Holocene oscillations of Southwest Atlantic shelf circulation based on planktonic foraminifera from an upwelling system (off Cabo Frio, Southeastern Brazil)

Abstract

International audienceThe Brazil Current (BC) is a relevant feature in the Atlantic Meridional Overturning Circulation (AMOC). Its behavior during slowdown or intense AMOC remains poorly known because of the lack of paleoceanographic records, especially for the Holocene. Here, we investigate changes in a western boundary upwelling system (Cabo Frio, off Southeastern Brazil) which are driven by variations in the BC and NE winds during the last 9 kyr. To assess the variability of the BC, we used δ¹⁸O, Mg/Ca, and assemblages of planktonic foraminifera. Our results indicate five oceanographic phases during the last 9 kyr. During Phase I (from 9.0 to 7.0 cal kyr BP), the BC diverged offshore from the modern upwelling area because of the low sea level, increasing the influence of shelf waters and coastal upwelling plumes on foraminifera assemblages. Phase II (7.0-5.0 kyr BP) was marked by the approach of the internal front of the BC with low intensity and episodes of strong productivity that were linked primarily to the upwelling of the South Atlantic Central Water (SACW) and/or Subpolar Shelf Waters (SPSWs) (cold). Phase III (5.0-3.5 kyr BP) was a transition, marking a large oceanographic and climatic change from the weakening of the AMOC. The internal front of the BC became warm and subsurface SACW upwelling was stronger. In Phase IV (3.5-2.5 kyr BP), the BC acquired its modern dynamics, but weak NE winds weakened the SACW's contribution to upwelling events. Finally, in Phase V (last 2.5 kyr BP), the NE winds reintensified, promoting frequent episodes of upwelling and intrusion by SPSWs during the Medieval Climate Anomaly

Mid–late Holocene event registered in organo-siliciclastic sediments of Lagoa Salgada carbonate system, southeast Brazil

“…Geochemical records from Lagoa Salgada in comparison with other climate records. (a) Gray scale (nm) from Cariaco Basin, Venezuela (Hughen et al, 1996); (b) Carbonate content (CaCO 3 ) (%) and (c) dolomite (%) from the Gulf of Oman (Cullen et al, 2000); (d) Carbonate content (CaCO 3 ) (%), (e) clay (%), and (f) Fe (%) from this study; (g) Mg/Ca, Globigerinoides ruber, from the southwest Brazilian coast (Lessa et al, 2016) and (h) Mg/Ca speleothem from Botuvera Cave, southwestern Brazil (Bernal et al, 2016). The nine black stars indicate the position of the radiocarbon dates measured in this study.…”

“…Upwelling during this period became limited to the subsurface with warm conditions on the surface waters, shown by increasing Mg/Ca ratio in planktonic foraminifera (Globigerinoides ruber) (Fig. 6f) (Lessa et al, 2016).…”

“…Some authors try to explain the drought and increase in aridity as a result of the weakening of the Asian monsoon (Giesche et al, 2019;Kathayat et al, 2018;Wang, 2005) due to the southward migration of the Intertropical Convergence Zone (ITCZ). Others suggest a prolonged northward shift of the mean position of the ITCZ (Li et al, 2018), being in contrast with the southward shift of the tropical rain belt. The irregular fluctuation of atmospheric pressure over the North Atlantic Ocean, changing the direction of the cy-A.…”

Branched GDGTs as Proxies in Surface Sediments From the South-Eastern Brazilian Continental Margin

“…The south-eastern Brazilian continental margin has proved to be a crucial region for paleoclimate studies (Rühlemann and Butzin, 2006;de Mahiques et al, 2009;Nagai et al, 2009Nagai et al, , 2016Souto et al, 2011;Pivel et al, 2013;Chiessi et al, 2014;Cordeiro et al, 2014;Evangelista et al, 2014;Lessa et al, 2014Lessa et al, , 2016Lazzari et al, 2018). The particular interest in the region stems from the relatively low input of continental material and enhanced autochthonous production due to cold and nutrient-rich deepwaters, a scenario contrasting with the overall oligotrophic conditions in the water column (Brandini et al, 2018).…”

Contrasting late-glacial paleoceanographic evolution between the upper and lower continental slope of the western South Atlantic

“…Most of these studies were based on intermediate to deep-water cores (> 1500 m) recovered from the mid-to lowercontinental slope, and investigations supported by shallower cores (< 1000 m) are generally limited to the Holocene (e.g., Albuquerque et al, 2016;Lessa et al, 2016;Nagai et al, 2014). Such a shorter-time range from shallow-water cores is due to the fact that wide regions of the continental shelf were exposed during the last glacial cycle, and that the sea-level rise of the last deglaciation provoked massive sedimentological disturbances, preventing the acquisition of well-organized chronological sequences.…”