Shallow Circulation Research Articles

Shallow circulation groundwater – the main type of water containing hazardous radon concentration

Abstract. The main factors affecting the value of 222Rn activity concentration in groundwater are the emanation coefficient of reservoir rocks (Kem), the content of parent 226Ra in these rocks (q), changes in the volume and flow velocity as well as the mixing of various groundwater components in the circulation system. The highest values of 222Rn activity concentration are recorded in groundwaters flowing towards an intake through strongly cracked reservoir rocks undergoing weathering processes. Because of these facts, waters with hazardous radon concentration levels, i.e. containing more than 100 Bq dm−3 222Rn, could be characterised in the way that follows. They are classified as radon waters, high-radon waters and extreme-radon waters. They belong to shallow circulation systems (at less than a few dozen metres below ground level) and are contemporary infiltration waters, i.e. their underground flow time ranges from several fortnights to a few decades. Because of this, these are usually poorly mineralised waters (often below 0.2–0.5 g dm−3). Their resources are renewable, but also vulnerable to contamination. Waters of this type are usually drawn from private intakes, supplying water to one or at most a few households. Due to an increased risk of developing lung tumours, radon should be removed from such waters when still in the intake. To achieve this aim, appropriate legislation should be introduced in many countries.

Distinction between epigenic and hypogenic maze caves

Certain caves formed by dissolution of bedrock have maze patterns composed of closed loops in which many intersecting fractures or pores have enlarged simultaneously. Their origin can be epigenic (by shallow circulation of meteoric groundwater) or hypogenic (by rising groundwater or production of deep-seated solutional aggressiveness). Epigenic mazes form by diffuse infiltration through a permeable insoluble caprock or by floodwater supplied by sinking streams. Most hypogenic caves involve deep sources of aggressiveness. Transverse hypogenic cave origin is a recently proposed concept in which groundwater of mainly meteoric origin rises across strata in the distal portions of large flow systems, to form mazes in soluble rock sandwiched between permeable but insoluble strata. The distinction between maze types is debated and is usually based on examination of diagnostic cave features and relation of caves to their regional setting. In this paper, the principles of mass transfer are applied to clarify the limits of each model, to show how cave origin is related to groundwater discharge, dissolution rate, and time. The results show that diffuse infiltration and floodwater can each form maze caves at geologically feasible rates (typically within 500ka). Transverse hypogenic mazes in limestone, to enlarge significantly within 1Ma, require an unusually high permeability of the non-carbonate beds (generally ≥10−4cm/s), large discharge, and calcite saturation no greater than 90%, which is rare in deep diffuse flow in sedimentary rocks. Deep sources of aggressiveness are usually required. The origin of caves by transverse hypogenic flow is much more favorable in evaporite rocks than in carbonate rocks.

Groundwater flow, transport, and residence times through topography‐driven basins with exponentially decreasing permeability and porosity

In this paper, we investigate the effects of systematic and local heterogeneity on groundwater flow, transport, and residence time distributions (RTDs) of basins where groundwater flow is topography driven. Systematic heterogeneity is represented by an exponentially depth‐decreasing hydraulic conductivity and porosity, and local heterogeneity is represented by the dispersivity. The RTDs for both a simple basin with one flow system and a basin with nested local and regional systems gradually evolve to a power law RTD with more pronounced systematic heterogeneity. Exponential decrease of poromechanical properties enhances shallow circulation and subdues deep and regional flows leading to longer flushing times for the large part of the domain, while the shallower portions flush solutes rapidly. Therefore, deeper basins lead to more persistent and pronounced power law RTDs when the poromechanical properties systematically decrease with depth. Separate contributions to the RTD due to stagnation zones associated with local flow cells and due to deeper immobile zones were identified; each leads to a different tailing behavior. Local heterogeneity slightly enhances the power law RTD by causing the tailing to begin earlier but does not affect the late time portion of the RTD. Systematic depth‐dependent heterogeneity is an important factor controlling the circulation and associated RTDs of subsurface fluids. It contributes significantly to generation of power law RTDs.

Observational Evaluation of a Convective Quasi-Equilibrium View of Monsoons

AbstractIdealized dynamical theories that employ a convective quasi-equilibrium (QE) treatment for the diabatic effects of moist convection have been used to explain the location, intensity, and intraseasonal evolution of monsoons. This paper examines whether observations of the earth’s regional monsoons are consistent with the assumption of QE. It is shown here that in local summer climatologies based on reanalysis data, maxima of free-tropospheric temperature are, indeed, nearly collocated with maxima of subcloud equivalent potential temperature, θeb, in all monsoon regions except the North and South American monsoons. Free-tropospheric temperatures over North Africa also exhibit a strong remote influence from the South Asian monsoon. Consistent with idealized dynamical theories, peak precipitation falls slightly equatorward of the maxima in θeb and free-tropospheric temperature in regions where QE seems to hold.Vertical structures of temperature and wind reveal two types of monsoon circulations. One is the deep, moist baroclinic circulation clearly seen in the South Asian monsoon. The other is of mixed type, with the deep moist circulation superimposed on a shallow dry circulation closely associated with boundary layer temperature gradients. While the existence of a shallow dry circulation has been documented extensively in the North African monsoon, here it is shown to also exist in Australia and southern Africa during the local summer. Analogous to moist QE theories for the deep circulation, the shallow circulation can be viewed in a dry QE framework in which shallow ascent occurs just equatorward of the peak boundary layer potential temperature, θb, providing a unified system where the poleward extents of deep and shallow circulations are bounded by maxima in θeb and θb, respectively.

Walfrid Ekman (1874-1954): Theoretical Oceanographer

The Swedish oceanographer Vagn Walfrid Ekman (1874-1954, see Figure 1) is best known for his work on shallow ocean circulation, especially his 1905 paper ‘On the influence of the Earth's rotation on ocean currents’. My paper examines Ekman's early work in its contemporary context, outlines his later work (after 1923) on horizontal circulation of ocean currents, and ends with a brief account of his contribution to oceanographic instruments, especially current meters.

Spatial and temporal dependence of clouds and their radiative impacts on the large‐scale vertical velocity profile

The structure of tropical large‐scale vertical velocity from the European Centre for Medium‐Range Weather Forecasts Re‐Analysis is compared with simultaneous satellite measurements of precipitation, top of atmosphere radiation, and clouds from the Tropical Rainfall Measuring Mission (TRMM) on timescales ranging from hours to months. The first two empirical orthogonal functions of the vertical velocity profile represent the traditional deep circulation (PC1) and a shallower circulation (PC2) associated with middle‐level divergence. Together they explain 90% of total variance and can distinguish two types of upward and downward motion: “top heavy” and “bottom heavy.” Cloud and radiation budget quantities measured on TRMM have coherent relationships to PC1 and PC2 on all timescales from simultaneous to long‐term means. The relative importance of PC2 is greater on short temporal and small spatial scales. “Top heavy” ascent is associated with deep cloud systems, more intense precipitation, lower outgoing long‐wave radiation, stronger cloud long‐wave forcing, and extensive anvils. Cloud short‐wave forcing depends primarily on PC1, while the net cloud forcing depends more on PC2. High‐thin clouds are less correlated with short‐term variations of the vertical velocity. Shallow precipitation measured by TRMM precipitation radar is associated with “bottom heavy” upward motion. Temporal compositing with respect to intense precipitating events shows that strong upward motions tend to develop in the lower atmosphere first and then change to the more “top heavy” type of upward motion. The associated cloud systems show consistent temporal changes in which high‐thick clouds develop first and extensive anvil clouds develop later. These results suggest that the elevated latent heating from stratiform precipitation and the development of “top heavy” upward motion profiles in the tropics are related to each other. The coherent relationships shown here between large‐scale vertical velocity and independently measured cloud and precipitation data can be used to test the performance of climate models.

Vertical-Mode Decompositions of 2-Day Waves and the Madden–Julian Oscillation

Abstract Vertical structures of 2-day waves and the Madden–Julian oscillation (MJO) are projected onto vertical normal modes for a quiescent tropical troposphere. Three modes capture the gross tropospheric structure of 2-day waves, while only two modes are needed to represent most of the baroclinic structure of the MJO. Deep circulations that project onto the first baroclinic mode are associated with deep cumulonimbus and stratiform rainfall. Shallow circulations that project onto higher wavenumber modes are associated with precipitating shallow cumulus and congestus and stratiform rainfall. For both disturbances the horizontal divergence contributed by shallow modes is an important factor in the column-integrated moist enthalpy budget. These modes converge moist static energy for a time prior to when deep circulations export moist static energy. These results highlight the importance of properly representing the effects of shallow cumulus, congestus, and stratiform precipitation in theories of convectively coupled waves and in atmospheric models.

Numerical simulations of single‐pass hydrothermal convection at mid‐ocean ridges: Effects of the extrusive layer and temperature‐dependent permeability

We develop numerical models of hydrothermal convection at oceanic spreading centers to understand the interplay between deeply circulating high‐temperature hydrothermal fluid and cooler seawater circulating in basalts of the upper crust. We assume the deep circulation follows an idealized single‐pass geometry and consider the effects of the thickness h and permeability of the extrusive layer kext both on the shallow circulation and on the temperature and heat output of the high‐temperature discharge. We also attempt to model the effect of mineral precipitation on mixing in the shallow crust by emplacing a low‐permeability vertical barrier in the extrusives to separate the high‐temperature discharge from the circulation in the extrusives. Finally, we investigate the effects of temperature‐dependent permeability on the mixing scenarios. The results show that maximum discharge temperature Tv is impacted more by the ratio kext/kd, where kd is the permeability of the deep discharge channel, than by h. Generally, high‐temperature discharge (Tv > 250°C) occurs provided kd > kext. In this case, the presence of a low‐permeability barrier further enhances Tv. Low‐temperature discharge (Tv < 150°C) can occur provided kext > 10kd. For systems such as the Galapagos Spreading Center, where vent temperatures are ∼20°C, kext/kd > 104, and the extrusive layer is likely to be thick. The results also suggest that sites of diffuse flow will occur either between high‐temperature vents along the ridge axis or off axis. The chemical composition of the fluid at these distal sites would be seawater, perhaps modified by low‐temperature water‐rock reactions. In contrast, the diffuse flow fluids near high‐temperature vents are mixture of seawater with high‐temperature hydrothermal fluid. Finally, the results show that the 150°C isotherm, which lies nearly horizontally at some distance from the discharge channel, may be within the extrusive layer, near the extrusive‐dike interface, or within the low‐permeability dike layer. This result supports the idea that the seismically defined layer 2A‐2B boundary within the oceanic crust may represent a mineral precipitation front rather than a lithologic boundary.

Onset and role of the Antarctic Circumpolar Current

For some time, onset of the Antarctic Circumpolar Current (ACC) was considered to have caused or stabilised full Antarctic glaciation. Recently, however, the importance of the ACC in this role has been questioned. In order to understand the relationship between the ACC and Antarctic glaciation, and thence the importance of ocean circulation to palaeoclimate, we need to determine the development history of both processes. To this end, we summarise all published estimates of ACC onset. The time of onset, of shallow circulation or deep, is uncertain, whether based on tectonic studies or the interpretation of changes in the sediment record. Two potential final barriers to circumpolar flow have been identified; south of Tasmania and south of South America. The former is well constrained by tectonics and marine geology to before 32 Ma for a deep gap, with a shallow gap in place by 35.5 Ma at the latest. These ages fit nicely with the onset of full Antarctic glaciation at 33–34 Ma, although some workers question the causality. Estimates of the time of opening of the latter range widely, whether based on tectonics or sedimentary geology, from as recently as 6 Ma to as early as 41 Ma, with the gap depth uncertain also. Resolution of the tectonics-based uncertainties by additional survey being most probably both time-consuming and inconclusive, and the geological estimates being open to alternative interpretations, we define an optimal strategy for additional sampling and measurement, designed to resolve the time of onset more certainly, possibly also resolving between deep and shallow opening, and thereby constraining the ACC role. Sample sites would have to be close to likely final barriers, to avoid extraneous influence, and within modern zones of ACC influence, ideally would form a depth transect, and would have continuous, mixed terrigenous and biogenic sections. A wide range of carefully selected parameters would be measured at each.

Caribbean sclerosponge radiocarbon measurements re-interpreted in terms of U/Th age models

Previously unpublished AMS radiocarbon measurements of a sclerosponge from tongue of the ocean (TOTO), Bahamas, as well as preliminary data from an investigation of the radiocarbon records of sclerosponges living at different depths in the adjacent Bahamas basin, Exuma Sound, are interpreted in terms of U-series age models. The data are compared to an existing Caribbean sclerosponge radiocarbon bomb curve measured using standard gas proportional beta counting and used to interpret a 210Pb age model. The Δ14C records from the sclerosponges illustrate a potential for use of radiocarbon both as a tracer of subsurface water masses or as an additional age constraint on recently sampled sclerosponges. By using an independent age model, this study lays the framework for utilizing sclerosponges from different locations in the tropics and subtropics and different depths within their wide depth range (0–250m) to constrain changes in production of subtropical underwater in the Atlantic Ocean. This framework is significant because the proxy approach is necessary to supplement the short and coarse time series being used to constrain variability in the formation of Caribbean subtropical underwater, the return flow of a shallow circulation cell responsible for nearly 10% of the heat transported poleward in the N. Atlantic.

Characteristics of secondary circulations over an inhomogeneous surface simulated with large-eddy simulation

Large-eddy simulation is used to study secondary circulations in the convective boundary layer modulated as a result of horizontally varying surface properties and surface heat fluxes over flat terrain. The presence of heat flux heterogeneity and its alignment with respect to geostrophic wind influences the formation, strength and orientation of organized thermals. Results show boundary-attached roll formation along heat flux maxima in the streamwise direction. The streamwise organization of the updrafts and downdrafts formed downwind of heterogeneities leads to counter-rotating secondary circulations in the crosswind plane. The distribution of resolved-scale pressure deviations shows large pressure gradients in the crosswind plane. Spanwise and vertical velocity variances and heat flux profiles depict considerable spatial variability compared to a homogeneous forest simulation. Secondary circulations are observed for various ambient wind scenarios parallel and perpendicular to heterogeneities. In the presence of increased wind speed, thermals emerging from the heat flux heterogeneity are elongated, and organize along and downwind of large-scale heterogeneity in the streamwise direction. Simulation with a reduced heat flux shows a shallower circulation with a lower aspect ratio. Point measurements of heat flux inside the roll circulation could be overestimated by up to 15–25% compared to a homogeneous case.

The role of freshwater fluxes in the thermohaline circulation: Insights from a laboratory analogue

Regime transitions in the meridional overturning circulation (MOC) and the rate of formation of deep and bottom waters are thought to be sensitive to changes in the freshwater flux at high latitudes. We model convective overturning in the presence of a surface freshwater input using laboratory experiments that are inverted relative to the ocean: we establish an equilibrium circulation forced by differential heating and cooling along the base of a box and perturb this flow by adding a stabilizing saltwater input at the ‘polar’ end of the box. An initially stable layer forms near the source of the salinity anomaly as a ‘polar halocline’. The subsequent circulation is governed largely by the ratio of salinity and thermal buoyancy supply. For small values of this ratio we observe periodic formation and breakdown of the halocline. Larger values of the flux ratio lead to subthermocline intrusions and stable layering laterally throughout the basin, isolating the bulk of the water column from the forcing boundary. The shutdown of deep overturning and formation of a shallow circulation occurs at a salinity buoyancy input of order 0.1 times the rate of loss of thermal buoyancy. This salinity buoyancy is then comparable to the buoyancy that forces the deep sinking plume below the thermocline in steady-state overturning. When the salinity buoyancy flux is removed, the circulation slowly returns to its original state.

Fracture-controlled fluid circulation and dissolutional weathering in sinkhole-prone carbonate rocks from central Italy

Fractures, karstic cavities, and calcite precipitates are analysed on Mesozoic, carbonate strata from the Cornicolani Mountains, central Italy, to quantify the relationships between fractures and related karstic cavities and to infer the fracture-controlled fluid pathways. The study area is characterized by active sinkholes and other karstic caves, which are among the deepest ones in the world. Results show a clear control of fractures on the process of dissolutional weathering and, therefore, on the fluid circulation. A model is proposed, in which two different modes of dissolutional weathering coexist: (i) a mode of diffuse weathering consisting in the dissolutional enlargement of closely spaced joints and bedding surfaces. This process affects the strata situated at depths of up to 5 m; (ii) a mode of more concentrated weathering active along highly permeable fault damage zones at depths of at least 70 m. These processes are mostly connected with the shallow circulation of calcite-dissolving meteoric waters, and are controlled by the disparity of permeability between the fault damage zones and the surrounding jointed strata. Some calcite precipitates along the studied fault damage zones may be connected with a dissolutional process operated by waters that interacted with the adjacent quiescent volcanic district. Such calcite-dissolving waters and the presence of high-permeable fault damage zones, along which the waters may ascent, are probably the cause for the development of some of the deep sinkholes along faults in the study area.

Origin and significance of clay‐coated fractures in mudrock fragments of the SAFOD borehole (Parkfield, California)

The clay mineralogy and texture of rock fragments from the SAFOD borehole at 3067 m and 3436 m measured depth (MD) was investigated by electron microscopy (SEM, TEM) and X‐ray‐diffraction (XRD). The washed and ultrasonically cleaned samples show slickenfiber striations and thin films of Ca‐K bearing smectite that are formed on polished fault surfaces, along freshly opened fractures and within adjacent mineralized veins. The cation composition and hydration behavior of these films differ from the Na‐montmorillonite of the fresh bentonite drilling mud, although there is more similarity with circulated mud recovered from 3479 m MD. We propose that these thin film smectite precipitates formed by natural nucleation and crystal growth during fault creep, probably associated with the shallow circulation of low temperature aqueous fluids along this shallow portion of the San Andreas Fault.

Hydrogeochemical properties of CO2-rich thermal–mineral waters in Kayseri (Central Anatolia), Turkey

The present study highlights the hydrogeological and hydrogeochemical characteristics of the CO2-rich thermal–mineral waters in Kayseri, Turkey. These waters of Dokuzpinar cold spring (DPS) (12–13°C), Yesilhisar mineral spring (YMS) (13–16°C), Acisu mineral spring (ACMS) (20–22.5°C), Tekgoz thermal spring (TGS) (40–41°C), and Bayramhaci thermal-mineral spring (BTMS) (45–46.5°C) have different physical and chemical compositions. The waters are located within the Erciyes basin in the Central Anatolian Crystalline complex consisting of three main rock units. Metamorphic/crystalline rocks occur as the basement, sedimentary rocks of Upper Cretaceous-Quaternary age form the cover, and volcanosedimentary rocks Miocene-Quaternary in age represent the extrusive products of magmatism acting in that period. All these units are covered unconformably by terrace and alluvial deposits, and travertine occurrences have variable permeability. Dokuzpinar cold spring, YMS and ACMS localized mainly along the faults within the region have higher Na+ and Cl− contents whereas TGS and BTMS have higher amounts of Ca2+ and HCO3−. The high concentrations of Ca2+ and HCO3− are mainly related to the high CO2 contents resulting from interactions with carbonate rocks. Whereas the high Na+ content is derived from the alkaline rocks, such as syenite, tuff and basalts, the Cl− is generally connected to the dissolution of the evaporitic sequences. These waters are of meteoric-type. BTMS deviates from meteoric water line. The content is related to the increases in the δ18O compositions due to mineral–water interaction (re-equilibrium) process. CO2-dominated YMS and ACMS with low temperatures have higher mineralizations. Yesilhisar mineral spring, ACMS, TGS and BTMS are oversaturated in terms of calcite, aragonite, dolomite, goethite and hematite, and undersaturated with respect to gypsum, halite and anhydrite. Yesilhisar mineral spring, ACMS and BTMS are also characterized by recent travertine precipitation. Dokuzpinar cold spring is undersaturated in terms of the above minerals. The higher ratios of Ca/Mg and Cl/HCO3, and lower ratios of SO4/Cl in BTMS than TGS suggest that TGS has shallow circulation compared to BTMS, and/or has much more heat-loss enroute the surface. The sequence of hydrogeochemical and isotopic compositions of the waters is in an order of DPS>YMS>ACMS>TGS>BTMS and this suggests a transition period from a shallow circulation to a deep circulation path.

Distribution of B, Cl and Their Isotopes in Pore Waters Separated from Gas Hydrate Potential Areas, Offshore Southwestern Taiwan

Boron (B) and chlorine (Cl) are widely distributed on the Earth's surface and show distinctive geochemical behaviors. Cl behaves rather conservatively in oceanic environments while B is an excess-volatile and its distribution is sensitive to sediment absorption and organic matter degradation. The distribution of B, Cl and their isotopes in pore waters provide useful information for distinguishing between shallow circulation and deep origin fluid sources. Thirty-six sediment cores 0-5 m in length were sampled from a foreland accretionary prism offshore Southwestern Taiwan where strong bottom simulating reflectors (BSRs) and an abundance of mud diapers were discovered. More than 350 pore water samples were separated and analyzed for B, Cl and other major ions. Four long cores were selected for B and Cl isotopic analysis. We found that the Cl in all cores varied less than 10%, suggesting no major hydrate dissolution or formation involvement at shallow depths in the study area. However, the B concentration changed greatly, ranging between 360 and 650 μM, indicating a possible sedimentary contribution during the early diagenesis stage. The B isotopic compositions were relatively depleted (~25 to 37‰) in these pore waters, implying the addition of sedimentary exchangeable B with low δ 11B. The Cl isotopes showed rather large variations, more than 8‰, possibly related to the addition of deep situated fluids. In summary, the chemical and isotopic characteristics of pore waters separated from piston cores off Southwestern Taiwan suggest strong influence from organic matter degradation during diagenesis at shallow depths and the possible addition of deep fluids advecting through mud diapir channels at greater depths, causing a minor degree of hydrate dissolution/formation to occur at shallow depths. Further systematic investigation of pore waters δ^18O and δ D are needed in a future study.

Controls on chemistry during fracture-hosted flow of cold CO 2-bearing mineral waters, Daylesford, Victoria, Australia: Implications for resource protection

Mineral springs from Daylesford, Australia discharge at ambient temperatures, have high CO 2 contents, and effervesce naturally. Mineral waters have high HCO 3 and Na concentrations (up to 4110 and 750 mg/L, respectively) and CO 2 concentrations of 620–2520 mg/L. Calcium and Mg concentrations are 61–250 and 44–215 mg/L, respectively, and Si, Sr, Ba, and Li are the most abundant minor and trace elements. The high P CO2 of these waters promotes mineral dissolution, while maintaining low pH values, and geochemical modelling indicates that the CO 2-rich mineral water must have interacted with both sediments and basalts. Amorphous silica concentrations and silica geothermometry indicate that these waters are unlikely to have been heated above ambient temperatures and therefore reflect shallow circulation on the order of several hundreds of metres. Variations in minor and trace element composition from closely adjacent spring discharges indicate that groundwater flows within relatively isolated fracture networks. The chemical consistency of individual spring discharges over at least 20 a indicates that flow within these fracture networks has remained isolated over long periods. The mineral water resource is at risk from mixing with potentially contaminated surface water and shallow groundwater in the discharge areas. Increased δ 2H values and Cl concentrations, and lower Na concentrations indicate those springs that are most at risk from surface contamination and overpumping. Elevated NO 3 concentrations in a few springs indicate that these springs have already been contaminated during discharge.

Large-Scale Atmospheric Forcing by Southeast Pacific Boundary Layer Clouds: A Regional Model Study*

Abstract A regional model is used to study the radiative effect of boundary layer clouds over the southeast Pacific on large-scale atmosphere circulation during August–October 1999. With the standard settings, the model simulates reasonably well the large-scale circulation over the eastern Pacific, precipitation in the intertropical convergence zone (ITCZ) north of the equator, and marine boundary layer stratocumulus clouds to the south. In a sensitivity experiment with the radiative effect of liquid clouds south of the equator over the eastern Pacific artificially removed, boundary layer clouds south of the equator almost disappear and precipitation in the ITCZ is reduced by 15%–20%, indicating that the stratocumulus clouds over the southeast Pacific have both local and cross-equatorial effects. Examination of the differences between the control and sensitivity experiments indicates that clouds exert a net diabatic cooling in the inversion layer. In response to this cloud-induced cooling, an in situ anomalous high pressure system develops in the boundary layer and an anomalous shallow meridional circulation develops in the lower troposphere over the equatorial eastern Pacific. At the lower branch of this shallow circulation, anomalous boundary layer southerlies blow from the boundary layer high toward the northern ITCZ where the air ascends. An anomalous returning flow (northerly) just above the cloud layer closes the shallow circulation. This low-level anomalous shallow circulation enhances the subsidence over the southeast Pacific above the cloud layer, helping to maintain boundary layer clouds and temperature inversion there. Meanwhile, the strengthened cross-equatorial flow near the surface enhances moisture convergence and convection in the ITCZ north of the equator. This in turn strengthens the local, deep Hadley circulation and hence the large-scale subsidence and boundary layer clouds over the southeast Pacific. This positive feedback therefore enhances the interhemispheric climate asymmetry over the tropical eastern Pacific.

Deposition and recirculation of tritium in the North Pacific Ocean

Tritium data, primarily from the GEOSECS and WOCE cruises of the 1970s and 1990s, are used to estimate the time‐evolving 3H inventory of the North Pacific basin. In the years between the two surveys, there have been changes both laterally and vertically in the distribution of 3H in the North Pacific that reflect the mean circulation and exchanges of the basin. We develop a simple multibox model of the shallow circulation of the North Pacific to explore the long‐term redistribution and changes in 3H inventories within the basin. To do this, we derived a new estimate of the delivery of bomb 3H to the North Pacific by precipitation for the period 1960–1997 and include other minor sources such as rivers. Vapor deposition dominates over direct precipitation of tritium to the basin, while inputs from continental runoff and the inflow from the south contribute over an order of magnitude less. The model predicted tritium budget of 25.1 ± 3.3 kg compares well with the estimated WOCE inventory of 23.4 ± 2.0 kg. We explore in detail the sensitivity of the budget calculations to model circulation and assumptions, as well as uncertainties in observations. We find that the ratio of tritium in vapor to that in precipitation is the most sensitive variable in the model budget, and the basin tritium inventory is consistent with a vapor‐to‐precipitation ratio of 0.67 (range 0.60–0.74), predictably somewhat less than the isotopic equilibrium value of 0.89. An inverse calculation shows that despite uncertainties in the tritium source function, the data also help constrain aspects of the basin circulation, including the Indonesian Throughflow.

Geochemical study of fluids on Lesbos island, Greece

Sixty-five water samples and seven associated gas samples have been collected on Lesbos island. The lithology and structural setting have resulted in two main types of hydrological circulation: a shallow circulation hosting low-salinity cold waters and a deeper one, hosting high-salinity hot waters that often emerge in thermal springs near the coast. The cold waters are characterized by Ca(Mg)-HCO 3(SO 4) composition, while the thermal waters generally have an Na-Cl composition. The chemical features of the former can be explained by their circulation in the ophiolite-bearing phyllitic basement and volcanic rocks. Waters circulating in the ultramafic layers of the basement are richer in Mg than the waters whose circulation is mainly within marble levels or volcanic rocks. The Na-Cl thermal waters are characterized by salinities ranging from 1910 to 35,700 mg/kg. As indicated by previous hydrogeochemical and isotopic studies, the Na-Cl composition of the thermal waters on Lesbos is the result of mixing between shallow meteoric waters and marine waters. While interacting with the minerals of the geothermal reservoir, the saline waters retain the Na/Cl sea water ratio but become enriched in Ca 2+ and depleted in Mg 2+ with respect to sea water. Processes of hydrothermal alteration at depth are activated by a gas phase enriched in CO 2, which reaches the geothermal reservoir by rising along the deep fractures of the basement. Thermodynamic calculations based on hydrothermal alteration processes occurring at the estimated temperatures of the geothermal reservoir (about 120 °C) indicate that the thermal waters of Lesbos are in equilibrium with talc and dolomite.