The Younger Dryas (YD; ~12.9 to 11.7 thousand years) marks an abrupt return to near-glacial conditions during the last deglaciation, yet its cause remains debated. One possible scenario, the YD impact hypothesis, proposes an extraterrestrial trigger. However, growing geochemical and stratigraphic evidence points toward a volcanic origin. This study presents the 187Os/188Os isotope and highly siderophile element (HSE) data from the Page-Ladson (PL) site (8JE591) in Florida, a well-dated, continuous sedimentary record spanning the YD onset. The onset in the PL profile is marked by unradiogenic osmium coincident with elevated Os and Re concentrations and a Cl-chondrite-normalized HSE pattern with a compositional range and signature closely matching volcanic aerosol patterns. When integrated with comparable records from Hall's Cave and the Debra L. Friedkin site in Texas, the unradiogenic 187Os/188Os ratios align across multiple depositional environments and correlate with a cluster of major bipolar volcanic eruptions (~12.98 to 12.87 thousand years) documented in Greenland and Antarctic ice cores whose cumulative radiative forcing exceeds the most volcanically active intervals of the Common Era. The magnitude and hemispheric asymmetry of this volcanic activity imply forcing of sufficient magnitude capable of disrupting the Atlantic Meridional Overturning Circulation and triggering rapid Northern Hemisphere cooling. These findings provide multiproxy, regionally consistent evidence for a volcanically driven perturbation at the onset of the YD, offering a robust alternative to impact-based explanations.

Volcanism-driven ocean acidification has repeatedly disrupted Earth's carbon cycle and ecosystems. Calcium isotopes (S44/40Ca) provide a potential means of tracking these perturbations, since both the weathering and burial of CaCO3 and the isotopic compositions associated with these fluxes are sensitive to changes in seawater pH. However, the use of S44/40Ca to reconstruct acidification and infer biocalcification stress is often complicated from overprinting by diagenetic processes. In this paper, we investigate stable Ca (S44/40Ca) and Sr (S88/86Sr) isotope responses to ocean acidification during Oceanic Anoxic Event 2 (OAE 2) in a pelagic marine carbonate succession from the Western Interior Seaway. The records are reconstructed using the Iona-1 core in Texas, which fully captures Os isotope evidence for the onset of LIP volcanism that is missing from a previously published Ca isotope record in the Western Interior Seaway in Colorado. Here we show that a negative Ca isotope excursion occupies the missing part of the previously published Western Interior Seaway record, and that it the begins with the onset of large igneous province volcanism as represented by the abrupt shift to lower initial 187Os/188Os isotopic ratios in the core. Box-model simulations show that the negative Ca isotope excursion could be produced through reduced Ca isotope fractionation affecting pelagic carbonate production under conditions of transient acidification stress, followed by rapid recovery through carbonate alkalinity feedback. S88/86Sr values broadly covary with S44/40Ca values but deviate from the expected kinetic slope, instead defining a mixing array consistent with seawater-buffered recrystallization. Together, the coupled stable Ca-Sr isotope systematics distinguishes primary acidification-driven signals from diagenetic modification and demonstrate the utility of dualisotope approaches for tracing volcanism-induced perturbations to the marine carbon cycle.

Estimating the sulfur isotope composition of Earth's fertile mantle can provide insight into Earth's constitution and the processes associated with establishing its highly siderophile element abundances. There is currently a disagreement in sulfur isotopic compositions between Ocean Island Basalts (OIB) (delta S-34 = 0) and Mid-Ocean Ridge Basalts (MORB) (delta S-34 approximate to -1) implying that their mantle sources are different. MORBs and OIBs are young, so it remains unknown whether sulfur isotopic composition differences stem from relatively ancient, perhaps primordial compositions inherited from Earth's accretion and differentiation or, if they reflect the effects of constant recycling of crust to the convective upper mantle. Subcontinental lithospheric mantle (SCLM) keels represent Earth's only ancient mantle reservoirs that have been preserved from mantle convection affecting MORBs and OIBs, and as such may retain important clues about the origin and evolution of its sulfur isotopic compositions. This study presents major and trace element compositions, and the isotope compositions of Sr, Nd, Os, and S from Kilbourne Hole, New Mexico peridotite xenoliths which confirm that KH xenoliths are associated with an ancient mantle reservoir (similar to 1.7 Ga). The average sulfur isotope composition of olivine and clinopyroxene hosted sulfides measured in-situ is delta S-34 = similar to-1 parts per thousand, which contrasts the bulk S isotope measurements that extend to values of similar to+2 parts per thousand. The difference in S isotope composition of in-situ vs. bulk measurements indicates that the phenocryst hosted sulfides likely record the composition of the ancient mantle, while the bulk S isotope composition records mixing with sulfur associated with subsequent metasomatism. Furthermore, the similarity in S isotope composition of individual KH sulfides and estimates for the mantle from MORB suggest that the process responsible for fractionating S-34/S-32 in Earth's mantle was likely widespread and occurred prior to similar to 1.7 Ga. Positive correlations among bulk delta S-34 and Sr-87/Sr-86 data, as well as bulk sulfide concentration and (Ce/Yb)(N,) illustrates that the bulk S reflects compositional influences from recycled continental crust materials. Similarly, variable Delta S-33 among the bulk KH measurements (-0.007 +/- 0.008 to +0.038 +/- 0.016) shows the compositional influence from mixing with recycled surface materials. The relationship among Delta S-33 and Delta S-36 values furthermore indicates that the metasomatic sulfur is of post-Archean age. Primary S isotope compositions can be preserved in the SCLM and can be discerned through a strategic bulk vs. in-situ S isotope measurement approach.

Anomalously high metal concentrations including iron enrichments are recorded in marine carbonates deposited during Ocean Anoxic Event 2 (OAE 2). These metal enrichments have been attributed to massive submarine eruptions during the formation of one or more large igneous provinces, the proposed trigger for OAE 2 (hydrothermal hypothesis), or to the release of metals from the reoxidation of formerly anoxic marine sediment during a period of temporary cooling during OAE 2 (sediment release hypothesis). Here we use iron stable isotopes to help distinguish between the two hypotheses for a trace metal enriched interval during OAE 2 in the Iona-1 core in the Western Interior Seaway, Texas. Our results show a two-step negative excursion during OAE 2 that is coincident with osmium isotope volcanic proxies measured in the same core, with peak negative values centered on a trace metal-enriched interval. After corrections for detrital and locally supplied iron to the study setting, the delta Fe-56 value of the remotely supplied iron is -0.28 +/- 0.05 parts per thousand, falling in the range of iron delta Fe-56 values observed in modern hydrothermal plumes (-0.1 to -0.5 parts per thousand), thus supporting the hydrothermal hypothesis as the source of iron and other associated trace metals enriched in the study core during OAE 2. By contrast, the sediment release hypothesis predicts much lower delta Fe-56 values, between -1.0 parts per thousand to -3.3 parts per thousand predicted for benthic supplies of iron from anoxic marine sediment overlying re-oxygenated bottom waters. This study shows that combining iron with other proxies for environmental change, particularly submarine volcanism, can distinguish hydrothermally supplied iron from dust, rivers, and shelf sediment supplies of iron despite iron's reputation for complicated cycling.

Constraining the heterogeneity of oxygen fugacity (fO2) in the upper mantle is important for understanding the role of the deep Earth in determining the redox state of the atmosphere. Measuring the fO2 of primitive, mantle-derived basalts is a means with which to constrain mantle fO2 heterogeneity. Trace element systematics of 721 primitive olivine crystals from fifteen Icelandic picrites are reported, and these data are used with previously published bulk rock compositions to constrain their magmatic fO2. These fO2 values were compared with previously reported ocean island basalt (OIB) fO2 data calculated using identical methods, showing that fO2 is negatively correlated with parental magma MgO and positively correlated with bulk rock La/Yb, trends which were not evident in previous, smaller, global ocean island basalt datasets. These correlations cannot reasonably be the result of fractional crystallization, differential volcanic degassing, or differential partial melting of a typical homogenous mantle peridotite. Instead, differential partial melting of a heterogeneous source which preferentially sampled oxidized fusible lithologies at low melt fractions and diluted such signatures with depleted peridotite melts at high melt fractions likely caused the observed fO2-MgO-isotopic trends. These conclusions further reinforce that the mantle should not be modeled as a homogenous peridotite, but instead as a heterogeneous, non-equilibrated mixture of enriched “blebs” in a matrix of depleted peridotite, and that degree of partial melting is an important control on what portions of the mantle are detectable in igneous rocks.

Galveston Bay is an anthropogenic-influenced estuary where industrial runoff, wastewater, and shipping vessel discharges enter the bay alongside natural freshwaters. Here, heavy metal concentrations in Galveston Bay surface sediment (2-year quarterly time-series) and a single sediment core are presented to explore the anthropogenic and geochemical controls on the spatiotemporal distributions, fluxes, sources, and potential toxicity of metals within this estuary. Samples were leached to distinguish authigenic sediment coatings from geogenic crystalline material. Spatial differences dominate the observed concentration variability, with higher metal concentrations in eastern vs. western bay sediments, as the eastern bay is where metals are flocculated from the dissolved phase and/or sediments are hydrodynamically trapped. Temporal variations are a secondary controlling factor, with sediment metal concentrations positively correlated with Trinity River discharge. Core data indicate stable Fe, Pb Ni, Cd and Hg levels during the 20th century but increasing Cu and Zn levels in recent years. Galveston Bay sediments are potentially toxic for As, Cd, Cr, Cu, Ni, Sb, Zn and Hg, based on federal toxicity standards. Enrichment factors and statistical analyses suggest that Ni and Cr originate from natural sources, while anthropogenic sources dominate supply of As, Cd, Hg, Ni, Pb, Sb, and Zn. This unique time-series shows that major flooding events, such as Hurricane Harvey in 2017, affect surface sediment metal distributions in Galveston Bay, but not any more than the natural geochemical controls on spatiotemporal distributions of metals in anthropogenic-influenced estuaries.

Chromium is a redox sensitive element that exhibits a large range of isotopic compositions in Earth's surface environments because of Cr(VI)-Cr(III) transformations. This property of Cr has been exploited as a tracer of Earth's oxygenation history using marine sediments. However, paleoredox applications using Cr are difficult to implement due to its complicated cycling, which creates spatial variability in seawater δ53Cr values. Applications are further hindered by the potential for variability in the major inputs of Cr, such as submarine volcanism, to mask redox processes. Two previous reports of negative excursions in sedimentary δ53Cr values during the middle Cretaceous Ocean Anoxic Event 2 (OAE 2) demonstrate these complications. Observed negative shifts in marine sediments conflict with the positive shifts expected in response to the increased drawdown of isotopically light Cr(III) prompted by the expansion of anoxic depositional sinks. In this study, a marine carbonate succession cored from the Eagle Ford Formation in Texas, USA, in the southern part of the Western Interior Seaway, depicts the negative 1.5‰ δ53Cr excursion occurring in two steps, with the second step reaching peak minimum values indistinguishable from isotopically unfractionated igneous sources. In contrast to published δ53Cr records, each step stratigraphically matches proxy evidence for increased eruption frequency and/or intensity of volcanic activity using combined 187Os/188Os, 87Sr/86Sr and Os concentration proxies previously measured from the same core, supporting higher inputs of volcanically sourced Cr to the oceans as the driver for the negative Cr isotope excursion.

Rare earth elements condense early during nebular cooling, and are present in pre-solar grains that have large variations in nucleosynthetic isotope compositions.Their abundances in materials that make up rocky bodies in the solar system are thus dependent on how early-formed materials and pre-solar grains are distributed in the protoplanetary disk.Neodymium isotopes track these materials from their variations in s-and r-process nucleosynthetic components.In addition, the 146 Sm-142 Nd decay system with a half-life of 103 million years, identifies potential differences in the Sm/Nd ratios of rocky body precursor materials, and once a rocky body has formed and begins to differentiate, consequent Sm/Nd fractionation can result in different present-day 142 Nd/ 144 Nd ratios relative to their precursor materials.For Earth, it is largely accepted that the convecting mantle today has a µ 142 Nd of 0, and different from chondrites ranging in µ 142 Nd from approximately -8 to -40.The open question remains on whether these differences are solely a function of a nucleosynthetic difference in Nd isotopes between Earth and chondrites, or if the present-day convecting mantle value for Earth represents a different Sm/Nd ratio in its precursor materials, or early internal differentiation and thus not representative of a bulk Earth.The current model favors the first scenario where the variations in μ 142 Nd reflect only nucleosynthetic differences.This would mean that the bulk Earth's Sm/Nd ratio was the same as that for chondrites.Here, new high precision Nd isotope data for enstatite chondrites are combined with data for chondrites and the Moon from the literature in order to re-evaluate this conclusion.An alternative model that employs Nd isotope constraints from lunar materials, and a scenario where the Earth and Moon are derived from a well-homogenized Nd reservoir resulting from the Moonforming giant impact, is considered.In this model, the Earth's Sm/Nd ratio is 2.4% higher than the average for chondrites and its initial 142 Nd/ 144 Nd ratio is closer to enstatite chondrites than previously proposed.These differences reflect mixing with the inner protoplanetary disk.Implications for an Earth with an elevated Sm/Nd ratio will be presented.

Reconstructing the building blocks that made Earth and the Moon is critical to constrain their formation and compositional evolution to the present. Neodymium (Nd) isotopes identify these building blocks by fingerprinting nucleosynthetic components. In addition, the 146 Sm– 142 Nd and 147 Sm– 143 Nd decay systems, with half-lives of 103 million years and 108 billion years, respectively, track potential differences in their samarium (Sm)/Nd ratios. The difference in Earth’s present-day 142 Nd/ 144 Nd ratio compared with chondrites 1 , 2 , and in particular enstatite chondrites, is interpreted as nucleosynthetic isotope variation in the protoplanetary disk. This necessitates that chondrite parent bodies have the same Sm/Nd ratio as Earth’s precursor materials 2 . Here we show that Earth and the Moon instead had a Sm/Nd ratio approximately 2.4 ± 0.5 per cent higher than the average for chondrites and that the initial 142 Nd/ 144 Nd ratio of Earth’s precursor materials is more similar to that of enstatite chondrites than previously proposed 1 , 2 . The difference in the Sm/Nd ratio between Earth and chondrites probably reflects the mineralogical distribution owing to mixing processes within the inner protoplanetary disk. This observation simplifies lunar differentiation to a single stage from formation to solidification of a lunar magma ocean 3 . This also indicates that no Sm/Nd fractionation occurred between the materials that made Earth and the Moon in the Moon-forming giant impact.

The emplacement of a Large Igneous Province (LIP) is implicated in the triggering of the Cenomanian-Turonian Ocean Anoxic Event 2 (OAE 2; ca 94 -95 Ma).However, evidence for a similar initiation mechanism for the Mid-Cenomanian Event (MCE; ca 96.5 Ma), a precursor to OAE 2 when comparable environmental conditions persisted, remains unclear.This shortterm carbon cycle perturbation during the MCE is associated with several changes which together may have ultimately cumulated to OAE 2. Thus, understanding the changes/process that led to the onset of the MCE could have implication for understanding the long term carbon cycling feedback that led to the initiation of OAE 2.Using Osmium isotope analysis, this study presents a reconstruction of mid-Cenomanian seawater 187 Os/ 188 Os from the Iona-1 core, SW Texas to tests the competing roles of LIP versus continental weathering activity in triggering the MCE.The absence of a prolonged unradiogenic Os isotope excursion (low 187 Os/ 188 Os) during the MCE interval argues against LIP involvement in the event's initiation.Rather, increased radiogenic 187 Os/ 188 Os at the onset of the MCE indicates that the event was triggered by increased continental weathering.However, the combination of a muted unradiogenic Os-isotope excursion coincident with a 40 Ar-39 Ar age of 96.4 Ma of basalts from Ellesmere Island, Canada, towards the end of the MCE, is consistent with High Arctic LIP-related volcanic activity that may have contributed to the demise of the MCE.

The emplacement of a Large Igneous Province (LIP) is implicated in the triggering of the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2).Evidence for a similar initiation mechanism for the mid-Cenomanian Event (MCE) is unclear.In this study, a reconstruction of mid-Cenomanian seawater 187 Os/ 188 Os, the first for the Western Interior Seaway, tests the competing roles of LIP versus continental weathering activity in triggering the MCE.The absence of a prolonged unradiogenic Os isotope excursion (low 187 Os/ 188 Os) at the onset of the MCE interval argues against LIP involvement in the event's initiation.Rather, more radiogenic 187 Os/ 188 Os at the onset, that continues to rise to the middle of the MCE, indicates that the event was triggered by increased continental weathering.The combination of decreasing 187 Os/ 188 Os from the middle of the MCE onward, coincident with a 40 Ar/ 39 Ar age of 96.4 Ma of basalts from Ellesmere Island, Canada, is consistent with High Arctic LIP-related volcanic activity that may have contributed to the end of the MCE.These new data on the MCE thus indicate that LIP activity is not always the trigger for carbon cycle perturbation and associated climate change.

Ocean island basalts (OIB) exhibit significant variations in their Sr-87/(86) Sr, Nd-143/Nd-144, Hf-176/Hf-177, Os-189,Os-187/Os-188, and Pb-206,Pb-207,Pb-208/Pb-204 compositions that indicate contributions from multiple mantle sources. To further constrain the relationships between diverse long-lived isotopic systems in OIB, new double-spike Pb and Hf isotope data, as well as isotope dilution highly sidemphile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundances are reported for nineteen Icelandic picrites and basalts for which He-3/He-4, Sr-87/Sr-86, Nd-143/Nd-144, and Os-186,Os-187/Os-188 data have been reported previously (Brandon et al., 2007. GCA). Hafnium-Nd-He isotope systematics for the sample set reveal the presence of an ancient refractory source with low-He-3/He-4, while He-Pb isotope correlations indicate that recycled oceanic crustal components with high-He-3/He-4 added from a less degassed mantle source are present in the Iceland plume. The highly depleted, low-He-3/He-4 signature is sampled in the Northern and Western Volcanic Zones, likely due to higher degrees of partial melting at those localities enabling sampling of this refractory endmember. The Os-187/Os-188 ratios show correlations with Pb, Hf, and Nd isotope ratios indicating binary mixing between depleted and enriched sources, with no evidence for decoupling due to sulfide metasomatism. The spread of Os-187/Os-188-Pb-206,Pb-207,Pb-208/Pb-204 data can be explained by addition of 1-11% young recycled oceanic crust to a depleted mantle source distinct from the source of Atlantic MORB. Abundances of the HSE in Northern and Western Volcanic Zone lavas overlap with those from other OIB with similar MgO contents, showing fractionated patterns that are enriched in the more incompatible Re, Pd, and Pt over the compatible Ru, Ir, and Os. Calculated parental melt HSE abundances of those lavas also overlap with those of other OIB. Eastern Volcanic Zone lavas are highly depleted in Pt and Pd relative to other Icelandic lavas. Abundances of Pt and Pd correlate with isotopic parameters, such as Pb-206/Pb-204, in Icelandic lavas, indicating that the isotopically enriched endmember sampled in the Eastern Volcanic Zone is depleted in the HSE, consistent with it being mafic in nature. The abundances of MgO in the lavas also roughly correlate with isotopic parameters, supporting the identity of the depleted endmember as ultramafic and the enriched endmember as mafic. New and previously published double-spike Pb isotope data show chiefly binary mixing with an additional component present in the vicinity of the volcano oraefajokull. Icelandic Pb-207/Pb-204 and Pb-208/Pb-204 data plot below and above the majority of Mid-Atlantic Ridge basalt data, respectively, indicating that both depleted and enriched end-members in the plume have a lower U depletion age and a higher Th/U than ambient mantle. This is consistent with the enriched and depleted end-members within the Iceland plume being <2 Ga recycled oceanic crust and lithospheric mantle, respectively.

Ocean anoxic events (OAE) are characterized by increased organic content of marine sediment on a global scale with accompanying positive excursions in sedimentary organic and inorganic δ 13C values. To sustain the increased C exports and burial required to explain the C isotope excursion, increased supplies of nutrients to the oceans are often invoked during ocean anoxic events. The potential source of nutrients in these events is investigated in this study for Oceanic Anoxic Event 2, which spans the Cenomanian-Turonian boundary. Massive eruptions of one or more Large Igneous Provinces (LIPs) are the proposed trigger for OAE 2. The global warming associated with volcanogenic loading of carbon dioxide to the atmosphere has been associated with increased continental weathering rates during OAE 2, and by extension, enhanced nutrient supplies to the oceans. Seawater interactions with hot basalts at LIP eruption sites can further deliver ferrous iron and other reduced metals to seawater that can stimulate increased productivity in surface waters and increased oxygen demand in deep waters. The relative importance of continental and submarine weathering drivers of expanding ocean anoxia during OAE 2 are difficult to disentangle. In this paper, a box model of the marine Sr cycle is used to constrain the timing and relative magnitudes of changes in the continental weathering and hydrothermal Sr fluxes to the oceans during OAE 2 using a new high-resolution record of seawater 87Sr/86Sr ratios preserved in a marl-limestone succession from the Iona-1 core collected from the Eagle Ford Formation in Texas. The results show that seawater 87Sr/86Sr ratios change synchronously with Os isotope evidence for the onset of massive LIP volcanism 60 kyr before the positive C isotope excursion that traditionally marks the onset of OAE 2. The higher temporal resolution of the seawater Sr isotope record presented in this study warrants a detailed quantitative analysis of the changes in continental weathering and hydrothermal Sr inputs to the oceans during OAE 2. Using an ocean Sr box model, it is found that increasing the continental weathering Sr flux by ∼1.8-times captures the change in seawater 87Sr/86Sr recorded in the Iona-1 core. The increase in the continental weathering flux is smaller than the threefold increase estimated by studies of seawater Ca isotope changes during OAE 2, suggesting that hydrothermal forcing may have played a larger role in the development of ocean anoxic events than previously considered

One of the prevailing hypotheses for the origin of the Younger Dryas (YD) cooling event is that it resulted from a bolide impact or airburst. Purported impact markers peak at or near the YD basal boundary layer at Northern Hemisphere locations. In this study, the Os-187/Os-188 ratios and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances in a well-dated sediment section through the Younger Dryas at the Debra L. Friedkin site, Texas are reported. Unradiogenic Os-187/Os-188 peaks, which could be mantle-derived or extraterrestrial, have been found above, within, and below the YD basal boundary layer. Mass balance mixing models using chondrites or iron meteorites with upper continental crust fail to duplicate the chondrite-normalized HSE patterns of the sediment samples. These HSE signatures in the Friedkin site section replicate those found in Hall's Cave, Texas. The new results here thus independently confirm that the HSE abundances in the unradiogenic Os layers are likely a fingerprint of volcanic gas aerosols derived from large Plinian eruptions and not extraterrestrial materials. To better constrain the lithological origins of YD sediments from the Friedkin and Hall's Cave sites, Texas, trace elements are presented here. The rare earth elements (REE) patterns and Ir, Ni, Ti and Zr abundances are also characterized with terrestrial signatures as opposed to impact melt rocks. An age profile correlation between the two study sites, further shows that three unradiogenic Os peaks overlap in time. The results are inconsistent with the extraterrestrial hypothesis and support instead an episodic and volcanic origin for the observed geochemical anomalies at the Debra L. Fried kin and Hall's Cave sites, Texas. (C) 2021 Elsevier Ltd. All rights reserved.

The geochemical behavior of Pb in terrestrial and coastal water systems significantly influences Pb biogeochemical cycling and pollutant exchange at the land-sea continuum. An ideal case study of Pb environmental geochemistry is Galveston Bay, an anthropogenic estuary exposed to industrial runoff, wastewater and shipping vessel spills but also fed by natural rivers. Here, sediments from Galveston Bay were measured for Pb isotope ratios and abundances to constrain Pb sources and fluxes and understand Pb pollution history in the bay. Lead isotopes have been established as source tracers of environmental pollution and allow Pb sources to be reliably fingerprinted and identified. Sediments were leached to distinguish authigenic sediment coatings from lithogenic residual sediments, in addition to bulk sediment digestions. Total Pb concentrations ranged from 1.76 µg/g–29.19 µg/g in bulk digests, which are below federal toxicity thresholds and aligns well with prior measurements of Pb in Galveston Bay sediments in the 20th century. Lead concentrations are spatially constrained by flocculation in eastern bay areas where the Trinity River enters the bay and positively temporally correlated to freshwater discharge. Sediment 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios range between 18.338–19.777±0.002, 15.557–15.755±0.002 and 37.913–43.340±0.005, respectively, and were used in an advanced Bayesian isotope mixing model to identify Pb sources in the Galveston Bay sediment fractions analyzed. Anthropogenic sources supply approximately 83.8%, 16.6% and 25.5% of Pb to the leachates, residues and bulk sediments, respectively. This study showcases the importance of estuaries in moderating terrestrial and marine Pb distribution and provides insight for future contaminant studies in Galveston Bay and other estuarine systems around the world.MAIN FINDINGS Pb isotope ratio and abundance measurements of Galveston Bay sediments demonstrate significant anthropogenic Pb inputs to the estuary despite low Pb levels.

Whether hydrogen incorporated in nominally anhydrous mantle minerals plays a role in the strength and longevity of the thick cratonic lithosphere is a matter of debate. In particular, the percolation of hydrogen-bearing melts and fluids could potentially add hydrogen to the mantle lithosphere, weaken its olivines (the dominant mineral in mantle peridotite), and cause delamination of the lithosphere's base. The influence of metasomatism on hydrogen contents of cratonic mantle minerals can be tested in mantle xenoliths from the Slave Craton (Canada) because they show extensive evidence for metasomatism of a layered cratonic mantle. Minerals from mantle xenoliths from the Diavik mine in the Lac de Gras kimberlite area located at the center of the Archean Slave craton were analyzed by FTIR for hydrogen contents. The 18 peridotites, two pyroxenites, one websterite and one wehrlite span an equilibration pressure range from 3.1 to 6.6 GPa and include samples from the shallow (<= 145 km), oxidized ultra-depleted layer; the deeper (similar to 145-180 km), reduced less depleted layer; and an ultra-deep (>= 180 km) layer near the base of the lithosphere. Olivine, orthopyroxene, clinopyroxene and garnet from peridotites contain 30-145, 110-225, 105-285, 2-105 ppm H2O, respectively. Within each deep and ultra-deep layer, correlations of hydrogen contents in minerals and tracers of metasomatism (for example light over heavy rare-earth-element ratio (LREE/HREE), high-field-strength-element (HFSE) content with equilibration pressure) can be explained by a chromatographic process occurring during the percolation of kimberlite-like melts through garnet peridotite. The hydrogen content of peridotite minerals is controlled by the compositions of the evolving melt and of the minerals and by mineral/melt partition coefficients. At the beginning of the process, clinopyroxene scavenges most of the hydrogen and garnet most of the HFSE. As the melt evolves and becomes enriched in hydrogen and LREE, olivine and garnet start to incorporate hydrogen and pyroxenes become enriched in LREE. The hydrogen content of peridotite increases with decreasing depth, overall (e.g., from 75 to 138 ppm H2O in the deep peridotites). Effective viscosity calculated using olivine hydrogen content for the deepest xenoliths near the lithosphere-asthenosphere boundary overlaps with estimates of asthenospheric viscosities. These xenoliths cannot be representative of the overall cratonic root because the lack of viscosity contrast would have caused basal erosion of lithosphere. Instead, metasomatism must be confined in narrow zones channeling kimberlite melts through the lithosphere and from where xenoliths are preferentially sampled. Such localized metasomatism by hydrogen-bearing melts therefore does not necessarily result in delamination of the cratonic root. (C) 2020 Elsevier Ltd. All rights reserved.

The Younger Dryas (YD) abrupt cooling event ca. 12.9 +/- 0.1 ka is associated with substantial meltwater input into the North Atlantic Ocean, reversing deglacial warming. One controversial and prevailing hypothesis is that a bolide impact or airburst is responsible for these environmental changes. Here, highly siderophile element (HSE; Os, Ir, Ru, Pt, Pd, and Re) abundances and Os-187/ Os-188 ratios were obtained in a well-dated sediment section at Hall's Cave, TX, USA to test this hypothesis. In Hall's Cave, layers below, above, and in the YD have Os-187/ Os-188 ratios consistent with incorporation of extraterrestrial or mantle-derived material. The HSE abundances indicate that these layers contain volcanic gas aerosols and not extraterrestrial materials. The most likely explanation is that episodic, distant volcanic emissions were deposited in Hall's Cave sediments. Coupled Os-187/ Os-188 ratios and HSE concentration data at close stratigraphic intervals are required to effectively differentiate between bolide and volcanic origins.