Alkenone isotopes show evidence of active carbon concentrating mechanisms in coccolithophores as aqueous carbon dioxide concentrations fall below 7 µmol L&amp;lt;sup&amp;gt;−1&amp;lt;/sup&amp;gt;

Marcus P S Badger

doi:10.5194/bg-18-1149-2021

Abstract

Abstract. Coccolithophores and other haptophyte algae acquire the carbon required for metabolic processes from the water in which they live. Whether carbon is actively moved across the cell membrane via a carbon concentrating mechanism, or passively through diffusion, is important for haptophyte biochemistry. The possible utilization of carbon concentrating mechanisms also has the potential to over-print one proxy method by which ancient atmospheric CO2 concentration is reconstructed using alkenone isotopes. Here I show that carbon concentrating mechanisms are likely used when aqueous carbon dioxide concentrations are below 7 µmol L−1. I compile published alkenone-based CO2 reconstructions from multiple sites over the Pleistocene and recalculate them using a common methodology, which allows comparison to be made with ice core CO2 records. Interrogating these records reveals that the relationship between proxy CO2 and ice core CO2 breaks down when local aqueous CO2 concentration falls below 7 µmol L−1. The recognition of this threshold explains why many alkenone-based CO2 records fail to accurately replicate ice core CO2 records, and it suggests the alkenone proxy is likely robust for much of the Cenozoic when this threshold was unlikely to be reached in much of the global ocean.

Highlights

Alkenones are long-chain (C37−39) ethyl and methyl ketones (Fig. 1; Brassell et al, 1986; Rechka and Maxwell, 1987) produced by a restricted group of photosynthetic haptophyte algae (Conte et al, 1994)
Two main proxy systems based on alkenone geochemistry exist: one allows reconstruction of sea surface temperature (SST) and relies on the changing degree of unsaturation of the C37 alkenone (UK37) (Brassell et al, 1986), whilst a second for atmospheric CO2 concentration is based on reconstructing the isotopic fractionation which takes place during photosynthesis (Laws et al, 1995; Bidigare et al, 1997)
Across the six sites included in this analysis, there are 217 CO2(εp−alk)-based estimates of atmospheric CO2 concentration over the past 260 kyr for comparison with the ice core records (Table 2; Bereiter et al, 2015)

Summary

Introduction

Alkenones are long-chain (C37−39) ethyl and methyl ketones (Fig. 1; Brassell et al, 1986; Rechka and Maxwell, 1987) produced by a restricted group of photosynthetic haptophyte algae (Conte et al, 1994). Two main proxy systems based on alkenone geochemistry exist: one allows reconstruction of sea surface temperature (SST) and relies on the changing degree of unsaturation of the C37 alkenone (UK37) (Brassell et al, 1986), whilst a second for atmospheric CO2 concentration is based on reconstructing the isotopic fractionation which takes place during photosynthesis (εp) (Laws et al, 1995; Bidigare et al, 1997) It is the second system using the stable carbon isotopic composition of the preserved alkenones for reconstructing atmospheric CO2 concentration (referred to throughout as CO2(εp−alk)) which is the focus of this study. Alkenones are produced primarily by two dominant coccolithophore species: Emiliania huxleyi and Gephyrocapsa oceanica.

Methods

Results

Discussion

Conclusion