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The AAPG/Datapages Combined Publications Database
GCAGS Transactions
Abstract
ABSTRACT: Using
Benthic
Foraminifera to Understand Seagrass Abundance,
Salinity, and Sea-Level Variation in Florida Bay
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Jie Cheng1, Laurel S. Collins1, Charles W. Holmes2, Lee Ann Hayek3, and Martin
A. Buzas4
1Department of Earth Sciences, Florida International University, Miami, Florida
33199
2U.S. Geological Survey, Center for Coastal and Watershed Studies,
600 4th St. S., St. Petersburg, Florida 33701
3Smithsonian Institution, NHB MRC-121, Washington, D.C. 20560
4Smithsonian Institution, Paleobiology, Washington, D.C. 20024
Benthic
foraminifera are used as a proxy for seagrass abundance and salinity
variation
over the past 150 years, and sea-level fluctuation through the late Holocene.
The
study area is Florida Bay, part of Everglades National Park. Current efforts
to restore
the park and surrounding ecosystems to an original state are being guided by
investigations
of the natural range of ecosystem variability, the purpose of this study.
Eight cores collected from four locations across Florida Bay were examined.
Ages
of sediments were determined with 210Pb for the last 100-150 years, and 14C
for older
samples. Benthic
foraminifera >63 µm were identified from 180 samples.
There are
three basic approaches: (1) The percent abundance of seagrass-associated foraminifera
is used to reconstruct historic fluctuations of seagrass abundance. (2) To
study salinity
change, the alternation between brackish (0-18 ppt) and marine (18-40 ppt)
species is
documented. (3) Sea-level change is interpreted from the variability in nearshore,
lake,
mudbank, and mangrove faunas associated with different water depths.
Preliminary results show the following: (1) Concurrent with a large seagrass
die-off
in the late 1980s – early 1990s, all cores show decreases in the percent
of seagrassassociated
benthic
foraminifera, as well as foraminifera in general, and increases after
1995. (2) All cores show decreasing salinity in the 1970s and increasing salinity
in the
1980s, possibly related to a multi-year drought. Before 1900, there were large
variations
in salinity between decades. In the 1900s, changes were more gradual and may
be related
to the construction of the Flagler Railroad, hurricanes, and canal construction. (3) The dominance and rapid decline of a mangrove fauna about 4000 years ago
reflect
the entry of seawater into Florida Bay. Further analysis is needed to identify
trends and
relate them to seagrass and salinity variations.