Laurentide Ice Sheet meltwater δ18O from paired LA-ICPMS and IRMS analyses on single foraminifers
Abstract
The oxygen isotope composition of past continental ice sheets can be estimated from modeling results and computations from whole-ocean deglacial δ18Owater change. Reconstructions of the glacial-interglacial rate of change in the δ18Owater of 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. Computing the oxygen isotopic value of meltwater from the LIS could provide a powerful tool for exploring the dynamics of ice sheet melting during the last deglaciation. Here we present a record of the oxygen isotopic composition of LIS meltwater entering the Gulf of Mexico during discrete time slices of the last deglaciation (18 - 11 ka). We employ a novel technique that combines laser ablation ICP-MS and oxygen isotope analyses on individual shells of the planktic foraminifer Orbulina universa to quantify the instantaneous δ18Owater value of ice sheet meltwater. For each individual O. universa shell we quantify Mg/Ca (a temperature proxy) and Ba/Ca (a salinity proxy) with LA-ICP-MS, and then analyze the same O. universa for δ18O using the remaining material from the same shell. From these proxies, we compute δ18Owater and salinity for each individual foraminifer. A regression of all the data points obtained from the same core interval yields a δ18Owater:salinity relationship whose y-intercept is the freshwater end-member. We analyzed 28 core intervals from 17.5 ka to 13 ka (10-40 individual foraminifers per interval) and one Holocene core top interval from Gulf of Mexico Orca Basin core MD02-2550. Our data suggest that prior to 15 ka, Mississippi River outflow into the Gulf of Mexico was dominated by regional precipitation, and that LIS meltwater did not play a significant role in salinity variation over the Orca Basin. From 15 to 13 ka, computed δ18Owater values of LIS meltwater from discrete core intervals range from -17‰ to -45‰. These values indicate a dynamic melting history of different parts of the LIS, with contributions from both the low-elevation, southern margin of the LIS and potentially from supraglacial lakes from the LIS interior. Between 13 ka and the Holocene, our data do not suggest Mississippi River influence over the Orca Basin. Rather, we hypothesize that open ocean conditions prevailed over the Orca Basin as sea level rose and the paleoshoreline at the southern margin of North America expanded northward.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.C51C0792V
- Keywords:
-
- 0774 CRYOSPHERE / Dynamics;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 4900 PALEOCEANOGRAPHY;
- 4926 PALEOCEANOGRAPHY / Glacial