We are pleased to share that our latest dusty paper has been finally published at Nature Scientific Reports! In this study, we have determined the coccolith‐Sr/Ca from a one‐year (2012–2013) time‐series sediment trap record in the western tropical North Atlantic (M4—49°N/12°W) to distinguish the biogeochemical effects of Saharan dust with respect to fertilisation and ballasting, and to gain a broader perspective on the coccolith calcite Sr/Ca in relation to the drivers of coccolith export production.
So, what is the difference between “production” and “export production”?
Coccolithophore production (number of cells per litre of seawater) is primarily a function of light and nutrients - through which these calcifying nannoplankton organisms perform photosynthesis in order to reproduce in the photic zone of the ocean -, further modulated by seasonal variations in hydrological parameters like temperature, salinity, turbulence and turbidity. In turn, coccolith export production, or coccolith fluxes (number of coccoliths per square meter, per day), results from this primary ecological signal, plus the effects from being further affected by several taphonomical phenomena (i.e., necrolysis and biostratinomy) that act upon the coccospheres/coccoliths before they sink out of the upper ocean. Sediment trap studies using this group of calcifying phytoplankton are based on the assumption that the settling of coccoliths in open-ocean areas is primarily related to the production occurring in the overlying photic layer, allowing for the assessment of the export fluxes and seasonal trends of distinct coccolithophore species during longer periods than most plankton studies.
Coccolith Sr/Ca ratios as a proxy to study the biogeochemical effects of Saharan dust deposition
The approach we used in this work is based on previous studies reporting higher coccolithophore Sr partitioning (i.e., higher amount of Sr incorporated into the calcite) as directly proportional to the coccolith calcification rate which, in turn, is a function of the coccolithophores’ growth rate (e.g., Stoll & Schrag, 2000; Rickaby et al., 2002). Our paper indeed shows that high coccolith Sr/Ca ratios were linked to enhanced coccolithophore export production in the upper part of the photic zone, most notably under windier, dry, and dustier conditions during spring. Attenuated Sr/Ca ratios in autumn were probably reflecting a combination of lower Sr‐incorporation by dominant but small‐size placolith‐bearing species and the presence of “aged” coccoliths rapidly scavenged during a highly productive and usually fast export event, likely added by (wet) dust ballasting.
This figure shows (a) the seasonal variation of the coccolith-Sr/Ca ratios in the bulk fraction (< 20 μm) and in the coccolith size fractions (small < 3 μm; intermediate 3–6 μm; large > 6 μm); (b) total coccolith- and coccolith-CaCO3 fluxes from sediment trap M4. Numbers refer to samples U2, U7, U12, U14, U18, U21 and U24, for which we performed a taxonomic analysis of all the studied coccolith size fractions. The light grey vertical bars indicate the periods during which co-increase in biogenic particle fluxes and Sr/Ca ratios was observed.
We noticed much higher Sr/Ca ratios in the large coccolith size fractions, supporting the existing notion that larger‐sized coccolithophores incorporate more Sr during calcification compared to smaller-sized species under the same environmental conditions. This was especially the case of the abnormally Sr‐rich species Scyphosphaera apsteinii, as shown in the separated large fraction of our Sr/Ca seasonal data, in line with observations from previous studies (Hermoso et al., 2017; Meyer et al., 2020).
Image of a coccosphere by Scyphosphaera apsteinii as seen under the Scanning Electron Microscope (credits: Jeremy Young, Nannotax). Despite its very low export productivity at out sediment trap site, and contributing less than 2% of the coccolith assemblages in all size fractions, this abnormally Sr-rich species produced disproportionally high percentages of carbonate both in the original trap samples and in the coccolith size fractions due to its large dimension (Guerreiro et al., 2021), resulting in unusually high Sr/Ca ratios. This is in line with previous studies which have also reported S. apsteinii to have unusually high, and still poorly understood, coccolith Sr/Ca ratios (Hermoso et al., 2017; Meyer et al., 2020).
While the Sr/Ca data confirm the occurrence of previously reported pulsed export productivity in the studied region, we recommend that these biogeochemical data should be interpreted taking into consideration the carbonate produced by size-distinct species within the coccolith sinking assemblages, and in the context of a multi-proxy framework. Our study suggests that the Sr/Ca ratio is less suitable to be used as a dust-related productivity proxy beyond a certain threshold of dust flux and/or when dust is deposited with rain, during which the dust ballasting effect may contribute to attenuate the coccolith biogeochemical “growth” original signal. Results from this study highlight the importance of doing more research addressing the physiological, biogeochemical, and abiotic drivers of Sr incorporation by ecologically- and morphologically distinct coccolithophore species.
If you want to know more about our study, click here.