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Radiocarbon and the deep ocean cycling of dissolved organic matter

A major challenge in marine chemistry is to understand how carbon cycles in the deep ocean. The deep ocean is cold and dark, but it is also where most ocean carbon resides. Our understanding of deep ocean carbon cycling is very limited, and we need a better appreciation of how deep sea biogeochemical cycles impact global climate and habitability.

Marine dissolved organic carbon (DOC) is one of the planet’s largest reservoirs of carbon, comparable in size to carbon stored in terrestrial plants, soil organic matter, and the atmosphere. Global oceanographic surveys carried out over the last two decades have provided a detailed picture of both the reservoir size and water column distribution of DOC, a synopsis of which is provided in Figure 1.  As shown in the figure, DOC concentrations are high in surface waters, but decrease with depth. This distribution tells us that DOC is produced in the surface, removal at intermediate depths, and removed very slowly in the deep ocean. We can learn more about how carbon is cycled in the ocean by using radiocarbon dating techniques to determine the ‘age’ of DOC. Radiocarbon dating of marine DOC yields an average age of 2000-3000 years in the surface ocean and approximately 5000-6000 years in the deep ocean (Figure 2). This suggests that a large amount of DOC survives for a long time before it is finally removed from the ocean.

However, this broad-brush picture of carbon cycling in the ocean overlooks many important aspects of how carbon is produced, transported, and ultimately sequestered. The simple profile of total DOC radiocarbon shown in Figure 2 masks a much more interesting and complex cycling of carbon, especially in the deep ocean where several different fractions of DOC with different radiocarbon values may co-exist. DOC radiocarbon measurements are made by irradiating seawater with high intensity ultraviolet light. UV radiation converts organic carbon to carbon dioxide, which is recovered and measured for radiocarbon (figs 3, 4). Joint Program student Chris Follett recognized that much more information about carbon cycling could be extracted from radiocarbon measurements if they were done in a step-wise fashion. If we adjust the intensity of ultraviolet light, or the irradiation time, we can selectively oxidize targeted fractions of DOC. We are beginning to learn how to manipulate the UV oxidation technique and with this new approach in hand, we are beginning to see the outlines of a deep sea carbon cycle (Fig. 5) that is far richer, and more complex than revealed by DOC concentration maps and total radiocarbon measurements alone.

The global distribution of dissolved organic carbon.
Figure 1. The global distribution of dissolved organic carbon. The trajectories of major deep-water masses are given in black arrows, silica in white contours. Surface DOC values are high in the tropics and subtropics (60-80 µM), and lower at the poles (40-50 µM). As water drifts from the deep North Atlantic Ocean (lower left panel) to the North Pacific Ocean (right side panel), there is a slow decrease in DOC from about 42 µM to 35 µM.
The distribution of DOC radiocarbon with depth in the water column.
Figure 2. The distribution of DOC radiocarbon with depth in the water column. Surface water DOC is a mixture of old “background” DOC with a radiocarbon value and concentration equal to deepwater values, and “new” DOC with a radiocarbon value equal to the measured surface water value for carbon-14 dioxide.
To make carbon-14 measurements of DOC, we use ultraviolet radiation to photo-oxidize DOC to carbon dioxide.
Figure 3. To make carbon-14 measurements of DOC, we use ultraviolet radiation to photo-oxidize DOC to carbon dioxide. Seawater is placed in the all quartz vessel shown in the picture and exposed to a high intensity UV light (immediately to the left of the vessel). Once the DOC is completely oxidized, we recover the carbon by purging the sample with helium and trapping CO2 with liquid nitrogen.
Chiara Sanitelli, a visiting scientist from the Institute of Biophysics in Pisa, Italy, works on a vacuum line
Figure 4. Chiara Sanitelli, a visiting scientist from the Institute of Biophysics in Pisa, Italy, works on a vacuum line at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS; www.whoi.edu/nosams) with Joint Program student Chris Follett to process samples from the deep Mediterranean Sea for radiocarbon measurements.
outlines of a new deep sea carbon cycle
Figure 5. With insights gained through mathematical modeling of DOC radiocarbon, we are beginning to see the outlines of a new deep sea carbon cycle. We believe DOC does not drift passively through the deep ocean, but is part of a dynamic cycle of carbon that is continually added and removed by a number of diverse processes.