FURTHER EVIDENCE FOR LATE PLEISTOCENE LAND SURFACE ADJUSTMENT IN RESPONSE TO A GLACIO-ISOSTATIC ADJUSTMENT IN THE MID-ATLANTIC

The Pleistocene substrate underlying the Blackwater National Wildlife Refuge (BNWR) on the Eastern Shore of the Chesapeake Bay preserves an extensive record of landscape response to climate forcing. Presently, this low-elevation region is experiencing high rates of relative sea level (RSL) rise. Tide gauge data collected ~15 km north of the field area in Cambridge, Maryland indicate RSL rise of approximately 3.48 mm/yr over the period from 1943-2006, almost twice that of the global rate of sea level rise (~1.8 mm/yr). Climate change, subsidence, and land use have all been cited as possible drivers of these accelerated rates. To better understand these observations, we place them in the context of previous responses of this region to climate cycling recorded in the Pleistocene substrate.

We use vibracoring, hollow-stem augering, and flight augering techniques to penetrate the Pleistocene stratigraphy (usually < 30 m) underlying BNWR in 60+ locations and collect samples for pollen analysis and geochronology in key locations. The lithology is used to correlate units and define the geologic framework. Pollen was collected from all units within selected cores to track changes in floral assemblages through time and sand samples were collected in light-safe core barrels inside hollow-stem augers for optically stimulated luminescence (OSL) dating.

Preliminary results suggest fluvio-estuarine deposition during marine isotope stage (MIS) 5a and MIS 4-3, when eustatic sea levels were >20 m and >80 m lower than present, respectively. These results corroborate findings from locations in Virginia down to North Carolina that suggest that a proglacial forebulge may have affected regional land surface elevations significantly during previous glacial cycles. More than 50 ky lapsed between the glacial maximum of MIS 6 and the continued subsidence necessary for deposition during MIS 5a in the region, and just ~20 ky separate the Last Glacial Maximum from the high rates of RSL rise observed in the vicinity of BNWR in this century. So whereas climate change and land use likely contribute to accelerated RSL rise and land subsidence, our data supports the idea that land subsidence may be caused in part by a collapsing forebulge today.