Bernhard Peucker-Ehrenbrink

The marine187Os/186Os record of the past 80 million years

We report new187Os/186Os data and Re and Os concentrations in metalliferous sediments from the Pacific to construct a composite Os isotope seawater evolution curve over the past 80 m.y. Analyses of four samples of upper Cretaceous age yield187Os/186Os values of between 3 and 6.5 and187Re/186Os values below 55. Mass balance calculations indicate that the pronounced minimum of about 2 in the Os isotope ratio of seawater at the K-T boundary probably reflects the enormous input of cosmogenic material into the oceans by the K-T impactor(s). Following a rapid recovery to187Os/186Os of 3.5 at 63 Ma, data for the early and middle part of the Cenozoic show an increase in187Os/186Os to about 6 at 15 Ma. Variations in the isotopic composition of leachable Os from slowly accumulating metalliferous sediments show large fluctuations over short time spans. In contrast, analyses of rapidly accumulating metalliferous carbonates do not exhibit the large oscillations observed in the pelagic clay leach data. These results together with sediment leaching experiments indicate that dissolution of non-hydrogenous Os can occur during the hydrogen peroxide leach and demonstrate that Os data from pelagic clay leachates do not always reflect the Os isotopic composition of seawater. New data for the late Cenozoic further substantiate the rapid increase in the187Os/186Os of seawater during the past 15 Ma. We interpret the correlation between the marine Sr and Os isotope records during this time period as evidence that weathering within the drainage basin of the Ganges-Brahmaputra river system is responsible for driving seawater Sr and Os toward more radiogenic isotopic compositions. The positive correlation between87Sr/86Sr and U concentration, the covariation of U and Re concentrations, and the high dissolved Re, U and Sr concentrations found in the Ganges-Brahmaputra river waters supports this interpretation. Accelerating uplift of many orogens worldwide over the past 15 Ma, especially during the last 5 Ma, could have contributed to the rapid increase in187Os/186Os from 6 to 8.5 over the past 15 Ma. Prior to 15 Ma the marine Sr and Os record are not tightly coupled. The heterogeneous distribution of different lithologies within eroding terrains may play an important role in decoupling the supplies of radiogenic Os and Sr to the oceans and account for the periods of decoupling of the marine Sr and Os isotope records.

A rain of ordinary chondritic meteorites in the early Ordovician

Forty fossil meteorites with a total original mass of V7.7 kg have been recovered in the first systematic search for fossil meteorites, pursued in an active quarry in Lower Ordovician (480 Ma) marine limestone in southern Sweden. The meteorites represent at least 12 different falls over a seafloor area of V6000 m 2 during 91.75 Myr, making the quarry one of the most meteorite dense areas known in the world. Geochemical analyses of relict chromite grains indicate that all or most of the meteorites are ordinary chondrites and probably L chondrites. Mechanisms for meteorite delivery from the asteroid belt to Earth were the same 480 Ma as today, however, the flux was one to two orders of magnitude higher, most likely reflecting the disruption of the L chondrite parent body at about that time. This is a major event in late solar-system history, which may also have led to an enhanced flux of asteroids to Earth during V30 Myr. fl 2001 Elsevier Science B.V. All rights reserved.

Mobility of rhenium, platinum group elements and organic carbon during black shale weathering

This study investigates the effects of black shale weathering on the Re–Os isotope system, platinum group element concentrations and the degradation of organic matter. Samples from a weathering profile in Late Devonian (∼365 Myr) Ohio Shale show a pronounced decrease (∼77%) in organic carbon (Corg) near the present soil surface, relative to the interior portion of the outcrop. A similar trend is observed for total N (∼67% loss). Conversely, organic phosphorus (Porg) concentrations increase by ∼59% near the soil surface. The decrease in Corg is accompanied by a pronounced decrease in Re (∼99%) and, to a lesser extent, Os (∼39%). Palladium and Pt do not appear to be significantly mobile. The effects of Re and Os mobility on the Re–Os isotope system are significant: none of the samples plots on a 365 Myr isochron. Rather, the samples define a trend with a slope corresponding to an age of ∼18 Myr with an initial 187Os/188Os of ∼6.1. This indicates recent disturbance of the Re–Os system. Isotope mass balance calculations imply that the labile fraction of Os is significantly more radiogenic (187Os/188Os of ∼7.8) than the average of the unweathered samples (187Os/188Os of ∼6.4). Based on data from this study, the molar ratio of labile Re to Corg in Ohio Shale is estimated at 7×10−8. We estimate the present-day riverine, black shale-derived Re flux to seawater using literature data on Re burial in anoxic marine sediments, and assuming steady-state between Re release during black shale weathering and Re burial in anoxic marine sediments. Then, the labile Re/Corg observed in this study implies that ∼0.5 Tmol of Corg is released annually from weathering of black shales, a trace lithology in the continental crust. This flux corresponds to ∼12% of the estimated annual CO2 flux from oxidative weathering of sedimentary rocks. The labile molar Re/Os of ∼270 indicates that black shale weathering releases ∼130 mol Os per year, which accounts for ∼7% of the riverine Os input to seawater. The data from this study support the notion that the crustal cycles of labile Corg, Re and Os are tightly coupled. Gray shales, which are less Corg-, Re- and Os-rich, but much more abundant in the continental crust than black shales, are likely to be even more important continental sources of Re and Os to seawater.

Global carbon export from the terrestrial biosphere controlled by erosion

Riverine export of particulate organic carbon (POC) to the ocean affects the atmospheric carbon inventory over a broad range of timescales. On geological timescales, the balance between sequestration of POC from the terrestrial biosphere and oxidation of rock-derived (petrogenic) organic carbon sets the magnitude of the atmospheric carbon and oxygen reservoirs. Over shorter timescales, variations in the rate of exchange between carbon reservoirs, such as soils and marine sediments, also modulate atmospheric carbon dioxide levels. The respective fluxes of biospheric and petrogenic organic carbon are poorly constrained, however, and mechanisms controlling POC export have remained elusive, limiting our ability to predict POC fluxes quantitatively as a result of climatic or tectonic changes. Here we estimate biospheric and petrogenic POC fluxes for a suite of river systems representative of the natural variability in catchment properties. We show that export yields of both biospheric and petrogenic POC are positively related to the yield of suspended sediment, revealing that POC export is mostly controlled by physical erosion. Using a global compilation of gauged suspended sediment flux, we derive separate estimates of global biospheric and petrogenic POC fluxes of and megatonnes of carbon per year, respectively. We find that biospheric POC export is primarily controlled by the capacity of rivers to mobilize and transport POC, and is largely insensitive to the magnitude of terrestrial primary production. Globally, physical erosion rates affect the rate of biospheric POC burial in marine sediments more strongly than carbon sequestration through silicate weathering. We conclude that burial of biospheric POC in marine sediments becomes the dominant long-term atmospheric carbon dioxide sink under enhanced physical erosion.

Rapid determination of Os isotopic composition by sparging OsO4 into a magnetic-sector ICP-MS

Abstract We present a method for the rapid determination of Os isotopic compositions and platinum group element (PGE) concentrations using ICP-MS on the same sample split. The method makes use of transfer of volatile OsO4 by an Ar gas stream into the torch of a magnetic sector ICP-MS for analysis. A variety of partial or complete sample dissolution methods can be used, namely (1) microwave oven digestion of samples in pressurized Teflon vessels under oxidizing acidic conditions, (2) NiS fire assay for complete sample dissolution and PGE preconcentration, (3) acid leaching techniques to extract labile PGE fractions, and (4) Carius tube digestion. Microwave oven sample digestion with HF–HNO3–HCl in an all-Teflon system as well as Carius tube digestion also allows the determination of Os isotopic composition and Os and Re concentrations on the same sample split. Spike–sample equilibration can be monitored on-line during sparging into the ICP-MS. The elimination of a nebulizer for Os isotope ratio measurements minimizes memory problems that are often associated with the determination of Os isotope ratios in liquid samples by ICP-MS. The external reproducibility of 187 Os/ 188 Os measurements of an in-house Os standard solution using a single-collector, magnetic sector ICP-MS (Finnigan ELEMENT) is 0.78% (1 S.D., n=13) for analyte amounts ranging from 81 pg to 1.22 ng total Os. The ion yield for 7-min data acquisitions is ∼5×10−5. Compared to negative thermal ionization mass spectrometry (N-TIMS), sample throughput is about on an order of magnitude higher. If NiS fire assay is used for PGE preconcentration, complementary PGE can be determined in the liquid residue by ICP-MS using a microconcentric nebulizer after the extraction of OsO4.

Effects of black shale weathering on the mobility of rhenium and platinum group elements

Radiogenic Os from the continents dominates the late Cenozoic marine Os budget. A variety of proposed radiogenic sources includes organic-rich sediments, Precambrian shields, and Re-rich sulfides. Identifying the dominant sources of radiogenic Os to the oceans has a strong impact on past climate models because weathering of silicates acts as a net sink of CO 2 , whereas weathering of organic matter acts as a source. Here we investigate the effects of black shale weathering on the integrity of the Re-Os isotope system and release of Re and platinum group elements. Time-correlative pairs of fresh (drill core) and weathered (outcrop) black shales of Late Ordovician age (Utica Shale magnafacies, Quebec) were analyzed for Re, Os, Ir, Pt, and Pd, as well as total S, N, and C (as C carb and C org ). The results indicate that 25%–64% of the initial Re budget, 45%–90% of the Os budget, 14%–65% of the Ir budget, 60%–77% of the Pt budget, 69%–86% of the Pd budget, and 44%–96% of the C org budget are lost during weathering. We use positive correlations of Os, Pt, and Pd with C org and N to calculate the respective platinum group element concentrations in the unweathered organic matter end member as ∼3 ng/g Os, ∼9 ng/g Pt, and 11–24 ng/g Pd. Although black shales make up

Hydrothermal lead transfer from mantle to continental crust: the role of metalliferous sediments

Abstract The amount of lead annually transferred from oceanic crust to metalliferous sediments was estimated in order to test the hypothesis that a non-magmatic flux of lead causes the Pb surplus in the continental crust. A Pb surplus has been inferred from global crust-mantle lead mass balances derived from lead concentration correlations with other trace elements and from lead isotope systematics in oceanic basalts. DSDP/ODP data on the amount of metalliferous sediments in the Pacific Ocean and along a South Atlantic traverse are used to calculate the mean worldwide thickness of 3(±1) m for purely metalliferous sediment componens. Lead isotope ratios of 39 metalliferous sediments from the Pacific define mixing lines between continent-derived (seawater) and mantle-derived (basaltic) lead, with the most metal-rich sediments usually having the most mantle-like Pb isotope composition. We used this isotope correlation and the Pb content of the 39 metalliferous sediments to derive an estimate of 130(±70) μg/g for the concentration of mantle-derived lead in the purely metalliferous end-member. Mass balance calculations show that at least 12(±8)% of the lead, annually transferred from upper mantle to oceanic crust at the ocean ridges, is leached out by hydrothermal processes and re-deposited in marine sediments. If all of the metalliferous lead is ultimately transferred to the continental crust during subduction, the annual flux of this lead from mantle to continental crust is 2.6(±2.0) × 106 kg. Assuming this transfer rate to be proportional to the rate of oceanic plate production, one can fit the lead transfer to models of plate production rate variations through time. Integrating over 4 Ga, hydrothermal lead transfer to the continental crust accounts for a significant portion of the Pb surplus in the continental crust. It therefore appears to be one of the main reasons for the anomalous behavior of lead in the global crust-mantle system.

Geochemistry of the end-Permian extinction event in Austria and Italy: No evidence for an extraterrestrial component

The end-Permian mass extinction (251 Ma) was the largest in Earths history, and the great extent of biospheric perturbation is recorded as dramatic shifts in carbon isotope ratios of sedimentary materials. Both terrestrial and extraterrestrial events are commonly invoked as causative mechanisms for the crisis, and the primary reason for the event remains the subject of controversy. Geochemical indicators sensitive to the influence of extraterrestrial material involve platinum group elements and osmium and helium isotope ratios. Analyses of extinction levels in two sections from Austria and Italy reveal no evidence of an extraterrestrial impact. The end-Permian crisis, it appears, was a homegrown catastrophe.

The effects of sampling artifacts on cosmic dust flux estimates: a reevaluation of nonvolatile tracers (Os, Ir)

We reevaluate the effects of sampling artifacts on estimates of the flux of cosmic dust to Earth by modeling the population of extraterrestrial particles as a function of surface–area and time–interval sampled. Results from a Monte Carlo simulation for nonvolatile tracers such as Os and Ir indicate that samples corresponding to ≥2.5 m2a adequately sample the incoming extraterrestrial particle population. This applies to nearly all extraterrestrial flux estimates based on Os isotope data for marine sediments. New and revised flux estimates based on Os isotopes in marine sediments constrain the flux of extraterrestrial matter to the sea floor to 30,000 ± 15,000 metric tons per year, within error identical to estimates of the flux of extraterrestrial matter to the top of Earth’s atmosphere. Care must be taken in the interpretation of iridium data for ice and snow samples because such samples are often too small to adequately sample the extraterrestrial particle population. The effects of systematic bias include large scatter in the data and underestimating the extraterrestrial matter flux.

Re-Os isotope systematics and weathering of Precambrian crustal rocks: Implications for the marine osmium isotope record

The effects of weathering of freshly exposed Precambrian rocks on the mobility of osmium were studied on soils developed on a chronosequence of glacial moraines from the western Wind River Range in Wyoming. The Os budget of the Precambrian granitoid source rocks is dominated by Os-rich trace phases, such as magnetite. Amongst the major silicates, biotite, or a trace phase in biotite, carries most of the Os and Re and is also the most radiogenic mineral with 187Os/186Os of ∼113 and 187Re/186Os of ∼2,000. Re-Os isotope systematics of source rocks and soils indicate that rapid oxidation of magnetite mobilizes Os with an isotopic composition similar to the isotopic composition of the bulk soils. A very radiogenic fraction of Os is mobilized through preferential weathering of biotite. Radiogenic runoff from Precambrian shields, inferred from osmium isotope analyses of freshwater Fe-Mn-nodules suggests that high-latitude Precambrian shields are important source areas of radiogenic Os in seawater. We propose that glacial scouring and weathering of glacial tills exposed after deglaciation of Precambrian shields surrounding the North Atlantic provides a mechanism for the slightly more radiogenic nature of North Atlantic seawater compared to other seawater masses. Glacial-interglacial variations in the osmium isotopic composition of seawater seem plausible and may be triggered by changes in weathering regimes on glacial-interglacial time scales in high-latitude shield areas surrounding the North Atlantic.