Geochemistry of benthic foraminifera as an enviornmental indicator:a study from multiple hydrographic regimes.

Abstract

The geochemistry(stable isotopes and trace elements) of living(stained)calcareous benthic foraminifera was compared with ambient bottom water stable isotope values to provide modern analog conditions and calibrations for enviornmental and paleoenvironmental assessments.Stable isotope values of live(stained)benthic foraminifera were investigated from push core and multicorer samples from the North Pacific(on the Aleutian margin,water depth 1988m)and the South Australian Bight(water depth 2476m and 1634m).Living benthic foraminifera specimens collected from contaminated sites in the Venice Lagoon were analysed for trace elements.Both the isotopic and the trace element study involved interpretation of modern live foraminiferal chemical reponses to different enviornments.The isotope analyses of living foraminifera from the North Pacific and the South Australian Bight provide calibration information for the evaluation of bottom water temperature and circulation of ancient oceans based on fossil foraminiferal geochemistry.Trace elements concentrations of Venice Lagoon foraminifera were used to assess the possibility of using foraminiferal geochemistry as a pollution indicator.Consistent with previous studies,shallow infaunal benthic foraminifera from the Aleutian and Australian argins were depleted in δ13 C with respect to bottom water dissolved inorganic carbon(DIC),and the deep infaunal foraminifera showed greater difference in values between foraminiferal carbon isotope values and DIC.The deep infaunal,Globobulimina pacifica,had δ18 O values that were in equilibrium with oxygen isotopic values at equilibrium calcite(δ18 Occ).Based on a few specimens that were divided in half,there was only minor isotopic heterogeneity in the test composition of benthic foraminifera genus Globobulimina.Differential foraminiferal uptake of Zn as indicated in initial laser ablation analyses showed marked differences between contaminated and less polluted sites in the Venice Lagoon.Higher incorporation of zinc in foraminiferal calcite from the more contaminated site was possibly the result of greater bioavailability of zinc in this environment.Differences also exist between the uptake of other trace metals such as Al,Mg and Mn by different foraminiferal genera.Differences in metal sequestration by benthic foraminifera suggest that the trace metal geochemistry of some foraminiferal taxa may be useful as a pollution indicator.