Sources of Sea-Level Rise and Freshwater Discharge during the Last Deglaciation

During the last deglaciation, changes in freshwater fluxes were associated with changes in ice-sheet melting or calving rates that caused changes in rates of sea-level rise, as well as with routing of continental runoff that changed the location of freshwater flux relative to sites of deepwater formation without necessarily changing sea level. Modeling studies have established the sensitivity of the Atlantic meridional overturning circulation (AMOC) to increases in the freshwater budget at the sites of deepwater formation. A large fraction of the Northern Hemisphere ice sheets drained into the North Atlantic, so that establishing variations in the freshwater flux derived from these ice sheets is important for establishing their role in causing changes in the AMOC. Here we review the geologic evidence constraining the sources of sea-level rise and locations of freshwater discharge between the Last Glacial Maximum (LGM) and the Holocene. The LGM was terminated by an abrupt 10-15 m sea-level rise at ~19 ka, which was likely sourced from widespread retreat of Northern Hemisphere ice sheets in response to high-northern-latitude insolation forcing. Subsequent sea-level rise of ~15 m between 19 ka and 14.5 ka can be attributed to continued retreat of the Laurentide (LIS) and Scandinavian (SIS) ice sheets, with an additional freshwater forcing was delivered by Heinrich event 1 at ~17 ka. The source of the abrupt acceleration in sea-level rise ~14.5 ka (meltwater pulse 1a of 15-25 m sea-level rise in less than 700 years) remains widely debated. Geochemical, ice margin and far-field sea-level records all point to a small Northern Hemisphere ice-sheet contribution (e.g., <1/2 of the sea-level rise). Antarctic land-records indicate the onset of West Antarctic Ice Sheet retreat from its LGM position at 14.0-15.0 ka, supported by geophysical modeling of far-field sea-level records, suggesting a significant contribution to this meltwater pulse. Likewise, the cause of the subsequent Younger Dryas cold event is much debated, with the original hypothesis, routing continental Lake Agassiz runoff from the Mississippi River to the St. Lawrence River, now questioned based on runoff records from the St. Lawrence Estuary, terrestrial outlet chronologies and ice sheet modeling of an Arctic freshwater forcing. We will show that 1) there is a clear signal of eastward freshwater routing in the St. Lawrence Estuary once caveats of various salinity proxies are considered. 2) Existing outlet chronologies are not in conflict with the routing hypothesis. 3) The modeled ice-sheet discharge to the Arctic is too small to have caused the Younger Dryas and there is no evidence for an Arctic freshwater forcing.