Title: Constraining marine refractory dissolved organic carbon cycling using carbon isotopes
Abstract: Dissolved organic carbon (DOC) is the largest exchangeable reservoir of organic carbon in the ocean, formed primarily through phytoplankton photosynthesis at the surface. Radiocarbon measurements show that deep ocean DOC can survive on average >6000 14 C years. This residence time greatly exceeds that of dissolved inorganic carbon (DIC), and indicates that a majority of DOC is refractory, or unreactive. The mechanisms that allow DOC to accumulate such high residence times remain under investigation. These mechanisms are important because the ocean’s role in the global carbon cycle may link closely to the strength of the biological pump and DOC storage.
Radiocarbon and stable carbon isotope analyses (∆ 14 C and 13 C) are extremely powerful tools to trace biogeochemical cycles and estimate residence times in different carbon reservoirs. Solid-phase extraction is a commonly used technique to isolate DOC (SPE-DOC) from the ocean for subsequent chemical or isotopic characterization using hydrophobic resins. This process typically isolates the low-molecular weight fraction of DOC that is carbon-rich with low ∆ 14 C, and is used to represent refractory DOC in the ocean.
First, this dissertation provides a rigorous assessment of how isotopic measurements ( 13 C and ∆ 14 C) are influenced by the SPE process. The mass and isotopic composition of the extraneous carbon “blank” is characterized, and an updated method is used to ensure complete elution of SPE-DOC. Next, this updated method is used to isolate SPE-DOC from three GO- SHIP Repeat Hydrography Cruises spanning the Pacific Ocean and a portion of the western Indian Ocean. SPE-DOC 13 C and ∆ 14 C values are significantly lower than those for total DOC. This novel isotopic dataset points toward the microbial carbon pump (MCP) as a major driver of the marine carbon cycle through the removal of biomolecules with high 13 C, and addition of
refractory products, driving low ∆ 14 C values (high ages/residence times).
The large meridional span in this SPE-DOC ∆ 14 C dataset also allows me to make the first regional estimates of refractory DOC relative abundance. These estimates support the simple yet powerful “two-pool” model of DOC cycling proposed in 1987. Although the importance of the MCP, and the two-pool model are not new ideas; historically, sampling has been spatially and temporally sparse. This dissertation research is novel due to the high-resolution isotopic perspective from the open ocean that provides rigorous support for these concepts.