Stability Analysis of the Climate-Vegetation System in the Northern High Latitudes

The stability of the climate-vegetation system in the northern high latitudesis analysed with three climate system models of different complexity: A comprehensive 3-dimensional model of the climate system, GENESIS-IBIS, and two Earth system models of intermediate complexity (EMICs), CLIMBER-2 andMoBidiC. The biogeophysical feedback in the latitudinal belt 60–70° N, although positive, is not strong enough to support multiple steady states: A unique equilibriumin the climate-vegetation system is simulated by all the models on a zonal scale for present-day climate and doubled CO₂ climate.EMIC simulations with decreased insolation also reveal a unique steady state. However, the climate sensitivity to tree cover, T_F, exhibits non-linear behaviour within the models. For GENESIS-IBIS and CLIMBER-2, T_F islower for doubled CO₂ climate than for present-day climate due to a shorter snow season and increased relative significance ofthe hydrological effect of forest cover. For the EMICs, T_F is higher for low tree fraction than for high treefraction, mainly due to a time shift in spring snow melt in response to changes in tree cover. The climate sensitivity to tree coveris reduced when thermohaline circulation feedbacks are accounted for in the EMIC simulations. Simpler parameterizations of oceanic processes have opposite effects on T_F: T_F is lower in simulations with fixed SSTs and higher in simulations with mixed layer oceans. Experiments with transient CO₂ forcing show climate and vegetation not in equilibrium in the northern high latitudes at the end of the 20thcentury. The delayed response of vegetation and accelerated global warming lead to rather abrupt changes in northern vegetation cover in the first halfof the 21st century, when vegetation cover changes at double the present day rate.     

L'unité ophiolitique de Pineto (Corse) : signification du détritisme continental dans sa couverture de flysch albo-cénomanien

The Jurassic N-MORB ophiolites of the Pineto Unit, which were unaffected by Alpine metamorphism, can be compared to the Apennine ophiolites. They are, however, distinguished by their cover rocks that include a silico-clastic flysch that we have dated as Albian–Cenomanian. Clastic deposits of the same type, but coarser grained, are known from the normal cover rocks of the Balagne Nappe E-MORB ophiolites, originally located on a thinned continental crust and/or near a continental margin. The Pineto Unit thus indicates that the detrital input of continental material was able to extend to a domain of clearly oceanic character in the Ligurian palaeo-ocean. To cite this article: M. Durand-Delga et al., C. R. Geoscience 337 (2005).     

Palaeoproterozoic arc magmatism and collision in Liaodong Peninsula (north-east China)

In the north‐eastern part of the North China Block, a mafic magmatic belt consisting of mafic–ultramafic rocks and marine sedimentary rocks crops out between the northern Archean Anshan Block and a southern Palaeoproterozoic Block. ⁴⁰Ar/³⁹Ar amphibole ages around 1.9 Ga from gabbros, and trace element analyses of gabbros, pyroxenite and shale show that these rocks formed along a Palaeoproterozoic active continental margin. The mafic magmatic belt is interpreted as an arc developed above a south‐directed subduction zone, which was subsequently overthrust to the north upon the Anshan Archean Block. This study provides a new example agreeing with increasing evidence supporting plate mobility and thrust tectonics during the Palaeoproterozoic. These new insights must be considered with regard to the formation of the North China Block by magmatic accretion and tectonic collision.     

Use of Systematic, Palaeoflood and Historical Data for the Improvement of Flood Risk Estimation. Review of Scientific Methods

The catastrophic floods recently occurring in Europe warn of the critical need forhydrologic data on floods over long-time scales. Palaeoflood techniques provideinformation on hydrologic variability and extreme floods over long-time intervals(100 to 10,000 yr) and may be used in combination with historical flood data (last1,000 yr) and the gauge record (last 30–50 yr). In this paper, advantages anduncertainties related to the reconstruction of palaeofloods in different geomorphologicalsettings and historical floods using different documentary sources are described.Systematic and non-systematic data can be combined in the flood frequency analysisusing different methods for the adjustment of distribution functions. Technical toolsintegrating multidisciplinary approaches (geologic, historical, hydraulic and statistical)on extreme flood risk assessment are discussed. A discussion on the potential theoreticalbases for solving the problem of dealing with non-systematic and non-stationary data ispresented. This methodology is being developed using new methodological approachesapplied to European countries as a part of a European Commission funded project (SPHERE).     

Mid-latitude Southern Indian Ocean response to Northern Hemisphere Heinrich events

The lack of temporal resolution and accurate chronology of Southern Ocean marine cores has hampered comparison of glacial millennial-scale oscillations between the Southern Ocean, Antarctic ice and other records from both hemispheres. In this study, glacial climate variability is investigated over the last 50 ka using a multi-proxy approach. A precise chrono-stratigraphy was developed on the high-sedimentation rate core MD94-103 (Indian Southern Ocean, 45°35′S 86°31′E, 3560 m water depth) by geomagnetic synchronization between the later core and NAPIS75, and ¹⁴C dates. High-resolution time-series of δ¹⁸O in planktonic foraminifera Globigerina bulloides and Neogloboquadrina pachyderma, and sea surface temperatures (SSTs) estimated from the alkenone U^K′₃₇ index and foraminifera assemblages have been generated. Temporal evolution of the two temperature proxy records is notably different during the last glacial period. While foraminifera data indicate a consistent cooling towards the last glacial maximum, anomalous warm glacial alkenone temperatures suggest a strong advection of warm “detrital” alkenones by surface waters of the Agulhas current. Superimposed to this general trend, during Heinrich events, foraminiferal SSTs point to warmer surface waters, while concurrent alkenone SSTs exhibit apparent coolings probably caused by enhanced local alkenone production. By analogy to modern observations, possible influence of ENSO-like conditions on the subantarctic Southern Ocean SSTs is discussed.     

Sm–Nd isotopic investigation of Neoproterozoic and Cretaceous igneous rocks from southern Brazil: A study of magmatic processes

Nd-evolutionary paths for diversified igneous suites from southern Brazil are here re-evaluated using published results. We interpret the ε_Nd paths considering the secondary fractionation of ¹⁴⁷Sm/¹⁴⁴Nd due to major petrogenetic processes. The inclusion of Nd isotopes and geochemical data for Precambrian and Mesozoic basic rocks allow improving the discussion on the subcontinental lithosphere beneath southern Brazil. Late Neoproterozoic rocks, mostly granitoids, are exposed in two regions of the southern Brazilian shield, an eastern collisional belt and a western foreland. The latter included two geotectonic domains amalgamated at this time, the São Gabriel Arc (900–700 Ma), and the Taquarembó cratonic block. Magma genesis mainly involved mixture of crustal and incompatible-element-enriched mantle components, both with a long residence time. Continental segments are the Neoarchaean–Paleoproterozoic lower crust (ca. 2.55 Ga) in the western foreland, and Paleoproterozoic–Neoproterozoic recycled crust (2.1–0.8 Ga) in the collisional belt. Granitoids with a single crustal derivation are limited in the southern Brazilian Shield. Mixing processes are well-registered in the western foreland, where the re-enriched old mantle was probably mixed with a 900–700 Ma-old subducted lithosphere and a 2.55 Ga-old lower crust. The contribution of the latter increased from the early 605–580 Ma to the later 575–550 Ma Neoproterozoic events, which may be due either to crustal thickening or to delamination of the lithosphere. Magma sources were diversified in the 660–630 Ma collisional belt. Initially, they involved the mixing between two components with similar Nd isotopic ratios, a 2.1–0.8 Ga-old recycled crust and a subduction-processed old mantle. Regional heating and abundant production of granitic melts, with diversified contribution of enriched mantle components, mark the end of the collisional period, at 630–580 Ma. We can also attribute this to the delamination of the lithosphere, so that the same geodynamic process may explain the magmatism in the whole shield at the end of the Dom Feliciano Orogeny. Mesozoic rocks include flood basalts from the Cretaceous Paraná Province and sub-coeval alkalic suites. Multiple processes of metasomatism affected the lithospheric mantle, resulting in some complexity but they mainly register two enriched-mantle components, both generated during Neoarchaean–Paleoproterozoic events. One end-member has a more pronounced subduction signature. The other one probably resulted from the re-enrichment of the first component at the end of the Camboriú collisional orogeny (2.0 Ga).     

SIMS analyses of oxygen isotopes: Matrix effects in Fe-Mg-Ca garnets

Large instrumental mass fractionation (IMF) may occur during measurements of oxygen isotope ratios by SIMS. Part of this fractionation depends on crystal structure and mineral composition. In order to improve the accuracy of SIMS measurements, we gathered 6 commonly used garnet standards and prepared 6 others to adequately cover the composition range Alm_0-73, Prp_0-99, Grs_0-20. Electron microprobe analyses were performed at UBP-Clermont to check the chemical homogeneity of these standards. Oxygen isotope compositions were determined by laser fluorination and mass spectrometry at UW-Madison. Ten SIMS sessions and 336 δ¹⁸O measurements at CRPG-Nancy, on a Cameca IMS1270 instrument, demonstrate that the standards are homogeneous with external reproducibility of 0.3‰ (1σ). In terms of δ¹⁸O, SIMS measurements indicate that, during a single session, IMF can vary up to 6.3‰ from one garnet standard to another. In most of the sessions, IMF can be correlated with the grossular content. However, for a satisfactory correction scheme, we suggest the combination of the 3 main components (Ca, Fe, Mg). This is done using a simple least square calculation routine. The correction coefficients determined for each session can be used to calculate the IMF and correct the measured isotopic ratio of a garnet of known chemical composition. This way, we were able to reproduce the δ¹⁸O values of most of the Fe-Mg-Ca garnet standards within ± 0.6‰. Interestingly, the use of only 3 end-member standards (AlmCMG, PrpMM, GrsSE) plus a standard of intermediate composition (e.g. UWG-2) is sufficient to reproduce δ¹⁸O within the same precision. Thus, linear interpolation among end-member standards is satisfactory in the case of the garnet solid-solutions. Two studies carried out on zoned garnets from the Alps and the Pyrenees indicate that matrix effects become significant when variations in grossular contents are important (> 10%). In order to obtain reliable isotope ratio measurements on Fe-Mg-Ca garnets using a SIMS, we suggest a correction scheme using at least 3 reliable end-member standards plus a standard of intermediate composition (a garnet standard closest to the average composition of the analysed garnet). This allows cross-checking and incorporates a correction based on the variations in composition of zoned crystals.     

Fractionation change of hydrogen isotopes in trees due to atmospheric pollutants

Atmospheric pollution effects on hydrogen fractionation in trees are demonstrated for the first time in this study. The δ²H ring-cellulose series obtained for black spruce trees at a site near a SO₂-emitting smelter display short-term fluctuations superimposed on a first order −11‰ shift coincident with the onset of smelter operations. The isotopic depletion in trees exposed to various levels of SO₂ diminish with increasing distance relative to the location of the smelter, and it is not detected at the most distant selected stand, 116 km downwind from the point source. Both the spatial and temporal isotopic trends likely result from the combined effects of leaf transpiration, biochemical processes and water uptake by the root system. The spatial and temporal trends of δ²H values are the reverse of the δ¹³C trends previously obtained for the same tree ring series. These coupled isotopic fractionations underline an important response of trees to pollution stresses likely generated by ambient atmospheric SO₂ concentrations. The degradation of global air quality during the last 150 yr likely generated large scale modifications in the way terrestrial vegetation grows. In this respect, hydrogen dendrogeochemistry combined with other tracers such as C isotope ratios constitutes a new tool to evaluate the past behavior of forest ecosystems in terms of C uptake and acclimation to various types of atmospheric pollution.