Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2014
Description:
At the land-ocean interface, terrestrial groundwater interacts with seawater to form a
subterranean estuary, which can play host to dynamic biogeochemical cycling of nutrients,
trace metals and radionuclides. This chemically altered groundwater enters the ocean through
submarine groundwater discharge (SGD), a process that is driven by a number of physical
processes acting on aquifers and the coastal ocean. In this thesis, seasonal variability in
chemical cycling and associated loading to the coastal ocean was observed in a monthly time
series within the Waquoit Bay (MA, USA) subterranean estuary. The position of the aquifer
mixing zone moved seaward with an increase in hydraulic gradient, resulting in low salinity
conditions and reduced mixing, while a decrease in gradient led to landward movement, high
salinity groundwater and enhanced mixing. At this location, seasonal variability in sea level,
not groundwater level, was the dominant variable driving the hydraulic gradient and
therefore SGD. Fluxes of sediment bound cations to the ocean increased coincidently with
sea level rise due to desorption. There was enhanced nitrogen attenuation during winter,
potentially due to longer groundwater residence times, with greater nutrient delivery to
coastal waters during the spring and summer bloom. Interannual climate fluctuations that
control sea level and precipitation may ultimately control the timing and magnitude of
chemical and water flux via SGD.
In addition to temporal variability, aquifer lithology influences chemical export. This
thesis also demonstrates that SGD from karst subterranean estuaries may play a role in local
and global element budgets. The potential for the chemical signature of SGD to be recorded
in the coral record was tested through a combination of coral culture experiments and field
and modeling studies in the Yucatan Peninsula. Coral barium was well correlated with
precipitation for a twelve-year record, with coral geochemistry reflecting the passage of a
hurricane in 2002. While additional complexities in deciphering coral records remain, this
proxy offers the potential to extend SGD records into the past.
Description:
This research was supported by a National Defense Science and Engineering Graduate Fellowship, a
National Estuarine Research Reserve Graduate Fellowship from the National Oceanic and Atmospheric
Administration, and grants from the U.S. Geological Survey (G10AC00210) and the U.S. National Science
Foundation (OCE-0425061, OCE-0751525 and OCE-0524994). Additional funds were provided by the
WHOI Academic Programs Office, WHOI Ocean and Climate Change Institute, and MIT endowed funds.
Keywords:
Biogeochemical cycles
;
Chemical oceanography
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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