The second pulse of the Late Ordovician mass extinction occurred around the Hirnantian-Rhuddanian boundary (~444 Ma) and has been correlated with expanded marine anoxia lasting into the earliest ...Silurian. Characterization of the Hirnantian ocean anoxic event has focused on the onset of anoxia, with global reconstructions based on carbonate δ
U modeling. However, there have been limited attempts to quantify uncertainty in metal isotope mass balance approaches. Here, we probabilistically evaluate coupled metal isotopes and sedimentary archives to increase constraint. We present iron speciation, metal concentration, δ
Mo and δ
U measurements of Rhuddanian black shales from the Murzuq Basin, Libya. We evaluate these data (and published carbonate δ
U data) with a coupled stochastic mass balance model. Combined statistical analysis of metal isotopes and sedimentary sinks provides uncertainty-bounded constraints on the intensity of Hirnantian-Rhuddanian euxinia. This work extends the duration of anoxia to >3 Myrs - notably longer than well-studied Mesozoic ocean anoxic events.
Uranium isotopes (238U/235U) have been used widely over the last decade as a global proxy for marine redox conditions. The largest isotopic fractionations in the system occur during U reduction, ...removal, and burial. Applying this basic framework, global U isotope mass balance models have been used to predict the extent of ocean floor anoxia during key intervals throughout Earth's history. However, there are currently minimal constraints on the isotopic fractionation that occurs during reduction and burial in anoxic and iron‐rich (ferruginous) aquatic systems, despite the consensus that ferruginous conditions are thought to have been widespread through the majority of our planet's history. Here we provide the first exploration of δ238U values in natural ferruginous settings. We measured δ238U in sediments from two modern ferruginous lakes (Brownie Lake and Lake Pavin), the water column of Brownie Lake, and sedimentary rocks from the Silurian‐Devonian boundary that were deposited under ferruginous conditions. Additionally, we provide new δ238U data from core top sediments from anoxic but nonsulfidic settings in the Peru Margin oxygen minimum zone. We find that δ238U values from sediments deposited in all of these localities are highly variable but on average are indistinguishable from adjacent oxic sediments. This forces a reevaluation of the global U isotope mass balance and how U isotope values are used to reconstruct the evolution of the marine redox landscape.
Key Points
δ238U values were measured in iron‐rich, anoxic (ferruginous) modern natural environments, and Paleozoic shales deposited under ferruginous conditions
δ238U fractionations in these environments are highly variable and generally indistinguishable from isotopic fractionations associated with oxic settings
δ238U fractionations in these environments are highly variable and generally indistinguishable from isotopic fractionations associated with oxic settings
An important hydrocarbon source rock (a ‘hot’ shale) has been identified from the middle Rhuddanian (Llandovery, lower Silurian) of a number of localities in North Africa and Arabia. It is unusual in ...having been deposited during a eustatic fall in sea-level. Evidence for this sea-level fall in Libya and Jordan is manifested in a wide variety of palynological proxy data (including reduced acritarch and chitinozoan total abundance, reduced acritarch diversity and changes in the relative abundances of various environmentally sensitive acritarch morphogroups) and from the presence of a minor positive δ13Corg excursion at the same stratigraphical level as the ‘hot’ shale. The environment of deposition was anoxic before, during and after deposition of the ‘hot’ shale, except for some very brief incursions of more oxygenated water that enabled the development of a very limited burrowing benthos and graptolite preservation as three-dimensional pyrite internal moulds. These incursions occurred just before and during deposition of the ‘hot’ shale and are interpreted as resulting from the influx of sediment-laden oxygenated currents from shallower water environments. Enhanced organic matter preservation within the ‘hot’ shale is attributed to greater productivity resulting from increased nutrient input, particularly from wind-blown dust, more rapid descent through the water column of organic matter in post-bloom marine snow macroaggregates, development of geochemically sealing microbial mats at the sea floor and more rapid burial of organic material than had occurred earlier in the Rhuddanian.
•‘Hot’ shale deposition in early Silurian occurred during regression not transgression.•Model supported by extensive palynological and carbon isotope data•Most detailed published study of a Silurian ‘hot’ shale core•Organic carbon preservation resulted from high productivity and rapid sinking and burial.
Two new species of graptolite are described from the lower part of the Spirograptus turriculatus Biozone (lower Telychian) of Kallholn Quarry, Dalarna, Sweden. Pristiograptus paradoxus sp. nov. is ...ventrally curved and is more similar to uppermost Wenlock-Ludlow Bohemograptus species than to other Llandovery Pristiograptus species. Torquigraptus loveridgei sp. nov. is stratigraphically intermediate between the well-known species, T. planus and T. proteus.
The Ordovician (∼487 to 443 Ma) ended with the formation of extensive Southern Hemisphere ice sheets, known as the Hirnantian glaciation, and the second largest mass extinction in Earth History. It ...was followed by the Silurian (∼443 to 419 Ma), one of the most climatically unstable periods of the Phanerozoic as evidenced by several large scale (>5‰) carbon isotope (δ13C) perturbations associated with further extinction events. Despite several decades of research, the cause of these environmental instabilities remains enigmatic. Here, we provide osmium (187Os/188Os) and lithium (δ7Li) isotope measurements of marine sedimentary rocks that cover four Silurian δ13C excursions. Osmium and Li isotope records resemble those previously recorded for the Hirnantian glaciation suggesting a similar causal mechanism. When combined with a new dynamic carbon-osmium-lithium biogeochemical model we suggest that astronomical forcing of the marine organic carbon cycle, as opposed to a decline in volcanic arc degassing or the rise of early land plants, resulted in drawdown of atmospheric CO2, triggering continental scale glaciation, intense global cooling and eustatic sea-level lows recognised in the geological record. Lower atmospheric pCO2 and temperatures during the Hirnantian and Silurian glaciations suppressed CO2 removal by silicate weathering, driving 187Os/188Os and δ7Li variability, supporting the existence of climate-regulating feedbacks.
•Osmium and Li isotope records are provided for four Silurian climate events.•A new C-Os-Li-model links these events to orbitally forced organic carbon burial.•Organic carbon burial drove CO2 sequestration, global cooling and glaciation.•A reduction in global silicate weathering rates helped reverse cooling trends.•Late Ordovician and Silurian climate reminiscent of “Cenozoic-style” glacial cycles.
•First described Laotian graptolites are from Llandovery of Truong Son Terrane.•Graptolites have higher latitude, Gondwanan not tropical South China affinities.•Fauna implies Truong Son was closer to ...Southern Europe-Arabia than to South China.
Graptolites are described from the Llandovery of the Sepon mine area, central Laos, part of the Truong Son Terrane. The palaeobiogeographical affinities of the Rhuddanian graptolites are with peri-Gondwanan Europe and Arabia rather than equatorial regions such as South China and Laurentia (Arctic Canada). A review of previous palaeontological research on the middle Palaeozoic of the Indochina terranes reveals an often contradictory picture with vertebrates suggesting close proximity of South China to the Truong Son Terrane in the Devonian and invertebrates and radiolarians providing evidence for a palaeogeographical barrier between the two. Graptolites from the Mojiang area, Yunnan are typical of low latitude Silurian faunas and suggest that this area (Simao or Loei Terrane) was separate from the Truong Son Terrane.
Palaeontological, sedimentological and geochemical data (particularly from LA-ICPMS analyses of pyrite) from a 12.4 m section through the lower Sheinwoodian Trewern Brook Mudstone Formation of ...Buttington Quarry, Wales are combined to reveal the environmental changes taking place during the early Wenlock Epoch of the Silurian, during the early part of the ESCIE (early Sheinwoodian carbon isotope excursion). The uppermost Cyrtograptus murchisoni through to lower Monograptus riccartonensis biozones show increased nutrient levels and peaks in palynomorph and graptolite abundance. At this time graptolite diversity globally was declining dramatically, possibly associated with metal (particularly zinc) concentrations attaining toxic levels and probably with the base of the euphotic zone being higher in the water column (resulting from blooms of phytoplankton) causing those graptolites that specialized in feeding on phytoplankton that were outcompeted in this diminished volume of illuminated surface waters to decline in numbers and eventually become extinct. Correlation of the graptolite and conodont biozonations enables the levels of the main stratigraphically higher Ireviken Event Datum Points to be proposed for the Buttington section and this in turn allows events recognised elsewhere (intervals of malformed palynomorphs and re-organization of polychaete faunas, together with an oceanic anoxic event) to be correlated with the lower part of the studied Buttington section. The commencement of the OAE, identified on Gotland at Ireviken Event Datum Point 4, appears to correlate rather well with a rapid transition, marking the top of the Butterley Mudstone Member at Buttington, from heavily bioturbated mudstones to laminated graptolitic mudstones. Surprisingly, the most rapid change in δ13Ccarb values within the Buttington section (from 0.6‰ at 8.19–8.25 m to 2.09‰ at 8.50–8.55 m) is not associated with any major faunal, palynological abundance or geochemical changes – these are all coincident with the initial rise in δ13Ccarb values, up to about the 5 m level in the Buttington section. Another unexplained feature of the graptolite record is a sudden change from Monograptus-dominated to Pristiograptus-dominated assemblages about half way through the studied section.
•Early Wenlock (Silurian) graptolite extinction at time of positive carbon isotope excursion.•Pyrite geochemistry provides a nutrient proxy indicating high nutrient abundance during decline in graptolite diversity.•High zinc concentrations may have contributed to graptolite extinctions.•Major lithological change to laminated shales correlated with beginning of Oceanic Anoxic Event.
Early Paleozoic bottom waters were mainly ferruginous, like the Neoproterozoic, with a shift to increased euxinia in the Devonian.
The extent to which Paleozoic oceans differed from Neoproterozoic ...oceans and the causal relationship between biological evolution and changing environmental conditions are heavily debated. Here, we report a nearly continuous record of seafloor redox change from the deep-water upper Cambrian to Middle Devonian Road River Group of Yukon, Canada. Bottom waters were largely anoxic in the Richardson trough during the entirety of Road River Group deposition, while independent evidence from iron speciation and Mo/U ratios show that the biogeochemical nature of anoxia changed through time. Both in Yukon and globally, Ordovician through Early Devonian anoxic waters were broadly ferruginous (nonsulfidic), with a transition toward more euxinic (sulfidic) conditions in the mid–Early Devonian (Pragian), coincident with the early diversification of vascular plants and disappearance of graptolites. This ~80-million-year interval of the Paleozoic characterized by widespread ferruginous bottom waters represents a persistence of Neoproterozoic-like marine redox conditions well into the Phanerozoic.
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