QUANTIFYING LAURENTIDE ICE SHEET MELTWATER GEOCHEMISTRY ACROSS THE LAST DEGLACIATION

The oxygen isotope composition of past continental ice sheets can be estimated from modeling results and computations from whole-ocean deglacial δ¹⁸O_water change. Reconstructions of the glacial-interglacial rate of δ¹⁸O_water change in the global ocean assume a homogeneous δ¹⁸O value for contributions from individual ice sheets such as the Laurentide Ice Sheet (LIS). However, observations of the modern Greenland and Antarctic ice sheets indicate dynamic and highly variable melting of different parts of these ice sheets. Quantifying the oxygen isotope variability of deglacial LIS meltwater could provide a powerful tool for exploring the dynamics of ice sheet melting during glacial terminations.

Here we present a record of the δ¹⁸O_water of LIS meltwater entering the Gulf of Mexico during discrete time slices of the last deglaciation (18–11 ka). We use a novel technique combining laser ablation ICP-MS and δ¹⁸O_c analyses on individual shells of the planktic foraminifer Orbulina universa to quantify the instantaneous δ¹⁸O_water value of ice sheet meltwater. For each individual O. universa, we quantify Mg/Ca (temperature) and Ba/Ca (salinity) with LA-ICP-MS, and then analyze the same individual for δ¹⁸O_c using the remaining shell material. From these proxies, we compute δ¹⁸O_w and salinity for each individual foraminifer. Regression of all data points from the same core interval yields a δ¹⁸O_w:salinity relationship whose y-intercept is the freshwater end-member. We analyzed 31 core intervals from 17.5—13 ka (10-40 individuals per interval), and one Holocene core top interval, from core MD02-2550, Orca Basin, Gulf of Mexico. Our data suggest that before 15.5 ka, Mississippi River geochemistry was dominated by regional precipitation, and LIS meltwater did not influence salinity variation over the Orca Basin. From 15.5—13 ka, computed δ¹⁸O_w values of LIS meltwater from discrete core intervals range from -17‰ to -62‰ (VSMOW), indicating a dynamic melting history of the LIS, with potential contributions from both the low-elevation, southern margin and high-elevation, high-latitude Keewatin and Labrador domes. After 13 ka, our data suggest no Mississippi River influence over the Orca Basin. Instead, we hypothesize open-ocean conditions, as sea level rose and the Gulf of Mexico paleoshoreline retreated northward.