Abstract
Using shells collected from a sediment trap series in the Madeira Basin, we investigate the effects of seasonal variation of temperature, productivity, and optimum growth conditions on calcification in three species of planktonic Foraminifera. The series covers an entire seasonal cycle and reflects conditions at the edge of the distribution of the studied species, manifesting more suitable growth conditions during different parts of the year. The seasonal variation in seawater carbonate saturation at the studied site is negligible compared to other oceanic regions, allowing us to assess the effect of parameters other than carbonate saturation. Shell calcification is quantified using weight and size of individual shells. The size–weight scaling within each species is robust against changes in environmental parameters, but differs among species. An analysis of the variation in calcification intensity (size-normalized weight) reveals species-specific response patterns. In Globigerinoides ruber (white) and Globigerinoides elongatus, calcification intensity is correlated with temperature (positive) and productivity (negative), whilst in Globigerina bulloides no environmental forcing is observed. The size–weight scaling, calcification intensity, and response of calcification intensity to environmental change differed between G. ruber (white) and G. elongatus, implying that patterns extracted from pooled analyses of these species may reflect their changing proportions in the samples. Using shell flux as a measure of optimum growth conditions, we observe significant positive correlation with calcification intensity in G. elongatus, but negative correlation in G. bulloides. The lack of a consistent response of calcification intensity to optimum growth conditions is mirrored by the results of shell size analyses. We conclude that calcification intensity in planktonic Foraminifera is affected by factors other than carbonate saturation. These factors include temperature, productivity, and optimum growth conditions, but the strength and sign of the relationships differ among species, potentially complicating interpretations of calcification data from the fossil record.
Highlights
Planktonic Foraminifera are important marine calcifiers, contributing 30–80% to the global pelagic carbonate flux [1]
From March to June 2002, G. bulloides generally showed the highest flux of all species, followed by G. ruber and G. elongatus
For the rest of the investigated time interval the flux values were generally reversed, with G. elongatus mostly showing the highest flux before G. ruber and G. bulloides (Fig 4)
Summary
Planktonic Foraminifera are important marine calcifiers, contributing 30–80% to the global pelagic carbonate flux [1]. Based on field observations [4,5,6,7,8,9] and laboratory culturing studies [10, 11], carbonate saturation of the ambient seawater appears to be the most promising parameter to explain variations in the calcification intensity of planktonic Foraminifera. A study based on Pliocene sediments [14] found no link between calcification in planktonic Foraminifera and atmospheric pCO2, but rather identified temperature as a potential factor explaining the observed variation in foraminiferal shell calcification Because both parameters are tightly linked, it is challenging to disentangle their relative contributions even in well constrained studies based on recent sediment-trap material [9]
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