Hydrogeology of Slow-Spreading Oceanic Crust: A Tracer Study in North Pond Borehole Observatories
Abstract
In 2011 the Integrated Ocean Drilling Program (IODP) commenced Expedition 336, a long-term biogeochemical and hydrologic study of ridge flank hydrothermal processes in slow-spreading, young (8 Ma) crust on the western flank of the Mid-Atlantic Ridge at North Pond. As part of this expedition a three-level instrumented borehole was deployed at IODP Hole U1383C. This borehole included several continuous fluid samplers (OsmoSamplers) and pressure sensors, each deployed on the wellhead yet connected to each of the three depth horizons via umbilical tubing. The sediment thickness at this site is 38.3 m and the three umbilicals penetrated an additional 61.7 m, 1123.7 m, and 164.7 m, respectively, of basalt. Another borehole (Hole U1383B) was drilled about 25 meters away but was abandoned on Exp. 336 after penetrating 52.8 m of sediment and 37 m of basalt. This borehole was sealed and instrumented with a CORK-Lite 6 months after Exp. 336. Pressure data indicate that the upper basaltic crust is slightly overpressured at Hole U1383C but slightly underpressured at Hole U1383B. In October 2017 we returned to these boreholes and injected a Cesium tracer into Hole U1383B. Over the next few days we extracted fluid samples from each of the three horizons in U1383C, but none of the samples contained excess Cs. Then the top plug was removed from U1383C, allowing the borehole to vent freely. A flow meter and fast-flow OsmoSampler were deployed to monitor this discharge. Less than 2 days later the flow meter and OsmoSampler were recovered, the hole was sealed, and fluid samples were collected a second time from each of the three horizons. Cs values were elevated in the OsmoSampler just prior to removal from the wellhead. Cs values also were elevated in samples collected from each of the three depth horizons after the borehole was sealed. The flow meter registered a near constant discharge rate. These results highlight the extent of connectivity in the oceanic crust, define rates of fluid flow within the crust, constrain hydrologic properties of the basaltic crust, and illustrate the potential for a single expedition to conduct tracer/injection tests to constrain subsurface microbial rates and processes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V43G0215W
- Keywords:
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- 0450 Hydrothermal systems;
- BIOGEOSCIENCESDE: 4825 Geochemistry;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICALDE: 8416 Mid-oceanic ridge processes;
- VOLCANOLOGYDE: 8424 Hydrothermal systems;
- VOLCANOLOGY