Carbon Isotope Dynamics in Oceanic Ecosystems

Carbon isotope dynamics in the world’s oceans underpin our understanding of both contemporary and past carbon cycling. Stable isotopes of carbon (notably 13C and 12C) and radiocarbon (14C) are incorporated into dissolved inorganic carbon, organic matter and biogenic carbonate through a suite of physical, chemical and biological processes. Fractionation during photosynthesis, calcification and air–sea exchange generates distinctive isotopic signatures that trace sources, sinks and the pathways of carbon through marine food webs and sedimentary archives. Marine primary producers—ranging from microalgae to coccolithophores—imprint vital effects on carbon isotope ratios that can be used to reconstruct past pCO2, ocean circulation patterns and biogeochemical pump efficiency. Advances in biogeochemical modelling and novel morphological proxies are now refining regional estimates of carbon export, offering critical insights into how climate change and ocean acidification influence the global carbon budget.

Research from Nature Portfolio

Recent work has produced a continuous record of sterane and phytane δ13C from marine sediments spanning the mid-Miocene to present, revealing a long-term decline in atmospheric pCO2 that parallels global cooling. Calculations of Earth system sensitivity based on these biomarkers suggest higher climate sensitivity than previously estimated, emphasising the importance of proxy calibration for near-future climate scenarios.

Seminal studies have charted a decrease in coccolithophore calcification since the Miocene, linking diminished cellular calcification to falling CO2 concentrations. Size-normalised coccolith thickness records demonstrate that past high-CO2 oceans promoted greater calcite production, with implications for the role of marine algae in modulating atmospheric CO2 over geological timescales.

Investigations into vital effects in coccolith calcite have established that the ratio of calcification to carbon fixation governs whether biogenic calcite is isotopically heavier or lighter than inorganic precipitates. This framework elucidates the cellular controls on isotopic fractionation and underpins emerging approaches to use coccoliths for CO2 paleobarometry.

Carbon Isotope Dynamics in Oceanic Ecosystems publication trend

The graph below shows the total number of articles in carbon isotope dynamics in oceanic ecosystems across all publications each year (not limited to Nature Index journals).

Technical terms

δ13C: The ratio of 13C to 12C in a sample relative to a standard, expressed in per mille (‰), used to trace sources and processes in the carbon cycle.

Biological pump: The mechanism by which CO2 is fixed by marine organisms and transported from the surface ocean to depth as sinking organic matter.

Carbon isotope fractionation: The preferential incorporation or removal of carbon isotopes during chemical or biological reactions, leading to distinctive isotopic signatures.

Coccolithophores: Microscopic marine algae that produce calcite platelets (coccoliths), serving as both major contributors to marine carbonate and proxies for past ocean conditions.

pCO2: The partial pressure of CO2, indicating its concentration in seawater or the atmosphere, and influencing biological uptake and isotopic fractionation.

References

1. Continuous sterane and phytane δ13C record reveals a substantial pCO2 decline since the mid-Miocene. Nature Communications (2024).
2. Decrease in coccolithophore calcification and CO2 since the middle Miocene. Nature Communications (2016).
3. The origin of carbon isotope vital effects in coccolith calcite. Nature Communications (2017).
4. A Circum‐Antarctic Plankton Isoscape: Carbon Export Potential Across the Summertime Southern Ocean. Global Biogeochemical Cycles (2024).
5. Carbon isotopes in the marine biogeochemistry model FESOM2.1-REcoM3. Geoscientific Model Development (2024).
6. Fossil coccolith morphological attributes as a new proxy for deep ocean carbonate chemistry. Biogeosciences (2023).