Seasonally abundant planktonic foraminifera of the Sargasso Sea; succession, deep-water fluxes, isotopic compositions, and paleoceanographic implications

Abstract

A suite of 31 two-month sediment-trap samples from the deep Sargasso Sea, collected over six years, provided data on seasonal variations in flux and oxygen and carbon isotopic composition of 12 species of seasonally abundant planktonic foraminifera. The order of succession in the course of the calendar year was Pulleniatina obliquiloculata, Globorotalia truncatulinoides, Globigerina bulloides, Globorotalia inflata, Globorotalia hirsuta, Globigerinita glutinata, Neogloboquadrina dutertrei, Globorotalia crassaformis, Globigerinoides sacculifer, Globigerinoides ruber (pink var.), Globigerina rubescens, and Globigerinoides conglobatus. The timing and succession of the flux peaks are very similar to population data derived from plankton tows two decades earlier. The pink variety of G. ruber occurs only during times of surface-water stratification and has several characteristics besides pigmentation, especially size and isotopic composition, which distinguish it clearly from the perennially abundant white variety. All 12 species occurred in significant numbers for two to six months each year and were very rare or completely absent during the remaining months. Peak fluxes of different species ranged from single tests to 300 tests per square meter and day. Most species underwent distinct annual cycles in average weight of their tests. Average annual flux of all planktonic foraminifera, including perennially abundant species, is close to 100,000 individuals or 1 g m-2. The total flux of foraminiferal calcite is about one-third that of coccolith calcite, but slightly greater than that of pteropod/heteropod aragonite. Comparisons of the oxygen isotopic composition of the eight more abundant species with equilibrium compositions for calcite, calculated from Station "S" temperature and salinity data for the upper 1,200 m, place tight constraints on their depth habitats and deviations from isotopic equilibrium. For paleoceanographic purposes, different species can be used as indicators of hydrographic conditions (mainly temperature) as follows: P. obliquiloculata, G. inflata, and N. dutertrei for winter mixed-layer (0-100 m); G. bulloides for 50-100 m throughout the year, but very rare during summer; G. conglobatus for 75-100 m during fall; G. ruber (pink) for surface water in summer. Whole tests of G. truncatulinoides and G. hirsuta appear to record average conditions at 200 and 600 m, respectively, but in reality those averages are the result of early-life test building near the surface and later calcite-crust deposition near 800 m for G. truncatulinoides and near 1,000 m for G. hirsuta. Analyses of specimens taken from the surface sediment at the study site yielded reasonable "paleotemperatures" for the seasons and depths characterized by the different species and confirmed the practicality of deriving past seasonality and mixed-layer depths from sedimentary assemblages. The seasonal variations of the carbon isotopic compositions of G. ruber (pink) and G. truncatulinoides appear to reflect the extent of photosynthetic carbon fixation near the surface and of its respiration above 800 m.