39 research outputs found
Thermally-induced clumped isotope resetting in belemnite and optical calcites: Towards material-specific kinetics
The application of carbonate clumped isotope (Î47) thermometry in deep-time is often limited by modification of
the original temperature signal by thermal resetting. New modeling approaches to estimate the initial isotopic
composition of partially reset calcites and maximal burial temperatures, however, open promising avenues in
temperature reconstruction. Such approaches strongly depend on laboratory-derived kinetic parameters of
calcite materials, which may differ in their microstructure, water content and distribution, and minor and trace
element composition, and thus may have different resetting kinetics. The rostra of belemnites, an extinct group of
mollusks with a wide temporal and spatial occurrence in the Mesozoic, have been extensively used for deep-time
paleoclimate reconstructions using oxygen isotope geochemistry. Belemnites are also important targets for
clumped isotope-based temperature reconstructions, but often are found to have reset Î47 compositions. Here,
we present results from heating experiments on belemnite rostral calcite and optical calcite and provide
belemnite-specific kinetic parameters for clumped isotope resetting. We show that belemnite calcite is altered
faster and at lower temperatures than optical calcite and all other calcites reported in previous studies. We
suggest that fast initial resetting results from oxygen isotope exchange of belemnite calcite with internal skeletal
water present as fluid inclusions or organic-derived water, a process completed within 2â4 min at the experimental
temperatures used here. Extrapolation to geological timescales using different solid-state bond reordering
models shows that belemnite calcite resetting starts at lower burial temperatures than brachiopod, spar, and
optical calcites. This susceptibility to thermal resetting results in a measurable (+3 âŠC) increase of the apparent
Î47 temperature even under shallow to moderate burial conditions (i.e., 40â50 âŠC for 106â107 years timescales).
Following the overprint to higher apparent Î47 temperatures during burial, the belemnite Î47 may further reequilibrate
during exhumation resulting in a decrease of apparent Î47 temperatures. Such âretrograde resettingâ
is similar to what is observed for carbonatites and marbles during cooling, and may cause underestimation
of the thermal resetting a sample experienced during its geological history. Overall, our results demonstrate the
importance of material-specific kinetic parameters and we urge caution when interpreting Î47-derived temperatures
of biogenic carbonates from deep-time archives.Swiss National Science Foundation
project number 200021_169849AF from Juan de la Cierva
Fellowship (IJC2019040065-I)Spanish Ministry of Science
and InnovationEuropean Development Fund
and the European Social FundThe European
Commission, Horizon 2020 (ICECAP; grant no. 101024218)The
Research Council of Norway Centre of Excellence funding schemeProject number 223272. EBSD data for the WA-CB-11 brachiopod provided
by the authors of Henkes et al. (2014)US National Science Foundation (EAR-1226832
Radiocarbon age offsets between two surface dwelling planktonic foraminifera species during abrupt climate events in the SW Iberian Margin
This study identifies temporal biases in the radiocarbon ages of the planktonic foraminifera species Globigerina bulloides and Globigerinoides ruber (white) in a sediment core from the SW Iberian margin (soâcalled Shackleton site). Leaching of the outer shell and measurement of the radiocarbon content of both the leachate and leached sample enabled us to identify surface contamination of the tests and its impact on their 14C ages. Incorporation of younger radiocarbon on the outer shell affected both species and had a larger impact downcore. Interspecies comparison of the 14C ages of the leached samples reveal systematic offsets with 14C ages for G. ruber being younger than G. bulloides ages during the last deglaciation and part of the Early and midâHolocene. The greatest offsets (up to 1,030 years) were found during Heinrich Stadial 1, the Younger Dryas, and part of the Holocene. The potential factors differentially affecting these two planktonic species were assessed by complementary 14C, oxygen and carbon isotopes, and species abundance determinations. The coupled effect of bioturbation with changes in the abundance of G. ruber is invoked to account for the large age offsets. Our results highlight that 14C ages of planktonic foraminifera might be largely compromised even in settings characterized by high sediment accumulation rates. Thus, a careful assessment of potential temporal biases must be performed prior to using 14C ages for paleoclimate investigations or radiocarbon calibrations (e.g., marine calibration curve Marine13, Reimer et al., 2013, https://doi.org/10.2458/azu_js_rc.55.16947).UID/Multi/04326/2019, IF/01500/2014info:eu-repo/semantics/publishedVersio
Radiocarbon Age Offsets Between Two Surface Dwelling Planktonic Foraminifera Species During Abrupt Climate Events in the SW Iberian Margin
[EN]This study identifies temporal biases in the radiocarbon ages of the planktonic foraminifera species Globigerina bulloides and Globigerinoides ruber (white) in a sediment core from the SW Iberian margin (soâcalled Shackleton site). Leaching of the outer shell and measurement of the radiocarbon content of both the leachate and leached sample enabled us to identify surface contamination of the tests and its impact on their 14C ages. Incorporation of younger radiocarbon on the outer shell affected both species and had a larger impact downcore. Interspecies comparison of the 14C ages of the leached samples reveal systematic offsets with 14C ages for G. ruber being younger than G. bulloides ages during the last deglaciation and part of the Early and midâHolocene. The greatest offsets (up to 1,030 years) were found during Heinrich Stadial 1, the Younger Dryas, and part of the Holocene. The potential factors differentially affecting these two planktonic species were assessed by complementary 14C, oxygen and carbon isotopes, and species abundance determinations. The coupled effect of bioturbation with changes in the abundance of G. ruber is invoked to account for the large age offsets. Our results highlight that 14C ages of planktonic foraminifera might be largely compromised even in settings characterized by high sediment accumulation rates. Thus, a careful assessment of potential temporal biases must be performed prior to using 14C ages for paleoclimate investigations or radiocarbon calibrations (e.g., marine calibration curve Marine13, Reimer et al., 201
A Reassessment of the Precision of Carbonate Clumped Isotope Measurements: Implications for Calibrations and Paleoclimate Reconstructions
Carbonate clumped isotopes offer a potentially transformational tool to interpret Earth's history, but the proxy is still limited by poor interlaboratory reproducibility. Here, we focus on the uncertainties that result from the analysis of only a few replicate measurements to understand the extent to which unconstrained errors affect calibration relationships and paleoclimate reconstructions. We find that highly precise data can be routinely obtained with multiple replicate analyses, but this is not always done in many laboratories. For instance, using published estimates of external reproducibilities we find that typical clumped isotope measurements (three replicate analyses) have margins of error at the 95% confidence level (CL) that are too large for many applications. These errors, however, can be systematically reduced with more replicate measurements. Second, using a Monte Carloâtype simulation we demonstrate that the degree of disagreement on published calibration slopes is about what we should expect considering the precision of Î47 data, the number of samples and replicate analyses, and the temperature range covered in published calibrations. Finally, we show that the way errors are typically reported in clumped isotope data can be problematic and lead to the impression that data are more precise than warranted. We recommend that uncertainties in Î47 data should no longer be reported as the standard error of a few replicate measurements. Instead, uncertainties should be reported as margins of error at a specified confidence level (e.g., 68% or 95% CL). These error bars are a more realistic indication of the reliability of a measurement.This study was funded by
the Swiss National Science Foundation
project 200020_160046,
200021_143485, 200021_169849, and
IZK022â160377, ETH research project
ETH-33 14-1, and by Australian
Research Council Australian Laureate
Fellowship FL12010005
Microstructural characterization of natural fractures and faults in the Opalinus Clay: insights from a deep drilling campaign across central northern Switzerland
Abstract The Middle-Jurassic Opalinus Clay is the foreseen host rock for radioactive waste disposal in central northern Switzerland. An extensive drilling campaign aiming to characterize the argillaceous formation resulted in a comprehensive drill core data set. The rheologically weak Opalinus Clay is only mildly deformed compared to the over- and underlying rock units but shows a variety of natural fractures. While these structures are hydraulically indistinguishable from macroscopically non-deformed Opalinus Clay today, their analysis allows for a better understanding of the deformation behaviour in the geological past. Here, we present an overview of the different fracture and fault types recorded in the Opalinus Clay and a detailed microstructural characterization of veinsânatural dilational fractures healed by secondary calcite and celestite mineralizations. Macroscopic drill core analysis revealed five different natural fracture types that encompass tension gashes of various orientations with respect to bedding and small-scale faults with displacements typically not exceeding the drill core diameter. The occurrence of different fault types generally fits well with the local tectonic setting of the different drilling sites and with respect to the neighbouring regional fault zones. The microstructural investigations of the various vein types revealed their often polyphase character. Fibrous bedding-parallel veins of presumable early age were found to be overprinted by secondary slickenfibres. The polyphase nature of fibrous bedding parallel veins and slickenfibres is supported by differing elemental compositions, pointing towards repeated fracturing and mineralization events. Direct dating of vein calcites with UâPb was unsuccessful. Nevertheless, age constraints can be inferred from structural orientations and fault slip kinematics. Accordingly, some of the veins already formed during sediment compaction in Mesozoic times, others possibly relate to Early Cenozoic foreland uplift. The youngest veins are most likely related to Late Cenozoic regional tectonic events, such as the Jura fold-and-thrust belt to the south and the Hegau-Lake Constance Graben to the northeast of the study area. During these latest tectonic events, previously formed veins acted as rheologically stiff discontinuities in the otherwise comparably weak Opalinus Clay along which deformation of the rock formation was re-localized
Marine temperatures underestimated for past greenhouse climate
AbstractUnderstanding the Earthâs climate system during past periods of high atmospheric CO2 is crucial for forecasting climate change under anthropogenically-elevated CO2. The Mesozoic Era is believed to have coincided with a long-term Greenhouse climate, and many of our temperature reconstructions come from stable isotopes of marine biotic calcite, in particular from belemnites, an extinct group of molluscs with carbonate hard-parts. Yet, temperatures reconstructed from the oxygen isotope composition of belemnites are consistently colder than those derived from other temperature proxies, leading to large uncertainties around Mesozoic sea temperatures. Here we apply clumped isotope palaeothermometry to two distinct carbonate phases from exceptionally well-preserved belemnites in order to constrain their living habitat, and improve temperature reconstructions based on stable oxygen isotopes. We show that belemnites precipitated both aragonite and calcite in warm, open ocean surface waters, and demonstrate how previous low estimates of belemnite calcification temperatures has led to widespread underestimation of Mesozoic sea temperatures by ca. 12 °C, raising estimates of some of the lowest temperature estimates for the Jurassic period to values which approach modern mid-latitude sea surface temperatures. Our findings enable accurate recalculation of global Mesozoic belemnite temperatures, and will thus improve our understanding of Greenhouse climate dynamics.</jats:p
Evaluating the reliability of UâPb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) carbonate geochronology: matrix issues and a potential calcite validation reference material
We document that the reliability of carbonate UâPb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is improved by matching the aspect ratio of the LA single-hole drilling craters and propagating long-term excess variance and systematic uncertainties. We investigated the impact of different matrices and ablation crater geometries using UâPb isotope analyses of one primary (WC-1) and two secondary reference materials (RMs). Validation RMs (VRMs) include a previously characterised one (ASH-15D) and a new candidate (JT), characterised by ID-TIMS (intercept age: 13.797±0.031âMa) with excellent agreement to pooled LA-ICP-MS measurements (13.75±0.11 | 0.36âMa), a U concentration of approx. 1â”gâgâ1 and 238Uâ206Pb ratios from 5 to 460, defining the isochron well. Differences in ablation crater depth to diameter ratios (aspect ratio) introduce an offset due to downhole fractionation and/or matrix effects. This effect can be observed either when the crater size between UâPb RM and the sample changes or when the ablation rate for the sample is different than for the RM. Observed deviations are up to 20â% of the final intercept age depending on the degree of crater geometry mismatch. The long-term excess uncertainty was calculated to be in the range of 2â% (ASH-15D) to 2.5â% (JT), and we recommend propagating this uncertainty into the uncertainty of the final results. Additionally, a systematic offset to the ID-TIMS age of 2â%â3â% was observed for ASH-15D but not for JT. This offset might be due to different ablation rates of ASH-15D compared to the primary RM or remaining matrix effects, even when the aspect ratios chosen are similar
Evaluating the reliability of UâPb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) carbonate geochronology: matrix issues and a potential calcite validation reference material
International audienceWe document that the reliability of carbonate U-Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is improved by matching the aspect ratio of the LA single hole drilling craters and propagating of long-term excess uncertainty and systematic uncertainties. We investigated the impact of different matrices and ablation crater geometries using U-Pb isotope analyses of one primary (WC-1) and two secondary reference materials (RMs). Validation RMs (VRM) include a previously characterized (ASH-15D) and a new candidate (JT), characterized by ID-TIMS (intercept age: 13.797 ± 0.031âMa) with excellent agreement to pooled LA-ICP-MS measurements (13.81 ± 0.11 Š 0.30âMa), U concentration of approx. 1 ÎŒg/g and 238U/206Pb ratios from 5 to 460, well defining the isochron. Differences in ablation crater depth to diameter ratios (aspect ratio) introduce an offset due to downhole fractionation and/or matrix effects. This effect can be observed either when the crater size between U-Pb RM and sample changes or when the ablation rate for the sample is different than for the RM. Observed deviations are up to 20â% of the final intercept age depending on the degree of crater geometry mismatch. The long-term excess uncertainty was calculated to be in the range of 2â% (ASH-15D) to 2.5â% (JT), and we recommend propagating this uncertainty into the uncertainty of the final results. Additionally, a systematic offset to the ID-TIMS age of 2â3â% was observed for ASH-15D but not for JT. This offset might be due to different ablation rates of ASH-15D compared to the primary RM or remaining matrix effects, even when chosen aspect ratios are similar