoflagellate biogeography (Bijl et al., 2018a, Bijl et al., 2018b; Sangiorgi et al., 2018) and temperature reconstructions (Hartman et al., 2018) suggest a different oceanographic configuration from that of today in this part of the Southern Ocean. These authors report from multiple lines of evidence warm-temperate sea surface temperatures (SST), limited sea ice expansion and reduced formation of Antarctic bottom waters, linked to a weaker oceanic frontal system, which allowed the intrusion of warmer waters from low latitudes towards the Antarctic margin. These data are consistent with Oligocene numerical simulations, which show weaker global overturning and gyre circulation because of weaker SWW (Herold et al., 2012). In addition, modeling results indicate a limited throughflow of the ACC due to the Australasian paleogeography during the Oligocene (Hill et al., 2013).
To decipher the characteristics and dynamics of the ACC and CDW that can then be related to East Antarctic Ice Sheet (EAIS) behavior off Wilkes-Adélie Land during the late Oligocene-Miocene, we report new data from a sediment record recovered by the Deep Sea Drilling Project (DSDP) Leg 28 at Site 269. This site was drilled on the Wilkes Land abyssal plain (Hayes et al., 1975) along the main pathway of the ACC. We focus on the study of the late Oligocene to earliest Miocene (~24.2–23 Ma) record. This record is partly compromised by debris flows at IODP Site U1356, located on the continental rise (Escutia et al., 2011), ~280 km landward from Site 269, and is missing in most sedimentary archives around the rest of Antarctica. Because of discontinuous drilling at Site 269, we investigate snapshots of the late Oligocene and early Miocene. Sediment, palynological, geochemical and isotopic data are used to describe and characterise the main changes in sedimentation related to proto-CDW dynamics. The findings at DSDP Site 269 are then compared to results from IODP Site U1356 (Escutia et al., 2011, 2014; Salabarnada et al., 2018) (Fig. 1). This latitudinal comparison provides important insights into changes in proto-ACC dynamics that can in turn be related to the evolution of the ice sheet in this region of the East Antarctic margin.
Section snippets
Site description and oceanographic setting
DSDP Leg 28 Site 269 is located on the abyssal plain off the Wilkes Land (61o40.57′S, 140o04.21′E, 4282 m water depth) (Hayes et al., 1975) (Fig. 1). Two holes were discontinuously drilled at this site. Our study focuses on Hole 269A, specifically the interval between 655 and 956 m below sea floor (mbsf) (cores 7R to 13R). Recovered sediments were interpreted shipboard to be mostly turbidites, but evidence of winnowing by bottom currents was also documented (Hayes et al., 1975). Facies were
Revising the initial age model
We established a new age model based on the integration of new magnetostratigraphic data, dinocyst and calcareous nannofossil biostratigraphy, calibrated using the GTS 2012 Astronomic Age Model (Gradstein et al., 2012) (Figs. 2; S1; S2 and Tables S1; S2). The presence of Operculodinium janduchenei in the sediment between ±753–955 (mbsf) (Cores 9R 3 W to 13R) is assigned to the lower Southern Ocean Dinocyst Zone (SODZ) SODZ8 (Bijl et al., 2018a). This suggests that the bottom of Hole 269A cannot
Facies analysis
A detailed facies analysis was performed on sediment from Hole 269A to determine depositional processes and aid paleoenvironmental reconstructions. We conducted a detailed description of the cores using standard sedimentological techniques (i.e., lithological characterization, contacts, sedimentary structures and textures) in order to produce the lithostratigraphic log in Fig. 3 and Supplementary Fig. 3. Visual descriptions were aided by high-resolution digital images obtained on the archive
Sedimentation at Hole 269A
Sediment from Hole 269A from 655 to 956 mbsf consists of alternations between bioturbated and laminated intervals of terrigenous-rich sediment (Figs. 3a, b, 4 and S3). Textural analyses show a low clay content (4 to 20%), a high silt fraction (40 to 80%) and a sand content between <5 and 60% (Fig. S3). Microfossil preservation is generally low throughout the study interval. Higher preservation of calcareous microfossils was found in the carbonate-cemented beds (Figs. 3a and 5) and within the
Glacial-interglacial sedimentation and short-term polar frontal system dynamics
Repeated alternations between laminated and bioturbated facies like those described at Hole 269A (Figs. 3c and 4), are common in deep-water settings around Antarctica and are interpreted to result from changes in sedimentation related to glacial-interglacial cycles, respectively (e.g., Hepp et al., 2006; Lucchi and Rebesco, 2007; Escutia et al., 2009, Escutia et al., 2011; Patterson et al., 2014; Salabarnada et al., 2018).
Terrigenous laminated deposits at Hole 269A (F1b, F2b, Figs. 3c and 4)
Conclusions
Our integrated sedimentological, geochemical, isotopic and palynological data sets from DSDP Site 269 provide new insights into the proto-ACC dynamics during the late Oligocene-early Miocene (~24.2 to 23 Ma) off the eastern Wilkes Land margin. We show that sedimentation at Site 269 is controlled by the persistent reworking of gravity flows and hemipelagic sedimentation by proto-CDW that is characterized by fluctuating current intensities driven by the migration of the frontal system in response
Acknowledgments
This research used samples provided by the International Ocean Discovery Program (IODP). We thank the staff at the Gulf Coast core repository (GCR) for assistance in core handling and shipping. We also thank David Houpt (GCR) for technical support with the XRF core scanning; Katharina Kreissig, Liam Holder, Barry Coles (Imperial College) and Katrina Kerr (Open University) for laboratory and technical support with the Nd isotopes and REE analyses; Emmanuelle Ducassou, Marie-Claire Perello (EPOC)
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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