The effect of population dynamics, growth and calcification of planktonic foraminifera on the pelagic carbonate flux

Abstract

The planktonic foraminifera calcite production represents a fundamental component of the pelagic carbon cycle by contributing up to half of the biogenic carbonate export flux to the deep ocean, whilst simultaneously the calcite biomineralisation releases CO2 from the surface waters. Due to its relevance for the pelagic carbonate budget and the biological pump, in terms of oceanic capacity of CO2 sequestration from the atmosphere, it is crucial to understand the mechanisms constraining the short-term (intra-and interannual) and long-term (centennial to millennial) variability in the planktonic foraminifera calcite flux. The planktonic foraminifera calcite flux is the product of the species individual calcite fluxes where variability in calcite flux can be caused by changes in the species i) individual flux, ii) shell size, and iii) calcification intensity. In the exported assemblages, where size and calcification intensity vary among the species, changes can be caused by the species composition as well. Previous research has not investigated these three mechanisms comprehensively in the context of the planktonic foraminifera calcite flux. Thus, it is uncertain how many of the potential controlling mechanisms are sufficient to be considered to constrain the variability in the planktonic foraminifera calcite flux and to derive an accurate planktonic foraminifera calcite budget. To assess the importance of these mechanisms in regulating the short-term variability in planktonic foraminifera calcite flux, record of species-resolved shell flux, shell size, and calcification intensity variations during two years of sedimentation (from 1990-1991 and 2007-2008) from the Cape Blanc upwelling area in the Atlantic Ocean was generated. In order to compare how the short-term variability in the regulating mechanisms compares to long-term trends and to assess how these mechanisms responded to large amplitude environmental changes, we investigated exceptionally preserved planktonic foraminifera communities from a well-dated sediment core (GeoB3104-1) covering the period of the last deglaciation in the tropical Atlantic and sediment samples from the mid-Miocene Devínska Nová Ves (Slovakia) site. The sediment trap time series revealed that on the intra-annual timescales, the variability in calcite flux is largely due to changes in shell flux, which can alone explain 82 % of the variability in the planktonic foraminifera calcite flux. On the interannual timescale, multiple mechanisms act in concert to shape the variability in the planktonic foraminifera calcite flux, and next to shell flux shell size also emerges as an important predictor of calcite flux. The sedimentary time series revealed that on the centennial to millennial timescales, shell flux is the most important mechanism shaping the variability in the planktonic foraminifera calcite flux. This conclusion is supported by observations from the Miocene site, which reveals that despite different background climate state and millions of years of evolution, the calcification intensity of the studied species remained similar. Thus, irrespective of the temporal scale, shell flux emerges as the most and calcification intensity the least important mechanism for predicting changes in the planktonic foraminifera calcite flux. Shell flux changed dramatically during large-scale environmental transition, whereas calcification intensity and shell size remained more similar during the glacial-interglacial cycle and between modern and mid-Miocene samples. Therefore, our analysis implies that in order to model/predict the global foraminifera calcite budget we first and foremost need to constrain the biotic and abiotic factors that control shell flux variability.