Ostermann Dorinda R.

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Ostermann
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Dorinda R.
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  • Article
    Sea surface temperature and salinity variability at Bermuda during the end of the Little Ice Age
    (American Geophysical Union, 2008-07-09) Goodkin, Nathalie F. ; Hughen, Konrad A. ; Curry, William B. ; Doney, Scott C. ; Ostermann, Dorinda R.
    We use geochemical and isotope measurements on a 225-year old brain coral (Diploria labyrinthiformis) from the south shore of Bermuda (64°W, 32°N) to construct a record of decadal-to-centennial-scale climate variability. The coral was collected alive, and annual density bands visible in X radiographs delineate cold and warm seasons allowing for precise dating. Coral skeletons incorporate strontium (Sr) and calcium (Ca) in relative proportions inversely to the sea surface temperature (SST) in which the skeleton is secreted. Previous studies on this and other coral colonies from this region document the ability to reconstruct mean annual and wintertime SST using Sr/Ca measurements ( Goodkin et al., 2007 , 2005). The coral-based records of SST for the past 2 centuries show abrupt shifts at both decadal and centennial timescales and suggest that SST at the end of the Little Ice Age (between 1840 and 1860) was 1.5° ± 0.4°C colder than today (1990s). Coral-reconstructed SST has a greater magnitude change than does a gridded instrumental SST record from this region. This may result from several physical processes including high rates of mesoscale eddy propagation in this region. Oxygen isotope values (δ 18O) of the coral skeleton reflect changes in both temperature and the δ 18O of seawater (δOw), where δOw is proportional to sea surface salinity (SSS). We show in this study that mean annual and wintertime δ 18O of the carbonate (δOc) are correlated to both SST and SSS, but a robust, quantitative measure of SSS is not found with present calibration data. In combination, however, the Sr/Ca and δOc qualitatively reconstruct lower salinities at the end of the Little Ice Age relative to modern day. Temperature changes agree with other records from the Bermuda region. Radiative and atmospheric forcing may explain some of the SST variability, but the scales of implied changes in SST and SSS indicate large-scale ocean circulation impacts as well.
  • Technical Report
    Automated system to measure the carbonate concentration of sediments
    (Woods Hole Oceanographic Institution, 1990-02) Ostermann, Dorinda R. ; Karbott, Darrell ; Curry, William B.
    We have developd a computer controlled system to measure the calcium carbonate content of sediment samples. A menu driven program controls the analysis of each sample. The system first communicates with a Mettler digital balance to record the weights of the 40 samples which must be loaded into each run. The sample boats are next loaded into the sample carousel which is then sealed from the atmosphere. The system is first pumped down to a vacuum of 0.04 torr. The valve to the pump closes and the stepping motor turns the carousel, moving a sample boat over the delivery slot and dropping the sample into 80°C 100% phosphoric acid under vigorous spinning action. During the reaction, carbonate is evolved into H2O and CO2 and the resulting pressure change within the closed system is measured by a pressure transducer and recorded into memory next to the sample identification and sample weight. The system is pumped once again to 0.04 torr and the process continues until all 40 samples have been analyzed. The data can then be uploaded and converted to percent carbonate values using a regression line produced from multiple analyses of varying weights of a 100% carbonate standard. Precision of the system, based upon 120 replicate analysis ranges from 0.49% to 0.88%.
  • Article
    Comparison of two methods to identify live benthic foraminifera : a test between Rose Bengal and CellTracker Green with implications for stable isotope paleoreconstructions
    (American Geophysical Union, 2006-12-01) Bernhard, Joan M. ; Ostermann, Dorinda R. ; Williams, David S. ; Blanks, Jessica K.
    The conventional method to distinguish live from dead benthic foraminifers uses Rose Bengal, a stain that reacts with both live and dead cytoplasm. CellTracker Green CMFDA is a fluorogenic probe causing live cells to fluoresce after proper incubation. To determine the more accurate viability method, we conducted a direct comparison of Rose Bengal staining with CellTracker Green labeling. Eight multicore tops were analyzed from Florida Margin (SE United States; 248-751 m water depths), near Great Bahama Bank (259-766 m), and off the Carolinas (SE United States; 220 m, 920 m). On average, less than half the Rose Bengal-stained foraminifera were actually living when collected. Thus, while Rose Bengal can significantly overestimate abundance, combined analyses of CellTracker Green and Rose Bengal can provide insights on population dynamics and effects of episodic events. Initial stable isotope analyses indicate that the CellTracker Green method does not significantly affect these important paleoceanographic proxies.