24 research outputs found
Stable isotope evidence for rapid uplift of the central Apennines since the late Pliocene
No full textThe central Apennines, an accretionary wedge overlying an area of slab detachment, are characterized by prominent topography, active normal faulting, and high uplift rates. However, previous studies have failed to resolve the surface uplift history, complicating efforts to link the topographic evolution with underlying geodynamic processes. We aim to better quantify orographic changes by using stable oxygen isotope paleoaltimetry. Modern surface water δ18O are 5‰ lower at high elevation than at sea level, reflecting orographic rainout over the Apennines. We present 262 new lacustrine and paleosol carbonate δ18O measurements collected from ten extensional intermontane basins—spanning both high and low elevations—and combine these with 1,166 published δ18O data, permitting us to constrain changes in δ18O both spatially and temporally. Since the Pliocene, δ18O in present-day high-elevation basins has continuously decreased, even as δ18O in lowland basins has remained constant over time. We attribute this continuous 5‰ shift to increased orographic rainout as the central Apennines were uplifted. We estimate an increase in mean elevation of approximately 1–2 km since the late Pliocene, and these estimates match the suggested timing and expected amplitude of slab break-off related uplift. This supports the hypothesis that the opening of the Adriatic slab window and associated mantle flow contributed significantly to building topography in the central Apennines. © 2020 Elsevier B.V
Neogene cooling driven by land surface reactivity rather than increased weathering fluxes
No full textThe long-term cooling, decline in the partial pressure of carbon dioxide, and the establishment of permanent polar ice sheets during the Neogene period1,2 have frequently been attributed to increased uplift and erosion of mountains and consequent increases in silicate weathering, which removes atmospheric carbon dioxide3,4. However, geological records of erosion rates are potentially subject to averaging biases5,6, and the magnitude of the increase in weathering fluxes—and even its existence—remain debated7–9. Moreover, an increase in weathering scaled to the proposed erosional increase would have removed nearly all carbon from the atmosphere10, which has led to suggestions of compensatory carbon fluxes11–13 in order to preserve mass balance in the carbon cycle. Alternatively, an increase in land surface reactivity—resulting from greater fresh-mineral surface area or an increase in the supply of reactive minerals—rather than an increase in the weathering flux, has been proposed to reconcile these disparate views8,9. Here we use a parsimonious carbon cycle model that tracks two weathering-sensitive isotopic tracers (stable 7Li/6Li and cosmogenic 10Be/9Be) to show that an increase in land surface reactivity is necessary to simultaneously decrease atmospheric carbon dioxide, increase seawater 7Li/6Li and retain constant seawater 10Be/9Be over the past 16 million years. We find that the global silicate weathering flux remained constant, even as the global silicate weathering intensity—the fraction of the total denudation flux that is derived from silicate weathering—decreased, sustained by an increase in erosion. Long-term cooling during the Neogene thus reflects a change in the partitioning of denudation into weathering and erosion. Variable partitioning of denudation and consequent changes in silicate weathering intensity reconcile marine isotope and erosion records with the need to maintain mass balance in the carbon cycle and without requiring increases in the silicate weathering flux
Decline of soil respiration in northeastern Tibet through the transition into the Oligocene icehouse
No full textInternational audienceSoil respiration (Rs), the production of carbon dioxide in soils, increases dramatically from deserts to forested ecosystems. Rs values thus provide a potential tool to identify past ecosystems if recorded in sedimentary archives. Here, we propose a quantitative method to reconstruct past Rs values from paleosols. This method reverses the soil paleobarometer, a proxy that estimates past atmospheric CO2 concentration values (CO2atm) from paleosols while considering a narrow range of variation for Rs. We use past CO2atm values from marine proxies to reconstruct soil respiration from a 20 million year-long isotopic record from northeastern Tibet covering the transition from the Eocene greenhouse to the Oligocene icehouse. We show that Rs dropped at least 4-fold through the transition into the Oligocene icehouse, marking the spread of boreal desert-steppes of Central Asia. We show that increasing aridity and the decline of monsoonal rainfall, in parallel with global cooling, caused the fall of soil respiration. These highly dynamic Rs emphasize the need for a systematic screening of paleosol isotopic data before using the soil paleobarometer to reconstruct CO2at
Silicate weathering as a feedback and forcing in Earth's climate and carbon cycle
No full textCurrent understanding of the long-term carbon cycle posits that Earth's climate is stabilized by a negative feedback involving CO2 consumption by chemical weathering of silicate minerals. This theory holds that silicate weathering responds to climate: when atmospheric pCO2 and surface temperatures rise, chemical weathering accelerates, consuming more atmospheric CO2 and cooling global climate; when pCO2 falls, weathering fluxes decrease, permitting buildup of CO2 and consequent warming. However, the functional dependence of global weathering rates on atmospheric pCO2 (Earth's “weathering curve”) remains highly uncertain, with a variety of mathematical formulations proposed in the literature. We explore the factors influencing this relationship, and how they may have changed over Earth history. We then revisit classic carbon cycle model experiments to demonstrate how the choice of weathering curve has dramatic consequences for the response of the Earth system to several types of climatic and carbon-cycle perturbations. First, the slope of the weathering curve determines the timescale of recovery and the “long tail” of elevated pCO2 following carbon release events. Second, the nature of Earth's weathering curve determines the response of pCO2 to changing volcanic CO2 degassing, which has varied significantly over geologic timescales. Finally, we demonstrate how changes to Earth's weathering curve over time driven by, for example, tectonic or evolutionary processes, can act as a forcing, in addition to a feedback, in the carbon cycle and climate. These examples highlight the importance of constraining Earth's weathering curve, both for improving our understanding of past carbon cycle perturbations and predicting the future impact of anthropogenic carbon release on long timescales
Spatial pattern of super-greenhouse warmth controlled by elevated specific humidity
No full textEarth’s climate sensitivity, defined as the temperature increase for a doubling of partial pressure of carbon dioxide (pCO2), and the mechanisms responsible for amplification of high-latitude warming remain controversial. The latest Palaeocene/earliest Eocene (LPEE; 57–55 million years ago) is a time when atmospheric CO2 concentrations peaked between 1,400 and 4,000 ppm, which allows us to evaluate the climatic response to high pCO2. Here we present a reconstruction of continental temperatures and oxygen isotope compositions of precipitation (reflective of specific humidity) based on clumped and oxygen isotope analysis of pedogenic siderites. We show that continental mean annual temperatures reached 41 °C in the equatorial tropics, and summer temperatures reached 23 °C in the Arctic. The oxygen isotope compositions of precipitation reveal that compared with the present day the hot LPEE climate was characterized by an increase in specific humidity and the average residence time of atmospheric moisture and by a decrease in the subtropical-to-polar specific humidity gradient. The global increase in specific humidity reflects the fact that atmospheric vapour content is more sensitive to changes in pCO2than evaporation and precipitation, resulting in an increase in the residence time of moisture in the atmosphere. Pedogenic siderite data from other super-greenhouse periods support the evidence that the spatial patterns of specific humidity and warmth are related, providing a new means to evaluate Earth’s climate sensitivity
Chemical versus mechanical denudation in metaclastic and carbonate bedrock catchments on Crete Greece and mechanisms for steep and high carbonate topography
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