Karen L. Bacon

THE TRIASSIC–JURASSIC BOUNDARY CARBON-ISOTOPE EXCURSIONS EXPRESSED IN TAXONOMICALLY IDENTIFIED LEAF CUTICLES

Abstract A negative stable carbon-isotope excursion (CIE) has been identified at sites across the globe in strata that span the Triassic–Jurassic boundary. Different studies have suggested that this negative CIE could be the result of either a change in vegetation or a massive perturbation in the global carbon cycle at this time. To determine which, 84 hand-picked leaf cuticle fragments from plant macrofossils previously identified to genus level were analyzed for stable carbon-isotope values. The samples were taken from known heights in nine plant beds spanning the Rhaetian–Hettangian (Upper Triassic–Lower Jurassic) at Astartekløft, East Greenland. We have constructed taxon-specific stable carbon-isotope curves for Ginkgoales and Bennettitales and compared these to an existing δ13C curve based on fossil wood from the same section. This study reveals that taxon-specific carbon-isotope curves based on the leaf data from these two seed-plant groups both record the same negative CIE as the fossil wood, despite having different ecological roles and different relative abundances in the section. Correspondence analysis of the macrofossil abundance data, where the plants are considered in their ecological groups, shows that the δ13C values bear no relationship to changes in vegetation. This result further suggests that vegetation change had little role in determining the δ13C values at this time. Considered together, the bulk cuticle and taxon-specific δ13C record indicate that the negative CIE at the Triassic–Jurassic boundary is likely to have been caused by a massive perturbation of the global carbon cycle and not by vegetation change.

Increased Atmospheric SO2 Detected from Changes in Leaf Physiognomy across the Triassic–Jurassic Boundary Interval of East Greenland

The Triassic–Jurassic boundary (Tr–J; ∼201 Ma) is marked by a doubling in the concentration of atmospheric CO2, rising temperatures, and ecosystem instability. This appears to have been driven by a major perturbation in the global carbon cycle due to massive volcanism in the Central Atlantic Magmatic Province. It is hypothesized that this volcanism also likely delivered sulphur dioxide (SO2) to the atmosphere. The role that SO2 may have played in leading to ecosystem instability at the time has not received much attention. To date, little direct evidence has been presented from the fossil record capable of implicating SO2 as a cause of plant extinctions at this time. In order to address this, we performed a physiognomic leaf analysis on well-preserved fossil leaves, including Ginkgoales, bennettites, and conifers from nine plant beds that span the Tr–J boundary at Astartekløft, East Greenland. The physiognomic responses of fossil taxa were compared to the leaf size and shape variations observed in nearest living equivalent taxa exposed to simulated palaeoatmospheric treatments in controlled environment chambers. The modern taxa showed a statistically significant increase in leaf roundness when fumigated with SO2. A similar increase in leaf roundness was also observed in the Tr–J fossil taxa immediately prior to a sudden decrease in their relative abundances at Astartekløft. This research reveals that increases in atmospheric SO2 can likely be traced in the fossil record by analyzing physiognomic changes in fossil leaves. A pattern of relative abundance decline following increased leaf roundness for all six fossil taxa investigated supports the hypothesis that SO2 had a significant role in Tr–J plant extinctions. This finding highlights that the role of SO2 in plant biodiversity declines across other major geological boundaries coinciding with global scale volcanism should be further explored using leaf physiognomy.

Spheroidal carbonaceous particles are a defining stratigraphic marker for the Anthropocene

There has been recent debate over stratigraphic markers used to demarcate the Anthropocene from the Holocene Epoch. However, many of the proposed markers are found only in limited areas of the world or do not reflect human impacts on the environment. Here we show that spheroidal carbonaceous particles (SCPs), a distinct form of black carbon produced from burning fossil fuels in energy production and heavy industry, provide unambiguous stratigraphic markers of the human activities that have rapidly changed planet Earth over the last century. SCPs are found in terrestrial and marine sediments or ice cores in every continent, including remote areas such as the high Arctic and Antarctica. The rapid increase in SCPs mostly occurs in the mid-twentieth century and is contemporaneous with the ‘Great Acceleration’. It therefore reflects the intensification of fossil fuel usage and can be traced across the globe. We integrate global records of SCPs and propose that the global rapid increase in SCPs in sedimentary records can be used to inform a Global Standard Stratigraphic Age for the Anthropocene. A high-resolution SCP sequence from a lake or peatland may provide the much-needed ‘Golden Spike’ (Global Boundary Stratotype Section and Point).

First discovery of Holocene cryptotephra in Amazonia

The use of volcanic ash layers for dating and correlation (tephrochronology) is widely applied in the study of past environmental changes. We describe the first cryptotephra (non-visible volcanic ash horizon) to be identified in the Amazon basin, which is tentatively attributed to a source in the Ecuadorian Eastern Cordillera (0–1°S, 78-79°W), some 500-600 km away from our field site in the Peruvian Amazon. Our discovery 1) indicates that the Amazon basin has been subject to volcanic ash fallout during the recent past; 2) highlights the opportunities for using cryptotephras to date palaeoenvironmental records in the Amazon basin and 3) indicates that cryptotephra layers are preserved in a dynamic Amazonian peatland, suggesting that similar layers are likely to be present in other peat sequences that are important for palaeoenvironmental reconstruction. The discovery of cryptotephra in an Amazonian peatland provides a baseline for further investigation of Amazonian tephrochronology and the potential impacts of volcanism on vegetation.

Palaeo leaf economics reveal a shift in ecosystem function associated with the end-Triassic mass extinction event

Climate change is likely to have altered the ecological functioning of past ecosystems, and is likely to alter functioning in the future; however, the magnitude and direction of such changes are difficult to predict. Here we use a deep-time case study to evaluate the impact of a well-constrained CO2-induced global warming event on the ecological functioning of dominant plant communities. We use leaf mass per area (LMA), a widely used trait in modern plant ecology, to infer the palaeoecological strategy of fossil plant taxa. We show that palaeo-LMA can be inferred from fossil leaf cuticles based on a tight relationship between LMA and cuticle thickness observed among extant gymnosperms. Application of this new palaeo-LMA proxy to fossil gymnosperms from East Greenland reveals significant shifts in the dominant ecological strategies of vegetation found across the Triassic–Jurassic transition. Late Triassic forests, dominated by low-LMA taxa with inferred high transpiration rates and short leaf lifespans, were replaced in the Early Jurassic by forests dominated by high-LMA taxa that were likely to have slower metabolic rates. We suggest that extreme CO2-induced global warming selected for taxa with high LMA associated with a stress-tolerant strategy and that adaptive plasticity in leaf functional traits such as LMA contributed to post-warming ecological success.

Could a potential Anthropocene mass extinction define a new geological period

A key aspect of the current debate about the Anthropocene focuses on defining a new geological epoch. Features of the Anthropocene include a biodiversity crisis with the potential to reach ‘mass extinction’ status alongside increasing global CO2 and temperature. Previous geological boundaries associated with mass extinctions, rises in atmospheric CO2 and rises in global temperature are more usually associated with transitions between geological periods. The current rapid increase in species extinctions suggest that a new mass extinction event is most likely imminent in the near-term future. Although CO2 levels are currently low in comparison with the rest of the Phanerozoic, they are rising rapidly along with global temperatures. This suggests that defining the Anthropocene as a new geological period, rather than a new epoch, may be more consistent with previous geological boundaries in the Phanerozoic.

Global peatland initiation driven by regionally asynchronous warming

Significance Peatlands are organic-rich wetlands that have acted as globally important carbon sinks since the Last Glacial Maximum. However, the drivers of peat initiation are poorly understood. Using a catalog of radiocarbon dates combined with simulations of past climates, we demonstrate that peat initiation in the deglaciated landscapes of North America, northern Europe, and Patagonia was driven primarily by warming growing seasons rather than by any increase in effective precipitation. In Western Siberia, which was not glaciated, climatic wetting was required to convert existing ecosystems into peatlands. Our findings explain the genesis of one of the world’s most important ecosystem types and its potentially fragile, distributed carbon store, with implications for understanding potential changes in peatland distribution in response to future warming. Widespread establishment of peatlands since the Last Glacial Maximum represents the activation of a globally important carbon sink, but the drivers of peat initiation are unclear. The role of climate in peat initiation is particularly poorly understood. We used a general circulation model to simulate local changes in climate during the initiation of 1,097 peatlands around the world. We find that peat initiation in deglaciated landscapes in both hemispheres was driven primarily by warming growing seasons, likely through enhanced plant productivity, rather than by any increase in effective precipitation. In Western Siberia, which remained ice-free throughout the last glacial period, the initiation of the world’s largest peatland complex was globally unique in that it was triggered by an increase in effective precipitation that inhibited soil respiration and allowed wetland plant communities to establish. Peat initiation in the tropics was only weakly related to climate change, and appears to have been driven primarily by nonclimatic mechanisms such as waterlogging due to tectonic subsidence. Our findings shed light on the genesis and Holocene climate space of one of the world’s most carbon-dense ecosystem types, with implications for understanding trajectories of ecological change under changing future climates.

Cuticle surfaces of fossil plants as a potential proxy for volcanic SO2 emissions: observations from the Triassic–Jurassic transition of East Greenland

Flood basalt volcanism has been implicated in several episodes of mass extinctions and environmental degradation in the geological past, including at the Triassic–Jurassic (Tr–J) transition, through global warming caused by massive outgassing of carbon dioxide. However, the patterns of biodiversity loss observed are complicated and sometimes difficult to reconcile with the effects of global warming alone. Recently, attention has turned to additional volcanic products as potential aggravating factors, in particular sulphur dioxide (SO2). SO2 acts both directly as a noxious environmental pollutant and indirectly through forming aerosols in the atmosphere, which may cause transient global dimming and cooling. Here, we present a range of morphological changes to fossil plant leaf cuticle surfaces of hundreds of Ginkgoales and Bennettitales specimens across the Tr–J boundary of East Greenland. Our results indicate that morphological structures of distorted cuticles near the Tr–J boundary are consistent with modern cuticle SO2-caused damage and supported by recent leaf-shape SO2 proxy results, thus identifying cuticle surface morphology as a potentially powerful proxy for SO2. Recording the timing and duration of SO2 emissions in the past may help distinguish between the driving agents responsible for mass extinction events and thus improve our understanding of the Earth System.

Economic value of trees in the estate of the Harewood House stately home in the United Kingdom

The estates of stately homes or manor houses are an untapped resource for assessing the ecosystem services provided by trees. Many of these estates have large collections of trees with clear value in terms of carbon storage, runoff prevention, and pollution removal along with additional benefits to biodiversity and human health. The estate of Harewood House in North Yorkshire represents an ideal example of such a stately home with a mixture of parkland and more formally planted gardens. The trees in each type of garden were analysed for height, diameter at breast height and light exposure. The data were then processed in iTrees software to generate economic benefits for each tree in both gardens. The analysis found that the larger North Front parkland garden had greater total benefits but the more densely planted formal West Garden had the greater per hectare value. In total, the trees on Harewood House estate are estimated to provide approximately £29 million in ecosystem service benefits. This study is the first to analyse the trees of stately homes for economic benefits and highlights that the trees are a valuable commodity for the estates. This should be considered in future planning and management of such estates.

Enhancing Student Employability through Urban Ecology Fieldwork

Abstract Students undertook a one-hour urban ecology activity based on the University of Leeds campus. The aims of the session were (1) to help students to link ecological theory to practice and (2) to encourage students to begin to think about and develop an online professional identity. Students were encouraged to tweet throughout the session and were surveyed four weeks after the session to determine if the aims had been met. A majority of students enjoyed the session and saw the links between the theory and practice of ecology. Most students also identified that an online professional identity is important and something that they should be developing. The session highlights that employability and professional development skills can be introduced to students within a subject-specific context early in their degree programme and still highlight the importance of generic transferable skills related to employability.