Mass spectrometric dating of flowstones from Stump Cross Caverns and Lancaster Hole,Yorkshire: palaeoclimate implications

A terrestrial flowstone sequence 97 cm thick from Stump Cross Caverns, Yorkshire, has been dated by both mass spectrometric and alpha spectrometric ²³⁸U-²³⁴U-²³⁰Th dating, and is demonstrated to have been deposited predominantly during interstadial periods over the last 170 ka, with no growth in periods of glaciation or within the Oxygen Isotope Stage 5 interglacial. Growth also occurs within Oxygen Isotope Stages 4 and 6, the former possibly correlating with interstadials recognised in high-resolution ice and ocean-core records. Comparison with the timing of growth of a mass spectrometrically dated Lancaster Hole flowstone, also from the same region, demonstrates only limited agreement. These differences are due to two factors limiting deposition at Stump Cross: flooding of the cave passage in warm periods, and the development of continuous permafrost or glacier cover in periods of severe climatic deterioration. Deposition at Lancaster Hole was controlled by limitations in ground-water supply rather than flooding of the cave passage. The results presented here demonstrate that in addition to regional palaeoclimate factors, local site conditions may limit speleothem growth. We conclude that in future palaeoclimate studies, the growth record from several coeval speleothem samples must be considered before a regional palaeoclimate interpretation can be made.     

Orbital-scale Asian summer monsoon variations:Paradox and exploration

The Asian summer monsoon(ASM) is a vast climate system, whose variability is critical to the livelihoods of billions of people across the Asian continent. During the past half-century, much progress has been made in understanding variations on a wide range of timescales, yet several significant issues remain unresolved. Of note are two long-standing problems concerning orbital-scale variations of the ASM.(1) Chinese loess magnetic susceptibility records show a persistent glacial-interglacial dominated ~100 kyr(thousand years) periodicity, while the cave oxygen-isotope(δ18 O) records reveal periodicity in an almost pure precession band(~20 kyr periodicity)—the "Chinese 100 kyr problem".(2) ASM records from the Arabian Sea and other oceans surrounding the Asian continent show a significant lag of 8–10 kyr to Northern Hemisphere summer insolation(NHSI), whereas the Asian cave δ18 O records follow NHSI without a significant lag—a discrepancy termed the "sea-land precession-phase paradox". How can we reconcile these differences? Recent and more refined model simulations now provide spatial patterns of rainfall and wind across the precession cycle, revealing distinct regional divergences in the ASM domain, which can well explain a large portion of the disparities between the loess, marine, and cave proxy records. Overall, we also find that the loess, marine, and cave records are indeed complementary rather than incompatible, with each record preferentially describing a certain aspect of ASM dynamics. Our study provides new insight into the understanding of different hydroclimatic proxies and largely reconciles the "Chinese 100 kyr problem" and "sea-land precession-phase paradox".     

Resolving Crustal and Mantle Contributions to Continental Flood Volcanism, Yemen; Constraints from Mineral Oxygen Isotope Data

Oxygen isotope ratios determined by laser fluorination analysison olivine, clinopyroxene and plagioclase separated from 31Oligocene flood basalts and rhyolites from Yemen display smallbut significant variations (5·1–6·2     

Petrogenesis of the Largest Intraplate Volcanic Field on the Arabian Plate (Jordan): a Mixed Lithosphere-Asthenosphere Source Activated by Lithospheric Extension

Miocene to Recent volcanism in northwestern Arabia producedthe largest intraplate volcanic field on the Arabian plate (HarratAsh Shaam, Jordan). The chemically and isotopically diversevolcanic field comprises mafic alkali basalts and basanites.The magmas underwent limited fractional crystallization of ol± cpx ± plag and rare samples have assimilatedup to 20% of Late Proterozoic crust en route to the surface.However, there are subtle Sr–Nd–Pb isotopic variations(⁸⁷Sr/⁸⁶Sr = 0·70305–0·70377, ¹⁴³Nd/¹⁴⁴Nd= 0·51297–0·51285, ²⁰⁶Pb/²⁰⁴Pb = 18·8–19·2),which exhibit marked correlations with major elements, incompatibletrace element ratios and abundances in relatively primitivebasalts (MgO >8·5 wt %), and cannot be explained byfractional crystallization and crustal contamination alone.Instead, the data require polybaric melting of heterogeneoussources. Semi-quantitative melt modelling suggests that thisheterogeneity is the result of small degree melts (2–5%)from spinel- and garnet-facies mantle, inferred to be shallowArabian lithosphere, that mixed with smaller degree melts (<1%)from a predominantly deep garnet-bearing asthenospheric(?) sourcewith ocean island basalt characteristics. The latter may bea ubiquitous part of the asthenosphere but is preferentiallytapped at small degrees of partial melting. Volcanism in Jordanappears to be the result of melting lithospheric mantle in responseto lithospheric extension. With time, thinning of the lithosphereallowed progressively deeper mantle (asthenosphere?) to be activatedand melts from this to mix with the shallower lithospheric mantlemelts. Although Jordanian intraplate volcanism is isotopicallysimilar to examples of Late Cenozoic volcanism throughout theArabian peninsula (Israel, Saudi Arabia), subtle chemical andisotopic differences between Yemen and Jordan intraplate volcanismsuggest that the Afar plume has not been channelled northwestwardsbeneath the Arabian plate and played no role in producing thenorthern Saudi Arabian and Jordan intraplate volcanic fields. KEY WORDS: asthenosphere; intraplate volcanism; Jordan; lithospheric mantle; Sr–Nd–Pb isotopes     

Recrystallization of dolomite: evidence from the Monterey Formation (Miocene), California

Dolomites from the upper calcareous-siliceous member of the Miocene Monterey Formation exposed west of Santa Barbara, California, were analysed for geochemical, isotopic and crystallographic variation. The data clearly document the progressive recrystallization of dolomite during burial diagenesis in marine pore fluids. Recrystallization is recognized by the following compositional and crystallographic variations. Dolomites have decreasing δ¹⁸O and δ¹³C compositions, decreasing Sr contents and increasing Mg contents with increasing burial depths and temperatures from east to west in the study area. δ¹⁸O values vary from 5·3‰ in the east to − 5·5‰ PDB in the west and are interpreted to reflect the greater extent and higher temperature of dolomite recrystallization in the west. δ¹³C values correlate with δ¹⁸O and decrease from 13·6‰ in the east to − 8·7‰ PDB in the west. Sr concentrations correlate positively with δ¹⁸O values and decrease from a mean of 750 ppm in the east to a mean of 250 ppm in the west. Mol% MgCO₃ values inversely correlate with δ¹⁸O values and increase from a minimum of 41·0 in the east to a maximum of 51·4 in the west. Rietveld refinements of powder X-ray diffraction data indicate that the more recrystallized dolomites have more contracted unit cells and increased cation ordering. The fraction of the Ca sites in the dolomites that are occupied by Ca atoms increases slightly with the approach to stoichiometry. The fraction of the Mg sites occupied by Mg atoms strongly correlates with mol% MgCO₃. Even in early diagenetic, non-stoichiometric dolomites, there is little substitution of Mg in Ca sites. During recrystallization, the amount of Mg substituting for Ca in Ca sites decreases even further. Most of the disorder in the least recrystallized, non-stoichiometric dolomites is related to substitution of excess Ca on Mg sites.     

HIGHER-ORDER FACTOR ANALYSIS AND ITS APPLICATION TO TRANSPORT NETWORKS

Owing to difficulties in interpretation, the application of higher-order factor analysis has been limited in geographical research, yet it is particularly appropriate to regionalization problems using transport-network data. A 162-node shortest-path matrix of the Australian internal air network is resolved into a two-higher-order factor system.     

Models for the tectonothermal evolution of the eastern Dalradian of Scotland

Abstract The preserved array of pressures in the eastern Dalradian indicates that considerable syn- to post-metamorphic differential uplift has occurred. This inferred differential uplift suggests that Buchan sillimanite zone rocks originally lay at higher structural levels than presently adjacent cooler kyanite zone rocks to the west. A number of features are believed to coincide with the western margin of the sillimanite zone. These are a maximum in temperature, sharp thermal features, a high strain zone, and a train of metabasites. These features are explained by invoking syn-metamorphic movement between the Buchan sillimanite zone and the kyanite zone to its west, involving some horizontal component of movement. It is suggested that the lateral, now eroded, equivalents of the Buchan area once provided part of the required tectonic thickening for other parts of the Dalradian. Areas surrounding the Buchan area suffered tectonic burial followed by metamorphism during uplift relative to the Buchan area.     

A TEST OF THE RADIATIVE ENERGY BALANCE OF THE SHAW MODEL FOR SNOWCOVER

Snow and ice present interesting challenges to hydrologists. Simulating the radiative balance over snow, which is an important part of surface–atmosphere interactions, is particularly challenging because of the decay in albedo over time and the difficulty in estimating surface temperature and incoming long-wave radiation fluxes. Few models are available that include a comprehensive energy and water balance for cold season conditions. The simultaneous heat and water model (SHAW) is a detailed, physical process model of a vertical, one-dimensional canopy–snow–residue–soil system which integrates the detailed physics of heat and water transfer through a plant canopy, snow, residue and soil into one simultaneous solution. Detailed provisions for metamorphosis of the snowpack are included. The SHAW model was applied to data for one winter/spring season (November to May) on a ploughed field in Minnesota without prior calibration to test the performance of the radiation components. Maximum snow depth during this period was 30 cm. For the nearly 100 days of snowcover, the model accounted for 69% of the variation in net solar radiation, 66% of the variation in incoming long-wave radiation, 87% of the variation in emitted long-wave radiation, 26% of the variation in net long-wave radiation and 55% of the variation in net radiation balance. Mean absolute error in simulated values ranged from 10 W m⁻² for emitted long-wave radiation to 27 W m⁻² for the entire net radiation balance. Mean bias error ranged from 8 W m⁻² for emitted long-wave radiation to −16 W m⁻² for the entire net radiation balance. When the entire 170 days of simulation, which included periods without snowcover, were included in the analysis, the variation in observed values increased greatly. As a result, the variation in observed values accounted for by the model increased to 97, 71, 93, 56 and 94%, respectively, while the mean absolute and mean bias errors in simulated values remained nearly the same. Model modifications and parameter adjustments necessary to improve winter-time simulation were investigated. Simulation results suggest that the SHAW model may be a useful tool in simulating the interactive influences of radiative transfer at the surface–atmosphere interface.