Abstract. Periods of high atmospheric CO2 levels during the Cretaceous–early Paleogene (∼ 140 to 34 Myr ago) were marked by very high polar temperatures and reduced latitudinal gradients relative to the Holocene. These features represent a challenge for most climate models, implying either higher-than-predicted climate sensitivity to atmospheric CO2 or systematic biases or misinterpretations in proxy data. Here, we present a reconstruction of marine temperatures at polar (> 80∘) and middle (∼ 40∘) paleolatitudes during the Early Jurassic (∼ 180 Myr ago) based on the clumped isotope (Δ47) and oxygen isotope (δ18Oc) analyses of shallow buried pristine mollusc shells. Reconstructed calcification temperatures range from ∼ 8 to ∼ 18 ∘C in the Toarcian Arctic and from ∼ 24 to ∼ 28 ∘C in Pliensbachian mid-paleolatitudes. These polar temperatures were ∼ 10–20 ∘C higher than present along with reduced latitudinal gradients. Reconstructed seawater oxygen isotope values (δ18Ow) of −1.5 ‰ to 0.5 ‰ VSMOW and of −5 ‰ to −2.5 ‰ VSMOW at middle and polar paleolatitudes, respectively, point to a significant freshwater contribution in Arctic regions. These data highlight the risk of assuming the same δ18Osw value for δ18O-derived temperature from different oceanic regions. These findings provide critical new constraints for model simulations of Jurassic temperatures and δ18Osw values and suggest that high climate sensitivity has been a hallmark of greenhouse climates for at least 180 Myr.
Abstract. Most of our knowledge of past ocean temperature history is based on the δ18O measurements of calcium carbonate fossil shells. However, the determination of past temperature using this proxy requires the knowledge of past ocean δ18O, which is generally poorly constrained. Other carbonate-based paleothermometers, such as Mg / Ca ratios, and clumped isotopes (Δ47), have been developed to estimate independently paleotemperatures, and allow past ocean δ18O to be calculated using various groups of calcifying organisms. Articulated brachiopods are some of the most commonly used in studies of past oceanic geochemistry and temperature. They are abundant in the fossil record since the Cambrian, and their low Mg-calcite mineralogy has been considered relatively resistant to diagenetic alteration for decades. Here, we investigate the potential of brachiopod shells as recorders of growing temperature and seawater δ18O using new brachiopod shell geochemical data, by testing multiple established or supposed carbonate-based paleothermometers. Modern articulated brachiopod shells covering a wide range of temperatures (-1.9 to 25.5 °C), depths (5 to 3431 m) and salinities (33.4 to 37.0 PSU), were analysed for their stable isotope compositions (δ13C, δ18O and Δ47), and elemental ratios (Mg / Ca, Sr / Ca, Na / Ca and Li / Ca). Our data allowed us to propose a revised oxygen isotope fractionation equation between modern brachiopod shell calcite and seawater: T=-5.2 (±0.3) (δ18Oc-δ18Osw) + 19.9 (±0.8) R2=0.95 (n=53) Where δ18Oc is in ‰VPDB, δ18Osw is in ‰VSMOW and T is in °C. The measured Δ47 values show a strong correlation with growing temperatures but are significantly offset relative to the canonical relationship established for other biogenic and abiogenic calcium carbonate minerals. Our results strongly support the use of clumped isotopes as an alternative temperature proxy and indicate that brachiopod Δ47 values can be used with δ18O to estimate past δ18Osw with a precision of about ±1 ‰ VSMOW. The obtained Mg / Ca ratios show no relationship with temperatures, indicating that this ratio is a poor recorder of past changes in temperatures, an observation at variance with several previous studies. Brachiopod shell Sr / Ca, Na / Ca and Li / Ca display relatively good and significant correlations with brachiopod living temperature, but data indicate the influence of environmental and biological factors unrelated to temperature. Our proposed revision of marine temperature and water δ18O proxies based on brachiopod shell geochemistry is promising to refine the record of these oceanic parameters in the Phanerozoic.
Abstract. Periods of high atmospheric CO2 levels during the Cretaceous-Early Paleogene (~140 to 33 My ago) were marked by very high polar temperatures and reduced latitudinal gradients relative to the Holocene. These features represent a challenge for most climate models, implying either higher-than-predicted climate sensitivity to atmospheric CO2, or systematic biases or misinterpretations in proxy data. Here, we present a reconstruction of marine temperatures at polar (>80°) and mid (~40°) paleolatitudes during the Early Jurassic (~180 My ago) based on the clumped isotope (Δ47) and oxygen-isotope (δ18Oc) analyses of mildly buried pristine mollusc shells. Reconstructed calcification temperatures range from ~8 to ~18 °C in the Toarcian Arctic and from ~24 to ~28 °C in Pliensbachian mid-paleolatitudes. These polar temperatures were ~10–20 °C higher than present along with reduced latitudinal gradients. Reconstructed seawater oxygen isotope values (δ18Ow) of −1.5 to 0.5 ‰ VSMOW and of −5 to −2.5 ‰ VSMOW at mid and polar paleolatitudes, respectively, point to a significant freshwater contribution in Arctic regions. This highlight the risk of assuming the same δ18Osw value for δ18O-derived temperature from different oceanic regions. These findings provide critical new constraints for model simulations of Jurassic temperatures and δ18Osw values and suggest that high climate sensitivity is a hallmark of greenhouse climates since at least 180 My.
Abstract. Most of our knowledge of past seawater temperature history is based on δ18O values of calcium carbonate fossil shells. However, the determination of past temperatures using this proxy requires the knowledge of past seawater δ18O values, which is generally poorly constrained. Other paleothermometers using carbonate archives, such as Mg/Ca ratios and clumped isotopes (Δ47), have been developed to allow for paleotemperatures to be estimated independently and to allow past ocean δ18O values to be calculated using various groups of calcifying organisms. Articulated brachiopod shells are some of the most commonly used archives in studies of past oceanic geochemistry and temperature. They are abundant in the fossil record since the Cambrian, and for decades, their low Mg–calcite mineralogy has been considered relatively resistant to diagenetic alteration. Here, we investigate the potential of brachiopod shells as recorders of seawater temperatures and seawater δ18O values using new brachiopod shell geochemical data by testing multiple well-established or suggested paleothermometers applied to carbonate archives. Modern articulated brachiopod shells covering a wide range of temperatures (−1.9 to 25.5 ∘C), depths (5 to 3431 m) and salinities (33.4 to 37.0 PSU) were analysed for their stable isotope compositions (δ13C, δ18O and Δ47) and their elemental ratios (Mg/Ca, Sr/Ca, Na/Ca and Li/Ca). Our data allowed us to propose a revised oxygen isotope fractionation equation between modern-brachiopod shell calcite and seawater: (1) T = - 5.0 ( ± 0.2 ) ( δ 18 O c - δ 18 O sw ) + 19.4 ( ± 0.4 ) , where δ18Oc is in ‰ VPDB, δ18Osw is in ‰ VSMOW, and T is in ∘C. Our results strongly support the use of clumped isotopes as an alternative temperature proxy but confirm significant offsets relative to the canonical relationship established for other biogenic and abiogenic calcium carbonate minerals. Brachiopod shell Mg/Ca ratios show no relationship with seawater temperatures, indicating that this ratio is a poor recorder of past changes in temperatures, an observation at variance with several previous studies. Despite significant correlations with brachiopod living temperature, brachiopod shell Sr/Ca, Na/Ca and Li/Ca values indicate the influence of environmental and biological factors unrelated to temperature, which undermines their potential as alternative temperature proxies. Kinetic effects (growth rates) could explain most of the deviation of brachiopod shell calcite from expected isotopic equilibrium with seawater and part of the distribution of Sr/Ca, Na/Ca and Li/Ca ratios.
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<p>Extreme and rapid climatic and environmental perturbations have punctuated Earth history. The causes and consequences of these past global-change events are relatively well constrained, but how the system can naturally recover through feedbacks remain largely unconstrained. The Toarcian in the Early Jurassic is an ideal time interval to understand the response of Earth system to rapid climate change. Indeed, it was marked by one of the most extreme hyperthermal events of the Phanerozoic accompanied by major environmental changes, named the Toarcian Oceanic Anoxic Event (T-OAE, ca. 183 Ma). Most studies have focused on the triggering mechanisms and the palaeoenvironmental response, whereas the recovery phase has been less studied. Increased chemical weathering of silicate rocks and burial of organic carbon are the two primary natural mechanisms generally proposed as negative feedbacks controlling the recovery. However, to date, the response of these feedbacks, their efficiency, and their timing are still uncertain, hampering an accurate view of the carbon cycle-climate dynamics. This study aims to tackle this lack of empirical data by providing a multi-proxy dataset combining sedimentological observations, mineralogical and geochemical analyses. Four worldwide distributed sites have been selected for this study: Fontaneilles in France (Grand Causses Basin), Vilyui in Siberia (Siberian Basin), Agua de la Falda in Chile (Andean Basin), and Ait Athmane in Morocco (High Atlas Basin). Our high-resolution carbon isotope records allow us to correlate the studied sites to trace the global carbon cycle dynamics in the aftermath of the Toarcian event. Lithium isotope ratios are used to trace global weathering rates and to understand processes that control the long-term carbon cycle. Our results indicate that higher silicate weathering rates during the Toarcian hyperthermal likely helped the climate system recover and return to cooler climatic conditions. High mercury and tellurium concentrations recorded after the T-OAE interval suggest that protracted Karoo-Ferrar volcanic activity may have played a role in the recovery.</p>
<p>One of the most dramatic warming episodes of the Mesozoic occurred near the Pliensbachian-Toarcian transition (Early Jurassic). The occurrence of abundant exotic clasts and glendonites in marine strata of Siberia suggests cold conditions during the late Pliensbachian, which may have led to the episodic growth of high latitude ice-sheets. These conditions ended abruptly during the early Toarcian when temperature rose rapidly across an episode of global biogeochemical perturbation known as the Toarcian Oceanic Anoxic Event (T-OAE). The rapid marine transgression coinciding with the T-OAE onset has been tentatively attributed to the rapid demise of these polar ice-sheets, which possibly released large amounts of methane in the atmosphere through permafrost thawing. Nevertheless, the scarce quantitative estimates of Pliensbachian-Toarcian temperatures have exclusively been obtained from low paleolatitude sites. Plus, existing temperature records are mostly based on oxygen isotope thermometry and hence remain equivocal in the absence of constraints on the ocean oxygen composition of Pliensbachian-Toarcian oceans and its temporal variability. Clumped isotope (&#916;<sub>47</sub>) data from aragonite bivalve shells from one NE Siberian site have recently provided the first quantitative evidence for extreme Toarcian polar warmth, with marine temperature estimates exceeding ~15&#176;C north of the Anabar shield. In this study, we present new &#916;<sub>47</sub> data from bivalve samples from Tyung River, south of the Anabar shield that allow to substantially expand this record both spatially and temporally. Clumped isotope data from aragonite shells confirm elevated marine temperatures (~13&#176;C) at the end of the T-OAE in polar areas some 850 km away from the previous record. Upper Pliensbachian calcite shells of <em>Harpax</em> collected from coastal to deltaic, boulder-bearing deposits of a nearby site record much lower temperature (~3&#176;C) and extreme <sup>18</sup>O-depletion of environmental waters (&#948;<sup>18</sup>O = -6.5&#8240;VSMOW). These results provide the first quantitative evidence for near-freezing polar temperatures during the Late Pliensbachian, which is a key prerequisite for the hypothesis of episodic ice-sheet growth prior to the T-OAE. Beyond glacio-eustasy, our new data offer a rare glimpse of extreme changes in polar temperatures across a transition from coldhouse to greenhouse climate and will certainly prove useful for future earth system simulations of Mesozoic climates.&#160;</p>
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