Aaron Mau

Date of Award

Fall 2021

Document Type

Open Access Thesis


Marine Science

First Advisor

Susan Lang


Marine dissolved organic carbon (DOC) is a carbon reservoir comparable in size to all atmospheric CO2 and is composed primarily of refractory material which can be thousands of years old. Circulation through hydrothermal systems has been demonstrated to profoundly alter this material, but the mechanisms that lead to addition, removal, and alteration of DOC are poorly understood. Consequently, the contribution of hydrothermal systems to organic carbon cycling in the deep ocean – and the biological and geochemical processes exhibited at different venting environments – 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. The Von Damm field is situated atop an oceanic core complex and is influenced by ultramafic rocks, whereas the mafic, neovolcanic Piccard field exhibits high temperature black-smoker fluids. Our goal was to determine the different fates of DOC in fluids from these fields and across a wide temperature range of 4 – 393 °C. DOC was characterized by multiple means including isotopically (13C, 14C), by Nuclear Magnetic Resonance (NMR), by Gas Chromatography Mass Spectrometry (GC-MS), and by determining the concentrations of specific compounds of interest such as the organic acid, formate. Our data indicate that at Von Damm, concentrations of DOC isolated by solid phase extraction (SPE-DOC) are inversely proportional to temperature, decreasing from background seawater values while becoming more depleted in 13C. Organic acid analysis indicates that formate is the dominant aqueous organic species in Von Damm vent fluids, with concentrations exceeding 800 μM. Dissolved inorganic carbon (DIC) meanwhile, becomes more enriched in 13C as formate concentrations increase. These data can be used to determine the isotopic fractionation factor between DIC and formate at Von Damm. At Piccard, most SPE-DOC samples have δ 13C values similar to background seawater and concentrations do not correlate with temperature. NMR analysis of these samples indicates that the hottest Piccard fluids (393 °C) contain a high concentration of a few compounds, unlike background seawater which is composed of thousands of compounds.

The synthesis of these multiple complimentary geochemical data with comparisons to other vent fields help us identify changes in DOC at different hydrothermal systems and the mechanisms at play in the deep carbon cycle. Particularly, that refractory or non-bioavailable DOC is lost during hydrothermal circulation at Von Damm, and labile DOC is being produced from DIC at Von Damm.

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