v1. Three ECHAM5-wiso model experiments for European region. For the first experiment (Europe_PI.nc), boundary conditions are set as pre-industrial and are identical to those in Botsyun et al., 2020. We also conducted two experiments with Middle Miocene boundary conditions reflecting two pCO2 settings (278 ppm and 450 ppm; Europe_Mio_278.nc and Europe_Mio_450.nc experiments) within current estimates of the Middle Miocene pCO2. v2. Global ECHAM5-wiso model experiments. geosp_simulations.nc contains surface geopotential for PI and Miocene experiments with modified Alpine topography and paleogeography.
v1. Three ECHAM5-wiso model experiments for European region. For the first experiment (Europe_PI.nc), boundary conditions are set as pre-industrial and are identical to those in Botsyun et al., 2020. We also conducted two experiments with Middle Miocene boundary conditions reflecting two pCO2 settings (278 ppm and 450 ppm; Europe_Mio_278.nc and Europe_Mio_450.nc experiments) within current estimates of the Middle Miocene pCO2. v2. Global ECHAM5-wiso model experiments. geosp_simulations.nc contains surface geopotential for PI and Miocene experiments with modified Alpine topography and paleogeography.
Abstract. Reconstructing Oligocene–Miocene paleoelevation contributes to our understanding of the evolutionary history of the European Alps and sheds light on geodynamic and Earth surface processes involved in the development of Alpine topography. Despite being one of the most intensively explored mountain ranges worldwide, constraints on the elevation history of the European Alps remain scarce. Here we present stable and clumped isotope measurements to provide a new paleoelevation estimate for the mid-Miocene (∼14.5 Ma) European Central Alps. We apply stable isotope δ–δ paleoaltimetry to near-sea-level pedogenic carbonate oxygen isotope (δ18O) records from the Northern Alpine Foreland Basin (Swiss Molasse Basin) and high-Alpine phyllosilicate hydrogen isotope (δD) records from the Simplon Fault Zone (Swiss Alps). We further explore Miocene paleoclimate and paleoenvironmental conditions in the Swiss Molasse Basin through carbonate stable (δ18O, δ13C) and clumped (Δ47) isotope data from three foreland basin sections in different alluvial megafan settings (proximal, mid-fan, and distal). Combined pedogenic carbonate δ18O values and Δ47 temperatures (30±5 ∘C) yield a near-sea-level precipitation δ18Ow value of -5.8±1.2 ‰ and, in conjunction with the high-Alpine phyllosilicate δD value of -14.6±0.3 ‰, suggest that the region surrounding the Simplon Fault Zone attained surface elevations of >4000 m no later than the mid-Miocene. Our near-sea-level δ18Ow estimate is supported by paleoclimate (iGCM ECHAM5-wiso) modeled δ18O values, which vary between −4.2 ‰ and −7.6 ‰ for the Northern Alpine Foreland Basin.
Abstract. The Paleocene-Eocene Thermal Maximum (PETM) offers insight into massive short-term carbon cycle perturbations that caused significant warming during a high-pCO2 world, affecting both marine and terrestrial ecosystems. PETM records from the marine-terrestrial interface (e.g. estuarine swamps and mire deposits) are, therefore, of great interest as their present-day counterparts are highly vulnerable to future climate and sea level change. Here, we assess paleoenvironmental changes of mid-latitudinal Late Paleocene-Early Eocene peat mire records along the paleo-North Sea coast. We provide carbon isotope data of bulk organic matter (δ13CTOC), organic carbon content (%TOC), and palynological data from an extensive peat mire deposited at a mid-latitudinal (ca. 41 °N) coastal site (Schöningen, Germany). The δ13CTOC data show a carbon isotope excursion (CIE) of −1.7 ‰ coeval with a conspicuous Apectodinium acme, calling for the presence of the PETM in this coastal section. Due to the exceptionally large stratigraphic thickness of the PETM at Schöningen (10 m of section) we established a detailed palynological record that indicates only minor changes in paleovegetation leading to and during the PETM. Instead, paleovegetation changes mostly follow natural successions in response to changes along the marine-terrestrial interface. Compared to other available peat mire records (Cobham, UK; Vasterival, France) it appears that wetland deposits around the Paleogene North Sea have a typical CIE magnitude of ca. −1.3 ‰ in δ13CTOC. Moreover, the Schöningen record shares major characteristics with the Cobham Lignite, including evidence for increased fire activity prior to the PETM, minor PETM-related plant species changes, a reduced CIE in δ13CTOC, and drowning of the mire (marine ingressions) during much of the PETM. This suggests that paleoenvironmental conditions during the Late Paleocene-Early Eocene, including the PETM, consistently affected major segments of the paleo-North Sea coast.
It is well known that a subtle nonlinearity can occur during clumped isotope analysis of CO2 that – if remaining unaddressed – limits accuracy. The nonlinearity is induced by a negative background on the m/z 47 ion Faraday cup, whose magnitude is correlated with the intensity of the m/z 44 ion beam. The origin of the negative background remains unclear, but is possibly due to secondary electrons. Usually, CO2 gases of distinct bulk isotopic compositions are equilibrated at 1000 °C and measured along with the samples in order to be able to correct for this effect. Alternatively, measured m/z 47 beam intensities can be corrected for the contribution of secondary electrons after monitoring how the negative background on m/z 47 evolves with the intensity of the m/z 44 ion beam. The latter correction procedure seems to work well if the m/z 44 cup exhibits a wider slit width than the m/z 47 cup. Here we show that the negative m/z 47 background affects precision of dual inlet-based clumped isotope measurements of CO2 unless raw m/z 47 intensities are directly corrected for the contribution of secondary electrons. Moreover, inaccurate results can be obtained even if the heated gas approach is used to correct for the observed nonlinearity. The impact of the negative background on accuracy and precision arises from small imbalances in m/z 44 ion beam intensities between reference and sample CO2 measurements. It becomes the more significant the larger the relative contribution of secondary electrons to the m/z 47 signal is and the higher the flux rate of CO2 into the ion source is set. These problems can be overcome by correcting the measured m/z 47 ion beam intensities of sample and reference gas for the contributions deriving from secondary electrons after scaling these contributions to the intensities of the corresponding m/z 49 ion beams. Accuracy and precision of this correction are demonstrated by clumped isotope analysis of three internal carbonate standards. The proposed correction scheme can be easily applied if the slit width of the m/z 49 Faraday cup is bigger than that of the m/z 47 cup.
High-precision analysis of the excess abundance (relative to the stochastic distribution) of mass 48 isotopologues in CO2 evolved from acid digestion of carbonates (∆48) has not been possible until recently due to the relatively low natural abundance of 18O. Here we show that the 253 Plus™ gas source mass spectrometer equipped with Faraday cups and 1013 Ω resistors can perform combined ∆47 and ∆48 analyses on carbonates with external reproducibilities (1SD) of 0.010 ‰ and 0.030 ‰, respectively. ~10 mg aliquots of five carbonate reference materials (ETH 1, ETH 2, ETH 3, ETH 4, and Carrara) are digested with phosphoric acid at 90 °C using a common acid bath. The evolved CO2 is purified using an automated gas preparation system (including cryotraps and a GC) and analyzed for its ∆47 and ∆48 compositions using the dual inlet system of a 253 Plus™ gas source mass spectrometer. Raw ∆47 and ∆48 values are finally normalized to the Carbon Dioxide Equilibrium Scale (CDES). In ∆47, CDES 90°C vs. ∆48, CDES 90°C space, calcite reference materials Carrara, ETH 3 and ETH 4 agree with the equilibrium curve for calcite after adding semi-empirically determined 90 °C acid fractionation factors of 0.196 ‰ (for ∆47) and 0.136 ‰ (for ∆48) to theoretical ∆63 and ∆64 data. Agreement between measured and theoretically expected ∆48, CDES 90°C highlights the accuracy of our high-precision clumped isotope analytical setup. Combined analysis of the abundances of mass 47 and mass 48 isotopologues in CO2 evolved from acid digestion of natural carbonates has excellent potential for the determination of accurate and highly precise paleotemperatures as well as for the identification of rate-limiting kinetic processes involved in biomineralization. A formation temperature of 15(±2) °C is obtained on the 95 % confidence level for the Upper Cretaceous chalk sample ETH 3.
Abstract Throughout the Late Pleistocene, millennial-scale cycles in the rate of poleward heat transport resulted in repeated heating and cooling of the Southern Ocean1. Ice sheet models2 suggest that this variation in Southern Ocean temperature can force fluctuations in the mass of the Antarctic ice sheet that transiently impact sea level by up to 15 meters. However, current geologic evidence for Antarctic ice response to this ocean thermal forcing is unable to calibrate these models, leaving large uncertainty in how Antarctica contributes to sea level on millennial timescales. Here we present a >100kyr archive of East Antarctic Ice Sheet response to Late Pleistocene millennial-scale climate cycles recorded by transitions from opal to calcite in subglacial precipitates. 234U-230Th dates for two precipitates define a time series for 32 mineralogic transitions that match Antarctic climate fluctuations, with precipitation of opal during cold periods and calcite during warm periods. Geochemical evidence indicates opal precipitation via cryoconcentration of silica in subglacial water and calcite precipitation from the admixture of meltwater flushed from the ice sheet interior. These freeze-flush cycles represent changes in subglacial hydrologic-connectivity driven by ice sheet thickness response to Southern Ocean temperature oscillations around the Ross Embayment. Changes in Ross Embayment ice mass require high ocean-ice heat exchange2, and would occur only after retreat of the West Antarctic Ice Sheet3 and large portions of the East Antarctic Ice sheet margin4. These results point to high Antarctic ice sheet sensitivity to millennial-scale ocean thermal forcing throughout the Late Pleistocene, and when combined with modeling results2, predict that an Antarctic ice volume of at least 2–5 meters sea level equivalent is vulnerable to millennial-scale climate forcing.
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