Oxygen isotope geochemistry of Laurentide ice-sheet meltwater across Termination I

We present a new method that quantifies the oxygen isotope geochemistry of Laurentide ice-sheet (LIS) meltwater across the last deglaciation, and reconstruct decadal-scale variations in the delta O-18 of LIS meltwater entering the Gulf of Mexico between similar to 18 and 11 ka. We employ a technique that combines laser ablation ICP-MS (LA-ICP-MS) and oxygen isotope analyses on individual shells of the planktic foraminifer Orbulina universa to quantify the instantaneous delta(18)Owater value of Mississippi River outflow, which was dominated by meltwater from the LIS. For each individual O. universa shell, we measure Mg/Ca (a proxy for temperature) and Ba/Ca (a proxy for salinity) with LA-ICP-MS, and then analyze the same O. universa for delta O-18 using the remaining material from the shell. From these proxies, we obtain delta(18)Owater and salinity estimates for each individual foraminifer. Regressions through data obtained from discrete core intervals yield delta(18)Ow vs. salinity relationships with a y-intercept that corresponds to the delta(18)Owater composition of the freshwater end-member. Our data suggest that from 15.5 through 14.6 ka, estimated delta O-18 values of Mississippi River discharge from discrete core intervals range from -11 parts per thousand to -21 parts per thousand VSMOW, which is consistent with delta O-18 values from both regional precipitation and the low-elevation, southern margin of the LIS. During the Bolling and Allerod (14.0 through 13.3 ka), estimated delta(18)Ow values of Mississippi River discharge from discrete core intervals range from -22 parts per thousand to -38 parts per thousand VSMOW. These values suggest a dynamic melting history of different parts of the LIS, with potential contributions to Mississippi River outflow from both the low-elevation, southern margin of the LIS and high-elevation, high-latitude domes in the LIS interior that were transported to the ablation zone. Prior to similar to 15.5 ka, the delta(18)Owater value of the Mississippi River was similar to that of regional precipitation or low-latitude LIS meltwater, but the Ba concentration in the Mississippi basin was affected by changes in weathering within the watershed, complicating Ba-salinity relationships in the Gulf of Mexico. After 13 ka, our data suggest Mississippi River outflow did not influence surface salinity above our Gulf of Mexico Orca Basin core site. Rather, we hypothesize that open ocean conditions prevailed as sea level rose and the paleoshoreline at the southern edge of North America retreated northward. (C) 2017 Published by Elsevier Ltd.