Thomas Steuber

Low-latitude seasonality of Cretaceous temperatures in warm and cold episodes

The Cretaceous period is generally considered to have been a time of warm climate. Evidence for cooler episodes exists, particularly in the early Cretaceous period, but the timing and significance of these cool episodes are not well constrained. The seasonality of temperatures is important for constraining equator-to-pole temperature gradients and may indicate the presence of polar ice sheets; however, reconstructions of Cretaceous sea surface temperatures are predominantly based on the oxygen isotopic composition of planktonic foraminifera that do not provide information about such intra-annual variations. Here we present intra-shell variations in δ18O values of rudist bivalves (Hippuritoidea) from palaeolatitudes between 8° and 31° N, which record the evolution of the seasonality of Cretaceous sea surface temperatures in detail. We find high maximum temperatures (∼35 to 37 °C) and relatively low seasonal variability (< 12 °C) between 20° and 30° N during the warmer Cretaceous episodes. In contrast, during the cooler episodes our data show seasonal sea surface temperature variability of up to 18 °C near 25° N, comparable to the range found today. Such a large seasonal variability is compatible with the existence of polar ice sheets.

Phanerozoic record of plate tectonic control of seawater chemistry and carbonate sedimentation

Sr concentrations in Phanerozoic biological calcite suggest a close link between fluctuations in the Sr/Ca ratio of seawater, aragonite versus calcite sedimentation, and the Mg/Ca ratio of seawater, the entire chain being driven by the rate of production and hydrothermal alteration of the oceanic crust. Model simulations suggest that variations in hydrothermal and continental weathering fluxes cannot be the sole reason for the observed Phanerozoic seawater Sr/ Ca trend. Changing Sr burial in marine carbonates is likely the most important mechanism that can explain the experimental data. During episodes of high seawater Mg/Ca ratios, aragonite was preferentially deposited, resulting in low seawater Sr/Ca ratios. At low Mg/ Ca ratios, calcite was the dominant carbonate sediment, and the Sr/ Ca ratio of seawater was high. The evidence for changing chemistry of seawater also has implications for the application of the Sr/Ca and Mg/Ca paleothermometers, in particular for pre-Quaternary samples, and for the assessment of diagenetic alteration of fossil skeletal carbonates by using Sr/Ca ratios.

A synoptical classification of the Bivalvia (Mollusca)

Preface This classification summarizes the suprageneric taxonomy of the Bivalvia for the upcoming revision of the Bivalvia volumes of the Treatise on Invertebrate Paleontology, Part N.

Plate tectonic control on the evolution of Cretaceous platform- carbonate production

Carbonate-producing biota on Cretaceous carbonate platforms changed from early Cretaceous aragonitic coral-algal and coral-rudist associations to Turonian-Maastrichtian calcite-dominated rudist associations. Abundance patterns of rudist bivalves examined from a large database show a shift from Barremian- Cenomanian aragonite-dominated to post-Cenomanian calcite- dominated shells. These skeletal-mineral changes agree with a decrease in the Mg/Ca ratio of seawater owing to high rates of production of oceanic crust during the Barremian−early Turonian and its effect on seawater saturation with respect to aragonite and calcite. Increasing Sr concentrations in biotic calcite from the Barremian to the Maastrichtian reflect the shift from aragonite-dominated to calcite-dominated platform sedimentation, because aragonite precipitation is a major sink for seawater Sr. It is speculated that a particularly low Mg/Ca ratio and the corresponding low supersaturation of seawater with respect to aragonite during the middle Cretaceous was a major cue for the radiation of calcite- dominated rudist bivalves, which became important carbonate producers on Turonian-Maastrichtian carbonate platforms. Changing Mg/Ca ratios of seawater are not considered to have caused middle Cretaceous crises in the evolution of carbonate platforms, but seawater composition could have selected for calcite-dominated biota when carbonate production resumed.

Catastrophic extinction of Caribbean rudist bivalves at the Cretaceous-Tertiary boundary

Strontium isotope ratios ( 87 Sr/ 86 Sr) in pristine low-Mg calcite of shells of rudist bivalves from the Titanosarcolites limestones exposed in the Central, Maldon, and Marchmont inliers of Jamaica indicate that species-rich rudist-coral associations persisted into the latest Maastrichtian (66–65 Ma). This finding contradicts the currently accepted hypothesis of stepwise extinction of rudist bivalves in the middle Maastrichtian and argues for a catastrophic, impact-related demise of Caribbean Cretaceous reefal ecosystems at the Cretaceous-Tertiary boundary.

Species richness and abundance patterns of Tethyan Cretaceous rudist bivalves (Mollusca: Hippuritacea) in the central-eastern Mediterranean and Middle East, analysed from a palaeontological database

Abstract Data about the stratigraphical and regional distribution of Cretaceous rudist bivalves in the central-eastern Mediterranean and Middle East from the Barremian to the Maastrichtian have been compiled in a taxonomic databank system. The investigated region comprises northern, central and southern regions of the Cretaceous Tethys. The regionally different quality of taxonomic data is critically discussed. Both species richness and abundance (number of species records) peak in the Early Aptian, Late Cenomanian and Early Campanian. Temporal patterns in species richness are examined in relation to evolutionary innovations of the group. Crises in species richness and abundance during the Early and mid-Cretaceous were coeval with oceanic anoxia associated with platform drowning. Also, regional emergence affected large parts of the Apulian Plate and other central Tethyan carbonate platforms during the mid-Cretaceous period of globally high sea level, so that these crises can be attributed to regional environmental perturbations, induced by either oceanic anoxia or tectonic movements. The Cenomanian–Turonian transition marks an important faunal turnover, as indicated by the demise of taxonomic groups which are characterised by recumbent ecological morphotypes and predominantly aragonitic shells. During the Turonian, they were replaced by associations dominated by largely calcitic Radiolitidae and Hippuritidae, among which elevator morphotypes dominated. The stratigraphy of many post-Turonian deposits in the studied region is too imprecise for a detailed analysis of Late Cretaceous distributional patterns, and the demise of rudists during the Maastrichtian in particular. The regional pattern of species richness and abundance is similar throughout most of the investigated region. A notable exception is the broad north-eastern African shelf from Libya to Lebanon, where rudists were almost completely absent after the Coniacian. This coincided with shallow-water phosphorite and black-shale formation on the extended shelf of this region, indicating the upwelling of nutrient-rich waters and stagnant conditions. During this time (Campanian–Maastrichtian), species-rich associations thrived along the north-eastern Afro-Arabian margin, now exposed in south-eastern Turkey, Iran, the United Arab Emirates (UAE) and Oman. This region forms the single distinct faunal province during the Cretaceous in the studied region, as indicated by an endemism of 40% at the genus level. The low species richness and abundance of Hippuritidae on Turonian–Maastrichtian central Tethyan carbonate platforms, when compared with regions having mixed calcareous–siliciclastic sedimentation, is interpreted to argue against a widespread photosymbiosis in the group. Several aspects of this study highlight the importance of regional tectonic and oceanographic controls of distributional, and perhaps also evolutionary, patterns of rudist bivalves. Their delineation still suffers from the regionally different quality of stratigraphical and taxonomic data.

Strontium isotope stratigraphy of Cretaceous hippuritid rudist bivalves: rates of morphological change and heterochronic evolution

Abstract Strontium isotope stratigraphy of 17 localities of rudist formations in the region of the former Mediterranean Tethys has provided a reliable and precise stratigraphical frame for the evaluation of morphological change in hippuritid rudist bivalves during the Coniacian–Campanian. The phyletic lineage Vaccinites cornuvaccinum (Bronn)–Vaccinites chaperi (Douville) evolved from the Early Coniacian until the Early Campanian and is characterized by phyletic size increase and allometric growth, as shown by morphometrical measurements of 102 shells. These chronospecies intergrade in the Late Coniacian so that V. cornuvaccinum is considered to be a reliable marker species for the Coniacian. The taxonomy of Vaccinites alpinus (Douville) is discussed and the species is recognized as a senior synonym of Vaccinites ultimus (Milovanovic). It appears first in the Late Santonian and the last appearance is probably in Late Campanian. Both lineages are characterized by phyletic size increase and peramorphic evolution involving hypermorphosis. A doubling of the length of the mantle margin occurred within 5 m.y. in both lineages. The results demonstrate that the combination of morphometric analyses and stratigraphical precision provides an important tool for the delineation of tempo and mode of evolution in rudist bivalves. Strontium isotope stratigraphy resulted in a considerable revision of the ranges of the species investigated. As the stratigraphy of many Tethyan carbonate platforms relies on the distribution of rudist bivalves, and the species investigated are abundant in many rudist formations, the history of many Late Cretaceous carbonate platforms must be re-evaluated.

Recent and historical discharge of a large European river system – oxygen isotopic composition of river water and skeletal aragonite of Unionidae in the Rhine

Abstract Seasonal variations in the oxygen isotopic composition of Rhine River water were analyzed in detail and compared with the oxygen isotopic record from recent and historical specimens of freshwater bivalves (Unionidae). The purpose of this study was to investigate the potential use of these aragonitic bivalves as proxy recorders for varying amounts and sources of discharge, and thereby infer climate change. Seasonal variations in the river water δ 18 O are on the order of 1–2‰. During summer, Alpine melt-water contributes significantly to the total discharge, resulting in average values of −10 to −10.5‰, whereas the non-Alpine contribution is higher during winter, as indicated by mean δ 18 O values of −8.5 to −9‰. The basic pattern of the modern seasonal variation of δ 18 O of river water can be described by a numerical mass balance approximation of the various contributions from the Alpine and non-Alpine catchments with their average δ 18 O composition. The δ 18 O of growth increments in the prismatic shell layer of Anodonta corresponds perfectly to what is predicted by known fractionation of 18 O between water and aragonite. Shell growth is restricted to water temperatures above 8–10°C, so variations in δ 18 O and the river water temperature are faithfully recorded by relatively large growth increments during summer. The distinctive isotopic signatures of individual flood events during summer and autumn are also recorded in the shells.

Stable isotope records (O, C) of Jurassic aragonitic shells from England and NW Poland: palaeoecologic and environmental implications

Abstract Shells of fully marine Middle to Upper Jurassic molluscs from England and north-western Poland were analysed with respect to their stable isotope (δ 18 O, δ 13 C) compositions, and palaeoecological and environmental life conditions of these molluscs were inferred from them. Light microscopical and SEM inspection and an analysis of the minor element content (Fe, Mn, Mg, Sr) suggest rather unaltered isotope signals. The δ 18 O and δ 13 C values show a characteristic distribution among three groups of co-occurring organisms. Benthic (adult) bivalves generally preserved higher δ 18 O and δ 13 C values than ammonites, whereas planktic bivalve larvae tend to possess the lowest δ 18 O but higher δ 13 C than adult bivalves. As this distribution pattern is found in numerous horizons and sections of Bathonian to Kimmeridgian age in NW Poland and England, it is thought to reflect real palaeoenvironmental parameters. All observations can be incorporated in a single model that assumes (i) seasonally induced temperature stratification of the water column, (ii) a correlation between phytoplankton blooms and reproduction season of planktic–planktotrophic bivalves, and (iii) insignificant vital effects with respect to the δ 13 C in bivalves, but strong biological control in ammonites. In addition, the δ 18 O evolution suggests that the Late Bajocian to Middle/Late Bathonian and Early Oxfordian to Late Kimmeridgian were considerably warmer than the latest Bathonian to Late Callovian time interval. The oxygen isotopic records from other European regions indicate a similar pattern of long-term palaeotemperature evolution. The comparatively high water temperatures during the Callovian to Oxfordian of the Isle of Skye (NW Scotland) are enigmatic, however. In the Early Oxfordian, sea surface and bottom temperatures began to rise in continental Europe and England. These changes coincide with a south-westward drift of the West European crustal plate, but a causal relationship remains to be demonstrated.

Platform environments, microfacies and systems tracts of the upper Cenomanian-lower Santonian of Sinai, Egypt

SummaryFactors controlling grain composition and depositional environments of upper Cenomanian—Santonian limestones of Sinai are discussed. The mainly shallow-water, inner-platform setting investigated is subdivided into five major facies belts, each represented by several microfacies types (MFTs). Their lateral distribution patterns and their composition underline aclear relation between depositional environment and platform position. The facies belts include sandstones and quartzose packstones of siliciclastic shorefaces, mudstones and bioclastic wackestones of restricted lagoons, shallow-subtidal packstones with diverse benthic foraminifera and calcareous algae, bioclastic and/or oolitic grainstones of inner-platform shoals, and wackestones of deep open-marine environments.The microfacies distribution patterns of the Cenomanian-Santonian strata are evaluated with respect to local and regional large-scale environmental changes. While protected shallow-subtidal environments with only subordinate ooids and oncoids prevail during the late Cenomanian, high-energy oolithic shoals and carbonate sands occur locally during the middle and late Turonian. They were probably related to a change of the platform morphology and a reorganisation of the platform after a late Cenomanian drowning. In the Coniacian-Santonian, the lack of ooids, oncoids, and the decrease of calcareous algae versus an increase in siliciclastics indicate a shift to lower water temperature and to a more humid climate. Especially in the Turonian, the interplay between sea-level changes, accommodation, hydrodynamics, and siliciclastic input is reflected by lithofacies and biofacies interrelation-ships that are elaborated within individual systems tracts. In particular, increasing accommodation intensified circulation and wave-agitation and controlled the distribution of high-energy environments of the middle and upper Turonian trans-gressive systems tracts. During highstands protected innerplatform environments prevailed.