RESEARCH REPORTS 581 Microbial Fabric Formation in Spring Mounds ("Microbialites") of AIkaline Salt Lakes in the Badain Jaran Sand Sea, PR China GERNOTARP* Institute and Museum ofGeology and Paleontology, University ofGöttingen, Goldschmidtstraße 3, D-37077 Göttingen, Germany JÜRGEN HOFMANN Institute of Geography, Free University of Berlin, Grunewaldstraße 35, D-12165 Berlin, Germany JOACHIM REITNER Institute and Museum of Geology and Paleontology, University of Göttingen, Goldschmidtstraße 3, D-37077 Göttingen, Germany PALAIOS, 1998, V. 13, p. 581-592 The large sandy desert of Badnin Jaran Shamo, Inner Mongolia, has several salt lakes located among megadunes with crests as high as 400 m. Most of the lakes are hypo- to hypersaline and alkaline, with pH values between 8.5 and 10. Because of their reef-like aragonitie pinnacles rising from the lake floor, Lake Nuoertu and Lake Huhejaran haue been investigated in detail. The porous limes tones of the tufa pinnacles ("spring mounds'') result {rom a hydrochem- ieally forced, exopolymer-mediated calcification of cyano- bacteria-dominated mierobial mats at sublacustrine springs. The development oftheir enigmatie fabries is doc- umented for the first time. Siekle-cell-like and bubble- shaped fabries reflect successive mineralization during deg- radation, shrinkage of the organie mucus, and gas forma- tion by bacteria. Fossil analogues are known from the Pleis- tocene and Miocene (Ries-crater lake), but older non-ma- rine, Phanerozoie examples are expected. 1b the extent that these textures are associated exclusively with alkaline envi- ronments, they can be used to test the hypothesis of an al- kali ne ocean early in Earth history. INTRODUCTION Spring mounds are reef-like carbonate build-ups that are linked to groundwater seeps, springs, or vents. This contribution focuses on non-marine, sublacustrine spring mounds. Analogues, known as "tufa pinnacles," are recog- nized from a number ofPleistocene to Recent alkaline salt lakes (Russei, 1889; Scholl, 1960; Scholl and Taft, 1964; Warren, 1982; Dean and Fouch, 1983; Gasse and Fontes, 1989; Kempe et al., 1991; Council and Bennett; 1993; Ben- son, 1994). Active pinnacles grow in a highly supersatu- rated mixing zone of HC03 -/C03z- -rich lake water and groundwater supplying divalent cations. Owing to the in- tensive development ofmicrobial mats, which are involved in carbonate precipitation and fabric formation, the result- * Author to whom correspondence should be addressed Copyright © 1998, SE PM (Society for Sedimentary Geology) ing carbonate rocks are now generally called "microbia- lites," or "stromatolites" if laminated fabries occur. Supersaturation with respect to calcium carbonate of the macroenvironment is generally aprerequisite for the formation of tufa, as it is for all non-enzymatically precip- itated carbonates. Abiotic parameters of carbonate equi- librium, such as physicochemical COz degassing by water agitation and elevated temperature (for fluvial tufa), and mixing of lake and spring water (for tufa pinnacles), are commonly regarded as controlling factors, that, in princi- pIe can sufficiently explain tufa formation. Microbial pro- cesses promoting CaC03 supersaturation are autotrophie COz-fixation, ammonification, sulfate reduction (Krum- bein, 1979) and methanotrophy (Ritger et al., 1987). Only photosynthetic COz removal has been considered in tufa systems. However, & 13C values indicate that tufa carbon- ate is linked to photosynthesis only to a minor extent (Us- dowski et al., 1979; Pentecost and Spiro, 1990). Because all substrate-water interfaces in nature are col- onized by various micro-organisms, and precipitation of CaC03 does not start before reaching moderate to high su- persaturation, one may consider the biological and bio- chemical properties of these biofilms as crucial factors. Re- duced diffusion within the microbially secreted extracel- lular polymerie substances (EPS) permits the establish- ment of microenvironments. Apart from the biotic influence on the carbonate ) equilibrium in micro-niches, biochemical properties of the EPS are expected to influ- ence precipitation significantly in biofilms and microbial mats. This should also be applicable in tufa systems. How- ever, few investigations (Emeis et al., 1987) have taken into account the effect of polysaccharides and amino acids on Caz+ accumulation, CaC03 nucleation, crystal growth, and fabric formation in tufa carbonates. The objective of this paper is to demonstrate the crucial influence of extracellular polymerie substances, and their microbial decay, on fabric formation within the hydro- chemically driven depositional system of spring-mounds. Further, the potential significance of these characteristic spring-mound carbonates for the reconstruction ofthe an- cient ocean chemistry is discussed. 0883-1351/9810013-0581/$3.00 582 CJ Sand desert (Shamo) 0 Gravel desert (Gobi) rn Outcropsand gravel desert - Exposed bedrock ,e-.:. ... Weiland area ,,=--;< ~ Lake Faults """' River """""" GreatWall [ZLi Studyarea + Anticlinal axis -+- Synclinal axis 1957 Elevation ( m a.s.l. ) FIGURE 1-Location 01 the study area in the Badain Jaran Shamo (Inner Mongolia Aut. Reg. , PR China). ENVIRONMENTAL SETTING During a Chinese-German expedition to the Badain Jaran Desert in 1995 one ofus (J.H.) discoveredpreviously unknown, active spring mounds in interdune salt lakes (Hofmann, 1996). The setting is devoid of artificial influ- ence and, hence, provides the rare opportunity to study undisturbed, actively growing spring mounds. Except for Lake Van and Mono Lake pinnacles, which clearly result from present-day carbonate precipitation, most spring mounds are fossil or subfossil structures (e.g., Walker Lake, Pyramid Lake, Winnemucca Lake, Lake Lahontan, Searles Lake. Scholl, 1960; Benson, 1994) With an area of approximately 44,000 square kilome- ters, the Badain Jaran Desert (39°20'N to 41°30'N; 1000 E to 103°E) is the third largest desert of China. It is situated on the northwestern part ofthe Alashan plateau (Fig. 1) at elevations between 1000 and 1500 m above sea level. The area is characterized by an extreme continental desert cli- mate with cold winters (Table 1). In contrast to other des- erts of Central Asia, more than 50% of the area is covered by megadunes that average 200-300 m in height and oc- casionally rise above 400 m (Fig. 2). They show a clear ori- ARP ET AL. TABLE 1-Climatic data 01 the study area, Badain Jaran Desert Annual rainfall Annual potential evaporation Mean annual air temperature Lowest monthly mean temperature Highest monthly mean temperature Winter minimum temperatures 113 mm (SE}- 38 min (NW) 3790 min (W}-2500 mm (E) +8.8°C - 9.9°C (January) +25.3°C (July) -30.0°C entation along an axis stretching SW-NE, roughly at right angles to the prevailing WNW wind direction. Although the megadunes are sparsely vegetated, they appear to be stable. The interdunal areas in the SE part of the desert are filled with salt plains, wetlands, and more than 100 perenniallakes of different sizes and salinities. There are neither rivers discharging into the lakes nor outlets drain- ing them. Inflow is assumed to be due exclusively to lat- eral and sublacustrine groundwater movement. The distribution of the lakes shows a chessboard-like pattern in the east and a more linear pattern in the west (Zheng et al., 1993). These patterns and the displacement of Cretaceous sedimentary strata at the outcrop of Yike- lierbao (39°42'N/102°18'E) in the vicinity ofthe lakes are evidence of young tectonic events along a fault running NE to SW. Lakes Nuoertu and Huhejaran, which are two of the largest in the Badain J aran Desert, were extensively stud- ied in 1995. Both are hypersaline lakes and exhibit pin- nacle-shaped spring mounds forming islands or reef-like structures below the lake surface (Fig. 3). In addition, fos- sil remnants indicating former lake-level highstands were recognized up to 15 m above the present lake level at Lake Nuoertu. For a general overview on Chinese salt lakes, the reader is referred toWilliams (1991) and references there- in. MATERIAL AND METHODS Water sampies for chemical analysis of spring water and lake brines were collected during September and Oc- tober 1995 in 250 ml and 125 ml polyethylene bottles that had been repeatedly rinsed with distilled water. Electrical FIGURE 2-LANDSAT-image (A) and sketchmap (B) 01 the study area showing the investigated lakes Nuoertu and Huhejaran situated between megadunes. MICROBIAL FABRIC FORMA TION IN SPRING MOUNDS 583 11,9 B Sampllng slle 01 bio111ms Shoreline 8 ,5 1927 '" ,. ,--_ . :,7 ~ _-- - - ~ '9 ~ __ ~ ----.--_______ _ / ·· · ~ ··· .. ·· t ··· :' A ~ B ~ 9,0 _' . :~: 10m 10,e( " (""1 , , , 10,4'" . " -', ' - C Cross section A - A' Shoreline 1995 -----... shallow water (depth < I ,5 m) Sediment cor. Vegetation (Phragmites spec.) j FIGURE 3-Loeation of sampling site at Lake Nuoertu. (A) Bathymetrie map with the loeation of spring mounds ("tufa pinnaeles"). (B) Detailed map of the sampling site and (C) sehematie eross seetion of the Mound. Note that an earthquake in 1927 eaused flooding of this shoreline seetion. conductivity (EC), pH, temperature, and oxygen concen- tration were measured in the field using an LF 95 temper- ature compensating conductivity meter, an Oxi 96 oxime- ter with salinity correction, and a pH 90 temperature com- pensating pH meter (WTW Co.). Nitrate, nitrite, and am- monium were analyzed in the field with a portable photometer (model LASA 2-plus, Dr. Lange Co.). Allother chemical analyses were carried out in the laboratory ofthe Institut für Wasser-, Boden- und Lufthygiene (Umwelt- bundesamt, Berlin) on unfiltered sampies, One subsampie of each sampie was treated with HgCl2 to avoid biochemi- cal alteration processes that might influence unstable pa- rameters (e.g., nitrate) during transport to the laboratory. The major cations and anions were analyzed using an ICP-OES (FISONS Inc,) and ion chromatograph (D!- ONEX D 12). Water sampies oflake brines had to be dilut- ed to 1:10,000 with distilled water because ofthe high salt content, The carbonate anions HCOa - and COa2- were de- termined by titration with HCI and NaOH to obtain the acid and base exchange capacity (Hütter, 1992). The total dissolved solids (TDS) were determined by evaporating to dryness at a specific temperature (105°C). Sampies of microbialites and adherent biofilms were fixed by a mixture consisting of 37% formol, 95% ethanol, and filtrated lake water (1:2:2). Owing to the natural car- bonate buffer of the lake water, an additional buffering was not necessary. Subsampies were stained in bulk with toluidin blue 0, basic fuchsin, or the fluorochromes cal- cein, tetracyclin-HCI, or acridine orange in the laboratory to obtain sufficient contrast. After dehydration in graded alcohol series (38--50-70-85-95-100%), the sampies were embedded in LR-White resin. Thin sections were cut with a Leica hardpart microtome. After mounting on glass slides, the sections were cut down to 5-10 IJ.m thickness and covered, The investigation and photographic documentation were carried out using a Zeiss Axiolab microscope with phase contrast and epifluorescence equipment, Nine sec- tions of 2 sampies from Lake Huhejaran, 47 sections of 7 spring-mound sampies ofLake Nuoertu, and 47 sections of 8 core sampies from Lake Nu,oertu were examined. The semiquantitative mineralogical composition of 3 sampies was determined by X-ray diffr~ctometry. WATER CHEl\lISTRY OF LAKE NUOERTU AND HUHEJARAN The concentration of total dissolved solids for the lake brines is 97,8 g/l for Lake Nuoertu and 107.1 g/l for Lake Huhejaran (Table 2). By contrast, the water of spring mounds in and around the lakes shows very low concen- trations of TDS in the range of 0,5 g/l, According to the classification system ofHammer (1983), both lakes are hy- persaline, while the springs belong to a freshwater regime. The physicochemical data (Table 3) show a very high al- kalinity of 624.5 meq/l (Lake Nuoertu) to 824,9 meq/l (Lake Huhejaran), and high pH values between 8.1 and 10 for both spring and lake water. Dissolved oxygen concen- 584 TABLE 2-Physieoehemieal dala 01 spring and lake waler 01 Lake Nuoertu (Oelober 6th , 1995). For loealilies 01 spring A-D see Fig. 3B. Lake Springwater brine (spring mound) lake spring spring spring spring water A B C D Temp. [OC] 17.8 17.1 16.8 17.2 16.5 TDS [g/l) 97 .8 0.45 0.43 0.39 0.44 Conductivity [mS/ cm] 90.3 0.62 0.59 0.53 0.59 pH 10.0 8.9 8.9 8.8 8.8 O2 [mg/l) 4.8 5.8 4.4 6.1 O2 [%] 73 69 58 73 0.255 discharge [1/ s] 0.1 0.15 0.5 0.1 trations range from 4.8 mg/l for lake water to 6.1 mg/l for spring water. The total discharge rates of the examined spring-mounds are not known, but isolated discharge rates between 0.1 and 0.5 l/s were measured at different outflows at the spring mound ofLake Nuoertu (Table 3). The chemical composition of hypersaline lake brines is dominated by sodium and chloride. In general, the ionic dominance is Na+ (92 meq%) > K+ (7 meq%) > Mg2+ (1 meq%) : Cl- (48 meq%) > C032- (33 meq%) > S042- (14 meq%) > HC03 - (5 meq%). The relative proportions ofthe major solutes do not vary remarkably between lakes, al- though there is a slight difference with regard to the total dissolved solids and alkalinity. High C032- and HC03 - concentrations are coupled with very low amounts of K +, Mg2+, and Ca2+. According to the nomenclature ofEugster and Hardie (1978), the brines of both lakes belong to the Na-CI-C03-(S04) type. The anion dominance shows nota- ble quantities of chloride and carbonate. Despite their ion- ic homogeneity, the Mg/Ca mole ratios are different in the two lakes, with values of20.8 (Nuoertu) and 2.6 (Huhejar- an). The high HCO/ Ca + Mg mole ratios are similar in ARP ET AL. both lakes. In addition to the major ions, a number of mi- nor ions have been found. In both lakes, dissolved boron (150-300 mg/l), which is normaHy present as a trace ele- ment only, is a significant component. Significant vertical differences in major ion composition are apparently ab- sent in both lakes, although meromictic lakes (e.g., Lake Yinderitu, see Fig. 2) exist in the vicinity. The major ions in spring and groundwater are also pre- dominantly Na+ and Cl- . In contrast to the lake water, Ca2+ and Mg2+ are more significant, with Mg/Ca mole ra- tios ~ 1. The pattern of ionic dominance in water of spring mounds is Na+ (66 meq%) » Mg2+ (15 meq%) > Ca2+ (14 meq%) > K+ (5 meq%) : Cl- (36 meq%) > S042- (27 meq%) > HC03 - (24 meq%) > N03 - (13 meq%). The total hard- ness of springwaters (calculated as the total amount of Mg2+ and Ca2+) is 0.79 mmol/l at Lake Huhejaran and 1.37 mmol/l at Lake Nuoertu. These waters can be termed soft water. The product of C032- + Ca2+ in the mixing zone of springwater and lake brine may lead to supersaturation and consequent precipitation of carbonates. The relatively high nitrate content of spring water is remarkable (0.865 mmol/l or 53.6 mg/l N03 - ). The source of nitrate is un- known. Nitrate contents of groundwater from weHs near Lake Nuoertu are already high. Bacterial decomposition processes within the spring mounds may additionaHy con- tribute to nitrate values, but there is no evidence to sup- port this. Minor chemical constituents such as phosphorus are also present. SPRING MOUNDS AND MICROBIAL MATS OF LAKE NUOERTU AND HUHEJARAN Spring Mounds of Lake Nuoertu: Structure of Biofilms and Fabric Development . Several spring mounds rising from the lake bottom have been observed in the southern part of Lake Nuoertu (Fig. 3). The largest one forms a small island approximately 30 TABLE 3-Chemieal eomposilion 01 lake brines, springs, and groundwaler (Lake Nuoertu and Huhejaran). Springwater (mmol/l) Huhejaran Groundwater Lake brines (mol/l) Nuoertu (spriI).gs ne ar (mmol/l) Nuoertu Huhejaran (spring mound th9< shore Nuoertu (n = 4) (n = 4) n = 2) n. = 4) (n = 2) I Density 20°C [g/cm3] 1.075 1.078 , 0.9978 0.9981 TDS [g/l) 97.8 107.1 0.5 0.4 0.4 Conductivity[mS/cm] 91.2 94.8 0.68 1.~9 0.64 pH 10.0 9.9 8.1 8.6 8.1 Na+ 1.495 1.486 3.255 2.593 2.625 K+ 0.112 0.112 0.245 0.115 0.255 Ca2+ 0.000295 0.001715 0.705 0.381 0.730 Mg2+ 0.00615 0.004483 0.740 0.405 0.790 NH. + 0.12 CI - 0.786 0.786 2.360 1.318 1.875 SOl- 0.113 0.111 1.760 0.505 0.690 N03 - 0.000365 0.00062 0.865 0.405 0.475 N02- 0.002 0.002 0.002 HC03 - 0.1015 0.1161 1.55 1.63 2.410 COl - 0.2614 0.3544 PO.3- 0.000006 0.000004 0.003 0.001 0.001 B 0.011976 0.01249 0.0216 0.0311 0.0211 MICROBIAL FABRIC FORMA TlON IN SPRING MOUNDS 585 FIGURE 4-Aragonite precipitation and labric development in spring mound biolilms 01 Lake Nuoertu. (A) Mucilagenous biolilm 01 the spring mound surface at the immediate water line, showing initial aragonite precipitates and their spatial concentration (arrows) around shrinkage voids and agas bubble. Plane light. Scale bar is 1 mm. (8) Formation 01 microcrystalline laminae at the border 01 shrinkage voids within the mucilagenous biolilm by spatial concentration and lurther growth 01 the initial ly precipitated aragonite crystals. Note the increased birelringence 01 successively cleaved polysaccharides in partly dehydrated areas (arrows) . Epitaxial " inorganic" precipitation 01 acicular to botryoidal aragonite is restricted to "mucus-Iree" surfaces within water-lilled voids. Cross polarized light. Scale bar is 250 f.l.m. (C) Complete section 01 a thin Phormidium-biolilm (arrows; stained with acridine orange) on top 01 spring-mound carbonate at less than 10 cm water depth. Note the highly porous Iramework composed 01 irregular stacks 01 laminae and voids. Plane light. Scale bar is 1 mm. (D) Fossil analogue 01 the spring-mound carbonates described herein. So-called "sickle-cell-limestones" lorm meter- to decameter-sized mounds in the Miocene Ries-crater lake, Southern Germany. Plane light. Scale bar is 1 mm. m in diameter. Active groundwater discharge is evident by a number of freshwater springs at fractures or seeps ne ar the shoreline. Submerged surfaces ofthis "tufa island" are covered by presently calcifying cyanobacterial biofilms. Tufa sampies were taken below water depths of 45 cm, 30 cm, 20 cm, and less than 10 cm to obtain lateral and ver- tical variations in biofilm structure and fabric of the pre- cipitated carbonate rocks. Sampies from 45-cm water depth show sparsely miner- alized biofilms of soft, mucilaginous-fibrous constitution. These are dominated by Phormidium, a filamentous cya- nobacterium. Small carbonate crystals, 0.25 to 1 f.Lm in size, are distributed in the EPS between cyanobacterial filaments and their sheaths. These small crystals are spin- dle- and needle-shaped, and are herein called "seed crys- tals". Detrital particles, such as quartz and feldspar grains, allochthonous carbonate crystals, partly calcified pellets, and arthropod remains, are bound in the biofilm. Early shrinkage of the EPS results in a spatial accumula- tion of seed crystals to form layers or patches separate from accumulated detrital components. At 20-30 cm water depth , parts of the biofilm between the living cyanobacterial plexus and the tufa substrate show a high degree of shrinkage and condensation of the EPS with numerous crystal aggregates. The seed crystals tend to be horizontally oriented and dispersed (Fig. 4A, B), but locally clotted or spheroidal aggregates with dark cen- ters also occur. Additionally, lenticular and bubble-shaped voids are developed within the shrinking mucus. The boundaries of the mucus towards the voids are extremely enriched in primary precipitated crystals, that coalesce by further growth to form micritic, irregular laminae (Fig. 4B). Sections from sampies taken below the immediate wa- ter line (less than 10 cm water depth) exhibit a similar de- velopment 50-350 f.Lm below the living Phormidium layer (Fig. 4C). In addition, colonies of Aphanocapsa (a coccoid cyanobacterium) are present in the living layer. These 586 w.ter depth 40cm o I dark green filamenrous I <::.-,--7 cyanob.cterial m.t I