Radiocarbon dating is a technique used in diverse disciplines including environmental science, archaeology, and biomedicine. In the geosciences, the processes by which radiocarbon is produced and cycled through the oceans, atmosphere, and biosphere are broadly understood but there is significant variability in radiocarbon concentrations over space and time. In an article recently published in Reviews of Geophysics, Alves et al.  present the latest status of research on the marine radiocarbon reservoir. Here the authors answer some questions about marine radiocarbon and its variability.
What are the different reservoirs of carbon on Earth and how does carbon move between them?
The Earth’s carbon reservoirs are distributed within the lithosphere, hydrosphere, and biosphere. In fact, even tiny microbes are carbon reservoirs, but scientists tend to group small reservoirs into larger categories (e.g., ocean, atmosphere, biosphere), important at the global scale.
Each compartment in the global carbon cycle stores and recycles carbon, but the magnitude of the storage and the rate of exchange vary enormously between them. Carbon stored in the atmosphere, surface ocean, biomass and soils, for example, is cycled relatively quickly via processes such as photosynthesis, respiration, decomposition, and air-sea gas exchange.
Inventories such as the deep ocean, sediments and rocks, on the other hand, store huge amounts of carbon accumulated over thousands to millions of years, but the fluxes of carbon in these reservoirs are comparatively very slow and occur via processes such as sedimentation and volcanic eruptions.
Is there a difference between terrestrial and marine radiocarbon levels?
Yes. Radiocarbon (14C) is produced in the atmosphere; the transfer of these atoms into the ocean occurs at the air-sea interface, and is controlled by factors such as wind speed and temperature. In the deep ocean, 14C can reside for about 1,000 years before upwelling returns carbon to the surface. Thus, air-sea gas exchange paired with slow internal mixing in the oceans lead to a disequilibrium in radiocarbon activity between the atmosphere and the ocean, which is known as the Marine Reservoir Effect (MRE).
It is important to mention that these processes are not uniform over the global ocean and thus the disequilibrium is not only between ocean and atmosphere, but there are also differences in radiocarbon levels within the ocean. The transfer of radiocarbon into the ocean can be favoured in some regions (CO2 sinks) and hindered in others (CO2 sources). Moreover, carbon makes its way into the ocean by a variety of other processes such as continental runoff, in which case the 14C levels of the estuary reflect a mix between ocean waters and freshwater.
How does ocean radiocarbon concentration vary over space and time?
The ocean radiocarbon concentration responds to the factors controlling the MRE, such as the air-sea gas exchange and ocean dynamics. These phenomena, in turn, respond to climate factors such as temperature, wind speed and sea-ice cover. Thus, whenever there is a spatial or temporal variation in any of these parameters, the MRE varies accordingly. Depending on the phenomenon considered, these variations can happen at different time and spatial scales. We know, for instance, that the increase of sea-ice cover in the North Atlantic during the Younger Dryas climatic event hampered the air-sea gas exchange, and was one of the causes for an increase in the magnitude of the MRE.
How do scientists take this difference into account?
It depends on the research objectives. The MRE can be used as a proxy for its controlling factors so scientists can measure the MRE magnitude to understand such phenomena and disentangle processes of ocean circulation and its spatiotemporal changes, for example. Archaeologists and other scientists, who are more interested in the radiocarbon dating tool per se, quantify the MRE in their study region and apply the correction in the calibration of marine radiocarbon ages from that particular place.
What are some of the unresolved questions in this field where additional research, data or modelling is needed?
The question of the dominant control on the MRE in different localities is still a subject of debate. More empirical data is needed to reconstruct spatiotemporal changes in the MRE, allowing model calibration and correlation with a changing climate. For the calculation of MRE offsets from archaeological contexts, there is a lack of robust protocols to assure sample suitability. Moreover, the radiocarbon community has suggested the use of local calibration curves to better account for regional MRE offsets in the heterogeneous ocean reservoir, but a reasonable method for their construction has not yet been proposed.
—Eduardo Queiroz Alves, Oxford Radiocarbon Accelerator Unit, University of Oxford, UK; email: [email protected]