Shifts in the Oligotrophic Warm Conditions of the Gulf of Mexico Over MIS-6 to MIS-1 Based on Planktonic Foraminifera Assemblages and Mg/Ca Ratios

Abstract

Collected from the southwestern Gulf of Mexico (GoM), the marine piston Core RC10-265 was used to reconstruct surface water masses, circulation and paleotemperature shifts at orbital scales over the last ∼180 ka. The chronology was constructed based on radiocarbon ages, planktonic foraminifera biostratigraphy and stable oxygen isotopes (δ18O). The ocean surface conditions were reconstructed based on planktonic foraminifera assemblages. Applying a Q-mode Factor Analysis, two scenarios were characterized through seven species. (a) The subtropical assemblage (Globigerinoides ruber—Globoconella inflata (positive scores) and the Globorotalia menardii group—Pulleniatina obliquiloculata (negative scores)) suggest that during the end of MIS-6, and MIS-4 to MIS-2, the surface waters were colder (∼22–25°C), with less oligotrophic and a more profound mixed layer depth (MLD) relative to interglacials. (b) The tropical assemblage (Globigerinoides ruber—Neogloboquadrina dutertrei—Globorotalia truncatulinoides (positive scores) and Globoconella inflata—Globigerina falconensis (negative scores)) suggest that during the early MIS-6, MIS-5e to 5b and MIS-1, the surface waters were warm (∼28–32°C), oligotrophic, with a shallow MLD like average modern conditions. High Fe/Ca and Mn/Ca values could result from early diagenesis or their incorporation in the primary calcite due to high Fe inputs to the GoM. Despite the former, Mg/Ca paleotemperatures are consistent with Caribbean and N Gulf reconstructions. This study confirms the relevance of the Loop Current (extended vs. contracted mode) and the associated variability in mesoscale eddies as a control of the paleoecology and paleotemperatures at orbital scales.