CHARLOTTE SJUNNESKOG and REED SCHERER, Institute of Earth Sciences, Uppsala University, Uppsala, Sweden
As a part of a project studying late Holocene climatic changes in the Antarctic Peninsula area, the stratigraphic resolution of the diatom record from bioturbated marine sediments recovered by trigger and box coring is examined. We are analyzing absolute diatom abundance and the diatom assemblage as proxies for diatom paleoproductivity and paleoenvironments. Determining stratigraphic resolution in box and trigger cores, which may recover the sediment-water interface, will aid in correlation between cores, including tying trigger and box cores to the longer records of piston cores, which do not recover an unaltered sediment-water interface. Correlating these records will allow calibration of paleoceanographic records with modern conditions. This study includes analysis of two trigger cores from the Bransfield Strait (TC 86-7, TC 86-9) and two box cores from the Gerlache Strait (BC 86-79, BC 86-82) (table).
The bottom of the Bransfield and Gerlache Straits is oxygenated, and the diatomaceous sediments are bioturbated typically to a depth of approximately 10 centimeters (cm) (Harden, DeMaster, and Nittrouer 1992). The sediments accumulate in the basins at rates of between 1 and 3 millimeters (mm) per year (Harden et al. 1992). Our hypothesis is that bioturbation in these high-accumulation-rate cores acts as a "low pass" filter, smoothing minor variation and highlighting subcentury-scale trends. This hypothesis holds only where bioturbation has been vigorous and mass sedimentation events absent. The samples, taken at 5-cm intervals down the cores, were prepared for identification and counting in the light microscope as described by Scherer (1994), allowing quantitative analyses of samples.
The slides were examined for total count of diatom valves, and also counted on a non- Chaetoceros basis, counting at least 400 valves excluding the genus Chaetoceros . Two separate counts are necessary because Chaetoceros resting spores dominate the assemblage, masking trends of other, less common species (Leventer et al. 1996). Several species of Chaetoceros are represented, but distinguishing between taxa in the light microscope is extremely difficult. To evaluate the stratigraphic resolution, the absolute abundance of diatoms was calculated, as well as ratios of different species. From the total count, the absolute abundance and a ratio of Chaetoceros resting spores vs. Chaetoceros vegetative cells (Cs/Cv) were calculated. From the Chaetoceros -free count, a ratio of sea-ice-related species ( Fragilariopsis curta + F. cylindrus ) versus a common neritic species ( Thalassiosira antarctica ) was calculated. These ratios are suggested to reflect the input of meltwater and a receding ice edge (Leventer et al. 1996).
The diatom assemblage preserved in the sediment is clearly dominated by Chaetoceros spp., making up between 80 and 98 percent of the assemblage (mean = 92 percent). The amount of vegetative Chaetoceros cells ranges from 1 to 16 percent (mean = 8 percent). On a Chaetoceros -free count, the diatom assemblages show some differences between the two straits, reflecting the different climatic/oceanic settings. The Gerlache Strait non- Chaetoceros assemblage is dominated by the neritic species Thalassiosira antarctica, with approximately 50 percent. The Bransfield Strait assemblage has approximately equal amounts of Fragilariopsis curta, F. kerguelensis, Thalassiosira antarctica, and T. gracilis , approximately 15-20 percent each, indicating a more diverse setting. The distribution of non- Chaetoceros species shows little variation downcore, and no obvious co-variation between species was found. Minor fluctuations occur, but often these cannot be observed in adjacent cores.
Of the four cores investigated, all but TC 86-7 show a general trend of increased diatom abundance downcore (figure 1). High concentrations of Chaetoceros resting spores in sediments traditionally have been interpreted as indicating high primary production (Leventer et al. 1996; Scherer 1992). The Chaetoceros abundance calculated from these cores [1x108 to 5x108 cells per gram dry sediment (c/gds)] compares well with those calculated by Leventer (1991), Scherer (1992), Zielinski and Gersonde (1997), and by Crosta, Pichon, and Labracherie (1997). Core TC 86-7 shows extremely high diatom abundance (>1x109 c/gds) at the 15-cm level. This peak is interpreted as a local mass-sedimentation event, which may not reflect decadal-scale trends. Because of this anomaly, this core is not included in the evaluation of the stratigraphic resolution of the well-mixed sediments, and it highlights the importance of careful examination of the cores as samples are selected.
The Chaetoceros ratio (Cs/Cv) also shows an increasing trend down the core (figure 2). In the samples from the Gerlache Strait (BC 86-79 and BC 86-82), increase in the Cs/Cv ratio is slightly out of phase, compared to the diatom abundance curve, whereas in TC 86-9 the ratio and valve abundance correlate well (figures 1 and 2). The ratio of ( F. curta + F. cylindrus ) /T. antarctica shows little or no variation in the samples from the Gerlache Strait, whereas the sample from the Bransfield Strait shows a down-core increase and good correlation with diatom abundance (figure 2).
Higher diatom abundance downcore and, coincidentally, higher Cs/Cv ratios are interpreted as indicating more favorable conditions for diatom growth. Lower diatom abundance today could also be influenced by increased terrigenous input, possibly caused by increased glacial melt.
With a stratigraphic sampling of only 5 cm, some fine-scale fluctuations in the diatom record can be observed. The signal from diatom abundance is distinct, despite bioturbation, suggesting a subcentury-scale trend. Tentative correlation between the Gerlache and Bransfield Straits, based on the diatom abundance, may be possible, if we exclude TC 86-7. Such a correlation implies a higher sediment accumulation rate in the Bransfield Strait than in the Gerlache Strait. This higher rate agrees with Harden et al. (1992), who estimated the accumulation rate in the northern Bransfield basin to be up to 3 mm/year, declining southward and about 2 mm/year in the Gerlache Strait.
The high resolution obtained from closely spaced sampling might be helpful in correlating trigger and box cores to piston cores and, in turn, enhancing the possibility of making correlations between piston cores. With better chronostratigraphy, correlation, and calibration with the modern sediment flux, the sediments of the Antarctic Peninsula will provide a detailed paleoclimate proxy record.
The material used in this investigation is provided by the Antarctic Research Facility, Florida State University. The project is funded by the Swedish Natural Science Research Council and Uppsala University.
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