Carbonate Preservation in Deep and Intermediate Water Masses in the South Atlantic: Evaluation and Geological Record (a Review)

Abstract

Evaluation of conventional dissolution proxies in South Atlantic surface sediments revealed broad applicability only in far offshore, rather oligotrophic regimes in the western basins. In contrast, they fail or produce misleading and incorrect results in the more productive eastern South Atlantic basins, due to the combined effects of variable dilution by non-carbonate material and fluctuating ecological conditions. Much more promising are the results from new dissolution proxies on the planktic foraminifer Globigerina bulloides (BDX') and the pteropod Limacina inflate (LDX) which were calibrated with carbonate saturation as indicated by GEOSECS data. In the western South Atlantic, the sedimentary calcite lysocline is encountered by the BDX' at the transition between AABW and LNADW. However, it rises up into the LNADW close to the equator due to additional supralysoclinal dissolution. In the eastern South Atlantic basins, supralysoclinal dissolution results in an elevation of the sedimentary calcite lysocline of several hundred metres to a maximum of 1600 m as compared to the position of the hydrographic lysocline, with aragonite preservation in the eastern South Atlantic being even poorer. At most sites investigated, the surface sediments are void of pteropods and thus LDX failure is indicated. However, in the western South Atlantic the LDX displays a double lysocline for aragonite, the upper lysocline at a water depth of 750 m and the lower at 2500 m. Aragonite and calcite preservation profiles indicate much weaker stratification of the water during the LGM. With 3200 m, the position of the calcite lysocline is encountered at the same level in the southern parts of the eastern and western basins dropping to 4000 m near the equator. Along the western continental margin no indication for aragonite-corrosive glacial AAIW was found, providing clear evidence for a strengthened GNAIW flow along the Brazil margin. The long-term history of carbonate dissolution in the equatorial Atlantic was reconstructed by a multiproxy approach combining benthic foraminifer stable isotopes and new proxies from silt analysis. For the first time, this allows a reconstruction of the chemical (nutrient content, carbonate corrosiveness) and physical (bottom current strength) properties of deep and intermediate water masses. The terrigenous silt records of ODP Site 927 at the Ceara Rise show rapid shifts from low to very high bottom-currents speeds for nearly all the isotopic transitions in the Brunhes epoch, indicating subsequent phases of shutdown and rapid reinstatement of LNADW circulation. A drastic reduction of glacial bottom-current strength at Site 927 is inferred after 2.75 Ma, synchronous with the first occurrence of larger continental ice shields and with a drastic decrease in deep convection in the Norwegian-Greenland Sea. After the mid-Pleistocene climate transition, progressively weaker bottom currents and poorer carbonate preservation during glacials indicate a progressive reduction of LNADW from the Late Pliocene to Pleistocene. On the contrary, an opposite trend with progressive improvement of preservation during glacials from Late Pliocene to the Pleistocene is observed in the Caribbean at Site 999. This indicates a contemporaneous progressive increase in the contribution of UNADW to the Atlantic in glacial periods. Altogether, a progressive weakening of the circulation in the LNADW loop and a contemporaneous strengthening of the UNADW loop are evident since the mid Pleistocene transition.