Palustrine calcretes of the Cretaceous Gyeongsang Supergroup, Korea: Variation and paleoenvironmental implications

Abstract Mostly siliciclastic lacustrine deposits from five stratigraphically different formations (Jinju Formation, Jindong Formation, Geoncheonri and correlative Hwasan Formations and Dadaepo Formation, in ascending order) in the Cretaceous Gyeongsang Basin, Korea, were examined for aspects of lithofacies and pedogenesis to evaluate the relative influence of geological controls on the development of palustrine calcretes (calcretes formed from palustrine deposits). The pedogenic carbonate development of palustrine deposits in the Gyeongsang Supergroup varies from formation to formation. The highest development is in the Dadaepo Formation and the second is in the Jindong Formation. The lowest development of palustrine calcretes is in the Geoncheonri and Hwasan Formations and the Jinju Formation shows intermediate development. The more negative d¹³C values and the less negative d¹⁸O values of the Dadaepo palustrine calcretes confirm greater pedogenic development in the Dadaepo Formation. That the highest development was in the Dadaepo Formation was attributed to it having the smallest lake size, indicating that lake size is critical to palustrine calcrete development in non‐carbonate lakes under semi‐arid climate. In spite of having the largest lake size, the higher development in the Jindong Formation could have resulted from its lowest lake gradient and most arid paleoclimate. The higher development of palustrine calcretes in the Late Cretaceous deposits (Jindong Formation) than the Early Cretaceous deposits (Jinju Formation) reflect overall increase in aridity throughout the period during the deposition of the Gyeongsang Supergroup. Consequently, the diverse development of the palustrine calcretes in the Gyeongsang Supergroup indicates that the lacustrine settings varied in time and space throughout the evolution of the Cretaceous Gyeongsang Basin. Such variation in palustrine calcrete development according to the change in paleoenvironments may provide a basis to interpret the relative paleoenvironmental condition of lacustrine deposits including paleoclimate, lake size and gradient.     

Diagenetic transformations and silcrete–calcrete intergrade duricrust formation in palaeo‐estuary sediments

The Boteti palaeo‐estuary in northern Botswana is located where the endoreic Boteti river, an overflow from the regional Okavango river system, enters the Makgadikgadi pans. The present work considers diagenetic silica and calcium carbonate dominated transformations. The aims are to help identify precursor conditions for the origin of microcrystalline silcrete–calcrete intergrade deposits while developing insight into pene‐contemporaneous silica and calcite matrix formation. General precursor conditions require the presence of cyclical endoreic freshwater inflow into a saline pan. The pan should be deep enough to sustain a permanent watertable under climatic conditions sufficient to cause carbonate fractionation within the groundwater. Freshwater inflow into a saline pan drives the geochemistry of the system (from freshwater to saline, from neutral to high pH). The geochemistry is controlled by the periodicity of inflow relative to salinity levels of phreatic groundwater in the receptor saline pan. The source of most silica and localized CaCO₃ is derived from the dissolution and precipitation of micro‐fossils, while more general CaCO₃ enrichment stems from saline pan based carbonate fractionation. Diagenetic change leads to colloidal then more consolidated bSiO₂/CaO aggregate formation (amorphous silica) followed by transformations into opaline silica over time. Irregular zones of siliceous sediment forming in otherwise calcareous deposits may relate to the irregular occurrence of biogenic silica in the source sediments, inferring a source for local silica mobilization in intergrade deposits. The distribution of calcareous micro‐fossils may have a similar converse effect. Diagenetic evidence from an intergrade deposit with a low SiO₂/CaO ratio suggests that transformation occurred more into the pan, while an intergrade deposit with a high SiO₂/CaO ratio more likely formed closer to a land margin and was frequently inundated by freshwater. Pene‐contemporaneous silcrete–calcrete intergrade formation under the above conditions may take place where dissolved silica crystallizes out in the vicinity of calcite crystals due to local decreases in pH. The continuing consolidation of bSiO₂/CaO aggregates may be facilitated by the presence of increasing amounts of calcite. It appears that CaCO₃ may act as a catalyst leading to pene‐contemporaneous bSiO₂/CaO aggregate formation. However the processes involved require further work. Copyright © 2013 John Wiley & Sons, Ltd.     

Calcrete profile development in Quaternary alluvial sequences,southeast Spain: implications for using calcretes as a basis for landform chronologies

A detailed study of the morphology and micro‐morphology of Quaternary alluvial calcrete profiles from the Sorbas Basin shows that calcretes may be morphologically simple or complex. The ‘simple’ profiles reflect pedogenesis occurring after alluvial terrace formation and consist of a single pedogenic horizon near the land surface. The ‘complex’ profiles reflect the occurrence of multiple calcrete events during terrace sediment aggradation and further periods of pedogenesis after terrace formation. These ‘complex’ calcrete profiles are consequently described as composite profiles. The exact morphology of the composite profiles depends upon: (1) the number of calcrete‐forming events occurring during terrace sediment aggradation; (2) the amount of sediment accretion that occurs between each period of calcrete formation; and (3) the degree of pedogenesis after terrace formation. Simple calcrete profiles are most useful in establishing landform chronologies because they represent a single phase of pedogenesis after terrace formation. Composite profiles are more problematic. Pedogenic calcretes that form within them may inherit carbonate from calcrete horizons occurring lower down in the terrace sediments. In addition erosion may lead to the exhumation of older calcretes within the terrace sediment. Calcrete ‘inheritance’ may make pedogenic horizons appear more mature than they actually are and produce horizons containing carbonate embracing a range of ages. Calcrete exhumation exposes calcrete horizons whose morphology and radiometric ages are wholly unrelated to terrace surface age. Composite profiles are, therefore, only suitable for chronological studies if the pedogenic horizon capping the terrace sequence can be clearly distinguished from earlier calcrete‐forming events. Thus, a detailed morphological/micro‐morphological study is required before any chronological study is undertaken. This is the only way to establish whether particular calcrete profiles are suitable for dating purposes. Copyright © 2003 John Wiley & Sons, Ltd.     

Duricrust development and valley evolution: Process–landform links in the kalahari

Duricrusts are an important landscape component of the Kalahari region of central southern Africa. Their exposures within the dry valleys (mekgacha) of the Kalahari provide some of the most widespread surface outcrops of the terrestrial Jurassic to Holocene Kalahari Group sediments. Exposures have been extensively used in the construction of lithostrati-graphic sequences, on the assumption that valley systems have incised their courses through a pre-existing duricrust sequence. Recent work, however, has identified the role of groundwater erosion processes in valley development, which may have influenced duricrust formation. Studies of duricrusts from boreholes drilled within two mekgacha show that duricrust type is intrinsically related to the presence of a valley. Analyses of calcretes and silcretes in a series of profiles and thin sections from the Letlhakeng area of Botswana also indicate extensive alteration and diagenesis in association with former higher water tables. Sedimentary sequences within duricrust host materials can be identified but there is no evidence for correlation of duricrust cements between exposures. Profile studies from the Auob Valley in Namibia, however, suggest that this valley has incised through a sequence of duricrusts. Caution is advised in future attempts to correlate duricrust types on the basis of valley exposures, with the recommendation that where such exposures are used in a lithostratigraphic context, only duricrust host material characteristics and not cementing materials should be considered.     

Pseudo-anticlines and other structures in some calcretes of Botswana and South Africa

Deformation structures, within some Quaternary calcretes of Botswana and South Africa, have been classified into five types. Type 1 folds are small-scale (< 2 m wavelength) anticlines in shale or sheet calcrete, separated by wedges of calcrete, polygonal in plan. The anticlines have resulted from horizontal expansion and buckling caused by the disruptive growth of the calcrete wedges. Type 2(a) folds are medium-scale, strongly convoluted features in sheet calcrete and bedrock and are attributed to the introduction of calcrete along major vertical joint planes. Type 2(b) folds are large wavelength (> 20 m), but low amplitude (< 1 m), anticlines in hardpan calcrete; structural evidence suggests dominantly horizontal compression within the hardpan due either to an overall increase in volume of the calcrete, or to the disruptive formation of calcrete in joints in the synclinal areas. The mineralogy of the calcretes is dominated by low-Mg calcite which may have inverted from a high-Mg form; the deformation in type 1 and 2 folds was probably caused by the disruptive and displacive growth of calcite during calcretization. Type 3 folds are saucer-shaped depressions in hardpan calcrete and may be due to the removal of soluble salts below the folded layer during or after calcretization. Finally, type 4 folds are small, diapiric anticlines resulting from the upward injection of swelling clays into calcrete or calcretized shale.     

Geochemical mapping of carbonate sediments in the Makgadikgadi basin,Botswana using moderate resolution remote sensing data

This paper maps the carbonate geochemistry of the Makgadikgadi Pans region of northern Botswana from moderate resolution (500 m pixels) remotely sensed data, to assess the impact of various geomorphological processes on surficial carbonate distribution. Previous palaeo‐environmental studies have demonstrated that the pans have experienced several highstands during the Quaternary, forming calcretes around shoreline embayments. The pans are also a significant regional source of dust, and some workers have suggested that surficial carbonate distributions may be controlled, in part, by wind regime. Field studies of carbonate deposits in the region have also highlighted the importance of fluvial and groundwater processes in calcrete formation. However, due to the large area involved and problems of accessibility, the carbonate distribution across the entire Makgadikgadi basin remains poorly understood. The MODIS instrument permits mapping of carbonate distribution over large areas; comparison with estimates from Landsat Thematic Mapper data show reasonable agreement, and there is good agreement with estimates from laboratory analysis of field samples. The results suggest that palaeo‐lake highstands, reconstructed here using the SRTM 3 arc‐second digital elevation model, have left behind surficial carbonate deposits, which can be mapped by the MODIS instrument. Copyright © 2006 John Wiley & Sons, Ltd.     

Topographically associated but chronologically disjunct late Quaternary floodplains and terraces in a partly confined valley,south‐eastern Australia

The Bellinger River catchment in the New England Fold Belt on the mid‐north coast of New South Wales is characterized by an assemblage of stepped late Quaternary alluvial units. Late Pleistocene terraces were formed by large, more competent rivers that eroded almost entire valley floors; however, a decline in discharge prior to the Holocene has resulted in the abandonment of these deposits as elevated terraces or residual alluvium, onlapped by contemporary floodplains. Intrinsic controls on floodplain formation appear to be superimposed over an early–mid‐Holocene climatic signature. A fluvially active period, known as the Nambucca Phase, from 10 to 4·5 ka, eroded Late Pleistocene terraces. Two floodplain surfaces, one higher than the other, both started to accrete vertically from 4 ka but with some valley locations remaining vulnerable to episodes of erosion, resulting in substantial units of even younger basal alluvium. The high floodplain is dominated by horizontally laminated, vertically accreted sequences, while the low floodplain, which overlaps in age, is characterized by pronounced cut‐and‐fill stratigraphy. Terraces and floodplains in partly confined settings can have similar elevations but be polycyclic, with very different basal ages. In such landscapes the classical assumption that individual terrace or floodplain profiles along a valley represent periods of coeval formation is shown to be frequently invalid. Copyright © 2007 John Wiley & Sons, Ltd.     

From mass‐wasting to slope stabilization – putting constrains on a tectonically induced transition in slope erosion mode: a case study in the Judea Hills,Israel

Calcrete‐coated remnants of landslide debris and alluvial deposits are exposed along the presently stable hillslopes of the Soreq drainage, Judea Hills, Israel. These remnants indicate that a transition from landslide‐dominated terrain to dissolution‐controlled hillslope erosion had occurred. This transition possibly occurred due to the significant decrease in tectonic uplift during the late Cenozoic. The study area is characterized by sub‐humid Mediterranean climate. The drainage hillslopes are typically mantled by thick calcrete crusts overlying Upper Cretaceous marine carbonate rocks. Using TT‐OSL dating of aeolian quartz grains incorporated in the calcrete which cements an ancient landslide deposit, we conclude that incision of ~100 m occurred from 1056 ± 262 to 688 ± 86 ka due to ~0·3° westward tilt of the region; such incision invoked high frequency of landslide activity in the drainage. The ages of a younger landslide remnant, alluvial terrace, and alluvial fan, all situated only a few meters above the present level of the active streambed, range between 688 ± 86 ka and 244 ± 25 ka and indicate that since 688 ± 86 the Soreq base level had stabilized and that landslide activity decreased significantly by the middle Pleistocene. Copyright © 2012 John Wiley & Sons, Ltd.     

The nature of calcareous deposits along pan margins in eastern central Namibia

In a region along the western margin of the Kalahari in eastern Namibia and western Botswana, many pan basins have mainly calcareous deposits along part of their margins. These are typically lined by low vertical scarps. In Namibia, these pans are mainly located in dry river beds. The petrographical study of these deposits demonstrates that they consist of lacustrine sediments that have to a varying extent been affected by early‐diagenetic processes and by the formation of late‐diagenetic features. The original composition of the deposits ranges from highly calcareous sediments, typically with ostracod, diatom and charophyte remains, to entirely non‐calcareous diatomites. The deposits generally show an upward increase in total carbonate content, which is mainly a synsedimentary feature. The early‐diagenetic processes that affected the deposits include the formation of orthic siliceous nodules. At a later stage, secondary calcite enrichment occurred, leaving only the silica‐impregnated sections unaffected. This enrichment partly accounts for the upward increase in total carbonate content in some profiles and often resulted in the development of a highly calcareous surface horizon. Sepiolite and amorphous silica that are part of the groundmass of the deposits may also partly have formed at this stage. Late‐diagenetic features include various forms of secondary calcite and silica. This study of pan basins in eastern central Namibia indicates that a lacustrine rather than purely pedogenic origin should also be considered for calcareous deposits that commonly occur along pan margins in other parts of southern Africa. Copyright © 2002 John Wiley & Sons, Ltd.     

Run‐off formation in a humid,temperate headwater catchment using a combined hydrological,hydrochemical and isotopic approach (Jizera Mountains,Czech Republic)

The formation of baseflow and stormflow was examined in the 1.18 km² part of the headwater catchment Uhlí?ská, Jizera Mountains, Czech Republic, over the period 2007–2011, by means of run‐off data and environmental tracers ¹⁸O and SiO₂. The baseflow, computed using the digital filter approach BFLOW, contributes 67% to total streamflow and has a mean residence time of 12.3 months. It is formed by groundwater discharge from the valley deluviofluvial granitic sediments, in combination with soil water in weathered layers on hillslopes during rainfall and snowmelt periods. The prevailing source of the groundwater is the infiltration of snowmelt water. Analysis of 20 run‐off events and their hysteretic patterns demonstrated that the stormflow water has a residence time of about 4 months and is generated by preferential flow on hillslopes combined by soil matrix drainage. Because of slower flow in the soil matrix, the enrichment of pore water in SiO₂ is more pronounced. The stormflow and snowmelt water flowing via preferential pathways of upslope minerals soils pushes the pre‐event groundwater through the pathways in wetlands to the stream, and the wetland can be therefore considered as groundwater supplied. This mechanism has been found to be typical for the groundwater‐supplied headwater catchments of the Jizera Mountains and can be also assumed in other mountainous headwaters of the granitic massif in Central Europe. The main methodological contribution of this study are the residence time calculations stratified by baseflow and event flow, identifying run‐off components of different travel times to streams and linking them with geochemical run‐off sources. This achievement was possible because of a comprehensive dataset on hydrology, stable isotopes and silica hydrochemistry in all relevant run‐off generation components. This concept indicates that a possible long‐term change in snowmelt may affect the run‐off regime of headwater catchments to climate or land‐use changes. Copyright © 2013 John Wiley & Sons, Ltd.     

Upper Pleistocene‐Holocene geomorphic changes dictating sedimentation rates and historical land use in the valley system of the Chifeng region,Inner Mongolia,northern China

This study focuses on the late Quaternary landscape evolution in the Chifeng region of Inner Mongolia, China, its relations to the history of the Pleistocene‐Holocene loess accumulation, erosion and redeposition, and their impact on human occupation. Based on 57 optically stimulated luminescence (OSL) ages of loess sediments, fluvial sand and floodplain deposits accumulated on the hill slopes and floodplains, we conclude that during most of the Pleistocene period the region was blanketed by a thick layer of aeolian loess, as well as by alluvial and fluvial deposits. The loess section is divided into two main units that are separated by unconformity. The OSL ages at the top of the lower reddish loess unit yielded an approximate age of 193 ka, roughly corresponding to the transition from MIS 7 to 6, though they could be older. The upper gray loess unit accumulated during the upper Pleistocene glacial phase (MIS 4–3) at a mean accumulation rate of 0·22 m/ka. Parallel to the loess accumulation on top of the hilly topography, active fans were operating during MIS 4–2 at the outlet of large gullies surrounding the major valley at a mean accumulation rate of 0·24 m/ka. This co‐accumulation indicates that gullies have been a long‐term geomorphic feature at the margins of the Gobi Desert since at least the middle Pleistocene. During the Holocene, the erosion of the Pleistocene loess on the hills led to the burial of the valley floors by the redeposited sediments at a rate that decreases from 3·2 m/ka near the hills to 1–0·4 m/ka¹ in the central part of the Chifeng Valley. This rapid accumulation and the frequent shifts of the courses of the river prevented the construction of permanent settlements in the valley floors, a situation which changed only with improved man‐made control of the local rivers from the tenth century AD. Copyright © 2010 John Wiley & Sons, Ltd.     

26Al and 10Be dating of late pleistocene and holocene fill terraces: a record of fluvial deposition and incision,Colorado front range

Cosmogenic ²⁶Al, ¹⁰Be, and ¹⁴C dating of fluvial fill terraces in steep canyons of the Colorado Front Range provides a temporal framework for analysing episodic aggradation and incision. Results from Boulder Canyon show that terrace heights above the modern channel (grade) can be divided into: (1) Bull Lake (≳100 ka; 20–15 m above grade); (2) Pinedale (32–10 ka; 15–4 m above grade); and (3) Holocene age (<4 m above grade). No pre‐Bull Lake deposits are preserved along Boulder Canyon, and only three small remnants >15 m above grade record Bull Lake deposition. Well‐preserved terraces of Pinedale age suggest that the range of terrace height above grade reflects short‐term fluctuations in the river profile during periods of rapidly changing stream load and power. Net river incision apparently occurred during transitions to interglacial periods. Soil development and stratigraphic position, along with limited cosmogenic and ¹⁴C dating, suggest that ∼130 ka terraces in Boulder Canyon correlate with the Louviers Alluvium, and that 32 to 10 ka fills in the canyon correlate with the Broadway Alluvium on the adjacent High Plains. Late Pleistocene incision rates (∼0·15 m ka⁻¹) along Boulder Canyon exceed pre‐late Pleistocene incision rates, and are higher than middle to late Pleistocene incision rates (∼0·04 m ka⁻¹) on the High Plains. This study provides an example of how modern geochronologic techniques allow us to understand better rivers that drain glaciated catchments. Copyright © 2002 John Wiley & Sons, Ltd.     

Multiple calcrete profiles in the Tabernas Basin,southeast Spain: their origins and geomorphic implications

The paper describes a sequence of Pliocene(? ) to Quaternary age calcretes developed within alluvial fan and fluvial gravels in the Tabernas Basin, Almería Province, southeast Spain. Calcrete profiles are described from sites adjacent to major tributaries of the Rambla de Tabernas. Six distinct calcrete units are identified within the basin. These have variable distributions but have developed in an identifiable evolutionary sequence. Two pairs of calcrete units are widely present across the basin preserving two former land surfaces. Each of the former land surfaces has been planated and subsequently buried by alluvial fan or fluvial gravels. A massive calcrete unit is present at the base of each gravel sequence, immediately in contact with the underlying bedrock, with a less well developed calcrete unit situated at the top of the gravel sequence. The lowest two calcrete units within the basin are more spatially restricted and are confined to the floors and flanks of incised drainage lines. The geochemistry, macro- and micromorphological properties and geomorphological positions of the calcrete units are outlined and, on the basis of this information, their mode of origin identified. Two main modes of calcrete genesis appear to be present: massive calcretes situated in direct contact with bedrock are suggested to have formed by groundwater processes, whilst calcretes situated at the top of gravel sequences are likely to have developed by pedogenic processes. Calcrete genesis is subsequently considered in the context of the reconstruction of the early phases of landscape development, and is suggested to have been controlled by phases of uplift and stability within the Tabernas Basin. © 1998 John Wiley & Sons, Ltd.     

Geomorphic response to late Quaternary tectonics in the axial portion of the Southern Apennines (Italy): A case study from the Calore River valley

The present study focuses on the morphotectonic evolution of the axial portion of the Southern Apennine chain between the lower Calore River valley and the northern Camposauro mountain front (Campania Region). A multidisciplinary approach was used, including geomorphological, field‐geology, stratigraphical, morphotectonic, structural, ⁴⁰Ar/³⁹Ar and tephrostratigraphical data. Results indicate that, from the Lower Pleistocene onwards, this sector of the chain was affected by extensional tectonics responsible for the onset of the sedimentation of Quaternary fluvial, alluvial fan and slope deposits. Fault systems are mainly composed of NW‐SE, NE–SW and W‐E trending strike‐slip and normal faults, associated to NW‐SE and NE–SW oriented extensions. Fault scarps, stratigraphical and structural data and morphotectonic indicators suggest that these faults affected the wide piedmont area of the northern Camposauro mountain front in the Lower Pleistocene–Upper Pleistocene time span. Faults affected both the oldest Quaternary slope deposits (Laiano Synthem, Lower Pleistocene) and the overlying alluvial fan system deposits constrained between the late Middle Pleistocene and the Holocene. The latter are geomorphologically and chrono‐stratigraphically grouped into four generations, I generation: late Middle Pleistocene–early Upper Pleistocene, with tephra layers ⁴⁰Ar/³⁹Ar dated to 158±6 and 113±7 ka; II generation: Upper Pleistocene, with tephra layers correlated with the Campanian Ignimbrite (39 ka) and with the slightly older Campi Flegrei activity (⁴⁰Ar/³⁹Ar age 48±7 ka); III generation: late Upper Pleistocene–Lower Holocene, with tephra layers correlated with the Neapolitan Yellow Tuff (~15 ka); IV generation: Holocene in age. The evolution of the first three generations was controlled by Middle Pleistocene extensional tectonics, while Holocene fans do not show evidence of tectonic activity. Nevertheless, considering the moderate to high magnitude historical seismicity of the study area, we cannot rule out that some of the recognized faults may still be active. Copyright © 2018 John Wiley & Sons, Ltd.     

Composition and origin of a lithalsa related to lake‐level recession and Holocene terrestrial emergence,Northwest Territories,Canada

Lithalsas of the Great Slave Lowland, Northwest Territories, occur within fine‐grained glaciolacustrine, lacustrine, and alluvial deposits. Detailed investigations of a lithalsa revealed that it is composed of ice‐rich sediments with ice lenses up to 0.2 m thick below 4 m depth. The observed ice accounted for about 2 m of the 4 m between the top of the lithalsa and adjacent terrain. The ice is isotopically similar to modern surface water, but enriched in δ¹⁸O relative to local precipitation. Total soluble cation concentrations are low in the basal, Shield‐derived and unweathered glaciolacustrine sediments of the lithalsa. Higher concentrations in the overlying Holocene‐aged lacustrine and alluvial deposits may be due to greater ion availability in Holocene surface waters. Increasing Cl^‐ and Na⁺ concentrations in clays at depth likely relate to exclusion and migration of these dissolved ions in pore water during ice lens formation though total soluble cations remain comparatively low. The lithalsa developed 700 to 300 cal yr BP. A conceptual model of lithalsa formation and landscape evolution illustrates that this feature and more than 1800 other lithalsas in the region have developed in association with Holocene terrestrial emergence following lake‐level recession. Copyright © 2017 John Wiley & Sons, Ltd.     

Probing deep weathering in the Shale Hills Critical Zone Observatory,Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface

Weathering is both an acid‐base and a redox reaction in which rocks are titrated by meteoric carbon dioxide (CO₂) and oxygen (O₂). In general, the depths of these weathering reactions are unknown. To determine such depths, cuttings of Rose Hill shale were investigated from one borehole from the ridge and four boreholes from the valley at the Susquehanna Shale Hills Observatory (SSHO). Pyrite concentrations are insignificant to depths of 23 m under the ridge and 8–9 m under the valley. Likewise, carbonate concentrations are insignificant to 22 and 2 m, respectively. In addition, a 5–6 m‐thick fractured layer directly beneath the land surface shows evidence for loss of illite, chlorite, and feldspar. Under the valley, secondary carbonates may have precipited. The limited number of boreholes and the tight folding make it impossible to prove that depth variations result from weathering instead of chemical heterogeneity within the parent shale. However, carbonate depletion coincides with the winter water table observed at ~20 m (ridge) and ~2 m depth (valley). It would be fortuitous if carbonate‐containing strata are found under ridge and valley only beneath the water table. Furthermore, pyrite and carbonate react quickly and many deep reaction fronts for these minerals are described in the literature. We propose that deep transport of O₂ initiates weathering at SSHO and many other localities because pyrite commonly oxidizes autocatalytically to acidify porewaters and open porosity. According to this hypothesis, the mineral distributions at SSHO are nested reaction fronts that overprint protolith stratigraphy. The fronts are hypothesized to lie subparallel to the land surface because O₂ diffuses to the water table and causes oxidative dissolution of pyrite. Pyrite‐derived sulfuric acid (H₂SO₄) plus CO₂ also dissolve carbonates above the water table. To understand how reaction fronts record long‐term coupling between erosion and weathering will require intensive mapping of the subsurface. Copyright © 2013 John Wiley & Sons, Ltd.     

Exposure age and erosional history of an upland planation surface in the US Atlantic Piedmont

The upland planation surface in the Piedmont of central New Jersey consists of summit flats, as much as 130 km² in area, that truncate bedding and structure in diabase, basalt, sandstone, mudstone and gneiss. These flats define a low‐relief regional surface that corresponds in elevation to residual hills in the adjacent Coastal Plain capped by a fluvial gravel of late Miocene age. A Pliocene fluvial sand is inset 50 m below the upland features. These associations suggest a late Miocene or early Pliocene age for the surface. To assess exposure age and erosional history, a 4·4 m core of clayey diabase saprolite on a 3 km² remnant of the surface was sampled at six depths for atmospherically produced cosmogenic ¹⁰Be. The measured inventory, assuming a deposition rate of 1·3 × 10⁶ atoms cm⁻² a⁻¹, yields a minimum exposure age of 227 000 years, or, assuming continuous surface erosion, a constant erosion rate of 10 m Ma⁻¹. Because the sample site lies about 60 m above the aggradation surface of the Pliocene fluvial deposit, and itself supports a pre‐Pliocene fluvial gravel lag, this erosion rate is too high. Rather, episodic surface erosion and runoff bypassing probably have produced an inventory deficit. Reasonable estimates of surface erosion (up to 10 m) and bypassing (up to 50 per cent of total precipitation) yield exposure ages of as much as 6·4 Ma. These results indicate that (1) the surface is probably of pre‐Pleistocene age and has been modified by Pleistocene erosion, and (2) exposure ages based on ¹⁰Be inventories are highly sensitive to surface erosion and runoff bypassing. Copyright © 2000 John Wiley & Sons, Ltd.     

Landscape response to tectonic and climatic forcing in the foredeep of the southern Apennines,Italy: insights from Quaternary stratigraphy,quantitative geomorphic analysis,and denudation rate proxies

We present new data about the morphological and stratigraphic evolution and the rates of fluvial denudation of the Tavoliere di Puglia plain, a low‐relief landscape representing the northernmost sector of the Pliocene‐Pleistocene foredeep of the southern Apennines. The study area is located between the easternmost part of the southern Apennine chain and the Gargano promontory and it is characterized by several orders of terraced fluvial deposits, disconformably overlying lower Pleistocene marine clay and organized in a staircase geometry, which recorded the emersion and the long‐term incision history of this sector since mid‐Pleistocene times. We used the spatial and altimetric distribution of several orders of middle to late Pleistocene fluvial terraces in order to perform paleotopographic reconstruction and GIS‐aided eroded volumes estimates. Then, we estimated denudation rates on the basis of the terraces chronostratigraphy, supported by published OSL and AAR dating. Middle to upper Pleistocene denudation rates estimated by means of such an approach are slightly lower than 0.1 mm yr^‐1, in good agreement with short‐term data from direct and indirect evaluation of suspended sediment yield. The analysis of longitudinal river profiles using the stream power erosion model provided additional information on the incision rates of the studied area. Middle to late Quaternary uplift rates (about 0.15 mm yr^‐1), calculated on the basis of the elevation above sea level of marine deposits outcropping in the easternmost sector of the study area, are quite similar to the erosion rates average value, thus suggesting a steady‐state fluvial incision. The approach adopted in this work has demonstrated that erosion rates traditionally obtained by quantitative geomorphic analysis and k_sn estimations can be successfully integrated to quantify rates of tectonic or geomorphological processes of a landscape approaching steady‐state equilibrium. Copyright © 2014 John Wiley & Sons, Ltd.     

Chemical weathering since the Little Ice Age recorded in lake sediments: a high‐resolution proxy of past climate

Because of the different geochemical behaviour of rubidium and strontium in earth surface processes, variations of the Rb/Sr ratios in lake sediments were used as a geochemical proxy of chemical weathering and past climate in a single watershed. Low magnetic susceptibility, low CaCO₃, low Sr concentration and, hence, high Rb/Sr ratio in the lake sediments indicate weak chemical weathering under a cold but wet climate during the Little Ice Age (LIA) in the closed Daihai Lake watershed. The concordant change in both Sr and CaCO₃ concentrations with δ¹⁸O values in the Dunde ice core suggests that weak chemical weathering during the wet LIA was controlled by air temperature. After the LIA, however, precipitation played a dominant role in chemical weathering. Copyright © 2001 John Wiley & Sons, Ltd.     

The silica supersaturated waters of northern Evia and eastern central Greece

The area of north Evia and eastern central Greece is characterized by strong geomorphological contrast and is built up mainly of consolidated rocks. Unconsolidated young sediments of Pleistocene to Holocene age cover the valley and basin flats, forming the most productive aquifers in this area. However, two more types of aquifers can be distinguished within the consolidated rock area. The first one is associated with karstified limestones and the second with strongly tectonized ultramafic rocks. The schist–chert formation, with intercalations of shales and cherts, seals the ultramafic masses underneath. Surface and spring waters associated with ultramafic rocks in north Evia and eastern central Greece were studied. Two types of water can be distinguished: (1) high Mg²⁺ and SiO₂ , bicarbonate as the dominant anion, pH 7·4–9·2, temperature 9·5–16·3 °C, low TDS (total dissolved solutes) (459–1037 mg/l), found both in peridotite and serpentinite areas, classified as Mg–HCO₃ type; (2) high Ca²⁺, low Mg²⁺ and SiO₂ , hydroxyl ion as the major anion, pH 11·2, temperature 28 °C, very low TDS (122 mg/l), found in peridotite areas, classified as Ca–OH type. The studied waters are highly supersaturated with respect to quartz, amorphous silica, brucite and most low temperature magnesium silicates (antigorite, sepiolite, talc, etc.). These waters show relatively narrow SiO₂ concentration ranges and a trend parallel to the amorphous silica saturation surface. The silica supersaturated waters have the potential to precipitate silica and consequently could affect the people of the local communities that use it as drinking water, causing health problems (kidney stones). Copyright © 1999 John Wiley & Sons, Ltd.