Abstract
Abstract. Coccolithophores contribute significantly to marine primary productivity and play a unique role in ocean biogeochemistry by using carbon for photosynthesis (soft-tissue pump) and for calcification (carbonate counter pump). Despite the importance of including coccolithophores in Earth system models to allow better predictions of the climate system's responses to planetary change, the reconstruction of coccolithophore productivity mostly relied on proxies dependent on accumulation and sedimentation rates and preservation conditions. In this study we used an independent proxy, based on the coccolith fraction (CF) Sr∕Ca ratio, to reconstruct coccolithophore productivity. We studied the marine sediment core MD03-2699 from the western Iberian margin (IbM), concentrating on glacial–interglacial cycles of Marine Isotopic Stage (MIS) 12 to MIS 9. We found that IbM coccolithophore productivity was controlled by changes in the oceanographic conditions, such as in sea surface temperature (SST) and nutrient availability, and by competition with other phytoplankton groups. Long-term coccolithophore productivity was primarily affected by variations in the dominant surface water mass. Polar and subpolar surface waters during glacial substages were associated with decreased coccolithophore productivity, with the strongest productivity minima concomitant with Heinrich-type events (HtEs). Subtropical, nutrient-poorer waters, increased terrigenous input, and moderate to strong upwelling during the deglaciation and early MIS11 are hypothesized to have attributed a competitive advantage to diatoms to the detriment of coccolithophores, resulting in intermediate coccolithophore productivity levels. During the progression towards full glacial conditions an increasing presence of nutrient-richer waters, related to the growing influence of transitional surface waters and/or intensified upwelling, probably stimulated coccolithophore productivity to maxima following the rapid depletion of silica by diatoms. We present conceptual models of the carbon and carbonate cycle components for the IbM in different time slices that might serve as a basis for further investigation and modelling experiments.
Highlights
Coccolithophores play a unique role in ocean biogeochemistry using carbon for both photosynthesis and calcification (e.g. Rost and Riebesell, 2004; Westbroek et al, 1993) and contributing up to 60 % to the total calcium carbonate in the ocean (Flores and Sierro, 2007)
We studied the marine sediment core MD03-2699 from the western Iberian margin (IbM), concentrating on glacial–interglacial cycles of Marine Isotopic Stage (MIS) 12 to MIS 9
We considered potential sources of detrital carbonate biasing the coccolith fraction (CF) Sr/Ca results: (1) ice-rafted debris (IRD) discharged by melting icebergs, (2) aeolian input, and (3) riverine discharge
Summary
Coccolithophores play a unique role in ocean biogeochemistry using carbon for both photosynthesis and calcification (e.g. Rost and Riebesell, 2004; Westbroek et al, 1993) and contributing up to 60 % to the total calcium carbonate in the ocean (Flores and Sierro, 2007). Several studies hypothesized a relevant role of coccolithophores in glacial–interglacial dynamics (Duchamp-Alphonse et al, 2018; McClelland et al, 2016; Omta et al, 2013; Rickaby et al, 2007; Saavedra-Pellitero et al, 2017), but there are uncertainties arising from the complex interactions between coccolithophore productivity and growth rates, nutrient dynamics and competition with non-calcifiers, seasonality and the export of both carbonate and organic carbon, and the final accumulation/burial rates (Balch, 2018; Duchamp-Alphonse et al, 2018; McClelland et al, 2016; Omta et al, 2013; Rickaby et al, 2007; Ridgwell and Zeebe, 2005; Saavedra-Pellitero et al, 2017). In order to provide useful information to modellers, it is fundamental to test the dynamic response of the various components of the system over different climatic scenarios through the use of multiproxy studies involving the reconstruction of coccolithophore growth rates and the export of both their organic and inorganic compounds, namely alkenones and nannofossil accumulation rate (AlkAR and NAR, respectively)
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