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Ecosystem History of Southern and Central Biscayne Bay: Summary Report on Sediment Core Analyses - Year Two
U.S. Geological Survey Open File Report 2004-1312
MATERIAL AND METHODS
Core Sites and Collection
This report discusses and/or includes results from three sets of cores collected between 1997 and 2003 (Table 1). The two wetland cores were collected in 1997 on either side of Military Canal (Figure 1) in collaboration with Dade County Department of Environmental Resources Management. Six cores were collected from three mudbanks located in a generally north-south transect through the Bay in 2002 (Figure 1; No Name Bank, Featherbed Bank, and Card Bank) (see Wingard and others, 2003 for complete description of cores and collecting methods). Three additional sites were selected in 2003 in areas within close proximity to historical freshwater outflow (Figure 1). Six cores were successfully collected from these three near-shore sites on June 19-20, 2003: 1) the basin west of Middle Key; 2) an inlet just north of Black Point; and 3) the southern end of Chicken Key. A and B cores were taken side by side at all three near-shore sites.
For the 2003 near-shore cores, the subset A cores were analyzed for 210-Pb, faunal remains (ostracodes, forams and mollusks), and sediment and shell geochemistry. Material from the A cores was archived for future diatom analyses. The subset B cores will be archived for any additional analyses that may be necessary. X-radiographs of the cores are shown in Figures 2-4 and descriptions are given in Appendix A.
In addition, this report will discuss results from cores collected between 1996 and 1997 by S.E. Ishman at Featherbed Bank (SEI297-FB1), Card Bank (SEI297-CB1) and Manatee Bay (SEI1196-MB1) (Table 1; Figure1). Detailed descriptions of these cores can be found in Ishman (1997). Ishman and others (1998) describe the faunal and floral patterns obtained from the SEI1196-MB1 core and Stone and others (2000) describe the faunal assemblages at SEI297-FB1. Mg/Ca data for SEI297-FB1 and ostracode data from SEI297-CB1 are reported in Wingard and others (2003). Interpretations and patterns in the 1996 and 1997 cores are discussed in light of the new findings reported here.
Faunal and Floral Analyses
Ostracodes, mollusks, foraminifers, and pollen were analyzed using the processing procedures described in Cronin and others (2001), Brewster-Wingard and others (2001), Ishman and others (1998), and Willard, Holmes, and Weimer (2001). All samples were taken in 2-cm segments for analyses. Sample spacing intervals for initial analyses varied from 2-cm to 16-cm depending on chronology, stratigraphy, and time constraints. Faunal and floral data were converted to percent abundance for analyses of faunal, floral, and ecosystem trends.
Ostracoda: One hundred ostracode specimens were picked, where possible, from the 150 and < 850 micron size fraction of the samples. If fewer than 100 ostracodes were present, all specimens were picked. Ostracode assemblages were examined from 55 samples (0-194 cm core depth) from 2002 Featherbed Bank core (GLW402-FBB) and from 6 samples (0-138 cm core depth) from 2002 Card Band core (GLW402-CBB). Eight samples were picked for ostracodes from the Middle Key core (GLW603-MKA) at 4-cm intervals (every other sample) down to 24 cm and separated into 16 taxonomic groups. Below 24 cm core depth the samples were essentially barren. Twenty two samples from Black Point North core (GLW603-BPN-A) were picked at 4-cm intervals and sorted into 18 taxonomic categories. Samples between 10 and 36 cm were barren. In Chicken Key core (GLW603-CKA) every sample was picked (2-cm intervals) down to 46 cm; below 38 cm core depth the samples were barren.
Mollusca: All mollusks 850 microns are picked and sorted into taxonomic groups and nine preservation categories. Samples containing extremely large quantities of mollusks are occasionally split and the size and number of splits are noted in the appendix data tables. The abundance data presented in this report exclude any worn mollusks and any fragments (defined as <50% of shell remaining) as these components are probably not indicative of the environment in which they are deposited. Worn and fragmented specimens do, however, provide important information about the depositional history of the sediments, so these data were utilized in interpretations.
Nineteen samples, at 8-cm intervals (every fourth sample), were analyzed for total molluscan faunal remains from the 1997 Card Bank Core (SEI297-CB1). Sixteen samples were picked for mollusks from Middle Key core (GLW603-MKA) at 8-cm intervals and were sorted into 41 taxonomic categories. Samples between 72 and 82 cm and from 96 cm to the bottom of the core were essentially barren. The lower most samples (below 112 cm) contained a mixed sample, contaminated by modern specimens and with recrystallized fossil material from the bedrock; these samples were not included in any analyses. Black Point North core (GLW603-BPNA) was examined at 8-cm intervals and the eleven samples yielded 54 taxonomic categories. No samples were barren in the Black Point North core, but the sample at 24-26 cm had very few mollusks in any preservation category. Ten samples were examined from Chicken Key core (GLW603-CKA), also at 8-cm intervals, but below 40 cm the samples essentially were barren.
Foraminifera: A total of 300 foraminifers were picked from the 63 and < 850 micron fraction of each sample where possible; otherwise all foraminifera present were picked. A number of barren samples were encountered in the near-shore cores as noted below. Twenty five samples were picked for foraminifera from the Featherbed Bank core (GLW402-FBB) at 8-cm intervals (every fourth sample) and separated into 69 taxonomic categories. Eight samples were examined from Middle Key core (GLW603-MKA) at 16-cm intervals (every eighth sample), but only two samples contained statistically significant numbers of foraminifera 0-2 cm and 16-18 cm samples the other samples were essentially barren. Black Point North core (GLW603-BPNA) contained 67 foraminiferal groups from twelve samples analyzed at 8-cm intervals. Eleven samples analyzed at 8-cm intervals from Chicken Key core (GLW603-CKA) contained 37 foraminiferal groups, but from 32 cm to the base of the core the samples contained twelve or fewer foraminifers total.
Pollen: Pollen samples were collected at 4, 6, or 10 cm intervals in the three near-shore cores (Middle Key core (GLW603-MKA), Black Point North core (GLW603-BPNA), and Chicken Key core (GLW603-CKA)) and at 2, 4, or 6 cm intervals in the two wetland cores (Military Canal cores SEI97-BW1 and SEI97-BW2) depending on depth in the core. No barren zones occurred but pollen concentrations did vary throughout the cores.
Ostracode Shell Chemistry Analyses
Geochemical analyses of metal/calcium ratios were conducted on ostracodes from the near-shore cores at Middle Key (GLW603-MKA), Black Point North (GLW603-BPNA), and Chicken Key (GLW603-CKA). Previous investigations have demonstrated that magnesium/calcium ratios can be effective proxies for estimating past salinity changes in Florida and Biscayne Bays (Dwyer and Cronin, 2001). The ostracodes utilized in the geochemical analyses were selected directly from samples also used in the analysis of ostracode faunal assemblages, and thus allowed for direct comparison of geochemical and faunal patterns.
Methods used on the near-shore cores collected in 2003 were identical to methods used on the 2002 mudbank cores, with the exception of the species used for shell chemistry analyses. The ostracode Loxoconcha matagordensis was analyzed in the 2002 cores, because of its abundance at sites in central Biscayne Bay. Malzella floridana shell chemistry is more suitable for analysis of the 2003 near-shore cores because this species is tolerant of a wide range of salinities and is common in cores from the Black Point, Chicken and Middle Key sites, where short term salinity variations are common. Wingard and others (2003) provided full details of the analytical procedures. Ideally a minimum of five adult valves of the ostracode species Malzella floridana were selected from each sample interval; however, in sparse intervals all valves available were used. These valves were processed and analyzed individually for Mg/Ca, Sr/Ca, and Na/Ca ratios by direct current plasma atomic emission spectrophotometry (DCP-AES) following procedures described in Dwyer and Cronin (2001, and references therein); results of analyses are presented in Appendix B. Precision on measurements is around two, four, and ten percent respectively. Some of the disparity in precision is because the instrument is optimized for Mg/Ca ratio analysis since Mg/Ca has proven to be the most useful paleoenvironmental indicator. While their usefulness is unclear, Sr/Ca and Na/Ca ratios were collected because with further study these data may provide additional paleoenvironmental information.
Conversion of the Mg/Ca ratios measured in the cores to a salinity value in ppt is based on analyses of the shell chemistry of living Biscayne Bay ostracodes. Live ostracodes were collected along with a water sample and the Mg/Ca of the shell was calibrated to the water chemistry (Cronin in Wingard, April 2003, unpublished report to SFWMD). Modern field collections were conducted in Biscayne Bay in 2002 and 2003 for the purpose of establishing the calibration index. However, it should be emphasized that sampling in low salinity waters was limited; fewer than ten samples were obtained in <10 ppt waters. Therefore, the Mg/Ca-based estimates of paleosalinity are most useful at salinities >10 ppt. Results of calibration of Mg/Ca values to actual salinity values are presented with the ostracode data results for each core in Appendix B.
Geochemical studies of sediments from dated cores collected in Biscayne Bay were conducted to examine historical changes in nutrient elements (C,N,P) and to correlate these changes with changes in faunal and floral indicators. Data on historical changes in nutrient elements in sediments reflect changes in nutrient load to the bay from natural and anthropogenic sources. For Biscayne Bay it is important to document the range of natural variability in nutrient load in order to evaluate the effects of recent anthropogenic activities (urbanization, canal discharge, agriculture).
Geochemical analyses were conducted on cores collected in 2002 from the mudbanks and cores collected in 2003 from the near-shore sites (Appendix C). Sub-samples of sediment for geochemistry were removed from the 2-cm core sample intervals, wet sieved (60 mesh) to remove coarse debris, dried, ground to a powder, and stored in clean glass vials prior to analysis. Total carbon (TC) and total nitrogen (TN) contents of sediments were determined using a Leco 932 CNS Analyzer (Leco Corporation, St. Joseph, MI, USA). Organic carbon (OC) was determined on the Leco analyzer after removal of inorganic carbon (IC, mostly carbonates) by an acid vapor method. Total phosphorus (TP) concentrations in sediments were determined by the method of Aspila and others (1976), slightly modified for work in Biscayne Bay sediments. Analytical precision (percentage relative standard deviation) was about 2% for (TC), 4% for OC, and 3% for TN and TP. IC is reported as the calculated difference between (TC) and OC (for example (%TC) - (%OC)). TP analyses have been completed on both the mudbank and near-shore estuarine cores; TC, TN, and OC analyses have been completed on the mudbank cores and are pending on the near-shore cores.
Development of Age Model for Cores
Preliminary age models were developed for the near-shore cores using three methods of dating where possible: (1) lead-210 analyses of the sediments; (2) first stratigraphic appearance of Casuarina (Australian pine); and (3) radiocarbon analyses of shell or wood material. The chronology of the upper segments of the cores was established using the lead-210 and pollen data. Decay of lead-210 isotopes into its daughter products is a reliable method for dating 20th century sediments (see Holmes and others, 2001, for detailed explanation of the methodology). The first occurrence of Casuarina pollen (Australian Pine), an exotic introduced into south Florida around the beginning of the 20th century (Langeland, 1990), provides an excellent stratigraphic marker for the early 1900s. In addition, disappearance of pine pollen and introduction of weedy species indicate land-clearing and agricultural practices, which can be correlated with historical records.
Carbon-14 analyses of individual shells or wood fragments were used in an attempt to establish ages for the lower portion of the cores (Table 2). Shell material was rare in these core segments, however, so we were unable to find well preserved specimens of the same species (the ideal situation) in the horizons we hoped to date. In addition, data are not currently available on the local carbon reservoir effect in Biscayne Bay. Conventional radiocarbon ages on marine shells must be corrected to account for the reservoir effect of ocean circulation on carbon. The global reservoir correction used in these analyses was 200-500 years, but local atmospheric and ocean processes can also affect the radiocarbon ages of marine and especially estuarine shells. The addition of terrestrial carbon into an estuary can produce older dates. The preliminary data presented here have not been corrected for these local effects.
The carbon-14 dates in the near-shore cores were calibrated using the procedure in Talma and Vogel (1993) and the INTCAL98 calibration dataset (Stuiver and others, 1998). Lead-210, carbon-14, and pollen stratigraphy also were utilized to establish the age models for the mudbank cores collected in 2002; details are presented in Wingard and others (2003). Radioisotopic analyses are in progress for the wetland cores; however, the first appearance of Casuarina and the decrease in pine pollen in the Military Canal cores provide a good estimate of the level at which 20th century sedimentation begins.
U.S. Department of the Interior, U.S. Geological Survey
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