Nd systematics of modern Indian Ocean Forminifera: implications for the use of Nd isotopes in palaeoceanography

The use of the Nd isotope composition of planktic foraminifera as a proxy for sea surface water is based on the assumption that Nd is incorporated during initial calcification and preserved after foraminifera death, subsequent transport through the water column and storage in the sediment pile. To test this assumption Nd concentrations are reported from foraminiferal tests collected from 20 to 3045 m depth (multinets and sediment traps) from offshore Somalia and in the Somali Basin of the Indian Ocean. Nd partition coefficients calculated for two species Globigerinoides ruber and Globigerina bulloides sampled in the uppermost water column (100 m) are 16-52. The measured Nd/Ca ratio of living foraminifera is ∼8 nmol/mol, which is 8-190 times lower than reported in cleaned foraminifera from sediment cores (61-1500 nmol/mol). Moreover, the Nd content of foraminiferal tests varies markedly within the water column (8-220 nmol/mol Ca) recording a positive correlation with Mn content (r