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The chemical composition of formation waters within the upper basaltic crust were calculated or measured at 24 sites on the northwest portion of the Juan de Fuca (JDF) Plate using data from sediment pore waters, scientific boreholes, and seafloor springs. Formation waters differ in composition across this ridge‐flank region because of variations in water‐rock reactions and residence times, exchange rates with overlying sediment pore waters, and microbial processes along flow paths. We interpret spatial variations in the solute composition of formation waters to resolve areas that are geochemically distinct or similar, lateral trends that result from water transport, areas where water‐rock reactions in the deeper crust are apparent, and sites of seawater recharge and formation water discharge. We provide evidence for large‐scale lateral flow associated with two (mostly) buried basement ridges on ∼1.4 and ∼3.5 Ma seafloor, which are subparallel to the JDF spreading axis to the west. Between these two ridges, where the seafloor and the sediment‐basement interface are relatively flat, formation waters have undergone extensive exchange with overlying sediment pore waters, consistent with a long residence time. Basaltic outcrops provide sites of seawater recharge and hydrothermal discharge, sometimes through the same feature, highlighting the heterogeneous nature of hydrogeologic conditions and processes. This work provides a blueprint for future plate‐scale studies to assess, for example, geologic controls of crustal age, spreading rate, and sedimentation on subsurface hydrologic patterns. Plain Language Summary Seawater flows through the upper volcanic crust like groundwater in aquifers on land. As seawater moves through basaltic rocks it becomes altered from reaction with crustal rocks and microbes and from mixing with other fluids. The magnitude of this flow is significant with discharge from the oceanic crust equivalent to the discharge of Earth's rivers to the ocean. This paper combines geochemical data from crustal formation waters at 24 locations across a vast area in the northeastern Pacific Ocean offshore of North America. These data characterize hydrologic processes including: subseafloor areas that are well connected or isolated from each other, places where crustal formation waters appear to travel tens of kilometers through the seafloor, and other areas where sites separated by just a few kilometers are poorly connected. We also document sites where basaltic outcrops allow cool seawater to recharge into the ocean crust, and other outcrops where reacted crustal waters discharge. By combining data collected over decades at multiple sites across a vast region, we developed a nuanced understanding of subsurface plate‐scale transport of crustal formation waters, improving our understand of how the oceanic crust evolves and the impact it has on the composition of seawater. Key Points Systematic differences in the chemical composition of ridge‐flank, crustal waters define patterns of solute transport within a 104 km2 area Some ridge‐flank regions are hydrogeologically isolated, and other areas are connected across distances of kilometers to tens of kilometers Hydrothermal discharge and recharge occur mainly though volcanic rock outcrops, with some outcrops both recharging and discharging

Marine dissolved organic carbon (DOC) is one of the largest active reservoirs of reduced carbon on Earth. In the deep ocean, DOC has been described as biologically recalcitrant and has a radiocarbon age of 4,000 to 6,000 years, which far exceeds the timescale of ocean overturning. However, abiotic removal mechanisms cannot account for the full magnitude of deep-ocean DOC loss. Deep-ocean water circulates at low temperatures through volcanic crust on ridge flanks, but little is known about the associated biogeochemical processes and carbon cycling. Here we present analyses of DOC in fluids from two borehole observatories installed in crustal rocks west of the Mid-Atlantic Ridge, and show that deep-ocean DOC is removed from these cool circulating fluids. The removal mechanism is isotopically selective and causes a shift in specific features of molecular composition, consistent with microbe-mediated oxidation. We suggest organic molecules with an average radiocarbon age of 3,200 years are bioavailable to crustal microbes, and that this removal mechanism may account for at least 5% of the global loss of DOC in the deep ocean. Cool crustal circulation probably contributes to maintaining the deep ocean as a reservoir of ‘aged’ and refractory DOC by discharging the surviving organic carbon constituents that are molecularly degraded and depleted in 14C and 13C into the deep ocean.

We explore the hydrogeologic and physical conditions necessary to sustain hydrothermal activity within the shallow subseafloors of ocean worlds, exploring wide ranges of multiple parameters using a Monte Carlo framework. We apply multiple analytical calculations to represent coupled fluid-thermal flow systems, as have been observed on Earth, using idealized representations that link lateral transport below a conductive boundary layer between sites of hydrothermal inflow (recharge) and outflow (discharge). These analytical calculations replicate outcomes and trends in results generated from more complex numerical simulations once we account for excess driving pressure that is consumed by secondary (local) convection during transport within the subseafloor. We investigate low heat flux scenarios that are expected to be limiting cases for sustaining hydrothermal flows (≤10 mW m −2 ) for which there is modest heating from radiometric decay and/or tidal dissipation within the interior of an ocean world. We explore limiting conditions appropriate for Europa’s deep subseafloor, and a subset of sustainable hydrothermal siphon calculations are identified that are especially efficient for mining interior heat. We identify parameters that can sustain a hydrothermal siphon for reaction temperatures of 80°C–120°C and water/rock mass ratios of ≤100 Gy –1 . These conditions should result in discharging fluids that are altered relative to inflowing water and a silicate interior that is more likely to retain the potential for geochemical reactions over geologic time. These conditions are of particular interest because they could help create habitable conditions below or at the seafloor of an ocean world and can provide a foundation for linked reaction modeling.

In many areas of the world, groundwater resources are increasingly stressed, and unsustainable use has become common. Where existing mechanisms for governing groundwater are ineffective or nonexistent, new ones need to be developed. Local level groundwater governance provides an intriguing alternative to top-down models, with the promise of enabling management to better match the diversity of physical and social conditions in groundwater basins. One such example is emerging in California, USA, where new state law requires new local agencies to self-organize and act to achieve sustainable groundwater management. In this article, we draw on insights from research on common pool resource management and natural resources governance to develop guidelines for institutional design for local groundwater governance, grounded in California’s developing experience. We offer nine criteria that can be used as principles or standards in the evaluation of institutional design for local level groundwater governance: scale, human capacity, funding, authority, independence, representation, participation, accountability, and transparency. We assert that local governance holds promise as an alternative to centralized governance in some settings but that its success will depend heavily on the details of its implementation. Further, for local implementation to achieve its promise, there remain important complementary roles for centralized governance. California’s developing experience with local level groundwater management in dozens of basins across the state provides a unique opportunity to test and assess the importance and influence of these criteria.

Most seafloor hydrothermal circulation occurs far from the magmatic influence of mid-ocean ridges, driving large flows of water, heat and solutes through volcanic rock outcrops on ridge flanks. Here we create three-dimensional simulations of ridge–flank hydrothermal circulation, flowing between and through seamounts, to determine what controls hydrogeological sustainability, flow rate and preferred flow direction in these systems. We find that sustaining flow between outcrops that penetrate less-permeable sediment depends on a contrast in transmittance (the product of outcrop permeability and the area of outcrop exposure) between recharging and discharging sites, with discharge favoured through less-transmissive outcrops. Many simulations include local discharge through outcrops at the recharge end of an outcrop-to-outcrop system. Both of these characteristics are observed in the field. In addition, smaller discharging outcrops sustain higher flow rates than larger outcrops, which may help to explain how so much lithospheric heat is extracted globally by this process. Much hydrothermal circulation occurs away from the mid-ocean ridges and out on ridge flanks, affecting lithospheric heat deficit, solute fluxes, and influencing the biosphere. Here, the authors use 3D simulations to look at what controls the circulation and flow rate between and through seamounts.

Oceanic crust comprises the largest hydrogeologic reservoir on Earth, containing fluids in thermodynamic disequilibrium with the basaltic crust. Little is known about microbial ecosystems that inhabit this vast realm and exploit chemically favorable conditions for metabolic activities. Crustal samples recovered from ocean drilling operations are often compromised for microbiological assays, hampering efforts to resolve the extent and functioning of a subsurface biosphere. We report results from the first in situ experimental observatory systems that have been used to study subseafloor life. Experiments deployed for 4 years in young (3.5 Ma) basaltic crust on the eastern flank of the Juan de Fuca Ridge record a dynamic, post-drilling response of crustal microbial ecosystems to changing physical and chemical conditions. Twisted stalks exhibiting a biogenic iron oxyhydroxide signature coated the surface of mineral substrates in the observatories; these are biosignatures indicating colonization by iron oxidizing bacteria during an initial phase of cool, oxic, iron-rich conditions following observatory installation. Following thermal and chemical recovery to warmer, reducing conditions, the in situ microbial structure in the observatory shifted, becoming representative of natural conditions in regional crustal fluids. Firmicutes, metabolic potential of which is unknown but may involve N or S cycling, dominated the post-rebound bacterial community. The archaeal community exhibited an extremely low diversity. Our experiment documented in situ conditions within a natural hydrological system that can pervade over millennia, exemplifying the power of observatory experiments for exploring the subsurface basaltic biosphere, the largest but most poorly understood biotope on Earth.

We quantified the distribution of hillslope runoff under different climate and land-use conditions in a coastal, mixed land-use basin, the Pajaro Valley Drainage Basin (PVDB), California, USA, in order to evaluate opportunities to improve groundwater supply. We developed dry, normal, and wet climate scenarios using high-resolution historic data and compared contemporary land use to pre-development land use under the different climate scenarios. Relative to pre-development conditions, urban and agricultural development resulted in more than twice as much simulated runoff generation, greater spatial variability in runoff, and less water available for recharge; these differences were most pronounced during the dry climate scenario. Runoff results were considered in terms of potential to support distributed stormwater collection linked to managed aquifer recharge (DSC-MAR), which routes excess hillslope runoff to sites where it can infiltrate and enhance groundwater recharge. In the PVDB, 10% of the annual groundwater deficit could be addressed by recharging 4.3% of basin-wide hillslope runoff generated during the normal scenario, and 10.0% and 1.5% of runoff during the dry and wet scenarios, respectively. Runoff simulation results were combined with an independent recharge suitability mapping analysis, showing that DSC-MAR could be effective in many parts of the PVDB under a range of climate conditions. These results highlight the importance of strategically locating DSC-MAR projects at the confluence of reliable supply and favorable subsurface hydrologic properties.

Decades of groundwater overuse in the Pajaro Valley have contributed to an estimated groundwater overdraft of 12,100 acre-feet per year (AFY) in the basin. In response, the Pajaro Valley Water Management Agency adopted a pilot groundwater recharge program, called Recharge Net Metering (ReNeM). ReNeM encourages development of infiltration projects on private or public land by offering a rebate on groundwater pumping fees based on the net increase in infiltration. The rebate uses the following equation: Rebate = W50x (Inftot − Infinc), where Infinc is the incidental infiltration that would have occurred without the project, Inftot is total measured infiltration, and W50 represents the proportion (50%) of the pumping fee assessed to the landowner based on location. The goal of Pajaro Valley’s ReNeM pilot program is to infiltrate 1,000 AFY to the aquifer by creating and operating infiltration projects at multiple sites. This effort will help reduce groundwater overdraft and associated undesirable consequences (seawater intrusion, disconnection with surface water, and degradation to water quality). This case study analyzes the local conditions and institutions that make the ReNeM pilot program feasible, including previously established groundwater pumping fees, metered wells, and the existence of third-party certifiers able to verify the results of project sites. The ReNeM pilot has enabled increased recharge by creating new incentives that have drawn PV Water, landowners, and tenant farmers to develop recharge projects. The ReNeM pilot is the first and thus far only application of this approach, but the methods used by ReNeM may have potential applicability elsewhere. This potential will hinge on whether the pilot can prove the effectiveness of the rebate scheme and demonstrate measurable benefits in the Pajaro Valley.

Leading deep-sea research expeditions requires a breadth of training and experience, and the opportunities for Early Career Researchers (ECRs) to obtain focused mentorship on expedition leadership are scarce. To address the need for leadership training in deep-sea expeditionary science, the Crustal Ocean Biosphere Research Accelerator (COBRA) launched a 14-week virtual Master Class with both synchronous and asynchronous components to empower students with the skills and tools to successfully design, propose, and execute deep-sea oceanographic field research. The Master Class offered customized and distributed training approaches and created an open-access syllabus with resources, including reading material, lectures, and on-line resources freely-available on the Master Class website (cobra.pubpub.org). All students were Early Career Researchers (ECRs, defined here as advanced graduate students, postdoctoral scientists, early career faculty, or individuals with substantial industry, government, or NGO experience) and designated throughout as COBRA Fellows. Fellows engaged in topics related to choosing the appropriate deep-sea research asset for their Capstone “dream cruise” project, learning about funding sources and how to tailor proposals to meet those source requirements, and working through an essential checklist of pre-expedition planning and operations. The Master Class covered leading an expedition at sea, at-sea operations, and ship-board etiquette, and the strengths and challenges of telepresence. It also included post-expedition training on data management strategies and report preparation and outputs. Throughout the Master Class, Fellows also discussed education and outreach, international ocean law and policy, and the importance and challenges of team science. Fellows further learned about how to develop concepts respectfully with regard to geographic and cultural considerations of their intended study sites. An assessment of initial outcomes from the first iteration of the COBRA Master Class reinforces the need for such training and shows great promise with one-quarter of the Fellows having submitted a research proposal to national funding agencies within six months of the end of the class. As deep-sea research continues to accelerate in scope and speed, providing equitable access to expedition training is a top priority to enable the next generation of deep-sea science leadership.

Integrated Ocean Drilling Program (IODP) Hole 1301A was drilled, cased, and instrumented with a long‐term, subseafloor observatory (CORK) on the eastern flank of the Juan de Fuca Ridge in summer 2004. This borehole is located 1 km south of ODP Hole 1026B and 5 km north of Baby Bare outcrop. Hole 1301A penetrates 262 m of sediment and 108 m of the uppermost 3.5 Ma basaltic basement in an area of warm (64°C) hydrothermal circulation. The borehole was instrumented, and those instruments were recovered 4 years later. Here we report chemical data from two continuous fluid samplers (OsmoSamplers) and temperature recording tools that monitored changes in the state of borehole (formation) fluids. These changes document the effects of drilling, fluid overpressure and flow, seawater‐basalt interactions, and microbial metababolic activity. Initially, bottom seawater flowed into the borehole through a leak between concentric CORK casing strings. Eventually, the direction of flow reversed, and warm, altered formation fluid flowed into the borehole and discharged at the seafloor. This reversal occurred during 1 week in September 2007, 3 years after drilling operations ceased. The composition of the formation fluid around Hole 1301A generally lies within bounds defined by springs on Baby Bare outcrop (to the south) and fluids that discharged from Hole 1026B (to the north); deviations likely result from reactions with drilling products. Simple conservative mixing of two end‐member fluids reveals reactions occurring within the crust, including nitrate reduction presumably by denitrifying microbes. The observed changes in borehole fluid composition provide the foundation for a conceptual model of chemical and microbial change during recharge of a warm ridge‐flank hydrothermal system. This model can be tested through future scientific ocean drilling experiments.