Revisiting marine isotope stage 3 and 5a (MIS3-5a) sea levels within the northwestern Gulf of Mexico

Based on the present elevation of shallow-marine environments, previous studies along the northwestern Gulf of Mexico including data from the Freeport Rocks Bathymetric High, offshore Texas, suggest that sea-level during marine isotope stage 3 (MIS3) was as high as - 15 m. This value is in conflict with oxygen-isotope proxies and other measurements of sea level from across the globe during this time period. We revisit this problem by examining 18 cores from the Freeport Rocks Bathymetric High and present new optically stimulated luminescence (OSL) ages on relict shoreline deposits to determine their numerical age. In addition, we also run glacio-hydro-isostatic models in order to determine what impact glacio-hydro-isostasy had on sea levels within the northwestern Gulf of Mexico between MIS3 and MIS5a. Our OSL ages show that the Freeport Rocks Bathymetric High, originally postulated as an MIS3 deposit, is a MIS5a deposit with an age of 90.7 +/- 5.5 ka. Based on these deposits and their relationship to former sea levels, we provide one of the first estimates for MIS5a sea levels within the northwestern Gulf of Mexico of around - 11 ± 2 m. This estimate is in good agreement with current glacio-hydro-isostasic model predictions for MIS5a. As the Freeport Rocks Bathymetric High provided the highest of a series of reported MIS3 sea-level indicators, more work is needed to determine the numerical ages of the other better chronostratigraphically-constrained reported MIS3 sea-level indicators, which remain as high as - 20 ± 4 m. If the other MIS3 sea levels from the Gulf of Mexico are truly MIS3 in age, this would require a highpoint in eustatic sea level of - 42.8 ± 6.7 m below present after correcting for local glacio-hydro-isostatic effects during MIS3.