GG7033 Marine Geochemistry (2023- ) Primary teacher
Master-level, Stockholm University
https://www.su.se/english/search-courses-and-programmes/gg7033-1.643000

GG4208 Sedimentary Systems (2022- ) Co-teach
Bachelor-level, Stockholm University
https://www.su.se/english/search-courses-and-programmes/gg4208-1.411544

GG7009 Ocean-climate interaction through geologic time​​​​​​​ (2024- ) Co-teach
Master-level, Stockholm University
https://www.su.se/english/search-courses-and-programmes/gg7009-1.673630

GG4090 Oceanography for geoscientists​​​​​​​​​​​​​​ (2024- ) Co-teach
Bachelor-level, Stockholm University
https://www.su.se/english/search-courses-and-programmes/gg4090-1.647699

GG4209 Global Geochemical Cycle (2021- 2023) Primary teacher
Bachelor-level, Stockholm University
https://www.su.se/english/search-courses-and-programmes/gg4209-1.411545

1) Silicate alteration in marine sediments: Chemical weathering and reverse weathering involving silicate minerals in terrestrial environments are likely the most important processes controlling the long-term CO2 concentration in the atmosphere and thereby, green house forcing. It is much less understood whether similar processes occur in marine environments and what is the scale of such processes.
2022-2023 Swedish Foundations' starting grant fellow-Ragnar Söderbergs stiftelse (info)
 2023-2028 ERC Consolidator grant (info)
Suggested reading: 
Hong, W-L., Torres, M. E., and Kutterolf, S. "Towards a global quantification of volcanogenic aluminosilicate alteration rates through the mass balance of strontium in marine sediments." Chemical Geology 550 (2020): 119743.  
Torres, M. E., Hong, W.-L., Solomon, E. A. et al. "Silicate weathering in anoxic marine sediment as a requirement for authigenic carbonate burial." Earth-Science Reviews 200 (2020): 102960.

2) Glacial-driven Submarine Groundwater Discharge (SGD): Submarine groundwater discharge (SGD) has been known for decades as an important process that, in addition to rivers, transports solutes from land to ocean. Ice sheet dynamics have been put forward to explain the SGD observed from places with past ice sheet coverage such as the Laurentide Ice Sheet in North America  as well as the present day Greenland and Antarctica ice sheets. What about the Fennoscandian ice sheet (FIS)? Is it possible that FIS also induced SGD in the past? Is there any trace left today to answer these questions?
2020-2024 GRIEG- EEA Grant and Norway Grant (info)
2022-2025 Swedish Research Council (VR).
Suggested reading:
Kim, Ji-Hoon, Jong-Sik Ryu, Wei-Li Hong, Kwangchul Jang, Young Ji Joo, Damien Lemarchand, Jin Hur et al. "Assessing the impact of freshwater discharge on the fluid chemistry in the Svalbard fjords." Science of the Total Environment 835 (2022): 155516.

Hong, W.‐L., Lepland, A., Himmler, T. et al. "Discharge of meteoric water in the eastern Norwegian Sea since the last glacial period." Geophysical Research Letters 46, no. 14 (2019): 8194-8204.

3) Methane dynamics in Arctic Ocean seeps: Emission of methane, a potent greenhouse gas, from cold seeps to overlying ocean has been a topic of interest in the Arctic region. I study the geochemical singals in pore fluids, seidments, and authigenic minerals to understand the mechanism, time scale, and consequence of methane emission. 
Suggested reading:
Hong, W.-L., Torres, M. E., Carroll, J. et al. "Seepage from an arctic shallow marine gas hydrate reservoir is insensitive to momentary ocean warming." Nature communications 8, no. 1 (2017): 1-14. 

4) Fluid flow and gas hydrate dynamics: Fluid flow and gas hydrate deposits commonly appear along continental margins. What can we learn from the pore fluid geochemistry? 
Suggested reading:
Hong, W‐L., Torres, M. E. , Portnov, A. et al. "Variations in gas and water pulses at an Arctic seep: fluid sources and methane transport." Geophysical Research Letters 45, no. 9 (2018): 4153-4162.
Peszynska, M., Hong, W.-L., Torres, M. E. and Kim, J.-H. "Methane hydrate formation in Ulleung Basin under conditions of variable salinity: Reduced model and experiments." Transport in Porous Media 114, no. 1 (2016): 1-27. 

A selection from Stockholm University publication database

• Morphology-Dependent Magnetic Properties in Shallow-Water Ferromanganese Concretions

  2024. Joonas Wasiljeff (et al.). Geochemistry Geophysics Geosystems 25 (5)

  Article

  Ferromanganese concretions commonly occur in shallow-water coastal regions worldwide. In the Baltic Sea, they can record information about past and present underwater environments and could be a potential source for critical raw materials. We report on their microstructural characteristics and magnetic properties and link them to their formation mechanisms and environmental significance. Microstructural investigations from nano- and micro-computed tomography, electron microscopy, and micro-X-ray fluorescence elemental mapping reveal diverse growth patterns within concretions of different morphologies. Alternating Fe- and Mn-rich growth bands indicate fluctuating redox conditions during formation. Bullet-shaped magnetofossils, produced by magnetotactic bacteria, are present, which suggests the influence of bacterial activity on concretion formation. Spheroidal concretions, which occur in deeper and more tranquil environments, have enhanced microbial biomineralization and magnetofossil preservation. Conversely, crusts and discoidal concretions from shallower and more energetic environments contain fewer magnetofossils and have a greater detrital content. Our results provide insights into concretion formation mechanisms and highlight the importance of diagenetic processes, oxygen availability, and bacterial activity in the Baltic Sea.

  Read more about Morphology-Dependent Magnetic Properties in Shallow-Water Ferromanganese Concretions

• Permafrost and groundwater interaction: current state and future perspective

  2023. Magdalena Diak (et al.). Frontiers in Earth Science 11

  Article

  This study reviews the available and published knowledge of the interactions between permafrost and groundwater. In its content, the paper focuses mainly on groundwater recharge and discharge in the Arctic and the Qinghai-Tibet Plateau. The study revealed that the geochemical composition of groundwater is site-specific and varies significantly within the depth of the aquifers reflecting the water-rock interactions and related geological history. All reviewed studies clearly indicated that the permafrost thaw causes an increase in groundwater discharge on land. Furthermore, progressing climate warming is likely to accelerate permafrost degradation and thus enhance hydrological connectivity due to increased subpermafrost groundwater flow through talik channels and higher suprapermafrost groundwater flow. In the case of submarine groundwater discharge (SGD), permafrost thaw can either reinforce or reduce SGD, depending on how much pressure changes affecting the aquifers will be caused by the loss of permafrost. Finally, this comprehensive assessment allowed also for identifying the lack of long-term and interdisciplinary in situ measurements that could be used in sophisticated computational simulations characterizing the current status and predicting groundwater flow and permafrost dynamics in the future warmer climate.

  Read more about Permafrost and groundwater interaction

• Separating Si phases from diagenetically-modified sediments through sequential leaching

  2023. Tzu-Hao Huang (et al.). Chemical Geology 637

  Article

  Silicon (Si) phases such as biogenic silica, lithogenic silicate and authigenic silica/silicate in marine sediments provide valuable information about past Si cycling. Wet-chemical sequential leaching methods are often applied to extract different Si phases from marine sediments to study Si diagenetic processes in shallow subsurface. The potential of this method to separate Si phases from deeply-buried and diagenetically-modified sediments has not been systematically examined. We applied a sequential leaching protocol to drill core sediments retrieved from the Ulleung Basin, East/Japan Sea. We performed geochemical (elemental abundance and stable Si isotopes, δ³⁰Si) and microscopic (X-ray diffraction and scanning electron microscope) analyses to monitor leaching efficiency in separating different Si phases. We show that, prior to alkaline leaching, applying weak acid is able to remove metal oxide and/or clay-like phases. The following Na₂CO₃ leaching, based on a commonly-adopted protocol, is able to dissolve some but not all diatoms. The results of elemental contents and δ³⁰Si values of leachates suggest that, in diagenetically-modified sediments, either a longer digesting time or a harsher alkaline leaching is needed to dissolve all diatoms. This is attributed to increased resistance of diatoms to Na₂CO₃ leaching as a result of reduced surface area and/or improved SiO₂ tetrahedron ordering during diagenetic processes over time and burial depths. Lithogenic silicate minerals can be dissolved by NaOH and potentially separated from diatoms if the latter is completely removed in the preceding leaching steps. Even if a trace amount of diatom is left undissolved in the NaOH leaching, it is still possible to separate the two through a mass balance calculation given the knowledge of composition for the two end-members. We conclude that a successful separation of Si phases in diagenetically modified sediments relies on the knowledge of elemental abundance and even δ³⁰Si values of the leachates, as well as information such as species of Si-skeleton organisms, contents and maturation degree of biogenic silica.

  Read more about Separating Si phases from diagenetically-modified sediments through sequential leaching

• Barium isotope fractionation in barite–fluid systems at chemical equilibrium

  2023. J. T. Middleton (et al.). Chemical Geology 627

  Article

  The barium isotope composition of sedimentary barite (BaSO₄, barium sulfate) is emerging as a powerful tracer of the sources and cycling of Ba in modern and ancient marine environments. To reliably use Ba isotopes to interrogate the marine Ba cycle, it is important to identify and constrain processes that fractionate the isotope composition of Ba in BaSO4. Of particular interest is ion exchange: micro-scale dissolution and precipitation that occurs in mineral–fluid systems at chemical equilibrium. This process is often important in systems where minerals, such as BaSO₄, and a fluid remain in contact for prolonged periods of time; however, the impact of ion exchange on Ba isotope compositions in BaSO₄ is unknown. To constrain the rate and isotopic effect associated with ion exchange in BaSO₄–fluid systems, we conducted a series of experiments under marine-relevant conditions and interpreted the results using a multi-phase time-dependent numerical reactor model. From a series of isotope-tracer experiments, we find that BaSO₄–fluid ion exchange progresses at a rate between 5 and 53 pmol m⁻² s⁻¹. In a parallel set of experiments used to assess mass-dependent isotope fractionation of Ba, the combined effect of BaSO₄ dissolution and precipitation while at chemical equilibrium was found to result in the continued evolution of Ba isotopes and produced a modeled offset of Δ¹³⁸Ba_barite–dBa = −0.10 ± 0.05 ‰ at isotopic equilibrium. We then constrained the magnitude of isotopic fractionation during BaSO₄ dissolution by fitting our data in the numerical reactor model and using previous estimates of Ba isotope fractionation during BaSO₄ precipitation (⍺_{precipitation} = 0.99968 ± 0.00002). At chemical equilibrium, we find our data are best explained by an ⍺_dissolution = 0.99978 ± 0.00006, implying that BaSO₄ dissolution releases isotopically ‘light’ Ba to solution. Since the magnitude of the isotope effects associated with BaSO₄ precipitation and dissolution are imbalanced, ion exchange will tend to alter the isotope composition of co-located BaSO₄ and fluids until the two phases are offset by ≈0.10 ‰. The importance of this effect on sedimentary BaSO₄ likely depends on several factors and we suggest multiple site-screening criteria to maximize the utility of this emerging proxy.

  Read more about Barium isotope fractionation in barite–fluid systems at chemical equilibrium

• Controlling factors on patterns of dissolved organic carbon and volatile fatty acids in a submarine mud volcano offshore southwestern Taiwan

  2023. Nai-Chen Chen (et al.). Frontiers in Earth Science 11

  Article

  Dissolved organic carbon (DOC) and volatile fatty acids (VFAs) play key roles in the carbon cycling of marine sediment. Both microbially or thermally activated cracking of organic matter often produces high quantities of DOC and VFAs. To uncover the distribution pattern of DOC and VFAs in sediments under both impacts, a submarine mud volcano (SMV), was chosen to denote a model system that could witness how microbial activities react under the mixing of seawater and deeply-sourced fluids in a subsurface environment. We examined the concentration profiles of DOC and several VFAs (lactate, formate, acetate, propionate, and butyrate) in pore water, covering both sulfate reduction and methanogenesis zones, and further numerically modeled six porewater species (DOC, bromide, calcium, magnesium, ammonium, and total alkalinity) to quantify their fluxes from depth as well as the rates of in-situ microbial processes. Apparently, bulk DOC concentrations fluctuated with depths, probably primarily controlled by in situ microbial processes. Lactate was detectable in some samples, while propionate and butyrate were under detection limit. Acetate and formate concentrations were consistently and uniformly low throughout all biogeochemical zones, with a slightly increasing trend with depth at the center of the SMV, suggesting active utilization and turnover by the terminal steps of organic matter mineralization. The numerical modeling suggests that most DOC patterns were primarily influenced by in-situ organic matter degradation, while the impact of upward migrating fluid become more significant at center sites. The calculation of the Gibbs energy of metabolic redox reactions reveals that acetoclastic sulfate reduction yields the highest energy throughout sediment columns and may co-exist with methanogenesis below sulfate reduction zone. In contrast, acetoclastic methanogenesis yields higher energy within sulfate reduction zone than below that region, suggesting it is thermodynamically feasible to co-occur with sulfate reduction in dynamic SMV environments.

  Read more about Controlling factors on patterns of dissolved organic carbon and volatile fatty acids in a submarine mud volcano offshore southwestern Taiwan

• Methane-derived authigenic carbonates – A case for a globally relevant marine carbonate factory

  2023. Sajjad A. Akam (et al.). Earth-Science Reviews 243

  Article

  Precipitation of methane-derived authigenic carbonates (MDAC) is an integral part of marine methane production and consumption, but MDAC's relative significance to the global marine carbon cycle is not well understood. Here we provide a synthesis and perspective to highlight MDAC from a global marine carbon biogeochemistry viewpoint. MDAC formation is a result and archive of carbon‑sulfur (C S) coupling in the shallow sulfatic zone and carbon‑silicon (C Si) coupling in deeper methanic sediments. MDAC constitute a carbon sequestration of 3.93 Tmol C yr⁻¹ (range 2.34–5.8 Tmol C yr⁻¹) in the modern ocean and are the third-largest carbon burial mechanism in marine sediments. This burial compares to 29% (11–57%) organic carbon and 10% (6–23%) skeletal carbonate carbon burial along continental margins. MDAC formation is also an important sink for benthic alkalinity and, thereby, a potential contributor to bottom water acidification. Our understanding of the impact of MDAC on global biogeochemical cycles has evolved over the past five decades from what was traditionally considered a passive carbon sequestration mechanism in a seep-oasis setting to what is now considered a dynamic carbonate factory expanding from deep sediments to bottom waters—a factory that has been operational since the Precambrian. We present a strong case for the need to improve regional scale quantification of MDAC accumulation rates and associated carbonate biogeochemical parameters, leading to their incorporation in present and paleo‑carbon budgets in the next phase of MDAC exploration.

  Read more about Methane-derived authigenic carbonates – A case for a globally relevant marine carbonate factory

• The phylogeography and ecology of Oligobrachia frenulate species suggest a generalist chemosynthesis-based fauna in the arctic

  2023. Arunima Sen (et al.). Heliyon 9 (3)

  Article

  We used ancient DNA (aDNA) extraction methods to sequence museum voucher samples of Oligobrachia webbi, a frenulate siboglinid polychaete described from a northern Norwegian fjord over fifty years ago. Our sequencing results indicate a genetic match with the cryptic seep species, Oligobrachia haakonmosbiensis (99% pairwise identity for 574 bp mtCOI fragments). Due to its similarity with O. webbi, the identity of O. haakonmosbiensis has been a matter of debate since its description, which we have now resolved. Furthermore, our results demonstrate that chemosynthesis-based siboglinids, that constitute the bulk of the biomass at Arctic seeps are not seep specialists. Our data on sediment geochemistry and carbon and nitrogen content reveal reduced conditions in fjords/sounds, similar to those at seep systems. Accumulation and decomposition of both terrestrial and marine organic matter results in the buildup of methane and sulfide that apparently can sustain chemosymbiotic fauna. The occurrence of fjords and by extension, highly reducing habitats, could have led to Arctic chemosymbiotic species being relatively generalist with their habitat, as opposed to being seep or vent specialists. Our stable isotope analyses indicate the incorporation of photosynthetically derived carbon in some individuals, which aligns with experiments conducted on frenulates before the discovery of chemosynthesis that demonstrated their ability to take up organic molecules from the surrounding sediment. Since reduced gases in non-seep environments are ultimately sourced from photosynthetic processes, we suggest that the extreme seasonality of the Arctic has resulted in Arctic chemosymbiotic animals seasonally changing their degree of reliance on chemosynthetic partners. Overall, the role of chemosynthesis in Arctic benthos and marine ecosystems and links to photosynthesis may be complex, and more extensive than currently known.

  Read more about The phylogeography and ecology of Oligobrachia frenulate species suggest a generalist chemosynthesis-based fauna in the arctic

• Lack of detectable chemosynthesis at a sponge dominated subarctic methane seep

  2023. Melina Sinner (et al.). Frontiers in Earth Science 11

  Article

  We used high-resolution imagery within a Geographic Information System (GIS), free gas and porewater analyses and animal bulk stable isotope measurements to characterize the biotic and abiotic aspects of the newly discovered Vestbrona Carbonate Field (VCF) seep site on the Norwegian shelf (63°28′N, 6° 31′E, ∿270 m water depth). Free gas was mainly composed of microbial methane. Sediment porewater sulfide concentrations were in the millimolar range and thus high enough to sustain seep chemosymbiotrophic animals. Nonetheless, the VCF lacked chemosymbiotrophic animals despite an abundance of methane-derived carbonate crusts which are formed by the same anaerobic processes that sustain chemosymbiotrophic animals at seeps. Furthermore, none of the sampled taxa, across various trophic guilds exhibited a detectable contribution of chemosynthetically fixed carbon to their diets based on bulk stable isotope values, suggesting a predominantly photosynthetic source of carbon to the VCF seep food web. We link the absence of chemosymbiotrophic animals to highly localized methane flow pathways, which may act as a “shunt-bypass” of the anaerobic oxidation of methane (AOM) and by extension sulfide generation, thus leading to sediment sulfide concentrations that are highly heterogeneous over very short lateral distances, inhibiting the successful colonization of chemosymbiotrophic animals at the VCF seep. Instead, the seep hosted diverse biological communities, consisting of heterotrophic benthic fauna, including long lived taxa, such as soft corals (e.g., Paragorgia arborea) and stony corals (i.e., Desmophyllum pertusum, formerly known as Lophelia pertusa). Compared to the surrounding non-seep seafloor, we measured heightened megafaunal density at the seep, which we attribute to increased habitat heterogeneity and the presence of a variety of hard substrates (i.e., methane-derived authigenic carbonates, dropstones and coral rubble), particularly since the most abundant taxa all belonged to the phylum Porifera. Compared to the surrounding non-seep seafloor, marine litter was denser within the VCF seep, which we link to the more variable local topography due to authigenic carbonates, which can rip off parts of bottom trawling nets thereby making the seep act as catchment area for marine litter.

  Read more about Lack of detectable chemosynthesis at a sponge dominated subarctic methane seep

• Volcanogenic aluminosilicate alteration drives formation of authigenic phases at the northern Hikurangi margin: Implications for subseafloor geochemical cycles

  2023. M. Luo (et al.). Chemical Geology 619

  Article

  Alteration of volcanogenic aluminosilicates (VAs) in marine sediments is recognized as critical in regulating geochemical cycles and sustaining the oceanic deep biosphere, but rates of VA alteration and its associated authigenic mineral formation are not commonly reported. Here we present results on analyses of sediments and pore water recovered from the upper 150 mbsf of four sites drilled on the northern Hikurangi margin during IODP Expeditions 372 and 375. Petrographic analyses show that volcanogenic materials (glass shards, feldspar, volcanic lithoclasts) constitute important components (15–45 wt%) of the hemipelagic mud, and reveal ongoing glass alteration with accompanying authigenic phase formation. A reaction-transport model constrained by pore water Sr, ⁸⁷Sr/⁸⁶Sr, Ca, Mg, and Si was applied to simulate VA diagenetic reactions. Our model results yield VA alteration rates of 0.047–0.64 mmol Sr m⁻² yr⁻¹, with substantially higher values at Sites U1517 and U1520 that experienced rapid sediment emplacement. In addition, our simulations show that >99% of the dissolved Si generated by VA alteration is fixed in silica cement and authigenic clay, and that ∼50% of Ca incorporated in the authigenic carbonate is supplied by VA alteration. First-order estimates suggest that, in addition to authigenic carbonate precipitation, authigenic clay formation may represent an important sink for dissolved Mg. This study quantitatively examines the linkage between VA alteration and formation of authigenic phases, highlights its role in subsurface geochemical cycles, and indicates that slope instability may play an important role in promoting VA diagenesis.

  Read more about Volcanogenic aluminosilicate alteration drives formation of authigenic phases at the northern Hikurangi margin

• Anaerobic oxidation has a minor effect on mitigating seafloor methane emissions from gas hydrate dissociation

  2022. Christian Stranne (et al.). Communications Earth & Environment 3 (1)

  Article

  Continental margin sediments contain large reservoirs of methane stored as gas hydrate. Ocean warming will partly destabilize these reservoirs which may lead to the release of substantial, yet unconstrained, amounts of methane. Anaerobic oxidation of methane is the dominant biogeochemical process to reduce methane flux, estimated to consume 90% of the methane produced in marine sediments today. This process is however neglected in the current projections of seafloor methane release from gas hydrate dissociation. Here, we introduce a fully coupled oxidation module to a hydraulic-thermodynamic-geomechanical hydrate model. Our results show that for seafloor warming rates > 1 degrees C century(-1), the efficiency of anaerobic oxidation of methane in low permeability sediments is poor, reducing the seafloor methane emissions by <5%. The results imply an extremely low mitigating effect of anaerobic oxidation of methane on climate warming-induced seafloor methane emissions. Microbial anaerobic oxidation of methane may not substantially mitigate projected warming-induced emissions of methane from marine hydrate-bearing sediments, according to a coupled hydraulic-thermodynamic-geomechanical hydrate model.

  Read more about Anaerobic oxidation has a minor effect on mitigating seafloor methane emissions from gas hydrate dissociation

• Assessing the impact of freshwater discharge on the fluid chemistry in the Svalbard fjords

  2022. Ji-Hoon Kim (et al.). Science of the Total Environment 835

  Article

  Changes in the cryosphere extent (e.g., glacier, ice sheet, permafrost, and snow) have been speculated to impact (bio)geochemical interactions and element budgets of seawater and pore fluids in Arctic regions. However, this process has rarely been documented in Arctic fjords, which leads to a poor systematic understanding of land-ocean interactions in such a warming-susceptible region. Here, we present the chemical and isotopic (δ¹⁸O, δD, δ¹¹B, and ⁸⁷Sr/⁸⁶Sr) compositions of seawater and pore fluids from five fjords in the Svalbard archipelago. Compared to bottom seawater, the low Cl⁻ concentrations and depleted water isotopic signatures (δ¹⁸O and δD) of surface seawater and pore fluids delineate freshwater discharge originating from precipitation and/or meltwater of the cryosphere (i.e., glacier, snow, and permafrost). In contrast, the high Cl⁻ concentrations with light water isotopic values in pore fluids from Dicksonfjorden indicate a brine probably resulted from submarine permafrost formation during the late Holocene, a timing supported by the numerical simulation of dissolved Cl⁻ concentration.

  The freshwater is influenced by the local diagenetic processes such as ion exchanges indicated by δ¹¹B signatures as well as interactions with bedrock during fluid migration inferred from pore fluid ⁸⁷Sr/⁸⁶Sr ratios. The interactions with bedrock significantly alter the hydrogeochemical properties of pore fluids in each fjord, yielding spatiotemporal variations. Consequently, land-ocean interactions in combination with the hydrosphere-cryosphere-lithosphere are critical factors for understanding and predicting the hydrology and elemental cycling during global climate change periods in the past, present, and future of the Svalbard archipelago.

  Read more about Assessing the impact of freshwater discharge on the fluid chemistry in the Svalbard fjords

• Boron concentrations and isotopic compositions in methane-derived authigenic carbonates: Constraints and limitations in reconstructing formation conditions

  2022. Wei-Li Hong (et al.). Earth and Planetary Science Letters 579

  Article

  The boron content and isotopic composition (δ¹¹B), of marine carbonates have the potential to constrain CO₂ chemistry during carbonate growth conditions. However, obtaining and interpreting boron compositions from authigenic carbonates in geological archives present several challenges that may substantially limit their application. In particular, contamination from non-carbonate phases during sample preparation must be carefully avoided, and a variety of controls on boron composition during authigenic growth conditions must be evaluated. To advance understanding of the use and limitations of boron in authigenic carbonates, we present data and modelling results on methane-derived authigenic carbonate (MDAC), a by-product of microbially mediated anaerobic oxidation of methane, taken from three cold seep sites along the Norwegian margin. We present a novel sequential leaching method to isolate the boron signals from the micritic (Mg-calcite) and cavity-filling (aragonitic) MDAC cements in these complex multi-phase samples. This method successfully minimizes contamination from non-carbonate phases. To investigate the factors that could potentially contribute to the observed boron signals, we construct a numerical model to simulate the evolution of MDAC δ¹¹B and B/Ca ratios over its growth history. We show that diagenetic fluid composition, depths of precipitation, the physical properties of sediments (such as porosity), and mineral surface kinetics all contribute to the observed boron compositions in the different carbonate cements. While broad constraints may be placed on fluid composition, the multiple competing controls on boron in these diagenetic settings limit the ability to place unique solutions on fluid CO₂ chemistry using boron in these authigenic carbonates.

  Read more about Boron concentrations and isotopic compositions in methane-derived authigenic carbonates

• Interactions between deep formation fluid and gas hydrate dynamics inferred from pore fluid geochemistry at active pockmarks of the Vestnesa Ridge, west Svalbard margin

  2021. Wei-Li Hong (et al.). Marine and Petroleum Geology 127

  Article

  Seafloor seepage sites along the Vestnesa Ridge off west-Svalbard have been, for decades, a natural laboratory for the studies of fluid flow and gas hydrate dynamics along passive continental margins. The lack of ground truth evidence for fluid composition and gas hydrate abundance deep in the sediment sequence however prohibits us from further assessing the current model of pockmark evolution from the region. A MARUM-MeBo 70 drilling cruise in 2016 aims to advance our understanding of the system by recovering sediments tens of meters below seafloor from two active pockmarks along Vestnesa Ridge. We report pore fluid composition data focusing on dissolved chloride, stable isotopes of water (delta O-18 and delta D), and the isotopic composition of dissolved boron (delta B-11). From one of the seepage sites, we detect a saline formation water with two layers where gas hydrates were recovered. This saline formation pore fluid is characterized by elevated chloride concentrations (up to 616 mM), high B/Cl ratios (9 x 10(-4) mol/mol), high delta O-18 and delta D isotopic signatures (+0.6 parts per thousand and +3.8 parts per thousand, respectively) and low 811B signatures (+35.0 parts per thousand), which collectively hint to a high temperature modification at great depths. Based on the dissolved chloride concentration profiles, we estimated up to 47% of pore space occupied by gas hydrate in the sediments shallower than 11.5 mbsf. The observation of bubble fabric in the recovered gas hydrates suggests formation during past periods of intensive gaseous methane seepage. The presence of these gas hydrates without associated positive anomalies in dissolved chloride concentrations however suggests that the decomposition of gas hydrate is as fast as its formation. Such a state of gas hydrates can be attributed to a relatively low methane supply transported by the saline formation water at present. Our findings based on pore fluid composition corroborate previous inferences along Vestnesa Ridge that fluids sustaining seepage have migrated from great depths and that the variable gaseous and aqueous phases through the gas hydrate stability zone control the distributions of authigenic carbonates and gas hydrates.

  Read more about Interactions between deep formation fluid and gas hydrate dynamics inferred from pore fluid geochemistry at active pockmarks of the Vestnesa Ridge, west Svalbard margin

• Distinct methane-dependent biogeochemical states in Arctic seafloor gas hydrate mounds

  2021. Scott A. Klasek (et al.). Nature Communications 12

  Article

  Archaea mediating anaerobic methane oxidation are key in preventing methane produced in marine sediments from reaching the hydrosphere; however, a complete understanding of how microbial communities in natural settings respond to changes in the flux of methane remains largely uncharacterized. We investigate microbial communities in gas hydrate-bearing seafloor mounds at Storfjordrenna, offshore Svalbard in the high Arctic, where we identify distinct methane concentration profiles that include steady-state, recently-increasing subsurface diffusive flux, and active gas seepage. Populations of anaerobic methanotrophs and sulfate-reducing bacteria were highest at the seep site, while decreased community diversity was associated with a recent increase in methane influx. Despite high methane fluxes and methanotroph doubling times estimated at 5–9 months, microbial community responses were largely synchronous with the advancement of methane into shallower sediment horizons. Together, these provide a framework for interpreting subseafloor microbial responses to methane escape in a warming Arctic Ocean.

  Read more about Distinct methane-dependent biogeochemical states in Arctic seafloor gas hydrate mounds

• A Pulse of Meteoric Subsurface Fluid Discharging Into the Chukchi Sea During the Early Holocene Thermal Maximum (EHTM)

  2021. Ji-Hoon Kim (et al.). Geochemistry Geophysics Geosystems 22 (8)

  Article

  The response of Arctic Ocean biogeochemistry to subsurface flow driven by permafrost thaw is poorly understood. We present dissolved chloride and water isotopic data from the Chukchi Sea Shelf sediments that reveal the presence of a meteoric subsurface flow enriched in cations with a radiogenic Sr fingerprint. This subsurface fluid is also enriched in dissolved inorganic carbon and methane that bear isotopic compositions indicative of a carbon reservoir modified by reactions in a closed system. Such fluid characteristics are in stark contrast with those from other sites in the Chukchi Sea where the pore water composition shows no sign of meteoric input, but reflect typical biogeochemical reactions associated with early diagenetic sequences in marine sediment. The most likely source of the observed subsurface flow at the Chukchi Sea Shelf is from the degradation of permafrost that had extended to the shelf region during the Last Glacial Maximum. Our data suggest that the permafrost-driven subsurface flow most likely took place during the 2-3 degrees C warming in the Early Holocene Thermal Maximum. This time scale is supported by numerical simulation of pore water profiles, which indicate that a minimum of several thousand years must have passed since the cessation of the subsurface methane-bearing fluid flow.

  Read more about A Pulse of Meteoric Subsurface Fluid Discharging Into the Chukchi Sea During the Early Holocene Thermal Maximum (EHTM)

Show all publications by Wei-Li Hong at Stockholm University