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.

Two expeditions to Dorado Outcrop on the eastern flank of the East Pacific Rise and west of the Middle America Trench collected images, video, rocks, and sediment samples and measured temperature and fluid discharge rates to document the physical and biogeochemical characteristics of a regional, low‐temperature (~15 °C) hydrothermal system. Analysis of video and images identified lava morphologies: pillow, lobate, and sheet flows. Glasses from collected lavas were consistent with an off‐axis formation. Hydrothermal discharge generally occurs through pillow lavas but is patchy, sporadic, and sometimes ceases at particular sites of discharge. Yearlong temperature measurements at five of these discharge sites show daily ranges that oscillate with tidal frequencies by 6 °C or more. Instantaneous fluid discharge rates (0.16 to 0.19 L/s) were determined resulting in a calculated discharge of ~200 L/s when integrated over the area defined by the most robust fluid discharge. Such discharge has a power output of 10–12 MW. Hydrothermal seepage through thin sediment adjacent to the outcrop accounts for <3% of this discharge, but seepage may support an oxic sediment column. High extractable Mn concentrations and depleted δ13C in the low but variable organic solid phase suggest that hydrothermal fluids provide a source for manganese accumulation and likely enhance the oxidation of organic carbon. Comparisons of the physical and geochemical characteristics at Dorado and Baby Bare Outcrops, the latter being the only other site of ridge‐flank hydrothermal discharge that has been sampled directly, suggest commonalities and differences that have implications for future discoveries.

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We measured dithionite-extractable iron and manganese along with a variety of bulk sedimentary solid and dissolved phases to constrain diagenetic reactions occurring within the sediment package of Grenada Basin, which is within the Lesser Antilles volcanic arc region. Core material was obtained during Integrated Ocean Drilling Program Expedition 340. For this report we focus primarily on five sites; three sites are located in the northern portion of the study area off the island of Montserrat (Sites U1394-U1396), and two sites are located farther south off the island of Martinique (Sites U1399 and U1400). Sediments throughout this region include tephra-rich volcanic sands, hemipelagic mud sequences, and carbonate-rich sequences, with widely variable proportions over short (centimeter scale) depth intervals. Regardless of the main sediment type, organic carbon contents are low with average values of 0.19 ± 0.1 wt% at Site U1394, 0.13 ± 0.08 wt% at Site U1395, 0.13 ± 0.06 wt% at Site U1396, 0.28 ± 0.08 wt% at Site U1399, and 0.23 ± 0.15 wt% at Site U1400. Carbonate contents are more variable, ranging between 0 and ~80 wt%, in cores from Sites U1394-U1396 and between 1 and 40 wt% at both Sites U1399 and U1400. These variations in carbonate content likely reflect variable dilution with volcanogenic sediment. Pore fluids reflect a range of diagenetic conditions from oxidizing (sulfate rich) to sulfate-reducing conditions. Reactive major elements (Ca and Mg) as well as a number of minor elements show a range of diagenetic behaviors from reactions likely related to carbonate dissolution or precipitation to exchange reactions between pore fluids and the volcanic-rich sedimentary substrate and clay formation. However, significant site-to-site variability is seen in the diagenetic behavior of these elements. Solid-phase reactive Fe ranges from 0.18 to 0.75 wt% at the northern sites and 0.4 to 1.5 wt% at the southern sites, whereas reactive Mn ranges from 0 to 0.1 wt% in the north and 0 to 0.3 wt% in the south. Sites U1399 and U1400 are located west of the island of Martinique in Grenada Basin in 2900 and 2745 m water depth, respectively (Fig. F1). The lithology of these two sites is similar and is composed of hemipelagic mud, interbedded tephra layers, volcaniclastic turbidites, and deformed sediments (see the "Expedition 340 summary" chapter [Expedition 340 Scientists, 2013a]); Le Friant et al., 2015). Site U1400 is located west of Martinique but closer in proximity to the island than Site U1399 (Fig. F1). Introduction Analytical methods Pore fluid extractionsPore fluids were obtained from all sediment cores presented in this report (Table T1). Sampling occurred every 10 m unless the sediment was unsuitable for pore fluid extraction (i.e., because of sands, debris, or other mission-specific rationales). Sampling procedures are described in detail in the "Methods" chapter (Expedition 340 Scientists, 2013b); however, we present a brief description for completeness. A 10 to 15 cm section of whole-round core was remo...

Highlights: Elevated carbonate concentrations coincide with maximum lake level Negative excursions in both  13 C and  15 N coincide with maximum lake level Results imply changes in lake chemistry during lake level oscillation centered at 1879