Coccolith-calcite Sr/Ca as a Biogeochemical Proxy to Study Dust-related Export vs. Ballasting

Atmospheric dust deposition can modulate the earth's climate and atmospheric carbon dioxide (CO₂) through fertilising the ocean (nutrient source) and by accelerating the biological carbon pump through fuelling the ballasting process. 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, we 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). 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. Results indeed show high Sr/Ca linked to enhanced export production in the upper part of the photic zone, most notably under windier, dry, and dustier conditions during spring. Attenuated Sr/Ca in the autumn were interpreted as 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. Higher Sr/Ca observed in the large coccolith size fractions support the existing notion that larger-sized coccolithophores incorporate more Sr during calcification under the same environmental conditions, as strikingly shown by the abnormally Sr‐rich species Scyphosphaera apsteinii. Therefore, we recommend that these biogeochemical data should be interpreted taking into consideration the carbonate produced by ecologically- and size-distinct species within the coccolith sinking assemblages, and in the context of a multi-proxy framework.

Reference: Guerreiro, C.V., Ziveri, P., Cavaleiro, C. & Stuut, J-B. (2024). Coccolith-calcite Sr/Ca as a proxy for transient export production related to Saharan dust deposition in the tropical North Atlantic. Sci. Rep.,14: 4295. https://doi.org/10.1038/s41598-024-54001-3