Evidence of supra-lysoclinal dissolution of pelagic calcium carbonate in the late Quaternary in the western South Atlantic

Abstract

Even at intermediate depths, where the environment is adequate for carbonate accumulations, a loss of calcareous microfossils may occur by the dissolution of CaCO3. Changes in the microfossil preservation indexes are a proxy for identifying oceanographic variations in the past. The goal of this study is to identify minor changes in the degree of preservation of foraminiferal assemblages in a supra-lysoclinal zone and their probable causes. Late Quaternary samples from three cores collected at intermediate water depths (1514, 1894 and 2091 m) on the Pelotas Basin continental slope were examined using the standard methodology for calcareous (foraminifera and coccolith) paleontological analyses. The age models were obtained based on AMS 14C dates and on the oxygen stable isotope (δ18O) record in foraminifera. Dissolution variations were identified using the coccolith-based CEX dissolution index, the carbonate content, foraminifera fragmentation index, the relationship between resistant and fragile planktonic foraminifera, and the abundance of fragile foraminifera Orbulina universa and Hoeglundina spp. Finally, the planktonic/benthic foraminifera ratio (P/B) was evaluated as a dissolution proxy, demonstrating non-applicability. Overall, the Marine Isotope Stages (MIS) 5 and MIS 1 (interglacials) show the best preservations, while MIS 4, 3 and 2 (glacials) show relatively poorer preservation. Although the cores were obtained well above the modern and the inferred glacial lysocline, periods of relative greater dissolution were identified which can be attributed to the advance of Upper Circumpolar Deep Water (UCDW). The best preservation indexes are attributed to the influence of North Atlantic Deep Water (NADW).