Quantification of Foraminifer and Coccolith Carbonate in South Atlantic Surface Sediments by Means of Carbonate Grain-Size Distributions

Abstract

This study presents a differentiated carbonate budget for marine surface sediments from the Mid-Atlantic Ridge of the South Atlantic, with results based on carbonate grain-size composition. Upon separation into sand, silt, and clay sub-fractions, the silt grain-size dis- tribution was measured using a SediGraph 5100. We found regionally characteristic grain-size distributions with an overall minimum at 8 mm equivalent spherical diameter (ESD). SEM observations reveal that the coarse particles (.8 mm ESD) are attributed to planktic for- aminifers and their fragments, and the fine particles (,8 mm ESD) to coccoliths. On the basis of this division, the regional variation of the contribution of foraminifers and coccoliths to the carbonate budget of the sediments are calculated. Foraminifer carbonate dominates the sed- iments in mesotropic regions whereas coccoliths contribute most car- bonate in oligotrophic regions. The grain size of the coccolith share is constant over water depth, indicating a lower susceptibility for car- bonate dissolution compared to foraminifers. Finally, the characteristic grain-size distribution in fine silt ( ,8 mm ESD) is set into context with the coccolith assemblage counted and biometrically measured using a SEM. The coccoliths present in the silt fraction are predominantly large species (length . 4 mm). Smaller species (length , 4 mm) belong to the clay fraction (,2 mm ESD). The average length of most frequent coccolith species is connected to prominent peaks in grain-size distri- butions (ESD) with a shape factor. The area below Gaussian distri- butions fitted to these peaks is suggested as a way to quantitatively estimate the carbonate contribution of single coccolith species more precisely compared to conventional volume estimates. The quantitative division of carbonate into the fraction produced by coccoliths and that secreted by foraminifers enables a more precise estimate for source/sink relations of consumed and released CO 2 in the carbon cycle. The allocation of coccolith length and grain size (ESD) suggests size windows for the separation or accumulation of distinct coccolith species in investigations that depend on non to slightly-mixed signals (e.g., isotopic studies).