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Sediment characteristics and mineralogy of salt mounds linked to underground spring activity in the Lop Nor playa, Western China
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Sediment characteristics and mineralogy of salt mounds linked to underground spring activity in the Lop Nor playa, Western China Ma Lichun a,∗ , Tang Qingfeng b , Li Baoguo c , Hu Yufei a , Shang Wenjun a a MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, 100037, China b Beijing Centre for Physical & Chemical Analysis, Beijing 100089, China c College of Resources and Environment, China Agricultural University, Beijing 100193, China
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Article history: Received 8 November 2015 Received in revised form 1 August 2016 Accepted 2 August 2016 Keywords: Lop Nor basin Saline playas Chemical sediment Mineralogy Sediment characteristics Salt mounds Underground spring Brine chemistry
a b s t r a c t Salt mounds are commonly distributed along playa margins and typically comprise alternating layers of loose fine sand and slightly hard halite-rich sediments as a result of long-term underground spring activity. A model of salt mound development was constructed for this study. It suggests that wind-blown sand supply and upward recharge of underground springs are two important factors in salt mound construction. Furthermore, it proposes that salt mound height is mainly controlled by the vertical transport range of underground springs and the thickness of the capillary fringe. A 1.5 m representative profile dug from the center of salt mound LP1 in the Lop Nor playa revealed a fairly complicated mineral assemblage including halite, gypsum, anhydrite, glauberite, epsomite, anhydrite, calcite, bischofite, polyhalite, schoenite, kieserite and carnallite. This matches closely with the assemblage predicted by the EQL/EVP model. The groundwater in the area is highly concentrated brine rich in Cl− and Na+ and poor in Ca2+ , displaying low alkalinity, and containing considerable amounts of SO4 2− , Mg2+ and K+ . Chemical analysis of groundwater revealed considerable variation in the salinity and chemical composition of groundwater over time. The Cs-137 technique was used to measure the accumulated ages of the salt mounds. This method may prove useful in the research of relatively young playa environments where carbon dating techniques are unworkable because of an absence of carbon-rich materials in recent saline sediments. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction Saline playas are widespread in the arid interior of Western China. These landforms commonly develop in topographic lows and regional groundwater discharge zones (Handford, 1982; Rosen, 1991) where shallow groundwater is saline to hyper-saline (Risacher et al., 2003). They are effective hydro-chemical sinks promoting elemental concentration and evaporite formation. The Lop Nor playa is situated at the junction of Xinjiang province and Gansu province in the eastern part of the Tarim Basin—China’s largest endorheic drainage system—with an area of 530,000 km2 (see Fig. 1). The playa itself has a surface area of approximately 20,000 km2 . As the lowest point and sole terminus of the Tarim drainage system (Li et al., 2008), the Lop Nor playa marks the discharge point of the basin’s local and regional ground-water and surface-water flow systems (Ma et al., 2010).
∗ Corresponding author. E-mail address: lichmafl[email protected] (L. Ma).
The present-day Lop Nor is a typical groundwater-discharge playa. It lacks surface inflows and contains concentrated underground brines beneath salt crusts with an extensive area of 5,500 km2 (Ma et al., 2010). Horizontal subsurface inflows from adjacent aquifers in the surrounding alluvial fans help to maintain a high evaporation rate and the overall playa system water balance. Down the regional hydrologic gradient, as groundwater moves from the outer fringes of the alluvial fans to the salt pans, its salinity gradually increase, reaching a maximum of roughly 350 g/l in the center of the playa (Ma et al., 2010). This groundwater commonly emerges as ascending springs along playa margins. Spring activity thus plays an important role in the hydro-chemical and geomorphological evolution of the playa margins. However, very little research has been done on this, especially in this particular area (Ma et al., 2010). There are many salt mounds of different sizes in the marginal areas of the Lop Nor playa. The main objectives of this study are: to describe the physico-chemical and sedimentary properties of these salt mounds, to investigate the process of salt mound formation,
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Table 1 Chemical analysis of underground brine from salt mound LP1 (ionic concentrations are expressed in ppm). 2006
2014
Na+ K+ Ca2+ Mg2+ HCO3 − + CO3 2− Cl− SO4 2−
53985 5113 277 8327 / 112001 28212
108372 8888 176 15732 / 177968 42211
Ratios
2006
2014
0.046 0.074 0.25
0.05 0.088 0.24
+
Fig. 1. Regional map showing the location of Lop Nor Basin and drainage system of Tarim Basin, Xinjiang province, China. XJ, Xinjiang Province; KM, Kuruktage Mountains; BM, Beishan Mountains. Modified from Ma (2010).
and to develop a depositional model for salt mound formation in playa environments. 2. Study area The Lop Nor depression is a Quaternary landform. This basin is in fault contact with the Kuruktage Mountains in the north, the Beishan Mountains in the east, and the Altyn Mountains in the south. To the west, the Tarim River’s old, dry delta gradually merges with the western margin of the Taklamakan Desert (Fig. 1). The study area (39◦ 50 −40◦ 40 N; 90◦ 10 −91◦ 30 E) is situated in the central-eastern sector of the Lop Nor basin, which is characterized in satellite images by alternating black and white concentric rings that closely resemble a “Great Ear” (Fig. 2). The actual surface of the basin is essentially flat, covered by polygonal salt crusts and desiccated saline mudflat deposits at an altitude of approximately 780 m above sea level. The study area is located in the driest part of central Asia, typically receiving an annual rainfall of <20 mm with a potential annual evaporation of >3500 mm. Hence, this is a typical arid continental climatic zone characterized by cold, dry winters, wide diurnal temperature ranges, hot summers with long periods of sunlight and year-round northeasterly winds. The mean annual wind velocity is 5 m/s, and severe sandstorms frequently occur in spring and summer. Wind is a dominant geomorphological agent in this region whose vast, wind-eroded landscapes incorporate meso-yardangs that are widely distributed along the northern, western, and eastern margins of the Lop Nor basin (Fig. 2). Field investigations conducted in the study area between 2007 and 2014 revealed that the playa margins, especially on the eastern side of the “Great Ear” salt pan, are characterized by an extensive distribution of active salt mounds (Zhong et al., 2008). The mounds have heights of 1–2 m and diameters of 3–15 m, with moist surfaces and dome-shaped tops (Fig. 3). They are generally represented by the dark tones in satellite images of the area (Fig. 2). The mounds’ moist surfaces and vertical sedimentary profiles imply underground spring activity. Because field investigations in most parts of the “Great Ear” salt pan are constrained by poor accessibility, a representative salt mound LP1 was selected for this study. It is approximately 1 m high with a diameter of about 12 m. Its location is presented in Fig. 2, and some of its geomorphological features are shown in Fig. 3. 3. Materials and methods This paper describes the mineralogy, petrology and geochemistry of the saline succession of salt mound LP1. Mineralogical identification was carried out by X-ray diffraction and analyses of 16 sediment samples under an environmental electron scan-
−
K /Cl Mg+ /Cl− SO4 2− /Cl−
ning microscope equipped with an energy dispersive spectroscopy (ESEM-EDS). The chemical compositions of both the solid sediments and the underground brines were analyzed in terms of Cl− , SO4 2− , Ca2+ , Mg2+ , K+ , and Na+ by Dionex DX-120 ion chromatography (RSD < 2%) and Perkin Elmer Optima-3300DV inductively coupled plasma optical emission spectrometry (RSD < 1%). The grain-size distribution was analyzed by Malvern Mastersizer 2000 laser diffractometry. Measurements of radioisotope levels (Cs-137 and Pb-210) were undertaken by the Nanjing Institute of Geography & Limnology (Chinese Academy of Sciences). 4. Results and discussion 4.1. Chemical characteristics of underground brine Two underground brine samples were obtained in September 2006 and August 2014, by digging a pit in the center of the salt mound where the groundwater level is encountered approximately 1.3 m below the surface (Fig. 3). Considerable variation was noted between the two samples in terms of salinity and chemical composition. It is possible that this is due to seasonal variation in precipitation and snow-melt recharge, but there are currently insufficient data to allow for rigorous investigation of this variation. The underground brine had a salinity value of 208 g/L in September 2006 and of 353 g/L in August 2014. Both samples were rich in Cl− and Na+ and poor in Ca2+ . They both had low alkalinity values and contained considerable amounts of SO4 2− , Mg2+ and K+ . The chemical characteristics of these underground brines are presented in Table 1. Calculations using the EQL/EVP computer program (Risacher and Clement, 2001) indicate that these underground brines are supersaturated with respect to calcite (CaCO3 ). Simulated evaporation was carried out in equilibrium mode under closed system conditions at 25 ◦ C. Pco2 was set at 10−3.4 using EQL/EVP code. The main minerals precipitated during simulated evaporation were halite (NaCl), bloedite (Na2 SO4 ·MgSO4 ·4H2 O), epsomite (MgSO4 ·7H2 O), hexahydrite (MgSO4 ·6H2 O), kieserite (MgSO4 ·H2 O), kainite (MgSO4 ·KCl·3H2 O), leonite (K2 SO4 ·MgSO4 ·4H2 O), glauberite (NaSO4 ·CaSO4 ), polyhalite (K2 SO4 ·MgSO4 ·2CaSO4 ·2H2 O), and carnallite (MgCl2 ·KCl·7H2 O). The total mass of other evaporite minerals in this system is <1% of the total. Both groundwater samples exhibited similar evaporation evolution pathways and precipitated comparable mineral species. Successive evaporation simulations led to Mg-Cl brines and caused the precipitation of a sequence of minerals of increasingly high solubilities including calcite (CaCO3 ), gypsum (CaSO4 ·2H2 O), glauberite (NaSO4 ·CaSO4 ), hydromagnesite (4MgCO3 ·Mg(OH) 2 ·4H2 O), halite (NaCl), bloedite (Na2 SO4 ·MgSO4 ·4H2 O), polyhalite (K2 SO4 ·MgSO4 ·2CaSO4 ·2H2 O), schoenite (K2 SO4 ·MgSO4 ·6H2 O),
Please cite this article in press as: Ma, L., et al., Sediment characteristics and mineralogy of salt mounds linked to underground spring activity in the Lop Nor playa, Western China. Chemie Erde - Geochemistry (2016), http://dx.doi.org/10.1016/j.chemer.2016.08.001
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Fig. 2. Locations of sampling sites and investigation routes in the “Great Ear” playa of the Lop Nor basin. (Background image: Landsat 5 TM image, 10/23/2006, pixel 30 m, R3, G2, B1).
Fig. 3. Field photograph showing salt mound LP1(a), its characteristic puffy surface (b), its vertical sediment profile (c), and its groundwater level(c).
leonite (K2 SO4 ·MgSO4 ·4H2 O), epsomite (MgSO4 ·7H2 O), kainite (MgSO4 ·KCl·3H2 O), hexahydrite (MgSO4 ·6H2 O), kieserite (MgSO4 ·H2 O) and carnallite (KCl·MgCl2 ·6H2 O). The simulated EQL/EVP results are shown in Fig. 4. 4.2. Sediment characteristics and mineralogy A typical pit of 1.5 m depth was dug to facilitate the studies of the sedimentary characteristics of salt mound LP1. The profile is dominated by an unconsolidated sand layer, but incorporates halite-rich layers deposited by concentrated pore water (Fig. 5). The accumulated salinity in the mound varies from 217 g kg−1 to
708 g kg−1 . The halite-rich layers have a grayish white appearance, varying thicknesses, and occur at depths of 20 cm, 38 cm, 52 cm and 61 cm (see Fig. 5). These layers have salinity values of >500 gkg−1 . The loose sand layer, however, has a salinity value of ∼300 g kg−1 , and presents with a light- to dark-brown tone depending on local water content. Sand grains in the profile are well-sorted and rounded (Fig. 6b), while halite crystals in the matrix are cubic (Fig. 6a, c, i, and j). The skeletal halite-rich layers are comprised of crystals that have merged to form interlocking fabrics within the sandy matrix. The halite is thought to precipitate directly due to capillary evaporation of supersaturated pore waters from the soft, brine-soaked fine-sand
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Fig. 4. Modeled underground brine evolution pathways by the EQL/EVP computer program under closed system conditions at 25 ◦ C, and Pco2 of 10−3.4 . (a) Sampling and analysis in 2006. (b) Sampling and analysis in 2014. The factor of concentration (FC) is defined as the ratio between the initial amount of free water and the amount of water in the solution (Risacher and Clement, 2001).
sediments. The microscopic morphological characteristics of quartz grains were studied by SEM. They have matte surfaces with dishand crescent-shaped concavities (Fig. 6b). These characteristics attest to aeolian weathering processes. Vertical variation in sand grain size distribution was nominal. The matrix was dominated by sand grains with diameters of 0.02–2 mm which constituted 80–90% of the total (Fig. 5). These grains are most likely derived from the aeolian dune and yardang landscapes to the north of the playa. The sandy texture facilitates capillary water transport to the mound surface, and the mound’s resistance to wind erosion is enhanced by the surface efflorescence crust resulting from saline pore-water evaporation of underground brines at the mound surface. The surficial crusts are commonly a few decimeters thick and contain a very complicated mixture of halite, schoenite, bischofite, gypsum, glauberite, calcite, and mostly detrital sediments. They exhibit several diagenetic overprints related to dehydration, dissolution, recrystallization, and sediment–water reactions. Halite is the major chemical mineral in the surficial efflorescence crust that is characteristically finely crystalline with a porous texture. The origin of the pores may be differential expansion of the ground, caused by irregular inter-sedimentary growth of salt crystals. Concentrated brine moves upwards by capillarity and evaporates, precipitating halite that covers the surface, and helps to maintain high moisture content in the salt mound even during the very dry season. Below the surface, the depositional sequence comprises an alternation of loose sand and slightly hard halite-rich layers. The sand layers are dominated by aeolian sand and a few evaporite minerals. The identified detrital minerals include quartz, albite, microcline, muscovite, ferrohornblende and kaolinite. Halite-rich sediment layers vary in thickness and are characterized by super high salinity of >500 gkg−1 . The positions and salinity values of these horizons probably reflect episodes of fluctuation in both the local geochemistry and the groundwater table. This fluctuation and alternation may promote sediment–water reactions and the diagenetic modification of minerals, resulting in rather complicated mineral assemblages. Thus, apart from halite,
Fig. 5. Stratigraphy, salinity and grain composition of salt mound (LP1) sediments.
the sediments in the salt mound also include gypsum, anhydrite, glauberite, epsomite, anhydrite, calcite, bischofite, polyhalite, schoenite, kieserite and carnallite. Halite crystals occur as cubes (Fig. 6a, c, i, and j), gypsum crystals are tabular (Fig. 6k, l), glauberite crystals are acicular (Fig. 6a), bischofite occurs as massive crystals with crack patterns (Fig. 6d, h), and polyhalite as sphere-like fibrous crystal clusters (Fig. 6e, f, and g). These are distributed throughout the sand matrix. On the whole, the observed mineral assemblages match closely with those predicted by the EQL/EVP computer program. Minor variations did occur with respect to potassium and magnesium salts, e.g., leonite, bloedite, hexahydrite and kainite, which were predicted by EQL/EVP but were not found in the sediment. This may be explained by evaporite dehydration, diagenesis and syndepositional alteration. 4.3. Mechanism of salt mound formation Based on field investigations and petrographic examination, this study proposes that salt mound accumulation is mainly due to longterm activity of underground springs. The supply of wind-blown sand also plays an important role in accumulation of mound sediments. The height of the salt mounds, however, is controlled by the vertical transport range of underground springs and the thickness of capillary fringe. A formation mode of salt mounds is shown in Fig. 7. In the early stages of salt mound development, active springs might have discharged at the surface along playa margins. Subsequently, the abundant salts present in the spring water are precipitated and the surface brine acts as a cohesion agent that tends to readily trap windblown sand. The long-term accumulation leads to the continued growth of the salt mounds. At a certain point, the spring is no longer able to reach the surface of the salt mound because the height of salt mound exceeds the vertical trans-
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Fig. 6. SEM micrographs of evaporite minerals and quartz grains from salt mound sediments (Mineral abbreviations codes: Hal, halite; Gla, glauberite; Bis, bischofite; Pol, polyhalite; Gy, gypsum).
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Fig. 6. (Continued)
Fig. 7. Formation mode of salt mounds in the Lop Nor playa.
port range of the spring. However, the brine film may still move to the surface by capillary forces due to high evaporation rates in this arid playa environment. Thus, the humid surface could continue to trap wind-blown sand, allowing mound development to progress. In the late stages of development, the height of the salt mound exceeds the capillary fringe. From this point onward, only free gaseous H2 O molecules continue to move up and be adsorbed onto the surfaces of post-deposited sand particles. A mound will develop progressively as long as the influx of water vapor from the groundwater exceeds the evaporation flux of the mound. Once this is no longer the case, mound growth will cease. Some end-member minerals, such as schoenite and halite have been found in the relatively humid surficial efflorescent crust at the top of salt mound LP1. This indicates that the capillary fringe is close to the surface, and hence that this salt mound is still active today. In general, if springs occur along the pans margin, hydraulic heads is higher in the center of the pans and hence would lead to
artesian waters and standing water. However, high precision elevation survey data indicate the “Great Ear” salt pans are essentially flat and horizontal with a very gentle slope of 0.07‰ from margin to the center (Li et al., 2008). In addition, these exposed salt pans have a rugged surface marked by pressure-ridges and well-developed hexagonal honeycomb polygons structures, and the micro-relief of salt crust ranges from 30 to 100 cm. During the field investigation, the wet salt pan and salt crystallization surface were observed in the center of the pans, where the salt crusts are saturated but no standing water is present. It indicated that the hydraulic head pressure is not high enough to produce artesian flow in present-day “Great Ear” playa system but might have been so in the past. The inflow volume response to climate change is the primary cause affecting the hydrologic balance and groundwater level fluctuation. However, in the high salinity playa environment, groundwater discharge is not a simple hydraulic conductive process. It depends on many factors such as permeability, brine density, minerals precipitation in the sediment, type of playa surface, cracks, bulk
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5. Conclusion
Fig. 8. Depth distribution of Cesium-137 concentration in the salt mound LP1.
density, porosity, sediment texture and structure, atmospheric conditions and solar radiation. The mechanisms are currently still poorly understood.
Lop Nor is a typical groundwater-discharge playa. As such, spring activity has played, and continues to play, an import role in its evolution and growth. Field investigations led to the discovery of a large number of salt mounds with moist surfaces in the marginal area of the Lop Nor playa. Wind-blown sand supply and upward recharge of underground springs are two important factors in salt mound construction. Furthermore, salt mound height is mainly controlled by the vertical transport range of underground springs and the thickness of the capillary fringe. A 1.5 m representative profile dug from the center of salt mound LP1 revealed an alternation of loose fine sand sediment and slightly hard halite-rich layers. The salt mound’s sediments comprise fairly complicated mineral assemblages that include halite, gypsum, anhydrite, glauberite, epsomite, anhydrite, calcite, bischofite, polyhalite, schoenite, kieserite and carnallite. Mineral assemblages predicted theoretically using the EQL/EVP computer program match closely with those encountered in actual salt mound sediments. The groundwater in the area is a highly concentrated brine dominated by Cl− and Na+ , containing considerable amounts of SO4 2− , Mg2+ and K+ , small amounts of Ca2+ , and having low alkalinity. There is considerable variation in the chemical composition of the ground water in this area over time. This may be due to seasonal variations with respect to precipitation and snow-melt across the recharge area. The Cs-137 technique was used to measure the accumulated age and to estimate deposition rates of salt mound LP1. This may represent a new way to estimate the age of relatively young saline sediments (Luly et al., 1986; Murray and Olley, 2002; Olley et al., 1997; Olsson, 1973) that do not contain carbon-rich materials.
4.4. Dating The Cs-137 technique was used to measure the accumulated age and estimated deposition rates of salt mound LP1. It is well known that radioactive Cs-137 does not occur naturally on earth and was formed atmospherically by nuclear testing that occurred from the mid 1940s to the 1970s (Zhiyanski et al., 2005). When Cs-137 settled out, it rapidly adhered to the surface soil in most environments. A useful spin-off of this situation is the provision of an effective “time marker” which facilitates the measurement of recent soil erosion/deposition rates (Aslani et al., 2003; Walling and He, 1998; Zhiyanski et al., 2005). Accumulated Cs-137 with radioactivity readings ranging from 0.83 to 5.48 Bq × g−1 was found in the top 9 cm of the salt mound (Fig. 8) (Zhong et al., 2008). The implication is that this salt mound has accumulated ∼9 cm since the mid-1940s when the first global fallout of bomb-derived radiocesium occurred. This equals a mean deposition rate of ∼0.16 mm per year. At that pace, a ∼1 m-high salt mound such as LP1 would have taken about 600 years to accumulate. Lop Nor is a large, active playa in which groundwater discharge commonly occurs accompanied by the accumulation of aeolianderived sediments. It constitutes a naturally steady depositional environment receiving very little precipitation (<20mm per year). Radioactive Cs-137 fallout, attributed to Chinese nuclear testing near the Lop Nor basin beginning in the 1960s, was deposited universally across the playa and is still present there in relatively high abundance. Therefore, it is feasible to use Cs-137 as a sedimentary marker when determining deposition rates and accumulated ages of landforms in this area. This study signifies a preliminary attempt to obtain the age of salt-mound sediments using the Cs137 method. This method could potentially represent a new way of dating young saline sediments with specific accretion cycles, most of which are related to groundwater and spring discharge in contemporary playa environments.
Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant No. 41002028),Central Public-Interest Scientific Institution Basal Research Fund (Grant No.YK1404), Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (A1406) and Chinese geologic survey project (N1611-9). References Aslani, M.A., Aytas, S., Akyil, S., Yaprak, G., Yener, G., Eral, M., 2003. Activity concentration of caesium-137 in agricultural soils. J. Environ. Radioact. 65, 131–145. Handford, C.R., 1982. Sedimentology and evaporite genesis in a Holocene continental-sabkha playa basin-Bristol Dry Lake, California. Sedimentology 29, 239–253. Li, B.G., Ma, L.C., Jiang, P.A., Duan, Z.Q., Qiu, H.L., Zhong, J.P., Wu, H.Q., 2008. High precision topographic data on Lake Lop Nur’s Great Ear and the timing of its becoming a dry salt lake. Chin. Sci. Bull. 53, 905–914. Luly, J.G., Bowler, J.M., Head, M.J., 1986. A radiocarbon chronology from the playa Lake Tyrrell Northwestern Victoria, Australia. Palaeogeogr. Palaeoclimatol. Palaeoecol. 54, 171–180. Ma, L.C., Lowenstein, T.K., Li, B.G., Jiang, P.A., Liu, C.L., Zhong, J.P., Qiu, H.L., Wu, H.Q., 2010. Hydrochemical characteristics and brine evolution paths of Lop Nor basin, Xinjiang province, Western China. Appl. Geochem. 25, 1770–1782. Murray, A.S., Olley, J.M., 2002. Precision and accuracy in the optically stimulated luminescence dating of sedimentary quartz: a status review. Geochronometria 21, 1–16. Olley, J.M., Roberts, R.G., Murray, A.S., 1997. A novel method for determining residence times of river and lake sediments based on disequilibrium in the thorium decay series. Water Resour. Res. 33, 1319–1326. Olsson, I.U., 1973. A critical analysis of 14C dating of deposits containing little carbon. In: Proceedings of the 8th International Conference of Radiocarbon Dating, Lower Hutt, New Zealand, 18–25 Oct. 1972. Royal Society of New Zealand, Wellingto, pp. 547–564. Risacher, F., Clement, A., 2001. A computer program for the simulation of evaporation of natural waters to high concentration. Comput. Geosci. 27, 191–201.
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Sediment characteristics and mineralogy of salt mounds linked to underground spring activity in the Lop Nor playa, Western China Ma Lichun a,∗ , Tang Qingfeng b , Li Baoguo c , Hu Yufei a , Shang Wenjun a a MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, 100037, China b Beijing Centre for Physical & Chemical Analysis, Beijing 100089, China c College of Resources and Environment, China Agricultural University, Beijing 100193, China
a r t i c l e
i n f o
Article history: Received 8 November 2015 Received in revised form 1 August 2016 Accepted 2 August 2016 Keywords: Lop Nor basin Saline playas Chemical sediment Mineralogy Sediment characteristics Salt mounds Underground spring Brine chemistry
a b s t r a c t Salt mounds are commonly distributed along playa margins and typically comprise alternating layers of loose fine sand and slightly hard halite-rich sediments as a result of long-term underground spring activity. A model of salt mound development was constructed for this study. It suggests that wind-blown sand supply and upward recharge of underground springs are two important factors in salt mound construction. Furthermore, it proposes that salt mound height is mainly controlled by the vertical transport range of underground springs and the thickness of the capillary fringe. A 1.5 m representative profile dug from the center of salt mound LP1 in the Lop Nor playa revealed a fairly complicated mineral assemblage including halite, gypsum, anhydrite, glauberite, epsomite, anhydrite, calcite, bischofite, polyhalite, schoenite, kieserite and carnallite. This matches closely with the assemblage predicted by the EQL/EVP model. The groundwater in the area is highly concentrated brine rich in Cl− and Na+ and poor in Ca2+ , displaying low alkalinity, and containing considerable amounts of SO4 2− , Mg2+ and K+ . Chemical analysis of groundwater revealed considerable variation in the salinity and chemical composition of groundwater over time. The Cs-137 technique was used to measure the accumulated ages of the salt mounds. This method may prove useful in the research of relatively young playa environments where carbon dating techniques are unworkable because of an absence of carbon-rich materials in recent saline sediments. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction Saline playas are widespread in the arid interior of Western China. These landforms commonly develop in topographic lows and regional groundwater discharge zones (Handford, 1982; Rosen, 1991) where shallow groundwater is saline to hyper-saline (Risacher et al., 2003). They are effective hydro-chemical sinks promoting elemental concentration and evaporite formation. The Lop Nor playa is situated at the junction of Xinjiang province and Gansu province in the eastern part of the Tarim Basin—China’s largest endorheic drainage system—with an area of 530,000 km2 (see Fig. 1). The playa itself has a surface area of approximately 20,000 km2 . As the lowest point and sole terminus of the Tarim drainage system (Li et al., 2008), the Lop Nor playa marks the discharge point of the basin’s local and regional ground-water and surface-water flow systems (Ma et al., 2010).
∗ Corresponding author. E-mail address: lichmafl[email protected] (L. Ma).
The present-day Lop Nor is a typical groundwater-discharge playa. It lacks surface inflows and contains concentrated underground brines beneath salt crusts with an extensive area of 5,500 km2 (Ma et al., 2010). Horizontal subsurface inflows from adjacent aquifers in the surrounding alluvial fans help to maintain a high evaporation rate and the overall playa system water balance. Down the regional hydrologic gradient, as groundwater moves from the outer fringes of the alluvial fans to the salt pans, its salinity gradually increase, reaching a maximum of roughly 350 g/l in the center of the playa (Ma et al., 2010). This groundwater commonly emerges as ascending springs along playa margins. Spring activity thus plays an important role in the hydro-chemical and geomorphological evolution of the playa margins. However, very little research has been done on this, especially in this particular area (Ma et al., 2010). There are many salt mounds of different sizes in the marginal areas of the Lop Nor playa. The main objectives of this study are: to describe the physico-chemical and sedimentary properties of these salt mounds, to investigate the process of salt mound formation,
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Table 1 Chemical analysis of underground brine from salt mound LP1 (ionic concentrations are expressed in ppm). 2006
2014
Na+ K+ Ca2+ Mg2+ HCO3 − + CO3 2− Cl− SO4 2−
53985 5113 277 8327 / 112001 28212
108372 8888 176 15732 / 177968 42211
Ratios
2006
2014
0.046 0.074 0.25
0.05 0.088 0.24
+
Fig. 1. Regional map showing the location of Lop Nor Basin and drainage system of Tarim Basin, Xinjiang province, China. XJ, Xinjiang Province; KM, Kuruktage Mountains; BM, Beishan Mountains. Modified from Ma (2010).
and to develop a depositional model for salt mound formation in playa environments. 2. Study area The Lop Nor depression is a Quaternary landform. This basin is in fault contact with the Kuruktage Mountains in the north, the Beishan Mountains in the east, and the Altyn Mountains in the south. To the west, the Tarim River’s old, dry delta gradually merges with the western margin of the Taklamakan Desert (Fig. 1). The study area (39◦ 50 −40◦ 40 N; 90◦ 10 −91◦ 30 E) is situated in the central-eastern sector of the Lop Nor basin, which is characterized in satellite images by alternating black and white concentric rings that closely resemble a “Great Ear” (Fig. 2). The actual surface of the basin is essentially flat, covered by polygonal salt crusts and desiccated saline mudflat deposits at an altitude of approximately 780 m above sea level. The study area is located in the driest part of central Asia, typically receiving an annual rainfall of <20 mm with a potential annual evaporation of >3500 mm. Hence, this is a typical arid continental climatic zone characterized by cold, dry winters, wide diurnal temperature ranges, hot summers with long periods of sunlight and year-round northeasterly winds. The mean annual wind velocity is 5 m/s, and severe sandstorms frequently occur in spring and summer. Wind is a dominant geomorphological agent in this region whose vast, wind-eroded landscapes incorporate meso-yardangs that are widely distributed along the northern, western, and eastern margins of the Lop Nor basin (Fig. 2). Field investigations conducted in the study area between 2007 and 2014 revealed that the playa margins, especially on the eastern side of the “Great Ear” salt pan, are characterized by an extensive distribution of active salt mounds (Zhong et al., 2008). The mounds have heights of 1–2 m and diameters of 3–15 m, with moist surfaces and dome-shaped tops (Fig. 3). They are generally represented by the dark tones in satellite images of the area (Fig. 2). The mounds’ moist surfaces and vertical sedimentary profiles imply underground spring activity. Because field investigations in most parts of the “Great Ear” salt pan are constrained by poor accessibility, a representative salt mound LP1 was selected for this study. It is approximately 1 m high with a diameter of about 12 m. Its location is presented in Fig. 2, and some of its geomorphological features are shown in Fig. 3. 3. Materials and methods This paper describes the mineralogy, petrology and geochemistry of the saline succession of salt mound LP1. Mineralogical identification was carried out by X-ray diffraction and analyses of 16 sediment samples under an environmental electron scan-
−
K /Cl Mg+ /Cl− SO4 2− /Cl−
ning microscope equipped with an energy dispersive spectroscopy (ESEM-EDS). The chemical compositions of both the solid sediments and the underground brines were analyzed in terms of Cl− , SO4 2− , Ca2+ , Mg2+ , K+ , and Na+ by Dionex DX-120 ion chromatography (RSD < 2%) and Perkin Elmer Optima-3300DV inductively coupled plasma optical emission spectrometry (RSD < 1%). The grain-size distribution was analyzed by Malvern Mastersizer 2000 laser diffractometry. Measurements of radioisotope levels (Cs-137 and Pb-210) were undertaken by the Nanjing Institute of Geography & Limnology (Chinese Academy of Sciences). 4. Results and discussion 4.1. Chemical characteristics of underground brine Two underground brine samples were obtained in September 2006 and August 2014, by digging a pit in the center of the salt mound where the groundwater level is encountered approximately 1.3 m below the surface (Fig. 3). Considerable variation was noted between the two samples in terms of salinity and chemical composition. It is possible that this is due to seasonal variation in precipitation and snow-melt recharge, but there are currently insufficient data to allow for rigorous investigation of this variation. The underground brine had a salinity value of 208 g/L in September 2006 and of 353 g/L in August 2014. Both samples were rich in Cl− and Na+ and poor in Ca2+ . They both had low alkalinity values and contained considerable amounts of SO4 2− , Mg2+ and K+ . The chemical characteristics of these underground brines are presented in Table 1. Calculations using the EQL/EVP computer program (Risacher and Clement, 2001) indicate that these underground brines are supersaturated with respect to calcite (CaCO3 ). Simulated evaporation was carried out in equilibrium mode under closed system conditions at 25 ◦ C. Pco2 was set at 10−3.4 using EQL/EVP code. The main minerals precipitated during simulated evaporation were halite (NaCl), bloedite (Na2 SO4 ·MgSO4 ·4H2 O), epsomite (MgSO4 ·7H2 O), hexahydrite (MgSO4 ·6H2 O), kieserite (MgSO4 ·H2 O), kainite (MgSO4 ·KCl·3H2 O), leonite (K2 SO4 ·MgSO4 ·4H2 O), glauberite (NaSO4 ·CaSO4 ), polyhalite (K2 SO4 ·MgSO4 ·2CaSO4 ·2H2 O), and carnallite (MgCl2 ·KCl·7H2 O). The total mass of other evaporite minerals in this system is <1% of the total. Both groundwater samples exhibited similar evaporation evolution pathways and precipitated comparable mineral species. Successive evaporation simulations led to Mg-Cl brines and caused the precipitation of a sequence of minerals of increasingly high solubilities including calcite (CaCO3 ), gypsum (CaSO4 ·2H2 O), glauberite (NaSO4 ·CaSO4 ), hydromagnesite (4MgCO3 ·Mg(OH) 2 ·4H2 O), halite (NaCl), bloedite (Na2 SO4 ·MgSO4 ·4H2 O), polyhalite (K2 SO4 ·MgSO4 ·2CaSO4 ·2H2 O), schoenite (K2 SO4 ·MgSO4 ·6H2 O),
Please cite this article in press as: Ma, L., et al., Sediment characteristics and mineralogy of salt mounds linked to underground spring activity in the Lop Nor playa, Western China. Chemie Erde - Geochemistry (2016), http://dx.doi.org/10.1016/j.chemer.2016.08.001
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Fig. 2. Locations of sampling sites and investigation routes in the “Great Ear” playa of the Lop Nor basin. (Background image: Landsat 5 TM image, 10/23/2006, pixel 30 m, R3, G2, B1).
Fig. 3. Field photograph showing salt mound LP1(a), its characteristic puffy surface (b), its vertical sediment profile (c), and its groundwater level(c).
leonite (K2 SO4 ·MgSO4 ·4H2 O), epsomite (MgSO4 ·7H2 O), kainite (MgSO4 ·KCl·3H2 O), hexahydrite (MgSO4 ·6H2 O), kieserite (MgSO4 ·H2 O) and carnallite (KCl·MgCl2 ·6H2 O). The simulated EQL/EVP results are shown in Fig. 4. 4.2. Sediment characteristics and mineralogy A typical pit of 1.5 m depth was dug to facilitate the studies of the sedimentary characteristics of salt mound LP1. The profile is dominated by an unconsolidated sand layer, but incorporates halite-rich layers deposited by concentrated pore water (Fig. 5). The accumulated salinity in the mound varies from 217 g kg−1 to
708 g kg−1 . The halite-rich layers have a grayish white appearance, varying thicknesses, and occur at depths of 20 cm, 38 cm, 52 cm and 61 cm (see Fig. 5). These layers have salinity values of >500 gkg−1 . The loose sand layer, however, has a salinity value of ∼300 g kg−1 , and presents with a light- to dark-brown tone depending on local water content. Sand grains in the profile are well-sorted and rounded (Fig. 6b), while halite crystals in the matrix are cubic (Fig. 6a, c, i, and j). The skeletal halite-rich layers are comprised of crystals that have merged to form interlocking fabrics within the sandy matrix. The halite is thought to precipitate directly due to capillary evaporation of supersaturated pore waters from the soft, brine-soaked fine-sand
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Fig. 4. Modeled underground brine evolution pathways by the EQL/EVP computer program under closed system conditions at 25 ◦ C, and Pco2 of 10−3.4 . (a) Sampling and analysis in 2006. (b) Sampling and analysis in 2014. The factor of concentration (FC) is defined as the ratio between the initial amount of free water and the amount of water in the solution (Risacher and Clement, 2001).
sediments. The microscopic morphological characteristics of quartz grains were studied by SEM. They have matte surfaces with dishand crescent-shaped concavities (Fig. 6b). These characteristics attest to aeolian weathering processes. Vertical variation in sand grain size distribution was nominal. The matrix was dominated by sand grains with diameters of 0.02–2 mm which constituted 80–90% of the total (Fig. 5). These grains are most likely derived from the aeolian dune and yardang landscapes to the north of the playa. The sandy texture facilitates capillary water transport to the mound surface, and the mound’s resistance to wind erosion is enhanced by the surface efflorescence crust resulting from saline pore-water evaporation of underground brines at the mound surface. The surficial crusts are commonly a few decimeters thick and contain a very complicated mixture of halite, schoenite, bischofite, gypsum, glauberite, calcite, and mostly detrital sediments. They exhibit several diagenetic overprints related to dehydration, dissolution, recrystallization, and sediment–water reactions. Halite is the major chemical mineral in the surficial efflorescence crust that is characteristically finely crystalline with a porous texture. The origin of the pores may be differential expansion of the ground, caused by irregular inter-sedimentary growth of salt crystals. Concentrated brine moves upwards by capillarity and evaporates, precipitating halite that covers the surface, and helps to maintain high moisture content in the salt mound even during the very dry season. Below the surface, the depositional sequence comprises an alternation of loose sand and slightly hard halite-rich layers. The sand layers are dominated by aeolian sand and a few evaporite minerals. The identified detrital minerals include quartz, albite, microcline, muscovite, ferrohornblende and kaolinite. Halite-rich sediment layers vary in thickness and are characterized by super high salinity of >500 gkg−1 . The positions and salinity values of these horizons probably reflect episodes of fluctuation in both the local geochemistry and the groundwater table. This fluctuation and alternation may promote sediment–water reactions and the diagenetic modification of minerals, resulting in rather complicated mineral assemblages. Thus, apart from halite,
Fig. 5. Stratigraphy, salinity and grain composition of salt mound (LP1) sediments.
the sediments in the salt mound also include gypsum, anhydrite, glauberite, epsomite, anhydrite, calcite, bischofite, polyhalite, schoenite, kieserite and carnallite. Halite crystals occur as cubes (Fig. 6a, c, i, and j), gypsum crystals are tabular (Fig. 6k, l), glauberite crystals are acicular (Fig. 6a), bischofite occurs as massive crystals with crack patterns (Fig. 6d, h), and polyhalite as sphere-like fibrous crystal clusters (Fig. 6e, f, and g). These are distributed throughout the sand matrix. On the whole, the observed mineral assemblages match closely with those predicted by the EQL/EVP computer program. Minor variations did occur with respect to potassium and magnesium salts, e.g., leonite, bloedite, hexahydrite and kainite, which were predicted by EQL/EVP but were not found in the sediment. This may be explained by evaporite dehydration, diagenesis and syndepositional alteration. 4.3. Mechanism of salt mound formation Based on field investigations and petrographic examination, this study proposes that salt mound accumulation is mainly due to longterm activity of underground springs. The supply of wind-blown sand also plays an important role in accumulation of mound sediments. The height of the salt mounds, however, is controlled by the vertical transport range of underground springs and the thickness of capillary fringe. A formation mode of salt mounds is shown in Fig. 7. In the early stages of salt mound development, active springs might have discharged at the surface along playa margins. Subsequently, the abundant salts present in the spring water are precipitated and the surface brine acts as a cohesion agent that tends to readily trap windblown sand. The long-term accumulation leads to the continued growth of the salt mounds. At a certain point, the spring is no longer able to reach the surface of the salt mound because the height of salt mound exceeds the vertical trans-
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Fig. 6. SEM micrographs of evaporite minerals and quartz grains from salt mound sediments (Mineral abbreviations codes: Hal, halite; Gla, glauberite; Bis, bischofite; Pol, polyhalite; Gy, gypsum).
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Fig. 6. (Continued)
Fig. 7. Formation mode of salt mounds in the Lop Nor playa.
port range of the spring. However, the brine film may still move to the surface by capillary forces due to high evaporation rates in this arid playa environment. Thus, the humid surface could continue to trap wind-blown sand, allowing mound development to progress. In the late stages of development, the height of the salt mound exceeds the capillary fringe. From this point onward, only free gaseous H2 O molecules continue to move up and be adsorbed onto the surfaces of post-deposited sand particles. A mound will develop progressively as long as the influx of water vapor from the groundwater exceeds the evaporation flux of the mound. Once this is no longer the case, mound growth will cease. Some end-member minerals, such as schoenite and halite have been found in the relatively humid surficial efflorescent crust at the top of salt mound LP1. This indicates that the capillary fringe is close to the surface, and hence that this salt mound is still active today. In general, if springs occur along the pans margin, hydraulic heads is higher in the center of the pans and hence would lead to
artesian waters and standing water. However, high precision elevation survey data indicate the “Great Ear” salt pans are essentially flat and horizontal with a very gentle slope of 0.07‰ from margin to the center (Li et al., 2008). In addition, these exposed salt pans have a rugged surface marked by pressure-ridges and well-developed hexagonal honeycomb polygons structures, and the micro-relief of salt crust ranges from 30 to 100 cm. During the field investigation, the wet salt pan and salt crystallization surface were observed in the center of the pans, where the salt crusts are saturated but no standing water is present. It indicated that the hydraulic head pressure is not high enough to produce artesian flow in present-day “Great Ear” playa system but might have been so in the past. The inflow volume response to climate change is the primary cause affecting the hydrologic balance and groundwater level fluctuation. However, in the high salinity playa environment, groundwater discharge is not a simple hydraulic conductive process. It depends on many factors such as permeability, brine density, minerals precipitation in the sediment, type of playa surface, cracks, bulk
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5. Conclusion
Fig. 8. Depth distribution of Cesium-137 concentration in the salt mound LP1.
density, porosity, sediment texture and structure, atmospheric conditions and solar radiation. The mechanisms are currently still poorly understood.
Lop Nor is a typical groundwater-discharge playa. As such, spring activity has played, and continues to play, an import role in its evolution and growth. Field investigations led to the discovery of a large number of salt mounds with moist surfaces in the marginal area of the Lop Nor playa. Wind-blown sand supply and upward recharge of underground springs are two important factors in salt mound construction. Furthermore, salt mound height is mainly controlled by the vertical transport range of underground springs and the thickness of the capillary fringe. A 1.5 m representative profile dug from the center of salt mound LP1 revealed an alternation of loose fine sand sediment and slightly hard halite-rich layers. The salt mound’s sediments comprise fairly complicated mineral assemblages that include halite, gypsum, anhydrite, glauberite, epsomite, anhydrite, calcite, bischofite, polyhalite, schoenite, kieserite and carnallite. Mineral assemblages predicted theoretically using the EQL/EVP computer program match closely with those encountered in actual salt mound sediments. The groundwater in the area is a highly concentrated brine dominated by Cl− and Na+ , containing considerable amounts of SO4 2− , Mg2+ and K+ , small amounts of Ca2+ , and having low alkalinity. There is considerable variation in the chemical composition of the ground water in this area over time. This may be due to seasonal variations with respect to precipitation and snow-melt across the recharge area. The Cs-137 technique was used to measure the accumulated age and to estimate deposition rates of salt mound LP1. This may represent a new way to estimate the age of relatively young saline sediments (Luly et al., 1986; Murray and Olley, 2002; Olley et al., 1997; Olsson, 1973) that do not contain carbon-rich materials.
4.4. Dating The Cs-137 technique was used to measure the accumulated age and estimated deposition rates of salt mound LP1. It is well known that radioactive Cs-137 does not occur naturally on earth and was formed atmospherically by nuclear testing that occurred from the mid 1940s to the 1970s (Zhiyanski et al., 2005). When Cs-137 settled out, it rapidly adhered to the surface soil in most environments. A useful spin-off of this situation is the provision of an effective “time marker” which facilitates the measurement of recent soil erosion/deposition rates (Aslani et al., 2003; Walling and He, 1998; Zhiyanski et al., 2005). Accumulated Cs-137 with radioactivity readings ranging from 0.83 to 5.48 Bq × g−1 was found in the top 9 cm of the salt mound (Fig. 8) (Zhong et al., 2008). The implication is that this salt mound has accumulated ∼9 cm since the mid-1940s when the first global fallout of bomb-derived radiocesium occurred. This equals a mean deposition rate of ∼0.16 mm per year. At that pace, a ∼1 m-high salt mound such as LP1 would have taken about 600 years to accumulate. Lop Nor is a large, active playa in which groundwater discharge commonly occurs accompanied by the accumulation of aeolianderived sediments. It constitutes a naturally steady depositional environment receiving very little precipitation (<20mm per year). Radioactive Cs-137 fallout, attributed to Chinese nuclear testing near the Lop Nor basin beginning in the 1960s, was deposited universally across the playa and is still present there in relatively high abundance. Therefore, it is feasible to use Cs-137 as a sedimentary marker when determining deposition rates and accumulated ages of landforms in this area. This study signifies a preliminary attempt to obtain the age of salt-mound sediments using the Cs137 method. This method could potentially represent a new way of dating young saline sediments with specific accretion cycles, most of which are related to groundwater and spring discharge in contemporary playa environments.
Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant No. 41002028),Central Public-Interest Scientific Institution Basal Research Fund (Grant No.YK1404), Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (A1406) and Chinese geologic survey project (N1611-9). References Aslani, M.A., Aytas, S., Akyil, S., Yaprak, G., Yener, G., Eral, M., 2003. Activity concentration of caesium-137 in agricultural soils. J. Environ. Radioact. 65, 131–145. Handford, C.R., 1982. Sedimentology and evaporite genesis in a Holocene continental-sabkha playa basin-Bristol Dry Lake, California. Sedimentology 29, 239–253. Li, B.G., Ma, L.C., Jiang, P.A., Duan, Z.Q., Qiu, H.L., Zhong, J.P., Wu, H.Q., 2008. High precision topographic data on Lake Lop Nur’s Great Ear and the timing of its becoming a dry salt lake. Chin. Sci. Bull. 53, 905–914. Luly, J.G., Bowler, J.M., Head, M.J., 1986. A radiocarbon chronology from the playa Lake Tyrrell Northwestern Victoria, Australia. Palaeogeogr. Palaeoclimatol. Palaeoecol. 54, 171–180. Ma, L.C., Lowenstein, T.K., Li, B.G., Jiang, P.A., Liu, C.L., Zhong, J.P., Qiu, H.L., Wu, H.Q., 2010. Hydrochemical characteristics and brine evolution paths of Lop Nor basin, Xinjiang province, Western China. Appl. Geochem. 25, 1770–1782. Murray, A.S., Olley, J.M., 2002. Precision and accuracy in the optically stimulated luminescence dating of sedimentary quartz: a status review. Geochronometria 21, 1–16. Olley, J.M., Roberts, R.G., Murray, A.S., 1997. A novel method for determining residence times of river and lake sediments based on disequilibrium in the thorium decay series. Water Resour. Res. 33, 1319–1326. Olsson, I.U., 1973. A critical analysis of 14C dating of deposits containing little carbon. In: Proceedings of the 8th International Conference of Radiocarbon Dating, Lower Hutt, New Zealand, 18–25 Oct. 1972. Royal Society of New Zealand, Wellingto, pp. 547–564. Risacher, F., Clement, A., 2001. A computer program for the simulation of evaporation of natural waters to high concentration. Comput. Geosci. 27, 191–201.
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