Ines Wendler

A stable and hot Turonian without glacial δ18O excursions is indicated by exquisitely preserved Tanzanian foraminifera

A shift from the icehouse climate in which humans evolved to a Late Cretaceous–like greenhouse climate is an often-repeated cautionary prediction of the consequences of continued anthropogenic CO 2 emissions. The corollary, that understanding the past might help predict the future, has justified many Late Cretaceous studies, but important questions remain about climate stability and sensitivity. New δ 18 O measurements of more than 1000 samples of exceptionally well preserved foraminifera (8 planktic and 11 benthic taxa) from two sites in Tanzania indicate that hot and remarkably stable conditions prevailed in the region during the Turonian, including during a proposed greenhouse glacial event. Planktic taxa have δ 18 O values largely between –4.0‰ and –5.0‰, suggesting surface-water temperatures between 30 and 35 °C. Estimates for seafloor temperatures are between 18 and 25 °C. No parallel shifts in δ 18 O values are observed among planktic and benthic taxa, contradicting an often-cited line of evidence for greenhouse glaciations and supporting an effectively ice-free Turonian world.

Production of calcareous dinoflagellate cysts in response to monsoon forcing off Somalia: a sediment trap study

Abstract To study the ecology of calcareous dinoflagellates we examined the impact of the SW and NE monsoons on cyst formation using sediment trap material, collected at 1032 m water depth, off Somalia from June 1992 to February 1993. The results do not confirm the relationship between cyst production and lower nutrient concentrations, as highest cyst fluxes were recorded during late SW monsoon under the relatively nutrient-rich and less agitated conditions of mature upwelled water. Lowest cyst fluxes were found under strongly stratified, nutrient-depleted surface waters during the inter-monsoon. Although all of the studied species seem to prefer a stratified water column, an elevated concentration of nutrients appears to be necessary to maintain high cyst production. Comparison of the mean cyst flux to the sediment trap with that into the underlying surface sediments reveals a loss of 81–96%, which can be attributed to calcite dissolution. The relatively small spheres of Thoracosphaera heimii are affected more than the cysts of the other species.

Oxygen availability effects on early diagenetic calcite dissolution in the Arabian Sea as inferred from calcareous dinoflagellate cysts

In oceanic regions with high primary production, such as the Arabian Sea, the primary signals of proxies are often altered by diagenetic processes. The present study aims at assessing the effects of early diagenesis on calcareous dinoflagellate cysts, which represent a relatively new tool for reconstructing the paleoenvironmental conditions within the photic zone. For this purpose, surface sediment samples from within and below the oxygen minimum zone (OMZ) of the northeastern and southwestern Arabian Sea have been analysed quantitatively for their calcareous dinoflagellate cyst content. The calculated cyst accumulation rates (ARs), the relative abundances and cyst fragmentation values were compared to bottom water oxygen (BWO) content and ARs of organic carbon at the sample positions. Different patterns were found in the northeastern and southwestern part of the Arabian Sea. In the SW, no relationship between cyst ARs and BWO is distinguishable, and the distribution of cyst ARs is thought to largely reflect primary cyst production. In the NE, much higher ARs of all species are found in samples from within the OMZ in comparison to samples from below it. This is interpreted to result from better calcite preservation within the OMZ, presumably due to reduced oxic degradation of organic matter. The differential drop of cyst ARs of the individual species at the lower boundary of the OMZ in the NE Arabian Sea, as well as the species-specific change in relative abundance and fragmentation, indicate different sensitivity to calcite dissolution of the different species. These results show that early diagenetic calcite dissolution can change both relative and absolute abundances of calcareous dinoflagellate cysts, which has to be considered if using them for paleoenvironmental reconstructions. Furthermore, it is shown that considerable calcite dissolution can occur above the carbonate saturation horizon in high productive areas. However, calcite preservation can be substantially increased, as soon as oxygen concentrations are too low for oxic degradation of OM. Under low oxic conditions (within and near the OMZ), the main factor controlling organic matter (OM) preservation appears to be BWO concentrations. Under higher oxygen levels (below f1500 m depth in the NE Arabian Sea) there seems to be an increasing influence of bioturbation and sedimentation rate on the preservation of OM by controlling its oxygen exposure time. This study presents an example of a highly productive basin in which differences in early diagenetic processes can lead to the preservation of a signal that is either dominated by primary production (off Somalia) or by secondary alteration (off Pakistan), although in both areas, an oxygen depleted zone is present. For estimating the effects of early diagenetic calcite dissolution in a sediment by metabolic CO2 (and probably by H2S oxidation), not only the content of organic carbon but also other geochemical proxies for paleoredoxconditions have to be included for paleoenvironmental reconstructions. D 2002 Elsevier Science B.V. All rights reserved.

The (palaeo) environmental significance of modern calcareous dinoflagellate cysts: a review

Increased interest in the environmental applicability of calcareous dinoflagellate cysts for palaeo-environmental studies arose in the last two decades, when it was discovered that they constitute a widespread and abundant group within the calcareous phytoplankton. As primary producers they are directly influenced by environmental Parameters of the surrounding water masses, and therefore extremely suitable for detailed palaeo-environmental and -oceanographical reconstructions. During the last years, detailed investigations on their distribution in surface sediments of the South Atlantic Ocean, the Mediterranean Sea and the Arabian Sea were carried out, in order to reveal which environmental parameters influence their distribution. This, in turn, formed the base for palaeo-environmental and -oceanographical reconstructions. The compilation of the available data on calcareous dinoflagellate ecology, morphology, taxonomy and palaeo-environmental application presented here demonstrates their ample application potential as palaeo-environmental tools.KurzfassungEin steigendes Interesse an der umweltrelevanten Einsetzbarkeit kalkiger Dinoflagellatenzysten für Paläoumweltstudien zeichnete sich in den letzten zwei Jahrzehnten ab, als man sie als weit verbreitete und häufige Gruppe innerhalb des kalkigen Phytoplanktons erkannte. Als Primärproduzenten werden sie direkt von den Umweltparametern der umgebenden Wassermassen beeinflusst und sind daher extrem nützlich für detaillierte Paläoumwelt- und Paläozeanographie-Rekonstruktionen. Während der letzten Jahre wurden detaillierte Untersuchungen der Verbreitung kalkiger Dinoflagellatenzysten in Oberflächensedimenten des Südatlantiks, des Mittelmeeres und des Arabischen Meeres ausgeführt, um herauszufinden, welche Umweltparameter ihre Verbreitung beeinflussen. Dies wiederum bildete die Basis für Paläoumwelt- und Paläozeanographie-Rekonstruktionen. Die hier präsentierte Quintessenz der zur Verfügung stehenden Daten über Ökologie, Morphologie, Taxonomie und Paläoumwelt-Anwendung kalkiger Dinoflagellaten zeigt ihr weitläufiges Anwendungspotential als Paläoumwelt-Werkzeug.

Upper Cretaceous inter-hemispheric correlation between the Southern Tethys and the Boreal: chemo- and biostratigraphy and paleoclimatic reconstructions from a new section in the Tethys Himalaya, S-Tibet

A new, 430 m long and mostly continuous Upper Cretaceous section from southern hemisphere low paleolatitudes in the Tethys Himalaya (Guru, Tibet) is presented. The lithology, microfacies and fossil contents of the Guru section indicate a continuous shallowing trend from open oceanic conditions at the slope during the Turonian to shallow marine inner shelf environments in the Maastrichtian, interrupted by short periods of stagnation or slight deepening. Strong variations in sedimentation rates and contents of carbonate, quartz, organic carbon and sulfur appear to reflect a combination of regional and global processes. On regional scale, the patterns of varying elastic sediment supply seem to be related to the fast northward drift of the Indian plate from the temperate climate belt in the Turonian and Coniacian, crossing the arid zone during the Santonian and early Campanian, and passing into the tropical humid belt in the late Campanian. On global scale, similar sedimentary changes as in Guru with a transition from clay-rich Turonian sediments to Coniacian carbonates are found in other regions of the world, suggesting an additional influence of global oceanic and climatic factors. Intervals of omission, erosion and reworking in upper Coniacian and lower Campanian sediments of the Guru section appear to coincide with global sea-level lowstands. Well preserved planktic foraminifera occur over most of the Guru section and provide good biostratigraphic control for correlation of stable carbon and oxygen isotopes with data from the reference section for the boreal and Maastrichtian carbon isotope fluctuations can be related to cyclic variations in carbonate showing detailed correlation of bulk sediment carbon isotopes and comparable trends in oxygen isotopes. The real white chalk in northern Germany at Lagerdorf-Kronsmoor and with the English Chalk reference section, content at Lagerdorf-Kronsmoor which are thought to be driven by long eccentricity. This indicates orbital forcing of both carbonate accumulation and carbon isotope signature, most likely mediated by global sea-level changes. The possibility of detailed delta C-13 correlation from boreal sections of the northern hemisphere to a low-latitude section of the southern hemisphere allows for global correlation with an accuracy not achieved by biostratigraphic methods so far. It further enables linking of microfossil and macrofossil biozones, facilitating precise comparison of shallow and deep water sections, which is essential for the improvement of our understanding of timing, causes and effects of climatic and oceanographic processes.

Revision and Evaluation of the Systematic Affinity of the Calcitarch Genus Pithonella Based on Exquisitely Preserved Turonian Material from Tanzania

Abstract Extraordinarily well-preserved pithonellid microfossils (calcitarchs, “calcispheres”) from the Turonian (upper Cretaceous) of Tanzania reveal previously unknown morphological traits, crystallographic patterns, and chemical signatures, providing new insight to this enigmatic group of microfossils. Using combined transmitted-reflected light microscopy, scanning electron microscope imagery, electron microprobe elemental analysis and stable isotope geochemistry, the present study reveals four new aspects of the genus Pithonella, notably, the following. An affinity with cyst-forming organisms, potentially the dinoflagellates, is indicated by the presence of a hatch opening and corresponding operculum. The pristine outer wall architecture consists of thin, smooth shingle-shaped plates with regular rows of slit-shaped pores and an apical sub-angular or circular pore. This primary surface pattern is significantly different from previous descriptions of an outer wall consisting of “parquet-shaped” prismatic crystal rows; this latter surface pattern is formed by secondary overgrowth. The crystallographic pattern of the inner wall is crypto-crystalline. Unaltered pithonellids reveal a calcite chemistry characterized by comparably high Mg-contents, relatively enriched stable carbon isotope values, and stable oxygen values indicating a surface water habitat. Based on these previously unseen traits, the diagnosis of the genus Pithonella is emended. A new species, Pithonella diconica, is described from the lower-middle Turonian sediments of Tanzania.

Using cathodoluminescence spectroscopy of cretaceous calcareous microfossils to distinguish biogenic from early-diagenetic calcite.

A comparative cathodoluminescence (CL) spectroscopic study of extraordinarily well-preserved versus diagenetically altered Turonian (∼92 Ma before present) calcitic and aragonitic microfossils was performed to document the cathodoluminescence characteristics of two common Cretaceous carbonate producers, foraminifera and calcareous dinoflagellates. Unaltered specimens reveal a conspicuous peak in the blue CL band at ≈ 400 nm that has rarely been previously reported for biogenic carbonates. We interpret this luminescence as an indicative feature of the primary bio-mineralized shells of calcareous dinoflagellates and foraminifera. Orange luminescence as the second important CL emission band (≈ 620 nm) in calcite generally increases with diagenetic cement overgrowth and recrystallization but can also be present in unaltered material. Thus, orange CL of biogenic calcite is not an unequivocal diagenetic indicator. Accordingly, spectroscopic investigation of both the ≈ 400 and ≈ 620 nm peaks represents a more objective criterion to evaluate the degree of diagenetic alteration. The ratio of relative intensities of the blue CL versus orange CL can provide a semiquantitative measure with relative intensity ratios blue:orange >2 occurring in the least diagenetically altered microfossils. Comparison of unaltered specimens of separate species reveals elemental differences that potentially indicate species-specific biomineralization or habitats.

Calcareous cyst-producing dinoflagellates: ecology and aspects of cyst preservation in a highly productive oceanic region

Abstract Absolute and relative abundances of calcareous dinoflagellate cyst species in surface sediment samples from the Arabian Sea are compared with environmental parameters of the upper 100 m of the water column to gain information on their largely unknown autecology. Ten species or morphotypes were encountered of which four occurred only as accessories. On the basis of the distribution patterns of the six more abundant species or morphotypes, the studied area is subdivided into three provinces, demonstrating a clear relationship to monsoon-controlled upper-ocean conditions. The two dominant species, Thoracosphaera heimii and Orthopithonella granifera, show opposite trends in distribution of both their absolute and relative abundances. In the NE Arabian Sea, low absolute and relative abundances of T. heimii are mainly attributed to enhanced dissolution of the small tests in this region, whereas elevated concentrations of O. granifera seem to be related to higher water temperatures and the influence of the Indus River. Sphaerodinella albatrosiana and Calciodinellum operosum are most abundant in the open ocean, associated with lower nutrient levels, relatively high temperatures and low seasonality. Spiny cysts (mainly represented by Scrippsiella trochoidea), in contrast, exhibit a more shelf-ward distribution and are most abundant in regions that are influenced by coastal upwelling, characterized by eutrophic and rather unstable conditions with seasonally lower temperatures and a shallow thermocline. A generally negative correlation of calcareous dinoflagellate cysts with primary productivity or high nutrient concentrations, as proposed by other workers, cannot be confirmed. Cyst accumulation rates off Somalia show that strong turbulence and high current speeds are unfavourable for calcareous dinoflagellates, suggesting that these organisms are more successful under rather stratified conditions.

Orthopithonella collaris sp. nov., a new calcareous dinoflagellate cyst from the K/T boundary (Fish Clay, Stevns Klint/Denmark).

A new calcareous dinoflagellate cyst species, Orthopithonella collaris sp. nov., is described from the Cretaceous/Tertiary (K/T) boundary clay (Fish Clay) of Stevns Klint, Denmark, on the basis of SEM studies and light-microscopic analyses of thin sections of single specimens. The species has been found exclusively in the Fish Clay and as such may be a potential marker for the K/T boundary. Its pulse-like occurrence is thought to be due to the abrupt, relatively short-term ecological catastrophe associated with the K/T boundary event.