Submarine Hydrothermal Activity Research Articles

Sedimentary paleoenvironment and its control on organic matter enrichment in the Mesoproterozoic hydrocarbon source rocks in the Ordos Basin, southern margin of the North China Craton

The black shale of the Mesoproterozoic Cuizhuang Formation in the Changcheng System in Yongji city, North China Craton, is a potential source rock. Understanding the organic matter enrichment mechanism is crucial for evaluating source rock resources and understanding oil and gas accumulation mechanisms. In this study, we evaluated the sedimentary paleoenvironment and organic matter enrichment mechanisms of shale using thin section observations, mineral composition analysis, organic geochemistry, and elemental geochemistry. We found significant differences in the sedimentary paleoenvironment and organic matter enrichment mechanisms between the lower Cuizhuang Formation and the Beidajian Formation shale. The Cuizhuang Formation was deposited in a late-stage, restricted basin environment during the rift phase, and elemental and geochemical indicators showed that the Cuizhuang Formation was in a suboxic–anoxic water environment, that was influenced by a warm and humid paleoclimate and submarine hydrothermal activities, which promoted the accumulation of organic matter. However, the enrichment of organic matter in the Cuizhuang Formation was mainly controlled by redox conditions. The formation of suboxic-anoxic water environments may be closely related to the warm and humid paleoclimate and submarine hydrothermal activities. Warm conditions promote continental weathering and increase marine productivity, thereby consuming oxygen in the bottom water. Moreover, acidic hydrothermal activity also helps to establish an anoxic environment. Our results reveal the effects controlling various coupled mechanisms dominated by redox conditions, which may explain the development of source rocks in the Cuizhuang Formation.

Submarine hydrothermal fluids facilitated the accumulation of cadmium in Lower Cambrian black shales, South China

The effects of Cd-rich venting fluids on the Cd contents and isotopic compositions of continental margin sediments remain poorly understood. Characterization of the Cd isotopic composition of sediment overprinted by venting fluids is critical, as this may provide insights into how mixing between the venting fluids and bottom waters affects the Cd isotopic composition. Our study determined the Cd isotopic compositions of Lower Cambrian metalliferous sediment deposited on the passive continental margin of the southeastern Yangtze Block, South China. The Cd is predominantly hosted in sphalerite of sulfide-bearing barite sediments, V deposits, and Ni-Mo−platinum group element (PGE) sulfide layers. The Cd isotopic compositions of samples from these three units indicate that the distribution of Cd at this type of continental margin was controlled by submarine hydrothermal activity. Following the venting of hydrothermal fluids onto the seafloor, Cd2+ reacted with a limited amount of H2S generated mainly by thermochemical sulfate reduction near the vents. This resulted in isotopically lighter Cd in the sulfide-bearing barite sediments and V deposits, which have δ114/110Cd values of 0.00‰ ± 0.13‰ and 0.00‰ ± 0.14‰, respectively. The residual dissolved Cd in the fluids migrated with the upwelling seawater and ultimately precipitated almost entirely as sphalerite in the sulfidic wedge, which resulted in Ni-Mo−PGE sulfide layers with a narrow range of high δ114/110Cd values (0.35‰ ± 0.03‰). The presence of Cd-rich venting fluids can alter the Cd contents and isotopic compositions of passive margin sediments, particularly those located within long-term stratified seawater and fault depressions. Previous studies may have underestimated the migration distance and affected areas of hydrothermal activity involving Cd-rich fluids.

Research on Submarine Hydrothermal Activity and Its Material Circulation, Magmatic Setting, and Seawater, Sedimentary, Biologic Effects

Submarine hydrothermal activity has been a focused research topic since its discovery in the 1970s [...]

Paleoenvironmental evolution and organic matter accumulation of the lower Cambrian Maidiping marine black shales in the intracratonic basin, western margin of Sichuan Basin, South China

ABSTRACT The Late Ediacaran to Early Cambrian marked a pivotal phase in paleoenvironmental and tectonic shifts. The Sichuan Basin, on the Yangtze platform’s western edge, developed the Mianyang-Changning intracratonic sag due to Xingkai taphrogenesis. The Maidiping Formation’s black shales, the initial fine-grained clastic deposits from the Early Cambrian, span the Sichuan Basin’s western margin. The influence of the intracratonic sag on organic matter accumulation and paleo-marine environment evolution remains unclear. Comprehensive petrological and geochemical analyses on the Qingping section revealed three distinct members within the Maidiping Formation: black shales and silicified dolomite with shales in a restricted basin (lower member), deep-water black shales (middle member), and phosphoritic turbidite sandstone with siliceous phosphorites (upper member). The lower member’s organic accumulation arose from restricted ocean circulation, leading to stratified waters and reduced oxygen at depth. The middle member’s organic build-up was influenced by enhanced intracratonic sag connectivity with the open ocean, sea-level rise, and increased productivity. The submarine hydrothermal activity might have induced anoxic conditions. The upper member saw a decline in organic matter due to phosphoritic debris and oxygenation. The Maidiping Formation augments the Lower Cambrian source rock, vital for oil and gas exploration in the Sichuan Basin’s western margin.

Emergence of the 2.1 Ga Francevillian biota was preceded by unprecedented hydrothermally driven seawater eutrophication

Recently, two independent studies suggest that the emergence of putative fossilized macro-eukaryotes in the Paleoproterozoic Francevillian Basin, ~2.1 billion years ago, may be related to a rise in seawater Zn bioavailability. This explanation is reliant on their extraordinary high Zn content and association with light Zn isotopes characteristic of eukaryotic enrichment. However, the trigger and origin of rising seawater Zn supply to the basin remains unknown. This study unravels a transient episode of intense submarine hydrothermal activity that triggered the weathering of a nutrient-rich oceanic crust reservoir, related to the collision of the Congo-São Francisco cratons during the Eburnean-Transamazonian orogeny, as the source of abundant seawater dissolved Zn, together with a suite of essential trace metals and phosphate to the continental margin waters. Surprisingly, the initiation of hydrothermal weathering coincided with the rapid onset of a rare Paleoproterozoic seawater eutrophication event. This transition is marked by basin-wide redox stratification, high sediment loading with organic carbon (Corg) and nitrogen, elevated C/N ratios, a steep negative Corg and positive bulk N isotope excursion, positive Ce anomalies, and low Mn/Fe ratios. Importantly, the transient eutrophication event ended with a reversal to lower seawater phosphate levels that coincided with rapid seawater ventilation and the appearance of macrofossil bearing sediments in Franceville. We suggest that these unexpected, localized conditions, set the stage for the emergence of the Francevillian biota.

Hydrothermal ferromanganese oxides around a petit-spot volcano on old and cold oceanic crust

Areas of old and cold oceanic plate lack conventional volcanism and have been assumed to be devoid of submarine hydrothermal activity. However, petit-spot volcanoes are common in areas of flexure of such oceanic plates. Here, we report hydrothermal ferromanganese oxides dredged from the vicinity of a petit-spot volcano at 5.7 km water depth in an area of oceanic plate flexure east of the Japan Trench. The bulk chemical, lead isotopic and mineralogical compositions of the samples indicate their formation by low-temperature hydrothermal activity, which can be interpreted as being caused by fluid–rock interactions at <200 °C. We propose that interaction of local marine sediments with volatile-rich petit-spot magmas may produce hydrothermal fluids containing not only iron and manganese but also enough amounts of carbon dioxide and methane to have implications for the global carbon cycle. However, contemporary hydrothermal activities at petit-spot volcanoes have not been confirmed yet.

Metamorphic Ages of the Jurassic Accretionary Complexes in the Kanto Mountains, Central Japan, Determined by K–Ar Dating of Illite: Implications for the Tectonic Relationship between the Chichibu and Sanbagawa Belts

To determine the metamorphic ages of the accretionary complexes in the Northern Chichibu Belt in SW Japan, K–Ar dating was conducted using weakly metamorphosed sedimentary rocks collected from the Kanto Mountains, Central Japan. Whole-rock ages were obtained for chert and red shale samples, and the mineral ages of fine-grained illite with a grain size of less than 4 μm were obtained for chert, red shale, mudstone, acidic tuff, and basic tuff. The K–Ar ages of chert and red shale presented large variations, with systematically older ages compared to those of mudstone and tuff in the same strata. The influence of submarine hydrothermal activities on chert and red shale before subduction is a possible cause of this deviation. The illite samples, which were fractionated into four grain-size classes using a suspension method, yielded older ages and higher illite crystallinity (i.e., smaller values of Kübler’s crystallinity index) for larger grain-size classes. The peak metamorphic ages were determined from the K–Ar ages of the 3–4 μm class illite in mudstone and tuff. The Late Jurassic to the Earliest Cretaceous accretionary complex of the lowest structural unit (Kashiwagi Unit) was dated within a small range between 117–110 Ma, which is distinctly older than the K–Ar ages of white mica reported from the Sanbagawa Belt. The peak metamorphic age of acidic tuff (113 Ma) at the type locality of the Mikabu Greenstones indicates that the subducted Mikabu seamount is a constituent of the Kashiwagi Unit. The peak metamorphic ages of the Manba and Kamiyoshida Units were obtained as 132–107 Ma and 163–144 Ma, respectively. Major structural discontinuity is suggested within the Middle Jurassic accretionary complexes.

Geochemistry of seafloor surface sediment and submarine hydrothermal signature of Tomini Bay, Indonesia

During the Indonesia–Australia Submarine Survey of Hydrothermal Activity (IASSHA) 2001 cruise, several seafloor surface sediment samples were collected from Tomini Bay. The samples were geochemically examined for major and trace-element geochemistry to study the hydrothermal signature fingerprint. The Tomini Bay seafloor surface sediment sampled by gravity coring is relatively enriched in Na, P, S, Ca, Ni, Sr, and Ba mainly from a terrigenous source. The elevated concentrations of Mg, S, Cr, Co and Ni at one site suggest a contribution from a mafic terrigenous source. A manganese-rich sediment layer was detected on the sediment surface with Mn up to 9474.43 ppm. A hydrothermal signature was successfully distinguished from the manganese-rich sediment layer. Despite the lack of hydrothermal precipitates, the geochemistry of the Tomini Bay seafloor surface sediments provides evidence for submarine hydrothermal activity. KEY POINTS Seafloor surface sediment is an alternative proxy for investigating submarine hydrothermal activity. Hydrothermal signature is well recorded in the Fe/Mn-rich sediment layer and can be distinguished from measured concentrations and elemental ratios of Fe, Mn, HFSE (Zr), trace elements (Cu, Ni, Zn, Co, Zr) and REY (Y, Ce). The Mn-rich sediment layer in Tomini Bay seafloor surface sediment preserved hydrothermal fingerprint and confirmed the evidence of an active hydrothermal system in the studied area.

Genesis of the Enkafela Mn deposit: a record of submarine hydrothermal activity in the Afar Depression, Northeast Ethiopia

ABSTRACT The genesis of the Enkafela Mn deposit and associated processes were investigated using field mapping, mineralogical and geochemical analyses by XRD, ICP-MS/AES and XRF. The geology is constituted by limestone, evaporites/gypsum, conglomerates and basalt. The orebody is hosted in the limestone and has two distinctive manganese layers: a massive bottom and powdery top. Pyrolusite, romanechite and birnessite are the major ore minerals. The geochemical results indicate elevated MnO (av. 70.25 wt-%), Ba (>10,000 ppm), Sr (>10,000 ppm) and depleted Fe2O3 (av. 0.37 wt-%) and other metals. The massive orebody has higher (172.9 ppm) REE content than the powdery ore (7.84 ppm). Chondrite normalised REE plot shows LREE enrichment for both orebodies. The massive manganese layer shows enrichment in Ce and no Eu anomaly. The powdery manganese is depleted in Ce and Eu. The geochemical and mineralogical studies indicate that the manganese mineralisation has dual sources: hydrogenous and hydrothermal sources (for the massive orebody), and hydrothermal (for the powdery). The data suggest that there was active submarine hydrothermal activity in Dallol associated with the regional geodynamic events while the area was inundated by the Red Sea during the Pleistocene.

Formation of Fe-Mn coatings on foraminifera from the ultraslow-spreading southwest Indian Ridge: Implications for hydrothermal and diagenetic overprints in sediments

Detailed fine-scale morphological, mineralogical, and geochemical analyses were conducted on the Si-(Fe) hydroxide formations, planktonic foraminifera (PF) and the Fe-Mn coatings within a typical sediment sample collected from near the newly-discovered active hydrothermal vent field at the ultraslow-spreading Southwest Indian Ridge (SWIR). The analyses were conducted with a combination of different techniques, including Laser Scanning Confocal Microscope (LSCM), Laser Raman, high-resolution XRD, SEM-EDS, C-O isotope measurement, EMPA, and in-situ LA-ICP-MS. The mixed-type hydrothermal-hydrogenetic Si-(Fe) hydroxide formations (commonly occur as cavity-fillings inside the PF chambers) contain a considerable amount of organic matter, and are mainly composed of nontronite, goethite, tridymite, and opal-A with the bulk compositions of 8.13–40.91 wt% SiO2, 6.85–54.50 wt% FeO, and 0.43–23.99 wt% Al2O3. Such high Al content can be mainly attributed to the precipitation and accumulation of hydrothermal-derived Al-rich materials, and the Si-Fe-Al assemblage reveals that it is mainly sourced from plagioclase dissolution/alteration during subseafloor fluid-rock interaction. Meanwhile, the relative enrichments of high field strength elements (HFSEs), heavy rare earth elements (HREE), and Cr-Ni-Sc also imply that these Si-(Fe) hydroxide formations have another source from pyroxene alteration. In addition, their distinct microtextures (filamentous, orbicular, or beaded-like) are indicative of bacterial-related precipitation. The hydrogenetic Fe-Mn coatings have a layered texture and exhibit no apparent fluorescence, and are mainly composed of vernadite and goethite with the bulk compositions of 23.98–30.06 wt% FeO and 8.55– 21.52 wt% MnO. Their average Fe/Mn ratios, growth rates, and Zn-As contents are slightly higher than those of the open-ocean SWIR Fe-Mn deposits, which suggests an additional Fe input from submarine hydrothermal activity. Moreover, the altered PF shell is mainly composed of calcite and have three color types (white/yellowish/yellow). The contents of Si, Al, Fe, Cr, Ni, Cu, Zn, Ba, HFSEs, and REE increase as the yellowish color deepens, which indicates the diagenetic contamination from basalt alteration. Consequently, a new diagenetic Fe-carbonate phase (ankerite) was formed in the inner layer of the altered PF shells, and close correlations were also found between their C-O isotope ratios. Both the δ13C and δ18O values tend to drop from white- to yellow-type PF shells, accompanied with the decreasing contents of Ca, Na, Mg, and Sr. All these parameters and signatures would help to unravel the hydrothermal influence and diagenetic overprinting, which could cause confusion when using foraminifera shell geochemistry (e.g., Sr/Ca and Mg/Ca ratios) as a marine tracer in paleoclimate reconstruction.

The significance of galena Pb model ages and the formation of large Pb-Zn sedimentary deposits

In an attempt to clarify the significance of Pb model ages in Pb-Zn sedimentary deposits, we report high-precision Pb isotopic compositions for 64 galenas and 52 K-feldspars, the former from ores and the latter separated from granites. All samples are from Spain and the French Pyrenees. Lead from galena ores is of unequivocal continental origin. With few exceptions, Pb model ages systematically exceed emplacement ages by up to 400 Ma, a gap which is well outside the uncertainties of ~ 30 Ma assigned to the model. The histogram of the new high-precision Pb isotope data shows prominent peaks of galena Pb model ages at 94 ± 38 Ma and 392 ± 39 Ma. When the data are consolidated with literature data and examined in 3-dimensional Pb isotope space, cluster analysis identifies five groups. The model ages of the peaks occur, in order of decreasing peak intensity, at 395 ± 40 (Middle Devonian), 90 ± 34 Ma (Middle Cretaceous), and 613 ± 42 Ma (Neoproterozoic), with two minor peaks at 185 + 26 Ma (Jurassic) and 313 ± 41 (Upper Carboniferous). To a large extent, the model ages centered around these peaks correspond to distinct localities. The ages of the peaks do not coincide with any of the Betic, Variscan, or Pan-African tectonic events, which are the main tectonic episodes that shaped Iberian geology, but instead match well-known global oceanic anoxic events. It is argued that surges of metals weathered from continental surfaces scorched during anoxic events accumulated and combined in anoxic water masses with unoxidized marine sulfide released by submarine hydrothermal activity to precipitate the primary Pb-Zn stock. Frozen Pb isotope compositions require that galenas from black shales are the source of the final ores. The sulfides were later remobilized by large-scale convective circulation of basinal and hydrothermal fluids. The peaks of K-feldspar Pb model ages are distinct from those of galenas and do not correlate with magmatic emplacement ages. It is suggested that they instead reflect local circulation in Paleozoic sediments surrounding individual plutons. While Pb isotopes can be used as a regional provenance tool, such an approach requires that the data are considered in a fully 3-dimensional space.

Shallow-water hydrothermalism at Milos (Greece): Nature, distribution, heat fluxes and impact on ecosystems

Submarine hydrothermal activity is responsible for heat and chemical exchanges through the seafloor. Shallow-water hydrothermal systems (SWHS), while identified around the globe, are often studied in a way that is less comprehensive than their deep-ocean counterparts (e.g., along ridges), where systematic optical and acoustic mapping is more prevalent and coupled to in situ observations and sampling. Using aerial drones, an AUV, and temperature measurements at 10–40 cm subseafloor, we investigated in 2019 one of the most extensive SWHS known to date, in Paleochori and nearby Spathi and Agia Kyriaki Bays (south of Milos, Greece). Hydrothermal venting, found from the shore to water depths of almost 500 m, shows emissions of gases and high-temperature fluids, often associated with bacterial mats and/or hydrothermal mineral precipitates. This study provides extensive drone mapping coupled with local AUV surveys for seafloor characterization and ground-truthing from the shore to ~20 m water depth. Seafloor photomosaics also provide a detailed context to samples, measurements and observations carried in situ. We interpret the photomosaics to define distinct seafloor types, linked to this hydrothermal activity. White hydrothermal patches (WHPs) often show a clear polygonal organization, together with outflow areas that are both more dispersed and distributed. Polygonal patterns likely result from fluid convection in a sandy porous medium heated from below. These WHPs display elevated subseafloor temperatures, typically >50°C, with maximum values of ~75°C. Photomosaics also display textures of biological origin, including seagrass and bioturbation patterns. Widespread bioturbation by burrowing shrimps is often associated with WHPs, bounding them, but also occurs on sandy seafloor away from hydrothermal patterns. Subseafloor temperatures at these bioturbated areas are of ~30–40°C, and are thus transitional between hot WHPs and sedimented seafloor unaffected by hydrothermal activity (~24°C). In addition to linking subseafloor temperature data and interpreted seafloor photomosaics, our results provide a comprehensive general overview of this SWHS, of the organization of its hydrothermal outflow through the seafloor, and of the underlying subseafloor fluid circulation. This paper also gives the first perspectives on the heat fluxes of the system, and constitutes a background for other studies on the nature and distribution of microbial communities, controlled by this hydrothermal activity.

Strontium isotope evolution of Middle Permian seawater in the Sichuan Basin, South China: Possible causes and implications

Middle Permian 87Sr/86Sr variations are recorded by the marine carbonates in the Qianfo section and the Yundadi3 well in the Sichuan Basin of the Yangtze Platform, China. Petrographic and geochemical examinations of micritic carbonates confirmed the preservation of near-primary 87Sr/86Sr signatures that can be utilized to construct a reliable high-resolution Sr isotope profile of the equatorial eastern Paleo-Tethys for global correlations. The high 87Sr/86Sr ratios associated with a eustatic regression event in the Late Capitanian (late Middle Permian) imply a predominant contribution from terrigenous materials. We observed extremely low 87Sr/86Sr ratios (ca. 0.7068 to 0.7069) in the 40 m-thick Capitanian interval, confirming the significant chemostratigraphic episode called the Capitanian Minimum. Globally low 87Sr/86Sr values in the Middle Permian were probably caused by weak continental weathering rates, suppressed riverine flux into the ocean during Pangean assembly, and increased hydrothermal flux into the ocean associated with the opening of the Neotethys during the Middle Permian. Based on the 87Sr/86Sr values of micritic carbonates from the Qianfo section and the Yundadi3 well, large amounts of nonradiogenic Sr may have been derived from strong submarine hydrothermal activity related to the Emeishan LIP, and areas closer to volcanic activity were more strongly affected. These combined factors were responsible for the observed low 87Sr/86Sr ratios in the Sichuan Basin.

Sedimentary environments and mechanism of organic matter enrichment of dark shales with low TOC in the Mesoproterozoic Cuizhuang Formation of the Ordos Basin: Evidence from petrology, organic geochemistry, and major and trace elements

Proterozoic strata have become a new target for hydrocarbon exploration worldwide. They exhibit the potential to become a resource replacement target. To provide a reference for hydrocarbon source rock evaluation, the sedimentary environment and mechanism of organic matter enrichment of dark shales in the Mesoproterozoic Cuizhuang Fm. were investigated for the first time by combining organic and inorganic research concepts. Although the total organic carbon (TOC) content in the Cuizhuang Fm. is relatively low (mean value of 0.59%), it still can be considered as a potential source rock. Thin-section observations, main and trace element contents, and biomarker compounds revealed the following: (1) the Cuizhuang Fm. was deposited in a neritic shelf sedimentary environment at a passive continental margin, and the climate was warm and humid. The hydrocarbon-generating organisms mainly comprised prokaryotes such as cyanobacteria and other bacteria. All these characteristics meet the conditions of source rock formation. (2) The low oxygen level, felsic source provenance, and absence of submarine hydrothermal activity led to low productivity, and the low productivity and low-grade hydrocarbon-generating organisms led to low TOC content in the Cuizhuang Fm. (3) The enrichment of V in the Cuizhuang Fm. is apparent, but does not reach the hyper-enrichment threshold of 500 ppm associated with euxinic conditions, revealing anoxic ferruginous conditions. Moreover, the V content is positively correlated with TOC content, indicating that organic matter enrichment was mainly controlled by the redox conditions of the bottom water. Therefore, the sedimentary areas with high productivity were related to basic source provenance and submarine hydrothermal activity, and anoxic bottom water may have promoted the development of better source rocks. This findings may represent the main direction for identifying effective source rocks in subsequent work.

Hydrothermal Fluid Particle Geochemistry of Submarine Vents in Kos Island, Aegean Sea East Mediterranean

The study aims to fill the gap of knowledge in the literature concerning submarine hydrothermal activity on the eastern part of Hellenic Volcanic Arc. Hydrothermal fluids were collected by scuba diving in active shallow hydrothermal venting areas on Kos Island in the Aegean Sea in East Mediterranean. Analysis was carried out in suspended particles for Fe, Mn, Cu, Pb, Cd, Ca, Ba, Sr, Li, Al and Si. During sampling, gas and water flux was measured and gas/water ratios were estimated. The data presented significant time and spatial variability. The gas flux values in Kephalos Bay varied from 2 to 126 L/h and water flux varied from 60 to 122 L/h. In Bros Thermi the gas flux values varied from 4.5 to 60 L/h and the water flux was found to be from 16.5 to 96 L/h. The suspended particulate matter (SPM) flux varied from 0.93 to 8.64 mg/L and between 0.21 and 20.94 mg/L respectively at two sites studied. Metal/Al ratios also varied significantly within a short distance. The pH of hydrothermal waters was from 5.50 to 5.95 in Kephalos Bay and from 6.09 to 6.53 in Bros Thermi suggesting either different mixing of fluids with seawater deeper in the substrate or perhaps that gases CO2 and H2S control pH values. The main scope of this research is to evaluate the fluxes of gas and water and study the main particulate geochemistry of hydrothermal venting fluids. Hydrothermal particles from the submarine vents studied were dominated by Fe, Ca, Si and Al and strongly enriched in Mn, Cu, Pb and Ba. Based on Metal/Al ratios, it is suggested that there are two distinct hydrothermal reservoirs in Kos. The chemistry of the particles emitted from seeps strongly influence both the marine sediment geochemistry and element distribution locally on the sea bottom while subjected to further transportation, hydrothermal particulates could also affect a much greater region.

Amino acids in surface sediments of the Zannone Island shelf (Western Mediterranean Sea): Possible bioindicators of submarine hydrothermal activity

This study shows results from amino acid analysis of seafloor sediment sampled between ~80 and ~130 m depth, within an active hydrothermal field located off Zannone Island (Tyrrhenian Sea). The total hydrolysable amino acid (THAA) content was determined at eight stations, three located within the main hydrothermal depression (i.e. the Zannone Giant Pockmark, ZGP) and five located in the surrounding seafloor. THAA concentration ranged between ~2 and ~11 nmol mg−1 dw and the lowest THAA concentrations were found at stations located within the ZGP. Amino acids resistant to degradation or originating from bacteria (β-alanine, histidine, leucine, arginine and lysine) had higher relative abundances inside the ZGP, whereas amino acids susceptible to degradation or originating from cellular plasma (proline, phenylalanine and tyrosine) showed higher relative abundances outside the ZGP. Degradation and reactivity indices were calculated, and the sedimentary organic matter in the three outside ZGP stations B07, B08 and B09 was found to be the most labile and reactive. Together with THAA concentrations, sediment grain size and carbon percentages were measured at 12 stations, five located within the ZGP and seven located outside the ZGP. At all stations, the percentage of sand was ~75% or higher. Carbon percentages were lower in samples located inside the ZGP than in the surrounding stations. The southern outside ZGP station B12 was found to be similar in terms of amino acids composition to the three inside ZGP stations, suggesting the influence of the hydrothermal fluids at this station.

Fe Isotope Compositions of Cyprus Umbers

AbstractSubmarine metalliferous sedimentary rocks are chemical precipitates resulted from hydrothermal exhalation near mid‐ocean ridge or faults. They record the submarine hydrothermal activity between lithosphere and hydrosphere and are critical for understanding Fe cycling in marine environment. Fe was expelled from the hydrothermal vent systems and was oxidized and precipitated in the ambient seawater, where the precipitation of hydrothermal Fe is largely controlled by oxidation state of seawater and is potentially revealed by its Fe isotope compositions. This hydrothermal process in modern hydrothermal vent systems have been well observed, but that for the ancient ones are still not well known. Umbers, or ferromanganoan sediments, overlying Troodos ophiolite in Cyprus of Mid‐Cretaceous age thus provides an excellent example for understanding the Fe cycles in ancient submarine hydrothermal process. Samples were collected from Margi village in Troodos and are mostly amorphous Fe‐Mn oxy‐hydroxides with very minor quartz, goethite, smectite and silicates such as clinopyroxene derived from the volcanic rocks. There is no terrestrial, detrital component. Samples were analyzed for their whole‐rock element and Fe isotope compositions. The results show that samples are composed mainly of SiO2 (13∼80 wt%), Fe2O3 (9∼54 wt%) and MnO (1.5∼10.4 wt%), with minor Al2O3 (0.7∼4.3 wt%). PAAS‐normalized REE patterns are near flat with significantly negative Ce anomalies (Ce/Ce* is from 0.2 to 0.5) and slightly positive Eu anomalies (Eu/Eu* is around 1.1), indicating a source from the oxidized seawater and the high‐temperature hydrothermal fluids. δ56FeIRMM‐014 values of samples are ‐0.32‰ to ‐0.15‰, with an average of ‐0.20‰, which are consistent with those of the hydrothermal fluids previously reported. The narrow Fe isotope compositions of Cyprus umbers that are close to those of submarine hydrothermal fluids indicates near complete oxidation of hydrothermal Fe2+ during its expulsion from the hydrothermal vent.

Influence of volcanic activities on redox chemistry changes linked to the enhancement of the ancient Sinian source rocks in the Yangtze craton

Volcanic eruptions and related submarine hydrothermal activities were very extensive during the Precambrian periods in the Yangtze craton in southern China. How these geological events affected redox chemistry changes linked to the enhancement of mudstone/shale hydrocarbon source rocks in ancient strata is an important concern for oil and gas exploration in the Precambrian strata. Samples were collected from two Sinian (Ediacaran) outcrops in the northern Sichuan Basin, the Zhaiziyan and Yanhe outcrops, which revealed Doushantuo Formation (Z1ds) shales approximately 30.5 m in thickness, and shales in the third member of the Deying Formation (Z2dn3) approximately 30.0 m in thickness. TOC contents of the black carbonaceous shales in the Doushantuo Formation of the Zhaiziyan outcrop are mostly greater than 5.0% (up to 12.9%). Positive correlations were observed between TOC contents and trace and rare earth element concentrations in the shales: samples with high concentrations of trace elements such as V, Ni, Cu, Zr, Rb, and Pb and total rare earth element (∑REE) concentrations had relatively high TOC contents. The shales are rich in volcanic ash interlayers and thin siliceous layers and have positive Eu anomalies, indicating that volcanic activities and related hydrothermal activities significantly impacted the development of black shale source rocks. In the rifting background of the Sinian cratonal marginal, extensive volcanic activities brought rich nutrients to the seawater, which may have promoted the breeding of cyanobacteria and multicellular algae. At the same time, the volcanic activities released large amounts of H2S and SO2, which could cause the seawater to become a euxinic environment in which the bottom seawater was sulfurized and anoxic. Massive amounts of organic matters formed in the surface seawater entered the anoxic bottom seawater and were well preserved in the shale sediments. Therefore, under the influence of volcanic activities, ancient Sinian organic-rich high-quality source rocks were widely developed and had a large hydrocarbon supply potential for the Sinian dolomite reservoirs.

The contribution of the Emeishan large igneous province to the strontium isotope evolution of the Capitanian seawater

ABSTRACTThe 87Sr/86Sr minimum of the Capitanian seawater is one of the most significant features in the Phanerozoic seawater 87Sr/86Sr history. In order to assess possible contribution of the Emeishan large igneous provinces (LIPs) to strontium isotope evolution of the Capitanian seawater, 87Sr/86Sr ratios were measured from the Capitanian limestones which are locally interlayered with the Emeishan basalts. The limestones underlying the Emeishan basalts have high 87Sr/86Sr ratios (0.7070–0.7074). However, extremely low 87Sr/86Sr ratios (0.7068–0.7070) were identified in the late Capitanian Jinogondolella prexuanhanensis–J. xuanhanensis zones, which correspond to the eruption time of the Emeishan LIP. The temporal coincidence of these two phenomena supports the idea of a potential linkage between Capitanian 87Sr/86Sr minimum and eruption of this igneous province. The strong submarine hydrothermal activity and erosion of the Emeishan LIP could have released large amounts of non-radiogenic Sr to the oceans and play an important role in strontium isotope evolution of the seawater.

Rare earth element systematics in boiled fluids from basalt-hosted geothermal systems

Hydrothermal processes that lead to REE fractionation and redistribution are important for understanding water-rock interactions in geothermal energy resources and mineral deposits, and for determining how submarine hydrothermal activity affects the composition of oceanic crust. Much previous work on REE transport and deposition has focused on submarine hydrothermal vents. We report REE concentrations in boiled fluids sampled from five subaerial, basalt-hosted geothermal fields, and explore controls on aqueous REE concentrations by ligand complexation and mineral supersaturation. Samples that boiled at pressures between 0.8 and 2.83 MPa were obtained from the Reykjanes, Svartsengi, Hellisheidi, and Nesjavellir geothermal systems in Iceland, and the Puna geothermal system in Hawaii. For comparison, we also report REE concentrations in hydrothermal fluids from the sediment hosted submarine Middle Valley hydrothermal system, which boiled at >250 MPa. The pH(25°C) values of the sampled subaerial geothermal fluids range from 3.94 to 6.77, and Cl concentrations range from near seawater (502 mmol/kg) to dilute (1.9 mmol/kg). La, Ce and Eu are the only REE present at levels above 5 picomole/kg (pmol/kg) in the boiled geothermal fluids; and there are notable CI chondrite normalized La and Eu anomalies in the saline fluids. REE concentrations in Middle Valley hydrothermal fluids fall within the typical range reported for submarine hydrothermal fluids and have around two orders of magnitude higher REE than the boiled subaerial geothermal fluids. Bulk samples of precipitates in pipes from the Reykjanes geothermal system have detectable REE, confirming that downhole fluids have lost REE during boiling and production of fluids for geothermal energy. Isenthalpic boiling models show that the proportions of La and Eu chloride complexes increase relative to other aqueous species as boiling progresses, attenuating the incorporation of La and Eu into precipitated well scale solids. Fluorapatite is calculated to precipitate on boiling of low pH and saline fluids and calcite is calculated to precipitate from dilute and near-neutral pH fluids, and these minerals likely sequester REE in boiled subaerial fluids. Submarine hydrothermal fluids are constrained to boiling at higher temperatures than subaerial geothermal fluids owing to pressure from overlying cold seawater, therefore secondary minerals and solids that incorporate REE are not extensively precipitated and REE concentrations in the fluids are higher.