This research will produce high-resolution reconstructions of sea-level change for the last 2000 years along the latitudinal gradient of the Atlantic Coast of the United States from Connecticut to Florida. Accurate estimates of sea-level rise in the pre-satellite era are needed to provide an appropriate context for 21st century projections and to validate geophysical and climate models. Documenting geographic trends in sea level is important because sea-level changes are not globally uniform owing to isostatic (land-level) movements of the solid Earth, gravitational and rotational changes driven by the exchange of mass between oceans and ice sheets, ocean density (steric) changes from temperature and salinity variations, and other factors. Analysis of microfossils from salt marsh sediments will generate transfer functions that document prehistoric sea level at a vertical resolution of ±0.1-0.3m. The age of the sediments will be determined from complementary dating methods. Sea-level reconstructions will be adjusted for land-level movements and ocean density influences to address the underlying mechanisms of sea-level change, focusing on the interplay between the contrasting influences of both the Greenland and Western Antarctic Ice Sheets. Whereas the Greenland Ice Sheet produces a significant latitudinal gradient in the magnitude of sea-level rise from north to south along the Atlantic Coast of the United States, the influence of Western Antarctic Ice Sheet is spatially uniform. Four specific research hypotheses related to the timing and magnitude of sea-level rise will be tested: (1) The timing of 20th century sea-level rise was synchronous between sites and the magnitude of this rise increased from north to south; (2) The magnitude of a deceleration in sea-level rise associated with the Little Ice Age (AD 1500-1850) increased from south to north; (3) The magnitude of sea-level rise during the Medieval Climate Anomaly (AD 1000-1500) increased from south to north; and (4) The rate of sea-level rise was constant from AD 0 to 1000.
Sea-level rise is a vital barometer of climate change, with potentially devastating consequences in the 21st century for coastal landforms, populations and infrastructure. Knowledge of sea-level variability during the past 2000 years is limited and the response to known climate deviations such as the Medieval Climate Anomaly, Little Ice Age and 20th century warming is unknown. This project will increase comprhension of the driving mechanisms of sea-level change and enhance the ability to predict 21st century sea-level rise. Such predictions must be validated against observations that constrain spatial and temporal variability in sea-level. The results of this project will contribute to the assessment of national hazards with respect to sea-level rise and coastal responses.
The purpose of this project was to develop precise sea-level records for the past 2000 yrs. We chose two locations along the eastern United States to look at how sea-level may have varied at different location at different times. An important influence on sea level is the amount of water it receives from the melting of glaciers in the Arctic and Antarctic. Ice melt has become an increasingly important component to sea level as the present climate warming continues to increase the rate at which ice is melting. An understanding of how ice melt has impacted sea level over the past 2000 yrs will provide a context for future scenarios. In general, large ice sheet such as the Greenland Ice Sheet (GIS) and Western Antarctic Ice Sheet (WAIS) actually attract the ocean toward them (because of the pull of gravity). In other words, when ice sheets melt the attraction of the ocean to the ice decreases and sea-level falls in areas near the ice sheet. In the reverse scenario, when ice sheets expand they increase the attraction between themselves and the ocean and sea-level rises in areas near the ice sheet. For example, if the GIS were to melt the water would accumulate near South America and on the coast of North America sea level would fall in the north and slowly increase toward Florida. We tested four general hypotheses to examine the impact of ice sheet melt at our study sites. Each hypothesis spans a timeframe of interest because of a stable or changing climate. (1) The recent rise in sea level occurs at each location at the same time but the magnitude of the rise in sea level decreases from north to south (WAIS ice melt). (2) A slowing of sea-level rise ~500 yrs to 150 yrs ago (Little Ice Age) increased from south to north (GIS ice sheet growth). (3) An acceleration in the rate of sea-level rise ~1000 yrs to 500 yrs ago (Medieval Climate Anomaly) had sea-levels rising more in the north than the south (WAIS melt). (4) From 2000 yrs to 1000 yrs ago the rate of sea-level was stable. To develop past sea-level records we used foraminifera, a single-celled organism that lives in all marine environments, in the tidal, near- and offshore. This bottom dweller generally lives in abundance in the surface sediment. Within salt marshes different species of foraminifera occupy different areas of a marsh, termed zones. The foraminifera are zoned with respect to the tidal frame with more salt tolerant species in the tidal flat to less salt tolerant species in the high marsh near the coastal forest (we test this using modern samples at each site). Depending on what foraminifera are found in the sediments we can assign a most likely zone of the salt marsh that has the most similarity (statistically). Foraminifera in core samples from past buried salt marsh sediment, up from 5.5m below the surface, were used to determine what zones occupied the marsh at different times. By dating these samples using radiocarbon analysis of plant remains we can develop a record of past sea-level change at each site. Sites at Georgia, New Jersey, and Delaware are being completed by our collaborators; Dr Ben Horton (UPenn) and Dr Andrew Kemp (Yale). For our results final adjustments to age models from Connecticut and Florida remain underway but the general trends are clear and are summarized in the following paragraphs. However, the sea level records presented are detrended for Glacial Isostatic Adjustment or GIA (the rebound or subsidence of the land related to the loading and subsequent unloading of large ice sheets during the most recent glaciation/deglaciation). For Connecticut this is 1 mm/yr and for Florida 0.3 mm/yr. In Connecticut, detrended sea level was relatively stable from 2200 yrs ago until ~150 yrs. From 150 yrs ago to present, sea level has risen at a rate of approximately 1.7 mm/yr. In Florida, detrended sea level over the past 2000 yrs has been stable. In the last ~100 yrs the rate of sea-level rise has increased to 1.8 mm/yr. However, over longer periods relative sea-level varied (we cannot assume that the GIA estimate is applicable back past 2000 yrs ago as it likely varied over longer timescales). From 8000 yrs ago to ~6000 yrs ago sea-level rose rapidly by > 0.6 mm/yr but by 6000 this slowed to 0.3 mm/yr. Result compared to the instrumental record (tide gauges) tests their validity as sea-level recorders (Fernandina beach tide gauge 2.0 mm/yr versus marsh records at 2.1 mm/yr). At individual sites this represents a local sea-level archive. When this technique is used in a regional analysis (along the US Atlantic coast), insight is gained into regional variability and differences in and between sites provides valuable information on past forcing mechanisms.
