Assessing the Recrystallisation of Planktonic Foraminifera in Pelagic Sediments and the Reliability of Mg/Ca Values With Laser Ablation (LA‐) ICP‐MS

Abstract The calcite tests of foraminifera used for reconstructions of past oceanic and climatic conditions are altered after deposition in pelagic sediments by recrystallisation but this process remains poorly understood. To investigate the controls and impacts of recrystallisation we compare laser ablation (LA‐) ICP‐MS‐derived Mg/Ca and Sr/Ca ratio depth profiles through test walls of the planktonic foraminifera Dentoglobigerina venezuelana from specific time intervals of Oligocene‐Miocene age sediments from different sites in the eastern equatorial Pacific. Fossil tests (between 13.9 and 26.8 Myrs old) exhibit intra‐test Mg/Ca heterogeneity comparable to recent foraminifera for which Sr/Ca ratios are homogenous (within 10%) across the test walls. However, for the majority of the tests from >20 Ma, Sr/Ca ratios decrease by >0.2 mmol/mol from the outer towards the inner test wall suggesting recrystallisation in a more open system with exchange with pore waters inside the test. Using this decreasing Sr/Ca metric, Site U1336 has the highest percentages of such recrystallised tests across the time intervals, presumably related to a strong geothermal gradient. Even though some tests were recrystallised in a more open system, the Mg/Ca heterogeneity across test walls and the mean values are similar to those of better preserved tests. This implies that these particular tests retain most of their original Mg/Ca signal and can reliably be used for temperature reconstructions. The laser ablation ICP‐MS technique employed provides a quantitative assessment of the chemical alteration of each test, allowing entire records to be screened for bias resulting from recrystallisation encountered in most marine sediments.