Early diagenesis effects on Mg/Ca thermometry during the MIS 9–5 in the Gulf of Mexico, evaluation on foraminifera tests and geochemical signals

Abstract

Analyses of Mg/Ca in planktonic foraminifera Globigerinoides ruber (white) tests were made to evaluate burial influences and preservation before reconstructing paleotemperatures. We studied samples from the eastern Gulf of Mexico marine sediment core EN32-18PC at a glacial-interglacial scale. The core chronology is based on planktonic foraminifera biostratigraphy and δ18OG. ruber stratigraphy, showing that the core spans the biozones V2 - Y and the Marine Isotope Stages (MIS) 9 to 4 (i.e., 319.3 to 66.0 ka). Based on Scanning Electron Microscopy (SEM) and LA-ICP-MS analysis of Mg/Ca, Sr/Ca, Mn/Ca, Fe/Ca, and Mn/Fe ratios, we found evidence of test alteration by early diagenesis. Some tests show material infillings, pore reduction, polygonal patterns between pores, and wall thickening by calcareous overgrowth. Values of Mg/Ca >6 mmol/mol and δ18O > 2‰ must be carefully evaluated before reconstructions. Values of Mn/Ca >0.6 mmol/mol and Fe/Ca >0.2 mmol/mol indicate the adherence of clays or coatings (Mn or Fe oxides). Nevertheless, we did not find evidence of severe diagenesis in the tests (like recrystallization or euhedral crystals grown by inorganic precipitates); thus, their alteration degree is incipient, and calcite overgrowth is the most severe diagenetic effect during MIS-6 and early MIS-5. After calculating paleotemperatures, we obtained SST values comparable to other cores in the region (i.e., 24 °C - 31 °C). The paleotemperature record fluctuates over MIS 9 to 5, in agreement with the insolation intensity cycle controlled by precession. However, suborbital trends could not be reconstructed due to the studied effects imprinted on the specimens.