Global Change Abstracts The Swiss Contribution - SCNAT

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182 Global Change Abstracts – The Swiss Contribution | Past Global Changes leaf area index (LAI), using both global vegetation models and palaeoecological data. Prior simulations indicate lowered LAIs at the Last Glacial Maximum (LGM), but this is the first attempt to corroborate predictions against observations. Location Eastern North America and eastern Beringia. Methods Using a dense surface pollen data set and remotely sensed LAIs from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument, we evaluate the ability of analogue-based techniques to reconstruct modern LAIs from pollen data. We then apply analogue techniques to LGM pollen records, calculate the ratio of LGM to modern LAIs (RLAI) and compare reconstructed RLAIs to RLAIs simulated by BIOME4. Sensitivity experiments with BIOME4 distinguish the effects of CO 2 and climate on glacial LAIs. Results Modern LAIs are skilfully predicted (r(2) = 0.83). Data and BIOME4 indicate that LAIs at the LGM were up to 12% lower than modern values in eastern North America and 60-94% lower in Beringia. In eastern North America, LGM climates partially counteracted CO 2-driven decreases in LAI, while in Beringia both contributed to lowered LAIs. Main conclusions In both regions climate is the primary driver of LGM LAIs. The decline in eastern North America LAIs is smaller than previously reported, so regional vegetation feedbacks to LGM climate may have been less significant than previously supposed. CO 2 exerts both physiological and community effects upon LAI, by regulating resource availability for leaf production and by influencing the competitive balance among species and hence the composition and structure of plant communities. Pollen-based reconstructions using analogue methods do not incorporate the physiological effect and so are upper estimates of full-glacial LAIs. BIOME4 sensitivity experiments indicate that the community and physiological effects together caused 10% to 20% decrease in LAIs at the LGM, so simulated RLAIs that are 80-100% of reconstructed RLAIs are regarded as consistent with data. Global Ecology and Biogeography, 2008, V17, N1, JAN, pp 122-134. 08.1-379 A matter of divergence: Tracking recent warming at hemispheric scales using tree ring data Wilson R, D’arrigo R, Buckley B, Büntgen U, Esper J, Frank D, Luckman B, Payette S, Vose R, Youngblut D Scotland, USA, Switzerland, Canada Paleontology , Meteorology & Atmospheric Sciences , Plant Sciences , Forestry No current tree ring (TR) based reconstruction of extratropical Northern Hemisphere (ENH) temperatures that extends into the 1990s captures the full range of late 20th century warming observed in the instrumental record. Over recent decades, a divergence between cooler reconstructed and warmer instrumental large-scale temperatures is observed. We hypothesize that this problem is partly related to the fact that some of the constituent chronologies used for previous reconstructions show divergence against local temperatures in the recent period. In this study, we compiled TR data and published local /regional reconstructions that show no divergence against local temperatures. These data have not been included in other large-scale temperature reconstructions. Utilizing this data set, we developed a new, completely independent reconstruction of ENH annual temperatures (1750-2000). This record is not meant to replace existing reconstructions but allows some degree of independent validation of these earlier studies as well as demonstrating that TR data can better model recent warming at large scales when careful selection of constituent chronologies is made at the local scale. Although the new series tracks the increase in ENH annual temperatures over the last few decades better than any existing reconstruction, it still slightly under predicts values in the post-1988 period. We finally discuss possible reasons why it is so difficult to model post- mid-1980s warming, provide some possible alternative approaches with regards to the instrumental target and detail several recommendations that should be followed in future large-scale reconstruction attempts that may result in more robust temperature estimates. Journal of Geophysical Research Atmospheres, 2007, V112, ND17, SEP 11 ARTN: D17103. 08.1-380 Relation between rock uplift and denudation from cosmogenic nuclides in river sediment in the Central Alps of Switzerland Wittmann H, von Blanckenburg F, Kruesmann T, Norton K P, Kubik P W Switzerland, Germany Geology , Paleontology , Cryology / Glaciology A north-south traverse through the Swiss Central Alps reveals that denudation rates correlate with recent rock uplift rates in both magnitude and spatial distribution. This result emerges from a study of in situ-produced cosmogenic Be-10 in riverborne quartz in Central Alpine catchments. As a prerequisite, we took care to investigate the potential influence of shielding from cosmic rays due to snow, glaciers, and topographic obstructions; to calculate a possible memory from Last Glacial Maximum (LGM) glaciation; and to identify a watershed size that is appropriate for systematic
Global Change Abstracts – The Swiss Contribution | Past Global Changes sampling. Mean denudation rates are 0.27 +/- 0.14 mm/ a for the Alpine foreland and 0.9 +/- 0.3 mm/ a for the crystalline Central Alps. The measured cosmogenic nuclide-derived denudation rates are in good agreement with post-LGM lake infill rates and are about twice as high as denudation rates from apatite fission track ages that record denudation from 9 to 5 Ma. In general, denudation rates are high in areas of high topography and high crustal thickness. The similarity in the spatial distribution and magnitude of denudation rates and those of rock uplift rates can be interpreted in several ways: (1) Postglacial rebound or climate change has introduced a transient change in which both uplift and denudation follow each other with a short lag time; (2) the amplitude of glacial to interglacial changes in both is small and is contained in the scatter of the data; (3) both are driven by ongoing convergence where their similarity might hint at some form of long-term quasi steady state; or (4) enhanced continuous Quaternary erosion and isostatic compensation of the mass removed accounts for the distribution of present-day rock uplift. Journal of Geophysical Research Earth Surface, 2007, V112, NF4, NOV 29 ARTN: F04010. 183 08.1-381 The prelude of the end-Permian mass extinction predates a postulated bolide impact Yin H, Feng Q, Baud A, Xie S, Benton M J, Lai X, Bottjer D J Peoples R China, Switzerland, England, USA Paleontology , Geology The mass extinction at the Permian-Triassic Boundary (PTB) is said to have been abrupt and probably caused by an extraterrestrial impact. However, evidence from the Global Stratotype Section and Point (GSSP) of the base of the Induan at Meishan, China, shows that the biotic crisis began prior to the level, in beds 25 and 26 at which the postulated impact event occurred. Evidence of such an earlier biotic crisis occurs in other sections in South China, and in central and western Tethyan regions. This event is characterized by the extinction of a range of faunas, including corals, deep-water radiolarians, most fusulinids and pseudotirolitidammonoids, and many Permian brachiopods. In all sections, this extinction level is usually a few decimeters to meters below that of the main mass extinction in the event beds (25 and 26) at Meishan, and their correlatives elsewhere. This earlier extinction event happened before the postulated bolide impact at the level of beds 25 and 26, and constrains interpretation of the mechanisms that brought about this greatest mass extinction. International Journal of Earth Sciences, 2007, V96, N5, OCT, pp 903-909.
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