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Hydrothermal sediments are a source of water column Fe and Mn in the Bransfield Strait, Antarctica
Full text linkShort sediment cores were collected from ~1100 m water depth at the top of Hook Ridge, a submarine volcanic edifice in the Central Basin of the Bransfield Strait, Antarctica, to assess Fe and Mn supply to the water column. Low-temperature hydrothermal fluids advect through these sediments and, in places, subsurface H2S is present at high enough concentrations to support abundant Sclerolinum sp., an infaunal tubeworm that hosts symbiotic thiotrophic bacteria. The water column is fully oxic, and oxygen penetration depths at all sites are 2-5 cmbsf. Pore water Fe and Mn content is high within the subsurface ferruginous zone (max. 565 µmol Fe L-1, >3 to 7 cmbsf) — 14-18 times higher than values measured at a nearby, background site of equivalent water depth. Diffision and advection of pore waters supply significant Fe and Mn to the surface sediment. Sequential extraction of the sediment demonstrates that there is a significant enrichment in a suite of reactive, authigenic Fe minerals in the upper 0-5 cm of sediment at one site characterised by weathered crusts at the seafloor. At a site with only minor authigenic mineral surface enrichment we infer that leakage of pore water Fe and Mn from the sediment leads to enriched total dissolvable Fe and Mn in bottom waters. The largest Eh anomaly observed from our Bransfield Strait survey is associated with the elevated total dissolvable metal content in the water column above this coring site.. We hypothesize that the main mechanism for Fe and Mn efflux from the sediment is breach of the surface oxic layer by the abundant Sclerolinum sp., along with episodic enhancements by physical mixing and resuspension of sediment in this dynamic volcanic environment. We propose that Hook Ridge sediments are an important source of Fe and Mn to the deep waters of the Central Basin in the Bransfield Strait, where concentrations are sustained by the benthic flux, and Fe is stabilised in the water column as either colloidal phases or ligand-bound dissolved species. Entrainment of this water mass into the Drake Passage and thereby the Antarctic Circumpolar Current could provide a significant metal source to this HNLC region of the Southern Ocean if mixing and upwelling occurs before removal of this metal pool to underlying sediments. Sediment-covered volcanic ridges are common within rifted margins and may play a previously overlooked role in the global Fe cycle
A multiproxy geochemical record of the early Aptian Selli event (OAE1a) from the platform carbonates of southern Italy
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Exploring the Texture of Ocean-Atmosphere Redox Evolution on the Early Earth
Full text linkThe evolution of oxygenic photosynthesis has dramatically reshaped the chemistry of the surface Earth, and the presence of significant quantities of O2 in the atmosphere and ocean now drives the fundamental dynamics of nearly all quantitatively significant biogeochemocal cycles (C, S, P, N, Fe). Whether by direct consumption through the metabolic demands of large, complex organisms, or through the recycling of essential substrates within microbial ecosystems, biologically produced O2 provides nearly all of the compounds used in metabolic electron transfer on a global scale. Although it is widely accepted that the partial pressure of O2 in Earth's atmosphere has increased through time (with attendant, although somewhat complex, changes in ocean ventilation), there is still much debate surrounding the timing of the emergence of oxygenic photosynthesis and little is known about the detailed tempo and mode with which this metabolic innovation came to shape early Earth surface chemistry. This dissertation explores the early oxygenation of Earth's atmosphere and the relationship between atmospheric oxygen levels and ocean ventilation from a variety of perspectives. First, empirical data based on an integrated suite of paleoredox proxies is used to suggest that biological oxygen production emerged and began exerting significant effects on Earth surface chemistry at least 100 million years prior to the initial accumulation of large quantities of O2 in the atmosphere (often referred to as the "Great Oxidation Event", approximately 2.4 billion years ago). The implications of this time lag between metabolic innovation and large-scale biogeochemical reorganization are explored through a series of quantitative models, focusing on the thermodynamics and kinetics of mineral reactions under various Earth surface conditions, regional oceanographic modeling of surface ocean O2 cycling, and a global sulfur isotope mass balance model that explicitly incorporates rare sulfur isotope systematics (33S, (36S) and the dynamics of sedimentary recycling on long timescales. Finally, the dynamics of ocean ventilation following the initial accumulation of oxygen in the atmosphere are explored by combining a large trace metal database with a spatially explicit mass balance model that exploits the differing redox behavior and surface cycling of molybdenum (Mo) and chromium (Cr)
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Illuminating Large Stable Isotope Variations in Alkaline Lakes
Full text linkStable isotopes ratios have yielded major geochemical insights since the mid-twentieth century, and yet there remains new ground to be broken when analyzing traditional isotopic systems in extreme environments. This dissertation relies on such analyses—including δ13C, δ18O, δ15N, and δ34S—at ancient and modern alkaline lakes. The specificity of these environments belies their broader significance, including for the earliest (> 3.5 Ga) Earth and Mars. For instance, impact craters, which were a vital feature of those systems, tend to produce alkaline lakes as a result of the weathering of mafic impact ejecta. Alkaline lakes are also among the most bioproductive natural aquatic environments, with chemistries favorable for accumulating bioessential compounds thought necessary for a de novo origin of life. The novel datasets within this dissertation investigate the capacity for alkaline lakes to exhibit distinct isotopic records. The exceptional nature of such signatures, particularly when combined with various contextual data, may allow them to act as effective proxies for past conditions. Notable examples include δ34S evidence of Rayleigh distillation during sulfate reduction within an impact-induced hydrothermal paleolake (Chapter 1), as well as the connection between pH and elevated δ15N via ammonia volatilization (Chapter 2); both of these topics are investigated at the Miocene Ries crater lake of southern Germany. To further elucidate δ15N increases via ammonia volatilization, multiple extant lakes of the Coorong lagoon are examined (Chapter 3). The results suggest hypersalinity is an important control on heightened δ15N dynamics and preservation in shallow alkaline systems. The dissertation work is then placed within a broader context (Chapter 4) by reviewing additional sites of ammonia volatilization; this includes a new stable isotope data set from the Eocene Green River Formation. A schematic for elevated δ15N in redox-stratified basins is proposed, based on the balance of ammonia volatilization and denitrification in response to spatial and temporal shifts in pH, salinity, and chemocline depth. In sum, alkaline lakes can exhibit remarkably distinct δ15N and δ34S signatures, which can help establish constraints on aqueous conditions at sites of high geologic and astrobiological value
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Animals, Oxygen, and the Mid-Proterozoic Earth System
Full text linkSince life took its earliest footholds, organisms have been in a perpetual arms race. Evolutionary trial and error has produced increasingly complex organisms. All with a single purpose, to utilize available resources as efficiently as possible and reproduce. This progression has led to things like photosynthesis and respiration, phagocytosis, multicellularity, differentiated cell types, neural networks, and apparently consciousness. This dissertation presents one account of the history of the push-and-pull relationship between life and the environment as both evolved together, centered around two case studies that contributed to the knowledgebase on which the larger argument is founded.Because the net outcome of biological activity is the concentration of reduced material, the net outcome is also environmental oxidation, so long as some of that material still exists. Oxygenic photosynthesis was not necessarily the path of least resistance to ecological success, but once the machinery was assembled, the substrates were nearly limitless in many environments. This intuitively leads to the utilization of O2 in respiration—is by far the best candidate for the job, assuming it is ambiently present. That statement suggests implicitly that evolution will continually exploit its environment to gain a competitive advantage.
This effectively frames one side of a long-standing debate concerning the role that environmental O2 concentrations played the evolution of the eukaryotic clade. The chapters herein elaborate on this argument. It begins with a case study of the 1.4-billion-year-old Xiamaling Formation, presenting iron speciation, sulfur isotope, trace metal, and molybdenum isotope data; concluding that while the Xiamaling Formation was likely deposited in a restricted setting, it still holds geochemical indications that global oceans could have been more well oxygenated at the time than many previously thought. The middle chapters explore these ideas further. The dissertation closes with another case study, Meiklejohn Peak. This study presents paired carbon isotope, sulfur isotope, and iodine concentration data, along with detailed sedimentology. The formation explored in this chapter was deposited during one of the largest pulses of diversification in the history of animal life, and multiple independent geochemical proxies suggest rising environmental O2 concentrations accompanied this event
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Biogeochemical Signatures in Precambrian Black Shales: Window Into the Co-Evolution of Ocean Chemistry and Life on Earth
Full text linkThe degradation of sedimentary organic matter drives a suite of biologically- mediated redox reactions that in turn reflect the chemical composition of pore waters and bottom waters on local to global scales. By analyzing the chemical and isotopic composition of modern sediments and ancient black shales, biogeochemists can track the evolution of ocean/atmosphere redox conditions, the chemical composition of the oceans, and the evolutionary course of life throughout Earth history. Chapter 1 introduces the concepts of suboxic versus euxinic depositional environments and details the cycling of the transition metal Mo in each of these environments. A method for using Mo enrichments in ancient suboxic black shales as an environmental paleoproxy is proposed. Chapter 2 is a temporal survey of Mo enrichments in euxinic sediments and ancient black shales, throughout Earth's history. The purpose of this study is to test the hypothesis that widespread euxinic conditions during the Proterozoic would lead to Mo drawdown on a global scale, affecting the global nitrogen cycle. Chapter 3 is a biogeochemical description of the Mesoproterozoic Newland Formation, a non-euxinic black shale. Chapter 4 is a biogeochemical description of the Archean Jeerinah Formation, Earth's earliest known euxinic basin
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Biogeochemical Evolution of the Rapidly Shrinking Salton Sea: California's Largest Lake in Crisis
No full textThe Salton Sea, California's largest lake (~800 km²), represents an extreme case study of anthropogenic transformation as lake level reduces at 0.3 m/year following the 2003 Quantification Settlement Agreement—the largest agricultural-to-urban water transfer in U.S. history. Designated as an "agricultural sump" since 1920s, the lake receives continuous drainage from agricultural run-off, creating hypereutrophic conditions with salinity twice that of the Pacific Ocean. This dissertation presents a comprehensive multi-decadal analysis (2000-2025) documenting a fundamental regime shift through integrated examination of nutrient dynamics, temperature stratification, oxygen cycling, and sulfur isotope geochemistry. Chapter 1 establishes nutrient cycling patterns showing persistent hypereutrophic phosphorus levels (>0.05 mg/L) despite N:P ratios exceeding 50:1, challenging phosphorus limitation assumptions. External nutrient loading from agricultural drainage remains consistent year-round, while internal cycling is substantial during summer reduced conditions. Chapter 2 documents significant shift in mixing patterns coinciding with 4-meter depth loss over 20 years through time-monitored and satellite data. The lake shifted from maintaining adequate seasonal stratification with brief summer anoxic events (2005-2007) to chronic dissolved oxygen depletion below regulatory standards of 5 mg/L from April through November (2020-2022). This represents a three-stage degradation sequence for shallowing eutrophic lakes: stratified oxic-anoxic layers, expanded year-round low-oxygen zones, and eventual surface-only oxygenation. Chapter 3 provides the first comprehensive isotopic characterization of sulfur cycling in this extreme sulfate-rich environment (~180 mM). Despite seasonal redox transitions from oxic to episodically euxinic conditions, δ³⁴S and δ¹⁸O of sulfate remain constant throughout the water column (4.3 ± 0.3‰ and 14.4 ± 0.2‰), with minimal fractionations between sulfate and sulfide (Δ³⁴SSO₄-H₂S = 0.6‰ and -0.3‰). This isotopic homogeneity contrasts sharply with typical stratified systems and reveals that rapid sulfur cycling can suppress traditional isotopic indicators despite active microbial processes. However, sediment porewaters show the more typical large isotopic offsets (Δ³⁴SSO₄-H₂S = 41.2–59.7‰), indicating active microbial sulfate reduction. Marginal evaporite crusts preserve the isotopic evolution from Colorado River dominance to agricultural sulfate loading, providing historical archives of anthropogenic impact. Three-component mixing models demonstrate that agricultural fertilizers (~31%), Colorado River water (~28%), and ancient anhydrite dissolution (~41%) dominate the sulfate budget, challenging existing hydrologic models and revealing bidirectional lake-groundwater interactions. 
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A Geochemical Study of the 1.4 Ga Roper Group, Northern Australia: A Window to Environmental Conditions and Life During the Mid-Proterozoic
Full text linkBy about 2.0 billion years ago (Ga), there is evidence for a period best known for its extended, apparent geochemical stability expressed famously in the carbonate-carbon isotope data. Despite the first appearance and early innovation among eukaryotic organisms, this period is also known for a rarity of eukaryotic fossils and organic biomarker fingerprints, suggesting low diversity and relatively small populations compared to the interval that followed. Nevertheless, the search for diagnostic biomarkers has not been performed within an independent paleoenvironmental context that should reveal the facies that were most likely hospitable to these oxygen-requiring organisms. Shales and mudstones obtained from drill core of the ca. 1.45 Ga Roper Group from the McArthur Basin of northern Australia provide one of our best windows into the mid-Proterozoic redox landscape. The group is well dated and minimally metamorphosed, and previous geochemical data show a relatively strong connection to the open ocean compared to other mid-Proterozoic records. Consequently, conditions captured in the Roper Group may better reflect the redox state of the ocean margin and may reveal eukaryote-favoring setting. Despite this potential, a comprehensive, multi-proxy study of the Roper Group had not been undertaken. Here we present one of the first integrated investigations of Precambrian biomarkers performed within a strict inorganic proxy context. Results show a textured paleoredox structure for the Velkerri Formation, vacillating between oxic and anoxic, even euxinic conditions in the water column. Despite this variability, including the likely presence of oxic bottom waters, we see no evidence for the sterane compounds that would point convincingly to the presence of eukaryotes in this marine basin roughly 1.45 Ga. Also missing are the aryl isoprenoids that would delineate shallow photic zone euxinia and thus the likelihood of appreciable anoxygenic primary production, even though the iron and trace metal data are consistent with the presence of euxinia deeper in the water column. The absence of eukaryotic signals--despite our search across oxic and anoxic facies that should favor their habitability and the preservation of their records, respectively--suggests that other controls such as long-term nutrient and oxygen deficiencies may have throttled the distributions and diversity of early complex life
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Exploring the Geochemical Fingerprints of an Oceanic Anoxic Event During the Late Cretaceous: the Global and Biological Implications
Full text linkUnderstanding the causes and consequences of oceanic anoxic events (OAEs) has been at the forefront of paleoceanography studies for the last several decades. The Mesozoic Era is noted for numerous OAEs that are diagnostically expressed by widespread organic-carbon deposition and coeval positive carbon-isotope excursions. OAEs have been extensively studied from angles but there is still minimal understanding of the global nature of these events. Through the work presented here I aim to quantify the global extent of euxinia (water column that is anoxic and contains sulfide) and seek to understand global extent of anoxia using a multi-geochemical-proxy approach. This work includes high-resolution studies spanning multiple sections with a global distribution for each proxy (sulfur isotopes, Mo, V and Cr trace metal, Fe isotopes) from OAE2, the Cenomanian-Turonian boundary event (~93.9 Ma). Coupled carbon and sulfur isotopes show positive isotope excursions at each locality during OAE2; although, the peak magnitudes of these shifts are offset by approximately a few hundred thousand years due to a waning burial of organic carbon and pyrite burial. Geochemical box modeling suggests 2 to 7% of the seafloor sediments were deposited under euxinic conditions. While, Mo trace metal geochemistry suggests similar results with values of ~10% euxinia and V and Cr depletions prior to euxinia imply increased global anoxia prior to the OAE. An organic carbon compilation suggest the known burial of organic carbon during OAE2 may account for the entire isotope excursion observed unless there is an major change increase in volcanic or weathering fluxes. However, Fe isotopes suggest there was not a pervasive increased signal for hydrothermal delivery of Fe except to the know euxinic basins. Quantitative consideration of these cycles is of paramount importance for constraining the budgets of carbon and sulfur, but also oxygen and other key biological elements, as we seek to improve our understanding of the mechanisms behind the initiation and termination of OAEs
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