Der Phanerozoische δ(88/86)Sr Datensatz mariner Karbonate - Implikationen zur Meerwasserchemie

In the light of increasing interest in the role of the world’s oceans in future global change, it is important to understand the underlying processes that were responsible for paleoceanographic changes in the geological past. Previous studies working on the geochemistry of ancient marine deposits agree that the seawater chemistry must have been changed throughout the Phanerozoic Eon. In particular, concurrent long-term changes in radiogenic and stable isotope systems, element ratios and concentrations and their correlation with sea-level changes, climate reconstructions, and global mass extinctions suggests common causative mechanisms. However, the geological processes being responsible for the observed changes are still being debated. Specifically, the role of mid ocean spreading rates, dolomitization and sea-level changes are thought to play a major role in paleo-seawater chemistry of major and trace elements. Within this study the first Phanerozoic stable strontium (Sr) isotope seawater record (δ(88/86)Sr-sw) is reconstructed, which is sensitive to imbalances in the Sr input and output fluxes. In a consequent model approach, the radiogenic Sr isotope record (87-Sr/86-Sr)sw and δ(88/86)Sr-sw are used to constrain the marine Phanerozoic Sr budget. On long timescales (~200Myr periodicity), δ(88/86)Sr-sw and modelled Sr carbonate burial rates (F(Sr)carb) follow times of proposed „aragonite seas“ and „calcite seas“, implying that the dominant carbonate mineralogy has an important effect on Sr burial rates. On shorter timescales, minima and maxima in F(Sr)carb are partly correlated to ocean anoxia and glaciations and related sea-level low stands, implying the importance of continental carbonate shelf weathering to the marine Sr budget. In particular, enduring high carbonate burial rates for ~21Myr could be related to seawater anoxia during the end-Permian mass extinctions. Here, bacterial sulphate reduction rates led to toxic and high alkaline deep waters that were intermittently upwelled to the surface ocean, causing massive carbonate precipitation on the seafloor as well as the largest biogeochemical crisis in the Phanerozoic Eon. Ultimately, insights from changes in δ(88/86)Sr-sw significantly improved our understanding of long-term changes in seawater chemistry and the relation of carbonate-related Sr fluxes to sea-level changes, mass extinctions, and global anoxia.In Anbetracht des zunehmenden Interesses an der Rolle der Weltmeere im zukünftigen globalen Wandel, ist es wichtig die zugrunde liegenden Prozesse für die paläozeanographischen Veränderungen der geologischen Vergangenheit zu verstehen. Die bisherigen geochemischen Arbeiten an alten marinen Ablagerungen stimmen dabei überein, dass sich die Meerwasserchemie im Phanerozoikum geändert haben muss. Insbesondere die langfristigen Veränderungen in radiogenen und stabilen Isotopensystemen, Element-verhältnissen und –konzentrationen, sowie deren Korrelation mit Meeresspiegeländerungen, Klimarekonstruktionen und globalen Massenaussterbeereignissen weisen auf gemeinsame kausale Mechanismen hin. Allerdings sind die dafür verantwortlichen geologischen Prozesse noch in der Diskussion. Insbesondere die Prozesse der Ozeankrustenproduktion an mittelozeanischen Rücken, der Dolomitisierung und Meeresspiegeländerungen werden in Betracht gezogen, eine wichtige Rolle in der Paläo-Meerwasserchemie der Haupt- und Spurenelementen zu spielen. Im Rahmen dieser Studie wurde der erste phanerozoische stabile Strontium (Sr)-Isotopen Datensatz des Meerwassers (δ(88/86)Sr-sw) erstellt, welcher empfindlich auf Ungleichgewichte zwischen den Sr Quellen und Senken reagiert. In einem darauf folgenden Modell-Ansatz wurden die radiogenen (87-Sr/86-Sr) und stabilen (δ(88/86)Sr-sw) Daten verwendet, um das marine phanerozoische Sr Budget zu bilanzieren. Auf langen Zeitskalen (~ 200Mio. Jahre Periodizität) folgen δ88/86Srsw und die modellierten Sr Sedimentationsflüsse (F(Sr)carb) den Perioden von „Aragonit Meeren“ und „Calcit Meeren“, was bedeutet, dass die dominante Karbonatmineralogie einen wichtigen Einfluss auf die Sr Sedimentation hat. Auf kürzeren Zeitskalen sind Minima und Maxima in F(Sr)carb teilweise mit marinen Anoxien sowie Vereisungen und den damit verbundenen Meeresspiegeländerungen korreliert, welches die Bedeutung der kontinentalen Karbonatschelfverwitterung für das Sr Budget anzeigt. Insbesondere die ~21Mio. Jahre anhaltendenden hohen Karbonatsedimentationsraten am Ende des Perms konnten zur Meerwasseranoxie während der Massenaussterbens in Bezug gesetzt werden. Hier führte die bakterielle Sulfatreduktion zu giftigen und stark alkalischen Tiefengewässern, die zeitweise zur Meeresoberfläche aufgetrieben wurden, was zu einer massiven Karbonatausfällung auf dem Meeresboden, sowie zur bisher größten biogeochemischen Krise im Phanerozoikum geführt hat. Letztendlich haben die Erkenntnisse aus den Veränderungen im δ(88/86)Sr-sw unser Verständnis von langfristigen Veränderungen in der Meerwasserchemie, sowie den Zusammenhang zwischen marine Karbonatflüssen zu Meeresspiegeländerungen, Massenaussterben und globaler Anoxie, deutlich verbessert

The isotope composition of selenium in chondrites constrains the depletion mechanism of volatile elements in solar system materials

Solar nebula processes led to a depletion of volatile elements in different chondrite groups when compared to the bulk chemical composition of the solar system deduced from the Sun's photosphere. For moderately-volatile elements, this depletion primarily correlates with the element condensation temperature and is possibly caused by incomplete condensation from a hot solar nebula, evaporative loss from the precursor dust, and/or inherited from the interstellar medium. Element concentrations and interelement ratios of volatile elements do not provide a clear picture about responsible mechanisms. Here, the abundance and stable isotope composition of the moderately- to highly-volatile element Se are investigated in carbonaceous, ordinary, and enstatite chondrites to constrain the mechanism responsible for the depletion of volatile elements in planetary bodies of the inner solar system and to define a δ(82/78)Se value for the bulk solar system. The δ(82/78)Se of the studied chondrite falls are identical within their measurement uncertainties with a mean of −0.20±0.26‰ (2 s.d., n=14n=14, relative to NIST SRM 3149) despite Se abundance depletions of up to a factor of 2.5 with respect to the CI group. The absence of resolvable Se isotope fractionation rules out a kinetic Rayleigh-type incomplete condensation of Se from the hot solar nebula or partial kinetic evaporative loss on the precursor material and/or the parent bodies. The Se depletion, if acquired during partial condensation or evaporative loss, therefore must have occurred under near equilibrium conditions to prevent measurable isotope fractionation. Alternatively, the depletion and cooling of the nebula could have occurred simultaneously due to the continuous removal of gas and fine particles by the solar wind accompanied by the quantitative condensation of elements from the pre-depleted gas. In this scenario the condensation of elements does not require equilibrium conditions to avoid isotope fractionation. The results further suggest that the processes causing the high variability of Se concentrations and depletions in ordinary and enstatite chondrites did not involve any measurable isotope fractionation. Different degrees of element depletions and isotope fractionations of the moderately-volatile elements Zn, S, and Se in ordinary and enstatite chondrites indicate that their volatility is controlled by the thermal stabilities of their host phases and not by the condensation temperature under canonical nebular conditions

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 Phanerozoic δ88/86Sr Record of Seawater: New Constraints on Past Changes in Oceanic Carbonate Fluxes

The isotopic composition of Phanerozoic marine sediments provides important information about changes in seawater chemistry. In particular, the radiogenic strontium isotope (87Sr/86Sr) system is a powerful tool for constraining plate tectonic processes and their influence on atmospheric CO2 concentrations. However, the 87Sr/86Sr isotope ratio of seawater is not sensitive to temporal changes in the marine strontium (Sr) output flux, which is primarily controlled by the burial of calcium carbonate (CaCO3) at the ocean floor. The Sr budget of the Phanerozoic ocean, including the associated changes in the amount of CaCO3 burial, is therefore only poorly constrained. Here, we present the first stable isotope record of Sr for Phanerozoic skeletal carbonates, and by inference for Phanerozoic seawater (δ88/86Srsw), which we find to be sensitive to imbalances in the Sr input and output fluxes. This δ88/86Srsw record varies from ∼0.25‰ to ∼0.60‰ (vs. SRM987) with a mean of ∼0.37‰. The fractionation factor between modern seawater and skeletal calcite Δ88/86Srcc-sw, based on the analysis of 13 modern brachiopods (mean δ88/86Sr of 0.176±0.016‰, 2 standard deviations (s.d.)), is -0.21‰ and was found to be independent of species, water temperature, and habitat location. Overall, the Phanerozoic δ88/86Srsw record is positively correlated with the Ca isotope record (δ44/40Casw), but not with the radiogenic Sr isotope record ((87Sr/86Sr)sw). A new numerical modeling approach, which considers both δ88/86Srsw and (87Sr/86Sr)sw, yields improved estimates for Phanerozoic fluxes and concentrations for seawater Sr. The oceanic net carbonate flux of Sr (F(Sr)carb) varied between an output of -4.7x1010mol/Myr and an input of +2.3x1010mol/Myr with a mean of -1.6x1010mol/Myr. On time scales in excess of 100Myrs the F(Sr)carb is proposed to have been controlled by the relative importance of calcium carbonate precipitates during the “aragonite” and “calcite” sea episodes. On time scales less than 20Myrs the F(Sr)carb seems to be controlled by variable combinations of carbonate burial rate, shelf carbonate weathering and recrystallization, ocean acidification, and ocean anoxia. In particular, the Permian/Triassic transition is marked by a prominent positive δ88/86Srsw-peak that reflects a significantly enhanced burial flux of Sr and carbonate, likely driven by bacterial sulfate reduction (BSR) and the related alkalinity production in deeper anoxic waters. We also argue that the residence time of Sr in the Phanerozoic ocean ranged from ∼1Myrs to ∼20Myrs

High-Precision Measurement of Os-187/Os-188 Isotope Ratios of Nanogram to Picogram Amounts of Os in Geological Samples by N-TIMS using Faraday Cups Equipped with 10(13) omega Amplifiers

N(Os-187)/N(Os-188) ratios of six geological reference materials were measured using static Faraday cups (FCs) with 10(13) omega amplifiers by N-TIMS. Our results show that the repeatability precision was 2-3 parts per thousand (2 RSD, n = 3), when taking ~ 1 g of BHVO-2 with 76 pg g(-1) of Os mass fraction and ~ 2 g of BCR-2 with 21 pg g(-1) of Os mass fraction for each sample, whether measured by FCs or by secondary electron multiplier. The repeatability precision measured by FCs was 1-0.2 parts per thousand (2 RSD, n = 3) when taking ~ 1 g of BIR-2 with 350 pg g(-1) of Os mass fraction, ~ 1 g of WGB-1 with 493 pg g(-1) of Os mass fraction or ~ 0.5 g of WPR-1 with 13.3 ng g(-1) of Os mass fraction for each sample, which is much better than those measured by secondary electron multiplier. Instead, when taking ~ 2 g of AGV-2 with 4 pg g(-1) Os mass fraction, the repeatability precision measured by secondary electron multiplier is 3-4 parts per thousand (RSD, n = 3), which is better than those measured by FCs. Of the six reference materials analysed, WPR-1 and BIR-1a are the most homogeneous with regard to Os isotopic composition (2 RSD of 0.08% and 0.23%, respectively) when test portion masses are 0.5-1 g