Carbonate Terrains Research Articles

Drainage evolution in porous carbonate terrain in semi-arid environments: The role of tectonics and climatic change – The case study of the Boukadir carbonate platform, Algeria

Porous carbonate terrains are widespread around the Mediterranean area and form non classical karstic landscapes. The related diffuse infiltration is expected to impede karstification and the establishment of dense drainage networks. In such terrain, the effect of tectonics and climate on drainage development and evolution remains poorly understood. To address this issue, we focus on a semi-arid area with a tuffaceous Messinian carbonate platform characterized by a well-established dendritic network in an active tectonic context. The platform is situated in the Chélif Basin in Algeria, affected by transpressive deformation in relation to the Africa-Eurasia collision. We combine geomorphological observations with drainage morphometric analyses using satellite images and DEM. The drainage network analysis reveals varying tectonic deformation and morphological anomalies resulting from instability in the drainage systems, expressed through (1) variable platform tilt and (2) the presence of knickpoints and convexities in river profiles. These anomalies are particularly pronounced in the west, where knickpoints are more numerous, and incisions deeper. We relate the origin of the tilt to the differential marl compaction and remobilization, and in the area with the highest tilt a recent propagation of the Boukadir thrust underneath the platform might also play a role. Finally, we propose a general model of drainage evolution which highlights the importance of weathering in the Maghreb area transforming the porous media in an indurated carbonate or calcrete.

Morphogenesis of a new landform responsive to superimposed sulphate and carbonate buried karsts in western Canada

AbstractThis case study documents the morphogenesis of a new karstic landform consisting of a calcareous tufa mound with a gypsum caprock that developed at La Saline Lake, an oxbow of the Athabasca River in northeast Alberta, Canada. The development of this novel tiered carbonate‐sulphate mound architecture resulted from emplacement above the largest known buried salt dissolution collapse‐subsidence karst system, which was superimposed by an overlying buried carbonate karst. Dissolution of halite‐anhydrite beds of the Middle Devonian Prairie Evaporite, 200 m below, resulted in extensive subsidence and brecciation of the overlying Upper Devonian carbonate karst. The development of the calcareous tufa mound resulted from meteoric‐driven groundwater mixed with glacial meltwater directed along the shallowly buried karstic carbonate terrain of Middle‐Upper Devonian limestone, which floors the McMurray Formation, the main reservoir unit of the Lower Cretaceous Athabasca Oil Sands. The flow of the carbonate‐saturated groundwater responsible for the emplacement of the tufa mound at the surface was abruptly redirected along a deeper pathway in response to dissolution collapse‐subsidence adjustments within the buried salt karst of the Prairie Evaporite. The redirected groundwater flow deeper encountered an anhydrite‐dominated segment of the salt dissolution trend within the Prairie Evaporite Formation, 200 m below. The migration up‐section of sulphate‐saturated brine onto the surface of the previously formed tufa mound resulted in the emplacement of a gypsum caprock as a new landform.

Controls on long-term denudation rate of carbonate terrains in the Eastern Mediterranean

Quantifying denudation rates under different climatic and tectonic settings is fundamental to understanding landscape evolution. Carbonate rocks may respond differently to climatic and tectonic forcings than other rock types because of their tendency to dissolve congruently. Yet, there is little information on the long-term denudation rate of carbonates and their controlling factors in different settings. Here, we present the first long-term denudation rates for carbonates in the south of the Central Anatolian Plateau. We quantified the outcrop denudation rates using in-situ cosmogenic 36Cl measured in 13 rock samples to evaluate spatial variations and potential controls on the style and rate of denudation. 36Cl-derived denudation rates range from 1.92 ± 0.31 to 45.77 ± 3.91 mm/ka. On the plateau, denudation rates are strongly correlated with precipitation and are lower than the theoretical maximum dissolution rates predicted assuming zero evaporation, implying that climate exerts the major control on carbonate denudation. These results indicate that chemical weathering is the prevalent denudation style. The denudation pattern shows spatial variability with a decreasing trend from SW to NE consistent with climate zones. High-to-moderate denudation rates occur in the continental climate, while the lowest denudation rates occur in the arid plateau interior. In the Mediterranean coastal region, measured denudation rates are higher than the predicted maximum dissolution rates, reflecting the mechanical removal of overlying rock or soil. The low denudation rates in the plateau interior indicate a slowly evolving landscape with an arid climate. The physical properties of rocks showed the weakest control on carbonate denudation.In the Central Taurides, significant infiltration of surface runoff resulted in strong subsurface karstification at the expense of surface drainage. This process caused disequilibrium between uplift and denudation and resulted in the preservation of high-elevation topography. Therefore, in the actively uplifting plateau margin, the influence of tectonics on carbonate denudation has been interfered with the impact of precipitation and subsurface karstification.

Seismic Fault Slip Behavior Predicted From Internal Microphysical Processes

AbstractEarthquake simulation and hazard prediction are strongly hampered by insufficient physical knowledge of the constitutive behavior of faults. Laboratory studies of carbonate fault friction suggest that seismogenic rupture on faults in carbonate terrains can be explained by a transition from high friction at low initial sliding velocities (V) to low friction at seismic slip velocities, that is, by rapid dynamic weakening. One proposed explanation for this weakening, invokes frictional heating resulting in deformation by grain boundary sliding accommodated by solid‐state diffusion (sometimes referred to as “viscous” or “superplastic” flow). We recently added this dynamic weakening mechanism to a microphysically based model addressing the (rate‐and‐state) frictional behavior of granular gouges undergoing low V shear characteristic of rupture nucleation and arrest. In the present study, we applied the full model to simulate seismic slip in laboratory carbonate faults. Assuming that slip localizes in a principal shear band within the fault (gouge) zone, and accounting for grain size evolution with velocity and temperature, the model reproduces the frictional, thermal and (micro‐)structural evolution observed during seismic slip experiments. In particular, it predicts spatial and temporal evolutions of grain size, porosity, and dominant deformation mechanisms, within and outside the assumed shear band, consistent with trends identified in the laboratory and natural fault zones.

Facies and early diagenesis of rainwater-fed paleospring calcareous tufas in the Kurkur oasis area (southern Egypt)

The Quaternary calcareous tufas precipitated in the Kurkur Oasis area in the southern Western Desert of Egypt were analyzed to determine their implications for the construction of environmental conditions during their formation. X-ray diffraction analysis showed that the tufas consist of low-Mg calcite, whereas macroscopic and microscopic analyses showed the presence of both allochthonous (clastic) and autochthonous components consisting predominantly of pisoliths, oncoids, intraclasts, lithoclasts, stromatolites and encrusted plant materials. These tufas form four facies associations that represent pisolitic intraclastic/lithoclastic oncoidal rudstones, phytohermal /bryophyte framestones, stromatolite-algal boundstones, and speleothem-like flowstones. These tufa associations were formed within a karstified carbonate terrain by a rainwater-fed paleospring system comprising waterfalls, slopes, dammed areas, lacustrines-paludal, and fluvial channel margin environments. Early diagenetic features are cementation, neomorphism and subaerial dissolution. Isotope-geochemical analysis indicated that the negative δ18O values (between – 13.26 and – 8.89‰ V-PDB) and the negative δ13C values (between – 3.16 and – 1.62‰ V-PDB) of the studied tufas are consistent with carbonates deposited from meteoric water in regions with much precipitation.

Microphysical Modeling of Carbonate Fault Friction at Slip Rates Spanning the Full Seismic Cycle.

Laboratory studies suggest that seismogenic rupture on faults in carbonate terrains can be explained by a transition from high friction, at low sliding velocities (V), to low friction due to rapid dynamic weakening as seismic slip velocities are approached. However, consensus on the controlling physical processes is lacking. We previously proposed a microphysically based model (the “Chen–Niemeijer–Spiers” [CNS] model) that accounts for the (rate‐and‐state) frictional behavior of carbonate fault gouges seen at low velocities characteristic of rupture nucleation. In the present study, we extend the CNS model to high velocities (1 mm/s ≤ V ≤ 10 m/s) by introducing multiple grain‐scale deformation mechanisms activated by frictional heating. As velocity and hence temperature increase, the model predicts a continuous transition in dominant deformation mechanisms, from frictional granular flow with partial accommodation by plasticity at low velocities and temperatures, to grain boundary sliding with increasing accommodation by solid‐state diffusion at high velocities and temperatures. Assuming that slip occurs in a localized shear band, within which grain size decreases with increasing velocity, the model results capture the main mechanical trends seen in high‐velocity friction experiments on room‐dry calcite‐rich rocks, including steady‐state and transient aspects, with reasonable quantitative agreement and without the need to invoke thermal decomposition or fluid pressurization effects. The extended CNS model covers the full spectrum of slip velocities from earthquake nucleation to seismic slip rates. Since it is based on realistic fault structure, measurable microstructural state variables, and established deformation mechanisms, it may offer an improved basis for extrapolating lab‐derived friction data to natural fault conditions.

Geological and Geophysical Study in Udheri Khola Area, Nalgad Hydroelectric Project, Jajarkot District, Lesser Himalaya, Western Nepal

Subsidence in carbonate rock is one of common and challenging action in terms of engineering construction. Geological study and geophysical investigation carried out in the intake area of Nalgad Hydroelectric Project Jajarkot, western Nepal Lesser Himalaya. The main objective was to identify the cause of subsidence in the intake area of Nalgad Hydroelectric Project, Jajarkot. Geological study of the area was carried to understand the lithology, thickness and structure of the area.
 The study area comprises two distinct rock units, namely, Dolomite Unit followed up by the Slate Unit. The Dolomite Unit is composed of light grey to grayish white stromatolitic dolomite which is thrusted over the Slate Unit near to Laikham village and Sepu Khola area. The Slate Unit is made up of grayish black to graphitic slate. A thin prominent calcareous horizon wasconfined between Slate Unit.
 2D-Electric Resistivity Tomography (ERT) measurements were deployed in four different lines to investigate the cause of the subsidence in the carbonate terrain. A concentric very high resistivity patch shown by Tomogram ER-D-01 survey line was identified and interpreted as dry cavity. The result of the 2D- ERT survey was correlated with core log data of geotechnical exploration in the suspicious point to ensure the presence of karst in the Dolomite Unit at right bank of Nalsyagu Khola near dam axis of Nalgad Hydroelectric Project. The 2D – ERT survey together with geotechnical investigation is capable of identifying subsurface karst feature as the cause of surface collapse in the area.

Controlling factors of riverine CO2 partial pressure and CO2 outgassing in a large karst river under base flow condition

Riverine CO2 outgassing constitutes the important vertical conduit in global carbon cycle and affects climate change. In order to investigate the factors that control riverine pCO2 and CO2 diffusion rate (FCO2), river water samples were collected in the mainstream and 42 tributaries of Xijiang River, a karst river during the dry season in 2015. The pCO2 varied from 260 to 6354μatm with a mean value of 1765μatm, indicating most of the samples were oversaturated with respect to CO2 compared to atmosphere. The pH, pCO2, and HCO3− exhibited clearly spatial distribution pattern that pH and HCO3− decreased along the flow direction while the pCO2 increased along the flow direction. Theoretical calculation indicated that the water temperature fluctuation, bio-degradation/respiration, photosynthetic activities and anthropogenic acidic inputs were not the reasonable explanations. The lithologic differences essentially predominated the riverine pH, pCO2, and HCO3−, the upper reaches of Xijiang River flow through karst carbonate terrain and thus exhibited high pH, HCO3− concentrations meanwhile low pCO2, the spatial distribution of pH, pCO2, and HCO3− became visible as the continuous addition of silicate weathering products with low pH and dissolved inorganic carbon (DIC) concentrations. The mixing calculation indicated the DIC concentrations of silicate weathering should lower than 0.5 mmol/L. The calculated FCO2 of Xijiang River ranged between −38.6 and 1579.8 mmol/m2/D, with a mean value of 574.8 mmol/m2/D and median value of 410.9 mmol/m2/D. FCO2 exhibited obviously lower values in karst terrain compared that in silicate terrains, because carbonate weathering results in high pH of karst rivers, which caused the low relative proportion of pCO2 in DIC. More estimations of CO2 outgassing rate from the karst rivers are required due to the unique characteristics and geochemical process in karst landform.

Toward a new conceptual model for groundwater flow in merokarst systems: Insights from multiple geophysical approaches

AbstractMerokarst aquifers — relatively thin (<1–2 m) karstified carbonate units interbedded between mudstone, shale, or sandstone — constitute a significant proportion of carbonate terrain and underlie a large portion of the west‐ and south‐central USA, yet few advances have been made in our understanding of porosity development and flow‐path generation in these complex systems in decades. Toward this end, we used a multi‐geophysical approach at the well‐studied Konza Prairie Biological Station (KPBS), a part of the larger Flint Hills (25,734 km2), underlain by thin limestone units (1–2 m thick) interbedded with mudstone/shale units (2–4 m thick), to elucidate hydrologic connectivity and potential controls on known groundwater flow directions. We combined electrical resistivity tomography (ERT), surface and borehole nuclear magnetic resonance (NMR), and ground penetrating radar (GPR) measurements across a low order catchment where over 25 boreholes and groundwater wells sampling perched aquifers could be used to constrain interpretation of lithology, potential flow paths, and permeability. Data revealed that groundwater export may be an unappreciated component of lateral‐flow‐dominated models used to represent merokarst in that: (a) potentiometric surfaces from two limestone units showed groundwater flows toward a hydrologic depression, opposite the direction of stream flow, in the upstream portion of the catchment, (b) long term measures of groundwater levels revealed a greater variance and overall water storage in this same upstream area compared to wells near the outlet, and (c) ERT and NMR results indicate pronounced lateral heterogeneity within a given unit, suggestive of a greater degree of vertical hydrological connectivity than usually considered for horizontally‐layered merokarst. Our data suggest vertical connectivity can shunt water to depth in these “sandwiched” merokarst aquifers, yielding atypical groundwater flow directions and unrealized deep export of weathering solutes and carbon.

Seasonal and Spatial Variation of Mo Isotope Compositions in Headwater Stream of Xijiang River Draining the Carbonate Terrain, Southwest China

The dissolved molybdenum (Mo) contents and Mo isotope in water samples from the upper Xijiang River (XJR), draining the carbonate terrain, southwest China, are reported to investigate the seasonal and spatial variations, sources, ion budget, and isotopic fractionation of dissolved Mo. The results show that the Mo concentrations (5.3–18.9 nmol/L) exhibit an extensive variation along the mainstream without significant spatial pattern, but the Mo concentrations are slightly higher in the dry season than in the wet season caused by the dilution effect. There is a slight spatial tendency for δ98/95Mo to become higher along the mainstream (0.51–1.78%), while the seasonal variations in δ98/95Mo values of NPR (Nanpanjiang River) reach and BPR (Beipanjiang River) reach can be identified higher in the dry season but lower in the wet season. Based on the hydro-geochemical analysis, the sources of dissolved Mo are identified as the carbonates and sulfide/sulfate minerals weathering with a seasonal contribution. Moreover, our results suggest there is no significant Mo isotopic fractionation during weathering and riverine transportation. The calculation of Mo budget demonstrates that the dissolved δ98/95Mo of river draining the carbonate terrain is underestimated, which could significantly influence the redox history of oceans by Mo isotope model.

Riverine Calcium End-Members Improve Coastal Saturation State Calculations and Reveal Regionally Variable Calcification Potential

Carbonate-rich groundwater discharged from springs, seeps, and spring-fed rivers on carbonate platforms creates environments of potential refuge for calcifying organisms in coastal waters by supplying higher [Ca2+] and [CO32-] along with typically lower nutrient concentrations. The benefits associated with carbonate terrains are maximized in the presence of submerged aquatic vegetation, especially seagrasses. To improve the accuracy of carbonate saturation state (Ω) determinations, calculated values of [CO32-] and Ksp* were paired with [Ca2+] values determined using a model that incorporates directly measured riverine calcium end-members (model A). This model results in Ω values larger than those calculated by assuming that [Ca2+] is directly proportional to salinity (model B; e.g., using CO2SYS, CO2calc). As an example, for salinity (S) between 13.5 and 24, improvements in saturation states calculated as differences (ΔΩ) between model A and model B saturation states in the tidal mixing zone of the Weeki Wachee River (Florida, USA) ranged from 0.39–1.00 (aragonite) and 0.61–1.65 (calcite). Saturation state ratios (Ω(A)/ Ω(B)) for coastal waters with enhanced [Ca2+] originating from carbonate-rich groundwater can be calculated from end-member calcium concentrations and salinity. Applied to several river systems in the conterminous United States, Ω(A)/ Ω(B) values calculated at S = 20 lead to Ω(A)/ Ω(B) ratios of 1.12 (Weeki Wachee), 1.09 (Anclote), 1.06 (Mississippi), and 1.03 (Columbia). These increases in saturation states can be used to identify potential calcification refugia for subsequent high resolution field studies that focus on, for example, the long-term viability of oyster communities and other calcifying organisms in brackish coastal waters.

Chemical weathering and CO2 consumption rate in a multilayered‐aquifer dominated watershed under intensive farming: The Orgeval Critical Zone Observatory, France

AbstractThe impact of intensive farming on chemical weathering in the Critical Zone is still an open question. Extensively instrumented and monitored over the last 50 years, the Orgeval Critical Zone Observatory (CZO) in France is an observation site impacted by intensive farming since the 1960s. The Orgeval observatory represents an ideal place to study the response and resilience capability of the Critical Zone under agricultural stress. This paper investigates the chemical composition of different water bodies in two nested catchments of the Orgeval CZO, including rainfall, springs, rivers, and rocks, over one and half hydrological year. We show that elemental and strontium isotopic ratios are powerful to constrain the origin of the elements. The results show that the river chemistry at the outlet of the two nested catchments is dominated by rain inputs (particularly atmospheric dust dissolution) and the chemical weathering of limestone and gypsum. Fertilizer input is clearly visible, although the distinction between gypsum dissolution and fertilizer inputs needs more investigation. The mixtures of water masses inferred from our data are in good agreement with the hydrological context of the watershed, that is, a multilayered aquifer structure. At the main outlet of the CZO, we estimate that the input of ocean‐derived solutes through rainfall represents 7 t km−2 year−1, on the same order of magnitude as the net fertilizer input (10 t km−2 year−1), and that rock weathering releases 50 t km−2 year−1. Including previously published physical erosion rates, we estimate that the total denudation rate (physical and chemical) of the Orgeval CZO is 20 mm (1,000 year)−1, which, along with the entire Seine watershed, is among the lowest chemical denudation rates for carbonate terrains under temperate climate. Chemical denudation is about 10 times higher than physical erosion in the Orgeval CZO. The consumption of CO2 by rock weathering is estimated to be between 265.103 and 360.103 molC km2 year−1, similar for the two nested catchments. Compared with the rivers, the springs show a higher CO2 consumption rate that suggests, as pointed out earlier, a enhancement of carbonate dissolution linked to nitrification and thus fertilizer application. The hyporheic zone appears to be a hot spot in the carbon cycle at the Orgeval CZO. This study sheds light on the complex, anthropocenic, interplay between geology, climate, and human activities that characterize and that take place in intensive agriculture regions.

How landscape heterogeneity governs stream water concentration-discharge behavior in carbonate terrains (Konza Prairie, USA)

Mounting evidence suggests ecosystem changes that alter subsurface water fluxes and carbon dioxide concentrations in carbonate terrains may drive measurable changes in chemical weathering rates, stream water chemistry, and flow path evolution on human timescales. We test this idea by exploring if the encroachment of woody vegetation into grasslands in a carbonate terrain landscape at the Konza Prairie (KS, USA) has resulted in differences in landscape-stream connectivity and, thus, the behavior of stream water solutes. Woody encroachment (up to 60% cover) at Konza has been observed on watersheds, particularly those that experience a fire return interval of four years or greater. We focus on three headwater catchments (two grassland and one woody-encroached) and a downstream confluence, and analyze stream water discharge and chemistry (major anions, cations, and dissolved nutrients) measured from 2015 to 2016.We observe that the woody-encroached watershed exhibits a greater area-normalized solute flux and greater degree of chemodynamic behavior for most geogenic species compared to the less encroached grassland watersheds. The downstream confluence exhibits the most chemostatic behavior for these same solutes compared to the low order watersheds. We interpret the chemodynamic behavior of the woody-encroached watersheds to arise from a greater diversity of flow paths and solute sources that contribute to this stream. End member mixing analysis (EMMA) supports this hypothesis but also indicates a possible “missing” end member which we interpret to be solutes likely derived from clay weathering along limestone-mudstone boundaries. We invoke differences in rooting systems between grass and woody species to explain the differences in flow paths and solute generation between these headwater sites given that they sit adjacent to each other, dissect the same nearly horizontal (dip 0.1–0.21°NW) lithologic units, and experience the same climate. If these processes hold true at other sites, then the globally observed encroachment of woody vegetation into grasslands may deepen flow paths and enhance chemical weathering fluxes from ecosystems, and over long-time periods alter the trajectory of soil development and landscape evolution.

Patterns and rates of 103–105 yr denudation in carbonate terrains under subhumid to subalpine climatic gradient, Mount Hermon, Israel

Patterns and rates of 103–105 yr denudation in carbonate terrains under subhumid to subalpine climatic gradient, Mount Hermon, Israel

Rare earth elements absorption patterns in grapevine “Vitis vinifera L.” cultivated in carbonate terrains (south-eastern Sicily, Italy)

Vitis vinifera L. vineyards grown on carbonate soil (Hyblean Plateau, SE Sicily) have been characterized in terms of rare earth elements (REEs) distribution. Results highlighted that the absorption of REEs by plants depends on the composition of the underlying soil, which in this case derives from limestone parent rock, allowing us to recognize the area of origin. Indeed, even slight differences in REEs content in soils may affect the absorption pattern of each grapevine cultivar. Importantly, the various parts of the plants showed differences in REEs absorption; such REEs fractionation is particularly evident in the leaf and juice samples. In general, the uptake and concentrations of REEs in plant tissues may be related to many factors such as geographical, climatic and lithological features. This is also pointed out by the statistical investigation, which took into account either the grapevine variety or each part of the plant. By taking into consideration both the grape variety and the type of soil, the present “multi-elemental” approach aims to provide a useful geochemical tool for assessing the geographical origin of the production area of wine.

Dust, impure calcite, and phytoliths: Modeled alternative sources of chemical weathering solutes in shallow groundwater

In highly reactive, carbonate terrains that constitute more than one-fifth of critical zone landscapes, quantifying bedrock weathering processes may require understanding the realities of carbonate mineral impurity on solubility, biotically-produced minerals as an integral part of leaky biogeochemical cycles, and dust-deposited minerals as important high-surface-area, first-contact solute sources. The potential impact of these processes has not been thoroughly investigated as groundwater solutes are mostly thought to be sourced from chemical reactions with soil and bedrock, although dust is often viewed as an important delivery mechanism of nutrients to other ecosystems, including those in mountain and tropical settings. We present results of computer and hand-calculated (manual) inverse modeling of two years of stream-water chemistry, spanning a dry and a wet year, for a groundwater-fed headwater stream at the Konza Tallgrass Prairie Long-Term Ecological Research Site in northeastern Kansas. Weathering of the limestone and shale bedrock at the site provide a possible source of solutes to the groundwater-fed stream, but our modeling suggests an alternate source considering the pathway of groundwater recharge likely encounters highly reactive phases before encountering bedrock. We used the mineralogy and geochemistry of local dust collected previously, estimates of a possible chemical composition of phytolith containing potassium, and an impure calcite representing measured limestone bedrock chemistry to show that chemical reactions with the current dust flux are adequate to account for the groundwater chemistry. Average annual amounts were about: 1) 380 kg ha−1 yr−1 dust dissolved, 2) 80 kg ha−1 yr−1 bedrock carbonate dissolved, and 3) 320 kg ha−1 yr−1 phytoliths precipitated. Small amounts of cation exchange were also required to balance the models. There were only small differences between the computer and manual inverse models; both methods resulted in up to 4.6 times more mass of dust dissolved than bedrock. We suggest that dust weathering may be a process that occurs widely, considering the ubiquitous dust flux in continental regions. Significance of researchSolutes in groundwater and in streams fed by groundwater are thought to be mostly derived from aquifer bedrock and soil water recharging the aquifer. This paradigm may fail for landscapes that have been subaerially exposed for 10's to 100's of millions of years in terrains where relatively rapid dissolution of minerals dominates, such as carbonate-mineral dominated rocks like limestone. Here we test whether dust, having high surface area and being the first inorganic solid encountered by meteoric precipitation, could provide enough solutes water to reduce the amount of bedrock that dissolves, extending the timeframe of landscape evolution. We found, through inverse modeling with PHREEQC, that dust can provide 4.6 times as much mass to stream water in merokarst than calcite and that the mass of dust dissolved was the same order of magnitude as local dust deposition. These results give permissive evidence that dust is important to understanding groundwater chemistry.

Radiocarbon dating of aquatic gastropod shells and its significance in reconstructing Quaternary environmental changes in the Alashan Plateau of northwestern China

Using landform evolution and lake sediment records to reconstruct Quaternary environmental changes requires reliable chronology. However, there is an ongoing debate regarding the fossil shells of aquatic gastropods in carbonate rock area to be used for radiocarbon dating. The focus of this debate is whether living gastropods incorporate the dead carbon, which would affect the accuracy of the dating results. Living aquatic gastropods were selected from habitats on carbonate terrain in arid region. In this study, 64 shell samples from living gastropods, including Cipangopaludina chinensis, Gyraulus convexiusculus, and Radix auricularia, collected from the Alashan Plateau of northwestern China, were dated by accelerator mass spectrometry (AMS) 14C dating. The results revealed that only five samples (~8%) from the same site contained no dead carbon. The other 59 samples (~92%) contained dead carbon in various amounts: ~59% samples contained 0–10% dead carbon, and ~33% samples contained more than 10% dead carbon. Of the samples, 56 were Radix auricularia and their 14C ages varied from present to 3253 cal yr BP. Two of the samples were Gyraulus convexiusculus samples dated at 379 and 290 cal yr BP which are highly consistent with 14C ages of Radix auricularia samples from the same site. The other six Cipangopaludina chinensis samples were dated at 407–863 cal yr BP. Thus, most of living aquatic gastropods from habitats on carbonate terrain contained dead carbon. The amount of dead carbon in the gastropod shells was nonselective among the three species. Radiocarbon dating of aquatic gastropod shells may not be useful for studying processes that occurred over multi-millennial timescales at different localities in areas of carbonate rock. It is difficult to establish high-resolution chronologies using 14C ages of gastropod shells in arid regions with carbonate rocks unless the limestone effect has been corrected.

Statistical pattern analysis of surficial karst in the Pleistocene Miami oolite of South Florida

A robust airborne light detection and ranging digital terrain model (LiDAR DTM) and select outcrops are used to examine the extent and characteristics of the surficial karst overprint of the late Pleistocene Miami oolite in South Florida. Subaerial exposure of the Miami oolite barrier bar and shoals to a meteoric diagenetic environment, lasting ca. 120 kyr from the end of the last interglacial highstand MIS 5e until today, has resulted in diagenetic alteration including surface and shallow subsurface dissolution producing extensive dolines and a few small stratiform caves.Analysis of the LiDAR DTM suggests that >50% of the dolines in the Miami oolite have been obscured/lost to urbanization, though a large number of depressions remain apparent and can be examined for trends and spatial patterns. The verified dolines are analyzed for their size and depth, their lateral distribution and relation to depositional topography, and the separation distance between them. Statistical pattern analysis shows that the average separation distance and average density of dolines on the strike-oriented barrier bar versus dip-oriented shoals is statistically inseparable. Doline distribution on the barrier bar is clustered because of the control exerted on dissolution by the depositional topography of the shoal system, whereas patterning of dolines in the more platform-ward lower-relief shoals is statistically indistinguishable from random. The areal extent and depth of dissolution of the dolines are well described by simple mathematical functions, and the depth of the dolines increases as a function of their size. The separation and density results from the Miami oolite are compared to results from other carbonate terrains. Near-surface, stratiform caves in the Miami oolite occur in sites where the largest and deepest dolines are present, and sit at, or near, the top of the present water table.

Carbonate minerals in the global carbon cycle

Carbonate minerals constitute Earth's largest C reservoir. This reservoir is considered unimportant to the global C cycle over long periods of time (e.g., >106yrs) because it provides balanced sources and sinks of atmospheric CO2 as carbonate minerals precipitate and dissolve from carbonic acid, respectively. However, carbonate mineral reactions may impact the carbon cycle at short time scales (102 to 105yrs) particularly when atmospheric pCO2 changes by several hundred ppm, as exemplified by the Paleocene-Eocene Thermal Maximum, at transitions between glacial-interglacial times, and over the past century from anthropogenic inputs. Here I review recent literature about mechanisms in which carbonate minerals may impact the global carbon cycle. Carbonate mineral dissolution, which increases dissolved inorganic carbon (DIC) concentrations, may represent a net atmospheric CO2 sink as primary productivity consumes DIC, resulting in organic C burial in fluvial systems of carbonate terrains. Alternatively, carbonate minerals dissolved by sulfuric and nitric acids may represent an atmospheric source of CO2 with no rapid balancing atmospheric sink. Sulfuric acid dissolves carbonate minerals of modern carbonate platform sediments and water-filled sinkholes, and in the deep sea at the base of carbonate platform scarps. In terrestrial landscapes, sulfuric acid dissolves carbonate minerals of karst terrains (hypogene caves) and in comminuted glacial sediments. Nitric acid forms from oxidation of reactive N, which has doubled in the past hundred years as humans fix atmospheric N2. Similar to some reactions involving sulfuric acid, nitric acid dissolution of carbonate minerals represents a net source of atmospheric CO2. This source is concentrated in fertilized agricultural fields in carbonate terrains. Each of these processes represents a small flux relative to major C exchanges, such as volcanic sources and silicate mineral weathering, but collectively they could impact the C cycle at magnitudes similar to these major processes. These impacts may increase as humans further alter elemental cycles of S and N.

Weathering-limited hillslope evolution in carbonate landscapes

Understanding topographic evolution requires integrating elementary processes acting at the hillslope scale into the long-wavelength framework of landscape dynamics. Recent progress has been made in the quantification of denudation of eroding landscapes and its links with topography. Despite these advances, data is still sparse in car-bonate terrain, which covers a significant part of the Earth's surface. In this study, we measured both long-term denudation rates using in situ-produced 36 Cl concentrations in bedrock and regolith clasts and surface convexity at 12 sites along ridges of the Luberon carbonate range in Provence, Southeastern France. Starting from ∼30 mm/ka for the lowering of the summit plateau surface, denudation linearly increases with increasing hilltop convexity up to ∼70 mm/ka, as predicted by diffusive mass transport theory. Beyond this point denuda-tion rates appear to be insensitive to the increase in hilltop convexity. We interpret this constant denudation as indicating a transition from a regime where hillslope evolution is primarily controlled by diffusive downslope regolith transport, toward a situation in which denuda-tion is limited by the rate at which physical and chemical weathering processes can produce clasts and lower the hilltop. Such an abrupt transition into a weathering-limited dynamics may prevent hillslope 1 denudation from balancing the rate of base level fall imposed by the river network and could potentially explain the development of high local relief in many Mediterranean carbonate landscapes.