Different Fates for Dissolved Organic Carbon in Mid-Cayman Rise Hydrothermal Systems Revealed through Novel Characterizations

Abstract:

Oceanic dissolved organic matter is a major carbon reservoir composed primarily of persistent, refractory material. Circulation through hydrothermal systems has been demonstrated to profoundly alter this material, but the conditions that lead to its addition, removal, and alteration are poorly constrained. As a consequence, the contribution of hydrothermal systems to organic carbon in the deep ocean, and the biological and geochemical processes responsible for this input, remain highly uncertain. In January 2020, samples were collected from two hydrothermal vent fields at the Mid-Cayman Rise that have substantial differences in host rock type, depth, and vent fluid temperature, allowing carbon fluxes and chemical transformations to be investigated over a range of physical and chemical conditions. The mafic, neovolcanic Piccard field hosts high temperature black-smoker fluids while the Von Damm field is situated atop an oceanic core complex and is influenced by ultramafic rocks that may facilitate abiotic organic matter production. Dissolved organic matter in fluids ranging from 4 - 393°C has been characterized by multiple means, including Nuclear Magnetic Resonance (NMR) and Isotope Ratio Mass Spectrometry. At Von Damm, concentrations of dissolved organic carbon isolated by solid phase extraction decreased from background seawater values proportionally with increasing temperature, while becoming more enriched in ¹³C. At Piccard, the dissolved organic carbon in fluids of supercritical pathways had no distinguishable difference in δ¹³C from background seawater, nor did concentrations correlate with temperature. NMR analysis of these samples indicates that Piccard fluids do not contain the thousands of compounds seen in background seawater, but instead contain a high concentration of a few compounds with specific characteristics detectable by NMR. Organic acid analysis indicates that formate is the dominant aqueous organic species in Von Damm vent fluids, with concentrations exceeding 800 μM. Our results suggest that there are radically different reaction pathways for DOC production and consumption at the Mid-Cayman Rise, which may have significant influences on deepwater carbon cycles and implications for extraterrestrial abiotic carbon synthesis.