Paleoredox Proxy Research Articles

Elemental geochemistry insights into source inputs, paleo-productivity, and paleo-depositional conditions of the Montney Formation

The analysis of the elemental composition of sediments provides valuable information about input sources, paleoredox conditions, or paleoproductivity. In this study, 1498 rock samples from six wells within the Triassic Montney Formation in Central Alberta, Western Canada, were analyzed using energy-dispersive X-ray fluorescence (ED-XRF). Wavelength dispersive X-ray fluorescence measurements were used to build and validate a Montney-specific calibration to interpret elemental concentrations from the ED-XRF dataset. The calibration accounted for heterogeneities, matrix variations and unique elemental associations that may be present within the Montney Formation. Sediment input sources are interpreted based on cross-plots of proxy elements to be terrigenous, carbonates and biogenic in origins. Analysis of multiple paleoredox proxies indicated the environment of deposition to be mainly oxic. The ratios of potassium (K) to rubidium (Rb) were used as a paleoclimate proxy. Results suggest the predominance of low chemical weathering attributed to a hot and arid paleoclimate during deposition of Montney Formation sediments. Phosphorous concentrations were determined to be strongly associated with Ca, as such it was interpreted to be inorganically sourced, thereby precluding it from being used to assess paleoproductivity. Excess Si interpreted to be biogenically sourced was used in interpreting moderate and high paleoproductivity for the Montney formation. This work demonstrates the versality of matrix-specific calibrated ED-XRF measurements as a valuable, cost efficient, standalone technique in geochemical analysis for characterizing geological formations.

Sediment-redox dynamics in an oligotrophic deep-water lake in Tierra del Fuego: insights from Fe isotopes

For long time in the history of Earth, ferruginous conditions governed the oceans. With the rise of oxygen during the Proterozoic era and the subsequent evolution of living organisms, worldwide deposition of iron formations occurred. These sedimentary units reveal the transition into oxic oceans, passing by local and transitory euxinic conditions, especially in coastal shelves. Constraining the iron cycle and the biogeochemical processes occurring in present and past ferruginous basins helps answering some of the question regarding global oxygenation, the evolution of life and past climate changes. Therefore, Fe speciation and Fe isotopes in both Proterozoic and recent sedimentary records have been widely used to reconstruct past basin dynamics and redox conditions in the sediment–water interface. However, sedimentation and early diagenesis can alter paleoredox proxies and their primary climate signals. In this work, we disentangled alteration processes occurring at the redox front below the sediment–water interface of a ventilated deep-water lake (Lago Fagnano, Argentina/Chile). A sequential extraction protocol was applied to characterize two reactive Fe pools: Fe oxyhydroxides and reduced iron. Subsequently, Fe isotopes were constrained to determine the main processes mobilizing Fe. At the redox front, ferric minerals reach a δ56Fe value of − 1.3‰ resulting from oxidation of dissolved Fe likely following a Rayleigh distillation effect. Dissolved Fe is produced right below via Fe reduction, as shown by the low ferric Fe content. Our observations delineate a redox cycle and a redox horizon undergoing constant upward migration, initiated by regular sedimentation. However, during events of increased rapid sedimentation (e.g., seismites) this dynamic cycle is interrupted inducing full or partial preservation of the Fe-rich redox front. In such case, oxidation of dissolved Fe is interrupted and can be recycled in ferrous minerals, such as Fe monosulfides and amorphous phases with δ56Fe values down to − 1.7 ‰. These findings have significant implications for the recording of biogeochemical cycles in the geological past, the use of Fe isotopes in freshwater-lake sediments for paleoclimate studies, and the progress of our knowledge regarding the geochemistry of past oceans.

Marine anoxia events across the upper Cambrian (stage 10) of eastern Gondwana: Implications from paleoenvironmental geochemical proxies

The uppermost Cambrian succession (Stage 10) of the Wa'ergang section (∼300 m of rhythmites, South China) records three globally correlated negative δ13Ccarb excursions (N1, N2, and the HERB- Helenmaria – Red Tops Boundary), overlying the SPICE event (Steptoean Positive Carbon Isotope Excursion), indicating perturbations in the global carbon cycle. The trigger and process of these events still remain elusive. The petrographic and chemical screening of the investigated carbonates proves well preservation of sedimentary fabrics and at least near-primary geochemical signatures. The paleoredox proxy (UEF/TOC, VEF/TOC, and MoEF/TOC) and δ15Norg profiles exhibit positive excursions that correlate with the negative δ13Ccarb excursions, indicating pulses of expanded oceanic anoxia potentially resulted from anoxic deep-water upwelling. The correlated negative shifts in TOC and paleoproductivity proxy (NiEF and CuEF) profiles suggest suppressed primary productivity, possibly by upwelling-induced oceanic toxic events, aligning with mass extinctions particularly during the HERB event. Independently, the paired δ13Ccarb and δ13Corg (Δ13C) variations reveal decreased biological fractionation that aligns with the δ13Ccarb excursions, implying pulses of declining atmospheric O2 levels, which are consistent with the suggested expanded oceanic anoxia and suppressed primary productivity. The peaks of the negative δ13Ccarb excursions are correlated with the base of upward-shallowing sequences, consistent with the evidence from TOC, δ13Corg, δ15Norg, and trace element proxies, support a scenario of upwelling deep toxic waters during sea-level rise that likely resulted in expanded oceanic anoxia, and ultimately led to the negative excursions in δ13Ccarb values and mass extinction.

Geochemical Record of Late Quaternary Paleodepositional Environment from Lacustrine Sediments of Soda Lake, Carrizo Plain, California

This study investigates the responses of the depositional environments of Soda Lake sediments to climatic shifts from the Last Glacial Maximum to the Holocene epoch based on the results of major and trace elements of the North Soda Lake (NSL) NSL1A core. The NSL1A core records the sedimentary evolution of the Soda Lake watershed since at least 25 cal ka BP. Element analyses provide evidence that Soda Lake sediments are mostly derived from marine sequences in the Southern Coast Ranges of California. Variation in proxies for paleoweathering, paleoclimate, paleosalinity, paleoproductivity, paleoredox, and water depth is utilized to reconstruct the evolution of the sedimentary environment. The Chemical Index of Alteration (CIA) values indicate low to moderate chemical weathering in the sediment source regions. Paleoredox proxies indicate that the NSL1A core formed in a mainly subreduction environment. The NSL1A core is divided into four zones based on the results of the proxies. Zone 4 (5.0–5.8 m) of the sediment core indicates stable hydroclimatic conditions with low and constant sand and silt content, suggesting a warm and relatively humid environment. Zone 3 (3.35–5.0 m) represents the early half of the Last Glacial Maximum interval and a high lake stand. The elevated sand content suggests postflood events due to the northerly migration of westerly storm tracks. Zone 2 (1.075–3.35 m) reveals nuanced changes, including decreasing salinity, slight increases in wetness, detrital trace metals, and paleoproductivity. These subtle shifts suggest a multifaceted environmental evolution: a trend toward wetter conditions alongside a prolonged shift from cooler to warmer periods. Zone 1 (0.15–1.075 m) spans the Lateglacial to Holocene transition as well as Early and Middle Holocene, marked by significant hydrologic and ecologic variability including rapid warming during the Bølling–Allerød and rapid cooling linked to the Younger Dryas.

Determining of an oceanic anoxic event and paleoenvironmental conditions of the Gulneri Formation Kirkuk Oilfield, Northern Iraq

Stable isotopic geochemistry in relation to major and trace elements geochemistry of core samples from the Gulneri Formation at Kirkuk Oilfield Northern Iraq has been conducted to determine the occurrence of the oceanic anoxic event (OAE2) and reconstruct the paleoenvironmental conditions during this event. Positive values of the stable carbon isotopic data have revealed that temperature is high during deposition accompanied with higher organic productivity and a high rate of organic carbon burial with a decrease in dissolved oxygen in marine environments, whereas, negative oxygen isotopic values indicate deposition in hot, dry, semi-arid environments with salty marine waters and high organic productivity. These results were also supported by geochemical evidence of paleoclimate through C-value and ratios of Sr/Cu, Rb/Sr, and Ga/Rb, paleosalinity using proxies from Ca/Ca+Fe and Sr/Ba ratios, and paleoproductivity using P/Ti, P/Al and barium ratios. The paleoredox proxies from V/(V+Ni), V/Ni, and U/Th ratios and Uδ indicate dominant anoxic conditions. The recorded oceanic anoxic event (OAE2) has been determined through significant excursion in carbon and oxygen isotopic values as well as geochemical proxies within the sequences of the upper part of the formation marking the final stage of the OAE event.

Productivity and organic carbon loading control uranium isotope behavior in ancient reducing settings: Implications for the paleoredox proxy

Uranium isotopes in ancient sedimentary rocks have emerged as a powerful proxy for the oxygenation history of Earth surface environments. Proper quantitative interpretation of δ238U data hinges, however, on an understanding of isotopic fractionation associated with uranium removal to sediments under different redox conditions. Whereas oxygenated environments are plentiful in the modern ocean, euxinic settings are scarce today, and persistently ferruginous settings are not a feature in the modern ocean, leaving lakes as our best analogs for iron-rich waters. These challenges to studying δ238U behavior under contemporaneous oxic, euxinic (sulfidic), and ferruginous (iron-rich) marine conditions limit our ability to apply the isotope proxy to ancient oceans. Here, we present δ238U data from five coeval cores through the Lower Mississippian Sunbury Shale of the semi-restricted Appalachian Basin (North America), which were deposited across a strong redox gradient from intermittently oxic conditions proximal to the Catskill Delta, to ferruginous conditions in the central basin axis and euxinic conditions along the basin-bounding Cumberland Sill. Overall, we find that both euxinic and ferruginous environments are capable of imparting high degrees of uranium isotope fractionation and that δ238U values covary more strongly with total organic carbon (TOC) and trace metal abundances than they do with the presence of euxinic versus ferruginous conditions. We suggest uranium sorption or incorporation into sinking organic matter in the water column as a mechanism for generating positive covariation between δ238U and TOC, particularly at lower TOC concentrations. We also compare our δ238U data to broadly coeval carbonates from Nevada, which can be used to estimate Early Mississippian seawater δ238U values. We find that Sunbury Shale cores deposited under stably anoxic conditions (both euxinic and ferruginous) and under consistently brackish salinities away from freshwater sources record an increasing δ238U trend through time that loosely mirrors the record for carbonates (and thus estimates for seawater) with an offset of ∼0.4‰ to 1.1‰, depending on the inferred offset between carbonate and seawater. In contrast, cores deposited close to the Catskill Delta and in a shallow oceanic-restricted basin transition across the Cumberland Sill record a decreasing δ238U trend through time, opposite the inferred seawater trend and suggestive of entirely local controls on δ238U values. Ultimately, these data suggest that highly productive environments with high rates of organic carbon loading can exert a strong control on the global δ238U mass balance regardless of whether euxinic or ferruginous conditions are present. These results necessitate a partial reassessment of the processes controlling δ238U variability in seawater and sedimentary rocks through Earth history.

Revisiting the applicability and constraints of molybdenum- and uranium-based paleo redox proxies: comparing two contrasting sill fjords

Abstract. Sedimentary molybdenum (Mo) and uranium (U) enrichments are often used as redox proxies to reconstruct bottom water redox changes. However, these redox proxies may not be equally reliable across a range of coastal settings due to varying depositional environments. Fjords vary greatly in their depositional conditions, due to their unique bathymetry and hydrography, and are highly vulnerable to anthropogenic and climatic pressures. Currently, it is unknown to what extent Mo and U sequestration is affected by variable depositional conditions in fjords. Here, we use pore water and sequential extraction data to investigate Mo and U enrichment pathways in sediments of two sill fjords on the Swedish west coast with contrasting depositional environments and bottom water redox conditions. Our data suggest that sedimentary authigenic Mo and U pools differ between the two fjords. At the (ir)regularly dysoxic (oxygen = 0.2–2 mL L−1) Gullmar Fjord, authigenic Mo largely binds to manganese (Mn) oxides and to a lesser extent to iron (Fe) oxides; Mo sulfides do not play a major role due to low sulfate reduction rates, which limits the rate of Mo burial. Authigenic U largely resides in carbonates. At the (ir)regularly euxinic (oxygen = 0 mL L−1; total hydrogen sulfide ≥ 0 mL L−1) Koljö Fjord, authigenic Mo is significantly higher due to binding with more refractory organic matter complexes and Mo-Fe-sulfide phases. Uranium is moderately enriched and largely bound to organic matter. We found no direct evidence for temporal changes in bottom water redox conditions reflected in Mo and U enrichments at either Gullmar Fjord or Koljö Fjord. While sulfidic bottom waters favor Mo sequestration at Koljö Fjord, enrichment maxima reflect a combination of depositional conditions rather than short-term low-oxygen events. Our data demonstrate that secondary pre- and post-depositional factors control Mo and U sequestration in fjords to such an extent that bottom water redox conditions are either not being systematically recorded or overprinted. This explains the large variability in trace metal enrichments observed in fjords and has implications for applying Mo and U as proxies for environmental redox reconstructions in such systems.

Exploring uranium isotopes in shark teeth as a paleo-redox proxy

The uranium isotope composition (δ238U) of seawater is a powerful proxy for the extent of marine anoxia. For paleoredox reconstructions, carbonates are the most popular U isotope archive, but they have recently come under increased scrutiny as their δ238U values are subject to diagenetic alteration after deposition. Therefore, there is a need to investigate other archives that may record and preserve the original seawater δ238U signal. In this study, we explore whether shark teeth provide such an archive. Shark teeth enameloid consisting of crystalline fluorapatite is more resistant to post-depositional alteration and less sensitive to isotopic exchange than marine carbonates due to the lower solubility of the crystalline fluorapatite. Since U is readily incorporated into phosphate, shark teeth could incorporate and preserve the original δ238U signature of seawater.To assess whether U isotopes in shark teeth can record seawater signatures, we measured the U isotopes (both δ238U and δ234Usec) in 39 fossil shark teeth from various locations, including Banks Island (Arctic), the Gulf of Mexico (GOM), and Pisco Basin (Peru), and ranging in age from modern to Cretaceous. Our results show that U concentrations are negligible in modern shark teeth (<1 ppb) but elevated in fossil samples (up to several hundred ppm), indicating that U is incorporated into shark teeth postmortem during burial. The δ238U values range from −0.72 to +0.57 ‰, and the δ234U values from −162.1 to +969.7 ‰. The data indicate that (i) diagenetic overprinting of seawater U isotope ratios is common among shark teeth, and (ii) δ238U data are influenced by local depositional environments. Nonetheless, the U isotope variations observed in shark teeth are comparable to those seen in marine carbonates, indicating that the samples with less diagenetic alterations might offer useful insight into the past extent of ocean anoxia.

Postdepositional Behavior of Molybdenum in Deep Sediments and Implications for Paleoredox Reconstruction

AbstractMolybdenum (Mo) is a trace element sensitive to oceanic redox conditions. The fidelity of sedimentary Mo as a paleoredox proxy of coeval seawater depends on the extent of Mo remobilization during postdepositional processes. Here we present the Mo content and isotope profiles for deep sediments from the Nankai Trough, Japan. The Mo signature suggests that these sediments have experienced extensive early diagenesis and hydrothermal alteration at depth. Iron (Fe)‐manganese (Mn) (oxyhydr)oxide alteration combined with Mo thiolation leads to a more than twenty‐fold enrichment of Mo within the sulfate reduction zone. Hydrothermal fluids and Mo adsorption onto Fe‐Mn (oxyhydr)oxides cause extremely negative Mo‐isotope values at the underthrust zone. These postdepositional Mo signals might be misinterpreted as expanded anoxia in the water column. Our findings highlight the importance of constraining postdepositional effects on Mo‐based proxies during paleoredox reconstruction.

Using a novel approach to characterize the surface reactivities of silica-rich ferrihydrite and biogenic cyanobacteria-ferrihydrite aggregates and the implications for Archean ocean geochemistry

Precambrian banded iron formations (BIF) are iron- and silica-rich chemical sedimentary rocks that are commonly used as paleo-redox proxies for Archean and Paleoproterozoic seawater geochemistry. At the onset of the Great Oxidation Event (herein GOE) around 2.4 Ga, cyanobacteria flourished with increasing nutrient fluxes due to oxidative weathering on land. In turn, this led to increased primary productivity that facilitated the permanent shift from a reducing Earth atmosphere to an oxidizing one. Interestingly, the duration of GOE also overlapped with one of the most prolific periods of BIF deposition.It is widely accepted that cyanobacteria were likely responsible for BIF formation during the GOE. Oxidation of dissolved Fe(II) by oxygen produced from cyanobacteria forms a metastable and amorphous mineral phase ferrihydrite, Fe(OH)3. As an essential component in both ancient BIF deposits and various modern ecosystems, the surface reactivity of ferrihydrite has been extensively studied under different conditions (i.e., pH and ionic strengths). Not only are the highly reactive surfaces of ferrihydrite particles important shuttles for trace element transport from the water column to the sediment pile, but previous studies have also demonstrated that cyanobacterial cells and ferrihydrite tend to aggregate at seawater pH. This means that ferrihydrite was also a vector for the transport of organic carbon to the seafloor. However, a complicating issue is how co-ions affect the surface reactivity of ferrihydrite, specifically dissolved silica which was abundant in ancient seawater. Although previous studies have demonstrated that silica can passivate the surface reactivity of ferrihydrite, what remains unclear is how silica impacts ferrihydrite-biomass aggregation. To fill this knowledge gap, we formed both silica-spiked ferrihydrite and cyanobacteria-ferrihydrite aggregates in situ and subsequently conducted empirical potentiometric acid-base titrations and Cd adsorption experiments on the fresh aggregate samples at three different ionic strengths (0.56 M, 0.1 M and 0.01 M). We minimized sample processing (i.e., drying and powdering) to a simple washing step, in which the aggregate pellets remained hydrated to avoid any mineral transformation thus altering their true surface reactivity in seawater. Experimental results were then fitted with non-electrostatic model to predict both surface charges and metal-adsorption behavior of ferrihydrite aggregates. Different from previous surface-complexation modelling studies, here we used a novel and more powerful modelling program called Phreefit. It utilizes the global optimization algorithms instead of more commonly used Newton-Raphson method in FITEQL program, which is often too limited for precisely modelling complex systems such as the two samples in this study. Furthermore, we also measured the surface charges of both samples over the pH range from 3 to 9 on a Malvern Zetasizer and characterized the surface functional groups through Fourier-Transform Infrared Spectroscopy to help with our interpretation of the experimental data.Preliminary results show that cyanobacteria-ferrihydrite aggregates formed primarily due to ionic bridging. Cyanobacterial cells likely facilitated the precipitation of dissolved silica. Findings from titration and Cd adsorption experiments indicate that the surface reactivity and capacity of both silica-rich ferrihydrite cyanobacteria-ferrihydrite aggregates to adsorb trace elements differ from their individual components, likely due to site blockage. This distinction is particularly prominent when considering the expected Archean seawater pH from 6 to 8. This disparity implies that the biogenic ferrihydrite aggregates do not exhibit an additive surface reactivity, which is in agreement with similar previous studies. Our combined results are crucial to accurately predict the adsorption of trace elements onto the aggregate surface and, ultimately, comprehend the archive of trace elements in sedimentary rocks used to reconstruct Precambrian ocean chemistry.

Significance of pH and iron-sulfur chemistry for molybdenum sequestration under sulfidic conditions

Molybdenum (Mo), a redox-sensitive trace metal, plays an important role in recording ancient oxygenation and deoxygenation events as a paleoredox proxy. The mobility and reactivity of Mo in aqueous conditions are closely tied to the chemistry of reduced sulfur and iron species. However, our current knowledge on the formation, structure, stability, and condensation pathways of FeMoS clusters in aqueous settings remains limited, which has driven the current study. In this study, we conducted systematic experiments investigating the interactions between dissolved Mo (initially introduced as molybdate, MoO42−, or tetrathiomolybdate, MoS42−), ferrous iron (Fe2+), and sulfide (ΣH2Saq) in variously defined abiotic sulfidic systems to determine the external conditions (i.e., pH, and reactant concentrations and ratios) necessary for the formation of solid-phase Fe-Mo sulfides. Solution samples of each system were monitored using ultraviolet-visible spectroscopy (UV–vis) to track the degree of thiolation of dissolved Mo species. Precipitates were analyzed using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) to determine their elemental compositions and valences, and structure (i.e., crystalline or amorphous), respectively. All FeMoS precipitates were amorphous and contained 76–90% Mo(IV) and 10–24% Mo(V) with a trend toward lower Mo(IV):Mo(V) ratios with increasing pH. The degree of Mo thiolation, which was strongly dependent on solution pH and Fe2+ concentrations, greatly affected the amount of Mo sequestered (i.e., an increased degree of Mo thiolation in solution led to an increased amount of Mo in the final FeMoS precipitate). These findings suggest that changes in pH and Fe2+ concentrations may be responsible for the sulfide-independent variations in Mo behavior observed in euxinic basins.

Organic matter preservation conditions in the third member of the Shahejie Formation (Dongpu Depression, China)

The third member of the Paleogene Shahejie Formation (Es3) in the Dongpu Depression, China, was studied to interpret its paleoenvironment through a series of trace element proxies, to reveal paleosalinity conditions, to elucidate on the hydrological conditions of the paleolake using carbon and oxygen stable isotopes, and to investigate the effects of paleoredox conditions on organic matter preservation. Most of the redox-sensitive proxies, such as U/Th, V/Cr, Ni/Co, Uauth, (Cu + Mo)/Zn, Cu/Zn, Th/U, and V/Sc, indicate that the Es3 was deposited under predominantly oxic and oxic/suboxic conditions, while V/(V + Ni) and δU and previous studies suggested that the formation was deposited under mainly anoxic conditions. Oxygen levels agreed with the trace element ratios and showed a decrease from the lower to the upper part of the Es3 and an increase in paleoproductivity in the same direction. Salt rocks interbedded within the formation were shown to have overall lower oxygen levels than their neighboring organic-rich layers, black shale, and mudstone intervals, thus have likely altered the paleoredox proxies to oxygen consumption. Although Sr/Ba values point to a saline lake environment, the relative fluctuations seen suggest that the above ratio is not an appropriate indicator of paleosalinity in salt-bearing strata and that the variations in paleoredox conditions in the formation are not a function of paleosalinity. Based on the negative δ13C and δ18O stable isotopic values, it is inferred that the shales in the Es3 were deposited in a hydrologically open lake environment. However, the presence of palynomorphs such as dinoflagellate cysts that are typically found in middle to outer neritic and open marine environments reinforces the assumption of a sea level transgression and inundation of the lake basin and mixing with freshwater algae and rare terrigenous phytoclasts such as cuticles, wood fragments, and suberin. The higher paleoproductivity and lower oxygen levels in the upper Es3 have contributed to greater OM preservation conditions in the absence of any variations in salinity or freshwater influx. The study also demonstrates that many of the paleoredox proxies and thresholds established are not valid to properly characterize certain formations deposited in lacustrine lake paleoenvironments such as the Shahejie, thus their applicability in certain situations must be re-evaluated.

A comparative study of the paleoclimate, paleosalinity and paleoredox conditions of Lower Jurassic-Lower Cretaceous sediments in northeastern Iraq

The findings of this study are considered to be of great importance given their singularity in northeastern (NE) Iraq, as the first broader study that discusses various paleoenviromental proxies (i.e., paleoclimate, paleosalinity and paleoredox proxies) which were predominant during the Early Jurassic-Early Cretaceous period of NE Iraq. To this end, a comparative study of the mineralogical and geochemical features of the Sarki, Sehkanyian, Sargelu, Naokelekan, Barsarin and Chia Gara formations from the Early Jurassic-Early Cretaceous-age Ranya section, was carried out together with a study of the same formations from the Warte section (both sections are located in NE Iraq). Petrographic and microfacies analyses show that all six formations in both sections were deposited in tidal flat (supratidal), restricted lagoons and shallow marine to bathyal depositional environments. The paleoclimate proxies (Ca, Al, Sr/Ba, Rb/Sr and Sr/Cu) and paleosalinity proxies (Sr, Ca, Al and Sr/Ba) together with oxygen isotopes, suggest that the climate was predominantly hot and arid during the period of deposition, contrasting with periods of warm and humid climates during the deposition of the Late Jurassic-Early Cretaceous-age Chia Gara Formation. This climatic change was revealed by a shift in clay mineralogy, from a dominance of illite to increasing amounts (and even dominance) of kaolinite marking the change to more humid conditions within the Late Jurassic-Early Cretaceous period. Geochemical proxies for redox conditions (U/Th, V/Cr, Ni/Co, V/Sc and V/(V+Ni) indicate that deposition of the sediments in the examined areas occurred under anoxic (or reduced oxygen) conditions. This difference in paleoclimate, paleosalinity and paleoredox conditions is likely to be controlled by tectonic activity in the region of northern Iraq, with the development of small silled sub-basins within the broader Zagros Basin/Zagros Fold Belt.

Stable chromium isotope compositions of hydrogenetic ferromanganese crusts potentially linked to primary productivity

Stable chromium isotope composition (δ53Cr) is a widely used paleo-redox proxy. However, modern seawater δ53Cr values do not correlate with dissolved O2 concentrations, suggesting that non-O2 factors may influence marine Cr cycling. We present new δ53Cr data for two hydrogenetic ferromanganese (FeMn) crusts deposited in the western- and central-north Pacific Ocean since the late Eocene, a period when the ocean remained largely oxic. Simple isotope mass balance and Rayleigh distillation models suggest that trivalent Cr is likely the major Cr species incorporated from seawater into the Fe phases of hydrogenetic FeMn crusts. The δ53Cr values of FeMn crusts decrease from −0.35‰ in the late Eocene to −0.74‰ in the middle Miocene, followed by increasing to modern values ranging from −0.33 to −0.47‰. The temporal trend of δ53Cr follows that of the regional pelagic sedimentation rate, suggesting that primary productivity may have played a role in shaping the δ53Cr signals recorded in the hydrogenetic FeMn crusts. Our data show that non-O2 factors may influence δ53Cr values recorded in Fe-rich sedimentary archives deposited in a largely oxic ocean, thus adding more complexities to the Cr isotope paleoproxy.

Uranium and molybdenum isotope evidence for globally extensive marine euxinia on continental margins and in epicontinental seas during the Devonian-Carboniferous Hangenberg Crisis

The end-Devonian Hangenberg Crisis was one of the biggest Phanerozoic mass extinctions. However, the mechanism(s) that triggered this event is still debated. In this study, multiple geochemical paleoredox proxies (redox-sensitive trace metals [e.g., Mo, U, Re, V] and isotope systems [Mo, U, S, C]) were applied to the Exshaw Formation black shales to infer ocean redox conditions during the Hangenberg Crisis. The measured δ13Corg values generally decrease upsection in the Exshaw Formation. The Exshaw black shales have increasing maturity levels from east to west in the Western Canada Sedimentary Basin. Large sulfur isotope fractionations (–15‰ to 65‰) between original seawater sulfate and pyrite is best explained by microbial sulfate reduction during deposition and early diagenesis rather than thermochemical sulfate reduction. Precise Re-Os ages previously reported for the Exshaw Formation suggest that metal geochemical signatures in these shales, including overmature shales, were minimally affected by hydrocarbon maturation and reflected depositional conditions. Both Sr/Ba and TS/TOC ratios (the latter only from immature shales affected minimally by hydrocarbon generation) suggest predominantly brackish and marine settings with significant water exchange between the basin and open ocean. The TOC contents, redox-sensitive trace metal concentrations (Mo, U, Re), and Fe speciation indicate local bottom waters ranged from suboxic (with sulfidic pore waters) to euxinic. The authigenic δ98Mo and δ238U values for the Exshaw shales range from 0.3‰ to 1.1‰, and from –0.23‰ to 0.39‰, respectively. The δ98Mo of the Exshaw Formation may have been offset to lower values than coeval seawater because of a local particulate Fe-Mn shuttle and/or local weakly euxinic bottom waters. Two different correlation patterns (positive correlation and no correlation) of δ238U and U enrichment factors (EF) were observed for Exshaw black shales deposited from locally euxinic bottom waters. The positive correlation group samples (−0.13‰ to 0.23‰) suggest U isotope fractionations (0.4‰ to 0.8‰) between sediments and seawater like those observed in modern euxinic basins. Higher δ238U values (>0.2‰) in the no correlation group suggest even larger U isotope offsets (0.8–1.2‰) from seawater, possibly associated with removal of U into organic floccule layers during deposition. Applying the above Mo and U isotope fractionations to the Exshaw shales, global seawater δ98Mo and δ238U at the Devonian-Carboniferous boundary (DCB) may have been in the range of 1.4‰ to 1.9‰ and −0.9‰ to −0.5‰, respectively. A Mo and U isotope mass balance model suggests a large areal extent of euxinic seafloor (6–10%) at the DCB, which could be pervasive along continental margins and in epicontinental seas during transgression. Therefore, our data support expanded ocean euxinia at the DCB as an important contributor to the Hangenberg Crisis.

Mercury isotope evidence for recurrent photic-zone euxinia triggered by enhanced terrestrial nutrient inputs during the Late Devonian mass extinction

Widespread oceanic anoxia marked by globally extensive deposition of organic-rich black shale during the Late Devonian was a major factor in the mass extinctions at the Frasnian-Famennian (FFB, ∼372 million years ago) and Devonian-Carboniferous boundaries (DCB, ∼359 million years ago), although the triggers for these deoxygenation events are still under debate. Here, we apply a novel paleoredox proxy, Hg isotopes, to investigate Late Devonian ocean redox variation and its causes. We found no Hg enrichments in North America across either the FFB or DCB, thus arguing against the hypothesis of global-scale volcanism as the trigger for Late Devonian environmental and biotic crises. Gradual negative shifts of both mass-independent fractionation (Δ199Hg) and mass-dependent fractionation (δ202Hg) occurred between the FFB and DCB, suggesting a progressive increase of Hg inputs associated with terrestrial organic matter. Moreover, multiple abrupt negative excursions of Δ199Hg (down to −0.19‰) along with concurrent positive shifts of δ202Hg occurred just above the FFB and across the DCB, providing strong evidence for recurrent photic-zone euxinia (PZE) that was preceded by increasing terrestrial inputs in the epicontinental seas of North America. We suggest that the increase of terrestrial inputs of nutrients, probably via expansion of vascular land plants, stimulated marine primary productivity and eventually PZE, which may have been a key kill mechanism for the Late Devonian mass extinction.

The large decline in carbonate δ238U from a PETM section at Tingri (South Tibet) was driven by local sea-level changes, not global oceanic anoxia

Uranium isotope compositions (δ238U) and Th/U in carbonates are being explored as paleoredox proxies to study global oceanic anoxia. However, the impact of changes to local depositional environments that might cause a decline in δ238U and an increase in Th/U has not been thoroughly investigated. Here we report a decline in δ238U (from +0.1 to −0.3‰) and an increase in Th/U from shallow-marine carbonates during the Paleocene-Eocene thermal maximum at Tingri (south Tibet). Changes of δ238U and Th/U are not fully coupled. The decline in δ238U coincided with a local sea-level fall, whereas the increase in Th/U occurred mainly during a sea-level rise. The decline in δ238U likely results from changes in redox conditions of pore waters and in primary carbonate mineralogy. The increase in Th/U is ascribed to reduced authigenic U(IV) accumulation. Our results suggest that changes in local depositional environments can cause behavior of δ238U and Th/U resembling that induced by expanded global oceanic anoxia. This finding calls for caution to consider local factors before applying δ238U and Th/U as global paleoredox proxies.

Technical comment on “Reexamination of 2.5-Ga ‘whiff’ of oxygen interval points to anoxic ocean before GOE”

Many lines of inorganic geochemical evidence suggest transient "whiffs" of environmental oxygenation before the Great Oxidation Event (GOE). Slotznick etal. assert that analyses of paleoredox proxies in the Mount McRae Shale, Western Australia, were misinterpreted and hence that environmental O2 levels were persistently negligible before the GOE. We find these arguments logically flawed and factually incomplete.

Protracted oxygenation across the Cambrian-Ordovician transition: A key initiator of the Great Ordovician Biodiversification Event?

Fluctuations in marine oxygen concentrations have been invoked as a primary driver for changes in biodiversity throughout Earth history. Expansions in reducing marine conditions are commonly invoked as key causal mechanisms for mass extinctions, while increases in marine oxygenation are becoming an increasingly common causal mechanism invoked for biodiversification events. Here we utilize a multiproxy approach to constrain local and global marine paleoredox conditions throughout the late Cambrian-Early Ordovician from two drill core successions in Baltoscandia. Local paleoredox proxies such as manganese concentrations and iron speciation reveal that both sites in the Baltic paleobasin had persistently anoxic and predominantly euxinic (anoxic and sulfidic) bottom water conditions throughout the study interval. Corresponding trace metal datasets indicate nuanced contraction and expansion of global anoxic and euxinic conditions along continental margins during the late Cambrian-Early Ordovician. Lastly, thallium isotope data from these locally reducing sections suggest a global expansion of oxygenated shelf and deeper marine environments from the late Cambrian into the Early Ordovician. This evidence for increasingly oxic marine environments coincides with increases in burrowing depth and tiering in marine animals, as well as diversification of body fossils throughout this ~8-million-year interval. The collective geochemical datasets provide some of the first direct paleoredox evidence for an increase in marine oxygen concentrations as a key mechanism for the Ordovician radiation of marine life.

Assessing the reliability of modern marine stromatolites as archives for the uranium isotope paleoredox proxy

Uranium isotopes (238U/235U) are commonly utilised in marine carbonates to reconstruct the redox evolution of ancient oceans and the atmosphere. However, questions remain concerning U isotope fractionation during carbonate precipitation and/or early sedimentary diagenesis. Moreover, the stromatolitic nature of many Precambrian limestones and dolostones brings further questions about U isotope fractionation during stromatolite formation due to microbial activity. Thus, it is unclear whether stromatolites are reliable archives for the U isotope composition of seawater. Here we measure the 238U/235U ratio of modern stromatolites from the hypersaline Shark Bay, Western Australia, a key study area that represents the largest modern example of stromatolite growth globally. A component-specific approach was utilised to analyse the uppermost crusts of modern stromatolites, older material from the deeper stromatolite laminae and the substrate upon which the stromatolites are forming. Our interpretations of δ238U data are aided by δ234U, δ18O, δ13C and trace element measurements, including some mineralogy and total organic carbon determinations for selected samples.Modern aragonitic crusts of stromatolites from the subtidal and intertidal zones around Shark Bay exhibited a narrow range of δ238U (−0.30 to −0.33‰), corresponding to a small offset of ca. +0.07‰ from seawater. This range of δ238U values overlaps with that of other primary carbonate precipitates in shallow marine environments such as corals but with a slightly higher mean δ238U. We investigated if this offset may be associated with the coprecipitation of U with aragonite from hypersaline seawater in Shark Bay, which has a higher proportion of dissolved U present as Ca2UO2(CO3)3 compared to open seawater. However, this measured offset is much smaller than that predicted using a speciation-dependent isotope fractionation model. Sub-recent material in the deeper stromatolite laminae exhibited higher and more variable δ238U (up to −0.15‰) compared to the modern aragonitic crusts of stromatolites. The strong inverse correlation between δ238U and the total organic carbon content (r = −0.83) and positive correlation with Mn concentrations (r = 0.91) suggest that 238U was preferentially reduced in the deeper laminae in association with organic carbon remineralisation under reducing conditions. The average δ238U offset of sub-recent stromatolite laminae from modern seawater was +0.13 ± 0.11‰ (1 s.d.), which is slightly lower than, but within uncertainty of, the offset measured for bulk carbonates from the Bahamas (+0.27 ± 0.14‰; 1 s.d.). These results demonstrate that stromatolites are valuable archives of global seawater δ238U, even in hypersaline restricted basins such as Shark Bay, but an offset to account for syndepositional diagenesis is likely required to accurately reconstruct seawater δ238U using ancient stromatolites.