Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Heat flow is estimated at eight sites drilled int the Guaymas Basin, Gulf of California, during the International Ocean Discovery Program Expedition 385. The expedition sought to understand the thermal regime of the basin and heat transfer between off‐axis sills intruding the organic‐rich sediments of the Guaymas Basin, and the basin floor. The distinct sedimentation rates, active tectonics, and magmatism make the basin interesting for scientific discoveries. Results show that sedimentation corrected heat flow values range 119–221 mW/m〈sup〉2〈/sup〉 in the basin and 257–1003 mW/m〈sup〉2〈/sup〉 at the site of a young sill intrusion, denominated Ringvent. Thermal analysis shows that heat in the Guaymas Basin is being dissipated by conduction for plate ages >0.2 Ma, whereas younger plate ages are in a state of transient cooling by both conduction and advection. Drilling sites show that Ringvent is an active sill being cooled down slowly by circulating fluids with discharge velocities of 10–200 mm/yr. Possible recharge sites are located ca. 1 km away from the sill's border. Modelling of the heat output at Ringvent indicates a sill thickness of ca. 240 m. A simple order‐of‐magnitude model predicts that relatively small amounts of magma are needed to account for the elevated heat flow in non‐volcanic, sediment‐filled rifts like the central and northern Gulf of California in which heating of the upper crust is achieved via advection by sill emplacement and hydrothermal circulation. Multiple timescales of cooling control the crustal, chemical and biological evolution of the Guaymas Basin. Here, we recognize at least four timescales: the time interval between intrusions (ca. 10〈sup〉3〈/sup〉 yr), the thermal relaxation time of sills (ca. 10〈sup〉4〈/sup〉 yr), the characteristic cooling time of the sediments (ca. 10〈sup〉5〈/sup〉 yr), and the cooling of the entire crust at geologic timescales.〈/p〉

Description: Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California http://dx.doi.org/10.13039/501100003089

Description: German Research Center for Geosciences

Description: https://web.iodp.tamu.edu/LORE/

Description: https://mlp.ldeo.columbia.edu/logdb/scientific_ocean_drilling/

Description: The movies show the evolution in time of 2-D numerical model calculations investigating hydrothermal fluid flow around a cooling sills in a sedimentary basin. Each movie shows the evolution of a single variable for a specific model setup. The reported variables are: water fraction in the host rock (fH2O), mass of methane (mCH4), permeability (Perm), porosity (Phi), temperature (T), and TOC content of the host rock (TOC). S0: five reference simulations in simplified setups S1-S3 : The basin has been dissected in three regions with different basin depths. Each of these regions was investigated in a specific 2D modelling section which is refered to as setup S1, S2 and S3, respectively.

Description: We document the geometry of a massive sill at the root of an approximately 20-m high and 800m-wide ring of hydrothermal formations, termed Ringvent, located 28.5 km off-axis on the northwestern flanking regions of the actively rifting Guaymas Basin (Gulf of California). Using petrophysical data collected during the IODP Expedition 385 and processed 2D seismic profiles, we present evidence on the mechanics of sill emplacement and how the related hydrothermal vent conduits were constructed. The currently active moderate-temperature hydrothermal vent field indicates that, despite being cold and crystallized, the magma plumbing system, is tapping into a deeper geothermal source of the basin. The vent system roots at the vertical end of the magma plumbing system with the top of the sill located at a depth range of 80 to 150 m below the seafloor. Our research aims at constraining how far deep the geothermal fluids are coming from, and identifying how close the hydrothermal system is from a steady-state condition, to draw implications for how frequently such a system may arise in nascent ocean basins.

Description: Highlights • Analytical and numerical methods are employed to investigate fluid flow in active mud volcanoes or SHHS. • The effects of conduit radius and fluid properties on the flow rate are presented. • Conduit radius of such piercement systems cannot exceed a few metres at depth. • Clasts, if not densely packed, will not affect the flow rate when they are smaller than a fifth of the conduit size. • A maximal exsolution depth between 1800 and 3200 m is inferred for CH4 and between 750 and 1000 m for CO2. Clastic eruptions involve the rapid ascension of sedimentary clasts together with fluids, gas and/or liquid phases that may further deform and brecciate the host rocks. These fluids transport the resulting mixture, called mud breccia, to the surface. Such eruptions are often associated with geological structures such as mud volcanoes, hydrothermal vent complexes and, more generally, piercement structures. They involve various processes, acting over a wide range of scales, which makes them a complex and challenging multi-phase system to model. Although piercement structures have been widely studied and discussed, only a few attempts have been made to model the dynamics of such clastic eruptions. The ongoing Lusi mud eruption, in the East Java back-arc basin, which began in May 2006, is a spectacular large scale clastic eruption. The Lusi eruptive behaviour has been extensively studied over the past decade and thus represents a unique opportunity to better understand ongoing clastic eruptions and thus fossil clastic systems. We use both analytical formulations and numerical models to investigate simple relationships between the mud breccia properties (density, viscosity, gas and clast content) and the volumetric flow rate. Our results show that the conduit radius of such piercement systems cannot exceed a few metres at depth, and that clasts, if not densely packed, will not affect the flow rate when they are smaller than a fifth of the conduit size. Using published data for the annual gas fluxes at Lusi, we infer a maximal depth at which exsolution starts. This occurs between 1800 m and 3200 m depth for methane and between 750 m and 1000 m for carbon dioxide. Based on annual gas fluxes, we estimate that the conduit radius should be no larger than 1.5 m to match the maximal mud discharge, recorded at Lusi.

Description: Large volumes of magma emplaced within sedimentary basins have been linked to multiple climate change events due to release of greenhouse gases such as CH4. Basin-scale estimates of thermogenic methane generation show that this process alone could generate enough greenhouse gases to trigger global incidents. However, the rates at which these gases are transported and released into the atmosphere are quantitatively unknown. We use a 2D, hybrid FEM/FVM model that solves for fully compressible fluid flow to quantify the thermogenic release and transport of methane and to evaluate flow patterns within these systems. Our results show that the methane generation potential in systems with fluid flow does not significantly differ from that estimated in diffusive systems. The values diverge when vigorous convection occurs with a maximum variation of about 50%. The fluid migration pattern around a cooling, impermeable sill alone generates hydrothermal plumes without the need for other processes such as boiling and/or explosive degassing. These fluid pathways are rooted at the edges of the outer sills consistent with seismic imaging. Methane venting at the surface occurs in three distinct stages and can last for hundreds of thousands of years. Our simulations suggest that although the quantity of methane potentially generated within the contact aureole can cause catastrophic climate change, the rate at which this methane is released into the atmosphere is too slow to trigger, by itself, some of the negative δ13C excursions observed in the fossil record over short time scales (〈 10,000 years).

Description: 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.

Description: Subvolcanic systems are characterized by complex combinations of intrusive units (dykes, sills, saucer-shaped sills, cone sheets, etc.) for which genetic relationships are unclear. This chapter explains how whole-rock geochemistry may be used to resolve the genetic relationships of such subvolcanic (and volcanic) systems. We start with a short introduction of the geochemical fingerprinting method with particular emphasis on the statistical refinement method called Forward Stepwise-Discriminant Function Analysis (FS-DFA). Combined with field mapping and structural analysis, geochemical fingerprinting based on major and trace elements and isotope ratios, is a very powerful tool to distinguish between igneous units (lavas, sills, dykes) with subtle (or not so subtle) geochemical differences. Different geochemical fingerprinting or signatures indicate derivation from distinct magma batches. The results from FS-DFA analyses may be used to reveal genetic relationships between geological units, or lack of such, which again may be used to throw light on subvolcanic plumbing systems, the feeding system in sill-dyke complexes, as well as other problems. The method is illustrated by studies of the Golden Valley Sill Complex in the Karoo Basin (South Africa).

Description: Volcanism in organic-rich sedimentary basins leading to thermogenic greenhouse gas generation has been documented as a strong forcing factor of past mass extinctions. However, quantitative studies fail to provide degassing rate estimates that would allow a direct comparison with anthropogenic warming. We are investigating different sill-emplacement sequences of a Large Igneous Province (LIP) plumbing system to identify their potential variable impact in terms of thermogenic degassing rates and cumulative amount of gas released at the basin top. We use a 2D finite element model that solves for hydrothermal fluid flow and thermal evolution around several cooling intrusions. Igneous sills are represented by horizontally dominated thermal anomalies that are sequentially placed within the sedimentary basin. We test different end-member scenarios of emplacement like bottom-to-top, top-to-bottom, and arbitrary emplacement order. Degassing pulses monitored during the simulations are recorded and compared for various end-member scenarios. The LIP emplaced in the Karoo Basin (South Africa, 183 Ma) is considered as a case study. We use basin lithostratigraphic properties (e.g. Total Organic Carbon content, sill to sediment proportion and structural data) to discuss results of our end-member models. This research potentially holds the key to demonstrate whether or not anthropogenic warming is in a comparable range to a documented paleo-environmental crisis and mass extinction triggered by degassing related volcanism

Description: Magma emplacement in organic‐rich sedimentary basins is a main driver of past environmental crises. Using a 2D numerical model, we investigate the process of thermal cracking in contact aureoles of cooling sills and subsequent transport and emission of thermogenic methane by hydrothermal fluids. Our model includes a Mohr‐Coulomb failure criterion to initiate hydrofracturing and a dynamic porosity/permeability. We investigate the Karoo Basin, taking into account host‐rock material properties from borehole data, realistic total organic carbon content, and different sill geometries. Consistent with geological observations, we find that thermal plumes quickly rise at the edges of saucer‐shaped sills, guided along vertically fractured high permeability pathways. Contrastingly, less focused and slower plumes rise from the edges and the central part of flat‐lying sills. Using a novel upscaling method based on sill‐to‐sediment ratio we find that degassing of the Karoo Basin occurred in two distinct phases during magma invasion. Rapid degassing triggered by sills emplaced within the top 1.5 km emitted ~1.6·103 Gt of thermogenic methane, while thermal plumes originating from deeper sills, carrying a 12‐times greater mass of methane, may not reach the surface. We suggest that these large quantities of methane could be re‐mobilized by the heat provided by neighboring sills. We conclude that the Karoo LIP may have emitted as much as ~22.3·103 Gt of thermogenic methane in the half million years of magmatic activity, with emissions up to 3 Gt/year. This quantity of methane and the emission rates can explain the negative δ13C excursion of the Toarcian environmental crisis. Key Points Sill geometry and emplacement depth as well as intruded host rock type are the main factors controlling methane mobilization and degassing Dehydration‐related porosity increase and pore‐pressure‐induced hydrofracturing are important mechanisms for a quick transport of methane from sill to the surface The Karoo Basin may have degassed ~22.3·103 Gt of thermogenic methane in the half million years of magmatic activity

Description: Der Magmatismus in Sedimentbecken verursachte globale Massenaussterben und ist die eng ste Analogie zum heutigen anthropogenen Klimawandel. Das Studium dieser natürlichen Prozesse ist oft schwierig, da die Magma-Aufstiegssysteme mit kaum sichtbarer Oberflächen expression verschüttet bleiben. Bei GEOMAR untersuchen wir diese Systeme mit Hilfe von marinen seis-mischen Daten und modernsten numerischen Modellen.