Online

DeJong, Benjamin David

Natural Resources

2015

PhD

Rising sea levels present an ongoing threat to communities and resources around the Chesapeake Bay, east coast, USA, where tide gauges indicate that the relative rise of sea level is approximately twice the rate of average, eustatic sea-level rise. This has significantly compromised the health and viability of salt marsh habitat on the Eastern Shore during the 20th century, and the biologists who are charged with managing coastal resources in the coming decades need to understand the nature and causes of high rates of regional sea-level rise to develop suitable adaptation plans. Dated geologic deposits and geophysical models suggest that sea-level rise is relatively high on mid-Atlantic coastlines because the land surface is subsiding due to a collapsing glacial forebulge following the Last Glacial Maximum (LGM). To fully understand this process, past sea-level indicators such as dated shoreline deposits are needed to reconstruct regional sea-level behavior in the past, but rigorous age control on geologic deposits is largely restricted to the Holocene and to marine isotope stage (MIS) 5, so the rates and timescales over which these processes operate remain unknown. This research provides long-term paleoenvironmental records from ancient environments under east-central Chesapeake Bay to place the current sea-level threats into the context of a long geologic history of sea-level fluctuations. First, the Pleistocene geologic framework of the region is reconstructed through borehole drilling. Sediments from boreholes provided material for interpreting depositional environments, and for establishing age control for deposits, so that the entire stratigraphy was constrained both in space and time. The geologic framework and ages indicate that Chesapeake Bay alternated between a deeply incised fluvial system and a filled estuary repeatedly in response to major climate fluctuations since at least the early Pleistocene, ~2 Ma. The ages and sedimentology indicate that the field area was submerged intermittently in a shallow estuary until nearly the end of marine isotope stage 3. Because global sea-level proxies suggest that sea level was ~40-80 meters lower than present at that time, these ages suggest that the penultimate glacial forebulge must have remained significantly lowered for nearly 100 ky following the retreat of ice. The implication of this time lag is that mid-Atlantic coastlines are still in a relatively early state of forebulge collapse, and subsidence following retreat of ice from the Last Glacial Maximum will likely continue for the foreseeable future. Ongoing subsidence will continue to exacerbate projected eustatic sea-level rise due to changing global climate, and coastal adaptation plans must remain focused on encouraging the migration of vital habitat toward higher elevations in the landscape.