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Description
The Carbonate Compensation Depth (CCD) is defined as the depth below which the CaCO3 content of surface sediments is less than 20 wt% due to dissolution. The CCD varies in its position over time due to changes in sea level, LCO2, and the weathering supply and burial removal of [Ca] and [CO{] ions. Regional CCD variations can be reconstructed from suites of Ocean Drilling Program (ODP) sediment cores of known age and paleodepth. The goal of this project was to determine Late Miocene to Early Eocene stratigraphic variations in wt% CaCO3 at ODP South Atlantic Site 1267 on Walvis Ridge, and compare this local high-resolution reconstruction of paleoCCD variations to more global low-resolution reconstructions. For each core sample, a dry powered split of knov\-'.n mass was acidified with a known volume of 0.5 M HNO3 solution to dissolve all carbonate, the resultant solution was analyzed by ICP-OES to determine [Ca], [Mg], and [Sr], and the given sediment mass, acid volume, and [Ca] were used to estimate wt% CaCO3. This ICP-OES-based method to calculate wt% CaCO3 was found to work as well as the standard carbonate-coulometry-based method. The Site 1267 magnetobiostratigraphic age-model was used to convert wt% CaCO3 data from the depth-domain to the time-domain, and the crustal age-depth relationship was applied to estimate Site 1267 paleodepths throughout the study interval. The sample population showed a significant and strong correlation between ICP-OES-determined wt% CaCO3 values and ship-board-determined natural gamma ray (NGR) counts, and this correlation was applied to the 2.5-cm-resolution NGR record to produce a high-resolution ( 103 yr) synthetic record of wt% CaCO3. Integration of sediment, time, and paleodepth data allowed determination of whether the paleoCCD was deeper or shallower than Site 1267's paleodepth through time. The Site 1267 record shows good general agreement with previous low-resolution(> 105"6 yr) paleoCCD reconstructions, correlates relatively well with eustatic sea level fluctuations. This approach can be applied to shallower and deeper ODP Leg 208 drill sites on Walvis Ridge to provide additional constraints on the regional history of the paleoCCD in the South Atlantic.
