Nd/Ca ratios in plankton-towed and core top foraminifera : confirmation of the water column adcquisition of Nd
Martínez Botí, M. A. (Universitat Autònoma de Barcelona. Institut de Ciència i Tecnologia Ambientals)
Vance, D. (University of Bristol. Department of Earth Sciences)
Mortyn, P. Graham (Universitat Autònoma de Barcelona. Institut de Ciència i Tecnologia Ambientals)

Fecha:	2009
Resumen:	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 μmol/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 μmol/mol) is essentially identical to that for the core tops (0. 1-0. 5 μmol/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.
Ayudas:	Ministerio de Ciencia e Innovación CTM2006-1193
Derechos:	Tots els drets reservats.
Lengua:	Anglès
Documento:	Article ; recerca ; Versió publicada
Materia:	Neodymium ; Ocean geochemistry ; Planktic foraminifera ; Plankton tows ; Proxy validation
Publicado en:	Geochemistry, geophysics, geosystems, Vol. 10, Issue 8 (August 2009) , ISSN 1525-2027

DOI: 10.1029/2009GC002701

16 p, 707.5 KB

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