The biogeochemical cycles of iron (Fe) and manganese (Mn) in lakes and reservoirs have predictable seasonal trends, largely governed by stratification dynamics and redox conditions in the hypolimnion. However, short-term (i.e., sub-weekly) trends in Fe and Mn cycling are less well-understood, as most monitoring efforts focus on longer-term (i.e., monthly to yearly) time scales. The potential for elevated Fe and Mn to degrade water quality and impact ecosystem functioning, coupled with increasing evidence for high spatiotemporal variability in other biogeochemical cycles, necessitates a closer evaluation of the short-term Fe and Mn cycling dynamics in lakes and reservoirs. We adapted a UV-visible spectrophotometer coupled with a multiplexor pumping system and PLSR modeling to generate high spatiotemporal resolution predictions of Fe and Mn concentrations in a drinking water reservoir (Falling Creek Reservoir, Vinton, VA, USA) equipped with a hypolimnetic oxygenation (HOx) system. We quantified hourly Fe and Mn concentrations during two distinct transitional periods: reservoir turnover (Fall 2020) and initiation of the HOx system (Summer 2021). Our sensor system was able to successfully predict mean Fe and Mn concentrations as well as capture sub-weekly variability, ground-truthed by traditional grab sampling and laboratory analysis. During fall turnover, hypolimnetic Fe and Mn concentrations began to decrease more than two weeks before complete mixing of the reservoir occurred, with rapid equalization of epilimnetic and hypolimnetic Fe and Mn concentrations in less than 48 hours after full water column mixing. During the initiation of hypolimnetic oxygenation in Summer 2021, we observed that Fe and Mn were similarly affected by physical mixing in the hypolimnion, but displayed distinctly different responses to oxygenation, as indicated by the rapid oxidation of soluble Fe but not soluble Mn. This study demonstrates that Fe and Mn concentrations are highly sensitive to shifting DO and stratification and that their dynamics can substantially change on hourly to daily time scales in response to these transitions.
Nickel (Ni) is a micronutrient that plays a role in nitrogen uptake and fixation in the modern ocean may have impacted rates of methanogenesis on geological timescales. Here we present the results of a diagnostic model of global ocean Ni fluxes which addresses key questions about the biogeochemical processes which cycle Ni in the modern oceans. Our approach starts with extrapolating the sparse available observations of Ni data from the GEOTRACES project into a global gridded climatology of ocean Ni concentrations. Three different machine learning techniques were tested, each relying on marine tracers with better observational coverage such as macronutrient concentrations and physical parameters. The ocean transport of this global Ni concentration field is then estimated using the OCIM2 ocean circulation inverse model, revealing regions of net convergence or divergence. These diagnostics are not based on any assumption about Ni biogeochemical cycling, but their spatial patterns can be interpreted as reflecting biogeochemical processes. We find that the spatial pattern of Ni uptake in the surface ocean is similar to phosphate (P) uptake, but not silicate (Si) uptake, suggesting that Ni is not incorporated into diatom frustules. We find that Ni:P ratios at uptake do not decrease with Ni concentrations approaching 2 nM, which challenges the hypothesis of a ~2 nM pool of non-bioavailable Ni in the surface ocean. Finally, the net regeneration of Ni occurs deeper in the ocean than P remineralization, which could be explained by reversible scavenging or the presence of a refractory Ni phase.
The isotopic composition of dissolved oxygen offers a family of potentially unique tracers of respiration and transport in the subsurface ocean. Uncertainties in transport parameters and isotopic fractionation factors, however, have limited the strength of the constraints offered by 18O/16O and 17O/16O ratios in dissolved oxygen. In particular, puzzlingly low 17O/16O ratios observed for some low-oxygen samples have been difficult to explain. To improve our understanding of oxygen cycling in the ocean’s interior, we investigated the systematics of oxygen isotopologues in the subsurface Pacific using new data and a 2-D isotopologue-enabled isopycnal reaction-transport model. We measured 18O/16O and 17O/16O ratios, as well as the “clumped” 18O18O isotopologue in the northeast Pacific, and compared the results to previously published data. We find that transport and respiration rates constrained by O2 concentrations in the oligotrophic Pacific yield good measurement-model agreement across all O2 isotopologues only when using a recently reported set of respiratory isotopologue fractionation factors that differ from those most often used for oxygen cycling in the ocean. These fractionation factors imply that an elevated proportion of 17O compared to 18O in dissolved oxygen―i.e., its triple-oxygen isotope composition―does not uniquely reflect gross primary productivity and mixing. For all oxygen isotopologues, transport, respiration, and photosynthesis comprise important parts of their respective budgets. Mechanisms of oxygen removal in the subsurface ocean are discussed.
The δ34S of seawater sulfate reflects processes operating at the nexus of sulfur, carbon, and oxygen cycles. However, knowledge of past seawater sulfate δ34S values must be derived from proxy materials that are impacted differently by depositional and post-depositional processes. We produced new timeseries estimates for the δ34S value of seawater sulfate by combining 6710 published data from three sedimentary archives—marine barite, evaporites, and carbonate-associated sulfate—with updated age constraints on the deposits. Robust features in multiple records capture temporal trends in the δ34S value of seawater and its interplay with other Phanerozoic geochemical and stratigraphic trends. However, high-frequency discordances indicate that each record is differentially prone to depositional biases and diagenetic overprints. The amount of noise, quantified from the variograms of each record, increases with age for all δ34S proxies, indicating that post-depositional processes obscure detailed knowledge of seawater sulfate’s δ34S value deeper in time.
Organic matter (OM) sulfurization can enhance the preservation and sequestration of carbon in anoxic sediments, and it has been observed in sinking marine particles from marine O2-deficient zones. The magnitude of this effect on carbon burial remains unclear, however, because the transformations that occur when sinking particles encounter sulfidic conditions remain undescribed. Here, we briefly expose sinking marine particles from the eastern tropical North Pacific O2-deficient zone to environmentally relevant sulfidic conditions (20C, 0.5 mM [poly]sulfide, two days) and then characterize the resulting solid-phase organic and inorganic products in detail. During these experiments, the abundance of organic sulfur in both hydrolyzable and hydrolysis-resistant solids roughly triples, indicating extensive OM sulfurization. Lipids also sulfurize on this timescale, albeit less extensively. In all three pools, OM sulfurization produces organic monosulfides, thiols, and disulfides. Hydrolyzable sulfurization products appear within ≤ 200-m regions of relatively homogenous composition that are suggestive of sulfurized extracellular polymeric substances (EPS). Concurrently, reactions with particulate iron oxyhydroxides generate low and fairly uniform concentrations of iron sulfide (FeS) within these same EPS-like materials. Iron oxyhydroxides were not fully consumed during the experiment, which demonstrates that organic materials can be competitive with reactive iron for sulfide. These experiments support the hypothesis that sinking, OM- and EPS-rich particles in a sulfidic water mass can sulfurize within days, potentially contributing to enhanced sedimentary carbon sequestration. Additionally, sulfur-isotope and chemical records of organic S and iron sulfides in sediments have the potential to incorporate signals from water column processes.
Water isotopes measured in Antarctic ice cores enable reconstruction at the first order of the past temperature variations. However, the seasonality of the precipitation and episodic events, including synoptic-scale disturbances, influence the isotopic signals recorded in ice cores. In this study, we adopted an isotope-enabled atmospheric general circulation model from 1981 to 2010 to investigate variations in climatic factors in δ18O of precipitation (δ18Op) at Dome Fuji, East Antarctica. The Southern Annular Mode (SAM), the primary mode of atmospheric circulation in the southern mid-high latitudes, significantly contributes to the isotope signals. Positive δ18Op anomalies, especially in the austral winter, are linked to the negative polarity of the SAM, which weakens westerly winds and increases the southward inflow of water vapor flux. Daily variations in temperature and δ18Op in Dome Fuji are significantly small in the austral summer, and their contribution to the annual signals is limited. The isotope signals driven by the SAM are a locational feature of Dome Fuji, related to the asymmetric component of the large-scale atmospheric pattern.
The composition of impurities in ice controls the stability of liquid water and thus the distribution of potential aqueous habitats. We present a framework for modeling the brine volume fraction in impure water ice as a polynomial function of temperature and bulk ice salinity, inspired by models originally developed for sea ice. We applied this framework to examine the distribution of brine within the thermally conductive layer of Europa’s ice shell, considering binary (NaCl and MgSO4) and multi-ion “analog” (Cl-dominated and SO4-dominated) endmember impurity compositions. We found the vertical extent of brine in a conductive ice layer, expressed as a fraction of the total layer thickness, to be <12% for NaCl, <2% for MgSO4, and <18% for both the analog endmember impurity compositions, suggesting that the depth where brine is stable in an ice shell is more sensitive to composition when only two ionic species are present. For the same temperature and bulk ice salinity, the brine volume fraction is higher in a Cl-dominated ice shell than a SO4-dominated ice shell. Pressure, governed by the ice thickness, was found to have only a minor effect on the vertical extent of brine within an ice shell, relative to temperature and bulk salinity. The minimum stable bulk ice shell salinity formed through freezing of an ocean was found to be insensitive to composition and ultimately governed by the magnitude of the assumed percolation threshold.
Clumped isotope thermometry can independently constrain the formation temperatures of carbonates, but a lack of precisely temperature-controlled calibration samples limits its application on aragonites. To address this issue, we present clumped isotope compositions of aragonitic bivalve shells grown under highly controlled temperatures (1‒18°C), which we combine with clumped isotope data from natural and synthetic aragonites from a wide range of temperatures (1‒850°C). We observe no discernible offset in clumped isotope values between aragonitic foraminifera, mollusks, and abiogenic aragonites or between aragonites and calcites, eliminating the need for a mineral-specific calibration or acid fractionation factor. However, due to non-linear behavior of the clumped isotope thermometer, including high-temperature (>100°C) datapoints in linear clumped isotope calibrations causes them to underestimate temperatures of cold (1‒18°C) carbonates by 2.7 ± 2.0°C (95% confidence level). Therefore, clumped isotope-based paleoclimate reconstructions should be calibrated using samples with well constrained formation temperatures close to those of the samples.
Gas hydrates stored in the continental margins of the world’s oceans represent the largest global reservoirs of methane. Determining the source and history of methane from gas hydrate deposits informs the viability of sites as energy resources, and potential hazards from hydrate dissociation or intense methane degassing from ocean warming. Stable isotope ratios of methane (13C/12C, D/H) and the molecular ratio of methane over ethane plus propane (C1/C2+3) have traditionally been applied to infer methane sources, but often yield ambiguous results when two or more sources are mixed, or when compositions were altered by physical (e.g., diffusion) or microbial (e.g., methanotrophy) processes. We measured the abundance of clumped methane isotopologue (13CH3D) alongside 13C/12C and D/H of methane, and C1/C2+3 for 46 submarine gas hydrate specimens and associated vent gases from 11 regions of the world’s oceans. These samples are associated with different seafloor seepage features (oil seeps, pockmarks, mud volcanoes, and other cold seeps). The average apparent equilibration temperatures of methane from the Δ13CH3D (the excess abundance of 13CH3D relative to the stochastic distribution) geothermometer increase from cold seeps (15 to 65 ℃) and pockmarks (36 to 54 ℃), to oil-associated gas hydrates (48 to 120 ℃). These apparent temperatures are consistent with, or a few tens of degrees higher than, the temperature expected for putative microbial methane sources. Apparent methane generation depths were derived for cold seep, pockmark, and oil seep methane from isotopologue-based temperatures and the local geothermal gradients. Estimated methane generation depths ranged from 0.2 to 5.3 kmbsf, and are largely consistent with source rock information, and other chemical geothermometers based on clay mineralogy and fluid chemistry (e.g., Cl, B, and Li). Methane associated with mud volcanoes yielded a wide range of apparent temperatures (15 to 313℃). Gas hydrates from mud volcanoes the Kumano Basin and Mediterranean Sea yielded δ13C-CH4 values from -36.9 to -51.0‰, typical for thermogenic sources. Δ13CH3D values (3.8 to 6.0‰) from these sites, however, are consistent with prevailing microbial sources. These mud volcanoes are located at active convergent plate margins, where hydrogen may be supplied from basement rocks, and fuel methanogenesis to the point of substrate depletion. In contrast, gas hydrate from mud volcanoes located on km-thick sediments in tectonically less active or passive settings (Black Sea, North Atlantic) yielded microbial-like δ13C-CH4 and C1/C2+3 values, and low Δ13CH3D values (1.6 to 3.3‰), which may be due to kinetic isotope effects. This study is the first to document the link between methane isotopologue-based temperature estimates and key submarine gas hydrate seepage features, and validate previous models about their geologic driving forces.
Previous studies have documented a weathering-limited regime in the upper reaches of the Ganges River Basin. Chemical weathering and element mobility at six sites in the lower reaches of the Ganges in the tidal floodplain of Southwest Bangladesh were investigated by comparing compositions of rice paddy soils, precursor tidal channel sediments, surface waters, and extract solutions, which represent the soluble fraction of solids. Little spatial variation in water and solid compositions is observed in each season, indicating similar processes are acting to transport elements across this region. Roughly one to several decades after deposition, rice paddy soils are not significantly different in mineralogy or composition from precursor tidal channel sediments, and both are similar to the composition of average upper continental crust. There is no detectable change in composition of tidal channel water between upstream and downstream sites. Rice paddy and tidal channel waters are saturated in the dominant minerals present in the silt-sized soils and sediments, including quartz and clay minerals. Together, these observations indicate the dominance of weathered material and weak chemical weathering in the tidal floodplain, consistent with a transport-limited regime. Multiple lines of evidence indicate a lack of exchange equilibrium between surface waters and coexisting solids, which may be a common feature in tidal river deltas where transport-limited regimes likely dominate.
Martian dust, which likely formed by non-aqueous chemical weathering [Huguenin, 1976] following broad-based support from recent Mars mission data, is susceptible to rapid diagenesis when exposed to macro-seepage from the sub-permafrost aqueous aquifer system on Mars . The modeled silicate components of the dust, derived from the non-aqueous weathering of primarily olivine and pyroxene, are Mg2HSiO4(OH) and Mg(HSiO3)(OH). These are M-S-H compounds, counterparts to the C-H-S compounds that form the commercial binder in concrete, forming an Mg3Si2O5(OH)4 counterpart binder on Mars upon exposure to liquid H2O macro-seepage from the aquifer below. Macro-seepage, triggered largely by geothermally heated water near impact sites, magmatic intrusions and volcanoes, is proposed to rapidly cement layers of regolith dust and fines into layers of M-S-H counterpart “concrete.” The matrix binder on Mars is predicted to be a member of the serpentine family (Mg/Si = 5), possibly having disordered Antigorite T structure. Layered sedimentary rock formations could have formed throughout geologic history up to the present time. Materials from the aquifer, transported by and introduced from the macro-seepage, including organic matter, may be contemporary rather than ancient. This contradicts the prevailing assumption that the sedimentary rocks were formed early in the planet’s history.
A key question pertaining to Europa’s habitability is whether hydrothermal activity could be sustained for long periods of time, enabling redox and nutrient exchange between the ocean and rocky interior [e.g. 1, 2]. Europa’s early ocean, if formed during differentiation, could have been infused with gases . A consequence of this initial infusion is that clathrate hydrates may have been stable within the ocean. These clathrates could then rise to the bottom of the ice shell, or blanket the seafloor, depending on their density relative to the ocean. Accumulations of floating and sinking clathrates would affect the geological and thermal evolution of Europa because of their high heat capacity and low thermal conductivity compared to ice Ih, but sinking clathrates could also inhibit chemical exchange between the ocean and the rocky interior. We calculate the stability and density of CH4 and CO2 clathrates, and predict the volumes precipitated at the seafloor or accumulated at the base of the ice shell, for ocean compositions evolved from the interior of Europa during metamorphism on the path towards formation of a metallic core . For a chemically reduced ocean derived from heating a mix of chondritic material near Jupiter , plus cometary volatiles, ~2 x 10^7 km^3 of methane clathrates form. These are less dense than the ocean (Fig. 1), and float to the base of the ice shell. However, for a CO2-rich ocean derived from CI or CM chondrites, ~3 x 10^8 – 2 x 10^9 km^3 of CO2 clathrates could form, i.e., sufficient feedstock to form a 13–77 km global layer on the seafloor. A salty ocean (e.g. 10 % MgSO4) or a warm seafloor (316 K) may be needed to prevent the accumulation of a CO2 clathrate blanket (Fig. 1), although the blanketing effect would thin the equilibrium thickness of the clathrate layer to ~500 m for allowable heat fluxes (~50 mW/m^2).  Vance, S. et al. (2007). Astrobiology, 7(6), 987–1005. https://doi.org/10.1089/ast.2007.0075  Klimczak, C. et al. (2019). 50th Lunar. Planet Sci. Conf., Abstract #2132, p. 2912. https://ui.adsabs.harvard.edu/abs/2019LPI….50.2912K  Melwani Daswani, M. et al. (2021). A metamorphic origin for Europa’s ocean (preprint). https://doi.org/10.1002/essoar.10507048.1  Desch, S. J. et al. (2018). Astrophys. J., Suppl. Ser., 238(1), 11. http://dx.doi.org/10.3847/1538-4365/aad95f
Pedogenic carbonates document a wealth of environmental information, but their seasonal variations may obscure long-term trends. Here we report evidence of changing seasonality of pedogenic carbonate growth from the Chinese Loess Plateau during the Quaternary glacial cycles, using the d18O of pedogenic carbonates (d18Oc). The glacial and interglacial d18Oc show negative and positive correlations with proxy-inferred rainfall amount, respectively. We explain this pattern using modern observations and modeling results, which show opposite correlations between d18Oc and rainfall amount in growing versus non-growing seasons. In glacial episodes under weak monsoon, pedogenic carbonate growth occurred within the growing season, inheriting a negative d18Oc-rainfall correlation. Conversely, pedogenic carbonate growth likely extended into the non-growing season during interglacials due to intensified monsoonal rainfall, incorporating a positive d18Oc-rainfall correlation. Our work links seasonal fluctuations of pedogenic carbonates with their long-term records, shedding new light on interpreting this paleoarchive.
GeoHealth as a research paradigm offers the opportunity to re-evaluate common research engagement models and science training practices. GeoHealth challenges are often wicked problems that require both transdisciplinary approaches and the establishment of intimate and long term partnerships with a range of community members. We examine four common modes of community engagement and explore how research projects are launched, who has the power in these relationships, and how projects evolve to become truly transformative for everyone involved.
Cenozoic growth of the Andes has been strongly influenced by subduction dynamics, reactivation of inherited crustal heterogeneities, and the superposed effects of climate. Subduction of the submarine Carnegie Ridge has fundamentally impacted late Cenozoic magmatism and tectonic activity in the northern Andes. Time-temperature inverse modeling of new thermochronological data from the Western Cordillera of Ecuador reveals two phases of cooling separated by isothermal conditions. The first cooling phase immediately postdates early and middle Miocene magmatism in the Western Cordillera and is attributed to post-magmatic thermal relaxation. The second cooling phase started after 6 Ma, which we infer to record the exhumation in the Western Cordillera, coeval with the last cooling phase in the Eastern Cordillera. Based on these findings we posit that the onset of subduction of the Carnegie Ridge at ~6-5 Ma increased plate coupling at the subduction interface and promoted shortening and regional rock uplift in the northern Andes. Overall, our results highlight the essential role of bathymetric anomalies in driving regional upper-plate deformation at non-collisional convergent plate margins.
Triple-oxygen isotope (δ18O and Δ17O) analysis of sulfate is becoming a common tool to assess several biotic and abiotic sulfur-cycle processes, both today and in the geologic past. Multi-step sulfur redox reactions often involve intermediate sulfoxyanions such as sulfite, sulfoxylate, and thiosulfate, which can rapidly exchange oxygen atoms with surrounding water. Process-based reconstructions therefore require knowledge of equilibrium oxygen-isotope fractionation factors (18α and 17α) between water and each individual sulfoxyanion. Despite this importance, there currently exist only limited experimental 18α data and no 17α estimates due to the difficulty of isolating and analyzing short-lived intermediate species. To address this, we theoretically estimate 18α and 17α for a suite of sulfoxyanions—including several sulfate, sulfite, sulfoxylate, and thiosulfate isomers—using quantum computational chemistry. We determine fractionation factors for sulfoxyanion “water droplets”; using the B3LYP/6-31G+(d,p) method; we additionally determine higher-order method (CCSD/aug-cc-pVTZ and MP2/aug-cc-pVTZ) and anharmonic zero-point energy (ZPE) scaling factors using a suite of gaseous sulfoxy compounds and test their impact on resulting sulfoxyanion fractionation-factor estimates. When including redox state-specific CCSD/aug-cc-pVTZ and anharmonic ZPE scaling factors, our theoretical 18α predictions for protonated isomers closely agree with all existing experimental data, yielding root-mean-square errors of 1.8 ‰ for SO3(OH)-/H2O equilibrium (n = 18 experimental conditions), 2.2 ‰ for SO2(OH)-/H2O (n = 27), and 3.9 ‰ for S2O2(OH)-/H2O (n = 3). This result supports the idea that oxygen exchange occurs via isomers containing oxygen-bound protons. By combining 18α and 17α predictions, we additionally estimate that SO3(OH)-, SO2(OH)-, SO(OH)-, and S2O2(OH) exhibit Δ17O values as much as 0.167 ‰, 0.097 ‰, 0.049 ‰, and 0.153 ‰ more negative than equilibrated water at Earth-surface temperatures (reference line slope = 0.5305). This theoretical framework provides a foundation to interpret experimental and observational triple-oxygen isotope results of several sulfur-cycle processes including pyrite oxidation, microbial metabolisms (e.g., sulfate reduction, thiosulfate disproportionation), and hydrothermal anhydrite precipitation. We highlight this with several examples.
The transport of methane from deep sediments towards the seafloor is widespread in ocean margins and has important biogeochemical implications for the deep ocean . A significant portion (>80%) of methane entering the shallow sediments from below at present is oxidized by microbially-driven anaerobic oxidation of methane (AOM), which mainly involves a microbial consortium of anaerobic methanotrophic archaea (ANME) and sulfate-reducing bacteria. Isoprenoid Glycerol dialkyl glycerol tetraethers (GDGTs) derived from core lipid membranes of ANMEs are often well preserved in sediment records. Methane Index (MI) is an organic geochemical proxy for methane seepage intensity which weighs in the relative proportion of GDGTs (GDGT-1,-2, and -3) preferentially synthesized by ANMEs with that of non-methane-related biomarker contribution from planktonic and benthic sources (Crenarchaeols) . This study analyzed the GDGT composition of sedimentary core lipids from IODP Site 1230 (Peru Margin) using two silica columns and a high-resolution and accurate mass Orbitrap Fusion Mass Spectrometer. Our results report novel GDGT isomers with concentration peaking at the Sulfate-Methane Transition Zones (SMTZ) with the highest AOM activity around 8 mbsf. Further, these isomers were almost absent above and below the SMTZ. Our observations suggest that these characteristic isomers of GDGT compounds preserved at the SMTZ depth are sourced from ANMEs. Identification of these novel isomers has important implications in refining the MI and additional GDGT based palaeoceanographic proxies like TEX86. 1. Akam et al. (2020), Frontiers in Marine Science 7, 206. 2. Y. G. Zhang et al. (2011), Earth and Planetary Science Letters 307, 525-534.
Studies of element partitioning between suspended sediment and water with increased seawater mixing are sparse, particularly in Bangladesh. However, these studies are important for understanding elemental cycling, pollutant transport, and impacts on aquaculture and sensitive ecosystems in estuaries and tidal deltas such as the Sundarbans mangrove forest in Southwest Bangladesh. Thus, water samples collected within the upper 1m of the water column along a transect of well-mixed tidal channels in Southwest Bangladesh during the dry season were analyzed for dissolved and suspended sediment element concentrations and other geochemical parameters. While most elements in the suspended load were close to or depleted relative to upper continental crust (UCC), several trace elements such as Sb, As, Cd and Se were slightly enriched. Additionally, most trace elements in the dissolved load were well above world average riverine concentrations, particularly Se and As. Dissolved load Ba and Se displayed mostly conservative mixing trends with seawater. Barium was likely originally sourced from sediment desorption and groundwater exfiltration, while Se may have been anthropogenically sourced from the city of Khulna or farther upstream. Dissolved As did not display conservative mixing trends, and may ultimately be geogenic in origin, possibly from groundwater. Ni and Co show trends consistent with desorption from competitive seawater cation exchange along the transect, similar to a study in the nearby Hooghly Estuary in West Bengal. Collectively, our results show that combined anthropogenic and natural influences on trace element distributions in coastal environments are important to quantify for continual protection of natural areas and better understanding of trace element discharge to global oceans.