Variability of shell mass, d18O and Mg/Ca in planktonic foraminifera: from production to preservation

Mg/Ca in planktonic foraminifera tests is now well established as a temperature proxy for surface and near-surface waters. Yet work remains to be carried out to define the post-depositional processes affecting foraminiferal Mg/Ca after water-column production. The advantage of Mg/Ca over other paleotemperature proxies is that it can be coupled with d18O measured on the same foraminiferal sample so as to determine seawater d18O. We present results of a study of the effect of dissolution on both Mg/Ca and d18O of several planktonic foraminiferal species along a depth transect (~450-4000 m) off Somalia, spanning the lysocline at ~2000 m. The planktonic foraminiferal species, Globigerinoides ruber, Globigerina bulloides, Globigerinoides sacculifer and Neogloboquadrina dutertrei were analysed for average shell mass, Mg/Ca, Sr/Ca, d18O and d13C from core-top samples from 250-300 um size fraction from a depth transect off Somalia (water depth range for samples: ~450-4000 m) . Average shell mass decreases by 10 % in G. ruber, 19 % in N. dutertrei, 29% in G. bulloides and 37 % in G. sacculifer below the lysocline. N. dutertrei is the only species that shows a significant decrease in both d18O (from -0.61 to -0.25 ‰) and Mg/Ca (from 2.57 to 1.10 mmol/mol). Only a small decrease in Mg/Ca is found in G. sacculifer (from 3.50 to 3.08 mmol/mol), G. bulloides (from 4.84 to 4.39 mmol/mol) and G. ruber (from 4.49 to 4.17 mmol/mol) below the lysocline. The d18O and Mg/Ca of G. ruber are least affected by dissolution and hence is the most useful species for surface water temperature reconstructions at this site. Data from the water column and the sediment record for G. ruber and N. dutertrei will serve to quantify the fate of shell mass, d18O and Mg/Ca proxies from production to preservation