HR: 1340h
AN: PP53B-1393    [Abstracts]
TI: Implications of Mn-Mg-rich Contaminant Phases for Mg/Ca Past Temperature Reconstructions
AU: * Pena, L D
EM: lpena@geo.ub.es
AF: GRC GeociŠncies Marines, Dept. Estrat. Paleont. i Geoc. Marines, University of Barcelona Marti Franques s/n, Barcelona, 08028 Spain
AU: Calvo, E
EM: eva.calvo@anu.edu.au
AF: Research School of Earth Sciences, The Australian National University, Camberra, ACT 0200 Australia
AU: Cacho, I
EM: icacho@ub.edu
AF: GRC GeociŠncies Marines, Dept. Estrat. Paleont. i Geoc. Marines, University of Barcelona Marti Franques s/n, Barcelona, 08028 Spain
AU: Eggins, S
EM: stephen.eggins@anu.edu.au
AF: Research School of Earth Sciences, The Australian National University, Camberra, ACT 0200 Australia
AU: Pelejero, C
EM: carles.pelejero@anu.edu.au
AF: Research School of Earth Sciences, The Australian National University, Camberra, ACT 0200 Australia
AB: In recent years, many paleoceanographic studies have employed the foraminiferal elemental composition as a proxy of sea water paleoenvironmental conditions. In particular, the foraminiferal Mg/Ca ratio has been widely used as a sea water paleotemperature estimator. Nevertheless, this technique is still in development and special attention needs to be focussed on those factors which can introduce potential biases in the Mg/Ca ratio and therefore leading to inaccurate reconstructions of past-oceanographic scenarios. This study aims to solve or reduce overestimations of past sea water temperatures derived from a contaminant Mg-rich phase associated to Mn-enrichments. We address this problem using samples from ODP 1240 (0§01.311'N, 86§27.758'W, 2921m water depth) from the Panama basin which is a Mn-enriched basin. By means of the LA-ICP-MS capabilities at the Research School of Earth Sciences (Australia) we have documented and identified the presence of Mn-Mg-rich contaminant phases at the inner part of the foraminifera walls. In order to asses the efficiency of the classical Mg/Ca cleaning protocols when dealing with the presence of these contaminants, we have selected a set of samples with different contents in Mn and cleaned them with both the oxidative and the full reductive methods. LA-ICP-MS and bulk ICP-MS results show that the reductive cleaning successfully removes the Mn-Mg rich layers from the samples whereas the oxidative cleaning does not effectively eliminate the contaminant phases from the selected samples. In order to test the efficiency of the different cleaning steps we have analyzed (ICP-MS) the residual fractions from each cleaning step using two sample sub-sets. The order in the reductive and oxidative step was reversed for each sub-set. Results suggest that the Mn-enrichment occurs in two different phases, one which is eliminated by either the oxidative or the reductive step but a second one which only is removed by the reductive step. This second one is also associated to Mg-enrichments. As a consequence of this contamination phase, temperature reconstructions can provide overestimations in the range of 0.5-4§C. In addition, our results clearly demonstrate that there are no significant differences in the Mg/Ca ratio when the order between reductive and oxidative steps is altered. A further insight on the nature of these Mn-Mg-rich contaminant phases has been obtained by means of complementary analytical techniques (XRD, SEM, EDX). The results obtained demonstrate that the contaminant phase is mainly a Mn-Mg carbonate known as Kutnahorite. This mineral has been previously identified in Mn-rich crusts of marine sediments but this study has been able to identify it for the first time within foraminiferal tests.
DE: 4267 Paleoceanography
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting