Past Carbonate Preservation Events in the Deep Southeast Atlantic Ocean (Cape Basin) and Their Implications for Atlantic Overturning Dynamics and Marine Carbon Cycling

Abstract

Micropaleontological and geochemical analyses reveal distinct millennial‐scale increases in carbonate preservation in the deep Southeast Atlantic (Cape Basin) during strong and prolonged Greenland interstadials that are superimposed on long‐term (orbital‐scale) changes in carbonate burial. These data suggest carbonate oversaturation of the deep Atlantic and a strengthened Atlantic Meridional Overturning Circulation (AMOC) during the most intense Greenland interstadials. However, proxy evidence from outside the Cape Basin indicates that AMOC changes also occurred during weaker and shorter Greenland interstadials. Here we revisit the link between AMOC dynamics and carbonate saturation in the deep Cape Basin over the last 400 kyr (sediment cores TN057‐21, TN057‐10, and Ocean Drilling Program Site 1089) by reconstructing centennial changes in carbonate preservation using millimeter‐scale X‐ray fluorescence (XRF) scanning data. We observe close agreement between variations in XRF Ca/Ti, sedimentary carbonate content, and foraminiferal shell fragmentation, reflecting a common control primarily through changing deep water carbonate saturation. We suggest that the high‐frequency (suborbital) component of the XRF Ca/Ti records indicates the fast and recurrent redistribution of carbonate ions in the Atlantic basin via the AMOC during both long/strong and short/weak North Atlantic climate anomalies. In contrast, the low‐frequency (orbital) XRF Ca/Ti component is interpreted to reflect slow adjustments through carbonate compensation and/or changes in the deep ocean respired carbon content. Our findings emphasize the recurrent influence of rapid AMOC variations on the marine carbonate system during past glacial periods, providing a mechanism for transferring the impacts of North Atlantic climate anomalies to the global carbon cycle via the Southern Ocean.

Southern Ocean link between changes in atmospheric CO2 levels and northern-hemisphere climate anomalies during the last two glacial periods

“…This is supported by an agreement of our saturation index with epibenthic δ 13 C- (Skinner et al, 2007;Margari et al, 2010) and SST changes at the Iberian Margin (Martrat et al, 2007; Fig. 7), the high-resolution 231 Pa/ 230 Th record from Bermuda Rise (Henry et al, 2016) and a carbonate saturation record from the deep Cape Basin (Gottschalk et al, 2018), which reflect similar AMOC changes in the Atlantic basin. This agreement corroborates a strong causal link between bottom water [O 2 ] changes in the deep South Atlantic and end-member changes of southern-sourced water masses related to variations in air-sea gas exchange-and/or the ventilation rate, which may be linked to variations in the southward extent of NADW.…”

Last glacial atmospheric CO2 decline due to widespread Pacific deep-water expansion

“…Below ~2.5 km, LGM North Atlantic [CO3 2-] values were up to ~20 mol/kg lower than today, consistent with greater mixing/advection of low-[CO3 2-] Glacial Antarctic Bottom Waters (GAABW) and/or increased respiration in the glacial ocean 1,2,19,20 . The boundary of ~2.5 km delineated by LGM upper and lower water masses is consistent with that found from other proxies ( 13 C, Cd/Ca, and Nd) and model results 1-3,19,21,22 . In the South Atlantic, deep-water [CO3 2-] in the 5 studied cores from ~3-4 km water depth are lower by ~20 mol/kg during the LGM than the Holocene, consistent with qualitative [CO3 2-] proxies from the same cores [23][24][25] ( Supplementary Fig. 2).…”

“…shows slightly higher deep-water [CO3 2-] values during the LGM than the Holocene, supported by multiple benthic B/Ca measurements in this core and qualitative proxies including %CaCO3 and foraminiferal fragmentation data for several South Atlantic cores at similar water depths 15,23,24 ( Supplementary Fig. 3).…”

Low terrestrial carbon storage at the Last Glacial Maximum: constraints from multi-proxy data

“…The results demonstrate that ocean sediments and the weathering-burial cycle are an integral part of the Earth system playing a fundamental role on glacialinterglacial timescales. The Bern3D EMIC features a three-dimensional geostrophic ocean (Müller et al, 2006;Edwards et al, 1998) with an isopycnal diffusion scheme and Gent-McWilliams parameterization for eddy-induced transport (Griffies, 1998). The model includes a thermodynamic sea-ice component coupled to a single-layer energy-moisture balance atmosphere (Ritz et al, 2011).…”