A comparison of benthic foraminiferal Mn / Ca and sedimentary Mn / Al as proxies of relative bottom-water oxygenation in the low-latitude NE Atlantic upwelling system

Abstract

Abstract. Trace element incorporation into foraminiferal shells (tests) is governed by physical and chemical conditions of the surrounding marine environment, and therefore foraminiferal geochemistry provides a means of palaeooceanographic reconstructions. With the availability of high-spatial-resolution instrumentation with high precision, foraminiferal geochemistry has become a major research topic over recent years. However, reconstructions of past bottom-water oxygenation using foraminiferal tests remain in their infancy. In this study we explore the potential of using Mn / Ca determined by secondary ion mass spectrometry (SIMS) as well as by flow-through inductively coupled plasma optical emission spectroscopy (FT-ICP-OES) in the benthic foraminiferal species Eubuliminella exilis as a proxy for recording changes in bottom-water oxygen conditions in the low-latitude NE Atlantic upwelling system. Furthermore, we compare the SIMS and FT-ICP-OES results with published Mn sediment bulk measurements from the same sediment core. This is the first time that benthic foraminiferal Mn / Ca is directly compared with Mn bulk measurements, which largely agree on the former oxygen conditions. Samples were selected to include different productivity regimes related to Marine Isotope Stage 3 (35-28 ka), the Last Glacial Maximum (28-19 ka), Heinrich Event 1 (18-15.5 ka), Bølling Allerød (15.5-13.5 ka) and the Younger Dryas (13.5-11.5 ka). Foraminiferal Mn / Ca determined by SIMS and FT-ICP-OES is comparable. Mn / Ca was higher during periods with high primary productivity, such as during the Younger Dryas, which indicates lowoxygen conditions. This is further supported by the benthic foraminiferal faunal composition. Our results highlight the proxy potential of Mn / Ca in benthic foraminifera from upwelling systems for reconstructing past variations in oxygen conditions of the sea floor environment as well as the need to use it in combination with other proxy records such as faunal assemblage data.

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

“…Therefore, the Mn/Ca intra-test variability should ideally not have a diagenetic contribution. The results for all specimens measured in this study showed an average Mn/Ca intra-test variability for A. tepida of 45±15 % (RSD average for all chambers measured on a single specimen; Table A1), which is comparable to that reported in some previous studies (30-50 %; Glock et al, 2012;McKay et al, 2015;Koho et al, 2017). Our results from the different sampling campaigns showed more specimens with higher variability in July compared to March and September (Figs.…”

“…So far, few studies have investigated the potential of using Mn/Ca ratios in benthic foraminifera as a proxy for bottom water redox state. These studies show that benthic foraminifera register environmental Mn 2+ concentrations in their tests (Reichart et al, 2003;Munsel et al, 2010;Glock et al, 2012;Groeneveld and Filipsson, 2013;Koho et al, 2015Koho et al, , 2017McKay et al, 2015;Barras et al, 2018). For example, Koho et al (2015Koho et al ( , 2017 demonstrated that foraminiferal species occupying a relatively deep microhabitat display higher Mn/Ca ratios than those living near the sediment surface.…”

“…Furthermore, the application of foraminiferal Mn/Ca ratios as a proxy for dissolved Mn has been tested in laboratory conditions, where calibration studies show a linear relation between seawater dissolved Mn 2+ concentrations and foraminiferal Mn/Ca ratios, though with species-dependent partition coefficients (Munsel et al, 2010;Barras et al, 2018). The use of foraminiferal Mn/Ca ratios as a palaeo-proxy for BWO has further been explored in several studies of deep-sea sediment records (Klinkhammer et al, 2009;Ní Fhlaithearta et al, 2010;McKay et al, 2015). Our study is the first to investigate benthic foraminiferal Mn/Ca ratios in a coastal ecosystem.…”

“…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.…”

“…Furthermore, by using microanalytical techniques, capable of measuring elemental concentrations in single foraminiferal chambers, short-term variability in oxygenation may be unravelled. Recent studies have shown the potential of intra-test variability in Mn/Ca to resolve vertical migration in the sediment and/or seasonal changes in oxygenation (Glock et al, 2012;McKay et al, 2015).…”