Michael D. Tryon

Fluid and chemical fluxes in and out of sediments hosting methane hydrate deposits on Hydrate Ridge, OR, I. Hydrological provinces

Extensive deposits of methane hydrate characterize Hydrate Ridge in the Cascadia margin accretionary complex. The ridge has a northern peak at a depth of about 600 m, which is covered by extensive carbonate deposits, and an 800 m deep southern peak that is predominantly sediment covered. Samples collected with benthic instrumentation and from Alvin push cores reveal a complex hydrogeologic system where fluid and methane fluxes from the seafloor vary by several orders of magnitude at sites separated by distances of only a few meters. We identified three distinct active fluid regimes at Hydrate Ridge. The first province is represented by discrete sites of methane gas ebullition, where the bulk of the flow occurs through channels in which gas velocities reach 1 m s−1. At the northern summit of the ridge the gas discharge appears to be driven by pressure changes on a deep gas reservoir, and it is released episodically at a rate of ∼6×104 mol day−1 following tidal periodicity. Qualitative observations at the southern peak suggest that the gas discharge there is driven by more localized phenomena, possibly associated with destabilization of massive gas hydrate deposits at the seafloor. The second province is characterized by the presence of extensive bacterial mats that overlay sediments capped with methane hydrate crusts, both at the northern and southern summits. Here fluid typically flows out of the sediments at rates ranging from 30 to 100 cm yr−1. The third province is represented by sites colonized by vesicomyid clams, where bottom seawater flows into the sediments for at least some fraction of the time. Away from the active gas release sites, fluid flows calculated from pore water models are in agreement with estimates using published flowmeter data and numerical model calculations. Methane fluxes out of mat-covered sites range from 30 to 90 mmol m−2 day−1, whereas at clam sites the methane flux is less than 1 mmol m−2 day−1.

Measurements of transience and downward fluid flow near episodic methane gas vents, Hydrate Ridge, Cascadia

Aqueous flux measurements within cold seep regions on northern Hydrate Ridge, Cascadia, indicate a high degree of variability, with extended periods of downflow and reversals of flow direction over periods of weeks to months. Local episodic venting of free methane gas was also observed. The instruments recorded similar changes in hydrologic flow patterns both on and off clam fields, the magnitude of the flow rates decreasing away from the clam field. The coincidence of episodic gas venting with nearby highly variable aqueous fluid flow suggests that they may be coupled. We propose that these observations are consistent with the action of a gas-driven pump that operates somewhat like a geyser. The hypothesis of gas-driven pumping of seawater through northern Hydrate Ridge has important ramifications for the mass fluxes through this region.

Pore fluid chemistry of the North Anatolian Fault Zone in the Sea of Marmara: A diversity of sources and processes

As part of the 2007 Marnaut cruise in the Sea of Marmara, an investigation of the pore fluid chemistry of sites along the Main Marmara Fault zone was conducted. The goal was to define the spatial relationship between active faults and fluid outlets and to determine the sources and evolution of the fluids. Sites included basin bounding transtensional faults and strike-slip faults cutting through the topographic highs. The basin pore fluids are dominated by simple mixing of bottom water with a brackish, low-density Pleistocene Lake Marmara end-member that is advecting buoyantly and/or diffusing from a relatively shallow depth. This mix is overprinted by shallow redox reactions and carbonate precipitation. The ridge sites are more complex with evidence for deep-sourced fluids including thermogenic gas and evidence for both silicate and carbonate diagenetic processes. One site on the Western High displayed two mound structures that appear to be chemoherms atop a deep-seated fluid conduit. The fluids being expelled are brines of up to twice seawater salinity with an exotic fluid chemistry extremely high in Li, Sr, and Ba. Oil globules were observed both at the surface and in cores, and type II gas hydrates of thermogenic origin were recovered. Hydrate formation near the seafloor contributes to increase brine concentration but cannot explain their chemical composition, which appears to be influenced by diagenetic reactions at temperatures of 75°C–150°C. Hence, a potential source for fluids at this site is the water associated with the reservoir from which the gas and oil is seeping, which has been shown to be related to the Thrace Basin hydrocarbon system. Our work shows that submerged continental transform plate boundaries can be hydrologically active and exhibit a diversity of sources and processes.

Complex flow patterns through Hydrate Ridge and their impact on seep biota

Our recent measurements of long-term fluid flux rates and output fluid chemistry at seep sites on Hydrate Ridge, Cascadia, suggest that the mechanisms driving fluid flow in gas hydrate bearing forearc settings may be significantly more complex than previously thought. Temporal changes in the polarity and magnitude of flow and in seep fluid chemistry were observed. The nature of flow appears to generate a strong response in the chemosynthetic seep community ecology. We document here: 1) the frequency and range of temporal variability in flow rates, 2) the relatively rapid changes in outflow composition, 3) strong outflow of seawater-like fluids, 4) the association of Calyptogena (sp) clams with oscillatory and inflow settings, and 5) microbial mat communities associated with strong advection of altered fluids. These results suggest that a reappraisal may be required of the nature of the hydrologic system and its effect on related processes such as benthic seep community ecology and biogeochemical cycles.

How far did the surface rupture of the 1999 İzmit earthquake reach in Sea of Marmara

An open problem concerning the Mw 7.4, 1999 Izmit earthquake along the North Anatolian Fault (NAF) system is the apparent conflict between estimates of strike-slip deformation based on field and remote sensing data. This is due to the fact that the main strand of the NAF west of the epicenter lies below the Sea of Marmara. Seismological evidence and models based on synthetic aperture radar interferometry suggest that coseismic and early postseismic displacement accumulated after the earthquake could have reached the western end of the Izmit Gulf and possibly the southern edge of the Cinarcik Basin, tapering off along the northern coast of the Armutlu Peninsula, more than 60 km from the epicenter. This scenario is not confirmed by onshore field observations that point toward a termination of the surface rupture around 30 km to the east. These discrepancies convey high uncertainties in the estimate of the tectonic load produced by the Izmit earthquake on the adjacent fault segment toward Istanbul. We analyzed data from different sources, including high-resolution marine geophysical surveys and two Nautile dives along the fault-controlled canyon that connects Izmit Cinarcik basins. Our observations suggest that the surface rupture of the 1999 Izmit earthquake propagated through the shallow Gulf but did not reach the deep Marmara basins. In fact, along the slope between Cinarcik and the western end of the Izmit Gulf, we do not observe fault-related ruptures affecting the sea-floor but rather a series of active gas seeps and black patches that mark the presence of known active faults. Our findings have implications for seismic risk assessment in the highly populated region of Istanbul, both for the estimate of tectonic load transferred to the next fault segments and the location of the next earthquake. Citation: Gasperini, L., A. Polonia, G. Bortoluzzi, P. Henry, X. Le Pichon, M. Tryon, N. Cagatay, and L...

Monitoring aseismic tectonic processes via hydrologic responses: An analysis of log-periodic fluid flow events at the Costa Rica outer rise

A log-periodic series of fluid inflow events from the outer rise off the Nicoya Peninsula, Costa Rica, was recorded on a flow meter deployed at the sediment surface. Similar periodic series have been seen in many failure processes in laboratory and field research, including seismic and chemical records preceding earthquakes. Each cycle has the appearance of an impulse and subsequent decay. The hypothesis that the recorded flow pattern is a result of a series of relatively small strain events resulting from aseismic extensional faulting of the outer rise basement is explored, and analytical results supporting this hypothesis are presented.

MATLAB scripts to calculate the water budget in the Kumano Basin, offshore Japan

Two matlab scripts which simulate water emission from the subducting plate and in the inner accretionary prism.

Seismic tremor event intervals from dual-frequency coherence

Slip occurring at plate boundaries creates seismic tremor as well as “normal” earthquakes. This nonvolcanic tremor appears to consist of swarms of low-frequency earthquakes which lack impulsive P and S arrivals. Tremor is accompanied by slip observed by GPS and can show anomalies in fluid flow. The seismic radiation resembles continuous microseismic noise more than discrete events. We report dual-frequency coherence (DFC) calculations on tremor and normal microseismic background noise observed on Ocean-Bottom Seismographs and land seismic stations around the Nicoya Peninsula, Costa Rica. Both the OBS and land tremor signals show a banded pattern in DFC that is absent in normal noise. The similarity in the DFC patterns between OBS and land tremor signals suggests a common source, eliminating the possibility that DFC is a property of the OBS or seafloor environment. Banded DFC patterns can be generated by repeated events with a repeat time equal to the reciprocal of the offset frequency between bands. If, a...