Late Eocene onset of the Proto-Antarctic Circumpolar Current

Abstract

The formation of the Antarctic Circumpolar Current (ACC) is critical for the evolution of the global climate, but the timing of its onset is not well constrained. Here, we present new seismic evidence of widespread Late Eocene to Oligocene marine diagenetic chert in sedimentary drift deposits east of New Zealand indicating prolonged periods of blooms of siliceous microorganisms starting ~36 million years ago (Ma). These major blooms reflect the initiation of the arrival and upwelling of northern-sourced, nutrient-rich deep equatorial Pacific waters at the high latitudes of the South Pacific. We show that this change in circulation was linked to the initiation of a proto-ACC, which occurred ~6 Ma earlier than the currently estimated onset of the ACC at 30 Ma. We propose that the associated increased primary productivity and carbon burial facilitated atmospheric carbon dioxide reduction contributing to the expansion of Antarctic Ice Sheet at the Eocene-Oligocene Transition.

Figure 1

Southern Ocean circulation and circumpolar oceanic fronts. (a) The Antarctic Circumpolar Current (ACC) marked by blue arrows and main oceanic fronts from north to south are represented by the sub-Tropical Front (STF), Subantarctic Front (SAF), the Polar Front (PF), and the Southern ACC Front (SACC). Figure adapted from ref.. (b) Simplified schematic representation of the present-day Southern Ocean overturning circulation^,. Ekman transport (ET) resulting from the Antarctic Coastal Current flowing counter-clockwise around Antarctica under the influence of the Polar easterlies causes waters to move towards Antarctica, while a northbound Ekman transport resulting from an eastbound ACC drives the waters away from Antarctica. It creates an area of divergence called the Antarctic Divergence Zone, where the Circumpolar Deep Water (CDW) upwells to the surface south of the Polar Front (PF). Northward advection of nutrient-rich upwelled water takes place by Ekman transport and references therein. Its subsequent subduction to intermediate depths forms the Antarctic Intermediate Water (AAIW). Part of the upwelled waters moves southwards forming the Antarctic Bottom Water (AABW). During the northward advection, some part of the Antarctic surface water mixes with subtropical surface water to form Subantarctic Mode Water (SAMW).

Figure 2

Study area map. Map showing the South Island of New Zealand, Campbell Plateau, sub-Tropical Front (STF), Antarctic Circumpolar Current (ACC), Pacific Deep Western Boundary Current (DWBC)^, and a modern cyclonic circulation (marked by blue arrows) in the Great South Basin (GSB) and the Bounty Trough (BT). Canterbury drifts (CD) and DWBC drifts are shown. Time thickness (two-way travel time in seconds) map of the mid-Eocene to Late Eocene interval in the GSB shows the NNE–SSW striking elongate sedimentary drift. Subsidiary drifts were deposited on the eastern offshore side of the main mound. Boreholes of ODP Leg 181 (1119, 1120 and 1122), IODP expedition 317 (U1352 and U1354), DSDP Leg 29 (275 and 276), Leg 90 (594) and oil exploration wells (Pukaki-1 and Pakaha-1) are marked.

Figure 3

Seismic characteristics of late Paleogene sedimentary drifts. (a) A seismic line shows a central mound between markers ME and TE with internal upslope prograding configuration. High amplitude reflections (HARS) occur towards the top of the mound. A moderate to strong positive amplitude reflection is identified as the opal-A to opal-CT reaction front (yellow dotted horizon). We correlated seismic horizons in the drift to Pukaki-1 (Fig. 5a). (b) Subsidiary mound developed offshore of the central mound (Location marked by a box in Fig. 3a). It shows internal convex reflection pattern and a landward moat. High amplitude reflections are seen above the marker that is assigned an age of 36 Ma.

Figure 4

Late Paleogene sedimentary drifts and opal-A/opal-CT reflector. (a) A lenticular shaped unit between markers ME and TE is identified as a buried plastered drift south of the Bounty Trough. Strong positive amplitude reflections are seen towards the top of the drift in the BT. (b) The opal-A/opal-CT reaction front shows negligible or smaller offset (blue arrow) than the offset of the host Late Eocene unit (cf. ref.). The diagenetic transformation could have post-dated the displacement across the faults, or the displacement rate of the faults was greater than the upward advancement of the diagenetic front. Some faults extending into the Oligocene and Miocene sequence also affected the diagenetic front, probably post-dating diagenetic transformation.

Figure 5

Results from the borehole Pukaki-1 and the spatial extent of an opal-A/opal-CT reflector. (a) On the seismic panel, bright reflections within Late Eocene and Early Oligocene strata are correlated with diagenetic chert as determined from recovered rock samples (ages calibrated to ref.). The top of cherty limestone clearly defines the opal-A/opal-CT conversion boundary and correlates with a sharp drop in sonic log response. (b) Variation of the depth of the opal-A/opal-CT reflector below the seabed.

Figure 6

Neodymium isotopic sections for different geologic time bins interpolated using natural neighbor interpolation (Table S1) and schematic late Paleogene southwest Pacific circulation showing the progressive development of a proto-ACC. (Top) The 38-36.5 Ma Nd isotopic section (left) reveals the extent and mixing of North and South Pacific deep waters (Fig. S2b)^,. At Site 1124 northbound flow of less radiogenic South Pacific deep waters is indicated by an arrow. Contourites in the GSB and BT are deposited by northbound bottom currents (centre). A westbound Antarctic Slope Current (ASC) existed north of Antarctica. At Site 1124, surface currents were influenced by the proto-East Australian Current (EAC); while deep northbound currents transported southern sourced deep waters. Cross-section (right) shows the northbound flow of South Pacific deep water (SPDW). (Middle) Equatorial/sub-equatorial deep waters with more radiogenic Nd isotopic signature arrive south of 30° S (36-34 Ma) and indicated by an arrow. A proto-ACC started to develop across the STR (centre) causing entrainment and upwelling (U) of proto-equatorial Pacific deep water (Proto-EPW) and subsequent northbound Ekman transport (right). The submerged Chatham Rise deflected the proto-EPW towards east. (Bottom) The 34-33 Ma Nd isotopic section (left) is similar to the 36-34 Ma section. A stronger proto-ACC caused upwelling (centre) and pronounced entrainment of proto-EPW (right). Colour codes for the paleogeographic maps in the middle column: black = land, dark grey = shelf, light grey = slope or submarine rise, white = deep ocean.