T.C. Atkinson

Diffuse flow and conduit flow in limestone terrain in the Mendip Hills, Somerset (Great Britain)

Abstract The hydrogeology of the karstic Carboniferous Limestone is described. Water tracing has established recharge areas for fifteen major springs and water budgets confirm the size of the areas found. Groundwater flow occurs in two modes: turbulent conduit flow and diffuse Darcian flow in fine fractures. Recharge is 50% quickflow via caves and closed depressions and 50% slower percolation. Active storage in the diffuse component ( S = 0.92%) is 30 times greater than in phreatic conduits. Diffuse hydraulic conductivity is 0.89 m day −1 and an average of 60–80% of groundwater is transmitted by conduits in this maturely karsted and steeply dipping aquifer.

Carbon dioxide in the atmosphere of the unsaturated zone: An important control of groundwater hardness in limestones

Weekly analyses of springwaters in the Mendip Hills, England, show constant PCO2. Soil air PCO2 (measured directly and calculated from soil water analyses) flucturates seasonally and is almost always less than PCO2 of springs. A source of CO2 in the unsaturated zone is proposed to account for this discrepancy, supported by direct measurements of CO2 in fractures in cave walls and analyses of drip waters. PCO2 increases with depth and values at the bottom of caves are similar to spring waters. A possible source is decay of down-washed soil organic matter.

Rates of tectonic uplift in the Corinth and Megara Basins, central Greece

We present evidence of the rates of late Quaternary tectonic uplift that have affected parts of central Greece during this period of active extension and basin formation. Dual lines of evidence indicate upper Pleistocene to Holocene uplift rates of the order of 0.3 mm yr−1 for the Corinth and Megara basins. First, U-series disequilibrium ages of Acropora sp. corals from uplifted marine sediments are used to derive minimum average rates of vertical displacement since deposition. Second, the geometries of cyclical transgressive sequences and of erosional terraces are consistent with the radiometric evidence and the known history of late Quaternary sea level fluctuations. Two types of uplift are distinguished on the basis of structural relationships: (1) fault block rotation about a horizontal axis (tilting) in response to footwall uplift on the active normal fault bounding the Alkyonides Gulf and (2) a more regional uplift which affects the Corinth Basin and Peloponnesos to the south. The results of U-series dating are discussed in the light of initial 234U/238U activity ratios, derived from the coral samples, which are higher than for average marine waters (1.17–1.31 compared to 1.14). These values may be related to a variable freshwater input to the structurally confined Gulf of Corinth or, alternatively, they may reflect a previously higher global 234U/238U activity ratio in marine waters, or post-mortem enrichment in 234U, although the mechanism of the latter is not yet understood.

The implications of groundwater velocity variations on microbial transport and wellhead protection – review of field evidence

Current strategies to protect groundwater sources from microbial contamination (e.g., wellhead protection areas) rely upon natural attenuation of microorganisms between wells or springs and potential sources of contamination and are determined using average (macroscopic) groundwater flow velocities defined by Darcys Law. However, field studies of sewage contamination and microbial transport using deliberately applied tracers provide evidence of groundwater flow paths that permit the transport of microorganisms by rapid, statistically extreme velocities. These paths can be detected because of (i) the high concentrations of bacteria and viruses that enter near-surface environments in sewage or are deliberately applied as tracers (e.g., bacteriophage); and (ii) low detection limits of these microorganisms in water. Such paths must comprise linked microscopic pathways (sub-paths) that are biased toward high groundwater velocities. In media where microorganisms may be excluded from the matrix (pores and fissures), the disparity between the average linear velocity of groundwater flow and flow velocities transporting released or applied microorganisms is intensified. It is critical to recognise the limited protection afforded by source protection measures that disregard rapid, statistically extreme groundwater velocities transporting pathogenic microorganisms, particularly in areas dependent upon untreated groundwater supplies.

Seasonal microclimate control of calcite fabrics, stable isotopes and trace elements in modern speleothem from St Michaels Cave, Gibraltar

Abstract Detailed monitoring of three drip sites in New St Michaels Cave, Gibraltar, reveals a strongly coherent seasonal pattern of dripwater chemistry despite each site having significantly different flow paths and discharge patterns. Calcite saturation is closely linked to regular seasonal variations in cave air pCO2 caused by seasonally reversing ventilation driven by temperature difference between the cave interior and the air outside. A coupled model of CO2 degassing and calcite precipitation links seasonal δ13C variations in coexisting dripwater, cave air CO2 and speleothem calcite to large variations in pCO2 that are driven by cave ventilation. The relationships between stable isotope ratios, Sr/Ca and speleothem fabrics across annually formed calcite laminae are consistent with a degassing–calcite precipitation process in which rapid degassing controls the δ13C of both drip water DIC and calcite whereas a much slower rate of calcite precipitation causes seasonal cycles of Sr in a more complex manner. By demonstrating the causes of laminated speleothem fabrics plus trace element and isotope cycles in modern speleothem from a closely monitored cave, this study provides clear links between the local microclimate and the proxy record provided by speleothem geochemistry. In Gibraltar, low cave air pCO2 in summer is unusual compared to what has been revealed by cave monitoring carried out elsewhere and shows that caution is needed when linking paired speleothem fabrics to specific seasons without knowledge of local processes operating in the cave.

The recovery and isotopic measurement of water from fluid inclusions in speleothems

The isotopic composition of speleothems is a useful palaeoclimatic indicator, but its value would be enhanced if information on the composition of the parent dripwaters could be recovered from fluid inclusions in the speleothem calcite. To develop a robust method for extracting and measuring oxygen and hydrogen isotopic composition of fluid inclusions we have used anhydrous Iceland Spar and microlitre glass capillaries of water as an analogue system. Crushing the capillary and calcite together in a high vacuum cell we have investigated the adsorbtive and isotopic behaviour of water when exposed to clean fracture surfaces. Significant water adsorption occurs at room temperature, accompanied by large negative isotopic shifts of both oxygen and hydrogen in the recovered foe water at H2O/CaCO3 mass ratios C for 60 min, totally desorbs the water and allows retrieval of the correct isotopic composition. Application of these methods to a British Late Holocene speleothem yields delta O-18 and delta H-2 compositions for the inclusion water which are closely comparable with the modern cave dripwaters and local precipitation. The results show that isotopic compositions can be recovered from inclusion samples of L (equivalent to approximately Ig of calcite) with precisions that are useful for palaeoclimatic research, +/-0.4% for delta O-18 and +/-3% for delta H-2. Greater precision than this will require replicate analysis for each speleothem growth increment. Copyright (C) 2001 Elsevier Science Ltd.

Dating of late Pleistocene interglacial and interstadial periods in the United Kingdom from speleothem growth frequency

Abstract The growth of speleothems is indicative of interglacial and interstadial conditions in the United Kingdom, since their growth is dependent on two factors. First, the occurrence of significant diffuse groundwater recharge and, second, the biogenic production of carbon dioxide in the soil, both are dependent on temperature and water availability. The growth frequency of speleothems is examined using a cumulative distributed error frequency method applied to 341 uncontaminated uranium-series age determinations. The curves derived are shown to be statistically stable, and the ages of the peaks are interpreted as the best estimates of the ages of interglacial and interstadial periods. Ten such periods are recognized during the interval 220,000 to 20,000 yr B.P., considerably more than are currently recognized in the UK pollen- and coleoptera-based Quaternary stratigraphy. Correlations between the speleothem growth frequency peaks and last interglacial (Ipswichian) sites can be made, but correlations with last glacial (Devensian) interstadial sites are limited because of the paucity of dates. The speleothem growth frequency record provides a well-dated terrestrial chronology for the past 140,000 yr B.P., which directly reflects regional palaeoclimatic conditions in Britain, and should prove very useful in unravelling the complex stratigraphy of the Devensian and Ipswichian stages.

Matrilocality during the prehistoric transition to agriculture in Thailand

Stable isotopes in teeth are providing important correlations between ancient people and the geographical location of their childhood homes. In an exciting new application, the authors measured the varying signatures of strontium, oxygen and carbon isotopes in the teeth of a sequence of people buried in Thailand during the period of the introduction and intensification of agriculture. Preliminary results point to the arrival of immigrant men, followed by a change in the relationship between the sexes: the women grow up on local food, the men have access to more widespread resources. This perhaps implies a matrilocal system, where forager men raised elsewhere marry into farming communities. It provides a likely antithesis to the social consequences of introducing agriculture into central Europe.

U-series dating of Hoxnian interglacial deposits at Marks Tey, Essex, England

A U-Th dating study was carried out on the authigenic carbonate component of interglacial lake sediments at Marks Tey, Essex, England. Isochron methods were necessary because of the presence of non-carbonate detritus. Calculated dates around the limit of the U-Th method were obtained for two horizons. Error limits were determined by Monte Carlo sampling from the normal distributions of errors in the isotope ratios. Comparison of the resulting empirical distribution functions with the marine oxygen isotope time-scale indicates with 87% confidence that the Marks Tey sediments correlate with OIS 11 or some older stage. As the Marks Tey deposits rest without stratigraphical break on Anglian till, and can be firmly correlated with the Hoxnian interglacial type site at Hoxne, Suffolk, England, this result implies that the Anglian glaciation took place in OIS 12 acid the Hoxnian Stage of the British chronostratigraphy commenced in OIS 11. Copyright (C) 1999 John Wiley & Sons, Ltd.

Geochemistry and radiometric dating of a Middle Pleistocene peat

Uranium, lead, and sulphur data for a Middle Pleistocene interglacial peat deposit from Norfolk, UK, suggest that uptake of these elements was synchronous and confined to a single early diagenetic episode, probably coeval with peat formation. Sulphur isotope data indicate that reducing conditions have been maintained within the deposit throughout its history. Both uranium and lead concentration profiles show a marked discontinuity near the middle of the bed, probably indicating an environmental change, possibly emergence. The lead isotope data are compatible with a single lead component below the discontinuity and two components above. Groundwater is thought to be the dominant source of lead with an additional airfall component present in the upper peat. The uranium and lead concentration profiles below the discontinuity and the sulfur isotope profile throughout the peat support the view that these elements were sequestered from upwelling groundwaters. The organic material is particularly suitable for 230Th238U dating because it contains a negligible allogenic mineral component and very low 232Th activity. A sequence of consistent ages through the peat profiel (mean 317 ± 14 ka) over a wide range of uranium concentrations (7–65 mg g−1, strongly suggests that a discrete, short-lived, uranium-uptake event has been dated and that subsequent differential isotopic migration has not occurred. One sample, from immediately below the discontinuity, has an infinite apparent age, but there is strong evidence for sequestration of uranium from the peat into adjacent wood fragments found along the discontinuity. Calculated initial 234U238U values of 1.2–1.3 support a groundwater origin for the uranium, rather than a marine origin resulting from a subsequent rapid transgression. The very restricted range of U/Pb ratios in the lower part of the peat bed, and the heterogeneity of the initial lead isotopic composition in the upper part, preclude UPb isochron dating. 210Po measurements (as a proxy for 210Pb) also indicate possible post-depositional migration of 222Rn which, if active over a significant period, would bias any UPb age estimate. The 230Th238U ages are consistent with deposition during oxygen isotope Stage 9.