Coccolithophore productivity at the western Iberian Margin during the middle Pleistocene (310–455 ka) – evidence from coccolith Sr/Ca data

Abstract

Abstract. Coccolithophores contribute significantly to the marine primary productivity and play a unique role in ocean biogeochemistry by using carbon for photosynthesis (biological pump) and also for calcification (carbonate pump). Despite the importance of including coccolithophores in global climate models to allow better predictions of the climate system's responses to planetary change, highly uncertain coccolithophore paleoproductivity past reconstructions 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 used the marine sediment core MD03-2699 from the western Iberian margin (IbM), spanning the glacial/interglacial cycles of Marine Isotope Stage (MIS) 12 to MIS 9. We found that IbM coccolithophore productivity was controlled by changes in the oceanographic conditions, such as in SST, the competition for nutrients with other phytoplankton groups and insolation. Long-term coccolithophore productivity was primarily affected by variations in the dominant water mass. Polar and subpolar surface waters during glacial substages were associated with decreased coccolithophore productivity, with strongest productivity minima being concomitant with Heinrich-type events (HtE). Subtropical, nutrient-poorer waters during interglacial substages, i.e. MIS 11c, might have lead to intensified competition for nutrients with diatoms resulting in intermediate levels of coccolithophore productivity. The transition from interglacial to glacial substages was likely associated with increasing presence of nutrient-richer waters, possibly with lower silica content than riverine discharges and mostly fed by either upwelling or surface waters of northern origin. This minimized the competition with diatoms and coccolithophores reached their productivity maxima. Climatic conditions during colder periods forced coccolithophores to change their phenology contributing to the dissonance between the CF Sr/Ca derived coccolithophore productivity and nannofossil accumulation rate and total alkenone flux, which is interpreted as a consequence of the narrowing yearly time-window for coccolithophore productivity.

Control Mechanisms of Primary Productivity Revealed by Calcareous Nannoplankton From Marine Isotope Stages 12 to 9 at the Shackleton Site (IODP Site U1385)

“…Ultimately, both interglacial sea‐level high stands during the early MIS 11c and MIS 9e (Rohling et al., 2009) exclude any offshore displacement of the coastal upwelling cells through the position of the Site U1385. A recently suggested increased influence of nutrient‐poorer subtropical waters during the full interglacial MIS 11 and 9 to the north of our Site (Cavaleiro et al., 2020) is consistent with our observations. As well, similar primary productivity scenario and associated upwelling conditions was described at the IbM during other more recent interglacial periods (i.e., MIS 5; Pailler & Bard, 2002) and the Holocene (Incarbona et al., 2010).…”

“…This observation is in agreement with nutrient‐richer surface waters favoring coccolithophore proliferation against diatoms discussed by Cavaleiro et al. (2020) at the nearby site MD03‐2699. Additionally, the strengthened atmospheric circulation could have resulted in substantial widening of the coastal upwelling (e.g., Pailler & Bard, 2002), benefiting eutrophication at Site U1385 due to greater proximity of the upwelled filaments (or area of influence of the upwelling).…”

“…Overall, productivity reduction and associated hydrological and paleoceanographic conditions observed during these abrupt cold events of different duration for the studied interval impacted nannoplankton assemblages on a regional level, as proven by the consistency of our record with that from core MD03‐2699 (Cavaleiro et al., 2020). The mechanism that reduces primary productivity is probably comparable to that proposed during the Heinrich episodes of the last glacial (e.g., Ausín et al., 2020; Incarbona et al., 2010).…”

“…Further work of Cavaleiro et al. (2020) observed similar patterns of coccolithophore productivity in response to G/I and millennial‐scale variability, primarily attributed to variations in the dominant surface water masses and their influence (i.e., changes in nutrient content) on the state of competition between phytoplankton groups, but suggested to be connected to wind‐driven upwelling. However, in these studies a clear determination of the mechanisms controlling vertical mixing dynamics and fertilization was severely hampered by the absence of direct information about prevailing wind regimes and atmospheric configuration.…”

Coccolith Sr/Ca is a Robust Temperature and Growth Rate Indicator that Withstands Dynamic Microbial Interactions

“…To study whether bacteria affect coccolith elemental composition, we analyzed Sr/Ca ratios. Ratios of coccoliths Sr/Ca have been recognized as reliable and sensitive indicators of a range of temperatures and growth rates (Stoll and Schrag 2000, Stoll and Schrag 2001, Stoll, Klaas et al 2002, Stoll, Rosenthal et al 2002, Stoll, Ziveri et al 2007, Saavedra-Pellitero, Baumann et al 2017, Mejía, Paytan et al 2018, Cavaleiro, Voelker et al 2019. From a microbial perspective, growth rates of both algae and bacteria largely depend on temperature.…”