Characteristics and Evolution of sill-driven off-axis hydrothermalism in Guaymas Basin – the Ringvent site

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.

The Guaymas Basin Subseafloor Sedimentary Archaeome Reflects Complex Environmental Histories

“…Contrasting with nearby core ContP10, Core RNVP11 shows the biogeochemical signatures of seawater inmixing at Ringvent, such as seawater sulfate concentrations, and previous hydrothermal alteration, as evidenced by silica dissolution and re-precipitation ( Teske et al., 2019 ). Subsurface-derived porewater methane in high concentrations of 1–1.5 mM coexists with porewater sulfate near seawater levels; sulfide is largely absent and reaches 10–100 μM only below 3 m depth.…”

“…We surveyed archaeal distribution at six sites on the northwestern and southeastern off-axis regions of Guaymas Basin and on the Upper Sonora Margin ( Figure 1 , Table S1 ). These locations represent four different environmental settings: (1) Sediments on the Guaymas Basin flanking regions without hydrothermal or seep activity, represented by cores ContP03, ContP10, and ContP13; (2) the oxygen minimum zone (OMZ) on the upper Sonora Margin ( Calvert, 1964 ), represented by core OMZP12, (3) compacted, highly reducing seep sediments from a pressure ridge, running along the transform fault that is cutting across the base of the Sonora Margin ( Paull et al., 2007 ; Simoneit et al., 1990 ), represented by core SeepP06, and (4) the Ringvent site, characterized by off-axis hydrothermal circulation ( Teske et al., 2019 ), represented by core RNVP11 ( Figure 1 ). At each site, sediment piston cores ranging from 5 to 486 cm below the seafloor (cmbsf) were collected and geochemically characterized ( Figure 2 ).…”

“…At each site, sediment piston cores ranging from 5 to 486 cm below the seafloor (cmbsf) were collected and geochemically characterized ( Figure 2 ). The sediments are geologically young, ranging in age between ~0.05K and ~20K calendar years, as determined by 14 C dating ( Teske et al., 2019 ). The different cores show distinct geochemical characteristics.…”

“…Many of these off-axis sites resemble cold seeps, where methane advection is linked to pathways formed by deeply buried magmatic sills ( Geilert et al., 2018 ). If the underlying sill is sufficiently shallow and hot, the hydrothermal underpinnings of these off-axis sites becomes visible; the recently described Ringvent site provides an example ( Teske et al., 2019 ).…”

“…Sampling areas include background sediments from the Guaymas Basin flanking regions, Sonora Margin sediment within the oxygen minimum zone, reducing sediment with cold seep characteristics from the base of the Sonora Margin, and sediment from the off-axis Ringvent site where hydrothermal circulation and methane seepage is driven by a gradually cooling, buried shallow sill. We expand a previous limited sequencing survey of these sediments focused on just one of these sites ( Teske et al., 2019 ) by (1) extending the geochemical analyses, (2) increasing the sampling resolution used for molecular sequencing (from one or two samples per site to ~1-m intervals for all sites), (3) providing a wide breadth of comparative ecological analyses, and (4) discussing the potential implications of our results at a basin-wide scale.…”