Mg/Ca and Mn/Ca ratios in benthic foraminifera: the potential to reconstruct past variations in temperature and hypoxia in shelf regions

Abstract

Abstract. Shelf and coastal regions are exceptionally important for many countries as they provide the main habitat for many economically important fish and shellfish species. With ongoing climate change and human-induced eutrophication the shelf regions are especially affected, resulting in increased temperatures and stratification as well as oxygen depletion of the bottom waters. In order to be able to predict the magnitude of these changes in the future, it is necessary to study how they varied in the past. Commonly used foraminiferal climate and environmental proxies, e.g., stable isotopes and trace metal/Ca ratios, that are applied in open-ocean settings are not necessarily applicable in shelf regions, either as faunas are significantly different or as conditions can change much faster compared to the open ocean. In this study we explore the use of Mg/Ca as paleothermometer and Mn/Ca as a potential proxy for changing dissolved oxygen conditions in bottom water on the benthic foraminifera Bulimina marginata and Globobulimina turgida. Living specimens were collected from the Skagerrak and the Gullmar Fjord (SW Sweden); the latter is hypoxic for several months a year. As the specimens were alive when collected, we assume it unlikely that any diagenetic coatings have already significantly affected the trace metal/Ca ratios. The Mg/Ca ratios are similar to previously published values but display much larger variation than would be expected from the annual temperature change of less than 2 °C. An additional impact of the difference in the calcite saturation state between the Skagerrak and the Gullmar Fjord could explain the results. Mn/Ca ratios from G. turgida can potentially be related to variations in dissolved oxygen of the habitat where the foraminifera calcify. Samples from the Skagerrak display increased Mn/Ca in specimens that lived deeper in the sediment than those that lived near the surface. G. turgida samples from the low-oxygen Gullmar Fjord showed significantly increased Mn/Ca, being highest when bottom water dissolved oxygen was at a minimum. Our study suggests that trace metal/Ca ratios in benthic foraminifera from shelf regions have the potential to record past variations in bottom water temperature and dissolved oxygen concentrations, but an additional impact of the inorganic carbonate chemistry cannot be excluded.

A comparison of benthic foraminiferal Mn / Ca and sedimentary Mn / Al as proxies of relative bottom-water oxygenation in the low-latitude NE Atlantic upwelling system

“…, utilising the trace-elemental composition of foraminiferal shells (tests) to reconstruct oxygen conditions is still in its infancy. One redox-sensitive element that has recently gained more interest is manganese (Mn), both as a trace element in biogenic foraminiferal calcite (Mn / Ca) and in bulk sediment samples (Reichart et al, 2003;Glock et al, 2012, Groeneveld andFilipsson, 2013;Lenz et al, 2014;Koho et al, 2015). Here we aim to explore the potential of Mn / Ca by analysing both benthic foraminiferal tests and comparing them to Mn / Al of bulk sediment samples from an upwelling record to determine if changes in oxygen conditions during different primary productivity regimes are detectable by these methods.…”

“…Therefore, under low-oxygen conditions, foraminiferal Mn / Ca is expected to be higher during hypoxic conditions than during anoxic conditions, but still higher than under oxic conditions. Whilst benthic foraminiferal Mn / Ca has been conventionally used as an indicator of contamination by Mn oxyhydroxide or Mn carbonate (Boyle, 1983;Barker et al, 2003), new studies are pioneering Mn / Ca as a potential proxy of related changes in bottom/pore-water oxygen and redox conditions (Ní Fhlaithearta et al, 2010;Glock et al, 2012;Groeneveld and Filipsson, 2013;Koho et al, 2015). Mn / Ca signatures of the ambient bottom water are recorded by benthic foraminifera.…”

“…Traditionally, trace element / Ca is analysed on solutionbased samples containing larger numbers of foraminifera specimens to give a representative result (Groeneveld and Filipsson, 2013). When a sufficient amount of specimens are not available for solution-based techniques, or if diagenesis has affected the tests, a micro-analytical technique upon single specimens such as secondary ion mass spectrometry (SIMS) is a valuable tool.…”

Reconstructing Holocene temperature and salinity variations in the western Baltic Sea region: a multi-proxy comparison from the Little Belt (IODP Expedition 347, Site M0059)

“…Bottom water temperatures can be reconstructed using Mg / Ca in benthic foraminifera (Raitzsch et al, 2008), but large gradients in salinity (and low absolute salinity) as well as parameters related to the carbonate system (e.g. alkalinity and carbonate saturation state) in the Baltic Sea may significantly influence the application of Mg / Ca ratios (Groeneveld and Filipsson, 2013).…”

Coastal primary productivity changes over the last millennium: a case study from the Skagerrak (North Sea)

“…It is likely that the Mn/Ca ratio increases during periods of lower bottom-water oxygen concentration (Groeneveld and Filipsson, 2013). It has to be noticed, however, that generally low Mn/Ca values (0.15 -0.8 mmol mol-1) in 5 our records suggest slight changes in dissolved bottom water oxygen in comparison to a severe oxygen depletion recorded in a NE Skagerrak fjord as indicated by high Mn/Ca values (0.20 -5.8 mmol mol-1) (Groeneveld and Filipsson, 2013). Thus, in our study we only use the Mn/Ca as a supportive indicator for seafloor oxygen conditions when the ratio corroborates changes seen in benthic foraminiferal assemblages.…”