Constraining Marine Refractory Dissolved Organic Carbon Cycling Using Carbon Isotopes
Dissolved organic carbon (DOC) is the largest exchangeable pool of organic carbon in the ocean, and is similar in size to atmospheric carbon. DOC is formed during surface ocean primary production by marine phytoplankton and can survive on average >6000 14C years in the deep ocean. This residence time greatly exceeds that of dissolved inorganic carbon (DIC), and indicates that a majority of DOC is refractory, or unreactive. Even though previous estimates of refractory DOC (RDOC) abundance is ~95% of total DOC, the mechanisms that facilitate its formation, cycling, and sinks are still poorly understood. A better understanding of these mechanisms is important because the ocean’s role in the global carbon cycle, and global physical climate, may link closely to the strength of the biological pump and DOC storage. This dissertation research characterizes the stable and radiocarbon isotopic compositions (13C and ∆14C) of solid-phase extracted DOC (SPE-DOC), which is representative of RDOC. First, the mass and isotopic composition of extraneous carbon (Cex) in the SPE method is analyzed. This study identifies that one SPE resin commonly used to extract DOC from seawater (Bond Elut PPL) elutes SPE-DOC with isotopic heterogeneity. Incomplete elution may exclude terrestrial-like organic matter that elutes later. An updated protocol including an extended elution is developed to ensure no fractionation of SPE-DOC occurs. This updated protocol is used to characterize SPE-DOC 13C and ∆14C values at the highest latitudinal resolution to date, using samples from three GO-SHIP Repeat Hydrography Cruises spanning the central and eastern Pacific Ocean and a portion of the western Indian Ocean. SPE-DOC 13C and ∆14C values are significantly lower than those for total DOC. Low 13C values relative to total DOC indicate preferential respiration of large biomolecules with high 13C values. An additional product of this heterotrophic respiration is carboxy-rich alicyclic matter, which is known to be extremely recalcitrant (C-rich and low in oxygen and hydrogen). These low SPE-DOC 13C and ∆14C values point to the microbial carbon pump (MCP) as a major driver of the marine carbon cycle through the removal of biomolecules with high 13C, and addition of refractory products that resist degradation. This increases DOC residence time and drives low ∆14C values. The large meridional span in this ∆14C dataset also allows for the first regional estimates of RDOC abundance. These estimates match the simple yet powerful “two-pool” model of DOC cycling proposed in 1987, in which the deep ocean is a well-mixed pool of old RDOC, and the surface is a mixture of RDOC from depth and labile DOC from recent photosynthesis. Although the importance of the MCP, and the two-pool model are not new ideas, sampling has been spatially and temporally sparse. This dissertation research provides a high-resolution isotopic perspective from under-sampled regions in the open ocean that supports these concepts.