Crustal Aquifer Research Articles

Evaluating the Physics of Outcrop‐To‐Outcrop Flow With Hydrothermal Flow Models

AbstractCold and diffuse hydrothermal circulation on mid‐ocean ridge flanks impacts heat and fluid fluxes between the seafloor and the ocean. One mode of this circulation is given by outcrop‐to‐outcrop flow, where seawater circulates through a crustal aquifer that connects two or more recharging and discharging seamounts or basement highs that outcrop through the less permeable sediment cover. The physical mechanism driving this flow is a lateral pressure gradient that is sustained by contrasting the hydrological properties of the recharging and discharging outcrops. To investigate the physical controls of this pressure gradient, we performed two‐dimensional numerical simulations of coupled heat transfer and fluid flow. We have modified aquifer permeability, outcrop permeability and width, outcrop distance, and sediment thickness to assess their mutual effects on the lateral pressure differences. We have also investigated how different flow patterns, resulting from changes in these parameters, manifest themselves in seafloor observables such as flow rates, aquifer temperatures, and heat flow. Our models show that outcrop‐to‐outcrop flow generally occurs for aquifer permeabilities ≥10−14 m2, depending on the basal heat input. High aquifer permeabilities correspond to fast flow rates and low fluid temperatures, whereas the maximum lateral pressure differences arise for lower permeabilities. The permeability and the geometric shape of the outcrops determine the flow direction, while the aquifer temperature is also affected by the distance between the outcrops. Thicker sediments increase the lateral pressure difference and the flow rate. Our models thus provide constraints for predicting subseafloor hydrothermal ridge flank flow behavior from regional field data.

The influence of permeability anisotropy in the upper ocean crust on advective heat transport by a ridge-flank hydrothermal system

In this study, we highlight the importance of permeability anisotropy on the hydrogeological regime of a ridge-flank hydrothermal system. Our study site, North Pond, is a marine sediment pond on ∼8 Ma seafloor in the North Atlantic, and represents a low-temperature, end-member ridge-flank hydrothermal system. Previous simulations of North Pond elucidated long-standing hypotheses concerning hydrothermal fluid and heat transport in the upper volcanic crust but failed to fully explain observed patterns of seafloor heat flux in this area. Here we use variography, a geostatistical method, to quantify relations between seafloor heat-flux measurements, and coupled numerical simulations of fluid and heat flow to simulate the hydrogeologic regime. Directional variography shows that heat-flux observations are correlated along-strike of the regional crustal fabric. Three-dimensional simulations that include permeability anisotropy are able to replicate seafloor heat-flux patterns across North Pond. The simulations that result in the best match to thermal data incorporate permeability anisotropy in the horizontal plane. We find that the feedback between permeability anisotropy and the asymmetric geometry of North Pond combine to promote advective removal of heat and mass within the crustal aquifer. These findings suggest that permeability anisotropy in the oceanic crust may influence ridge-flank hydrothermal circulation more broadly.

Numerical Simulation of Cool Hydrothermal Processes in the Upper Volcanic Crust Beneath a Marine Sediment Pond: North Pond, North Atlantic Ocean

AbstractLow temperature hydrothermal systems hosted in the volcanic oceanic crust are responsible for ∼20% of Earth's global heat loss. Marine sediment ponds comprise an important type setting on young ridge flanks where hydrothermal circulation advectively extracts lithospheric heat, but the nature of coupled fluid‐heat transport in these settings remains poorly understood. Here we present coupled (fluid‐heat) numerical simulations of ocean crustal hydrogeology in and below North Pond, a sediment pond on ∼8 Ma seafloor of the North Atlantic Ocean. Two‐ and three‐dimensional simulations show that advective transport beneath North Pond is complex and time varying, with multiple spatial and temporal scales, consistent with seafloor and borehole observations. A unidirectional, single‐pass flow system is neither favored nor needed to match the spatial distribution of seafloor heat flux through North Pond sediments. When the permeability of the crustal aquifer is relatively high (10−10–10−9 m2), simulations can replicate much of the observed variability and suppression of seafloor heat flux and can explain basement overpressures and transient perturbations in pressure and temperature in the upper volcanic crust. Simulation results can also help explain heterogeneity in pore fluid chemistry and microbiology in the crust. Although driven by the same physical processes, the dynamics of hydrothermal circulation below North Pond are different from those seen on "discharge‐dominated" ridge flanks, where the permeability and exposed area of isolated basement outcrops control the extent of regional heat extraction.

Metagenomic Insights Into the Microbial Iron Cycle of Subseafloor Habitats

Microbial iron cycling influences the flux of major nutrients in the environment (e.g., through the adsorptive capacity of iron oxides) and includes biotically induced iron oxidation and reduction processes. The ecological extent of microbial iron cycling is not well understood, even with increased sequencing efforts, in part due to limitations in gene annotation pipelines and limitations in experimental studies linking phenotype to genotype. This is particularly true for the marine subseafloor, which remains undersampled, but represents the largest contiguous habitat on Earth. To address this limitation, we used FeGenie, a database and bioinformatics tool that identifies microbial iron cycling genes and enables the development of testable hypotheses on the biogeochemical cycling of iron. Herein, we survey the microbial iron cycle in diverse subseafloor habitats, including sediment-buried crustal aquifers, as well as surficial and deep sediments. We inferred the genetic potential for iron redox cycling in 32 of the 46 metagenomes included in our analysis, demonstrating the prevalence of these activities across underexplored subseafloor ecosystems. We show that while some processes (e.g., iron uptake and storage, siderophore transport potential, and iron gene regulation) are near-universal, others (e.g., iron reduction/oxidation, siderophore synthesis, and magnetosome formation) are dependent on local redox and nutrient status. Additionally, we detected niche-specific differences in strategies used for dissimilatory iron reduction, suggesting that geochemical constraints likely play an important role in dictating the dominant mechanisms for iron cycling. Overall, our survey advances the known distribution, magnitude, and potential ecological impact of microbe-mediated iron cycling and utilization in sub-benthic ecosystems.

Time-series transcriptomics from cold, oxic subseafloor crustal fluids reveals a motile, mixotrophic microbial community

The oceanic crustal aquifer is one of the largest habitable volumes on Earth, and it harbors a reservoir of microbial life that influences global-scale biogeochemical cycles. Here, we use time series metagenomic and metatranscriptomic data from a low-temperature, ridge flank environment representative of the majority of global hydrothermal fluid circulation in the ocean to reconstruct microbial metabolic potential, transcript abundance, and community dynamics. We also present metagenome-assembled genomes from recently collected fluids that are furthest removed from drilling disturbances. Our results suggest that the microbial community in the North Pond aquifer plays an important role in the oxidation of organic carbon within the crust. This community is motile and metabolically flexible, with the ability to use both autotrophic and organotrophic pathways, as well as function under low oxygen conditions by using alternative electron acceptors such as nitrate and thiosulfate. Anaerobic processes are most abundant in subseafloor horizons deepest in the aquifer, furthest from connectivity with the deep ocean, and there was little overlap in the active microbial populations between sampling horizons. This work highlights the heterogeneity of microbial life in the subseafloor aquifer and provides new insights into biogeochemical cycling in ocean crust.

Ecology of Subseafloor Crustal Biofilms.

The crustal subseafloor is the least explored and largest biome on Earth. Interrogating crustal life is difficult due to habitat inaccessibility, low-biomass and contamination challenges. Subseafloor observatories have facilitated the study of planktonic life in crustal aquifers, however, studies of life in crust-attached biofilms are rare. Here, we investigate biofilms grown on various minerals at different temperatures over 1–6 years at subseafloor observatories in the Eastern Pacific. To mitigate potential sequence contamination, we developed a new bioinformatics tool – TaxonSluice. We explore ecological factors driving community structure and potential function of biofilms by comparing our sequence data to previous amplicon and metagenomic surveys of this habitat. We reveal that biofilm community structure is driven by temperature rather than minerology, and that rare planktonic lineages colonize the crustal biofilms. Based on 16S rRNA gene overlap, we partition metagenome assembled genomes into planktonic and biofilm fractions and suggest that there are functional differences between these community types, emphasizing the need to separately examine each to accurately describe subseafloor microbe-rock-fluid processes. Lastly, we report that some rare lineages present in our warm and anoxic study site are also found in cold and oxic crustal fluids in the Mid-Atlantic Ridge, suggesting global crustal biogeography patterns.

The Thermal Effects of Plate‐Bending‐Related Thickening of the Oceanic Crustal Aquifer in the Nankai Trough and Japan Trench Subduction Zones

AbstractFor the Nankai Trough and Japan Trench subduction zones, we examine the combined thermal effects of lateral heat exchange by fluid circulation in an oceanic crustal aquifer and the thickening of that aquifer due to plate‐bending‐related faulting. Faults induced by the bending of a plate entering a subduction zone are hypothesized to increase the depth over which vigorous hydrothermal circulation can redistribute heat in the oceanic crust. Previous 1‐D (vertical) thermal models have examined how aquifer thickening can mine heat from deep in the crust seaward of the trench. Here we construct 2‐D thermal models that include aquifer thickening and lateral heat exchange in the aquifer. We vary the maximum aquifer thickness and the landward extent to which hydrothermal circulation persists within the subducted crust. For the Nankai margin, models most consistent with heat flux data require vigorous fluid circulation extending up to 150 km landward of the trench; aquifer thickening is permitted but not required for models to be consistent with the heat flux observations. Conversely, for the Japan Trench, preferred models include substantial aquifer thickening (maximum aquifer thickness of 1.8–5 km); vigorous circulation extending landward of the trench is permitted but is not required. For hot subduction zones, the thermal effects of aquifer thickening are modest relative to the large lateral advective heat redistribution. For cold subduction zones, where small lateral temperature gradients limit the amount of lateral heat redistribution, aquifer thickening can be the dominant process generating thermal anomalies.

Three-dimensional models of hydrothermal circulation through a seamount network on fast-spreading crust

We present results from three-dimensional, transient, fully coupled simulations of fluid and heat transport on a ridge flank in fast-spread ocean crust. The simulations quantify relationships between rates of fluid flow, the extent of advective heat extraction, the geometry of crustal aquifers and outcrops, and crustal hydrologic parameters, with the goal of simulating conditions similar to those seen on 18–24 M.y. old seafloor of the Cocos plate, offshore Costa Rica. Extensive surveys of this region documented a ∼14,500 km2 area of the seafloor with heat flux values that are 10–35% of those predicted from conductive cooling models, and identified basement outcrops that serve as pathways for hydrothermal circulation via recharge of bottom water and discharge of cool hydrothermal fluid. Simulations suggest that in order for rapid hydrothermal circulation to achieve observed seafloor heat flux values, upper crustal permeability is likely to be ∼10−10 to 10−9 m2, with more simulations matching observations at the upper end of this range. These permeabilities are at the upper end of values measured in boreholes elsewhere in the volcanic ocean crust, and higher than inferred from three-dimensional modeling of another ridge-flank field site where there is less fluid flow and lower advective power output. The simulations also show that, in a region with high crustal permeability and variable sized outcrops, hydrothermal outcrop-to-outcrop circulation is likely to constitute a small fraction of total fluid circulation, with most of fluid flow occurring locally through individual outcrops that both recharge and discharge hydrothermal fluid.

A dynamic microbial community with high functional redundancy inhabits the cold, oxic subseafloor aquifer

The rock-hosted subseafloor crustal aquifer harbors a reservoir of microbial life that may influence global marine biogeochemical cycles. Here we utilized metagenomic libraries of crustal fluid samples from North Pond, located on the flanks of the Mid-Atlantic Ridge, a site with cold, oxic subseafloor fluid circulation within the upper basement to query microbial diversity. Twenty-one samples were collected during a 2-year period to examine potential microbial metabolism and community dynamics. We observed minor changes in the geochemical signatures over the 2 years, yet the microbial community present in the crustal fluids underwent large shifts in the dominant taxonomic groups. An analysis of 195 metagenome-assembled genomes (MAGs) were generated from the data set and revealed a connection between litho- and autotrophic processes, linking carbon fixation to the oxidation of sulfide, sulfur, thiosulfate, hydrogen, and ferrous iron in members of the Proteobacteria, specifically the Alpha-, Gamma- and Zetaproteobacteria, the Epsilonbacteraeota and the Planctomycetes. Despite oxic conditions, analysis of the MAGs indicated that members of the microbial community were poised to exploit hypoxic or anoxic conditions through the use of microaerobic cytochromes, such as cbb3- and bd-type cytochromes, and alternative electron acceptors, like nitrate and sulfate. Temporal and spatial trends from the MAGs revealed a high degree of functional redundancy that did not correlate with the shifting microbial community membership, suggesting functional stability in mediating subseafloor biogeochemical cycles. Collectively, the repeated sampling at multiple sites, together with the successful binning of hundreds of genomes, provides an unprecedented data set for investigation of microbial communities in the cold, oxic crustal aquifer.

Nanocalorimetric Characterization of Microbial Activity in Deep Subsurface Oceanic Crustal Fluids.

Although fluids within the upper oceanic basaltic crust harbor a substantial fraction of the total prokaryotic cells on Earth, the energy needs of this microbial population are unknown. In this study, a nanocalorimeter (sensitivity down to 1.2 nW ml-1) was used to measure the enthalpy of microbially catalyzed reactions as a function of temperature in samples from two distinct crustal fluid aquifers. Microorganisms in unamended, warm (63°C) and geochemically altered anoxic fluids taken from 292 meters sub-basement (msb) near the Juan de Fuca Ridge produced 267.3 mJ of heat over the course of 97 h during a step-wise isothermal scan from 35.5 to 85.0°C. Most of this heat signal likely stems from the germination of thermophilic endospores (6.66 × 104 cells ml-1FLUID) and their subsequent metabolic activity at temperatures greater than 50°C. The average cellular energy consumption (5.68 pW cell-1) reveals the high metabolic potential of a dormant community transported by fluids circulating through the ocean crust. By contrast, samples taken from 293 msb from cooler (3.8°C), relatively unaltered oxic fluids, produced 12.8 mJ of heat over the course of 14 h as temperature ramped from 34.8 to 43.0°C. Corresponding cell-specific energy turnover rates (0.18 pW cell-1) were converted to oxygen uptake rates of 24.5 nmol O2 ml-1FLUID d-1, validating previous model predictions of microbial activity in this environment. Given that the investigated fluids are characteristic of expansive areas of the upper oceanic crust, the measured metabolic heat rates can be used to constrain boundaries of habitability and microbial activity in the oceanic crust.

A distinct and active bacterial community in cold oxygenated fluids circulating beneath the western flank of the Mid-Atlantic ridge.

The rock-hosted, oceanic crustal aquifer is one of the largest ecosystems on Earth, yet little is known about its indigenous microorganisms. Here we provide the first phylogenetic and functional description of an active microbial community residing in the cold oxic crustal aquifer. Using subseafloor observatories, we recovered crustal fluids and found that the geochemical composition is similar to bottom seawater, as are cell abundances. However, based on relative abundances and functional potential of key bacterial groups, the crustal fluid microbial community is heterogeneous and markedly distinct from seawater. Potential rates of autotrophy and heterotrophy in the crust exceeded those of seawater, especially at elevated temperatures (25 °C) and deeper in the crust. Together, these results reveal an active, distinct, and diverse bacterial community engaged in both heterotrophy and autotrophy in the oxygenated crustal aquifer, providing key insight into the role of microbial communities in the ubiquitous cold dark subseafloor biosphere.

Three‐dimensional modeling of outcrop‐to‐outcrop hydrothermal circulation on the eastern flank of the Juan de Fuca Ridge

AbstractWe present three‐dimensional simulations of coupled fluid and heat transport in the ocean crust, to explore patterns and controls on ridge‐flank hydrothermal circulation on the eastern flank of the Juan de Fuca Ridge. Field studies have shown that there is large‐scale fluid flow in the volcanic ocean crust in this region, including local convection and circulation between two basement outcrops separated by ~50 km. New simulations include an assessment of crustal permeability and aquifer thickness, outcrop permeability, the potential influence of multiple discharging outcrops, and a comparison between two‐dimensional (profile) and three‐dimensional representations of the natural system. Field observations that help to constrain new simulations include a modest range of flow rates between recharging and discharging outcrops, secondary convection adjacent to the recharging outcrop, crustal permeability determinations made in boreholes, and the lack of a regional seafloor heat flux anomaly as a consequence of advective heat loss from the crust. Three‐dimensional simulations are most consistent with field observations when models use a crustal permeability of 3 × 10−13 to 2 × 10−12 m2, and the crustal aquifer is ≤300 m thick, values consistent with borehole observations. We find fluid flow rates and crustal cooling efficiencies that are an order of magnitude greater in three‐dimensional simulations than in two‐dimensional simulations using equivalent properties. Simulations including discharge from an additional outcrop can also replicate field observations but tend to increase the overall rate of recharge and reduce the flow rate at the primary discharge site.

The role of sedimentation history and lithology on fluid flow and reactions in off-axis hydrothermal systems: A perspective from the Troodos ophiolite

Off-axis hydrothermal systems are thought to carry globally significant chemical fluxes but different ocean crust sections show widely differing extents of alteration making the quantification of these fluxes complex. With the aim of better understanding the origin of this diversity in alteration extent we have studied two sections of the lava pile in the Troodos ophiolite with distinct sedimentation history and volcanic architecture. The Akaki section is dominated by pillow lavas and the oldest sediments overlying the crust are ~20Myr younger than the ophiolite. Here there is a 300m thick zone at the top of the lavas that is enriched in CO2 and alkali elements, and has high 87Sr/86Sr and δ7Li. These features indicate extensive chemical exchange with seawater. In contrast to the Akaki area, the Onophrious section is dominated by sheet flows and the oldest sediments are of the same age as the ophiolite. Here the CO2 and alkali element enriched zone is much thinner (<100m), is less enriched in these elements (e.g. by a factor of three for CO2), and has lower 87Sr/86Sr and δ7Li. The O-isotopic compositions of calcites from these CO2- and alkali-enriched zones were precipitated from fluids with bottom water temperatures (~10°C). Maintaining such low temperatures to 300m depth in the crust in the Akaki area suggests that this was a region of recharge. Below these CO2- and alkali-enriched zones temperatures increase with depth such that calcite precipitation in the Onophrious area occurred at ~10°C higher temperature, at any given depth, than in the Akaki area. The increase in precipitation temperature with depth indicates poor thermal mixing within the crustal aquifer, likely due to laterally continuous sheet flows restricting the permeability. The chemical and thermal constraints suggest that both timing of the onset of sedimentation and volcanic architecture play important roles in controlling fluid and chemical fluxes. The same signal can be seen in drill core data from the modern ocean basins, with higher sedimentation rates leading to lower fluid fluxes and higher temperatures in the crustal aquifer.

Fluid seepage velocities through marine sediments constrained by a global compilation of interstitial water SO42−, Mg2+, and Ca2+ profiles

AbstractThe depth dependence of the SO42−, Mg2+, and Ca2+ contents of interstitial waters extracted from sediments in 140 Deep Sea Drilling Project/Ocean Drilling Program/Integrated Ocean Drilling Program drill holes on oceanic crust has been fit using a model of transport (advection and diffusion) and reaction. These drill holes come from a range of crustal ages, sediment types, sediment thicknesses, and sediment accumulation rates. The best fitting specific discharge through the sediment at these locations is estimated to be generally &lt;500 m Myr−1 (0.05 cm yr−1), where sediments are thicker than 100 m, although an order of magnitude faster seepage is estimated for some locations with sediment tens of meters thick. Assuming that the drill holes are globally representative, then seepage of fluids through marine sediments at specific discharges of a few hundred m Myr−1 is estimated to be only a few percent of the total ridge flank hydrothermal fluid flux. This is consistent with the previous suggestion that hydrothermal fluid exchange between the ocean and the crustal aquifer primarily occurs through basement outcrops. Chemical fluxes of SO42−, Mg2+, and Ca2+ across the sediment‐water interface globally (excluding continental margins) are estimated to be less than 10% of the riverine input to the ocean.

The origin of methanethiol in midocean ridge hydrothermal fluids.

Simple alkyl thiols such as methanethiol (CH3SH) are widely speculated to form in seafloor hot spring fluids. Putative CH3SH synthesis by abiotic (nonbiological) reduction of inorganic carbon (CO2 or CO) has been invoked as an initiation reaction for the emergence of protometabolism and microbial life in primordial hydrothermal settings. Thiols are also presumptive ligands for hydrothermal trace metals and potential fuels for associated microbial communities. In an effort to constrain sources and sinks of CH3SH in seafloor hydrothermal systems, we determined for the first time its abundance in diverse hydrothermal fluids emanating from ultramafic, mafic, and sediment-covered midocean ridge settings. Our data demonstrate that the distribution of CH3SH is inconsistent with metastable equilibrium with inorganic carbon, indicating that production by abiotic carbon reduction is more limited than previously proposed. CH3SH concentrations are uniformly low (∼10(-8) M) in high-temperature fluids (>200 °C) from all unsedimented systems and, in many cases, suggestive of metastable equilibrium with CH4 instead. Associated low-temperature fluids (<200 °C) formed by admixing of seawater, however, are invariably enriched in CH3SH (up to ∼10(-6) M) along with NH4(+) and low-molecular-weight hydrocarbons relative to high-temperature source fluids, resembling our observations from a sediment-hosted system. This strongly implicates thermogenic interactions between upwelling fluids and microbial biomass or associated dissolved organic matter during subsurface mixing in crustal aquifers. Widespread thermal degradation of subsurface organic matter may be an important source of organic production in unsedimented hydrothermal systems and may influence microbial metabolic strategies in cooler near-seafloor and plume habitats.

Phylogenetic diversity of microorganisms in subseafloor crustal fluids from Holes 1025C and 1026B along the Juan de Fuca Ridge flank.

To expand investigations into the phylogenetic diversity of microorganisms inhabiting the subseafloor biosphere, basalt-hosted crustal fluids were sampled from Circulation Obviation Retrofit Kits (CORKs) affixed to Holes 1025C and 1026B along the Juan de Fuca Ridge (JdFR) flank using a clean fluid pumping system. These boreholes penetrate the crustal aquifer of young ocean crust (1.24 and 3.51 million years old, respectively), but differ with respect to borehole depth and temperature at the sediment-basement interface (147 m and 39°C vs. 295 m and 64°C, respectively). Cloning and sequencing of PCR-amplified small subunit ribosomal RNA genes revealed that fluids retrieved from Hole 1025C were dominated by relatives of the genus Desulfobulbus of the Deltaproteobacteria (56% of clones) and Candidatus Desulforudis of the Firmicutes (17%). Fluids sampled from Hole 1026B also contained plausible deep subseafloor inhabitants amongst the most abundant clone lineages; however, both geochemical analysis and microbial community structure reveal the borehole to be compromised by bottom seawater intrusion. Regardless, this study provides independent support for previous observations seeking to identify phylogenetic groups of microorganisms common to the deep ocean crustal biosphere, and extends previous observations by identifying additional lineages that may be prevalent in this unique environment.

Global analysis of the effect of fluid flow on subduction zone temperatures

Knowledge of the controls on temperature distributions at subduction zones is critical for understanding a wide range of seismic, metamorphic, and magmatic processes. Here, we present the results of ∼220 thermal model simulations covering the majority of known subduction zone convergence rates, incoming plate ages, and slab dips. We quantify the thermal effects of fluid circulation in the subducting crust by comparing results with and without advective heat transfer in the oceanic crustal aquifer. We find that hydrothermal cooling of a subduction zone is maximized when the subducting slab is young, slowly converging, steeply dipping, and the crustal aquifer is ventilated near the trench. Incoming plate age is one of the primary controls on the effectiveness of advective heat transfer in the aquifer, and the greatest temperature effects occur with an incoming plate &lt;50 Ma. The thermal effects of fluid circulation decrease dramatically with increasing age of the incoming plate. Temperatures in the Cascadia, Nankai, southern Chile, Colombia/Ecuador, Mexico, and Solomon Islands subduction zones are likely strongly affected by fluid circulation; for these systems, only thermal models of Cascadia and Nankai have included fluid flow in subducting crust.

The role of outcrop‐to‐outcrop fluid flow in off‐axis oceanic hydrothermal systems under abyssal sedimentation conditions

It has been proposed that ridge flank hydrothermal circulation by outcrop‐to‐outcrop (lateral) flow may be the dominant mode of oceanic hydrothermal circulation globally. In this model, the upper igneous crust is an aquifer overlain by low permeability sediments, and aquifer‐ocean fluid exchange occurs through basement outcrops. Thermally induced pressure gradients drive fluid laterally from recharge outcrops to discharge outcrops. To test the global applicability of outcrop‐to‐outcrop flow, models of synthetic basement bathymetry representative of crust formed at different spreading rates, pelagic sediment supply and post depositional transport, and sediment hydraulic impedance are used to quantify the time‐varying distribution of sediment and basement outcrops globally. Results suggest that basement outcrops may be 40–50% closer together than previously estimated. The modeled sediment and outcrop results are coupled with a two‐dimensional model of outcrop‐to‐outcrop fluid flow and heat exchange in a vertically isothermal crustal aquifer to predict the spatial distributions of seafloor heat flow within the simulation region under this mode of hydrothermal circulation. It is found that both the time‐varying average and standard deviation of modeled seafloor heat flow required by outcrop‐to‐outcrop flow simultaneously fit the global heat flow data if the average aquifer permeability decreases from ∼10−9 m2 to ∼10−11 m2 over the duration of the global heat flow deficit (to ∼65 Myr). This permeability range is consistent with other estimates of upper crustal permeability on comparable spatial scales, and supports the proposition that outcrop‐to‐outcrop fluid flow may be the dominant mode of off‐axis hydrothermal circulation globally.

Thermal effects of hydrothermal circulation and seamount subduction: Temperatures in the Nankai Trough Seismogenic Zone Experiment transect, Japan

[1] We examine the thermal effects of seamount subduction. Seamount subduction may cause transient changes in oceanic crust hydrogeology and plate boundary fault position. Prior to subduction, seamounts provide high-permeability pathways between the basaltic crustal aquifer and overlying ocean that can focus fluid flow and efficiently cool the oceanic crust. As the seamount is subducted, the high-permeability pathway is closed, shutting down the advective transfer of heat. If significant fluid flow occurs, it would be restricted after seamount subduction and would result in a redistribution of heat warming the trench and cooling landward parts of the system. Additionally, subducting seamounts can influence the position of the plate boundary fault that has thermal consequences by locally controlling the proportions of incoming sediment that subduct and accrete. Shifting the decollement to the seafloor at the trench in the wake of seamount subduction causes limited cooling focused at the toe of the margin wedge. We apply these features of seamount subduction to a thermal model for the Nankai Trough Seismogenic Zone Experiment transect on the margin of Japan. Models with hydrothermal circulation provide an explanation for anomalously high surface heat flux observations near the trench. They yield temperatures of ∼100°C−295°C for the rupture area of the 1944 Tonankai earthquake. Temperatures in the region of episodic tremor and slip are estimated at ∼290°C–325°C, ∼70°C cooler than a model with no fluid circulation.

Advanced instrument system for real-time and time-series microbial geochemical sampling of the deep (basaltic) crustal biosphere

Integrated Ocean Drilling Program borehole CORK (Circulation Obviation Retrofit Kit) observatories provide long-term access to hydrothermal fluids circulating within the basaltic crust (basement), providing invaluable opportunities to study the deep biosphere. We describe the design and application parameters of the GeoMICROBE instrumented sled, an autonomous sensor and fluid sampling system. The GeoMICROBE system couples with CORK fluid delivery lines to draw large volumes of fluids from crustal aquifers to the seafloor. These fluids pass a series of in-line sensors and an in situ filtration and collection system. GeoMICROBE's major components include a primary valve manifold system, a positive displacement primary pump, sensors (e.g., fluid flow rate, temperature, dissolved O2, electrochemistry-voltammetry analyzer), a 48-port in situ filtration and fluid collection system, computerized controller, seven 24V–40A batteries and wet-mateable (ODI) communications with submersibles. This constantly evolving system has been successfully connected to IODP Hole 1301A on the eastern flank of the Juan de Fuca Ridge. Also described here is a mobile pumping system (MPS), which possesses many of the same components as the GeoMICROBE (e.g., pump, sensors, controller), but is directly powered and controlled in real time via submersible operations; the MPS has been employed repeatedly to collect pristine basement fluids for a variety of geochemical and microbial studies.