Johan van der Burgh

Episodic fresh surface waters in the Eocene Arctic Ocean

It has been suggested, on the basis of modern hydrology and fully coupled palaeoclimate simulations, that the warm greenhouse conditions that characterized the early Palaeogene period (55–45 Myr ago) probably induced an intensified hydrological cycle with precipitation exceeding evaporation at high latitudes. Little field evidence, however, has been available to constrain oceanic conditions in the Arctic during this period. Here we analyse Palaeogene sediments obtained during the Arctic Coring Expedition, showing that large quantities of the free-floating fern Azolla grew and reproduced in the Arctic Ocean by the onset of the middle Eocene epoch (∼50 Myr ago). The Azolla and accompanying abundant freshwater organic and siliceous microfossils indicate an episodic freshening of Arctic surface waters during an ∼800,000-year interval. The abundant remains of Azolla that characterize basal middle Eocene marine deposits of all Nordic seas probably represent transported assemblages resulting from freshwater spills from the Arctic Ocean that reached as far south as the North Sea. The termination of the Azolla phase in the Arctic coincides with a local sea surface temperature rise from ∼10 °C to 13 °C, pointing to simultaneous increases in salt and heat supply owing to the influx of waters from adjacent oceans. We suggest that onset and termination of the Azolla phase depended on the degree of oceanic exchange between Arctic Ocean and adjacent seas.

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.

New Observations and Synthesis of Paleogene Heterosporous Water Ferns

Premise of research. Reproductive structures of modern genera of heterosporous water ferns (Marsileaceae and Salviniaceae) are widespread and abundant in plant mesofossil assemblages from the Paleogene. For Salviniaceae, whole fertile fossil plants give a good understanding of morphology. These fossils can be applied in paleoenvironmental analysis and to study water fern origin, evolution, and diversification. Methodology. New specimens were examined by SEM and TEM. Synchrotron x-ray tomographic microscopy (SRXTM) is evaluated as a nondestructive tool for investigating Azolla Lam. morphology. Pivotal results. Azolla anglica Martin and Salvinia cobhamii Martin (earliest Eocene, United Kingdom) are fully characterized using SEM and TEM. SRXTM enables digital rendering of the float system in Azolla, but individual floats are difficult to distinguish. Modern water fern genera characterize the Paleogene, but extinct sister taxa characterize the Cretaceous. Literature review documents that water ferns are intolerant of salinity over 5 psu. Conclusions. The oldest fully documented Salvinia Séguier sori and spores occur in earliest Eocene deposits at Cobham, United Kingdom, probably linked to warm climates. An unusual co-occurrence of Salvinia with Azolla is preserved at this site. The Azolla species differs from those present in the same region during other Eocene warm-climate intervals. SRXTM offers potential to retrieve taxonomically useful information on internal structures of Azolla. There is a major turnover in water ferns (dominantly extinct to almost entirely modern genera) across the Cretaceous-Paleogene transition. The utility of water ferns as indicator taxa is exemplified by recognition of freshwater ocean surfaces and widespread continental wetlands during the latest Early to earliest Middle Eocene in and around the Arctic and Nordic Seas.

New Ginkgophytes from the Upper Triassic–Lower Cretaceous of Spitsbergen and Edgeøya (Svalbard, Arctic Norway): The History of Ginkgoales on Svalbard

Premise of research. During the ongoing investigation of Upper Triassic–Lower Cretaceous plant macrofossils from Svalbard, Norway, some ginkgoalean leaf fossils were found from Carnian and Aptian deposits of Spitsbergen and Edgeøya that represent new ginkgophyte species. One new species is described as Baiera aquilonia sp. nov., and one ginkgophyte leaf is assigned to Ginkgoites sp. Along with the description of the new material, an overview of the presence and distribution of ginkgophytes in the high-latitude ecosystems of Svalbard through time is provided. Methodology. The plant macrofossils have been analyzed with transmitted-light and epifluorescence microscopy. Attempts to isolate cuticles were made. Pivotal results. The investigation resulted in the description of one species new to science, Baiera aquilonia sp. nov., and one specimen assigned to Ginkgoites sp. The presence of ginkgophytes on Svalbard changed significantly through time: periods of dominance and wide distribution interchanged with periods of very low diversity and abundance. Conclusions. Ginkgophytes were thriving in Svalbard, which was already located above 60°N by the Carnian, from the Late Triassic to the Cenozoic in varying abundance and were finally extirpated, probably as a result of dramatic climatic changes at the end of the Paleogene.