Characterization of contaminant phases in foraminifera carbonates by electron microprobe mapping

Abstract

[1] The advent of new microanalytical techniques such as electron microprobe mapping (EMP) and laser ablation microsamplers coupled to mass spectrometers (LA-ICP-MS) provides a new array of possibilities to explore in great detail the trace elements distribution in foraminiferal carbonates. Here we apply these techniques to characterize diagenetic phases present in foraminiferal shells from Ocean Drilling Program Site 1240 in the Panama Basin, a region characterized by the presence of manganese-rich minerals in the sediments. The combined application of these techniques allows us to characterize the elemental and spatial distribution on the surface and across the foraminiferal shells. Results illustrate the presence of at least two different Mn-rich contaminant phases in the foraminiferal carbonates: Mn-rich carbonates and ferromanganese oxides. Elemental maps also highlight the relevance of the foraminifera shell texture and porosity in the distribution and formation of these contaminant phases. In the case of Neogloboquadrina dutertrei, Mn phases form a continuous thin layer in the inner part of the chambers, whereas for Globigerinoides ruber, Mn phases have a rather patchy distribution and are usually found within the pores. Significant high magnesium concentrations are always associated with these Mn-rich phases. These new findings support the need of removing these Mn-rich contaminant phases in order to measure accurately the Mg/Ca ratios in the foraminifera shell and therefore obtain reliable Mg/Ca paleotemperature reconstructions.

Seawater nutrient and carbonate ion concentrations recorded as P/Ca, Ba/Ca, and U/Ca in the deep-sea coral Desmophyllum dianthus

“…It has been reported that Mn-rich phases can contain trace element contaminants, compromising elemental ratio reconstructions in foraminifera (Pena et al, 2008). In corals, such phases may be Fe-Mn oxide and hydroxide inclusions, contaminant particles (e.g.…”

Mn∕Ca intra- and inter-test variability in the benthic foraminifer <i>Ammonia tepida</i>

“…In the case of a high intrinsic variability, independent of environmental parameters, more spot measurements will be necessary to obtain a reliable mean value for one specimen or several specimens from the same stable environment (Sadekov et al, 2005;de Nooijer et al, 2014a). Although contamination is an important issue in Mn/Ca measurements (Boyle, 1983;Barker et al, 2003;Pena et al, 2005Pena et al, , 2008, pre-treatment cleaning and a precise targeting of the measurement interval (when using secondary ion mass spectrometry, SIMS, or a LA-ICP-MS), should largely eliminate the potential influence of contaminant phases and/or diagenetic overgrowths (Glock et al, 2012;Koho et al, 2015;McKay et al, 2015). Therefore, the Mn/Ca intra-test variability should ideally not have a diagenetic contribution.…”

Reactivity of neodymium carriers in deep sea sediments: Implications for boundary exchange and paleoceanography

“…The REE budget of uncleaned sedimentary planktonic foraminifera is dominated by authigenic coatings derived from bottom water (Palmer, 1985;Palmer and Elderfield, 1986;Roberts et al, 2012) that likely comprise iron (oxyhydr)oxides, manganese oxides and/or manganese carbonates (e.g. Boyle, 1983;Haley et al, 2005;Pena et al, 2008;Tachikawa et al, 2013). We suggest that such Mn coatings associated with foraminifera (or with other finer-grained carbonate material within the core) represent the phase that is rapidly dissolved during experiment (iii), and that these are removed by the acetic acid leaching during stage 1 of experiment (ii).…”