Axel Munnecke

The Ireviken Event in the lower Silurian of Gotland, Sweden – relation to similar Palaeozoic and Proterozoic events

Abstract For a long time, the Silurian was thought to represent a time of stable environmental conditions in the greenhouse period that followed the Late Ordovician glaciation. During the past decade, knowledge about the Silurian has increased markedly and today it is known that the conditions in the Silurian were much more variable than previously assumed. Detailed isotopic investigations have revealed several distinct positive excursions in both carbon and oxygen isotope values. In low latitudes, these periods of high C- and O-isotope values are in many cases characterised by the growth of reefs and the formation of extended carbonate platforms. The sediments deposited during these excursions contain impoverished fossil assemblages, especially with respect to conodonts, graptolites, and trilobites. A conspicuous isotope excursion coincident with facies changes and a marked mass extinction is observed near the Llandovery/Wenlock boundary. This event is called the ‘Ireviken Event’ after its type locality on the island of Gotland, Sweden (Jeppsson, L., 1987. In: Palaeobiology of Conodonts. Ellis Horwood Ltd., Chichester, pp. 129–145). Here, isotope data from nine sections at the NW coast of Gotland are presented that cover the time interval of the Ireviken Event. The δ13C mean values rise from +1.4‰ to +4.5‰, and the δ18O values increase from −5.6‰ to −5.0‰. The relative timing of stable isotope development, extinctions, and facies development is discussed. It is shown that first extinctions precede the isotope excursion. This indicates that extinction events and stable isotope development are only indirectly connected but might reflect the same causes. Other events characterised by similar relationships between positive isotope excursions, mass extinctions, and facies development are found in younger parts of the Silurian (late Wenlock and Ludlow), in the late Ordovician, the late Cambrian, and, with some reservations, in the Proterozoic. The similarities between these events indicate analogous controlling mechanisms. For the Silurian, climatic changes between humid and arid conditions in low latitudes were postulated in an earlier study of the authors. A palaeoceanographic/climatic model was postulated which is consistent with most of the sedimentological, palaeontological, and geochemical data. Here we demonstrate that this model may be applicable also for the older events.

Paleoenvironmental changes in the Silurian indicated by stable isotopes in brachiopod shells from Gotland, Sweden

Ratios of stable carbon and oxygen isotopes in brachiopod shells (more than 370 specimens, esp. Atrypa reticularis) from the Silurian of Gotland, Sweden, have been analysed. Preservation of biological skeletal ultrastructures, observed in SEM-micrographs, and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs. The Silurian of Gotland consists of 440 m carbonate deposits, spanning the late Llandovery to late Ludlow epochs (431-411 m.y.). Repeatedly, uniform sequences of micritic limestones and marks are interrupted by complex-structured reefs and adjacent platform sediments. Previously, the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses. The Silurian sequence of Gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units. Lower values occur in periods dominated by deposition of marly sequences. Higher values are observed in periods dominated by reefs and extended carbonate platforms. The isotope ratios are influenced by local as well as global factors. The oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input, rather than changes in paleotemperature. Consequently, the facies distribution of Gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and freshwater runoff rather than by sea level fluctuations. Periods of arid climate and, therefore, anti-estuarine downwelling of oxygenated surface water appear as short episodes of reef growing (≤1.5 m.y.) in an epoche characterized by a tropic humid climate, which causes an estuarine circulation and the upwelling of CO2-rich deep water. Carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13C-rich surface water (arid episodes) or upwelling of 13C-depleted deep water (humid episodes) of a Silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales.

Understanding the Great Ordovician Biodiversification Event (GOBE): Influences of paleogeography, paleoclimate, or paleoecology?

“The Great Ordovician Biodiversification Event” (GOBE) was arguably the most important and sustained increase of marine biodiversity in Earth’s history. During a short time span of 25 Ma, an “explosion” of diversity at the order, family, genus, and species level occurred. The combined effects of several geological and biological processes helped generate the GOBE. The peak of the GOBE correlates with unique paleogeography, featuring the greatest continental dispersal of the Paleozoic. Rapid sea-floor spreading during this time coincided with warm climates, high sea levels, and the largest tropical shelf area of the Phanerozoic. In addition, important ecological evolutionary changes took place, with the “explosion” of both zooplankton and suspension feeding organisms, possibly based on increased phytoplankton availability and high nutrient input to the oceans driven by intense volcanic activity. Extraterrestrial causes, in the form of asteroid impacts, have also been invoked to explain this remarkable event.

The formation of micritic limestones and the development of limestone-marl alternations in the Silurian of Gotland, Sweden

SummaryMicritic limestone-marl alternations make up the major part of the Silurian strata on Gotland (Sweden). Their position on the stable Baltic Shield protected them from deep burial and tectonic stress and allowed the preservation of early stages of burial diagenesis, including lithification. In the micritic limestones certain characteristics have been preserved (e.g., pitted microspar crystals, sharp boundaries between microspar and components, lack of deformation phenomena) that offer insights into their formation. We suppose the formation of these micritic limestones and limestone-marl alternations to be based on a rhythmic diagenesis within an aragonite solution zone (ASZ) close below the sediment surface. The micritic limestones are the product of a poikilotopic cementation of carbonate muds which consisted of varying portions of aragonitic, calcitic and terrigenous matter. Their microspar crystals show the primary size and shape of the cements lithifying the original carbonate mud. Dissolution of aragonite in the marls provided the carbonate for the lithification of the limestones. By cementation, the limestone beds evaded further compaction. The marls, which already underwent a volume decrease by aragonite depletion, lacked cement and became more and more compacted due to increasing sedimentary overburden. Although field observations show that primary differences in material influence the development of limestone-marl alternations they are not required for their formation.

Development of facies and C/O-isotopes in transects through the Ludlow of Gotland: Evidence for global and local influences on a shallow-marine environment

SummaryThe Silurian of Gotland is characterized by repeated changes in depositional facies development. The deposition of uniform sequences of micritic limestones and marls was interrupted four times by the growth of reef complexes and the formation of expanded carbonate platforms. Coinciding with these, often abrupt, facies changes extinction events occurred which predominantly affected nektonic and planktonic organisms. Ratios of carbon- and oxygen-isotopes covary with the facies development. Periods in which the deposition of limestonemarl alternations prevailed are characterized by relatively low C- and O-isotope values. During periods of enhanced reef growth isotope values are high. For these changes,Bickert et al. (1997) assume climatic changes between humid “H-periods”, with estuarine circulation systems and cutrophic surface waters with decreased salinity in marginal seas, and arid “A-periods”, with an antiestuarine circulation and oligotrophic, stronger saline surface waters.In order to separate local and regional influences on the isotopic development from the global trend, the interactions between facies formation and isotope record have to be clarified. For this purpose, the patterns of isotope values in the upper part of the Silurian sequence on Gotland (upper Wenlock —upper Ludlow) has been determined and stratigraphically correlated along four transects through different facies areas. Facies formation during this time interval was investigated by differentiation and mapping of twelve facies complexes in the southern part of Gotland. These include shelf areas, reef complexes with patch reefs and biostromes, backreef facies, and marginal-marine deposits. The good correspondence between the carbon-isotope records of the four transects suggests that local environmental conditions in the different facies areas did not influence the δ13C values. Therefore, a supra-regional or even global mechanism for the C-isotope variations is likely.In contrast to carbon istopes, the oxygen-isotope values of the four transects generally show parallel trends, but higher variabilities and in parts distinctly deviating developments with a trend to more negative values. These are interpreted as an effect of local warming in small shallow-water areas which developed during arid periods in reef complexes and backreef areas.The boundaries between A-periods and H-periods, as defined by δ13C values, which are interpreted as isochrones, can be mapped. From the upper Homerian to the Pridolian six parastratigraphic isotope zones are defined which only partly match the stratigraphic division ofHede (1942, 1960). The isotope stratigraphy results in an improved correlation between the shallow and marginal-marine areas in the eastern part of Gotland and the uniform shelf areas at the west coast of the island.Furthermore, a detailed relationship between the development of carbon and oxygen isotope ratios, the carbonate facies formation, and the succession of palaeontological events could be observed. At the transition from H-periods to A-periods, major extinction events occurred prior to the first increase of δ13C and δ18O values. Extinction events affected conodonts, graptolites, acritarchs, chitinozoans, and vertebrates and resulted in impoverished nektonic and planktonic communities. The reef-building benthos was less affected. Parallel to a first slight increase of isotope values, facies began to change, and reefs developed in suitable locations. The subsequent rapid increase of C- and O-isotope values occurred contemporarily with strong facies changes and a short-term drop of sea-level. Oligotrophic conditions in the later stages of A-periods led to strong reef growth and to an expansion of carbonate platforms.The transitions from A-periods to H-periods were more gradual. The δ13C values decreased slowly, but reef growth continued. Later the reefs retreated and were covered by the prograding depositional facies of shelf areas. The diversity of planktonic and nektonic communities increased again.The close relationship between facies formation, palaeontological events and isotope records in the Silurian suggests common steering mechanisms but gives no indication of the causes for the repeated extincion events related to H-period/A-period transitions. Especially the observation, that strong extinctions occurred prior to changes of isotope values and facies, points to causes that left no signals in the geological record. Hypothetical causes like collapse of trophical nets, anoxias, or cooling events are not evident in the sediment record or do not fit into the regular succession of period transitions.

Shell succession, assemblage and species dependent effects on the C/O-isotopic composition of brachiopods — examples from the Silurian of Gotland

Brachiopods are the most widely used biological constituents for the determination of stable C- and O-isotopic compositions in Paleozoic sediments. In the present study, critical precautions for obtaining reliable isotope values and maximum possible reliability are discussed. Samples were taken in the tectonically undisturbed and exceptionally well-preserved Silurian sequence on Gotland (Sweden). The state of preservation of brachiopod shells was examined by cathodoluminescence (CL) microscopy and by comparison of shell ultrastructures with those of recent brachiopods. Nine types of ultrastructure are distinguished and attributed either to biological differences or to diagenetic alterations. For isotope analyses, only largely unaltered areas of the secondary, fibrous shell layer should be used. Shell layers with other types of ultrastructure (primary, tertiary, laminar) may have isotopic compositions different from the assumed seawater value even when diagenetically unaltered. Only brachiopod taxa with a shell succession that includes a sufficiently thick secondary shell layer (e.g., Spiriferida, Rhynchonellida, Orthida) give reliable, mutually corresponding isotope values, indicating that the shell calcite precipitated at or near equilibrium with ambient seawater. Brachiopods with other kinds of shell successions (Strophomenida, Pentamerida) as well as other calcite-shelled organisms (trilobites, ostracodes) give non-equilibrium isotopic results and therefore should not be used. By restricting the samples to one species or to a group of closely related species, even minor differences in primary isotopic composition can be excluded. In the Silurian of Gotland, the stratigraphically widely distributed spiriferid Atrypa reticularis was selected for study. Considering the type of record, four different taphonomic assemblages are defined. In autochthonous assemblages of specimens, which lived contemporarily, original values of carbon as well as oxygen isotopes can be evaluated with a reliability of <±0.4‰. This allows the recognition of minor, hydrographically produced differences between neighbouring, contemporary facies areas and the determination of small (<1‰) isotopic excursions. Thus, besides being indicators of hydrographic conditions in ancient oceans, stable carbon- and oxygen-isotope compositions can be used as a stratigraphic tool that provides correlations, sometimes even within biozones.

Differential Diagenesis of Rhythmic Limestone Alternations Supported by Palynological Evidence

Alternating cemented and uncemented, fine-grained layers from Pliocene periplatform carbonates of the Great Bahama Bank have fundamentally different diagenetic features. The cemented layers consist predominantly of microspar, interpreted as an early marine, shallow-burial cement. The intercalated, uncemented, softer layers are devoid of microspar cement and exhibit signs of mechanical compac- tion. Precursor sediments of both cemented and uncemented layers apparently consisted largely of aragonite needles. The needles in the compacted layers show signs of dissolution, suggesting that the calcium carbonate required for cementation of the uncompacted layers was provided by aragonite dissolution within the compacted layers. The lack of compaction in the cemented layers shows that cementation, and hence aragonite dissolution in the adjacent compacted layers, took place in the shallow-burial realm. The dissolved carbonate was trans- ported by diffusion to adjacent layers and reprecipitated as calcite cement, thereby preventing significant compaction of these layers. These processes are not yet complete in the material examined, and some aragonite remains in the compacted layers. The sedimentary composition of the two rock types (compacted and uncompacted) is similar, indicating a similar precursor sediment for both. Between 1.5 and 7 times the concentration of palynomorphs oc- curs in the compacted layers, apparently the result of passive diage- netic enrichment. Although the trigger for diagenetic differentiation has not been determined, the pure limestone succession studied here appears to serve as a clay-free analog to limestone-marl alternations.

Palaeozoic Reefs and Bioaccumulations: Climatic and Evolutionary Controls.

The geological record contains a fascinating diversity of reefs and shell accumulations. As my other biosedimentary structures, their facies characterization requires careful observation at outcrop and sample scale, and in thin-section to provide information about the global geometries, fabrics and textures respectively. This collection of papers encompasses the breadth of sedimentary geometries and facies displayed by Palaeozoic reefs, shell accumulations, and transitional composite deposits. The definition of reefs and shell concentrations has given rise to variations in nomenclature. The papers in this volume cover specific problems regarding the nomenclature and facies characterization of reefs, shell accumulations and transitional composite deposits. However, rather than attempt a complete revision of terms, the authors have touched on some of the important issues at this stage of development in the field: the main climatic, environmental and evolutionary factors that controlled the Palaeozoic development of shell accumulation and reefs.

Limestone-marl alternations: A warm-water phenomenon?

Ancient limestone-marl alternations are concentrated in settings analogous to loci of aragonite accumulation in the modern world. They typically occur on shelves in the tropical-subtropical climate belt, are far more abundant on passive continental margins than on active ones, and are rare in upwelling zones. In recent studies, aragonite was proposed to play an important role in differential diagenesis typical of most limestone-marl alternations. The coincidence of depositional settings of ancient limestone-marl alternations and modern aragonite accumulation is a strong case for this hypothesis. If confirmed, it could provide a valuable tool for broad-scale paleoenvironmental interpretations. An additional, different type of limestone-marl alternations resulted from the Cretaceous explosion in productivity of calcitic plankton: these pelagic ones are fundamentally different in their style of diagenesis.

NEW SEM OBSERVATIONS OF KERIOTHECAL WALLS: IMPLICATIONS FOR THE EVOLUTION OF FUSULINIDA

This study presents new scanning electron microscopy (SEM) observations of Paleozoic foraminifera. It focuses especially on the wall of three Fusulinida, Sakmarella moelleri, Paraskinnerella skinneri, and Taiyuanella furoni, that belong to the Schwagerinoidea, which are fusulinids with keriothecal test walls. In addition, Eostaffella sp. is investigated as an example of the microstructure of the microgranular imperforate tectum. The fillings of the keriothecal alveoles of Sakmarella, Paraskinnerella, and Taiyuanella yield diverse, characteristic flower-like structures that indicate a gradual evolution of the microstucture of the tests. Sakmarella and Paraskinnerella have flowers with a narrow center, and Taiyuanella has a larger center. The true keriothecal structure, described, for example, in Triticites, is distinguished from these structures by simply filled alveoles. The term ‘‘anthotheca’’ is introduced for the structure with narrow-centered flowers. It can be distinguished from the large-centered flower structure identified as Zhuang’s (1989) stalactotheca. Following a literature review of previous work on keriothecal structures, several morphofunctional hypotheses are discussed, including the housing of symbiotic algae or cyanobacteria, which leads to the new interpretation that the flowers of the antho- and stalactothecal structures are induced by algal endosymbionts. In addition, we discuss the monophyly of keriothecal fusulinids. The transition from diaphanothecal to keriothecal forms recognized since Thompson (1964) is reinterpreted in terms of relationships with symbiotic algae or cyanobacteria. The Schwagerinoidea constitute an important group in understanding the evolution of larger foraminifera in the late Paleozoic.