Carbon Isotope Fractionation in Noelaerhabdaceae Algae in Culture and a Critical Evaluation of the Alkenone Paleobarometer

Abstract

Anthropogenic CO 2 emissions are perturbing the global carbon cycle more rapidly than at any time in the last 66 million years (Zeebe et al., 2016). A key component of the natural carbon cycle is coccolithophores, which modify oceanic dissolved inorganic carbon and alkalinity inventories through photosynthetic carbon fixation and the biogenic production of calcium carbonate plates (Balch et al., 1992). These eukaryotic algae are abundant and widely distributed throughout the global oceans today (

Controls on Alkenone Carbon Isotope Fractionation in the Modern Ocean

“…Blanco-Ameijeiras et al (2020) measured cell membrane permeability to CO 2 in E. huxleyi and found CO 2 permeability was approximately 1e −4 m s −1 . However, here we apply an effective permeability of 2.0e −5 m s −1 so τ values used here are comparable to those reported in recent culture work (Phelps et al, 2021).…”

“…Recent syntheses of culture data have elucidated the quantitative relationships between various physiological and environmental influences, and confirmed that ε p does indeed decline as [CO 2(aq) ] decreases (e.g., Phelps et al, 2021;Stoll et al, 2019). However, culture experiments suggest that light energy, independent of its effect on growth rate, is nearly as important a control on ε p as physiology and CO 2 , with higher ε p occurring at higher irradiance (Phelps et al, 2021;Stoll et al, 2019;Wilkes & Pearson, 2019). Though alkenone ε p (ε p37:2 , subscript representing the diunsaturated ketone with 37 carbon atoms) has been extensively used to generate paleo-CO 2 records, only two studies have quantitatively considered cell size, growth rate, and irradiance when interpreting alkenone ε p in down-core sediment sequences (Stoll et al, 2019;Tanner et al, 2020).…”

“…The lower permeability that we use accounts for additional restriction of diffusion by the chloroplast and pyrenoid (see Phelps et al [2021] for further discussion). We treat permeability as a constant in all of our ensuing analysis;…”

“…Recent syntheses of culture data have elucidated the quantitative relationships between various physiological and environmental influences, and confirmed that ε p does indeed decline as [CO 2(aq) ] decreases (e.g., Phelps et al, 2021;Stoll et al, 2019). However, culture experiments suggest that light energy, independent of its effect on growth rate, is nearly as important a control on ε p as physiology and CO 2 , with higher ε p occurring at higher irradiance (Phelps et al, 2021;Stoll et al, 2019;Wilkes & Pearson, 2019).…”

“…We examine the relationship between alkenone ε p and carbon demand and carbon supply, as well as the direct influence of irradiance and nutrients on alkenone ε p . We evaluate whether a multiple linear regression model of ε p developed from cultures (Phelps et al, 2021) is applicable to the ocean, and test the theory of the conventional diffusive alkenone ε p model using growth rate data.…”