Stratigraphic Signals Research Articles

Fossilized autogenic responses of grain‐size transition to sediment supply and water discharge: Alluvial fan experiments

ABSTRACTAutogenic feedbacks can produce large‐scale, organized stratigraphic patterns in alluvial fans, but autogenic depositional signatures of specific upstream boundary conditions remain challenging to interpret. Here, a combination of theory, experiment and field application is used to explore how autogenic lithofacies changes can be interpreted as stratigraphic indicators of upstream boundary conditions. Six experiments were conducted to test the effects of sediment supply and water discharge rates on autogenic advance and retreat of the lithofacies boundary (grain‐size transition) in an alluvial fan with two dominant grain sizes. Migration of the grain‐size transition caused a short‐term zigzag pattern in the grain‐size transition position in the dip‐directional deposit section. For each experiment, time‐lapse images and laser topographic scans of the fan surface and stratigraphic cross‐sections of the final deposits were used to quantify characteristic timescales of autogenic processes. Timescales for fan‐margin migration, surface wet‐fraction change and grain‐size transition migration generally shorten as sediment supply rate increases and water discharge rate decreases. Increasing the sediment supply rate shortens the duration of the fluvial sediment storage and release cycle, producing higher frequency zigzags in the grain‐size transition trajectory. Increasing the water discharge tends to widen channels and lengthens the duration of the fluvial sediment storage and release cycle, constructing lower frequency zigzags in the grain‐size transition trajectory. Increasing the water discharge also enables more sediment to transport further downstream during release events, leading to higher magnitude zigzags in the grain‐size transition trajectory. These relationships between upstream boundary conditions and the grain‐size transition trajectory demonstrate how autogenic stratigraphic signals could be used as a tool to infer relative changes in boundary conditions.

Overview on GPlates: focus on plate reconstruction

Plate tectonic reconstructions have been employed in geosciences since 1970s, in the context of hydrocarbon exploration, regional geology and paleobiology. Such studies have given valuable inputs for climate and geodynamic computations, present-day mantle structure, models of plate motion, and the interpretation of the drift of hotspots, true polar wander (TPW), sea level and stratigraphic signals. However, geodynamic models generated in the past by incorporating global plate tectonic reconstructions have limitations. To overcome this, GPlates software brings forward a new era of interactive plate tectonic reconstruction software integrated with GIS databases that incorporates a wide variety of geological and geophysical data. Besides modelling tectonic and crustal evolution, GPlates has also been used in visualizing paleogeography and paleobathymetry, in understanding deep carbon cycle, subduction zone initiation, mantle evolution, investigating earthquakes and predicting future supercontinents. the software has been widely used in hydrocarbon exploration along the passive conjugate margins such as from the margins of South America and Africa and has provided promising results in acquiring new and more reliable prospecting criteria for petroleum systems. Additionally, it has become an integral tool for paleolatitude calculations, modelling of paleoclimate and paleoenvironment. This article reviews key plate reconstructions that have been carried out using GPlates, the typical constraints and the set of input parameters.

Computer rendering of high-frequency stacking patterns in cross-sectional stratigraphic simulations using the line integral convolution

Here, we show a novel use of a mathematical technique in stratigraphic visualization, which has been effectively used in other fields, to generate high-resolution strata texture maps to improve the rendering of two-dimensional forward stratigraphic simulations during post-processing. The texture maps are obtained by convolving stratigraphic timelines with a random dotted pattern that produces realistic layerings similar to those observed in seismic cross sections. The texture mappings are scale-independent and are not affected by artifacts like moiré patterns. Thus, the stratigraphy in poorly resolved areas is rendered at the same level of detail as those zones with high numerical resolution. The texture maps can also be combined with other attributes generated by numerical simulations like sedimentation rate or grain size to bring into the foreground subtle details that otherwise could be swamped by large amplitude or high frequency stratigraphic signals.

Where the Meghalayan meets the Anthropocene: Stratigraphic signals of human-environmental interactions on the periphery of Indian civilisation

The aims of this study were to review human-environment interactions during the Meghalayan and to search for the stratigraphic boundary of a new epoch, informally termed the Anthropocene, as well as to determine whether the stratigraphic signals of human activity on the Meghalaya Plateau in Northeast India can be correlated globally. This plateau is the base of the Meghalayan Age that was determined from a speleothem in a cave located on it. Review indicates that study region developed on the periphery of ancient Indian civilisation, with stratigraphic signals of human activity being apparent in only the last few thousand years; that is, substantially later than the neighbouring ancient Indian civilisation. The stratigraphic signals are heterogeneous and diachronous, not only as a result of various human activities, but also in the effect of the diverse sensitivities of the environment to anthropogenic disturbances. A discrete and visible cultural layer that relates to the development of settlements and the production of new materials is still being formed and reworked. The only synchronous stratigraphic signal with a global range seems to be associated with the artificial radionuclide fallout from nuclear weapons testing, which covers a topsoil layer of up to tens of centimetres thick

RADIONUCLÍDEOS COMO MARCADORES DE UM NOVO TEMPO: O ANTROPOCENO

On May 2019, the members of the Anthropocene Working Group completed a binding vote to affirm some of the key questions that have been agreed in 2016: the Anthropocene should be considered as a formal chronostratigraphic unit, defined by a Global Boundary Stratotype Section and Point (GSSP), using one of the stratigraphic signals around the mid-twentieth century of the Common Era. The fallout from nuclear weapons testing appears as the most suitable GSSP. In this article, the main anthropogenic markers are discussed as well as the sources or human-made radiation and the signature of nuclear weapons testing. The appearance of 239+240Pu in the early 1950s and the clear peak, in 1963, makes a good marker. High resolution archives include layers of sedimentary rocks, lacustrine and marine sediments and corals and polar ice cores. A coupled application of other radioactive isotopes, as 137Cs, 14C and 241Am, may be useful to categorize distinct fallout signatures. A site to define the Anthropocene (golden spike) would ideally be located between 30o and 60o north of the equator. Nevertheless, results obtained in sediments on the Brazilian coast showed that both 239+240Pu and 137Cs signatures could be detected and quantified as markers of this “atomic age”.

Tectonic processes, variations in sediment flux, and eustatic sea level recorded by the 20 Myr old Burdigalian transgression in the Swiss Molasse basin

Abstract. The stratigraphic architecture of the Swiss Molasse basin, situated on the northern side of the evolving Alps, reveals crucial information about the basin's geometry, its evolution, and the processes leading to the deposition of the siliciclastic sediments. Nevertheless, the formation of the Upper Marine Molasse (OMM) and the controls on the related Burdigalian transgression have still been a matter of scientific debate. During the time period from ca. 20 to 17 Ma, the Swiss Molasse basin was partly flooded by a shallow marine sea striking SW–NE. Previous studies have proposed that the transgression occurred in response to a rise in global sea level, a reduction of sediment flux, or an increase in tectonically controlled accommodation space. Here, we readdress this problem and extract stratigraphic signals from the Burdigalian molasse deposits that can be related to changes in sediment supply rate, variations in the eustatic sea level, and subduction tectonics. To achieve this goal, we conducted sedimentological and stratigraphic analyses of several sites across the entire Swiss Molasse basin. Field investigations show that the transgression and the subsequent evolution of the Burdigalian seaway was characterized by (i) a deepening and widening of the basin, (ii) phases of erosion and non-deposition during Lower Freshwater Molasse (USM), OMM, and Upper Freshwater Molasse (OSM) times, and (iii) changes in along-strike drainage reversals. We use these changes in the stratigraphic record to disentangle tectonic and surface controls on the facies evolution at various scales. As the most important mechanism, rollback subduction of the European mantle lithosphere most likely caused a further downwarping of the foreland plate, which we use to explain the deepening and widening of the Molasse basin, particularly at distal sites. In addition, subduction tectonics also caused the uplift of the Aar massif. This process was likely to have shifted the patterns of surface loads, thereby resulting in a buckling of the foreland plate and influencing the water depths in the basin. We use this mechanism to explain the establishment of distinct depositional settings, particularly the formation of subtidal shoals wherein a bulge in relation to this buckling is expected. The rise of the Aar massif also resulted in a reorganization of the drainage network in the Alpine hinterland, with the consequence that the sediment flux to the basin decreased. We consider this reduction in sediment supply to have amplified the tectonically controlled deepening of the Molasse basin. Because the marine conditions were generally very shallow, subtle changes in eustatic sea level contributed to the formation of several hiatuses that chronicle periods of erosion and non-sedimentation. These processes also amplified the tectonically induced increase in accommodation space during times of global sea level highstands. Whereas these mechanisms are capable of explaining the establishment of the Burdigalian seaway and the formation of distinct sedimentological niches in the Swiss Molasse basin, the drainage reversal during OMM times possibly requires a change in tectonic processes at the slab scale, most likely including the entire Alpine range between the Eastern and Central Alps. In conclusion, we consider rollback tectonics to be the main driving force controlling the transgression of the OMM in Switzerland, with contributions by the uplift of individual crustal blocks (here the Aar massif) and by a reduction of sediment supply. This reduction of sediment flux was likely to have been controlled by tectonic processes as well when basement blocks became uplifted, thereby modifying the catchment geometries. Eustatic changes in sea level explain the various hiatuses and amplified the deepening of the basin during eustatic highstand conditions.

Early mining and smelting lead anomalies in geological archives as potential stratigraphic markers for the base of an early Anthropocene

This article reviews possible lower boundaries for an early Anthropocene period. Although a noticeable environmental impact of humans, caused by hunting, the use of fire, forest clearance, animal domestication and agriculture had already occurred in the Neolithic, these early signals are strongly diachronous and localised. Here, we examine early significant, synchronous and regional stratigraphic signals indicating an anthropogenic influence as consequences of mining and smelting-related trace metal contamination. A first regional lead contamination event in the Northern Hemisphere is recognized during the (Eastern Mediterranean) Late Bronze Age to Early Iron Age, between 3500 and 2800 BP, with a peak at around 3000 BP. Another pronounced anthropogenic lead peak is recorded around 2000 BP, during the Roman period. These events, as defined by lead enrichment and changes in lead isotope ratios, accompanied by other trace metal enrichments, are found in several types of archives, such as Arctic ice-cores and European peat-bogs, speleothems as well as fluvial, lake and marine records. Potential stratigraphic correlations and secondary markers may be present using tephrochronology, climate events, and magnetostratigraphy. Such a definition of the base of a formally defined (early) Anthropocene stage/period allows the application of the GSSP (Global Stratotype Section and Point) concept by using a point in a physical archive, and, in contrast to the late Anthropocene, includes a significant quantity of anthropogenic strata as evidence for an Anthropocene chronostratigraphic unit.

About this title ‐ Geology and Geomorphology of Alluvial and Fluvial Fans: Terrestrial and Planetary Perspectives

Alluvial and fluvial fans are the most widespread depositional landform bordering the margins of highland regions and actively subsiding continental basins, across a broad spectrum of tectonic and climatic settings. They are significant to the local morphodynamics of mountain regions and also to the evolution of sediment-routing systems, affecting the propagation and preservation of stratigraphic signals of environmental change over vast areas. The volume presents case studies discussing the geology and geomorphology of alluvial and fluvial fans from both active systems and ancient ones preserved in the stratigraphic record. It brings together case studies from a range of continents, climatic and tectonic settings, some introducing innovative monitoring and analysis techniques, and it provides an overview of current debates in the field. This volume will be of particular interest to geologists, geomorphologists, sedimentologists and the general reader with an interest in Earth science.

Geology and geomorphology of alluvial and fluvial fans: current progress and research perspectives

Abstract Alluvial and fluvial fans are the most widespread depositional landforms bordering the margins of long-lived highland regions and actively subsiding continental basins, across a broad spectrum of tectonic and climatic settings. Their significance is relevant not only to the local morphodynamics of mountain regions and proximal basinal sectors, but also to the long-term evolution of sediment-routing systems, affecting the propagation of stratigraphic signals of environmental change and the preservation potential of stratal successions over much larger spatial scales than those they occupy. Subaerial fan systems archive information on the palaeoclimate, local tectonic history and landscape response to various allogenic factors, although our ability to decipher such information is still limited. Early recognition of alluvial fans dates from the late nineteenth century, but a coordinated research community on these systems has been active only over the last few decades and the full relevance of fluvial fan systems to the geomorphology of present day continental basins and to the interpretation of ancient stratigraphic successions has been convincingly demonstrated only over the last decade. This introductory chapter summarizes advances in our knowledge of alluvial and fluvial fans, identifies potential new lines of future inquiry, and presents the contributions to this volume in the context of the current state of research.

The stratigraphical signature of the Anthropocene in England and its wider context

The Anthropocene deposits of England, here regarded as those formed after ∼1950 CE, are now extensive, take various forms, and may be characterized and recognized by a number of stratigraphic signals, such as artificial radionuclides, pesticide residues, microplastics, enhanced fly ash levels, concrete fragments and a novel variety of ‘technofossils’ and neobiotic species. They include the uppermost parts of both ‘natural’ deposits such as the sediment layers formed in lakes and estuaries, and more directly human-made or human-influenced ones such as landfill deposits and the ‘artificial ground’ beneath urban areas and around major constructions. ‘Negative deposits’ include the worked areas of quarries and regions such as the English Fenland, where thick peat deposits have ablated to leave a strongly modified underlying landscape, and extend beneath into the subterranean realm as mine workings, metro systems and boreholes. The production of these is still rapidly increasing and evolving in character, while the early signs of global change, such as warming, sea level rise, and modifications to biotic assemblages, are beginning to further modify the emerging geology of this new phase of Earth history.

Stratigraphy of the north polar layered deposits of Mars from high‐resolution topography

AbstractThe stratigraphy of the layered deposits in the polar regions of Mars is theorized to contain a record of recent climate change linked to insolation changes driven by variations in the planet's orbital and rotational parameters. In order to confidently link stratigraphic signals to insolation periodicities, a description of the stratigraphy is required based on quantities that directly relate to intrinsic properties of the layers. We use stereo digital terrain models (DTMs) from the High Resolution Imaging Science Experiment to derive a characteristic of north polar layered deposit (NPLD) strata that can be correlated over large distances: the topographic protrusion of layers exposed in troughs, which is a proxy for the layers' resistance to erosion. Using a combination of image analysis and a signal‐matching algorithm to correlate continuous depth‐protrusion signals taken from DTMs at different locations, we construct a stratigraphic column that describes the upper ~500 m of at least 7% of the area of the NPLD and find accumulation rates that vary by factors of up to 2. We find that, when coupled with observations of exposed layers in images, the topographic expression of the strata is consistently continuous across large distances in the top 300–500 m of the NPLD, suggesting that it is better related to intrinsic layer properties than the brightness of exposed layers alone.

Assessing pelagic palaeoenvironments using foraminiferal assemblages — A case study from the late Campanian Radotruncana calcarata Zone (Upper Cretaceous, Austrian Alps)

Two upper Campanian sections in the Austrian Alps representing the north western Tethyan biogeographic realm from either sides of the Penninic Ocean (Alpine Tethys) have been examined aiming at a high-resolution assessment of foraminiferal assemblages: the Postalm section from the Northern Calcareous Alps (southern active margin) and the Oberhehenfeld section from the Ultrahelvetics (northern passive margin). This study focuses on plankton biostratigraphy and foraminiferal palaeoecology of the Radotruncana calcarata Total Range Zone.The Postalm section displays cyclic red deposits with marls and marly limestones, while we find uniform grey marls at Oberhehenfeld. The Oberhehenfeld section from the Ultrahelvetics can be correlated stratigraphically to the Postalm section using foraminifera, calcareous nannoplankton and stable isotope stratigraphy, and provides a point of comparison from the northern margin of the Penninic Ocean.The two sections show minimal difference in faunal composition and few distinct local stratigraphic signals. Palaeoenvironmental trends from the late Campanian can be recognised relating the two sections from the Austrian Alps. The depositional water depth can be reconstructed as some 500–800m. Plankton assemblages show a remarkable stability despite the sudden appearance and disappearance of R. calcarata, hinting at the late Campanian as a time interval of general foraminiferal stasis without significant evolutionary events. We speculate that the origin and extinction of R. calcarata are related to the prolonged evolution of ocean stratification during the Campanian from the mid-Cretaceous sluggish hothouse during a time of general slow greenhouse climate decline.

Geomorphic and stratigraphic signals of postglacial meltwater pulses on continental shelves

Selective development and preservation of major shoreline complexes on the continental shelf provide geomorphic evidence in support of alternating periods of Holocene sea-level stillstand and rapid rise during meltwater pulses. On a tectonically stable far-fi eld setting (the southeast African shelf), regionally developed, well-preserved shoreline complexes occur at ~‐100 m and ~‐60 m within an overall shelf sequence dominated by transgressive ravinement (non-preservation of shorelines). The development of geomorphically mature submerged shorelines with equilibrium forms is attributed to extended periods of sea-level stability, and their preservation is the result of early cementation followed by very rapid sea-level rise (several centimeters per year) that caused overstepping. Meltwater Pulses 1A and 1B are recorded in the shelf stratigraphy and geomorphology. Setting aside local infl uences of topography and sediment supply, we hypothesize that shelf stratigraphies should preferentially preserve shorelines at water depths associated with the stillstands or slowstands immediately preceding meltwater pulses. A reappraisal of shelf stratigraphy and geomorphology, in particular the preservation potential of former shorelines, is required in the light of contemporary understanding of postglacial sea-level history.

A stratigraphical basis for the Anthropocene?

Abstract Recognition of intimate feedback mechanisms linking changes across the atmosphere, biosphere, geosphere and hydrosphere demonstrates the pervasive nature of humankind's influence, perhaps to the point that we have fashioned a new geological epoch, the Anthropocene. To what extent will these changes be evident as long-lasting signatures in the geological record? To establish the Anthropocene as a formal chronostratigraphical unit it is necessary to consider a spectrum of indicators of anthropogenically induced environmental change, and to determine how these show as stratigraphic signals that can be used to characterize an Anthropocene unit and to recognize its base. It is important to consider these signals against a context of Holocene and earlier stratigraphic patterns. Here we review the parameters used by stratigraphers to identify chronostratigraphical units and how these could apply to the definition of the Anthropocene. The onset of the range of signatures is diachronous, although many show maximum signatures which post-date 1945, leading to the suggestion that this date may be a suitable age for the start of the Anthropocene.

Eumalacostracan Evolution: Conflict between Three Sources of Data

There is no consensus on the inter-ordinal relationships of eumalacostracans, despite the recent synthesis of several morphological matrices with data from four molecular markers. Signals from different molecules confl ict with each other, and all are conspicuously at odds with morphology. Can fossils help to resolve the problem? Here, we utilize palaeontological data in two ways. Firstly we coded a selection of fossil taxa into our morphological matrix, and assessed their impact upon inferred phylogeny relative to that of their living counterparts (fi rst order jackknifing). This revealed that our morphological tree is very sensitive to the precise taxon sample (a problem that must be addressed in future studies), but that our fossil groups were not disproportionately infl uential. Secondly, we asked whether the order in which groups appear in the fossil record provides a means to choose between competing trees. The congruence between morphological and stratigraphic signals was extremely weak and non-signifi cant in most cases, precluding the use of fossil dates in this way. Many trees imply ghost ranges of duration near the theoretical maximum, and worse than for the majority of other animal groups so far investigated. An incomplete fossil record and fragile/weakly-supported trees combine with considerable molecular rate heterogeneity to make the Eumalacostraca extremely poorly suited to molecular clock studies. Future insights into their phylogeny are likely to come from the development of new molecular markers, as well as hard-won data on internal anatomy and ultrastructure.

Sequence stratigraphy of experimental strata under known conditions of differential subsidence and variable base level

Sequence stratigraphy has been applied from reservoir to continental scales, providing a scale-independent model for predicting the spatial arrangement of depositional elements. We examine experimental strata deposited in the Experimental EarthScape facility at St. Anthony Falls Laboratory, focusing on stratigraphic surfaces defined by discordant contact geometries, surfaces analogous to those delineated in the original work on seismic sequence stratigraphy. In this controlled setting, we directly evaluate critical sequence-stratigraphic issues, such as stratigraphic horizon development and time significance, as well as the internal geometry and migration of the bounded strata against the known boundary conditions and depositional history. Four key stratigraphic disconformities defined by marine downlap, marine onlap, fluvial erosion, and fluvial onlap are mapped and vary greatly in their relative degree of time transgression. Marine onlap and downlap contacts closely parallel topographic surfaces (time surfaces) and, prior to burial, approximate the instantaneous offshore topography. These stratal-bounding surfaces are also robust stratigraphic signals of relative base-level fall and rise, respectively. Marine onlap surfaces are of special interest. They tend to be the best preserved discordance, where widespread, allogenic-based onlap surfaces subdivide otherwise amalgamated depositional cycles amidst cryptic stacks of marine foresets; however, local, autogenic-based marine onlap discordances are present throughout the fill. A critical distinguishing feature of allogenic onlap is the greater lateral persistence of the discordance. Surfaces defined by subaerial erosional truncation and fluvial onlap do not have geomorphic equivalence because channel processes continually modify the surface as the stratigraphic horizons are forming. Hence, they are strongly time transgressive. Last, the stacking arrangement of the preserved bounded strata is found to be a good time-averaged representation of the mass-balance history.

Soft computing methodologies for spectral analysis in cyclostratigraphy

In this paper we present some soft computing methodologies for time-series analysis applied to cyclostratigraphy. An application to some stratigraphic signals to detect Earth orbital (Milankovic’) periodicities which are expected to be recorded in Cretaceous shallow water carbonate sequences outcropping in Southern Apennines (Italy), is described. The results obtained with classical spectral analysis techniques, based on the modified periodogram, are compared to the results of our methods based on neural nets and genetic algorithms. The aim of these cross comparisons is to find the most reliable, fast and accurate methodology to identify orbital periodicities in noisy and segmented stratigraphic signals.

New Perspectives on Orbitally Forced Stratigraphy

This survey of the current status of research into Earth’s orbitally forced paleoclimatic record summarizes recent developments in the theory of Earth’s orbital parameters, and reviews how various techniques of data collection and analysis have fared in the search and recovery of orbital signals in ancient stratigraphy. The emerging significance of the quasi-periodicity of Earth’s orbital variations as a principal tool in the analysis of orbitally forced stratigraphy is discussed in detail. Five case studies are presented that illustrate new directions in research: (a) time series analysis of discontinuous strata; (b) measurement of ultra-high resolution stratigraphic signals; (c) new perspectives on the 100 kyr Pleistocene glaciation problem; (d) stratigraphic evidence for solar system resonance modes; and (e) evaluating Phanerozoic length of day from orbitally forced stratigraphy.

Pleistocene fluctuations in the Agulhas Current Retroflection based on the calcareous plankton record

Piston core (PS2487-6), recovered south of Cape Town, and sediment surface samples, recovered in the area of the Agulhas Current retroflection, were used to construct paleoceanographic scenarios for the late Quaternary in a region with an important role in global water mass transfer. Coccolithophore (calcareous nannofossil) and planktonic foraminifera assemblages and oxygen isotope data were collected. Stratigraphic control is based on calibration of the δ 18O stratigraphic signals with calcareous nannofossil events that are thought to be synchronous over a broad range of latitudes. Study of the surface sediments permits the characterisation of the Agulhas Current, Subtropical Convergence and Subantarctic coccolithophore assemblages. The Agulhas Current assemblage has relatively high proportions of Florisphaera profunda, Gephyrocapsa oceanica and Umbilicosphaera spp. These species are absent or present in low proportions in subantarctic waters. The abundance of coccolithophores during isotope stages 1, 5 and 7 is characteristic of relatively warm, stratified surface waters, with a deep nutricline and chlorophyll maximum, which strongly suggests that the area was under the influence of the Agulhas Current retroflection. The incursion of Globigerinoides ruber, abundant today in the Agulhas Current, also supports this interpretation. Conversely, during glacial stages 2–4 and 6, a strong reduction in warm and stratified water indicators can be observed, together with an increase in cold-eutrophic species. The interval from isotope stages 8 to 12 displays an assemblage dominated by Gephyrocapsa caribbeanica, the highest values being seen in the so-called Mid-Brunhes event, accompanied by a clear reduction in subtropical Holocene species. During the glacial interval from isotope stages 10–12, G. caribbeanica dominated the assemblage. The ecological interpretation of this species is controversial due to a clear evolutionary overprint. Isotope stage 12 is here interpreted as having been the coldest one in the period studied. It is characterised by a strong increase in Neogloboquadrina pachyderma (dextral and sinistral) and a remarkable decrease in tropical and subtropical planktonic foraminifera. From isotope stage 13 to 18, a hiatus is interpreted. Below this hiatus the stratigraphic resolution is poorer, although the glacial–interglacial cyclicity is well defined in the Agulhas Current Retroflection area (Core PS2487-6) for the last 25 isotope stages. The glacial–interglacial cyclicity is thought to be due to a fluctuation in the Subtropical Convergence Zone, probably linked to the eastward and westward displacement of the Agulhas Current retroflection. In any case, the core studied was always in a subtropical environment, under the influence of the Agulhas Current, which was enhanced during interglacial periods. For the whole of the interval studied, increases in Calcidiscus leptoporus, Umbilicosphaera spp., and Syracosphaera spp. among the calcareous nannofossils, and in G. ruber and Globigerinoides sacculifer within the planktonic foraminifera, clearly respond to interglacial pulses, reaching maximum values during short periods close to the major deglaciations.

Reconstruction of climatic changes during the Late Pleistocene, based on sediment records from the Konya Basin (Central Anatolia, Turkey)

Climatic changes during the last climatic cycle have been studied using three sediment cores from the Konya plain, a now dry, closed and semi-arid lacustrine basin at 1000 m altitude in central Anatolia, Turkey. The reconstruction of regional climatic characteristics and evolution is based on mineral, diatom, pollen and molluscan content of sediments. Correlations are made between cores using 14C and U-Th ages, stratigraphy and drought levels shown by changes in mineral contents. Environmental responses to local and regional climatic changes are traced by trends in authigenic carbonates, evaporites, detrital mineral content and by diatom-inferred salinity levels. Our data have shown that, during the period covering the end of the previous Glacial (Termination II) and the last Interglacial (between c. 150 and 117 ka), peat and freshwater shallow lakes expanded. From 117 to 66 ka, the plain was occupied by lakes, the salinity and existence of which varied in time and space. Specific events are marked by mineralogic and stratigraphic signals at (i) c. 101 ka and (ii) 66 ka. From 66 ka to 30 ka, desiccation of the lake is marked by a hiatus. At c. 27 ka, milder climatic conditions led to the extension of freshwater marshes and lakes in the central depressions while palaeosols developed on the emerged parts of the plain. From 25 to 20·5 ka 14C cal., the sediments of a freshwater to brackish lake are present in one core only. From 20·5 ka 14C cal. onwards, strong evaporitic conditions occurred, the lake edges being transformed into playas. Upper parts of the sequences registered other lacustrine short phases, both before the Younger Dryas and during the Holocene. Copyright © 1999 John Wiley & Sons, Ltd.