Drought, vegetation change, and human history on Rapa Nui (Isla de Pascua, Easter Island)

Stratigraphic records from lake sediment cores and slope deposits on Rapa Nui document prehistoric human impacts and natural environmental changes. A hiatus in sedimentation in Rano Raraku suggests that this lake basin dried out sometime after 4090-4410 cal yr BP and refilled only decades to centuries before AD 1180-1290. Widespread ecosystem changes caused by forest clearance by Polynesian farmers began shortly after the end of this drought. Terrestrial sections show a chronology of burning and soil erosion similar to the lake cores. Although changing sediment types and shifts in the pollen rain suggest that droughts occurred earlier in the Holocene, as yet there is no evidence for droughts occurring after AD 1180-1290. The timing of the agricultural colonization of Rapa Nui now seems well established at ca. AD 1200 and it was accompanied by rapid deforestation that was probably exacerbated by the island's small size, its droughty climate, and the rarity of primeval fires. Detailed records of a large interval of Rapa Nui's ecological history remain elusive due to the drought hiatus in the Rano Raraku sediment record. We find no evidence for a “rat outbreak impact” on Rapa Nui's vegetation preceding anthropogenic forest clearance.     

Permian magmatism and metamorphism in the Dent Blanche nappe: constraints from field observations and geochronology

In the Dent Blanche Tectonic System, the Mont Morion biotite-bearing granite is a km-scale intrusion preserved in a low-strain volume. Zircon saturation thermometry suggests that it crystallised from a melt that reached about 800 °C. U–Pb zircon and allanite geochronology indicates crystallization of the magma in the Permian (290 ± 3 Ma; 280 ± 8 Ma, respectively). Migmatitic biotite-gneiss and amphibolite are found as xenoliths within the Mont Morion granite and constitute its country-rocks. In two samples of migmatitic biotite-gneiss zircon has metamorphic overgrowths that yield U–Pb ages of 285 ± 3 Ma and 281 ± 4 Ma, and are thus contemporaneous with the intrusion of the granite. The Mont Morion granite with its country-rocks of migmatitic biotite-bearing gneiss and amphibolite was thus emplaced at middle crustal levels while amphibolite facies metamorphism affected its country rocks. The magmatic and metamorphic record in the Mont Morion area reflects the high-temperature regime and lithospheric thinning of the Adriatic continental margin during Permian.     

Water-table and discharge changes associated with the 2016–2017 seismic sequence in central Italy: hydrogeological data and a conceptual model for fractured carbonate aquifers

A seismic sequence in central Italy from August 2016 to January 2017 affected groundwater dynamics in fractured carbonate aquifers. Changes in spring discharge, water-table position, and streamflow were recorded for several months following nine Mw 5.0–6.5 seismic events. Data from 22 measurement sites, located within 100 km of the epicentral zones, were analyzed. The intensity of the induced changes were correlated with seismic magnitude and distance to epicenters. The additional post-seismic discharge from rivers and springs was found to be higher than 9 m³/s, totaling more than 0.1 km³ of groundwater release over 6 months. This huge and unexpected contribution increased streamflow in narrow mountainous valleys to previously unmeasured peak values. Analogously to the L’Aquila 2009 post-earthquake phenomenon, these hydrogeological changes might reflect an increase of bulk hydraulic conductivity at the aquifer scale, which would increase hydraulic heads in the discharge zones and lower them in some recharge areas. The observed changes may also be partly due to other mechanisms, such as shaking and/or squeezing effects related to intense subsidence in the core of the affected area, where effects had maximum extent, or breaching of hydraulic barriers.

     

Snow water equivalent estimation in the Mallero basin using snow gauge data and MODIS images and fieldwork validation

Snow accumulation and ablation rule the temporal dynamics of water availability in mountain areas and cold regions. In these environments, the evaluation of the snow water amount is a key issue. The spatial distribution of snow water equivalent (SWE) over a mountain basin at the end of the snow accumulation season is estimated using a minimal statistical model (SWE‐SEM). This uses systematic observations such as ground measurements collected at snow gauges and snow‐covered area (SCA) data retrieved by remote sensors, here MODIS. Firstly, SWE‐SEM calculates local SWE estimates at snow gauges, then the spatial distribution of SWE over a certain area using an interpolation method; linear regressions of the first two order moments of SWE with altitude. The interpolation has been made by both confining and unconfining the spatial domain by SCA. SWE‐SEM is applied to the Mallero basin (northern Italy) for calculating the snow water equivalent at the end of the winter season for 6 years (2001–2007). For 2007, SWE‐SEM estimates are validated through fieldwork measurements collected during an ‘ad hoc’ campaign on March 31, 2007. Snow‐surveyed measurements are used to check SCA, snow density and SWE estimates. Copyright © 2009 John Wiley & Sons, Ltd.     

Crystal structure of columbite under high pressure

The structural evolution of two columbites under pressure, one ferrocolumbite from Raode (Africa) and one manganocolumbite from Kragero (Norway), has been determined by single-crystal X-ray diffraction. Structural investigations at high pressure have been carried out on samples which were preliminarily annealed to attain the complete cation-ordered state. For each crystal, five complete datasets have been collected from room pressure up to ca. 7 GPa. Structure refinements converged to final discrepancy factors R ranging between 5.2 and 5.8% for both the crystals. Structure refinements of X-ray diffraction data at different pressures allowed characterisation of the mechanisms by which the columbite structure accommodates variations in pressure. A and B octahedral volumes in both samples decrease linearly as pressure increases, with a larger compression of the larger A site. The difference in polyhedral bulk moduli of the A sites for the two samples does not appear to relate directly to the octahedral sizes, the A site being more compressible in the Fe-rich sample than in the Mn-rich one. By far the most compressible direction in both the analysed samples is along b. The cations are in fact free to move along this direction, thus allowing the octahedral chains to slide over each other; this effect is particularly evident in the manganocolumbite sample which shows a steep shortening of interchain A–B distances along b.     

First-principles calculation of H/D isotopic fractionation between hydrous minerals and water

Hydrogen fractionation laws between selected hydrous minerals (brucite, kaolinite, lizardite, and gibbsite) and perfect water gas have been computed from first-principles quantum-mechanical calculations. The β-factor of each phase was calculated using the harmonic phonon dispersion curves obtained within density functional theory. All the fractionation laws show the same shape, with a minimum between 200 °C (brucite) and 500 °C (gibbsite). At low temperatures, the mineral/liquid water fractionation laws have been obtained using the experimental gas/liquid water fractionation laws. The resulting fractionation laws systematically overestimate measurements by 15‰ at low temperatures to 8‰ at ≈400 °C. Based on this general agreement, all calculated laws were empirically corrected with reference to brucite/water data. These considerations suggest that the experimental or natural calibrations by Xu and Zheng (1999) and Horita et al. (2002) (brucite/water), Gilg and Sheppard (1996) (kaolinite/water), Wenner and Taylor (1973) (lizardite/water), and in some extents Vitali et al. (2001) (gibbsite/water) are representative of equilibrium fractionations. Besides, internal isotopic fractionation of hydrogen between inner-surface and inner hydroxyl groups has been computed for kaolinite and lizardite. The obtained fractionation is large, of opposite sign for the two systems (respectively, −23‰ and +63‰ at 25 °C) and is linear in T^-2. Internal fractionation of hydrogen in TO phyllosilicates might thus be used in geothermometry.