Anoxia and euxinia

Late Early Permian-lowermost Triassic carbonate, siliceous (spiculites) and clastic marine sediments in the Marmierfjellet area (Isfjorden, central Spitsbergen) contain a relatively diverse and abundant trace fossil assemblage providing important information about the depositional processes. The Vøringen Member (Late ArtinskianÁKungurian) of the Kapp Starostin Formation (Late Artinskian-?Changhsingian) contains trace fossils (Nereites, Phycosiphon, Zoophycos and Arenicolites common in tempestites) typical of the proximal archetypal Cruziana ichnofacies, which indicates lower shoreface. Nereites, Phycosiphon and Zoophycos, accompanied by other rare trace fossils, characterize the Svenskegga and Hovtinden members of the Kapp Starostin Formation. They are interpreted as the distal Cruziana ichnofacies, possibly transitional to the Zoophycos ichnofacies typical of the lower offshore zone. However, the sporadic occurrences of Arenicolites and Macaronichnus can point to episodic shallowing to upper offshore - lower shoreface. The lowest part of the Triassic Vikinghøgda Formation (Induan-Olenekian) contains a very low-diverse ichnoassemblage composed of a few simple and branched forms ascribed to the impoverished Cruziana ichnofacies (lower to upper offshore environment), which is attributed to the early recovery stage after the Permian-Triassic extinction. The trace fossils and loss of primary sedimentary structures caused by intense bioturbation throughout most of the section point to generally oxygenated pore waters on the sea floor. However, some horizons, especially laminated black shales, display reduced or no bioturbational activity. These horizons also show high V/(V+Ni) ratios, which indicate oxygen-depleted sediments with periods of anoxic conditions. A remarkable black shale unit deposited under anoxic and sulphidic conditions occurs at the Permian-Triassic transition.

Explaining the ~5-million-year delay in marine biotic recovery following the latest Permian mass extinction, the largest biotic crisis of the Phanerozoic, is a fundamental challenge for both geological and biological sciences. Ocean redox perturbations may have played a critical role in this delayed recovery. However, the lack of quantitative constraints on the details of Early Triassic oceanic anoxia (for example, time, duration, and extent) leaves the links between oceanic conditions and the delayed biotic recovery ambiguous. We report high-resolution U-isotope (d 238 U) data from carbonates of the uppermost Permian to lowermost Middle Triassic Zal section (Iran) to characterize the timing and global extent of ocean redox variation during the Early Triassic. Our d 238 U record reveals multiple negative shifts during the Early Triassic. Isotope mass-balance modeling suggests that the global area of anoxic seafloor expanded substantially in the Early Triassic, peaking during the latest Permian to mid-Griesbachian, the late Griesbachian to mid-Dienerian, the Smithian-Spathian transition, and the Early/Middle Triassic transition. Comparisons of the U-, C-, and Sr-isotope records with a modeled seawater PO 4 3− concentration curve for the Early Triassic suggest that elevated marine productivity and enhanced oceanic stratification were likely the immediate causes of expanded oceanic anoxia. The patterns of redox variation documented by the U-isotope record show a good first-order correspondence to peaks in ammonoid extinctions during the Early Triassic. Our results indicate that multiple oscillations in oceanic anoxia modulated the recovery of marine ecosystems following the latest Permian mass extinction.

Molybdenum isotopes are considered to be a promising paleoredox proxy in reducing facies for evaluating either global-ocean redox conditions (given quantitative Mo uptake) or local redox conditions (given non-quantitative Mo uptake). Here, we examine the Mo-isotopic composition of black to gray shales from the Permian-Triassic boundary (PTB) section at Chaohu, eastern China, in order to assess the utility of this proxy under a range of paleoredox conditions. We evaluated local redox conditions using a combination of redox-sensitive trace-element concentrations (Mo, U) and Fe T /Al ratios, recognizing six intervals of more reducing conditions at the PTB and Induan-Olenekian boundary (IOB), in the mid-Smithian, at the Smithian-Spathian boundary (SSB), and in the mid-and upper Spathian. These six intervals are associated with variably weak to moderate enrichments of Mo, and they show variable shifts away from background δ 98 Mo values of ca. −0.4 to +0.1‰ (recording detrital Mo-isotopic compositions) toward δ 98 Mo values as high as +2.3 ± 0.2‰. Based on an analysis of local redox proxies versus δ 98 Mo as well as of sample-to-sample variance in δ 98 Mo values, we conclude that only one of the six reducing intervals in the study section is likely to record contemporaneous seawater δ 98 Mo (as a result of quasi-quantitative Mo uptake), i.e., the interval from the Hindeodus typicalis conodont Zone close to the PTB, for which δ 98 Mo values of +2.3 ± 0.2‰ suggest average global-ocean redox conditions similar to those of the modern ocean. For the remaining five reducing intervals, non-quantitative Mo uptake led to Mo-isotope signatures reflecting local, mostly suboxic redox conditions. The highest Mo concentrations (17–23 ppm) were yielded by the mid-and upper Spathian intervals, and the lack of concurrent U enrichment in the mid-Spathian interval suggests the local operation of a Mn-Fe particulate shuttle, an inference that is consistent with its relatively low δ 98 Mo values (0 ± 0.3‰). The lack of evidence for strong oceanic anoxia in the South China Craton following the PTB crisis may have been a factor in the relatively rapid recovery of marine ecosystems in this region during the Early Triassic. The present contribution serves as a case study of the application of Mo-isotopes as a paleoredox proxy in low-[Mo] sediments, which requires a careful assessment of the relative importance of global versus local redox influences based on additional proxies and evaluation criteria.

Sedimentary burial of the essential nutrient phosphorus (P) under anoxic and sulfidic conditions is incompletely understood. Here, we use chemical and micro-scale spectroscopic methods to characterize sedimentary P burial along a water column redox transect (six stations, 78–2107 m water depth) in the Black Sea from the shelf with its oxygenated waters to the anoxic and sulfidic deep basin. Organic P is an important P pool under all redox regimes, accounting for up to 60% of P burial. We find a general down-core increase in the relative importance of organic P, especially on the shelf where P bound to iron (Fe) and manganese (Mn) (oxyhydr)oxides is abundant in the uppermost sediment but rapidly declines in concentration with sediment depth. Our chemical and spectroscopic data indicate that the carbonate-rich sediments (Unit I, $3000 years, $0– 30 cm depth) of the sulfidic deep Black Sea contain three major P pools: calcium phosphate (apatite), organic P and P that is strongly associated with CaCO 3 and possibly clay surfaces. Apatite concentrations increase from 5% to 25% of total P in the uppermost centimeters of the deep basin sediments, highlighting the importance of apatite formation for long-term P burial. Iron(II)-associated P (ludlamite) was detected with X-ray absorption spectroscopy but was shown to be a minor P pool ($5%), indicating that lateral Fe–P transport from the shelf (''shuttling ") likely occurs but does not impact the P burial budget of the deep Black Sea. The CaCO 3 –P pool was relatively constant throughout the Unit I sediment interval and accounted for up to 55% of total P. Our results highlight that carbonate-bound P can be an important sink for P in CaCO 3-rich sediments of anoxic, sulfidic basins and should also be considered as a potential P sink (and P source in case of CaCO 3 dissolution) when reconstructing past ocean P dynamics from geological records.

May, A. (1996a): Relationship among sea-level fluctuation, biogeography, and bioevents of the Devonian: an attempt to approach a powerful, but simple model for complex long-range control of biotic crises. - Geolines, 3: 38-49, 2 figs.; Praha.

ABSTRACT. Connections among changing sea-level, development of marine biogeography and appearance of bioevents in Devonian times are emphasised. Decreasing Devonian provincialism of the marine fauna reflects rising sea-level of global ocean. Whereas the Emsian sea-level lowstand was accompanied by strong provincialism of the marine communities, the Frasnian highstands were linked to an unusual decrease of provincialism. As generally documented, the partial but intensive transgressional events (often related to bioevents) were usually associated with a decrease of the provincialism. The best explanation for the majority of Devonian bioevents is the effect of oceanic anoxia. A model suggested herein operates with a fast transgression related to fast ocean sea-level rise. A direct consequence of this fast transgression was the increased primary biomass production (caused by both increased infiux of nutrients and increased total surface of shallow seas). Decreased oxygen supply of oceans continued this process being caused by the global temperature increase which resulted from the albedo decrease. These consequences of these combined factors produced an oceanic anoxic event which started the extinction processes of the bioevent. The well-documented pattern of intensified biological crises during the Devonian, time-slice by time-slice (from Emsian to Frasnian), is explained as a direct result of decreased provincialism of the marine fauna. Both decrease of the provincialism and increased intensity of the Devonian bioevents can be deduced directly or indirectly from changes of global sea-level. This simply constructed but very complex explanation yields two advantages: first, it needs only very few infiuencing factors, second, it can basically renounce incalculable influences (e.g. meteorite impacts). The suggested concept of oceanic anoxic events can explain also other major bioevents of the Earth history.

KURZFASSUNG: Der Artikel versucht, Zusammenhänge zwischen den Veränderungen des Meeresspiegels, der Entwicklung der marinen Biogeographie und den Bioevents im Devon herauszuarbeiten und zu erklären. Die Entwicklung des Faunen-Provinzialismus im Devon ist ein Ergebnis der Entwicklung des globalen Meeresspiegels. Im Emsium war der Meeresspiegel niedrig und der Provinzialismus der Meeresfauna gross, im Frasnium war der Meeresspiegel hoch und der Provinzialismus sehr gering. Insbesondere einige Trangressions-Schübe, die sich auch als Bioevents bemerkbar machten, führten zu schnellen Abnahmen der Grösse des Provinzialismus. Die beste Erklärung für die meisten devonischen Bioevents ist das Auftreten von ozeanischen anoxischen Events. Das hier vorgestellte Modell setzt bei schnellen Transgressionen an, die durch schnelle Meeresspiegel-Anstiege verursacht wurden. Die Konsequenzen einer schnellen Transgression waren gesteigerte Primär produktion organischer Substanz und geringere Sauerstoff- Versorgung der Ozeane, was zusammen zum ozeanischen anoxischen Event und zum Bioevent führte. Der Umstand, dass die Intensität der Bioevents vom Emsium zum Frasnium zunahm, wird durch die Abnahme des Provinzialismus der Meeresfauna in dieser Zeit erklärt. Sowohl die Entwicklung des Provinzialismus der marinen Fauna als auch das Auftreten und die Intensität der Bioevents im Devon lassen sich direkt oder mittelbar aus der Entwicklung des globalen Meeresspiegels ableiten. Dieser sehr umfassende Erklärungsansatz hat den Vorteil, mit sehr wenigen Einfiussfaktoren auszukommen und auf unkalkulierbare Einwirkungen (wie z.B. Meteoriten-Einschläge) völlig verzichten zu können. Vermutlich ist das hier entwickelte Konzept eines ozeanischen anoxischen Events auch auf andere Bioevents übertragbar.

KEYWORDS: Devonian, Kellwasser Event, ecosystems, biogeography, sea level fiuctuation.

Abstract Molybdenum isotopes are considered to be a promising paleoredox proxy in reducing facies for evaluating either global-ocean redox conditions (given quantitative Mo uptake) or local redox conditions (given non-quantitative Mo uptake). Here, we examine the Mo-isotopic composition of black to gray shales from the Permian-Triassic boundary (PTB) section at Chaohu, eastern China, in order to assess the utility of this proxy under a range of paleoredox conditions. We evaluated local redox conditions using a combination of redox-sensitive trace-element concentrations (Mo, U) and FeT/Al ratios, recognizing six intervals of more reducing conditions at the PTB and Induan-Olenekian boundary (IOB), in the mid-Smithian, at the Smithian-Spathian boundary (SSB), and in the mid- and upper Spathian. These six intervals are associated with variably weak to moderate enrichments of Mo, and they show variable shifts away from background δ98Mo values of ca. −0.4 to +0.1‰ (recording detrital Mo-isotopic compositions) toward δ98Mo values as high as +2.3 ± 0.2‰. Based on an analysis of local redox proxies versus δ98Mo as well as of sample-to-sample variance in δ98Mo values, we conclude that only one of the six reducing intervals in the study section is likely to record contemporaneous seawater δ98Mo (as a result of quasi-quantitative Mo uptake), i.e., the interval from the Hindeodus typicalis conodont Zone close to the PTB, for which δ98Mo values of +2.3 ± 0.2‰ suggest average global-ocean redox conditions similar to those of the modern ocean. For the remaining five reducing intervals, non-quantitative Mo uptake led to Mo-isotope signatures reflecting local, mostly suboxic redox conditions. The highest Mo concentrations (17–23 ppm) were yielded by the mid- and upper Spathian intervals, and the lack of concurrent U enrichment in the mid-Spathian interval suggests the local operation of a Mn-Fe particulate shuttle, an inference that is consistent with its relatively low δ98Mo values (0 ± 0.3‰). The lack of evidence for strong oceanic anoxia in the South China Craton following the PTB crisis may have been a factor in the relatively rapid recovery of marine ecosystems in this region during the Early Triassic. The present contribution serves as a case study of the application of Mo-isotopes as a paleoredox proxy in low-[Mo] sediments, which requires a careful assessment of the relative importance of global versus local redox influences based on additional proxies and evaluation criteria.

The evolution of complex life over the past 600 million years was disrupted by at least five mass extinctions, one of which occurred at the close of the Triassic period. The end-Triassic extinction corresponds to a period of high atmospheric-CO 2 concentrations caused by massive volcanism and biomass burning; most extinction scenarios invoke the resulting environmental perturbations in accounting for the loss of marine and terrestrial biodiversity. Here we reconstruct changes in Tethyan shallow marine ecosystems and ocean redox chemistry from earliest Jurassic (Hettangian)-aged black shales from Germany and Luxemburg. The shales contain increased concentrations of the biomarker isorenieratane, a fossilized pigment from green sulphur bacteria. The abundance of green sulphur bacteria suggests that the photic zone underwent prolonged periods of high concentrations of hydrogen sulphide. This interval is also marked by the proliferation of green algae, an indicator of anoxia. We conclude that the redox changes in the entire water column reflect sluggish circulation in marginal regions of the Tethys Ocean. We suggest that the resultant repeated poisoning of shallow epicontinental seas-hotspots of Mesozoic biodiversity-with hydrogen sulphide may have slowed the recovery of marine ecosystems during the Early Jurassic.

May, A. (1997g): Ein Modell zur Erklärung der Bio-Events und der biogeographischen Entwicklung im Devon durch den Meeresspiegel. - Dortmunder Beiträge zur Landeskunde, Naturwissenschaftliche Mitteilungen, 31: 137-174, 5 figs.; Dortmund.

Abstract: The attempt is made, to show and to explain connections between the changing sea-Ievel, marine biogeography, and bio-events within the Devonian. The Devonian development of marine faunal provincialism is the result of changes in global sea-Ievel. Within the Emsian sea-Ievel was low and marine faunal provincialism was strong, while within the Frasnian sea-Ievel was high and marine faunal provincialism was weak. Some particular transgressive intervals made themselves felt as bio-events, conducive to fast decreases in the intensity of provincialism. Starting with the Daleje Event of the Emsian and ending with the Kellwasser Event of the Frasnian, the bio-events are discussed in detail. In this context new data concerning the Middle Devonian of the Eastern Rhenish Massif are presented. The best explanation for most Devonian bio-events is the occurrence of oceanic anoxic events. The model presented here uses fast transgressions caused by rapid sea-Ievel rises (see fig. 1). Fast transgression increased primary biomass production (caused by an increased influx of nutrients and an increased area of shallow sea) and decreased oxygen supply in the oceans (caused by global temperature increase resulting from albedo decrease). Both factors together resulted in the oceanic anoxic event conducive to the bio-event. The fact, that bio-event intensity increased from the Emsian to the Frasnian is explained by the decrease of marine faunal provincialism within this time. Both the development of provincialism and the occurrence and intensity of bio-events within the Devonian can be deduced directly or indirectly from the changes in global sea-Ievel. This comprehensive attempt at explanation has the advantage of needing only a few influencing factors and renouncing incalculable influences (e. g. meteorite impacts). Possibly, the model of oceanic anoxic events presented is also transferable to other bio-events (e. g.: bio-events of the Famennian, Cenomanian/Turonian Event). Kurzfassung: Der Artikel versucht, Zusammenhänge zwischen den Veränderungen des Meeresspiegels, der Entwicklung der marinen Biogeographie und den Bio-Events im Devon herauszuarbeiten und zu erklären. Die Entwicklung des Faunen-Provinzialismus im Devon ist ein Ergebnis der Entwicklung des globalen Meeresspiegels. Im Emsium war der Meeresspiegel niedrig und der Provinzialismus der Meeresfauna groß, im Frasnium war der Meeresspiegel hoch und der Provinzialismus sehr gering. Insbesondere einige Trangressions-Schübe, die sich auch als Bio-Events bemerkbar machten, führten zu schnellen Abnahmen der Größe des Provinzialismus. Die Bio-Events vom Daleje-Event des Emsiums bis zum Kellwasser-Event des Frasniums werden detailliert besprochen. In diesem Zusammenhang werden auch neue Daten zum Mitteldevon des Rechtsrheinischen Schiefergebirges gegeben. Die beste Erklärung für die meisten devonischen Bio-Events ist das Auftreten von ozeanischen anoxischen Events. Das hier vorgestellte Modell setzt bei schnellen Transgressionen an, die durch schnelle Meeresspiegel-Anstiege verursacht wurden (siehe Abb. 1). Die Konsequenzen einer schnellen Transgression waren gesteigerte Primärproduktion organischer Substanz und geringere Sauerstoff-Versorgung der Ozeane, was zusammen zum ozeanischen anoxischen Event und zum Bio-Event führte. Der Umstand, daß die Intensität der Bio-Events vom Emsium zum Frasnium zunahm, wird durch die Abnahme des Provinzialismus der Meeresfauna in dieser Zeit erklärt. Sowohl die Entwicklung des Provinzialismus der marinen Fauna als auch das Auftreten und die Intensität der Bio-Events im Devon lassen sich direkt oder mittelbar aus der Entwicklung des globalen Meeresspiegels ableiten. Dieser sehr umfassende Erklärungsansatz hat den Vorteil, mit sehr wenigen Einflußfaktoren auszukommen und auf unkalkulierbare Einwirkungen (wie z. B. Meteoriten-Einschläge) völlig verzichten zu können. Vermutlich ist das hier entwickelte Konzept eines ozeanischen anoxischen Events auch auf andere Bio-Events übertragbar.

The evolution of complex life over the past 600 million years was disrupted by at least five mass extinctions, one of which occurred at the close of the Triassic period. The end-Triassic extinction corresponds to a period of high atmospheric-CO2 concentrations caused by massive volcanism and biomass burning; most extinction scenarios invoke the resulting environmental perturbations in accounting for the loss of marine and terrestrial biodiversity. Here we reconstruct changes in Tethyan shallow marine ecosystems and ocean redox chemistry from earliest Jurassic (Hettangian)-aged black shales from Germany and Luxemburg. The shales contain increased concentrations of the biomarker isorenieratane, a fossilized pigment from green sulphur bacteria. The abundance of green sulphur bacteria suggests that the photic zone underwent prolonged periods of high concentrations of hydrogen sulphide. This interval is also marked by the proliferation of green algae, an indicator of anoxia. We conclude that the redox changes in the entire water column reflect sluggish circulation in marginal regions of the Tethys Ocean. We suggest that the resultant repeated poisoning of shallow epicontinental seas—hotspots of Mesozoic biodiversity—with hydrogen sulphide may have slowed the recovery of marine ecosystems during the Early Jurassic.