Beschreibung: During opening of a new ocean magma intrudes into the surrounding sedimentary basins. Heat provided by the intrusions matures the host rock creating metamorphic aureoles potentially releasing large amounts of hydrocarbons. These hydrocarbons may migrate to the seafloor in hydrothermal vent complexes in sufficient volumes to trigger global warming, e.g. during the Paleocene Eocene Thermal Maximum (PETM). Mound structures at the top of buried hydrothermal vent complexes observed in seismic data off Norway were previously interpreted as mud volcanoes and the amount of released hydrocarbon was estimated based on this interpretation. Here, we present new geophysical and geochemical data from the Gulf of California suggesting that such mound structures could in fact be edifices constructed by the growth of black-smoker type chimneys rather than mud volcanoes. We have evidence for two buried and one active hydrothermal vent system outside the rift axis. The vent releases several hundred degrees Celsius hot fluids containing abundant methane, mid-ocean-ridge-basalt (MORB)-type helium, and precipitating solids up to 300 m high into the water column. Our observations challenge the idea that methane is emitted slowly from rift-related vents. The association of large amounts of methane with hydrothermal fluids that enter the water column at high pressure and temperature provides an efficient mechanism to transport hydrocarbons into the water column and atmosphere, lending support to the hypothesis that rapid climate change such as during the PETM can be triggered by magmatic intrusions into organic-rich sedimentary basins.

Beschreibung: As offshore petroleum exploration and development move into deeper water, industry must contend increasingly with gas hydrate, a solid compound that binds water and a low-molecular-weight gas (usually methane). Gas hydrate has been long studied in industry from an engineering viewpoint, due to its tendency to clog gas pipelines. However, hydrate also occurs naturally wherever there are high pressures, low temperatures, and sufficient concentrations of gas and water. These conditions prevail in two natural environments, both of which are sites of active exploration: permafrost regions and marine sediments on continental slopes. In this article we discuss seismic detection of gas hydrate in marine sediments. Gas hydrate in deepwater sediments poses both new opportunities and new hazards. An enormous quantity of natural gas, likely far exceeding the global inventory of conventional fossil fuels, is locked up worldwide in hydrates. Ex-traction of this unconventional resource presents unique exploration, engineering, and economic challenges, and several countries, including the United States, Japan, Canada, India, and Korea, have initiated joint industry-academic-governmental programs to begin studying those challenges. Hydrates also constitute a potential drilling hazard. Because hydrates are only stable in a restricted range of pressure and temperature, any activity that sufficiently raises temperature or lowers pressure could destabilize them, releasing potentially large volumes of gas and decreasing the shear strength of the host sediments. Assessment of the opportunities and hazards associated with hydrates requires reliable methods of detecting hydrate and accurate maps of their distribution and concentration. Hydrate may occur only within the upper few hundred meters of deepwater sediment, at any depth between the seafloor and the base of the stability zone, which is controlled by local pressure and temperature. Hydrate is occasionally exposed at the seafloor, where it can be detected either visually or acoustically by strong seismic reflection amplitudes or high backscatter …

Beschreibung: International Ocean Discovery Program (IODP) Expedition 385 drilled organic-rich sediments with sill intrusions on the flanking regions and in the northern axial graben in Guaymas Basin, a young marginal rift basin in the Gulf of California. Guaymas Basin is characterized by a widely distributed, intense heat flow and widespread off-axis magmatism expressed by a dense network of sill intrusions across the flanking regions, which is in contrast to classical mid-ocean ridge spreading centers. The numerous off-axis sills provide multiple transient heat sources that mobilize buried sedimentary carbon, in part as methane and other hydrocarbons, and drive hydrothermal circulation. The resulting thermal and geochemical gradients shape abundance, composition, and activity of the deep subsurface biosphere of the basin. Drill sites extend over the flanking regions of Guaymas Basin, covering a distance of ~81 km from the from the northwest to the southeast. Adjacent Sites U1545 and U1546 recovered the oldest and thickest sediment successions (to ~540 meters below seafloor [mbsf]; equivalent to the core depth below seafloor, Method A [CSF-A] scale), one with a thin sill (a few meters in thickness) near the drilled bottom (Site U1545), and one with a massive, deeply buried sill (~356–430 mbsf) that chemically and physically affects the surrounding sediments (Site U1546). Sites U1547 and U1548, located in the central part of the northern Guaymas Basin segment, were drilled to investigate a 600 m wide circular mound (bathymetric high) and its periphery. The dome-like structure is outlined by a ring of active vent sites called Ringvent. It is underlain by a remarkably thick sill at shallow depth (Site U1547). Hydrothermal gradients steepen at the Ringvent periphery (Holes U1548A–U1548C), which in turn shifts the zones of authigenic carbonate precipitation and of highest microbial cell abundance toward shallower depths. The Ringvent sill was drilled several times and yielded remarkably diverse igneous rock textures, sediment–sill interfaces, and hydrothermal alteration, reflected by various secondary minerals in veins and vesicles. Thus, the Ringvent sill became the target of an integrated sampling and interdisciplinary research effort that included geological, geochemical, and microbiological specialties. The thermal, lithologic, geochemical, and microbiological contrasts between the two deep northwestern sites (U1545 and U1546) and the Ringvent sites (U1547 and U1548) form the scientific centerpiece of the expedition. These observations are supplemented by results from sites that represent attenuated cold seepage conditions in the central basin (Site U1549), complex and disturbed sediments overlying sills in the northern axial trough (Site U1550), terrigenous sedimentation events on the southeastern flanking regions (Site U1551), and hydrate occurrence in shallow sediments proximal to the Sonora margin (Site U1552). The scientific outcomes of Expedition 385 will (1) revise long-held assumptions about the role of sill emplacement in subsurface carbon mobilization versus carbon retention, (2) comprehensively examine the subsurface biosphere of Guaymas Basin and its responses and adaptations to hydrothermal conditions, (3) redefine hydrothermal controls of authigenic mineral formation in sediments, and (4) yield new insights into many geochemical and geophysical aspects of both architecture and sill–sediment interaction in a nascent spreading center. The generally high quality and high degree of completeness of the shipboard datasets present opportunities for interdisciplinary and multidisciplinary collaborations during shore-based studies. In comparison to Deep Sea Drilling Project Leg 64 to Guaymas Basin in 1979, sophisticated drilling strategies (for example, the advanced piston corer [APC] and half-length APC systems) and numerous analytical innovations have greatly improved sample recovery and scientific yield, particularly in the areas of organic geochemistry and microbiology. For example, microbial genomics did not exist 40 y ago. However, these technical refinements do not change the fact that Expedition 385 will in many respects build on the foundations laid by Leg 64 for understanding Guaymas Basin, regardless of whether adjustments are required in the near future.

Beschreibung: Seismic reflection and refraction data from the SE Greenland margin provide a detailed view of a volcanic rifted margin from Archean continental crust to near-to-average oceanic crust over a spatial scale of 400 km. The SIGMA III transect, located ∼600 km south of the Greenland-Iceland Ridge and the presumed track of the Iceland hot spot, shows that the continent-ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup. Initially, 18.3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8–10 km thick crust. Changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system, which was close to sea level for at least 1 m.y. of subaerial spreading despite a reduction in productivity from 17 to 13.5 km thick crust over this time interval. This could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system. The thermal anomaly must be exhaustible, consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet. Exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial, to shallow marine, and finally to deep marine extrusion in ∼2 m.y. is shown by the morphological changes. In addition, comparison to the conjugate Hatton Bank shows a clear asymmetry in the early accretion history of North Atlantic oceanic crust. Nearly double the volume of material was emplaced on the Greenland margin compared to Hatton Bank and may indicate east directed ridge migration during initial opening.

Beschreibung: During opening of a new ocean, magma intrudes into the surrounding sedimentary basins. Heat provided by the intrusions matures the host rock, creating metamorphic aureoles potentially releasing large amounts of hydrocarbons. These hydrocarbons may migrate to the seafloor in hydrothermal vent complexes in sufficient volumes to trigger global warming, e.g., during the Paleocene-Eocene Thermal Maximum (PETM). Mound structures at the top of buried hydrothermal vent complexes observed in seismic data off Norway were previously interpreted as sediment volcanoes, and the amount of released hydrocarbon was estimated based on this interpretation. Here, we present new geophysical and geochemical data from the Gulf of California suggesting that such mound structures could in fact be edifices constructed by the growth of black smoker–type chimneys rather than sediment volcanoes. We have evidence for two buried and one active hydrothermal vent systems outside the rift axis. The active vent releases fluids of several hundred degrees Celsius containing abundant methane, mid-ocean ridge basalt–type helium, and precipitating solids up to 300 m high into the water column. Our observations challenge the idea that methane is emitted slowly from rift-related vents. The association of large amounts of methane with hydrothermal fluids that enter the water column at high pressure and temperature provides an efficient mechanism to transport hydrocarbons into the water column and atmosphere, lending support to the hypothesis that rapid climate change such as during the PETM can be triggered by magmatic intrusions into organic-rich sedimentary basins.

Beschreibung: A multichannel seismic (MCS) experiment spanning 600 km across the Alarcón Rise and its conjugate rifted margins in the southern Gulf of California (western North America) provides insight into the spatial and temporal evolution of extension between Baja California and the mainland (Mexico). Stratigraphic analysis of multiple rift basins within the Alarcón spreading corridor indicates an initial stage of oblique extension starting ca. 14–12 Ma. This initial phase of extension was characterized by the formation of several large basins in the center of the gulf and on the southeast margin with negligible synrift sedimentation. A second phase of oblique extension, likely synchronous with large-scale basin opening in the central and northern Gulf of California, began ca. 8–5 Ma and was characterized by the formation of smaller half-grabens distributed across the conjugate margins that contain both synrift and postrift deposits. A key feature imaged within the MCS data is a highly reflective, ropey layer at the top of basement, interpreted to be either volcanic rocks from the 25–12 Ma Comondú Group, and/or early rifting volcanic rocks that are between 11 and 9 Ma, or younger. This volcanic layer is extensively faulted, suggesting that it predates the episode of early extension. Upper crustal extension appears to be equally distributed across conjugate margins, forming a symmetrical continental rift. Two styles of rifted basin are observed; older basins (estimated as 14–11 Ma using sedimentation rates) show distributed extension with extensive basement faulting. In contrast, the younger basins (likely post–6 Ma) are asymmetrical with synrift deposits thickening into the basin-bounding faults. The northeast-southwest geomorphic expression of the Tamayo bank and trough and other features provides additional evidence that northwest-southeast oblique extension began ca. 12 Ma. These new spatial and temporal constraints, when combined with a crustal thickness profile obtained across the entire Alarcón corridor, suggest that significant northwest-southeast oblique extension within the Gulf of California started well before 6 Ma, in contrast to earlier models.

Beschreibung: Multichannel seismic transects reveal an ~2-km-thick, ~50 x 100 km evaporite body under the shelf on the eastern margin of the Guaymas Basin, central Gulf of California (Mexico). These thick newly discovered evaporites appear to be correlated with well-known gypsum beds near Santa Rosalía to the northwest, on the Baja California peninsula. Closing the Gulf of California along kinematic flow lines suggests that the thin, scattered, ca. 7 Ma Santa Rosalía gypsum beds formed on the fringe of the much thicker evaporite deposit. This correlation, and the large volume of the Guaymas evaporates, implies that substantial marine incursions and subsequent evaporite deposition occurred during the Late Miocene and prior to lithospheric rupture. Furthermore, the shape of the Guaymas evaporite is indicative of a transtensional basin, suggesting that oblique extension existed in the central Gulf of California ca. 7 Ma.