Reliable reconstructions of atmospheric carbon dioxide concentrations (pCO2) are required at high... more Reliable reconstructions of atmospheric carbon dioxide concentrations (pCO2) are required at higher resolution than currently available to help resolve the relationship between mass extinctions and changes in palaeo-pCO2 levels. Such reconstructions are needed: 1, at a high temporal resolution for constraining the pre- and post-extinction atmospheres; and 2, at a sufficient spatial resolution to constrain potential inter-hemispheric differences. Here we estimate pCO2 based on fossil Lauraceae leaf cuticle specimens derived from three localities with strata spanning the latest Cretaceous to the mid-Paleocene, including a new Cretaceous–Paleogene boundary (K–Pg) locality, in New Zealand. We use two independent methods of stomatal density-based pCO2 reconstructions; a transfer function calibrated using herbarium material and the stomatal ratio method, producing three calibration sets. Our results based on the mean values of each of the three calibration methods indicate pCO2 ranging between ca. 460 and 650 ppm during the latest Cretaceous, falling precipitously to average values between ca. 360 and 430 ppm across the K–Pg boundary, and further to ca. 305–320 ppm in the mid-Paleocene. A ‘spike’ of extremely high pCO2at the K–Pg could not be confirmed, but our results are, nonetheless, consistent with previously published pCO2 records from the Northern Hemisphere, and show that stomatal density worldwide was responding to significant changes in pCO2 across the K–Pg.
ABSTRACT The fossil diatom record from the Hässeldala Port palaeolake, southeastern Sweden, offer... more ABSTRACT The fossil diatom record from the Hässeldala Port palaeolake, southeastern Sweden, offers an excellent opportunity to investigate how past climatic shifts influenced catchment conditions and early lake development. The record, dating to between 13 900 and 11 200 cal. a BP, covers a climatically dynamic period, starting with deglaciation followed by oscillations between warmer and colder climate states. The stratigraphical changes in the fossil diatom assemblages show a trend of less open-water taxa and a successively more complex periphytic community as the lake shallows and the aquatic habitat structure develops. A diatom-based reconstruction of lake water pH indicates a natural acidification trend early in the record from 13 900 to 12 500 cal. a BP. From 12 500 cal. a BP, coincident with the start of climate cooling, to 11 300 cal. a BP this trend is disrupted and lake waters become more alkaline. A cooler and drier climate most likely resulted in reduced soil organic matter build-up as well as more frozen ground that impeded hydrological flow and decreased the input of dissolved organic matter and organic acids into the lake system. This study demonstrates the importance of the hydrological system as a link between terrestrial and aquatic ecosystems during early lake ontogeny.
Reliable reconstructions of atmospheric carbon dioxide concentrations (pCO2) are required at high... more Reliable reconstructions of atmospheric carbon dioxide concentrations (pCO2) are required at higher resolution than currently available to help resolve the relationship between mass extinctions and changes in palaeo-pCO2 levels. Such reconstructions are needed: 1, at a high temporal resolution for constraining the pre- and post-extinction atmospheres; and 2, at a sufficient spatial resolution to constrain potential inter-hemispheric differences. Here we estimate pCO2 based on fossil Lauraceae leaf cuticle specimens derived from three localities with strata spanning the latest Cretaceous to the mid-Paleocene, including a new Cretaceous–Paleogene boundary (K–Pg) locality, in New Zealand. We use two independent methods of stomatal density-based pCO2 reconstructions; a transfer function calibrated using herbarium material and the stomatal ratio method, producing three calibration sets. Our results based on the mean values of each of the three calibration methods indicate pCO2 ranging between ca. 460 and 650 ppm during the latest Cretaceous, falling precipitously to average values between ca. 360 and 430 ppm across the K–Pg boundary, and further to ca. 305–320 ppm in the mid-Paleocene. A ‘spike’ of extremely high pCO2 at the K–Pg could not be confirmed, but our results are, nonetheless, consistent with previously published pCO2 records from the Northern Hemisphere, and show that stomatal density worldwide was responding to significant changes in pCO2 across the K–Pg.
The flora of the Cenomanian–Turonian (ca. 96–90Ma) Tupuangi Formation, Chatham Islands, New Zeala... more The flora of the Cenomanian–Turonian (ca. 96–90Ma) Tupuangi Formation, Chatham Islands, New Zealand, was inhabiting a region well within the south polar circle (~70–80° S) during the early Late Cretaceous, an interval characterised by extreme global greenhouse conditions. The Tupuangi flora offers a unique perspective into an ecological and environmental setting which has no extant analogue, whilst providing proxies of polar palaeoclimatic conditions during a phase of extreme global warming. Ginkgoites waarrensis Douglas, 1965 (emended herein), a species known previously from a single occurrence in Australia, is an abundant element of the Tupuangi flora. Forty-five leaf samples from three localities are reported, and a systematic treatment of this species revealed a wide morphological range. In contrast to the exclusively riparian niche of more recent members of Ginkgoales, associated sedimentological and palaeoecological data suggest that this species had an ecological preference for regularly disturbed, coastal deltaic settings. Herein,we review the geographic and stratigraphic distributions of Cretaceous Gondwanan ginkgoalean leaf taxa. An increasing diversity of this group from the Early Cretaceous to the early Late Cretaceous supports a broader trend of floral provincialisation throughout this interval, most likely driven by concurrent global transgression and active tectonic extension across southern Gondwana. Carbon dioxide has been inferred as a primary proximate cause of the mid-Cretaceous global greenhouse. The leaf cuticles of Ginkgoites waarrensis were utilised to approximate atmospheric carbon dioxide (pCO2) during the Cenomanian. Stomatal index (SI) data were collected from ten specimens, and the stomatal ratio method yielded a semi-quantitative pCO2 estimate of 1150–1350 ppmv, which is consistent with modelled and proxy estimates of the Cenomanian. The present study explores the inherent limitations of the transfer function method for estimating CO2 when applied to taxa with very low SI values, such as G. waarrensis. In addition to pCO2, temperature and irradiance are identified as environmental variables which may have systematically promoted the low SI of G. waarrensis, but their combined influence is likely mitigated by the relatively high temperature of this region during the mid-Cretaceous and the high summer insolation at polar latitudes.
During a study of macroflora from the Astartekløft locality in Jameson Land, East Greenland, endo... more During a study of macroflora from the Astartekløft locality in Jameson Land, East Greenland, endophytic insect ovipositions (egg traces) belonging to ichnogenus Paleoovoidus were recorded for the first time in ginkgoalean (Ginkgoites, Sphenobaiera, and Baiera) fossil leaves across the Triassic–Jurassic (Tr–J) transition (ca. 200 Ma). The ovipositions may have been produced by insects in the order Odonata (dragonflies and damselflies) and are relatively more abundant before than after the Tr–J transition, possibly reflecting changes in plant-insect association. Fossil clitellate annelid (leech) cocoons were also discovered in a macerated sample from a single bed within the Tr–J transition. The cocoons belong to two species: Dictyothylakos pesslerae and Pilothylakos pilosus, extending the range of the latter genus from the Early Cretaceous to the Early Jurassic. This new evidence suggests that the ecosystem and food webs were profoundly affected by the environmental degradation surrounding the end-Triassic event (ETE), which was marked by faunal mass extinctions and floral turnover. Invertebrate ichno- and body fossils may add significantly to paleoenvironmental information provided by plant fossil assemblages, and therefore a protocol for recording evidence of invertebrate activity in paleobotanical research is suggested, including analyzing a standardized number of specimens for fossil traces and bulk maceration for discovery of invertebrate body fossils. More well-designed studies on Mesozoic plant-invertebrate associations are needed and will provide deeper knowledge about the structure and evolution of complex ecosystems.
We thank Köhler et al. for their interest in our research and welcome discussions about the metho... more We thank Köhler et al. for their interest in our research and welcome discussions about the methods and possible interpreta- tions involved in Lateglacial climate change research. We respond to each main criticism below, but at the onset briefly point out sig- nificant misunderstandings in the comment. Firstly, Köhler et al. concentrate their criticism on the statistical robustness of one re- cord, while our interpretation of a large fluctuation in CO2 concen- tration (pCO2) at the onset of the Younger Dryas (YD) is based on four distinct and geographically separate stomatal records and a parallel radiocarbon fluctuation. Secondly, the authors are confusing an artificial ocean flushing experiment, which we repeat- edly emphasised was a conceptual study of how much pCO2 the ocean can release if it were severely flushed, with an actual simu- lation of the real event. They pay no attention to the hypothesis of a more realistic ocean flushing scenario that is provided.
Just before the onset of the Younger Dryas (YD) cold event, several stomatal proxy-based pCO2 rec... more Just before the onset of the Younger Dryas (YD) cold event, several stomatal proxy-based pCO2 records have shown a sharp increase in atmospheric CO2 concentration (pCO2) of between ca 50 and 100 ppm, followed by a rapid decrease of similar or even larger magnitude. Here we compare one of these records, a high-resolution pCO2 record from southern Sweden, with the IntCal13 record of radiocarbon (D14C). The two records show broadly synchronous fluctuations at the YD onset. Specifically, the IntCal13 record documents decreasing D14C just before the YD onset when pCO2 peaks, consistent with a source of “old” CO2 from the deep ocean. We propose that this fluctuation occurred due to a major ocean flushing event. The cause of the flushing event remains speculative but could be related to the hypothesis of the glacial ocean as a thermobaric capacitor. We confirm that the earth system can produce such large multi- decadal timescale fluctuations in pCO2 through simulating an artificial ocean flushing event with the GENIE Earth System Model. We suggest that sharp transitions of pCO2 may have remained undetected so far in ice cores due to inter-firn gas exchange and time-averaging. The stomatal proxy record is a powerful complement to the ice core records for the study of rapid climate change.
The Triassic–Jurassic boundary (TJB) coincides with major disruption to the carbon cycle and glob... more The Triassic–Jurassic boundary (TJB) coincides with major disruption to the carbon cycle and global warming as the Central Atlantic Magmatic Province developed. This resulted in both marine and terrestrial extinctions, with terrestrial plants thought to experience thermal stress as global temperatures and atmospheric CO2 levels rose. As plant compression fossils typically only preserve external morphological features, it has not been possible to reconstruct plant paleophysiology in order to elucidate the mechanisms underlying plant stress and extinction. Here we present a new approach allowing us to infer the photosynthetic performance and stress physiology of fossil plants, applied to fossil Ginkgoales across the TJB. We use correlations between the adaxial epidermal cell density of extant Ginkgo biloba and photosynthetic and protective stress physiology to infer the paleophysiological condition of Late Triassic–Early Jurassic-aged plants from Astartekløft, East Greenland. The density of fossil leaf adaxial epidermal cells indicates that photosynthetic performance of Ginkgoales became increasingly impaired towards the latter stages of the Triassic, before improving into the Early Jurassic. This is consistent with d13C isotope values, paleo-[CO2] levels, and global mean temperatures, suggesting that photosynthetic performance was influenced by the prevailing environmental conditions during the TJB event. Dissipation of absorbed energy as heat would also have risen towards the boundary as plant stress increased, in order to protect the photosynthetic physiology. The increase in dissipation of energy as heat, associated with a reduction in convective heat loss due to reduced transpiration rates, would have exacerbated plant thermal stress at the TJB, thus contributing to sudden biodiversity loss and ecological change.
The stomatal index (a measure of stomatal density) of an extinct Australian Early Jurassic arauca... more The stomatal index (a measure of stomatal density) of an extinct Australian Early Jurassic araucariacean conifer species, Allocladus helgei Jansson, is used to reconstruct the atmospheric carbon dioxide concentration (pCO2) in the Early Jurassic. The fossil leaves are preserved in a single bed, palynologically dated to late Pliensbachian (~185–183 Mya). Atmospheric pCO2 is estimated from the ratios between the stomatal index of A. helgei and the stomatal indices of three modern analogs (nearest living equivalent plants). CO2 concentration in the range of ~750–975 ppm was calibrated from the fossil material, with a best-estimated mean of ~900 ppm. The new average pCO2 determined for the late Pliensbachian is thus similar to, although ~ 10% lower, than previously inferred minimum concentrations of ~1000, based on data from the Northern Hemisphere, but may help con- strain pCO2 during this period. Our results are the first pCO2 estimates produced using Jurassic leaves from the Southern Hemisphere and show that i) paleo-atmospheric pCO2 estimates are consistent at a global scale, though more investigations of Southern Hemisphere material are required, and ii) the stomatal proxy method can now be used without the context of relative change in pCO2 when applying the correct methodology.
A new stomatal proxy-based record of CO2 concentrations ([CO2]), based on Betula nana (dwarf birc... more A new stomatal proxy-based record of CO2 concentrations ([CO2]), based on Betula nana (dwarf birch) leaves from the Hässeldala Port sedimentary sequence in south-eastern Sweden, is presented. The record is of high chronological resolution and spans most of Greenland Interstadial 1 (GI-1a to 1c, Allerød pollen zone), Greenland Stadial 1 (GS-1, Younger Dryas pollen zone) and the very beginning of the Holocene (Preboreal pollen zone). The record clearly demonstrates that i) [CO2] were significantly higher than usually reported for the Last Termination and ii) the overall pattern of CO2 evolution through the studied time period is fairly dynamic, with significant abrupt fluctuations in [CO2] when the climate moved from interstadial to stadial state and vice versa. A new loss-on-ignition chemical record (used here as a proxy for temperature) lends independent support to the Hässeldala Port [CO2] record. The large-amplitude fluctuations around the climate change transitions may indicate unstable climates and that “tipping-point” situations were involved in Last Termination climate evolution. The scenario presented here is in contrast to [CO2] records reconstructed from air bubbles trapped in ice, which indicate lower concentrations and a gradual, linear increase of [CO2] through time. The prevalent explanation for the main climate forcer during the Last Termination being ocean circulation patterns needs to re-examined, and a larger role for atmospheric [CO2] considered.► A stomatal proxy-based CO2 record from the Last Termination is presented. ► The stomata based CO2 record is much more dynamic than ice core-based CO2 records. ► CO2 first increases abruptly before decreasing at cooling transitions and vice versa.
The physiological effects of high CO2 concentrations, i.e., [CO2], on plant stomatal responses
m... more The physiological effects of high CO2 concentrations, i.e., [CO2], on plant stomatal responses
may be of major importance in understanding the consequences of climate change, by caus- ing increases in runoff through suppression of plant transpiration. Radiative forcing by high [CO2] has been the main consideration in models of global change to the exclusion of plant physiological forcing, but this potentially underestimates the effects on the hydrological cycle, and the consequences for ecosystems. We tested the physiological responses of fossil plants from the Triassic–Jurassic boundary transition (Tr–J) succession of East Greenland. This interval marks a major high CO2-driven environmental upheaval, with faunal mass extinc- tions and significant floral turnover. Our results show that both stomatal size (expressed in fossil material as SL, the length of the stomatal complex opening) and stomatal density (SD, the number of stomata per mm2) decreased significantly during the Tr–J. We estimate, using a leaf gas-exchange model, that the decreases in SD and SL resulted in a 50%–60% drop in stomatal and canopy transpiration at the Tr–J. We also present new field evidence indicating simultaneous increases in runoff and erosion rates. We propose that the consequences of sto- matal responses to elevated [CO2] may lead to locally increased runoff and erosion, and may link terrestrial and marine biodiversity loss via the hydrological cycle.
Although progress has been made in recent years in reconstructing the environmental conditions at... more Although progress has been made in recent years in reconstructing the environmental conditions at the Triassic/Jurassic Boundary (TJB), published records of atmospheric CO2 concentrations have been of low resolution and/or based on multi-taxon estimates. This is addressed here by reconstructing CO2 concentrations across the TJB using stomatal frequencies of four phylogenetically and ecologically distinct plant groups from two depositionally, geographically and taphonomically separate boundary sections in East Greenland and Northern Ireland, with stomatal proxy methods and regression analysis. The resulting CO2 records then are compared with an additional existing TJB record from a geological section in Sweden. The final results indicate that pre-TJB (Rhaetian), the CO2 concentration was approximately 1000 ppm, that it started to rise steeply pre-boundary and had doubled to around 2000–2500 ppm at the TJB. The CO2 concentration then remained elevated for some time post-boundary, before returning to pre-TJB levels in the Hettangian. These results are in very good accordance with published C-isotope, fire and leaf dissection records, and clearly indicate steeply rising and lingering CO2 concentration at the TJB.
Bennettites are an abundant and frequently well- preserved component of many Mesozoic fossil flor... more Bennettites are an abundant and frequently well- preserved component of many Mesozoic fossil floras, often playing an important ecological role in flood plain vegetation communities. During a recent study focusing on stomatal indices of Triassic–Jurassic fossil plants, it became evident that the leaf fragments of two bennettite genera Anomoza- mites Schimper (1870) emend. Harris (1969) and Pterophyl- lum Brongniart (1825) display a significant overlap of leaf shape as well as cuticular characters. Owing to the preference of recognition of single taxa (ideally species) for the stomatal method, we use a database of 70 leaf fragments of Anomoza- mites and Pterophyllum compressions from five isotapho- nomic Late Triassic sedimentary beds of Astartekløft in East Greenland to test whether leaf and cuticle fragments of the two genera can be separated using a range of quantitative and qualitative morphological and statistical analyses. None of the observed characters – including stomatal frequencies – could be applied to separate the fragments of the two genera into well-defined groups. Our results therefore indicate that fragmented material and dispersed cuticles cannot be utilised to distinguish between Anomozamites or Pterophyllum at the genus level, but that instead these cuticle fragments may be used interchangeably as stomatal proxies. Classification of fossil leaves into either of these genera is thus only possible given adequate preservation of macro-morphology and is not possible based solely on cuticle morphology. We suggest that this large inter- and intra-generic morphological variation in both leaf and cuticle traits within Anomozamites and Ptero- phyllum may be related to the bennettites’ role as understory plants, experiencing a range of micro-environmental condi- tions, perhaps depending mainly on sun exposure. Based on the results obtained in this study, we conclude that Anomozamites and Pterophyllum cuticle fragments can be employed interchangeably in palaeo [CO2] reconstructions based on the stomatal method, thus potentially annexing a plethora of bennettitalean fossil plant material as CO2 proxies, including dispersed cuticles.
Abstract
The Triassic/Jurassic boundary (TJB, ~200 million years ago) is associated with one of t... more Abstract The Triassic/Jurassic boundary (TJB, ~200 million years ago) is associated with one of the five major mass extinction events of the Phanerozoic. Considerable effort has in recent years been spent on reconstructing the environmental conditions at the TJB and a consensus is forming that the upheaval event was caused by global climate change, driven by CO2 emmissions from large scale flood basalt volcanism in connection with the break-up of the supercontinent Pangaea. However, published records of atmospheric CO2 concentrations [CO2] have been of low resolution and/or based on multi-taxon estimates. In this thesis, the [CO2] across the TJB is reconstructed using stomatal proxy, transfer function and regression analysis methods on multiple single-taxon proxy records. The stomatal frequencies of four phylogenetically and ecologically distinct plant groups from two depositionally and taphonomically seperate boundary sections in East Greenland and Northern Ireland are recorded. The fossil plant goups include orders Ginkgoales, Bennettitales, Coniferales and possibly Pteridospermales and stomatal frequency is tracked at genus or multi-genus level. Bennettitaleans, here genera Anomozamites and Pterophyllum, were for the first time found to respond to [CO2] fluctuations and also to be interchangeable due to overlapping cuticular traits. This discovery may lead to a great increase in potential palaeo-[CO2] reconstructions using the stomatal method, given the abundance of bennettitaleans in Mesozoic strata worldwide. Nearest living equivalent (NLE) analyses were undertaken and two fern species established as best NLE for bennettitaleans, whereas three species of cycads and two tree ferns, theoretically better NLE material for bennettitaleans, were found to be unresponsive to CO2. The compiled stomatal proxy CO2 records indicate that pre-TJB (Rhaetian, Triassic), the [CO2] was ~1000 ppm, that it started to rise steeply pre-boundary and had doubled to ~2000 - 2500 ppm at the TJB. The [CO2] then remained elevated for some time post-boundary, before returning to pre-TJB levels in the Hettangian (Jurassic). These results are in good accordance with existing TJB records from two geological sections in Sweden and Germany and with published C-isotope, fire and leaf dissection records, and clearly indicate steeply rising and lingering CO2 concentration at the TJB. Stomatal length, as well as stomatal frequency, was found to have decreased at the TJB, perhaps resulting in an up to 50% drop in stomatal conductance and/or canopy transpiration as calibrated using a new stomatal model. Decline in transpiration in turn is predicted to increase runoff, which may lead to increased flooding, erosion and leaching of soil nutrients on land, and consequently to eutrophication and anoxia in the oceans. Increase in runoff at the TJB may thus have caused many of the adverse environmental effects and loss of biodiversity that are evident in the geological record. Stomatal responses to CO2 may be an underestimated but potentially important factor in shaping patterns of biodiversity in past and future climate change scenarios.
Tectonic displacement and small-scale tsunamis apparently affected deposition of the Kolymbia lim... more Tectonic displacement and small-scale tsunamis apparently affected deposition of the Kolymbia limestone, Cape Vagia, Rhodes, Eastern Mediterranean. Coarse beds interrupt the sequential build-up of this Pliocene–Pleistocene bryomol limestone. Celleporid bryozoans, bivalves, and brachiopods dominate these beds. The palaeoecology of the thicket-forming Celleporaria palmata is re-evaluated and subsequently revised. The limestone comprises two parasequences in a transgressive systems tract, and deposition occurred at palaeodepths between 30 and 120 m. At intervals, tectonic movements lowered relative sea level and sent slumps of shallow-water fauna downslope. The depositional history was validated using independent sets of data: sediment structure and grain size, palaeobathymetry using bryozoan growth forms and occurrences of modern representatives of bryozoans and other taxa, basin configuration, and regional tectonics. Concordance of these lines of evidence provides a means of evaluating confidence in palaeoenvironmental inferences.
Reliable reconstructions of atmospheric carbon dioxide concentrations (pCO2) are required at high... more Reliable reconstructions of atmospheric carbon dioxide concentrations (pCO2) are required at higher resolution than currently available to help resolve the relationship between mass extinctions and changes in palaeo-pCO2 levels. Such reconstructions are needed: 1, at a high temporal resolution for constraining the pre- and post-extinction atmospheres; and 2, at a sufficient spatial resolution to constrain potential inter-hemispheric differences. Here we estimate pCO2 based on fossil Lauraceae leaf cuticle specimens derived from three localities with strata spanning the latest Cretaceous to the mid-Paleocene, including a new Cretaceous–Paleogene boundary (K–Pg) locality, in New Zealand. We use two independent methods of stomatal density-based pCO2 reconstructions; a transfer function calibrated using herbarium material and the stomatal ratio method, producing three calibration sets. Our results based on the mean values of each of the three calibration methods indicate pCO2 ranging between ca. 460 and 650 ppm during the latest Cretaceous, falling precipitously to average values between ca. 360 and 430 ppm across the K–Pg boundary, and further to ca. 305–320 ppm in the mid-Paleocene. A ‘spike’ of extremely high pCO2at the K–Pg could not be confirmed, but our results are, nonetheless, consistent with previously published pCO2 records from the Northern Hemisphere, and show that stomatal density worldwide was responding to significant changes in pCO2 across the K–Pg.
ABSTRACT The fossil diatom record from the Hässeldala Port palaeolake, southeastern Sweden, offer... more ABSTRACT The fossil diatom record from the Hässeldala Port palaeolake, southeastern Sweden, offers an excellent opportunity to investigate how past climatic shifts influenced catchment conditions and early lake development. The record, dating to between 13 900 and 11 200 cal. a BP, covers a climatically dynamic period, starting with deglaciation followed by oscillations between warmer and colder climate states. The stratigraphical changes in the fossil diatom assemblages show a trend of less open-water taxa and a successively more complex periphytic community as the lake shallows and the aquatic habitat structure develops. A diatom-based reconstruction of lake water pH indicates a natural acidification trend early in the record from 13 900 to 12 500 cal. a BP. From 12 500 cal. a BP, coincident with the start of climate cooling, to 11 300 cal. a BP this trend is disrupted and lake waters become more alkaline. A cooler and drier climate most likely resulted in reduced soil organic matter build-up as well as more frozen ground that impeded hydrological flow and decreased the input of dissolved organic matter and organic acids into the lake system. This study demonstrates the importance of the hydrological system as a link between terrestrial and aquatic ecosystems during early lake ontogeny.
Reliable reconstructions of atmospheric carbon dioxide concentrations (pCO2) are required at high... more Reliable reconstructions of atmospheric carbon dioxide concentrations (pCO2) are required at higher resolution than currently available to help resolve the relationship between mass extinctions and changes in palaeo-pCO2 levels. Such reconstructions are needed: 1, at a high temporal resolution for constraining the pre- and post-extinction atmospheres; and 2, at a sufficient spatial resolution to constrain potential inter-hemispheric differences. Here we estimate pCO2 based on fossil Lauraceae leaf cuticle specimens derived from three localities with strata spanning the latest Cretaceous to the mid-Paleocene, including a new Cretaceous–Paleogene boundary (K–Pg) locality, in New Zealand. We use two independent methods of stomatal density-based pCO2 reconstructions; a transfer function calibrated using herbarium material and the stomatal ratio method, producing three calibration sets. Our results based on the mean values of each of the three calibration methods indicate pCO2 ranging between ca. 460 and 650 ppm during the latest Cretaceous, falling precipitously to average values between ca. 360 and 430 ppm across the K–Pg boundary, and further to ca. 305–320 ppm in the mid-Paleocene. A ‘spike’ of extremely high pCO2 at the K–Pg could not be confirmed, but our results are, nonetheless, consistent with previously published pCO2 records from the Northern Hemisphere, and show that stomatal density worldwide was responding to significant changes in pCO2 across the K–Pg.
The flora of the Cenomanian–Turonian (ca. 96–90Ma) Tupuangi Formation, Chatham Islands, New Zeala... more The flora of the Cenomanian–Turonian (ca. 96–90Ma) Tupuangi Formation, Chatham Islands, New Zealand, was inhabiting a region well within the south polar circle (~70–80° S) during the early Late Cretaceous, an interval characterised by extreme global greenhouse conditions. The Tupuangi flora offers a unique perspective into an ecological and environmental setting which has no extant analogue, whilst providing proxies of polar palaeoclimatic conditions during a phase of extreme global warming. Ginkgoites waarrensis Douglas, 1965 (emended herein), a species known previously from a single occurrence in Australia, is an abundant element of the Tupuangi flora. Forty-five leaf samples from three localities are reported, and a systematic treatment of this species revealed a wide morphological range. In contrast to the exclusively riparian niche of more recent members of Ginkgoales, associated sedimentological and palaeoecological data suggest that this species had an ecological preference for regularly disturbed, coastal deltaic settings. Herein,we review the geographic and stratigraphic distributions of Cretaceous Gondwanan ginkgoalean leaf taxa. An increasing diversity of this group from the Early Cretaceous to the early Late Cretaceous supports a broader trend of floral provincialisation throughout this interval, most likely driven by concurrent global transgression and active tectonic extension across southern Gondwana. Carbon dioxide has been inferred as a primary proximate cause of the mid-Cretaceous global greenhouse. The leaf cuticles of Ginkgoites waarrensis were utilised to approximate atmospheric carbon dioxide (pCO2) during the Cenomanian. Stomatal index (SI) data were collected from ten specimens, and the stomatal ratio method yielded a semi-quantitative pCO2 estimate of 1150–1350 ppmv, which is consistent with modelled and proxy estimates of the Cenomanian. The present study explores the inherent limitations of the transfer function method for estimating CO2 when applied to taxa with very low SI values, such as G. waarrensis. In addition to pCO2, temperature and irradiance are identified as environmental variables which may have systematically promoted the low SI of G. waarrensis, but their combined influence is likely mitigated by the relatively high temperature of this region during the mid-Cretaceous and the high summer insolation at polar latitudes.
During a study of macroflora from the Astartekløft locality in Jameson Land, East Greenland, endo... more During a study of macroflora from the Astartekløft locality in Jameson Land, East Greenland, endophytic insect ovipositions (egg traces) belonging to ichnogenus Paleoovoidus were recorded for the first time in ginkgoalean (Ginkgoites, Sphenobaiera, and Baiera) fossil leaves across the Triassic–Jurassic (Tr–J) transition (ca. 200 Ma). The ovipositions may have been produced by insects in the order Odonata (dragonflies and damselflies) and are relatively more abundant before than after the Tr–J transition, possibly reflecting changes in plant-insect association. Fossil clitellate annelid (leech) cocoons were also discovered in a macerated sample from a single bed within the Tr–J transition. The cocoons belong to two species: Dictyothylakos pesslerae and Pilothylakos pilosus, extending the range of the latter genus from the Early Cretaceous to the Early Jurassic. This new evidence suggests that the ecosystem and food webs were profoundly affected by the environmental degradation surrounding the end-Triassic event (ETE), which was marked by faunal mass extinctions and floral turnover. Invertebrate ichno- and body fossils may add significantly to paleoenvironmental information provided by plant fossil assemblages, and therefore a protocol for recording evidence of invertebrate activity in paleobotanical research is suggested, including analyzing a standardized number of specimens for fossil traces and bulk maceration for discovery of invertebrate body fossils. More well-designed studies on Mesozoic plant-invertebrate associations are needed and will provide deeper knowledge about the structure and evolution of complex ecosystems.
We thank Köhler et al. for their interest in our research and welcome discussions about the metho... more We thank Köhler et al. for their interest in our research and welcome discussions about the methods and possible interpreta- tions involved in Lateglacial climate change research. We respond to each main criticism below, but at the onset briefly point out sig- nificant misunderstandings in the comment. Firstly, Köhler et al. concentrate their criticism on the statistical robustness of one re- cord, while our interpretation of a large fluctuation in CO2 concen- tration (pCO2) at the onset of the Younger Dryas (YD) is based on four distinct and geographically separate stomatal records and a parallel radiocarbon fluctuation. Secondly, the authors are confusing an artificial ocean flushing experiment, which we repeat- edly emphasised was a conceptual study of how much pCO2 the ocean can release if it were severely flushed, with an actual simu- lation of the real event. They pay no attention to the hypothesis of a more realistic ocean flushing scenario that is provided.
Just before the onset of the Younger Dryas (YD) cold event, several stomatal proxy-based pCO2 rec... more Just before the onset of the Younger Dryas (YD) cold event, several stomatal proxy-based pCO2 records have shown a sharp increase in atmospheric CO2 concentration (pCO2) of between ca 50 and 100 ppm, followed by a rapid decrease of similar or even larger magnitude. Here we compare one of these records, a high-resolution pCO2 record from southern Sweden, with the IntCal13 record of radiocarbon (D14C). The two records show broadly synchronous fluctuations at the YD onset. Specifically, the IntCal13 record documents decreasing D14C just before the YD onset when pCO2 peaks, consistent with a source of “old” CO2 from the deep ocean. We propose that this fluctuation occurred due to a major ocean flushing event. The cause of the flushing event remains speculative but could be related to the hypothesis of the glacial ocean as a thermobaric capacitor. We confirm that the earth system can produce such large multi- decadal timescale fluctuations in pCO2 through simulating an artificial ocean flushing event with the GENIE Earth System Model. We suggest that sharp transitions of pCO2 may have remained undetected so far in ice cores due to inter-firn gas exchange and time-averaging. The stomatal proxy record is a powerful complement to the ice core records for the study of rapid climate change.
The Triassic–Jurassic boundary (TJB) coincides with major disruption to the carbon cycle and glob... more The Triassic–Jurassic boundary (TJB) coincides with major disruption to the carbon cycle and global warming as the Central Atlantic Magmatic Province developed. This resulted in both marine and terrestrial extinctions, with terrestrial plants thought to experience thermal stress as global temperatures and atmospheric CO2 levels rose. As plant compression fossils typically only preserve external morphological features, it has not been possible to reconstruct plant paleophysiology in order to elucidate the mechanisms underlying plant stress and extinction. Here we present a new approach allowing us to infer the photosynthetic performance and stress physiology of fossil plants, applied to fossil Ginkgoales across the TJB. We use correlations between the adaxial epidermal cell density of extant Ginkgo biloba and photosynthetic and protective stress physiology to infer the paleophysiological condition of Late Triassic–Early Jurassic-aged plants from Astartekløft, East Greenland. The density of fossil leaf adaxial epidermal cells indicates that photosynthetic performance of Ginkgoales became increasingly impaired towards the latter stages of the Triassic, before improving into the Early Jurassic. This is consistent with d13C isotope values, paleo-[CO2] levels, and global mean temperatures, suggesting that photosynthetic performance was influenced by the prevailing environmental conditions during the TJB event. Dissipation of absorbed energy as heat would also have risen towards the boundary as plant stress increased, in order to protect the photosynthetic physiology. The increase in dissipation of energy as heat, associated with a reduction in convective heat loss due to reduced transpiration rates, would have exacerbated plant thermal stress at the TJB, thus contributing to sudden biodiversity loss and ecological change.
The stomatal index (a measure of stomatal density) of an extinct Australian Early Jurassic arauca... more The stomatal index (a measure of stomatal density) of an extinct Australian Early Jurassic araucariacean conifer species, Allocladus helgei Jansson, is used to reconstruct the atmospheric carbon dioxide concentration (pCO2) in the Early Jurassic. The fossil leaves are preserved in a single bed, palynologically dated to late Pliensbachian (~185–183 Mya). Atmospheric pCO2 is estimated from the ratios between the stomatal index of A. helgei and the stomatal indices of three modern analogs (nearest living equivalent plants). CO2 concentration in the range of ~750–975 ppm was calibrated from the fossil material, with a best-estimated mean of ~900 ppm. The new average pCO2 determined for the late Pliensbachian is thus similar to, although ~ 10% lower, than previously inferred minimum concentrations of ~1000, based on data from the Northern Hemisphere, but may help con- strain pCO2 during this period. Our results are the first pCO2 estimates produced using Jurassic leaves from the Southern Hemisphere and show that i) paleo-atmospheric pCO2 estimates are consistent at a global scale, though more investigations of Southern Hemisphere material are required, and ii) the stomatal proxy method can now be used without the context of relative change in pCO2 when applying the correct methodology.
A new stomatal proxy-based record of CO2 concentrations ([CO2]), based on Betula nana (dwarf birc... more A new stomatal proxy-based record of CO2 concentrations ([CO2]), based on Betula nana (dwarf birch) leaves from the Hässeldala Port sedimentary sequence in south-eastern Sweden, is presented. The record is of high chronological resolution and spans most of Greenland Interstadial 1 (GI-1a to 1c, Allerød pollen zone), Greenland Stadial 1 (GS-1, Younger Dryas pollen zone) and the very beginning of the Holocene (Preboreal pollen zone). The record clearly demonstrates that i) [CO2] were significantly higher than usually reported for the Last Termination and ii) the overall pattern of CO2 evolution through the studied time period is fairly dynamic, with significant abrupt fluctuations in [CO2] when the climate moved from interstadial to stadial state and vice versa. A new loss-on-ignition chemical record (used here as a proxy for temperature) lends independent support to the Hässeldala Port [CO2] record. The large-amplitude fluctuations around the climate change transitions may indicate unstable climates and that “tipping-point” situations were involved in Last Termination climate evolution. The scenario presented here is in contrast to [CO2] records reconstructed from air bubbles trapped in ice, which indicate lower concentrations and a gradual, linear increase of [CO2] through time. The prevalent explanation for the main climate forcer during the Last Termination being ocean circulation patterns needs to re-examined, and a larger role for atmospheric [CO2] considered.► A stomatal proxy-based CO2 record from the Last Termination is presented. ► The stomata based CO2 record is much more dynamic than ice core-based CO2 records. ► CO2 first increases abruptly before decreasing at cooling transitions and vice versa.
The physiological effects of high CO2 concentrations, i.e., [CO2], on plant stomatal responses
m... more The physiological effects of high CO2 concentrations, i.e., [CO2], on plant stomatal responses
may be of major importance in understanding the consequences of climate change, by caus- ing increases in runoff through suppression of plant transpiration. Radiative forcing by high [CO2] has been the main consideration in models of global change to the exclusion of plant physiological forcing, but this potentially underestimates the effects on the hydrological cycle, and the consequences for ecosystems. We tested the physiological responses of fossil plants from the Triassic–Jurassic boundary transition (Tr–J) succession of East Greenland. This interval marks a major high CO2-driven environmental upheaval, with faunal mass extinc- tions and significant floral turnover. Our results show that both stomatal size (expressed in fossil material as SL, the length of the stomatal complex opening) and stomatal density (SD, the number of stomata per mm2) decreased significantly during the Tr–J. We estimate, using a leaf gas-exchange model, that the decreases in SD and SL resulted in a 50%–60% drop in stomatal and canopy transpiration at the Tr–J. We also present new field evidence indicating simultaneous increases in runoff and erosion rates. We propose that the consequences of sto- matal responses to elevated [CO2] may lead to locally increased runoff and erosion, and may link terrestrial and marine biodiversity loss via the hydrological cycle.
Although progress has been made in recent years in reconstructing the environmental conditions at... more Although progress has been made in recent years in reconstructing the environmental conditions at the Triassic/Jurassic Boundary (TJB), published records of atmospheric CO2 concentrations have been of low resolution and/or based on multi-taxon estimates. This is addressed here by reconstructing CO2 concentrations across the TJB using stomatal frequencies of four phylogenetically and ecologically distinct plant groups from two depositionally, geographically and taphonomically separate boundary sections in East Greenland and Northern Ireland, with stomatal proxy methods and regression analysis. The resulting CO2 records then are compared with an additional existing TJB record from a geological section in Sweden. The final results indicate that pre-TJB (Rhaetian), the CO2 concentration was approximately 1000 ppm, that it started to rise steeply pre-boundary and had doubled to around 2000–2500 ppm at the TJB. The CO2 concentration then remained elevated for some time post-boundary, before returning to pre-TJB levels in the Hettangian. These results are in very good accordance with published C-isotope, fire and leaf dissection records, and clearly indicate steeply rising and lingering CO2 concentration at the TJB.
Bennettites are an abundant and frequently well- preserved component of many Mesozoic fossil flor... more Bennettites are an abundant and frequently well- preserved component of many Mesozoic fossil floras, often playing an important ecological role in flood plain vegetation communities. During a recent study focusing on stomatal indices of Triassic–Jurassic fossil plants, it became evident that the leaf fragments of two bennettite genera Anomoza- mites Schimper (1870) emend. Harris (1969) and Pterophyl- lum Brongniart (1825) display a significant overlap of leaf shape as well as cuticular characters. Owing to the preference of recognition of single taxa (ideally species) for the stomatal method, we use a database of 70 leaf fragments of Anomoza- mites and Pterophyllum compressions from five isotapho- nomic Late Triassic sedimentary beds of Astartekløft in East Greenland to test whether leaf and cuticle fragments of the two genera can be separated using a range of quantitative and qualitative morphological and statistical analyses. None of the observed characters – including stomatal frequencies – could be applied to separate the fragments of the two genera into well-defined groups. Our results therefore indicate that fragmented material and dispersed cuticles cannot be utilised to distinguish between Anomozamites or Pterophyllum at the genus level, but that instead these cuticle fragments may be used interchangeably as stomatal proxies. Classification of fossil leaves into either of these genera is thus only possible given adequate preservation of macro-morphology and is not possible based solely on cuticle morphology. We suggest that this large inter- and intra-generic morphological variation in both leaf and cuticle traits within Anomozamites and Ptero- phyllum may be related to the bennettites’ role as understory plants, experiencing a range of micro-environmental condi- tions, perhaps depending mainly on sun exposure. Based on the results obtained in this study, we conclude that Anomozamites and Pterophyllum cuticle fragments can be employed interchangeably in palaeo [CO2] reconstructions based on the stomatal method, thus potentially annexing a plethora of bennettitalean fossil plant material as CO2 proxies, including dispersed cuticles.
Abstract
The Triassic/Jurassic boundary (TJB, ~200 million years ago) is associated with one of t... more Abstract The Triassic/Jurassic boundary (TJB, ~200 million years ago) is associated with one of the five major mass extinction events of the Phanerozoic. Considerable effort has in recent years been spent on reconstructing the environmental conditions at the TJB and a consensus is forming that the upheaval event was caused by global climate change, driven by CO2 emmissions from large scale flood basalt volcanism in connection with the break-up of the supercontinent Pangaea. However, published records of atmospheric CO2 concentrations [CO2] have been of low resolution and/or based on multi-taxon estimates. In this thesis, the [CO2] across the TJB is reconstructed using stomatal proxy, transfer function and regression analysis methods on multiple single-taxon proxy records. The stomatal frequencies of four phylogenetically and ecologically distinct plant groups from two depositionally and taphonomically seperate boundary sections in East Greenland and Northern Ireland are recorded. The fossil plant goups include orders Ginkgoales, Bennettitales, Coniferales and possibly Pteridospermales and stomatal frequency is tracked at genus or multi-genus level. Bennettitaleans, here genera Anomozamites and Pterophyllum, were for the first time found to respond to [CO2] fluctuations and also to be interchangeable due to overlapping cuticular traits. This discovery may lead to a great increase in potential palaeo-[CO2] reconstructions using the stomatal method, given the abundance of bennettitaleans in Mesozoic strata worldwide. Nearest living equivalent (NLE) analyses were undertaken and two fern species established as best NLE for bennettitaleans, whereas three species of cycads and two tree ferns, theoretically better NLE material for bennettitaleans, were found to be unresponsive to CO2. The compiled stomatal proxy CO2 records indicate that pre-TJB (Rhaetian, Triassic), the [CO2] was ~1000 ppm, that it started to rise steeply pre-boundary and had doubled to ~2000 - 2500 ppm at the TJB. The [CO2] then remained elevated for some time post-boundary, before returning to pre-TJB levels in the Hettangian (Jurassic). These results are in good accordance with existing TJB records from two geological sections in Sweden and Germany and with published C-isotope, fire and leaf dissection records, and clearly indicate steeply rising and lingering CO2 concentration at the TJB. Stomatal length, as well as stomatal frequency, was found to have decreased at the TJB, perhaps resulting in an up to 50% drop in stomatal conductance and/or canopy transpiration as calibrated using a new stomatal model. Decline in transpiration in turn is predicted to increase runoff, which may lead to increased flooding, erosion and leaching of soil nutrients on land, and consequently to eutrophication and anoxia in the oceans. Increase in runoff at the TJB may thus have caused many of the adverse environmental effects and loss of biodiversity that are evident in the geological record. Stomatal responses to CO2 may be an underestimated but potentially important factor in shaping patterns of biodiversity in past and future climate change scenarios.
Tectonic displacement and small-scale tsunamis apparently affected deposition of the Kolymbia lim... more Tectonic displacement and small-scale tsunamis apparently affected deposition of the Kolymbia limestone, Cape Vagia, Rhodes, Eastern Mediterranean. Coarse beds interrupt the sequential build-up of this Pliocene–Pleistocene bryomol limestone. Celleporid bryozoans, bivalves, and brachiopods dominate these beds. The palaeoecology of the thicket-forming Celleporaria palmata is re-evaluated and subsequently revised. The limestone comprises two parasequences in a transgressive systems tract, and deposition occurred at palaeodepths between 30 and 120 m. At intervals, tectonic movements lowered relative sea level and sent slumps of shallow-water fauna downslope. The depositional history was validated using independent sets of data: sediment structure and grain size, palaeobathymetry using bryozoan growth forms and occurrences of modern representatives of bryozoans and other taxa, basin configuration, and regional tectonics. Concordance of these lines of evidence provides a means of evaluating confidence in palaeoenvironmental inferences.
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Papers by Margret Steinthorsdottir
We use two independent methods of stomatal density-based pCO2 reconstructions; a transfer function calibrated using herbarium material and the stomatal ratio method, producing three calibration sets. Our results based on the mean values of each of the three calibration methods indicate pCO2 ranging between ca. 460 and 650 ppm during the latest Cretaceous, falling precipitously to average values between ca. 360 and 430 ppm across the K–Pg boundary, and further to ca. 305–320 ppm in the mid-Paleocene. A ‘spike’ of extremely high pCO2at the K–Pg could not be confirmed, but our results are, nonetheless, consistent with previously published pCO2 records from the Northern Hemisphere, and show that stomatal density worldwide was responding to significant changes in pCO2 across the K–Pg.
may be of major importance in understanding the consequences of climate change, by caus- ing increases in runoff through suppression of plant transpiration. Radiative forcing by high [CO2] has been the main consideration in models of global change to the exclusion of plant physiological forcing, but this potentially underestimates the effects on the hydrological cycle, and the consequences for ecosystems. We tested the physiological responses of fossil plants from the Triassic–Jurassic boundary transition (Tr–J) succession of East Greenland. This interval marks a major high CO2-driven environmental upheaval, with faunal mass extinc- tions and significant floral turnover. Our results show that both stomatal size (expressed in fossil material as SL, the length of the stomatal complex opening) and stomatal density (SD, the number of stomata per mm2) decreased significantly during the Tr–J. We estimate, using a leaf gas-exchange model, that the decreases in SD and SL resulted in a 50%–60% drop in stomatal and canopy transpiration at the Tr–J. We also present new field evidence indicating simultaneous increases in runoff and erosion rates. We propose that the consequences of sto- matal responses to elevated [CO2] may lead to locally increased runoff and erosion, and may link terrestrial and marine biodiversity loss via the hydrological cycle.
The Triassic/Jurassic boundary (TJB, ~200 million years ago) is associated with one of the five major mass extinction events of the Phanerozoic. Considerable effort has in recent years been spent on reconstructing the environmental conditions at the TJB and a consensus is forming that the upheaval event was caused by global climate change, driven by CO2 emmissions from large scale flood basalt volcanism in connection with the break-up of the supercontinent Pangaea. However, published records of atmospheric CO2 concentrations [CO2] have been of low resolution and/or based on multi-taxon estimates. In this thesis, the [CO2] across the TJB is reconstructed using stomatal proxy, transfer function and regression analysis methods on multiple single-taxon proxy records. The stomatal frequencies of four phylogenetically and ecologically distinct plant groups from two depositionally and taphonomically seperate boundary sections in East Greenland and Northern Ireland are recorded. The fossil plant goups include orders Ginkgoales, Bennettitales, Coniferales and possibly Pteridospermales and stomatal frequency is tracked at genus or multi-genus level. Bennettitaleans, here genera Anomozamites and Pterophyllum, were for the first time found to respond to [CO2] fluctuations and also to be interchangeable due to overlapping cuticular traits. This discovery may lead to a great increase in potential palaeo-[CO2] reconstructions using the stomatal method, given the abundance of bennettitaleans in Mesozoic strata worldwide. Nearest living equivalent (NLE) analyses were undertaken and two fern species established as best NLE for bennettitaleans, whereas three species of cycads and two tree ferns, theoretically better NLE material for bennettitaleans, were found to be unresponsive to CO2.
The compiled stomatal proxy CO2 records indicate that pre-TJB (Rhaetian, Triassic), the [CO2] was ~1000 ppm, that it started to rise steeply pre-boundary and had doubled to ~2000 - 2500 ppm at the TJB. The [CO2] then remained elevated for some time post-boundary, before returning to pre-TJB levels in the Hettangian (Jurassic). These results are in good accordance with existing TJB records from two geological sections in Sweden and Germany and with published C-isotope, fire and leaf dissection records, and clearly indicate steeply rising and lingering CO2 concentration at the TJB.
Stomatal length, as well as stomatal frequency, was found to have decreased at the TJB, perhaps resulting in an up to 50% drop in stomatal conductance and/or canopy transpiration as calibrated using a new stomatal model. Decline in transpiration in turn is predicted to increase runoff, which may lead to increased flooding, erosion and leaching of soil nutrients on land, and consequently to eutrophication and anoxia in the oceans. Increase in runoff at the TJB may thus have caused many of the adverse environmental effects and loss of biodiversity that are evident in the geological record. Stomatal responses to CO2 may be an underestimated but potentially important factor in shaping patterns of biodiversity in past and future climate change scenarios.
We use two independent methods of stomatal density-based pCO2 reconstructions; a transfer function calibrated using herbarium material and the stomatal ratio method, producing three calibration sets. Our results based on the mean values of each of the three calibration methods indicate pCO2 ranging between ca. 460 and 650 ppm during the latest Cretaceous, falling precipitously to average values between ca. 360 and 430 ppm across the K–Pg boundary, and further to ca. 305–320 ppm in the mid-Paleocene. A ‘spike’ of extremely high pCO2at the K–Pg could not be confirmed, but our results are, nonetheless, consistent with previously published pCO2 records from the Northern Hemisphere, and show that stomatal density worldwide was responding to significant changes in pCO2 across the K–Pg.
may be of major importance in understanding the consequences of climate change, by caus- ing increases in runoff through suppression of plant transpiration. Radiative forcing by high [CO2] has been the main consideration in models of global change to the exclusion of plant physiological forcing, but this potentially underestimates the effects on the hydrological cycle, and the consequences for ecosystems. We tested the physiological responses of fossil plants from the Triassic–Jurassic boundary transition (Tr–J) succession of East Greenland. This interval marks a major high CO2-driven environmental upheaval, with faunal mass extinc- tions and significant floral turnover. Our results show that both stomatal size (expressed in fossil material as SL, the length of the stomatal complex opening) and stomatal density (SD, the number of stomata per mm2) decreased significantly during the Tr–J. We estimate, using a leaf gas-exchange model, that the decreases in SD and SL resulted in a 50%–60% drop in stomatal and canopy transpiration at the Tr–J. We also present new field evidence indicating simultaneous increases in runoff and erosion rates. We propose that the consequences of sto- matal responses to elevated [CO2] may lead to locally increased runoff and erosion, and may link terrestrial and marine biodiversity loss via the hydrological cycle.
The Triassic/Jurassic boundary (TJB, ~200 million years ago) is associated with one of the five major mass extinction events of the Phanerozoic. Considerable effort has in recent years been spent on reconstructing the environmental conditions at the TJB and a consensus is forming that the upheaval event was caused by global climate change, driven by CO2 emmissions from large scale flood basalt volcanism in connection with the break-up of the supercontinent Pangaea. However, published records of atmospheric CO2 concentrations [CO2] have been of low resolution and/or based on multi-taxon estimates. In this thesis, the [CO2] across the TJB is reconstructed using stomatal proxy, transfer function and regression analysis methods on multiple single-taxon proxy records. The stomatal frequencies of four phylogenetically and ecologically distinct plant groups from two depositionally and taphonomically seperate boundary sections in East Greenland and Northern Ireland are recorded. The fossil plant goups include orders Ginkgoales, Bennettitales, Coniferales and possibly Pteridospermales and stomatal frequency is tracked at genus or multi-genus level. Bennettitaleans, here genera Anomozamites and Pterophyllum, were for the first time found to respond to [CO2] fluctuations and also to be interchangeable due to overlapping cuticular traits. This discovery may lead to a great increase in potential palaeo-[CO2] reconstructions using the stomatal method, given the abundance of bennettitaleans in Mesozoic strata worldwide. Nearest living equivalent (NLE) analyses were undertaken and two fern species established as best NLE for bennettitaleans, whereas three species of cycads and two tree ferns, theoretically better NLE material for bennettitaleans, were found to be unresponsive to CO2.
The compiled stomatal proxy CO2 records indicate that pre-TJB (Rhaetian, Triassic), the [CO2] was ~1000 ppm, that it started to rise steeply pre-boundary and had doubled to ~2000 - 2500 ppm at the TJB. The [CO2] then remained elevated for some time post-boundary, before returning to pre-TJB levels in the Hettangian (Jurassic). These results are in good accordance with existing TJB records from two geological sections in Sweden and Germany and with published C-isotope, fire and leaf dissection records, and clearly indicate steeply rising and lingering CO2 concentration at the TJB.
Stomatal length, as well as stomatal frequency, was found to have decreased at the TJB, perhaps resulting in an up to 50% drop in stomatal conductance and/or canopy transpiration as calibrated using a new stomatal model. Decline in transpiration in turn is predicted to increase runoff, which may lead to increased flooding, erosion and leaching of soil nutrients on land, and consequently to eutrophication and anoxia in the oceans. Increase in runoff at the TJB may thus have caused many of the adverse environmental effects and loss of biodiversity that are evident in the geological record. Stomatal responses to CO2 may be an underestimated but potentially important factor in shaping patterns of biodiversity in past and future climate change scenarios.