holocene palaeoenvironmental reconstruction in norteastern brazi inferred from polen charcoal and carboon isotope records soils in the barreirinhas region maranhao state were sampled for 613 c analysis and buried charcoal fragments in the soils were radiocarbon dated three soil profiles collected in forested areas around the lagoa do caqo and one in a woody savanna mixture of non arboreal and arboreal species located approximately 10 km southeast of the lagoa were studied a high resolution pollen record was obtained from lake sediments showing that forest vegetation was predominant in the area in the early holocene from approximately 10 000 14c yr bp the pollen spectrum gradually changed suggesting the dominance of open savanna communities these were transformed to a more forested landscape woody savanna from approximately 7500 yr bp the lake sediments also record evidence of fire indicated by buried charcoal particles at several soil depths during the holocene the 613 c analysis of soil organic matter som indicates that from between approximately io0000 yr bp and 9000 yr bp to 4000 yr bp a woody savanna

did fires drive holocene carbon sequestration in boreal ombrotrophic peatlands of eastern canada wildfire is an important factor on carbon sequestration in the north american boreal biomes being globally important stocks of organic carbon peatlands may be less sensitive to burning in comparison with upland forests especially wet unforested ombrotrophic ecosystems as found in northeastern canada we aimed to determine if peatland fires have driven carbon accumulation patterns during the holocene to cover spatial variability six cores from three peatlands in the eastmain region of quebec were analyzed for stratigraphic charcoal accumulation results show that regional holocene peatland fire frequency was 2 4 fires 1000 yr 1 showing a gradually declining trend since 4000 cal yr bp although inter and intra peatland variability was very high charcoal peak magnitudes however were significantly higher between 1400 and 400 cal yr bp possibly reflecting higher charcoal production driven by differential climatic forcing aspects carbon accumulation rates generally declined towards the late holocene with minimum values of 10 g m 2 yr 1 around 1500 cal yr bp the absence of a clear correlation between peatland fire regimes and carbon accumulation indicates that fire regimes have not been a driving factor on carbon sequestration at the millennial time scale

nitrogen in rock occurrences and biogeochemical implications 1 there is a growing interest in the role of bedrock in global nitrogen cycling and potential for increased ecosystem sensitivity to human impacts in terrains with elevated background nitrogen concentrations nitrogen bearing rocks are globally distributed and comprise a potentially large pool of nitrogen in nutrient cycling that is frequently neglected because of a lack of routine analytical methods for quantification nitrogen in rock originates as organically bound nitrogen associated with sediment or in thermal waters representing a mixture of sedimentary mantle and meteoric sources of nitrogen rock nitrogen concentrations range from trace levels 200 mg n kg 1 in granites to ecologically significant concentrations exceeding 1000 mg n kg 1 in some sedimentary and metasedimentary rocks nitrate deposits accumulated in arid and semi arid regions are also a large potential pool nitrogen in rock has a potentially significant impact on localized nitrogen cycles elevated nitrogen concentrations in water and soil have been attributed to weathering of bedrock nitrogen in some environments nitrogen released from bedrock may contribute to nitrogen saturation of terrestrial ecosystems more nitrogen available than required by biota nitrogen saturation results in leaching of nitrate to surface and groundwaters and where soils are formed from ammonium rich bedrock the oxidation of ammonium to nitrate may result in soil acidification inhibiting revegetation in certain ecosystems collectively studies presented in this article reveal that geologic nitrogen may be a large and reactive pool with potential for amplification of human impacts on nitrogen cycling in terrestrial and aquatic ecosystems

modeling relative frost weathering rates at geomorphic scales abstract frost damage is a powerful agent of geomorphic change cracks can grow when the ice pressure in pores reaches a threshold that depends on matrix properties and crack geometry mineral surfaces that are preferentially wetted by liquid water rather than ice are coated by premelted liquid at a pressure that is lower than the ice pressure because this pressure difference increases as the temperature cools when the ice pressure is effectively pinned at the cracking threshold temperature gradients induce gradients in liquid pressure that draw water towards colder temperatures porosity increases and frost damage accumulates in regions where water supplies crack growth to apply this understanding over the large spatial and temporal scales that are relevant to evolving landscapes we develop a simple model that tracks porosity changes our central assumption is that frost damage is correlated with porosity increases under conditions where frost cracking takes place accordingly we account for the permeability reductions with decreased temperature that accompany ice growth along porous pathways and derive general expressions for the porosity change through time at particular depths as well as the total porosity increase through all depths beneath a point at the ground surface over the time during which cracking occurs each year to illustrate the resulting patterns of frost weathering we consider a general case in which the permeability has a power law dependence on temperature and the annual surface temperature variation is sinusoidal we find that the degree of frost damage generally decreases with depth except at localized depths where damage is elevated because the rock spends longer times near the threshold for cracking leading to enhanced water supply in comparison with neighboring regions the magnitude of the net expansion that results from porosity changes at all depths beneath the ground surface is increased for seasonal thermal cycles with larger amplitudes with a broad maximum centered on a mean annual temperature near the threshold required for crack growth warmer mean annual temperatures lead to less damage because of the reduction in time during which it is cold enough for cracking whereas colder mean annual temperatures are accompanied by reduced water supply due to the temperature dependence of permeability all of the controlling parameters in our model are tied explicitly to physical properties that can in principle be measured independently which suggests promise for informing geomorphic interpretations of the role of frost weathering in evolving landforms and determining erosion rates

tracing the provenance of fine grained dust deposited on the central chinese loess plateau eolian dust deposits in north china provide an excellent means of determining past variations in continental paleoclimate and atmospheric circulation however debate still exists on which deserts in east asia are the dominant sources of chinese loess and whether the dust provenance has shifted significantly at different time scales here we present new constraints on the provenance of fine grained dust deposited on the central chinese loess plateau clp by combining electron spin resonance signal intensity and crystallinity index of fine grained quartz contained in samples from two loess paleosol sequences our results show that the fine grained dust deposits on the clp originate mainly from the gobi desert in southern mongolia and the sandy deserts in northern china primarily the badain juran and tengger deserts rather than from the taklimakan desert in western china at least during the last climatic cycle the dominant source of fine grained dust varied significantly from southern mongolia during cold periods to northern china during warm periods the glacial interglacial provenance fluctuations are strongly coupled with changes in the intensity of the near surface northwesterly winter monsoon

black carbon concentration and isotopic composition of surface sand from deserts and dune fields in northern china abstract black carbon bc is a continuum of incomplete combustion products of vegetation and fossil fuels it can be used as a proxy for wildfires and paleovegetation reconstruction nevertheless bc can be transported by wind and water via atmospheric and fluvial transport to research the characteristics of bc in deserts and dune fields that are directly connected with the chinese loess plateau clp in northern china and to evaluate the possibility that bc in deserts and dune fields occurs as exogenous bc in the clp we investigated surface sand from the mu us dune field the tengger desert and the badain jaran desert and analyzed the stable carbon concentration and carbon isotopes of organic carbon soc 13 c soc and bc bc 13 c bc the 13 c soc 13 c bc values ranged from 27 4 to 22 5 and 28 2 to 23 0 respectively which mainly reflected 13 c changes in surface c 3 grasses a positive correlation was observed between 13 c bc and 13 c soc suggesting that local vegetation mainly controls 13 c bc the difference between 13 c bc and 13 c soc ranged from 0 2 to 3 5 and the difference is likely to be due to carbon isotope discrimination occurring during burning and soc decomposition the soc and bc in these deserts and dune fields are very small averaging 0 048 and 0 011 the bc of surface sand in these areas is about 5 5 of bc in the clp surface soils and the 13 bc value between surface sand in deserts and surface soils in clp ranged from 0 3 to 6 3 so the change of 13 bc in the clp surface soils deduced by exogenous bc from deserts is less than 0 35 all these findings suggest that bc in deserts has a limited impact on bc in the clp and is unlikely to affect interpretations of the bc record of wildfires and paleovegetation information in the clp

bedrock fracture by ice segregation in cold regions the volumetric expansion of freezing pore water is widely assumed to be a major cause of rock fracture in cold humid regions data from experiments simulating natural freezing regimes indicate that bedrock fracture results instead from ice segregation fracture depth and timing are also numerically simulated by coupling heat and mass transfer with a fracture model the depth and geometry of fractures match those in arctic permafrost and ice age weathering profiles this agreement supports a conceptual model in which ice segregation in near surface permafrost leads progressively to rock fracture and heave whereas permafrost degradation leads episodically to melt of segregated ice and rock settlement

stable carbon isotope ratios from archaeological charcoal as palaeoenvironmental indicators abstract the potential to provide environmental proxies using stable carbon isotopes from modern and archaeological charcoal is explored experiments on modern podocarpus yellowwoods show that 13 c values of stems branches and charcoal preserve proxy environmental conditions including rainfall humidity and temperature an additional experiment showed that combustion temperature affects the carbon isotope signature of charcoal burning at 450 c to 500 c depletes 13 c values with respect to the original wood but the charcoal retains the seasonal and inter annual isotopic trends recorded during the growth of the tree the 13 c of podocarpus charcoal from three levels from the middle stone age site of sibudu cave kwazulu natal south africa was compared with modern analogues from two different environments seaton park kwazulu natal and the baviaans kloof eastern cape other environmental proxies from levels dated from 70 ka and 48 ka show that environmental conditions changed from warmer and wetter to colder and drier and finally becoming warmer and drier the isotope data is consistent with this reconstruction the results from this series of experiments indicate that it is possible to obtain meaningful palaeoenvironmental information from 13 c values of archaeological charcoal

nitrogen release from rock and soil under simulated field conditions abstract a laboratory study was performed to simulate field weathering and nitrogen release from bedrock in a setting where geologic nitrogen has been suspected to be a large local source of nitrate two rock types containing nitrogen slate 1370 mg n kg 1 and greenstone 480 mg n kg 1 were used along with saprolite and bc horizon sand from soils derived from these rock types the fresh rock and weathered material were used in batch reactors that were leached every 30 days over 6 months to simulate a single wet season nitrogen was released from rock and soil materials at rates between 10 20 and 10 19 mol n cm 2 s 1 results from the laboratory dissolution experiments were compared to in situ soil solutions and available mineral nitrogen pools from the bc horizon of both soils concentrations of mineral nitrogen no 3 nh 4 in soil solutions reached the highest levels at the beginning of the rainy season and progressively decreased with increased leaching this seasonal pattern was repeated for the available mineral nitrogen pool that was extracted using a kcl solution estimates based on these laboratory release rates bracket stream water no 3 n fluxes and changes in the available mineral nitrogen pool over the active leaching period these results confirm that geologic nitrogen when present may be a large and reactive pool that may contribute as a non point source of nitrate contamination to surface and ground waters

magnetic susceptibility evidence of monsoon variation on the loess plateau of central china during the last 130 000 years the magnetic susceptibility of loess and paleosols in central china represents a proxy climate index closely related to past changes of precipitation and vegetation and thus to summer monsoon intensity time series of magnetic susceptibility constructed for three loess paleosol sequences in the southern part of the chinese loess plateau document the history of summer monsoon variation during the last 130 000 yr they correlate closely with the oxygen isotope record of stages 1 to 5 in deep sea sediments soils were forming during intervals of strong summer monsoon whereas loess units were deposited at times of reduced monsoon intensity the chinese loess paleosol sequence can thus be viewed as a proxy record of asian monsoon variability extending over the last 2 5 myr

the magnetic properties of loess and paleosol samples from the chinese loess plateau spanning the last 22 million years abstract magnetic measurements susceptibility isothermal and anhysteretic remanence and quotients derived from these are presented for a suite of loess and paleosol samples from sites on the chinese loess plateau the three sites xifeng dongwan and qinan qa i together span most of the last 22 million years the results provide the first multi parameter magnetic measurements from pre pleistocene loess paleosol sequences that began accumulation in the early miocene mass specific measurements have been corrected for carbonate concentrations which peak in the loess layers and range up to 57 all the magnetic properties recorded indicate variability on both orbital and supra orbital timescales the changing magnetic properties allow the record to be divided into a sequence of eight sample groups that broadly parallel the main supra orbital changes in magnetic susceptibility the pre pleistocene parts of the record include intervals mainly late miocene and pliocene during which loess as well as paleosol layers are characterized by high values for properties such as frequency dependent susceptibility fd usually associated with strong weathering most of the miocene record prior to 8 5 ma with the exception of a brief interval from ca 14 4 to 15 9 ma shows reduced values for indicators of pedogenic ferrimagnetic concentrations in both the loess and paleosol layers as well as a low amplitude of variability between loess and paleosol values the palaeoenvironmental implications of this part of the sequence are unclear since they hinge on the type of model of pedogenic magnetic mineral formation adopted high field remanence measurements indicate that the coercivity of the antiferromagnetic minerals represented declined progressively throughout the whole period in both the loess and the paleosol samples though there is no unambiguous evidence for long term diagenetic alteration of the magnetic properties on timescales beyond the orbital ones that drive the loess paleosol alternations the present results suggest caution in applying existing climofunctions based on magnetic susceptibility to deposits spanning the whole of the last 22 ma

ice growth in a spherical cavity of a porous medium we consider an idealized problem of a sphere of ice growing symmetrically in a spherical cavity within a porous rock in order to identify and quantify different physical mechanisms that can result in fracturing the rock we show that if the permeability of the rock is very small then high pressures can develop in the cavity as the water inside it expands on freezing however given typical permeabilities of most rocks the pressure is relieved by flow out of the cavity through the rock pores when ice fills the cavity there remains a microscopic film of water separating the ice from the rock owing to disjoining forces and these forces can stress the rock and have the potential to fracture it the elastic pressure in the rock depresses the freezing temperature which can limit the potential for fracturing this simple example reveals the important interactions between disjoining forces elasticity and fluid flow in determining the pressure exerted during freezing of water saturated cavities in rocks

geologic nitrogen and climate change in the geochemical budget of karkevagge swedish lapland abstract while nitrogen is an essential nutrient for plant growth in the terrestrial environment and generally fixed by biological processes in soils in arctic and alpine environments n is a severely limited nutrient because of both climatic and soil constraints it is widely believed that geologic nitrogen is such a small component of the nitrogen cycle that this source is generally not viewed as being a significant component however it has been estimated that in fact as much as 20 of the total nitrogen budget may be stored in geological materials and where n is severely limited such as in arctic and alpine environments geologic n may in fact be a dominant source of n for biological processes nitrogen saturation may therefore have substantial impacts on geochemical cycles involving both soil and water chemistry in order to determine the potential for the rocks and soils of karkevagge to be significant sources of n samples of the principal rock units of the valley along with soil developed on a variety of rock types were sampled ammonia ammonium contents were determined using ion chromatography ammonia ammonium contents of the sampled rock materials display a wide range of abundances with marble containing a mean nh4 abundance of 4300 mg n kg quartz schist 1600 mg n kg biotite schist 4300 mg n kg and garnet mica schist 4500 mg n kg soil ammonia ammonium abundances were found to be 1600 mg n kg n pool abundances range from 2 5 kg m3 for marble to 8 7 for garnet mica schist soil n pools were found to be 3 0 kg m3 the inorganic n present as ammonia ammonium measured in bedrock represents substantial n input into the geochemical budget the n in bedrock originates from organically bound nitrogen associated with sediment or geothermal waters representing a mixture of sedimentary mantle and meteoric sources nitrogen in the soil originates from the weathering of n rich rocks the presence of unweathered nitrate minerals or fluid inclusions released to the soil and ultimately to plants by assimilation by biota or nitrification geologic nitrogen in karkevagge rocks and soils may represent a large and reactive pool with the potential for considerable impact on the geochemical system to investigate the potential responses of nitrogen release as well as other nutrients to increasing co2 concentration scenarios flow through leaching experiments were conducted on crushed rock samples and soils from the study area leaching experiments revealed two to three fold increases in both n and k release with a tripling of carbon dioxide concentrations ammonium shows more rapid release than potassium which suggests substantial changes in nitrogen cycling with increasing co2 enrichment of the atmosphere

stable carbon isotope records of black carbon on chinese loess plateau since last glacial maximum an evaluation on their usefulness for paleorainfall and paleovegetation reconstruction abstract stable carbon isotope composition of soil organic matter 13 c som has been widely used to infer past environmental or vegetation c 3 c 4 changes however as a carbonaceous product of incomplete burning of biomass the carbon isotope signature of black carbon bc has received little study and a limited usage on this purpose moreover the environmental or vegetation indication of bc carbon isotope composition 13 c bc in sedimentary records remains ambiguous although the 13 c bc is supposed to reflect carbon isotope composition of vegetation being burnt this deserves site specific studies here we analyzed 13 c bc and 13 c som on loess paleosol samples from lijiayuan and yangling sections on chinese loess plateau clp spanning the last glacial maximum to decipher the environmental or vegetation meaning of 13 c bc at each study site opposite changing patterns were observed on the 13 c bc values for the two sections the 13 c bc at yangling in southern part of the clp varied from 19 18 to 21 93 mean 20 62 with more positive values occurred during the middle holocene than those during the lgm demonstrating more c 4 plants occupied in the region during the warm humid middle holocene this is consistent with the changing pattern widely documented in 13 c som records over the clp during the same period by contrast the 13 c bc at lijiayuan in northwestern part of the clp changed within the range of 21 83 24 64 mean 23 34 and displayed more negative values during the early middle holocene with respect to the lgm period the 13 c bc at lijiayuan were about 2 5 lower than those at yangling indicating a northward decrease of c 4 plants on the clp the anti phased changes of 13 c bc at lijiayuan were considered to reflect variations in paleorainfall because the vegetation is dominated by c 3 plants at the study site and carbon isotope compositions of c 3 plants decrease as rainfall increases compared with 13 c som record at the same profile 13 c bc seem biased against capturing c 4 signal during mid holocene possibly due to dominance of c 3 plants at the study site during fire seasons and a potentially extra source of bc from forest fires occurred on surrounding mountains by contrast 13 c bc tend to exaggerate c 4 signal during the lgm attributed to a possible shift of fire season from spring to summer these findings suggest that we probably underestimate the abundance of c 4 plants during mid holocene and overestimate c 4 biomass during the lgm using bc carbon isotope composition

a theoretical model of the fracture of rock during freezing we present a mathematical model for the breakdown of porous rock by the growth of ice within cracks the model is founded upon well established principles of fracture mechanics and recent advances in soil physics ijong with the assumption that progressive crack growth results from water migrating to ice bodies in cracks much as water migrates to ice lenses in freezing soil our model predicts crack growth rates compatible with empirical data calculations for a granite and a marble indicate that sustained freezing is most effective in producing crack growth when temperatures range from 4 c to 15 c at higher temperatures thermodynamic limitations prevent ice pressure from building up sufficiently to produce significant crack growth at lower temperatures the migration of water necessary for sustaining crack growth is strongly inhibited in hydraulically open systems in which pore water pressure remains near atmospheric pressure during the freezing process crackgrowth rates during continuous cooling will generally be greatest at low rates of cooling less than 0 1 0 5 c h at higher rates of cooling the influx of water to growing cracks is significantly inhibited the model delineates clearly the role of material parameters elastic moduli fracture mechanical properties grain size and shape and crack size environmental conditions temperature temperature gradient water pressure and time in frost damage to rocks our calculations along with recent experimental work on water migration in freezing rocks fukuda 1983 lead us to question the widely accepted importance of two phenomena freezing of water in sealed cracks and freeze thaw cycling in the fracture of rock exposed to natural freezing conditions

evidence of frost cracking inferred from acoustic emissions in a high alpine rock wall ice formation within rock is known to be an important driver of near surface frost weathering as well as of rock damage at the depth of several meters which may play a crucial role for the slow preconditioning of rock fall in steep permafrost areas this letter reports results from an experiment where acoustic emission monitoring was used to investigate rock damage in a high alpine rock wall induced by natural thermal cycling and freezing thawing the analysis of the large catalog of events obtained shows i robust power law distributions in the time and energy domains a footprint of rock micro fracturing activity induced by stresses arising from thermal variations and associated freezing thawing of rock ii an increase in ae activity under sub zero rock temperatures suggesting the importance of freezing induced stresses ae activity further increases in locations of the rock wall that are prone to receiving melt water these results suggest that the framework of further modeling studies theoretical and numerical should include damage elastic interaction and poro mechanics in order to describe freezing related stresses

direct effects of co2 and temperature on silicate weathering possible implications for climate control a critical uncertainty in models of the global carbon cycle and climate is the combined effect of organic activity temperature and atmospheric co2 on silicate weathering here we present new dissolution rates of anorthite and augite which indicate that silicate weathering in organic rich solutions is not directly affected by soil co2 but is very sensitive to temperature apparently co2 accelerates silicate weathering indirectly by fertilizing organic activity and the production of corrosive organic acids the weathering dependencies highlight the ability of silicate weathering to act as a global thermostat and damp out climate change when used as input in steady state carbon cycle and climate models

chinese loess and the paleomonsoon abstract this paper presents a general review of the recent research advances of the east asian paleomonsoon based mainly on studies of the loess soil sequences in the chinese loess plateau in the last 2 6 million years the paleomonsoonal history may be divided into about 166 events on a time scale of the earth s orbital variations during the last glacial period millenial scale oscillations of the monsoon system were prominent which can be fairly well correlated with the dansgaard oeschger cycles recognized in the greenland ice cores the monsoonal rainfall belt has experienced a wide repeated advance retreat change during the glacial interglacial cycles of the pleistocene both temporal and spatial changes of the monsoon system in the quaternary could have been linked closely to global ice volume variations

vegetation fire climate and human disturbance history in the southwestern mediterranean area during the late holocene detailed pollen charcoal isotope and magnetic susceptibility data from an alpine lake sediment core from sierra nevada southern spain record changes in vegetation fire history and lake sedimentation since ca 4100 cal yr bp the proxies studied record an arid period from ca 3800 to 3100 cal yr bp characterized by more xerophytic vegetation and lower lake levels a humid period is recorded between ca 3100 and 1850 cal yr bp which occurred in two steps 1 an increase in evergreen quercus between 3100 and 2500 cal yr bp indicating milder conditions than previously and 2 an increase in deciduous quercus and higher lake levels between ca 2500 and 1850 cal yr bp indicating a further increase in humidity and reduction in seasonal contrast humid maxima occurred during the roman humid period previously identified in other studies in the mediterranean region intensified fire activity at this time could be related to an increase in fuel load and or in human disturbance an arid period subsequently occurred between 1850 and 650 cal yr bp though a decrease in quercus and an increase in xerophytes the alternation of persistent north atlantic oscillation modes probably played an important role in controlling these humid arid cycles

elemental delta c 13 at allen s cave nullarbor plain australia assessing post depositional disturbance and reconstructing past environments the degree to which post depositional disturbance can be assessed in archaeological sequences is critical for the development of robust chronologies for such deposits a common approach is the identification of conjoined artefacts distributed through the sequence as a measure of disturbance following deposition although this method is relatively labour intensive and does not provide a continuous record of disturbance through the entire sequence here we present 13c values for different size fractions 125 m of chemically resistant charcoal distributed through a sedimentary profile at allen s cave south australia the curves generated from both size fractions parallel one another indicating minimal disturbance through the entire profile and providing greater confidence in the luminescence and radiocarbon chronologies reported previously for the site in addition the elemental 13c values provide an indication of greater aridity compared with present persisting in this area from ca 45 to 12 ka consistent with other proxy records the onset of the holocene is marked by a shift to wetter conditions probably as a result of post glacial marine transgression and closer proximity to the coastline copyright 2001 john wiley sons ltd

changes in terrestrial ecosystem since 30 ma in east asia stable isotope evidence from black carbon in the south china sea a 30 m y stable isotopic record of marine deposited black carbon from regional ter restrial biomass burning from the northern south china sea reveals photosynthetic path way evolution for terrestrial ecosystems in the late cenozoic this record indicates that c3 plants negatively adjusted their isotopic discrimination and c 4 plants appeared grad ually as a component of land vegetation in east asia since the early miocene a long time before sudden c4 expansion occurred during the late miocene to the pliocene the changes in terrestrial ecosystems with time can be reasonably related to the evolution of east asian monsoons which are thought to have been induced by several intricate mechanisms during the late cenozoic and could contribute significantly to the post miocene marine carbonate isotope decline

black carbon in soils and sediments analysis distribution implications and current challenges this review highlights the ubiquity of black carbon bc produced by incomplete combustion of plant material and fossil fuels in peats soils and lacustrine and marine sediments we examine various definitions and analytical approaches and seek to provide a common language bc represents a continuum from partly charred material to graphite and soot particles with no general agreement on clear cut boundaries formation of bc can occur in two fundamentally different ways volatiles recondense to highly graphitized soot bc whereas the solid residues form char bc both forms of bc are relatively inert and are distributed globally by water and wind via fluvial and atmospheric transport we summarize chronologically the ubiquity of bc in soils and sediments since devonian times differentiating between bc from vegetation fires and from fossil fuel combustion bc has important implications for various biological geochemical and environmental processes as examples bc may represent a significant sink in the global carbon cycle affect the earth s radiative heat balance be a useful tracer for earth s fire history build up a significant fraction of carbon buried in soils and sediments and carry organic pollutants on land bc seems to be abundant in dark colored soils affected by frequent vegetation burning and fossil fuel combustion thus probably contributing to the highly stable aromatic components of soil organic matter we discuss challenges for future research despite the great importance of bc only limited progress has been made in calibrating analytical techniques progress in the quantification of bc is likely to come from systematic intercomparison using bcs from different sources and in different natural matrices bc identification could benefit from isotopic and spectroscopic techniques applied at the bulk and molecular levels the key to estimating bc stocks in soils and sediments is an understanding of the processes involved in bc degradation on a molecular level a promising approach would be the combination of short term laboratory experiments and long term field trials

effect of human controlled hydrological regime on the source transport and flux of particulate organic carbon from the lower huanghe yellow river evaluating the role of fluvial transfer of terrestrial organic carbon oc and subsequent burial in the global carbon cycle requires the sources and fluxes of fluvial oc to be assessed which remains poorly constrained in the huanghe yellow river here we report the elemental stable isotopic and radiocarbon activity of particulate organic carbon poc sampled at the outlet of huanghe in 2012 2013 we show that the huanghe riverine poc can be explained by binary mixing of fossil pocfossil and non fossil pocnon fossil components the former may reach 40 of the total poc the huanghe pocnon fossil is mostly sourced from c3 plants with a mean residence time of c 2200 years the current human controlled hydrological regime strongly influenced the poc sources transport modes and fluxes in 2012 2013 the huanghe delivered 0 73 tg 1 tg 1012 g of poc to the sea and about 28 of the annual poc flux occurred within a short human induced flood event globally the huanghe should be one of the largest rivers in the transfer and re burial of fossil oc however the fate of huanghe fossil oc is still unconstrained and needs to be further investigated copyright 2015 john wiley sons ltd

environmental controls of frost cracking revealed through in situ acoustic emission measurements in steep bedrock 1 frost cracking the breakdown of rock by freezing is one of the most important mechanical weathering processes acting on earth s surface insights on the mechanisms driving frost cracking stem mainly from laboratory and theoretical studies transferring insights from such studies to natural conditions involving jointed bedrock and heterogeneous thermal and hydrological properties is a major challenge we address this problem with simultaneous in situ measurements of acoustic emissions used as proxy of rock damage and rock temperature moisture content the 1 year data set acquired in an alpine rock wall shows that 1 liquid water content has an important impact on freezing induced rock damage 2 sustained freezing can yield much stronger damage than repeated freeze thaw cycling and 3 that frost cracking occurs over the full range of temperatures measured extending from 0 down to 15 c these new measurements yield a slightly different picture than previous field studies where ice segregation appears to play an important role

stable carbon isotopic composition of black carbon in surface soil as a proxy for reconstructing vegetation on the chinese loess plateau abstract black carbon bc is a continuum of carbon produced by incomplete combustion whether its stable carbon isotopic composition expressed as 13 c bc values can be used directly for paleovegetation reconstruction is still in debate in the present study we investigated surface soil from the chinese loess plateau and collected samples from the following three transects the ruicheng jingbian lantian huanxian and qin an dingxi from east to west the 13 c bc values for surface soil ranged from 27 9 to 21 9 the stable carbon isotopic composition of soil organic carbon expressed as 13 c soc values ranged from 26 8 to 20 9 a strong positive correlation was observed between 13 bc and 13 c soc r 0 88 p 0 000 13 bc and 13 c soc 13 soc bc values ranged from 1 5 to 1 3 with an average of 0 2 the characteristics of vegetation on the chinese loess plateau and its variation indicated by 13 bc and 13 c soc values were consistent with those of modern plants it suggests that 13 bc values for surface soil are controlled mainly by surface plants and that therefore 13 bc can indicate the vegetation effectively within a precision range of 1 5

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

determination of black carbon by chemical oxidation and thermal treatment in recent marine and lake sediments and cretaceous tertiary clays abstract several sample treatments and analytical methods for the measurement of black carbon in a wide variety of sediments were considered in which the removal of organic carbon is a critical step in the procedure a comparison between chemical oxidation and thermal treatment was conducted and the latter was shown to be inappropriate due to the polymerisation of organic matter using acid dichromate oxidation reactive kerogen was separated from black carbon in a variety of lake and marine and recent and ancient sediments with relatively small and quantifiable losses 1 6 of black carbon under carefully controlled reaction conditions

late pliocene pleistocene expansion of c4 vegetation in semiarid east asia linked to increased burning plants using the c 4 photosynthetic pathway commonly tropical and subtropical grasses increased in abundance in east asia dur ing the late cenozoic determining the exact timing and likely fac tors leading to this major vegetation change requires region specific studies here variations in pyrogenic carbon mass accumulation rate pyc mar and isotope composition 13 c pyc from an 7 m y long depositional sequence from the central loess plateau china suggest increased biomass burning and an increased contribution to combusted material from c 4 taxa from 2 6 ma changes in the composition of pyc after 0 6 ma likely reflect the effects of lower temperatures particularly during glacial periods and changes in seasonality of precipitation increased pyc mar without concomi tant changes in 13 c pyc at ca 0 15 ma appears to indicate a decou pling of feedbacks between changes in climate fire regime and veg etation and may mark the onset of anthropogenic burning in the region these new data suggest that c 4 taxa were present on the loess plateau from at least the late miocene rising to prominence at ca 2 6 ma following changes in climate and critically an increase in biomass fires

diagenesis of ammonium during hydrocarbon maturation and migration wilcox group louisiana u s a abstract as organic matter matures n is released in the reduced form as nh4 and substitutes for k in clay minerals as oil forms an abundance of n is generated coincident with the formation of authigenic illite this study suggests that nh4 released from organic matter during maturation migrates through sandstones together with the pore fluids and volatile phases generated from the migrating crude oil this causes anomalously high nh4 substitution in authigenic illites fixed nh4 concentrations in illite are a product of both hydrocarbon maturation and migration concentrations of fixed nh4 from organic rich mudstones in two wilcox group oil fields show a general increase with depth coincident with increasing organic maturation reservoir sandstones containing crude oils have fixed nh4 concentrations as much as two times those of the mudstones when values are normalized to the quantity of clay minerals fixed nh4 concentrations in the gaseous portion of the reservoir are three times as high two out of five sandstones from a dry hole adjacent to a producing oil field contain anomalously high 2 above mean concentrations of fixed nh4 with respect to total organic centent this together with a high production index and hydrogen index in those units suggests that migrating hydrocarbons may have influenced the fixed nh4 concentrations

stable carbon isotope of black carbon in lake sediments as an indicator of terrestrial environmental changes an evaluation on paleorecord from daihai lake inner mongolia china abstract we measured the carbon isotope ratio of black carbon bc from the daihai lake sediment core dh99a in north central china with an objective to examine the effectiveness and sensitivity of the 13c values of bc 13cbc as a potential indicator of terrestrial environmental changes we first performed a statistical study on the available data regarding carbon isotope fractionation cif during the conversion of c3 and c4 vegetation to bc and observed that the mean cif for bc produced from c3 plants is 0 3 whereas that for bc from c4 plants is 1 7 this result provides a solid reference for reconstructing vegetation and environmental changes using the 13cbc values the 13cbc record in the dh99a sediment core spanning the last ca 10 000 years displayed large variations from 23 7 to 29 2 which suggests that c3 plants dominantly occupied the daihai lake region during the holocene the most negative 13cbc peaks coincided with high values of tree percentages and grain sizes which occurred under relatively wetter climatic conditions during the middle holocene ca 6500 3200 cal yr bp and an interval between 1700 and 1350 cal yr bp in contrast the least negative 13cbc values corresponded to low values of tree percentages and grain sizes during relatively drier phases of the early and late holocene the generally negative correlation of the 13cbc values with the tree percentages and grain sizes was thought to reflect a negative correlation of the 13cbc values with the monsoon precipitation this correlation is consistent with the response of carbon isotope in modern c3 plants to precipitation in north china therefore we developed a computational model to reconstruct the changes in annual precipitation over the daihai lake region using the 13cbc values the results indicated that the annual precipitation was highly variable ranging from 170 mm lower to 310 mm higher than that at present during the middle holocene whereas the annual precipitation was generally 70 mm lower than that at present during the early and late holocene the general features of the inferred precipitation changes using the 13cbc values are generally consistent with those reconstructed using pollen data of the same sediment core meanwhile the 13cbc values tend to register some extreme variations of monsoon precipitation which were not reflected in the pollen assemblages we conclude that the 13cbc values in the daihai lake sediments may serve as a sensitive and reliable proxy for monitoring monsoon precipitation

evolution of the mechanical behaviour of limestone subjected to freeze thaw cycles the frost weathering of three french limestones lens ls migne mi and savonnieres sa with different physico mechanical properties during the freeze thaw cycles was studied the weathering evolution in stone samples undergoing the freeze thaw cycles was monitored by measurement of following properties total porosity p wave velocity static elastic moduli uniaxial compressive strength and toughness the thermo mechanical behaviour of stones was also studied by measurement of the strain and temperature in surface and centre of sample during one freeze thaw cycle these results mainly the weathering observed for sa 73 with a degree of saturation of 73 and a lack of weathering for ls with a degree of saturation of 90 show that it is impossible to define a unique critical degree of saturation for different stones and this varies from stone to stone the shrinkage and or the expansion of samples during the crystallization phase were determined the shrinkage is related to the migration of water away from tiny pores towards larger cavities where ice has already formed the expansion is attributed on the one hand to the rise in pressure that comes from the water which flows towards the ice front and on the other hand to the relatively important quantity of ice formed when thermal contraction and the contraction due to migration dominate the elastic moduli static moduli and dynamic moduli was found to be affected the most by frost weathering and is a real sensor of first micro crack initiation whereas the uniaxial compressive strength and the porosity were found to be affected the least

increased seasonality and aridity drove the c4 plant expansion in central asia since the miocene pliocene boundary abstract continuous and high resolution records of the content mass accumulation rate mar and 13 c values of black carbon obtained from integrated ocean drilling program iodp site u1430 in the southwestern japan sea have been established and combined with previous results obtained from central asia the main objective of this work is to reconstruct the historical changes in vegetation types c3 c4 and to constrain the driving force of c4 plant expansion over the last 13 ma the stable carbon isotope value of black carbon 13 c bc shows a major shift since the miocene pliocene boundary 5 3 ma suggesting significant expansion of c4 plants in broad areas of central asia including the inland basins of northwestern china and the loess plateau however a decline in the content and mar of black carbon reveals the absence of any link between fire and c4 plant expansion in central asia due to the dramatic decrease in biomass under a drying regime on a global scale asynchronous expansion of c4 plants suggests that regional hydroclimatic change rather than decline in co 2 concentration was the most important factor to influence c4 expansion we propose that the increased seasonality and the enhanced long term aridity driven by the concurrent decline in winter westerly vapor and increase in east asian summer monsoon precipitation were the main driving forces of c4 plant expansion in broad areas of central asia variations in winter westerly moisture have played a significant role in changes of regional climate and vegetation in central asia since the late miocene