Petroleum-derived dissolved organic matter from natural seepage in deep sea environments

Abstract

Natural petroleum seepage discharges approximately 600,000 tons of oil every year into the marine environment. A fraction of the released petroleum is water-soluble and becomes part of oceanic dissolved organic matter (DOM), one of the largest and most complex pools of organic matter on Earth’s surface. Despite the constant discharge of petroleum by natural seepage, the environmental implications and persistence of petroleum-derived DOM are vastly unknown. This thesis investigates molecular transformations of DOM in natural petroleum seepages and assesses the potential release of petroleum-compounds to oceanic deep-sea DOM. In a first step, the release of oil-derived DOM from natural deep sea asphalt seeps was studied using controlled laboratory incubation experiments. Fresh asphalt samples collected at the Chapopote asphalt volcano in the Southern Gulf of Mexico were incubated aerobically in artificial seawater over four weeks. The compositional changes in the water-soluble fraction of asphalt-derived DOM were determined with ultrahigh-resolution mass spectrometry (Fourier-transform ion cyclotron resonance mass spectrometry, FT-ICR-MS) and by excitation-emission matrix spectroscopy to characterize fluorescent DOM (FDOM) applying parallel factor (PARAFAC) analysis. Result showed that highly reduced aliphatic asphalt-derived DOM was readily biodegraded, while aromatic and sulfur-containing DOM (DOS) appeared to be less bioavailable and accumulated in the aqueous phase. This indicates that natural asphalt and potentially other petroleum seepages can be marine sources of recalcitrant dissolved black carbon (DBC) which has apparent radiocarbon ages older than 20,000 years in the deep sea. In order to evaluate this laboratory finding in the natural environment, petroleum-derived DOM was investigated in the Guaymas Basin. The Guaymas Basin in the Gulf of California is a young rift system where hot basaltic sill intrusions into organic-rich sediments lead to the generation of large amounts of complex petroleum compounds. The effect of hydrothermal heating and the resulting presence of petroleum compounds on the porewater DOM composition were investigated. Sediment samples were retrieved from sites with in situ temperatures ranging from 4 to >106 °C that exhibited a strong petroleum smell and partially contained oil droplets. A strong correlation of sediment temperature to both composition and quantity of porewater DOM was observed, driven by enhanced microbial transformation of organic matter at temperatures below ~60 °C and increasingly sulfurized DOM at high-temperature sites. DOM associated with hydrothermal heating had elevated contributions of highly unsaturated, reduced, sulfur-containing DOM and petroleum-associated PARAFAC components. A considerable DOM fraction of hydrothermal origin was present both in overlying bottom waters and in recalcitrant deep-sea DOM, suggesting hydrothermal sediments as a source of recalcitrant DOM to the water column. To assess elemental fluxes in DOM of hydrothermal porewaters, composition and quantities of DOM from sedimentary porewaters and from the hot-water soluble fraction by Soxhlet extraction of the same sediments were evaluated. Results showed highly elevated concentrations of DBC and potentially recalcitrant DOS in porewater, suggesting the release of both DOM fractions from hydrothermal sediments. DBC in porewaters and hot-water extracts originated from two distinct sources: hydrothermal petroleum and re-dissolved presumably terrestrial-derived, pre-aged DBC. This study further assessed quantities of DBC and dissolved organic nitrogen, sulfur and phosphorus that could be discharged into the water column in case of basin-wide hydrothermal heating. Results indicate that hydrothermal alteration and subsequent petroleum impregnation of sediments can be sources of recalcitrant DOS and DBC to deep sea environments. This thesis provides important novel information about the transformation and release of water-soluble petroleum compounds from natural deep-sea seepage into the marine environment. By introducing radiocarbon-depleted DOM to the marine environment, natural petroleum seepages and hydrothermal alteration of sediments with subsequent petroleum impregnation may be an explanation for observed old radiocarbon ages of recalcitrant DOM in the deep ocean.