Publication Date:
2017-07-21
Description:
The adaptation of some benthic foraminiferal species to low oxygen conditions provides the prospect of using the chemical composition of their tests as proxies for bottom water oxygenation. Manganese may be particularly suitable as such a geochemical proxy, because this redox element is soluble in reduced form (Mn2+), and hence can be incorporated into benthic foraminiferal tests under low oxygen conditions. Therefore, intra- and inter-test differences in foraminiferal Mn / Ca ratios may hold important information about short term variability in pore water Mn2+ concentrations and sediment redox conditions. Here, we studied Mn / Ca inter- and intra-test variability of living individuals of the shallow infaunal foraminifer Ammonia tepida sampled in Lake Grevelingen (The Netherlands) in three different months of 2012. The deeper parts of this lake are characterised by seasonal hypoxia/anoxia with associated shifts in microbial activity and sediment geochemistry, leading to seasonal Mn2+ accumulation in the pore water. Earlier laboratory experiments with similar seawater Mn2+ concentrations as encountered in the pore waters of Lake Grevelingen suggest that intrinsic intra-test variability in A. tepida (11–25 % RSD) is responsible for a considerable portion of the observed variability in Mn / Ca. Our results show that the seasonally highly dynamic environmental conditions in the study area lead to a strongly increased Mn / Ca intra- and inter-test variability (average of 45 % RSD). Within single specimens, both increasing and decreasing trends in Mn / Ca ratios with size are observed. Our results suggest that the variability of successive single chamber Mn / Ca ratios reflects the temporal variability of pore water Mn2+. Additionally, active or passive migration of the foraminifera in the surface sediment may explain part of the observed Mn / Ca variability.
Print ISSN:
1810-6277
Electronic ISSN:
1810-6285
Topics:
Biology
,
Geosciences
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