The Late Eocene 187Os / 188Os excursion : chemostratigraphy, cosmic dust flux and the early Oligocene glaciation

Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 241 (2006): 477-492, doi:10.1016/j.epsl.2005.11.035.High resolution records (ca. 100 kyr) of Os isotope composition (187Os/188Os) in bulk sediments from two tropical Pacific sites (ODP Sites 1218 and 1219) capture the complete Late Eocene 187Os/188Os excursion and confirm that the Late Eocene 187Os/188Os minimum, earlier reported by Ravizza and Peucker-Ehrenbrink [Earth Planet. Sci. Lett. 210 (2003) 151-165], is a global feature. Using the astronomically tuned age models available for these sites, it is suggested that the Late Eocene 187Os/188Os minimum can be placed at 34.5±0.1 Ma in the marine records. In addition, two other distinct features of the 187Os/188Os excursion that are correlatable among sections are proposed as chemostratigraphic markers which can serve as age control points with a precision of ca. ±0.1 Myr. We propose a speculative hypothesis that higher cosmic dust flux in the Late Eocene may have contributed to global cooling and early Oligocene glaciation (Oi-1) by supplying bio-essential trace elements to the oceans and thereby resulting in higher ocean productivity, enhanced burial of organic carbon and draw down of atmospheric CO2. To determine if the hypothesis that enhanced cosmic dust flux in the Late Eocene was a cause for the 187Os/188Os excursion can be tested by using the paired bulk sediment and leachate Os isotope composition, 187Os/188Os were also measured in sediment leachates. Results of analyses of leachates are inconsistent between the south Atlantic and the Pacific sites, and therefore do not yield a robust test of this hypothesis. Comparison of 187Os/188Os records with high resolution benthic foraminiferal δ18O records across the Eocene-Oligocene transition suggests that 187Os flux to the oceans decreased during cooling and ice growth leading to the Oi-1 glaciation, whereas subsequent decay of ice-sheets and deglacial weathering drove seawater 187Os/188Os to higher values. Although the precise timing and magnitude of these changes in weathering fluxes and their effects on the marine 187Os/188Os records are obscured by recovery from the Late Eocene 187Os/188Os excursion, evidence of the global influence of glaciation on supply of Os to the ocean is robust as it has now been documented in both Pacific and Atlantic records.This study was supported by NSF awards OCE-0118380, EAR-0215297 and EAR-0215297

Source apportionment of atmospheric trace gases and particulate matter: comparison of log-ratio and traditional approaches

In this paper we compare multivariate methods using both traditional approaches, which ignore issues of closure and provide relatively simple methods to deal with censored or missing data, and log-ratio methods to determine the sources of trace constituents in the atmosphere. The data set examined was collected from April to July 2008 at a sampling site near Woods Hole, Massachusetts, along the northeastern United States Atlantic coastline. The data set consists of trace gas mixing ratios (O3, SO2, NOx, elemental mercury [Hgo], and reactive gaseous mercury [RGM]), and concentrations of trace elements in fine (<2.5 μm) particulate matter (Al, As, Ba, Ca, Cd, Ce, Co, Cs, Fe, Ga, Hg, K, La, Mg, Mn, Na, P, Pb, Rb, Sb, Sr, Th, Ti, V, Y, and Zn) with varying percentages of censored values for each species. The data were separated into two subcompositions: s1, which is comprised by RGM and particulate Hg (HgP), which are both highly censored; and s2 which includes all of the trace elements associated with particulate matter except Hg, and the trace gases O3, SO2, NOx, and Hgo. Principal factor analysis (PFA) was successful in determining the primary sources for constituents in s2 using both traditional and log-ratio approaches. Using the traditional approach, regression between factor scores and RGM and particulate Hg concentrations suggested that none of the sources identified during PFA led to positive contributions of either reactive mercury compound. This finding is counter to most conventional thinking and is likely specious, resulting from removal of censored data (up to >80% of the entire dataset) during the analysis. Log-ratio approaches to find relationships between constituents comprising s2 with RGM and HgP (i.e., s1) focused on log-ratio correlation and regression analyses of alr-transformed data, using Al as the divisor. Regression models accounted for large fractions of the variance in concentrations of the two reactive mercury species and generally agreed with conceptualizations about the formation and behavior of these species. An analysis of independence between the subcompositions demonstrated that the behavior of the two constituents comprising s1 (i.e., RGM and HgP) is dependent on changes in s2. Our findings suggest that although problems related to closure are largely unknown or ignored in the atmospheric sciences, much insight can be gleaned from the application of log-ratio methods to atmospheric chemistry data

Constraining the marine strontium budget with natural strontium isotope fractionations (⁸⁷Sr/⁸⁶Sr*, δ^88/86Sr) of carbonates, hydrothermal solutions and river waters

We present strontium (Sr) isotope ratios that, unlike traditional 87Sr/86Sr data, are not normalized to a fixed 88Sr/86Sr ratio of 8.375209 (defined as δ88/86Sr = 0 relative to NIST SRM 987). Instead, we correct for isotope fractionation during mass spectrometry with a 87Sr–84Sr double spike. This technique yields two independent ratios for 87Sr/86Sr and 88Sr/86Sr that are reported as (87Sr/86Sr*) and (δ88/86Sr), respectively. The difference between the traditional radiogenic (87Sr/86Sr normalized to 88Sr/86Sr = 8.375209) and the new 87Sr/86Sr* values reflect natural mass-dependent isotope fractionation. In order to constrain glacial/interglacial changes in the marine Sr budget we compare the isotope composition of modern seawater ((87Sr/86Sr*, δ88/86Sr)Seawater) and modern marine biogenic carbonates ((87Sr/86Sr*, δ88/86Sr)Carbonates) with the corresponding values of river waters ((87Sr/86Sr*, δ88/86Sr)River) and hydrothermal solutions ((87Sr/86Sr*, δ88/86Sr)HydEnd) in a triple isotope plot. The measured (87Sr/86Sr*, δ88/86Sr)River values of selected rivers that together account for not, vert, similar18% of the global Sr discharge yield a Sr flux-weighted mean of (0.7114(8), 0.315(8)‰). The average (87Sr/86Sr*, δ88/86Sr)HydEnd values for hydrothermal solutions from the Atlantic Ocean are (0.7045(5), 0.27(3)‰). In contrast, the (87Sr/86Sr*, δ88/86Sr)Carbonates values representing the marine Sr output are (0.70926(2), 0.21(2)‰). We estimate the modern Sr isotope composition of the sources at (0.7106(8), 0.310(8)‰). The difference between the estimated (87Sr/86Sr*, δ88/86Sr)input and (87Sr/86Sr*, δ88/86Sr)output values reflects isotope disequilibrium with respect to Sr inputs and outputs. In contrast to the modern ocean, isotope equilibrium between inputs and outputs during the last glacial maximum (10–30 ka before present) can be explained by invoking three times higher Sr inputs from a uniquely “glacial” source: weathering of shelf carbonates exposed at low sea levels. Our data are also consistent with the “weathering peak” hypothesis that invokes enhanced Sr inputs resulting from weathering of post-glacial exposure of abundant fine-grained material

The spatial distribution of aeolian dust and terrigenous fluxes in the tropical Atlantic Ocean since the Last Glacial Maximum

© 2021. American Geophysical Union. All Rights Reserved. The flux of terrestrial material from the continents to the oceans links the lithosphere, hydrosphere, and biosphere through physical and biogeochemical processes, with important implications for Earth's climate. Quantitative estimates of terrigenous fluxes from sources such as rivers, aeolian dust, and resuspended shelf sediments are required to understand how the processes delivering terrigenous material respond to and are influenced by climate. We compile thorium-230 normalized 232Th flux records in the tropical Atlantic to provide an improved understanding of aeolian fluxes since the Last Glacial Maximum (LGM). By identifying and isolating sites dominated by aeolian terrigenous inputs, we show that there was a persistent meridional gradient in dust fluxes in the eastern equatorial Atlantic at the LGM, arguing against a large southward shift of the intertropical convergence zone during LGM boreal winter. The ratio of LGM to late-Holocene 232Th fluxes highlights a meridional difference in the magnitude of variations in dust deposition, with sites 700 km away, characterized by 232Th fluxes approximately twice as large as aeolian-dominated sites in the east

A fossil winonaite-like meteorite in Ordovician limestone: A piece of the impactor that broke up the L-chondrite parent body?

AbstractAbout a quarter of all meteorites falling on Earth today originate from the breakup of the L-chondrite parent body ∼470 Ma ago, the largest documented breakup in the asteroid belt in the past ∼3 Ga. A window into the flux of meteorites to Earth shortly after this event comes from the recovery of about 100 fossil L chondrites (1–21 cm in diameter) in a quarry of mid-Ordovician limestone in southern Sweden. Here we report on the first non-L-chondritic meteorite from the quarry, an 8 cm large winonaite-related meteorite of a type not known among present-day meteorite falls and finds. The noble gas data for relict spinels recovered from the meteorite show that it may be a remnant of the body that hit and broke up the L-chondrite parent body, creating one of the major asteroid families in the asteroid belt. After two decades of systematic recovery of fossil meteorites and relict extraterrestrial spinel grains from marine limestone, it appears that the meteorite flux to Earth in the mid-Ordovician was very different from that of today

Seasonal hydrology drives rapid shifts in the flux and composition of dissolved and particulate organic carbon and major and trace ions in the Fraser River, Canada

© The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Biogeosciences 12 (2015): 5597-5618, doi:10.5194/bg-12-5597-2015.Rapid changes in the volume and sources of discharge during the spring freshet lead to pronounced variations in biogeochemical properties in snowmelt-dominated river basins. We used daily sampling during the onset of the freshet in the Fraser River (southwestern Canada) in 2013 to identify rapid changes in the flux and composition of dissolved material, with a focus on dissolved organic matter (DOM). Previous time series sampling (at twice monthly frequency) of dissolved inorganic species in the Fraser River has revealed smooth seasonal transitions in concentrations of major ions and tracers of water and dissolved load sources between freshet and base flow periods. In contrast, daily sampling reveals a significant increase in dissolved organic carbon (DOC) concentration (200 to 550 μmol L−1) occurring over a matter of days, accompanied by a shift in DOM optical properties, indicating a transition towards higher molecular weight, more aromatic DOM composition. Comparable changes in DOM composition, but not concentration, occur at other times of year, underscoring the role of seasonal climatology in DOM cycling. A smaller data set of total and dissolved Hg concentrations also showed variability during the spring freshet period, although dissolved Hg dynamics appear to be driven by factors beyond DOM as characterized here. The time series records of DOC and particulate organic carbon (POC) concentrations indicate that the Fraser River exports 0.25–0.35 % of its annual basin net primary productivity. The snowmelt-dominated hydrology, forested land cover, and minimal reservoir impoundment of the Fraser River may influence the DOC yield of the basin, which is high relative to the nearby Columbia River and of similar magnitude to that of the Yukon River to the north. Anticipated warming and decreased snowfall due to climate changes in the region may cause an overall decrease in DOM flux from the Fraser River to the coastal ocean in coming decadesThis work was partially supported by a WHOI Ocean Ventures Fund award to BMV and NSF grants EAR-1226818 to BPE, OCE-0851015 to TIE, BPE, and VG, and OCE-0851101 to RGMS, and support to BPE from Jane and James Orr

Tracing river chemistry in space and time : dissolved inorganic constituents of the Fraser River, Canada

Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 124 (2014): 283-308, doi:10.1016/j.gca.2013.09.006.The Fraser River basin in southwestern Canada bears unique geologic and climatic features which make it an ideal setting for investigating the origins, transformations and delivery to the coast of dissolved riverine loads under relatively pristine conditions. We present results from sampling campaigns over three years which demonstrate the lithologic and hydrologic controls on fluxes and isotope compositions of major dissolved inorganic runoff constituents (dissolved nutrients, major and trace elements, 87Sr/86Sr, δD). A time series record near the Fraser mouth allows us to generate new estimates of discharge-weighted concentrations and fluxes, and an overall chemical weathering rate of 32 t km-2 y-1. The seasonal variations in dissolved inorganic species are driven by changes in hydrology, which vary in timing across the basin. The time series record of dissolved 87Sr/86Sr is of particular interest, as a consistent shift between higher (“more radiogenic”) values during spring and summer and less radiogenic values in fall and winter demonstrates the seasonal variability in source contributions throughout the basin. This seasonal shift is also quite large (0.709 – 0.714), with a discharge-weighted annual average of 0.7120 (2 s.d. = 0.0003). We present a mixing model which predicts the seasonal evolution of dissolved 87Sr/86Sr based on tributary compositions and water discharge. This model highlights the importance of chemical weathering fluxes from the old sedimentary bedrock of headwater drainage regions, despite their relatively small contribution to the total water flux.This work was supported by the WHOI Academic Programs Office and MIT PAOC Houghton Fund to BMV, a WHOI Arctic Research Initiative grant to ZAW, NSF-ETBC grant OCE-0851015 to BPE and TIE, and NSF grant EAR-1226818 to BPE

Climate control on terrestrial biospheric carbon turnover

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Eglinton, T. I., Galy, V. V., Hemingway, J. D., Feng, X., Bao, H., Blattmann, T. M., Dickens, A. F., Gies, H., Giosan, L., Haghipour, N., Hou, P., Lupker, M., McIntyre, C. P., Montluçon, D. B., Peucker-Ehrenbrink, B., Ponton, C., Schefuß, E., Schwab, M. S., Voss, B. M., Wacker, L., Wu, Y., & Zhao, M. Climate control on terrestrial biospheric carbon turnover. Proceedings of the National Academy of Sciences of the United States of America, 118(8), (2021): e2011585118, htps://doi.org/ 10.1073/pnas.2011585118.Terrestrial vegetation and soils hold three times more carbon than the atmosphere. Much debate concerns how anthropogenic activity will perturb these surface reservoirs, potentially exacerbating ongoing changes to the climate system. Uncertainties specifically persist in extrapolating point-source observations to ecosystem-scale budgets and fluxes, which require consideration of vertical and lateral processes on multiple temporal and spatial scales. To explore controls on organic carbon (OC) turnover at the river basin scale, we present radiocarbon (14C) ages on two groups of molecular tracers of plant-derived carbon—leaf-wax lipids and lignin phenols—from a globally distributed suite of rivers. We find significant negative relationships between the 14C age of these biomarkers and mean annual temperature and precipitation. Moreover, riverine biospheric-carbon ages scale proportionally with basin-wide soil carbon turnover times and soil 14C ages, implicating OC cycling within soils as a primary control on exported biomarker ages and revealing a broad distribution of soil OC reactivities. The ubiquitous occurrence of a long-lived soil OC pool suggests soil OC is globally vulnerable to perturbations by future temperature and precipitation increase. Scaling of riverine biospheric-carbon ages with soil OC turnover shows the former can constrain the sensitivity of carbon dynamics to environmental controls on broad spatial scales. Extracting this information from fluvially dominated sedimentary sequences may inform past variations in soil OC turnover in response to anthropogenic and/or climate perturbations. In turn, monitoring riverine OC composition may help detect future climate-change–induced perturbations of soil OC turnover and stocks.This work was supported by grants from the US NSF (OCE-0928582 to T.I.E. and V.V.G.; OCE-0851015 to B.P.-E., T.I.E., and V.V.G.; and EAR-1226818 to B.P.-E.), Swiss National Science Foundation (200021_140850, 200020_163162, and 200020_184865 to T.I.E.), and National Natural Science Foundation of China (41520104009 to M.Z.)