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Sea-level Changes along the Atlantic Coast of the United States: Implications for Glacial Isostatic Adjustment Models and Current rates of Sea-level Change

The aim of this NSF sponsored proposal is to establish a relative sea-level (RSL) database from the Atlantic Coast of the United States and combine it with data from Atlantic Canada, the United States Gulf Coast and the Caribbean since the Last Glacial Maximum (LGM). These data will be used to validate and refine Glacial Isostatic Adjustment (GIA) models. There is an urgent need for a re-assessment of the quality of the observational evidence of former sea levels from the Atlantic Coast of the United States, as well as concepts inherent in the interpretation of the data. Furthermore, other factors such as sediment compaction and tidal range variations have rarely been taken into account. Such deficiencies in RSL histories represent a significant gap in our understanding of Late Quaternary sea level, its driving mechanisms and spatially variable expression. They also constitute an extremely important limitation to the dynamical models of the GIA process that are currently employed to filter tide-gauge and satellite records of sea-level change so as to isolate the contribution due to climate warming. The specific research objectives of this proposal are: 1) critical re-assessment of (un)published RSL data since the LGM from the Atlantic Coast of the United States; 2) combine the United States Atlantic Coast data with sea-level reconstructions from Atlantic Canada, the Caribbean and the United States Gulf Coast; 3) isolate the effects of tidal regime change and sediment consolidation from differential crustal movements; 4) validate and refine GIA models; and 5) determine rates of RSL change and differential crustal movements along the Atlantic Coast of Canada and the United States, as well as the Gulf Coast and Caribbean.

 


Millennial-scale records of sea-level change along the Atlantic coast of the United States

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. Exploring geographic trends in sea level is of critical importance because sea-level changes are not globally uniform due 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. This NSF sponsored 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. Innovative microfossil-based transfer functions from salt marsh sediments will generate sea-level records at an unprecedented vertical resolution (±0.1-0.3m). Combining this approach with a suite of complementary dating methods provides the ability to precisely constrain the chronology (decadal to centennial age resolution) of subtle changes in sea level.


Advanced regional and decadal predictions of coastal inundation for the U.S. Atlantic and Gulf coasts

The rate of sea-level rise along the US Atlantic and Gulf coasts increased through the 20th century and will almost certainly continue to accelerate during the 21st century and beyond, although significant uncertainty surrounds the magnitude and geographic distribution. Key uncertainties include the role of the Greenland and West Antarctic ice sheets, mountain glaciers and ocean density (steric) changes. Insufficient understanding of these physical processes has precluded accurate prediction of sea-level rise. New approaches using semi-empirical models that relate instrumental records of climate and sea-level rise have projected up to 2 m of sea-level rise by AD 2100. But the duration of instrumental records is insufficient to adequately constrain the climate-sea-level relationship. In this NOAA sponsored proposal we will produce new high resolution proxy data of sea-level and employ global temperature reconstructions spanning the alternation between the so-called “Medieval Climate Anomaly” and “Little Ice Age” and their estimated uncertainties to provide crucial additional constraints to the parameters in semi-empirical models of sea-level rise. Before the models can provide appropriate data for coastal management and planning, they must be complemented with regional estimates of sea-level rise. The proxy sea-level data collected from six study areas (Massachusetts, New Jersey, North Carolina, Georgia and Atlantic and Gulf coasts of Florida) will expose regional variability due to glacial isostatic adjustment of the solid Earth and gravitational and rotational changes driven by the exchange of mass between oceans and ice sheets. The Atlantic and Gulf coasts of the US are also at risk from changes in the frequency and magnitude of tropical cyclones; particularly when superimposed on background sea-level rise. The historical and observational records are insufficient for characterizing the nature and recurrence of extreme and rare events. We will couple regional sea-level rise projections with hurricane simulations and storm surge models to map coastal inundation for the current climate and the best and worst case climate scenarios of the IPCC AR4.

 


Megathrust paleogeodesy at the central Cascadia subduction zone

Our NSF and USGS sponsored research will test various hypotheses that explain how strain accumulates along the great megathrust fault between continental and oceanic plates at the Cascadia subduction zone, and is then suddenly released during great (magnitude 8 to 9) earthquakes on the fault.  We measure subduction-zone strain indirectly by inferring coastal land-level changes from small changes in relative sea level.  This new information about how the Cascadia plate boundary deforms will help us understand deformation at other plate boundaries and improve assessments of earthquake and tsunami hazards in central western North America.

 


A paleoseismic record of repeated great earthquakes on the Sunda Subduction megathrust, Northern Sumatra

The 2004 Andaman-Aceh earthquake and tsunami focused global attention on the northernmost part of the Sumatran subduction zone and raised questions about the past geologic history of the Sunda megathrust offshore of northernmost Sumatra. The objective of the proposed research is to understand and chronicle the long-term seismic behavior of the Sunda megathrust of northernmost Sumatra through a study that integrates forefront techniques of paleoseismology, geodesy, geochronology and paleoenvironmental reconstruction using microfossils. We propose to establish a subduction zone chronology of earthquakes and vertical and level changes for the northern most past of the Sumatran subduction zone offshore of northern Sumatra that extends back in time through the latter half of the Holocene (approximately five thousands years). Our reconstructed subduction zone history, documented at the coast arcward of the inboard extent of the coseismic rupture, will include investigation of several critical components that help define the nature of subduction zone tectonics at a convergent margin. These components include recurrence interval between earthquakes, amounts and relative timing of coseismic and interseismic vertical land level changes, documentation of any subsidence precursory to the main coseismic subsidence event, documentation of uplift after the main coseismic subsidence event caused by slow, down dip afterslip on the megathrust and, finally, development of a chronology of tsunami that invades the Sumatran coast.

 


Subduction Zone Segmentation over Multiple Seismic Cycles, South-Central Chile

The possible segmentation of subduction zone faults presents one of the most significant questions to the physics and geology of earthquake occurrence. Advances in instrumentation and modeling in recent decades have produced important insights into fault behavior, yet these measurements span only fractions of most earthquake cycles. We propose to investigate the magnitude and distribution of coastal land-level changes and tsunami deposits associated with subduction zone earthquakes along the coast of south-central Chile (37-39°S) to address the question of whether segmentation of subduction zone faults is maintained over multiple earthquake cycles. The combination of paleoseismic evidence for the past 4000 years, frequent large subduction zone earthquakes, and an unusually long and comprehensive historic record of earthquakes in south-central Chile creates a natural laboratory to address fundamental questions about the seismic segmentation. Prior to 2010, the Arauco Peninsula of south-central Chile was one of the most accepted hard segment boundaries along the Peru-Chile trench; it was believed to bound several historical ruptures, including the Great Chilean earthquake of 1960. The Maule, Chilean earthquake in February 2010 called this hypothesis into question, when the rupture extended south of this boundary and only partially overlapped with the seismic gap that had been building since the AD 1835 earthquake. The unforeseen extent of the ruptures in the 2004 Sumatra-Andaman and 2011 Tohoku-Oki earthquakes underscore the practical importance of this problem for assessment of earthquake hazards in all subduction zones.

 


Geologic evidence of tsunamis originating from the Japan Trench’s southern segment

In the wake of the 2011 Tohoku-oki tsunami, studies modelling rupture scenarios for the Japan Trench have identified areas of uncertainty, particularly along the southern segment. The accuracy of these seismic models and the understanding of fault movement along the southern Japan Trench can be greatly improved by locating and mapping prehistoric tsunamis deposits. Records of tsunamis developed from the sedimentary deposits they leave behind, improve our understanding by expanding the age range of events available for study. This NSF sponsored EAGER study aims to locate geologic evidence of past tsunamis originating from the southern segment of the Japan Trench, which to date remains undocumented. The study will investigate the beach ridges and coastal ponds of Chiba region of Japan using the state-of-the-art litho-, bio-, and chronostratigraphical techniques.