Articles | Volume 14, issue 12
Biogeosciences, 14, 3067–3082, 2017
Biogeosciences, 14, 3067–3082, 2017

Research article 22 Jun 2017

Research article | 22 Jun 2017

Benthic foraminiferal Mn / Ca ratios reflect microhabitat preferences

Karoliina A. Koho1,2,3, Lennart J. de Nooijer1, Christophe Fontanier4,5,6, Takashi Toyofuku7, Kazumasa Oguri7, Hiroshi Kitazato7,8, and Gert-Jan Reichart1,3 Karoliina A. Koho et al.
  • 1Department of Ocean Systems, NIOZ-Royal Netherlands Institute for Sea Research and Utrecht University, Den Burg, the Netherlands
  • 2Department of Environmental Sciences, P.O. Box 65 (Viikinkaari 1), 00014 University of Helsinki, Helsinki, Finland
  • 3Department of Earth Sciences – Geochemistry, Faculty of Geosciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, the Netherlands
  • 4Ifremer, Géosciences marines, Z.I. Pointe du Diable CS10070, 29280 Plouzané, France
  • 5Université of Bordeaux, Environnements et Paléo-environnements Océaniques et Continentaux, UMR 5805, 33600 Talence, France
  • 6Laboratoire de planétologie et de géodynamique – bio-indicateurs actuels et fossiles, Université of Angers, 49035 Angers, France
  • 7Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan
  • 8Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato-ku 108-8477, Tokyo, Japan

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.

Short summary
Here we report Mn / Ca ratios in living benthic foraminifera from the NE Japan margin. The results show that the Mn incorporation directly reflects the environment where the foraminifera calcify. Foraminifera that live deeper in sediment, under greater redox stress, generally incorporate more Mn into their carbonate skeletons. As such, foraminifera living close to the Mn reduction zone in sediment appear promising tools for paleoceanographic reconstructions of sedimentary redox conditions.
Final-revised paper