Radiocarbon in the oceans – Eos

Radiocarbon dating is a technique used in various disciplines, including environmental science and archaeology. In the geosciences, the processes by which radiocarbon is produced and cycled in the oceans, atmosphere, and biosphere are widely understood, but there is significant variability in radiocarbon concentrations over space and time. In an article recently published in Geophysics Opinion, Alves et al. [2018] present the current state of research on the marine radiocarbon reservoir. The authors answer here some questions about marine radiocarbon and its variability.

What are the different carbon pools on Earth and how does carbon move between them?

The carbon pools of the Earth are distributed in the lithosphere, the hydrosphere and the biosphere. In fact, even tiny microbes are carbon sinks, but scientists tend to group small sinks into larger categories (eg, ocean, atmosphere, biosphere) that are globally important.

Each compartment of the global carbon cycle stores and recycles carbon, but the extent of storage and the rate of exchange vary greatly between them. Carbon stored in the atmosphere, ocean surfaces, biomass and soils, for example, is cycled relatively rapidly through 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 carbon fluxes in these reservoirs are comparatively very slow. and occur via processes such as sedimentation and volcanic eruption. eruptions.

Is there a difference between terrestrial and marine radiocarbon levels?

Yes. Radiocarbon (14C) is produced in the atmosphere; the transfer of these atoms to the ocean occurs at the air-sea interface and is controlled by factors such as wind speed and temperature. In the deep ocean, 14The carbon can reside for about 1,000 years before upwelling brings the carbon back to the surface. Thus, air-sea gas exchange coupled with slow internal mixing in the oceans leads to an imbalance in radiocarbon activity between the atmosphere and the ocean, known as the marine reservoir effect (EMR).

Some of the mechanisms impacting the marine radiocarbon reservoir effect in a coastal region. Credit: Alves et al., 2018, figure 4

It is important to mention that these processes are not uniform over the global ocean and therefore the imbalance is not only between the ocean and the atmosphere, but there are also differences in the levels of radiocarbon in the ocean. . Radiocarbon transfer to the ocean may be favored in some regions (CO2 wells) and hindered in others (CO2 sources). In addition, carbon enters the ocean through a variety of other processes such as continental runoff, in which case the 14C levels in the estuary reflect a mixing between ocean water and fresh water.

How does ocean radiocarbon concentration vary in space and time?

Ocean radiocarbon concentration responds to factors controlling MRE, such as air-sea gas exchange and ocean dynamics. These phenomena, in turn, react to climatic 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 occur at different temporal and spatial scales. We know, for example, that increased sea ice cover in the North Atlantic during the Younger Dryas climatic event impeded air-sea gas exchange and was one of the causes of the increase in the magnitude of the EMR.

How do scientists account for this difference?

It depends on the research objectives. The MRE can be used as a proxy for its controlling factors so that scientists can measure the magnitude of the MRE to understand these phenomena and unravel the processes of ocean circulation and its spatio-temporal changes, for example. Archaeologists and other scientists, who are more interested in the radiocarbon dating tool per se, quantify the ERM in their region of study and apply the correction in the calibration of marine radiocarbon ages from that location particular.

What are some of the unresolved questions in this area that require additional research, data, or modeling?

The issue of dominant control over MREs in different localities is still debated. More empirical data is needed to reconstruct spatio-temporal changes in the MRE, allowing model calibration and correlation with climate change. For calculating EMR offsets from archaeological contexts, there is a lack of robust protocols to ensure sample relevance. Additionally, the radiocarbon community has suggested the use of local calibration curves to better account for regional compensations of EMRs 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]

Correction, April 26, 2018: In an earlier version of this article, the editor incorrectly stated that radiocarbon is used in biomedicine for dating purposes; this has been removed.


Alves, EQ (2018), Radiocarbon in the oceans, Eos, 99, Posted April 17, 2018.

Text © 2018. The authors. CC BY-NC-ND 3.0
Unless otherwise stated, images are subject to copyright. Any reuse without the express permission of the copyright holder is prohibited.

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