GMPV9.7

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.

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.

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.

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.

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.

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.

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.