The Mesozoic Era featured emplacement of a number of Large Igneous Provinces (LIPs), formed by the outpouring of millions of cubic kilometres of basaltic magma. The radiometric ages of several ...Mesozoic LIPs coincide with the dates of Oceanic Anoxic Events (OAEs). As a result of these coincidences, a causal link has been suggested, but never conclusively proven. This study explores the use of mercury as a possible direct link between the Karoo–Ferrar LIP and the coeval Toarcian OAE (T-OAE). Mercury is emitted to the atmosphere as a trace constituent of volcanic gas, and may be distributed globally before being deposited in sediments. Modern marine deposits show a strong linear correlation between mercury and organic-matter content. Results presented here indicate departures from such a simple linear relationship in sediments deposited during the T-OAE, and also during the Pliensbachian–Toarcian transition (an event that saw elevated benthic extinctions and carbon-cycle perturbations prior to the T-OAE). A number of depositional settings illustrate an increased mercury concentration in sediments that record one or both events, suggesting a rise in the depositional flux of this element. Complications to this relationship may arise from very organic-rich sediments potentially overprinting any Hg/TOC signal, whereas environments preserving negligible organic matter may leave no record of mercury deposition. However, the global distribution of coevally elevated Hg-rich levels suggests enhanced atmospheric mercury availability during the Early Toarcian, potentially aided by the apparent affinity of Hg for terrestrial organic matter, although the relative importance of aquatic vs terrestrial fixation of Hg in governing these enrichments remains uncertain. A perturbation in atmospheric Hg is most easily explained by enhanced volcanic output. It is suggested that extrusive igneous activity caused increased mercury flux to the Early Toarcian sedimentary realm, supporting the potential of this element as a proxy for ancient volcanism. This interpretation is consistent with a relationship between LIP formation and a perturbed carbon cycle during the Pliensbachian–Toarcian transition and T-OAE. The recording of these two distinct Hg excursions may also indicate that the Karoo–Ferrar LIP released volatiles in temporally distinct episodes, due either to multiple phases of magmatic emplacement or sporadic release of thermogenic gaseous products from intrusion of igneous material into volatile-rich lithologies.
•Analysis of seven Toarcian stratigraphic sections for Hg concentrations and Hg/TOC ratios.•Control analysis of an upper Jurassic section not associated with volcanism for comparison.•Evidence for two pulses of increased atmospheric mercury levels during the Early Toarcian.•Hg/TOC spikes coincide with record of Pliensbachian–Toarcian extinction and Toarcian OAE.•A global perturbation of volcanic mercury is inferred, but with locally derived complexities.
Widespread marine anoxia triggered by the runoff and recycling of nutrients was a key phenomenon associated with the Frasnian–Famennian (FF) mass extinction. However, the relative importance of ...global‐scale processes versus local influences on site‐specific environmental change remains poorly understood. Here, nitrogen‐isotope (δ15N) trends are combined with organic‐biomarker, phosphorus, and Rock‐Eval data in FF sites from the USA (H‐32 core, Iowa), Poland (Kowala Quarry), and Belgium (Sinsin). Up‐to‐date cyclostratigraphic age models for all three sites allow the nature and timing of changes to be precisely compared across the globe. Negative δ15N excursions across the FF interval from the H‐32 core and Kowala correlate with geochemical evidence for euxinic, phosphorus‐rich, water columns, and possible cyanobacterial activity, suggestive of increased diazotrophic N fixation, potentially coupled with ammonium assimilation at the latter site. By contrast, previously studied sites from Western Canada and South China document enhanced water‐column denitrification around the onset of the Upper Kellwasser (UKW) Event, re‐emphasizing the geographical heterogeneity in environmental perturbations at that time. Moreover, environmental degradation began >100 kyr earlier in Poland, coeval with a major increase in bioavailable phosphorus supply, than in Iowa, where no such influx is recorded. These regional differences in both the timing and nature of marine perturbations during the FF interval likely resulted from the variable influx of terrigenous nutrients to different marine basins at that time, highlighting the importance of local processes such as terrestrial runoff in driving environmental degradation during times of climate cooling such as the UKW Event.
Plain Language Summary
The Frasnian–Famennian mass extinction, ∼372 million years ago, marked one of the most severe biological crises in Earth's history. The extinction has been linked to rapid climate changes and reduced seawater oxygen levels across the global ocean. However, the degree to which environmental stress was globally versus locally controlled remains unclear. This study presents geochemical markers of water‐column oxygenation and nutrient cycling (nitrogen isotopes, phosphorus contents, organic biomarkers) at three localities, the H‐32 core (Iowa, USA), the Kowala Quarry (Poland), and Sinsin (Belgium). The unique feature of these records is the existence of precise age‐depth models, allowing direct comparison of the timing of environmental changes between these sites, and with other key sections from Western Canada and South China. It is shown that whilst the H‐32 core and Kowala indicate possible increases in cyanobacterial nitrogen fixation under phosphorus‐rich, oxygen‐ and nitrate‐depleted conditions, other sites show markedly different nitrogen‐cycle disturbances, such as enhanced water‐column denitrification. Additionally, environmental stress commenced earlier in Kowala than elsewhere, coincident with elevated phosphorus influx to that setting. These regional variations in the timing and nature of environmental perturbations emphasize the importance of local processes such as terrestrial nutrient runoff in causing the Frasnian–Famennian extinction.
Key Points
Nitrogen‐isotope records of globally variable environmental change in the Frasnian–Famennian crisis
Combination with age models highlights further variability in the onset of those changes
Multi‐proxy geochemistry highlights nutrient runoff as trigger of earliest anoxia
Abstract
The Devonian Frasnian–Famennian (F–F) boundary marks one of the five main extinction intervals of the Phanerozoic Aeon. This time was characterized by two pulses of oceanic anoxia, named the ...Lower and Upper Kellwasser events, during which massive marine biodiversity losses occurred. This paper presents high-resolution magnetic susceptibility, X-ray fluorescence elemental geochemistry and carbon isotope datasets obtained from the Steinbruch Schmidt F–F boundary section (Germany). These records lead to an astronomical time calibration of the environmental changes associated with the two ocean anoxia pulses. Cyclostratigraphic interpretation indicates deposition of the black argillaceous Lower and Upper Kellwasser horizons over ~ 90 and ~ 110 kyr, respectively; approximately equivalent to the duration of one short eccentricity cycle. This study confirms that the succession of events within the Upper Kellwasser event is paced by obliquity, under a low-eccentricity orbit. Hence, astronomical insolation forcing likely contributed to the expansion of ocean anoxia and other environmental perturbations associated with these two crises. The new floating chronology established for the Steinbruch Schmidt section is anchored in numerical time by means of a radio-isotopic date, obtained from a bentonite layer interbedded between the two Kellwasser horizons. After anchoring, this time scale gives a high-precision age of 371.870 ± 0.108 Ma for the F–F boundary.
Oceanic Anoxic Event 2 (OAE 2), during the Cenomanian‐Turonian transition (∼94 Ma), was the largest perturbation of the global carbon cycle in the mid‐Cretaceous and can be recognized by a positive ...carbon‐isotope excursion in sedimentary strata. Although OAE 2 has been linked to large‐scale volcanism, several large igneous provinces (LIPs) were active at this time (e.g., Caribbean, High Arctic, Madagascan, Ontong‐Java) and little clear evidence links OAE 2 to a specific LIP. The Mid‐Cenomanian Event (MCE, ∼96 Ma), identified by a small, 1‰ positive carbon‐isotope excursion, is often referred to as a prelude to OAE 2. However, no underlying cause has yet been demonstrated and its relationship to OAE 2 is poorly constrained. Here we report sedimentary mercury (Hg) concentration data from four sites, three from the southern margin of the Western Interior Seaway and one from Demerara Rise, in the equatorial proto‐North Atlantic Ocean. We find that, in both areas, increases in mercury concentrations and Hg/TOC ratios coincide with the MCE and the OAE 2. However, the increases found in these sites are of a lower magnitude than those found in records of many other Mesozoic events, possibly characteristic of a marine rather than atmospheric dispersal of mercury for both events. Combined, the new mercury data presented here are consistent with an initial magmatic pulse at the time of the MCE, with a second, greater pulse at the onset of OAE 2, possibly related to the emplacement of LIPs in the Pacific Ocean and/or the High Arctic.
Key Points
Elevated Hg and Hg/TOC values occur at the stratigraphic levels of the Mid‐Cenomanian Event and Oceanic Anoxic Event 2
Small Hg signals are consistent with Hg dispersal by a submarine plume, following a subaqueous LIP emplacement, rather than atmospheric dispersion
Anomalously high rates of continental weathering have frequently been proposed as a key stimulus for the development of widespread marine anoxia during a number of Late Devonian environmental and ...biospheric crises, which included a major mass extinction during the Frasnian–Famennian transition (marked by the Upper and Lower Kellwasser horizons). Here, this model is investigated by presenting the first stratigraphic record of osmium-isotope trends (187Os/188Os) in upper Devonian strata from the Kowala Quarry (Holy Cross Mountains, Poland). Changes in reconstructed 187Os/188Os seawater values to more radiogenic compositions are documented at the base of both the Lower (~0.42 to ~0.83) and Upper (~0.31 to ~0.81) Kellwasser horizons characteristic of the Frasnian–Famennian transition, and additionally within upper Famennian shales that record a more minor environmental perturbation known as the Annulata Event (~0.20 to ~0.53). These shifts indicate the occurrence of extremely enhanced continental weathering rates at the onsets of the Kellwasser crises and during the later Annulata Event. The similarity of 187Os/188Os values in this study from Frasnian–Famennian boundary and lower Famennian strata (between 0.4 and 0.5) to those from North American stratigraphic equivalents suggests that the 187Os/188Os values record global trends. These findings support a causal relationship between increased continental weathering (and thus, nutrient supply to the marine shelf) and the environmental perturbations that occurred during numerous Late Devonian events, including both of the biospherically catastrophic Kellwasser crises as well as other, less severe, oceanic anoxic events.
•First stratigraphic record of Late Devonian osmium-isotope trends.•Record includes the Frasnian–Famennian extinction and later Annulata anoxic event.•Enhanced continental weathering rates shown to occur during all the studied crises.•Supports the runoff of terrestrial nutrients as a trigger of Late Devonian anoxia.
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