Undersaturated Water Research Articles

Physics of Hydrophobic Cavities

It has been long believed that modern physics fails in explaining the forces measured between hydrophobic surfaces. We show that the dispersion attraction and cavitation between two uncharged strongly hydrophobic surfaces agree with the classical capillarity and Lifshitz theories. The fluorinated macroscopic glass spheres, with contact angles over 90°, show several times weaker van der Waals attraction across water compared to measurements in air. The aqueous film, stable down to distances shorter than 3 nm, breaks on contact. A bridging vacuum cavity rapidly fills with diffusing air. The long-range capillary force is affected by contact angle hysteresis and pressure regulation effects. The dispersion attraction and cavitation are similar in air-supersaturated and undersaturated water; the stability of the bubbles forming on breaking the meniscus critically depends on the allocation of the system with respect to the coexistence boundary.

Modeling the dissolution of settling CaCO3 in the ocean

The production, transport, and dissolution of carbonate shells plays an important role in the global carbon cycle. For instance, these processes affect the ocean's CO32− concentration, which, on glacial‐interglacial timescales, is involved in the regulation of atmospheric pCO2. The saturation state of the ocean with respect to calcium carbonate suggests that any dissolution of calcium carbonate takes place not in the water column but at the seafloor. On the other hand, several authors suggest that a significant part of the dissolution takes place in the upper part of the water column. In the present paper, a simple model is presented to investigate under which circumstances dissolution of calcium carbonate in the water column may be plausible. Our analysis also includes dissolution facilitated by CO2 release from organic carbon remineralization, which may generate a microenvironment that is undersaturated with respect to CaCO3. In marine snow aggregates, however, respiration‐driven dissolution is shown to be insignificant due to the size and settling velocities of the aggregates. With respect to aragonite, dissolution in the water column attributable to the bulk saturation state alone is significant and may well contribute to the alkalinity maximum in the undersaturated intermediate waters of the North Pacific Ocean. The suggested large loss of calcium carbonate dissolved in the water column (60–80% of CaCO3 production), however, cannot be explained by the mechanisms investigated here.

Ba distribution in surface Southern Ocean sediments and export production estimates

We present excess Ba (Baxs) data (i.e., total Ba corrected for lithogenic Ba) for surface sediments from a north‐south transect between the Polar Front Zone and the northern Weddell Gyre in the Atlantic sector and between the Polar Front Zone and the Antarctic continent in the Indian sector. Focus is on two different processes that affect excess Ba accumulation in the sediments: sediment redistribution and excess Ba dissolution. The effect of these processes needs to be corrected for in order to convert accumulation rate into vertical rain rate, the flux component that can be linked to export production. In the Southern Ocean a major process affecting Ba accumulation rate is sediment focusing, which is corrected for using excess 230Th. This correction, however, may not always be straightforward because of boundary scavenging effects. A further major process affecting excess Ba accumulation is barite dissolution during exposure at the sediment‐water column interface. Export production estimates derived from excess 230Th and barite dissolution corrected Baxs accumulation rates (i.e., excess Ba vertical rain rates) are of the same magnitude but generally larger than export production estimates based on water column proxies (234Th‐deficit in the upper water column; particulate excess Ba enrichment in the mesopelagic water column). We believe export production values based on excess Ba vertical rain rate might be overestimated due to inaccurate assessment of the Baxs preservation rate. Barite dissolution has, in general, been taken into account by relating it to exposure time before burial depending on the rate of sediment accumulation. However, the observed decrease of excess Ba content with increasing water column depth (or increasing hydrostatic pressure) illustrates the dependence of barite preservation on degree of saturation in the deep water column in accordance with available thermodynamic data. Therefore correction for barite dissolution would not be appropriate by considering only exposure time of the barite to some uniformly undersaturated deep water but requires also that regional differences in degree of undersatuation be taken into account.

A decision-logic framework for investigating subsidence problems potentially attributable to gypsum karstification

Karst regions, especially gypsum ones, are prone to subsidence; this can cause severe problems in urban areas. However, this subsidence may have causes other than active karstification. A decision-logic framework designed to tackle this issue is presented. It comprises subsidence description identification of causal mechanisms; construction and evaluation of conceptual models; evaluation and parameterization of fundamental processes and development of a management strategy. This framework is applied to an area of active subsidence in the UK underlain by gypsiferous rocks. In this example, particular attention is paid to the evaluation of gypsum dissolution using four criteria: presence of evaporite; presence of undersaturated water; energy to drive water through the system; and an outlet for the water. Gypsum palaeokarst was identified from borehole evidence and contemporary karstification is indicated by groundwaters containing up to 1800 mg/l of dissolved sulphate. Strontium/sulphate ratios enabled the discrimination of gypsum and non-gypsum-derived sulphate ions and correlation with the hydrostratigraphy. Continuous measurement of groundwater levels showed differential potentiometric surfaces between stratigraphical horizons and indicated a complex pattern of groundwater movement. Integration of these data in a physically and chemically based groundwater model, incorporating a void evolution capability, is suggested.

Precipitation and Dissolution of Calcite in a Swiss High Alpine Lake

Water chemistry and sediment trap data from the 18.9-m-deep, high-altitude, hardwater lake Hagelseewli (2339 m asl.) indicates that biogenic induced calcite precipitation occurs in a water depth of 6 to 9 m at a temperature of 4°C. Our data indicates that calcite precipitation takes place as a short pulse that lasts less than 14 d in response to photosynthetic CO2 uptake in late summer probably by cyanobacterial picoplankton. In up to 8.4 times supersaturated water 30 μm large calcite crystals precipitate that show surface features typical for eutrophic lakes (rough surface and enhanced columnar growth). Subsequently, during the long period of ice coverage calcite is completely dissolved in strongly undersaturated bottom waters. Sediment calcite concentrations therefore are below 0.3%, although the comparison of trap fluxes and sediment accumulation rates indicates that they could be as high as 25%. The theoretical annual accumulation of calcite in Hagelseewli is similar in magnitude to a single spring precipitation event in a low-altitude temperate, hardwater lake. In the most recent sediments the occurrence of higher amounts of organic carbon and sulfur indicates increasing bottom water anoxia during the last 30 to 40 yr.

Precipitation and Dissolution of Calcite in a Swiss High Alpine Lake

Water chemistry and sediment trap data from the 18.9-m-deep, high-altitude, hardwater lake Hagelseewli (2339 m asl.) indicates that biogenic induced calcite precipitation occurs in a water depth of 6 to 9 m at a temperature of 4°C. Our data indicates that calcite precipitation takes place as a short pulse that lasts less than 14 d in response to photosynthetic CO2 uptake in late summer probably by cyanobacterial picoplankton. In up to 8.4 times supersaturated water 30 μm large calcite crystals precipitate that show surface features typical for eutrophic lakes (rough surface and enhanced columnar growth). Subsequently, during the long period of ice coverage calcite is completely dissolved in strongly undersaturated bottom waters. Sediment calcite concentrations therefore are below 0.3%, although the comparison of trap fluxes and sediment accumulation rates indicates that they could be as high as 25%. The theoretical annual accumulation of calcite in Hagelseewli is similar in magnitude to a single spring precipitation event in a low-altitude temperate, hardwater lake. In the most recent sediments the occurrence of higher amounts of organic carbon and sulfur indicates increasing bottom water anoxia during the last 30 to 40 yr.

Small‐scale oxygen fluxes and remineralization in sinking aggregates

Sinking aggregates are the major component of the vertical particulate flux in most regions of the ocean. Controlling factors for aggregate remineralization rates and solute exchange with the surrounding water, however, are poorly quantified because of few empirical data. To study the role of flow and diffusion on aggregate remineralization rates, oxygen distributions were mapped within and around aggregates by use of microelectrodes in a flow field similar to that experienced by sinking aggregates. The oxygen distribution was asymmetrical with a wake of undersaturated water at the rear (downstream) of the aggregates. Oxygen concentrations within the aggregates were >80% of air saturation. The diffusive fluxes of oxygen at the aggregate‐water interface were similar along the equator and at the downstream pole for a wide range of different aggregate sources (field‐sampled diatom aggregates, lab‐made diatom aggregates, aggregates formed from freeze‐thawed diatoms, and zooplankton detritus aggregates) measured at various temperatures. Remineralization rates were reaction limited and, hence, determined by substrate quality and quantity rather than by transport‐limited oxygen supply during sedimentation at ambient oxygen concentrations above ~25 µM.

Depth of conduit flow in unconfined carbonate aquifers

The locus of formation of cave conduits in carbonate aquifers is dependent on hydraulic, structural, and solubility factors, and these can facilitate flow deep below the water table. Geothermal heating results in increasing temperatures and decreasing viscosity with depth. This favors deep conduit development for flow paths with lengths >3 km. Steeply dipping strata aid the flow of undersaturated water to depth along bedding planes. These factors indicate that flow deep below the water table should be associated with steep dips and long flow paths. Empirical evidence strongly supports this model and demonstrates that the flow depth of conduits is directly proportional to flow-path length and stratal dip.

Benthic foraminifera and sediment grain size variability at intermediate water depths in the Northeast Atlantic during the late Pliocene–early Pleistocene

Abstract Sedimentological and faunal records from the transitional period marking the onset of widespread northern hemisphere glaciation have been investigated at Ocean Drilling Program Site 984. The late Pliocene interglacial sediments of the northeast Atlantic are carbonate rich and show evidence of vigorous bottom water circulation at intermediate water depths. Contrasting this, the late Pliocene glacial sediments are characterised by carbonate dissolution and slower bottom current velocities. Weak or “leaky” Norwegian Sea overflows, undersaturated with respect to carbonate, influenced this region during the late Pliocene glacials. The early Pleistocene pattern of intermediate water circulation appears to have changed radically in the northeast Atlantic. At this time, interglacial carbonate values and inferred bottom current velocities are low. This suggests slow-flowing, undersaturated Norwegian Sea water bathing the site. The overflow increased during the early Pleistocene interglacials as the exchange between the Atlantic and Norwegian-Greenland Seas improved. The most significant feature of the early Pleistocene glacials is the increase in inferred bottom current velocity. These changes reflect a switch in deep North Atlantic convection to shallower depths during glacial periods, possibly in a manner similar to the increasing contribution of glacial intermediate water to the North Atlantic during the late Pleistocene glacials. Our results suggest that the late Pleistocene climate variability of the North Atlantic is a pervasive feature of the late Pliocene–early Pleistocene record.

The radiocarbon age of calcite dissolving at the sea floor: estimates from pore water data

We measured the radiocarbon content and stable isotopic composition of pore water and bottom water ΣCO 2, sedimentary organic carbon, and CaCO 3 at two sites on the Ceara Rise, one underlying bottom water that is supersaturated with respect to calcite (Site B), the other underlying undersaturated bottom water (Site G). The results were combined with pore water O 2, ΣCO 2, and Ca 2+ profiles (Martin and Sayles, 1996) to estimate the radiocarbon content of the CaCO 3 that is dissolving in the sediment mixed layer. At Site G, the CaCO 3 that is dissolving in the upper 2 cm of the sediments is clearly younger (richer in 14C) than the bulk sedimentary CaCO 3, indicating that nonhomogeneous CaCO 3 dissolution occurs there. The case for nonhomogeneous dissolution is much weaker at the site underlying supersaturated bottom water. The results indicate that nonhomogeneous dissolution occurs in sediments underlying undersaturated bottom water, that the dissolution is rapid relative to the rate of homogenization of the CaCO 3 in the mixed layer by bioturbation, and that the dissolution rate of CaCO 3 decreases as it ages in the sediment mixed layer. The results support the hypothesis, based on solid phase analyses, that the preferential dissolution of young (i.e., radiocarbon-rich) CaCO 3 leads to a pattern of increasing radiocarbon age of mixed-layer CaCO 3 as the degree of undersaturation of bottom water increases (Keir, 1984; Broecker et al., 1991).

Solubility of chlorargyrite (AgCl) in water vapor at elevated temperatures and pressures

The solubility of chlorargyrite (AgCl) in undersaturated water vapor was investigated at temperatures of 300 to 360 oC and pressures up to 180 bars. It was shown that the presence of water vapor increases the concentration of AgCl in the gas (vapor) phase by between 1.5 and 2 orders of magnitude. This phenomenon is attributed to the formation of hydrated gaseous particles. Silver chloride dissolved in water vapor without changing its stoichiometry (congruent dissolution, Ag:Cl = 1:1). On the basis of the experimental data obtained in this study, the process of chlorargyrite dissolution, and the formation of hydrated gaseous particles in water vapor can be described by the reaction: AgCl cryst. +3· H 2 O gas = AgCl·( H 2 O) 3 gas Considering that Ag is coordinated by three molecules of water and one molecule of chlorine in the AgCl · (H 2 O) 3 gas particle, it was assumed that the silver atom is in fourfold coordination. The properties of the AgCl · (H 2 O) 3 particle were refined using ab initio molecular orbital calculations, and the stable geometry of the particle was deduced to have C3 symmetry. The temperature dependence of the equilibrium constant for the reaction controlling the formation of AgCl · (H 2 O) 3 gas is described by the equation: logK ( P=1 bar) =(22.578±5.505)−(0.0255±0.0045)· TK−(11987.6±658.5)/ TK Preliminary calculations suggest that water vapor can transport significant quantities of silver, and that such transport may play an important role in mobilizing silver in natural hydrothermal systems.

On the stability relations of hydrous minerals in water-undersaturated magmas

Reaction of a hydrous mineral with a water-bearing silicate melt is analyzed in terms of the sto- ichiometry of (in)congruent dissolution. Application of the law of mass action provides an explana- tion for the presence of an isobaric thermal maximum that manifests itself in terms of enhanced stability of the phase under conditions of water undersaturation. It is shown that the water content of the melt at which the maximum develops is independent of pressure and water speciation, but is strongly dependent on the identity of other (anhydrous) melt species and upon the stoichiometric numbers of additional solid phases involved in the reaction. Utilizing MELTS, the analysis is applied to the cummingtonite-bearing rhyolitic magmas of the Taupo volcanic zone of New Zealand.

Simulation of the development of karst aquifers: role of the epikarst

The evolution of a karst aquifer is modelled taking into account the karst groundwater flow as well as the dissolution kinetics of calcite. In particular, infiltration of water from the epikarst is simulated which controls the temporal and spatial distribution of recharge to the phreatic zone. The results show that the evolution of karst conduits is initiated in the spring. The existence of preferential flow paths leads to the evolution of highly conductive so-called dendritic cave systems, i.e., single passages which concentrate the flow and drain the catchment. With time, the amount of undersaturated water flowing directly into the conduit system is increased leading to an acceleration of the conduits enlargement. Three phases are identified for the evolution of karst aquifers: (a) an initiation stage; (b) an enlargement stage; and (c) a stagnation phase.

Seasonal patterns of carbonate diagenesis in nearshore terrigenous muds: Relation to spring phytoplankton bloom and temperature

Pore water saturation state with respect to calcite and aragonite minerals in Long Island Sound sediments fluctuates from saturated and near saturated conditions during late fall, to undersaturated during winter, before slowly changing to supersaturated conditions during late spring. Undersaturation occurs during cold, winter periods when lower rates of ΣCO 2 production (low rates of heterotrophic metabolism) and oxidation of reduced minerals such as FeS lower calcium carbonate saturation states. Direct evidence that dissolution of both calcites and aragonite are occurring during this season comes from the simultaneous increases in excess pore water carbonate dissolution products Ca 2+ , F - , and Sr 2+ during periods of pore water undersaturation. Higher ΣCO 2 production rates during warmer periods cause the CO 3 2- concentration to become supersaturated for both calcite and aragonite. ΣCO 2 production is controlled by both temperature and substrate availability so that benthic deposition of organic matter produced during the spring bloom accelerates the seasonal progression of pore waters to supersaturation. These patterns control carbonate dynamics in temperate, nearshore regions, and result in a regularly observed, yearly cycling of calcium carbonate dominated by alternating periods of net dissolution and net precipitation.

Seasonal and vertical variations of sinking particle fluxes in the West Caroline Basin

A sediment trap experiment was carried out in the West Caroline Basin, located in the equatorial western Pacific between influences of the Asian monsoon and the open ocean. Annual mass flux at the shallow trap at Site 1 was 57.10 g m -2 yr -1. Generally, the higher flux of organic matter was associated with higher activities of biogenic opal-producing and carbonate-producing plankton communities. In addition, as the organic matter content increases, the organic carbon/carbonate carbon ratio shows a tendency to increase. Carbonate-producing plankton was predominant during periods 1 and 3 (May to July and November to the beginning of December), which could be due to limited silica supply to the euphotic zone. On the other hand, surface sea water was more nutrient-rich during periods 2 and 4 (August to October and the end of December to April) at Site 1. These high total mass fluxes could be stimulated by wind. The amount of biogenic components collected in the sediment traps and the accumulation in surface sediments at Site 1 could be compared with primary productivity values. Carbonate and biogenic opal fluxes were 99% and 90% less, respectively, in the surface sediments compared to those in the shallow sediment trap. This could be due to the reaction of sinking particles with undersaturated deep sea water just above the sea floor, rather than with the water column during sinking. About 20% of the organic matter was decomposed between the shallow and deep sediment traps and more than 98% between the deep sediment trap and final burial in the surface sediments. The relative amount of organic carbon preserved in surface sediments was about 0.10% of annual primary productivity.

Climatic variability in upper ocean ventilation rates diagnosed using chlorofluorocarbons

The chlorofluorocarbon CFC‐12 (CCl2F2) distributions from two occupations of a meridional hydrographic section in the eastern North Atlantic are used to describe the oceanic penetration of CFCs and change in the integrated ventilation patterns over the five years from 1988 to 1993. The CFC‐12 water‐column inventories increased by 30–40%, despite a slowing atmospheric growth rate (14%), because of continuing uptake by undersaturated subsurface water masses whose response is lagged by the ventilation time‐scales. After removing the long‐term CFC temporal trend using a tracer age based normalization technique, we observe a distinct dipole pattern in upper ocean ventilation, with reduced convection in the subpolar gyre and enhanced production of saline subtropical underwater in 1993. These differences are discussed in relation to interannual variability in atmospheric surface forcing, upper ocean anomalies, and convection patterns associated with the North Atlantic Oscillation.

Mobility and Immobility of Redox-Sensitive Elements in Deep-Sea Turbidites During Shallow Burial

During the first few tens of thousands of years after an organic-rich turbidite is emplaced on an abyssal plain, surficial oxidation of the unit occurs by downwards diffusion of bottom water O2. From work on piston cores, it is known that a suite of redox-sensitive elements form characteristic relocation peaks in such units around the deepest reaches (upper few dm) achieved by oxidation (fossil oxic/post-oxic boundaries). Four individual turbidites now buried at depths of 130-230 m below seafloor (mbsf), obtained by ODP Leg 157 drilling on the Madeira Abyssal Plain (MAP), have been investigated to determine the effects of burial diagenesis on these geochemical signals over 3-7 My. Porewater data indicate that post-oxic conditions persist to around 130 mbsf on the MAP, below which sulphate reduction predominates. All four units have retained high total organic carbon (Corg) contents (∼1.4%) in their lower reaches, despite the fact that bottom waters have previously oxidised this same organic matter in the turbidite tops to ∼0.2%. The apparent inertness of the turbidite Corg contrasts with the reactivity of organic matter observed in hemipelagic sediment from other ODP investigations. In all four cases, low initial CaCO3 contents have been quantitatively removed along with Corg oxidation, with additional dissolution of carbonate by undersaturated bottom waters. Diagenetic fractions of the elements Se, Cd, Sb, Tl, and V are present in highly-localised peaks and show no evidence of migration. In different combinations, the elements Cu, Co, Ni, and Zn appear to have migrated over short (<5 cm) distances in thin (mm) dark-coloured sulphide bands, probably sourced by reduction of a Mn-oxyhydroxide host phase during early (70 ky) burial. Two examples of local diagenetic pyritisation (up to 20 wt% FeS2) are observed in the bodies of the two turbidites buried to ∼130 mbsf, confirming that sulphate reduction is active at this level. This diagenetic pyrite is associated with high concentrations of As, and lesser enrichments of Ni, Zn, Co, Se, Sb, and possibly Mo.

Disrupted coal and carbonate facies within two Pennsylvanian cyclothems, southern Illinois basin, United States

Two cyclothems contain an unusual suite of disrupted fabrics. This study9s interdisciplinary integration of coal maceral and sedimentological information offers significant new interpretations of coal-limestone interchange, both depositionally and diagenetically. Disrupted fabrics within the Herrin Coal and Providence Limestone are interpreted as representing previously undocumented interactive processes between coals and limestones. The Herrin Coal contains brecciated beds that are the result of marine transgression over a coastal-plain mire, representing high-energy conditions at the swamp-beach interface. This interface has rarely been documented in detail. Peat is inherently unstable prior to its conversion to coal, and we document this by interpreting conglomeratic beds in overlying strata. Differential compaction of Herrin peats generated subaqueous debris flows in the overlying Providence Limestone. This mechanism for syndiagenetic and early diagenetic sediment deformation has not been previously recognized. In addition, near-surface and surface processes produced other disrupted fabrics in the Providence Limestone, such as in situ mosaic breccias, nodular fabrics, and calcrete horizons. This study also documents that stratigraphically adjacent, alkaline carbonate sediments were further altered by undersaturated acidic pore waters, derived from compacting peats during early diagenesis. Our interdisciplinary approach to distinguishing and interpreting all of these disrupted fabrics led to a new appraisal of the importance of depositional and postdepositional interactions between coal and limestone. The presence of mire deposits can have a significant influence on the thickness, distribution, and fabrics in overlying strata. In cyclothems, climatic and eustatic changes may control overall depositional patterns, but peat compaction may determine certain facies.

CaCO 3 dissolution in California continental margin sediments: The influence of organic matter remineralization

In situ benthic flux chamber and oxygen microelectrode and shipboard porewater results have been used to quantify sea floor dissolution of CaCO 3 on the continental rise adjacent to central California, USA. The porewater distributions and benthic fluxes of O 2, NO 3 −, TA, Ca 2+, δ 13C, and TIC are interpreted using a numerical simulation of organic matter remineralization and CaCO 3 dissolution in marine sediments. The processes considered in the simulation include: organic matter oxidation by O 2, NO 3, and SO 4 2−; CaCO 3 dissolution and precipitation; HS − and NH 4 + oxidation; and sediment mixing and sediment accumulation. Calculated benthic fluxes of O 2, NO 3 −, TA, TIC, δ 13C, and Ca 2+; porewater concentrations of O 2, NO 3 −, and NH 4 +; and sediment distributions of organic carbon, CaCO 3, excess 210Pb, and 14C agree well with the measurements. Benthic fluxes of alkalinity and inferred CaCO 3 dissolution rates cannot be explained on the basis of dissolution driven solely by bottom water undersaturation. If the influence of metabolically-produced CO 2 is included, benthic fluxes are fully reconciled, however. This is in agreement with benthic chamber Ca 2+ and δ 13C results that independently imply substantial CaCO 3 dissolution in these sediments. The above observations are in contrast to those reported by Jahnke et al. (1994) for the west African continental rise and the western equatorial Pacific where 1-G diagenetic models predict dissolution fluxes larger than observed with benthic flux chambers. We conclude that the extent of metabolic CaCO 3 dissolution may vary regionally. Numerous factors, such as the depth of metabolic CO 2 production and CaCO 3 dissolution kinetics, are known or predicted to influence metabolic dissolution. Among the factors that should be considered in reconciling these observations are: (l) the extent to which sulfate reduction and reoxidation reactions may influence acid-base properties in surface sediments and (2) the total sedimentary CaCO 3 content of the sites that may influence porewater acidity through surface exchange reactions at the mineral surface.

Geotechnical studies associated with decommisioning the strategic petroleum reserve facility at Weeks Island, Louisiana: A case history

The first sinkhole at the Weeks Island Strategic Petroleum Reserve (SPR) site was initially observed in May 1992. Concurrent with the increasing dissolution of salt over the mined oil storage area below, it has gradually enlarged and deepened. Beginning in 1994 and continuing to the present, the injection of saturated brine directly into the sinkhole throat some 76 m beneath the ground surface essentially arrested further dissolution, providing time to make adequate preparation for the safe and orderly transfer of crude oil to other storage facilities. This mitigation measure marked the first time that such a control procedure has been used in salt mining; previously all control has been achieved by either in-mine or from-surface grouting. A second and much smaller sinkhole was noticed in early 1995 on an opposite edge of the SPR mine, but with a very similar geological and mine mechanics setting. Both sinkholes occur where the edges of upper 152 m and lower 213 m mined storage levels are nearly vertically aligned. Such coincidence maximizes the tensional stress development, leading to fracturing in the salt. This cracking takes 20 or more years to develop. The cracks then become flow paths for brine incursion, which after time progress into the mined openings. Undersaturated ground water gradually enlarges the cracks in salt through dissolution, leading to eventual collapse of the overlying sand to form sinkholes. Other geologic conditions may also be secondary factors in controlling both mining extent and sinkhole location.