Subseafloor Cross‐Hole Tracer Experiment Reveals Hydrologic Properties, Heterogeneities, and Reactions in Slow‐Spreading Oceanic Crust

Abstract

The permeability, connectivity, and reactivity of fluid reservoirs in oceanic crust are poorly constrained, yet these reservoirs are pathways for about a quarter of the Earth's heat loss, and seawater-rock exchange within them impact ocean chemical cycles. We present results from the second ever cross-hole tracer experiment within oceanic crust and the first conducted during a single expedition and in slow-spreading crust west of the Mid-Atlantic Ridge at North Pond. Here we employed boreholes that were drilled by the Integrated Ocean Drilling Program (Sites U1382 and U1383) that were instrumented and sealed. A cesium salt solution and bottom seawater tracer experiment provided a measure of the minimum Darcy fluid velocity (2 to 41 m/day) within the upper volcanic crust, constraining the minimum permeability of 10 −11 to 10 −9 m 2 . We also document chemical heterogeneities in crustal fluid compositions, rebound from drilling disturbances, and nitrification within the basaltic crust, based on systematic differences in borehole fluid compositions over a 5-year period. These results also show heterogeneous fluid compositions with depth in the borehole, indicating that hydrothermal circulation is not vigorous enough to homogenize the fluid composition in the upper permeable basaltic basement, at least not on the time scale of 5 years. Our work verifies the potential for future manipulative experiments to characterize hydrologic, biogeochemical, and microbial process within the upper basaltic crust. Plain Language SummarySeawater flows within the oceanic crust, much like groundwater flows though permeable aquifers within continental crust. As seawater flows through the oceanic crust it dissolves some minerals while precipitating others. These biogeochemical reactions coupled with subsurface seawater flow affect the distribution of heat, solutes, and microbial populations within the oceanic crust, and such reactions and transport can impact oceanic processes. To assess hydrologic and biogeochemical processes that occur within the oceanic crust west of the Mid-Atlantic Ridge, we conducted the second ever borehole-to-borehole tracer experiment within oceanic crust; however, similar experiments are commonplace in continental settings. On the basis of this tracer experiment and prior sampling of these boreholes, we determined that the upper several hundred meters of volcanic oceanic crust are highly permeable and support microbial nitrification; however, fluids within the upper oceanic crust are not well mixed at this location. Our results provide the foundation to conduct future manipulative experiments within the oceanic crust to characterize hydrologic and biogeochemical processes within this poorly constrained, yet globally significant, environment.

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

“…Geochemical and heat-flow data also show that U1382A experiences greater connectivity to the open ocean than U1383C, although all fluids are geochemically very similar to deep seawater [9,47]. Heat-flow data [48,49] and tracer experiments [12] indicate rapid lateral fluid flow from U1382A to U1383C through the porous crust, while radiocarbon data suggests potential residence times on the order of hundreds (U1382A) or thousands (U1383C) of years [10]. Within these important geochemical and hydrogeochemical contexts, we therefore focused our analyses on understanding the microbial community in 2017, the samples furthest removed from the disturbances caused by drilling, while also allowing us to examine how the community has changed over time by comparing 2017 samples to those collected in 2012-2014 [9,13].…”

“…Time series sampling at North Pond from 2012-2017 has therefore been critical to resolving when the crustal aquifer returned to its pre-drilling state. Geochemical measurements taken over this 6-year period indicate the fluid chemistry rebounded by 2017 [12], and cell count data in 2017 are the lowest since the CORKs were first sampled ( Table 1, Trembath-Reichert et al In Review). Geochemical and heat-flow data also show that U1382A experiences greater connectivity to the open ocean than U1383C, although all fluids are geochemically very similar to deep seawater [9,47].…”

“…Observatory U1383C contains three sampling depth horizons below the sediment-crust interface separated by packer seals: Shallow (~70-146 mbsf), Middle (146-200 mbsf), and Deep (~200-332 mbsf). In all years, umbilical lines were flushed before sampling began and both temperature and oxygen were monitored throughout sampling [9,12,13]. To collect microbial biomass for -omics analyses in 2017, crustal fluid was pumped at a rate of ~0.5 LPM for 80 min through a 0.22 µm, 47 mm GWSP filter (Millipore).…”

“…Experiments using fluids collected in 2012 incubated with 13 C-labeled bicarbonate and acetate at 5°C and 25°C detected both autotrophic and heterotrophic activity at a higher level than in bottom seawater, [9], but low rates of metabolic activity were detected in these same fluids at 4°C using nanocalorimetry [21]. However, all of these samples were taken before it was clear the North Pond aquifer had chemically rebounded from the initial drilling disturbances, and time series geochemical data suggests that by 2017, the North Pond aquifer had recovered [12]. Therefore, fluids collected from North Pond in 2017 are likely the most representative of microbial communities in the cold, oxic aquifer.…”

“…However, much of the remaining crustal pore volume is composed of cold, oxygenated deep ocean water that enters and exits the basaltic crust through seafloor exposures [7,8]. These crustal fluids are chemically similar to seawater, but slightly enriched in DIC and depleted in DOC and O2 [9,10,11,12]. Microbial communities in the crustal fluids are distinct from those in the surrounding bottom water [9,13], but the lifestyle and adaptive strategies of microbial communities and the biogeochemical processes they mediate in the oligotrophic, oxic subseafloor aquifer remain poorly constrained.…”