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Understanding the natural spatial and temporal variability that exists within an ecosystem is a critical component of efforts
to restore systems to their natural state. Analysis of benthic foraminifers and molluscs from modern monitoring sites within
Florida Bay allows us to determine what environmental parameters control spatial and temporal variability of their assemblages.
Faunal assemblages associated with specific environmental parameters, including salinity and substrate, serve as proxies for
an interpretation of paleoecologic data. The faunal record preserved in two shallow (<2 m) cores in central Florida Bay (Russell
Bank and Bob Allen Bank) provides a record of historical trends in environmental parameters for those sites. Analysis of these
two cores has revealed two distinct patterns of salinity change at these sites: 1) a long-term trend of slightly increasing
average salinity; and 2) a relatively rapid change to salinity fluctuations of greater frequency and amplitude, beginning
around the turn of the century and becoming most pronounced after 1940. The degree of variability in substrate types at each
locality limits interpretations of substrate trends to specific sites. A common sequence of change is present in the Russell
Bank and Bob Allen Bank cores: from mixed grass and bare-sediment indicators at the bottom of the cores, to bare-sediment
dwellers in the center, to a dominance of vegetative-cover indicators at the top of the cores. Changes in interpreted salinity
patterns around the turn of the century are consistent with the timing of the construction of the Flagler Railroad from 1905
to 1912, and the Tamiami Trail and the canal and levee systems between 1915 and 1928. Beginning around 1940, the changes in
the frequency and amplitude of salinity fluctuations may be related to changes in water management practices, meteorologic
events (frequent hurricanes coupled with severe droughts in 1943 and 1944), or a combination of factors. The correspondence
of these changes in Florida Bay with changes in the terrestrial Everglades suggests factors affecting the entire ecosystem
are responsible for the salinity and substrate patterns seen in Florida Bay. 相似文献
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Joint investigations of the Middle Pliocene climate I: PRISM paleoenvironmental reconstructions 总被引：1，自引：0，他引：1
Harry Dowsett Robert Thompson John Barron Thomas Cronin Farley Fleming Scott Ishman Richard Poore Debra Willard Thomas Holtz Jr. 《Global and Planetary Change》1994,9(3-4)
The Pliocene epoch represents an important transition from a climate regime with high-frequency, low-amplitude oscillations when the Northern Hemisphere lacked substantial ice sheets, to the typical high-frequency, high-amplitude Middle to Late Pleistocene regime characterized by glacial—interglacial cycles that involve waxing and waning of major Northern Hemisphere ice sheets. Analysis of middle Pliocene (3 Ma) marine and terrestrial records throughout the Northern Hemisphere forms the basis of an integrated synoptic Pliocene paleoclimate reconstruction of the last significantly warmer than present interval in Earth history. This reconstruction, developed primarily from paleontological data, includes middle Pliocene sea level, vegetation, land—ice distribution, sea—ice distribution, and sea-surface temperature (SST), all of which contribute to our conceptual understanding of this climate system. These data indicate middle Pliocene sea level was at least 25 m higher than present, presumably due in large part to a reduction in the size of the East Antarctic Ice Sheet. Sea surface temperatures were essentially equivalent to modern temperatures in tropical regions but were significantly warmer at higher latitudes. Due to increased heat flux to high latitudes, both the Arctic and Antarctic appear to have been seasonally ice free during the middle Pliocene with greatly reduced sea ice extent relative to today during winter. Vegetation changes, while more complex, are generally consistent with marine SST changes and show increased warmth and moisture at higher latitudes during the middle Pliocene. 相似文献
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Alexander W. Karlsen Thomas M. Cronin Scott E. Ishman Debra A. Willard Randy Kerhin Charles W. Holmes Marci Marot 《Estuaries and Coasts》2000,23(4):488-508
Environmentally sensitive benthic foraminifera (protists) from Chesapeake Bay were used as bioindicators to estimate the timing
and degree of changes in dissolved oxygen (DO) over the past five centuries. Living foraminifers from 19 surface samples and
fossil assemblages from 11 sediment cores dated by210Pb,137Cs,14C, and pollen stratigraphy were analyzed from the tidal portions of the Patuxent, Potomac, and Choptank Rivers and the main
channel of the Chesapeake Bay.Ammonia parkinsoniana, a facultative anaerobe tolerant of periodic anoxic conditions, comprises an average of 74% of modern Chesapeake foraminiferal
assemblages (DO-0.47 and 1.72 ml l−1) compared to 0% to 15% of assemblages collected in the 1960s. Paleoecological analyses show thatA. parkinsoniana was absent prior to the late 17th century, increased to 10–25% relative frequency between approximately 1670–1720 and 1810–1900,
and became the dominant (60–90%) benthic formaniferal species in channel environments beginning in the early 1970s. Since
the 1970s, deformed tests ofA. parkinsoniana occur in all cores (10–20% ofAmmonia), suggesting unprecedented stressful benthic conditions. These cores indicate that prior to the late 17th century, there
was limited oxygen depletion. During the past 200 years, decadal scale variability in oxygen depletion has occurred, as dysoxic
(DO=0.1–1.0 ml l−1), perhaps short-term anoxic (DO<0.1 ml l−1) conditions developed. The most extensive (spatially and temporally) anoxic conditions were reached during the 1970s. Over
decadal timescales, DO variability seems to be linked closely to climatological factors influencing river discharge; the unprecedented
anoxia since the early 1970s is attributed mainly to high freshwater flow and to an increase in nutrient concentrations from
the watershed. 相似文献
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Camerlenghi Angelo Domack Eugene Rebesco Michele Gilbert Robert Ishman Scott Leventer Amy Brachfeld Stefanie Drake Allison 《Marine Geophysical Researches》2001,22(5-6):417-443
We present the results of a marine geophysical investigation of the northern Prince Gustav Channel. By comparative analysis of multibeam bathymetric data, single channel seismic reflection profiles, underway chirp sonar data, ADCP current data and sediment coring, we define the main morphological elements of the area. In particular we define the glacial morphogenesis in relation to the excavation of inner shelf basins and troughs along structural discontinuities and lithologic boundaries. We identify streamlined surfaces that testify to the grounding of ice and past ice flow directions. These glacial forms are found only on glacial tills preserved in the deepest part of the basins, while net erosion to bedrock has occurred elsewhere. Since the Last Glacial Maximum (LGM), the relict glacial morphology has been draped by hemipelagic and diatomaceous mud, and bottom currents have played a major role in focusing sedimentation within small depocentres, that we define as contouritic drifts. Based on shallow sediment architecture and supported by direct measurements, we propose that the direction of bottom water flow is from the outer shelf into the Prince Gustav channel as a result of a combination of tidal currents and ice shelf-related thermohaline circulation. 相似文献
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