Earth’s oceans absorb, store, and cycle vast amounts of carbon in the form of dissolved organic carbon (DOC). Marine DOC consists of a diverse array of molecules that move through the global carbon cycle at different rates—from days to millennia—by various biological and biogeochemical processes. However, the multitude of molecules in marine DOC is far from fully cataloged.
In a new study, Broek et al. present the first direct investigation of the molecules that make up a key subcategory of DOC: low–molecular weight DOC. Low–molecular weight DOC tends to be less suitable for biological consumption, so it is thought to accumulate and store carbon in the ocean for thousands of years.
To investigate these important but elusive molecules, the researchers applied a new technique that enabled them to separate low–molecular weight DOC from other forms of DOC. They examined the characteristics of low–molecular weight DOC in seawater samples collected from aboard research vessels in the north central Pacific and central North Atlantic from the surface to a depth of 2,500 meters.
Radiocarbon analysis confirmed that on average, low–molecular weight DOC is older than both high–molecular weight DOC and the entire DOC pool as a whole. The composition of molecules found in the low–molecular weight DOC was remarkably consistent throughout the ocean and was in line with existing hypotheses about the kinds of DOC molecules that might persist in the ocean for thousands of years.
The two-site investigation also enabled novel observations about the fate of DOC as it slowly circulates around Earth in the deep ocean. For instance, the authors found evidence that some low–molecular weight DOC is consumed in the deep ocean, despite its reputation for lingering.
The authors call for further investigation into the structures of low–molecular weight DOC molecules and differences in their reactivity. Elucidating the properties of these molecules and their role in carbon storage in the deep ocean could improve understanding of global climate change. (Global Biogeochemical Cycles, https://doi.org/10.1029/2020GB006547, 2020)
—Sarah Stanley, Science Writer