Stable isotopes in planktonic and benthic foraminifera from Arctic Ocean surface sediments

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Abstract
Oxygen and carbon isotopic analysis have been performed on the tests of Planulina wuellerstorfi and three size fractions of sinistral Neogloboquadrina pachyderma recovered from 33 Arctic Ocean surface-sediment samples. Stable isotopic compositions of N. pachyderma are found to be dependent on the test size: larger specimens show considerable enrichment in both .delta.18O and .delta.13C. The difference between the isotopic compositions of the 63-125 and 125-250 .mu.m size fractions in N. pachyderma can be explained by biogenic fractionation effects during foraminiferal test growth. Larger (250-500 .mu.m) N. pachyderma displayed accretions of secondary calcite, i.e., the outermost shell contained significant amounts of inorganically precipitated magnesium calcite. Thus, larger foraminifera may not be suited for down-core stable isotopic studies. There is a difference of .apprx. 2.permill. between .delta.18O values of surface samples from the eastern and western Arctic Ocean, reflecting large differences between surface-water salinity in these regions. Therefore, oxygen isotopic data may have limited use as a chronostratigraphic tool in down-core studies in the Arctic Ocean, but we can use them to infer past variations in surface-water salinities. Planulina wuellerstorfi also showed depletions of both .delta.18O and .delta.13C in its calcite tests relative to calcite precipitated in isotopic equilibrium with ambient sea water; these depletions ranged from -0.8 to -0.9.permill. in .delta.18O and -1.2 to -0.9.permill. in .delta.13C. This taxon is found to deposit its shell very close to the .delta.13C of .SIGMA. CO2 of bottom waters.