Orbital cycle-related benthic-pelagic fluctuations in Foraminifera during the last glacial-interglacial interval in the western South Atlantic

Abstract

<p>Paleoceanographic studies reconstructing surface paleoproductivity and benthic conditions allow us to measure the effectiveness of the biological pump, an important mechanism in the global climate system. In order to assess surface productivity changes and their effect on the sea-floor environment, a multiproxy paleoceanographic analysis was conducted on the core SAT-048A (1542 m.b.s.l.), recovered from the continental slope of the southernmost Brazilian continental margin, western South Atlantic. We assessed sea surface productivity using different planktonic foraminiferal proxies: (1) the relative abundances of the species <em>Globigerina bulloides</em> and <em>Globigerinita glutinata</em> and (2) the δ<sup>13</sup>C signal of shells of the species <em>Globigerinoides ruber ruber</em>. To assess the organic matter (OM) flux to the seafloor, the foraminiferal planktonic:benthic ratio and the δ<sup>13</sup>C signal of shells of the benthic foraminifer <em>Uvigerina</em> spp. were used. To study dissolution effects occurring at the sea-floor, the Fragmentation Intensity (i.e., the proportion of fragments and broken foraminiferal shells), the number of planktonic foraminiferal tests per gram of dry sediment, and the CaCO<sub>3</sub> and Sand contents of the sediment were measured. Superimposed on the climate-induced changes related to the last glacial-interglacial transition, the reconstruction indicates paleoproductivity changes synchronized with the precessional cycle. From the reconstructed data, it was possible to identify the glacial and postglacial stages: surface productivity, flux to the seafloor, and dissolution rates of planktonic foraminiferal tests where high during the glacial and low during the postglacial. Furthermore, within the glacial, enhanced productivity was associated with higher insolation values, which can be explained by increased NE summer winds that strengthened the Brazil Current transport and, in turn, promoted meandering and upwelling of the nutrient rich South Atlantic Central Water. Changes in the Atlantic Meridional Overturning Circulation and the reorganization of bottom water masses may change the CO<sub>3</sub><sup>2-</sup> saturation levels and, consequently, influence carbonate preservation. However, the δ<sup>13</sup>C values from shells of <em>Uvigerina</em> spp. are different from present-day δ<sup>13</sup>C values from dissolved inorganic carbon for the Upper Circumpolar Deep Water and the North Atlantic Deep Water, which is likely linked to varying OM fluxes. Future studies (e.g., εNd in benthic Foraminifera) must quantify the effect of the reorganization of the bottom water masses on the dissolution of the planktonic foraminiferal tests, to better understand the effect of the biological pump removing carbon from the seawater and its subsequent sequestration in the seafloor sediments.</p>