During the Paleocene-Eocene Thermal Maximum (PETM), ∼56 Mya, thousands of petagrams of carbon were released into the ocean-atmosphere system with attendant changes in the carbon cycle, climate, ocean chemistry, and marine and continental ecosystems. The period of carbon release is thought to have lasted <20 ka, the duration of the whole event was ∼200 ka, and the global temperature increase was 5-8 • C. Terrestrial and marine organisms experienced large shifts in geographic ranges, rapid evolution, and changes in trophic ecology, but few groups suffered major extinctions with the exception of benthic foraminifera. The PETM provides valuable insights into the carbon cycle, climate system, and biotic responses to environmental change that are relevant to long-term future global changes.

Fractionation of carbon isotopes by plants during CO 2 uptake and fixation (Δ leaf ) varies with environmental conditions, but quantitative patterns of Δ leaf across environmental gradients at the global scale are lacking. This impedes interpretation of variability in ancient terrestrial organic matter, which encodes climatic and ecological signals. To address this problem, we converted 3,310 published leaf δ 13 C values into mean Δ leaf values for 334 woody plant species at 105 locations (yielding 570 species-site combinations) representing a wide range of environmental conditions. Our analyses reveal a strong positive correlation between Δ leaf and mean annual precipitation (MAP; R 2 ¼ 0.55), mirroring global trends in gross primary production and indicating stomatal constraints on leaf gas-exchange, mediated by water supply, are the dominant control of Δ leaf at large spatial scales. Independent of MAP, we show a lesser, negative effect of altitude on Δ leaf and minor effects of temperature and latitude. After accounting for these factors, mean Δ leaf of evergreen gymnosperms is lower (by 1-2.7‰) than for other woody plant functional types (PFT), likely due to greater leaf-level water-use efficiency. Together, environmental and PFT effects contribute to differences in mean Δ leaf of up to 6‰ between biomes. Coupling geologic indicators of ancient precipitation and PFT (or biome) with modern Δ leaf patterns has potential to yield more robust reconstructions of atmospheric δ 13 C values, leading to better constraints on past greenhouse-gas perturbations. Accordingly, we estimate a 4.6‰ decline in the δ 13 C of atmospheric CO 2 at the onset of the Paleocene-Eocene Thermal Maximum, an abrupt global warming event ∼55.8 Ma.biogeochemistry | ecophysiology | fractionation | PETM H uman perturbation of the global carbon (C) cycle is potentially far greater in rate and magnitude than variations in the recent past, pushing predictions of future climate beyond the calibration range of models based on modern and near-modern observations. Robust predictions of future impacts of rising CO 2 require not only extrapolation of ecological patterns along modern environmental gradients but also insights gained from changing ecological patterns at times of high CO 2 and hot climate in the geologic past (1 and 2). Global patterns of variation in leaf carbon isotope (δ 13 C leaf ) values potentially record climate-driven changes in modern plant physiology and biogeochemistry. An understanding of factors controlling plant fractionation (Δ leaf ) at the global scale will improve interpretations of past changes in climate and ecology recorded in ancient terrestrial sedimentary organic carbon (2). Patterns in δ 13 C leaf of living plants at the global scale, however, are unresolved in spite of abundant published data at smaller spatial scales.In living plants, δ 13 C leaf values reflect the balance of photosynthesis and stomatal conductance and their coupled response to the environment (3). Edaphic factors (e.g., water availability, alt...

One of the great debates about extinction is whether humans or climatic change caused the demise of the Pleistocene megafauna. Evidence from paleontology, climatology, archaeology, and ecology now supports the idea that humans contributed to extinction on some continents, but human hunting was not solely responsible for the pattern of extinction everywhere. Instead, evidence suggests that the intersection of human impacts with pronounced climatic change drove the precise timing and geography of extinction in the Northern Hemisphere. The story from the Southern Hemisphere is still unfolding. New evidence from Australia supports the view that humans helped cause extinctions there, but the correlation with climate is weak or contested. Firmer chronologies, more realistic ecological models, and regional paleoecological insights still are needed to understand details of the worldwide extinction pattern and the population dynamics of the species involved.