A New Seismic Stratigraphy in the Indian‐Atlantic Ocean Gateway Resembles Major Paleo‐Oceanographic Changes of the Last 7 Ma

Abstract

The exchange of water masses between the Indian Ocean and the Atlantic constitutes an integral interocean link in the global thermohaline circulation. Long‐term changes in deep water flow have been studied using seismic reflection profiles but the seismic stratigraphy was poorly constrained and not resolved for the time period from the late Miocene onward. Here we present results from International Ocean Discovery Program Site U1475 (Agulhas Plateau) located over a sediment drift proximal to the entrance of North Atlantic Deep Water into the Southern Ocean and South Indian Ocean. Site U1475 comprises a complete carbonate‐rich stratigraphic section of the last ~7 Ma that provides an archive of climate‐induced variations in ocean circulation. Six marker reflectors occurring in the upper 300 m of the drift are identified here for the first time. The formation of these reflectors is mainly due to density changes that are mostly caused by changes in biogenic versus terrigenous sediment deposition. Synthetic seismograms allow age assignments for the horizons based on biostratigraphy and magnetostratigraphy. Prominent reflectors are related to late Pleistocene glacial/interglacial variability, the middle and early Pleistocene transitions, and the onset of the northern hemisphere glaciation. A peculiar early Pliocene interval (~5.3–4.0 Ma) bounded by two reflectors is characterized by fourfold elevated sedimentation rates (>10 cm/kyr) and the occurrence of sediment waves. We argue that this enhanced sediment transport to the Agulhas Plateau was caused by a reorganization of the bottom current circulation pattern due to maximized inflow of North Atlantic Deep Water.

Antarctic icebergs reorganize ocean circulation during Pleistocene glacials

“…IRD MAR was determined by counting detrital mineral grains in the >150µm sediment fraction (subsampled using a micropaleontology splitter to yield 500-1000 entities) before normalization to sample weight (IRD concentration in #g) and then multiplication by apparent bulk mass accumulation rate (Extended Data Figure 2c), derived from estimates of dry bulk density from 57 and linear sedimentation rate. The temporal evolution, spectral characteristics, and timing of peaks in the IRD record are largely unaffected by conversion from concentration to Mass Accumulation Rate (MAR).…”

Expedition 392 Scientific Prospectus: Agulhas Plateau Cretaceous Climate

“…The seismostratigraphic model developed for the proposed Expedition 392 drill sites (Figures F3, F4, F5, F6, F7, F8) is tentative because the uppermost part of the sedimentary sequence of the Agulhas Plateau has previously been sampled by scientific drilling at only one location (IODP Expedition 361 Site U1475 on the southwest Agulhas Plateau; Hall et al, 2017), and therefore the interpretations for ages of reflectors should be applied with caution. The seismostratigraphic models for the Agulhas Plateau and Transkei Basin sequences are based on analysis of reflection characteristics (e.g., amplitude, frequency content of units, and continuity of reflections) and are tied to the ages of conventional gravity/piston cores (Tucholke and Carpenter, 1977; Table T1) where possible and to Site U1475 for the post-Miocene sequences (Gruetzner et al, 2019).…”

“…A regional hiatus of late Miocene-early Pliocene age (~10-6 Ma) is attributed to erosion and redeposition of sediments by Circumpolar Deep Water in the ACC (Tucholke and Embley, 1984) and represented by a strong reflection (Reflector LP), which is often found very close to, and thus indistinguishable from, the seafloor. Reflector LP is dated to 5.7 Ma (Gruetzner et al, 2019) and interpreted to represent the onset of a period of increased Atlantic Meridional Overturning Circulation. A relatively thin veneer of Pliocene-Pleistocene sediments (~0-50 m) rests on top of the lower Pliocene reflection across most areas of the Agulhas Plateau, forming thicker drift packages only in localized areas.…”

“…A relatively thin veneer of Pliocene-Pleistocene sediments (~0-50 m) rests on top of the lower Pliocene reflection across most areas of the Agulhas Plateau, forming thicker drift packages only in localized areas. Several reflections in the Pliocene-Pleistocene sequence have been associated with climate-induced changes in circulation (Gruetzner et al, 2019).…”

“…Proposed sites (A: red stars = primaries, B: pink stars = alternates) and existing sediment cores (triangles) on seismic lines (black lines) collected in 1998, 2005(Uenzelmann-Neben, 1998, 2005, 2014. Green diamond = Expedition 361 Site U1475 (Gruetzner et al, 2019;Hall et al, 2016). 24˚30' Table T1.…”