LATE HOLOCENE AND 20th CENTURY SEA-LEVEL RISE OF THE EASTERN UNITED STATES

Accurate estimates of global sea-level rise in the pre-satellite era provide a context for 21^st century sea-level predictions, but the use of tide-gauge records is complicated by the contributions from changes in land level due to glacial isostatic adjustment (GIA). We have constructed a rigorously quality-controlled database of late Holocene sea-level indices from the US Atlantic Coast, which we use to elucidate the current rates of GIA. The US Atlantic Coast exhibits subsidence rates of less than 0.8 mm yr^-1 in Maine, increasing to rates of 1.7 mm yr^-1 in Delaware, and a return to rates less than 0.9 mm yr^-1 in the Carolinas. There is an absence of appropriate data in Georgia and eastern Florida. This pattern can be attributed to ongoing GIA due to the demise of the Laurentide Ice Sheet.

Our data allow us to define the geometry of the associated collapsing proglacial forebulge with a level of resolution unmatched by any other currently available method. The corresponding rates of relative sea-level rise serve as “background” rates on which future sea-level rise must be superimposed. We further employ the geological data to remove the GIA component from tide-gauge records to estimate a mean 20^th century sea-level rise rate for the U.S. Atlantic Coast of 1.8 ± 0.2 mm yr^-1, which is similar to the global average. However, we find a distinct spatial trend in the rate of 20^th century sea-level rise, increasing from Maine to South Carolina. This is the first evidence of this phenomenon from observational data alone. We suggest this may be related to either the melting of the Greenland Ice Sheet, and/or ocean steric effects.