Abstract
The global ocean contains a massive reservoir (662±32 Pg C) of dissolved organic carbon (DOC), and its dynamics, particularly in the deepest zones, are only slowly being understood. DOC in the deep ocean is ubiquitously low in concentration (~35 to 48 μmol kg-1) and aged (4000 to 6000 years), persisting for multiple meridional overturning circulation cycles. Deep waters relatively enriched in DOC form in the North Atlantic. They migrate to the Southern Ocean (SO) to mix with waters from Antarctic shelves and the deep Pacific and Indian Oceans, in turn forming the voluminous circumpolar waters. The latter ultimately feed back into the Atlantic, Pacific, and Indian basins as bottom and intermediate waters. The goal of this dissertation is to assess evidence for local versus remote processes in influencing the distribution of DOC in the deep ocean. We use both large-scale datasets and isotopic and molecular composition data to understand the dynamics of the DOC pool on a global scale, though with an emphasis on the southern hemisphere. Chapter 2 focuses on DOC in Antarctic shelf systems and Antarctic Bottom Water (AABW) formation; Chapter 3 looks at the mostly conservative behavior of DOC in the deep SO; Chapter 4 considers the radiocarbon content of DOC in the South Indian Ocean (SIO) to assess the age of DOC; and Chapter 5 looks at the radiocarbon and molecular composition of DOC in the far North Pacific. In Chapter 6, findings are summarized. One potential local source of DOC to the deep SO is export from Antarctic shelf systems. Antarctica’s continental shelves generate the densest waters in the world ocean and are responsible for the formation of AABW. AABW has the potential to sequester carbon in the deep ocean for millennia. DOC enrichment in dense shelf waters (DSW) in the Ross Sea was quantified and the potential for DOC to be sequestered into AABW was assessed. Ross Sea DSW was enriched in DOC by ~7 μmol kg-1 relative to the incoming source waters (initial conditions), primarily due to deep vertical mixing of DOC-enriched surface waters. The total DOC excess in DSW suggests that 4.0±0.6 Tg DOC y-1 is exported off the shelf. However, this exported fraction does not appear to persist in newly formed AABW and is likely remineralized, sequestering this carbon as TCO2 in the deep ocean. A test of the conservative behavior of DOC in the Southern Ocean was assessed by considering its transport within deep water masses (North Atlantic Deep Water (NADW), Indian Deep Water (IDW), and Pacific Deep Water (PDW)). Multiple approaches (multiple linear regression, mass transport, and mass balance calculations) were used with data from CLIVAR Repeat Hydrography sections to evaluate the system. DOC concentrations in the deep SO and CDW largely reflect the linear mixing of those several deep waters entering the system from the north. Mass balance suggests too that the relatively depleted DOC radiocarbon content in the deep SO is a conserved property, but limited radiocarbon profiles in the SIO prevent a full analysis of the conserved behavior of 14C age of DOC. Consequently, to further test for the conservative behavior of the 14C age of DOC in the SO, the 14C age of DOC in the SIO was assessed; these data complete a survey of ages in the major deep water masses entrained into the SO. Four profiles of the 14C content of DOC spanning the SIO were collected, ranging from the Polar Front (56ºS) to the subtropics (29ºS). Surface waters had bulk radiocarbon ages of ~4400 years at the Polar Front and ~2000 14C years in the subtropics. At depth, mean radiocarbon age of DOC in CDW was 5302±160 14C years, while that in IDW was significantly older at 5557±81 14C years. A high-salinity remnant of NADW intruding into the deep SIO had a distinctly younger radiocarbon age for DOC (~5100 14C years). Multiple linear regression of 14C ages was used to assess the transport of DOC in the SIO. These results additionally confirm the conservative behavior of DOC in the deep Southern Ocean. To assess how the conserved behavior of DOC can be related to the molecular composition of dissolved organic matter (DOM), the 14C age of DOC was coupled with DOM composition in PDW in the far deep North Pacific (FNP), where the ocean’s oldest water masses are located. DOM in this region has an old radiocarbon age (~6400 years old) and an overall molecular composition comprised of intrinsically stable compounds with low reactivities. By correlating radiocarbon age with each molecular formulae in solid-phase extracted DOM, the majority of DOM in PDW was found to have low reactivity and slow removal rates, especially in comparison with the DOM composition from the deep North Atlantic. Overall, this dissertation uses large-scale biogeochemical distributions as well as isotopic and molecular composition techniques to show that DOC cycling in the deep SO and SIO are due to mixing. As Antarctic shelf systems do not export DOC to the deep SO, instead DOC distributions and 14C age of DOC in the SO are largely controlled by mixing of NADW, IDW, and PDW. In addition, the SIO exhibits 14C ages of DOC consistent with water mass transports into that basin. Finally, the degraded molecular characteristic and old age of PDW in the FNP suggests that DOM in that region is low in reactivity and has not been renewed for long timescales.