Diagenetic Effect on Planktic Foraminiferal Mg/Ca Records in the Arctic Ocean

Mg/Ca ratios in planktic foraminifera have long been used as a thermometer to reconstruct the past climate. However, this study revealed that in certain intervals, the Mg/Ca ratios of the planktic foraminifer Neogloboquadrina pachyderma, a widespread species in the Arctic Ocean, displayed anomalously high values which could severely bias paleo-temperature reconstructions. Therefore, in this study, we analyzed the spatial and temporal distribution of Mg/Ca ratios of N. pachyderma in sediment cores of the Arctic Ocean to address the causes of these anomalies. Authigenic carbonate overgrowth was revealed under Scanning Electron Microscope, while Mg intensities from Energy Dispersive Spectroscopy and Electron Probe Micro Analyzer indicated high-Mg bands on the inner and outer surfaces of N. pachyderma causing the non-ontogenetic anomalously high Mg/Ca ratios. The distribution of these high-Mg bands on N. pachyderma did not match descriptions of biological alteration or methanogenic calcite overgrowth. In contrast, the authigenic overgrowth on foraminiferal tests was most likely due to the dissolution and reprecipitation of detrital high-Mg calcite within the marine sediment. Our work thus adds support for the observation that dissolution and reprecipitation of calcite from Mg/Ca-rich ice rifted debris can be an important process altering the calcium carbonate chemistry of foraminiferal shells in the Arctic Ocean. This authigenic overgrowth is difficult to remove without damaging the original foraminiferal shell during the cleaning processes. Consequently, specimens exhibiting authigenic overgrowth should be excluded from Mg/Ca temperature reconstructions; alternatively, in situ micro-domain analytical methods might be used to specifically target the unaltered parts of the foraminiferal shell.