Calcification intensity in planktonic Foraminifera reflects ambient conditions irrespective of environmental stress

[摘要] Planktonic Foraminifera are important marine calcifiers,and the ongoing change in the oceaniccarbon system makes it essential to understand the influence of environmental factors on thebiomineralization of their shells. The amount of calcite deposited by planktonic Foraminiferaduring calcification has been hypothesized to reflect a range of environmental factors. However,it has never been assessed whether their calcification only passively responds to the conditionsof the ambient seawater or whether it reflects changes in resource allocation due to physiologicalstress. To disentangle these two end-member scenarios, an experiment is required where the twoprocesses are separated. A natural analogue to such an experiment occurred during the depositionof the Mediterranean sapropels, where large changes in surface water composition andstratification at the onset of the sapropel deposition were decoupled from local extinctions ofplanktonic Foraminifera species. We took advantage of this natural experiment and investigated thereaction of calcification intensity, expressed as mean area density (MAD), of fourspecies of planktonic Foraminifera to changing conditions during the onset of Sapropel S5(126–121 ka) in a sediment core from the Levantine Basin. We observed a significantrelationship between MAD and surface water properties, as reflected by stable isotopesin the calcite of Foraminifera shells, but we failed to observe any reaction of calcificationintensity on ecological stress during times of decreasing abundance culminating in localextinction. The reaction of calcification intensity to surface water perturbation at the onset ofthe sapropel was observed only in surface-dwelling species, but all species calcified morestrongly prior to the sapropel deposition and less strongly within the sapropel than at similarconditions during the present-day. These results indicate that the high-salinity environment ofthe glacial Mediterranean Sea prior to sapropel deposition induced a~more intense calcification,whereas the freshwater injection to the surface waters associated with sapropel depositioninhibited calcification. The results are robust to changes in carbonate preservation andcollectively imply that changes in normalized shell weight in planktonic Foraminifera shouldreflect mainly abiotic forcing.

[学科分类] 地球化学与岩石