Nd/Ca ratios in plankton-towed and core top foraminifera: Confirmation of the water column acquisition of Nd

Martínez-Botí, M.A., Vance, D. and Mortyn, P.G. (2009) Nd/Ca ratios in plankton-towed and core top foraminifera: Confirmation of the water column acquisition of Nd. Geochemistry, Geophysics, Geosystems, 10 (8), Q08018. (doi:10.1029/2009GC002701).

Abstract

Planktic foraminifera have been used as recorders of the neodymium (Nd) isotopic composition of
seawater, although there is still controversy over the precise provenance of the Nd signal. We present an
extensive, multispecific plankton tow Nd/Ca data set from several geographic locations (SE Atlantic, NE
Atlantic, Norwegian Sea, and western Mediterranean), together with core top samples from the
Mediterranean region. The range of Nd/Ca ratios in plankton-towed foraminifera, cleaned only of organic
material, from all regions (0.01–0.7 umol/mol), is similar to previously published analyses of sedimentary
foraminifera cleaned using both oxidative and reductive steps, with distribution coefficients (Kd) ranging
between 4 and 302. For the Mediterranean, where core top and plankton tow data are both available, the
range for plankton tows (0.05–0.7 umol/mol) is essentially identical to that for the core tops (0.1–
0.5 umol/mol). Readsorption of Nd during cleaning is ruled out by the fact that the plankton tow samples
underwent only an oxidative cleaning process. We find a relationship between manganese (Mn) and Nd in
plankton tow samples that is mirrored by a similar correlation in core top samples. This relationship
suggests that Fe-Mn coatings are of negligible importance to the Nd budgets of foraminifera as the Nd/Mn
ratio it implies is over an order of magnitude greater than that seen in other Fe-Mn oxide phases. Rather,
since both plankton tows and core tops present a similar behavior, the Nd/Mn relationship must originate in
the upper water column. The data are consistent with the acquisition of Nd and Mn from the water column
by binding to organic material and the fact that intratest organic material is shielded from both aggressive
cleaning and diagenetic processes. Collectively, the results help to explain two abiding puzzles about Nd in
sedimentary planktic foraminifera: their high REE contents and the fact that they record a surface water Nd
isotopic signal, regardless of the cleaning procedure used.

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