Marine productivity largely controls the oceanic uptake of atmospheric carbon dioxide and contributed to the global climate changes that led to the termination of the last glacial cycle. Past changes in marine productivity were presumably associated with disturbances in the Atlantic meridional overturning circulation (AMOC). In the South Atlantic, however, the evolution of marine productivity throughout the last glacial–interglacial cycle is still poorly constrained mainly due to the scarcity of records with high temporal resolution. Here we present high‐resolution records of paleoproductivity and upper‐water‐column properties from the western subtropical South Atlantic covering the last 40,000 years. Our records are based on faunal and stable oxygen isotopic analyses of planktonic foraminifera from a marine sediment core collected from an upwelling region off southeastern South America (27°S). We used the relative abundance of eutrophic planktonic foraminifera (i.e., Globigerinita glutinata and Globigerina bulloides) as proxies of primary productivity. Our findings reveal, for the first time, enhanced primary productivity during Heinrich Stadials along the last glacial, when the AMOC showed reduced strength. Additionally, our results reveal decreased primary productivity over the Last Glacial Maximum, a period of markedly lower sea level; and the Younger Dryas, when the AMOC showed only moderate reductions. The most outstanding productivity decline, however, is depicted at the onset of the Holocene, when the AMOC recovers its strength. We hypothesize that the observed changes were triggered by the dynamics of the Brazil Current primarily driven by disturbances in the AMOC.

Chapter 2 Last Glacial-Interglacial Cycle 6 Chapter 3 Regional Setting 17 3.1 Continental climate 18 3.2 Upper water column properties 19 3.3 Upper ocean circulation 21 3.4 Primary productivity 24 Chapter 4 Methodology 4.1 Marine sediment core: Preliminary procedures 4.2 Age model 4.3 Micropaleontological analyses: Planktonic foraminiferal assemblages 4.3.1 Reconstruction of paleotemperature 4.3.2 Reconstruction of paleoproductivity 4.3.3 Reconstruction of Agulhas leakage 4.4 Geochemical analyses: Stable oxygen isotope 4.4.1 Reconstruction of δ 18 O ivc-ssw : a proxy for paleosalinity 4.4.2 Reconstruction of past upper ocean stratification ii

Within the context of anthropogenic warming, rainfall oscillations may induce especially important societal impacts worldwide. In this article, we propose to study potential underlying mechanisms related to precipitation changes on a planetary scale by taking advantage of the recent theory of Rossby waves of long periods winding around subtropical gyres, the Gyral Rossby Waves (GRWs). The stable oxygen isotopic compositions of speleothems are used to regionalize and reconstruct the evolution of long-term rainfall oscillation during the Holocene. The method applied here consists in estimating the wavelet power of dated series of stable oxygen isotopic composition (δ18O) in speleothems within period bands representative of subharmonic modes. Our findings highlight: (1) hydrological processes resulting from friction between the North Equatorial Current (NEC) and the North Equatorial Counter Current (NECC) to explain the weakening of ENSO activity in mid-Holocene, and (2) the quasi-resonance of the equatorward migration of the summer Inter Tropical Convergence Zone (ITCZ) during the Holocene, because of the progressive decrease of the thermal gradient between the low and high latitudes of the gyres. The results of this study suggest that the spatial and temporal variations in the amplitude of the rainfall oscillations are related both on the acceleration/deceleration phases of the western boundary currents and on the shrinkage of the Hadley cell. The latitudinal shift of the summer ITCZ in response to changes in the thermal gradient is of the utmost importance in predicting the expansion of deserts resulting from anthropogenic warming.

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