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| Figure 2. Graphic of key sediment cores from Hillsborough Bay and Moses Hole. |
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Three vibracores were taken in Moses Hole from which 2.5-3.0 meter long sediment cores were recovered. Gray carbonate clay was recovered in the lowest part of each core and it is overlain by shell hash representing a transgression of the sea over the area, as part of the termination of the last sea-level rise or as a result of a major storm deposit.
Overlying the shell hash is a 1.5 meter sand deposit containing minor fragments of shells and evidence of extensive bioturbation or burrowing by organisms living on and below the sea floor. Pollen and microfauna studies indicate that the top meter of sand was deposited in historic times, suggesting that it was deposited rapidly, perhaps as the result of a major storm.
Biomarker studies on organic matter demonstrate that submergent vascular plants dominate the organic matter, but contributions from upland terrestrial plants were observed in the upper part of the core.
Salinity values of pore water throughout the core were at open marine levels, but on land, away from the bay salinities of pore water in the sediment is over twice marine levels. These results agree with geophysical measurements of more deeply buried groundwater salinities from depths of 10-40 meters conducted in the same area. Pore-water measurements taken from onshore cores in the Mariposa Point area provide valuable information for biologists studying flora.
The Hillsborough Bay core contained more organic material than the core taken from Moses Hole and yielded lead and carbon isotopes suitable for dating. Biomarker studies on organic matter in the core documented the transition from prehistorical, dominantly submergent vascular plants to upland species. This information suggests changes in river flow and groundwater input to the bay. In addition, recent conditions in the bay changed from aerobic to anaerobic, which is coincident with enhanced input of algal, zooplankton and sewage sources.
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