Microscale neodymium distribution in sedimentary planktonic foraminiferal tests and associated mineral phases

Abstract

Neodymium isotopic ratios recorded in calcareous foraminiferal tests (shells) and associated authigenic minerals were used to trace past water masses, although the origin of preserved Nd signals is still a matter of debate. We determined, for the first time, the microscale Nd distribution in two planktonic foraminifera species (Globigerinoides ruber and Neogloboquadrina dutertrei) and coexisting authigenic minerals from two selected time slices (15.6 and 129kyr) of a marine sediment core in the Panama Basin. Elemental mapping of Nd, Ca, Fe, Mn, and Si was performed using NanoSIMS ion probe and electron probe microanalysis (EPMA) on uncleaned tests together with scanning electron microscopy (SEM) imagery. EPMA and SEM images revealed the presence of Fe-rich framboidal minerals inside test pores and inner chambers, particularly in the old samples. The young sample presented Fe- and Mn-rich patches on the inner test wall. The Nd intensity in Fe–Mn-rich patches and in some framboids was much higher than in foraminiferal calcite, where the Nd distribution was randomly heterogeneous with no systematic features such as an ontogenic Nd-rich layer or species-specific differences. The relationship between Nd, Fe, and Mn confirmed a significant role of Fe–Mn oxides as a Nd carrier. The high Nd enrichment in some framboids could be explained by Nd adsorption onto oxidized surface of Fe sulfides particles rather than direct incorporation. Since the authigenic precipitates are major Nd carrier phases, the Nd isotopic signals in sedimentary foraminiferal tests likely reflect bottom/pore water values rather than surface water ones.