Andrew T. Fisher

Quantifying surface water–groundwater interactions using time series analysis of streambed thermal records: Method development

[1] We present a method for determining streambed seepage rates using time series thermal data. The new method is based on quantifying changes in phase and amplitude of temperature variations between pairs of subsurface sensors. For a reasonable range of streambed thermal properties and sensor spacings the time series method should allow reliable estimation of seepage rates for a range of at least ±10 m d � 1 (±1.2 � 10 � 2 ms � 1 ), with amplitude variations being most sensitive at low flow rates and phase variations retaining sensitivity out to much higher rates. Compared to forward modeling, the new method requires less observational data and less setup and data handling and is faster, particularly when interpreting many long data sets. The time series method is insensitive to streambed scour and sedimentation, which allows for application under a wide range of flow conditions and allows time series estimation of variable streambed hydraulic conductivity. This new approach should facilitate wider use of thermal methods and improve understanding of the complex spatial and temporal dynamics of surface water–groundwater interactions.

Permeability within basaltic oceanic crust

Water-rock interactions within the seafloor are responsible for significant energy and solute fluxes between basaltic oceanic crust and the overlying ocean. Permeability is the primary hydrologic property controlling the form, intensity, and duration of seafloor fluid circulation, but after several decades of characterizing shallow oceanic basement, we are still learning how permeability is created and distributed and how it changes as the crust ages. Core-scale measurements of basaltic oceanic crust yield permeabilities that are quite low (generally 10−22 to 10−17 m²), while in situ measurements in boreholes suggest an overlapping range of values extending several orders of magnitude higher (10−18 to 10−13 m²). Additional indirect estimates include calculations made from borehole temperature and flow meter logs (10−16 to 10−11 m²), numerical models of coupled heat and fluid flow at the ridge crest and within ridge flanks (10−16 to 10−9 m²), and several other methods. Qualitative indications of permeability within the basaltic oceanic crust come from an improved understanding of crustal stratigraphy and patterns of alteration and tectonic modification seen in ophiolites, seafloor samples and boreholes. Difficulties in reconciling the wide range of estimated permeabilities arise from differences in experimental scale and critical assumptions regarding the nature and distribution of fluid flow. Many observations and experimental and modeling results are consistent with permeability varying with depth into basement and with primary basement lithology. Permeability also seems to be highly heterogeneous and anisotropic throughout much of the basaltic crust, as within crystalline rocks in general. A series of focused experiments is required to resolve permeability in shallow oceanic basement and to directly couple upper crustal hydrogeology to magmatic, tectonic, and geochemical crustal evolution.

Channelized fluid flow in oceanic crust reconciles heat-flow and permeability data

Hydrothermal fluid circulation within the sea floor profoundly influences the physical, chemical and biological state of the crust and the oceans. Circulation within ridge flanks (in crust more than 1 Myr old) results in greater heat loss and fluid flux than that at ridge crests and persists for millions of years, thereby altering the composition of the crust and overlying ocean. Fluid flow in oceanic crust is, however, limited by the extent and nature of the rocks permeability. Here we demonstrate that the global data set of borehole permeability measurements in uppermost oceanic crust defines a trend with age that is consistent with changes in seismic velocity. This trend—which indicates that fluid flow should be greatly reduced in crust older than a few million years—would appear to be inconsistent with heat-flow observations, which on average indicate significant advective heat loss in crust up to 65 Myr old. But our calculations, based on a lateral flow model, suggest that regional-scale permeabilities are much higher than have been measured in boreholes. These results can be reconciled if most of the fluid flow in the upper crust is channelized through a small volume of rock, influencing the geometry of convection and the nature of fluid–rock interaction.

Tectonics and hydrogeology of the northern Barbados Ridge: Results from Ocean Drilling Program Leg 110

Drilling near the deformation front of the northern Barbados Ridge cored an accretionary prism consisting of imbricately thrusted Neogene hemipelagic sediments detached from little-deformed Oligocene to Campanian underthrust deposits by a decollement zone composed of lower Miocene to upper Oligocene, scaly radiolarian claystone. Biostrati-graphically defined age inversions define thrust faults in the accretionary prism that correlate between sites and are apparent on the seismic reflection sections. Two sites located 12 and 17 km west of the deformation front document continuing deformation of the accreted sediments during their uplift. Deformational features include both large- and small-scale folding and continued thrust faulting with the development of stratal disruption, cataclastic shear zones, and the proliferation of scaly fabrics. These features, resembling structures of accretionary complexes exposed on land, have developed in sediments never buried more than 400 m and retaining 40% to 50% porosity. A single oceanic reference site, located 6 km east of the deformation front, shows incipient deformation at the stratigraphic level of the decollement and pore-water chemistry anomalies both at the decollement level and in a subjacent permeable sand interval. Pore-water chemistry data from all sites define two fluid realms: one characterized by methane and chloride anomalies and located within and below the decollement zone and a second marked solely by chloride anomalies and occurring within the accretionary prism. The thermogenic methane in the decollement zone requires fluid transport many tens of kilometers arcward of the deformation front along the shallowly inclined decollement surface, with minimal leakage into the overlying accretionary prism. Chloride anomalies along faults and a permeable sand layer in the underthrust sequence may be caused by membrane filtration or smectite dewatering at depth. Low matrix permeability requires that fluid flow along faults occurs through fracture permeability. Temperature and geochemical data suggest that episodic fluid flow occurs along faults, probably as a result of deformational pumping.

Sediment deformation and hydrogeology of the Nankai Trough accretionary prism: Synthesis of shipboard results of ODP Leg 131

The main objective of Leg 131 was to provide data on the deformational processes and associated hydrogeology of the Nankai prism toe. Drilling succeeded, for the first time in the history of ocean drilling, in penetrating the complete sedimentary sequence to basaltic basement, reaching 1327 mbsf (metres below seafloor) with good core recovery (55%). Excellent correlation of the lithology and structure, including the frontal thrust and the decollement, with seismic reflection images was also determined. Bedding dips, faults and shear bands analyzed in the cores confirm the pattern of deformation to be mainly due to NW-SE shortening, as expected from the plate tectonic convergence vector. Below the decollement, no significant deformation features were observed, indicating that the decollement is a sharp discontinuity in stress transmission. Physical properties data show major discontinuities at the decollement, notably an increase in porosity below the later. This may indicate excess pore pressure in the subducted section and decollement zone. A less marked increase in porosity below the frontal thrust may reflect the youthfulness of this feature. Attempts to make downhole measurements were severely hampered by unstable hole conditions, but useful constraints have been placed on the thermal regime, and some calibration of laboratory physical properties toin-situ conditions has been provided, andin-situ stress and pore pressure were measured in the uppermost sediments. Evidence of channelized fluid flows is inconclusive. No sharp geochemical signatures or unequivocal geochemical anomalies indicative of channelized fluid flow were found. Thermal measurements are not significantly different from those predicted by a purely conductive heat flow model. A signature of low chloride pore water near the decollement may partly be related to smectite diagenesis but may also be due to episodic fluid flow events. We conclude that dewatering probably occurred dominantly through diffuse flow throughout the accreted sediments at this site.

Abnormal fluid pressures and fault-zone dilation in the Barbados accretionary prism: Evidence from logging while drilling

Logs collected while drilling measured density in situ, through the accretionary prism and decollement zone of the northern Barbados Ridge. Consolidation tests relate void ratio (derived from density) to effective stress and predict a fluid pressure profile, assuming that the upper 100 m of the prism is at a hydrostatic pressure gradient. The calculated fluid pressure curve rises to >90% of lithostatic below thrusts in the prism, presumably due to the increase in overburden and lateral tectonic loading. Thin (0.5–2.0 m) intervals of anomalously low density and resistivity in the logs through the basal decollement zone suggest dilation and perhaps hydrofracturing. A peak in hydraulic head in the upper half of the decollement zone requires lateral influx of fluid, a conclusion consistent with previous geochemical studies. Although the calculated fluid-pressure profile is model dependent, its inherent character ties to major structural features.

Inferred pore pressures at the Costa Rica subduction zone: implications for dewatering processes

Drilling on Ocean Drilling Program (ODP) Leg 170, offshore Costa Rica indicates that the entire incoming sedimentary section is underthrust. Thus, observed changes in the thickness of underthrust sediments as they are progressively buried beneath the margin wedge provide a direct measure of the rate and magnitude of sediment dewatering. Laboratory consolidation tests indicate that in situ excess pore-fluid pressures within the underthrust section range from 1.3 MPa at the top of the section to 3.1 MPa near the base. The inferred pore pressure profile implies that fluids escape the uppermost sediments most rapidly, whereas the basal sediments remain essentially undrained. This interpretation suggests that the sedimentary and underlying ocean crustal hydrologic systems are decoupled. We use a simple model of fluid flow to demonstrate that dewatering of the underthrust sediments can occur via lateral flow only if sediment permeability is strongly anisotropic, or if flow is focused along permeable stratigraphic layers. If significant dewatering occurs by vertical fluid flow, it must occur within closely spaced, high-permeability conduits. fl 2000 Elsevier Science B.V. All rights reserved.

Colonization of subsurface microbial observatories deployed in young ocean crust.

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.

Fluid flow paths in the Middle America Trench and Costa Rica margin

The hydrology of the subducting plate and its dewatering behavior through the shallow subduction zone is linked to the structure and deformation of the forearc prism, the nature of the seismogenic zone, the composition of seawater for selected elements, and the composition of the residual slab subducted to depths of magma generation at the volcanic arc. Two locally independent systems of fluid flow govern the transport of heat and chemistry through the Costa Rica subduction complex, a dominantly nonaccretionary subduction zone. One fluid system is the margin wedge, decollement, and underthrust sediment section. Fluid sources include local sediment compaction and mineral dehydration at depth. A second flow system occurs in basement, beneath the sedimentary sequence on the incoming plate. This region is characterized by extremely low conductive heat flow, and the sediment overlying basement has pore-water geochemistry similar to that of seawater. Flow nearly parallel to the trench could be directed by permeability associated with faults and driven by a combination of differential heating and earthquake strain cycling.

Deep sea bottom-simulating-reflectors: calibration of the base of the hydrate stability field as used for heat flow estimates *

Ocean Drilling Program and Deep Sea Drilling Project downhole data from three areas, the southwestern Japan Nankai margin, the continental slope off Peru, and the Blake-Bahama Outer Ridge, provide temperature calibrations for bottom simulating reflectors (BSR) that mark the base of a clathrate hydrate stability field. The inferred temperatures at BSRs provide an important reference for the mapping of geothermal gradient and heat flow from subduction zone accretionary sedimentary wedges. The borehole results provide information on which stability field is applicable for the BSRs and thus calibrate the heat flow estimates. While an ideal calibration has not been possible, the BSR temperatures at the three sites in the temperature range 25–27°C, have been estimated with uncertainties of ±0.7 to ±2.0°C. The temperatures correspond closely to the laboratory dissociation temperatures for pure water-pure methane hydrate at equivalent pressures. No laboratory data are available for seawater salinity and methane at equivalent pressures, but extrapolation from lower pressures gives temperatures 1–2°C lower, which is just significantly different. The data also could be explained by the stability curve for seawater salinity and methane with about 7% CO2, or with a small amount of higher hydrocarbons, but most hydrate samples that have been recovered by deep sea drilling have contained almost pure methane. The uncertainties in the temperature at the BSR should contribute no more than ±5% error in heat flow estimates from BSR depths if the pure water-methane stability field is used.