Benthic foraminiferal Mn / Ca ratios reflect microhabitat preferences

Karoliina A Koho,Takashi Toyofuku,Lennart J De Nooijer,Christophe Fontanier,Hiroshi Kitazato,Kazumasa Oguri,Gert-Jan Reichart

doi:10.5194/bg-14-3067-2017

Abstract

Abstract. The Mn / Ca of calcium carbonate tests of living (rose-Bengal-stained) benthic foraminifera (Elphidium batialis, Uvigerina spp., Bolivina spissa, Nonionellina labradorica and Chilostomellina fimbriata) were determined in relation to pore water manganese (Mn) concentrations for the first time along a bottom water oxygen gradient across the continental slope along the NE Japan margin (western Pacific). The local bottom water oxygen (BWO) gradient differs from previous field study sites focusing on foraminiferal Mn / Ca and redox chemistry, therefore allowing further resolution of previously observed trends. The Mn / Ca ratios were analysed using laser ablation inductively coupled plasma-mass spectrometer (ICP-MS), allowing single-chamber determination of Mn / Ca. The incorporation of Mn into the carbonate tests reflects environmental conditions and is not influenced by ontogeny. The inter-species variability in Mn / Ca reflected foraminiferal in-sediment habitat preferences and associated pore water chemistry but also showed large interspecific differences in Mn partitioning. At each station, Mn / Ca ratios were always lower in the shallow infaunal E. batialis, occupying relatively oxygenated sediments, compared to intermediate infaunal species, Uvigerina spp. and B. spissa, which were typically found at greater depth, under more reducing conditions. The highest Mn / Ca was always recorded by the deep infaunal species N. labradorica and C. fimbriata. Our results suggest that although partitioning differs, Mn / Ca ratios in the intermediate infaunal taxa are promising tools for palaeoceanographic reconstructions as their microhabitat exposes them to higher variability in pore water Mn, thereby making them relatively sensitive recorders of redox conditions and/or bottom water oxygenation.

Highlights

Benthic foraminifera, single-celled, testate eukaryotes, are common proxies used in palaeoceanographic studies
Mn / Ca ratios were always lower in the shallow infaunal E. batialis, occupying relatively oxygenated sediments, compared to intermediate infaunal species, Uvigerina spp. and B. spissa, which were typically found at greater depth, under more reducing conditions
Our results suggest that partitioning differs, Mn / Ca ratios in the intermediate infaunal taxa are promising tools for palaeoceanographic reconstructions as their microhabitat exposes them to higher variability in pore water Mn, thereby making them relatively sensitive recorders of redox conditions and/or bottom water oxygenation

Summary

Introduction

Benthic foraminifera, single-celled, testate eukaryotes, are common proxies used in palaeoceanographic studies. One of the most commonly applied geochemical foraminifera-based proxies is the Mg / Ca ratio of the test carbonate, which has been shown to primarily reflect seawater temperatures (e.g. Nürnberg et al, 1996; Elderfield et al, 2006). In the absence of oxygen the solid Mn (hydr)oxides are reduced to aqueous Mn2+ (Froelich et al, 1979), subsequently leading to the build-up of bioavailable Mn2+ in pore water Foraminifera calcifying under such conditions are expected to show elevated Mn / Ca ratios, the concentration depending on the actual in situ aqueous Mn concentrations (Munsel et al, 2010). Exceptions are environments such as oxygen minimum zones, where bottom waters have been oxygendeprived for extended periods. In sediments, where bottom waters and surficial sediments are oxygenated and deeper sediments are anoxic, and Mn is retained in sediments, the incorporation of Mn in different species of foraminifera is expected to depend on the species-specific in-sediment living depth

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