eemian cooling in the norwegian sea and north atlantic ocean preceding continental ice sheet growth changing conditions in the north atlantic region may drive global climate changes1 2 according to previous reconstructions of the last interglacial the eemian north atlantic sea surface tempera tures ssts were similar to present day values3 in the norwegian sea even warmer conditions appeared as a single pulse of short duration4 5 whereas the greenland ice record suggests that the warm interglacial air temperatures were interrupted by several cold periods6 here we use faunal and stable isotope analyses of foraminifera in two sediment cores from the north atlantic ocean and norwegian sea to reconstruct high resolution records of sst and sea surface salinity sss during the eemian interglacial our results which differ significantly from the greenland record6 show a sharp decrease in sst and sss of the norwegian sea associated with a more moderate cooling and freshening of the north atlantic at the middle of isotope substage 5e several millennia before the beginning of continental ice sheet growth changes in the norweg ian sea surface conditions appear to have represented an important climate change affecting global atmospheric and thermohaline circulations

the oxygen isotopic compositions of silica phytoliths and plant water in grasses implications for the study of paleoclimate abstract information about climatic conditions during plant growth is preserved by the oxygen isotope composition of biogenic silica phytoliths deposited in grasses the oxygen isotope composition of phytolith silica is dependent on soil water 18 o values relative humidity and evapotranspiration and temperature during plant growth phytolith and plant water 18 o values for c3 a breviligulata and c4 c longifolia grasses from natural and greenhouse sites in southwestern ontario were used to compare the isotopic fractionation between biogenic silica and water in various parts of these living plants for non or weakly transpiring tissues rhizomes stems sheaths in both grass species the 18 o silica plant water remained constant at 34 and the 18 o and d values of plant water collected from pre dawn and mid day samplings showed little variation these plant waters were only slightly enriched in 18 o and d relative to water provided to the grasses isotopic temperatures calculated from the silica and plant water isotopic data matched measured growing temperatures for the region by comparison the upper leaf water was extremely enriched in oxygen 18 and deuterium at maximum rates of transpiration relative to water from non transpiring tissues as were the calculated steady state values for leaf water 18 o and d silica produced in the transpiring tissues leaf inflorescence has higher 18 o values than silica from non transpiring tissues but the enrichment is modest compared to upper leaf water under mid day conditions leaf phytoliths have formed from plant water typical of average conditions in the lower leaf where the extreme 18 o enrichment is not encountered c longifolia was also collected from alberta and nebraska where growing conditions are different from southwestern ontario phytoliths at all three sites have a similar pattern of 18 o values within the plants but the isotopic separation between leaf and stem silica increases from 4 to 8 as average relative humidity decreases the difference between actual growing temperature and that calculated using measured 18 o values for stem silica and local meteoric water became progressively larger as relative humidity decreased likely because of evaporative 18 o enrichment of soil water such effects are most pronounced in arid environments and pertinent in grasslands where much of the active rooting zone can be situated at the shallower depths most affected by the 18 o enrichment of soil water

silica diagenesis ii general mechanisms abstract amorphous silica phases opal a precipitate in nature due to the formation of dense colloids in supersaturated alkaline aqueous solutions with low relative concentrations of other ions opal a dissolves and yields solutions of still relatively high silica content in pore waters containing abundant cations open framework polymers form which flocculate to yield opal ct opal ct becomes increasingly ordered primarily due to preferential growth of cristobalite relative to tridymite and crystal size increase opal ct dissolves to yield pore waters of low silica concentration which allows slow growth of quartz crystals from monomeric solution the quartz crystals then slowly increase in size and crystallinity carbonates appear to enhance opal ct formation possibly due to the activity of positively charged hydroxyl complexes hence polymerization in relatively pure systems is involved in opal a formation and polymerization in impure systems is involved in opal ct formation and slow growth from monomeric low silica concentration solutions is involved in precipitation of quartz in sedimentary realms

insight on gem opal formation in volcanic ash deposits from a supereruption a case study through oxygen and hydrogen isotopic composition of opals from lake tecopa california u s a

organic matter metabolizability andd calcium carbonate dissolution in nearshore marine muds abstract examination of intertidal mud flat and tidal creek sediments from sapelo island georgia and indian neck connecticut indicates that variations in the degree of calcium carbonate preservation exist between these localities although the preservation of calcium carbonate in modern nearshore terrigenous marine muds has previously been shown to be strongly dependent on the physical and biological attributes of the depositional environment berger and soutar 1970 sholkovitz 1973 aller 1982 the phenomenon documented in this study occurs in the absence of significant variations in benthic faunas or physical conditions evidence for variations in the degree of calcium carbonate preservation is provided by analyses of sediment chemistry and observations of mollusc shells recovered from he sediments the pattern of sediment chemistry and calcium carbonate preservation is shown to be related to seasonal climatic variations and the metabolizability of organic matter supplied to the sediments during the summer the presence of readily metabolizable organic matter in the connecticut sediments results in high rates of microbial decay establishment of anoxic sulfidic conditions at the sediment water interface and the production of large amounts of ferrous sulfide minerals during periods of reduced microbial activity winter ferrous sulfide minerals in the surficial sediments are oxidized resulting in dramatically lower in situ ph values and undersaturation with respect to calcium carbonate in georgia temperatures remain relatively warm throughout the year and therefore macrofau al and microbial activity do not exhibit the extreme seasonal variations present in connecticut because the destruction of reactive organic matter continues throughout the year the organic matter supplied to the georgia sediments is less readily metabolizable as a result the inception of anoxic sulfidic conditions is depressed below the sediment water interface only limited amounts of ferrous sulfide minerals are formed in the surficial sediments and saturation or supersaturation with respect to calcium carbonate is maintained throughout the year this relationship between organic matter metabolizability and calcium carbonate preservation has the potential to be used as an indicator of the environmental conditions present at the time of sediment deposition application of the results of this modern sediment study to facies dependent variations in calcium carbonate preservation in ancient marine shales suggests that prior interpretations of the paleoenvironmental significance of these variations may be in error

piecing together the puzzle of the ediacara biota excavation and reconstruction at the ediacara national heritage site nilpena south australia abstract patterns of origination and evolution of early complex life on this planet are interpreted largely from the fossils of the precambrian soft bodied ediacara biota excavation and reconstruction of beds of the ediacara member of the rawnsley quartzite at the national heritage ediacara fossil site nilpena in the flinders ranges of south australia has exposed nearly 300 square meters of fossiliferous bedding planes as a result the taphonomy and sedimentology of the succession are well constrained rendering it possible to disentangle ecological from environmental and taphonomic signals the fossil record that is preserved on these beds is a function of a number of factors including 1 sedimentary environment 2 initial colonization 3 nature of textured organic surfaces 4 duration of time between episodes of disturbance and 5 biostratinomic processes the excavation and reconstruction of beds at nilpena yield an exceptional and unique opportunity to examine not only the taxonomic composition of ediacara communities but also their ecological character at various stages of development preserved ecological snapshots of fossil assemblages range from immature communities of small bodied individuals associated with poorly developed organic mats to communities characterized by a high diversity of macrofaunal taxa wide range of body sizes and the presence of dense textured organic surfaces

a comparison of the palaeoclimate signals from diatom oxygen isotope ratios and carbonate oxygen isotope ratios from a low latitude crater lake the analysis of oxygen isotope ratios 18o from authigenic lake carbonates has become a well established palaeoclimate technique less common is the use of 18o in biogenic silica e g diatoms sponge spicules phytoliths but the technique is being increasingly utilised for non carbonate lakes this paper aims to compare 18odiatom with 18ocarbonate in a low latitude closed basin lake due to the influence of ph rarely are both carbonate and diatoms preserved in lake sediments in concentrations high enough to measure oxygen isotopes the 18o composition of both carbonate and silica should reflect the isotope composition and temperature of the lake water at the time of precipitation and hence theoretically they should be equivalent only one comparative study of 18odiatom and 18ocarbonate currently exists and that found differences between their 18o compositions probably because they precipitated in different seasons unless there is evidence to suggest that silica and carbonate precipitate at the same time their 18o records are likely to be different but complementary in this study we show that 18odiatom and 18ocalcite from lake tilo ethiopia show some similar broad climate trends however the 18odiatom curve is generally more variable and does not record two regional arid events picked up by the 18ocalcite data several possible reasons for this are discussed the precision of the 18odiatom extraction technique contamination of the diatom samples from tephra volcanic glass vital effects in the diatom samples differences in their respective equilibrium isotope fractionation rates and the possibility that the dominant diatom aulacoseira granulata and calcite precipitated in different seasons during this time currently the problems associated with the cleaning and extraction of 18odiatom suggests that details found in 18ocalcite records may be lost in 18odiatom records where contaminants such as tephra and clay minerals are difficult to remove

oxygen isotopes in fresh water biogenic opal northeastern us allerod younger dryas temperature shift the first oxygen isotope analysis of biogenic opal from lake sediments from the allerod younger dryas transition in a core from linsley pond connecticut gives an average estimate of a 6 c drop in temperature during the younger dryas this shift represents temperatures during the bloom season and may be less than the winter temperature drop the sharp transition itself with a duration of about 200 years suggests that the temperature decrease may have been as large as 12 c previous estimates of the allerod younger dryas temperature shifts are controversial and range from 3 20 c suggesting that further interdisciplinary research on the same samples is warranted

late glacial to middle holocene climatic record of lacustrine biogenic silica oxygen isotopes from a southern ocean island late glacial to middle holocene climatic record of lacustrine biogenic silica oxygen isotopes from a southern ocean island

mineralogical characterization of silica sinters from the el tatio geothermal field chile silica sinters deposited from alkali chloride waters in the el tatio geothermal field in northern chile 22 20 s 68 01 w have been characterized by xrd sem tg dta and ftir the dominant silica phase is opal a minor contents of opal a ct and opal ct are also present together with halite nacl sylvite kcl and realgar ass accessory phases include teruggite ca4mgas2b12o22 oh 12 12 h2o sassolite h3bo3 and detrital quartz sio2 fwhm values reflect the immature nature of the studied opal a dta heating experiments of opal a show cristobalite crystallization at 1000 c whereas dta cooling experiments show the cristobalite transformation at 200 c the total weight loss is related to changes in the sinter microtextures mineral phases and organic matter contents ftir spectra show the effects of silica maturation as consequence of the loss of trapped and absorbed water attached to silanols

oxygen isotope ratios of sedimentary biogenic silica reflect the european transcontinental climate gradient oxygen isotope ratios of biogenic silica offer considerable potential in tracing past climate change using both marine and lacustrine sediment records despite recent improvements and standardisation in preparatory and analytical methods evidence for a systematic relationship between climate parameters and the oxygen isotope composition of sedimentary diatom silica 18osilica is contradictory in particular observed differences between the 18osilica of living and sedimentary diatom silica in both lake and marine environments question the validity of applying isotope fractionation models derived from living diatoms to reconstructing climate from sediment records in order to address this issue oxygen isotope ratios of biogenic silica 18osilica are examined from a suite of lake surface sediment samples which span a large climatic gradient across western europe 18osilica values are strongly correlated with air temperature and the isotope composition of precipitation 18op when evaporation of lake waters is accounted for the time averaged surface sediment data suggest a fractionation of 35 between water and biogenic silica however the effect of temperature appears less marked than that observed between contemporary diatoms and lake water over short timescales suggesting that diagenetic processes may lead to a reduction in the temperature sensitivity of sediment 18osilica records comparison of 18osilica data with d analysed from n alkanes of aquatic origin n c17 n c21 and n c23 demonstrates that the slope of the regional 18o d water line is preserved within the sediment record offering exciting potential for palaeoclimate research

quantitative bedrock geology of brazil 1 we quantitatively analyze the area age distribution of sedimentary igneous metamorphic and ultramafic bedrock on the basis of data from the digital geologic map of brazil published as a gis map by the brazilian geological survey bedrock units exclusively encompassing sedimentary rocks igneous rocks or metamorphic rocks cover 40 4 31 5 and 17 7 respectively of the total bedrock area these numbers have to be considered minimum estimates of the areal abundance of sedimentary igneous and metamorphic bedrock because polygons defined by mixed lithologies cover 8 5 9 5 of the total bedrock area these mixed units are sedimentary rocks with igneous and or metamorphic contributions 1 4 metamorphic rocks with sedimentary contributions 1 2 metamorphic rocks with igneous contributions 1 5 igneous rocks with sedimentary and or metamorphic contributions 4 4 and ultramafic units with sedimentary igneous and or metamorphic contributions 1 2 the average ages of major lithologic units weighted according to bedrock area are as follows sedimentary rocks average stratigraphic age of 248 5 1 myr median stratigraphic age of 87 5 myr igneous rocks 1153 13 1 myr metamorphic rocks 1678 30 1 myr and ultramafic rocks 1227 25 1 myr the average bedrock age of brazil is 946 7 1 myr the range in lithologic composition and age structure of the various bedrock units reflects the complex tectonic makeup of brazil that ranges from neogene sedimentary cover in the amazon basin to precambrian cratons guyana and brazilian shields and transamazonian greenstone belts the average spatial resolution of the data is 232 km2 polygon 1 and is sufficient to perform area age analyses of individual river drainage basins larger than 5 000 km2

oxygen isotopes in marine diatoms a comparative study of analytical techniques and new results on the isotope composition of recent marine diatoms oxygen isotope analyses of marine diatoms were performed in two independent ways stepwise fluorination of hydrous opal a results in plateau 180 values representing the isotopic composition of the silica frame oxygen a method of controlled isotope exchange before silica dehydration also produces reliable results although the exchangeability of the silica was variable consequently a calibration of the isotope exchange method using the results from stepwise fluorination experiments is very useful and sometimes essential in order to select a water vapor of an appropriate isotopic composition to be used for equilibration sediment diatom samples ethmodiscus rex and thalassiothrix longissima from the antarctic and the north atlantic ocean respectively show strong 180 enrichments of 46 8 and 44 1 which are caused by large isotope fractionation occurring at the low temperature prevailing during silica water isotope exchange reactions however phytoplankton samples from surface waters of the norwegian greenland sea and the bellingshausen sea antarctica have 180 values between 30 4 and 35 0 thus the true silica water isotopic fractionation is approximately 3 to 10 lower than the temperature dependent silica water equilibrium published in the literature for sedimentary diatoms our results indicate that successive isotope exchange reactions of diatomaceous silica with ambient seawater and or pore water determine the isotope values of diatoms in sediments

mg ca ratios in the planktonic foraminifer neogloboquadrina pachyderma sinistral in the northern north atlantic nordic seas in core top samples in the nordic seas mg ca ratios of n pachyderma sin are generally consistent with previous high latitude calibration data but do not reflect the modern calcification temperature gradient from 2 c in the northwest to 8 c in the southeast this is because mg ca ratios in foraminiferal shells from the central nordic seas are 0 4 mmol mol higher than expected from calibrations of nurnberg 1995 and elderfield and ganssen 2000 the enhanced mg ca ratios are observed in an area with low sedimentation rates 5 cm kyr possible factors that may cause this include bioturbation holocene variability in old core tops dissolution pore water chemistry occurrence of volcanic ash and other natural variability the enhanced foraminiferal mg ca ratios in areas of the nordic seas and the northern north atlantic may also be linked with secondary factors related to the presence of fresher and colder water masses possibly combined with pore water chemistry in low sedimentation areas differing from high sedimentation areas

palaeo seasonality of the last two millennia reconstructed from the oxygen isotope composition of carbonates and diatom silica from nar golu central turkey carbonates and diatoms are rarely deposited together in lake sediments in sufficient quantities for the oxygen isotope composition 18o to be investigated simultaneously from both hosts here 18ocarbonate are compared to 18odiatom data from the varved sediments of nar golu a closed lake in central turkey over the last 1710 years lake monitoring suggests carbonate is probably precipitated during may june and 18ocarbonate is a proxy for regional water balance diatom activity is mainly weighted towards the spring at times between 301 and 561 ad while 18ocarbonate values are the highest for the entire 1710 year period suggesting summer drought 18ocorrected diatom values are among the lowest 18olakewater values estimated for the times of diatom growth and carbonate precipitation show large differences we suggest this could be explained by increased snowmelt that formed a freshwater lid on the lake at the time of peak diatom growth increased snowmelt is also inferred 561 801 ad from 801 ad to the present precipitation is less winter dominated although increased snowmelt is inferred 921 1071 ad and in the latter part of the little ice age i e the mid to late 1800s ad by combining oxygen isotope data from hosts that form in lakes at different times of the year we show that such analyses can provide insights into palaeo seasonality

silica diagenesis i solubility controls abstract the diagenetic sequence of silica is examined in terms of several thermodynamic and kinetic concepts the relationship between solubility and surface area or particle size is sufficient to explain simple opal a opal ct quartz transformations suggesting that the role of temperature and other physical parameters may not be as critical as previously thought complexation and adsorption in an impure system when added to the model allow for possibilities of opal a transformation directly to quartz of clay and zeolite formation from an opaline silica source and of variations in silica transformation rates due to presence of impurities in the system

seasonally resolved diatom δ18o records from the west antarctic peninsula over the last deglaciation understanding the response of the antarctic ice sheets during the rapid climatic change that accompanied the last deglaciation has implications for establishing the susceptibility of these regions to future 21st century warming a unique diatom 18o record derived from a high resolution deglacial seasonally laminated core section off the west antarctic peninsula wap is presented here by extracting and analysing single species samples from individual laminae season specific isotope records were separately generated to show changes in glacial discharge to the coastal margin during spring and summer months as well as documenting significant intra annual seasonal variability during the deglaciation with increased discharge occurring in summer relative to spring further intra seasonal variations are apparent between individual taxa linked to the environment that individual diatom species live in whilst deglacial 18o are typically lower than those for the holocene indicating glacial discharge to the core site peaked at this time inter annual and inter seasonal alternations in excess of 3 suggest significant variability in the magnitude of these inputs these deglacial variations in glacial discharge are considerably greater than those seen in the modern day water column and would have altered both the supply of oceanic warmth to the wap as well as regional marine atmospheric interactions in constraining changes in glacial discharge over the last deglaciation the records provide a future framework for investigating links between annually resolved records of glacial dynamics and ocean climate variability along the wap

holocene oxygen isotope record of diatoms from lake kotokel southern siberia russia and its palaeoclimatic implications the oxygen isotope composition of diatom silica d 18 odiatom from marine and lake sediments is helpful for the interpretation of the past climate and environments especially when complemented by other proxy records this paper presents a holocene oxygen isotope record of diatoms from lake kotokel located 2 km east of lake baikal in southern siberia russia the isotope record displays variations in d 18 odiatom from 23 7 to 30 3 from about 11 5 ka bp until today comparing the isotope composition of recent lake kotokel water mean d 18 o 12 to that of the most recent diatom sample d 18 o 27 5 an isotope fractionation in the right order of magnitude was calculated the kotokel d 18 o diatom record is controlled by changes in the isotopic composition of the lake water rather than by

quantitative bedrock geology of the conterminous united states of america 1 we quantitatively analyze the area age distribution of bedrock based on data from the most recent geologic map of the conterminous united states of america king and beikman 1974a 1974b made available in digital form by the united states geologic survey the area age distribution agrees surprisingly well with older data higgs 1949 but provides much higher temporal resolution the mean stratigraphic age of all sedimentary bedrock is 134 myr that of tertiary cambrian sediments is 104 myr the analysis also reveals area coverage of some minor lithologies such as ultramafic rocks that cover 0 15 of the conterminous united states area coverage of 162 lithostratigraphic units is made available as an excel data sheet

rapid coastal change during the mid to late holocene the record of barrier estuary sedimentation in the romney marsh region southeast england a common problem facing sea level researchers lies in determining the cause of rapid changes observed in coastal stratigraphic sequences such changes are commonly ascribed to the interaction of processes that operate over differing temporal and spatial scales i e rapid local and regional sea level trends and storm magnitude frequency at least some of the difficulty in distinguishing between these processes lies in the often limited stratigraphic database upon which palaeoenvironmental reconstruction is based here we present the results of detailed morphostratigraphic and micropalaeontological investigations from a series of sites located at the interface between the protective gravel barrier complex of dungeness and the barrier estuary sediments of romney marsh england a period of rapid coastal regression is identified between c 4500 and 3000 cal yrs bp during which time the sea level continued to rise but was outpaced by sediment accretion and peat accumulation a subsequent acceleration in th

seawater mediated interactions between diatomaceous silica and terrigenous sediments results from long term incubation experiments reactors containing frustules of the cultured diatom thalassiosira punctigera suspended in seawater were incubated with or without added sediment from the mississippi river delta or the congo river fan the diatom frustules were separated from the terrigenous sediments by a dialysis membrane thereby only allowing the exchange of dissolved species one series of incubations was carried out in the laboratory at room temperature 21 c and for a period of 10 months another series of reactors was deployed along a mooring in the mozambique channel at three water depths 500 1250 and 2000 m for a period of 22 months chemical analyses after total destruction of frustules collected at the end of the incubations showed elemental transfer from seawater mg and k and the sediments al fe mn p and ca to the frustules in the presence of the terrigenous sediments the dissolved silicate concentrations at the end of the incubations were systematically lower that those measured in the incubations without the sediments in addition electron microscopy revealed the formation of new mineral precipitates these included amorphous deposits on the frustules containing si fe al mg k and p as well as euhedral clay crystallites differences were observed between the incubations performed in the laboratory and those deployed at sea likely as a result of differences in redox conditions temperature and reaction time overall the interactions between biogenic silica seawater and lithogenic minerals reduce the regeneration of nutrient silicon fixed by siliceous organisms these interactions take place on relatively short time scales months to years and affect not only the marine cycle of silicon but also those of other major and minor elements such as al fe mn k and mg

tectonism and exhumation in convergent margin orogens insights from ore deposits abstract three widespread and abundant types of ore deposits epithermal silver gold ag au porphyry copper cu and orogenic gold au form by hydrothermal activity during tectonic convergence along continental margins and their preservation provides new insights into rates of those processes that form and destroy ore deposits as well as overlying rock ages for each of the three deposit types define lognormal age frequency distributions that are nearly identical in form but differ in their modal ages epithermal ag au porphyry cu and orogenic au deposits exhibit age modes at 3 11 and 199 ma respectively these ore types form at depths of 0 5 1 9 and 10 km respectively such that modal age increases with depth of ore emplacement qualitatively the dearth of deposits younger than their modal age reflects the fact that ore deposits form at depth within the crust and time is required for exhumation to expose them continued erosion serves to destroy ore bodies giving rise to fewer deposits

understanding the transfer of contemporary temperature signals into lake sediments via paired oxygen isotope ratios in carbonates and diatom silica problems and potential abstract although the oxygen isotope composition 18o of calcite 18ocalcite and to a lesser extent diatom silica 18odiatom are widely used tracers of past hydroclimates especially temperature and surface water hydrology the degree to which these two hosts simultaneously acquire their isotope signals in modern lacustrine environments or how these are altered during initial sedimentation is poorly understood here we present a unique dataset from a natural limnological laboratory to explore these issues this study compares oxygen and hydrogen isotope data 18o 2h of contemporary lake water samples at 2 weekly intervals over a 2 year period 2010 12 with matching collections of diatoms 18odiatom and calcite 18ocalcite from sediment traps at 10 m and 25 m at rostherne mere maximum depth 30 m a well monitored eutrophic seasonally stratified monomictic lake in the uk the epilimnion shows a seasonal pattern of rising temperature and summer evaporative enrichment in 18o and while there is a temperature imprint in both 18odiatom and 18ocalcite there is significant inter annual variability in both of these signals the interpretation of 18odiatom and 18ocalcite values is complicated due to in lake processes e g non equilibrium calcite precipitation especially in spring leading to significant 18ocalcite depletion and for 18odiatom by post mortem depositional and possibly dissolution or diagenetic effects for 2010 and 2011 respectively there is a strong temperature dependence of 18ocalcite and 18odiatom in fresh trap material with the fractionation slope for 18odiatom of ca 0 2 c in agreement with several other studies the 18odiatom data indicate the initiation of rapid post mortem secondary alteration of fresh diatom silica within 6 months with some trap material undergoing partial maturation in situ diatom 18o of the trap material is also influenced by resuspension of diatom frustules from surface sediments notably in summer 2011 with the net effect seen as an enrichment of deep trap 18odiatom by about 0 7 relative to shallow trap values contact with anoxic water and anaerobic bacteria are potentially key to initiating this silica maturation process as deep trap samples that were removed prior to anoxia developing do not show enrichment dissolution perhaps enhanced by anaerobic bacterial communities may also be responsible for changes to 18odiatom that lead to increasing but potentially predictable error in inferred temperatures using this proxy high resolution multi year monitoring can shed light on the complex dynamics affecting 18odiatom and 18ocalcite and supports the careful use of sedimentary 18odiatom and 18ocalcite as containing valuable hydroclimatic signals especially at a multi annual resolution although there remain substantial challenges to developing a reliable geothermometer on paired 18odiatom and 18ocalcite in particular 18odiatom needs cautious interpretation where silica post mortem secondary alteration is incomplete and diatom preservation is not perfect and we recommend dissolution be routinely assessed on diatom samples used for isotopic analyses

quantitative bedrock geology of east and southeast asia brunei cambodia eastern and southeastern china east timor indonesia japan laos malaysia myanmar north korea papua new guinea philippines far eastern russia singapore south korea taiwan thailand vietnam 1 we quantitatively analyze the area age distribution of sedimentary igneous and metamorphic bedrock based on data from the most recent digital geologic maps of east and southeast asia coordinating committee for coastal and offshore geosciences programmes in east and southeast asia ccop and the geologic survey of japan 1997 1 2 000 000 published as digital geoscience map g 2 by the geological survey of japan sedimentary rocks volcanic rocks plutonic rocks ultramafic rocks and metamorphic rocks cover 73 3 8 5 8 8 0 9 and 8 6 of the surface area respectively the average ages of major lithologic units weighted according to bedrock area are as follows sedimentary rocks average stratigraphic age of 123 myr median age of 26 myr volcanic rocks 84 myr 20 myr intrusive rocks 278 myr 195 myr ultramafic rocks unknown and metamorphic rocks 1465 myr 1118 myr the variability in lithologic composition and age structure of individual countries reflects the complex tectonic makeup of this region that ranges from precambrian cratons e g northeast china and north korea to mesozoic cenozoic active margins e g japan the philippines indonesia and new guinea the spatial resolution of the data varies from 44 km2 per polygon japan to 1659 km2 per polygon taiwan and is on average 490 km2 polygon similar to our previous analyses of the united states of america and canada the temporal and spatial resolution is sufficiently high to perform age area analyses of individual river basins larger than 10 000 km2 and to quantitatively evaluate the relationship between bedrock geology and river chemistry as many rivers draining tropical mountainous islands of east and southeast asia have a disproportionate effect on the dissolved and particulate load delivered to the world oceans bedrock geology in such river drainage basins disproportionately affect ocean chemistry

holocene sea level change in the severn estuary southwest england a diatom based sea level transfer function for macrotidal settings the recent growth in the use of microfossil based transfer functions in late quaternary sea level reconstructions reflects their potential to accurately quantify palaeo sea level changes this study details the development of a diatom based sea level transfer function for the severn estuary southwest england a macrotidal setting that experiences the second highest tidal range in the world this setting presents difficulties in representing the full tidal range from mean sea level msl to highest astronomical tide hat however two separate transects were merged successfully and a statistically significant relationship between contemporary diatom assemblages and altitude m o d was established a diatom based transfer function for palaeoaltitude was developed using weighted averaging wa tolerance downweighted weighted averaging wa tol and weighted averaging partial least squares wa pls wa tol produced the lowest prediction errors for altitude and the transfer function was applied to a fossil diatom data set from gordano valley a site adjacent to the severn estuary 2007 sage publications

a laminated sediment sequence from the northern north atlantic ocean and its climatic record a remarkable 3 m thick sequence of laminated diatom ooze ldo has been recovered in the north atlantic ocean the ldo is monospecific and consists of the pennate diatom thalassiothrix longissima biostratigraphy and benthic oxygen isotope stratigraphy indicate that the ldo was deposited during substage 5e eem in analogy to recent observations of massive surface concentrations of diatoms in the equatorial pacific and neogene examples we suggest that the ldo represents numerous depositional events of thalassiothrix mats that were physically accumulated at a subarctic convergence front the occurrence of the ldo combined with other climate proxies indicates substantial geographic stability of ocean fronts in the north atlantic during substage 5e

diatom oxygen isotopes evidence of a species effect in the sediment record diatom oxygen isotope measurements are commonly made on bulk mixed species assemblages due to the difficulty in purifying and separating individual taxa as such it is essential to understand processes in diatoms which may lead to isotope offsets both between and within individual species existing studies have suggested that mechanisms which may lead to isotopes offset in diatoms such as vital effects are either nonexistent or negligible here we present a suite of diatom oxygen isotope data from the onset of major northern hemisphere glaciation at odp site 882 in the northwest pacific ocean which display large offsets mean 1 23 max 3 51 error 0 84 between two different size fractions 75 150 um and 150 um that are dominated by only two species coscinodiscus marginatus and coscinodiscus radiatus these offsets are most likely size related although additional interspecies and intraspecies effects may also be important in determining the exact magnitude of the offsets consequently considerable care is needed when interpreting bulk diatom oxygen isotope data in relation to paleoenvironmental change especially when the amount of stratigraphical change within the isotopes is small

a review of the oxygen isotope composition of lacustrine diatom silica for palaeoclimate reconstruction diatom silica frustules are a form of biogenic opal and contain oxygen isotopes that are being increasingly used in palaeoclimate studies frustules are comprised of an inner tetrahedrally bonded silica skeleton si o si with an outer hydrous layer the hydrous layer is freely exchangeable and must be removed prior to oxygen isotope measurement using oxidising reagents and or high temperatures analysis of the oxygen isotope composition of diatom silica requires samples that are almost pure diatomite since extraction techniques will liberate oxygen from all the components in the sediment there is a generally acceptable protocol involving chemistry sieving and settling techniques and more recently laminar flow separation recent studies of lacustrine diatoms have shown that even a small proportion of contaminant can have a significant influence on the oxygen isotope value all lake sediments require their own specific procedure and every sample must be scrutinised by microscopy to check for the level of contamination prior to analysis where sediment cannot be purified sufficiently a semi quantitative assessment of the diatom content can facilitate mass balance techniques when robust preparation methods are used diatom silica offers an important palaeoclimate proxy providing an additional complementary rather than an alternative host of oxygen isotopes in carbonates in lacustrine isotope studies it is often assumed but rarely proven that diatom silica form in isotope equilibrium and hence empirically derived palaeotemperature equations usually made in vitro can be used most studies lack the detailed investigation of contemporaneous materials from natural waters of the region under study to test these equations further investigations need to be conducted in order to understand the systematic relationship between temperature water isotope composition and diatom isotope composition to enable quantitative interpretation of the sediment record experiments to test for vital effects and inter specific differences have so far revealed little variation it is useful to know when and where the diatom silica is formed in the modern environment so details of diatom ecology are important in the interpretation of isotope data here we show that valuable palaeoclimate data can be gained from the oxygen isotope composition of diatom silica 18odiatom especially since diatoms are abundant in many lakes sensitive to climate variation where other hosts e g carbonates are absent to date the most successful studies have been conducted in areas where the 18odiatom registers changes in the oxygen isotope composition of the lake water rather than temperature which is then related to other aspects of climate in these studies the range of values obtained is often greater than the error introduced by contaminating materials

ftir spectroscopic studies of rock sediments in namakkal tamil nadu south india for vegetations minerals of 15 rock sediments of kollimalai hills namakkal district tamil nadu south india were determined using ftir spectroscopy crystallinity extinction coefficient and relative distribution for different types of vegetation were analyzed owing to its importance and the suitability and those parameters were compared with those of many reported relevant data the possible vegetation of each city is interpreted in light of qualitatively and quantitatively observed measurements of relevant parameters such as crystallinity index relative distribution and extinction coefficients

concavity orientations of bivalve shells in estuarine and nearshore shelf sediments georgia box core x ray radiographs from estuarine and closely adjacent nearshore shelf sediments of georgia were used to study interrelationships among detrital valve orientations sediment textures and physical and biogenic sedimentary structures the cores represent preserved sediment sequences and thus should be more indicative of the rock record than shell orientations observed on substrate surfaces with little bioturbation physical processes mainly govern shell configurations horizontal concave down valves predominate in large and small scale crossbedded sands laminated sands and thin shell beds inclined concave down valves also are common together with lesser numbers of horizontal concave up valves vertical and inclined concave up valves predominate in thick shell beds shell filled burrows and thin shell beds consisting of shell ornamented diopatra tube fragments horizontal concave up valves also are common in each of these and inclined concave down valves are abundant in thick shell beds with progressively more intense bioturbation as is commonplace in estuarine and nearshore shelf sediments proportions of horizontal and inclined concave down valves decrease as those of vertical and inclined concave up valves increase the latter two becoming dominant the abundance of horizontal concave up valves increases initially but decreases between 90 and 100 percent bioturbation some of these results especially the predominance of vertical and inclined concave up valves differ from those of previous workers small sizes of dominant georgia bivalves probably account for much of the difference because of pronounced local variations large numbers of measurements also are imperative

physical and biogenic aspects of backbarrier sedimentary sequences georgia coast u s a abstract configuration of the present georgia coast with its broad expanses of salt marsh and intricate tidal drainages behind pleistocene holocene barrier islands implies the filling of an extensive backbarrier lagoonal system however riverine and salt marsh estuaries dominate the modern depositional setting and reconstructed vertical sequences suggest that backbarrier filling was accomplished by estuarine processes backbarrier sediments exhibit considerable diversity in grain size and physical and biogenic sedimentary structures principal sediment textures include clean to muddy coarse and fine sand mud and to a lesser extent shells pebbles and mud and bedrock clasts physical sedimentary structures many modified by bioturbation include large scale cross bedding trough and planar with graded sets small scale crossbedding ripple lamination and flaser wavy and lenticular bedding laminated to interbedded sand and mud and rare pebble or shell beds ebb oriented structures predominate over flood oriented ones except in upper reaches of riverine estuaries biogenic sedimentary structures typically are diverse and abundant characteristic lebensspuren include polychaete dwelling tubes decapod and polychaete burrows ray holes and bioturbate textures imparted by amphipods ophiuroids bivalves and anemones patterns of bioturbation are controlled to some extent by sediment type salinity levels and rates of deposition and physical reworking of sediments although intensity of bioturbation generally increases seaward bioturbate textures are more readily preservable in the lower energy middle reaches of estuaries because of numerous local pleistocene sediment sources and local variations in hydrographic regimes the stratigraphic record of salt marsh estuaries differs relatively little from that of riverine estuaries the basic sequence in ascending order typically consists of 1 channel lag and or channel fill deposits 2 estuarine accretionary beds 3 point bar tidal flat and shoal sediments and 4 channel margin and salt marsh deposits

isotope offsets in marine diatom δ18o over the last 200 ka previous work has suggested that a species effect may be present in diatom oxygen isotope ratios 18odiatom while predominantly attributed to be a size related species effect currently the precise mechanism remains unknown here two size fractions of diatoms 38 75 m and 100 m covering the last 200 ka are analysed for 18o from odp site 882 in the north west pacific ocean synchronous variations of up to 13 occur in both size fractions however large isotope offsets mean 2 02 exist between the two fractions with no relationship between their magnitude and the overlying palaeoenvironmental conditions in contrast to earlier work from the same site no one size fraction is constantly higher or lower in 18o relative to the other as such the dominant mechanism is most likely separate to the size effect previously detected in addition no relationship exists between the magnitude of the offsets and the species composition of the two size fractions the presence of these isotope offsets imposes significant constraints upon the future use of 18odiatom in palaeoceanographic reconstructions and reiterates the need to extract and analyse only species and size specific diatom samples

transfer and early diagenesis of biogenic silica oxygen isotope signals during settling and sedimentation of diatoms in a temperate freshwater lake lake holzmaar germany abstract we have investigated the transfer of oxygen isotope signals of diatomaceous silica 18odiatom from the epilimnion 0 7 m through the hypolimnion to the lake bottom 20 m in freshwater lake holzmaar germany sediment traps were deployed in 2001 at depths of 7 and 16 m to harvest fresh diatoms every 28 days the 7 m trap collected diatoms from the epilimnion being the main zone of primary production while the 16 m trap collected material already settled through the hypolimnion also a bottom sediment sample was taken containing diatom frustules from approximately the last 25 years the 18odiatom values of the 7 m trap varied from 29 4 in spring autumn to 26 2 in summer according to the temperature dependence of oxygen isotope fractionation and represent the initial isotope signal in this study remarkably despite the short settling distance 18odiatom values of the 7 and the 16 m trap were identical only during spring and autumn seasons while from april to september 18odiatom values of the 16 m trap were roughly 1 5 enriched in 18o compared to those of the 7 m trap isotopic exchange with the isotopically lighter water of the hypolimnion would shift the 18odiatom value to lower values during settling from 7 to 16 m excluding this process as a cause for the deviation dissolution of opal during settling with intact organic coatings of the diatom cells and near neutral ph of the water should only cause a minor enrichment of the 16 m values nevertheless opal from the bottom sediment was found to be 2 5 enriched in 18o compared to the weighted average of the opal from the 7 m trap thus resuspension of bottom material must have contributed to the intermediate 18odiatom signal of the 16 m trap during summer dissolution experiments allowed further investigation of the cause for the remarkably enriched 18odiatom value of the bottom sediment experiments with different fresh diatomaceous materials show an increase of opaline 18o at high ph values which is remarkably reduced when organic coatings of the cells still exist or at near neutral ph in contrast high ph conditions do not affect the 18odiatom values of sub fossil and even fossil opal ir analyses show that the 18o enrichment of the sedimentary silica is associated with a decrease in si oh groups and the formation of si o si linkages this indicates a silica dehydroxylation process as cause for the isotopic enrichment of the bottom sediment silica dissolution and dehydroxylation clearly induce a maturation process of the diatom oxygen isotope signal presumably following an exponential behaviour with a rapid initial phase of signal alteration the dynamics of this process is of particular importance for the quantitative interpretation of sedimentary 18odiatom values in terms of palaeothermometry

new isotopic temperature estimates for early silica diagenesis in bedded cherts when analyzed by a stepwise fluorination technique that reduces undesired oxygen contributions from hydroxyl and water opal ct samples from the siliceous member of the miocene monterey formation california yield 18 o values that contribute to mounting evidence that silica diagenesis in bedded chert can occur at much lower temperatures than previously supposed monterey opal ct 18 o values of 37 5 and 36 3 point to diagenetic temperatures at least as low as 17 21 c rather than the 35 50 c values normally inferred our results suggest that hydration waters can significantly reduce the bulk 18 o of opal ct and that special analytical techniques and consideration of water rock ratios are warranted for isotope thermometry off diagenetic silica transformations

from diatom opal a δ18o to chert δ18o in deep sea sediments abstract 18o signature of marine deep sea cherts was previously used to reconstruct past ocean temperature and oceanic bottom water 18o through the cenozoic and mesozoic periods oxygen isotopes in deep sea cherts which were never exposed to meteoric water exhibit a wide range of values indicating that the evolution and maturation of biogenic amorphous opal opal a to opal ct and microquartz chert is accompanied by isotopic changes we measured 18o of diatom opal a opal ct and microquartz chert in downcore profiles of sediment cores odp 795 and 799 from the sea of japan diatom opal a opal ct and microquartz chert were separated and purified from the bulk sediment this was followed by xrd sem eds and xps to identify and quantify the mineral phases the qualitative elemental concentrations and the amounts of authigenic clays diatom opal a transforms to opal ct and microquartz chert when it is deposited in sediment originally rich in diatoms relative to other lithologic constituents such as clay diatom opal a 18o is in the range of 34 38 18o of opal ct is 25 at the first phase transformation at 330 mbsf in odp 795 and 420 mbsf in odp 799 this phase transformation occurs at a temperature of about 40 c 18o of opal ct remains essentially unchanged until it undergoes the second phase transformation to microquartz chert at 470 mbsf in odp 795 and at 580 mbsf in odp 799 at which point its 18o decreases abruptly to 20 this phase transformation occurs at a temperature of about 60 c the 18o values of opal ct and microquartz chert appear to reflect equilibrium formation temperatures of silica corresponding to the geothermal gradient and the local porewater 18o the 18o of opal ct and microquartz chert are controlled by the geothermal temperatures and compositions of pore waters during polymorphic transformations deep within the sediment indicating that the 18o of these phases cannot be used to determine temperature or composition of seawater during diatom growth

oxygen isotopes of marine diatoms and relations to opal a maturation in order to develop the potential tool of diatom oxygen isotopes for paleoenvironmental studies we compared oxygen isotopes of natural marine diatoms sampled from ocean surface water sediment traps and surface sediments with oxygen isotopic fractionations determined for laboratory diatom cultures freshly grown natural diatoms phytoplankton samples and sediment trap material and cultured diatoms reveal similar oxygen isotope fractionation factors the fresh diatoms have 3 to 10 parts per thousand lower isotope fractionation factors than fossil sedimentary diatoms a temperature related oxygen isotope fractionation could not be established for the laboratory cultures and the natural phytoplankton samples and there is evidence that diatom growth rate until reaching the stationary growth state also controls the measured silica water oxygen isotope fractionation factor it is possible however that slow diatom growth in sea surface water may well lead to a temperature dependent silica water oxygen isotope fractionation which is the prerequisite for a use of diatom oxygen isotopes in palco surface water studies ftir spectroscopic analyses of various diatomaceous materials revealed that the ratio of integrated peak intensities for si o si si oh correlates with the 3 to 10 parts per thousand delta o 18 silica increase from fresh to fossil diatoms open system flow through silica dissolution experiments suggest that the diatom frustules are isotopically homogenous and that the increase in o 18 is therefore not due to dissolution of isotopically light surficial si oh groups it is concluded that slow internal condensation reactions during silica maturation in surface sediments cause both an increase in the intensity ratio of si o si si oh and the o 18 content of framework oxygen these findings also indicate that the oxygen isotope compositions of marine sediment diatoms do not indicate sea surface water temperature but rather reflect variable o 18 contents of surface sediments copyright c 2001 elsevier science ltd

silica gel formation during fault slip evidence from the rock record dynamic reduction of fault strength is a key process during earthquake rupture many mechanisms for causing coseismic weakening have been proposed based on theory and laboratory experiments including silica gel lubrication however few have been observed in nature here we report on the fi rst documented occurrence of a natural silica gel coating a fault surface the corona heights fault slickenside in san francisco california is covered by a shiny layer of translucent silica microstructures in this layer show fl ow banding armored clasts and extreme comminution compared to adjacent cataclasites the layer is composed of 100 nm to 1 m grains of quartz hydrous crystalline silica and amorphous silica with 10 100 nm inclusions of fe oxides and ellipsoidal silica colloids kinematic indicators and mixing with adjacent cataclasites suggest the shiny layer was fl uid during fault slip the layer therefore represents a relict silica gel that formed during fault motion and which could have resulted in frictional instability these observations confi rm that the silica gels formed in rock friction experiments do occur in natural faults and therefore that silica gel formation can act as a dynamic weakening mechanism in faults at shallow crustal conditions

biodeposition by the ribbed mussel geukensia demissa in a salt marsh sapelo island georgia abstract quantitative aspects of biodeposition by a common salt marsh invertebrate the ribbed mussel geukensia demissa were evaluated in a coastal salt marsh seasonal biodepositional rates calculated for the ponded water and transitional marsh two habitats in which mussels are typically abundant were based on 1 mussel density 2 duration of tidal inundation time available for suspension feeding and biodeposition and 3 in situ individual mussel biodepositional rates per season these rates compared with both contemporaneous physiochemical deposition and previous measurements of net seasonal sediment accumulation in nearby transitional and ponded water marsh habitats suggest that mussel biodeposition can account for nearly all net summer sediment accumulation there and for significant fractions of spring and autumn accumulation as well winter mussel biodeposition is negligible considerable local recycling of biodeposits and other sediment also is suggested by the study mussel biodeposition therefore constitutes a large proportion of the annual marsh sediment budget and is geologically important in the accretion of georgia salt marshes biodeposition probably is an important process in many other salt marshes as well

application of fourier transform infrared spectroscopy to silica diagenesis the opal a to opal ct transformation abstract an important goal in silica diagenesis research is to understand the kinetics of opal transformation from noncrystalline opal a to the disordered silica polymorph opal ct because the conventional technique for monitoring the transformation powder x ray diffraction xrd is applicable only to phases with long range order we used fourier transform infrared spectroscopy ftir to monitor the transformation we applied this technique combined with xrd and tem to experimental run products and natural opals from the monterey formation and from siliceous deposits in the western pacific ocean using a ratio of two infrared absorption intensities i472 cm 1 i500 cm 1 the relative prop rtions of opal a and opal ct can be determined the progress of the transformation is marked by changes in slope of vs depth or time when a sufficient stratigraphic profile is available there are three stages in the opal a to opal ct reaction 1 opal a dissolution 2 opal ct precipitation whose end point is marked by completion of opal a dissolution and 3 opal ct ordering during which tridymite stacking is eliminated in favor of cristobalite stacking

temporal record of osmium concentrations and 187os 188os in organic rich mudrocks implications for the osmium geochemical cycle and the use of osmium as a paleoceanographic tracer the final publication is available at elsevier via https doi org 10 1016 j gca 2017 06 046 2017 this manuscript version is made available under the cc by nc nd 4 0 license http creativecommons org licenses by nc nd 4 0

temperature dependence of the oxygen isotopic fractionation between diatom silica and water abstract we present a new paleotemperature scale based on the oxygen isotopic ratio of the non exchangeable fraction of the oxygen from diatom silica the equation t 17 2 2 4 18osilica 18owater 40 0 2 18osilica 18owater 40 2 was derived using recent sediment samples from different oceanic areas the temperature and isotopic composition of the local surface water comparison of our results with other relationships established for quartz water or amorphous silica water at higher temperature suggests no difference in isotopic fractionation between quartz water and biogenic silica water couples

selective dissolution of siliceous microfossils observed in a box core from the north east equatorial pacific a box core taken at 11 50 3 n and 137 28 2 w in the central pacific manganese nodule province was studied to determine the pattern of diatom and radiolarian preservation with depth in the sediment as well as to observe downcore variations in clay mineralogy we observed marked deterioration of the siliceous microfossils within the upper 30 cm of this sediment over this depth interval the quaternary diatoms disappear first followed deeper downcore by the dissolution of quaternary radiolarians tertiary microfossils in general were the most corrosion resistant and the residual microfossil assemblage in the lower part of the core consisted of fragmented robust tertiary forms consequently the apparent biostratigraphical age of the sediment appeared much greater than the age suggested by mineralogical and radioisotopic data

cryogenic opal a deposition from yellowstone hot springs sub zero winter temperatures on the yellowstone plateau alter the opal a precipitation pathway offluid erupting from hot springs and geysers frozen fluid often only meters from boiling pools contains abundant opal a particles comprising sheet and filamentlike aggregations of opal a microspheres which are formed by opal a precipitation in brine pockets channels and veins by natural cryogelling unconsolidated cryogenic opal a sediment accumulates in and below water ice where it is locked until spring thaw conditions sediment is then either remobilized contributing large volumes of opal a particulate to geothermally influenced wetlands or becomes adhered in situ by dehydration and cementation this strongly seasonal opal a precipitation regime has been overlooked in investigations of sinter deposition accretion rates and microbe mineral interactions natural opal a textures recorded fromyellowstonemaybereplicatedsimplybyfreezingandthawingsyntheticsilica saltsolutioninthelaboratory cryogenicprocess may have influenced mineral precipitation and sediment accumulation in many other geothermal areas particularly active terrestrial springs located at high altitude latitude fossil systems influenced by ancient glaciations plus potential astrobiological targets e g mars and europa 2007 elsevier b v all rights reserved

a 13 600 year diatom oxygen isotope record from the south carpathians romania reflection of winter conditions and possible links with north atlantic circulation changes this study provides a continuous lateglacial and holocene record of diatom silica oxygen isotope changes 18odiat in a subalpine lake sediment sequence obtained from the retezat mts taul dintre brazi 1740 m a s l this through flow shallow high altitude lake with a surface area of only 0 4 ha has short water residence time and is predominantly fed by snowmelt and rainwater its 18odiat record principally reflects the oxygen isotope composition of the winter and spring precipitation as diatom blooms occur mainly in the spring and early summer hence changes in 18odiat are interpreted as seasonal scale changes in the amount of winter precipitation low oxygen isotope values 27 28 5 occurred during the lateglacial until 12 300 cal bp followed by a sharp increase thereafter in the holocene 18odiat values ranged from 29 to 31 until 3200 cal bp followed by generally lower values during the late holocene 27 30 short term decreases in the isotopic values were found between 10 140 9570 9000 8500 7800 7300 6300 5800 5500 5000 and at 8015 4400 4000 cal bp after 3200 cal bp a decreasing trend was visible with the lowest values between 3100 2500 and after 2100 cal bp the general trend in the record suggests that contribution of winter precipitation was generally lower between 11 680 and 3200 cal bp followed by increased contribution during the last millennia the late holocene decrease in 18odiat shows good agreement with the speleothem 18o lake level and testate amoebae records from the carpathian mountains that also display gradual 18o decrease and lake level mire water table level rise after 3200 cal bp strong positive correlation with north atlantic circulation and solar activity proxies such as the austrian and hungarian speleothem records furthermore suggested that short term increases in the isotopic ratios in the early and mid holocene are likely connectable to high solar activity phases and high frequency of positive north atlantic oscillation indexes that may have resulted in decreased winter precipitation in this region

a laser fluorination method for oxygen isotope analysis of biogenic silica and a new oxygen isotope calibration of modern diatoms in freshwater environments abstract an empirical calibration for the oxygen isotope fractionation between biogenic silica and water was determined for diatom frustules sampled from living diatom communities in the jemez mountains of northern new mexico usa over a temperature range from 5 1 to 37 8 c the silica water fractionation is defined by the equation 1000 ln silica water 2 39 0 13 106t 2 4 23 1 49 this relationship is in close agreement with other published silica water fractionation factors for laboratory cultured diatom samples however it is as much as 8 lower than equilibrium quartz water fractionations and 3 4 lower than observed silica water fractionations in diatomaceous silica collected from sediment traps and sediment cores there are three possible explanations for the disparate silica water fractionation factors observed in diatom silica 1 silica does not precipitate in equilibrium with ambient water 2 silica does precipitate in equilibrium with ambient water but the silica water fractionation factor for diatom silica is considerably less than the equilibrium fractionation factor for quartz water or 3 silica precipitation is influenced by a vital effect where the 18o value of the water inside the diatom cell walls is lower than the 18o values of ambient water post mortem loss of organic material results in an alteration or maturation of diatom silica in which silica reequilibrates with a silica water fractionation closer to the equilibrium quartz water fractionation alteration is likely to occur rapidly after the diatom frustule loses its organic coating either as it settles through the water column or at the sediment water interface 18o values recorded by paleo diatom silica therefore do not record growing conditions but more likely record conditions at the sediment water interface in the case of lacustrine environments where the bottom water remains at a nearly constant 4 c the reequilibration of diatom silica with bottom conditions could reduce or remove the conflating effects of temperature on 18o values recorded by paleo diatom silica and provide direct information on the 18o value of the lake water

palaeoclimate interpretation of stable isotope data from lake sediment archives the isotope composition of authigenic and biogenic carbonates and diatom silica are commonly used as palaeoclimate proxies from lake sediments this article reviews the controls on the isotope composition of lacustrine skeletal and non skeletal deposits and illustrates how stable isotope studies contribute to an understanding of changes in temperature precipitation patterns evaporation and the carbon cycle it highlights the differences in the palaeoclimate potential of a wide range of lakes ranging from open to closed lake basins a large number of the case histories but not all are drawn from studies of temperate lakes from europe large closed lake systems in the tropics and elsewhere lose water predominantly through evaporation and contain sediments with variable and generally high 18o values fluctuations in the isotope composition of authigenic or biogenic minerals are mainly a function of long term changes in the precipitation evaporation ratio in contrast small open lakes which have a degree of through flow typically contain sediments with 18o values that vary by no more than a few these variations are generally ascribed to variations in temperature or the isotope composition of precipitation p from which either an annual or seasonally specific signal can be gained these types of lakes are common in northern europe and at high altitudes the interpretation of isotope data from a lacustrine succession requires a knowledge of the local processes that might control and modify the signal their effects need to be quantified and a robust calibration using the modern lake system is necessary to establish the relationship between the measured signal the isotopic composition of the host waters and climate

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

dehydroxylation and diagenetic variations in diatom oxygen isotope values abstract numerous studies have documented changes in the dissolution and reactivity of biogenic silica as it is transferred from the water column to sediment archives here we present the first experimental data that demonstrate a physical mechanism by which the oxygen isotope 18osil values of biogenic silica diatoms are altered during early diagenesis the 18osil value of diatom silica cultured at 19 3 c was 31 9 0 2 n 6 the same silica experimentally aged in an artificial seawater media at near silica saturation at 85 c had an average 18osil value of 27 1 0 6 n 20 the most significant change in the 18osil value was coincident with an initial reduction in the total silanol abundance indicating that the timing of dehydroxylation reactions in natural sedimentary environments is associated with diagenetic changes in the recorded 18osil values the rate of change in the experimental aging environment at 85 c was rapid with significant changes in both silanol abundance and 18osil values additionally the silica water fractionation relationship recorded by the experimentally aged samples approaches the equilibrium quartz water fractionation factor the linear rate law was used to estimate the timing of these changes in low temperature environments the initial and most significant change in silica reactivity and 18osil values is likely to occur on the order of 10 s of years at 4 c published silica water fractionation factors for sedimentary diatoms most likely represent a combination of growth and diagenetic environments and the 18o value of diagenetic water needs to be addressed when using 18osil values to reconstruct paleoceanographic and paleoenvironmental conditions

combined oxygen and silicon isotope analysis of diatom silica from a deglacial subarctic pacific record we present an sif4 separation line coupled to a laser fluorination system which allows for an efficient combined silica d18o and d30si analysis 50 min per sample the required sample weight of 1 5 2 0mg allows for high resolution isotope studies on biogenic opal besides analytical tests the new instrumentation set up was used to analyse two marine diatom fractions 63 mm 10 20mm with different diatom species compositions extracted from a bolling allerod holocene core section md01 2416 north west nw pacific to evaluate the palaeoceanographic significance of the diatom isotopic signals and to address isotopic effects related to contamination and species related isotope effects vital and environmental effects while d30si offsets between the two fractions were not discernible supporting the absence of species related silicon isotope effects systematic offsets occur between the d18o records although small these offsets point to species related isotope effects as bias by contamination can be discarded the new records strengthen the palaeoceanographic history during the last deglaciation in the nw pacific characterized by a sequence of events with varying surface water structure and biological productivity with such palaeoceanographic evolution it becomes unlikely that the observed systematic d18o offsets signal seasonal temperature variability this calls for reconsideration of vital effects generally excluded to affect d18o measurements

online oxygen isotope analysis of sub milligram quantities of biogenic opal using the inductive high temperature carbon reduction method coupled with continuous flow isotope ratio mass spectrometry we present a method for determining sub milligram quantities of biogenic opal the method employs the inductive high temperature carbon reduction method for dehydration of opal and reduction of silica and a continuous flow isotope ratio mass spectrometry system for direct analysis of the oxygen isotope ratio in the evolved carbon monoxide the accuracy and precision of the online analytical system were evaluated by isotopic analysis of various quantities of standard sio2 in the range 40 538 g the time required to analyse a single sample was relatively short 50 min hence our method is suitable for routine analysis for paleoenvironmental studies that require large amounts of time series data the method was validated for samples in the sub milligram range and can be applied to oxygen isotope analysis of various types of biogenic opal that have not been analysed because of their small amounts in natural samples we successfully applied our method to i the first oxygen isotope analysis of monospecific radiolarian skeletons and ii high resolution oxygen isotope analysis of holocene diatom frustules from the southern ocean

an excess 226ra chronology for deep sea sediments from saanich inlet british columbia abstract to further explore the efficacy of 226 ra excess dating for deep sea sediments previously dated varve sediments from saanich inlet were investigated ages obtained using 226 ra excess are comparable to the varve ages in the upper 20 25 m of the sedimentary record but radiometric ages for those sediments older than c 4000 yr bp are significant underestimates this results from major changes in sedimentation within saanich inlet around 4000 yr bp linked to rising sea levels with younger sediments characterised by a higher biogenic contribution resulting from the establishment of an anoxic fjord environment the older sediments were deposited in a shallow water inlet characterised by variable ra mass balance and non radiogenic losses therefore while 226 ra excess can produce reliable dates its application may be limited where the relative significance of authigenic and allogenic input and bottom water anoxia has been variable and where closed system behaviour is compromised

century to millennial scale climatic variability in lake malawi revealed by isotope records diatom based oxygen isotope data 18odiatom from lake malawi show multi centennial scale wet dry intervals spaced approximately every 2 3 ka throughout a 25 ka sequence the 18odiatom record is supported by a lower resolution deuterium dpa isotope curve derived from palmitic acid we interpret these isotope data in terms of major shifts in precipitation and evaporation moderated by seasonal controls on the host organisms dry periods marked by relatively positive isotope values represent the extension of abrupt holocene events noted from northern and equatorial africa to 10 15 s these events in lake malawi correspond to cool episodes in greenland thereby demonstrating teleconnections generated by meridional temperature gradients sea surface temperatures are likely to be the primary transmitter of deglacial climate changes although trade wind strength and direction is critical in controlling precipitation patterns in tropical regions conversely the global hydrological cycle driven by low latitude regions represents an important positive feedback amplifying deglacial processes

lingulid brachiopods and lingulichnus from a silurian shelf slope carbonate sequence delorme group mackenzie mountains northwest territories structures assigned to the ichnogenus lingulichnus commonly containing the burrow forming brachiopod were collected from an argillaceous unit in a silurian shelf dolostone sequence delorme group the sediments benthic fauna and inclined orientation of all lingulid burrows indicate the presence of a bottom current and restricted conditions on the margin of the selwyn basin high abundance low diversity faunas of lingulids and lingulichnus are indicative of limiting conditions at the substrate surface under low energy conditions they seem to be indicative of the zoophycos ichnofacies

experimental diatom dissolution and the quantification of microfossil preservation in sediments abstract four laboratory experiments on fresh modern diatoms collected from lakes in the northern great plains of north america were carried out to assess the effects of dissolution on diatom abundance and composition marked differences in mean dissolution susceptibility exist between species despite sometimes significant intra specific variation between heterovalves twenty four taxa were ranked according to susceptibility to dissolution using an exponential decay model of valve abundance this dissolution ranking was used to derive two weighted indices of sample preservation a third index f was based on a simple binary classification of valve morphology into dissolved and pristine categories as distinguished by light microscopy lm when compared against rank indices and a measure of species diversity this diatom dissolution index was found to be the best predictor of the progress of dissolution as estimated by total valve abundance or biogenic silica bisi loss strong empirical relationships between f index values and diatom abundance r2 0 84 n 32 and bisi r2 0 89 n 32 were developed and applied to a diatom sequence from a short core from devils lake north dakota and compared to diatom inferred and observed salinity at this site the f index is a simple effective diagnostic tool to assess important aspects of diatom preservation the index can provide insight into si cycling and record changes in conditions pertinent to diatom dissolution and has a role in validation of transfer functions or other inferences derived from compositional data

rapid post mortem maturation of diatom silica oxygen isotope values 1 oxygen isotope values of diatom silica 18odiatom are increasingly used as paleoclimate proxies however the magnitude and timing of post mortem alteration of 18odiatom values has been unclear in freshwater diatom silica from a human made pond in northern new mexico post mortem alteration of 18odiatom values occurs within one year of sediment burial diatom silica collected antemortem has an average 18odiatom value of 21 5 vsmow 1 3 n 23 whereas diatom silica from two sediment cores from the same freshwater environment records significantly higher 18odiatom values 28 9 0 8 n 13 the difference in ante and post mortem 18odiatom values indicate post mortem alteration of diatom silica oxygen that results in a 7 increase in 18o values this study demonstrates that 18odiatom values reach mature values within 0 5 years of frustule death initial diatom 18o values that record growing conditions are rapidly overwritten during silica maturation and the mature diatom 18o values approach isotopic equilibrium for quartz water the rapid post mortem alteration of diatom 18o values explains much of the disparate data regarding silica water fractionation for diatom silica and has a profound effect on the use of diatom silica 18o values as a paleoclimate proxy in lacustrine and marine environments

deglacial subarctic pacific surface water hydrography and nutrient dynamics and links to north atlantic climate variability and atmospheric co2 the glacial to holocene evolution of subarctic pacific surface water stratification and silicic acid si dynamics is investigated based on new combined diatom oxygen 18odiat and silicon 30sidiat isotope records along with new biogenic opal subsurface foraminiferal 18o alkenone based sea surface temperature sea ice diatom and core logging data from the ne pacific our results suggest that 18odiat values are primarily influenced by changes in freshwater discharge from the cordilleran ice sheet cis while corresponding 30sidiat are primarily influenced by changes in si supply to surface waters our data indicate enhanced glacial to mid heinrich stadial 1 hs1 ne pacific surface water stratification generally limiting the si supply to surface waters however we suggest that an increase in si supply during early hs1 when surface waters were still stratified is linked to increased north pacific intermediate water formation the coincidence between fresh surface waters during hs1 and enhanced ice rafted debris sedimentation in the north atlantic indicates a close link between cis and laurentide ice sheet dynamics and a dominant atmospheric control on cis deglaciation the bolling allerod b a is characterized by destratification in the subarctic pacific and an increased supply of saline si rich waters to surface waters this change toward increased convection occurred prior to the bolling warming and is likely triggered by a switch to sea ice free conditions during late hs1 our results furthermore indicate a decreased efficiency of the biological pump during late hs1 and the b a possibly also the younger dryas suggesting that the subarctic pacific has then been a source region of atmospheric co2

dissolution and preservation of antarctic diatoms and the effect on sediment thanatocoenoses abstract comparison of southern ocean diatom populations from i surface ocean production ii underlying antarctic sediments and iii laboratory dissolution experiments demonstrates that dissolution can account for the temporal and spatial variations in sedimentary diatom assemblages observed in southern ocean sediments increasing dissolution causes relative depletions in n kerguelensis k enrichments in t lentiginosa l and slight enrichments in e antarctica a this reflects the relative susceptibility to dissolution of the three species that dominate antarctic sediments we have devised a preservation index for the southern ocean based on the ratio k k l to estimate relative extents of dissolution and applied it to natural assemblages holocene southern ocean sediments display increasing opal preservation toward higher latitudes but south of the antarctic polar front preservation decreases in the order well preserved se indian s atlantic sw indian se pacific poorly preserved dissolution also accounts for the pattern of diatom assemblages in the last glacial maximum lgm sediments of the indian and pacific sectors but in the atlantic increased e antarctica abundances at lgm must have resulted from an increase in surface ocean production of this species holocene and lgm diatoms in atlantic and pacific sector sediments are equally well preserved but in the indian sectors holocene sediments are better preserved than those of lgm age paleoceanographic and paleoclimatic transfer functions derived from factor analyses of variations in the sedimentary abundances of these three diatoms have ignored the effects of differential dissolution on thanatocoenosis and thus should be interpreted with caution

the silicon isotope record of early silica diagenesis the heavy isotopes of silicon are strongly enriched in some of the youngest early diagenetically formed porcellanite layers from the southwest indian ridge pleistocene and the maud rise pliocene these porcellanite layers are composed of opal ct and were formed by the conversion of amorphous silica opal a from siliceous sediment via dissolution reprecipitation their bulk si30 values range between 1 7 and 2 3 detritus poor siliceous sediment surrounding these layers is significantly lower at 0 3 to 1 5 sequential chemical extractions of bulk siliceous sediment show i preferential dissolution of diatoms featuring higher si30 than radiolaria and al si components the detailed investigation of porcellanite layers by micro scale si isotope and al si analyses using uv femtosecond laser ablation icp mass spectrometry show that ii precipitation of authigenic aluminum silicates enriched in light si isotopes drives pore waters to even higher si30 we suggest that the same processes redistributed stable silicon isotopes in precursor siliceous sediments of ancient chert we infer that past environmental conditions can be reconstructed with high fidelity from the stable si isotope composition of chert when initial seawater si concentrations were high such as in the precambrian exchange of si between layers during phase transformation from opal a to opal ct and from opal ct to quartz is impeded when variable amounts of detrital minerals are present because they control rates of silica phase transformation and hence the timing of dissolution reprecipitation during burial

diatom δ18o evidence for the development of the modern halocline system in the subarctic northwest pacific at the onset of major northern hemisphere glaciation establishing a time frame for the development of the modern halocline and stratified water column in the subarctic north pacific has significant paleoclimatic implications here we present a d18o diatom record consisting of only two species that represents autumn winter conditions in the region across the onset of major northern hemisphere glaciation boundary at circa 2 73 ma a decrease in d18o diatom of 4 6 occurs whereas previously published d18o foram results show a 2 6 increase the d18o diatom and uk37 sea surface reconstructions indicate both a significant freshening of 2 4 practical salinity units and an increase in surface temperatures in the summer to early winter period from circa 2 73 ma onward in contrast the concomitant increase in d18o foram is likely to be reflective of conditions beneath the mesothermal structure and or spring conditions when warmer sea surface temperatures are not present in the region these results are consistent with the development of the modern halocline system at 2 73 ma with year round stratification of the water column and a strengthened seasonal thermocline during the summer to early winter period resulting in one of the largest summer to winter temperature gradients in the open ocean the onset of stratification would also have led to a warm pool of surface water from circa 2 73 ma which may have provided a potential source of extra moisture needed to supply the growing north american ice sheets at this time

quantitative bedrock geology of alaska and canada 1 we quantitatively analyze the area age distribution of sedimentary igneous and metamorphic bedrock on the basis of data from the most recent geologic maps of alaska beikman 1980 1 2 500 000 and canada douglas 1969 updated 1 5 000 000 made available in digital form by the u s geologic survey and the geologic survey of canada national resources canada respectively sediments cover 72 9 and 52 4 of the surface of alaska and canada volcanic rocks comprise 11 6 and 6 of the surface area in alaska and canada respectively whereas intrusive rocks cover 6 6 and 24 of the surface respectively ultramafic rocks account for 0 20 and 0 08 of the bedrock area whereas metamorphic rocks cover 3 4 and 16 1 the average ages of major lithologic units weighted according to bedrock area are significantly younger in alaska than in canada marine sediments stratigraphic age of 206 myr in alaska versus 599 myr in canada continental sediments stratigraphic age of 81 myr versus 150 myr volcanic rocks 126 myr versus 1377 myr intrusive rocks 114 myr versus 2109 myr ultramafic rocks 261 myr versus 874 myr and metamorphic rocks 474 myr versus 2545 myr this age difference reflects the contrast between the young active margin influence on the geology of alaska and the old cratonic nature of much of the bedrock exposed in canada glacial erosion and the resulting exposure of precambrian bedrock primarily in canada contributes to this age contrast the average temporal and spatial resolution of the digital data is sufficiently high to perform age area analyses on individual river basins larger than 20 000 km2 and to evaluate the relationship between bedrock geology and river chemistry

fe si oxyhydroxide deposits at a slow spreading centre with thickened oceanic crust the lilliput hydrothermal field 9 33 s mid atlantic ridge diffuse and focused low temperature fluids emanate at 9 33 s mid atlantic ridge and precipitate fe sioxyhydroxides that form chimneys mounds and flat lying deposits this extensive vent field named lilliput lies at the axial zone of a spreading segment with a significantly thickened crust 11 km theoretically much more heat needs to be removed from a thick crust spreading center compared to a spreading center with typical thickness of 6 km therefore settings with thickened crust should be favourable for supporting very powerful hydrothermal systems capable of producing large mineral deposits this is the first report on the composition of seafloor hydrothermal deposits at abnormally thickened oceanic crust due to hotspot ridge interaction our studies revealed that generally the lilliput hydrothermal deposits are very similar in morphology structure composition and lateral extent to other low temperature hydrothermal deposits of mid ocean ridges and intraplate volcanoes deposits at the lilliput vent field are composed of si containing goethite and ferrihydrite have very low contents of a number of transition and rare earth elements and show ree distribution patterns with negative ce and eu anomalies the speciation and precipitation of the main deposit forming elements fe and si at the hydrothermal field appear to be partially controlled by live microbes and exuded organic compounds the 18o values of the precipitated silica containing feoxyhydroxides point to low temperature formation and sr nd pb isotope variations suggest that the hydrothermal precipitates scavenged metals predominantly from the ambient seawater these findings are in agreement with the biogeochemical scenario for their precipitation

quantitative bedrock geology of the continents and large scale drainage regions we quantitatively analyze the area age distribution of sedimentary extrusive volcanic and endogenous plutonic and or metamorphic bedrock on the basis of data from the most recent digital geological map of the world at a scale of 1 25 000 000 the spatial resolution of the digital bedrock data averages 13 905 km2 per polygon comparison of certain regions of the world previously analyzed at higher spatial resolution with the low resolution world data reveals general consistency in the areal exposure of major rock types as well as a minor systematic bias toward older average bedrock ages in the global data set application of the global bedrock data to 19 large scale drainage regions and three large internally drained regions reveals considerable regional variability of earth s bedrock geology that is consistent with the dominant geotectonic setting of the respective drainage region

diagenetic cycling of nutrients in seafloor sediments and the carbonate silica balance in a paleozoic cool water carbonate system sverdrup basin canadian arctic archipelago abstract seafloor processes are often destructive of biogenic remains while at the same time promoting growth of authigenic minerals such as hematite fluorapatite glauconite and chalcedony middle to late permian sedimentary rocks in the sverdrup basin are cool water marine sandstones shales limestones and cherts rich in carbonate and siliceous biotic components the authigenic minerals hematite fluorapatite glauconite and chalcedony are abundant from the interactions of nutrient elements silica phosphorus and iron with carbonate and siliceous biotas in ramp environments of variable energy and sediment accumulation rate these authigenic phases represent in part faunas that are no longer preserved and should not be viewed simply as diagenetic products early middle permian siliciclastic rocks the assistance formation contain carbonate heterozoan biotas with widespread siderite and hematite and local void filling phosphate deposited under moderate sediment accumulation rates sponge spicules were dissolved unless encased in hematite or siderite and incorporated in silica cement in contrast more slowly deposited younger middle permian inboard cherty limestones the lower trold fiord formation underwent frequent physical and biological reworking resulting in dissolution of biogenic silica and precipitation of glauconite and carbonate fluorapatite sediment reworking also excavated and produced rounded glauconite and phosphate clasts much of the glauconite was then oxidized to hematite on the seafloor again unless encased in phosphate sponge spicules were dissolved and the silica precipitated as glauconite or chalcedony coeval deeper water phosphatic cherty limestones the degerbols formation do however contain sponge spiculites reduced sediment reworking led to retention of silica in pore fluids and slowed dissolution of sponge spicules resulting in local preservation of sponge spiculite facies late permian organic rich cherts the lindstrom formation with prolific siliceous sponges and a less plentiful carbonate biota accumulated relatively slowly sponge spicules were preserved and carbonate biota partially dissolved in these organic rich deposits implying that the biota had to be overwhelmingly dominated by siliceous components to overcome ambient silica undersaturation in pore fluids each succession has a distinct composition depending upon the rate of sediment accumulation the original benthic biota and the nature and abundance of silica phosphate and iron in seawater at any given time these characteristics controlled the formation of authigenic minerals and the preservation or dissolution of carbonate and silica biotas

a combined oxygen and silicon diatom isotope record of late quaternary change in lake el gygytgyn north east siberia determining the response of sites within the arctic circle to long term climatic change remains an essential pre requisite for assessing the susceptibility of these regions to future global warming and arctic amplification to date existing records from north east russia have demonstrated significant spatial variability across the region during the late quaternary here we present diatom 18o and 30si data from lake el gygytgyn russia and suggest environmental changes that would have impacted across west beringia from the last glacial maximum to the modern day in combination with other records the results raise the potential for climatic teleconnections to exist between the region and sites in the north atlantic the presence of a series of 2 3 decreases in 18odiatom during both the last glacial and the holocene indicates the sensitivity of the region to perturbations in the global climate system evidence of an unusually long holocene thermal maximum from 11 4 7 6 ka bp is followed by a cooling trend through the remainder of the holocene in response to changes in solar insolation this is culminated over the last 900 years by a significant decrease in 18odiatom of 2 3 which may be related to a strengthening and easterly shift of the aleutian low in addition to possible changes in precipitation seasonality

the effect of progressive dissolution on the oxygen and silicon isotope composition of opal a phytoliths implications for palaeoenvironmental reconstruction abstract the 30 si and 18 o values of silica phytoliths precipitated in plants can be applied to palaeoenvironmental reconstructions this study examines the effect of partial dissolution of phytoliths on their isotopic compositions and physical characteristics specific surface area mean particle size and discusses problems with paleoclimate reconstructions using the 18 o and 30 si values of phytoliths that have been modified in soils dissolution experiments were conducted in batch reactors under a range of ph 4 10 and temperature 4 44 c conditions the 18 o and 30 si values of phytoliths behaved similarly as dissolution progressed with values increasing until the solutions were approximately 30 40 saturated with silicic acid during this phase the isotopic composition of the remaining silica was primarily affected by dissolution reactions that preferentially removed the light isotopes 16 o and 28 si partial dissolution resulted in phytolith 18 o values that were up to 3 9 higher and 30 si values that were up to 0 63 higher than unaltered material after 30 40 saturation is reached precipitation of new silica begins to reduce the 18 o and 30 si values of remaining silica despite a net dissolution the 30 si values of precipitated silica were determined by the 30 si value of silicic acid previously released by dissolution the 18 o values of precipitated silica were determined by the 18 o value of water and the temperature of the experiment silica precipitated in isotopic equilibrium with water acted to reduce phytolith 18 o values as the solution approached saturation in soil environments such reactions may result in a silica coating on the phytolith that has a 18 o value in equilibrium with soil water confounding paleoclimate estimates this study demonstrates that assessment of the extent of post depositional alteration of soil phytolith assemblages is essential prior to the use of their isotopic compositions in paleoclimate models

distinguishing peperite from other sediment matrix igneous breccias lessons from the iberian pyrite belt abstract breccias composed of coarse monomictic porphyritic igneous clasts and fine grained matrix are common in subaqueous volcanic successions we use the descriptive name sediment matrix igneous breccia for this facies and have recognized at least five different origins among examples in the iberian pyrite belt 1 peperite 2 sediment infill volcanic breccia 3 mud matrix resedimented hyaloclastite 4 mud rich water settled fiamme breccia and 5 apparent sediment matrix igneous breccia because the components and textures are similar discriminating among the different origins can be very difficult both peperite and sediment infill volcanic breccia can occur along top contacts of thick intervals of felsic coherent and monomictic breccia facies the presence of peperite indicates that the contact is intrusive whereas the presence of sediment infill volcanic breccia indicates that the contact is depositional hence correct distinction between peperite and sediment infill volcanic breccia is an important means of discriminating felsic intrusions from felsic lavas and domes the distinction underpins the reconstruction of volcanic centers and facies architecture and the ordering of volcanic intrusive sedimentary and mineralizing events in ancient submarine volcanic successions in addition in volcanic hosted massive sulfide districts such as the iberian pyrite belt paleoseafloor positions are considered highly prospective for massive sulfide ore bodies correct interpretation of sediment matrix igneous breccias is also important in this context because sediment infill volcanic breccia mud matrix resedimented hyaloclastite and mud rich water settled fiamme breccia all form or are deposited at the seafloor and hence delineate paleoseafloor positions in contrast peperite and apparent sediment matrix igneous breccias form subsurface and do not delineate paleoseafloor positions

enhancement of diatom frustule dissolution by iron oxides abstract several reports in the literature indicate the possibility that iron oxyhydroxides in sediments enhance the dissolution of diatom frustules we tested this possibility by conducting a laboratory experiment in which various amounts of iron were added to sediment spiked with undissolved frustules a visual dissolution index was developed to assess the extent of dissolution a concentration of 300 mg fe g sediment was found to accelerate dissolution relative to 0 3 and 30 mg fe g sediment additions such abnormally high exposure to iron oxyhydroxides may be encountered by frustules over time particularly in aluminosilicate sediments subjected to frequent redox transitions

mesotidal estuarine sequences a perspective from the georgia bight abstract although much has been written about estuaries and estuarine sediments very few facies models have been proposed to date and considerable ambiguity remains in coastal classification schemes current stratigraphic concepts fail to elucidate not only the differences between certain deltaic estuarine and lagoon fill sequences but also the pronounced similarities between many riverine and salt marsh estuarine sequences these distinctive latero vertical successions recent and ancient warrant increased scrutiny and conceptual refinement preliminary stratigraphic models and definitions presented here based largely upon modern georgia estuaries should provide a broad framework for further evaluation and clarification of the above facies relationships from that standpoint an estuarine sequence consists of complex intertidal to subtidal mostly channel form facies dominated to some extent by tidal effects typically displaying abrupt variations in sediment texture and composition and in physical and biogenic sedimentary structures subtle to pronounced salinity gradients may be discernible nevertheless the features most diagnostic of the overall sequence are 1 tidally influenced channel form deposits and 2 characteristic bioturbate textures and trace fossils

monospecific pliocene oyster buildups murray basin south australia brackish water end member of the reef spectrum abstract paleoestuarine monospecific oyster bioherms and biostromes formed by the extant soft bottom oyster ostrea angasi are prolific in the pliocene norwest bend formation murray basin south australia these buildups are pioneer communities that record arrested ecological succession comprising only the stabilization and colonization stages of reef development each of these stages is directly linked to a different phase of marine inundation stabilization is restricted to the early transgressive systems tract when pre existing topographic highs were flooded and populated by dense aggregations of oyster clusters colonization is manifest throughout the transgressive and highstand systems tracts when increased accommodation allowed the vertical and lateral accretion of juvenile oyster communities into large buildups bioherms formed in areas of low terrigenous clastic sedimentation and seafloor disturbance both of which encouraged rapid community growth biostromes in contrast record buildup development on an unstable seafloor where episodic catastrophic community burial by storm sedimentation created repeated conditions of seafloor stabilization and colonization subaqueous dunes formed as part of the falling stage systems tract when river and tidal currents reworked shell material from bioherms and biostromes into large scale subaqueous bedforms brackish water conditions in the palaeoestuary a turbid water column mesotrophic nutrient levels and high spatial homogeneity of oyster buildups are interpreted to have prevented recruitment of specialized stenohaline organisms that typify the diversification stage of other metazoan reefs comparison with modern low diversity oyster communities imply that high fecundity and growth rates as well as exogenic nutrient cycling also made oysters in the norwest bend formation highly adapted competitors capable of thriving in this nutrient rich setting

the effect of species on lacustrine δ18odiatom and its implications for palaeoenvironmental reconstructions the oxygen isotope composition of diatom silica 18odiatom is increasingly being used to reconstruct climate from marine and lacustrine sedimentary archives although diatoms are assumed to precipitate their frustule in isotopic equilibrium with their surrounding water it is unclear whether internal processes of a given species affect the fractionation of oxygen between the water and the diatom we present 18odiatom data from two diatom size fractions 3 38 and 38 m characterized by different species in a sediment core from heart lake alaska differences in 18odiatom between the two size fractions varies from 0 to 1 2 with a mean offset of 0 01 n 20 fourier transform infrared spectroscopy confirms our samples consist of pure biogenic silica sio2 and 18odiatom trends are not driven by contamination the maximum offset is outside the range of error but the mean is within analytical error of the technique 1 06 demonstrating no discernible species dependent fractionation in 18odiatom we conclude that lacustrine 18odiatom measurements offer a reliable and valuable method for reconstructing 18owater considering the presence of small offsets in our two records we advise interpreting shifts in 18odiatom only where the magnitude of change is greater than the combined analytical error

rheotaxis and distribution of oysters and mussels georgia tidal creeks and salt marshes u s a abstract azimuth measurements on commissures of the oyster crassostrea virginica and the mussel geukensia demissa reveal preferred alignments by these animals to tidal current flow in beds along tidal creeks where ebb current velocities exceed flood current velocities most oysters orient parallel to channel trend and exhibit a slight preference for the ebb direction in contrast mussels nestling among these oysters tend to orient transverse to channel trend small clumps of oysters and mussels on the salt marsh surface also exhibit discernible orientations most tend to be aligned with tidal flood which is stronger than tidal ebb across the marsh locally however these trends may be difficult to relate to large scale components of marsh hydrography because of pronounced microtropography and its influences on current flow overall configuration of oyster biostromes evidently is controlled by a combination of slope areal extent and elevation of habitable substrates and competition among individual oysters valve orientations per se seem to be governed primarily by hydraulic pressure gradients including adaptations by the animals to both drag forces and passive disposition of waste materials the latter also is considered to be the most important stimulus affecting the rheotaxis of salt marsh oysters and mussels

late pleistocene oxygen isotope records of biogenic silica from the atlantic sector of the southern ocean we determined the isotopic composition of oxygen in marine diatoms in eight deep sea cores recovered from the atlantic sector of the southern ocean the analytical reproducibility and core to core consistency of the isotopic signal suggests that diatom 18o can be used as a new paleocenographic tool to reconstruct past variations in surface water characteristics and to generate 18o isotope based stratigraphy for the southern ocean the data indicate that diatom 18o reflects sea surface temperature and seawater isotopic composition and that diatoms retain their isotopic signal on timescales of a least 430 ka the 18o analyses of different diatom assemblages reveal that the isotopic signal is free of species effects and that the common antarctic species have the same water opal fractionation the transition from the last glacial maximum lgm to the holocene is fully recorded in high sedimentation rate cores an 18o enrichment during the lgm a post lgm meltwater spike and an input of meltwater during the late holocene are the main isotopic features observed in down core records the origin of this meltwater was very likely melting icebergs and or continental ice or by melting sea ice that had accumulated snow the most pronounced meltwater effects are recorded in cores that are associated with the weddel gyre our results provide the basis for extending isotope studies to oceanic regions devoid of carbonate further isotopic stratigraphies may be constructed for records and regions where they were previously not possible

reactive surface area control of the dissolution kinetics of biogenic silica in deep sea sediments abstract flow through dissolution experiments demonstrate that the specific reactive surface area of biogenic silica decreases significantly with depth in surficial sediments of the southern ocean specific surface areas obtained from nitrogen gas bet adsorption isotherms however do not exhibit the systematic changes of the specific reactive surface area inferred from the kinetic experiments furthermore the decrease in surface reactivity of the siliceous skeleton fragments continues at depths in the sediments where there is no longer a net release of silicic acid to the pore waters thus the specific reactive surface changes most likely reflect a progressive reduction of the surface density of reactive sites aging during burial rather than the preferential dissolution of a more reactive fraction of siliceous debris deposited from the water column the observed aging of the silica surfaces has important consequences for the benthic regeneration of nutrient silicic acid from deep sea sediments

holocene glacial discharge fluctuations and recent instability in east antarctica article history antarctica holds the largest ice sheet in the world the east antarctic ice sheet eais and plays a significant role in both local and global climate through the interactions between ice sheets ocean sea ice and atmosphere our understanding of east antarctica holocene climate variability relies mainly on ice cores that however do not document glacial discharge history here we present the first high resolution 18 odiatom record derived from two marine sediment cores retrieved on the east antarctic continental shelf to reconstruct glacial discharge off adelie land and george v land al gvl over the last 11 000 years from decadal to centennial resolution our results suggest multi centennial glacier advances and retreats until 2000 cal yr bp followed by a period of relative instability marked by two major glacial retreats centered at 1700 cal yr bp and 1980 ce we suggest that the multi centennial oscillations during the early mid holocene reflect glacier fluctuations in response to long term local seasonal insolation and short term solar variability we also propose that 18

global geologic maps are tectonic speedometers rates of rock cycling from area age frequencies relations among ages and present areas of exposure of volcanic sedimentary plutonic and metamorphic rock units lithosomes record a complex interplay between depths and rates of formation rates of subsequent tectonic subsidence and burial and or rates of uplift and erosion thus they potentially serve as efficient deep time geologic speedometers providing quantitative insight into rates of material transfer among the principal rock reservoirs processes central to the rock cycle areal extents of lithosomes exposed on all continents from two map sources geological survey of canada gsc and the food and agricultural organization fao of the united nations educational scientific and cultural organization unesco indicate that volcanic sedimentary plutonic and metamorphic rocks occupy 8 73 7 and 12 of global exposures respectively plots of area versus age of all mapped rock types display a power law relation where 6 5 of continental area is resurfaced with younger 10 volcanic 90 sedimentary units every million years and where areas of rock exposure decrease by 0 86 for each 1 increase in outcrop age r 2 0 90 area age relations for volcanic and sedimentary lithosomes are similar to the power law distribution defined by all rock units because 81 of mapped area consists of these two lithologies and reflect progressive decrease in amount of exposure with increasing age over the long term continental surfaces are blanketed by new volcanic rocks and sediments at rates of 1 5 and 12 1 10 6 km 2 ma respectively in contrast to power law distributed volcanic and sedimentary rocks that form at the earth9s surface age frequency distributions for plutonic and metamorphic rocks exhibit lognormal relations with modes at ca 154 and 697 ma respectively a dearth of younger exposures of plutonic and metamorphic rocks reflects the fact that these rock types form at depth and some duration of tectonism is therefore required for their exposure increasing modal ages from quaternary for volcanic and sedimentary successions to early mesozoic for intrusive rocks to neoproterozoic for metamorphic rocks demonstrate that greater amounts of geologic time are required for uplift to bring more deeply formed rocks to the earth9s surface the two different age frequency distributions observed for these major rock types a general power law age distribution for volcanic and sedimentary rocks and a lognormal distribution for plutonic and metamorphic rock ages reflect the interplay between depths of formation and mean rates of vertical tectonic displacement age frequency distributions for each of the major rock types are closely replicated by a model that presumes that individual crustal elements behave as a large population of random walks in geologic time and crustal depth and where the processes of surficial erosion associated with tectonic uplift serve to impose an absorbing boundary on this random walk space comparisons between model predicted age frequencies and those apparent in global map data suggest that mean rates of crustal subsidence and uplift are approximately equal in magnitude with mean rates of vertical tectonic diffusion of lithosomes from crustal depths of formation of about half a kilometer per million years rates of uplift and subsidence are strongly dependent on durations of tectonic dispersion lithosome ages however mean rates on the order of hundreds of meters per million years suggested by map age frequencies are the same as would be anticipated on the basis of hundreds of published rates of erosional uplift and exhumation determined by more conventional geochronometers this agreement suggests that geologic maps serve as effective deep time speedometers for the geologic rock cycle

changes in lake stratification and oxygen distribution inferred from two contrasting records of magnetotactic bacteria and diatoms 1 magnetotactic bacteria mtb and diatoms produce hard parts on seasonal timescales that continuously becomes embedded in various soft sediment archives consequently they can and are frequently used to reconstruct past environmental conditions in oceans fjords and freshwater basins in this study we observe how these two cosmopolitan creatures can be used to track vertical shifts in the distribution of oxygen in a small mountain lake situated in western norway several short and long sediment cores have been retrieved from the lake allowing for a detailed reconstruction that spans the last 2500 years during periods of enhanced runoff from the catchment the oxic anoxic transition zone oatz is lowered down to the lake sediment interface due to increased convection we posit that this condition reduces the concentration of microaerophilic magnetite producing mtb an interpretation partly supported by rock magnetic analyzes preservation of diatom frustules is however improved whenever this oxygen rich scenario prevails similarly when runoff is low and the oatz is lifted above the lake sediment interface mtb concentration seems to increase whereas dissolution of diatoms is more common this consistent antiphase relationship between mtb concentration and diatom preservation occurring on a multidecadal timescale during lowest reconstructed lake water ph in the lake s history suggest a coupled response to subtle changes in external environmental variables that affect catchment and lake hydrology

the use of multivariate statistics to resolve multiple contamination signals in the oxygen isotope analysis of biogenic silica analysis of the oxygen isotope composition 18o of diatom silica is a commonly used tool for palaeoclimate reconstruction that recent studies have demonstrated may be complicated by the presence of non diatom detrital material such contamination can mask any true climate driven signal leading to spurious results analysis of the 2 6 ma barsemoi diatomites from the east african rift valley highlights the presence of both tephra and clay in purified samples here we present a new method for assessing the relative contribution and geochemical composition of contamination components where sedimentary samples may be affected by more than one type of contamination this approach shows that the incorporation of analytical techniques such as x ray fluorescence spectrometry coupled with statistical modelling can be used to develop a three end member model to successfully resolve climate driven changes in 18odiatom mass balance corrections made to 18odiatom data demonstrate the importance of adopting quantitative geochemical analysis in tandem with the 18o analysis of biogenic silica to obtain accurate and meaningful results for palaeoclimate reconstruction

biogeochemical controls on palaeoceanographic environmental proxies a review scientific observations of our oceans and climate go back no more than a couple of hundred years most of our information about the evolution of earth s ocean climate system relies instead on proxies primarily measurements of sediment components that respond to changes in environmental parameters this paper provides an overview of some of the most important biological and geochemical proxies and outlines their contribution to our understanding of the ocean climate system we also discuss some of the challenges that need to be overcome to obtain accurate records these include better understanding of the controls on the mechanisms of biomineralization the impacts of post depositional dissolution and diagenesis on primary proxy relationships proxy validation and analytical considerations

the dilemma of disappearing diatoms incorporating diatom dissolution data into palaeoenvironmental modelling and reconstruction abstract taphonomic issues pose fundamental challenges for quaternary scientists to recover environmental signals from biological proxies and make accurate inferences of past environments the problem of microfossil preservation specifically diatom dissolution remains an important but often overlooked source of error in both qualitative and quantitative reconstructions of key variables from fossil samples especially those using relative abundance data a first step to tackling this complex issue is establishing an objective method of assessing preservation here diatom dissolution that can be applied by different analysts and incorporated into routine counting strategies here we establish a methodology for assessment of diatom dissolution under standard light microscopy lm illustrated with morphological criteria for a range of major diatom valve shapes dissolution data can be applied to numerical models transfer functions from contemporary samples and to fossil material to aid interpretation of stratigraphic profiles and taphonomic pathways of individual taxa using a surface sediment diatom salinity training set from the northern great plains ngp as an example we explore a variety of approaches to include dissolution data in salinity inference models indirectly and directly results show that dissolution data can improve models with apparent dissolution adjusted error rmse up to 15 lower than their unadjusted counterparts internal validation suggests improvements are more modest with bootstrapped prediction errors rmsep up to 10 lower when tested on a short core from devils lake north dakota which has a historical record of salinity dissolution adjusted models infer higher values compared to unadjusted models during peak salinity of the 1930s 1940s dust bowl but nonetheless significantly underestimate peak values site specific factors at devils lake associated with effects of lake level change on taphonomy preservation and re working implied by dissolution data may override model improvements incorporating dissolution dissolution adjusted salinity models are also applied to a 150 year sediment record from spiritwood lake north dakota which suggests that this lake has a damped and lagged response to major regional climate forcing of salinity during the dust bowl at this site dissolution data also suggest different taphonomic behaviour of taxa related to their seasonal patterns of growth and sedimentation thus dissolution data can improve models and aid interpretation of sedimentary profiles as records of limnological ecological and environmental change filtered by taphonomy