Short and long-term sea-level controls on coastal peat/gravel barrier systems
Abstract
Submarine groundwater discharge (SGD) is an important component of the coastal hydrologic cycle because it affects mixing and biogeochemistry in the subterranean estuary. SGD operates at a variety of temporal scales ranging from years (e.g., interannual, seasonal) to days (e.g., tidal) to seconds (e.g., waves). Correlations between freshwater SGD and interannual climate oscillations such as the El Niño-Southern Oscillation (ENSO) have been demonstrated for sandy barrier-island aquifers; however, these correlations break down in low and high permeability or low-recharge aquifers. Peat and gravel aquifer systems, typical of the southwest coast of Britain, contain two units that have contrasting hydraulic properties: the freshwater peats are low permeability and low recharge units that are overlain by high permeability and high recharge gravel aquifers. Although these dual-aquifer systems have characteristics that preclude correlations with interannual climate oscillations, they are influenced by other temporal variations. For example, the high permeability of the gravel allows them to react nearly instantaneously to tidal oscillations and wave runup. At the other extreme, the peat components of these systems react much more slowly and respond to long-term changes in boundary conditions (e.g., sea-level change). As a case study, we use the peat/gravel aquifer system in Hallsands, Devon, UK. Macrotidal boundary conditions influence this aquifer with spring and neap tidal amplitudes up to 2.5 m and 1.0 m, respectively. We calibrate the hydraulic properties of the two units to tidal efficiencies observed in monitoring wells. We use simulations of a generic, two-layer system to demonstrate the effects of the relatively short temporal variations of the tidal oscillations. Because the spring and neap oscillations are so different, two separate zones of high salinity occur within the gravel above the main saline/fresh interface, one at the location of the spring high tide and the other midway between the spring high and low tides. The location of the upper interface has been detected with ground-penetrating radar measurements. In addition, the main mixing zone in the gravel moves up-gradient during neap conditions and down-gradient during spring conditions. The peat in the freshwater marsh responds to much larger temporal scales and its surface acts as a proxy for sea-level change over the past 7,000 years. The groundwater modeling demonstrates that the water table in the reed marsh is controlled by the permeability of the peats and tidal oscillations. We radiocarbon dated five basal peat samples along the slope of the hard substrate where they have not been affected by sediment compaction. These age-depth points match the sea-level curve produced by traditional methods. Whereas previously freshwater peats were considered to provide only limiting sea-level index points, this result indicates that peat from backbarrier freshwater marsh systems can be used for accurate reconstructions of Holocene sea-level change.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.H34E..01A
- Keywords:
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- 1807 HYDROLOGY / Climate impacts;
- 1829 HYDROLOGY / Groundwater hydrology;
- 1830 HYDROLOGY / Groundwater/surface water interaction;
- 1847 HYDROLOGY / Modeling