What is That Gunk on My Artifact

Chinese Science Bulletin, 2011

Nearly 18 years after the proposal of the weathering-related carbon sink concept (Berner R A. Weathering, plants and the long-term carbon cycle. Geochim Cosmochim Acta, 1992, 56: 3225-3231), it is an appropriate timing to re-evaluate its geological context with the updated dataset. Ryskov et al. (Ryskov Ya G, Demkin V A, Oleynik S A, et al. Dynamics of pedogenic carbonate for the last 5000 years and its role as a buffer reservoir for atmospheric carbon dioxide in soils of Russia. Glob Planet Change, 2008, 61: 63-69) lately claimed that in the course of soil formation for the last 5000 years the soils of Russia fixed atmospheric carbon dioxide as pedogenic carbonate during the arid periods at a rate of 2.2 kg C/(m 2 a) in chernozem, 1.13 kg C/(m 2 a) in dark-chestnut soil, 0.86 kg C/(m 2 a) in light-chestnut soil, on the basis of carbon isotopic data; however, their interpretations of the data do not appear straightforward nor persuading, and thus their claim is likely misleading. Their interpretations are also contrary to the conclusions drawn by Dart et al. (Dart R C, Barovich K M, Chittleborough D J, et al. Calcium in regolith carbonates of central and southern Australia: Its source and implications for the global carbon cycle. Palaeogeogr Palaeoclimatol Palaeoecol, 2007, 249: 322-334) who found that Australian regolith carbonates did not capture any additional CO 2 ; instead the carbonate was simply being remobilized from one pool to another. Here we raise comments to these explanations on the following two issues: (1) origin of pedogenic carbonate: silicate weathering vs. carbonate weathering, and (2) problems in using carbon isotopic technique to distinguish carbonates formed by silicate weathering and carbonate weathering. It is concluded that pedogenic carbonate may not be an important atmospheric CO 2 sink at all, i.e. carbonate weathering-related pedogenic carbonate does not capture any additional CO 2 , while the CO 2 capture in silicate weathering-related pedogenic carbonate is small in short-term time scales due to the slow kinetics of silicate weathering. pedogenic carbonate, carbonate weathering (carbonate dissolution-reprecipitation), silicate weathering, atmospheric CO 2 sink

Carbonates are important constituents of many soils throughout the world (IUSS Working Group WRB, 2006); they play an active role in the sequestration of atmospheric C and are extremely useful for paleoenvironmental reconstructions (Sheldon and Tabor, 2009). However, soil carbonates may have different origins and it is therefore important to distinguish and quantify pedogenic and lithogenic material for the correct assessment of their role as C sink, and for the interpretation of past climatic conditions. For this purpose, the most reliable methods used are those based on carbon isotopic fractionation proposed by Salomons and Mook (1976) and Nordt at al. (1998). A complex geologic setting may however bias the interpretation of the results and the isotopic methods may be not sufficient for a correct interpretation. The precipitation of pedogenic carbonates deeply affect soil structure, by changing the pore volumes, and surface properties, through the increased presence of low specifi...

Geoderma, 2012

Stable isotope C analysis is the most reliable method used for the distinction and understanding of soil carbonates origin. However, in soils with a complex geological setting the carbonate δ 13 C signature could lead to incorrect interpretations if used alone. Thus coupling this technique to other methods may be necessary. In this work we evaluated advantages and disadvantages of several methods, some of which are well known while others are still unused, to distinguish among carbonates of different origins in a soil developed on "Valle Versa Chaotic Complex", a marly geological formation in North-western Italy. For a better evaluation of their potentialities the methods were also applied to simpler situations used as a reference for carbonate of pedogenic and lithogenic origins. Thin sections analysis revealed the presence of three kinds of carbonates in the investigated complex soil: one was pedogenic, while two showed clear lithogenic origin. The lithogenic carbonate that showed a low δ 13 C (about − 9‰) was interpreted as freshwater while isotopic signature increased up to − 4‰ with the presence of marls, thus no evidence of pedogenic precipitations could be obtained with isotopic analysis. The mean crystallite domain (L 104 ) was highly variable and related to the amounts of co-precipitated impurities in the carbonates. Thus these methods provided important information about the formation environment. These rarely used techniques permitted to distinguish between pedogenic and lithogenic material in simple systems, but did not adequately support the presence of pedogenic carbonates in the complex soil. Surface areas and porosity evaluated by N 2 adsorption are particularly influenced by the processes occurring during calcification such as the development of coatings and pore infillings. The comparison between additive models and measured specific surface area, indeed allowed us to observe the effect of pedogenic carbonate on the physical properties, although it did not permit any quantification. These results indicated that, although all the methods were able to distinguish between pedogenic and lithogenic origins in simple systems, only micromorphology and N 2 adsorption techniques allowed for the identification of pedogenic carbonate in a more complex soil system.

This article presents a research study on the deposition process of Ca 2C induced by Synechocystis sp. PCC6803 in BG11 liquid medium with different Ca 2C concentrations and different pH. The changes of Ca 2C concentrations were measured by using atomic absorption method and the corresponding dynamical models were studied. Minerals and cells were analyzed by high resolution transmission electron microscope, selected area electron diffraction, scanning electron microscope, energy dispersive X-ray spectroscope, X-ray diffraction. The selected area electron diffraction patterns were analyzed by Digital Micrograph 3.7 software. The result showed that Ca 2C concentrations decreased faster in the experimental group. The changes of calcium carbonate precipitation were fitting to an exponential model. PH 7 and Ca 2C concentration of 1.5 g/L were most conducive to calcium carbonate precipitation in the corresponding gradient range. The result of high-resolution transmission electron microscopy showed that minerals in the experimental group differed obviously from that of the control group in the surface morphology, but both of them were calcites. It also showed that a certain number of minute calcites adhesion to the outer surfaces of S. PCC6803 cells. The result of scanning electron microscopy displayed that many sunken holes emerged on the surfaces of the prismatic calcium carbonate minerals. The results of X-ray diffraction proved that minerals induced by S. PCC6803 were calcites with preferential orientation. This article discusses the process of carbonate formation and the possible role played by S. PCC6803. It may be useful to further study the mechanism of microbial carbonates deposition in the field of geology.

Geology, 1991

Pedogenic calcite in desert soils has become increasingly important as an indicator of paleoclimate, landscape stability, and landscape age. This study indicates that calcic and petrocalcic horizons in desert soils are not simply the result of inorganic precipitation of calcite. Soil microorganisms were found to be involved in calcite precipitation in a typical desert soil near Las Cruces, New Mexico. Fossilized remains of calcified fungal hyphae and Microcodium structures are abundant in the petrocalcic horizon. Soil bacteria and fungi precipitated calcite when cultured on a Ca-rich medium. In an experiment where soil columns were irrigated with Ca-rich solutions, calcite formed in soils containing soil microorganisms, but no calcite formed in sterile soils. Thus, biomineralization of calcite by soil microorganisms appears to be an important mechanism of unknown magnitude.

Chemical Geology

Significant amounts of pedogenic Ca-carbonate nodules have been observed in the Far North Region of Cameroon in the carbonate-free watershed of the Mayo Tsanaga, thus a priori not favourable for carbonate nodules accumulations. These nodules are associated with a Clay-Rich Parent Material (CRPM), covering either a granitic (upstream) or a greenstone bedrock (downstream). In this peculiar context, the amount of pedogenic carbonate nodules represents large quantities of Ca and C. Therefore, determining the Ca sources for pedogenic carbonate nodules is a key point regarding the mechanisms leading to carbonate nodule precipitation and their role in the biogeochemical cycle of Ca. Three sites, two on granite and one on greenstone, were studied by combining Sr et Nd isotope systematics in order to assess the Ca sources of carbonate nodules and the origin of the CRPM associated with nodules. Carbonate nodules have a distinct Sr isotopic composition in each profile, pointing to the contribution of a local source for Ca. Sr isotopic compositions of plagioclases and carbonate nodules display a concomitant variation in each profile, indicating that plagioclases acted as a Sr, and thus Ca, source for the carbonate nodules. Nevertheless, carbonate nodules have a higher Sr isotopic signature than plagioclases, implying the contribution of another more radiogenic Sr source. Sr and Nd isotope data from the CRPM show that it is a mixture of weathered bedrock compounds and Saharan dust. This result highlights two other potential Sr and Ca sources: biotite and Saharan dust. Calculations of their respective contributions demonstrate that in situ weathering significantly adds to the Ca source of nodules. This result contrasts with many other studies from similar settings that highlight the importance of allochthonous-marine, aeolian-inputs. This conclusion points out that a substantial part of Ca is transferred from the local bedrock to the carbonate nodules. This mechanism seems to have been possible as peculiar edaphic conditions preserved the primary Ca sources, making in situ weathering a significant process for providing Ca.

Geoderma, 2003

Limited information is available about inorganic carbon stocks in the boreal regions. The objective of this study was to determine the amount and accumulation rate of pedogenic carbonate (PC) in soils of the boreal grassland and forest regions of Saskatchewan, Canada. The storage of pedogenic carbonate increases from 134 kg m À 2 in semiarid grassland (Brown soils) in the southwest to 165 kg m À 2 in the northeast, under forest (Gray soils), within the time decreasing from 17,000 years in the southwest to 11,500 years in the northeast. The rate of pedogenic carbonate accumulation likewise increases from 8.3 to 14.3 g m À 2 year À 1 in the same direction. The results show that the soils of the prairies and forests have sequestered 1.4 times more C in the form of pedogenic carbonates than as organic matter. Stable carbon isotope values of pedogenic carbonate decreases from southwest to northeast. This is consistent with decreasing representation of C4 plants in the vegetation in the same direction. The rate of pedogenic carbonate accumulation increases with increasing annual precipitation. This suggests that the rate-limiting factor to precipitate with CO 2 is Ca in the boreal region of Canada. Silicate weathering is more significant in Luvisols (Alfisols), suggesting that they may be most effective in truly sequestering additional amount of C in the soil.

Geoderma, 2013

Soil carbonates are key features in soils of arid and semiarid environment, playing an important role from pedogenetic, landscape history, paleoclimatic and environmental points of view. The objectives of this work were (i) to study pathways of pedogenic carbonate (PC) formation, (ii) to distinguish between lithogenic and pedogenic inorganic C by using the natural C isotope abundance, and (iii) to estimate the soil C pools in a gypsiferous semiarid Mediterranean environment (Sicily, Italy). Five soil pedons developed on calcareous and non-calcareous parent materials from Holocene (10,000 years BP) to Upper Tortonian (7.2-5.3 Ma BP) in age were surveyed. During field soil description, the highest stage of carbonate morphology was found in soils developed on non-calcareous Holocene colluvial deposits (youngest deposits in age) which also showed the highest amount of PC. The great amount of PC in soils developed on youngest deposits was ascribed to a soil-landscape relationships. Being located in a doline overhung by gypsum outcrops, precipitation of Ca 2+ from gypsum dissolved by rainfall and biogenic CO 2 is reliable. The significant positive relationship between soil organic C and pedogenic carbonates δ 13 C values confirms that PC was formed from biogenic CO 2 . Organic C pool in the first cubic meter of soil ranged from 17 to 42 kg, whilst pedogenic inorganic C pool from 2.8 to 30.7 kg. The estimated rate of inorganic C accumulation in soils developed on youngest deposits was 2.5 g m −3 y −1 , whereas the rate was negligible on older parent material. The hypothesized pathways of PC formation were ex-novo precipitation of gypsum-Ca 2+ and biogenic CO 2 and dissolution of lithogenic CaCO 3 and re-precipitation of Ca 2+ with biogenic CO 2 . From an environmental prospective, investigated soils may act as a sink of C when Ca 2+ from gypsum is available for the formation of pedogenic carbonates.

Management of Environmental Quality An International Journal

Carbon isotopic analysis is a useful tool for investigating paleoenvironments, as the pedogenic carbonate δ13C is related to δ13CSOM and to the proportions of C3/C4 plants. In this work we interpreted the paleoenvironmental conditions at the time of carbonate precipitation in soils formed under different climates and during different geological ages. Samples were taken from a Bk (PR1, Holocene) and from two Bkm horizons (PR2 and PR3, Pleistocene). When the mean δ13C plant values and the most plausible paleotemperatures were used in the evaluation, PR1 showed a lower percentage of C4 plants (48%) than Pleistocene soils (~53%), in agreement with paleoclimate changes. When instead the δ13C values of current plants were used for PR1, C4 plants ranged from 59 (12°C) to 66% (18°C), suggesting two possible interpretations: either plant species changed during the Holocene, or the plant mean values normally used in the literature are not suitable for Pleistocene reconstructions.

Encyclopedia of Earth Sciences Series, 2011