Seasonal variations in isotopic composition and deep-water fluxes of the tests of perennially abundant planktonic foraminifera of the Sargasso Sea; results from sediment-trap collections and their paleoceanographic significance

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Abstract
A six-year series of sediment-trap samples from the deep Sargasso Sea provides data on seasonal stable-isotope and flux variations of the tests of planktonic foraminifera. Three species-Globigerinoides ruber (white var.), Globigerinella aequilateralis, and Orbulina universa- are abundant throughout the year and undergo consistent annual cycles in number, mass, average weight and oxygen and carbon isotopic composition of tests approaching the seafloor. Comparisons of their oxygen isotopic composition with that calculated from hydrographic data for Station "S" for the same six-year period allows the assignment of average depths of origin of the settling tests to within 25 m. Tests of G. ruber form in the upper 25 m and deviate from isotopic equilibrium with ambient seawater by an average of -0.20.permill.. Tests of G. aequilateralis form between 50 and 75 m, and thin-walled O. universa tests form between 25 and 50 m. Secondary wall thickening of O. universa takes place in deeper water. Both G. aequilateralis and O. universa tests form in or very close to oxygen isotopic equilibrium. The annual flux variations of the three species do not significantly bias the isotopic composition of multiannual accumulations of their tests toward certain seasons. Therefore, all three species are good paleoceanographic indicators: isotopic analyses of multispecimen samples give information on mean annual hydrography, whereas the spread among analyses of individual tests can indicate the amplitude of the annual temperature-salinity variation. (Effects of salinity variations below 50 m are negligible in the Sargasso Sea). The carbon isotope composition of the tests of these three species appears largely controlled by internal biological factors, rather than external factors. Thus the annual carbon isotope cycles of G. aequilateralis and O. universa are out of phase even though their tests form in the same water during winter. The .delta.13C of G. ruber tests may respond to 12C and nutrient depletion in the surface water and, therefore, might be useful as an indicator of the intensity of past primary productivity. The large tests of G. aequilateralis have a .delta.13C which is correlated to the flux of organic carbon to the deep water. It is conceivable that these tests record the intensity of photosynthetic carbon fixation at the depth of the chlorophyll maximum, but the utility of these relationships for paleocenoagraphic purposes needs to be established by studies in other oceanographic settings.

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