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1.
Few global syntheses of oxygen and carbon isotope composition of pedogenic carbonates have been attempted,unlike marine carbonates.Pedogenic carbonates represent in-situ indicators of the climate conditions prevailing on land.The δ~(18)O and δ~(13)C values of pedogenic carbonates are controlled by local and global factors,many of them not affecting the marine carbonates largely used to probe global climate changes.We compile pedogenic oxygen and carbon isotopic data(N= 12,167) from Cretaceous to Quaternary-aged paleosols to identify potential trends through time and tie them to possible controlling factors.While discrete events such as the PaleoceneEocene Thermal Maximum are clearly evidenced,our analysis reveals an increasing complexity in the distribution of the δ~(18)O vs δ~(13)C values through the Cenozoic.As could be expected,the rise of C_4 plants induces a shift towards higher δ~(13)C values during the Neogene and Quaternary.We also show that the increase in global hypsometry during the Neogene plays a major role in controlling the δ~(18)O and δ~(13)C values of pedogenic carbonates by increasing aridity downwind of orographic barriers.Finally,during the Quaternary,an increase of 3‰ inδ~(18)O values is recorded both by the pedogenic carbonates and the marine foraminifera suggesting that both indicators may be used to track global climate signal.  相似文献   

2.
Ana M. 《Earth》2003,60(3-4):261-298
Interest in palustrine carbonates and calcretes has increased over the last 20 years since they contain significant environmental information. Much of the work performed in this area has focused on either of two types of terrestrial carbonate—palustrine carbonates or calcretes (pedogenic and groundwater)—yet their simultaneous study shows there may be a gradual transition from one form to the other, revealing the interplay between pedogenic, sedimentary, and diagenetic processes. Three main factors control the formation of these carbonates: the position of the water table, the host rock, and the period of sub-aerial exposure. In pedogenic calcretes, precipitation of carbonate takes places mostly in the vadose zone above the water table, and within a previous host rock or sediment. In groundwater calcretes, the precipitation of carbonate also occurs within a previous host rock and around the groundwater table. In palustrine carbonates, however, the precipitation of lime mud occurs in a lacustrine water body. Palustrine carbonates necessarily form on previous lacustrine mud, whereas both types of calcretes may form on any type of sediment or soil. The sub-aerial exposure time needed to form palustrine carbonates may by relatively short (even a season), whereas pedogenic calcretes need more time (several years to millions of years). Groundwater calcretes do not form on the topographic surfaces, so there is no need of sub-aerial exposure. However, stable surfaces favour the development of thick groundwater calcretes. Small fluctuations in the water table cause gradual transitions of these three types of terrestrial carbonates and the subsequent mixture of their characteristic features, causing difficulties in the interpretation of these carbonates.

The formation of these carbonates is controlled by palaeoenvironmental factors. Both commonly form in semi-arid climates. Arid climates are also suitable for calcretes, but sub-humid conditions are more suitable for palustrine carbonates. More indications of climatic conditions may be obtained through the analysis of the δ18O content of both calcretes and palustrine carbonates, and from the depth of the horizon containing carbonate nodules in pedogenic calcretes. Vegetation is also important in the formation of these types of carbonates. Data on the prevailing vegetation can be obtained from the analysis of the micro and macrofabric as well as from the δ13C signal of the primary carbonates, which, in pedogenic carbonates, has also been used to estimate atmospheric pCO2 during the Phanerozoic. These terrestrial carbonates are widely distributed on floodplains and distal areas of alluvial basins. Their presence and characteristics can be used as indicators of aggradation, subsidence or accommodation rates, and therefore as indicators of different tectonic regimes.

Even though the study of these carbonates has notably increased in recent years, much less is known about them than about marine carbonates. Presently, there is much emphasis on obtaining a general model for sequence stratigraphy in terrestrial basins, with a need to include the carbonates analysed in this paper.  相似文献   


3.
In contrast to well-studied paleoenvironments of Plio-Pleistocene hominin sites in East Africa, little is known about the ecology of the earliest North African human occupation sites. The recent studies at Ain Hanech and El-Kherba in northeastern Algeria have broadened the range of Plio-Pleistocene hominin ecology to include the earliest known archaeological sites documented in North Africa. Ain Hanech and El-Kherba are significant for yielding savanna-like faunas associated with Oldowan stone tools dated back to approximately 1.8 million years ago. This paper focuses on reconstructing the ecology of Ain Hanech and El-Kherba based on excavated faunas and stable carbon isotope of pedogenic carbonates from El-Kherba stratigraphic profile. The results point out to an overall open paleolandscape and a C3 predominantly paleovegetation both woody and grasses. In addition, the stable-carbon-isotopic evidence shows a clear environmental change through time at El-Kherba, which likely impacted hominin foraging activities in level A.  相似文献   

4.
Pedogenic carbonates were collected from Early Cretaceous strata in Sichuan and Liaoning, China. These paleosol carbonates and calcareous paleosols were evaluated in order to reconstruct atmospheric CO2 concentrations during the Early Cretaceous using a paleosol barometer. Using the isotopic ratios of pedogenic carbonates from Early Cretaceous (early-middle Berriasian, early Valanginian) strata in Sichuan Basin, averaged atmospheric pCO2 is estimated to have been 360 ppmv in the early-middle Berriasian and a mean value of 241 ppmv in the early Valanginian. In the late Barremian in western Liaoning, however the average was 530 ppmv, with a range of 365 ppmv and 644 ppmv, lower than previous estimates of pCO2 for these time periods, consistent with the suggestion of overall climate cooling and paleotemperature fluctuation during the Early Cretaceous. This indicates that not all of the Cretaceous was a high or continuous CO2 greenhouse, especially during Early Cretaceous.  相似文献   

5.
土壤次生碳酸盐碳氧稳定同位素古环境意义及应用   总被引：9，自引：1，他引：9  
土壤次生碳酸盐碳氧稳定同位素特征是反映古气候与古环境的重要代用指标,其碳氧稳定同位素组成分别受土壤CO2中C同位素组成和大气水的O同位素控制。在一定深度的土壤中,土壤次生碳酸盐δ^13C就主要受当地植物类型(C3植物和C4植物等)控制。土壤次生碳酸盐样预处理中剔除土壤中原生碳酸盐以及有机物污染尤为重要。土壤中次生碳酸盐C、O稳定同位素地球化学在土壤发生学、古气候恢复、古生态重建以及全球变化研究中应用日益广泛,但解译时可能受应用年代范围、成岩作用、原生和次生碳酸盐混杂、土壤碳酸盐多元发生等因素影响,其应用机理和范围还需进一步探讨。  相似文献   

6.
The influence of the pedogenic and climatic contexts on the formation and preservation of pedogenic carbonates in a climosequence in the Western Ghats (Karnataka Plateau, South West India) has been studied. Along the climosequence, the current mean annual rainfall (MAR) varies within a 80 km transect from 6000 mm at the edge of the Plateau to 500 mm inland. Pedogenic carbonates occur in the MAR range of 500-1200 mm. In the semi-arid zone (MAR: 500-900 mm), carbonates occur (i) as thick hardpan calcretes on pediment slopes and (ii) as nodular horizons in polygenic black soils (i.e. vertisols). In the sub-humid zone (MAR: 900-1500 mm), pedogenic carbonates are disseminated in the black soil matrices either as loose, irregular and friable nodules of millimetric size or as indurated botryoidal nodules of centimetric to pluricentimetric size. They also occur at the top layers of the saprolite either as disseminated pluricentimetric indurated nodules or carbonate-cemented lumps of centimetric to decimetric size.Chemical and isotopic (87Sr/86Sr) compositions of the carbonate fraction were determined after leaching with 0.25 N HCl. The corresponding residual fractions containing both primary minerals and authigenic clays were digested separately and analyzed. The trend defined by the 87Sr/86Sr signatures of both labile carbonate fractions and corresponding residual fractions indicates that a part of the labile carbonate fraction is genetically linked to the local soil composition. Considering the residual fraction of each sample as the most likely lithogenic source of Ca in carbonates, it is estimated that from 24% to 82% (55% on average) of Ca is derived from local bedrock weathering, leading to a consumption of an equivalent proportion of atmospheric CO2. These values indicate that climatic conditions were humid enough to allow silicate weathering: MAR at the time of carbonate formation likely ranged from 400 to 700 mm, which is 2- to 3-fold less than the current MAR at these locations.The Sr, U and Mg contents and the (234U/238U) activity ratio in the labile carbonate fraction help to understand the conditions of carbonate formation. The relatively high concentrations of Sr, U and Mg in black soil carbonates may indicate fast growth and accumulation compared to carbonates in saprolite, possibly due to a better confinement of the pore waters which is supported by their high (234U/238U) signatures, and/or to higher content of dissolved carbonates in the pore waters. The occurrence of Ce, Mn and Fe oxides in the cracks of carbonate reflects the existence of relatively humid periods after carbonate formation. The carbonate ages determined by the U-Th method range from 1.33 ± 0.84 kyr to 7.5 ± 2.7 kyr and to a cluster of five ages around 20 kyr, i.e. the Last Glacial Maximum period. The young occurrences are only located in the black soils, which therefore constitute sensitive environments for trapping and retaining atmospheric CO2 even on short time scales. The maximum age of carbonates depends on their location in the climatic gradient: from about 20 kyr for centimetric nodules at Mule Hole (MAR = 1100 mm/yr) to 200 kyr for the calcrete at Gundlupet (MAR = 700 mm/yr, Durand et al., 2007). The intensity of rainfall during wet periods would indeed control the lifetime of pedogenic carbonates and thus the duration of inorganic carbon storage in soils.  相似文献   

7.
MIS 11 is often considered to be the best climatic analogue for the Holocene. Many studies have suggested, however, that it is a period of extreme climate warmth comparable in temperatures to the Middle and Late Pliocene. In Britain deposits of the Hoxnian interglacial are correlated to MIS 11 and multi-proxy techniques can be used to reconstruct the climate of this interglacial. Soil, groundwater and freshwater carbonates are common in Hoxnian deposits and the stable isotopic composition of these precipitates can be used to increase our understanding of MIS 11 environments in Britain. Carbonates from Marks Tey, Clacton, Swanscombe, Elveden and West Stow are studied, the stratigraphic context of which indicates that their formation is broadly synchronous (in the mid-Hoxnian, pollen zones Ho II to Ho III). The carbon isotopic composition of groundwater and pedogenic carbonates is typically depleted with respect to δ13C (ca −9 to −8‰ VPDB) reflecting uptake of plant respired CO2 during water migration/recharge. The carbon isotopic composition of lacustrine carbonate is more enriched with respect to δ13C (ca 0-1‰VPDB) reflecting the equilibration of lake waters with atmospheric CO2. The δ18O of groundwater and pedogenic carbonates is slightly more enriched than modern soil carbonates but not as enriched as soil carbonates formed under interglacials that were warmer than the Holocene (i.e. the Cromerian). The stable isotopic composition of Hoxnian carbonates does not, therefore, indicate that this interglacial was characterised by uniquely warm climates in the context of other Middle Pleistocene interglacials and the Holocene. This is contrary to many marine and littoral records from around the world but consistent with environmental records from Britain and Europe.  相似文献   

8.
定量评价半干旱岩溶区土壤次生碳酸盐比例和来源有助于认识土壤系统影响岩溶作用的机理。选取山西晋中盆地西南,吕梁山东侧的半干旱岩溶区马跑神泉小流域为研究对象,通过对林地、退耕地、灌丛地土壤剖面进行分层取样并测定碳酸盐含量及其δ13C、CO2浓度及其δ13C值,分析其随深度的变化规律和控制因素;并结合研究区碳酸盐岩的δ13C值计算3个剖面各层土壤次生碳酸盐所占比例。研究结果表明:3个土壤剖面的碳酸盐含量、CO2浓度在0~50 cm土层随深度增加而增加,在50~70 cm土层随深度增加而减少;土壤碳酸盐δ13C值、δ13CCO2值在0~50 cm土层随深度增加而偏负,在50~70 cm土层随深度增加而偏重;土壤碳酸盐含量及其δ13C值主要受次生碳酸盐比例控制,而土壤CO2及其δ13CCO2值在上层主要受大气CO2和土壤有机质分解生成的CO2共同影响,下层还受土-岩界面岩溶作用过程制约;退耕地、林地、灌丛剖面次生碳酸盐所占比例的均值分别为52%、42%和32%,证实北方半干旱岩溶区土壤中存在原生碳酸盐向次生碳酸盐转化过程。   相似文献   

9.
Stable isotopes were measured in the carbonate and organic matter of palaeosols in the Somma–Vesuvius area, southern Italy in order to test whether they are suitable proxy records for climatic and ecological changes in this area during the past 18000 yr. The ages of the soils span from ca. 18 to ca. 3 kyr BP. Surprisingly, the Last Glacial to Holocene climate transition was not accompanied by significant change in δ18O of pedogenic carbonate. This could be explained by changes in evaporation rate and in isotope fractionation between water and precipitated carbonate with temperature, which counterbalanced the expected change in isotope composition of meteoric water. Because of the rise in temperature and humidity and the progressive increase in tree cover during the Holocene, the Holocene soil carbonates closely reflect the isotopic composition of meteoric water. A cooling of about 2°C after the Avellino eruption (3.8 ka) accounts for a sudden decrease of about 1‰ in δ18O of pedogenic carbonate recorded after this eruption. The δ13C values of organic matter and pedogenic carbonate covary, indicating an effective isotope equilibrium between the organic matter, as the source of CO2, and the pedogenic carbonate. Carbon isotopes suggest prevailing C3 vegetation and negligible mixing with volcanogenic or atmospheric CO2. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

10.
The Berriasian Rupelo Formation of the W Cameros Basin consists of a 2–200 m thickness of marginal and open lacustrine carbonate and associated deposits. Open lacustrine facies contain a non-marine biota with abundant charophytes (both stems and gyrogonites), ostracods, gastropods and rare vertebrates. Carbonate production was mainly biogenic. The associated marginal lacustrine (‘palustrine’) facies show strong indications of subaerial exposure and exhibit a wide variety of pedogenic fabrics. Silicified evaporites found near to the top of the sequence reflect a short hypersaline phase in the lake history. The succession was laid down in a low gradient, shallow lake complex characterized by wide fluctuations of the shoreline. Carbon and oxygen stable isotope analyses from the carbonates show non-marine values with ranges of δ13 from ? 7 to ? 11‰and δ18 from ? 3 to ? 7.5‰. Differences in the isotopic composition of open lacustrine carbonates are consistent with sedimentary evidence of variation in organic productivity within the lake. Analyses from the entire sample suite plot on a linear trend; isotopic compositions become lighter with increasing evidence of pedogenic modification. This suggests progressive vadose zone diagenesis and influence of meteoric waters rich in soil-derived CO2. The stable isotope data thus support evidence from petrography and facies relations that ‘palustrine’limestones form through pedogenic modification of lake carbonates.  相似文献   

11.
In the Mesozoic–Cenozoic continental deposits of the Tian Shan area, two main levels containing pedogenic carbonates have been identified on both the southern and northern foothills of the range: one in the Upper Jurassic series and one in the Upper Cretaceous–Lower Palaeocene series. In order to reconstruct the palaeoenvironmental and palaeotopographic characteristics of the Tian Shan area during these two periods, we measured the oxygen and carbon isotope composition of these pedogenic carbonates (calcrete and nodules). The stable isotope compositions are homogeneous: most δ18O values are between 21 and 25‰ and most δ13C values are between −4 and −6‰. No distinction can be made between the calcrete and nodule isotopic compositions. The constancy of isotopic values across the Tian Shan is evidence of a development of these calcification features in similar palaeoenvironmental conditions. The main inference is that no significant relief existed in that area at the Cretaceous−Palaeogene boundary, implying that most of the present relief developed later, during the Cenozoic. In addition to the pedogenic carbonates, few beds of limestones interstratified in the Jurassic series of the southern foothills display oxygen and carbon isotope compositions typical of lacustrine carbonates, ruling out brackish water incursion at that period in the region.  相似文献   

12.
Stable carbon and oxygen isotope measurements have been obtained from ten calcareous soil profiles developed on noncalcareous parent materials in Nevada and New Mexico. Despite the number of variables that can affect the isotopic composition of soil carbonate, most of these profiles show remarkably little variation in δ 13C and δ 18O with depth. This tendency towards uniformity is attributed to the fact that pedogenic carbonate forms by the incremental accumulation of a large number of small aliquots of CaCO3. As the number of aliquots of CaCO3 increases, the composition of their sum, which is the calcareous horizon itself, tends to converge to the long-term average of the individual aliquots. A mathematical model is presented to show how the time required for convergence is affected by the amplitude and period of aliquot variation and the extent of dissolution and reprecipitation of previously deposited carbonate. Calculations further indicate that in some cases the variation in δ 18O of soil carbonates may be moderated by the opposing effects of temperature change on the H2OCaCO3 fractionation factor and on the extent of evapotranspiration.The data of this study indicate that soil carbonates from sites within a homogeneous climatic region show relatively little site-to-site variation in δ 18O as compared to geographical variation in δ 13C. This suggests that the δ 18O of soil carbonates is primarily controlled by broad climatic factors (rainfall composition and temperature) whereas site factors (e.g., soil CO2) have a strong effect on δ 13C. In Nevada and New Mexico the δ 18O of Pleistocene calcareous profiles is 2.0–4.0‰ lighter than that of nearby Holocene soil carbonate, suggesting that old well-developed profiles may have formed dominantly under cooler pluvial conditions.  相似文献   

13.
Based on the organic and carbonate carbon levels of the top 1 m of soil from desertified soils of Northern People's Republic of China, climatic and vegetative cover zones have been derived for some 334000 km2 of desertification-prone lands. Regional accumulations of pedogenic carbonates were examined relative to precipitation, altitude, and temperature. The largest accumulations of pedogenic carbonates were found in Calcic soils in warm, arid areas. Accumulated organic carbon predominated in soils under Betula platyphylla. In the naturally desertified lands of China, for example, the top 1.0-m soil layer contains some 7.84 Pg of organic carbon and 14.9 Pg of carbonate carbon. Total stored carbon, including carbonate carbon, is 1.8-fold more than organic carbon alone. The carbon released through land desertification in China may be an important factor affecting changes in concentrations of greenhouse gases worldwide.  相似文献   

14.
The relationship between the stable C-isotope composition of the soil environment and modern climate and vegetation was determined empirically along a present-day climatic transect in the eastern Mojave Desert. The δ13C of the soil CO2 and carbonates decreased with increasing elevation and plant density, even though plant assemblages at all elevations were isotopically similar. Several factors, including differences in the ratios of pedogenic of limestone calcite and differences in past vegetation, were considered as explanations of this trend, However, it appears that in the sparsely vegetated Mojave Desert, the δ13C of pedogenic carbonate is controlled by differences in plant density and biological activity. This relationship may provide a tool for assessing past vegetational densities, as long as the vegetation is isotopically homogeneous.  相似文献   

15.
The Mullipallam creek in Muthupet mangroves region is the only E-W trending coastal strip in the SE coast of India and is very important, as the mangrove acts as a barrier to natural diasters. Natural, anthropogenic signals and accumulation of elements were made by collecting sediment samples at various depths in a core. All sediments were analyzed for carbonates (CaCO3), organic carbon (OC), major (Si, Al, Fe, Na, K, Ca, Mg, P), and trace (Mn, Cr, Cu, Ni, Co, Pb, Zn). Normalization with Al values has been done for all the major and trace elements and enrichment factors have been calculated. The calculated enrichment factors and comparison indicate that the trace metals (especially Pb) are enriched mainly due to the external (anthropogenic) activities in the land as well as in the coastal zone (Palk Strait).  相似文献   

16.
Injection carbonatites of volcanic-plutonic origin with Au-bearing base-metal sulfide mineralization were established in 2006–2007 along the southern slope of the Byrranga Range in the Russian continental sector of the Arctic. According to thermobarogeochemical data, carbonatites were formed at a temperature not lower than 380–410°C from boiling gas-liquid H2O-CO2-salt fluids under decompression. The REE concentrations and patterns of sedimentary and magmatic carbonates are sharply distinct. Carbonatites are enriched in REE as compared to carbonates from limestone and are characterized by an REE distribution close to MORB. The geochronological and isotopic geochemical study of zircons has shown that carbonatite bodies (lavas?) were formed 238–219 Ma ago (Middle-Late Triassic); zircon crystallized at a high temperature. Detrital zircons from the Lower Carboniferous crinoid limestone are not younger than 594 Ma. The absolute predominance of Precambrian zircons, which are variable in age, indicates that they were captured from the crust, when carbonatitic magma ascended from subcrustal depths.  相似文献   

17.
Abundant Lower Cretaceous (Berriasian–Hauterivian) paleosols have been recognized in the Sichuan Basin, along with the preserved pedogenetic features, e.g., soil horizons, soil structure, root traces and pedogenic nodules. Chemical, geochemical and mineralogical analyses were used to examine the paleosols. These paleosols were classified as Entisols, Inceptisols, Aridisols and Alfisols in terms of the modern soil taxonomic system. Early Cretaceous paleoprecipitation and paleotemperature in the Sichuan Basin were estimated from the degree of chemical weathering for non-calcareous paleosols, and from the depth to the calcic horizon and stable oxygen isotopic composition of pedogenic carbonates in calcareous paleosols, respectively. A temperate semi-arid climate generally prevailed in the Sichuan Basin as a part of the South China Block (SCB) and was controlled by subtropical high-pressure and a rain-shadow effect because the humid air masses from the Paleo-Pacific were impeded by the highlands of the South China Block. Further, several intervals of sub-humid paleoclimate occurred due to strengthened monsoonal circulation in the Early Cretaceous. Using the paleosol barometer, the paleoatmospheric CO2 levels of the Early Cretaceous are estimated to range from ∼120 to ∼520 ppmv, with a mean of 305 ppmv. Regional temperature is generally coupled with atmospheric CO2 concentration and is roughly consistent with the sea level fluctuation.  相似文献   

18.
Radiocarbon dating of the organic-rich sediments of Lake Illisarvik in the outer Mackenzie Delta indicates that formation of the lake occurred approximately 9500yr BP, with maximum expansion around 6000 yr BP. Sedimentation rates have remained relatively constant at an average of 0.3mm/yr. 13C results on biogenic and inorganic carbonates and organics indicate a change from dominantly terrestrial organics (?27 to ?28%0) to submerged aquatic vegetation or plankton (?18 to ?23%0) upon formation of the lake (9500yr BP), and a dramatic return to dominantly terrestrial organics at 5800yr BP (δ13C = ?27 to ?30%0). This latter shift is accompanied by a drastic reduction in the macroflora and fauna populations. 18O results suggest that a warmer climate than today existed prior to the shift at 5800yr BP.  相似文献   

19.
Cenozoic atmospheric circulation, climatic changes, sedimentation and weathering over the Indian sub-continent were mainly influenced by the northward drift of the Indian Plate, the shrinking Paratethys, India-Asia collision and the rise of the Himalayas. This study is aimed at exploring the fluvial sedimentary record of the north-west part of the Himalayan Foreland Basin to interpret weathering and pedogenesis during early Oligocene to Mid-Miocene time. Palaeopedological investigation of a 3.1 km thick succession from Kangra sub-basin of the Himalayan Foreland Basin shows that the lower 2 km part of the succession is characterized by the red (10R hue) and the upper 1.1 km part of the succession by the yellow (2.5Y hue) palaeosols with varying intensity of weathering and pedogenesis. The association of sedimentary rocks and pedogenic expression in palaeosols indicate four (Type-A to Type-D) pedofacies in the entire Oligocene–Miocene succession. The pedofacies are defined by a decrease in the intensity of palaeopedogenic development from strongly-developed palaeopedofeatures in Type-A, moderately-developed palaeopedofeatures in Type-B, weakly-developed palaeopedofeatures in Type-C and to the only incipient stage of palaeopedogenesis in Type-D pedofacies. The palaeolatitudinal shift during the convergence of the Indian Plate played a major role in weathering and palaeopedogenesis with the inception of seasonality during the early Oligocene, which is demonstrated by the formation of the red palaeosols with pedogenic CaCO3 and vertic features in tropical conditions. The transition to yellow palaeosols at about 20 Ma is marked by increased humidity, rapid aggradation, pronounced uplift and enhanced erosion of the hinterland. These yellow palaeosols are characterized by the abundance of weakly-developed Bw and Bss horizons, pure clay pedofeatures and absence of any pedogenic CaCO3 during short pedogenic intervals in subtropical conditions.  相似文献   

20.
The formation of pedogenic carbonate (calcrete) in terrestrial environments is commonly mediated by microorganisms. In Australia, Au-anomalous calcrete is an important sampling medium for geochemical exploration, but current models describing its formation do not include a confirmed microbial component. This study demonstrates that bacterial communities in calcareous sands from dunes overlying the Barns Gold Deposit in semi-arid South Australia, are capable of mediating the biomineralisation of Au-anomalous carbonates. Bacterial enrichment cultures obtained from calcareous sands at three depths (0.1, 0.64 and 2.1 m, plus abiotic control) were incubated in urea and Ca2+-containing growth media (pH 8), unamended and amended with Au (100 parts-per-billion, ppb) as Au–aspartic-acid complex. During the incubation of the enrichment cultures urea was turned over to NH4+ within 96 h to 220 h. The solution pH increased concurrently by approximately 1.2 units, and Au-anomalous Ca-carbonate crystallites were precipitated on cells, which functioned as nucleation sites; no carbonate precipitation was observed in abiotic controls. Compared to the medium, Au was strongly enriched in these carbonates and appeared to be uniformly dispersed in the individual crystallites, as shown using LA-ICP-MS; a similar distribution is present in naturally occurring Au-anomalous calcrete. Phylogenetic 16S rRNA PCR DGGE analyses, shotgun cloning and functional microbial analyses (BioLog, ureC quantitative PCR) demonstrated that naturally occurring and culture-enriched bacterial communities were dominated by alkaliphylic, halotolerant Bacillus spp. The indigenous bacterial communities were capable of utilising amino acids (including l-aspartic acid) and urea, which appears to lead to the destabilisation of the Au–amino acid complexes and concomitant co-precipitation of Au in the Ca-carbonates. In conclusion, a model combining geomicrobial– with evapotranspiration– and plant-based components is likely to best describe the formation of (Au-anomalous) calcrete in semi-arid and arid zones.  相似文献   

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