Early Jurassic large igneous province carbon emissions constrained by sedimentary mercury

Large igneous province eruptions and their carbon emissions often coincide with, and are hypothesized to have driven, severe environmental perturbations in the geological past. However, the vast scale of large igneous provinces and uncertainties in magmatic volatile contents and radioisotopic dates limit our ability to resolve gas emissions in detail over time. Here we employ high-resolution (~5–200 kyr) sedimentary mercury data from the Llanbedr (Mochras Farm) borehole, Wales, to derive quantitative large igneous province degassing estimates over a 20-million-year-long Early Jurassic interval (195–175 million years ago). Intervals of relatively elevated sedimentary mercury coincide with episodes of carbon-cycle change, including the Toarcian Oceanic Anoxic Event (183–182 million years ago). We use excess mercury loading to estimate large igneous province-associated carbon emissions, revealing that multi-millennial episodes of activity plausibly drove recognized pCO2 and temperature increases. However, previous carbon-cycle model-based carbon emission scenarios require faster and larger carbon inputs than our proposed emissions. Resolving this discrepancy may require climate–carbon-cycle feedbacks or co-emitted gases to substantially exacerbate the carbon-cycle response, processes potentially underestimated in current models. Our long and near-continuous record of Early Jurassic large igneous province activity demonstrates mercury’s potential as a tool to resolve past carbon fluxes

Assessment of Hg speciation changes in the sedimentary rock record from thermal desorption characteristics

Sedimentary mercury (Hg) has become a widely used proxy for paleo-volcanic activity. However, scavenging and drawdown of Hg by organic-matter (OM) and sulfides are important non-volcanic factors determining variability in such records. Most studies, therefore, normalize total Hg (HgT) to a Hg “host-phase” proxy (e.g., HgT/TOC for OM, HgT/TS for sulfides), with the dominant host-phase determined based on the strongest observed (linear) correlations. This approach suffers from various non-linearities in Hg-host-phase behavior and does not account for succession-level, let alone sample-level, Hg speciation changes. Thermal desorption characteristics or “profiles” (TDPs) for many Hg species during pyrolysis analysis are well-established with applications including distinguishing between OM-bound Hg and different Hg sulfides and oxides in (sub-)recent sediments. We explore the use of TDPs for geological sediment (rock) samples and illustrate the presence of multiple release phases (Hg species)—correlated to geochemical host-phase—in (almost) all the 65 analyzed Tithonian (146–145 Ma) silt and mudrock samples. By quantifying the Hg in each release phase for every sample, we find TOC concentration may determine ∼60% of the variability in the first (lower temperature) Hg TDP release phase: a stark difference with the total Hg released from these samples, where ∼20% of variation is explained by TOC variability. TDPs provide insight on sample-level Hg speciation and demonstrate that, while the common assumption of single-phase Hg speciation in sedimentary rocks is problematic, differences in Hg speciation can be detected, quantified, and accounted for using commonly applied techniques—opening potential for routine assessment

Investigating the behavior of sedimentary mercury (Hg) during burial-related thermal maturation

Understanding the behavior of mercury (Hg) in organic-rich sediments as they undergo thermal maturation is important, for example, because enrichment of Hg in sedimentary deposits has become a widely used proxy for volcanism from Large Igneous Provinces (LIPs). In this study, we evaluate the effects of such processes on sedimentary Hg concentrations by investigating a common stratigraphic interval in three drill cores with different levels of thermal maturity (immature, mature and post-mature) in Toarcian sediments (Posidonienschiefer Formation) from the Lower Saxony Basin, Germany. We present Hg concentrations, bulk organic geochemistry, and total sulfur data. Mercury concentrations in the mature and post-mature sediments are increased >2-fold relative to the immature material, which is greater than any potential differences in original Hg concentrations in the studied successions prior to burial. Organic-carbon and host-rock mass loss during thermal maturation may have concentrated Hg in the mature sediments to some extent, provided Hg is considered effectively immobile. The increased Hg, TOC-normalized Hg, and TS-normalized Hg are most likely linked to the “closed system” behavior of Hg in sedimentary basins and the relatively low temperatures (70–260°C) during maturation that resulted in limited Hg mobility. More speculatively, a certain degree of redistribution of Hg within the mature sediments is suggested by its enrichment in distinct stratigraphic levels. Regardless of the exact mechanisms at play, the elevated Hg concentrations in mature sediments amplify both Hg/TOC and Hg/TS, implying that thermal effects must be considered when using normalized Hg as a proxy for far-field volcanic activity

Investigating the behavior of sedimentary mercury (Hg) during burial-related thermal maturation

Understanding the behavior of mercury (Hg) in organic‐rich sediments as they undergo thermal maturation is important, for example, because enrichment of Hg in sedimentary deposits has become a widely used proxy for volcanism from Large Igneous Provinces (LIPs). In this study, we evaluate the effects of such processes on sedimentary Hg concentrations by investigating a common stratigraphic interval in three drill cores with different levels of thermal maturity (immature, mature and post‐mature) in Toarcian sediments (Posidonienschiefer Formation) from the Lower Saxony Basin, Germany. We present Hg concentrations, bulk organic geochemistry, and total sulfur data. Mercury concentrations in the mature and post‐mature sediments are increased >2‐fold relative to the immature material, which is greater than any potential differences in original Hg concentrations in the studied successions prior to burial. Organic‐carbon and host‐rock mass loss during thermal maturation may have concentrated Hg in the mature sediments to some extent, provided Hg is considered effectively immobile. The increased Hg, TOC‐normalized Hg, and TS‐normalized Hg are most likely linked to the “closed system” behavior of Hg in sedimentary basins and the relatively low temperatures (70–260°C) during maturation that resulted in limited Hg mobility. More speculatively, a certain degree of redistribution of Hg within the mature sediments is suggested by its enrichment in distinct stratigraphic levels. Regardless of the exact mechanisms at play, the elevated Hg concentrations in mature sediments amplify both Hg/TOC and Hg/TS, implying that thermal effects must be considered when using normalized Hg as a proxy for far‐field volcanic activity

Tracing North Atlantic volcanism and seaway connectivity across the Paleocene–Eocene Thermal Maximum (PETM)

There is a temporal correlation between the peak activity of the North Atlantic Igneous Province (NAIP) and the Paleocene–Eocene Thermal Maximum (PETM), suggesting that the NAIP may have initiated and/or prolonged this extreme warming event. However, corroborating a causal relationship is hampered by a scarcity of expanded sedimentary records that contain both climatic and volcanic proxies. One locality hosting such a record is the island of Fur in Denmark, where an expanded pre- to post-PETM succession containing hundreds of NAIP ash layers is exceptionally well preserved. We compiled a range of environmental proxies, including mercury (Hg) anomalies, paleotemperature proxies, and lithium (Li) and osmium (Os) isotopes, to trace NAIP activity, hydrological changes, weathering, and seawater connectivity across this interval. Volcanic proxies suggest that NAIP activity was elevated before the PETM and appears to have peaked during the body of the δ13C excursion but decreased considerably during the PETM recovery. This suggests that the acme in NAIP activity, dominated by flood basalt volcanism and thermogenic degassing from contact metamorphism, was likely confined to just ∼ 200 kyr (ca. 56.0–55.8 Ma). The hundreds of thick (> 1 cm) basaltic ashes in the post-PETM strata likely represent a change from effusive to explosive activity, rather than an increase in NAIP activity. Detrital δ7Li values and clay abundances suggest that volcanic ash production increased the basaltic reactive surface area, likely enhancing silicate weathering and atmospheric carbon sequestration in the early Eocene. Signals in lipid biomarkers and Os isotopes, traditionally used to trace paleotemperature and weathering changes, are used here to track seaway connectivity. These proxies indicate that the North Sea was rapidly cut off from the North Atlantic in under 12 kyr during the PETM recovery due to NAIP thermal uplift. Our findings reinforce the hypothesis that the emplacement of the NAIP had a profound and complex impact on Paleocene–Eocene climate, both directly through volcanic and thermogenic degassing and indirectly by driving regional uplift and changing seaway connectivity

Changes in the high latitude Southern Hemisphere through the Eocene-Oligocene Transition:a model-data comparison

International audienceAbstract. The global and regional climate changed dramatically with the expansion of the Antarctic Ice Sheet at the Eocene–Oligocene transition (EOT). These large-scale changes are generally linked to declining atmospheric pCO2 levels and/or changes in Southern Ocean gateways such as the Drake Passage around this time. To better understand the Southern Hemisphere regional climatic changes and the impact of glaciation on the Earth's oceans and atmosphere at the EOT, we compiled a database of 10 ocean and 4 land-surface temperature reconstructions from a range of proxy records and compared this with a series of fully coupled, low-resolution climate model simulations from two models (HadCM3BL and FOAM). Regional patterns in the proxy records of temperature show that cooling across the EOT was less at high latitudes and greater at mid-latitudes. While certain climate model simulations show moderate–good performance at recreating the temperature patterns shown in the data before and after the EOT, in general the model simulations do not capture the absolute latitudinal temperature gradient shown by the data, being too cold, particularly at high latitudes. When taking into account the absolute temperature before and after the EOT, as well as the change in temperature across it, simulations with a closed Drake Passage before and after the EOT or with an opening of the Drake Passage across the EOT perform poorly, whereas simulations with a drop in atmospheric pCO2 in combination with ice growth generally perform better. This provides further support for previous research that changes in atmospheric pCO2 are more likely to have been the driver of the EOT climatic changes, as opposed to the opening of the Drake Passage

Single-species dinoflagellate cyst carbon isotope fractionation in core-Top sediments: Environmental controls, CO2 dependency and proxy potential

Sedimentary bulk organic matter and various molecular organic components exhibit strong CO2-dependent carbon isotope fractionation relative to dissolved inorganic carbon sources. This fractionation (p) has been employed as a proxy for paleo-pCO2. Yet, culture experiments indicate that CO2-dependent p is highly specific at genus and even species level, potentially hampering the use of bulk organic matter and non-species-specific organic compounds. In recent years, significant progress has been made towards a CO2 proxy using controlled growth experiments with dinoflagellate species, also showing highly species-specific p values. These values were, however, based on motile specimens, and it remains unknown whether these relations also hold for the organic-walled resting cysts (dinocysts) produced by these dinoflagellate species in their natural environment. We here analyze dinocysts isolated from core tops from the Atlantic Ocean and Mediterranean Sea, representing several species (Spiniferites elongatus, S. (cf.) ramosus, S. mirabilis, Operculodinium centrocarpum sensu Wall and Dale (1966) (hereafter referred to as O. centrocarpum) and Impagidinium aculeatum) using laser ablation-nano-combustion-gas-chromatography-isotope ratio mass spectrometry (LA/nC/GC-IRMS). We find that the dinocysts produced in the natural environment are all appreciably more 13C-depleted compared to the cultured motile dinoflagellate cells, implying higher overall p values, and, moreover, exhibit large isotope variability. Where several species could be analyzed from a single location, we often record significant differences in isotopic variance and offsets in mean 13C values between species, highlighting the importance of single-species carbon isotope analyses. The most geographically expanded dataset, based on O. centrocarpum, shows that p correlates significantly with various environmental parameters. Importantly, O. centrocarpum shows a CO2-dependent p above g1/4g€¯240g€¯μatm pCO2. Similar to other marine autotrophs, relative insensitivity at low pCO2 is in line with active carbon-concentrating mechanisms at low pCO2, although we here cannot fully exclude that we partly underestimated p sensitivity at low pCO2 values due to the relatively sparse sampling in that range. Finally, we use the relation between p and pCO2 in O. centrocarpum to propose a first pCO2 proxy based on a single dinocyst species

Paleocene–Eocene age glendonites from the Mid-Norwegian Margin – indicators of cold snaps in the hothouse?

The International Ocean Discovery Program (IODP) Expedition 396 to the mid-Norwegian margin recovered > 1300 m of pristinely preserved, volcanic-ash-rich sediments deposited during the late Paleocene and early Eocene from close to the centre of the North Atlantic Igneous Province (NAIP). Remarkably, many of these cores contain glendonites, pseudomorphs after the purported cold-water mineral ikaite, from sediments dated to the late Paleocene and early Eocene. These time intervals span some of the hottest climates of the Cenozoic, including the Paleocene–Eocene Thermal Maximum (PETM). Global deep-ocean temperatures are not thought to have dropped below 10 ∘C at any point during this time, making the occurrence of supposedly cold-water (near-freezing temperature) glendonite pseudomorphs seemingly paradoxical. This study presents a detailed sedimentological, geochemical, and microscopic study of the IODP Exp. 396 glendonites and presents an updated model for the ikaite-to-calcite transformation for these glendonites. Specifically, we show that early diagenesis of basaltic ashes of the NAIP appear to have chemically promoted ikaite growth in the sediments in this region. Together with existing knowledge of late Paleocene and early Eocene glendonites from Svalbard to the north and early Eocene glendonites from Denmark to the south, these new glendonite finds possibly imply episodic, short-duration, and likely localized cooling in the Nordic Seas region, which may have been directly or indirectly linked to the emplacement of the NAIP

No evidence for a volcanic trigger for late Cambrian carbon-cycle perturbations

The early Paleozoic was marked by several carbon-cycle perturbations and associated carbon-isotope excursions (CIEs). Whether these CIEs are connected to significant (external) triggers, as is commonly considered to be the case for CIEs in the Mesozoic and Cenozoic, or result from small carbon-cycle imbalances that became amplified through lack of efficient silicate weathering or other feedbacks remains unclear. We present concentration and isotope data for sedimentary mercury (Hg) and osmium (Os) to assess the impact of subaerial and submarine volcanism and weathering during the late Cambrian and early Ordovician. Data from the Alum Shale Formation (Sweden) cover the Steptoean positive carbon-isotope excursion (SPICE; ca. 497–494 Ma), a period marked by marine anoxia and biotic overturning, and several smaller CIEs extending into the early Ordovician. Our Hg and Os data offer no strong evidence that the CIEs present in our record were driven by (globally) elevated volcanism or continental weathering. Organic-carbon and Hg concentrations covary cyclically, providing further evidence of an unperturbed Hg cycle. Mesozoic and Cenozoic CIEs are commonly linked to enhanced volcanic activity and weathering, but similar late Cambrian–early Ordovician events cannot easily be connected to such external triggers. Our results are more consistent with reduced early Paleozoic carbon-cycle resilience that allowed small imbalances to develop into large CIEs

The Eurasian epicontinental sea was an important carbon sink during the Palaeocene-Eocene thermal maximum

The Palaeocene-Eocene Thermal Maximum (ca. 56 million years ago) offers a primary analogue for future global warming and carbon cycle recovery. Yet, where and how massive carbon emissions were mitigated during this climate warming event remains largely unknown. Here we show that organic carbon burial in the vast epicontinental seaways that extended over Eurasia provided a major carbon sink during the Palaeocene-Eocene Thermal Maximum. We coupled new and existing stratigraphic analyses to a detailed paleogeographic framework and using spatiotemporal interpolation calculated ca. 720–1300 Gt organic carbon excess burial, focused in the eastern parts of the Eurasian epicontinental seaways. A much larger amount (2160–3900 Gt C, and when accounting for the increase in inundated shelf area 7400–10300 Gt C) could have been sequestered in similar environments globally. With the disappearance of most epicontinental seas since the Oligocene-Miocene, an effective negative carbon cycle feedback also disappeared making the modern carbon cycle critically dependent on the slower silicate weathering feedback.</p