Serial Mg/Ca and Sr/Ca chronologies across single benthic foraminifera tests

We used secondary ion mass spectrometry (SIMS) to analyse Mg/Ca and Sr/Ca across four individual tests of the benthic foraminifera Hyalinea balthica, collected ‘live’ from a site on the NW European shelf with seasonal variations in bottom water temperature (> 4 °C). Test growth is episodic and we have produced chronologies of Mg/Ca and Sr/Ca variations between consecutive chambers. Chronologies do not correlate well between replicate specimens although in one case the seasonal Mg/Ca trend is in reasonable agreement with estimated seawater temperature variations. We also observe significant intra-chamber hetereogeneity even within the outermost test chamber which is deposited nominally as a single event and has not been thickened during the addition of subsequent chambers. Mg/Ca and Sr/Ca also vary significantly between tests of similar size which were collected at the same time. SIMS can be used to analyse single specimens from sediment samples in which foraminifera are rare or to selectively target pristine calcite in diagenetically altered individuals. Our observation that Mg and Sr can vary significantly between foraminifera tests of the same size indicates a potential pitfall of this approach.