Search

The stable isotopic records of ostracode valves deposited during thelast interglaciation in Raymond Basin, Illinois, have delta13C and delta18O values as high as +16.5% and +9.2% respectively, the highest values yet reported from continental ostracodalcalcite. Located in south-central Illinois, Raymond, Pittsburg, BaldKnob, and Hopwood Farm basins collectively have yielded important long pollen and ostracode records that date from about 130 000 years ago to the present. Although fossils from the present-day interglaciation are not well preserved, these records constitute the only described, conformable, fossiliferous successions of this age from the interior of glaciated North America. The high delta13C values from Raymond Basin are attributed to the residual effects of methane loss either by ebullition or by emission through the stems of senescentemergent aquatic vegetation. A mass balance model suggests that an increase in delta13C of dissolved inorganic carbon on the order of +15% is possible within a few hours given modest rates of methanogenesis of about 0.02 mol m-2 d-1. The delta13C records from other studies of ostracode valves have values approaching, but not exceeding about +14% suggesting a limitingvalue to delta13C enrichment due to simultaneous inputs and outputs of dissolved inorganic carbon. Values of delta18O in ostracodal calcite are quite variable (-4 to +9%) in sediment from the late Sangamon subepisode. A model of isotopic enrichment in adesiccating water body implies that a reduction in reservoir volume of 20% could produce this range of isotopic values. High humidity andevaporation probably account for most of the delta18O variability. [ABSTRACT FROM AUTHOR]

An essential procedure to increase the analytical efficiency of Δ 47 measurements requires raising the temperature of phosphoric acid digestion for carbonate materials. This temperature change introduces a fractionation offset in Δ 47 that must be accounted for prior to calculation of temperatures of carbonate formation and to allow interlaboratory comparison of results. We measured the phosphoric acid fractionation factor relative to reaction at 25 °C for calcite, aragonite, and dolomite across a temperature range from 25 to 90 °C. Significantly, all three minerals behave similarly during phosphoric acid digestion, allowing for a single temperature dependent acid fractionation relationship: 1000 ln ∝ C O 2 Acid − Δ 47 = 0.022434 ± 0.001490 ∗ 10 6 T 2 − 0.2524 ± 0.0168 where α is the phosphoric acid fractionation factor, and T is in degrees Kelvin. Mineralogical or isotopic compositional effects on the fractionation factor were not observed, suggesting that this acid fractionation factor may be valid for all carbonate minerals. We also present inorganic temperature calibrations for both calcite and aragonite at low temperatures (5–70 °C) and find them to agree with prior published data. Using the new acid fractionation factor, published Δ 47 –temperature calibrations are recalculated. This analysis confirms a statistically significant Δ 47 –temperature calibration difference between data analyzed at 25 °C versus higher temperatures. The origin of the discrepancy remains unknown, but it appears that the acid fractionation factor is not the cause. [ABSTRACT FROM AUTHOR]

An asteroid impact at the end of the Cretaceous caused mass extinction, but extinction mechanisms are not well-understood. The collapse of sea surface to sea floor carbon isotope gradients has been interpreted as reflecting a global collapse of primary productivity (Strangelove Ocean) or export productivity (Living Ocean), which caused mass extinction higher in the marine food chain. Phytoplankton-dependent benthic foraminifera on the deep-sea floor, however, did not suffer significant extinction, suggesting that export productivity persisted at a level sufficient to support their populations. We compare benthic foraminiferal records with benthic and bulk stable carbon isotope records from the Pacific, Southeast Atlantic, and Southern Oceans. We conclude that end-Cretaceous decrease in export productivity was moderate, regional, and insufficient to explain marine mass extinction. A transient episode of surface ocean acidification may have been the main cause of extinction of calcifying plankton and ammonites, and recovery of productivity may have been as fast in the oceans as on land. [ABSTRACT FROM AUTHOR]

Marine and continental records show an abrupt negative shift in carbon isotope values at ?55.8?Myr ago. This carbon isotope excursion (CIE) is consistent with the release of a massive amount of isotopically light carbon into the atmosphere and was associated with a dramatic rise in global temperatures termed the Palaeocene-Eocene thermal maximum (PETM). Greenhouse gases released during the CIE, probably including methane, have often been considered the main cause of PETM warming. However, some evidence from the marine record suggests that warming directly preceded the CIE, raising the possibility that the CIE and PETM may have been linked to earlier warming with different origins. Yet pre-CIE warming is still uncertain. Disentangling the sequence of events before and during the CIE and PETM is important for understanding the causes of, and Earth system responses to, abrupt climate change. Here we show that continental warming of about 5?°C preceded the CIE in the Bighorn Basin, Wyoming. Our evidence, based on oxygen isotopes in mammal teeth (which reflect temperature-sensitive fractionation processes) and other proxies, reveals a marked temperature increase directly below the CIE, and again in the CIE. Pre-CIE warming is also supported by a negative amplification of ?13C values in soil carbonates below the CIE. Our results suggest that at least two sources of warming-the earlier of which is unlikely to have been methane-contributed to the PETM. [ABSTRACT FROM AUTHOR]

Abstract: 87Sr/86Sr measurements were carried out on samples of various macrofossil taxa recovered from the SMS AND-2A core with the main goal of improving age control of the core, and provide insight on marine climate at the time of carbonate precipitation. Shell material was carefully screened using cathodoluminescence (CL) to discern optimum areas for microdrilling. Powders thus obtained were in addition analyzed for Ca, Mg, Sr, Mn, and Fe contents to further evaluate the possibility of diagenetic alteration of the skeletal carbonate before measuring their Sr isotope compositions. Bivalves turned out to be the best candidates for isotopic determinations. Unaltered calcitic bivalves produced reliable 87Sr/86Sr age ranges. Aragonitic material, on the other hand, produced less radiogenic 87Sr/86Sr than anticipated, an unexpected result. Preliminary oxygen isotope determinations in calcite samples confirm contrasting marine climate conditions between the late Early Miocene (16.5–16.0 Ma), and the early Late Miocene (~11 Ma). [Copyright &y& Elsevier]

Reports stable oxygen isotope measurements of aragonite in fish otoliths, or ear stones, collected across the Eocene/Oligocene boundary. Reconstruction of palaeotemperatures, which show little change during the interval studied; Finding that winters became about 4 degrees Celsius colder across the boundary; Suggestion that seasonal variation in temperature, rather than change in mean annual temperature, helped to cause faunal turnover during this transition.

This study uses stable noble gases' (He, Ne, Ar, Kr, Xe) volume fractions and isotopic ratios from Antrim Shale natural gas to assess compositional variability and vertical fluid migration within this reservoir, in addition to distinguishing between the presence of thermogenic versus biogenic methane. R/Ra values, where R is the measured 3 He/ 4 He ratio and Ra is the atmospheric value of 1.384 ± 0.013 × 10 − 6 , vary from 0.01 to 0.34 suggesting a largely dominant crustal 4 He component with minor atmospheric and mantle contributions. Crustal 21 Ne, 40 Ar and 136 Xe contributions are also present but the atmospheric component is largely dominant for these gases. Crustal contributions for 21 Ne, 40 Ar and 136 Xe vary between 1.1% and 12.5%, between 0.7% and 19% and between 0.1% and 2.7%, respectively. A few samples present higher than atmospheric 20 Ne/ 22 Ne ratios pointing to the presence of a small mantle Ne component. High horizontal and vertical variability of noble gas signatures in the Antrim Shale are observed. These are mainly due to variable noble gas input from deep brines and, to a smaller extent, variable in-situ production within different layers of the Antrim Shale, in particular, the Lachine and Norwood members. Estimated 4 He ages, considering external 4 He input for Antrim Shale water, vary between 0.9 ka and 238.2 ka and match well for most samples with the timing of the major Wisconsin glaciation, suggesting that Antrim Shale water was influenced by glaciation-induced recharge. Consistency between measured and predicted 40 Ar/ 36 Ar ratios assuming Ar release temperatures ≥ 250 °C supports a thermogenic origin for most of the methane in these samples. This thermogenic methane is likely to originate at greater depths, either from the deeper portion of the Antrim Shale in the central portion of the Michigan Basin or from deeper formations given that the thermal maturity of the Antrim Shale in the study area is rather low. [ABSTRACT FROM AUTHOR]

Abstract: Significant atmospheric noble gas excesses in aquifer systems have systematically been linked to increased hydrostatic pressure, either due to increased water table levels or due to the development of ice cover. Measured noble gases (Ne, Ar, Kr, and Xe) in the shallow Saginaw aquifer in the Michigan Basin display both moderate (∼20–60% Ne excess) and large (∼80−>120% Ne excess) excesses of atmospheric noble gases with respect to air saturated water for modern recharge conditions. All large atmospheric noble gas excesses are located in the main discharge area of the Michigan Basin, in the Saginaw Lowlands region. Here, through a step-by-step analysis, we first show that large atmospheric noble gas excesses in the Saginaw aquifer do not result from increased hydrostatic pressure but, instead, are the result of vertical transport of atmospheric noble gases that are believed to have escaped from deep Michigan Basin brines following a past thermal event of mantle origin. Subsequently, we show that the atmospheric noble gas pattern of the entire Michigan Basin strata appears to result from two distinct end-members: (a) an end-member represented by the deepest, most depleted brines from which most of the atmospheric noble gases escaped; and (b) an end-member with excess atmospheric noble gas values above those displayed by the Saginaw samples. The latter is unconstrained due to the dilution effect exerted by recharge water. Using a Rayleigh distillation model we further show that the greater enrichment of lighter relative to heavier noble gases in the Saginaw aquifer in the Saginaw Lowlands area is compatible with either diffusion or solubility related mechanisms. These findings reinforce the notion that a past thermal event is indeed responsible for the atmospheric noble gas excesses found in the Saginaw aquifer in the Saginaw Lowlands area. They are also consistent with and reinforce previous findings with respect to the occurrence of a thermal event of mantle origin in the Michigan Basin. [Copyright &y& Elsevier]

Geochemical and modeling studies suggest that the transition from the "greenhouse" state of the Late Eocene to the "icehouse" conditions of the Oligocene 34-33.5 Ma was triggered by a reduction of atmospheric pCO2 that enabled the rapid buildup of a permanent ice sheet on the Antarctic continent. Marine records show that the drop in pCO2 during this interval was accompanied by a significant decline in high-latitude sea surface and deep ocean temperature and enhanced seasonality in middle and high latitudes. However, terrestrial records of this climate transition show heterogeneous responses to changing pCO2 and ocean temperatures, with some records showing a significant time lag in the temperature response to declining pCO2. We measured the Δ47 of aragonite shells of the freshwater gastropod Viviparus lentus from the Solent Group, Hampshire Basin, United Kingdom, to reconstruct terrestrial temperature and hydrologic change in the North Atlantic region during the Eocene-Oligocene transition. Our data show a decrease in growing-season surface water temperatures (~10 °C) during the Eocene-Oligocene transition, corresponding to an average decrease in mean annual air temperature of ~4-6°C from the Late Eocene to Early Oligocene. The magnitude of cooling is similar to observed decreases in North Atlantic sea surface temperature over this interval and occurs during major glacial expansion. This suggests a close linkage between atmospheric carbon dioxide concentrations, Northern Hemisphere temperature, and expansion of the Antarctic ice sheets. [ABSTRACT FROM AUTHOR]

Water resources management in Maui, Hawaii, remains a challenge due to uneven distribution of rainfall, water scarcity in populated areas, and an ill-defined hydrological system. Based on data collected in June 2014, Niu et al. (2017) suggested that noble gases in this rapid groundwater infiltration system record seasonality and may shed light on water sources, recharge locations, and groundwater flow paths. This study goes one step further by analyzing rainwater, basal and perched (springs) aquifer data collected in June 2014 and February 2016. It shows for the first time that both rain and spring water display sharp temporal variations which are related to either seasonal (summer/winter) variations or to the strong 2015–2016 El Niño year. These spring water temporal variations also suggest that perched aquifer groundwater is extremely young and likely unreliable as a long-term water supplier. In contrast, noble gas temporal variations are absent in the basal aquifer. This, in turn, is consistent with tritium derived groundwater ages on the order of a few decades for the basal aquifer. This study further demonstrates that Ne is an excellent candidate to estimate the source elevation of rain and thus, to distinguish between orographic and synoptic-scale rain. Water stable isotope data are also presented for both years with 2016 samples exhibiting greater deuterium excesses than those of 2014, an observation consistent with the drier 2015–2016 El Niño climate. Estimated water source elevations based on δ18O for rainwater display a positive correlation with those based on Ne concentrations. • Both rain and spring water display sharp noble gas temporal variations. • Perched aquifers are very young and likely unreliable as a long-term water supplier. • Ne is an excellent candidate to distinguish orographic rain from synoptic-scale rain. [ABSTRACT FROM AUTHOR]