Abstract
Abstract. 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 intra- and inter-test variability in 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 characterized 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 intra-test variability due to ontogenetic trends (i.e. size-related effects) and/or other vital effects occurring during calcification in A. tepida (11–25 % relative SD, RSD) is responsible for part of the observed variability in Mn∕Ca. Our present 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 in successive single-chamber Mn∕Ca ratios reflects the temporal variability in pore water Mn2+. Additionally, active or passive migration of the foraminifera in the surface sediment may explain part of the observed Mn∕Ca variability.
Highlights
In many coastal ecosystems, high summer temperatures and eutrophication lead to seasonally occurring hypoxia ([O2] < 63 μM; Rabalais et al, 2002; Diaz and Rosenberg, 2008), linked to the emergence of water column stratification in combination with lower oxygen solubility and higher respiration rates in warmer waters (e.g. Keeling et al, 2010)
Mn/Ca ratios of individual chambers ranged from 0.03 to 0.57 mmol mol−1 for the 10 specimens sampled in March 2012 (Fig. 2 (03/2012); Table A1), from 0.04 to 0.63 mmol mol−1 for the 16 specimens collected in July 2012 (Fig. 2 (07/2012); Table A1) and from 0.04 to 0.48 mmol mol−1 for the 18 specimens sampled in September 2012 (Fig. 2 (09/2012); Table A1)
The results for A. tepida show an average Mn/Ca ratio of 0.17±0.08 mmol mol−1 and a range of 0.08 ± 0.04 to 0.39 ± 0.19 mmol mol−1 for the average Mn/Ca per specimen (Table A1). 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)
Summary
High summer temperatures and eutrophication lead to seasonally occurring hypoxia ([O2] < 63 μM; Rabalais et al, 2002; Diaz and Rosenberg, 2008), linked to the emergence of water column stratification in combination with lower oxygen solubility and higher respiration rates in warmer waters (e.g. Keeling et al, 2010). When the bottom waters of coastal water bodies are oxygenated, Mn oxides are present in the oxic surface layer of the sediment and are reduced to soluble Mn2+ in the deeper anoxic sediment layers. Dissolved Mn2+ can diffuse upwards in the pore water across the oxic–anoxic boundary, where it precipitates again in the form of Mn oxides, leading to a continuous cycling of Mn within the upper sediment (Aller, 1994; Slomp et al, 1997). When eutrophication and stratification of the water column lead to (seasonally) hypoxic bottom water conditions, the oxygen penetration depth is reduced, causing an upward movement of the Mn redox front and diminishing the possibility that pore water Mn2+ is oxidized. Mn2+ may diffuse from the pore water into the water column (Sundby and Silverberg, 1985; Thamdrup et al, 1994; Dellwig et al, 2007; Konovalov et al, 2007; Pakhomova et al, 2007; Kowalski et al, 2012)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
