Global Elemental Cycles Research Articles

Viruses—Agents of Change in the Oceans

Viruses are usually thought of as the cause of countless diseases. However, in the oceans, viruses are part of the natural cycle of life and death. This article discusses marine viruses that infect phytoplankton—the tiny micro-algae that form the base of the marine food web and affect Earth’s climate. Through an ongoing “arms race” between viruses and the cells they infect, viruses can promote the evolution of their hosts, and even help their hosts acquire genes that can help them survive. By killing phytoplankton species that become very abundant, viruses can allow other species to grow, promoting biodiversity. Finally, viruses affect global cycles of carbon and other elements, indirectly influencing the climate of our planet.

The evolution of trait correlations constrains phenotypic adaptation to high CO2 in a eukaryotic alga.

Microbes form the base of food webs and drive biogeochemical cycling. Predicting the effects of microbial evolution on global elemental cycles remains a significant challenge due to the sheer number of interacting environmental and trait combinations. Here, we present an approach for integrating multivariate trait data into a predictive model of trait evolution. We investigated the outcome of thousands of possible adaptive walks parameterized using empirical evolution data from the alga Chlamydomonas exposed to high CO2. We found that the direction of historical bias (existing trait correlations) influenced both the rate of adaptation and the evolved phenotypes (trait combinations). Critically, we use fitness landscapes derived directly from empirical trait values to capture known evolutionary phenomena. This work demonstrates that ecological models need to represent both changes in traits and changes in the correlation between traits in order to accurately capture phytoplankton evolution and predict future shifts in elemental cycling.

Genomic insights into diverse bacterial taxa that degrade extracellular DNA in marine sediments

Extracellular DNA is a major macromolecule in global element cycles, and is a particularly crucial phosphorus, nitrogen and carbon source for microorganisms in the seafloor. Nevertheless, the identities, ecophysiology and genetic features of DNA-foraging microorganisms in marine sediments are largely unknown. Here, we combined microcosm experiments, DNA stable isotope probing (SIP), single-cell SIP using nano-scale secondary isotope mass spectrometry (NanoSIMS) and genome-centric metagenomics to study microbial catabolism of DNA and its subcomponents in marine sediments. 13C-DNA added to sediment microcosms was largely degraded within 10 d and mineralized to 13CO2. SIP probing of DNA revealed diverse ‘Candidatus Izemoplasma’, Lutibacter, Shewanella and Fusibacteraceae incorporated DNA-derived 13C-carbon. NanoSIMS confirmed incorporation of 13C into individual bacterial cells of Fusibacteraceae sorted from microcosms. Genomes of the 13C-labelled taxa all encoded enzymatic repertoires for catabolism of DNA or subcomponents of DNA. Comparative genomics indicated that diverse ‘Candidatus Izemoplasmatales’ (former Tenericutes) are exceptional because they encode multiple (up to five) predicted extracellular nucleases and are probably specialized DNA-degraders. Analyses of additional sediment metagenomes revealed extracellular nuclease genes are prevalent among Bacteroidota at diverse sites. Together, our results reveal the identities and functional properties of microorganisms that may contribute to the key ecosystem function of degrading and recycling DNA in the seabed.

A decade of progress in understanding cycles of trace elements and their isotopes in the oceans

We present an overview of a Virtual Special Issue (VSI) entitled “Cycles of trace elements and isotopes in the ocean - GEOTRACES and beyond.” In addition to hosting the overview of the GEOTRACES Intermediate Data Product 2017, we highlight the contents of 30 other articles that comprise the VSI. These articles contribute to the International GEOTRACES mission in four areas of discovery: developments in marine transition metal isotope analysis; the role of the Southern Ocean in driving global trace element cycling; advances in understanding of the utility of rare-earth elements and natural and anthropogenic radionuclides as tracers of ocean processes; and using aerosols, particles, and elemental speciation to interrogate processes driving trace element cycles. We close by highlighting the future direction of developments in the field.

MOSAIC (Modern Ocean Sediment Archive and Inventory of Carbon): a (radio)carbon-centric database for seafloor surficial sediments

Abstract. Mapping the biogeochemical characteristics of surficial ocean sediments is crucial for advancing our understanding of global element cycling, as well as for assessment of the potential footprint of environmental change. Despite their importance as long-term repositories for biogenic materials produced in the ocean and delivered from the continents, biogeochemical signatures in ocean sediments remain poorly delineated. Here, we introduce MOSAIC (Modern Ocean Sediment Archive and Inventory of Carbon; https://doi.org/10.5168/mosaic019.1, http://mosaic.ethz.ch/, last access: 1 March 2021; Van der Voort et al., 2019), a (radio)carbon-centric database that seeks to address this information void. The goal of this nascent database is to provide a platform for development of regional-to-global-scale perspectives on the source, abundance and composition of organic matter in marine surface sediments and to explore links between spatial variability in these characteristics and biological and depositional processes. The database has a continental margin-centric focus given both the importance and complexity of continental margins as sites of organic matter burial. It places emphasis on radiocarbon as an underutilized yet powerful tracer and chronometer of carbon cycle processes, with a view to complementing radiocarbon databases for other Earth system compartments. The database infrastructure and interactive web application are openly accessible and designed to facilitate further expansion of the database. Examples are presented to illustrate large-scale variabilities in bulk carbon properties that emerge from the present data compilation.

Structure and Dynamics of Soil Microbial Communities of Natural and Transformed Ecosystems

Soil microbial communities play an important role in ecosystems functioning and are on the field scale essential for plant nutrition and health. On a larger scale, they contribute to global element cycling. Furthermore, they are involved in the turnover processes of organic matter, the breakdown of xenobiotics and the formation of soil aggregates. An ecological state of soils depends on the structure and activity of soil microorganisms. The results of soil monitoring in various ecosystems in different climatic zones of Ukraine showed a clear trend for the correlation between the agroecological conditions and activity of microbiocenosis. The most significant influence of agricultural activity on the soil microbiota can be observed on the poorly soddy-podzolic and gray forest soils, where the crop cultivation without fertilization resulted in a decrease in the total count of microorganisms by 2.2-4.5 times. Application of agricultural measures aimed at achieving maximum productivity, specifically the combination of mineral, organic and biological fertilizers, contributes to an average 1.3-4.1 times increase in the total count of microorganisms in the soil, compared with non-fertilized variants. Soils with low content of organic matter and acidic medium, soddy-podzolic and gray forest soils were characterized by a high number of micromycetes, 136-185×103 CFU·g-1 soil, and a relatively low number of eutrophic and nitrogen-fixing microorganisms. The soil of natural ecosystems is characterized by a high total count of the microorganisms with a balanced structure of various ecological-trophic groups and balanced processes of mineralization-immobilization, organic matter decomposition, and humus accumulation.

Barium stable isotopes as a fingerprint of biological cycling in the Amazon River basin

Abstract. The biological cycle of rock-derived nutrients on the continents is a major component of element transfer between the Earth's surface compartments, but its magnitude currently remains elusive. The use of the stable isotope composition of rock-derived nutrients, which can be fractionated during biological uptake, provides a promising path forward with respect to quantifying biological cycling and its overall contribution to global element cycling. In this paper, we rely on the nutrient-like behaviour of the trace element barium (Ba) and use its elemental and stable isotope compositions in dissolved and sediment load river samples to investigate biological cycling in the Amazon Basin. From these measurements, we show that dissolved Ba mainly derives from silicate rocks, and a correlation between dissolved Ba and K abundances suggests that biological cycling plays a role in the Ba river budget. Furthermore, the isotope composition of Ba (δ138Ba) in the dissolved load was found to be significantly different from that of the parent silicate rocks, implying that dissolved Ba isotopic signatures are affected by (i) the precipitation of soil-forming secondary phases as well as (ii) biological uptake and release from dead organic matter. Results from an isotope mass balance method applied to the river dissolved load data indicate that, after its release to solution by rock weathering, Ba is partitioned between the river dissolved load, secondary weathering products (such as those found in soils and river sediments), and the biota. In most sub-catchments of the Amazon, river Ba abundances and isotope compositions are significantly affected by biological cycling. Relationships between estimates of Ba cycled through biota and independent metrics of ecosystem dynamics (such as gross primary production and terrestrial ecosystem respiration) allow us to discuss the role of environmental parameters such as climate or erosion rates on the biological cycling of Ba and, by extension, the role of major rock-derived nutrients. In addition, catchment-scale mass and isotope budgets of Ba show that the measured riverine export of Ba is lower than the estimated delivery of Ba to the Earth surface through rock alteration. This indicates the existence of a missing Ba component, which we attribute to the formation of Ba-bearing particulate organics (possibly accumulating as soil organic matter or currently growing biomass within the catchments) and to organic-bound Ba exported as “unsampled” river particulate organic matter. Given our findings on the trace element Ba, we explore whether the river fluxes of most major rock-derived nutrients (K, Mg, Ca) might also be significantly affected by biological uptake or release. A first-order correction of river-derived silicate weathering fluxes from biological cycling shows that the carbon dioxide (CO2) consumption by silicate weathering at the mouth of the Amazon could be several times higher than the previously reported value of 13 × 109 mol CO2 yr−1 (Gaillardet et al., 1997). Overall, our study clearly shows that the chemical and isotope compositions of rivers in the Amazon – and most likely in other large river basins – bear a biological imprint, thereby challenging common assumptions made in weathering studies.

Microbial biomodification of clay minerals.

Clay minerals are important reactive centers in the soil system. Their interactions with microorganisms are ubiquitous and wide-ranging, affecting growth and function, interactions with other organisms, including plants, biogeochemical processes and the fate of organic and inorganic pollutants. Clay minerals have a large specific surface area and cation exchange capacity (CEC) per unit mass, and are abundant in many soil systems, especially those of agricultural significance. They can adsorb microbial cells, exudates, and enzymes, organic and inorganic chemical species, nutrients, and contaminants, and stabilize soil organic matter. Bacterial modification of clays appears to be primarily due to biochemical mechanisms, while fungi can exhibit both biochemical and biomechanical mechanisms, the latter aided by their exploratory filamentous growth habit. Such interactions between microorganisms and clays regulate many critical environmental processes, such as soil development and transformation, the formation of soil aggregates, and the global cycling of multiple elements. Applications of biomodified clay minerals are of relevance to the fields of both agricultural management and environmental remediation. This review provides an overview of the interactions between bacteria, fungi and clay minerals, considers some important gaps in current knowledge, and indicates perspectives for future research.

Organic carbon and microbial activity in marine sediments on a global scale throughout the Quaternary

Microbial degradation of organic carbon in marine sediments is a key driver of global element cycles on multiple time scales. However, it is not known to what depth microorganisms alter organic carbon in marine sediments or how microbial rates of organic carbon processing change with depth, and thus time since burial, on a global scale. To better understand the connection between the dynamic carbon cycle and life’s limits in the deep subsurface, we have combined a number of global data sets with a reaction transport model (RTM) describing first, organic carbon degradation in marine sediments deposited throughout the Quaternary Period and second, a bioenergetic model for microbial activity. The RTM is applied globally, recognizing three distinct depositional environments – continental shelf, margin and abyssal zones. The results include the masses of particulate organic carbon, POC, stored in three sediment-depth layers: bioturbated Holocene (1.7 × 1017 g C), non-bioturbated Holocene (2.5 × 1018 g C) and Pleistocene (1.4 × 1020 g C) sediments. The global depth-integrated rates of POC degradation have been determined to be 1.3 × 1015, 1.3 × 1014 and 3.0 × 1014 g C yr−1 for the same three layers, respectively. A number of maps depicting the distribution of POC, as well as the fraction that has been degraded have also been generated. Using POC degradation as a proxy for microbial catabolic activity, total heterotrophic processing of POC throughout the Quaternary is estimated to be between 10−11 and 10−6 g C cm−3 yr−1, depending on the time since deposition and location. Bioenergetic modeling reveals that laboratory-determined microbial maintenance powers are poor predictors of sediment biomass concentration, but that cell concentrations in marine sediments can be accurately predicted by combining bioenergetic models with the rates of POC degradation determined in this study. Our model can be used to quantitatively describe both the carbon cycle and microbial activity on a global scale for marine sediments less than 2.59 million years old.

Bacterial chemolithoautotrophy via manganese oxidation.

Manganese is among Earth’s most abundant elements. Its oxidation had long been theorized1, yet undemonstrated2–4, to fuel chemolithoautotrophic microbial growth. Here, an enrichment culture exhibiting Mn(II)-oxidation-dependent, exponential growth was refined to a two species co-culture. Oxidation required viable bacteria at permissive temperatures, resulting in the generation of small Mn oxide nodules to which the cells associated. The majority member of the culture, ‘Candidatus Manganitrophus noduliformans’, affiliates within phylum Nitrospirae (Nitrospirota) but is distantly related to known Nitrospira and Leptospirillum species. The minority member has been isolated, but does not oxidise Mn(II) alone. Stable isotope probing revealed Mn(II)-oxidation-dependent, 13CO2-fixation into cellular biomass. Transcriptomics reveals candidate pathways for coupling extracellular manganese oxidation to aerobic energy conservation and to autotrophic CO2-fixation. These findings expand the known diversity of inorganic metabolisms supporting life, while completing a biogeochemical energy cycle for manganese5,6, one that may interface with other major global elemental cycles.

Niche Differentiation of Sulfate- and Iron-Dependent Anaerobic Methane Oxidation and Methylotrophic Methanogenesis in Deep Sea Methane Seeps.

Methane seeps are widespread seafloor ecosystems shaped by complex physicochemical-biological interactions over geological timescales, and seep microbiomes play a vital role in global biogeochemical cycling of key elements on Earth. However, the mechanisms underlying the coexistence of methane-cycling microbial communities remain largely elusive. Here, high-resolution sediment incubation experiments revealed a cryptic methane cycle in the South China Sea (SCS) methane seep ecosystem, showing the coexistence of sulfate (SO42–)- or iron (Fe)-dependent anaerobic oxidation of methane (AOM) and methylotrophic methanogenesis. This previously unrecognized methane cycling is not discernible from geochemical profiles due to high net methane consumption. High-throughput sequencing and Catalyzed Reporter Deposition-Fluorescence in situ Hybridization (CARD-FISH) results suggested that anaerobic methane-oxidizing archaea (ANME)-2 and -3 coupled to sulfate-reducing bacteria (SRB) carried out SO42–-AOM, and alternative ANME-2 and -3 solely or coupled to iron-reducing bacteria (IRB) might participate in Fe-AOM in sulfate-depleted environments. This finding suggested that ANME could alter AOM metabolic pathways according to geochemical changes. Furthermore, the majority of methylotrophic methanogens belonged to Methanimicrococcus, and hydrogenotrophic and acetoclastic methanogens were likely inhibited by sulfate or iron respiration. Fe-AOM and methylotrophic methanogenesis are overlooked potential sources and sinks of methane in methane seep ecosystems, thus influencing methane budgets and even the global carbon budget in the ocean.

Into the deep and beyond: Carbon and nitrogen subduction recycling in secondary peridotites

Understanding the volatile cycles at convergent margins is fundamental to unravel the Earth's evolution from primordial time to present. The assessment of fluid-mobile and incompatible element uptake in serpentinites via interaction with seawater and subduction-zone fluids is central to evaluate the global cycling of the above elements in the Earth's mantle. Here, we focus on the carbon (C), nitrogen (N) and C isotope compositions of chlorite harzburgites and garnet peridotites deriving from subduction-zone dehydration of former oceanic dehydration of serpentinite – i.e., metaperidotites (Cima di Gagnone, Swiss Central Alps) with the aim of evaluating the contribution of these rocks to the global C-N cycling. These ultramafic rocks, enclosed as lenses in a metasedimentary mélange, represent the destabilization of antigorite and chlorite at high-pressure/temperature (P/T) along a slab-mantle interface. Chlorite- and garnet-bearing rocks have similar ranges in C concentration ([C] = 210 – 2465 ppm and 304 – 659 ppm, respectively), with one magnesite-bearing chlorite harzburgite hosting 11000 ppm C. The average N concentrations ([N]) of the garnet peridotites (54 ± 15 ppm, one standard deviation indicated) are higher than those of the chlorite harzburgites (29 ± 6 ppm). The δ13C of total C (TC) and total organic C (TOC) values of the Gagnone metaperidotites range from -12.2 to -17.8‰ and from -27.8 to -26.8‰, respectively, excluding the magnesite-bearing chlorite harzburgites with higher values of -7.2‰ (TC) and -21.2‰ (TOC). The [C] of these rocks are comparable to those of serpentinites form modern and ancient oceanic environments and with [C] of high-P serpentinites. However, the lack of preserved serpentinite precursors makes it difficult to determine whether release of H2O during high-P breakdown of antigorite and chlorite is coupled with significant C release to fluids. The δ13C values appear to reflect mixing between seawater-derived carbonate and a reduced C source and a contribution from the host metasedimentary rocks ([C] = 301 ppm; [N] = 33 ppm; TC δ13C = -24.4‰; TOC δ13C = -27.0‰) cannot be completely excluded. The C-O isotope composition of the carbonate in magnesite-bearing chlorite harzburgites is compatible with progressive devolatilization at oxidized conditions, whereas the signatures of the majority of the other Gagnone samples appear to reflect different degree of interaction with sedimentary fluids. The [N] of the Gagnone metaperidotites are higher than those of oceanic and subducted serpentinites and show a range similar to that of high-P antigorite-serpentinites from mantle wedges. This enrichment is compatible with fluid-mediated chemical exchange with the surrounding metasedimentary rocks leading to strong modification of the Gagnone metaperidotites' geochemistry during prograde subduction along the slab-mantle interface. Comparing the δ13C data reported in this study with published δ13C values for diamonds, we suggest that the volatile recycling via Gagnone-like metaperidotites in subduction zones could contribute to deep-Earth diamond genesis and in particular to the formation of blue boron (B)-bearing diamonds. Our results highlight that the subduction of secondary peridotites evolved along the slab-mantle interface is a viable mechanism to inject volatiles into the deep mantle, particularly in hotter geothermal regimes such as the ones active during the early Earth's history.

A first-order geochemical budget for suspended sediment discharge to the Bay of Bengal from the Ganges-Brahmaputra river system

The Ganges-Brahmaputra-Meghna (G-B) river system transports >1 × 109 t/yr of sediment, with an estimated 0.7 × 109 t/yr reaching the Bay of Bengal (BoB). This discharge represents a major input of sediment and associated elements to the global ocean, but quantification of the sediment-element mass reaching the BoB has yet to be fully explored. Published geochemical and suspended sediment data are used to calculate a first-order budget for the modern sediment supply of geochemical elements to the BoB. River profile bulk sediment-element concentrations are calculated based on suspended sediment and element measurements taken in the Ganges and Brahmaputra rivers. A Monte Carlo analysis is applied to account for variable sediment and geochemical contributions from each river. Results show that on average, the G-B system contributes ~5% of the global riverine discharge of solid-phase elements from sediment to the oceans. G-B sediments transport >10% of the global element supply of Hf and Zr. For others, like As and Cu, contributions from the G-B are <5%. Results also show that sediment reaching the BoB is relatively enriched in Hf, Zr, Th, REEs, Sn, and Bi, and majorly depleted in Na and Sr compared to UCC elemental concentrations. While limited by data availability and necessary simplifying assumptions, this study nevertheless provides a reasonable first-order budget for the modern discharge of solid-phase elements to the BoB. Insights from this work are significant for understanding the role of the G-B river system in global elemental cycling, and for providing a basis of comparison for future sediment-element discharge in light of rapid environmental change taking place in the region.

Origin of Rodingite Forming Fluids Constrained by Calcium and Strontium Isotope Ratios in the Leka Ophiolite Complex

Alteration of oceanic lithosphere plays an important role in the global element cycling due to its dominant abundance on the Earth's surface. The formation of rodingites, a Ca rich lithology that occurs intimately associated with Ca depleted serpentinite complexes, is a widespread process during strong alteration of ocean-floor lithologies and may represent a key process that influences the global Ca cycle. Stable Ca and radiogenic Sr isotope compositions of rodingites and associated lithologies from the Leka ophiolite complex, Norway, preserve mantle signatures and thus provide evidence that the Ca and Sr sources for rodingitisation are dominantly mantle derived, with no detectable seawater input. The difference of ~0.7‰ in δ44/40Ca of different phases found in the rocks is overall consistent with inter-mineral fractionation at high temperatures related to the CaO bonding environment, but there are also indications for isotopic disequilibria between non-cogenetic mineral phases.While the water required for rodingite formation may originate from seawater, the dissolved Ca and Sr involved in the mineral reactions during rodingitisation (Plg + Ca - > Gros + Clz + Al) can be balanced at a local scale, being derived from the serpentinisation of mantle peridotites (Cpx1 + H2O + Al = Srp + Chlorite + Ca + Cpx2). Consequently, rodingites do not act as a sink for oceanic Ca. Instead, they are a sink for Ca released during serpentinisation of the oceanic lithospheric mantle.

Bioturbation may not always enhance the metabolic capacity of organic polluted sediments

Marine sediments are a major sink of organic matter, playing a crucial role in the global cycling of major elements. Macrofauna, through the reworking of particles and movement of solutes (bioturbation), enhances oxic conditions and the sediment metabolic capacity. Increases in the inputs of organic matter can lead to profound changes in the seabed and impact benthic ecological functions. Through a microcosm experiment, the effect of bioturbation of the polychaete Lumbrineris latreilli on biogeochemical fluxes under scenarios of increasing loads of organic matter was quantified. We found that bioturbation can buffer the negative consequences of anoxic conditions produced by organic enrichment, preventing the build-up of toxic by-products derived from anaerobic metabolic pathways by maintaining oxic conditions. However, the maintenance of oxic conditions by bioturbation is at the expense of limiting the sediment metabolic capacity. The maintenance of oxic conditions may limit anaerobic metabolic pathways, and consequently, the metabolic capacity of sediment. Thus, under organic matter pollution conditions, bioturbation may lessen the metabolic capacity of the sediment.

Effect of Acidic Conditions on Surface Properties and Metal Binding Capacity of Clay Minerals

Clays are one of the most abundant minerals on the Earth’s surface, and they play a significant role in global trace element cycling because of their ability to adsorb and incorporate a range of metal cations. However, to what extent clay surface reactivities are affected by changes in pH of the surrounding environment remains poorly understood. To address this uncertainty, we investigated three common clay minerals (kaolinite, illite, and montmorillonite) and compared their surface properties and metal binding capacity prior to and after acidic treatment (pH ranges from 0 to 7). We report that (1) the morphologies and crystal structures of the three studied clay minerals do not differ markedly after exposure to an acidic solution, (2) all three clays display increases in surface area after the pH 0 acidic treatment, and (3) cadmium adsorption to all three clays decreased after the pH 0 acidic treatment. Our results suggest that the variation in metal adsorption capacity is related to acidic modification of clay surface properties. Importantly, this work shows the relevance of acidic environments on surface-controlled reactions of trace metals to clay minerals.

Microbial metabolite fluxes in a model marine anoxic ecosystem.

Permanently anoxic regions in the ocean are widespread and exhibit unique microbial metabolic activity exerting substantial influence on global elemental cycles and climate. Reconstructing microbial metabolic activity rates in these regions has been challenging, due to the technical difficulty of direct rate measurements. In Cariaco Basin, which is the largest permanently anoxic marine basin and an important model system for geobiology, long-term monitoring has yielded time series for the concentrations of biologically important compounds; however, the underlying metabolite fluxes remain poorly quantified. Here, we present a computational approach for reconstructing vertical fluxes and in situ net production/consumption rates from chemical concentration data, based on a 1-dimensional time-dependent diffusive transport model that includes adaptive penalization of overfitting. We use this approach to estimate spatiotemporally resolved fluxes of oxygen, nitrate, hydrogen sulfide, ammonium, methane, and phosphate within the sub-euphotic Cariaco Basin water column (depths 150-900m, years 2001-2014) and to identify hotspots of microbial chemolithotrophic activity. Predictions of the fitted models are in excellent agreement with the data and substantially expand our knowledge of the geobiology in Cariaco Basin. In particular, we find that the diffusivity, and consequently fluxes of major reductants such as hydrogen sulfide, and methane, is about two orders of magnitude greater than previously estimated, thus resolving a long-standing apparent conundrum between electron donor fluxes and measured dark carbon assimilation rates.

Deep Sulfate-Methane-Transition and sediment diagenesis in the Gulf of Alaska (IODP Site U1417)

Sediment samples from the Gulf of Alaska (GOA, IODP Expedition 341, Site U1417) have been analyzed to understand present and past diagenetic processes that overprint the primary sediment composition. No Sulfate-Methane Transition Zone (SMTZ) was observed at the zone of sulfate depletion, but a >200 m thick sulfate- and methane-free sediment interval occurred between the depth of sulfate depletion (~200 m) and the onset of methanogenesis (~440 m). We suggest that this apparent gap in biogeochemical processing of organic matter is caused by anaerobic oxidation of methane fueled by sulfate which is released during dissolution of barites at the upper boundary of the methane rich layer. Beneath the methanogenic zone, at ~650 m depth, pore-water sulfate concentrations increase again, indicating sulfate supply from greater depth feeding into a deep, inverse SMTZ. A likely explanation for the availability of sulfate in the deep sub-seafloor at U1417 is the existence of a deep aquifer related to plate bending fractures, which actively transports sulfate-rich water to, and potentially along, the interface between sediments and oceanic crust. Such inverse diagenetic zonations have been previously observed in marine sediments, but have not yet been linked to subduction-related plate bending. With the discovery of a deep inverse SMTZ in an intra-oceanic plate setting and the blocking of upward methane diffusion by sulfate released from authigenic barite dissolution, Site U1417 provides new insights into sub-seafloor pore-fluid and gas dynamics, and their implications for global element cycling and the deep biosphere.

Enhanced chemical weathering of albite under seawater conditions and its potential effect on the Sr ocean budget

Abstract Currently, knowledge about the dissolution kinetics of silicate minerals under seawater conditions in general and about feldspar minerals specifically is lacking, even though feldspars are the most common minerals in the earth’s crust. Consequently, global geochemical cycles of elements and isotopes overlook the potential effects of silicate dissolution in seawater and its contribution to mass balances. The aim of the present study is to quantify the effect of salinity on albite dissolution kinetics under seawater conditions and the possible impact of salinity on Sr concentrations and isotopic compositions in seawater. A series of albite dissolution experiments with synthetic NaCl solutions (0.0001–1 molal) at pH 5 and 25 °C were conducted using multi-point batch experiments (MPBEs). Above the Na concentration threshold (0.01 molal), the albite dissolution rate was found to increase with the NaCl concentration and with the ionic strength. Based on the present study and previous experimental studies, a combined model that describes the effects of pH, Na concentration and ionic strength on the albite dissolution rate was developed. The proposed model suggests that albite dissolution under natural seawater conditions is mainly controlled by the effect of ionic strength. Considering that seawater solutions are under far-from-equilibrium conditions, the dissolution rate of albite is more than order of magnitude faster in seawater than in the continental, abiotic environment. Hence, the potential of silicate dissolution in seawater to contribute elements to the oceans is significant. In addition, a MPBE was conducted with a Bancroft, Ontario, Canada albite sample using synthetic seawater (SSW). Based on the changes in Si, Al, Sr, Ca and 87Sr/86Sr with time in this experiment, it is suggested that the relative dissolution rates of albite and apatite inclusions in the albite sample control the Sr concentrations and isotopic compositions. The estimated contribution of Sr release due to feldspar dissolution in seawater under far-from-equilibrium conditions is 2.67∙108 mol y−1. This result is significant if considering other forms of Al-Si mineral dissolution on the ocean floor.

Fungus-initiated catalytic reactions at hyphal-mineral interfaces drive iron redox cycling and biomineralization

The ability of fungi to weather a wide range of minerals influences plant nutrition and enhances global biogeochemical cycles of life-essential elements. The fungus-mineral interface plays a key role in weathering, but the specific mechanisms underlying these processes remain poorly understood. Here, we examined fungal-mineral weathering using hematite and Trichoderma guizhouense. We showed that hematite dissolution increased over cultivation time, with the formation of secondary minerals up to ∼3000 µm −2 at the interfaces after 66 h cultivation. Of the hematite associated with hyphae, approximately 15% was converted to the secondary mineral ferrihydrite. Importantly, superoxide radicals were detected at the hyphal tips and along the whole hyphae. During cultivation, a high concentration (∼1000 nM) of hydroxyl radical was also detected. Synchrotron radiation based spectromicroscopies at fungus-mineral interfaces suggest that fungus hyphae alter the local redox state of iron and thus are redox-active. These findings indicate that fungus-initiated catalytic reactions occur at hyphal-mineral interfaces, in view of the fact that superoxide does not diffuse far from the site of formation. Furthermore, these results also suggest that the catalytic reactions may serve as a new strategy for microbial iron uptake. Together, these findings constitute a significant step forward in understanding the ways that fungi make minerals available to biological systems.