Neodymium in modern foraminifera from the Indian Ocean: implications for the use of foraminiferal Nd isotope compositions in paleo-oceanography

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 (r2=0.91). The observed changes in the Nd concentration after foraminiferal death appear to reflect Nd addition from deeper in the water column and imply a major disturbance of the primary Nd isotope composition formed at the surface. The high Nd/Ca reported for cleaned foraminifera from sediment cores are interpreted to be a consequence of current cleaning techniques. Although cleaning successfully removes organics and secondary Mn coatings into solution any released rare earth elements (REE) are subsequently re-adsorbed onto the calcite. Cleaning removes elements with a 2+ valency state into solution (e.g. Cd and Mn) but more reactive elements with higher valency states (e.g. REE3+, Th4+) are re-absorbed onto residual carbonate. It is concluded that, although foraminifera clearly record temporal variations in Nd isotope composition, the relationship between these variations and changes in the sea surface Nd isotope composition has yet to be established unambiguously. Further work is required to rule out the possibility that the Nd in foraminifera represents a mixed signal derived from surface sediments and bottom and pore waters.