“Climate tipping elements” often refer to large-scale earth systems with the potential to respond nonlinearly to anthropogenic climate change by transitioning towards substantially different long-term states upon passing key thresholds, frequently referred to as “tipping points.” In some but not all cases, such changes could produce additional greenhouse gas emissions or radiative forcing that could compound global warming. Improving understanding of tipping elements is important for predicting future climate risks. Here we review mechanisms, predictions, impacts, and knowledge gaps associated with ten notable earth systems proposed to be climate tipping elements. We evaluate which tipping elements are more imminent and whether shifts will likely manifest rapidly or over longer timescales. Some tipping elements are significant to future global climate and will likely affect major ecosystems, climate patterns, and/or carbon cycling within the 21st century. However, assessments under different emissions scenarios indicate a strong potential to reduce or avoid impacts associated with many tipping elements through climate change mitigation. Most tipping elements do not possess the potential for abrupt future change within years, and some proposed tipping elements may not exhibit tipping behavior, rather responding more predictably and directly to the magnitude of forcing. Nevertheless, significant uncertainties remain associated with many tipping elements, highlighting an acute need for further research and modeling to better constrain risks.
Antarctic landfast sea ice (fast ice) is stationary sea ice that is attached to the coast, grounded icebergs, ice shelves, or other protrusions on the continental shelf. Fast ice forms in narrow (generally up to 200 km wide) bands, and ranges in thickness from centimeters to tens of meters. In most regions, it forms in autumn, persists through the winter and melts in spring/summer, but can remain throughout the summer in particular locations. Despite its relatively limited horizontal extent (comprising between about 4 and 13 \% of overall sea ice), its presence, variability and seasonality are drivers of a wide range of physical, biological and biogeochemical processes, with both local and far-ranging ramifications for various Earth systems. Antarctic fast ice has, until quite recently, been overlooked in studies, likely due to insufficient knowledge of its distribution, leading to its reputation as a “missing piece of the Antarctic puzzle”. This review presents a synthesis of current knowledge of the physical, biogeochemical and biological aspects of fast ice, based on the sub-domains of: fast ice growth, properties and seasonality; remote-sensing and distribution; interactions with the atmosphere and the ocean; biogeochemical interactions; its role in primary production; and fast ice as a habitat for grazers. Finally, we consider the potential state of Antarctic fast ice at the end of the 21st Century, underpinned by Coupled Model Intercomparison Project model projections. This review also gives recommendations for targeted future work to increase our understanding of this critically-important element of the global cryosphere.
Between 81º30’ E and 83ºE the Himalayan range’s “perfect” arcuate shape is interrupted by an embayment. We hypothesize that thrust geometry and duplexing along the megathrust at mid-lower crustal depths plays a leading role in growth of the embayment as well the southern margin of the Tibetan plateau. To test this hypothesis, we conducted thermokinematic modeling of published thermochronologic data from the topographic and structural embayment in the western Nepal Himalaya to investigate the three-dimensional geometry and kinematics of the megathrust at mid-lower crustal depths. Models that can best reproduce observed cooling ages suggest that the megathrust in the western Nepal Himalaya is best described as two ramps connected by a long flat that extends further north than in segments to the east and west. These models suggest that the high-slope zone along the embayment lies above the foreland limb of an antiformal crustal accretion zone on the megathrust with lateral and oblique ramps at mid-lower crustal depths. The lateral and oblique ramps may have initiated by ca. 10 Ma. This process may have controlled along-strike variation in Himalayan-plateau growth and therefore development of the topographic embayment. Finally, we analyze geological and morphologic features and propose an evolution model in which landscape and drainage systems across the central-western Himalaya evolve in response to crustal accretion at depth and the three-dimensional geometry of the megathrust. Our work highlights the importance of crustal accretion at different depths in orogenic-wedge growth and that the mid-lower crustal accretion determines the location of plateau edge.
Although adequately detailed kerosene chemical-combustion Arrhenius reaction-rate suites were not readily available for combustion modeling until ca. the 1990’s (e.g., Marinov ), it was already known from mass-spectrometer measurements during the early Apollo era that fuel-rich liquid oxygen + kerosene (RP-1) gas generators yield large quantities (e.g., several percent of total fuel flows) of complex hydrocarbons such as benzene, butadiene, toluene, anthracene, fluoranthene, etc. (Thompson ), which are formed concomitantly with soot (Pugmire ). By the 1960’s, virtually every fuel-oxidizer combination for liquid-fueled rocket engines had been tested, and the impact of gas phase combustion-efficiency governing the rocket-nozzle efficiency factor had been empirically well-determined (Clark ). Up until relatively recently, spacelaunch and orbital-transfer engines were increasingly designed for high efficiency, to maximize orbital parameters while minimizing fuels and structural masses: Preburners and high-energy atomization have been used to pre-gasify fuels to increase (gas-phase) combustion efficiency, decreasing the yield of complex/aromatic hydrocarbons (which limit rocket-nozzle efficiency and overall engine efficiency) in hydrocarbon-fueled engine exhausts, thereby maximizing system launch and orbital-maneuver capability (Clark; Sutton; Sutton/Yang). The rocket combustion community has been aware that the choice of Arrhenius reaction-rate suite is critical to computer engine-model outputs. Specific combustion suites are required to estimate the yield of high-molecular-weight/reactive/toxic hydrocarbons in the rocket engine combustion chamber, nonetheless such GIGO errors can be seen in recent documents. Low-efficiency launch vehicles (SpaceX, Hanwha) therefore also need larger fuels loads to achieve the same launched/transferred mass, further increasing the yield of complex hydrocarbons and radicals deposited by low-efficiency rocket engines along launch trajectories and into the stratospheric ozone layer, the mesosphere, and above. With increasing launch rates from low-efficiency systems, these persistent (Ross/Sheaffer ; Sheaffer ), reactive chemical species must have a growing impact on critical, poorly-understood upper-atmosphere chemistry systems.
Mass recycling from subduction to magmatic extrusion shapes our habitable environment and Earth’s interior. Subducted igneous crust may form pyroxenites before participating magmatism, but the deep journey of associated carbonates remains unclear. Here we report new Mg-isotope data for ~89 to 81 Ma basaltic rocks in Langshan area, central Asia (δ26Mg = -0.391 to -0.513 ‰) with a synthesis for post-110 Ma basalts across eastern Asian continent. The merged low-δ26Mg basaltic province normally interpreted as derivations from carbonated sources paradoxically displays geochemical signatures (low Ca/Al and high K2O contents) resembling partial melts of uncarbonated sources. Negative correlations of δ26Mg vs TiO2 and FCKANTMS, the proxy of pyroxenitic melts, and adiabatic melting modeling suggest presence of Mg-isotopically light source pyroxenites transformed from decarbonated altered oceanic crust. This may explain ubiquitous pyroxenitic contributions in many low-δ26Mg basaltic suites and has significant implication for deep carbon cycling.
While Hg in sediments is increasingly used as a proxy for deep-time volcanic activity, the behaviour of Hg in OM-rich sediments as they undergo thermal maturation is not well understood. In this study, we evaluate the effects of thermal maturation on sedimentary Hg contents and, thereby, the impact of thermal maturity on the use of the Hg/TOC proxy for large igneous province (LIP) volcanism. We investigate three cores (marine organic matter) with different levels of thermal maturity in lowermost Toarcian sediments (Posidonienschiefer) from the Lower Saxony Basin in Germany. We present Hg content, bulk organic geochemistry, and total sulfur in three cores with different levels of thermal maturity. The comparison of Hg data between the three cores indicates that Hg content in the mature/overmature sediments have increased > 2-fold compared to Hg in the immature deposits. Although difficult to confirm with the present data, we speculate that redistribution within the sedimentary sequence caused by the mobility and volatility of the element under relatively high temperatures may have contributed to Hg enrichment in distinct stratigraphic levels of the mature cores. Regardless of the exact mechanism, elevated Hg content together with organic-carbon loss by thermal maturation exaggerate the value of Hg/TOC in mature sediments, suggesting that thermal effects have to be considered when using TOC-normalised Hg as a proxy for far-field volcanic activity.
Pulsing seepages of native hydrogen (H2) have been observed at the surface on several emitting structures. It is still unclear whether this H2 pulsed ﬂux is controlled by deep migration processes, atmosphere/near-surface interactions or by bacterial fermentation. Here, we investigate mechanisms that may trigger pulsating fluid migration at depth and the resulting periodicity. We set up a numerical model to simulate the migration of a deep constant fluid flow. To verify the model’s formulation to solve complex fluid flows, we first simulate the morphology and amplitude of 2D thermal anomalies induced by buoyancy-driven water ﬂow within a fault zone. Then, we simulate the H2 gas flow along a 1-km draining fault, crosscut by a lower permeable rock layer to investigate the conditions for which a pulsing system is generated from a deep control. For a constant incoming flow of H2 at depth, persistent bursts at the surface only appear in the model if: (I) a permeability with an effective-stress dependency is used, (II) a strong contrast of permeability exists between the different zones, (III) a sufficiently high value of the initial effective stress state at the base of the low permeable layer exists, and (IV) the incoming and continuous fluid flow of H2 at depth remains low enough so that the overpressure does not “open” instantly the low permeability layer. The typical periodicity expected for this type of valve-fault control of H2 pulses at the surface is at a time scale of the order of 100 to 300 days.
Although adequately detailed kerosene chemical-combustion Arrhenius reaction-rate suites were not readily available for combustion modeling until ca. the 1990’s (e.g., Marinov ), it was already known from mass-spectrometer measurements during the early Apollo era that fuel-rich liquid oxygen + kerosene (RP-1) gas generators yield large quantities (e.g., several percent of total fuel flows) of complex hydrocarbons such as benzene, butadiene, toluene, anthracene, fluoranthene, etc. (Thompson ), which are formed concomitantly with soot (Pugmire ). By the 1960’s, virtually every fuel-oxidizer combination for liquid-fueled rocket engines had been tested, and the impact of gas phase combustion-efficiency governing the rocket-nozzle efficiency factor had been empirically well-determined (Clark ). Up until relatively recently, spacelaunch and orbital-transfer engines were increasingly designed for high efficiency, to maximize orbital parameters while minimizing fuels and structural masses: Preburners and high-energy atomization have been used to pre-gasify fuels to increase (gas-phase) combustion efficiency, decreasing the yield of complex/aromatic hydrocarbons (which limit rocket-nozzle efficiency and overall engine efficiency) in hydrocarbon-fueled engine exhausts, thereby maximizing system launch and orbital-maneuver capability (Clark; Sutton; Sutton/Yang). The combustion community has been aware that the choice of Arrhenius reaction-rate suite is critical to computer engine-model outputs. Specific combustion suites are required to estimate the yield of high-molecular-weight/reactive/toxic hydrocarbons in the rocket engine combustion chamber, nonetheless such GIGO errors can be seen in recent documents. Low-efficiency launch vehicles also need larger fuels loads to achieve the same launched mass, further increasing the yield of complex hydrocarbons and radicals deposited by low-efficiency rocket engines along launch trajectories and into the stratospheric ozone layer, the mesosphere, and above. With increasing launch rates from low-efficiency systems, these persistent (Ross/Sheaffer ; Sheaffer ), reactive chemical species must have a growing impact on critical, poorly-understood upper-atmosphere chemistry systems.
Self-organizing diffusion-reaction systems naturally form complex patterns under far from equilibrium conditions. A representative example is the rhythmic concentration pattern of Fe-oxides in Zebra rocks; these patterns include reddish-brown stripes, rounded rods, and elliptical spots. Similar patterns are observed in the banded iron formations which are presumed to have formed in the early earth under global glaciation. We propose that such patterns can be used directly (e.g., by computer-vision-analysis) to infer basic quantities relevant to their formation giving information on generalized chemical gradients. Here we present a phase-field model that quantitatively captures the distinct Zebra rock patterns based on the concept of phase separation that describes the process forming Liesegang stripes. We find that diffusive coefficients (i.e., the bulk self-diffusivities and the diffusive mobility of Cahn-Hilliard dynamics) play an essential role in controlling the appearance of regular stripe patterns as well as the transition from stripes to spots. The numerical results are carefully benchmarked with the well-established empirical spacing law, width law, timing law and the Matalon-Packter law. Using this model, we invert for the important process parameters that originate from the intrinsic material properties, the self-diffusivity ratio and the diffusive mobility of Fe-oxides, with a series of Zebra rock samples. This study allows a quantitative prediction of the generalized chemical gradients in mineralized source rocks without intrusive measurements, providing a better intuition for the mineral exploration space.
Minerals are information-rich materials that offer researchers a glimpse into the evolution of planetary bodies. Thus it is important to extract, analyze, and interpret this abundance of information in order to improve our understanding of the planetary bodies in our solar system and the role our planet’s geosphere played in the origin and evolution of life. Over the past decades, data-driven efforts in mineralogy have seen a gradual increase. The development and application of data science and analytics methods to mineralogy, while extremely promising, has also been somewhat ad-hoc in nature. In order to systematize and synthesize the direction of these efforts, we introduce the concept of “Mineral Informatics”. Mineral Informatics is the next frontier for researchers working with mineral data. In this paper, we present our vision for Mineral Informatics, the X-Informatics underpinnings that led to its conception, the needs, challenges, opportunities, and future directions. The intention of this paper is not to create a new specific field or a sub-field as a separate silo, but to document the needs of researchers studying minerals in various contexts and fields of study, to demonstrate how the systemization and increased access to mineralogical data will increase cross- and interdisciplinary studies, and how data science and informatics methods are a key next step in integrative mineralogical studies.
Here, we explore the importance of export productivity versus anoxia in the formation of sedimentary layers with enhanced total organic carbon (TOC) content. We use geochemical, sedimentological and micropaleontological records from two SW Sicily outcropping successions, Lido Rosello (LR) and Punta di Maiata (PM), over three mid-Pliocene precession-forced climate cycles (4.7 – 4.6 million years ago [Ma]). Grey marls, deposited during precession minima, show enhanced TOC in both records. We suggest that basin-wide, low-oxygenated bottom-waters, resulting from freshwater-induced stratification during precession minimum, was integral to preserving grey marl TOC. Furthermore, prolonged eastern Mediterranean stratification may have produced a deep chlorophyll maximum (DCM), leading to ‘shade-flora’ dominated productivity. The LR succession displays two unique laminated layers containing enhanced TOC. These laminations do not occur at specific times in the precession cycle or in time-equivalent PM samples. They are likely to have been produced by an intermittent dysoxic/anoxic pool at LR, caused by a local depression, which enhanced TOC preservation. Consequently, the laminations provide a rare window into ‘true’ eastern Mediterranean productivity conditions during precession maxima, as organic matter is typically poorly preserved during these period due to enhanced ventilation. The laminated ‘windows’ indicate that eastern Mediterranean export productivity may not have been significantly lower during precession maxima compared to precession minima, as previously thought. During these periods, productivity conditions are likely to have been comparable to the modern eastern Mediterranean, with a spring-bloom caused by enhanced winter/spring deep-water mixing preceding a summer ‘shade-flora’ bloom caused by a summer-stratification induced DCM.
Drilling and multi-stage hydraulic fracturing bring a large amount of water into the formation, and clay-bearing shale reservoirs interact with water, which may lead to reduction of gas production, attenuation of fracturing effects, and even wellbore instability. Because of the complex fabric of shale, a thorough understanding of changes in shale micromechanics and corresponding mechanisms when exposed to water remains unclear. In this work, representative terrestrial and marine shale samples were selected for experiments based on clay enrichment. Then, contact resonance (CR) technique was performed to characterize micromechanics of shale after exposure to water. Visual phenomena provided by environmental scanning electron microscopy (ESEM) assisted to explain the underlying mechanisms. It was found that the hydration effect lowered both the storage modulus and stiffness of samples, but with different contributions from brittle minerals and clay, as well as variations depending on bedding plane orientation. Owing to the difference in composition, terrestrial shale exhibited stronger water sensitivity and anisotropy, with a general 15%-25% decrease in modulus, while marine shale changed relatively little (-5%-15%). Moreover, microscopic observation experiments revealed that complex interaction mechanisms may have existed that produced the mechanical changes. The reduction of capillary force and the interlaminar swelling of clay particles after water adsorption weakened the strength-related behavior of shale. However, the swelling-caused confining effect or void space closure during the water imbibition process might have offset this weakening effect, and even increased mechanical properties. At mesoscale, excessive shrinkage caused the growth of micro-cracks, which significantly attenuated overall mechanical behavior.
Mountain System Recharge (MRS) processes are the natural recharge pathways in arid and semi-arid mountainous regions. However, MSR processes are often poorly understood and characterized in hydrologic models. Mountains are the primary source of water supply to valley aquifers via multiple pathways including lateral groundwater flow from the mountain block (Mountain-block Recharge, MBR) and focused recharge from mountain streams contributing to mountain front recharge (MFR) at the piedmont zone. Here, we present a multi-tool isogeochemical approach to characterize mountain flow paths and MSR processes in the northern Tulare basin, California. We used groundwater chemistry data to delineate hydrochemical facies and explain the chemical evolution of groundwater from the Sierra Nevada to the Central Valley aquifer. Isotope tracers helped to validate MSR processes. Novel application of End-Member Mixing Analysis (EMMA) using conservative chemical components revealed three MSR end-members: (1) evaporated Ca-HCO3 water type associated with MFR, (2) non-evaporated Ca-HCO3 and Na-HCO3 water types with short residence times associated with shallow MBR, and (3) Na-HCO3 groundwater type with long residence time associated with deep MBR. We quantified the contribution of each MSR process to the valley aquifer using mixing ratio calculation (MIX). Our results show that deep MBR is a significant component of recharge representing more than 50% of the valley groundwater. Greater hydraulic connectivity between the Sierra Nevada and Central Valley has significant implications for parameterizing Central Valley groundwater flow models and improving groundwater management. Our framework is useful for understanding MSR processes in other snow-dominated mountain watersheds.
During the first 2934 sols of the Curiosity rover’s mission 33,468 passive visible/near-infrared reflectance spectra were taken of the surface by the mast-mounted ChemCam instrument on a range of target types. ChemCam spectra of bedrock targets from the Murray and Carolyn Shoemaker formations on Mt. Sharp were investigated using principal component analysis (PCA) and various spectral parameters including the band depth at 535 nm and the slope between 840 nm and 750 nm. Four endmember spectra were identified. Passive spectra were compared to Laser Induced Breakdown Spectroscopy (LIBS) data to search for correlations between spectral properties and elemental abundances. The correlation coefficient between FeOT reported by LIBS and BD535 from passive spectra was used to search for regions where iron may have been added to the bedrock through oxidation of ferrous-bearing fluids, but no correlations were found. Rocks in the Blunts Point-Sutton Island transition that have unique spectral properties compared to surrounding rocks, that is flat near-infrared (NIR) slopes and weak 535 nm absorptions, are associated with higher Mn and Mg in the LIBS spectra of bedrock. Additionally, calcium-sulfate cements, previously identified by Ca and S enrichments in the LIBS spectra of bedrock, were also shown to be associated with spectral trends seen in Blunts Point. A shift towards steeper near-infrared slope is seen in the Hutton interval, indicative of changing depositional conditions or increased diagenesis.
Rising temperatures and an acceleration of the hydrological cycle due to climate change are increasing river discharge, causing permafrost thaw, glacial melt, and a shift to a groundwater-dominated system in the Arctic. These changes are funnelled to coastal regions of the Arctic Ocean where the implications for the distributions of nutrients and biogeochemical constituents are unclear. In this study, we investigate the impact of terrestrial runoff on marine biogeochemistry in Inuit Nunangat (the Canadian Arctic Archipelago) --- a key pathway for transport and modification of waters from the Arctic Ocean to the North Atlantic --- using sensitivity experiments from 2002-2020 with an ocean model of manganese (Mn). The micronutrient Mn traces terrestrial runoff and the modification of geochemical constituents of runoff during transit. The heterogeneity in Arctic runoff composition creates distinct terrestrial fingerprints of influence in the ocean: continental runoff influences Mn in the southwestern Archipelago, glacial runoff dominates the northeast, and their influence co-occurs in central Parry Channel. Glacial runoff carries micronutrients southward from Nares Strait in the late summer and may help support longer phytoplankton blooms in the Pikialasorsuaq polynya. Enhanced glacial runoff may increase micronutrients delivered downstream to Baffin Bay, accounting for up to 18% of dissolved Mn fluxes seasonally and 6% annually. These findings highlight how climate induced changes to terrestrial runoff may impact the geochemical composition of the marine environment, and will help to predict the extent of these impacts from ongoing alterations of the Arctic hydrological cycle.
Organic matter is an important constituent in organic-rich shale, which influences the hydrocarbon generation, as well as the mechanical behavior, of shale reservoirs. The physical, chemical, and mechanical properties of organic matter depend on the source material and the thermal evolution process. Previous works attempted to investigate the impact of thermal maturation on the mechanical properties of organic matter. However, owing to the lack of maceral classification and the limitation of data volume during the mechanical measurement, no consistent trend has been identified. In this work, vitrinite reflectance test, scanning electron microscope observation, nanoindentation, and micro-Raman analysis were combined for geochemical and mechanical characterization. A total of 114 test areas were selected for testing, enhancing reliability of the test results. The Young’s moduli of organic matter are from 3.57 GPa to 8.32 GPa. With the same thermal maturity, inertinite has the highest Young’s modulus, while the modulus of bitumen is the lowest. The Young’s moduli of different organic types all increase with vitrinite reflectance. When vitrinite reflectance increases from 0.62% to 1.13%, the modulus of inertinite and vitrinite is increased by 57% and 78%, respectively. In addition, with the increase of thermal maturity, the micro-Raman test results show a decrease of intensity ratio of D peak to G peak, indicating an increase of the ordered structure in organic matter. Organic type and thermal maturity reflect the diversity of the source material and chemical structure change during the thermal evolution process, and together they influence the mechanical properties of organic matter.
Extraction of sulfides from the partially molten mantle is vital to elucidate the cycling of metal and sulfur elements between different geochemical circles but has not been investigated systematically. Using laboratory experiments and theoretical calculations, this study documents systematical variations in lithologies and compositions of silicate minerals and melts, which are approximately consistent with the results of the thermodynamically-constrained model. During a melt-peridotite reaction, the dissolution of olivine and precipitation of new orthopyroxene generate an orthopyroxene-rich layer between the melt source and peridotite. With increasing reaction degree, more melt is infiltrated into and reacts with upper peridotite, which potentially enhances the concomitant upward transport of dense sulfide droplets. Theoretical analyses suggest an energetically focused melt flow with a high velocity (~ 170.9 μm/h) around sulfide droplets through the pore throat. In this energic melt flow, we, for the first time, observed the mechanical coalescence of sulfide droplets, and the associated drag force was likely driving upward entrainment of fine μm-scale sulfide. For coarse sulfide droplets whose sizes are larger than the pore throat in the peridotite, their entrainment through narrow constrictions in crystal framework seems to be physically possible only when high-degree melt-peridotite reaction drives high porosity of peridotite and channelized melt flows with extremely high velocity. Hence, the melt-rock reaction could drive and enhance upward entrainment of μm- to mm-scale sulfide in the partially molten mantle, potentially contributing to the fertilization of the sub-continental lithospheric mantle and the endowment of metal-bearing sulfide for the formation of magmatic sulfide deposits.
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.