Wolfram M. Kürschner

The impact of Miocene atmospheric carbon dioxide fluctuations on climate and the evolution of terrestrial ecosystems.

The Miocene is characterized by a series of key climatic events that led to the founding of the late Cenozoic icehouse mode and the dawn of modern biota. The processes that caused these developments, and particularly the role of atmospheric CO2 as a forcing factor, are poorly understood. Here we present a CO2 record based on stomatal frequency data from multiple tree species. Our data show striking CO2 fluctuations of ≈600–300 parts per million by volume (ppmv). Periods of low CO2 are contemporaneous with major glaciations, whereas elevated CO2 of 500 ppmv coincides with the climatic optimum in the Miocene. Our data point to a long-term coupling between atmospheric CO2 and climate. Major changes in Miocene terrestrial ecosystems, such as the expansion of grasslands and radiations among terrestrial herbivores such as horses, can be linked to these marked fluctuations in CO2.

Paleoatmospheric Signatures in Neogene Fossil Leaves

An increase in the atmospheric carbon dioxide (CO2) concentration results in a decrease in the number of leaf stomata. This relation is known both from historical observations of vegetation over the past 200 years and from experimental manipulations of microenvironments. Evidence from stomatal frequencies of fossil Quercus petraea leaves indicates that this relation can be applied as a bioindicator for changes in paleoatmospheric CO2 concentrations during the last 10 million years. The data suggest that late Neogene CO2 concentrations fluctuated between about 280 and 370 parts per million by volume.

Oak leaves as biosensors of late Neogene and early Pleistocene paleoatmospheric CO2 concentrations

Abstract Complementary to the interpretation of δ 13 C values of biogenic carbonate and sedimentary organic carbon in marine sediments, paleoatmospheric CO 2 levels can be estimated by considering the inverse relationship between atmospheric CO 2 concentration and stomatal parameters (frequency, size) on leaves of land plants. In woody plants, the significance of this (species-specific) physiological response to changing CO 2 regimes is now repeatedly confirmed, both experimentally and from historical sequences of leaves collected since the onset of industrialization. A corollary of this relationship is that analysis of stomatal parameters on fossil leaves has the potential of determining changes in paleoatmospheric CO 2 levels at different time scales. Well-preserved cuticle remains of oak leaves from late Miocene, Pliocene and early Pleistocene sediments of the Lower Rhine Embayment (Germany, The Netherlands) give promise of extending the record of stomatal frequency response to the last 10 Ma. During intervals with warm-temperate to subtropical climatic conditions, oak leaves are characterized by a high stomatal resistance (or low conductance) to CO 2 diffusion and low stomatal frequencies; during cooler intervals we observe an opposite picture. Comparison with historical relations between CO 2 concentration and stomatal properties suggests that paleoatmospheric CO 2 concentrations were not significantly higher than during the last 200 years and fluctuated several times between 280 and 370 ppmv in covariation with contrasting regional climatic conditions. On a global scale, intervals with reduced CO 2 levels match glacial pulses characterized by the occurrence of ice-rafted detritus in high-latitude oceanic sediments.

The anatomical diversity of recent and fossil leaves of the durmast oak (Quercus petraea Lieblein/Q. pseudocastanea Goeppert) — implications for their use as biosensors of palaeoatmospheric CO2 levels

Abstract Detailed anatomical studies on leaves of Quercus petraea enable the recognition of distinct sun and shade morphotypes based on epidermal and mesophyll characteristics. Epidermal cell properties, like the cell area and the corresponding epidermal cell density, the anticlinal wall undulation and the structure of the palisade parenchyma are good parameters to differentiate between sun and shade leaves. These distinguishing leaf anatomical characteristics also reveal distinct sun and shade morphotypes among 50 leaf remains of Q. pseudocastanea , the fossil representative of Q. petraea , from two Late Miocene clay intercalations in the open-cast mine Hambach (Germany). Like their recent equivalents, fossil sun morphotypes show higher epidermal density as a result of a restricted amount of lateral epidermal cell expansion. Correspondingly, the stomatal density in sun leaves is considerably increased by about 60%, whereas the stomatal index is only slightly higher. Fossil sun leaves are also characterized by straight to rounded epidermal cell walls and a thicker palisade parenchyma due to a second cell layer. Fossil shade leaves in turn show a pronounced undulation of the epidermal cell walls and only a single layered-palisade parenchyma. The rich fossil oak leaf assemblages are characterized by a high abundance (90%) of sun morphotypes, which is mainly the result of taphonomic processes. This remarkable predominance of sun morphotypes evokes a confident reproducibility of former analysis of stomatal parameters as a measure of palaeoatmospheric CO 2 levels. However, the stomatal density with its high variability should only be used as a bioindicator of palaeoatmospheric CO 2 levels in large data sets or from one particular leaf morphotype, preferably from sun leaves. In the case of small sample sets, the application of the stomatal index is highly recommended.

Atmospheric Carbon Injection Linked to End-Triassic Mass Extinction

The end-Triassic mass extinction coincided with a massive release of carbon to the atmosphere and rapid climate change. The end-Triassic mass extinction (~201.4 million years ago), marked by terrestrial ecosystem turnover and up to ~50% loss in marine biodiversity, has been attributed to intensified volcanic activity during the break-up of Pangaea. Here, we present compound-specific carbon-isotope data of long-chain n-alkanes derived from waxes of land plants, showing a ~8.5 per mil negative excursion, coincident with the extinction interval. These data indicate strong carbon-13 depletion of the end-Triassic atmosphere, within only 10,000 to 20,000 years. The magnitude and rate of this carbon-cycle disruption can be explained by the injection of at least ~12 × 103 gigatons of isotopically depleted carbon as methane into the atmosphere. Concurrent vegetation changes reflect strong warming and an enhanced hydrological cycle. Hence, end-Triassic events are robustly linked to methane-derived massive carbon release and associated climate change.

Stomatal frequency adjustment of four conifer species to historical changes in atmospheric CO2

The species-specific inverse relation between atmospheric CO(2) concentration and stomatal frequency for many woody angiosperm species is being used increasingly with fossil leaves to reconstruct past atmospheric CO(2) levels. To extend our limited knowledge of the responsiveness of conifer needles to CO(2) fluctuations, the stomatal frequency response of four native North American conifer species (Tsuga heterophylla, Picea glauca, Picea mariana, and Larix laricina) to a range of historical CO(2) mixing ratios (290 to 370 ppmV) was analyzed. Because of the specific mode of leaf development and the subsequent stomatal patterning in conifer needles, the stomatal index of these species was not affected by CO(2). In contrast, a new measure of stomatal frequency, based on the number of stomata per millimeter of needle length, decreased significantly with increasing CO(2). For Tsuga heterophylla, the stomatal frequency response to CO(2) changes in the last century is validated through assessment of the influence of other biological and environmental variables. Because of their sensitive response to CO(2), combined with a high preservation capacity, fossil needles of Tsuga heterophylla, Picea glauca, P. mariana, and Larix laricina have great potential for detecting and quantifying past atmospheric CO(2) fluctuations.

Triassic palynology of central and northwestern Europe: a review of palynofloral diversity patterns and biostratigraphic subdivisions

Abstract We document palynofloral trends through the Triassic in the Germanic and Alpine facies with an emphasis on diversity trends and possibly related palaeoenvironmental changes. As a first order approximation of palynofloral diversity, we used the range through method of the software package PAST based on a range chart compiled from several Triassic palynological studies and reviews. Our analysis suggests that during the entire Triassic the diversity of plants producing spores was largely controlled by the availability of water, while diversity among gymnosperms was also affected by other environmental and biotic factors. In general, palynofloral diversity declines by some 50% between the early Carnian and the Norian, mainly as a result of a decrease in the number of pollen species. This is the second most severe loss in pollen species after the Permian–Triassic biotic crisis. In comparison to the marked palynofloral turnover at the Permian–Triassic transition and the end-Carnian decrease in palynofloral diversity, the end-Triassic biotic crisis appears to have little affected palynofloral species diversity in Europe. A study of the palynostratigraphy of NW Europe recognizes nine zones (and nine subzones) that encompass the Triassic, most of which have their boundaries based on the first occurrences of marker species. The palynostratigraphic zones and subzones in Europe are correlated to the marine Triassic stages based on various data, including numerous palynological records in marine Alpine Triassic strata.

An explanation for conflicting records of Triassic–Jurassic plant diversity

Macrofossils (mostly leaves) and sporomorphs (pollen and spores) preserve conflicting records of plant biodiversity during the end-Permian (P-Tr), Triassic–Jurassic (Tr-J), and end-Cretaceous (K-T) mass extinctions. Estimates of diversity loss based on macrofossils are typically much higher than estimates of diversity loss based on sporomorphs. Macrofossils from the Tr-J of East Greenland indicate that standing species richness declined by as much as 85% in the Late Triassic, whereas sporomorph records from the same region, and from elsewhere in Europe, reveal little evidence of such catastrophic diversity loss. To understand this major discrepancy, we have used a new high-resolution dataset of sporomorph assemblages from Astartekløft, East Greenland, to directly compare the macrofossil and sporomorph records of Tr-J plant biodiversity. Our results show that sporomorph assemblages from the Tr-J boundary interval are 10–12% less taxonomically diverse than sporomorph assemblages from the Late Triassic, and that vegetation composition changed rapidly in the boundary interval as a result of emigration and/or extirpation of taxa rather than immigration and/or origination of taxa. An analysis of the representation of different plant groups in the macrofossil and sporomorph records at Astartekløft reveals that reproductively specialized plants, including cycads, bennettites and the seed-fern Lepidopteris are almost absent from the sporomorph record. These results provide a means of reconciling the macrofossil and sporomorph records of Tr-J vegetation change, and may help to understand vegetation change during the P-Tr and K-T mass extinctions and around the Paleocene–Eocene Thermal Maximum.

Multiple phases of carbon cycle disturbance from large igneous province formation at the Triassic-Jurassic transition

The end-Triassic mass extinction (ca. 201.4 Ma) coincided with a major carbon cycle perturbation, based on an ∼5‰−6‰ negative excursion in δ 13 C TOC (total organic carbon) records. Both events coincided directly with the onset of massive flood basalt volcanism in the Central Atlantic Magmatic Province (CAMP). Organic carbon isotope data from the western Tethys Ocean (Austria) and the Germanic basin (UK and Germany), however, demonstrate earlier disruption of the global carbon cycle, preceding CAMP eruptions. A 2‰−3‰ late Rhaetian precursor negative excursion in marine and continental δ 13 C TOC records is matched by a negative perturbation in δ 13 C leaf data, suggesting multiple events of Rhaetian atmospheric 13 C depletion. Intruding dike and sill systems, preceding CAMP eruptive volcanic activity, may have released ∼3000–7000 Gt of isotopically light carbon as thermogenic methane from subsurface organic-rich strata. This possibly caused an end-Triassic atmospheric p CO 2 increase and reduced ecosystem stability before the actual onset of eruptive volcanic activity in the CAMP region. We present a model that identifies three phases of disturbances in global biogeochemical cycles related to the formation of this large igneous province.

Vegetation history, diversity patterns, and climate change across the Triassic/Jurassic boundary

Abstract High-resolution palynological data sets from shallow marine Triassic-Jurassic (Tr/J) boundary beds of two principal sections in Europe (Hochalplgraben in Austria and St. Audries Bay in the United Kingdom) were analyzed to reconstruct changes in vegetation, biodiversity, and climate. In Hochalplgraben, a hardwood gymnosperm forest with conifers and seed ferns is replaced by vegetation with dominant ferns, club mosses and liverworts, which concurs with an increased diversification of spore types during the latest Rhaetian. Multivariate statistical analysis reveals a trend to warmer and wetter conditions across the Tr/J boundary in Hochalplgraben. The vegetation changes in St. Audries Bay are markedly different. Here, a mixed gymnosperm forest is replaced by monotonous vegetation consisting mainly of Cheirolepidiaceae (80–100%). This change is caused by a transition to a warmer and more arid climate. The observed diversity decrease in St. Audries Bay affirms this interpretation. Although both sections show major vegetation changes, neither of them demonstrates a distinctive floral mass extinction. A compilation of Tr/J boundary sections across the world demonstrates the presence of Cheirolepidiaceae-dominated forests in the Pangaean interior and increases in abundance of spore-producing plants adjacent to the Tethys Ocean. We propose that the non-uniform vegetation changes reflected in the Tr/J palynological records are the result of environmental changes caused by Central Atlantic Magmatic Province volcanism. The increase in greenhouse gases caused a warmer climate and an enhanced thermal contrast between the continent and the seas. Consequently, the monsoon system got stronger and induced a drier continental interior and more intensive rainfall near the margins of the Tethys Ocean.