Carbon Isotopic Fractionation of Alkenones and Gephyrocapsa Coccoliths Over the Late Quaternary (Marine Isotope Stages 12–9) Glacial‐Interglacial Cycles at the Western Tropical Atlantic

Abstract

Coccolithophores, single-celled marine phytoplankton, play a unique role in the marine carbon cycle because their primary production contributes to both the operation of the carbonate counter pump and the biological organic carbon pump during their lifecycle. There is considerable interest in understanding their past role in the carbon cycle and how their growth and calcification may have been affected by changing oceanographic conditions and changing CO 2 availability (e.g.,

Multi-proxy estimates of sea surface temperature and CO2 in the western Atlantic during the Late Miocene

“…This correlation is consistent with less stratification, higher nutrient supply and faster growth rates in the 7.2 to 7.8 Ma interval, leading to suppressed  p and lower intercepts. This interpretation is also quantitatively consistent with the different intercepts implied by the Late Miocene and Mid and Late Quaternary  p differences at Site 926, since the higher intercept of the Pleistocene samples also coincide with evidence for greater stratification (González-Lanchas et al, 2021). The increased water column stratification after 6.6 Ma appears only partly controlled by salinity, since the  18 O sw suggests low salinities occur only between 6.4 and 6 Ma (Figure 2) but the water column remained stratified when salinity recovers to previous levels.…”

“…The calculation of CO 2 follows the statistical model proposed by Stoll et al (2019), as applied in the Late Quaternary glacial-interglacial study in which non-CO 2 influences were aggregated to a single term (González-Lanchas et al, 2021). The simplified equation would be written as follows:…”

“…When CO 2 is independently constrained, such as in the late Pleistocene from ice core records, then the non-CO 2 effects on  p can be distinguished and evaluated (e.g. Badger et al (2013), Hernández-Almeida et al (2023), González-Lanchas et al (2021)).…”

“…In this study, we evaluate the consistency of late Miocene CO 2 proxy records by providing a new high resolution record of  p during the interval from 7.8 to 5.5 Ma at Site 926 on the Ceara Rise in the Western tropical Atlantic. This low latitude oligotrophic setting has been a mainstay for long-term Cenozoic CO 2 records based on the  p approach (Y. G. Zhang et al, 2013) (González-Lanchas et al, 2021Pagani et al, 2011;Stoll et al, 2019) with the assumption of constant non-CO 2 influences on  p .…”

“…)). After initial application of the culture-based regression with slope and updated intercept equal toGonzález-Lanchas et al (2021), we further discuss the variations of the intercept independently.2.8 Calculation of non-CO 2 effects on  p Changes in the intercept (I, non-CO 2 factors) in the equation above can be back-calculated using paired δ 11 B measurements (Site 926; R. M Brown et al (2022). and this study) for the Late Miocene and published CO 2 estimations from the Late Miocene based on  p (Site 1088;Tanner et al (2020)).…”