experimental low grade alteration of biogenic magnetite indicates microbial involvement in generation of banded iron formations during the deposition of banded iron formation bif the downward flux of ferric oxyhydroxides and phytoplankton biomass should have facilitated fe iii reduction during burial with the end product being ferrous iron containing minerals including magnetite although earlier studies have attempted to quantify the significance of this pathway based on models of the ancient fe cycle the only direct evidences of a biological role in magnetite formation in bif are their iron isotope compositions and unique crystallography which are reminiscent of biologically generated magnetite however the biogenesis hypothesis lacks an explanation as to why modern biogenic magnetite crystals are generally a few hundred nm or smaller in size yet the magnetite crystals in bif are mostly tens of micrometers or larger in size in this study we demonstrate that biogenic magnetite crystals can grow in size upon reaction between oxyhydroxide and microbial biomass after compression and heating to 1 kbar and 150 1c respectively the magnetite crystals previously produced by thermoanaerobacter spp tor39 reach sizes in excess of 700 nm after the p t experiments while new magnetite grains 4 400 nm formed from the superparamagnetic magnetite dominated end product of shewanella sp culture this study indicates that the large magnetite crystals observed in bif can be derived through a three stage sequence beginning with dissimilatory iron reduction of an initial ferric iron rich sediment coupled to the oxidation of dead phytoplankton biomass followed by magnetite crystal aging and ultimately pressure temperature induced abiotic alteration of the biogenic magnetite during metamorphism

high resolution stalagmite climate record from the yucatan peninsula spanning the maya terminal classic period the decline of the classic maya civilization was complex and geographically variable and occurred over a 150 year interval known as the terminal classic period tcp c e 800 950 paleoclimate studies based on lake sediments from the yucatan peninsula lowlands suggested that drought prevailed during the tcp and was likely an important factor in the disintegration of the classic maya civilization the lacustrine evidence for decades of severe drought in the yucatan peninsula however does not readily explain the long 150 year socio political decline of the classic maya civilization here we present a new absolute dated highresolution stalagmite 18o record from the northwest yucatan peninsula that provides a much more detailed picture of climate variability during the last 1500 years direct calibration between stalagmite 18o and rainfall amount offers the first quantitative estimation of rainfall variability during the terminal classic period our results show that eight severe droughts lasting from 3 to 18 years occurred during major depopulation events of classic maya city states during these droughts rainfall was reduced by 52 to 36 the number and short duration of the dry intervals help explain why the tcp collapse of the mayan civilization occurred over 150 years

effects of aridity and vegetation on plant wax δd in modern lake sediments we analyzed the deuterium composition of individual plant waxes in lake sediments from 28 watersheds that span a range of precipitation d h vegetation types and climates the apparent isotopic fractionation ea between plant wax n alkanes and precipitation differs with watershed ecosystem type and structure and decreases with increasing regional aridity as measured by enrichment of 2 h and 18 o associated with evaporation of lake waters the most negative ea values represent signatures least affected by aridity these values were 125 5 for tropical evergreen and dry forests 130 for a temperate broadleaf

pre aged plant waxes in tropical lake sediments and their influence on the chronology of molecular paleoclimate proxy records sedimentary records of plant wax hydrogen ddwax and carbon d 13 cwax stable isotopes are increasingly applied to infer past climate change compound specific radiocarbon analyses however indicate that long time lags can occur between the synthesis of plant waxes and their subsequent deposition in marginal marine sediments the influence of these time lags on interpretations of plant wax stable isotope records is presently unconstrained and it is unclear whether such time lags also affect lacustrine sediments we present compound specific radiocarbon 14 cwax data for n alkanoic acid plant waxes n c26 to n c32 from 1 a sediment core from lake chichancanab yucatan peninsula mexico 2 soils in the lake chichancanab catchment and 3 surface sediments from three other lakes in southeastern mexico and northern guatemala 14 cwax ages in the surface sediments are consistently older than modern and may be negatively correlated with mean annual precipitation and positively correlated with lake catchment area 14 cwax ages in soils surrounding lake chichancanab increase with soil depth consistent with deep subsoil horizons being the primary source of lacustrine aged plant waxes which are likely delivered to lake sediments through subsurface transport plant waxes in the lake chichancanab core are 350 1200 years older than corresponding ages of bulk sediment deposition determined by 14 c dates on terrestrial plant macrofossils in the core a ddwax time series is in closer agreement with other regional proxy hydroclimate records when a plant wax 14 c age model is applied as opposed to the macrofossil based core chronology inverse modeling of plant wax age distribution parameters suggests that plant waxes in the lake chichancanab sediment core derive predominantly from millennial age soil carbon pools that exhibit relatively little age variance 200 years our findings demonstrate that high temporal resolution climate records inferred from stable isotope measures on plant waxes in lacustrine sediments may suffer from possible chronologic distortions as a consequence of long residence times of plant waxes in soils they also underscore the importance of direct radiocarbon dating of these organic molecules

a 4000 year lacustrine record of environmental change in the southern maya lowlands peten guatemala a 4000 yr sediment core record from lake salpeten guatemala provides evidence for maya induced forest clearance and consequent soil erosion between 1700 cal yr b c and 850 cal yr a d radiocarbon ages of wood seeds and charcoal support an age depth model with average errors of 110 cal yr relatively low carbonate 18o values between 1300 and 400 cal yr b c coincide with pollen evidence for forest loss consistent with increased surface and groundwater flow to the lake minimum 18o values between 400 cal yr b c and 150 cal yr a d suggest a high lake level as do 14c dated aquatic gastropods as much as 7 5 m above the present lake stage high lake levels resulted from reduced evaporation to precipitation ratios increased hydrologic input caused by anthropogenic deforestation or both the preclassic abandonment 150 a d and early classic late classic boundary 550 a d are marked by relatively high 18o values indicating reduced lake levels oxygen isotope composition increased further coincident with the terminal classic maya demographic decline between 800 and 900 a d this period of high 18o may have been caused by the greater aridity that has been documented in northern yucatan lakes or by decreased hydrologic input to the lake as a consequence of forest recovery reduced soil erosion after 850 cal yr a d coincided with the terminal classic maya demographic decline and permitted forest recovery and resumption of organic sedimentation

the formation and failure of natural dams of the numerous kinds of dams that form by natural processes dams formed from landslides glacial ice and late neoglacial moraines present the greatest threat to people and property landslide dams form in a wide range of physiographic settings the most common types of mass movements that form landslide dams are rock and debris avalanches rock and soil slumps and slides and mud debris and earth flows the most common initiation mechanisms for dam forming landslides are excessive rainfall and snowmelt and earthquakes landslide dams can be classified into six categories based on their relation with the valley floor type i dams 11 of 184 landslide dams from around the world that we were able to classify do not reach from one valley side to the other type ii dams 44 span the entire valley floor in some cases depositing material high on opposite valley sides type iii dams 41 move considerable distances both upstream and downstream from the landslide failure type iv dams 1 are rare and involve the contemporaneous failure of material from both sides of a valley type v dams 1 also are rare and are created when a single landslide sends multiple tongues of debris into a valley and forms two or more landslide dams in the same reach of river type vi dams 3 involve one or more failure surfaces that extend under the stream or valley and emerge on the opposite valley side many landslide dams fail shortly after formation in our sample of 73 documented landslide dam failures 27 of the landslide dams failed less than 1 day after formation and about 50 failed within 10 days over topping is by far the most common cause of failure the timing of failure and the magnitude of the resulting floods are controlled by dam size and geometry material characteristics of the blockage rate of inflow to the impoundment size and depth of the impoundment bedrock control of flow and engineering controls such as artificial spill ways diversions tunnels and planned breaching by blasting or conventional excavation glacial ice dams can produce at least nine kinds of ice dammed lakes the most dangerous are lakes formed in main valleys dammed by tributary glaciers failure can occur by erosion of a drainage tunnel under or through the ice dam or by a channel over the ice dam cold polar ice dams generally drain supraglacially or marginally by downmelting of an outlet channel warmer temperate ice dams tend to fail by sudden englacial or subglacial breaching and drainage late neoglacial moraine dammed lakes are located in steep mountain areas affected by the advances and retreats of valley glaciers in the last several centuries these late neoglacial dams pose hazards because 1 they are sufficiently young that vegetation has not stabilized their slopes 2 many dam faces are steeper than the angle of repose 3 these dams and lakes are immediately downslope from steep crevassed glaciers and near vertical rock slopes and 4 downstream from these dams are steep canyons with easily erodible materials that can be incorporated in the flow and increase flood peaks the most common reported failure mechanism is overtopping and breaching by a wave or series of waves in the lake generated by icefalls rockfalls or snow or rock avalanches melting of ice cores or frozen ground and piping and seepage are other possible failure mechanisms natural dams may cause upstream flooding as the lake rises and downstream flooding as a result of failure of the dam although data are few for the same potential energy at the dam site ownstream flood peaks from the failure of glacier ice dams are smaller than those from landslide moraine and structed earth fill and rock fill dam failures moraine dam failures appear to produce some of the largest downstream flood peaks for potential energy at the dam site greater than 1011 1012 joules differences in flood peaks natural dam failures appear to be controlled by dam characteristics and failure mechanisms

evolution of the raman spectrum of carbonaceous material in low grade metasediments of the glarus alps switzerland terra nova 22 354 360 2010 abstract the raman spectrum of carbonaceous material cm from advanced diagenesis 200 c to low grade metamorphism 320 c is documented in the helvetic flysch of the glarus alps switzerland the spectrum is complex with several defect bands at 1200 d4 1350 d1 1500 d3 and 1620 d2 cm 1 we document the evolution of these bands relative to the graphite g band with increasing metamorphic grade and we show that this qualitative evolution may be used as a proxy for temperature in the glarus alps we develop a robust peak fitting method and propose quantitative parameters that may be used as proxies for thermal metamorphism in this region further work in other geological contexts is needed to assess whether the spectral evolution observed in the glarus alps may be generalized and to critically assess the potential for calibrating a general empirical and quantitative thermometer based on the raman spectrum of cm in low grade rocks

metallic ion binding by bacillus subtilis implications for the fossilization of microorganisms the silicification of bacterial cells has been followed in a laboratory simulation by electron microscopy and energy dispersive x ray spectroscopy structural degradation of the bacteria was not influenced by silica availability but the remains of degraded cells did promote the precipitation of silica in contrast cells reacted with ferric iron before aging remained intact and structures conforming to the original morphology of the bacteria were preserved by silicification the binding of metallic ions by microbial cells in particular the retention of iron is therefore considered to be an important contributing factor to the fossilization of microorganisms

early entombment within silica minimizes the molecular degradation of microorganisms during advanced diagenesis most ancient organic microfossils delicately preserved in 3d have been found in cherts although entombment within silica has been shown to promote morphological preservation the impact of early silicification on the molecular evolution of fossilized microorganisms during burial remains poorly constrained here we report results of advanced fossilization experiments performed under pressure 250 bars and temperature 250 c conditions typical of sub greenschist facies metamorphism for different durations up to 100 days on microorganisms experimentally entombed or not within a silica gel the experimental residues have been characterized using xrd and xanes spectroscopy the present study demonstrates that entombment within silica limits the degradation of microorganism molecular signatures likely through specific chemical interactions despite the progressive conversion of silica into quartz during the experiments extrapolation of the present results suggests that such protection may persist during geological timescales the present experimental study provides molecular evidence that in addition to morphologies cherts may support the chemical preservation of remains of ancient life the present results thus constitute a step forward towards the reconstruction of the original chemistry of pu tative fossilized microorganisms

biosignatures and microbial fossils in endolithic microbial communities colonizing ca sulfate crusts in the atacama desert abstract since the description of microbial communities colonizing ca sulfate crusts in the atacama desert there has been much interest in the mechanisms that could lead to the formation and preservation of biosignatures or microbial fossils of these communities a key to understanding physico chemical processes of taphonomy and early diagenesis is to examine microfossils in their natural environment in this study we characterize organomineral traces and microbial fossils found around microbial communities present in these ca sulfate crusts through scanning electron microscopy microanalytical eds and raman spectroscopy techniques calcium carbonate precipitates were detected around remnants of cryptoendolithic algae beneath the crust surface as what seems to be the final step in the organomineralization of these cryptoendolithic communities we also observed alga cell remains permineralized by mg si rich minerals inside gypsum crystals additionally mg si bearing minerals formed a web like structure within the hypoendolithic cyanobacterial habitat via permineralization of extracellular polymeric substances our observations indicate that despite the extremely hyperarid environment microenvironmental conditions may be appropriate for the formation of biosignatures and microbial fossils of extinct endolithic microbial communities a model of the possible organomineralization processes involved is presented

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

alternative origin of aliphatic polymer in kerogen the origin of sedimentary organic matter kerogen has been attributed to random recombination reactions of biological components in sediments or to selective preservation of decay resistant macromolecules neither hypothesis explains the aliphatic composition of the cuticle of fossil arthropods thermal maturation experiments on modern arthropods involving confined pyrolysis at 250 360 c degrade the chitin protein complex of the cuticle and transform free aliphatic components into a polymeric structure the results of the application of electron microscopy and spectroscopic methods to modern thermally matured and fossil arthropod cuticles indicate that in situ polymerization of free and ester bound cuticular lipids can lead to kerogen formation thus fossil arthropod fragments can contribute to sedimentary organic matter

hydrogen isotope ratios of recent lacustrine sedimentary n alkanes record modern climate variability abstract hydrogen isotope ratios were measured on n alkanes n c12 to n c31 extracted from recent lake surface sediments along a n s european transect to test if modern climate variability is recorded in these biomarkers d values of the n alkanes are compared to d values of meteoric water from the iaea gnip database spanning a range from 119 in northern sweden to 41 in southern italy to lake water d values and to mean annual temperatures varying between 2 0 c in the north and 13 7 c in the south d values of the short chained n alkanes n c12 to n c20 excluding algal derived n c17 and n c19 are higher in the north and lower in the south the isotopic fractionation e for hydrogen between meteoric water and the short chained n alkanes is increasing from n to s by more than 100 and is significantly correlated to mean annual temperature for n c16 and n c18 this suggests that these n alkanes may originate from a different source in the northern lakes possibly due to petroleum contamination or are synthesized using a different biochemical pathway the n c17 and n c19 alkanes of algal origin the n c21 and n c23 alkanes originating from water plants and the long chain n alkanes n c25 n c27 n c29 and n c31 of terrestrial origin clearly correlate with d values of meteoric water lake water and mean annual temperature indicating that they excellently record the d value of meteoric water the mean hydrogen isotope fractionation ec17 w of 157 sd 13 between n c17 and meteoric water is fairly constant over the wide range of different climates and lake environments suggesting only minor influence of environmental factors on this biochemical fractionation this suggests that d values of n c17 are suitable to reconstruct the isotopic composition of source water the mean fractionation between the long chain n alkanes and water is 128 sd 12 the mean difference of 31 between both e values is likely due to evaporative enrichment of deuterium in the leaf water if this is the only influence on the enrichment the difference between the d values of terrestrial and aquatic compounds might be suitable to reconstruct terrestrial evapotranspiration of the lake environment

forcing of wet phases in southeast africa over the past 17 000 years a continuous record of hydrologic variability for the past 17 000 years at the mouth of the zambezi river shows that hydrologic conditions in southeast africa were controlled by variations in local insolation and migrations of the intertropical convergence zone rather than by indian ocean temperature records of hydroclimatological variability in southern africa since the last glacial maximum have produced a conflicting picture of the relative influences of events in the remote northern hemisphere and the more local indian ocean this paper presents a continuous record of hydrological variability for the past 17 000 years from marine sediment core drilled off the mouth of the zambezi river the record shows that hydrological conditions in the region were influenced by variations in local insolation and migrations of the intertropical convergence zone which were probably driven by events in the northern hemisphere rather than the indian ocean intense debate persists about the climatic mechanisms governing hydrologic changes in tropical and subtropical southeast africa since the last glacial maximum about 20 000 years ago in particular the relative importance of atmospheric and oceanic processes is not firmly established1 2 3 4 5 southward shifts of the intertropical convergence zone itcz driven by high latitude climate changes have been suggested as a primary forcing2 3 whereas other studies infer a predominant influence of indian ocean sea surface temperatures on regional rainfall changes4 5 to address this question a continuous record representing an integrated signal of regional climate variability is required but has until now been missing here we show that remote atmospheric forcing by cold events in the northern high latitudes appears to have been the main driver of hydro climatology in southeast africa during rapid climate changes over the past 17 000 years our results are based on a reconstruction of precipitation and river discharge changes as recorded in a marine sediment core off the mouth of the zambezi river near the southern boundary of the modern seasonal itcz migration indian ocean sea surface temperatures did not exert a primary control over southeast african hydrologic variability instead phases of high precipitation and terrestrial discharge occurred when the itcz was forced southwards during northern hemisphere cold events such as heinrich stadial 1 around 16 000 years ago and the younger dryas around 12 000 years ago or when local summer insolation was high in the late holocene that is during the past 4 000 years

terminal classic drought in the northern maya lowlands inferred from multiple sediment cores in lake chichancanab mexico we present new density records from sediment cores taken along a depth transect in lake chichancanab mexico the data reveal in great detail the climatic events that comprised the terminal classic drought tcd and coincided with the demise of classic maya civilization between ca 750 and 1050 ad in shallow water cores the tcd is marked by a condensed gypsum horizon in deeper water sections it is expanded and represented by numerous interbedded gypsum and organic rich strata the tcd was not a single two century long megadrought but rather consisted of a series of dry events separated by intervening periods of relatively moister conditions we estimate the tcd occurred between ca 770 and 1100 ad and included an early phase ca 770 870 ad and late phase ca 920 1100 ad the intervening 50 year period ca 870 920 ad was relatively moister each dry phase is represented by multiple gypsum density bands interbedded with organic rich sediment that indicate alternating dry and wet conditions spectral analysis revealed significant periods around 213 50 and 27 years despite uncertainty regarding the ages of these events their pattern is robust and generally consistent with other proxy records under the same climate regime such as the marine cariaco basin off northern venezuela science 299 2003 1731

biomineralization crystallography and magnetic properties of bullet shaped magnetite magnetosomes in giant rod magnetotactic bacteria article i nfo magnetosomes produced by magnetotactic bacteria are of great interest for understanding bacterial biomineralization along with sedimentary magnetism and environmental magnetism one of the most intriguing species magnetobacterium bavaricum can synthesize hundreds of bullet shaped magnetite magnetosomes per cell which contribute significantly to magnetic properties of sediments however the biomineralization mechanism and magnetic properties of such magnetosomes remain unknown in this paper we have conducted a comprehensive study of the crystallography and magnetic properties of bullet shaped magnetosomes formed by uncultivated giant rod magnetotactic bacteria referred to as myr 1 recently discovered in lake miyun beijing china transmission electron microscopy observations reveal that each myr 1 cell contains hundreds of bullet shaped magnetite magnetosomes which are arranged into 3 5 braid like bundles of chains the formation of the bullet shaped magnetosomes can be divided into two stages initial isotropic growth up to 20 nm followed by elongation along the 100 direction which is unusual compared with the expected 111 magnetic easy axis although the 100 orientation is the hard axis of the face centered cubic magnetite the shape anisotropy of bullet shaped magnetosomes and intra bundle magnetostatic interactions confine the magnetization direction of the chain along the long axis of the cell bundle due to each bundle of magnetosome chains effectively behaving as an elongated single domain particle the myr 1 cells show high coercivity weak intra bundle magnetostatic interaction and high ratio these results provide new insights into the biomineralization process and magnetic properties of bullet shaped magnetite magnetosomes

compound specific radiocarbon dating of the varved holocene sedimentary record of saanich inlet canada 1 the radiocarbon contents of various biomarkers extracted from the varve counted sediments of saanich inlet canada were determined to assess their applicability for dating purposes calibrated ages obtained from the marine planktonic archaeal biomarker crenarchaeol compared favorably with varve count ages the same conclusion could be drawn for a more general archaeal biomarker gdgt 0 although this biomarker proved to be less reliable due to its less specific origin the results also lend support to earlier indications that marine crenarchaeota use dissolved inorganic carbon dic as their carbon source the average reservoir age offset r of 430 years determined using the crenarchaeol radiocarbon ages varied by 110 years this may be caused by natural variations in ocean atmosphere mixing or upwelling at the ne pacific coast but variability may also be due to an inconsistency in the marine calibration curve when used at sites with high reservoir ages

a multitracer study of radionuclides in lake zurich switzerland 2 residence times removal processes and sediment focusing residence and settling times of particles and particle reactive nuclides evaluated from in situ tracer studies can be used as diagnostic indicators of trace element pathways in lakes natural e g 210pb 7be and 10be and artificial i e chernobyl 137cs radionuclide fluxes through lake zurich at 50 and 130 m depth from 1983 to 1987 atmospheric fluxes of the same nuclides and fluxes of 10be during 1987 allowed the calculation of nuclide residence times as well as particle settling and transit velocities in lake zurich the residence time of 210pb in the lake is approximately 1 month and hence of the same order as the residence times of particles and stable pb steady state residence times for 7be were calculated as 150 170 days while non steady state instantaneous removal residence times ranged from 50 to 800 days polonium 210 was removed from lake zurich with removal times of 10 26 months indicating slow removal processes or efficient recycling in the lake water transit velocities of particles calculated from the attenuation of chernobyl 137cs fluxes at 50 m and 130 m depth during early may 1986 were 17 m d 1 while 2 4 5 m d 1 was calculated as an effective settling velocity from the attenuation of 7be fluxes at 130 m depth as compared to 50 m depth model calculations reveal that the overall removal process of atmospherically deposited 7be from the water column to the sediments is mainly controlled by the adsorption coagulation step and not by the rate of particle settling extra inputs of 10be 210pb or 137cs into the lower trap i e rebound flux originating from episodic lateral inputs of fine particles with a high content of 210pb 10be and 137s were observed during the lake stratification period i e summer this novel observation of summertime radionuclide and sediment focusing is different from previously described wind generated resuspension and focusing effects caused by river plumes and lake sediments during the stagnation period boundary and focusing effects for radionuclide bearing particles are proposed to explain observed summertime enhancements of longer lived nuclide fluxes at 130 m depth as well as the decrease in calculated effective settling velocities

climate variability on the yucatan peninsula mexico during the past 3500 years and implications for maya cultural evolution climate variability on the yucatan peninsula during the past 3500 yrs is reconstructed from the measurement of 18o in monospecific ostracods and gastropods in a 6 3 m sediment core from lake punta laguna mexico this late holocene record is divided into three periods based on changes in mean 18o values from 3310 to 178514c yr b p period i low mean 18o values indicate relatively wet conditions i e low evaporation to precipitation ratio e p mean oxygen isotopic values increased 178514c yr b p and the interval between 1785 and 93014c yr b p period ii was distinctly drier than the periods before or after the climate during the latter part of period ii was persistently dry with exceptionally arid events centered at 1171 1019 and 94314c yr b p equivalent to 862 986 and 1051 a d this interval of frequent drought was recorded at several other localities in mexico and central america and coincided with the collapse of classic maya civilization following the last arid event 18o values decreased abruptly at 93014c yr b p beginning of period iii signaling a return to wetter conditions that have generally prevailed to the present with the exception of a dry episode centered at 55914c yr b p 1391 a d the paleoclimatic record from punta laguna provides evidence that multi decadal and millennial scale changes ine poccurred on the yucatan peninsula during the late holocene these wet dry episodes may have influenced cultural evolution in mesoamerica

impact of biomineralization on the preservation of microorganisms during fossilization an experimental perspective abstract the biogenicity of fossil microbial biomorphs is often debated because their morphologies are poorly informative and the chemical structural and isotopic signatures of putative biogenic organic molecules have been altered during their incorporation into the sediments and the geological history of the host rock here we investigated the effect of encrustation by biominerals on the morphological and chemical degradation of escherichia coli cells during experimental thermal treatments non calcified e coli cells and e coli cells encrusted by calcium phosphates were exposed to heating under an argon atmosphere at two different temperatures 300 c 600 c for 20 h two additional experiments were performed on non calcified e coli cells at 300 c for 2 h and 100 h to discuss the influence of experiment duration organic residues of all experiments were characterized at a multiple length scale using a combination of scanning electron microscopy transmission electron microscopy raman microspectroscopy electron paramagnetic resonance epr spectroscopy fourier transform infrared ft ir spectroscopy and x ray absorption near edge structure spectroscopy xanes at the carbon k edge in the absence of encrusting biominerals the morphological structure of the organic residues of e coli cells was completely lost after heating at 300 or 600 c even after short 2 h long heating experiments the content of aromatic functional groups of the organic residues of non calcified e coli cells increased during heat treatment at 600 c for 20 h while the amide functional groups were lost as indicated by ft ir spectroscopy consistently the epr spectrum of these organic residues indicated important transformation as a comparison this spectrum appeared similar to epr spectra of ancient organic carbons such as carbons from the apex chert ca 3460 myr indicating a similar concentration of aromatic moieties in contrast calcified e coli exposed to the same conditions showed only limited morphological alteration as observed by electron microscopy as well as lower chemical transformation as detected by ft ir and epr spectroscopies despite the difficulties to relate experimental conditions directly to geological conditions these experiments evidence the influence of cell encrustation by minerals on their chemical and morphological preservation potential during fossilization processes

carbon isotope composition of lake sediments in relation to lake productivity and radiocarbon dating abstract carbon 13 profiles and radiocarbon dates were obtained from two canadian kettle basins having similar geological and hydrological characteristics to develop criteria for evaluating the validity of radiocarbon dates on lake sediment from basins in calcareous terrain radiocarbon dates from a site in alberta show a variable hard water effect related to local hydrological changes during postglacial history of the lake basin whereas radiocarbon dates from the other site in ontario show no noticeable influence of old carbon during its history these differences are mainly related to lake water residence time which has influenced carbon isotopic exchange between atmospheric co2 and dissolved inorganic carbon in lake water 13c values for bulk organic sediment and terrestrial and aquatic macrofossils reveal that the main component of lake sediment at both sites is autocthonous in origin furthermore each site supported different submerged aquatic plant communities that used different sources of carbon for photosynthesis thereby imprinting the organic sediments with a characteristic 13c composition both sites reflect a clear relationship between 13c values and paleoproductivity this study shows the individualistic response of the developing lake system to the hydrology lake biota and local geology and demonstrates the problem of using 13c in lake sediments as a single criterion to recognize the validity of radiocarbon dates of lake sediment without supporting paleoecological information

can sedimentary leaf waxes record d h ratios of continental precipitation field model and experimental assessments abstract d h ratios of leaf waxes d wax derived from terrestrial plants and preserved in lake sediments can provide important information on past continental hydrology ideally d wax can be used to reconstruct precipitation d h ratios d p which is a well established paleoclimate proxy however many other factors such as vegetation and relative humidity rh also affect d wax variation how the combination of these factors affects sedimentary d wax is unclear here we use a transect of 32 lake surface sediments across large gradients of precipitation relative humidity and vegetation composition in the southwestern united states to study the natural factors affecting sedimentary d wax d values of c 28 n alkanoic acids show significant correlation with d p values r 2 0 76 with an apparent isotopic enrichment of 99 8 indicating that sedimentary d wax values track overall d p variation along the entire transect leaf waxes produced by plants grown under controlled conditions rh 80 60 40 show a small increase in d h ratios as rh decreases consistent with prediction from the craig gordon model however the isotopic effect of rh on d wax along the natural transect is partially countered by the opposing influence of vegetation changes the correlation between d wax and d p values is significantly higher r 2 0 84 in the drier portions of the transect than in the wetter regions r 2 0 64 this study suggests that d h ratios of sedimentary leaf waxes can be used as a proxy for precipitation d variations with particularly high fidelity in dry regions although more studies in other regions will be important to further test this proxy

carbonaceous cherts of the barberton greenstone belt south africa isotopic chemical and structural characteristics of individual microstructures abstract carbonaceous matter occurring in chert deposits of the 3 4 3 2 ga old barberton greenstone belt bgb south africa has experienced low grade regional metamorphism and variable degrees of local hydrothermal alteration here a detailed study is presented of in situ analysis of carbonaceous particles by lrs laser raman spectroscopy and sims secondary ion mass spectrometry reporting degree of structural disorder carbon isotope ratio and nitrogen to carbon ratio this combination of in situ analytical tools is used to interpret the 13 c values of only the best preserved carbonaceous remains enabling the rejection of non indigenous unmetamorphosed material as well as the exclusion of strongly hydrothermally altered carbonaceous particles raman spectroscopy confirmed that all carbonaceous cherts studied here have experienced a regional sub to lower greenschist facies metamorphic event although this identifies these organics as indigenous to the cherts it is inferred from petrographic observations that hydrothermal alteration has caused small scale migration and re deposition of organics this suggest that morphological interpretation of these carbonaceous particles and in general of putative microfossils or microlaminae in hydrothermally altered early archean cherts should be made with caution a chert in the hooggenoeg formation which is older than and has been hydrothermally altered by a volcanic event 3445 ma ago contains strongly altered carbonaceous particles with a uniform n c ratio of 0 001 and a range of 13 c that is shifted from its original value cherts of the kromberg formation post date this volcanic event and contain carbonaceous particles with a n c ratio between 0 002 and 0 006 both the buck reef chert and the footbridge chert of the kromberg formation have retained fairly well preserved 13 c values with ranges from 34 to 24 and 40 to 32 respectively abiologic reactions associated with hydrothermal serpentinization of ultramafic crust such as fischer tropsch synthesis were an unlikely source for carbonaceous material in these cherts the carbonaceous matter in these cherts has all the characteristics of metamorphosed biologic material

organic matter preservation on continental slopes importance of mineralogy and surface area abstract theoretical considerations calculations and data reported by keil et al 1994a were used to assess the recent hypotheses that mineral surface roughness controls the specific surface area of continental margin sediments and that the matrix linked organic carbon content of continental margin sediments occurs as a monolayer or monolayer equivalent coating of organic compounds on the surfaces of detrital minerals and other detrital nonorganic grains mayer 1994 results of our analysis indicate that it is not the surface roughness of the terrigenous detrital framework grains that controls the specific surface area of most continental margin sediments even in the sand and silt fractions but rather the presence of nonspherical high surface area to volume particles primarily clays but also oxy hydroxides and ultra structured nonorganic bioclasts such as diatom frustules analysis of the mineralogy organic carbon content surface area and in situ microfabric of continental margin sediments off california in the mendocino and san luis obispo areas and off the state of washington indicate a strong correlation between the amount of matrix linked organic matter present and the suite of clay minerals data indicate that organic carbon appears to be preferentially sequestered in smectite rich sediments compared to those whose clay fractions are dominated by chlorite we suggest that this association is a function of differences in the site density and chemistry of the clays and differences in their flocculation behavior our data also indicate that organic carbon preservation on the three california transects all of which have a suboxic oxygen minimum zone that impinges on the sea floor is not significantly influenced by differences in bottom water oxygen concentration and carbon stable isotope data suggest no preferential preservation of continental organic matter over that of marine origin

terminal proterozoic cyanobacterial blooms and phosphogenesis documented by the doushantuo granular phosphorites i in situ micro analysis of textures and composition abstract in order to ascertain the origin of granular phosphorites and the roles of microorganisms in phosphogenesis we conducted comprehensive petrographic surveys and correlated in situ micro analyses of granular phosphorites from the doushantuo formation near yichang south china phosphatic granules display organically zoned internal structures often associated with abundant cyanobacteria like microfossils the internal ultrastructure of the granules as documented by raman microspectroscopy scanning electron microscopy sem and transmission electron microscopy tem is characterized by randomly oriented apatite nano crystals embedded with ubiquitous carbonaceous particles in the apatite groundmass these represent primary textures formed by the rapid growth of apatite provided with abundant nucleation sites within microbial biofabrics nanosims elemental mapping revealed close correspondence of carbon and nitrogen with microfossil structures at the cellular and sub cellular level we propose that the doushantuo granules themselves were formed by microbially mediated accretionary growth followed by rapid phosphatization occurring at the sediment water interface extracellular polymeric substances eps produced by cyanobacteria would have played crucial roles in these processes by promoting aggregated granule growth in addition to providing nucleation sites for apatite crystallization while previous studies have suggested a dominant role of sulfur metabolizing microorganisms in the precipitation of phosphate in phosphorites new observations indicate that the emplacement of most sulfur bearing minerals in the doushantuo phosphorites postdate phosphatization itself our new model of phosphorite formation thus places cyanobacterial eps as an earlier key component of the mineralization of the doushantuo granular phosphorites

microscopy evidence of bacterial microfossils in phosphorite crusts of the peruvian shelf implications for phosphogenesis mechanisms abstract phosphorites are sedimentary formations enriched in ca phosphate minerals the precipitation of these minerals is thought to be partly mediated by the activity of microorganisms the vast majority of studies on phosphorites have focused on a petrological and geochemical characterization of these rocks however detailed descriptions are needed at the sub micrometer scale at which crucial information can be retrieved about traces of past or modern microbial activities here scanning electron microscopy sem analyses of a recent phosphorite crust from the upwelling style phosphogenesis area off peru revealed that it contained a great number of rod like and coccus like shaped micrometer sized 1 1 m and 0 5 m respectively objects referred to as biomorphs some of these biomorphs were filled with carbonate fluoroapatite cfa a calcium phosphate phase common in phosphorites some were empty some were surrounded by one or two layers of pyrite transmission electron microscopy tem and energy dispersive x ray spectrometry edxs analyses were performed on focused ion beam fib milled ultrathin foils to characterize the texture of cfa and pyrite in these biomorphs at the few nanometer scale non pyritized phosphatic biomorphs were surrounded by a thin 5 15 nm thick rim appearing as a void on tem images bundles of cfa crystals sharing the same crystallographic orientations aligned along their c axis were found in the interior of some biomorphs pyrite formed a thick 35 115 nm layer with closely packed crystals surrounding the pyritized biomorphs whereas pyrite crystals at distance from the biomorphs were smaller and distributed more sparsely scanning transmission x ray microscopy stxm analyses performed at the c k edge provided maps of organic and inorganic carbon in the samples inorganic c mainly present as carbonate groups in the cfa lattice was homogeneously distributed whereas organic c was concentrated in the rims of the phosphatic biomorphs finally stxm analyses at the fe l 2 3 edges together with tem edxs analyses revealed that some pyritized biomorphs experienced partial oxidation the mineralogical features of these phosphatic biomorphs are very similar to those formed by bacteria having precipitated phosphate minerals intra and extracellularly in laboratory experiments similarly pyritized biomorphs resemble bacteria encrusted by pyrite we therefore interpret phosphatic and pyritized biomorphs present in the peruvian phosphorite crust as microorganisms fossilized near the boundary of zones of sulfate reduction the implications of these observations are then discussed in the light of the different possible and non exclusive microbially driven phosphogenesis mechanisms that have been proposed in the past i organic matter mineralization in particular mediated by iron reducing bacteria and or sulfate reducing bacteria srb ii reduction of iron oxyhydr oxides by iron reducing bacteria and or srb and iii polyphosphate metabolism in sulfide oxidizing bacteria possibly associated with srb

atmospheric circulation patterns during late pleistocene climate changes at lake malawi africa abstract the climate of tropical africa transitioned from an interval of pronounced orbitally paced megadroughts to more humid and stable conditions approximately 70 000 years ago scholz et al 2007 the regional atmospheric circulation patterns that accompanied these climatic changes however are unclear due to a paucity of continental paleoclimate records from tropical africa extending into the last interglacial we present a new 140 kyr record of the deuterium hydrogen isotopic ratio of terrestrial leaf waxes d wax from drill cores from lake malawi southeast africa that spans this important climatic transition d wax shifts from highly variable and relatively d depleted to more stable and d enriched around 56 ka contemporary with the onset of more humid conditions in the region moisture source and transport history dominate the d wax signal at lake malawi with local rainfall amount playing a secondary role for much of the paleorecord analysis of modern moisture sources for lake malawi suggests that d depletion of waxes during the megadroughts may have been caused by an enhanced contribution of the drier d depleted air mass currently located in central southern africa to the lake malawi catchment this d depleted air mass is associated with the descending limb of the hadley cell which implies significant changes in the hadley circulation during the megadroughts and related changes in the position of the intertropical convergence zone over africa these findings demonstrate the ability of d wax to serve as an atmospheric tracer when used in conjunction with additional proxy records for moisture balance and elucidate potential mechanisms for pronounced hydrological change in southeast africa during the late pleistocene

hydrogen isotope ratios of lacustrine sedimentary n alkanes as proxies of tropical african hydrology insights from a calibration transect across cameroon abstract hydrogen isotope values d of sedimentary aquatic and terrestrial lipid biomarkers originating from algae bacteria and leaf wax have been used to record isotopic properties of ancient source water i e precipitation and or lake water in several mid and high latitude lacustrine environments in the tropics however where both processes associated with isotope fractionation in the hydrologic system and vegetation strongly differ from those at higher latitudes calibration studies for this proxy are not yet available to close this gap of knowledge we sampled surface sediments from 11 lakes in cameroon to identify those hydro climatological processes and physiological factors that determine the hydrogen isotopic composition of aquatic and terrestrial lipid biomarkers here we present a robust framework for the application of compound specific hydrogen isotopes in tropical africa our results show that the d values of the aquatic lipid biomarker n c17 alkane were not correlated with the d values of lake water carbon isotope measurements indicate that the n c17 alkane was derived from multiple source organisms that used different hydrogen pools for biosynthesis we demonstrate that the d values of the n c29 alkane were correlated with the d values of surface water i e river water and groundwater which on large spatial scales reflect the isotopic composition of mean annual precipitation such a relationship has been observed at higher latitudes supporting the robustness of the leaf wax lipid d proxy on a hemispheric spatial scale in contrast the d values of the n c31 alkane did not show such a relationship but instead were correlated with the evaporative lake water d values this result suggests distinct water sources for both leaf wax lipids most likely originating from two different groups of plants these new findings have important implications for the interpretation of long chain n alkane d records from ancient lake sediments in particular a robust interpretation of palaeohydrological data requires knowledge of the vegetation in the catchment area as different plants may utilise different water sources our results also suggest that the combination of carbon and hydrogen isotopes does help to differentiate between the metabolic pathway and or growth form of organisms and therefore the source of hydrogen used during lipid biosynthesis

the provenance of vegetation and environmental signatures encoded in vascular plant biomarkers carried by the ganges brahmaputra rivers organic matter carried by rivers and deposited in continental and marine sediments contains valuable information on past environmental conditions and their impact on the terrestrial biosphere in order to use sedimentary records to reconstruct past environmental conditions on the continents such as vegetation cover and type or precipitation intensity we need to understand the provenance of the organic signatures and how they are transferred and potentially modified by fluvial transport to the sedimentary reservoir in particular we need to understand how environmental conditions are imprinted in the composition of riverine particulate organic carbon here we investigate the stable carbon and hydrogen isotopic signatures of vascular plant leaf wax biomarkers in the modern day ganges and brahmaputra river complex one of the largest fluvial systems on earth the distribution of n alkanes and n alkanoic acids in ganges brahmaputra river sediments suggests that vascular plant inputs are consistently a significant component of the organic carbon pool molecular 13c measurements reveal that c3 vegetation inputs delivered by the himalayan rivers are partly degraded and replaced by mixed c3 c4 vegetation input in the floodplain the hydrogen isotopic composition of long chain c24 n alkanoic acids in combination with their stable c isotopic composition provides constraints on the isotopic composition of the environmental water used by higher plants within the drainage basin calculated compositions compare well with the depleted d isotopic ratios of the river water during the summer monsoon suggesting that h isotopic compositions of long chain alkanoic acids are a valuable proxy for the composition of summer monsoon moisture in the ganges brahmaputra basin detailed studies of the isotopic anatomy of modern river systems are critical for informed interpretation of marine sedimentary records in river influenced continental margins

ftir absorption indices for thermal maturity in comparison with vitrinite reflectance r0 in type ii kerogens from devonian black shales abstract ftir absorbance signals in kerogens and macerals were evaluated as indices for thermal maturity two sets of naturally matured type ii kerogens from the new albany shale illinois basin and the exshaw formation western canada sedimentary basin and kerogens from hydrous pyrolysis artificial maturation of the new albany shale were characterized by ftir good correlation was observed between the aromatic aliphatic absorption ratio and vitrinite reflectance r 0 ftir parameters are especially valuable for determining the degree of maturity of marine source rocks lacking vitrinite with increasing maturity ftir spectra express four trends i an increase in the absorption of aromatic bands ii a decrease in the absorption of aliphatic bands iii a loss of oxygenated groups carbonyl and carboxyl and iv an initial decrease in the ch 2 ch 3 ratio that is not apparent at higher maturity in naturally matured samples but is observed throughout increasing r 0 in artificially matured samples the difference in the ch 2 ch 3 ratio in samples from natural and artificial maturation at higher maturity indicates that short term artificial maturation at high temperatures is not fully equivalent to slow geologic maturation at lower temperatures with increasing r 0 the carboxyl carbonyl aromatic carbon ratio generally decreases except that kerogens from the exshaw formation and from hydrous pyrolysis experiments express an intermittent slight increase at medium maturity ftir derived aromaticities correlate well with r 0 although some uncertainty is due to the dependence of ftir parameters on the maceral composition of kerogen whereas r 0 is solely dependent on vitrinite

an overview of biomineralization processes and the problem of the vital effect biomineralization links soft organic tissues which are compositionally akin to the atmosphere and oceans with the hard materials of the solid earth it provides organisms with skeletons and shells while they are alive and when they die these are deposited as sediment in environments from river plains to the deep ocean floor it is also these hard resistant products of life which are mainly responsible for the earth s fossil record consequently biomineralization involves biologists chemists and geologists in interdisciplinary studies at one of the interfaces between earth and life leadbeater and riding 1986 biomineralization refers to the processes by which organisms form minerals the control exerted by many organisms over mineral formation distinguishes these processes from abiotic mineralization the latter was the primary focus of earth scientists over the last century but the emergence of biogeochemistry and the urgency of understanding the past and future evolution of the earth are moving biological mineralization to the forefront of various fields of science including the earth sciences the growth in biogeochemistry has led to a number of new exciting research areas where the distinctions between the biological chemical and earth sciences disciplines melt away of the wonderful topics that are receiving renewed attention the study of biomineral formation is perhaps the most fascinating truly at the interface of earth and life biomineralization is a discipline that is certain to see major advancements as a new generation of scientists brings cross disciplinary training and new experimental and computational methods to the most daunting problems it is however by no means a new field the first book on biomineralization was published in 1924 in german by w j schmidt schmidt 1924 and the subject has continued to intrigue a dedicated community of scientists for many years until the early 1980s the field was known as

geochemical evolution of organic rich shales with increasing maturity a stxm and tem study of the posidonia shale lower toarcian northern germany hydrocarbon generation and retention processes occurring within gas shales as a response to increases in thermal maturation are still poorly constrained while efforts have been directed at unravelling the resource potential composition and textures of these economically important unconventional systems their spatial variability in chemistry and structure is still poorly documented at the sub micrometer scale here we have characterized samples of the lower toarcian posidonia shale samples from northern germany at varying stages of thermal maturation using a combination of compositional organic geochemistry and spectromicroscopy techniques including synchrotron based scanning transmission x ray microscopy stxm we document geochemical and mineralogical heterogeneities down to the nanometer scale within the investigated samples as a function of their level of thermal maturity in particular authigenic albite crystals containing nanometric halite inclusions have been documented within the investigated mature and overmature samples the presence of such tracers of palaeobrine carbonate interactions supports a maturation scenario for the lower toarcian posidonia shale intimately related to ascending brine fluids rather than a maturation scenario solely resulting from high heat flows in addition various types of asphaltene and nso rich bitumen have been detected within the same samples very likely genetically derived from thermally degraded organic precursors furthermore the formation of nanoporous pyrobitumen has been inferred for samples of gas window maturity likely resulting from the formation of gaseous hydrocarbons by providing in situ insights into the fate of bitumen and pyrobitumen as a response to the thermal evolution of the macromolecular structure of kerogen the results reported here constitute an important step towards better constraining hydrocarbon generation processes during natural shale gas maturation

age discrepancy between molecular biomarkers and calcareous foraminifera isolated from the same horizons of northwest pacific sediments abstract recent observations show that establishment of molecular level radiocarbon stratigraphy is increasingly critical for paleoceanographic studies especially to enable comparable discussion about past ocean and climate changes inferred from multiple proxy records based on lipid biomarkers and calcareous foraminiferal microfossils so far although we have interesting evidence that there are temporal and spatial age offsets between the algal alkenone biomarker used for sst sea surface temperature estimates and planktonic foraminifera in regions with high sedimentation rates less is known about such age offsets in the settings of lower sedimentation in this study to investigate the potential of biomarkers as an alternative dating proxy to foraminifera in a low sedimentation rate setting we measured radiocarbon ages of 17 different lipid biomarkers fatty acids fas n alkanes and alkenones bulk total organic matter toc and foraminifera from two sections 12 15 cm and 21 24 cm depth of a surface sediment core in the northwest pacific the sedimentation rate estimated from the toc ranged from 10 5 cm kyr in the core top to 0 9 cm kyr in the lower part of the core the ages of the fas detected at 12 15 cm depth ranged from 530 yr bp c 18 to 3250 yr bp c 28 the 14 c analysis of the fas could be divided into two groups fas c 16 c 18 derived from marine organisms and those c 24 c 26 c 28 derived from terrestrial higher plants the high molecular weight hmw fas ages were older 2550 yr bp c 24 to 3250 yr bp c 28 than those of the low molecular weight lmw fas 530 yr bp c 18 to 1820 yr bp c 16 at 21 24 cm depth the alkenone ages were 7100 7300 yr younger than those of the planktonic foraminifera in this horizon the alkenone 14 c age 7500 yr bp and the alkenone estimated sst 15 3 c suggest that these alkenones were produced in this region during the holocene the alkenone 14 c age was also in good agreement with those of the lmw fas c 14 ic 15 aic 15 c 16 and c 18 1 derived from marine plankton and bacteria similarly the toc age was 5700 yr younger than that of the planktonic foraminifera these age differences are large compared with those estimated for the horizon at 12 15 cm depth which was characterized by a relatively higher sedimentation rate these trends between ages of alkenones toc and planktonic foraminifera are not comparable to the results from sites with high sedimentation rates such as the benguella upwelling system mollenhauer g eglinton t i ohkouchi n schneider r r muller p j grootes p m et al 2003 asynchronous alkenone and foraminifera records from the benguela upwelling system geochim cosmochim acta 67 2157 2171 or the bermuda rise drift deposit ohkouchi n eglinton t i keigwin l d hayes j m 2002 spatial temporal offsets between proxy records in a sediment drift science 289 1224 1227 which are significantly different sedimentary settings from our site the radiocarbon results from these high sedimentation rate regions suggest that the alkenones are several thousand years older than the coexisting planktonic foraminifera on the other hand the age offsets between alkenones and planktonic foraminifera at our site are larger than those from the other sites and the alkenones are younger than the planktonic foraminifera

northern hemisphere controls on tropical southeast african climate during the past 60 000 years the processes that control climate in the tropics are poorly understood we applied compound specific hydrogen isotopes d and the tex86 tetraether index of 86 carbon atoms temperature proxy to sediment cores from lake tanganyika to independently reconstruct precipitation and temperature variations during the past 60 000 years tanganyika temperatures follow northern hemisphere insolation and indicate that warming in tropical southeast africa during the last glacial termination began to increase 3000 years before atmospheric carbon dioxide concentrations d data show that this region experienced abrupt changes in hydrology coeval with orbital and millennial scale events recorded in northern hemisphere monsoonal climate records this implies that precipitation in tropical southeast africa is more strongly controlled by changes in indian ocean sea surface temperatures and the winter indian monsoon than by migration of the intertropical convergence zone

kerogen origin evolution and structure kerogen commonly defined as the insoluble macromolecular organic matter om dispersed in sedimentary rocks is by far the most abundant form of om on earth this fossil material is of prime importance as the source of oil and natural gas moreover kerogen can provide essential information on major topics such as past environments climates and biota this review reports the main advances in kerogen studies since the comprehensive synthesis edited by durand durand b ed kerogen insoluble organic matter from sedimentary rocks editions technip paris 1980 it is organized into eight sections the first two are concerned with the successive definitions of kerogen and the definition used here the different techniques used for kerogen isolation without loss or degradation and basic kerogen analysis the third and fourth focus on sedimentary om sources and preservation processes in relation to depositional environment including sedimentation conditions favourable for kerogen accumulation and extrapolation to past geological time great strides have been made in the latter topics over the last 25 years owing to a combination of classical studies in organic geochemistry and studies in other domains such as biogeochemistry oceanography hydrology and soil science along with the development of powerful analytical tools the next two sections deal with the different kerogen classifications by type and kerogen evolution and maturation upon burial in sediments structural modelling of coal and kerogen based on physical and or chemical structural analysis is described in the following section although only statistical the models thus derived provide a synthetic view of the main structural resemblances and differences among various samples in relation to source maturity or physicochemical properties finally the last section explores some of the advances in kerogen understanding expected for the near future the review includes a list containing about 500 references

a new look at old carbon in active margin sediments recent studies suggest that as much as half of the organic carbon oc undergoing burial in the sediments of tectonically active continental margins may be the product of fossil shale weathering these estimates rely on the assumption that vascular plant detritus spends little time sequestered in intermediate reservoirs such as soils freshwater sediments and river deltas and thus only minimally contributes to the extraneously old 14c ages of total organic matter often observed on adjacent shelves here we test this paradigm by measuring the 14c and 13c values of individual higher plant wax fatty acids as well as the 13c values of extractable alkanes isolated from the eel river margin california the isotopic signatures of the long chain fatty acids indicate that vascular plant material has been sequestered for several thousand years before deposition a coupled molecular isotope mass balance used to reassess the sedimentary carbon budget indicates that the fossil component is less abundant than previously estimated with pre aged terrestrial material instead composing a considerable proportion of all organic matter if these findings are characteristic of other continental margins proximal to small mountainous rivers then the importance of petrogenic oc burial in marine sediments may need to be reevaluated 2009 the geological society of america

late quaternary behavior of the east african monsoon and the importance of the congo air boundary both atlantic and indian ocean climate dynamics exert influence over tropical african hydroclimate producing complex patterns of convergence and precipitation to isolate the indian ocean influence on african paleohydrology we analyzed the deuterium hydrogen ratio of higher plant leaf waxes delta d wax in a 25 000 year sediment record from lake challa 3 degrees s 38 degrees e in the easternmost area of the african tropics whereas both the seismic record of inferred lake level fluctuations and the branched and isoprenoidal tetraether bit index proxy record changes in hydrology within the challa basin delta d wax as a proxy for the isotopic composition of precipitation delta d p is interpreted as a tracer of large scale atmospheric circulation that integrates the history of the moisture transported to the lake challa area specifically based on modern day isotope rainfall relationships we argue that lake challa delta d wax reflects the intensity of east african monsoon circulation the three hydrological proxy records show generally similar trends for the last 25 000 years but there are important differences between them primarily during the middle holocene we interpret this deviation of delta d wax from local hydrological history as a decoupling of east african monsoon intensity which heavily influences the isotopes of precipitation in east africa today from rainfall amount in the challa basin in combination the hydrological proxy data from lake challa singularly highlight zonal gradients in tropical african climate that occur over a variety of timescales suggesting that the congo air boundary plays a fundamental role in controlling hydroclimate in the african tropics

human impacts climate change and aquatic ecosystem response during the past 2000 yr at lake wandakara uganda analyses of carbon and hydrogen isotope ratios of terrestrial leaf waxes and the carbon and nitrogen abundance ratio and isotopic composition of bulk sediments from lake wandakara a crater lake in western uganda east africa document human and climatic controls on the aquatic system and on the surrounding terrestrial vegetation during the past two millennia our data indicate that wandakara was a relatively stable productive lake surrounded by c3 vegetation from ad 70 to 1000 abrupt changes in the 13c of terrestrial leaf waxes indicate a series of abrupt shifts in the relative abundance of c3 and c4 vegetation caused by a combination of climate change and human activities around wandakara beginning at ad 1000 abrupt shifts in bulk sediment organic geochemistry particularly c n ratios and 15n indicate that human activities at this time caused permanent changes in the limnology of lake wandakara including eutrophication our results suggest that the biogeochemistry of lake wandakara was more sensitive to shifting human impacts than to climate variations during the past millennium highlighting the importance of understanding the intensity of pre colonial human impacts on africa s aquatic ecosystems

δd values of individual n alkanes from terrestrial plants along a climatic gradient implications for the sedimentary biomarker record abstract the compound specific hydrogen isotope composition d of sedimentary n alkanes is increasingly used as a palaeoclimate proxy to explore the origin of the hydrogen isotope signal of n alkanes from higher terrestrial plant biomass in sedimentary organic matter of small lakes we have analysed the n alkane composition and d values of 31 plant biomass samples birch betula beech fagus oak quercus alder alnus hornbeam carpinus and myrtle myrtus as well as non stomata containing sphagnum cladonia and moss from 14 sites along a climatic gradient from northern finland to southern italy eleven of the sites were previously investigated for the n alkane d values in adjacent lake sediments and are compared to the leaf wax n alkane d values the distribution of terrestrial n alkanes in lake sediments was not found to be characteristic of the dominant vegetation around the lake evidently n alkane distributions within the same species at different sites were found to be variable these findings complicate the use of certain n alkane distributions in sediments as a palaeoenvironmental indicator d values of plant biomass n alkanes show a significant positive correlation with the precipitation d values biomass n alkane d values are in general less negative than the associated sedimentary n alkanes of terrestrial origin possibly due to the stronger evapotranspiration during the summer when sampling took place this suggests that n alkanes in leaves show seasonal variations and the sedimentary organic matter receives the lighter isotope signal of the autumn when most leaves are incorporated into the sediment the variability of the d values from non stomata containing plants is much higher than for broadleaf trees since stomata can regulate water loss through evapotranspiration the average chain length acl of n alkanes from deciduous tree leaves increases from north to south with increasing acl the hydrogen isotopic fractionation e between source water and n alkanes also increases our results prove that terrestrial n alkane d values record the d value of precipitation modified by the amount of isotope enrichment in the leaf water due to plant anatomy stomata vs no stomata and site specific meteorological conditions such as evapotranspiration relative humidity and soil moisture therefore terrestrial n alkanes in lake sediments can deliver information on changes in these parameters over geological timescales in lake sediments where aquatic n alkanes such as n c 17 record the lake water d value the isotopic difference between terrestrial and aquatic n alkanes could therefore be used to reconstruct changes in evapotranspiration

role of water in hydrocarbon generation from type i kerogen in mahogany oil shale of the green river formation abstract hydrous and anhydrous closed system pyrolysis experiments were conducted on a sample of mahogany oil shale eocene green river formation containing type i kerogen to determine whether the role of water had the same effect on petroleum generation as reported for type ii kerogen in the woodford shale the experiments were conducted at 330 and 350 c for 72 h to determine the effects of water during kerogen decomposition to polar rich bitumen and subsequent bitumen decomposition to hydrocarbon rich oil the results showed that the role of water was more significant in bitumen decomposition to oil at 350 c than in kerogen decomposition to bitumen at 330 c at 350 c the hydrous experiment generated 29 more total hydrocarbon product and 33 more c 15 hydrocarbons than the anhydrous experiment this is attributed to water dissolved in the bitumen serving as a source of hydrogen to enhance thermal cracking and facilitate the expulsion of immiscible oil in the absence of water cross linking is enhanced in the confines of the rock resulting in formation of pyrobitumen and molecular hydrogen these differences are also reflected in the color and texture of the recovered rock despite confining liquid water pressure being 7 9 times greater in the hydrous experiments than the confining vapor pressure in the anhydrous experiments recovered rock from the former had a lighter color and expansion fractures parallel to the bedding fabric of the rock the absence of these open tensile fractures in the recovered rock from the anhydrous experiments indicates that water promotes net volume increase reactions like thermal cracking over net volume decrease reactions like cross linking which results in pyrobitumen the results indicate the role of water in hydrocarbon and petroleum formation from type i kerogen is significant as reported for type ii kerogen

the link between biomineralization and fossilization of bacteria insights from field and experimental studies abstract fossil biominerals and fossils of microorganisms in ancient rocks contain important biogeochemical signals decoding this record may reveal crucial information about the evolution of life on earth and terrestrial paleoenvironments however the identification of traces of life especially in very old rocks is extremely challenging because the morphological and chemical signatures of biominerals and microfossils are subtle of microscopic size and inevitably altered with aging in this review we stress on the fact that biomineralization is often the first step of fossilization and produces particular chemical structural and morphological features that can be preserved in fossil biominerals or microfossils with a special focus on fe biomineralization the taphonomic processes affecting biominerals and microfossils and altering their morphology and or chemical composition over time are then discussed in light of experimental fossilization simulations and field sample analyses we suggest that taxonomic biases observed in the fossil record may be related to differential abilities of species to trigger biomineralization this calls for studies of the effective biomineralizing activity and fossilization potential of each species present in highly diverse natural microbial communities finally recent analytical developments leave little doubt that very substantial progress in the study of biomineralization processes and ancient biostructures will be achieved in the near future