LATE PLEISTOCENE AND HOLOCENE GLACIAL MELTWATER INPUT TO THE GULF OF MEXICO

Situated at the outlet of the Mississippi River system, the Gulf of Mexico (GOM) is ideally located to record meltwater input from the Laurentide Ice Sheet (LIS) during the last glacial cycle. Injection of low-salinity meltwater into the North Atlantic via eastern outlets (e.g., St. Lawrence Seaway and Hudson River systems) is thought to have reduced thermohaline circulation and thereby triggered rapid regional to global climate change. Meltwater input to the GOM has only been expected when the eastern outlets were covered by Laurentide ice. However, re-examination of the late Pliocene to present history of meltwater input to the GOM, including new evidence for episodic meltwater input during the last glacial cycle and early Holocene, suggests that the GOM faithfully records LIS meltwater history even during minimal to moderate ice volume.

Based on oxygen isotope data on planktic foraminifera, meltwater first entered the GOM following glacial stages 92 and 90 (ca. 2.34-2.30 Ma) during the late Pliocene, when LIS ice volume was roughly half that of the last glacial maximum (Joyce et al., 1990). For the late Pleistocene, we capitalize on new sediment cores collected by the R/V Marion Dufresne in 2002 on IMAGES cruise VIII from laminated Orca Basin. To isolate changes in the oxygen isotopic composition of seawater from sea-surface temperature (SST), we use paired oxygen isotope and Mg/Ca data on Globigerinoides ruber. Episodic decreases of >1.0‰ to seawater values of <0.5‰ (modern value is 1.2‰) in middle marine isotope stage (MIS) 3 (45-30 ka; Hill et al., in prep.) indicate freshwater input during moderate ice volumes of MIS 3, with a timing that does not match Dansgaard-Oeschger interstadials. Furthermore, a major excursion of >1.0‰ to values reaching 0.4‰ is seen from ca. 8.47-8.35 ka (LoDico et al., in prep.), which is synchronous with draining of Lakes Agassiz and Ojibway (Barber et al., 1999). Ongoing molecular organic geochemical and isotopic work is also assessing freshwater influence on GOM SST (Uk37), biological production, terrestrial plant input, and continental hydrologic and vegetation changes. Catastrophic flooding at these times indicates that southward routing may not be sufficient to allow episodic intensification of thermohaline circulation and regional warming during the last glacial cycle.