Mn/Ca, Mg/Ca and Sr/Ca ratios of multiple chambers of specimens of Ammonia tepida from Lake Grevelingen@en

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.部分底栖有孔虫（benthic foraminifera）物种对低氧环境的适应性，使其壳体（test）的化学成分可作为底层水氧化还原状态的地球化学代用指标（geochemical proxy）。锰元素或许尤其适合作为此类代用指标，因为该氧化还原元素（redox element）以还原态（Mn²+）形式可溶，因此可在低氧环境下被结合到底栖有孔虫的壳体中。因此，有孔虫锰钙比值（Mn/Ca ratios）在壳内与壳间（intra- and inter-test）的差异，或许蕴含着孔隙水（pore water）Mn²+浓度与沉积物氧化还原条件（sediment redox conditions）短期变化的重要信息。本研究针对2012年三个不同月份采自荷兰格雷弗林根湖（Lake Grevelingen）的浅内栖有孔虫（shallow infaunal foraminifer）钝顶希望虫（Ammonia tepida）活体个体，分析了其壳体锰钙比值的壳内与壳间变异特征。该湖深水区呈现季节性低氧/缺氧（seasonal hypoxia/anoxia）状态，伴随微生物活动与沉积物地球化学特征的转变，最终导致孔隙水中Mn²+出现季节性富集。此前开展的实验室模拟实验显示，当海水Mn²+浓度与格雷弗林根湖孔隙水浓度相近时，钝顶希望虫钙化作用（calcification）过程中由个体发育趋势（ontogenetic trends）（即尺寸相关效应（size-related effects））及其他生命效应（vital effects）引发的壳内变异（相对标准偏差（RSD）为11%~25%），仅能解释部分观测到的锰钙比值变异。本研究结果表明，研究区域季节动态极强的环境条件，使锰钙比值的壳内与壳间变异显著升高（平均相对标准偏差达45%）。在单个壳体内部，可观测到锰钙比值随壳体尺寸同时出现上升与下降的趋势。研究结果显示，连续单房室（single-chamber）的锰钙比值变异可反映孔隙水Mn²+的时间动态变化。此外，有孔虫在表层沉积物中的主动或被动迁移，也能解释部分观测到的锰钙比值变异。