EVIDENCE FOR MELTWATER PULSE 1A IN THE GULF OF MEXICO BASED ON RADIOGENIC ISOTOPES OF LEACHATES

Meltwater pulse 1a (MWP-1a) is associated with a rapid sea level rise of more than 20 m in less than 500 yrs during the last deglaciation. This event has been dated at between 14.6-13.9 ka based on new Barbados drillcores (Mortlock et al., 2010). The contribution of inferred meltwater from the Antarctic ice sheet and Northern Hemisphere ice sheets is under debate. Relative contributions from the northern versus southern hemisphere have implications for variations in ocean surface salinity, North Atlantic Deep Water formation and related climate responses. A study of bulk sediment δ¹⁸O values from core MD02-2550 from the anoxic Orca Basin in the Gulf of Mexico produced peak δ¹⁸O values of -5.5 ‰ during a foraminifera-barren interval dated at 14.54-14.35 ka based on an age model using >40 AMS ¹⁴C dates on Globigerinoides ruber. This δ¹⁸O value is unusual for marine sediments and has been interpreted to represent Paleozoic Canadian Sediments eroded by the Laurentide Ice Sheet and delivered to the Gulf of Mexico during MWP-1a (Flower, 2010). For this study, we evaluated radiogenic isotopes of Nd, Pb, and Sr on dilute 0.1N HCl leachates of powdered bulk sediments from MD02-2550 to identify changes in input sources during the foraminifera-barren interval. Measured ²⁰⁶Pb/²⁰⁴Pb values are ~19.0 before and after the foraminifera-barren interval, with peak values of 19.7 during the foraminifera-barren interval that coincide with the δ¹⁸O peak. Similarly ⁸⁷Sr/⁸⁶Sr isotopes reveal variable background values of 0.70892-0.70927 with a peak value of 0.70943 coinciding with the δ¹⁸O and ²⁰⁶Pb/²⁰⁴Pb peaks. We interpret these background values to represent typical Mississippi river outflow, while the maximum Pb and Sr isotopic values are associated with a flux of material derived from the Laurentide Ice Sheet (LIS). Nd isotopes do not produce the same pattern, possibly due to the composition of material contributing the more radiogenic signals. The Sr and Pb isotopic results support the idea that the foraminifera-barren interval and δ¹⁸O peak in the Orca Basin sediments represent the LIS, and indicate a contribution to MWP-1a by the LIS during the Bǿlling warm interval.