Oceanic Dissolved Organic Carbon: The World Tour

Abstract:

At 660 PgC, oceanic dissolved organic matter is one of Earth’s major, bio-reactive carbon reservoirs. Three decades ago, the ocean science community turned its attention to the pool because our understanding of it as a sink/source of carbon was wholly inadequate. The central question emerging then and still pursued today: “What is the role of the pool in the ocean carbon cycle?” Global scale observations, emerging over the intervening years within the GO-SHIP and predecessor programs, have illuminated the spatial distributions and thereby large-scale dynamics of the pool. In this presentation, we take a visual tour of the global ocean, investigating temporal and spatial variabilities of dissolved organic carbon (DOC) in both the upper and deep systems. From these, we infer the cycling of carbon through this very dilute yet very large reservoir. We consider the net production of DOC in the upper ocean, with the goal of making its seasonal accumulation predictable; we then consider the slow removal of DOC upon export via overturning circulation. Observations will show exported biogenic particles contributing DOC to the bathypelagic ocean, in turn supporting the deep microbes. Even with these advances, the central question remains far from answered. The observably dynamic portion of the oceanic DOC pool, at only 20 PgC, is not the fraction with the greatest mass. The apparently long-lived and less reactive balance of the DOC pool holds 640 PgC. We remain uncertain as to the ‘refractory’ nature of that carbon, with its bulk radiocarbon ages of several thousands of years. Just how dynamic is that ‘old’ carbon?