Christian Samtleben

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.

Plankton in the Norwegian-Greenland Sea: from living communities to sediment assemblages —an actualistic approach

A synoptic study is carried out to reconstruct the development of the plankton community in the late Quaternary in the Norwegian-Greenland Sea. It comprises quantitative analyses of coccolithophores, dinoflagellate cysts, diatoms and radiolarians. An actualistic approach is applied to evaluate the different fossil records of these plankton groups. The preliminary results of the current investigation are reported here.The composition and distribution of living communities of coccolithophores are presented as an example. A close relationship between the distribution of regional groups and surface water masses is observed. Seasonal vertical fluxes of coccolithophores and radiolarians through the water column show similar patterns within different years. However, diatoms are highly variable, both in absolute fluxes and species composition. The differentiation of sporadic and periodic processes is evident only after several years of observation. During settling and sedimentation biotic and abiotic processes such as grazing, dissolution and lateral transport alter the assemblages.Investigation of death assemblages in surface sediments reveals that in spite of these alteration processes the abundance and species distribution are related to surface water masses. Higher abundances and diversities are usually found in sediments underlying the warm Norwegian Current. Concentrations decrease to the north-west towards the cold polar water masses.The sediment assemblages of all groups are strongly altered relicts of former living communities. They are characterized by distinct changes in species composition and absolute abundances related to palaeo-oceanographic development. Their variation through the sedimentary record is used to distinguish four ecostratigraphic units during the late Weichselian and Holocene.

Coccolithophores in the Nordic Seas : comparison of living communities with surface sediment assemblages

Coccolithophores were investigated in both numerous plankton samples and surface sediments from across the entire Nordic Seas. Cell numbers of living coccolithophore communities are consistently higher during the high-production period (summer) than during the low-production period (fall–early summer). Particularly high cell numbers are found far to the west during the high-production period. High cell densities in general are mainly caused by Emiliania huxleyi, which often dominates the communities in the eastern Norwegian Sea. In contrast, during the low-production period, many of the collected samples contain no coccolithophores. The original composition of the living communities is obscured in the surface sediment assemblages. Samples from surface sediments contain significantly fewer coccolithophore species than the living communities of the same area. The most frequently occurring species are Emiliania huxleyi and Coccolithus pelagicus. In addition, Calcidiscus leptoporus and Gephyrocapsa muellerae are regularly found in small amounts. This is remarkable since these species are rarely found in living communities. Based on the diversity and concentration of the preserved species and on the ratio change between E. huxleyi and C. pelagicus, distinct surface water masses in this area are clearly reflected.

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.

Controls on modern carbonate sedimentation on warm-temperate to arctic coasts, shelves and seamounts in the Northern Hemisphere: Implications for fossil counterparts

SummaryIn contrast to the well studied tropical carbonate environments, interest in non-tropical carbonate deposition was rather low until the basic ideas of theForamol-concept were outlined byLees & Buller (1972). In the following two decades studies on non-tropical carbonate settings evolved as a new and exciting branch of carbonate sedimentology (seeNelson 1988). This is archieved in a great number of publications dealing on temperate carbonate deposits from numerous coastal and open shelf settings on both hemispheres. The existence of wide extended carbonate depositional systems and even reefal frameworks in Subarctic and Arctic seas which are in focus by our research group made it possible to study modern non-tropical carbonate settings along a latitudinal transect from the warm-temperate Mediterranean Sea to the cold Nordic Seas. Because of increasing seasonality in environmental conditions towards high latitudes, the major controls in biogenic carbonate production can be more clearly addressed in these areas. After the initiation of the priority program “Global and regional controlling processes of biogenic sedimentation-evolution of reefs” by the German Science Foundation four years ago, a set of modern case studies were comparatively analysed specifically with regard to their principle controlling processes:(1)Modern and Holocene coralline algal reefs and rhodolith pavements formed in wave-protected shallow waters along the coast of the Brittany and northern Norway. Their finetuned interaction with herbivores resulted in the development of widespread but low-diverse, slowly growing coralline algal frameworks with high competitive value against the rapid-growing phaeophytic communities.(2)The MediterraneanCladocora caespitosa-banks provide an instructive example of non-tropical hermatypic coral framework construction out of the subtropical-tropical coral reef belt.(3)The geometry and environmental controls of several kilometer long coral reefs formed by the azooxanthellateLophelia pertusa andMadrepora oculata are studied in more than 250 m water depth in mid and northern Norway.(4)ModernBryomol-sediments are widely distributed on non-tropical deeper shelf settings. The formational processes converting bryozoan-thickets into huge piles of sand and gravel dunes are recently studied on the outer shelves off northern Brittany and off northern Norway.(5)Arctic sponge-bryozoan buildups on the seamount Vesterisbank in the Greenland Sea and(6)balanid-dominated open shelf carbonates on the Spitsbergen Bank form the Arctic endmembers of modernForamol-deposits. Seasonalice-edge phytoplankton blooms and efficient mechanisms of pelagic-benthic food transfer characterize these depositional settings. Fossil counterparts of each of these modern case studies are discussed in context with their paleoceanographic and environmental settings.

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.

Distribution, Export and Alteration of Fossilizable Plankton in the Nordic Seas

The present study gives an overview of recent research results concerning the distribution of living fossilizable plankton groups (coccolithophores, diatoms, dinoflagellate cysts, radiolarians and planktic foraminifers) in the water column of the Nordic Seas. Information is combined with results of sediment trap investigations on fluxes and the species composition of these fossilizable plankton groups in the Nordic Seas and their relation to oceanographic conditions. In order to validate spatial and temporal occurrence patterns of the plankton assemblages, their fluxes were monitored with sediment traps over several years in three different oceanic regions, characterized by the major surface water masses of the Nordic Seas. In addition, the alteration processes on settling plankton assemblages were investigated at three water depths.

Distribution of Calcareous, Siliceous and Organic-Walled Planktic Microfossils in Surface Sediments of the Nordic Seas and their Relation to Surface-Water Masses

The distribution of calcareous, siliceous and organic-walled planktic microfossils has been investigated in surface sediments from the Nordic Seas, Planktic foraminifers, coccolithophores, radiolarians, diatoms and dinoflagellate cysts generally reflect major oceanographic domains in the distribution of sediment assemblages, The occurrence of fossilizable plankton in the water column and in sediments has been compared to describe processes which alter the composition of assemblages.