Person: Ryan J. Sibert

Abstract

The Guaymas Basin spreading center, at 2000 m depth in the Gulf of California, is overlain by a thick sedimentary cover. Across the basin, localized temperature anomalies, with active methane venting and seep fauna exist in response to magma emplacement into sediments. These sites evolve over thousands of years as magma freezes into doleritic sills and the system cools. Although several cool sites resembling cold seeps have been characterized, the hydrothermally active stage of an off-axis site was lacking good examples. Here, we present a multidisciplinary characterization of Ringvent, an ~1 km wide circular mound where hydrothermal activity persists ~28 km northwest of the spreading center. Ringvent provides a new type of intermediate-stage hydrothermal system where off-axis hydrothermal activity has attenuated since its formation, but remains evident in thermal anomalies, hydrothermal biota coexisting with seep fauna, and porewater biogeochemical signatures indicative of hydrothermal circulation. Due to their broad potential distribution, small size and limited life span, such sites are hard to find and characterize, but they provide critical missing links to understand the complex evolution of hydrothermal systems.

Light hydrocarbon gas mixtures are commonly found in organic-rich marine sediments. Methane (C1) is typically the dominant constituent in these mixtures, but ethane (C2) and propane (C3) are nearly always present in trace amounts. C1 dynamics are typically associated with either thermal cracking of deeply buried organic matter or the metabolic end-product of organic matter degradation. Ethane and propane production had typically been associated with thermocatalytic processes in deeply buried sediments, but limited studies suggested C2/C3 production in biogenic C1 gas mixtures was likely attributable to the activity of methanogenic archaea. However, very few of these studies looked at C1/C2 production in deep-sea sediments, and quantification of rates had either not been attempted, or were absent from the literature. We attempted to use organic-rich, cold seep sediments from the Green Canyon area of the Gulf of Mexico (GC600) to determine C2/C3 dynamics in the first ten meters of sediment (i.e. 0 – 10 m). We found C2/C3 production in near surface cold-seep sediments to be indistinguishable from the background degassing signatures of clay minerals. Surface sediments (i.e. < 4 m) are hypothesized to be dominated by communities of organisms that oxidize C2/C3 compounds, rather than communities that produce them. Experiments determining the controls and magnitude of C2/C3 oxidation in surface sediments in cold-seep environments are ongoing. We hypothesized that C2/C3 production likely occurs deeper in the sediment column (i.e. > 4 m), based primarily on ethane and propane profiles of similar environments. Such material proved difficult to acquire; efforts are ongoing to obtain deep piston cores (i.e. >10 m) for environmental profiling and experimental manipulation in the deeper sediment layers where C2/C3 production likely occurs.