Mid- to late Holocene environmental and climatic changes in New Caledonia, southwest tropical Pacific, inferred from the littoral plain Gouaro-Déva

Multiproxy analysis of three littoral cores from western New Caledonia supports the hypothesis that the main controlling factors of environmental changes are sea-level change, ENSO variability and extra-tropical phenomena, such as the Medieval Warm Period (MWP) marked by a tendency for La Niña-like conditions in the tropical Pacific. The record starts during the late Holocene sea-level rise when the terrestrial vegetation indicated wet and cool conditions. The site was a coastal bay definitely transformed into a freshwater swamp at around 3400 cal yr BP, after the rapid drawdown of sea level to its current level. Sediments and foraminiferal assemblages indicated subsequent episodes of freshwater infillings, emersion or very high-energy conditions, likely related to climatic changes and mostly controlled by ENSO variability. Between 2750 and 2000 cal yr BP, relatively dry and cool climate prevailed, while wetter conditions predominated between ca. 1800 and 900 cal yr BP. The Rhizophoraceae peak between ca. 1080 and 750 cal yr BP, coeval with the MWP, may indicate a global phenomenon. Microcharcoal particles present throughout the record increased after 1500 cal yr BP, suggesting an anthropogenic source. From ca. 750 cal yr BP the appearance of current type of vegetation marks the human impact.     

Oxygen isotope variability in calcite shells of the ostracod Cyprideis torosa in Akyatan Lagoon,Turkey

The euryhaline ostracod Cyprideis torosa lives in Akyatan Lagoon, Turkey, which is exposed to large spatial and seasonal variations in water salinity, δ¹⁸O, and temperature. Hydrogen and oxygen isotope measurements of waters reveal that the large range of salinity (15–80 g L^?1) in the lagoon results from a combination of evaporation and mixing between Mediterranean seawater and Seyhan River input. Round sieve-pore relative abundance in C. torosa provides a robust proxy for water salinity (S) from 15 to 80 g L^?1, according to the equation: S = 161.41 (±4.52) * log₁₀(% rounded pores) ? 94.04 (±3.44) (R² = 0.937; p = 10^?31). Seasonal sampling and isotope analysis of C. torosa in waters of known δ¹⁸O values (?4.7 to +6.9 ‰ V-PDB) and temperatures (15–35 °C) yielded a weak positive correlation (r = 0.71) between 1000 lnα_{(calcite–water)} ‰ V-SMOW) and 10³ * T^?1. Specimens of C. torosa collected during the mild and warm seasons have oxygen isotope compositions close to those of inorganic calcite precipitated in equilibrium with ambient water. The large oxygen-isotope variability observed during any season of the year most likely results from shell calcification in water bodies of highly variable salinity, alkalinity, Mg/Ca and water saturation relative to calcite. Indeed, distinct water bodies in the Akyatan Lagoon are generated by mixing of fresh and marine waters, which are exposed to different evaporation rates at the seasonal scale.     

Species composition and dietary relationships in a brackish shallow water fish assemblage in the Bothnian Sea,Sweden

A nearshore fish assemblage inhabiting a shallow bay in the southern Bothnian Sea, Sweden, with demanding environmental conditions (c. 5‰; >15°C during 4 months 1980 and 1 month 1981) was studied during a two-year period, in 1980 and 1981. Seasonal distribution patterns, dietary relationships, and growth rates were studied in Pungitius pungitius (L.), Pomatoschistus minutus (Pallas.), Gasterosteus aculeatus (L.), Phoxinus phoxinus (L.), Pomatoschistus microps (Krøyer) and Gobius niger L. The structure of the juvenile populations changed both over seasons and years but the adult populations remained constant. Lower water temperature, fewer individuals, lower and delayed fish growth, and lower interspecific food overlaps were found in 1981 compared to 1980. Few significant correlations were found in both years between the following community parameters: diversity, niche width, food overlap, and the proportion of each species in the fish assemblage.According to these results it is suggested that food competition and resource partitioning were of minor importance for the structuring of the fish community in the present area. Rather, the extreme salinity and the fluctuations in temperature regulated the fish populations. One may conclude that the populations of juveniles were more influenced by short-term changes in the environment than those of the adults.     

Climate and environmental change at the end of the Holocene Humid Period: A pollen record off Pakistan

Pollen studies from core SO90-56KA recovered from the Arabian Sea off the Makran Coast (24° 509N, 65° 559E; 695 m depth) show that the end of the Holocene Humid Period, linked to the weakening of Indian monsoon fluxes, took place between 4700 and 4200 BP. Two periods of strong summer monsoon activity are identified between 5400–4200 BP and 2000–1000 BP during which the montane pollen taxa coming from the Himalayas reached the Makran coast due to increased fluvial activity of the Indus River. A contrasting period, dominated by the winter monsoon between 4200 and 2000 BP, is identified based on the presence of pollen taxa from the Baluchistan plateaus. The regional vegetation of the low- and midaltitudes, arid and semiarid, are remarkably stable from 4500 BP to the present.     

The crystal and melt structure of spinel and alumina at high temperature: An in-situ XANES study at the Al and Mg K-edge

We present structural information obtained on spinel and alumina at high temperature (298-2400 K) using in-situ XANES at the Mg and Al K-edges. For spinel, ^[4](Al_x,Mg_1−x)^[6](Al_2−x,Mg_x)O_4, with increasing temperature, a substitution of Mg by Al and Al by Mg in their respective sites is observed. This substitution corresponds to an inversion of the Mg and Al sites. There is a significant change in the Al K-edge spectra between crystal and liquid, which can be attributed to a change of the ^[6]Al normally observed in corundum at room temperature, to a mixture of ^[6]Al-^[4]Al in the liquid state. This conclusion is in good agreement with previous ²⁷Al NMR experiments. Furthermore, both experiments at the Al and Mg K-edges are in good agreement with XANES calculation made using FDMNES code.     

Textural evidence for recent co-seismic circulation of fluids in the Nojima fault zone, Awaji island, Japan

The Hirabayashi borehole (Awaji Island, Japan) was drilled by the Geological Survey of Japan (GSJ) 1 year after the Hyogo-ken Nanbu (Kobe) earthquake (1995, M_JMA=7.2). This has enabled scientists to study the complete sequence of deformation across the active Nojima fault, from undeformed granodiorite to the fault core. In the fault core, different types of gouge and fractures have been observed and can be interpreted in terms of a complex history of faulting and fluid circulation. Above the fault core and within the hanging wall, compacted cataclasites and gouge are cut by fractures which show high apparent porosity and are filled by 5–50 μm euhedral and zoned siderite and ankerite crystals. These carbonate-filled fractures have been observed within a 5.5-m-wide zone above the fault, but are especially abundant in the vicinity (1 m) of the fault. The log-normal crystal size distributions of the siderite and ankerite suggest that they originated by decaying-rate nucleation accompanied by surface-controlled growth in a fluid saturated with respect to these carbonates. These carbonate-filled fractures are interpreted as the result of co-seismic hydraulic fracturing and upward circulation of fluids in the hanging wall of the fault, with the fast nucleation of carbonates attributed to a sudden fluid or CO₂ partial pressure drop due to fracturing. The fractures cut almost all visible structures at a thin section scale, although in some places, the original idiomorphic shape of carbonates is modified by a pressure-solution mechanism or the carbonate-filled fractures are cut and brecciated by very thin gouge zones; these features are attributed to low and high strain-rate mechanisms, respectively. The composition of the present-day groundwater is at near equilibrium or slightly oversaturated with respect to the siderite, calcite, dolomite and rhodochrosite. Taken together, this suggests that these fractures formed very late in the evolution of the fault zone, and may be induced by co-seismic hydraulic fracturing and circulation of a fluid with a similar composition to the present-day groundwater. They are therefore potentially related to recent earthquake activity (<1.2 Ma) on the Nojima fault.     

Early Cretaceous megaspore assemblages from southeastern Australia

Twenty-nine megaspore species including six new taxa (Bacutriletes otwayensis sp. nov.,Erlansonisporites cerebrus sp. nov., Erlansonisporites decisum sp. nov., Hughesisporites coronatus sp. nov., Hughesisporites dettmanniae sp. nov., and Verrutriletes depressus sp. nov.) are documented from Aptian and Albian strata of the Gippsland and Otway basins, southeastern Australia. Together with six taxa known only from underlying Neocomian strata, these megaspores are used to establish four provisional biozones for the Lower Cretaceous that complement existing biostratigraphic schemes based on miospores and plant macrofossils. Megaspores are best represented in silty floodbasin facies and it is likely that the parent plants predominantly occupied moist understorey to fully aquatic habitats on the floodplain. Megaspores are sparsely represented in most other fluvial facies chiefly due to reworking of floodbasin sediments into higher energy channel and crevasse deposits. The relatively high diversity of lycophyte and fern megaspores contrasts with the scarcity of these plant groups in macrofossil assemblages. The megaspore record suggests that heterosporous cryptogams may have been significantly more prominent in the vegetation of this region than previously suggested. Several megaspores from southeastern Australia are closely comparable to forms from India and Argentina indicating broad similarities between Early Cretaceous heterosporous fern and lycophyte communities across Gondwana. These similarities also suggest that megaspores may be useful for inter-continental biostratigraphic correlation.     

Sensitivity of UT1 determined by single-baseline VLBI to atmospheric delay model, terrestrial and celestial reference frames

 At the present time, the daily VLBI observations on the Westford-Wettzell baseline is the only continually running VLBI project for studies of high-frequency Earth rotation variations. An analysis of this experiment with regard to the potential errors in the atmospheric delay model and in adopted celestial and terrestrial reference frames is presented in the paper. A new VLBI geometric delay model is applied and an algorithm for global adjustment for this specific single-baseline VLBI developed. The results over three years show discrepancies at the milliarcsecond level between the daily observations and the adopted atmospheric model as well as the combined celestial reference frame. A significant number of these discrepancies are removed by the global adjustment. Received: 19 August 1996; Accepted: 13 September 1996