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The Central Atlantic Magmatic Province (CAMP) eruptions are generally regarded as the main driver of major environmental change and mass-extinction across the Triassic-Jurassic (T-J) boundary (~201.3 Ma). CAMP emissions have been invoked as the main trigger for the formation of abnormal pollen and spores during the end-Triassic crisis that may have led to forest dieback and proliferation of pioneer species. Proposed scenarios include extensive climate change leading to wildfire activity, acid rain, and increased UV-B radiation due to ozone depletion. More recently, volcanogenic mercury (Hg) has been implicated in the occurrence of mutations in fern spores. However, Hg-dynamics in deep-time remain poorly understood and require further examination. Here, we explore a new long-term (Rhaetian to Sinemurian) bulk Hg-concentration record combined with Hg-isotope data to understand the link between floral turnovers and the Hg-cycle.Shallow marine sediments sampled from the Schandelah-1 core in northern Germany contain a record of cyclical shifts in malformed fern spores coinciding with fluctuations in carbon isotopes, increased levels of weathering, and Hg-enrichments. Similarly, increased mutagenic spore abundances with accompanying Hg-isotope records confirm the volcanogenic origin of Hg at the T-J boundary, showing a sharp positive excursion in mass-independent fractionation (MIF) of odd-numbered Hg-isotopes. Hettangian cyclicity is clearly reflected in the Hg-isotopic signals, showing positive excursions in mass-dependent/independent fractionation records (d202Hg and D199Hg) during periods of sedimentary Hg-enrichment. In addition, the Hettangian Hg-isotopic signature clearly deviates from Rhaetian signatures, which hints at climate-controlled mechanisms being responsible. Atmospheric Hg-loading via volcanism can explain the synchronous enrichments of Hg concentrations at the T-J boundary interval in multiple sites across the globe. In contrast, the origin of this periodic Hg-loading is more difficult to pinpoint, but it becomes clear that Hg is showing shifts in speciation and closely tied to terrestrial vegetation development. Orbitally induced changes to the regional hydrological regime, resulting in increased wildfire activity, monsoonal intensity, and soil erosion, potentially redistributed Hg stored in soil and/or bedrock reservoirs causing a shift to more mobile Hg-species. Overall, this shows a more dominant role of climate-induced Hg-remobilisation, rather than direct volcanic emissions, to disturbance in terrestrial vegetation.

Deccan Traps flood basalt volcanism affected ecosystems spanning the end-Cretaceous mass extinction, with the most significant environmental effects hypothesized to be a consequence of the largest eruptions. The Rajahmundry Traps are the farthest exposures (~1,000 km) of Deccan basalt from the putative eruptive centers in the Western Ghats and hence represent some of the largest volume Deccan eruptions. Although the three subaerial Rajahmundry lava flows have been geochemically correlated to the Wai Subgroup of the Deccan Traps, poor precision associated with previous radioisotopic age constraints has prevented detailed comparison with potential climate effects. In this study, we use new 40Ar/39Ar dates, paleomagnetic and volcanological analyses, and biostratigraphic constraints for the Rajahmundry lava flows to ascertain the timing and style of their emplacement. We find that the lower and middle flows (65.92 ± 0.25 and 65.67 ± 0.08 Ma, ±1σ systematic uncertainty) were erupted within magnetochron C29r and were a part of the Ambenali Formation of the Deccan Traps. By contrast, the uppermost flow (65.27 ± 0.08 Ma) was erupted in C29n as part of the Mahabaleshwar Formation. Given these age constraints, the Rajahmundry flows were not involved in the end-Cretaceous extinction as previously hypothesized. To determine whether the emplacement of the Rajahmundry flows could have affected global climate, we estimated their eruptive CO2 release and corresponding climate change using scalings from the LOSCAR carbon cycle model. We find that the eruptive gas emissions of these flows were insufficient to directly cause multi-degree warming; hence, a causal relationship with significant climate warming requires additional Earth system feedbacks.

Flood basalts are some of the largest magmatic events in Earth history, with intrusion and eruption of millions of km$^3$ of basaltic magma over a short time period ($\sim$ 1-5 Ma). A typical conti...

Over the past few years, mercury (Hg) concentrations in (predominantly) marine sediments have gained widespread attention as a far-field, high-temporal resolution proxy for deep-time enhanced volcanic activity. The primary focus of these Hg studies has been a range of events in the past 500 million years; mostly larger and smaller mass extinctions and periods of high-amplitude climate change. As a result, sedimentary Hg data reinforced the notion many of these events are indeed coeval with and hypothesized causally connected to large igneous provinces (LIPs). However, relatively poor constraints on long-term dispersal of emissions through the marine and terrestrial biosphere, accumulation and preservation mechanisms of Hg pose difficulties for its use as a qualitative proxy for enhanced volcanic emissions. As a result, using sedimentary Hg for detailed modeling of Hg cycling or past gaseous emissions of magmatic volatiles, e.g. carbon and sulfur, and by extension environmental impact, remains speculative.The use of Hg normalization to common Hg-binding sedimentary components such as organic carbon (TOC), Fe or Al provides a basic means of comparing relative Hg loading within a sedimentary sequence. Yet, normalizing Hg to these major sedimentary components relies on simple linear relations and this approach often leaves substantial variance. While the high Hg concentrations have usually been ascribed to variability in volcanic activity, there are likely other factors that may invoke changes in the Hg concentrations in sediments, or mask Hg emitted by volcanism such as amount or type and flux of organic matter being deposited in basins and oxygenation of water and local sediments.To evaluate potential confounding factors, we compiled published Hg, TOC and bulk and trace element data, modern and deep-time events, periods with and without known anomalous volcanic activity and cover a range of depositional settings. We find that the depositional setting, as inferred from lithology and bulk sediment chemistry exerts a major control on the overall concentrations of Hg. Differences in Hg loading between time-correlative deposits persist after normalization to major sedimentary components, likely as a result of a complex interplay between various spatial and environmental factors. Our data compilation further allows us to explore the potential of establishing a range for background Hg values and variability through different periods of geological deep-time. Collectively, such constraints can aid the understanding of changes induced by environmental factors or volcanic emissions and inform Hg-cycling models.

The Cretaceous-Paleogene boundary (KPB) in the Hell Creek area of Montana is recognized in several places by an iridium anomaly, which is typically identified at or very near the lithological contact between the Hell Creek Formation and the Tullock Member of the Fort Union Formation. Previous work in the area has argued that organic carbon isotope (δ13Corg) excursions can be used for chemostratigraphic correlation within these continental depositional environments, most importantly for the identification of the KPB where impact evidence is unavailable. However, it is unclear how modern surficial weathering affects δ13Corg values, particularly in terrestrial depositional settings, and whether standard sampling methods are sufficient to obtain unaltered rock material. We tested the fidelity of the terrestrial δ13Corg record with respect to surficial alteration and contamination through investigation of different field sampling techniques, including hand trenches, a backhoe-excavated trench, and sediment coring. We find that δ13Corg values in hand and backhoe trenched sections are more positive than δ13Corg values in cored sections, implying that modern surficial alteration affects δ13Corg values but not overall trends. A negative δ13Corg excursion associated with the KPB is present in most sections we analyzed, but it does not appear to be unique within our sections. The KPB excursion occurs within a coal layer, and we observe similar excursions within other carbon-rich lithologies. Given that we cannot disentangle local lithological effects from global atmospheric changes, we conclude that a negative δ13Corg excursion is not an unequivocal indicator of the KPB in the Hell Creek area.

Deccan Traps continental flood basalt eruptions spanned the Cretaceous-Paleogene mass extinction, erupting over a million cubic kilometers of basalt over a total duration of approximately a million years. The environmental consequences of flood basalt eruptions depend on the timing and amount of volatile release; eruption rates are thus needed to evaluate their potential to cause climate change. Radioisotopic dates are not currently sufficient to resolve sub-ten thousand year eruptive tempos, necessary for constraining the effects of short-lifetime volatiles including sulfur dioxide. Recent studies have demonstrated that increases in mercury concentration in sedimentary records correlate with flood basalt eruptions under some circumstances. However, mercury concentrations have primarily been used to show the presence or absence of flood basalt eruptions. We show that this proxy can be used to quantitatively estimate eruptive rates using a mercury geochemical cycle framework. We illustrate this using new mercury chemostratigraphic records from terrestrial Cretaceous-Paleogene boundary sections in eastern Montana, USA, with multiple high-resolution chronologic constraints. We estimate that Deccan eruptions lasted on the order of centuries and released 500–3000 megagrams (Mg) of mercury per year, corresponding to ∼50–250 km3/a of lava. The box model framework highlights the importance of carefully accounting for differences in sedimentation rate and sampling resolution when comparing mercury records from different locations and depositional environments. While there are uncertainties in the box model estimates due to possible variation in flood basalt mercury emissions and sedimentation rates, they provide a useful framework to quantitatively evaluate the global mercury budget change indicated by changing concentration in sedimentary records. Eruptions of the estimated size would have released enough SO2, if it reached the stratosphere, to cause significant cooling for the duration of the eruption. However, given our constraints on the duration of individual eruptions, these colder periods are likely too brief to be clearly visible in most existing paleoclimate records.