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Magma-sediment interaction at the subseafloor: mechanisms and role in the Earth CPL-Cycles (Carbon-, Plate-, Life-Cycles)

The impact of magma-sediment interactions in the top wet sediments of active sedimentary basins is poorly quantified. Yet this accelerator of diagenetic processes, impacts important fields of geology ranging from potential ocean-climate disorder due to petroleum mobilization and emission, to catabolic power supply for deep microbial activity. With one end-member killing life and the other fueling it, can we derive the implications at the scale of the plate cycle? This session calls for contributions that help to understand the magma-sediment interaction processes in the first 500 m of active sedimentary basins and how it may play a major role the Earth CPL-Cycles (Carbon-, Plate-, Life-Cycles).

Convener: Christophe Galerne | Co-conveners: Tara StephensECSECS, Tobias Hoefig, Ivano Aiello
Presentations
| Wed, 25 May, 17:00–18:12 (CEST)
 
Room -2.16

Wed, 25 May, 17:00–18:30

Chairpersons: Christophe Galerne, Tobias Hoefig

17:00–17:06
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EGU22-3879
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ECS
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Highlight
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Virtual presentation
Tara L. Stephens et al.

Magmatic sheet intrusions are commonly segmented, across multiple scales, with preserved segments typically interpreted as representing an early stage of intrusion growth. Intrusion propagation has long been associated with linear elastic host rock deformation, associated with tapered or elliptical tip zones. Many studies have identified intrusive segments with non-tapered (e.g., superelliptical) tip geometries, associated with a variety of non-brittle host rock deformation. This has led to development of several anelastic propagation models, including fluidisation, viscous indentation, brittle faulting, and ductile faulting and flow. These models are commonly inferred to represent the propagation mechanism throughout intrusion growth, in host rocks with constant material properties. However, non-brittle host rock deformation may overprint earlier emplacement mechanisms, hence tip geometries observed in the field may not be indicative of the entire emplacement process.

Here we present a quantitative field study of segment tip geometry and associated host rock deformation using a segmented basaltic sill network at Neist Point, Isle of Skye, UK (part of the Little Minch Sill Complex), and static Finite Element (FE) Models of intrusion tip stress distributions, to define a new conceptual model for intrusion growth. The FE models highlight that as tip geometry changes from elliptical to rectangular, tensile and shear stress maxima move increasingly out-of-plane following the positions of maximum tip curvature, as would be expected for non-brittle propagation and matching field observations of host rock deformation. The studied sill segments are hosted in interbedded limestone, sandstone, siltstone, and mudstone units, and are mostly thin (<2 m thickness) with each hosted in a single unit; two of the studied intrusions are thick (>2 m) and their tips transect multiple units. We identified 39 segments in total, 26 of which were geometrically characterised, and a total of 43 tips were measured. Segments with tapered tips were commonly associated with host rock bending (elastic-brittle emplacement), while superelliptical segments show a variety of host rock deformation (e.g., brecciation, faulting). Notably, this deformation is limited to the preserved segment tips, with no such features recorded along the length of the intrusions. Tip geometry and host rock deformation style are not linked to host rock lithology: local conditions of emplacement evolve to facilitate varying deformation mechanisms within a single intrusive network. Changes to magma viscosity (via crystallisation, volatile/heat loss) and host rock properties (heating, brecciation, fluidisation) may inhibit elastic-brittle fracture, and promote segment inflation and non-brittle propagation.

We propose a multiphase conceptual model for basaltic segments in an initially brittle host. Segments are emplaced initially via elastic-brittle fracture followed by a transitional phase of segment inflation, tip rounding, and modification to the conditions of emplacement leading to a non-brittle propagation phase. Our model accounts for multiple segment geometries and styles of host rock deformation observed across many intrusive complexes and across an array of host rock lithologies. Intrusive segments preserved in outcrop primarily represent the final conditions of emplacement, rather than their growth.

How to cite: Stephens, T. L., Walker, R. J., Healy, D., Bubeck, A., Greenfield, C., Gill, S. P. A., and Poppe, S.: Segment tip geometry of sheet intrusions: a dynamic model for the evolving conditions of emplacement, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3879, https://doi.org/10.5194/egusphere-egu22-3879, 2022.

17:06–17:12
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EGU22-6464
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Highlight
Ivano Aiello et al.

The diagenesis of biogenic silica is arguably one of the most significant early diagenetic processes in the shallow subseafloor and can also have profound implication for the routing of magma in rift basins. The transformation of glass-like amorphous silica (opal-A) in diatoms and radiolarians into crystalline forms (opal-CT) converts soft and watery biogenic oozes into harder sedimentary rocks. The silica diagenetic front is extensive (km-scale) forming prominent seismic reflectors, causes regional-scale differential compaction, subsidence, and the expulsion of pore fluids. Most importantly, it corresponds to changes in sediment rheology from ductile oozes to brittle, fracture-prone and more permeable sedimentary rocks. The relationships between silica diagenesis and subseafloor magmatism have been investigated by Expedition 385 of the Integrated Ocean Discovery Program (IODP) in the off-axis region Guaymas Basin (GB) of the Gulf of California at the neighboring Sites U1545 and U1546, the latter including a  ~70 m-thick sill intrusion, and at Site U1547, where the top of a massive sill was recovered at shallower depths. The lithostratigraphic and mineralogic analyses of the cores recovered by the expedition combined with interstitial water geochemistry and physical properties unveil a new and somewhat unexpected picture of the GB’s subseafloor environment. The first discovery is that despite the super-fast sedimentation/burial (up to 1m/kyr) and the very high geothermal gradients, the transformation of opal-A to opal-CT occurs at ~75 ºC or higher in situ temperatures which is much hotter (and deeper) than expected based on previous deep-sea core studies, outcrop studies, laboratory experiments or calculated by models. We hypothesize that the apparent ‘sluggishness’ of silica transformation is the result of the current kinetic model not being able to predict this transformation when burial rates are much faster than typical biogenic sediments in open ocean conditions for which they were originally created. The second important finding relates the Opal CT-zone with magma intrusions.  The massive sill  at Site U1546 splits the opal-CT zone, though the latter has identical characteristics (e.g. total thickness, gradual increase in silica crystallization with depth) as the opal-CT zone at the nearby Site U1545, located just outside the extent of the sill. Moreover, the sill intrusion is much shallower at Site U1547 where the opal-CT zone is also shallower due to the higher geothermal gradient (~510 Cº/km as opposed to ~100 ºC/km at Site U1546). Not only these observations suggest that the sill formation postdates the silica phase change, but also that this diagenetic interface controls the way magma moves in the GB subseafloor whereby the opal-A/opal-CT transition zone acts as major physical anisotropy in the sedimentary column to reroute magma from vertical to lateral movement. In conclusion, this study greatly expands the range of depths/temperatures at which amorphous silica can persists in the subseafloor and establishes fascinating connections between seemingly disconnected processes in the natural world: surface water biological productivity and crustal architecture of a newborn ocean. 

How to cite: Aiello, I., Hofig, T., Riboulleau, A., Galerne, C., and Buatier, M.: Relationships between routing of magma and biosilica diagenesis in the shallow subseafloor of a nascent ocean basin (Guaymas Basin, Gulf of California), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6464, https://doi.org/10.5194/egusphere-egu22-6464, 2022.

17:12–17:18
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EGU22-6679
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Highlight
Christophe Galerne et al.

Magma-sediment mingling occurring in shallow porous sediments is mainly investigated through field observations of old exhumed rift basins. During the IODP Expedition 385 we have drilled through the shallow sills emplaced in the active rift of the Guaymas Basin, Gulf of California. The results of this expedition enable a pioneer study of the impact of recent magma-sediment mingling processes leading to peperite formation. Furthermore, it provides a present-day geological context to investigate and quantify the impact of this mingling process on the element cycles in subsurface sediments, and temporal evolution of the microbial habitat associated with epithermal hydrothermal fluid circulation. Our approach of exploring magma-sediment mingling processes includes laboratory experiments, numerical modelling, and identification of specific field analogues in addition to petrological and geochemical constraints.

Using these modern techniques, we review here the discoveries made during the IODP Expedition 385 and present preliminary results from our post-cruise research from the perspective of the peperite formation. We report here petrographic and geochemical evidence of magma sediment hybridization indicative of an intense mingling process inferred to occur during the emplacement phase. The rheology of the soft, unconsolidated sediment controls and explains the various intrusion shapes and dimensions. Numerical simulation results indicate that heat dissipation in this context is much less efficient, which in turn considerably decreases the amount of thermogenic gas mobilized through thermal cracking in the contact aureole of sills. Additionally, we observe that hydrothermal pipe systems established during the cooling phase of sill emplacement can remain active at moderate- to low-temperature state after the heat of the sill has vanished. Using 2D seismic information and IODP drilling results, we were able to reconstruct the 3D structure of the sill at depth. It is funnel-shaped and roots in a depth where geothermal fluids can ascend from. The temperature found at these depths is consistent with the background geothermal gradient, suggesting that the large heat flow anomaly found at Hole U1548C is the mere expression of the active hydrothermal circulation fuelled by deeply sourced geothermal fluids.

These potentially long-lasting hydrothermal systems provide preferable temperature and energetic conditions for microbial activity to thrive, with mildly degraded petroleum components from below and water recharge from above. Moreover, evidence indicates that the sill at Site U1547 is non-unique at the scale of the Guaymas Basin. How many of these catabolic reactors form at the early rifting phase? Can this process perhaps trigger peaks in subsurface biomass production associated with new continental margin formation? Our research heralds the dawn of a new paradigm. We suggest that in the context of a nascent ocean, sill emplacement in the first 500 m of sediments may power life instead of suppressing it.

How to cite: Galerne, C., Bach, W., Berndt, C., Schwark, L., Höfig, T., Buatier, M., Cheviet, A., Kahl, W.-A., Hasenclever, J., Lizarralde, D., Stockhausen, M., Wiggers, C., Karstens, J., and Monien, P. and the IODP Expedition 385 Scientists: Magma-sediment mingling processes, control and longevity of related hydrothermal systems – Implications for the Earth’s Carbon-, Plate-, Life-Cycles (IODP Exp 385, Guaymas Basin, Gulf of California), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6679, https://doi.org/10.5194/egusphere-egu22-6679, 2022.

17:18–17:24
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EGU22-9637
Wolf-Achim Kahl et al.

Magma emplacement in basin sediments at shallow levels initiates a plethora of processes, proceeding both simultaneously and sequential, and on variable length scales. On the grain scale, uptake and heating of sediment and pore fluids during magma emplacement, as well as the fractionation of a highly mobile fluid phase upon crystallization induce distinct rock microfabrics.  

 

IODP Expedition 385 drilled through the shallow sills emplaced in the active rift of the Guaymas Basin, Gulf of California. Here we present first results of a high-resolution X-ray µ-CT survey on drill core material recovered from Hole U1546, covering a shallow basaltic sill emplaced within 300 m below the seafloor. Comprising sample material from just a few meters from top and bottom contacts, from the center of the ca. 75 m thick sill, and a distinctly gabbroic region in the upper third of the sill, several characteristic features of the rock microfabric have been observed. In general, 3D quantitative digital image analysis of the porosity matches the trend determined by onboard moisture and density analyses. Detailed size and shape analysis reveal a bimodal distribution of the pores near the sill margins: Near the top contact, porosity is constituted by small interstitial pores and large spherical vesicles; Near the bottom contact, scarce pores comprise minute interstitial voids and dendritic cavities within granular concretions. While in the center layer of the sill pores are minuscule, porosity within the gabbroic region occurs as almost mm-sized interstitial cavities. The spherically-shaped large vesicles near the top contact feature a complex history: Ductile environment is required in the course of formation, and subsequent precipitation of zeolite or/and calcite can be observed. In places, iron sulfide is present near or at the vesicle walls. In addition, consideration of the grain phase allows further constraints. While plagioclase phenocrysts are abundant throughout the entire sill thickness, grain sizes of matrix pyroxenes and plagioclase near the center are larger than near the contact. In the gabbroic region the presence of hornblende is indicative of a water content of 2 to 3 wt.%. The induced lowering of the solidus temperatures in this region explains why very large crystal sizes are allowed to develop morphologies of pyroxene and plagioclase intergrowth resulting in a coarser gabbroic texture.

How to cite: Kahl, W.-A., Galerne, C., and Bach, W.: Assessing mechanisms and timing in magma-sediment interaction in the subseafloor: True 3-D microfabric observations provide insights into shallow sill emplacement processes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9637, https://doi.org/10.5194/egusphere-egu22-9637, 2022.

17:24–17:30
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EGU22-9301
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ECS
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Virtual presentation
Christin Wiggers et al.

Magma emplacement and the mingling of sediment with magma (forming peperite facies) in unconsolidated, near-surface basin sediments have been described in fossil systems but have not been reported in active settings. The IODP Expedition 385 drilled through the soft sediment (first 500 m) of the Guaymas Basin, a young marginal rift basin in the Gulf of California. This basin is characterized by an unusually high spatial density of sill intrusions emplaced off-axis. One of the drilled sills, inferring an emplacement within 200 m depth below the seafloor (Site U1547), displays an extraordinarily high porosity (12–20%) in the form of large spherical vesicles. Despite being cold and crystallized, this sill is at the root of an active ring of hydrothermal system that outlines the edges of the sill at depth. Here we explore the hypothesis that this high porosity originates from the magma-sediment mingling process occurring during the magma emplacement. Understanding this process has direct implications for better constraining the different stages of epithermal activity in the Guaymas Basin and will provide clues to possible mass transfers and longevity of the hydrothermal system.

To test the above hypothesis, we performed laboratory experiments in a gas-mixing furnace to determine how the primary porosity is affected by the liberation of thermogenic gas at high temperature from the organic-rich sediment. Experiments were conducted at 1200 ℃ and fO2 corresponding to -8 (QFM). We quantified element segregation associated with textural variations using scanning electron microscopy and electron microprobe analysis and compared the results with observations in samples from IODP Expedition 385. Our results show that large vesicles form when the magma mix with organic-rich sediments. Natural samples from Site U1547 display increasing porosity towards the top of the sill consistent with an immiscibility process of the thermogenic free gas similar to that demonstrated in the laboratory experiments. Additionally, the absence of shear forces applied to the large vesicles (perfectly spherical) most likely reflect an initial increasing amount of sediment uptake into the magma towards the top contact. This suggests that the sediment uptake during the emplacement process is reflected and pre-condition the final porosity profile of the sill. Calcite precipitation in some of the vesicles and iron sulfide presence near or at the vesicle walls are good support of a sediment-derived origin of the vesicles.

Our investigation demonstrates the process by which large porosity forms and develops in the post-emplacement phase of a sill. This occurs through a seeding process associated with magma-sediment mingling resulting in the assimilation of wet and organic-rich sediment during the emplacement phase. A numerical simulation of the regional setting of Guaymas Basin will show how gas accumulation might have been encountered during the emplacement phase, thereby considerably enhancing the porosity growth potential inside the sill. We propose that the high porosity of the sill could be instrumental in driving the current hydrothermal stage of the system, as it enables channelling of deeply sourced geothermal fluids along and through the existing magma plumbing system.

How to cite: Wiggers, C., Galerne, C., Acosta, M., Pistone, M., Bach, W., Kahl, W.-A., Burwicz-Galerne, E., Monien, P., Baumgartner, L. P., Höfig, T., and Klügel, A.: Microstructural and chemical investigation of magma-sediment mingling in natural and laboratory samples, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9301, https://doi.org/10.5194/egusphere-egu22-9301, 2022.

17:30–17:36
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EGU22-6922
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Virtual presentation
Martine Buatier et al.

The Guaymas Basin in the Gulf of California represents the nascent stage of an ocean, characterized by siliceous organic-rich sediments (diatom ooze) deposited at very high sedimentation rates. The basin is also characterized by a dense network of shallow mid-Pleistocene to recent sills that have intruded the subseafloor over a distance of several tens of kilometers from the spreading axis. These magmatic intrusions produce heat anomalies and sediment transformations in the contact aureoles that affect both organic and inorganic compounds, such as cracking of sediment OM, dehydration and dissolution of mineral phases, creation of porosity and convection of hydrothermal fluids. We focus our investigations on the phyllosilicates (clay minerals) that formed at the contact between the sill and the sediment (IODP Expedition 385, Sites U1546 and U1550): overall our results support the intepretation that these hydrated minerals can be used as proxies for fluid-rock interactions. In our study, we investigate sediment and sill samples from the contact zones by combining XRD analysis of oriented samples (< 2 and <16 µm fractions) with optical and SEM observations.  The XRD patterns from the clay fractions of the siliceous claystones suggest that the background (non metamorphosed) sediment of the Guaymas basin is mainly composed of detrital smectite, kaolinite and illite. However, in both the lower and upper aureole, the contact sediment displays a different clay assemblage characterized by the occurrence of mixed-layer smectite and chlorite. In the sediment located just above the sill contact at Sites U1546 and U1550 the corrensite (a regular smectite-chlorite mixed-layered) is also present. SEM images support the interpretation that this mineral is authigenic and occurs together with with euhedral pyrrhothite and quartz suggesting that it precipitated from hydrothermal fluid circulation. The magmatic sill is associated with two types of phyllosilicates. One is mica (biotite composition) which occurs as large crystals (~50 to ~100 µm) intergrowths with magmatic plagioclase suggesting a magmatic origin. The other one, smectite aggregates filling vacuoles and replacing magmatic glass, is widespread in most basaltic samples. The latter is a trioctahedral Mg-Fe smectite (Saponite) with K or Na in the interlayer. The formation of the smectites suggest that basalt further interacted with connate fluids or seawater and that a significant hydratation of the basalt is located at sediment contact.

Authigenic phyllosilicates are abundant at sediment-sill interface, they registered the last stages of the fluid-rock interaction probably related to hydrothermal fluid circulation.

How to cite: Buatier, M., Cheviet, A., Gameiro, M., Choulet, F., and Aiello, I.: Phyllosilicate precipitation in sediment and sill from the Guaymas basin : proxies for post magmatic and hydrothermal fluid circulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6922, https://doi.org/10.5194/egusphere-egu22-6922, 2022.

17:36–17:42
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EGU22-6261
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ECS
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Virtual presentation
Alban Cheviet et al.

Igneous sill intrusions into young organic-rich sedimentary basins have major impact on the carbon cycle but also on the transfer of major and trace element between deep and superficial geological reservoirs. The Guaymas Basin in the Gulf of California represents the nascent stage of an ocean characterized by siliceous organic-rich sediments (diatom ooze) deposited at high sedimentation rates. A very dense network of shallow sill intrusions recently invaded the basin. We focused on Site U1546 (Holes A and C) located at about ~51 km northwest of the axial graben of the northern Guaymas Basin spreading segment; this site recovered 540m of sediments and  an 80m-thick sill located at 350-430 meters below the seafloor (mbsf). The relatively high geothermal gradient (> 200 °C/km) induces measurable diagenetic transformations in sediments, involving sulfides, carbonates and silica (and clay minerals). Based on retrieved materials from IODP Expedition 385, we present here geochemical and mineralogical characterization of the sedimentary intervals at sill contacts. Our results indicate that sulfides and silica polymorphs are the main phases impacted by contact metamorphism. The transition between opal CT-quartz and pyrite-pyrrhotite is observed in the contact aureoles. In the upper aureole, authigenic quartz and disseminated 20-50 micron pyrrhotite partly fill secondary pores and detrital feldspars are partially dissolved. Patchy carbonate also fills primary interparticle sediment pores just above the contact. In the lower contact aureole, quartz and 200-micron-size euhedral crystals of pyrrhotite are also present. Additionally, a significant metasomatism is observed in the lower contact-aureole meta-sediments with authigenic plagioclase precipitated around detrital feldspars and locally euhedral pyroxenes included in patches of carbonate cement; this suggests precipitation by late to post magmatic fluids at T>300°C. The lower contact aureole is also more enriched in CaO, Na2O, Fe2O3 and trace elements (Cu, As, Zn…). Based on these petrological investigations a new conceptual model of magma sediment fluid interactions will be proposed.

How to cite: Cheviet, A., Buatier, M., Choulet, F., Galerne, C., Riboulleau, A., Aiello, I., and Marsaglia, K.: Contact metamorphic reactions related to magmatic sill intrusion in the Guaymas basin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6261, https://doi.org/10.5194/egusphere-egu22-6261, 2022.

17:42–17:48
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EGU22-6433
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ECS
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Virtual presentation
Paleomagnetism of the Guaymas Basin, Gulf of California: New Results from IODP Expedition 385
(withdrawn)
Louise M.T. Koornneef et al.
17:48–17:54
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EGU22-4385
Swanne Gontharet et al.

   During the International Ocean Discovery Program (IODP) Expedition 385, diagenetic carbonates were recovered at different depths (from 10 to 534 mbsf) in six drilling sites (U1545, U1546, U1547, U1548, U1550 and U1552) distributed from the northern spreading segment of the Guaymas Basin (Gulf of California) to ~52 km away from the axial graben. As this basin is a site of active sea floor spreading, hemipelagic diatomaceous and organic-rich sediments rapidly accumulating at all these drilling sites are influenced by magmatic intrusions, at depths greater than 90 mbsf. The geothermal gradient observed ranges from 135 °C/km to 682 °C/km. Except in sites U1547 and U1548, both located close to a circular hydrothermal mound (called Ringvent), sills are in thermal equilibrium with the surrounding sediments.

   Carbonate samples (over fifty) collected during this expedition are composed of fine-grained micritic sediments and weakly to strongly lithified carbonate concretions. The petrographic observations and XRD analyses show that stoichiometric dolomite is the dominant authigenic carbonate phase, with minor contribution of Fe-rich dolomite, and low- to high-Mg calcite. Centimeter-sized concretions composed of ankerite and Mg-calcite also occur in the sediments collected from the northern axial graben (site U1550) and a site with subsurface gas hydrates (U1552), respectively. Other authigenic minerals are often associated with these carbonates including pyrite, barite, anhydrite, zeolites and crystalline opal (opal-CT). The carbon isotopic compositions of the bulk carbonate from these samples exhibit large variations ranging from -28.9 to +12.0‰ VPDB. Very low d 13C values, only measured in a centimetre-seized concretion associated with cold methane seepage (site U1552), indicate that the bicarbonate used for carbonate precipitation is derived from anaerobic oxidation of methane (AOM) coupled with bacterial sulfate reduction (BSR). The very high d13C values of diagenetic carbonates indicate the use of biogenic 13C-rich CO2 reservoirs related to active methanogenesis or other autotrophic microbial pathways. The oxygen isotopic compositions of these carbonates cover a large range from -5.3 to +4.0‰ VPDB. The decreasing d18O values with increasing depth reflect the effect of high geothermal gradients due to the close proximity of magmatic sills.  

How to cite: Gontharet, S., Pierre, C., Pelleter, E., Caquineau, S., Boudouma, O., Demange, J., Teske, A. P., Lizarralde, D., and Höfig, T. W. and the Expedition 385 Scientists: Diagenetic carbonates from deep sub-seafloor organic-rich sediments influenced by magmatic sill intrusions (IODP Exp 385-Guaymas Basin, Gulf of California), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4385, https://doi.org/10.5194/egusphere-egu22-4385, 2022.

17:54–18:00
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EGU22-1504
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ECS
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Virtual presentation
Florian Neumann et al.

Heat flow is estimated at eight sites drilled during Integrated Ocean Drilling Program (IODP) Expedition 385 in the sedimented Guaymas Basin, Gulf of California. One of the expedition objects was designed to understand the thermal regime of the basin and to better understand heat transfer mechanisms from sill intrusions into organic-rich sediment. Sedimentation corrections are significant and increase basin values of heat flow values on average by 12% and range from 119 to 221 mW/m2 in the basin. Thermal analysis suggests that heat flow in the basin is distributed equally between conductive and advective heat transfer for plate ages older than 0.2 Ma. At Ringvent, Site U1547 a young sill intrusion is related to locally elevated heat flow displaying values between 257 and 1000 mW/m2. Thermal analysis of the five holes drilled at Site U1547 suggests that the sill structure hosts an active hydrothermal system. Our study suggests that rapidly cooling intrusion led to discharge velocities between 15 – 40 mm/yr and possibly recharge of the system may occur through normal faults. To be consistent with the heat output, we estimate the sill intrusion thickness to be ~240 m. The highly three-dimensional nature of the sill intrusion at Site U1547 and the question of its thickness add considerable complications which are currently investigated in complementary studies.

How to cite: Neumann, F., Negrete-Aranda, R., Harris, R. N., Contreras, J., Galerne, C., Peña-Salinas, M. S., Spelz-Madero, R., Lizarralde, D., Teske, A., and Hoefig, T. and the Expedition 385 Scientists: Heat flow and thermal regime in the Guaymas Basin, Gulf of California: Estimates of conductive and advective heat transport, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1504, https://doi.org/10.5194/egusphere-egu22-1504, 2022.

18:00–18:06
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EGU22-3312
Yuki Morono et al.

Guaymas Basin is a young marginal rift basin in the Gulf of California characterized by active seafloor spreading and rapid sediment deposition, including organic-rich sediments derived from highly productive overlying waters and terrigenous sediments from nearby continental margins. The combination of active seafloor spreading and rapid sedimentation within a narrow basin results in a dynamic environment where linked physical, chemical, and biological processes regulate the cycling of sedimentary carbon and other elements. This continuum of interrelating processes from magma to microbe motivated International Ocean Discovery Program Expedition 385 and is reflected in its title, “Guaymas Basin Tectonics and Biosphere.”

During IODP Expedition 385, organic-rich sediments with sill intrusions on the flanking regions and in the northern axial graben of Guaymas Basin (in eight sites) were drilled and core samples were recovered. Those cored samples were examined for their microbial cell abundance in a highly sensitive manner by density-gradient cell separation at the super clean room of Kochi Core Center, Japan, followed by direct counting on fluorescence microscopy. Cell abundance in surficial seafloor sediment (~109 cells/cm3) was roughly 1000 times higher than the bottom seawater (~106 cells/cm3) and gradually decreased with increasing depth and temperature. In contrast to the cell abundance profile observed at Nankai Trough (IODP Exp. 370), the gradual decrease of cell abundance was observed up to around 75ºC, and we detected microbial cells even at hot horizons above 100ºC.

We will present the overview of the microbial cell distribution in the Guaymas Basin and discuss its relation to the current and past environmental conditions, e.g., temperature and sill-intrusion, etc.

How to cite: Morono, Y., Teske, A., Galerne, C., Bojanova, D., Edgcomb, V., Meyer, N., Schubert, F., and Toffin, L. and the IODP Expedition 385 Scientists: Microbial cell distribution in the Guaymas Basin subseafloor biosphere, a young marginal rift basin with rich organics and steep temperature gradient, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3312, https://doi.org/10.5194/egusphere-egu22-3312, 2022.

18:06–18:12
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EGU22-3278
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Highlight
Andreas, P. Teske et al.

Guaymas Basin is a young marginal rift basin in the Gulf of California characterized by active seafloor spreading and rapid deposition of organic-rich sediments derived from highly productive overlying waters and terrigenous sediments from nearby continental margins. Catalyzed by the steep geothermal gradient, sedimentary organic material of photosynthetic origin turns into a diverse spectrum of hydrocarbons that accumulate especially in deep, hot sediments, and might supply substrates for hydrocarbon-degrading microorganisms. The “from magma to microbe” perspective on these processes motivated International Ocean Discovery Program Expedition 385“Guaymas Basin Tectonics and Biosphere.”

Hydrocarbon concentrations were determined in sediment samples selected from eight drilling sites on the flanking regions and in the northern axial graben of Guaymas Basin. Total petroleum hydrocarbon (C9-C44) concentrations increased from ca. 50-250 mg/kg towards >2000 mg/kg at in-situ temperatures above 80°C. A similar increase from ca. 10 mg/kg towards >100 mg/kg was observed for total saturated hydrocarbons. These gradients are shaped by abiotic hydrocarbon generation above 80°C at depth, and possibly by microbial hydrocarbon degradation at cooler temperatures in the upper sediment column. In a two-pronged approach, we are currently investigating the activity of bacterial/fungal consortia, isolated from Guaymas Basin surficial sediments, in the oxidation of selected alkanes and polyaromatics that occur at Guaymas Basin. In parallel, we explore the diversity, depth range and in-situ temperature range of bacteria, archaea and fungi in the Guaymas Basin subsurface sediments using PCR and metagenomic sequencing, to constrain microbial hydrocarbon cycling in the deep subsurface. Updates on these ongoing investigations will be presented.

How to cite: Teske, A. P., Mara, P., Vázquez, M., Baker, B., Morono, Y., and Edgcomb, V. and the Expedition 385 Scientists: Investigating microbial constraints on hydrocarbon processing in Guaymas Basin subseafloor sediments with sill intrusions , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3278, https://doi.org/10.5194/egusphere-egu22-3278, 2022.