Surface features of the Salt Basin of Lancaster County, Nebraska

Catena 34 Ž1999. 243–275

Surface features of the Salt Basin of Lancaster County, Nebraska R.M. Joeckel a

a,b,)

, B. Ang Clement

c

Department of Natural Sciences, BelleÕue UniÕersity, 1000 GalÕin Road South, BelleÕue, NE 68005-3098, USA b UniÕersity of Nebraska State Museum, Lincoln, NE, USA c Biology Department, Doane College, 1014 Boswell AÕenue, Crete, NE, 68333-2430, USA Received 26 January 1998; revised 8 September 1998; accepted 16 September 1998

Abstract The Salt Basin of Lancaster County, Nebraska is distinguished by the presence of ephemeral to semi-permanent saline wetlands, salt flats, surface accumulations of salt, zones of bacterial sulfate reduction in wetland soils and stream sediments, and soil cryptogam layers. Salt flat soils are unique in the region and have laminated surface horizons, which probably result from a combination of soil crusting, salt crusting, and microbial binding of grains, with vesicular horizons characteristic of desert soils directly underneath. Soil-surface salt accumulations are dominated by halite ŽNaCl. and contain minor amounts of thenardite ŽNa 2 SO4 .; they range in morphology from thin, powdery, and very transient efflorescences to thicker, more persistent, soil-cementing crusts. The salt crusts form by the upward wicking of Naq- and Cly-dominated groundwaters and their subsequent surface evaporation. Although it has been largely ignored by geologists for over a century, the Salt Basin can now be viewed as an unusual occurrence of inland sebkhas. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Salt; Halite; Soils; Groundwater; Nebraska

1. Introduction Inland salt springs Ž‘salt licks’. in the eastern half of the United States were a significant local to regional source of commercial salt well into the 19th century, and have long been perceived as unique features on the physical landscape. During the period 1750–1830, salt works were established from New York to Illinois ŽMulthauf, )

Corresponding author. Tel.: q1-402-293-3733; Fax: q1-402-293-2023; E-mail: [email protected] 0341-8162r99r$19.00 q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 4 1 - 8 1 6 2 Ž 9 8 . 0 0 1 1 4 - 3

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1996., and a keen interest in salt occurring at the land surface subsequently followed the later westward expansion of the United States across the Missouri River. Salt springs and efflorescences in the Salt Basin of Lancaster County, Nebraska ŽFig. 1. were known to Euramericans as early as 1835, and by the late 1850s they had become the main impetus for the settlement of the area ŽMorton, 1906; Brown et al., 1980; McKee, 1984.. Economic exploitation of the Salt Basin failed within 30 years of Euramerican settlement, but salt maintains prominence in the identity and folklore of the region ŽMcKee, 1984.. A brief discussion in Hayden’s report on a geological reconnaissance of Nebraska ŽHayden, 1872. is the only geologic description made of the Salt Basin in nearly pristine condition, and since then it has been discussed in even less detail in only a few publications. No comprehensive scientific description of the Salt Basin was ever published. This paucity of basic information was the original impetus for the present study, yet the Salt Basin should be of broader interest to pedologists and geologists for at least three reasons. First, the geology, pedology, and geomicrobiology of inland groundwater-related salt accumulations have received less attention than have perennial or ephemeral saline lakes. There are many published accounts of lacustrine evaporites, including some from the Great Plains of western North America Že.g., Last, 1989a,b, 1993., but there are relatively few accounts of surface salt accumulations like those found in the Salt Basin Že.g., Driessen and Schoorl, 1973; Skarie et al., 1987; Mees and Stoops, 1991; Smoot and Castens-Seidell, 1994.. Second, most studies of surficial salt accumulations have been in semiarid and arid regions, particularly in the subtropics, but the Salt Basin has a subhumid climate Ž680–690 mm rainfall. and lies just above 408 N latitude, farther north than other well-known surface occurrences of salt near the eastern margin of the Great Plains Že.g., Johnson, 1988.. Also, the longitudinal position Ž96843X . of the Salt Basin is far eastward from the major surface occurrences of evaporite precipitation in central North America. Third, the overwhelming dominance of halite in Salt Basin evaporites differs from many other groundwater-derived surface deposits, which typically contain several salts and are not dominated by halite, or lack halite altogether Že.g., Skarie et al., 1987; Mees and Stoops, 1991; Kohut and Dudas, 1993.. The Salt Basin presents an interesting methodological challenge because it lies within a major regional population center ŽFig. 1. and has been affected by intense human land use for over a century. Thus, both modern scientific data and historical observations suggesting pre- or early-settlement conditions must be used to formulate an accurate interpretation. The purposes of this study are to: Ž1. examine the macromorphology and micromorphology of surface accumulations of salts in the Salt Basin; Ž2. describe the pedological, chemical, and biological features associated with surface accumulations of salt; Ž3. interpret the origins of salts crusts, salt flats, and saline wetlands using modern scientific data and historical accounts; and Ž4. compare pedological features of the Salt Basin with other soils in the surrounding area and with similar surface salt occurrences worldwide. 1.1. Geologic and geographic setting Nineteenth-century accounts recognized three main areas of surface salt accumulations in the Salt Basin: the Chester Basin Žnow Salt Lake or Capitol Beach Lake within

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Fig. 1. Study area in Lancaster County, Nebraska, showing Salt Basin, defined here as the area of greatest manifestation of saline wetlands and springs and salt flats along Salt Creek and its tributaries.

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the city limits of Lincoln; Figs. 1 and 2., the Lancaster Basin Žnow Oak Lake and environs, also in Lincoln; Figs. 1 and 3., and the Kanosha Basin, a name applied to salt flats in the valley of Little Salt Creek, immediately north of Lincoln ŽHayden, 1872; Morton, 1906.. The Kanosha Basin is not as clearly defined in early writings as are the other two basins, but it almost certainly included the area of modern Arbor Lake ŽFig. 3.. These three large basins all lie below the 1150 ft Ž350 m. MSL contour, on the lowest parts of the local landscape. In addition to the Chester, Lancaster, and Kanosha basins, many smaller areas of salt flats and saline wetlands occurred in historic times in central to northcentral Lancaster County, from Lincoln to Waverly on the floodplain of Salt Creek and on the immediately adjoining floodplains of the terminal reaches of tributary streams, particularly Oak Creek, Middle Creek, and Little Salt Creek ŽBurgess and Worthen, 1908; see Fig. 1.. Small wetlands with lower salinities are present elsewhere in Lancaster County and in adjacent parts of Saunders County to the north and northeast ŽNebraska Game and Parks Commission, 1991.. Most of the salt flats and wetlands in the Salt Basin were eventually converted for urban, residential, or recreational uses, and are now extinct or severely degraded. In their pre-settlement condition, the salt basins were usually dry and the

Fig. 2. Historical Chester Basin ŽCB. and Lancaster Basin ŽLB. in modern Lincoln, Nebraska Žsee Fig. 1 for general location., showing partial flooding of former salt basins by man-made recreational lakes ŽOak Lake and Salt Lake.. Early-settlement position, names, and extent of salt basins from Morton Ž1906..

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Fig. 3. Arbor Lake and environs Žsee Fig. 1 for general location., showing salt flats in 1980 and salt flats in 1908 according to soil survey by Burgess and Worthen Ž1908.. Salt flats in valley of Little Salt Creek are probably Kanosha Basin of historical literature.

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wetlands probably experienced seasonal changes in water level, but since 1895 many of the existing wetlands and parts of all three of the original salt basins were impounded to retain surface water. All of the salt basins described in the early literature lie atop Holocene alluvial deposits of the DeForest Formation ŽMandel and Bettis, 1995., which fill the valleys of the major streams in the region, the Drift Hills physiographic region of Condra Ž1934.. The stratigraphically highest of these alluvial deposits clearly postdate the original Euramerican settlement of the area in 1856 ŽMandel and Bettis, 1995.. The valleys are cut in sandstones and mudrocks of the Dakota Formation ŽCretaceous.; pre-Illinoian tills, Pleistocene fluvial deposits, and Wisconsinan loesses of the Gilman Canyon and Peoria Formations form the valley walls of local drainages ŽMandel and Bettis, 1995.. The ultimate source of the salts accumulating in the Salt Basin has never been firmly established, but two common interpretations are: Ž1. the leaching of glacial sediments in the northern and western parts of the county drained by the major streams Že.g., Burgess and Worthen, 1908. and Ž2. intrusion of saline waters from Cretaceous marine shales to the west Že.g., Mechling, 1931., andror the local bedrock of the Cretaceous Dakota Formation. No local deposits of rock salt or other evaporites have been found. It is plausible that sulfate ion could have been concentrated in Salt Basin groundwaters by the oxidation and leaching of pyrite-bearing glacial tills underlying adjacent uplands, but the origin of high chloride ion concentrations remains problematic, and perhaps can only be explained by the leakage of saline groundwaters from Cretaceous sedimentary rocks. 2. Materials and methods Measurements of pH and Eh were made in the field with a KClrAgCl electrode Žcalibrated with standards., and electrical conductivities of soil pastes, surface waters, and groundwaters were measured with a drop salinity meter. Iron sulfides were detected in the field with 1 N HCl, ferric iron with a–aX dipiridyl dye ŽChilds, 1981; Soil Survey Staff, 1992.. Groundwater chlorides were field tested with 1 M AgNO 3 and groundwater sulfates with 1 M BaCl 2 . The presence of sulfate-reducing bacteria in soil materials was verified by culturing samples in the laboratory using methods outlined by Postgate Ž1984.. Salt crust samples were powdered and analyzed by X-ray diffraction ŽXRD. using back-loading powder mounts. Soil samples were examined under the scanning electron microscope ŽSEM. after coating with Pt–Pd, and biological materials were fixed with OsO4 before coating and SEM examination. Color aerial photos from the Farm Service Agency and the Lancaster County Engineer Žboth in Lincoln, NE. were used to trace recent changes in salt flats and saline wetlands. Water samples were analyzed at the Soil and Plant Analysis Laboratory of the Institute of Agriculture and Natural Resources at the University of Nebraska-Lincoln. 3. Observations: characteristics of the Salt Basin Four distinctive features characterize the Salt Basin region around Lincoln: Ž1. salt flats and patches of salty ground alongside drainages, where salt accumulates at the

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surface; Ž2. ephemeral to semipermanent wetlands, which are generally rimmed by salt flats that lie slightly higher ŽF 2 m. than the high-water surface of the wetlands; Ž3. saline springs and seeps, which were noted even in the first government land survey of 1856–1857; and Ž4. zones of sulfate reduction Žblack sulfidic materials or BSM. in soil profiles beneath the standing water of ephemeral wetlands, in creek sediments, and around saline springs and seeps. 3.1. Salt flats On the modern Salt Basin landscape, salt occurs on tracts of land ranging in area from a few tens of square meters to a few tens of hectares. At the onset of European settlement, individual tracts of salt-covered land were possibly as large as 240 ha in area ŽMorton, 1906, p. 286.. The largest salt flats are on 0–2% slopes, but salt also occurs as smaller patches alongside or within the banks of streams that have become entrenched since Euramerican settlement, namely Salt Creek, Oak Creek, and Little Salt Creek. All of the salt lands in the Salt Basin have been altered directly, usually severely, by land use practices since 1856. Water tables beneath the salt flats lie at depths of 45–120 cm, but most commonly at 90–120 cm. Electrical conductivity in salt flat soils may be as high as 16.4 mmho cmy1 , and generally increases upwards to the soil surface. In comparison, almost all other soils in the region have electrical conductivities of - 2 mmho cmy1 ŽBrown et al., 1980.. The pH of salt flat and marsh soils ranges from mildly alkaline to very strongly alkaline, pH 7.3 to 9.4, but it is almost always moderately alkaline to very strongly alkaline Ž8–9.4.. Other surface soils in the region have pH values - 7.8, and range from moderate acid to mildly alkaline in reaction ŽBrown et al., 1980.. Individual salt flats in the modern Salt Basin range from 0.5 to 30 ha in area, but vary considerably in area from year to year ŽFig. 4.. They are dominated by silty clay loam to silt loam soils, which are usually mapped as Cumulic Haplaquolls of the Salmo Series ŽBrown et al., 1980.. Although human-accelerated erosion or scalping have removed the uppermost layers of the soil over large areas, natural physical and biological processes on salt flats have modified even disturbed horizons and leave a strong imprint characteristic of local pedogenic and geologic conditions. The salt flats are further characterized by various distinctive features. Vertical desiccation cracks and soil polygons 2–10 cm wide appear on the salt flats, particularly in the summer. Desiccation cracks are regularly filled in by sheet wash and by soil swelling after rainfall events, but traces of them are visible throughout the summer and into the fall. Even when cracks have filled or swollen shut, the faint outlines of gently domed ŽF 5 mm high. polygons are still visible on the surface. Salt persists in and around desiccation cracks even after a rainfall event, because of the mechanical concentration of salt crystals by sheetwash andror enhanced capillary movement of groundwater up the cracks. Ant mounds and the spoil piles from tiger beetle larvae burrows Žeach - 10 cm in diameter. are prominent periodically throughout the summer. The upward movement of soil material by these animals buries surface salt crusts by as much as 1 cm per event and thereby contributes to the near-surface cycling of salts. Earthworm casts were not

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Fig. 4. Areas covered by salt flats at Arbor Lake from September 1980 to August 1996, as measured from soil conservation airphotos. Salt flat areas measured in a 1955 airphoto are considerably greater than the 1981 value.

identified and no earthworms were encountered in soil sampling on salt flats, presumably because of high soil salinity. Occasionally, very low ŽG 5 mm high., elongate Ž; 0.5–5 m. ridges with wavelengths of 4–8 cm are visible on the larger salt flats. These features have never been observed in the process of formation, but given their dimensions and their appearance after episodes of persistent winds, it seems likely that they are wind adhesion ripples produced shortly after rainfall events. Raindrop impressions are preserved in abundance on salt flats for days up to a few weeks because the soil surface is denuded and because a very thin layer of dry, loose, light-colored silt is present at the surface. A characteristic surface ridge-and-valley microtopography on salt flats is produced by the wetting and drying of near-surface cyanobacterial layers. The upper 7–10 cm of salt flat soil profiles ŽFig. 5. consist almost entirely of medium to coarse silt and typically contain the following salient horizons, in descending order: Ž1. a laminated surface horizon, which is usually 1–1.5 cm thick, and contains a thin subsurface cryptogam layer containing cyanobacteria; Ž2. a horizon containing up to 20% Žby volume. vesicles with circular cross-sections, each 1–3 mm in diameter; Ž3. a

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Fig. 5. Soil microhorizonation in A horizons of salt flat soils on east salt flats of Arbor Lake Žsee Fig. 3.. Note disappearance of laminar horizon and vesicular horizon under continuous vascular plant cover.

horizon containing up to 20% Žby volume. ovoid to prolate, horizontal Ž‘flattened’. vesicles up to 12 mm long, a few fine Ž3–5 mm in diameter. burrows and moderate fine, platy soil structure; and Ž4. a horizon dominated by moderate, medium platy soil structure and lacking vesicles. Horizons 3 and 4 both show significant mixing of lighter Že.g., 2.5 YR 6r2. surface soil with darker Že.g., 10 YR 3r2. subsurface soil, apparently through insect bioturbation. The presence of vesicular horizons in salt flats soils is apparently unique in eastern Nebraska, if not in a larger area of the American Midwest. There are distinguishing patterns in soil structure and micro-horizonation in salt flat soils, which can change significantly across as little as 2 m moving outward from bare salt flat surfaces to vegetated salt flat margins. Vesicles and lamination disappear completely at the vegetated margins of salt flats, and microprismatic soil structure appears under vegetation ŽFig. 5.. Microprismatic soil structure on salt flats results from the tendency of surface soil under field moisture conditions to fracture readily along conspicuous, fine, vertical cracks up to 10 cm deep, thereby producing vertically-elongate soil aggregates 3–5 mm wide. The SEM observations of surface soil horizons show a variety of features, particularly in laminated horizons. Some samples from the soil surface lack significant amounts of clay, yet other samples from the laminated horizon below the surface show very thin and discontinuous clay coatings on silt grains and some bridging of silt grains by very

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small amounts of clay ŽFig. 6a.. These clay bridges are almost all - 1 mm, but a few reach 2 mm in thickness. Discontinuous to continuous, subhorizontal and undulating, finely-layered, 0.5–8 mm-thick sedimentary microlaminae of well-oriented clay appear within laminated horizons ŽFig. 6b., and the thickest clay accumulations between grains are directly continuous with these laminae, indicating that the clay infiltrated downward before deposition. Although such microstratification by grain size is readily apparent in some samples of laminated horizons under SEM, some of the distinction between laminae visible in hand specimens seems to result from the presence of dead cyanobacterial filaments, which color certain laminae darker than others and no doubt also have a binding effect on soil particles. Living cyanobacterial filaments, and possibly fungal hyphae as well, bind grains just under the soil surface ŽFig. 7.. SEM and field observations indicate that, at field moisture, surface laminated horizons are probably held together weakly by clay bridges, microbial binding, and no doubt capillary water as well, allowing them to undergo deformation upon desiccation. When completely desiccated, these horizons also become weakly cemented with salt. Under the SEM, vesicles in surface soils from the salt flats lack clay coatings. The only difference between soil material around vesicles and that in the remainder of the soil profile is that the silt grains around the walls of vesicles are slightly more closely packed than those of the soil as a whole, particularly the soil surface. 3.2. Surface occurrence of salt Two main types of salt accumulation characterize the salt flats of the Salt Basin: Ž1. white, powdery efflorescences or very thin ŽF 3 mm. crusts of halite ŽNaCl., by far the dominant constituent of Salt Basin salts; and Ž2. dark gray to gray Ž10 YR 4r1–5r1. weakly cemented crusts, which are dominated by halite ŽNaCl., but also contain thenardite ŽNa 2 SO4 . ŽFig. 8.. Both types can be present on the same salt flat, albeit in different positions andror at different times of the year. In addition to the salt accumulations that form at the soil surface, salt is also present below the surface in desiccated soil. This particular form of salt binds soil grains together in the dry state and it is thus referred to as salt cement. Type 1 accumulations are and transient: they can be completely dissolved in a single, moderate ŽF 2.5 cm. rainfall event, yet they can begin to re-form on the same spot within 1 week in the late spring, and within two days in the hottest part of the summer Žmid-July through early September.. Type 2 accumulations are more persistent than type 1 accumulations, and are also considerably thicker Ž5–10 mm.. In type 2 accumulations, precipitated salts serve as a weak cement in the uppermost layers of the soil, typically near the margins of salt flats, binding soil particles into a crust. Type 2 accumulations form coherent crusts with rumpled and roughened upper surfaces consisting of 3–5 mm wide tuberosities and intervening shallow pits, yielding a total surface relief of up to 4 mm; the undersides of these crusts are smoother and may incorporate many fine Ž- 1 mm. vesicles. The morphology of type 2 accumulations Žcrusts. results from: Ž1. their association with a thin cyanobacterial cryptogam layer on the soil surface, which undergoes shrinkage as it periodically desiccates Žsee discussion below.; Ž2. periodic partial dissolution by rainfall; and, possibly, Ž3. bioturbation by ants and beetle larvae,

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Fig. 6. Ža. Scanning electron microscope ŽSEM. photo of thin salt crust Žsc. atop soil surface on salt flat at Arbor Lake. Note abrupt contact Žsmall arrows. between salt crust and underlying silty soil. Small amounts of clay Žcc. partially coat and bridge silt grains. Žb. SEM photo of laminated horizon at top of soil profile from salt flat at Arbor lake, showing at least three discrete, subhorizontal, convoluted layers Ž1, 2, 3. of finely-laminated clay. Note voids Žv. where clay-engulfed silt grains have fallen out of soil matrix. Unnumbered white arrow points near lower right to clay layer that is highly convoluted and cracked.

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Fig. 7. SEM photo of uppermost part of soil profile from west salt flat at Arbor Lake Žsee Fig. 3., showing small Ž1. and large Ž2. filaments representing both fungal hyphae and cyanobacterial filaments. Salt crust Žs. at land surface is at top of photo.

which brings granular soil particles to the surface. Of these factors, the first seems to have the strongest effect. Under the SEM, the morphology of individual salt crystals varies with weather conditions, the type of salt crust, and the growth position of crystals on either the upper or lower surfaces of salt crusts. Individual cubic salt crystals vary in size from less than 0.5 mm to about 50 mm wide, but coalesced crystals form roundish clumps up to about 1000 mm in diameter. On the upper surfaces of crusts Žtype 2., or in efflorescences Žtype 1., the central regions of cubic halite crystals are frequently hollow due to partial dissolution, and some crystals become subhedral as dissolution proceeds around crystal

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Fig. 8. X-ray diffractogram of relatively thick crust Žtype 2 accumulation. from soil surface on west salt flat at Arbor Lake Žsee Fig. 3.. Crust is dominated by halite, with lesser amounts of quartz silt, and minor thenardite.

exteriors. Hollows within crystals create a striking boxwork effect ŽFig. 9a., and margins of salt crystals are incomplete because of dissolution, leaving ‘C’-shaped or ‘L’-shaped remnants. Solution in surficial salt crystals may result from hygroscopy Žabsorption of atmospheric humidity. as well as dissolution by throughflowing meteoric water, as some of the samples examined by SEM were collected between major rains and shortly after the reappearance of efflorescences on salt flat surfaces. On the undersides of undisturbed crusts Žtype 1., salt crystals typically show a complex ‘stairstep’ pattern from the growth of intact 15–30 mm-wide halite crystals rather than boxwork or incomplete crystals ŽFig. 9b.. Very rarely, salt Žpresumably halite. appears in the form of elongate Ž3–10 mm wide and up to 150 mm long. fibrous crystals ŽFig. 10. just beneath the soil surface. Many of these crystals have 408 to 908 bends along their length, and are equivalent to the ‘capillarites’ of some authors Že.g., Sonnenfeld, 1984, 1989., which are produced by crystallization under the pull of capillary action. These elongate crystals have angular to rounded termini and usually contain long grooves parallel to their long axes ŽFig. 10.;

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Fig. 9. SEM photos of upper Ža. and lower Žb. surfaces of a salt crust. Upper surface Ža. shows partially-dissolved halite crystals with voids Žv. creating boxwork effect; light-colored crystal cluster at lower center is probably thenardite. Lower surface Žb. shows intact ‘stairstep’ pattern of intergrown halite crystal faces.

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Fig. 10. SEM photo of fiber-like salt Žpresumably halite. crystals with angular bends Žwhite arrows.. Crystals have grown from soil surface Ž2. upward toward surface salt crust Ž1..

the grooves are probably analogous to the solution hollows visible in cubic halite crystals. Salt cement appears as crusts that engulf and bridge silt grains in dry subsurface soil material. It is generally very thin and appears amorphous at low magnification under the SEM, but closer inspection reveals cubic growth traces, like those seen on the undersides of salt crusts, wherever salt cement is a continuous phase. 3.3. Wetlands Wetlands at Arbor Lake ŽAL. and near a drainage ditch ŽID. along Interstate 80 crossing the old Chester Basin are ephemeral, usually being filled in hours to a few days by overland flow following late spring and summer rainstorms, and subsequently drying up over a period of two to eight weeks in the summer months. The rapidity of their filling and the regularity and near totality of their desiccation is evident from direct observation of the wetlands and from a 16 year Ž1980–1996. sequence of aerial photos at the Farm Service Agency ŽFSA. office in Lincoln, Nebraska. Between May and November 1997, AL was filled twice after major rainstorms and subsequently desiccated. It was then full over a long period from November 1997 until the time of revision of this manuscript in June, 1998, halfway through a wet year. The original Chester Basin at ID was full for a period of a few days only once during the same period. In most of the 1980–1996 FSA photos, taken yearly in the summer Žand after 1986 in the fall as well., AL was dry except for deep channels at its southern end, and the remnant of the old Chester Basin was always dry.

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These observations of the wetlands at AL and ID indicate a direct linkage with runoff and no significant contribution of groundwater discharge to filling. Furthermore, salt accumulates on the floors of wetlands only after they are completely dry Žusually in the mid to late summer. and the wicking of groundwater has begun. Thus, very little salt deposition by wetlands can be attributed to the evaporation of standing surface waters. Rather, as on the salt flats, the capillary rise of groundwater and its evaporation at the surface during periods when the wetlands have dried up are responsible for nearly all of the salt accumulation on the floors of the wetlands. Observations at AL demonstrate that weak and very spotty salt efflorescences Žirregular patches ranging approximately from 10 cm2 to 1 m2 . formed on the floor of the lake only in early September 1997, in the driest part of the summer and after almost all of the lake had disappeared through evaporation, by which time much of the lake floor had been colonized by vascular plants. As long as water stands in the wetlands, however, surface evaporation cannot drive the upward flux of groundwater and dissolved ions. Salinity in standing marsh waters is very low after rainfall events and increases as evaporation proceeds, but most or all of the dissolved ions in the water must derive from the in-situ dissolution of existing salt carried into AL by overland flow through upstream basins. Polygonal cracking Žforming polygons 15–30 cm wide, significantly larger than those found on salt flats. is prominent on marsh floors in the aftermath of summer desiccation. After long periods of desiccation, polygons on the floor of AL develop upturned edges as well as horizontal Žsheet. cracks in the upper 2.5 cm of the underlying soil. Sheet cracks are particularly prominent beside vertical desiccation cracks. The vertical desiccation cracks delineating polygons extend to a maximum depth of 10 cm in the summer. Soil profiles under marshes exhibit some features characteristic of both wetland soils and better-drained soils. When AL is full of water, its floor is covered by a 2–3 mm cryptogam layer underlain by a thin Ž0.5–5 cm., black sulfidic horizon Žsee discussion below., which thickens under deeper, permanently water-filled channels at the south end of the lake. The sulfidic horizon ŽFig. 11. shows no soil structure, and remains massive even after desiccation. Even when the lake is full and the soil profile is saturated, however, the horizons directly beneath the sulfidic zone exhibit moderate to strong fine angular blocky structure. The retention of subsoil structure reflects the highly ephemeral nature of surface-ponded runoff water in AL and its failure to affect subsurface soil properties below 5 cm depth. However, mottling appears in sub-wetland soils at minimum depths of 30–50 cm, indicating the influence of upward-moving groundwater on soil chemistry. Grain corrosion can be seen under SEM in samples of surficial soil from the marsh floor at AL. The corroded grains ŽFig. 12. appear to be feldspars, and their partial dissolution is inferred to result from the transient high pH environment generated in the active sulfidic layer that exists episodically just below the lake bottom when AL is full

Fig. 11. Occurrence of black sulfidic materials ŽBSM. in soil profiles at Ža. I-80 ditch at Salt Lake Žsee Figs. 1, 2, 14. and Žb, c. at Arbor Lake; soil colors in Ža. and Žb. are for wet Žw. materials. After long periods of desiccation, polygonal mudcracks on floor of Arbor Lake Žc. have upturned edges and are associated with horizontal Žsheet. cracks in the underlying soil.

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Fig. 12. SEM photo of corroded grain from ephemeral BSM zone underneath cryptogam mat on floor of Arbor Lake, wherein high pH values Žup to 10. were recorded. Grain is partially surrounded by incomplete uncorroded shell Žs..

of water. The corroded grains are imbedded in the cryptogam mat that forms during wet periods and which subsequently desiccates to form a leathery crust. 3.4. Saline springs and seeps The surface appearance of saline groundwater in the Salt Basin attracted the attention of the earliest Euramerican settlers, particularly in the areas of the Chester and Lancaster Basins. Today, saline springs and seeps appear in the banks of natural drainages and man-made ditches that cross salt lands, as well as along the channelized streams that drain the basin ŽSalt, Little Salt, and Oak creeks.; the two best examples encountered in the study were springs associated with man-made drainage improvements on Little Salt Creek near AL and springs or seeps associated with a man-made drainage at ID. Springs and seeps in both of these localities produced enough flow to saturate sands and muds adjacent to them with saline waters, and black sulfidic sediments were generated. 3.5. Black sulfidic materials Black sulfidic materials ŽBSM. are waterlogged sandy to muddy soil horizons or sediments exhibiting active bacterial sulfate ŽSO42y . reduction ŽFigs. 11 and 13.. BSM are possibly the most unexpected feature of the Salt Basin, and are distinguished by the following features: Ž1. direct association with standing water, or with continuous

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Fig. 13. Ža. Occurrence of BSM in colluvialralluvial sediment apron along Little Salt Creek near Arbor Lake Žsee Fig. 3 for location of springs along creek.. Žb. Occurrence of BSM in drainage ditch along I-80 crossing the former Chester Basin Žnow, in part, Salt Lake; see Figs. 2 and 14..

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saturation by throughflowing groundwater from saline seeps and springs; Ž2. an immediate reaction with 1 N HCl, which produces a moderate Ždetectable. to strong Žnauseating. H 2 S odor and accompanying strong effervescence, particularly in the wet state Žthe normal field technique for detecting iron sulfides.; Ž3. weak to strong reaction with a–aX dipiridyl dye ŽChilds, 1981; Soil Survey Staff, 1992., which produces a pinkish stain indicating the presence of Fe ŽII.; Ž4. characteristic black color Žusually N 2r0. in the wet state, which distinguishes them immediately from any other soil materials and sediments at or near the land surface in the region; Ž5. pronounced exothermic reaction with 30% H 2 O 2 , particularly in the wet state, resulting in copious discharge of gas and water vapor, and an immediate color change from black ŽN 2r0 wet. to brown or dark brown Žapproximately 10 YR 4r3–3r3 wet.; Ž6. high pH values, ranging from 7.8 to 12.1 Žcommonly 8–11., and generally low Eh values Žas low as y200 mV. relative to nearby near-surface environments; Ž7. highly labile sulfidic components that oxidize readily when water tables drop or when surface water evaporates, leaving behind only faint, irregular, yellowish brown Že.g., 10 YR 4r4–4r6. mottles attesting to the transformation of FeŽII. in sulfides to FeŽIII. in oxyhydroxides. Not surprisingly Žcf. Cornwell and Morse, 1987., attempts to distinguish discrete iron monosulfide minerals such as mackinawite Že.g., Ivarson and Hallberg, 1976. in BSM by XRD of whole-sediment samples failed. It is very likely, therefore, that the sulfides present in the BSM are

Fig. 14. Possible appearance of BSM as dark areas in Salt Lake Žflooded former Chester Basin. before construction of I-80 Žcurrent route of which is shown., based on July, 1955 airphoto in files of Lancaster County ŽNebraska. County Engineer’s Office. Darkenings in lake may also be due to reflective properties of lake water. Salt Lake is now a residential area, and salt flats shown at margins of lake are no longer present.

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amorphous. Bacterial sulfate reduction in BSM was verified in the laboratory by culturing aliquots of BSM in Medium E of Postgate Ž1984., a culturing medium containing iron and sodium sulfates that is formulated especially for the detection of sulfate reducing bacteria. Cultures were incubated at 308C for 3–5 days, after which time black colonies, direct evidence for sulfate reduction, appeared. At Little Salt Creek near AL, bands of black ŽN 2r0. sulfidic sand and mud and grayish non-sulfidic sand and mud alternate within a thin ŽF 50 cm thick. apron of colluvial and fluvial sediment along the lowest part of the banks of the channelized and entrenched stream. At ID, the drainage ditch into which seeps and springs discharge contains gelatinous, almost thixotropic, black sulfidic mud. The surface sedimentsrsoils at both localities are oxidized to brown Ž10 YR 5r3–4r3. to a depth of 3 mm or less, but there is no outward trace of oxidation below this depth in muds that remain saturated ŽFig. 11.. The thin surficial oxidized zones are always associated with a microbial mat of some kind. Where muds have desiccated, yellowish brown Ž10 YR 5r6–5r8. mottling is present in former sulfidic layers, indicating the oxidation of ferrous iron, the precipitation of iron oxyhydroxides, and the removal of sulfur as the sulfate ion. At ID and in the sediment apron along Little Salt Creek near AL, the slight discharge of groundwater through small ŽF 3 cm in diameter. springs brings BSM to the surface above the oxidized layer, resulting in 1–10 cm-diameter black spots atop the surficial oxidized zone and cryptogam layer. Bird and insect trackways or trails produce a similar effect, exposing underlying BSM for several days after they are made. Black spots seem to persist as long as groundwater discharge continues. A July 1955 airphoto of Salt Lake, taken prior to the construction of Interstate 80 across the western half of the lake, shows distinct, irregularly-shaped darkenings on the floor of the lake under what was then very shallow water ŽFig. 14.. These darkenings could very well be BSM brought to the surface by the discharge of groundwater, in which case they would be compatible with pre-1895 accounts of abundant springs and BSM-producing wetlands in the same area.

4. Cryptogam layers Cryptogam layers develop in diverse habitats in and around salt flats and are integral parts of soil surface morphology and processes. They are subdivided according to location: Ž1. layers developed immediately underneath salt efflorescences on salt flats and at the margins of salt flats, Ž2. crusts forming on the floors of marshes, and Ž3. microbial communities growing in the immediate vicinity of active saline seeps and springs. 4.1. Cryptogam layers underneath salt efflorescences A thin ŽF 1 mm. layer of filamentous cyanobacteria is present just under the soil surface at all of the salt flats studied; this layer is visible as a horizontal green streak in freshly cut soil blocks, and after summer rains the soil surface is bright greenish from rejuvenated cyanobacterial growth. The fungus Cladosporidium was also cultured from

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this layer. In SEM, the cyanobacterial layer under the salt flat surface appears as a mass of filaments binding soil particles together ŽFig. 7.. These filaments are probably both cyanobacterial and fungal in origin. At the AL salt flats, the uppermost 5–15 mm layer of soil under the cyanobacterial zone exhibits up to 15 fine ŽF 1 mm thick. horizontal to strongly convoluted alternating dark- and light-colored layers of clay, silt, and cyanobacterial filaments ŽFig. 15.. This layered horizon may develop in a manner analogous to that of marine stromatolites Že.g., Scoffin, 1987., that is by ongoing microbial growth as sedimentation occurs at the salt flat surface, resulting in sediment trapping and binding. Microbial binding of grains is visible in Fig. 7, and cyanobacteria do episodically grow through to the soil surface on the salt flats. However, sedimentation is minimal and in the form of periodic sheetwash, as opposed to settling of sediment from a permanent water column in the case of aquatic stromatolites. The layering in the uppermost part of salt flat soils may form merely through the phenomenon of abiotic crusting, which is common in bare agricultural and desert soils ŽWest et al., 1992.. The layered horizons of salt flat soils commonly have a stromatolite-like appearance, as their laminae are contorted and folded by the episodic desiccation of the cyanobacterial layer just below the soil surface. The salt flat ‘stromatolites’ are expressed at the soil surface as a series of irregular, pustulous microridges about 0.5 cm high, 2–40 cm wide, and up to several cm long, with rounded crests and wavelengths of 0.5 cm to several cm. In cross-section, the microridges appear as asymmetrical folds, which are presumably created as cyanobacterial mats contract upon desiccation. Desiccation of clay microlayers within laminated horizons may also contribute to this effect. Surface relief appears to be persistent but not absolutely permanent. Its best expression follows multiple wetting and drying cycles. Following summer rains, cyanobacteria colonies are directly visible as small green patches on the soil surface atop the microridges. In contrast, during intervening dry periods the microridges develop a surficial white salt efflorescence and microbial growth is no longer visible at the soil surface.

Fig. 15. Schematic view of ‘stromatolitic’ structure from laminated layer of soil under salt flat at Arbor Lake; sedimentary laminae of soil crust Žsee Fig. 6b. and cyanobacterial layers are deformed by desiccation, forming microridges Žmr. at soil surface. Laminated zone flakes off the soil as coherent crust when dry.

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Fig. 16. SEM photos of cryptogam layers from bottom of Arbor Lake Ža. and from I-80 ditch Žb.. Arbor Lake cryptogam layer Ža. consists mostly of matted, large, cyanobacterial filaments Žc. and included diatoms Žd.. I-80 ditch cryptogam layer Žb. consists of green algal filaments partially surrounded by reddish-brown Žpresumably iron-oxide- or iron-hydroxide-enriched. sheaths Žwhite arrows. which include scattered diatoms and bacteria.

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4.2. Cryptogam layers in other settings Cyanobacteria layers are prominent at or just below the soil surface on the margins of salt flats ŽFig. 5., and may thicken to 2–3 mm. In the presence of a normal cover of well-rooted vascular plants, however, they have no effect on soil surface microtopography. Cryptogam layers on the wetland floor at AL desiccate to form a black ŽN 2r0. leathery crust 1–2 mm thick that persists for months after water disappears from AL. In SEM, this layer is a dense, fibrous mat of large cyanobacterial filaments and diatoms ŽFig. 16a.. While actively growing when the lake is full, this mat directly overlies the thin zone of sulfide reduction ŽBSM. in the soil profile under the marsh floor. It is not known whether a direct relationship exists between the mat and the process of sulfate reduction underneath it, although it is presumed that sulfate-reducing bacteria are sequestered below the mat. Cryptogam layers with more diverse assemblages of microbes are prominent atop zones of BSM around springs and wherever saline groundwaters issue from seeps. Cryptogam layers at ID exceed 10 mm in thickness, grow almost continuously along the line of saline seep discharge, and are dominated by green algae. Algal filaments from

Fig. 17. Chemistry of groundwaters Žcollected at springs and seeps. and surface waters encountered in study.

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the ID cryptogam layer frequently have 0.5–1.5 mm-thick reddish sheaths of iron oxide ŽFig. 16b., which presumably result from the oxidation of Fe 2q concentrated in BSM below the cryptogam mat. 5. Surface water and groundwater chemistry Groundwater samples ŽFigs. 17 and 18a. taken directly from springs near AL and at ID showed very high levels of Naq Ž10,400–17,950 ppm. and Cly Ž15,250–22,600

Fig. 18. Ža. Ranges of electrical conductivity in Salt Basin spring waters, surface waters, and soils. Žb. Patterns of electrical conductivity in soil profiles on east ŽEAL. and west ŽWAL. salt flats, under shallow-water marsh after late-May rain Žsta. 1. and under desiccating marsh in June Žsta. 12. at Arbor Lake. Upward increases in conductivity at salt flats and sta. 12 indicate salt concentration in soils by capillary wicking and surface evaporation.

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ppm. and high electrical conductivity. A sample from a pool of standing water within 1 m of a seep at ID in August 1997 showed even greater Naq and Cly concentrations Ž82,000 ppm and 113,600 ppm, respectively. because of solar evaporation. Ca2q is the next most abundant cation Ž220–320 ppm in unevaporated groundwaters and 1220 ppm in the evaporation-concentrated sample.. SO42y Ž660–840 ppm in unevaporated groundŽ400–770 waters and 7200 ppm in the evaporation-concentrated sample. and HCOy 3 ppm in unevaporated groundwaters and 386 ppm in the evaporation-concentrated sample. have similar ranges as the second and third most abundant anions, though HCOy 3 is not concentrated by evaporation as it probably maintains an equilibrium with atmospheric carbon dioxide. Soils under salt marshes and salt flats show upward increases in electrical conductivity ŽFig. 18b., resulting from the surface evaporation of upward-moving saline groundwaters. Surface waters from the southern Žpermanently filled. end of AL and from Little Salt Creek have the same pattern of abundance of ions in local groundwaters, but ionic concentrations are significantly less ŽFigs. 17 and 18a., indicating the diluting effect of meteoric waters accumulated by overland flow, particularly in AL. Nineteenth-century data for saline springs of the Salt Basin are limited to a few almost anecdotal remarks. These data range from 10 to 30% NaCl ŽAughey, 1880, p. 53; Morton, 1906, pp. 291–293., which would be much greater than the concentrations measured from springs at AL and ID. The reliability of these data is uncertain.

6. Discussion 6.1. Hydrology and behaÕior of the wetlands Early accounts Že.g., Cox, 1888; Morton, 1906. indicate that the Salt Basin wetlands and salt accumulations in and around them were ephemeral in the settlement period, so they are not a product of post-settlement environmental degradation, stream entrenchment, and depression of local water tables Žcf. Nebraska Game and Parks Commission, 1991.. Environmental changes have definitely occurred, and their effects have been ecologically detrimental, but they have not completely recast the hydrologic behavior of the Salt Basin. The most convincing evidence of the originally ephemeral to semipermanent nature of Salt Basin wetlands is the 1895 decision to impound the Chester Basin, which had earlier been artificially filled with saline groundwaters pumped from depth ŽMorton, 1906.. This act created Salt Lake, now a residential development ŽCapitol Beach. within the city limits of Lincoln ŽMcKee, 1984.. The later damming of Arbor Lake and the former Lancaster Basin Žnow Oak Lake. in the early 1900s also attest indirectly to the ephemeral nature of standing waters in low-lying areas of the Salt Basin. Early accounts of the Lancaster, Chester, and Kanosha basins ŽHayden, 1872, pp. 36–37; Aughey, 1880, p. 55; fide Morton, 1906, pp. 275–293. pointedly refer to multiple springs Ž‘hundreds’ according to Aughey, 1880. having marked diurnal changes in flow, sometimes referred to as ‘tidal’ in nature, yet discharging only small quantities of water Ža few gallons per minute.. According to some accounts, the basins supposedly

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filled with water at night, but evaporated completely in the daytime, leaving salt crusts behind. More credible accounts, however, remark that upon first sight from a distance, the salt flats presented only the illusion of standing water. There is no indication that anyone ever actually saw the basins fill with spring water, although diurnal fluctuations in the flow of springs because of evaporation and changes in barometric pressure can occur in regard to both confined and unconfined aquifers ŽFreeze and Cherry, 1979, pp. 229–234.. Rather, as at AL today, the salt basins were filled with water only episodically, and then by overland flow rather than groundwater, and had to be diked to hold water for any extended period of time, or even filled with groundwater pumped from depth. The conspicuous failure of an 1885 attempt at salt concentration in the Chester Basin because of flooding after a major rainfall event ŽMorton, 1906, pp. 288–289. attests to the role of overland flow, rather than groundwater discharge, in creating episodic standing water in the salt basins in early settlement times. 6.2. Origin and interpretation of surface salt accumulations The range of depth to water table in Salt Basin salt flats corresponds closely with those Ž22–145 cm; average of 93 cm. measured by Timpson et al. Ž1986. under groundwater-supplied surface salt efflorescences in agricultural lands of North Dakota. This correspondence, particularly the frequency of water table depth measurements between 90 and 120 cm, reflects physical constraints on the capillary movement of groundwater in soils of particular textures: under ideal conditions, the maximum capillary rise of water in pure silt should be approximately 100 cm ŽHeath, 1989., near the middle of the range of common water table depth observations. Timpson et al. Ž1986. interpreted salt efflorescences in North Dakota soils as the result of leaching in the root zone on high landscape positions, followed by lateral movement of the resulting solute-enriched groundwater, and, finally, by precipitation of salts at the soil surface in low-lying areas via the surface evaporation of groundwater moved upward by capillary action. This and other occurrences of salt efflorescences on the land surface appear to be associated with land use ŽSkarie et al., 1987., particularly intensive irrigation, yet there are several documented cases of salt efflorescences forming in completely natural settings Že.g., Driessen and Schoorl, 1973; Gavish, 1980; Smoot and Castens-Seidell, 1994., as was the case in the Lancaster County salt basins in pre-settlement times. Widespread natural salt efflorescences appear to be rare in subhumid or humid climates, however, making the Lancaster County examples noteworthy. In the Salt Basin, upward increases in soil electrical conductivity under marshes and salt flats Žalso noted by Gavish, 1980., the observation that salt flats are rarely or never covered at all by standing saline waters, and the seasonal nature of salt efflorescences on desiccated marsh floors all provide irrefutable evidence for the persistent generation of salt crusts by capillary movement and the evaporation of groundwater at the land surface, i.e., the ‘per ascensum’ model Že.g., Watson, 1979; Goudie and Pye, 1983; Sonnenfeld, 1989., rather than from the evaporation of standing surface waters. The presence of highly soluble halite on the surface is further characteristic of per ascensum evaporite crystallization; otherwise, leaching would tend to produce a zone of halite crystallization beneath the soil surface near the permanent water table ŽWarren, 1989, pp. 152–153.. Capillary wicking occurs only with subaerial exposure, and therefore Salt

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Basin wetlands receive most of the recharge of their salts by this mechanism after seasonal desiccation only. The description of Lancaster County salt flats provided by Hayden Ž1872. suggests that before Euramerican settlement the actual surface discharge of saline waters was more prominent. Nonetheless, Hayden’s description ŽHayden, 1872, p. 36. of ‘‘depressions in the surface nearly destitute of vegetation and wwithx white incrustations wsicx of salt’’ reflects the same mechanism of capillary wicking and evaporation visible today. Also, the transient nature of salt crusts produced by capillary wicking was indicated in the early accounts Že.g., Cox, 1888, p. 9. of failed seasonal attempts at salt harvesting by individuals or families during the 1860s. The type 2 crusts from Salt Basin salt flats are similar to ‘salcretes’ or salt-cemented crusts described by Pye Ž1980. from coastal sand dunes, but the Salt Basin crusts are thinner, appear in silty soils rather than in dune sands, affect only a very small land area, and form by evaporation of waters introduced by capillary rise, rather than by sea spray. Moreover, the geomorphic role played by Salt Basin salt crusts appears to be minimal, although they do appear to survive multiple rainfall events, and technically they probably warrant the term ‘salcrete’ as soil-cementing phenomena. Other authors Že.g., Glennie and Evans, 1976; Watson, 1979. working elsewhere around the world have noted inland salt crusts up to 75 cm thick cemented by various soluble salts, including halite, sodium sulfate and gypsum. These crusts are typically more long-lived features found in semiarid to arid regions, however, and they are formed by a variety of processes, including evaporation of lacustrine waters, lateral flow of groundwater, and the descent of meteoric waters, as well as the capillary rise of groundwater ŽWatson, 1979; Sonnenfeld, 1989.. 6.3. Significance and comparison of salt flat features The salt flats of the Salt Basin change in areal extent from year to year, but the early accounts, including detailed soil survey maps ŽBurgess and Worthen, 1908. reveal that they have appeared in approximately the same locations throughout recorded Euramerican history. Thus, despite the severe effects of land use and anthropogenic environmental change at the land surface, the spatial pattern of groundwater discharge in the local flow system has not changed. Characteristic surface and subsurface features of salt flats Žand desiccated marsh floors, to some degree., such as desiccation cracks and associated horizontal Žsheet. cracks, fine lamination and cyanobacteria layers, have parallels in modern carbonate-mud marine tidal flats such as those described by Shinn Ž1983. and others from the tropics and subtropics. The features of Salt Basin salt flat soils, such as vesicular horizons, laminated surface crusts, platy subsoil horizons, and subsurface cyanobacterial layers, are also similar to features in many arid-region soils Že.g., Evenari et al., 1974; Figueira and Stoops, 1983.. There are at least three conspicuous genetic similarities between these environments: Ž1. consistent exposure to sun, Ž2. cyclic wetting and drying, Ž3. lack of a widespread vascular plant cover. Laminated horizons at the tops of salt flat soil profiles are morphologically similar to crusts on agricultural soils from a variety of geographic locales ŽWest et al., 1992..

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However, laminated horizons are more natural in origin than soil crusts, which are commonly associated with human land use. The laminated horizons are most similar to sedimentary soil crusts, which form by small-scale sedimentation on the soil surface, although clearly there is also a microbial sediment-binding effect present as well. The laminated layers document soil surface horizon development in the absence of all of the following: widespread vascular plant cover, significant organic matter accumulation, and a diverse soil fauna. The most interesting features of salt flat soils, however, are the conspicuous near-surface vesicular horizons, which resemble the birdseye to laminar fenestral porosity described in muddy marine carbonate tidal flat sediments Že.g., Shinn, 1983; Scoffin, 1987.. They are more clearly analogous, however, to the vesicular A horizons ŽAv horizons. of many Aridisols Že.g., Springer, 1958; Evenari et al., 1974; Brewer, 1976; Figueira and Stoops, 1983; Sullivan and Koppi, 1991; Blank et al., 1996.. The vesicles are unusual in that they are absent from soils adjacent to the Salt Basin salt flats and any others on the surrounding regional landscape. Brown et al. Ž1980. did not describe vesicular horizons in any Lancaster County soils. The abundance of vesicles just below the surface of the salt flat soils, and their tendency to elongate, flatten, and then disappear with depth ŽFig. 5. suggests that they form by soil-surface processes and under minimal overburden pressures. The birdseye fenestral fabrics found in marine tidal carbonate flat muds Že.g., Shinn, 1983; Scoffin, 1987. are usually attributed to gas pockets formed by organic decay, but the vesicles in salt flat soils always appear under cyanobacterial layers ŽFig. 5., rather than within them, so a similar explanation for the soil features is untenable. Furthermore, no evidence was found that would suggest vesicle formation by salt dissolution. Salt crystals were never found as fillings within vesicles, even in soil specimens collected in dry periods. Instead, the vesicles in salt flat soils probably form by repeated wetting and drying of soils and the pressure of trapped air in saturated surface soil ŽMiller, 1971; Evenari et al., 1974; Figueira and Stoops, 1983; Sullivan and Koppi, 1991.. This explanation is supported by the observation of open air bubbles directly at the soil surface on salt flats at AL for short periods of time after major rainfalls or snowmelts have saturated the uppermost part of the soil. Evenari et al. Ž1974. photo 2, made similar observations in soils of the Sinai Desert. The fine platy structure observed below vesicular layers in Salt Basin salt flat soils probably results from cyclic wetting and drying because the same kind of structure was observed by Miller Ž1971. and Figueira and Stoops Ž1983. in wetting–drying experiments with soil materials in the laboratory. 6.4. BSM and sulfate reduction Natural sulfate reduction was never described in earlier accounts of the Salt Basin, probably because previous scientific observations were very superficial. The anecdotal reference to ‘foul black mud’ in Salt Creek, its tributaries, and adjacent wetlands by Brook et al. Ž1892. is likely to represent BSM, however. Also, the original government report on the Salt Basin ŽHawn, 1862, fide Morton, 1906, pp. 291–293. describes the Chester Basin salt springs as originating in an ‘impenetrable morass,’ a description reminiscent of the nearly-fluidized BSM currently present in the ditch at ID. These

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statements almost certainly reflect the long-term existence of BSM and sulfate reduction in the wetlands of the Salt Basin. In fact, microbial sulfate reduction and the precipitation of finely disseminated iron monosulfides is common in alkaline lakes and seeps under semiarid or arid climates worldwide Že.g., Hume, 1925; Abd-el-Malek and Rizk, 1963, Lyons et al., 1994..

7. Conclusions Surface accumulations of salt in the Salt Basin may be either very ephemeral efflorescences of halite or longer-lived crusts containing both halite and thenardite. The morphology of halite crystals in these accumulations varies with position in the crust Župper surface vs. underside. and degree of dissolution by hygroscopy or descending meteoric waters. Laminated horizons and cryptogam layers in salt flat soils suggest an unusual set of processes in comparison with other soils nearby: sheetwash, crust-like microsedimentation, microbial binding, microbial shrinkage, rapid wetting–drying, and vesicle formation seem to dominate in salt flat soils. The formation of vesicular horizons under natural conditions in the subhumid climate of the Salt Basin broadens the pedological understanding of vesicle formation, which has been associated almost exclusively with dry-region soils or particular agricultural soils under irrigation. Microbial groundwater sulfate reduction and iron sulfide precipitation are prominent aspects of the biogeochemistry of the Salt Basin, yet the precipitated iron sulfides are very labile, being subject to rapid and complete oxidation with the decline of standing water levels or the water table. Together, these observations indicate that, despite degradation by human land use for nearly a century and a half, the Salt Basin still has characteristic and highly dynamic biogeochemical and pedological processes. Upward increases in electrical conductivity and other observations suggest capillary wicking of groundwater as the dominant mechanism of deposition of surface salts in the Salt Basin. Therefore, it is appropriate to redefine the Salt Basin salt flats and wetlands as continental sebkhas, that is, natural depressions in which salt accumulates at the surface by the capillary evaporation of groundwater ŽSonnenfeld, 1984, p. 158.. Most inland sebkhas are in semiarid to arid regions, and the Salt Basin sebkhas stand out as an important, isolated example of natural salt accumulation in a subhumid region with a continental climate. The upward movement of salt by capillary rise of groundwater, recycling of salt by rainfall throughflow, and movement of dissolved salt by runoff are periodic processes in the Salt Basin. The regular procession of these events, which is largely determined by the subhumid climate of the area, makes the Salt Basin unique in comparison to the classic examples of inland sebkhas described from drier climates. Unlike salt accumulations at the land surface in drier regions, those in the Salt Basin are entirely ephemeral, forming, disappearing and re-forming on a time scale of days to weeks. Many of the characteristic features of the Salt Basin salt flats, such as salt crusts and desiccation cracking, appear at scales at least an order of magnitude smaller than analogous features in semiarid to arid continental sebkha equivalents, wherein water table depths can also be several times deeper than they are in the Salt Basin ŽSonnenfeld,

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1984; Warren, 1989.. The thicknesses of vesicular layers and underlying platy horizons are, however, very similar in both Salt Basin soils and in analogous soils in arid regions around the world ŽSpringer, 1958; Evenari et al., 1974; Figueira and Stoops, 1983.. Thus, in terms of scale, surface evaporation features are more climate-dependent than subsurface soil features.

Acknowledgements We thank D. Feely ŽCollege of Dentistry, University of Nebraska-Lincoln. for the use of SEM facilities and M.A. Holmes ŽDepartment of Geological Sciences, University of Nebraska-Lincoln. for her assistance in XRD analysis of salt crust specimens. W. Lynn and M. Wilson ŽNational Soil Survey Laboratory, Lincoln, Nebraska. provided us with the use of a drop salinity meter and also gave helpful technical advice. L. Render ŽDepartment of Natural Sciences, Bellevue University. supplied us with soil sampling equipment. Field assistance was provided by J. Clement and A.K. Joeckel. Discussions with M. Thompson ŽDepartment of Agronomy, Iowa State University. and D. Loope ŽDepartment of Geological Sciences, University of Nebraska-Lincoln. were very helpful. We also greatly appreciate the support of J. Scholar, chair of the Department of Natural Sciences, Bellevue University. J. Warren, J. Catt, and P. Sonnenfeld provided useful critiques.

References Abd-el-Malek, Y., Rizk, S.G., 1963. Bacterial sulphate reduction and the development of alkalinity: III. Experiments under natural conditions in the Wadi Natrun. ˆ J. Appl. Bacteriol. 26, 20–26. Aughey, S., 1880. Sketches of the Physical Geography and Geology of Nebraska. Daily Republican Book and Job Office, Omaha, 326 pp. Blank, R.R., Young, J.A., Lugaski, T., 1996. Pedogenesis on talus slopes, the Buckskin Range, Nevada, USA. Geoderma 71, 121–142. Brewer, R., 1976, Fabric and Mineral Analysis of Soils. Robert E. Krieger Publishing, Huntington, New York. Brook, W.E., Porterfield, J.C., Yates, B.C., 1892. The Salt Creek Rectification Scheme. B.C.E. Thesis, University of Nebraska-Lincoln, Lincoln, unpublished handwritten manuscript. Brown, L.E., Quandt, L., Scheinost, S., Wilson, J., Witte, D., Hartung, S., 1980. Soil survey of Lancaster County, Nebraska. United States Department of Agriculture, Soil Conservation Service, in cooperation with University of Nebraska, Conservation and Survey Division, 174 pp. Burgess, J.L., Worthen, E.L., 1908. Soil Survey of Lancaster County, Nebraska. U.S. Department of Agriculture. Government Printing Office, Washington, DC, 24 pp. Childs, C.W., 1981. Field tests for ferrous iron and ferric–organic complexes Žon exchange sites or in water-soluble forms. in soils. Austr. J. Soil Res. 19, 175–180. Condra, G.E., 1934. Geography, Agriculture, Industries of Nebraska. University Publishing, Lincoln, 307 pp. Cornwell, J.C., Morse, J.W., 1987. The characterization of iron sulfide minerals in anoxic marine sediments. Mar. Chem. 22, 193–206. Cox, W.W., 1888. History of Seward County, Nebraska. State Journal, Lincoln, 290 pp. Driessen, P.M., Schoorl, R., 1973. Mineralogy and morphology of salt efflorescences on saline soils in the Great Konya Basin, Turkey. J. Soil Sci. 24, 436–442. Evenari, M., Yaalon, D.H., Gutterman, Y., 1974. Note on soils with vesicular structure in deserts. Zeit. Geomorph. 18, 162–172.

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