Cause of severe droughts in Southwest China during 1951–2010

The cause of severe droughts over the Southwest China (SWC) during the local dry season is investigated based on the station rainfall data and the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data during 1951–2010. The droughts are in general consistent with local anomalous descent in the middle troposphere. The diagnosis of the vertical motion (omega) equation indicates that the local descent is primarily maintained by the anomalous cold temperature advection processes. Both the advection of anomalous temperature by mean wind and the advection of mean temperature by anomalous wind contribute to maintaining the anomalous descent over the SWC region. A composite analysis shows that the circulation anomaly over SWC is induced by remote forcing from the tropical Pacific and North Atlantic Oceans. During La Niña years, enhanced heating over the Maritime Continent induces anomalous downward motion over SWC through the connection of local Hadley circulation. Adiabatic warming associated with the downward motion helps to set up and maintain the local anomalous anticyclone. Another possible route is through the North Atlantic-Asia teleconnection, in which downstream Rossby wave energy propagation plays a crucial role. A negative-phase North Atlantic Oscillation may trigger a large-scale wave train pattern that induces an anomalous anticyclone over the subtropical Asia and promotes the dry condition over SWC.     

An Inter-Laboratory Assessment of the Thorium Isotopic Composition of Synthetic and Rock Reference Materials

We present a concerted international effort to cross-calibrate five synthetic Th isotope reference materials (UCSC Th "A", OU Th "U", WUN, IRMM-35 and IRMM-36), and six rock reference materials (UCSC TML, Icelandic ATHO, USGS BCR-2, USGS W-2, USGS BHVO-2, LV18) using multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). We then compare our new values with a compilation of literature mass spectrometric data for these reference materials and derive recommended "consensus"²³⁰Th/²³²Th values for each. We also present isotope dilution U and Th concentration data for four rock reference materials (UCSC TML, Icelandic ATHO, USGS BCR-2, USGS W-2).     

Hydrologic modeling of the Heihe watershed by DLBRM in Northwest China

Water shortage is a chronic problem in arid Northwest China.The rapid population growth and expanding urbanization as well as potential climate change impacts are likely to worsen the situation,threatening domestic,irrigation,and industrial supplies and even the survival of the ecosystems in Northwest China.This paper describes the preliminary work of adapting the Distributed Large Basin Runoff Model(DLBRM) to the Heihe watershed(the second largest inland river in arid Northwestern China,with a drainage area of 128,000 km2) for understanding distribution of glacial-snow melt,groundwater,surface runoff,and evapotranspi-ration,and for assessing hydrological impacts of climate change and glacial recession on water supply in the middle and lower reaches of the watershed.Preliminary simulation results show that the Qilian Mountain in the upper reach area produces most runoff in the Heihe watershed.The simulated daily river flows during the period of 1990-2000 indicate that the Heihe River dis-charges about 1×109 m3 of water from the middle reach(at Zhengyixia Station) to lower reach,with surface runoff and interflow contributing 51 and 49 percent respectively.The sandy lower soil zone in the middle reach has the highest evapotranspiration rate and also contributes nearly half of the river flow.Work underway focuses on the DLBRM model improvement and incorporation of the climate change and management scenarios to the hydrological simulations in the watershed.     

A detailed isotopic and petrological study of a single garnet from the Gassetts Schist,Vermont

Garnets, up to 1.2 cm across, from a metapelite in the Acadian metamorphic terrain of eastern Vermont have been analysed for major elements and segregated into different fractions for isotopic analysis. The garnets preserve abundant inclusions of minerals present during garnet growth which allow a nearly complete reaction history to be established. The Sm-Nd and U-Pb isotopic analyses yield concordant ages of 380 Ma for the rim of one garnet and this is interpreted as the formation age. The changing mineral assemblages through garnet growth, their evolving compositions and the thermodynamic dataset of Holland and Powell (1990) are used to put constraints on the P-T evolution during growth. These imply growth during heating from 540 to 635° C and decompression from 9.7 to 7.2 kbar, representing a temperature increase of 95° C and an uplift of 7 km during growth of the garnet. While growth during heating and decompression is consistent with both field evidence and analysis of garnet microstructures and is predicted by theoretical models of regional metamorphism, the extent of the temperature increase requires either very slow uplift (0.15 mm a^-1) or an additional magmatic heat input. Slow uplift is precluded by existing constraints on both the duration of the uplift event and that of garnet growth and it is concluded that an external magmatic heat input is required. Comparison with published data on the timing of metamorphism in other parts of the terrain suggests that the peak occurred earlier in lower grade regions, a conclusion that is again supported by theoretical studies. Following the peak, cooling and uplift occurred at a modest rate consistent with simple isostatic recovery.     

A Case Study of Carbon Pools Under Three Different Land-Uses in Panamá

This paper examines changes in carbon (C) pools associated with land-use, synthesizing data from two experiments dealing with different aspects of tree plantation establishment in Central Panamá. First, we analysed soil profiles in a grazed pasture and an adjacent 5-year-old teak (Tectona grandis) plantation. There were small differences in soil C mass in the top 10 cm of the pasture and the plantation, though analysis of paired profiles suggested larger differences at greater depth. Analysis of the ¹³C signatures in the pasture soils and litter showed that 90% to 95% of the organic matter in the surface 5 cm was derived from C₄ pasture plants, over the 45 years since the pasture was converted from forest. Comparison of the ¹³C signatures in the pasture and teak plantation profiles indicated substantial replacement of C₄—derived organic matter with the dominantly C₃—derived plantation tissues. Organic matter turnover times in the upper 10 cm of the soils ranged from 8 to 34 years and from 11 to 58 years in the upper 30 cm, depending on topographic location. We also present preliminary results, and technical challenges, for an eddy covariance experiment set up to provide a direct comparison between a grazed pasture and a native tree plantation. The two ecosystems studied are estimated to be small CO₂ sinks, 92 g,C,m^–2 yr^–1 for the pasture, and 57 g,C,m^–2 yr^–1 for native species plantation in the first year after establishment. The pastures response to seasonal change was more pronounced, both in term of CO₂ fluxes and in term of herbaceous productivity, than the plantations response. The storage below ground systems contained up 40% of the total sapling biomass.     

Mass flow sedimentology within the HYC Zn–Pb–Ag deposit,Northern Territory,Australia: evidence for syn-sedimentary ore genesis

Laterally continuous mass-flow deposits are an important feature of the HYC stratiform sediment-hosted Zn–Pb–Ag deposit, which reveal more about the HYC mineralising system than has been previously recognised. Mass flow deposits are interbedded with sheet-like mineralised lenses in a carbonaceous dolomitic siltstone host rock. Sedimentological processes of mass-flow deposit emplacement are proposed that constrain stratiform mineralisation to the top metre of the sediment pile, based on mass-flow geometry and detailed clast petrology. Four distinct sedimentary facies are identified within the mass-flow units: framework-supported polymictic boulder breccia; matrix-supported pebble breccia; and gravel-rich and sand-rich graded turbidite beds. The boulder breccias are weakly reverse graded and show rapid lateral transition into the other facies, all of which are distal manifestations of the same sedimentary events. The flow geometry and relationships between these facies are interpreted to reflect mass-flow initiation as clast-rich debris flows, with transformation via the elutriation of fines into a subsequent turbulent flow from which the turbidite and matrix-supported breccia facies were deposited. All the mass-flow facies contain clasts of the common and minor components of the in-situ laminated base-metal mineralised siltstone. Texturally these are identical to their in-situ counterparts, and are clearly distinct from other sulphidic clasts that are of unequivocal replacement origin. In the boulder breccias, intraclasts may be the dominant clast type and the matrix may contain abundant fine-grained sphalerite and pyrite. Dark coloured sphaleritic and pyritic breccia matrices are distinct from pale carbonate-siliclastic matrices, are associated with high abundance of sulphidic clasts, and systematically occupy the lower part of breccia units. Consequently, clasts that resemble in-situ ore facies are confirmed as genuine intraclasts that were incorporated into erosive mass flows prior to complete consolidation. Disaggregation and assimilation of sulphidic sediment in the flow contributed to the sulphide component of the dark breccia matrices. The presence of laminated sulphidic intraclasts in the mass-flow facies constrains mineralisation at HYC to the uppermost part of the seafloor sediment pile, where this material was susceptible to erosion by incoming clast-rich mass flows.Editorial handling: N. White