Benthic foraminiferal Mn / Ca ratios reflect microhabitat preferences

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 singlechamber 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.

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

“…This latter difference can be explained by the much higher pore water Mn 2+ concentrations in our study (< 310 µmol L −1 ) and in the study of Koho et al (2017; < 5 µmol L −1 ) compared to the study of Glock et al (2012;< 0.1 µmol L −1 ). Despite the different pore water Mn 2+ concentrations in our study compared to Koho et al (2017), we found rather similar foraminiferal Mn/Ca ratios, and this contrast could be partly resolved by higher partition coefficients in deep-sea species compared to coastal species (Barras et al, 2018). For A. tepida, the range measured in specimens from Lake Grevelingen in our study compares well with measured average Mn/Ca ratios in the study of Barras et al (2018;0.13 and 0.86 mmol mol −1 ) for a similar range of pore water Mn 2+ concentrations compared to the concentration of dissolved Mn in the seawater of the culturing experiments (10 to 100 µmol L −1 ).…”

“…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, by using microanalytical techniques, capable of measuring elemental concentrations in single foraminiferal chambers, short-term variability in oxygenation may be unravelled.…”

“…Recently, Mn/Ca ratios in benthic foraminiferal tests have been proposed as a potential palaeo-proxy for BWO. This suggestion is based on several observations made in recent continental margin and deep-sea ecosystems (Reichart et al, 2003;Glock et al, 2012;Groeneveld and Filipsson, 2013;Koho et al, 2015Koho et al, , 2017. 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).…”

“…This range is comparable to that found in living specimens of some deepsea infaunal species from the NE Japan margin (E. batialis, B. spissa, U. cf. graciliformis, U. akitaensis, N. labradorica, C. fimbriata; 0.0020 to 0.277 mmol mol −1 ; Koho et al, 2017) but is elevated compared to Mn/Ca ratios measured in single tests of living benthic foraminifera from the Peruvian oxygen minimum zone (OMZ) (B. spissa; 0.0021 to 0.010 mmol mol −1 ; Glock et al, 2012). This latter difference can be explained by the much higher pore water Mn 2+ concentrations in our study (< 310 µmol L −1 ) and in the study of Koho et al (2017; < 5 µmol L −1 ) compared to the study of Glock et al (2012;< 0.1 µmol L −1 ).…”

“…graciliformis, U. akitaensis, N. labradorica, C. fimbriata; 0.0020 to 0.277 mmol mol −1 ; Koho et al, 2017) but is elevated compared to Mn/Ca ratios measured in single tests of living benthic foraminifera from the Peruvian oxygen minimum zone (OMZ) (B. spissa; 0.0021 to 0.010 mmol mol −1 ; Glock et al, 2012). This latter difference can be explained by the much higher pore water Mn 2+ concentrations in our study (< 310 µmol L −1 ) and in the study of Koho et al (2017; < 5 µmol L −1 ) compared to the study of Glock et al (2012;< 0.1 µmol L −1 ). Despite the different pore water Mn 2+ concentrations in our study compared to Koho et al (2017), we found rather similar foraminiferal Mn/Ca ratios, and this contrast could be partly resolved by higher partition coefficients in deep-sea species compared to coastal species (Barras et al, 2018).…”