Carbon isotopic composition of marine organic matter
Coccolithophore bloom imaged by the MERIS sensor on Envisat (17th August 2011). Image credit: MERIS/ESA
Carbon uptake in photosynthetic marine algae
There are 2 stable isotopes of carbon: 12C (98.9%) and 13C (1.1%). Variations are expressed in standard delta notation as δ13C.
Photoautotrophs, such as plants and algae, convert CO2 to simple sugars during photosynthesis. Marine algae acquire this CO2 from carbon dissolved in seawater (CO2[aq]). During photosynthesis, 13CO2 reacts more slowly than 12CO2, resulting in an isotopic fractionation (εp) between CO2[aq] and the resulting organic matter of the algae. Thus, εp can be calculated using the initial δ13C of CO2 (which can be estimated via the δ13 C of carbonate in the shells of planktonic foraminifera) and the δ13C of organic matter preserved in the same sediment record.
A cartoon of a cell showing the key processes that dictate carbon isotope discrimination during photosynthesis: diffusion of carbon dioxide into the cell and carbon dioxide fixation by rubisco. Adapted from Badger et al (2011)
When CO2[aq] is limited, less isotopic discrimination occurs. This results in a clear relationship between εp and the concentration of CO2[aq]in the seawater, which culture experiments have allowed us to define as:
where εf is the maximum potential isotopic fractionation in algae and b reflects various physiological factors that may influence how CO2 is transported from the environment into the cell (Bidigare et al., 1997). Given that CO2[aq] is in equilibrium with atmospheric CO2 in many parts of the surface ocean, atmospheric CO2 can then be calculated using Henry’s law.
EARLY HISTORY
The δ13C value of organic matter derived from marine phytoplankton has been used to reconstruct atmospheric CO2 concentrations for over three decades. The first studies explored the possibilities with bulk organic matter (Hayes et al., 1989; Hayes, 1999) and chlorophyll-derived geoporphyrins (Popp et al., 1989; Freeman and Hayes, 1992). Since then, compound-specific analysis has been preferred over bulk organic matter (see below); by restricting the source of the organic matter, this can help reduce the number of uncertainties and assumptions in calculating CO2.
Alkenones
The most widely studied and applied organic compound for calculating εp is the unsaturated C 37 alkenone compounds produced by select Haptophyte algae (e.g. Jasper et al., 1994; Pagani et al., 2011; Super et al., 2018) that span the past ca. 50 million years (Brassell, 2014).
The C37 alkenone with two double bonds
As alkenones are produced by a restricted number of species, laboratory cultures can be readily conducted and have provided insights on isotopic fractionation mechanisms, such as cell size, growth rate, and membrane permeability (e.g. Bidigare et al., 1997; Wilkes et al., 2018).
General algal biomarkers (e.g. phytane)
An (old) schematic showing chlorophyll-a and its diagenetic products (e.g. pristane/phytane)..
General algal biomarkers, compounds representative of the overall phytoplankton community, have also been applied. Although less source-specific, this gives the additional benefits of ubiquitous spatial and temporal presence throughout the geologic record. For example, chlorophyll-a (the pigment essential for photosynthesis) is universally found in sediments and oils, and its diagenetic products have been used to reconstruct CO2 using both its porphyrin head (e.g. Popp et al., 1989; Freeman and Hayes, 1992) and the diagenetic product of its sidechain phytane.
Due to the ubiquity and longevity of chlorophyll-a products, phytane has been particularly helpful in clarifying periods such as the Cretaceous (Bice et al., 2006; Sinninghe Damsté et al., 2008; Naafs et al., 2016). Phytane-based estimations of CO2 span 455 million years (Witkowski et al., 2018), the oldest CO2-proxy record to date.
Potential Complications
There are a number of critical assumptions to calculate CO2 from carbon isotopic composition of algal organic matter:
- Some organisms actively uptake CO2 via carbon-concentrating mechanisms, especially when CO2 is limited. The above model assumes CO2 is acquired exclusively through diffusion. A glacial-interglacial study suggests that there may be a lower limit to CO2 reconstructions of ca. 250 ppm due to this upregulation (Badger et al., 2019; Stoll et al. 2019).
- Accurate temperature reconstructions are required. Alkenones can also be used to estimate temperature via the Uk ’37 proxy (Marlowe et al., 1984; Brassell et al., 1986).
- Algal growth rate needs to be assumed to be stable or reconstructed. Typically low growth rate (e.g. oligotrophic) ocean sites are chosen where growth rates ought to be stable and low (e.g. Pagani et al. 1999; Badger et al. 2013).
- Cell geometry can affect the diffusion of CO2 across the cell membrane (Popp et al. 1998). Alkenone-producers are generally small and close to spherical but can change in cell size (e.g. Henderiks & Pagani 2007) so coccoliths have now been used to better estimate this (e.g. Zhang et al., 2019) General biomarkers presumably average the community (Witkowski et al., 2019) but require further investigation.