Deep Equatorial Atlantic Ocean Circulation over the Last 25,000 Years – Insights from 231Pa/230Th, 14C and Sortable Silt Grain Size

Abstract:

Evidence points to a key role of the Atlantic circulation in controlling climate changes during the last deglacial (20-10 ka), though quantifying the rate of past ocean circulation remains an open challenge. Existing Atlantic rate proxy records over the last deglacial exhibit pronounced differences both in timing and magnitudes at both deep and intermediate depths. This uncertainty calls for more rate proxy data at key sites to improve understanding of the driving mechanisms behind the Atlantic observations. This study examines three complementary rate proxies (²³¹Pa/²³⁰Th, Δ¹⁴C and sortable silt grain size) of Equatorial Atlantic sediment cores from 2.7 to 4.1 km depth from the east and west of the Mid-Atlantic Ridge.

Sediment cores from the Ceara Rise in the western basin (4.1 km depth; 3.3 km depth) support a picture of relatively ¹⁴C-depleted deep water with higher Pa/Th values during the Last Glacial Maximum compared to the modern day. During Heinrich Stadial 1, there is a marked increase in Pa/Th values. Measured opal fluxes indicate that opal scavenging is not the main driver of the observed Pa/Th changes. At the same time, radiocarbon data indicate that the most ¹⁴C-depleted signature occurs at around 3 km water depth. Sortable silt mean size also shows distinctive changes around this period, but with some offset in timing potentially related to changes in circulation rate at different scales. Results from two new cores collected on a recent cruise (JC094) will be used to extend our understanding of these Equatorial Atlantic glacial-deglacial rate proxy dynamics and their relative timings. Both cores were collected far from the continental margin, in the central and eastern Atlantic (7^o26.10’N, 21^o47.78’W, 3.4 km depth; 15^o27.86’N, 50^o59.49’W, 2.7 km depth) providing a synoptic basin scale view of these important rate tracers in the Atlantic.