Matthew J. Pruis

Fluxes of fluid and heat from the oceanic crustal reservoir

Abstract Recent discoveries define a global scale fluid reservoir residing within the uppermost igneous oceanic crust, a region of seafloor that is both warm and may harbor a substantial biosphere. This hydrothermal fluid reservoir formed initially within volcanic rocks newly erupted at mid-ocean ridges, but extends to the vastly larger and older ridge flanks. Upper oceanic crust is porous and permeable due to the presence of lava drainbacks, fissuring, and inter-unit voids, and this porosity and permeability allows active fluid circulation to advect measurable quantities of lithospheric heat from the crust to an average age of 65 Myr. A compilation of crustal porosities shows that this fluid reservoir contains nearly 2% of the total volume of global seawater. Heat flow and sediment thickness data allow calculation of reservoir temperatures, predicting 40°C mean temperatures in Cretaceous crust. Utilizing these temperature estimates, heat flow measurements and models for the thermal structure and evolution of the oceanic lithosphere, we have computed mean hydrothermal fluxes into the deep ocean as a function of plate age. The total hydrothermal volume flux into the oceans approaches 20% of the total riverine input and may contribute to the global seawater mass balance.

Tapping into the sub-seafloor: examining diffuse flow and temperature from an active seamount on the Juan de Fuca Ridge

Abstract A unique sampling strategy involving the cementing of a permanent fluid sampler directly to the seafloor on Axial Seamount, Juan de Fuca Ridge, has recently allowed the first long-term direct coupling to a low-temperature hydrothermal vent on a mid-ocean ridge. Using the hydrologically sealed sampler, direct measurement of fluid volume and heat flux from a diffuse hydrothermal vent on the seafloor was obtained over a period of 206 days. Enhanced variance at both tidal and lower frequencies was recorded with a high coherence between the temperature and flow data. We estimate a volume flux of 48 m 3 /yr and a heat flux of 260 W/m 2 for the square meter of seafloor sampled by the instrument. Measurement of the Darcy flow velocities of the effluent fluid are approximately 1.5×10 −6 m/s and indicate fluid velocities within cracks in the substrate of 1–4 m/day, with an effective upper crustal permeability of 10 −11 –10 −12 m 2 . While the measured variability in fluid flow is driven primarily by changes in the thermal buoyancy of upwelling fluid, there is also a significant (above hydrostatic) pressure gradient contribution to the measured flux. This overpressure produces roughly four times the driving force compared to that attributed to thermal buoyancy alone. Small variations in the volume flux and the effluent fluid temperature (∼1.5×10 −8 m/s and ∼0.5°C, respectively) also occurred on approximately tidal time scales and appear to be related to poroelastic control of the velocity of hydrothermal fluid through the seafloor boundary (i.e. tidal pumping).

Survey studies hydrothermal circulation on the Northern Juan de Fuca Ridge

The circulation of hydrothermal fluid within upper oceanic crust constrains the global composition of seawater and is also responsible for many of the dynamic chemical and biological processes that alter the underlying volcanic rocks that form the sea floor. The heat of crustal formation drives this fluid circulation, and the impact on the overlying ocean is most easily observed at mid-ocean ridge spreading centers. Previous efforts to quantify the heat associated with crustal formation have lacked information regarding the partitioning of thermal energy between discrete, high-temperature vent fields, ubiquitous low-temperature diffuse venting, and the pervasive conductive heat flux through the volcanic rocks.

Density and porosity of the upper oceanic crust from seafloor gravity measurements

The exposure of 1300 meters of upper oceanic crust at the Blanco Fracture Zone allows near-bottom gravity measurements to determine the in situ density of the seafloor as a function of depth. Gravity measurements along the north wall of the Blanco Depression indicate an outcrop density of 2530 ± Kg/m³ for the upper 800 meters of crust and a calculated porosity of 23%. The lower 500 meters of crust (800 to 1300 meters below the sea floor) has a measured density of 2710 ± 130 Kg/m³ and a porosity of 14%. These data indicate that most of the extrusive volcanic oceanic crust is highly porous and can act as an aquifer and large-scale reservoir for hydrothermal fluids. These direct crustal density measurements also support previous interpretations that low seismic velocities observed in Layer 2 are due to the high porosity of the upper extrusive section.

Earthquakes' impact on hydrothermal systems may be far‐reaching

Recent work has linked earthquake activity with changes in flow and temperature due to hydrothermal venting at mid-ocean ridges. These intriguing relationships are important motivation for modeling marine hydrothermal systems. However, a re-examination of some earlier vent monitoring data from the Juan de Fuca Ridge, combined with analysis of recently reprocessed SOSUS (SOund Surveillance System) hydrophone data (Figure 1), suggest that such activity may be linked over considerable distances of greater than 200 km and reaction intervals of over a month. The available observational data are sparse, so the direct association between earthquakes and changes in crustal fluid circulation are difficult to verify. However, the response times and distance scales are consistent with other observations, including earthquakes in land-based settings [Hill et al., 1993] and modeling of flow in porous media [Pruis et al., 2000]. If true, these associations imply that marine hydrothermal systems are extremely complex and may be sensitive to very subtle environmental changes.

Porosity of very young oceanic crust from sea floor gravity measurements

Porosity and density are two of the most important, but under-determined parameters in studies of igneous oceanic crust. Lacking quantitative measurements of crustal densities, but with plausible expectations of substantial variation with age, previous investigators have used estimates that have varied widely. To address this problem more rigorously, a recent ALVIN program obtained 133 on-bottom gravity stations to calculate crustal densities for three recent volcanic eruptions on the Juan de Fuca Ridge. In the summer of 1995, a Bell-Aerospace BGM-3 gravity meter was mounted in ALVIN and made fixed-station gravity measurements on 11 transect lines from three distinct volcanic environments including (1) the recent CoAxial Segment volcanic eruption that occurred in 1993, (2) two nearby flows with less well-determined ages but dated post-1981, and (3) several older adjacent areas on the Juan de Fuca axial ridge. Densities for the recent eruptive units ranged between 2280 and 2450 kg/m³, while the upper 50 meters of the older, unfractured surrounding flows were approximately 2800 kg/m³. Laboratory grain densities measured from rocks collected in the study area allow crustal porosities to be calculated, which are anomalously high for the recent flows in comparison to the surrounding axial ridge. Porosities for the three youngest flows range between 29–36%, while the adjacent unfractured regions of the surrounding axial region had porosities near 10%. Two localized regions of intense surficial crustal fracturing in older crust, located just south of the 1993 eruption site, also had high porosities of 26–28%. This high variability indicates that the evolution of crustal porosities from young to older oceanic crust is not related only to the collapse and infilling of macroscopic voids, but that tectonic activity can significantly modify the porosity of older crustal structures.

Role of Ice Dynamics in the Sea Ice Mass Balance

Over the past decade, the Arctic Ocean and Beaufort Sea ice pack has been less extensive and thinner than has been observed during the previous 35 years [e.g., Wadhams and Davis, 2000; Tucker et al., 2001; Rothrock et al., 1999; Parkinson and Cavalieri, 2002; Comiso, 2002]. During the summers of 2007 and 2008, the ice extents for both the Beaufort Sea and the Northern Hemisphere were the lowest on record. Mechanisms causing recent sea ice change in the Pacific Arctic and the Beaufort Sea are under investigation on many fronts [e.g., Drobot and Maslanik, 2003; Shimada et al., 2006]; the mechanisms include increased ocean surface warming due to Pacific Ocean water inflow to the region and variability in meteorological and surface conditions. However, in most studies addressing these events, the impact of sea ice dynamics, specifically deformation, has not been measured in detail.