sedimentary rhythm of mn carbonate laminae induced by east asian summer monsoon variability and human activity in lake ohnuma southwest hokkaido northern japan abstract past and ongoing precipitations of mn carbonate rhodochrosite have occurred in the freshwater lake ohnuma which is situated about 8 km south of the hokkaido komagatake volcano southwest hokkaido northern japan lake sediments from a surface and a longer 4 m core contain the sedimentary rhythms of rhodochrosite laminae and volcanic deposits which were identified by nondestructive micro xrf fluorescence analyses the three tephra ash layers from the hokkaido komagatake volcano characterized by peaks of al si k ca and ti correspond to regionally recognized tephras 1640 1929 and 1996 ce based on ashfall isopachs and 137cs 210pb and 14c ages determined from the two cores the sedimentation rate showed a drastic change around 1929 ce 0 3 cm yr for 1640 1929 ce and 2 7 cm yr for 1929 2012 ce the mn content record of lake ohnuma sediments composed of rhodochrosite laminae showed a periodic variation with a decadal centennial scale over the interval 1640 2012 ce sediment petrography and hydrochemical measurements suggest that the metastable phase of rhodochrosite was deposited during summer after being precipitated followed by the crystal growth of rhodochrosite within the anoxic sediments on the basis of these results high mn content is interpreted as an indicator of strengthened summer stratification associated with reduced rainfall runoff the ti si ratios are representative of the relative abundance of detrital inputs from the watershed the combined signals of elevated mn content and ti si ratio for 1935 2012 ce indicate eutrophication and hypoxia in the lake and enhanced land erosion in the watershed associated with the post 1970s intensive land use such as livestock farming for 1640 1935 ce the lake ohnuma record shows an antiphase relationship between the mn content and ti si ratio which implies that the detrital inputs reflected the strength of the east asian summer monsoon easm and the reduced rainfall runoff resulted in weaker mixing followed by depleted o2 in the water column the antiphase variation in mn content and ti si ratio suggest weak easm during the little ice age 1850 ce followed by strong easm between 1860 and 1929 ce

a higher moisture level in the early holocene in northern mongolia as evidenced from sediment records of lake hovsgol and lake erhel abstract paleoclimatic environmental changes of northern mongolia were investigated by chemical and mineralogical analyses of sediment cores from lake hovsgol and lake erhel back to 18 9 and 34 3 cal ka bp respectively the climate of this region was dry in the glacial period and wet in the holocene desalination of lake hovsgol occurred at 13 2 11 6 cal ka bp i e during the transition from the late glacial to early holocene at the same time ca 12 82 cal ka bp deposition in the lake erhel area changed from fluvial to lacustrine climate of northern mongolia was humid during the late glacial to the early holocene period ca 10 57 7 24 cal ka bp for the hovsgol and from ca 12 82 to 7 8 cal ka bp for the erhel this reconstruction differs from that for the more northern lake baikal region which humidification continued from the last glacial period until mid holocene this difference could be attributed to longer period of glacier melting and permafrost thawing around the baikal and on its watershed associated with increased summer insolation

origin of the lethal gas burst from lake monoun cameroun abstract on 15 august 1984 a lethal gas burst issued from a submerged 96 m deep crater in lake monoun in cameroun western africa killing 37 people the event was associated with a landslide from the eastern crater rim which slumped into deep water waters below 50 m are anoxic dominated by high fe 2 600 mg l and hco 3 1900 mg l anoxic and supersaturated with siderite which is a major component of the crater floor sediments the unusually high fe 2 levels are attributed to reduction of laterite derived ferric iron gradually brought into the lake as loess and in river input sulfur compounds are below detection limits in both water and gas gases effervescing from depressurized deep waters are dominantly co 2 with minor ch 4 having 13 c of 7 18 and 54 8 per mil respectively bacterial decomposition of organic matter may account for the methane but 14 c of lake water indicates that only 10 of the carbon is modern giving an apparent age of 18 000 years the dominant source of carbon is therefore attributed to long term emission of co 2 as volcanic exhalation from vents within the crater which led to gradual build up of hco 3 in the lake the density stratification of the lake may have been upset by an earthquake and underwater landslide on 15 august which triggered overturn of the lake and caused nucleation of co 2 in the deep water the resultant ebullition of co 2 from deep lake waters led to a gas burst at the surface and locally generated a water wave up to 5 m high people travelling through the gas cloud were asphyxiated presumably from co 2 and suffered skin discoloration from unidentified components

the climate data toolbox for matlab climate science is highly interdisciplinary by nature so understanding interactions between earth processes inherently warrants the use of analytical software that can operate across the disciplines of earth science toward this end we present the climate data toolbox for matlab which contains more than 100 functions that span the major climate related disciplines of earth science the toolbox enables streamlined entirely scriptable workflows that are intuitive to write and easy to share included are functions to evaluate uncertainty perform matrix operations calculate climate indices and generate common data displays documentation is presented pedagogically with thorough explanations of how each function works and tutorials showing how the toolbox can be used to replicate results of published studies as a well tested well documented platform for interdisciplinary collaborations the climate data toolbox for matlab aims to reduce time spent writing low level code let researchers focus on physics rather than coding and encourage more efficacious code sharing

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

formation of diagenetic siderite in modern ferruginous sediments ferruginous conditions prevailed in the world s deep oceans during the archean and proterozoic eons sedimentary iron formations deposited at that time may provide an important record of environmental conditions yet linking the chemistry and mineralogy of these sedimentary rocks to depositional conditions remains a challenge due to a dearth of information about the processes by which minerals form in analogous modern environments we identified siderites in ferruginous lake towuti indonesia which we characterized using high resolution microscopic and spectroscopic imaging combined with microchemical and geochemical analyses we infer early diagenetic growth of siderite crystals as a response to sedimentary organic carbon degradation and the accumulation of dissolved inorganic carbon in pore waters we suggest that siderite formation proceeds through syntaxial growth on preexisting siderite crystals or possibly through aging of precursor carbonate green rust crystal growth ultimately leads to spar sized 50 m mosaic single siderite crystals that form twins bundles and spheroidal aggregates during burial early formed carbonate was detectable through microchemical zonation and the possible presence of residual phases trapped in siderite interstices this suggests that such microchemical zonation and mineral inclusions may be used to infer siderite growth histories in ancient sedimentary rocks including sedimentary iron formations

a 1600 year multiproxy record of paleoclimatic change from varved sediments in lake xiaolongwan northeastern china 1 a multiproxy record from lake xiaolongwan northeastern china reveals distinct stratigraphical patterns in organic carbon total nitrogen organic carbon isotope nitrogen isotope clastic content and dinocyst concentration over the past 1600 years high atomic c n ratios in the sediment suggest that a large amount of organic matter in the sediments is derived from vascular plants and soil in the catchment based on the good agreement between cyclical variations of 13corg in the sediments and the spring drought index from historical documents we interpret that the variation of 13corg in the sediments is an indicator of humidity or precipitation in the study area several notable drought periods with heavier 13corg occurred during ad 490 570 ad 780 990 ad 1360 1450 ad 1590 1670 and the last 150 years these century scale drought episodes correspond to solar activity it suggests that solar forcing plays an important role for the 13corg variation in the sediments through driving the changes in humidity vapor pressure deficit dinocyst concentration is high during most of the medieval warm period and low during the european dark age and the little ice age the increase in dinocyst concentration is generally synchronous with decrease of 15n value the 15n values in the studied lake are most likely influenced by the duration and intensification of lake overturn which regulates the amount of depleted 15n nutrients dissolved organic nitrogen and ammonium and dinoflagellate blooms

itrax description and evaluation of a new multi function x ray core scanner a new automated multi function core scanning instrument named itrax has been developed that records optical radiographic and elemental variations from sediment half cores up to 1 8 m long at a resolution as fine as 200 m an intense micro x ray beam focused through a flat capillary waveguide is used to irradiate samples to enable both x radiography and x ray fluorescence xrf analysis data are acquired incrementally by advancing a split core via a programmable stepped motor drive through the flat rectangular section x ray beam traditional xrf determination of element composition in sediments provides high quality data but it takes a considerable time and normally consumes gram quantities of material that is often only available in limited quantities the itrax core scanner non destructively collects optical and x radiographic images and provides high resolution elemental profiles that are invaluable for guiding sample selection for further destructive detailed sampling this paper presents a description of the construction characteristics and capabilities of the itrax system high resolution itrax data obtained from sediment cores are also presented and compared with results from traditional wavelength dispersive xrf analysis at lower resolution finally some recent technical developments linked to the second generation itrax are presented

massive mn carbonate formation in the landsort deep baltic sea hydrographic conditions temporal succession and mn budget calculations abstract the sediments of the landsort deep and gotland basin in the central baltic sea are strongly enriched in mn carbonate however conceptual models attempting to explain the intense mn carbonate precipitation in both basins are in part conflicting in the gotland basin model deposited mn oxides are converted to mn carbonate after the oxygenation of euxinic bottom waters enriched in dissolved mn by major baltic inflows huckriede and meischner 1996 by contrast according to the landsort deep model mn carbonate precipitation occurs independent of oxygenation events below a euxinic water column lepland and stevens 1998 lenz et al 2015 in this study we investigated mn solid phase signatures in recent sub recent well dated sediments from the landsort deep and compared them to long term observations to identify the hydrographic conditions favoring mn carbonate formation the comparisons of water column o2 and sulfide time series with sedimentary mn carbonate enrichments identified long lasting bottom water oxygenation as an important environmental factor in the enhancement of mn carbonate precipitation up to 32 wt mn in the landsort deep thus by preventing the escape of dissolved mn from still reducing sediments into the open water column these conditions allow the accumulation of large amounts of mn oxide particles at the sediment water interface with their subsequent conversion to mn carbonate the euxinic conditions that have prevailed almost continuously in the landsort deep since roughly ad 2000 do not favor mn enrichment maximum 0 9 wt mn highlighting the importance of the recurring oxygenation of bottom water most likely via medium intensity inflows the mn abundances in seven sediment cores from water depths of 190 437 m together with mn balance calculations indicated that the mn inventory in the water column in response to porewater mn reflux and detrital mn input is sufficient for mn carbonate enrichment only in the deepest part of the basin

methods used in the u s s r to reduce near surface inhomogeneity effects on deep magnetotelluric sounding abstract the effects of near surface inhomogeneities on deep magnetotelluric sounding are considered methods are suggested allowing for and ruling out the near surface effects examples for practical application of these methods are given

climate system transition from glacial to interglacial state around the beginning of the last termination evidence from a centennial to millennial scale climate rhythm a periodic variation on a centennial to millennial scale was observed in the manganese and iron contents of lake suigetsu sediments in japan dating from 19 0 to 5 7 cal kyr b p calibrated thousand years before present the increased manganese and iron contents which mainly indicate the relative abundance of siderite result from a greater vertical circulation in the lake during a cold period with a higher mass of manganese and iron oxide being precipitated the cold periods inferred from the increased manganese and iron contents coincide with the timing of ice rafting debris ird events in the north atlantic which span from the last termination to the holocene suggesting that the climate shift was hemispheric or probably global in nature the manganese variation resembles the residual atmospheric 14c production 14c data of the holocene that reflect variation in solar activity and its periodicities correspond with those of solar related climate changes from paleoproxy records in the north atlantic and east asia from the last cold stage to the holocene in particular the mn variation was larger in amplitude and shorter in timescale during the last cold stage when compared with those during the last termination and the holocene thereby possibly showing a significant difference in climate response to solar irradiance the transition of the climate system from a glacial to an interglacial state corresponds to the beginning of the last termination

hydroclimatic changes in china and surroundings during the medieval climate anomaly and little ice age spatial patterns and possible mechanisms abstract investigating hydroclimatic changes during key periods such as the medieval climate anomaly mca 1000 1300 ad and the little ice age lia 1400 1900 ad is of fundamental importance for quantifying the responses of precipitation to greenhouse gas induced warming on regional and global scales this study synthesizes the most up to date and comprehensive proxy moisture precipitation records during the past 1000 years in china and surroundings the proxy data collected include 34 records from arid central asia aca and 37 records from monsoonal asia our results demonstrate a pattern of generally coherent regional moisture variations during the mca and lia in mid latitude asia north of 30 n monsoonal northern china north china and northeast china was generally wetter while aca northwest china and central asia was generally drier during the mca than in the lia a west east mode the boundary between wetter northern china and drier aca was roughly consistent with the modern summer monsoon boundary in monsoonal china to the east of 105 e the northern part was generally wetter while the southern part was generally drier during the mca than in the lia a north south mode with a boundary roughly along the huai river at about 34 n these spatial patterns of moisture precipitation variations are also identified by instrumental data during the past 50 years in order to understand the possible mechanisms related to the moisture variations during the mca and lia we investigate the major sst and atmospheric modes e g the el nino southern oscillation enso the atlantic multidecadal oscillation amo and the north atlantic oscillation nao which affect the moisture precipitation variations in the study region using both the instrumental data and the reconstructed time series it is found that the enso may play an important role in determining hydroclimatic variability over china and surroundings on a multi centennial time scale and that the foregoing spatial patterns could be attributed to the la nina like el nino like condition during the mca lia in addition amo and nao may also have their own contributions

a 21 000 cal years paleoclimatic record from caco lake northern brazil evidence from sedimentary and pollen analyses abstract sedimentological studies including seismic profiles mineralogy and organic geochemistry on two cores from the center and margin of caco lake maranhao state northern brazil revealed variable climatic and environmental conditions during the last 21 cal kyr bp between 21 and 17 cal kyr bp during the late glacial maximum regional climate was predominantly dry interrupted by short humid phases as reflected by a succession of very thin layers of sand and organic matter the late pleistocene climate was relatively wet as is suggested by rapid lake level rise and forest expansion the late pleistocene humid climate differed significantly from present conditions we suggest that late glacial humid conditions were the consequence of intensification of the inter tropical convergence zone or shifts of its position resulting in antarctic cold front occurrences the abrupt climatic changes during this period were marked by siderite deposition into caco lake which appears to be related to regional hydrologic changes linked to global northern hemisphere events the holocene was characterized by lower moisture availability and a distinct dry period until 7 cal kyr bp in response to south american insolation conditions

role of precursory less viscous mixed magma in the eruption of phenocryst rich magma evidence from the hokkaido komagatake 1929 eruption during the 1929 activity of hokkaido komagatake volcano the plinian eruption of a phenocryst rich andesite was preceded by a small eruption of more mafic magma formed by magma mixing a similar eruption sequence has been reported for some other eruptions pallister et al 1996 venezky and rutherford 1997 suggesting that eruption of a mixed magma is a precursor of phenocryst rich magmas for the purpose of understanding the tapping processes of the phenocryst rich magma chamber we investigated the temporal variation in the erupted magma and estimated the viscosity and density of the end member and mixed magmas with constraints drawn from petrography for the precursory mixed magma we estimate 33 4 vol phenocrysts andesitic dacitic melt composition 3 wt h2o content and temperature of 1040 c in comparison for the climactic silicic end member magma we estimate 48 3 vol phenocryst high silica rhyolitic melt 3 wt h2o and temperature of 950 c respectively the mafic end member magma which was not erupted is thought to be an almost aphyric basaltic andesitic magma based on mass balance calculation of the phenocryst content the proportion of the mafic end member magma component in the mixed magma was calculated to be 20 40 wt on the basis of these data we estimate magma viscosities of 103 9 106 9 and 102 0 pa s for the mixed silicic end member and mafic end member magmas respectively the calculated density differences among these magmas are inconsequential when possible errors are considered we calculate the minimum excess pressure required for dike propagation to be 31 mpa for the silicic end member magma and 8 mpa for the mixed magma using the estimated viscosity and dike propagation model of rubin 1995 if we assume that excess pressure is limited by the wall rock strength of the magma chamber excess pressure retainable in the magma chamber is less than ca 20 mpa this suggests that the mixed magma was able to ascend to the surface without freezing whereas the viscous silicic end member magma could not the formation and precursory eruption of the mixed magma are therefore effective and necessary initiation processes for the phenocryst rich viscous magma eruption

southern hemispheric westerlies control sedimentary processes of laguna azul south eastern patagonia argentina multiproxy investigations of lacustrine sediments from laguna azul 52 s document multi millennial holocene influences of southern hemispheric westerlies shw on the hydroclimatic variability of

a new geochemical classification of sedimentary environments because of the relative constancy of ph in most subaqueous sediments and the general lack of measurability of eh these parameters are not practically useful for classifying sedimentary environments in their place a new classification is offered which derived from studies of modern sediments is based on the presence or absence of dissolved oxygen and dissolved sulfide in the sediments at the time of authigenic mineral formation sedimentary environments are first of all divided into oxic and anoxic depending upon the presence or absence of measurable dissolved oxygen anoxic environments in turn are divided into sulfidic and non sulfidic depending upon the presence of measurable dissolved sulfide anoxic nonsulfidic environments are further divided into postoxic or resulting from oxygen removal without sulfate reduction weakly reducing and methanic or resulting from complete sulfate reduction with consequent methane formation strongly reducing the environments are shown to succeed one another during early diagenesis in the order oxic post oxic sulfidic methanic iron and manganese minerals characteristic and stable in each environment are listed and discussed so that they may be used to distinguish the environments when studying the ancient geological record

sedimentary geochemistry of manganese implications for the environment of formation of manganiferous black shales the sedimentary geochemistry of manganese is dominated by the redox control of its speciation higher oxidation states mn super 3 and super 4 occurring as insoluble oxyhydroxides in well oxygenated environments and the lower oxidation state mn super 2 being much more soluble in oxygen deficient settings its geochemical behavior is therefore quite different in oxic and anoxic environments and where oxic and anoxic conditions are juxtaposed mn is recycled between the two environments in modern marine sediments mn is present above its crustal abundance as an oxyhydroxide in all slowly accumulating pelagic sediments of the deep ocean and in surficial deposits of continental margin environments diagenetic recycling of mn in the latter causes surficial deposits to have larger mn enrichments than in many pelagic sediments bottom sediments of permanently anoxic basins show no enrichments and have mn concentrations that are controlled solely by the aluminosilicate fraction manganese carbonates kutnohorite and calcic rhodochrosite are found only in anoxic sediments accumulating beneath surface oxic horizons and therefore under oxygenated bottom waters in many nearshore environments such enrichments are due to delivery of mn by burial of surface oxyhydroxides into the subsurface anoxic environment where they are dissolved pore water mn levels can reach saturation with respect to a mixed mn ca carbonate phase in such sediments the diagenetic origin for these phases is shown by their carbon isotope compositions which typically indicate a carbon source from decomposing organic matter the presence of mn carbonates therefore signifies that the host sediment must have accumulated under oxygenated bottom waters on the basis of this information it is proposed that in contrast to several current explanations for the formation of mn carbonates kutnohorite and rhodochrosite in ancient organic rich shales limestone and marl sequences and in many mn ore deposits the occurrence of these mineral phases indicates that the sediments originally accumulated beneath oxygenated bottom waters by analogy with the present mn carbonates could not have formed in the bottom waters of anoxic basins these diagenetic phases however did form where mn was supplied at a high rate namely by the burial of oxyhydroxide enriched surface sediments to a subsurface anoxic environment this situation could only have occurred under oxygenated bottom waters the presence of mn carbonates in ancient black shales and in some carbonate rich rocks lends strong support to the notion that these rocks did not necessarily form in anoxic basins but owe their carbon richness to a high supply of organic matter to sediments deposited under oxygenated bottom waters probably in continental margin settings

varve microfacies and varve preservation record of climate change and human impact for the last 6000 years at lake tiefer see ne germany the holocene sediment record of lake tiefer see exhibits striking alternations between well varved and non varved intervals here we present a high resolution multi proxy record for the past 6000

double the dates and go for bayes impacts of model choice dating density and quality on chronologies abstract reliable chronologies are essential for most quaternary studies but little is known about how age depth model choice as well as dating density and quality affect the precision and accuracy of chronologies a meta analysis suggests that most existing late quaternary studies contain fewer than one date per millennium and provide millennial scale precision at best we use existing and simulated sediment cores to estimate what dating density and quality are required to obtain accurate chronologies at a desired precision for many sites a doubling in dating density would significantly improve chronologies and thus their value for reconstructing and interpreting past environmental changes commonly used classical age depth models stop becoming more precise after a minimum dating density is reached but the precision of bayesian age depth models which take advantage of chronological ordering continues to improve with more dates our simulations show that classical age depth models severely underestimate uncertainty and are inaccurate at low dating densities and also perform poorly at high dating densities on the other hand bayesian age depth models provide more realistic precision estimates including at low to average dating densities and are much more robust against dating scatter and outliers indeed bayesian age depth models outperform classical ones at all tested dating densities qualities and time scales we recommend that chronologies should be produced using bayesian age depth models taking into account chronological ordering and based on a minimum of 2 dates per millennium

stable isotopes in recent sediments of lake arendsee ne germany response to eutrophication and remediation measures abstract recent sediments of lake arendsee a small eutrophic lake in ne germany were examined in high temporal resolution in order to investigate the applicability of stable isotopes as proxies to trace paleoenvironmental conditions the age model of the sediment core is based on varve counting and 137cs 210pb dating and reveals that the sedimentary record covers the last 70 years during which the lake has undergone major changes in productivity and eutrophication in response to nutrient loading since 1965 toc tic tn and tp accumulation rates in general reflect the change from oligo mesotrophic to eutrophic level the isotopic compositions of autochthonous organic matter and authigenic calcite whitings demonstrate that carbon isotope fractionation takes place in a co2 limited pool in accordance with the historical limnological observations enhanced eutrophication is reflected by 13ctoc increases of over 3 from 1934 to 1985 as a result of less 12c 13c fractionation by photosynthesis during intense primary productivity decreasing 13ctic values indicate that biogenically induced calcite formation occurred during enhanced primary production accompanied by co2 limitation the gradual trend to more light nitrogen isotopic signatures towards the sediment surface reflects the dominance of air nitrogen fixation of cyanobacteria blooms during the last decades the 18otic distribution suggests that plankton blooms between 1934 and 1978 had shifted towards the early season with lower temperatures as a result of increasing nutrient level which subsequently favor plankton blooms even in colder waters since the mid 1980s decreasing 13ctoc and increasing 13ctic values since 1985 indicate a more efficient 12c 13c fractionation during photosynthesis with no co2 limitation this improvement in water quality is in accordance with several remediation measures carried out since the late 1970s linear isotopic trends are interrupted by high tic toc tn and tp accumulation rates in the early 1970s which can be clearly attributed to the artificial drainage of a nearby wetland carried out during that time this event can be considered an important external source for nutrient loaded water and particulate matter and is displayed by significantly heavier 13ctic and 18otic values in the sedimentary record

complex trajectories of aquatic and terrestrial ecosystem shifts caused by multiple human induced environmental stresses large shifts in the isotopic compositions of organic matter om in lake sediments over the last few hundred years are commonly interpreted as representing changes in photosynthetic productivity corresponding to eutrophication or in the input of terrestrial om due to human disturbances based on multiple proxy data c n ratio d 13 c and d 15 n of om d 13 c of calcite lithology and fossil pollen from a 700 year sediment core at white lake new jersey usa we propose a new explanation that relates these large shifts in om d 13 c and d 15 n to human induced changes in aquatic om producers combined records of geochronology fossil pollen and lithology from white lake reveal that the upland forest was cleared by european settlers for farmland beginning around 1745 a d and has gradually reforested since 1930 after the abandonment of the farmlands for the pre agricultural period om had relatively constant but extremely low d 13 cvpdb 35 8 to 34 5 and d 15 nair 3 5 to 2 5 and high atomic c n ratios 13 7 to 16 7 indicating a stable anoxic lake environment with prominent microbial producers following the human disturbance since 1745 high om mass accumulation rates and abundances of the green alga pediastrum indicate an increase in aquatic photosynthetic productivity due to enhanced nutrient input from disturbed uplands however carbonate d 13 c remains constant or even decreases during this period implying that increasing productivity did not elevate the d 13 c of dissolved inorganic carbon and thus cannot explain the observed large increase in om d 13 c 7 4 and d 15 n 5 8 over this period instead d 13 c d 15 n and c n ratios of om and differences in d 13 c between calcite and om suggest that the large increase in om d 13 c and d 15 n can be attributed to a human induced ecological shift in the predominant organic source from anaerobic bacteria to autotrophic phytoplankton during the post agricultural period mass accumulation rates of om carbonate and silicate and the d 13 c of om and calcite all decreased significantly corresponding to stabilization of the uplands however over the last 70 years an intensifying aquatic stress from the deposition of 15 n enriched industrial pollutants has resulted in a steady increase of 1 9 in d 15 n proxy records for lake d 13 c and d 15 n

microbial mounds prior to the frasnian famennian mass extinctions hantang guilin south china late frasnian mounds of the yunghsien formation guilin south china developed as part of the guilin platform mostly in reef flat and platform margin settings microbial mounds in platform margin settings at hantang about 10 km west of guilin contain frasnian biota such as stachyodes and kuangxiastraea and thus occur below the frasnian famennian mass extinction boundary platform margin facies were dominated by microbes algae and receptaculitids massive corals and stromatoporoids are not common and rarely show reef building functions as they did in givetian time the margin mounds are composed of brachiopod receptaculitid cementstone and a variety of boundstones that contain rothpletzella renalcis thrombolite and stromatolite other microbial communities include girvanella izhella ortonella and wetheredella solenoporoid algae are abundant locally zebra structures and neptunian dykes are well developed at some intervals pervasive early cementation played an important role in lithification of the microbial boundstones and rudstones frasnian reefs of many regions of the world were constructed by stromatoporoids and corals although a shift to calcimicrobe dominated frameworks occurred before the famennian however the exact ages of many frasnian margin outcrops are poorly constrained owing to difficulties dating shallow carbonate facies the hantang mounds represent a microbe dominated reef building community with rare skeletal reef builders consistent with major late devonian changes in reef composition diversity and guild structure occurring before the end of the frasnian a similar transition occurred in the canning basin of western australia but coeval successions in north america western europe and the northern urals are either less well known or represent different bathymetric settings the transition in reef building style below the frasnian famennian boundary is documented here in the two best exposed successions on two continents which may have been global set in the larger context of late devonian and mississippian microbial reef building the hantang mounds help to demonstrate that controls on microbial reef communities differed from those on larger skeletal reef biota calcimicrobes replaced stromatoporoids as major reef builders before the frasnian famennian extinction event and increasing stromatoporoid diversity towards the end of the famennian did not result in a resurgence of skeletal reef frameworks calcimicrobes dominated margin facies through the famennian but declined near the devonian carboniferous boundary stromatolite and thrombolite facies which occurred behind the mound margin at hantang rose to dominate mississippian shallow water reef frameworks with only a minor resurgence of the important frasnian calcimicrobe renalcis in the visean when well skeletonized organisms corals also became volumetrically significant frame builders again

a 700 year record of climate change based on geochemical and palynological data from varved sediments lac pavin france abstract the fe flux budget of the meromictic crater lac pavin french massif central is re evaluated in the light of new results from detailed sediment investigations geochemistry sedimentology pollen varve counting sedimentation of lac pavin during the last 700 years is characterised by high net accumulation of biogenic silica 100 500 g m 2 yr 1 sio 2 and autochthonous fe precipitates f fe exc 2 200 g m 2 yr 1 fe at overall high sediment accumulation e g ad 1530 1890 3 7 mm yr 1 high net accumulation of biogenic silica and fe exc is sustained by nutrient and fe influx via sub lacustrine springs the feeding of the groundwater inflow is closely correlated with the seepage of meteoric waters which declines in winter due to freezing of the ground in the context of the derived local vegetation dynamics balanced fe exc flux is interpreted as a proxy for rainfall during the growing season meteorological monitoring data document a significant inverse correlation between monthly temperature and rainfall between may and october which reflects enhanced cloudiness for precipitation above average beech growing under sub optimal conditions at the high altitude of the study site represents a major tree component in the catchment of lac pavin minor temperature decrease in the growing season can explain reduced flowering of beech in the following year distinct short term variability of beech characterised by an inverse correlation with the balanced f fe exc supports the data interpretation two periods with prevailing of overall wetter and cooler conditions in the growing season were derived ca ad 1525 1600 and ad 1630 1660 climate change to drier and warmer conditions is implied around ad 1450 and ad 1620 the late maunder minimum ad 1675 1715 is characterised by distinct short term variability of the palaeo hydrological conditions at the study site

varves in lake sediments a review abstract downcore counting of laminations in varved sediments offers a direct and incremental dating technique for high resolution climatic and environmental archives with at least annual and sometimes even seasonal resolution the pioneering definition of varves by de geer 1912 had been restricted to rhythmically deposited proglacial clays one century later the meaning of varve has been expanded to include all annually deposited laminae in terrestrial and marine settings under favourable basin configurations and environmental conditions limnic varves are formed due to seasonality of depositional processes from the lake s water column and or transport from the catchment area subsequent to deposition of topmost laminae the physical preservation of the accumulating varved sequence requires the sustained absence of sediment mixing for example via wave action or macrobenthic bioturbation individual sub laminae in varved lake sediments typically express contrasting colours always differ in terms of their organic chemical and or mineralogical compositions and often also differ with regard to grain size various predominating climatic and depositional conditions may result in clastic biogenic or endogenic incl evaporitic varved sediments and their mixtures to reliably establish a varve chronology the annual character of laminations needs to be determined and verified in a multidisciplinary fashion sources and influences of possible errors in varve chronologies are best determined and constrained by repeated varve counts and by including radioisotopes and correlation with historically documented events a well established varve chronology greatly enhances the scientific value of laminated limnic archives by securely anchoring the wealth of multi proxy palaeoenvironmental information in the form of time series for multidisciplinary investigations applications of varved records are discussed with special reference to advances since the 1980s these span fields like calibrating radiometric dating methods reconstructing past changes of the earth s magnetic field or detecting fluctuations in solar forcing once a varve chronology is established it can be applied to precisely date events like volcanic ash layers earthquakes or human impact as well as short and long term climate temperature precipitation wind hydroclimatic conditions or flooding and environmental changes eutrophication pollution due to their exceptional high temporal resolution and in combination with their robust and accurate internal time scale in calendar years annually laminated sediments can be regarded as one of the most precious environmental archives on the continents these records are necessary to extend temporally limited instrumental records back in time as such they have societal relevance with regard to risk assessments related to natural hazards arising from e g flooding or volcanic eruptions

preservation potential of seasonal laminated deposits as a useful tool for environmental analysis in mesotrophic lake kizaki central japan abstract in this paper discusses the aquatic environment at lake kizaki central japan during the last ca 60 years and describes the importance of laminated sediments in environmental analyses the flux of total algal carbon increased between 1965 and 1996 while the bsi flux which can be used in diatom production showed a larger increase between 1960 and 1998 than typically occurs on average the flux of dinosterol an indicator of dinoflagellate production also increased from 1966 to 1998 and the sedimentary dinoflagellate record correlated with observations of freshwater red tides caused by dinoflagellate blooms parallel laminated sediment was determined to have appeared between 1960 and 1995 corresponding to a period of high total algal carbon levels and bsi fluxes for lamina formation and its preservation process the thickness of the oxygen poor layer depended on excessive total algal production during stagnant periods and appeared to be important because nutrients were diffused and accumulated in the oxygen poor layer from bottom sediments this could result in diatom blooms at the next overturn period the development of the thick oxygen poor layer appeared to be triggered primarily by the presence of excessive nutrients generated by human activities in general the occurrence of laminated sediments can provide a useful tool for understanding the eutrophic state in lake kizaki

upper devonian reefs and microbialite at maoying south china implications for paleoenvironmental changes abstract reefal carbonate deposits in south china preserve a record of community structure and environmental change across the frasnian famennian boundary and through the late devonian detailing a regional response to one of the largest mass extinction events in earth history the upper devonian frasnian to famennian limestone outcrops in the maoying area of northern ziyun county south china record a carbonate platform depositional setting with some patch reef development the upper devonian carbonate deposits in maoying area are distinctive from those in correlative sections in dushan county with rare argillaceous limestone and dolomitic limestone as well as less dolostone exposures to the south in the guilin area of northeastern guangxi province are similar to those seen in the study area upper devonian carbonates in the maoying area are divided into two lithologic groups reef limestones or dolostone beds composed of five lithologically distinctive intervals the upper devonian strata have a cumulative thickness of 530 m the upper devonian reefs in the maoying area were developed on a shell pavement of brachiopod and bivalve shell shoal packstones and evolved into patch reefs in the interior of the carbonate platform patch reef development was limited to early frasnian time reef building organisms are dominated by massive and tabular stromatoporoids amphipora sp stachyodes sp and coral thamnopora sp while brachiopods and algae accumulated between stromatoporoid skeletal frameworks three evolutionary stages of the frasnian patch reefs including an initial development stage maturity stage and demise stage can be recognized according to sedimentary structures and fossil assemblage characteristics the frasnian strata overlying the patch reefs represent a well bedded back reef depositional setting and are mainly composed of amphipora floatstone and packstone containing a few bulbous stromatoporoids and small fasciculate corals famennian strata are characterized by laminated limestone and fenestrate limestone that are interbedded with infrequent mudstone and argillaceous limestone horizons a prominent microbialite bed developed in the laminated limestone of the famennian carbonate platform consists of stromatolite and thrombolite framestone this bed can be correlated in horizon and age with microbial reefs and mounds in the famennian offshore carbonate platform in the huanan epeiric sea such as famennian microbial reefs and mounds in guilin this represents a biogenic carbonate formation characterized by microbial fabrics that accumulated in a nearshore carbonate platform setting after the global frasnian famennian biotic extinction event which promoted the growth of microbialites during the latest devonian in south china

varved lacustrine sediments of lake tougou ike western japan with reference to holocene sea level changes in japan abstract eighteen metres of laminated sediments were recovered in a holocene sediment core taken from lake tougou ike in 1992 the lamination evident from the alternation of light and dark coloured laminae records cyclic deposition of biochemical sediments annual environmental changes in and around the lake caused seasonal variations in sediment flux to the lake floor explaining these cyclic variations in sedimentary facies ams 14 c dating of organic remains from 40 horizons in the succession supports the interpretation of each light to dark couplet as an annual increment each laminated couplet is identified as a non glacial varve formed mainly by seasonal changes in diatom productivity varve counting and 14 c dating provide a detailed holocene chronological framework with time resolution on the order of individual years within which to reconstruct paleoenvironmental changes recorded in the sediment core holocene relative sea level changes at lake tougou ike were estimated with 10 yr precision using the variable authigenic iron mineral content of the sediment as a proxy the results demonstrate that the relative sea level at lake tougou ike rose abruptly at about 8900 7700 5600 4000 3600 and 2700 cal yr bp and decreased gradually in the intervening intervals these relative rises in sea level are demonstrated to be coeval with ice sheet discharge events recorded in sediment cores from the north atlantic ocean abrupt coastal environmental changes caused by sea level changes likely took place throughout the japanese islands and will probably be identified in both the holocene and the last glacial interval the latter being a time when dansgaard oeschger cycles were clearly active in the north atlantic region

stable nitrogen isotope record of lacustrine sediments in lake onuma northern japan indicates regional hydrological variability during the past four centuries abstract discriminating natural and anthropogenic forcing on hydroclimatic changes is of fundamental importance for projecting future precipitation distribution under the warming climate a sediment core on12c obtained from lake onuma is used to study the evolution of precipitation in northern japan the age depth model for the core is based on three identified tephra layers combined with accelerator mass spectrometry 14 c measurements of plant remnants 137 cs dating and lithology multi proxy records including pollen concentration alnus pollen percentages and c n atomic ratios indicate that the 15 n record reflects nitrogen availability in the catchment where precipitation may exert a significant impact the 15 n record shows that the precipitation was gradually increasing from 1640 ad reaching peak strength in the middle of the 19th century followed by a dry interval during the 20th century the reconstructed variability in the precipitation from lake onuma is supported by other precipitation records from the asian monsoon region solar irradiance is likely responsible for the changes in the summer monsoon precipitation on the centennial timescales during the late little ice age and the winter monsoon may also have played an important role in snow precipitation however the 20th century anomaly indicates that the asian summer monsoon precipitation variability might be affected by anthropogenic forcing

centennial scale east asian summer monsoon intensity based on δ18o values in ostracode shells and its relationship to land ocean air temperature gradients over the past 1700 years centennial scale variations in the east asian summer monsoon easm remain ambiguous in the present study the 18o of shells from the ostracode bicornucythere bisanensis from lake nakaumi japan were examined to identify the bottom salinity changes caused by easm variations over the past 1700 yr four weak easm periods were identified in lake nakaumi at a d 600 1000 1350 and 1900 we detected discordance among the centennial scale easm variations in japan and the 18o of stalagmites 18ostalagmite in southern china furthermore both total solar irradiance and atmospheric 14c concentration which were previously considered to be linked with the centennial scale easm variations were asynchronous with the easm variations conversely we observed significant positive and negative correlations between the easm from japan and the 18ostalagmite in north central china and the relationship was associated with the asia pacific oscillation apo index centennial scale easm periods dominated during the high apo period these synchronous variations imply that the strength of the easm variations is tied to temperature gradients between east china and the north central pacific ocean

carbon and oxygen stable isotopic signatures of methanogenesis temperature and water column stratification in holocene siderite varves abstract manganoan siderite fe mn co 3 occurs in abundance of up to 19 dry weight as the sole endogenic carbonate within a succession of holocene organic rich varved sediments from freshwater otter lake ol michigan radiocarbon dating and varve counts from a 7 m piston core constrain periods of major siderite accumulation to sediments older than 1200 cal yr bp sediment petrography suggests that siderite was a seasonal precipitate confined to the summer layer of the varve couplet bulk sediment chemistry reveals cycles in abundance of manganese and aluminum coincident with centennial scale cycles of siderite accumulation siderite 13 c and 18 o are enriched when the mineral is abundant and depleted when it is least abundant samples with high abundance of siderite precipitated in oxygen isotopic equilibrium with modern lake water but are 13 c enriched relative to modern waters samples from intervals of low siderite abundance are 13 c and 18 o depleted relative to equilibrium with modern lake water these data suggest that abundant siderite precipitation occurred when the ol water column was ferruginous iron meromictic allowing for enhanced ferrous iron concentrations and dissolved inorganic carbon dic enriched in 13 c below the chemocline where methanogenesis in waters and sediments influenced dic composition seasonal siderite precipitation was triggered by water column alkalinity fluctuations driven by summer calcite dissolution manganese substitution in siderite lowered kinetic barriers to low temperature mineral precipitation climate changes and basin filling influenced lake stratification and the rate of siderite precipitation the siderite occurrence reported here displays remarkable similarity to its occurrence in paleogene maar lake deposits suggesting that further studies of holocene lacustrine siderites may provide insight into ancient sedimentary systems and environments

evidence for lake formation as a response to an inferred holocene climatic transition in brazil abstract geochemical mineralogical and palynological records from a holocene sediment core 12 7 m long from lake silvana southeastern brazil served to identify the source of detrital sediments within catchment soils the lake basin was first flooded in the early holocene 9400 14 c yr b p when a gibbsite rich b soil horizon started to accumulate four distinct hydrologic phases are consistent with vegetational changes indicated by pollen data phases of slope instability colluviation and low lake level correspond to pollen free intervals and point to a severe retreat of the vegetation cover the rapid sedimentation of slope wash sediments alternating with alluvial sediments appears to have formed a tributary fan 7 8 m thick that dammed the lake silvana basin 8500 14 c yr b p probably as a response to a drastic increase in precipitation rates

a new opportunity to detect paleo earthquake events dating back to the past 10 millennia a record from lacustrine sediment bulk density and grain density profiles of the uppermost 90 cm portion of lacustrine sediment from lake suigetsu one of the five major lakes of lake mikatagoko located in central japan indicate the occurrence of at least 10 depositional events characterized by anomalously high density spikes two events are accompanied by turbidites the dates of the sedimentary events estimated here are correlated with the occurrence of destructive earthquakes since 1100 a d close examination of the sedimentological signatures recorded in the lake sediment and historical literature describing destructive earthquake phenomena provides a new opportunity to detect paleo earthquake events in a high seismic area having a long history

origin and environment of manganese rich sediments within black shale basins abstract sediments with unusually high manganese contents occur in anoxic basins of the central baltic sea we present a new model for the enrichment of manganese which is based on sedimentological and geochemical analyses of holocene sediments from the gotland deep and on published hydrographic data due to salinity stratification and high primary production bottom waters of the central baltic sea are anoxic and rich in dissolved mn 2 episodically bottom waters become oxygenated by inflows of denser water from the north sea mixing results in oxidation of mn 2 to fine grained particulate oxides which accumulate in the deepest parts of the basins where currents are mostly absent here anoxic conditions are re established within a few months or years and manganese oxides are again dissolved causing high concentrations of mn 2 in the near bottom layer alkalinity is high as a consequence of bacterial reduction of manganese oxides and sulphate under these conditions manganese is reprecipitated and forms a layer of ca rich rhodochrosite on the top of laminated sapropels occasionally periods of such extremely and rapidly changing redox conditions are followed by long term oxygenation of bottom waters in that case a layer rich in manganese oxides forms on the top of a bioturbated layer due to dissolution of rhodochrosite and oxidation of manganese in the near surface sediment return of anoxic conditions results again in the transformation of manganese oxides into rhodochrosite varying rhodochrosite contents in the basinal sediments therefore record a detailed history of redox conditions in bottom waters the manganese rich sediments of the baltic sea share many features with some occurrences of lower jurassic and lower carboniferous manganese ores in western and central europe we therefore suggest that these manganese enrichments were formed by similar processes

a complete terrestrial radiocarbon record for 11 2 to 52 8 kyr b p radiocarbon 14c provides a way to date material that contains carbon with an age up to 50 000 years and is also an important tracer of the global carbon cycle however the lack of a comprehensive record reflecting atmospheric 14c prior to 12 5 thousand years before the present kyr b p has limited the application of radiocarbon dating of samples from the last glacial period here we report 14c results from lake suigetsu japan 35 35 n 135 53 e which provide a comprehensive record of terrestrial radiocarbon to the present limit of the 14c method the time scale we present in this work allows direct comparison of lake suigetsu paleoclimatic data with other terrestrial climatic records and gives information on the connection between global atmospheric and regional marine radiocarbon levels

chronology and sedimentation process of varved lacustrine sediment in lake fukami central japan abstract continuous varve sequences were taken from lake fukami a monomictic lake southern part of nagano prefecture central japan in 2000 total 196 varve units were recognized in the upper 2 m horizon of the sediment core sedimentation rates in the core have some high peaks indicating increments of clastic particles supplied to the lake owing to natural disasters including floods and earthquakes and the artificial construction around lake fukami the results of the correlation between the meteorological records in iida meteorological station and the total carbon flux change based on the varve chronology indicates that the total carbon flux shows an inverse relationship to february mean temperature and precipitation february mean temperature controlled the vertical mixing of lake water column that affects the productivity of phytoplankton in the epilimnion or decomposition of organic matter in the hypolimnion february precipitation probably has affected the activity of denitrifying bacteria in the stagnation period pyrite and siderite minerals are recognized in the core by observation of thin sections and x ray powder diffraction analysis the pyrite was deposited in summer and the siderite was deposited in summer and winter these iron minerals indicate the decomposition processes of organic matters in the water column in each season

continental scale temperature variability during the past two millennia past global climate changes had strong regional expression to elucidate their spatio temporal pattern we reconstructed past temperatures for seven continental scale regions during the past one to two millennia the most coherent feature in nearly all of the regional temperature reconstructions is a long term cooling trend which ended late in the nineteenth century at multi decadal to centennial scales temperature variability shows distinctly different regional patterns with more similarity within each hemisphere than between them there were no globally synchronous multi decadal warm or cold intervals that define a worldwide medieval warm period or little ice age but all reconstructions show generally cold conditions between ad 1580 and 1880 punctuated in some regions by warm decades during the eighteenth century the transition to these colder conditions occurred earlier in the arctic europe and asia than in north america or the southern hemisphere regions recent warming reversed the long term cooling during the period ad 1971 2000 the area weighted average reconstructed temperature was higher than any other time in nearly 1 400 years

tracing bottom water oxygenation with sedimentary mn fe ratios in lake zurich switzerland abstract redox dynamics of manganese mn were studied in the sediment of lake zurich using precise sediment core age models monthly long term oxygen o 2 monitoring data of the water column 1936 2010 and high resolution xrf core scanning the age models were based on bi annual lamination and calcite precipitation cycles if present mn exhibits distinct maxima which coincide with the annual maximum deep water o 2 concentrations in spring according to the monitoring data in contrast the iron fe signal is mainly the result of calcite dilution as indicated by a strong negative correlation between fe and calcium ca xrf data the mn fe ratio in the core from the maximum lake depth zh10 15 137 m revealed a moderate correlation with o 2 measurements in the lake bottom water confirming the successful application of the mn fe ratio to semi quantitatively reconstruct bottom water oxygenation in the lake mostly low ratios were observed between 1895 and the mid 1960s as a result of eutrophication however geochemical focusing and sedimentological factors can reduce the applicability of the mn fe ratio in reconstructing o 2 concentrations in the bottom water of lakes

evaluating a primary carbonate pathway for manganese enrichments in reducing environments abstract most manganese mn enrichments in the sedimentary rock record are hosted in carbonate minerals which are assumed to have formed by diagenetic reduction of precursor mn oxides and are considered diagnostic of strongly oxidizing conditions here we explore an alternative model where mn carbonates form in redox stratified water columns linked to calcium carbonate dissolution in ferruginous brownie lake in minnesota usa we document mn carbonates as an hcl extractable phase present in sediment traps and in reducing portions of the water column mn carbonate becomes supersaturated in the brownie lake chemocline where dissolved oxygen concentrations fall below 5 m and mn oxide reduction increases the dissolved mn concentration supersaturation is enhanced when calcite originating from surface waters dissolves in more acidic waters at the chemocline in the same zone sulfate reduction and microaerobic methane oxidation add dissolved inorganic carbon dic with negative 13 c these observations demonstrate that sedimentary mn enrichments may 1 develop from primary carbonate phases and 2 can occur in environments with dissolved oxygen concentrations 200 m and where mn and fe are partitioned by s cycling photoferrotrophy or microaerophilic fe oxidation a shallow lysocline enhances mn carbonate production by providing additional dic and nucleation sites for crystal growth this carbonate model for mn enrichments is expected to be viable in both euxinic and ferruginous environments and provides a more nuanced view of the relationships between mn and carbon cycling with applications throughout the rock record

an abrupt wind shift in western europe at the onset of the younger dryas cold period the younger dryas cold reversal during the last glacial termination is one of the most abrupt climate change events observed in the northern hemisphere analyses of varved lake sediments from western germany suggest that storminess in the north atlantic region increased within a single year at this time providing a mechanistic link between a decrease in the meridional overturning circulation and western european cooling the younger dryas cooling 12 700 years ago is one of the most abrupt climate changes observed in northern hemisphere palaeoclimate records1 2 3 4 annually laminated lake sediments are ideally suited to record the dynamics of such abrupt changes as the seasonal deposition responds immediately to climate and the varve counts provide an accurate estimate of the timing of the change here we present sub annual records of varve microfacies and geochemistry from lake meerfelder maar in western germany providing one of the best dated records of this climate transition5 our data indicate an abrupt increase in storminess during the autumn to spring seasons occurring from one year to the next at 12 679 yr bp broadly coincident with other changes in this region we suggest that this shift in wind strength represents an abrupt change in the north atlantic westerlies towards a stronger and more zonal jet changes in meridional overturning circulation alone cannot fully explain the changes in european climate6 7 we suggest the observed wind shift provides the mechanism for the strong temporal link between north atlantic ocean overturning circulation and european climate during deglaciation

quantitative pollen based climate reconstruction in central japan application to surface and late quaternary spectra abstract climate reconstruction in japan by the best modern analogues method was tested with 285 surface pollen spectra and then applied to the 47 000 years pollen profile from lake mikata central japan both pollen taxa percentages and plant functional type affinity scores were used in the reconstruction of six climate variables the reconstructed values were compared with the estimated modern climate at each surface pollen site in order to evaluate the accuracy of the pollen based reconstruction the reconstruction of thermal parameters is very reliable correlation coefficient r between 0 86 and 0 89 the reconstruction of annual precipitation is less accurate r 0 7 we also proposed a new technique to calibrate reconstructed values into both probability distribution and or confidence regions of observed values the specialized programs for this calibration polcalib and polhist are available at qsr website http www elsevier nl locate quascirev the reconstruction of the late quaternary climate changes around lake mikata shows that the summer and winter temperatures were 5 c lower than present around 40 uncal kyr bp reached a minimum around 20 uncal kyr bp 10 c lower than present and became similar to present day at 8 uncal kyr bp the estimates for annual precipitation show that it was driest around 20 uncal kyr bp

manganese and iron geochemistry in sediments underlying the redox stratified fayetteville green lake abstract manganese and iron are redox sensitive elements that yield clues about biogeochemistry and redox conditions both in modern environments and in the geologic past here we investigated mn and fe bearing minerals preserved in basin sediments underlying fayetteville green lake a redox stratified lake that serves as a geochemical analogue for paleoproterozoic oceans synchrotron source microprobe techniques xrf xanes and xrd and bulk geochemical analyses were used to examine the microscale distribution and speciation of mn fe and s as a function of depth in the top 48 cm of anoxic lake sediments manganese was primarily associated with calcite grains as a manganese rich carbonate that precipitated in the chemocline of the water column and settled through the euxinic basin to collect in lake sediments iron was preserved in framboidal iron sulfides that precipitated in euxinic bottom waters and underwent transformation to pyrite and marcasite in the sediments previous studies attribute the formation of manganese rich carbonates to the diagenetic alteration of manganese oxides deposited in basins underlying oxygenated water our study challenges this paradigm by providing evidence that mn bearing carbonates form in the water column and accumulate in sediments below anoxic waters consequently manganoan carbonates preserved in the rock record do not necessarily denote the presence of oxygenated bottom waters in ocean basins