A Glimpse at South Atlantic Deep and Surface Water Evolution over the Last 40 Ky

Abstract:

We present sub-millennially resolved time series from core MD08-3167, retrieved at 23°S and 1950 m water depth on the Namibian margin during 2008 RETRO cruise. At present, this region is characterised by upwelling of eastern South Atlantic central water from 150-500 m depth. Stable isotopes (δ¹⁸O and δ¹³C) and Mg/Ca were measured on the planktonic foraminifer Globigerina bulloides, which may be assumed to thrive in upwelled waters. Stable isotopes were also measured on the benthic foraminifer Cibicides wuellerstorfi. Core MD08-3167 records were dated based on 23 ¹⁴C dates covering the last 30 ky.

Water δ¹⁸O was computed from G. bulloides δ¹⁸O and Mg/Ca measurements. A sharp decrease in G. bulloides δ¹⁸O paralleled by a sharp increase in G. bulloides Mg/Ca temperature from ~10 to 14 °C takes place over 1000 y between ~18.5 and 17.5 calendar ky BP (ka). After a wide maximum at ~14°C centred about 15 ka, G. bulloides Mg/Ca temperature decreases to 13°C at 13 ka, in phase with the Antarctic cold reversal, then increases to reach an Early Holocene optimum at 16 °C at 9 ka, before decreasing to reach ~15°C, i.e. modern temperatures at 50 m depth. Upwelled water δ¹⁸O (δ¹⁸O_w) decreases by 1.8‰ from the last glacial maximum (LGM) to the Late Holocene. The evolution of δ¹⁸O_w is marked by a sharp 0.9‰ decrease in parallel with the 4°C warming recorded by G. bulloides Mg/Ca, then essentially follows the decrease in global mean ocean δ¹⁸O inferred from sea level rise. Our records thus indicate that upwelled waters were both ~4°C colder and 0.9‰ enriched during the LGM relative to the modern ocean after correction for sea level rise. LGM low SSTs can thus not be attributed to enhanced advection of subpolar waters into the Benguela current. In contrast, our records suggest enhanced upwelling of colder water along the continental margin during the LGM that could originate from a mixture of re-circulated South Atlantic central water, glacial North Atlantic intermediate water, and circumpolar deep water.

We further compare core MD08-3167 records with benthic isotopic and surface records from other well-dated South Atlantic cores of comparable temporal resolution, i.e. western tropical Atlantic core GeoB3910, and Sub-Antarctic cores MD07-3076Q and TNO57-21 (Barker and Diz, 2014).