Grzegorz Racki

Water column anoxia, enhanced productivity and concomitant changes in δ13C and δ34S across the Frasnian–Famennian boundary (Kowala — Holy Cross Mountains/Poland)

The investigation of the trace element and organic geochemistry of the Frasnian–Famennian boundary section at Kowala (Holy Cross Mountains/Poland) shows that the lower water column was oxygen-deficient during late Frasnian and early Famennian times. The abundance and carbon isotopic composition of diaryl isoprenoids, biomarkers indicative for green sulfur bacteria, prove that euxinic waters reached into the photic zone, at least episodically. Total organic carbon (TOC) contents show two maxima that are time-equivalent to the Kellwasser horizons deposited in shallower water settings. Enhanced TOC concentrations are explained by a higher primary productivity, presumably as a consequence of an enhanced nutrient supply from the continent. The increase in the abundance of hopanes and bituminite suggests that the bacterial contribution to TOC increased at the Frasnian–Famennian transition. The sulfur isotopic composition of pyritic- and organically bound sulfur shows a +27‰ excursion across the boundary. The observation that the δ34S values of organic-bound sulfur closely resemble that of pyrite sulfur indicates a common sulfur source, likely early diagenetic sulfide. A change in the δ13C of total dissolved inorganic carbon as a consequence of an enhanced burial of 12C-enriched organic carbon is indicated by a +3‰ excursion measured for TOC as well as for individual n-alkanes and isoprenoids. The burial of large amounts of organic carbon is expected to result in a decrease in pCO2 and should affect the photosynthetic carbon isotope fractionation (ep). The fact that we observe no change in ep can be explained by the circumstance that ep was most probably at maximum values, as a consequence of high atmospheric and oceanic-dissolved CO2 concentrations during the Devonian.

The Frasnian/Famennian boundary interval in the South Polish–Moravian shelf basins: integrated event-stratigraphical approach

Abstract The Late Devonian mass extinction event near the Frasnian/Famennian (F/F) boundary has been analysed using conodont biostratigraphy and biofacies, sedimentology, magnetic susceptibility and geochemistry in reference sections of the South Polish–Moravian shelf (Holy Cross Mountains, Cracow and Brno areas). High-resolution biostratigraphic study revealed difficulties in the precise recognition of this ‘natural’ stage boundary, but confirmed the occurrence of a major (third-order) sequence boundary in the F/F transition in an active synsedimentary tectonic setting, marked by erosional discontinuities, hardgrounds and brecciation or omission surfaces. Conclusive evidence of an extraterrestrial impact has not been found. Among Earth-bound factors, the main devastating role in the shelf habitats is ascribed to fluctuating anoxia and/or nutrient dynamics in a disturbed greenhouse climatic setting. The long-term facies changes were determined by a conspicuous break in carbonate production, accompanied by replacement of mature stromatoporoid–coral reefs by pioneer shelly-crinoid banks, microbial mounds and localised oolitic bars. The key F/F passage interval was marked by intermittent but generally accelerated periplatform ooze/debris input and severe storm events, as well as by probably highly fluctuating oxygenation and biological overproduction, best manifested in radiolarian–silicisponge and cephalopod acmes. Eutrophication phenomena, at least partly stimulated by various hydrothermal and volcanic processes, were one of the major biogeochemical processes during this climax of the Late Devonian biotic crisis. However, significant oceanographic perturbations were also linked with the onset of the transgressive–hypoxic Upper Kellwasser Event, still within the latest Frasnian linguiformis Zone, i.e., well before the F/F conodont collapse. Implied variations in the redox state of seawater through the Kellwasser timespan, probably in regionally highly variable temporal scales, are in agreement with recent palaeoecological and biogeochemical inferences, in particular suggesting recovering oxygenation just prior to the F/F boundary in the other Laurussian intrashelf basins. All the data support a long-time, multicausal Earth-bound crisis instead a worldwide cosmic catastrophe.

Radiolarian palaeoecology and radiolarites: is the present the key to the past?

Abstract Radiolarian productivity pulses and related radiolarite deposition are phenomena difficult to understand from an exclusively actualistic viewpoint. Evolutionary selection pressure among silica-secreting marine plankton, both radiolarians and diatoms, has led toward more economic usage of rapidly shrinking nutrient resources, including dissolved silica, of the photic zone in the late Cenozoic oceans, and, in particular, a substantial modification of oceanic cycle by the diatom explosive radiation. Even if there is a proved link between biomineralization and dissolved silica loading among the phytoplankton only, the relative independence of modern siliceous planktic biotas from the available silica pool reflects mainly their progressive physiological specialisation during evolutionary history. Oceanic chemistry and productivity, as well as patterns of circulation/upwelling have changed radically during the Phanerozoic. Radiolarites apparently represent an ‘anachronistic’ facies, as exemplified by their long-lived and ocean-wide distribution in palaeo-Pacific, and hitherto, highlighted actualistic models of localized intra-oceanic wind-driven upwelling loci are of largely questionable applicability. In addition to plate drift, hypersiliceous domains and intervals are explainable mostly by a large-scale volcano–hydrothermal activity during major plate-boundary reconfigurations, which, in many ways, favoured siliceous biotas acme, and their skeletal remains accumulation and preservation. Factors tied to rapid, voluminous submarine eruptions, such as thermal buoyant megaplumes and basin overturns, offer a viable alternative for traditional climatic/circulation scenarios in case of hypersiliceous high productivity events irrelevant to greenhouse-to-icehouse climatic change. The evolving carbon and silica cycles were coupled through the greenhouse effect and enhanced chemical weathering. Volcano-hydrothermal and tectonic uplift events, related mostly to extensive rifting and/or accelerated oceanic spreading, were the endogenous driving force that created this perturbation of the exogenous system. The present biogeochemical cycle is representative only for the overall silica-depleted post-Eocene oceanic ecosystems, which broadly correlates with a major expansion of diatoms groups extremely efficient in silica removal, and closely linking the silica budget with phosphorus and nitrogen cycles. Thus, an orthodox uniformitarian approach to biosiliceous sedimentation, based on a silica-starved vigorous ocean, is of limited significance when applied to the pre-Neogene settings, especially in the peculiar planktic habitats of epeiric seas, as well as during biotic crises marked by strong geotectonic overprint. The major turnovers in marine siliceous biota composition, in particular after the end-Permian radiolarite gap, may have been coupled with discernible changes in an increasing biological control on the long-term oceanic silica cycling (‘punctuated equilibrium’). The evolutionary turnovers have induced a stepdown decrease of dissolved silica levels through the Phanerozoic, contemporaneously with the general secular trend of upward scaling of nutrient-related ecological processes and increased effectiveness of resource utilization.

Frasnian–Famennian biotic crisis: undervalued tectonic control?

The prime cause of the Late Devonian Kellwasser crisis, culminating in a mass extinction event near the Frasnian–Famennian (F–F) boundary, remains conjectural. Nevertheless, rapid sea-level fluctuations of uncertain origin during tectono-eustatic highstand, paired with repeated oceanic anoxia and climatic changes, are usually thought at present to be one of the main immediate triggers. The Cathles–Hallam model of stress-induced changes in plate density, accompanying rapid rift formation, furnishes an alternative for understanding the enigmatic sudden eustatic variations in the non-glacial time. Late Devonian tectonic extension, causing rifting and volcanicity, appears to be strongly marked in several regions of Eurasia, particularly in Kazakhstan and eastern Laurussia. All larger Devonian continents were more or less tectonically affected. A subtle record of this tectonic rearrangement is implied even for distant and apparently quiet carbonate platforms in local extensional block faulting and tilting, hydrothermal mineralisation, geochemical anomalies, and localized blooms of siliceous biota. Interpreting the late Frasnian regressive–transgressive pattern in terms of the Cathles–Hallam tectono-eustatic model, two major rifting events are hypothesized: one at about the beginning of the rhenana Zone, and a second principal pulse in the late linguiformis Zone, that encompassed the F–F transition. Developing the Veimarn–Milanovsky scenario of the global extensional pulse, it is assumed that the key endogenous factors were related to episodic (super)plume activity. The tectonically triggered changes climaxed in thermal and nutrient pulses, and induced the stepdown ecosystem destabilization observed in the F–F bio-crisis. Minor cometary strike(s) might have eventually participated in this prolonged multicausal environmental stress, mainly due to additional thermal shocks, but perhaps effective on a regional scale only.

Large sulphur isotopic perturbations and oceanic changes during the Frasnian-Famennian transition of the Late Devonian

The Frasnian–Famennian transition of the Late Devonian was one of the most critical intervals in the Phanerozoic. Sulphur isotopic pairs of carbonate-associated sulphate and pyrite sulphide from coeval sections in South China and Poland reveal frequent perturbations of sulphur cycling during this time interval. These data suggest a sudden oceanic overturn during a rapid sea-level fall probably induced by jerky block tilting in the latest Frasnian. This event was followed by long-lasting photic-zone euxinia during a rapid sea-level rise in the earliest Famennian. Large increases in continental nutrient fluxes, and subsequent primary productivity and organic burial, could have greatly enhanced bacterial sulphate reduction, producing excessive sulphide through the water columns owing to iron depletion. Subsequently, rapid ventilation of oceanic basins occurred, during which direct aerobic oxidation of sulphide into sulphate predominated in bottom waters and even surface sediments with minimal fractionation. This oxygenation was probably induced by intensive climatic cooling and/or large-scale sea-level fall. The temporal coincidence of two extinction phases with the oceanic overturn and succeeding photic-zone euxinia suggests that these extreme oceanic events played an important role in the severe biotic crisis. Furthermore, photic-zone euxinia coupled with subsequent climatic cooling may have delayed post-extinction recovery of some taxa. Supplementary materials: Two supplementary tables (S1 and S2) indicating pyrite and CAS contents and sulphur isotopic results (δ34SCAS, δ34Spy and Δ34S) across the F-F boundary at Fuhe, South China and Kowala, Poland, respectively, and a supplementary figure showing systematic sulphur isotopic variations across the F-F boundary and their correlations between the two studied sections are available at www.geolsoc.org.uk/SUP18593.

Carbon isotope stratigraphy across the Silurian–Devonian transition in Podolia, Ukraine: evidence for a global biogeochemical perturbation

The carbon and oxygen isotope composition of marine carbonates (δ 13 C and δ 18 O, respectively) are studied in the fossiliferous, stratigraphically well-constrained and remarkably expanded successions of Podolia, SW Ukraine, spanning the Silurian–Devonian transition. Significant isotopic shifts are directly comparable to previously published global secular trends in well-preserved brachiopod calcite isotopic ratios from this region, and therefore may be taken as a reliable primary record of seawater δ 13 C changes. The sections reveal a major positive δ 13 C excursion, with an amplitude above 6 ‰, beginning in the upper Pridoli and reaching peak values as heavy as +4.2 ‰ in the lowermost Lochkovian. This turnover in carbon cycling is followed by a general trend toward more negative δ 13 C values in the upper Lochkovian. The Podolian isotopic signals provide strong support for the previously inferred global biogeochemical perturbation across the Silurian–Devonian transition, reflecting a complex combination of palaeogeographical, biogeochemical and evolutionary processes in the late Caledonian geodynamic setting, with a likely undervalued role of the expanding vegetation in vast near-coastal shallows and deltas.

Late Devonian marine anoxia challenged by benthic cyanobacterial mats

Mass occurrence of benthic cyanobacterial mats in a sequence of Late Devonian black shales and bituminous limestones of the Holy Cross Mts. (central Poland), enclosing the famous Kellwasser and Hangenberg extinction horizons, is reported. The microbiota forming the mats is compared with some modern benthic chroococcalean cyanobacteria. Similarly to their extant counterparts, the Devonian cyanobacteria must had been phototrophic and oxygenic aerobes which could, however, tolerate slightly sulfidic conditions characterizing the near-bottom waters of the Late Devonian epicontinental sea. The cyanobacterial mats successfully colonized the oxygen-deficient and H(2)S-enriched seabed otherwise unfavorable for most other benthic biota. The redox state of this sluggish Late Devonian sea, ascribed previously mostly to anoxic or euxinic conditions, is reassessed as probably pulsating between anoxic, dysoxic, and weakly oxic conditions. The redox state was dependent on the rate of oxygen production by the cyanobacterial mats, the intensity of H(2)S emissions from the decaying mat biomass, and the rate of planktonic production.

Chapter 8Productivity and bottom water redox conditions at the Frasnian-Famennian boundary on both sides of the Eovariscan Belt: constraints from trace-element geochemistry

Abtract The Late Devonian Frasnian-Famennian (F-F) mass extinction event coincides in many places with the depositionof C org -rich “Kellwasser” facies. Four F-F boundary sections representative of platform and basin environments from widely separated locations (Morocco, Germany, and France) were analysed for inorganic geochemistry, especially trace elements (redox and productivity proxies), in order to describe paleodepositional environments for the Kellwasser horizons. Ni/Co, V/Cr, U/Th, and V/(V+Ni) ratios, as well as redox trace metal concentrations indicate that oxygen-depleted conditions existed during the times of Kellwasser facies deposition. In platform settings, dysoxic conditions seem to be limited to the Late Frasnian. In basinal settings, oxygen depletion was stronger and persisted into the Early Famennian. Enrichments of Ba, Cu, Ni, that are limited to the Late Frasnian, show that surface productivity was relatively high and organic matter could accumulate, especially in the deeper environments. The stratigraphical distribution of several geochemical markers are linked with two positive excursions of the δ 13 C carb signal that result from enhanced organic matter burial. Reducing conditions likely resulted from high productivity of Late Devonian marine ecosystems. Intense nutrient supply resulted probably from the biogeochemical recycling of nutrients, and/or runoff from emerged lands. Coupled with other factors, such as rapid sea-level fluctuations and climatic changes, oxygen-depleted conditions and eutrophication would have modified Late Devonian environments and could be possible factors in the F-F mass mortality.

Faunal Dynamics Across the Silurian—Devonian Positive Isotope Excursions (δ13C, δ18O) in Podolia, Ukraine: Comparative Analysis of the Ireviken and Klonk Events

Two global isotopic events, the early Sheinwoodian (early Wenlock) and that at the Silurian—Devonian transition, have been comprehensively studied in representative carbonate successions at Kytayhorod and Dnistrove, respectively, in Podolia, Ukraine, to compare geochemistry and biotic changes related correspondingly to the Ireviken and Klonk events. These two large-scale isotope excursions reveal different regional ecosystem tendencies. The well-defined increasing trend across the Llandovery—Wenlock boundary in siliciclastic input, redox states and, supposedly, bioproductivity, was without strict correlative relations to the major 13C enrichment event. The environmental and biotic evolution was forced by eustatic sea-level fluctuations and two-step climate change toward a glaciation episode, but strongly modified by regional epeirogeny movements due to location near the mobile Teisseyre-Törnquist Fault Zone. Thus, the global early Sheinwoodian biogeochemical perturbation was of minor depositional significance in this epeiric sea, as in many other Laurussian domains. Conversely, the Podolian sedimentary record of the Klonk Event exhibits temporal links to the abrupt &dgr;13C anomaly, overprinted by a tectonically driven deepening pulse in the crucial S–D boundary interval. This carbon cycling turnover was reflected in the regional carbonate crisis and cooling episodes, paired with a tendency towards eutrophication and recurrent oxygen deficiency, but also with major storms and possible upwelling. Faunal responses in both Podolian sections follow some characters of the Silurian pattern worldwide, as manifested by conodont changeover prior to the major early Sheinwoodian isotopic/climatic anomaly. This contrasts with the relative brachiopod and chitinozoan resistances in the course of the Ireviken Event. Also, during the Klonk Event, a moderate faunal turnover, both in benthic and pelagic groups, occurred only near the very beginning of the prolonged 13C-enriched timespan across the system boundary, possibly due to progressive dysoxia and temperature drop. The characters point to a peculiarity of the Klonk Event by comparison with the Silurian global events, and some similarity already to the succeeding Devonian transgressive/anoxic episodes.

The Weathering-Modified Iridium Record of a New Cretaceous—Palaeogene Site at Lechówka Near Chełm, SE Poland, and Its Palaeobiologic Implications

In the light of integrated biostratigraphic and geochemical data, a complete shallow-marine succession across the Cretaceous—Palaeogene (K—Pg) boundary, with the critical boundary clay coupled with a burrowed siliceous chalk (“opoka” in Polish geological literature), possibly equivalent of the basal Danian Cerithium Limestone in Denmark, has been discovered at Lechówka near Chelm, SE Poland. An extraterrestrial signature marking the K—Pg boundary is confirmed by anomalously high amounts of iridium (up to 9.8 ppb) and other siderophile elements (especially Au and Ni), as well as by an elevated Ir/Au ratio consistent with a chondrite meteoritic composition. The major positive iridium spike surprisingly occurs in Maastrichtian marls, 10 cm below the boundary clay interval, which can be explained by diagenetic mobilisation and re-concentration of the impact-derived components. Thus, intensively infiltrating, humic acid-rich ground waters during the long-lasting Palaeogene weathering in tropical humid regimes were probably responsible not only for the large-scale decalcification of the Lechówka section, but also for both downward displaced position of the iridium enrichment, a dispersed profile of this anomaly and its significantly lessened value, but still approaching an increase by a factor of 100. This modified record of the K—Pg boundary event points to a careful reconsideration of the iridium anomaly as a trustworthy marker for studying the extinction patterns across the K—Pg boundary, as supported by the recent data from New Jersey, USA.