Sea-level changes along the U.S. Atlantic coast: Implications for glacial isostatic adjustment models and current rates of sea-level change

• Main Author: Engelhart, Simon E
• Format: Text
• Language: English
• Published: ScholarlyCommons 2010
• Subjects: Geology|Physical oceanography|Geomorphology|Paleoclimate Science; Greenland; Ice Sheet
• Online Access: https://repository.upenn.edu/dissertations/AAI3431115

This study develops the first database of Holocene sea-level index points for the U.S. Atlantic coast using a standardized methodology. The database will help further understanding of the temporal and spatial variability in relative sea-level (RSL) rise, provide constraints on geophysical models and document ongoing crustal movements due to Glacial Isostatic Adjustment (GIA). I sub-divided the U.S. Atlantic coast into 16 areas based on distance from the center of the Laurentide Ice Sheet. Rates of RSL change were highest during the early Holocene and have been decreasing over time, due to the continued relaxation response of the Earth’s mantle to GIA and the reduction of ice equivalent meltwater input around 7 ka. The maximum rate of RSL rise (c. 20 m since 8 ka) occurred in New Jersey and Delaware, which is subject to the greatest forebulge collapse. The rates of early Holocene (8 to 4 ka) rise were 3 - 5.5 mm a-1. I employed basal peat index points, which are subject to minimal compaction, to constrain models of GIA. I demonstrated that the current ICE-5G/6G VM5a models cannot provide a unique solution to the observations of RSL during the Holocene. I reduced the viscosity of the upper mantle by 50%, removing the discrepancy between the observations and predictions along the mid-Atlantic coastline. However, misfits still remain in Maine, northern Massachusetts and the Carolinas. Late Holocene (4 ka to present) RSL data are a proxy for crustal movements as the eustatic component was minimal during this time. Land subsidence is less than 0.8 mm a -1 in Maine, increasing to 1.7 mm a-1 in Delaware, and a return to rates lower than 0.9 mm a-1 in the Carolinas. This pattern results from the ongoing GIA due to the demise of the Laurentide Ice Sheet. I used these rates to remove the GIA component from tide gauge records to estimate a mean 20th century sea-level rise rate for the U.S. Atlantic coast of 1.8 ± 0.2 mm a-1. I identified a distinct spatial trend, increasing from Maine to South Carolina, which may be related to either the melting of the Greenland Ice Sheet, and/or ocean steric effects.