Tectonic controls on metamorphism, partial melting, and intrusion: timing and duration of regional metamorphism and magmatism in the Ni de Massif, Turkey

Mafic high-pressure granulite, eclogite and pyroxenite xenoliths have been collected from a Mesozoic volcaniclastic diatreme in Xinyang, near south margin of the Sino-Korean Craton (SKC). The high-pressure granulite xenoliths are mainly composed of fine-grained granoblasts of Grt+Cpx+Pl+Hbl±Kfs±Q±Ilm with relict porphyritic mineral assemblage of Grt+Cpx±Pl±Rt. P–T estimation indicates that the granoblastic assemblage crystallized at 765–890 °C and 1.25–1.59 GPa, corresponding to crustal depths of ca. 41–52 km with a geotherm of 75–80 mW/m². Calculated seismic velocities (V_p) of high-pressure granulites range from 7.04 to 7.56 km/s and densities (D) from 3.05 to 3.30 g/cm³. These high-pressure granulite xenoliths have different petrographic and geochemical features from the Archean mafic granulites. Elevated geotherm and petrographic evidence imply that the lithosphere of this craton was thermally disturbed in the Mesozoic prior to eruption of the host diatreme. These samples have sub-alkaline basaltic compositions, equivalent to olivine– and quartz–tholeiite. REE patterns are flat to variably LREE-enriched (La_N/Yb_N=0.98–9.47) without Eu anomaly (Eu/Eu*=0.95–1.11). They possess 48–127 ppm Ni and 2–20 ppm Nb with Nb/U and La/Nb ratios of 13–54 and 0.93–4.75, respectively, suggesting that these high-pressure granulites may be products of mantle-derived magma underplated and contaminated at the base of the lower crust. This study also implies that up to 10 km Mesozoic lowermost crust was delaminated prior to eruption of the Cenozoic basalts on the craton.     

Photosynthesis-irradiance relationships for three species of submersed macrophytes in the tidal freshwater Hudson River

Net photosynthesis under a range of natural light intensities was determined for three common macrophytes of the tidal freshwater Hudson River:Vallisneria americana Michx.,Potamogeton perfoliatus L., andMyriophyllum spicatum L. Light-saturated net photosynthetic rates did not differ among species, nor were there differences among species in the light intensity at which respiration balanced photosynthesis. The initial slope (α) of the photosynthesis-irradiance (P-I) curve was greater forV. americana than the other two species. As a result, maximum photosynthetic rates were reached inV. americana at significantly lower irradiances than those required to saturate photosynthesis inP. perfoliatus andM. spicatum. This characteristic would give this species a competitive advantage under conditions in which growth is limited by light availability. Macrophyte biomass distribution showed no clear correlation with depth.V. americana was the most abundant of the three species, both in frequency of occurrence and absolute biomass. The low irradiance required for the saturation of photosynthesis inV. americana may contribute to its predominance in the turbid Hudson River.     

Nontidal circulation and mixing processes in the lower Potomac estuary

An analysis of the vertical structure of nontidal longitudinal currents and salinity in a reach of the lower Potomac River Estuary suggests that values for vertical eddy viscosity and eddy diffusivity scale with water depth H, tidal current amplitude U and bulk Richardson number according to conventional empirical formulas. However, the constant which relates the vertical eddy coefficients under conditions of neutral stability to UH is found to be an order of magnitude less than that expected for tidal conditions. Analyses also suggest that the degree of enhancement of longitudinal dispersion by the shear effect associated with the nontidal currents is a strong function of bulk Richardson number.     

Plesiosaurs

Plesiosaurs are an unusual and intriguing group of extinct aquatic reptiles ( Fig. 1 ). They are sauropterygians, a group known from an array of semi‐aquatic forms during the Triassic period: placodonts, pachypleurosaurs and nothosaurs. The first plesiosaurs are known from the very latest Triassic, but by the Early Jurassic plesiosaurs were cosmopolitan in distribution and lasted successfully to the latest Cretaceous, when they became victims of the K‐T extinction event. Plesiosaurs were predominantly marine organisms, although their fossils are not uncommon in brackish or even fresh water deposits. We know that all plesiosaurs were carnivorous; many of them were top predators in their respective ecosystems. But with no living descendants (or analogues) plesiosaurs are mysterious fossil organisms—as we will see, many questions regarding their biology remain unanswered or contentious. However, plesiosaurs are currently undergoing renewed scientific attention.

Figure 1 Open in figure viewer PowerPoint The beautifully preserved skeleton of the plesiosaur Rhomaleosaurus victor seen in ventral view, from the Lower Jurassic (Toarcian) of Holzmaden, Germany (total length 3.44 m). Redrawn from Fraas (1910).     

Dams on the Mekong River: Lost fish protein and the implications for land and water resources

Proposed dam construction in the Lower Mekong Basin will considerably reduce fish catch and place heightened demands on the resources necessary to replace lost protein and calories. Additional land and water required to replace lost fish protein with livestock products are modelled using land and water footprint methods. Two main scenarios cover projections of these increased demands and enable the specific impact from the main stem dam proposals to be considered in the context of basin-wide hydropower development. Scenario 1 models 11 main stem dams and estimates a 4–7% increase overall in water use for food production, with much higher estimations for countries entirely within the Basin: Cambodia (29–64%) and Laos (12–24%). Land increases run to a 13–27% increase. In scenario 2, covering another 77 dams planned in the Basin by 2030 and reservoir fisheries, projections are much higher: 6–17% for water, and 19–63% for land. These are first estimates of impacts of dam development on fisheries and will be strongly mediated by cultural and economic factors. The results suggest that basic food security is potentially at a high risk of disruption and therefore basin stakeholders should be fully engaged in strategies to offset these impacts.     

Aspects of wave behaviour in the mid‐ and upper troposphere of the southern Hemisphere

Abstract

The zonal wavenumber spectra of the geopotential heights of the 300‐ and 500‐mb surfaces in the Southern Hemisphere were determined for each month between May 1972 and November 1979 using daily operational analyses produced by the Australian Bureau of Meteorology. During over one‐quarter of the “summer” months (November through March) there are very prominent peaks at zonal wavenumber five in the region of the mid‐latitude jet (～35–60°S). Frequently wavenumber five totally dominates the eddy fields in individual daily maps so that height contours in mid‐latitudes take on virtually pentagonal shapes. During periods when wavenumber 5 is prominent, it is observed to propagate eastward in a very regular manner with a period of about eleven days. All these findings are consistent with Salby's (1982) earlier results concerning the Southern Hemisphere height fields during the first few months of the FGGE experiment.

There is little evidence for a similar phenomenon in the winter circulation of the Southern Hemisphere.     

Modelling tropical cyclone risks for present and future climate change scenarios using geospatial techniques

Tropical cyclones and their devastating impacts are of great concern to coastal communities globally. An appropriate approach integrating climate change scenarios at local scales is essential for producing detailed risk models to support cyclone mitigation measures. This study developed a simple cyclone risk-modelling approach under present and future climate change scenarios using geospatial techniques at local scales, and tested using a case study in Sarankhola Upazila from coastal Bangladesh. Linear storm-surge models were developed up to 100-year return periods. A local sea level rise scenario of 0.34?m for the year 2050 was integrated with surge models to assess the climate change impact. The resultant storm-surge models were used in the risk-modelling procedures. The developed risk models successfully identified the spatial extent and levels of risk that match with actual extent and levels within an acceptable limit of deviation. The result showed that cyclone risk areas increased with the increase of return period. The study also revealed that climate change scenario intensified the cyclone risk area by 5–10% in every return period. The findings indicate this approach has the potential to model cyclone risk in other similar coastal environments for developing mitigation plans and strategies.     

Evolution of permafrost in China during the last 20 ka

The formation and evolution of permafrost in China during the last 20 ka were reconstructed on the basis of large amount of paleo-permafrost remains and paleo-periglacial evidence, as well as paleo-glacial landforms, paleo-flora and paleofauna records. The results indicate that, during the local Last Glacial Maximum(LLGM) or local Last Permafrost Maximum(LLPMax), the extent of permafrost of China reached 5.3×106-5.4×106 km2, or thrice that of today, but permafrost shrank to only0.80×106-0.85×106 km2, or 50% that of present, during the local Holocene Megathermal Period(LHMP), or the local Last Permafrost Minimum(LLPMin). On the basis of the dating of periglacial remains and their distributive features, the extent of permafrost in China was delineated for the two periods of LLGM(LLPMax) and LHMP(LLPMin), and the evolution of permafrost in China was divided into seven periods as follows:(1) LLGM in Late Pleistocene(ca. 20000 to 13000-10800 a BP)with extensive evidence for the presence of intensive ice-wedge expansion for outlining its LLPMax extent;(2) A period of dramatically changing climate during the early Holocene(10800 to 8500-7000 a BP) when permafrost remained relatively stable but with a general trend of shrinking areal extent;(3) The LHMP in the Mid-Holocene(8500-7000 to 4000-3000 a BP)when permafrost degraded intensively and extensively, and shrank to the LLPMin;(4) Neoglaciation during the late Holocene(4000-3000 to 1000 a BP, when permafrost again expanded;(5) Medieval Warming Period(MWP) in the late Holocene(1000-500 a BP) when permafrost was in a relative decline;(6) Little Ice Age(LIA) in the late Holocene(500-100 a BP), when permafrost relatively expanded, and;(7) Recent warming(during the 20 th century), when permafrost continuously degraded and still is degrading. The paleo-climate, geography and paleopermafrost extents and other features were reconstructed for each of these seven periods.