Synchronism of Holocene East Asian monsoon variations and North Atlantic drift-ice tracers

Eighteen radiocarbon-dated eolian and paleosol profiles within a 1500-km-long belt along the arid to semi-arid transition zone of north-central China record variations in the extent and strength of the East Asian summer monsoon during the Holocene. Dated paleosols and peat layers represent intervals when the zone was dominated by a mild, moist summer monsoon climate that favored pedogenesis and peat accumulation. Brief intervals of enhanced eolian activity that resulted in the deposition of loess and eolian sand were times when strengthened winter monsoon conditions produced a colder, drier climate. The monsoon variations correlate closely with variations in North Atlantic drift-ice tracers that represent episodic advection of drift ice and cold polar surface water southward and eastward into warmer subpolar water. The correspondence of these records over the full span of Holocene time implies a close relationship between North Atlantic climate and the monsoon climate of central China.     

Characteristics of tin-streaming channels on Dartmoor,UK

The remnants of former tin-streaming channels on the North Teign River, Dartmoor have been investigated using techniques adopted from palaeohydrology, sedimentology, and environmental chemistry. the pattern of multiple streaming channels in the lower reaches, and a single channel in the upper reaches coincides with a reduction of potential stream power from 250 Wm⁻² at a drainage area of 8.7 km². the streaming sections were engineered to maximize the efficiency to separate cassiterite (SnO₂) from its gravel matrix, and the palaeohydraulic properties of channels suggests that sorting was most efficient for the sand and fine gravel size ranges. This was confirmed when the residual tin content of streaming-sediments was assessed. Residual levels of tin were found only in the silt fraction (<63 μm).     

On the causes of 18O-depletion and 18O/16O homogenization during regional metamorphism; the East Humboldt Range core complex,Nevada

 Previous stable isotope studies at Lizzies Basin revealed that metasedimentary rocks are ¹⁸O-depleted relative to protolith values, particularly in the lower parts of the section (Lower Zone) where the rocks are also isotopically homogeneous on a scale of hundreds of meters (quartz δ¹⁸O=+9.0 to +9.6 per mil). In contrast, metasedimentary rocks at higher levels at Lizzies Basin (Upper Zone) are less ¹⁸O-depleted and more heterogeneous in δ¹⁸O. In order to understand more fully the isotopic evolution of this terrane, a series of detailed, meter-scale traverses across various metamorphic and igneous lithologies were completed at Lizzies Basin, and at the structurally higher Angel Lake locality. Traverses in the Lizzies Basin Lower Zone and in the lower parts of Angel Lake (Angel Lake Lower Sequence) across various silicate lithologies, including abundant granitoids, reveal similar degrees of homogeneity, although the average δ¹⁸O values are higher at Angel Lake. In contrast, traverses which include substantial thicknesses of marble and calc-silicate gneiss and very little granitoid have more heterogeneous quartz δ¹⁸O values (+11.9 to +13.4 per mil), and also have a higher average δ¹⁸O (+12.9 per mil), than observed elsewhere. The scale of ¹⁸O/¹⁶O homogeneity in quartz observed at Lizzies Basin and Angel Lake (meters to hundreds of meters) requires fluid-mediated isotope exchange, which accompanied Tertiary metamorphism. There is a correlation between the degree of ¹⁸O-depletion in metasedimentary rocks, ¹⁸O/¹⁶O homogenization between lithologies, and the proportion of granitoids (leucogranites in particular) within any part of the section, and a corresponding anticorrelation with the proportion of marble. This points to a causal relationship, whereby the leucogranites (as well as the Tertiary hornblende diorite and biotite monzogranite) acted as both a relatively low-¹⁸O reservoir and a source of fluids to enhance exchange, while the marbles hindered isotope depletion and homogenization by acting as relatively high-¹⁸O reservoirs and impermeable layers. Material balance calculations help delineate the plausible mechanisms of exchange between granitoids and metasediments. Single-pass infiltration of magmatic fluids from the granitoids is not capable of reproducing all of the observations. Fluid-mediated exchange by convective recirculation of magmatic fluids on a scale of meters is the mechanism which explains all of the observations. The generalized model for the isotopic evolution of the East Humboldt Range core complex provides an excellent opportunity to establish the main causes and controlling factors of ¹⁸O-depletion and ¹⁸O/¹⁶O homogenization during regional metamorphism. Received: 27 July 1993 / Accepted: 1 July 1994     

Optical system research of multi-object fiber spectroscopic survey telescope

In the first part of this paper,four different Cassegrain optical systems with their correctors are designed and studied for multi-object fiber slit spectroscopic survey.The aperture in 6.5 m and field of view 3°are taken for these optical systems.Assuming observation wavelength range is 0.365–0.95μm,the maximum zenith distance for observing is 60°,the maximum diameter of these lenses is 1.66 m,the altitude of the telescope site is 2500 m,two correctors are composed of 4-piece lenses and the other two are 5-piece lenses.The results obtained are:f-ratio about 3.7,the image quality for all four systems with EE80D≤0.60′′,the linear diameter of the focal surface is about 1.2 m and 11 000 fibers can be set on it.Considering the limit of size of fused silica and optical glass,the maximum diameter for lens is about 1.7 m.Such a 6.5 m telescope is about the largest one if using the above correctors.Considering the multiobject spectroscopic survey is greatly important,we also studied some telescope optical systems having their aperture near or larger than 10 m used for the multi-object fiber spectroscopic survey.Such ideas are introduced in the last section of this paper.