The origin of carbon isotope vital effects in coccolith calcite

Abstract

Calcite microfossils are widely used to study climate and oceanography in Earth's geological past. Coccoliths, readily preserved calcite plates produced by a group of single-celled surface-ocean dwelling algae called coccolithophores, have formed a significant fraction of marine sediments since the Late Triassic. However, unlike the shells of foraminifera, their zooplankton counterparts, coccoliths remain underused in palaeo-reconstructions. Precipitated in an intracellular chemical and isotopic microenvironment, coccolith calcite exhibits large and enigmatic departures from the isotopic composition of abiogenic calcite, known as vital effects. Here we show that the calcification to carbon fixation ratio determines whether coccolith calcite is isotopically heavier or lighter than abiogenic calcite, and that the size of the deviation is determined by the degree of carbon utilization. We discuss the theoretical potential for, and current limitations of, coccolith-based CO2 paleobarometry, that may eventually facilitate use of the ubiquitous and geologically extensive sedimentary archive.

Understanding Bulk Sediment Stable Isotope Records in the Eastern Equatorial Pacific, From Seven Million Years Ago to Present Day

“…Once recognized that vital effects impact coccolith stable isotope composition, bulk sediment δ 13 C and δ 18 O records at Site 851 (Figures ) become more complicated and more intriguing. This is because the impact of vital effects may change through time through either different coccolithophorid species assemblages, different pathways for coccolith carbonate precipitation, or both (Hermoso, ; McClelland et al, ; Rickaby et al, ; Steinmetz & Anderson, ). This broad view includes temporal variations in coccolith size or growth rate, which have been suggested to modify the stable isotope composition of coccolith carbonate (Bolton & Stoll, ; Jin et al, , and above references).…”

“…This can arrive to the cell by diffusion of CO 2(aq) into an internal "CO 2 pool" or by active uptake of HCO 3 − (aq) , which then can be converted to CO 2 (Anning et al, 1996;Bolton & Stoll, 2013;Buitenhuis et al, 1999;Paasche, 2001;Rickaby et al, 2010). Indeed, the transfer of protons (H + ) from the coccolith vesicle to the cytosol has been described as a primary means to elevate CO 2 available for photosynthesis (Anning et al, 1996;Bolton et al, 2012;McClelland et al, 2017;Rickaby et al, 2010). This mechanism may become preferential at relatively low CO 2 (aq) concentrations.…”

“…This is because different pathways for converting dissolved inorganic carbon to calcite fractionate isotopes differently (McConnaughey, 1989 and above references), which can lead to calcite δ 13 C and δ 18 O isotope compositions out of equilibrium with the chemistry of ambient seawater. These are termed "vital effects" and can be particularly large for certain coccolithophorids (Bolton et al, 2012;Bolton & Stoll, 2013;McClelland et al, 2017;Rickaby et al, 2010;Ziveri et al, 2003).…”

Selective Preservation of Coccolith Calcite in Ontong‐Java Plateau Sediments

“…Unlike foraminifera, there is little evidence that coccolith calcite is heterogeneous in composition, particularly for any given species. However, coccolithophores exhibit a large range in carbon isotope vital effects (e.g., Bolton & Stoll, 2014;McClelland et al, 2017). In general, large coccolithophores demonstrate negative 13 C and smaller coccolithophores demonstrate higher 13 C (Bolton et al, 2016).…”

Enhancing Our Palaeoceanographic Toolbox Using Paired Foraminiferal and Coccolith Calcite Measurements From Pelagic Sequences

“…The mechanistic and causal link between CO 2 availability and the magnitude of the vital effects of the coccoliths reflects several intracellular controls by: (i) cell size; (ii) cellular growth rate (µ); and (iii) the relative allocation of the internal DIC pool to calcification and photosynthesis that might be approximated by their particulate inorganic to organic carbon ratio (PIC/POC ratio). Culture studies have provided decisive constraints on the changing intensity of the vital effect with the environment (Rickaby et al, 2010;Hermoso et al, 2014Hermoso et al, , 2016McClelland et al, 2017). These biogeochemical controls are detailed in Hermoso (2014) and McClelland et al (2017), and overall explain that the coccolith intrinsically bear valuable environmental information in their geochemistry (carbon and oxygen isotope systems), and in particular for retracing the evolution of [CO 2aq ].…”