A global reassessment of the controls on iron speciation in modern sediments and sedimentary rocks: A dominant role for diagenesis

The speciation of iron in sediments and sedimentary rocks is a widely used proxy for the chemistry and oxidation state of ancient water bodies. Specifically, the fraction of reactive iron out of the total iron (FeHR/FeT) and the fraction of pyrite iron out of the reactive iron pool (FePYR/FeHR) are thought to constrain the oxidation state and the presence of sulfide in the water column, respectively. This approach was developed and tested against modern core-top sediments, but application to sedimentary rocks requires consideration of the effects of diagenesis and lithification on iron speciation. Furthermore, the effects of deep burial, metamorphism, and late-stage alteration during exhumation or sampling (e.g., oxidative weathering) have not been systematically explored. To bridge this gap, we combined new data from four sediment cores (n = 54) with an extensive literature compilation of modern sediments (2936 measurements from 316 cores) and ancient sedimentary rocks (12,173 measurements spanning the Neoarchean to Quaternary). The modern data include both surface and buried sediments, allowing an investigation of the effects of diagenesis on iron speciation. Depending on the thresholds used to distinguish oxic from anoxic environments and ferruginous from euxinic environments, interpretation of the modern sedimentary iron speciation data within the existing framework yields incorrect environmental classifications up to ≈70% of the time. In modern sediments, diagenesis is the main reason that iron speciation does not represent the chemistry and oxidation state of the water column. We find that iron speciation correlates with porewater chemistry and that it changes with progressive burial along three distinctive FeHR/FeT–FePYR/FeHR arrays, each of which represents a different set of diagenetic processes. We suggest that similarly to modern sediments, stratigraphic variation in iron speciation in sedimentary rocks primarily reflects progressive burial diagenesis or variation in depositional conditions rather than temporal variation in water-column chemistry and oxidation state. Indeed, analysis of the geologic iron speciation data reveals no statistically significant trends in either FeHR/FeT or FePYR/FeHR from the Archean to the present day. The diagenetic FeHR/FeT–FePYR/FeHR arrays that we identified in modern marine sediments suggest that under certain conditions, iron speciation analyses may be used to constrain FeHR/FeT in the local sediment source(s). Hence, we suggest that iron speciation data, together with complementary petrographic, mineralogical and geochemical constraints, may be used to constrain the local iron source(s) and early and late diagenetic processes, but rarely the chemistry or oxidation state of ancient water columns

Spatially and temporally variable sulfur cycling in shallow-sea hydrothermal vents, Milos, Greece

Shallow-sea hydrothermal systems are ideal for studying the relative contributions to sedimentary sulfur archives from ambient sulfur-utilizing microbes and from fluxes of hydrothermally derived sulfur. Here we present data from a vent field in Palaeochori Bay, Milos, Greece using a suite of biogeochemical analytical tools that captured both spatial and temporal variability in biotic and abiotic sulfur cycling. Samples were collected along a transect from a seagrass meadow to an area of active venting. The abundance and isotopic composition of sulfide captured in situ, together with geochemistry from sedimentary porewaters and the overlying water column and solid phase sulfide minerals, record evidence of ephemeral activity of microbial sulfate reduction as well as sulfide oxidation. The sulfur and oxygen isotope composition of porewater sulfates indicate active sulfate reduction within the transition zone between the vents and seagrass, rapid recycling of biologically produced sulfide within non-vent sediments, and reoxidation of abiotic sulfide within the vent field. A phylogenetic survey of sediments also indicates the pervasive presence of a suite of putative sulfur-metabolizing bacteria, including sulfate reducers and sulfide oxidizers, many of which can utilize intermediate valence sulfur compounds. The isotopic composition of pyrite in these sediments consistently records a microbially influenced signature (δ 34 S py of −4.4 to −10.8‰) relative to the hydrothermal endmember (δ 34 S ~ + 2.5‰), independent of distance from the vent source. The narrow range of pyrite δ 34 S across sediments with a highly variable hydrothermal influence suggests that physical mixing (e.g., by storm events) homogenizes the distribution of biogenic and hydrothermal Fe-sulfides throughout the region, overprinting the spatially and temporally variable interplay between biological and hydrothermal sulfur cycling in these environments. © 2018 Elsevier B.V

Boron isotopes in Central American volcanics indicate a key role for the subducting oceanic crust

The geochemistry of arc magmas can shed light on chemical outfluxes from subducting slabs to the overlying mantle. Boron (B) abundances and isotope ratios are valuable tracers of slab-derived components due to the distinct compositions of the mantle and subducting materials and distinctive isotopic fractionation during dehydration. New Be/B and δ11B measurements in olivine-hosted melt inclusions (MIs) from three Nicaraguan volcanic centers (Telica, Cerro Negro, and Masaya) are consistent with a B-rich slab component that has δ11B ranging from +2.9‰ to +5.9‰, slightly higher than new measurements of hemipelagic (δ11B = +0.7‰±0.03 and +2.1‰±0.08; 1σ n = 3) and carbonate (δ11B = +2.9‰±0.06 and 3.7±0.09; 1σ n = 3) sediments sampled by DSDP Hole 495 on the Cocos plate. A thermochemical model of the Nicaraguan subduction zone is used to quantitatively model B loss and isotopic fractionation during slab dehydration and melting. In contrast to previous studies regarding B systematics in Central America and elsewhere, this model reproduces the range of δ11B preserved in Nicaraguan olivine-hosted MIs without the involvement of serpentinite-derived fluids. The model indicates that Nicaraguan MI δ11B signatures are primarily controlled by input from subducted altered oceanic crust (AOC), with a minor contribution from subducted sediments. This finding implies that the volatile element budget delivered from the slab to the volcanic arc is also mostly derived from the ocean crust, and that volatiles carried in deeper layers of the slab may be recycled beyond the arc into the deeper mantle beneath Central America

Metastable Iron (Mono)sulfides in the Shallow-Sea Hydrothermal Sediments of Milos, Greece

Metastable iron sulfides are involved in a series of biotic and abiotic processes in the marine environment, including the mineralization of organic matter. However, naturally occurring metastable iron (mono)sulfide minerals are rarely reported in marine sediments, and current information about their formation and characteristics comes from synthetic sulfides. Here, we studied sulfur speciation and mineralogy in a submarine surface core (0-22 cm depth) from an active, shallow-sea hydrothermal system (Milos, Greece) that is dominated by sulfur-metabolizing microorganisms. Geochemical analysis results showed S-Fe-As enrichment in the bottom layers of the core, which were further characterized using a suite of techniques. Powder X-ray diffraction and Synchrotron-based μ-X-ray diffraction did not show crystalline Fe-S compounds whereas scanning electron microscopy and Synchrotron-based X-ray fluorescence mapping indicated the presence of Fe-S(-As) phases and sulfur particles. Sulfur microspeciation by X-ray absorption near-edge structure spectroscopy showed a mixture of oxidation states, including organic sulfur species, indicative of active sulfur biogeochemical cycling. Ultimately, transmission electron microscopy was used for the identification of the Fe-S mineral assemblage in the samples that included arsenic-bearing pyrite and the metastable mackinawite, monoclinic pyrrhotite and greigite, alongside elemental sulfur nanoparticles. Previous studies on the mineralogy of Milos hydrothermal sediments omitted the presence of metastable iron sulfides, that were up to now known to form in marine sediments from estuaries and anoxic/euxinic basins. Our results highlight that the use of standard microscopic, spectroscopic and diffraction techniques may overlook the presence of metastable iron sulfides in natural samples. Considering that metastable iron sulfides are implicated in critical biogeochemical processes for the marine ecosystems, their role in sulfur, iron, and carbon cycling in modern and ancient marine sediments might be underrated. © 2022 American Chemical Society. All rights reserved