a new estimate for present day cocos caribbean plate motion implications for slip along the central american volcanic arc velocities from 153 continuously operatinggps sites on the caribbean north american and pacic plates arecombinedwith61newlyestimatedpacic cocosseafloor spreading rates and additional marine geophysical data to deriveanewestimate of present daycocos caribbean plate motion acomparisonofthepredictedcocos caribbean di rectiontoslipdirectionsofnumerousshallow thrustsubduc tion earthquakes from the middle america trench between costa rica and guatemala shows the slip directions to be deflected 10 clockwise from the plate convergence direc tion supporting thehypothesisthatfrequentdextralstrike slip earthquakes along the central american volcanic arc result from partitioning of oblique cocos caribbean plate convergence linear velocity analysis for forearc locations in nicaragua and guatemala predicts 14 2m m yr 1 of northwestward trench parallel slip of the forearc relative to thecaribbean plate possibly decreasing inmagnitude inel salvador and guatemala where extension east of the vol canic arc complicates the tectonic setting

the isotopic climatic records in the allerod bolling younger dryas and post younger dryas events the combined isotopic records in the greenland ice cores in modern and ancient wood samples and in marine foraminifera provide a climatic record for the allerod bolling younger dryas and post younger dryas periods that is not compatible with the usual climatic interpretation of the 2h 1h ratios in the greenland ice cores for example the younger dryas was not solely a north atlantic event because evidence for it is also found in the 2h 1h record in wood samples which grew on the north slope of alaska the younger dryas was not necessarily a time of cooling over the ice caps moreover it might have been a warming period in the temperate zones of the northern hemisphere continents a better understanding of the climatic scenarios during these periods of time could be ascertained from additional isotopic data in plant remains which grew during these intervals and which cover a wider climatic range on the continents

abrupt climatic events during the last glacial interglacial transition in alaska 1 evidence is mounting that abrupt climatic shifts occurred during the last glacial interglacial transition lgit in the north atlantic and other regions however few high resolution climatic records of the lgit exist from the high latitudes of the north pacific rim we analyzed lake sediments from southwestern alaska for biogenic silica organic carbon organic nitrogen diatom assemblages and compound specific hydrogen isotopes results reveal climatic changes coincident with the younger dryas intra allerod cold period and pre boreal oscillation however major discrepancies exist in the paleoclimate patterns of the bolling allerod interstadial between our data and the gisp2 18o record from greenland and causes are uncertain these data suggest that the north pacific and north atlantic experienced similar reversals during climatic warming of the lgit but that the bolling allerod cooling trend in the gisp2 18o record is probably not a hemispheric or global pattern

high resolution record of climate stability in france during the last interglacial period the last interglacial period 127 110 kyr ago has been considered to be an analogue to the present interglacial period the holocene which may help us to understand present climate evolution but whereas holocene climate has been essentially stable in europe variability in climate during the last interglacial period has remained unresolved because climate reconstructions from ice cores1 2 continental records3 4 and marine sediment cores5 6 give conflicting results for this period7 here we present a high resolution multi proxy lacustrine record of climate change during the last interglacial period based on oxygen isotopes in diatom silica diatom assemblages and pollen climate transfer functions from the ribains maar in france contrary to a previous study8 our data do not show a cold event interrupting the warm interglacial climate instead we find an early temperature maximum with a transition to a colder climate about halfway through the sequence the end of the interglacial period is clearly marked by an abrupt change in all proxy records our study confirms that in southwestern europe the last interglacial period was a time of climatic stability and is therefore still likely to represent a useful analogue for the present climate

quantification of biotic responses to rapid climatic changes around the younger dryas a synthesis to assess the presence or absence of lags in biotic responses to rapid climatic changes we 1 assume that the 18o in biogenically precipitated carbonates record global or hemispheric climatic change at the beginning and at the end of the younger dryas without any lag at our two study sites of gerzensee and leysin switzerland 2 derive a time scale by correlating the 18o record from these two sites with the 18o record of the grip ice core 3 measure 18o records in ostracods and molluscs to check the record in the bulk samples and to detect possible hydrological changes 4 analyse at gerzensee and leysin as well as at two additional sites that lack carbonates and hence a 18o record pollen plant macrofossils chironomids beetles and other insects and cladocera 5 estimate our sampling resolution using the grip time scale for the isotope stratigraphies and the biostratigraphies and 6 summarise the major patterns of compositional change in the biostratigraphies by principal component analysis or correspondence analysis we conclude that at the major climatic shifts at the beginning and end of the younger dryas hardly any biotic lags occur within the sampling resolution of 8 30 years and that upland vegetation responded as fast as aquatic invertebrates we suggest that the minor climatic changes associated with the gerzensee and preboreal oscillations were weakly recorded in the biostratigraphies at the lowland site but were more distinct at higher altitudes individualistic responses of plant and animal species to climatic change may reflect processes in individuals e g productivity and phenology in populations e g population dynamics in spatial distributions e g migrations and in ecosystems e g trophic state we suggest that biotic responses may be telescoped together into relatively short periods 50 to 150 years perhaps disrupting functional interactions among species and thus destabilising ecosystems

a 14 000yr paleoenvironmental record from windmill lake central alaska lateglacial and holocene vegetation in the alaska range abstract a 14 000 yr high resolution pollen and lake level record from windmill lake in central alaska provides new evidence for the late pleistocene and holocene paleoenvironments of this region at 14 000 14 c yr bp sparse herb tundra and low lake levels indicate a cold arid environment about 11 800 14 c yr bp the vegetation abruptly shifted to a shrub tundra and lake levels increased suggesting warmer and more mesic conditions ca 10 500 10 200 14 c yr bp herbaceous taxa increased at the expense of the shrubs suggesting a transient episode of climatic deterioration nearly contemporaneous with the younger dryas chronozone by 8000 14 c yr bp spruce was present in the watershed and alder grew locally by 6500 14 c yr bp the ams radiocarbon chronology indicates later expansions of betula picea and alnus than at most sites in central alaska which are conventionally dated

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

the nicoya convergent margin a region of exceptionally low heat flow over 100 measurements of seafloor heat flow reveal that the accretionary complex adjacent to the nicoya peninsula is characterized by remarkably low heat flow values over the accretionary prism average 28 mw m and values in the trench and the ocean crust seaward of the trench average 14 mw m we attribute the low heat flow to effective hydrothermal cooling of the upper crust on the subducting plate and suggest that extensional faults created by flexure of the lithosphere enhance hydrothermal circulation thermal models show that subduction of low temperature crust combined with significant frictional heating at the decollement can explain the low and uniform heat flow disparity between heat flow values observed on the lower trench slope with model results suggests upward advection of heat by porewater flux through broadly distributed conduits

porosity of sediments in accretionary prisms and some implications for dewatering processes rapid dewatering largely through loss of porosity is one of the most striking changes affecting deformed sediments in accretionary prisms analysis of porosity from sample and seismic data confirms that porosity in accretionary prisms is significantly less than that in basinal strata of similar lithology more specifically the porosity depth gradient increases arcward across the prism until porosities less than 10 occur within a few kilometers of the surface in deeper parts of the prism dewatering and volume reduction occur not only during loss of porosity but also during dehydration and metamorphic reactions calculations involving volume change lead us to define a function which reflects total volume reduction the distribution of the relative intensity of dewatering within the prism can be calculated from the porosity contours and trajectories of sediment elements an initial attempt indicates that in the toe region the rate of dewatering is accelerated at depth relative to the basinal section and may be greatest near the base of the prism toward the rear of the prism the maximum rate of dewatering occurs at progressively shallower depths several lines of evidence suggest that in addition to gradual diffuse dewatering localized zones of strong dewatering are associated with zones of intense shear this model of shear dewatering based on critical state soil mechanics suggests that sediments adjacent to the basal decollement should be strongly dewatered

apatite fission track analysis of oligocene strata in south texas u s a testing annealing models analysis of apatite fission track data from forty seven surface and subsurface samples of fine grained sandstone to pebbly conglomerate from the oligocene catahoula gueydan frio and vicksburg formations documents the response of fission track ages and track length distributions to simple post depositional thermal histories characterized by monotonic heating data from outcrop samples document that the apatite component was derived from source areas characterized by rapid cooling through the temperature range from 120 to 50 c during the early tertiary the apatite population is characterized by a log normal chlorine distribution mean 0 3 wt 0 3 wt consisting predominantly of low cl apatites 80 is 0 5 wt mean track lengths from core samples decrease systematically with increasing depth and temperature the decrease in apatite fission track ages with increasing depth and temperature is less systematic due to the scatter in inherited ages at the time of deposition no tracks were observed in apatite from samples at temperatures above 130 c mean track lengths for the south texas data agree well with those for a similar data set from the otway basin southeastern australia for present day temperatures of 70 c at higher temperatures mean lengths are 1 to 9 m longer than those observed from the otway basin this discrepancy is interpreted to reflect up to 40 ma longer residence times at maximum burial temperatures for the otway basin samples standard deviations about mean lengths are also lower than observed in the otway basin study consistent with the wider variation of cl contents mean 0 8 wt 0 7 wt that characterize apatite from the otway basin recently published apatite fission track annealing models predict significantly different mean track length vs present day temperature profiles for thermal histories believed to be representative of those experienced by sampled south texas strata because of the large uncertainties attributed to present day temperatures 10 c the predicted mean length vs temperature trends for these models essentially fall within or on the uncertainty envelope for the observed trend residuals of predicted minus observed mean lengths suggest that the previously published carlson composite parameter model using an initial mean length of 15 8 m instead of 16 5 m or the crowley et al model for track length reduction down to 11 m provide the best estimates of mean track length reduction over geologic time scales for near end member f apatite populations

evidence for a variable and wet younger dryas in southern alaska pollen macro and micro fossils and sedimentologic indicators in sediment cores from discovery pond dp in south central alaska indicate that the coldest interval of the last deglacation was coincident with the onset of the younger dryas yd around 12 8 cal ka the multi proxy record from dp together with a compilation of recently published yd records from southern alaska and the adjacent northern pacific ocean shows that during the course of the yd temperatures increased then reached a maximum sometime around 11 calka at dp a pronounced increase in the abundance of isoetes and pediastrum including species associated with oligotrophic lakes and known to respond to increased precipitation combined with a reduction in wetland aquatics and an increase in the minerogenic component of the sediment all indicate a shift from wetland to open water conditions at around 12 2 cal ka similar to other evidence from southern alaska our proxy record from dp indicates an increase in temperature and effective moisture during the second half of the yd an increase in winter precipitation might be associated with a deepening of the aleutian low pressure system and a northward shift in winter storm tracks consistent with recent simulations by climate system models

fluid budgets of subduction zone forearcs the contribution of splay faults 1 geochemical and geophysical evidence indicate that splay faults cutting subduction zone forearcs are a key hydraulic connection between the plate boundary at depth and the seafloor existing modeling studies have generally not included these structures and therefore a quantitative understanding of their role in overall fluid budgets the distribution of fluid egress at the seafloor and advection of heat and solutes has been lacking here we use a two dimensional numerical model to address these questions at non accretionary subduction zones using the well studied costa rican margin as an example we find that for a range of splay fault permeabilities from 10 16 m2 to 10 13 m2 they capture between 6 and 35 of the total dewatering flux simulated flow rates of 0 1 17 cm yr are highly consistent with those reported at seafloor seeps and along the decollement near the trench our results provide a quantitative link between permeability architecture fluid budgets and flow rates and illustrate that these features play a fundamental role in forearc dewatering and in efficiently channeling heat and solutes from depth

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

holocene climatic change in swedish lapland inferred from an oxygen isotope record of lacustrine biogenic silica holocene climatic variability was studied in a 9500 year lake sediment sequence from the abisko region in swedish lapland using the oxygen isotope ratio in diatom biogenic silica d 18 osi oxygen and hydrogen isotope ratios of waters from the abisko area suggest that in this region the evaporative flux is small and the isotopic composition of most lakes reflects that of the local precipitation the hydrological setting of the region and sensitivity analysis of isotopic response to changing climatic parameters such as humidity inflow and evaporation show that the downcore diatom d18osi record is primarily controlled by changes in the summer isotopic composition of the lake water the overall 3 5 depletion in d 18 osi since the early holocene is interpreted as an increase in the influence of the arctic polar continental air mass that carries depleted precipitation we estimate that this change is associated with a 2 5 4 c cooling that has occurred since the early holocene in general the diatom d 18 osi record resembles the average annual air temperature reconstructed for the greenland ice core gisp2 especially during the past 4000 years with a pronounced cooling starting at 2000 years bp

the younger dryas cold interval as viewed from central greenland abstract greenland ice core records provide an exceptionally clear picture of many aspects of abrupt climate changes and particularly of those associated with the younger dryas event as reviewed here well preserved annual layers can be counted confidently with only 1 errors for the age of the end of the younger dryas 11 500 years before present ice flow corrections allow reconstruction of snow accumulation rates over tens of thousands of years with little additional uncertainty glaciochemical and particulate data record atmospheric loading changes with little uncertainty introduced by changes in snow accumulation confident paleothermometry is provided by site specific calibrations using ice isotopic ratios borehole temperatures and gas isotopic ratios near simultaneous changes in ice core paleoclimatic indicators of local regional and more widespread climate conditions demonstrate that much of the earth experienced abrupt climate changes synchronous with greenland within thirty years or less post younger dryas changes have not duplicated the size extent and rapidity of these paleoclimatic changes

crenarchaeotal membrane lipids in lake sediments a new paleotemperature proxy for continental paleoclimate reconstruction paleoclimate studies of continental environments have been hampered by the lack of an independent paleotemperature proxy a novel sea surface temperature proxy has been proposed for marine systems based on membrane lipids of marine crenarchaeota this proxy will provide an independent continental paleotemperature tool that will allow paleoclimatologists to address fundamental questions about temperature variability in continental environments and its relationship to climate change

long term hydrogeochemical records in the oceanic basement and forearc prism at the costa rica subduction zone abstract two sealed borehole hydrologic observatories corks were installed in two active hydrogeochemical systems at the costa rica subduction zone to investigate the relationship between tectonics fluid flow and fluid composition the observatories were deployed during ocean drilling program odp leg 205 at site 1253 0 2 km seaward of the trench in the upper igneous basement and at site 1255 0 5 km landward of the trench in the decollement downhole instrumentation was designed to monitor formation fluid flow rates composition pressure and temperature the two year records collected by this interdisciplinary effort constitute the first co registered hydrological chemical and physical dataset from a subduction zone providing critical information on the average and transient state of the subduction thrust and upper igneous basement the continuous records at odp site 1253 show that the uppermost igneous basement is highly permeable hosting an average fluid flow rate of 0 3 m yr and indicate that the fluid sampled in the basement is a mixture between seawater 50 and a subduction zone fluid originating within the forearc 50 these results suggest that the uppermost basement serves as an efficient pathway for fluid expelled from the forearc that should be considered in models of subduction zone hydrogeology and deformation three transients in fluid flow rates were observed along the decollement at odp site 1255 two of which coincided with stepwise increases in formation pressure these two transients are the result of aseismic slip dislocations that propagated up dip from the seismogenic zone over the course of 2 weeks terminating before reaching odp site 1255 and the trench the nature and temporal behavior of strain and the associated hydrological response during these slow slip events may be an analog for the response of the seaward part of the subduction prism during or soon after large subduction zone earthquakes

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

postglacial climates inferred from a lake at treeline southwest yukon territory canada abstract pollen chironomid and ostracode records from a lake located at alpine treeline provide regional paleoclimate reconstructions from the southwest yukon territory canada the pollen spectra indicate herbaceous tundra existed on the landscape from 13 6 11 ka followed by birch shrub tundra until 10 ka although picea pollen dominated the assemblages after 10 ka low pollen accumulation rates and picea percentages indicate minimal treeline movement through the holocene chironomid accumulation rates provide evidence of millennial scale climate variability and the chironomid community responded to rapid climate changes ostracodes were found in the late glacial and early holocene but disappeared due to chemical changes of the lake associated with changes in vegetation on the landscape inferred mean july air temperature total annual precipitation and water depth indicate a long term cooling with increasing moisture from the late glacial through the holocene during the younger dryas 12 9 11 2 ka cold and dry conditions prevailed the early and mid holocene were warm and dry with cool wet conditions after 4 ka and warm dry conditions since the end of the little ice age

sensitivity analysis in thermal and maturity modelling abstract knowledge about the present and past thermal structure in sedimentary basins is central to understanding the evolution of sedimentary basins and for success in hydrocarbon exploration the possibilities and limitations in resolving the palaeothermal structure by inversion of downhole temperature and vitrinite reflectance data are discussed by means of an example from the danish central trough the prediction of petroleum generation is sensitive to uncertainties in source rock temperature history and the kinetic parameters of kerogen degradation the effects of uncertainties in surface temperature heat flow depth of burial thermal resistivity structure and kinetic parameters are investigated using a previously developed stochastic method regarding the state of petroleum generation the number of safe statements based on numerical modelling essentially are reduced to three 1 the source rock has not generated significant amounts of petroleum 2 the source rock has released most of its petroleum potential or 3 the source rock has released some of its petroleum potential in the latter case the state of petroleum generation is best described by a cumulative distribution function of source rock transformation which reflects the effects of uncertainties in the basic modelling parameters the probability of occurrence of a particular transformation ratio value like those derived in traditional deterministic modelling is zero an example of application from the danish central trough is given

the relationship between track shortening and fission track age reduction in apatite combined influences of inherent instability annealing anisotropy length bias and system calibration abstract the relationship between reduced track density 0 and reduced mean confined track length l l 0 during annealing has been studied for both spontaneous and induced tracks in four apatite samples with different chemical composition compositional range and origin and with spontaneous tracks giving mean confined track lengths varying from 14 5 m down to 12 3 m in all four apatites induced tracks show a relationship between l l 0 and 0 composed of two regions at higher values the two appear to be equal while at lower values l l 0 is greater than 0 this can be understood qualitatively in terms of the anisotropy of annealing and the bias against revelation of short tracks data from three apatites of uniform composition show near identical relationships in a sample showing a range of apatite compositions the point of divergence from the 1 1 relationship is somewhat higher than in the uniform composition apatites because individual grains anneal at different rates and the presence of highly annealed grains affects the track density more strongly than the mean track length spontaneous tracks show similar relationships to those observed for induced tracks in each sample when the data are renormalised to allow for the natural annealing present in the spontaneous tracks previous studies which led to the idea of length reduction without density reduction for spontaneous tracks did not renormalise density data as well as length data thereby distorting the apparent relationship data for the multi compositional apatite sample where annealing has taken place in geological conditions also agrees with the laboratory annealing data for both spontaneous and induced tracks this consistency suggests that a single process is responsible in all three cases implying that a laboratory based description of annealing can be applied to geological conditions with confidence recognition that spontaneous tracks in apatite are commonly annealed suggests that in general the fission track age should be interpreted as a reflection of both the time over which tracks have been retained and the distribution of etchable track lengths since the latter is a manifestation of the thermal history the fission track age should be interpreted in similar terms

exceptionally steep north south gradients in lake temperatures during the last deglaciation during the transition from the last glacial to the present interglacial climate late glacial period high summer insolation1 combined with the presence of the laurentide ice sheet is thought to have promoted the development of strong summer thermal gradients in north america south of the ice margin2 here we use palaeoecological methods to obtain quantitative evidence for the existence of these gradients our palaeoclimate reconstructions are based on water temperatures inferred from fossil assemblages of aquatic midge larvae in five lakes along a 240 km transect extending from central new brunswick canada to southeastern maine usa we show that the temperature gradient shifted during the killarney oscillation ko and younger dryas yd cooling events and that water temperature differences of 9 and 11 c existed over distances of 55 and 240 km respectively during parts of the late glacial period these gradients are much stronger than the current north south trends of lake summer surface water temperatures for these five lakes and for lakes across the northern tree line the rapid motion of such steep temperature gradients may have affected the progressive development of aquatic and terrestrial ecosystems in the region

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

diatom oxygen isotopes in pro glacial lake sediments from northern sweden a 5000 year record of atmospheric circulation we use a pro glacial oxygen isotope record of diatom silica 18odiatom and a sedimentary proxy for glacier flutuations to determine centennial millennial scale climate change during the last 5000

late pleistocene cosmogenic 36cl glacial chronology of the southwestern ahklun mountains alaska thirty two cosmogenic 36cl surface exposure ages constrain the timing of two late pleistocene glacial advances in the western ahklun mountains southwestern alaska boulders were sampled from one early wisconsin sensu lato and six late wisconsin moraines deposited by ice cap outlet glaciers and local alpine glaciers four moraine boulders deposited during an extensive early wisconsin ice cap outlet glacier advance have a mean surface exposure age of 60 300 3200 yr a moraine deposited by an ice cap outlet glacier during the restricted late wisconsin advance has a mean surface exposure age of 19 600 1400 yr five moraines deposited by late wisconsin alpine glaciers have mean ages that range between 30 000 and 17 000 yr the 36cl ages are consistent with limiting 14c and thermoluminescence ages from related deposits and indicate that ahklun mountains glaciers reached their most extensive position of the last glaciation early during the late pleistocene in contrast to the deep sea isotopic record of global ice volume 1

submarine weathering of silicate minerals and the extent of pore water freshening at active continental margins in order to investigate how submarine weathering processes may affect the water balance of sediments at convergent plate margins six sediment cores were retrieved off central chile at water depth between 800 and 4000 m the sediment solid phase was analyzed for its major element composition and the pore fluids were analyzed for dissolved sulfate sulfide total alkalinity major cations chloride bromide iodide hydrocarbons as well as the carbon isotopic composition of methane because of negligible weathering on land surface sediments off central chile are rich in reactive silicate minerals and have a bulk composition similar to volcanic rocks in the adjacent andes deep sourced fluxes of alkalinity cations and chloride indicate that silicate minerals are subject to weathering in the forearc during burial comparison of deep sourced signals with data from nearby ocean drilling program sites reveals two different types of weathering processes in shallow tens of meters methanic sediments of slope basins with high organic carbon burial rates reactive silicate minerals undergo incongruent dissolution through reaction with co2 from methanogenesis at greater burial depth hundreds of meters silicate weathering is dominated by authigenic smectite formation this process is accompanied by uptake of water into the clay interlayers thus leading to elevated salinities in the surrounding pore water deep seated smectite formation is more widespread than shallow silicate dissolution as it is independent from the availability of co2 from methanogenesis although solute transport is not focused enough to form cold seeps in the proper sense tectonically induced diffuse fluid flow transfers the deep seated signal of smectite formation into the shallow sediments the temperature controlled conversion of smectite to illite is considered the most important dehydration process in marine forearc environments depth of kilometers however in agreement with other studies at active margins e g aleutians cascadia nankai trough and despite ubiquitous evidence for smectite formation little evidence for seafloor seepage of dehydration fluids could be found off central chile we argue that the circular process of pore water uptake during smectite formation and release upon illitization implies a balanced freshwater budget and therefore a rather limited potential for net pore water freshening on a margin wide scale according to this rationale pore water freshening at seafloor seeps preferentially occurs at lower latitudes central america barbados mediterranean ridge where terrestrial weathering is more intense thus leading to external i e detrital smectite and thus freshwater inputs to the subduction system

links between clay transformation and earthquakes along the costa rican subduction margin we investigate the depth distribution of smectite clay transformation and its along strike variability at the middle america trench offshore costa rica we take advantage of recent well constrained thermal models that refine our understanding of the margin s thermal structure and which capture significant along strike variability using these thermal models together with sediment compositions defined by drilling we compute the distribution of smectite transformation and associated fluid production we show that the hypocenters of large m 6 9 well located megathrust earthquakes lie consistently down dip of peak fluid production we suggest that silica cementation associated with smectite transformation promotes lithification and slip weakening behavior that in combination with declining fluid pressures facilitate the initiation of unstable slip the earthquake ruptures extend up dip into the region of peak reaction possibly due to excess pore pressures that facilitate their propagation these results are consistent with the hypothesis that smectite transformation contributes to the onset of stick slip behavior and acts as an important control on earthquake nucleation and propagation

pleistocene glacial history of the southern ahklun mountains southwestern alaska soil development morphometric and radiocarbon constraints abstract four new ams 14 c ages glacial mapping and measurements of soil development loess thickness and moraine morphology constrain the age and extent of at least three quaternary advances by outlet lobes of an ice cap over the ahklun mountains of southwestern alaska the relative age data are from 107 sites correlated to 25 ice marginal positions in the kanektok goodnews togiak and kulukak river valleys and along the southeastern flank of the range radiocarbon ages provide minimum ages for six and a maximum age for one of the former ice margins soil and morphometric parameters subdivide the ice limits into three relative age groups one to three pre wisconsin advances probably middle pleistocene in age are represented by drift with relatively thick b horizons 60 5 cm with bt horizons thick loess caps 80 12 cm and broad moraines 135 134 m with gentle side slopes 7 5 an extensive early wisconsin sensu lato s l advance 39 9 ka and three associated stillstands or readvances are characterized by intermediate soil and morphometric parameters bw and weak bt horizons with thicknesses of 40 11 cm loess thicknesses of 69 46 cm crest widths of 38 13 m and slope angles of 14 4 the maximum late wisconsin advance 16 9 ka and two readvances or stillstands are associated with thin 20 5 cm weakly to moderately developed bw horizons thin loess caps 28 8 cm narrow crest widths 28 9 m and steep slope angles 18 3 the data confirm that early wisconsin glaciers in southeast beringia were much more extensive than late wisconsin glaciers which were apparently limited by availability of moisture

three dimensional seismic imaging of the costa rica accretionary prism structural diversity in a small volume of the lower slope conventional two dimensional seismic reflection investigations have been generally relied upon to provide images of large to medium scale structural features in accretionary prisms we undertook a three dimensional seismic reflection survey of a small part of a prism arcward of the middle america trench off costa rica to more correctly image structure and to use the improved structural information to examine the processes of accretion this survey reveals small features with dimensions of hundreds of meters while also defining features thousands of meters in lateral extent both of which were underappreciated in conventional two dimensional data from the same area we have imaged active off scraping at the trench and both duplexing and out of sequence faulting a few kilometers arcward of the trench fault spacing and reflector geometry vary dramatically over a space of several hundred meters some of these variations are related to visible changes in morphology of the underlying oceanic basement but others are not so easily documented fault surface reflections define an architecture which may control gross fluid motion through the prism this architecture is apparently formed by duplexing and out of sequence faulting and has been maintained by periodic motion on some of the out of sequence faults the slope sediment apron records multiple phases of deformation abundant small offset reverse faults break the seafloor and indicate recent shortening of a broad region of the underlying prism a primary result of this survey is appreciation of the structural diversity across a small width of an accretionary prism

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

the jameson land basin east greenland a fission track study of the tectonic and thermal evolution in the cenozoic north atlantic spreading regime abstract the late mesozoic and cenozoic thermal history of the jameson land basin is constrained by new apatite and zircon fission track ft data of surface permian to jurassic sedimentary rocks the results show a general regional thermal evolution related to burial to temperatures close to and in excess of the maximum temperatures of the apatite annealing interval 125 c followed by cooling mainly due to cenozoic uplift and erosion faulting and differential movements in the basin generally occurred after cooling below the apatite partial annealing zone paz 75 125 c however in the northern part of the basin the data suggest a thicker sediment cover or localized heating related to an earlier fracture zone both apatite ft analysis and vitrinite reflectance values reveal a postmature signature for the studied rocks in the northeastern jameson land and premature to mature for the western central and southern jameson land rocks with respect to generation of hydrocarbons the chemical variations of apatite enhance the possibility of recognizing sample positions near maximum temperatures in the paz furthermore the pb zn mineralization pattern closely follows the tertiary maturity trend given by the ft data the type and distribution of mineralization suggest that it was influenced by the regional thermal evolution of the basin in the northeast domain circulating fluids may have overprinted the regional thermal record before ca 20 ma basaltic dyke and sill intrusions 55 45 ma locally caused resetting of apatite ft ages but generally the direct influence from upper crustal magmatic activity played only a minor role the thermal evolution in northeast jameson land is related to the late tectonic evolution of the northeast atlantic involving a change in ridge position at ca 25 ma which followed the passage of the proto icelandic mantle plume at 63 40 ma

hydrogeology and mechanics of subduction zone forearcs fluid flow and pore pressure at subduction zones fluid flow pore pressure and tectonic processes are tightly interconnected excess pore pressure is driven by tectonic loading and fluids released by mineral dehydration and it has profound effects on fault and earthquake mechanics through its control on effective stress the egress of these overpressured fluids which is in part governed by the presence of permeable fault zones is a primary mechanism of volatile and solute transport to the oceans recent field measurements new constraints gained from laboratory studies and numerical modeling efforts have led to a greatly improved understanding of these coupled processes here we summarize the current state of knowledge of fluid flow and pore pressure in subduction forearcs and focus on recent advances that have quantified permeability architecture fluxes the nature and timing of transience and pressure distribution thus providing new insights into the connections between fluid metamorphic mechanical and fault slip proc

fluid venting in the eastern aleutian subduction zone fluid venting has been observed along 800 km of the alaska convergent margin the fluid venting sites are located near the deformation front are controlled by subsurface structures and exhibit the characteristics of cold seeps seen in other convergent margins the more important characteristics include 1 methane plumes in the lower water column with maxima above the seafloor which are traceable to the initial deformation ridges 2 prolific colonies of vent biota aligned and distributed in patches controlled by fault scarps over steepened folds or outcrops of bedding planes 3 calcium carbonate and barite precipitates at the surface and subsurface of vents and 4 carbon isotope evidence from tissue and skeletal hard parts of biota as well as from carbonate precipitates that vents expel either methane or sulfide dominated fluids a biogeochemical approach toward estimating fluid flow rates from individual vents based on oxygen flux measurements and vent fluid analysis indicates a mean value of 5 5 0 7 l m 2 d 1 for tectonics induced water flow wallmann et al 1997b a geophysical estimate of dewatering from the same area von huene et al 1997 based on sediment porosity reduction shows a fluid loss of 0 02 l m 2 d 1 for a 5 5 km wide converged segment near the deformation front our video guided surveys have documented vent biota across a minimum of 0 1 of the area of the convergent segment off kodiak island hence an average rate of 0 006 l m 2 d 1 is estimated from the biogeochemical approach the two estimates for tectonics induced water flow from the accretionary prism are in surprisingly good agreement

fluid budgets along the northern hikurangi subduction margin new zealand the effect of a subducting seamount on fluid pressure we estimate fluid sources around a subducted seamount along the northern hikurangi subduction margin of new zealand using thermomechanical numerical modelling informed by wedge structure and porosities from multichannel seismic data calculated fluid sources are input into an independent fluid flow model to explore the key controls on overpressure generation to depths of 12 km in the thermomechanical models sediment transport through and beneath the wedge is calculated assuming a pressure sensitive frictional rheology the change in porosity pressure and temperature with calculated rock advection is used to compute fluid release from compaction and dehydration our calculations yield more precise information about source locations in time and space than previous averaged estimates for the hikurangi margin the volume of fluid release in the wedge is smaller than previously estimated from margin averaged calculations 14 m3 yr 1 m 1 and is exceeded by fluid release from underlying subducting sediment 16 m3 yr 1 m 1 clay dehydration contributes only a small quantity of fluid by volume 2 m3 yr 1 m 1 from subducted sediment but the integrated effect is still significant landward of the seamount fluid source terms are used to estimate fluid pressures around a subducting seamount in the fluid flow models using subducted sediment permeability derived from porosity and testing two end members for decollement permeability models in which the decollement acts as a fluid conduit predict only moderate fluid overpressure in the wedge and subducting sediment however if the subduction interface becomes impermeable with depth significant fluid overpressure develops in subducting sediment landward of the seamount the location of predicted fluid overpressure and associated dehydration reactions is consistent with the idea that short duration shallow slow slip events sses landward of the seamount are caused by anomalous fluid pressures alternatively it may result from frictional effects of changing clay content along the subduction interface

cyclic variation and solar forcing of holocene climate in the alaskan subarctic high resolution analyses of lake sediment from southwestern alaska reveal cyclic variations in climate and ecosystems during the holocene these variations occurred with periodicities similar to those of solar activity and appear to be coherent with time series of the cosmogenic nuclides 14c and 10be as well as north atlantic drift ice our results imply that small variations in solar irradiance induced pronounced cyclic changes in northern high latitude environments they also provide evidence that centennial scale shifts in the holocene climate were similar between the subpolar regions of the north atlantic and north pacific possibly because of sun ocean climate linkages

three dimensional numerical simulation of fluid flow and heat transport within the barbados ridge accretionary complex 1 the thickness of the barbados accretionary prism varies along strike the increased thickness of the prism to the south is believed to affect the transport of heat by fluids resulting in anomalously high heat flow at the deformation front we utilized a coupled three dimensional numerical fluid flow and heat transport model to investigate the effect of variable accretionary prism thickness on fluid flow patterns and heat transport in the decollement zone the variation in sediment thickness produced along strike fluid flow in the more permeable decollement although surface heat flow is higher for simulations where flow occurs along both the decollement and underthrust sediments the resultant advective heat transport was not sufficient to appreciably raise steady state surface heat flow at the latitude of the ocean drilling program transect simulated temperatures in the decollement were consequently much lower than in situ borehole measurements suggesting an additional source of warmer fluid we then investigated the transient hydrologic and thermal response of the system to enhanced fault permeability model results provide insight into the degree to which episodic faulting may drive fluid flow and heat transport within subduction complexes a transient scenario in which decollement permeability was increased two orders of magnitude with fluid flow along both the decollement and underthrust sediments caused the temperature gradient at the location of the drill sites to increase by a factor of two within 7100 years the results of the three dimensional numerical simulations indicate that while both along strike advective heat transport and transient fluid flow events should be considered in the heat flow distribution of the northern barbados accretionary complex the effects of episodic fluid flow are more significant

regional atmospheric circulation change in the north pacific during the holocene inferred from lacustrine carbonate oxygen isotopes yukon territory canada analyses of sediment cores from jellybean lake a small evaporation insensitive groundwater fed lake provide a record of changes in north pacific atmospheric circulation for the last 7500 yr at 5 to 30 yr resolution isotope hydrology data from the southern yukon indicate that the oxygen isotope composition of water from jellybean lake reflects the composition of mean annual precipitation 18op recent changes in the 18o of jellybean sedimentary calcite 18oca correspond to changes in the north pacific index npi a measure of the intensity and position of the aleutian low al pressure system this suggests that 18op variability was related to the degree of fractionation during moisture transport from the gulf of alaska across the st elias mountains and that holocene shifts were controlled by the intensity and position of the al following this model between 7500 and 4500 cal yr b p long term trends suggest a predominantly weaker and or westward al between 4500 and 3000 cal yr b p the al shifted eastward or intensified before shifting westward or weakening between 3000 and 2000 cal yr b p rapid shifts eastward and or intensification occurred 1200 and 300 cal yr b p holocene changes in north pacific atmospheric circulation inferred from jellybean lake oxygen isotopes correspond with late holocene glacial advances in the st elias mountains changes in north pacific salmon abundance and shifts in atmospheric circulation over the beaufort sea

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

late pleistocene temperature history of southeast africa a tex86 temperature record from lake malawi we present a tex86 derived surface water temperature record for lake malawi that provides the first continuous continental record of temperature variability in the continental tropics spanning the past similar to 74 kyr with millennial scale resolution average temperature during marine isotope stage mis 5a was 26 5 degrees c with a range from 25 7 to 27 3 degrees c comparable to holocene temperatures mis 4 was a relatively cold period with temperatures generally decreasing from 25 5 degrees c at 68 ka to a minimum of 20 degrees cat similar to 60 ka 1 5 2 degrees c colder than the last glacial maximum lgm termination of mis 4 is characterized by a rapid increase of 3 4 degrees c in only similar to 0 5 kyr temperatures were relatively stable throughout mis 3 at the resolution of this study with an average of 23 8 degrees c and a range from 25 1 to 22 9 degrees c the lack of millennial scale temperature variability during mis 3 suggests that lake malawi s documented response to the bipolar seesaw brown et al 2007 is not reflected in its thermal history our temperature estimates for the lgm and holocene are consistent with a previously published tex86 record from lake malawi with a temperature of similar to 22 6 degrees c for the lgm similar to 25 26 degrees c in the mid holocene and similar to 25 28 for the late holocene in general the present extended tex86 record indicates that temperature variability in tropical east africa during late mis 5 and mis 4 was as great as that associated with the deglaciation and holocene a decrease in southern hemisphere insolation between 70 and 60 ka may have played an important role in forcing temperatures during mis 4 but after 60 ka other factors such as the extent of the polar ice sheets or atmospheric co2 may have forced temperature in tropical africa to a greater extent than local summer insolation

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

silicon isotope composition of diatoms as a paleoenvironmental proxy in lake huguangyan south china abstract silicon is essential for the growth of diatoms which utilize dissolved silicic acid in lake water and form opaline silica sio 2 n h 2 o during the uptake of dissolved silicic acid there is a preferential incorporation of light silicon isotope 28 si into biogenic silica resulting in the enrichment of heavy silicon isotope 30 si in dissolved silicic acid the silicon isotope composition of diatom silica 30 si diatom may thus record changes in the percentage utilization of dissolved silicic acid by diatoms which can be then related to other aspects of climate environment with the aim of exploring the potential of 30 si diatom as an indicator of lacustrine environment here we made the first measurements of 30 si diatom in the sediment core from lake huguangyan a closed crater lake in china the result shows that 30 si diatom varies from 0 6 to 1 1 and displays broad similarities to variations in contents of biogenic silica and organic carbon throughout the sediment core 30 si diatom is a reliable paleotemperature proxy in lake huguangyan which is supported by good correlation between 30 si diatom and available temperature records heavier 30 si diatom indicates greater dissolved silicic acid utilization at higher temperature while lighter 30 si diatom reflects decreased utilization at lower temperature the most negative 30 si diatom values in the sediment core occur between ad 1580 and 1920 which suggests ad 1580 1920 was the coldest period in lake huguangyan over the past 2000 years thus providing evidence for the existence of the lia in tropical south china there are few means by which to reconstruct the history of temperature changes in tropical terrestrial region 30 si diatom in this study has proven to be a new promising paleotemperature proxy in lacustrine sediments and may play important role in reconstructing past temperature changes at low latitude in the future detailed investigations on the silicon isotopes of diatoms in more lakes would be desirable in further research

variability of apatite fission track annealing kinetics ii crystallographic orientation effects a method is presented that permits the length of any horizontal confined fission track inclined at a specified angle to the crystallographic c axis in apatite to be converted to an equivalent track length parallel to the crystallographic c axis the model is based on the results of annealing experiments for six selected apatites five calcian fluorapatites and durango apatite representing a subset of the 15 total apatite specimens studied an iterative process of calculation is required to project fission track lengths onto the c axis and computer source code implementing the solution to this problem is presented this method of projecting apatite fission track lengths onto the crystallographic c axis is shown to remove effectively fission track length variation within single fission track populations due to anisotropic track length reduction for all 15 apatites studied in addition a model is developed that offers predictions that closely reproduce published experimental data concerning the relationship between fission track density etched fission tracks per unit area of apatite surface and the arithmetic mean fission track length finally it is shown that natural fission track populations exhibit fission track length anisotropy similar to that of fission track populations created and annealed in the laboratory this observation implies that the same process by which apatite fission tracks anneal in the laboratory is responsible for annealing of apatite fission tracks in the geological environment

hydrogeological modeling of porous flow in the oregon accretionary prism venting of pore fluid was recently reported near the front of the oregon accretionary prism and was directly measured at one of the venting sites during alvin dives in this study the authors model the flow of pore fluid in the oregon accretionaly prism they use seismic reflection sections to constrain subsurface structures drill hole results from deep sea drilling project leg 18 to constrain sediment properties and regional tectonic information to constrain the timing and the rate of plate convergence and sediment imbrication and thickening the results of two dimensional time element finite element modeling show that high pore pressures are generated beneath the decollement due to the load of imbricated and thickened sediments diffusive flow at a rate of several millimetres per year is predicted within the accretionary prism if fault zone permeability is discounted the presence of a decollement or permeable fault penetrating the accretionary prism as imaged on the seismic reflection section can drastically alter the patterns of fluid flow and produce focused venting of fluid as observed on the other hand fluid venting along faults may not significantly alter the high pore pressures below the decollement

fluid flow at the toe of convergent margins interpretation of sharp pore water geochemical gradients abstract fluid expulsion greatly impacts chemical and mass budgets at subduction zones particularly if it is focused along faults and stratigraphic conduits geochemical anomalies centered at decollement zones such as pore water freshening and the presence of thermogenic hydrocarbons indicate long distance focused flow of deeply sourced fluids the sharp gradient of these anomalies below the decollement zone has been interpreted as evidence for recent pulses of fluid flow initiating a few to tens of ka however this interpretation does not consider that underthrust sediments are moving arcward beneath the decollement zone in addition upward flow from the compacting underthrust sediments can modify chemical profiles here we use a simple model that couples fluid flow and solute transport to evaluate these sharp chemical gradients we find that observed geochemical anomalies at the northern barbados and costa rican subduction zones can be explained either by recent pulses of flow or by sustained flow along the decollement zone coupled with modest vertical fluid expulsion from consolidating underthrust sediments the latter interpretation is consistent with estimates of upward flow rate at costa rica based on estimated pore pressure gradients and measured permeabilities within the underthrust sediments one important implication is that recent pulses of flow along fault conduits may not be required to explain the geochemical anomalies furthermore mixing of locally derived fluids flowing upward from the underthrust sediments and deeply sourced fluids flowing along the decollement zone provides an explanation for the observed changes in pore water freshening along the decollement at costa rica

late quaternary history of tundra vegetation in northwestern alaska pollen analysis of a new core from joe lake indicates that the late quaternary vegetation of northwestern alaska was characterized by four tundra and two forest tundra types these vegetation types were differentiated by combining quantitative comparisons of fossil and modern pollen assemblages with traditional qualitative approaches for inferring past vegetation such as the use of indicator species although imprecisely dated the core probably spans at least the past 40 000 yr a graminoid salix tundra dominated during the later and early portions of the glacial record the middle glacial interval and the transition from glacial to interglacial conditions are characterized by a graminoid betula salix tundra a populus forest betula shrub tundra existed during the middle potion of this transition being replaced in the early holocene by a betula alnus shrub tundra the modern picea forest shrub tundra was established by the middle holocene these results suggest that the composition of modem tundra communities in northwestern alaska developed relatively recently and that throughout much of the late quaternary tundra communities were unlike the predominant types found today in northern north america although descriptions of vegetation variations within the tundra will always be restricted by the innate taxonomic limitations of their herb dominated pollen spectra the application of multiple interpretive approaches improves the ability to reconstruct the historical development of this vegetation type

hydrological instability during the last interglacial in central asia a new diatom oxygen isotope record from lake baikal last interglacial variability is commonly used as an analogue for variability in a future warmer world pervasive cycles are increasingly apparent in last interglacial archives although studies in continental regions are under represented here we provide a new isotopic record of diatom silica 18odiatom spanning c 127 5 115 ka bp from lake baikal in central asia peak rain fed discharge occurred c 125 4 ka bp shortly after july insolation maximum and initiation of siberian soil development between 127 and 119 7 ka bp there are six marked fluctuations in 18odiatom values with a pacing of approximately 1 26 0 3 ka similar to fluctuations of within lake productivity fluctuations in 18odiatom values show good agreement with patterns in atlantic meridional overturning circulation amoc supporting hypothesis of strong teleconnections via the westerlies between the north atlantic and central asia two periods of low 18odiatom values are especially notable the earliest between c 126 5 and 126 ka bp is concurrent with the final stages of the heinrich 11 the second between 120 5 and 119 7 ka bp is also concurrent with an increase in ice rafted debris in the north atlantic aquatic productivity in lake baikal increased between 119 7 and 117 4 ka bp before declining to the top of the record 115 ka bp concomitant with a shift to predominately cool steppe catchment vegetation however isotopic composition of discharge into lake baikal provides evidence for strong penetration of westerlies into central asia during the latter stages of the last interglacial variability in 18odiatom values was compared between the last interglacial and the holocene millennial scale variability was significantly more stable during the last interglacial possibly linked to diminished influence of freshwater discharge on amoc during periods of higher global mean temperatures

glacier readvance during the late glacial younger dryas in the ahklun mountains southwestern alaska an expansion of alpine glaciers during the latest pleistocene produced an extraordinarily well defined end moraine system in the ahklun mountains southwestern alaska these moraines deposited during the mount waskey advance are several kilometers beyond modern glacier termini and 80 km upvalley of the late wisconsin ahklun mountains ice cap terminal moraine eleven cosmogenic 10 be and 26 al exposure ages on moraine boulders combined with radiocarbon ages from a lake core upvalley of a moraine deposited during the mount waskey advance suggest that the advance culminated between 12 4 and 11 0 ka sometime during or shortly following the younger dryas event ca 12 9 11 6 ka we believe that the mount waskey advance was a consequence of cooling during the younger dryas these data further strengthen emerging evidence for younger dryas age cooling of the north pacific region

an oxygen isotope record of holocene climate change in the south central brooks range alaska abstract understanding the ecological and socio economic impacts of climatic warming requires knowledge of associated changes in moisture balance reconstructions of holocene moisture balance variation offer indispensible baseline information against which recent changes can be evaluated we analyzed chara stem encrustations in the sediments of takahula lake located in the south central brooks range of alaska for oxygen and carbon isotope composition to infer climatic change over the past 8000 years to help constrain climatic interpretations of the sediment 18 o record we also analyzed water samples from takahula and other lakes in the region for oxygen and hydrogen isotope composition results show that winter precipitation dominates the water balance of these lakes and that post input evaporation is a key control of lake water isotope composition of takahula lake stratigraphic patterns in chara 18 o supplemented by those in 13 c and sediment lithology reveal distinct changes in effective moisture precipitation minus evaporation over the past 8000 years effective moisture was relatively high from 8000 to 5000 cal bp with marked fluctuations between 6800 and 5000 cal bp it then decreased to reach a minimum around 4000 cal bp and increased with fluctuations from 4000 to 2500 cal bp followed by a decreasing trend toward the present that was interrupted by a wet little ice age centered at 400 cal bp aridity during the 20th century was among the highest of the entire 8000 year record at the millennial timescale the temporal patterns of moisture balance shifts at takahula lake are broadly coherent with those inferred from previous paleoclimate records from the region the chara 18 o values around 5600 cal bp and during the little ice age are up to 5 lower than at present and 3 6 lower than that of the modern input water to the lake these exceptionally low values suggest that factors other than effective moisture must have contributed to the pronounced variations in the takahula lake 18 o record increased winter precipitation associated with a westerly aleutian low position may account for 1 of the 18 o decrease other factors leading to the 18 o depletion during these periods probably include decreased temperatures as well as increased lake ice cover and associated reductions in evaporation

climate catchment runoff and limnological drivers of carbon and oxygen isotope composition of diatom frustules from the central andean altiplano during the lateglacial and early holocene diatom based carbon and oxygen isotope analyses 13cdiatom and 18odiatom were performed on diatom rich laminated sediments of lake chungara andean altiplano northern chile deposited during the lateglacial and early holocene 12 400 8300 cal years bp to reconstruct climate change and environmental response across this major climate transition the 13cdiatom data show both centennial to millennial scale changes related to fluctuations in lake productivity and co2 concentration in the lake water and variations in carbon sources to the lake through time the 18odiatom data reflect changes in lake hydrology and climate the combination of 13cdiatom and 18odiatom data reveals interactions between the internal lake processes and its catchment runoff during wet periods low 18odiatom values 13cdiatom indicates an enhanced contribution of allochthonous carbon whereas during dry periods high 18odiatom 13cdiatom values suggest more autochthonous carbon production and recycling these decadal to centennial oscillations are not recognized after 10 000 cal years bp possibly as a result of enso like phenomenon weakening humid conditions during the lateglacial early holocene transition 12 400 10 100 cal years bp were possibly due to the establishment of la nina like conditions in the tropical south pacific whereas dry conditions in the early holocene 10 100 9100 cal years bp may be caused by the northward migration of the itcz due to both enso like weakening and an insolation minimum finally a return to humid conditions at the end of the early holocene 9100 8300 cal years bp is coincident with an increase in summer insolation

new evidence on the neogene uplift of south tianshan constraints from the u th he and aft ages of borehole samples of the tarim basin and implications for hydrocarbon generation we identified a neogene rapid uplift denudation event of the south tianshan based on apatite u th he and apatite fission track aft ages in tertiary rocks of the tarim basin using borehole samples the u th he thermochronology can be used to reveal the tectono thermal events with lower temperature than that of aft thermochronology and has not been used previously to study the uplift of the tianshan mountain using these data we show the relationship between the uplift of the south tianshan and the subsidence deposition of the northern tarim basin during the neogene the apatite helium ages reveal the migration of uplift erosion and deposition in the northern tarim basin a rapid uplift of the south tianshan during the miocene and a corresponding rapid subsidence in the northern tarim basin occurred however in the pliocene the kuqa depression and south tianshan uplifted and eroded at the same time and in turn provided the detrital source rocks for the northern uplift of the tarim basin in contrast to earlier studies we arrive at the conclusion that the south tianshan experienced rapid uplift in the miocene based on u th he data of apatite obtained from borehole samples collected in the tarim basin itself and not from the bordering mountain chain combined apatite u th he and fission track thermochronometry enables reconstruction of thermal histories of sedimentary rocks between 40 and 120 c and this has implications for the generation of liquid hydrocarbon in the 65 120 c range in the basin thermal and burial histories of typical samples were also modelled to show the rapid uplift in our study our works not only provide a new evidence for the south tianshan uplift but also indicate that there is a coupling between uplift and subsidence in the south tianshan and adjacent northern part of the tarim basin which controlled hydrocarbon accumulation in the kuqa depression and northern uplift of the tarim basin

low temperature thermochronology of the southern east greenland continental margin evidence from apatite u th he and fission track analysis and implications for intermethod calibration abstract the southern coast of east greenland is characterized by high topographic relief and deep fjords but the evolution of the landscape and the low temperature thermochronology of the region is not well understood here we present apatite fission track and u th he ages that suggest several important features of the long term geomorphic history of this region but which also illustrate an important discrepancy in the thermal histories derived from each technique apatite from bedrock of the southern coast of east greenland between 62 n and 67 n has fission track ages ranging from 60 to 840 ma and u th he ages ranging from 21 to 250 ma the ages generally increase with elevation and distance from the coast and fission track analyses show significant differences in thermal histories for the different regions in the kangertittivatsiaq area c 66 67 n apatite fission track data and models suggest two separate periods of slow cooling prior to c 200 ma and between c 160 ma and the late cenozoic more recently than c 20 ma each of which was followed by a period of rapid cooling and inferred exhumation apatite he data in the kangertittivatsiaq region including crystal size age correlations in low elevation samples are most simply interpreted as recording an incision event of at least 1 5 km later than 20 ma near the coast this may have been caused by glacial erosion the u th he data also indicate an earlier phase of rapid exhumation at c 250 ma in the skjoldungen kap mosting area c 62 64 n approximately 200 km south of the kangertittivatsiaq apatite fission track data suggest slow exhumation from c 200 ma into the neogene followed by fast exhumation the similarity of fission track ages 200 ma at sea level in the fjords in the skjoldungen area c 62 64 n do not suggest tilting in the hinterland related to the breakup of the east greenland continental margin furthermore the cenozoic fission track ages and modeling fission track data suggest that pre breakup basins may have covered the outer coast despite the broadly similar topographic implications of the fission track and u th he data the thermal histories derived from these systems are inconsistent fission track data require thermal histories that predict he ages younger than observed and he data require thermal histories that predict fission track ages older than observed similar discrepancies have also been observed in other settings characterized by long term low temperature thermal histories and may reflect changes in annealing or diffusion behavior or both that either develop or become more apparent in such cases

temperature and precipitation history of the arctic as the planet cooled from peak warmth in the early cenozoic extensive northern hemisphere ice sheets developed by 2 6 ma ago leading to changes in the circulation of both the atmosphere and oceans from not vert similar2 6 to not vert similar1 0 ma ago ice sheets came and went about every 41 ka in pace with cycles in the tilt of earth s axis but for the past 700 ka glacial cycles have been longer lasting not vert similar100 ka separated by brief warm interglaciations when sea level and ice volumes were close to present the cause of the shift from 41 ka to 100 ka glacial cycles is still debated during the penultimate interglaciation not vert similar130 to not vert similar120 ka ago solar energy in summer in the arctic was greater than at any time subsequently as a consequence arctic summers were not vert similar5 c warmer than at present and almost all glaciers melted completely except for the greenland ice sheet and even it was reduced in size substantially from its present extent with the loss of land ice sea level was about 5 m higher than present with the extra melt coming from both greenland and antarctica as well as small glaciers the last glacial maximum lgm peaked not vert similar21 ka ago when mean annual temperatures over parts of the arctic were as much as 20 c lower than at present ice recession was well underway 16 ka ago and most of the northern hemisphere ice sheets had melted by 6 ka ago solar energy reached a summer maximum 9 higher than at present not vert similar11 ka ago and has been decreasing since then primarily in response to the precession of the equinoxes the extra energy elevated early holocene summer temperatures throughout the arctic 1 3 c above 20th century averages enough to completely melt many small glaciers throughout the arctic although the greenland ice sheet was only slightly smaller than at present early holocene summer sea ice limits were substantially smaller than their 20th century average and the flow of atlantic water into the arctic ocean was substantially greater as summer solar energy decreased in the second half of the holocene glaciers re established or advanced sea ice expanded and the flow of warm atlantic water into the arctic ocean diminished late holocene cooling reached its nadir during the little ice age about 1250 1850 ad when sun blocking volcanic eruptions and perhaps other causes added to the orbital cooling allowing most arctic glaciers to reach their maximum holocene extent during the warming of the past century glaciers have receded throughout the arctic terrestrial ecosystems have advanced northward and perennial arctic ocean sea ice has diminished here we review the proxies that allow reconstruction of quaternary climates and the feedbacks that amplify climate change across the arctic we provide an overview of the evolution of climate from the hot house of the early cenozoic through its transition to the ice house of the quaternary with special emphasis on the anomalous warmth of the middle pliocene early quaternary warm times the mid pleistocene transition warm interglaciations of marine isotope stages 11 5e and 1 the stage 3 interstadial and the peak cold of the last glacial maximum

seafloor structural observations costa rica accretionary prism by studying seafloor morphology we can make associations between near surface deformation fluid flow and the overall structural framework of accretionary prisms in february 1994 a ds rv alvin program to the costa rica accretionary prism investigated the relationship of fluid seepage and sediment deformation by using the distribution of chemosynthetic communities and heat flow anomalies as indicators of fluid flow the active normal faults that cut the hemipelagic section on the cocos plate may provide conduits for fluids that cause the regional heat flow to be extremely low these normal faults intersect the toe of the prism at an oblique angle creating localized regions of increased deformation positive heat flow anomalies observed at the deformation front indicate diffuse fluid flow however we discovered no seep communities indicative of focused flow the seaward most seep communities discovered are in a region of active out of sequence thrusts that cut a sediment apron which covers the complex to within 3 km of the prism toe vents occur consistently at the base of the fault scarps dives on a mud diapir show extensive seep communities pock marks and authigenic carbonates evidence of fluid release is on the crest which implies a low viscosity fluid migrating upward in the center of the structure normal faults on the upper slope can be seen in cross section in the walls of a submarine canyon the faults cut the slope apron and displace the seafloor actively maintaining the critical taper of the prism

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

a biogenic silica δ18o record of climatic change during the last glacial interglacial transition in southwestern alaska abstract despite growing evidence for environmental oscillations during the last glacial interglacial transition from high latitude terrestrial sites of the north pacific rim oxygen isotopic records of these oscillations remain sparse the lack of data is due partially to the paucity of lakes that contain carbonate sediment suitable for oxygen isotopic analysis we report here the first record of oxygen isotopic composition in diatom silica 18osi from a lake in that region 18osi increases gradually from 19 0 to 23 5 between 12 340 and 11 000 14c yr b p reflecting marked climatic warming at the end of the last glaciation around 11 000 14c yr b p 18osi decreases by 1 7 suggesting a temperature decrease of 3 5 8 9 c at the onset of the younger dryas yd in southwestern alaska climatic recovery began ca 10 740 14c yr b p as inferred from the increase of 18osi to a maximum of 23 9 near the end of the yd our data reveal that a yd climatic reversal in southwestern coastal areas of alaska occurred but the yd climate did not return to full glacial conditions

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

large arctic temperature change at the wisconsin holocene glacial transition analysis of borehole temperature and greenland ice sheet project ii ice core isotopic composition reveals that the warming from average glacial conditions to the holocene in central greenland was large approximately 15 c this is at least three times the coincident temperature change in the tropics and mid latitudes the coldest periods of the last glacial were probably 21 c colder than at present over the greenland ice sheet

apatite fission track results from ireland and the porcupine basin and their significance for the evolution of the north atlantic abstract apatite fission track analysis was applied to samples from the irish mainland and exploration boreholes 13 3 1 26 26 1 and 26 28 1 in the northern porcupine and donegal basins to study the regional thermal and tectonic history of the irish continental margin samples from the irish mainland have apatite ages ranging from 301 25 to 56 5 ma with mean track lengths in the range 13 89 0 25 to 11 19 0 22 m and indicate higher temperatures during the middle jurassic about 160 180 ma and during the late cretaceous to early tertiary about 50 80 ma these two periods of increased palaeotemperatures correlate with major reorganizations in plate movements and suggest that uplift and erosion after the maximum palaeotemperatures was achieved as a consequence of a variation in the regional pattern of crustal stress the results from borehole 26 28 1 indicate that within the resolution of the data all samples are presently at their maximum post depositional burial temperatures to produce the mean track length results in the range 11 24 0 49 to 11 81 0 20 m from borehole 26 26 1 it is necessary to invoke post palaeocene geothermal gradients in excess of 50 c km increased relative to the present day gradient 28 c km similar increased geothermal gradients are recognized during the early tertiary in adjacent sedimentary basins and may be explained as a result of heat advection by gravity driven fluid flow established in response to tectonic uplift of the basin margins

radiocarbon geochemistry of modern and ancient arctic lake systems baffin island canada the accuracy of arctic lake chronologies has been assessed by measuring the14c activities of modern carbon sources and applying these isotopic mass balances to dating fossil lake materials small 1 km2 shallow 25 m arctic lakes with watersheds 12 km2have soil and peat stratigraphic sections with14c activities ranging from 98 to 51 modern the14c activity of particulate organic carbon dissolved organic carbon and dissolved inorganic carbon from lake and stream waters ranges from 121 to 95 modern the sediment water interface of the studied lakes shows consistent14c ages of 100014c yr although the14c activity of living aquatic vegetation is 115 modern radiocarbon measurements of components of the lacustrine carbon pool imply that the 100014c yr age of the sediment water interface results from deposition of14c depleted organic matter derived from the watershed

a re evaluation and spatial analysis of evidence for a younger dryas climatic reversal in beringia abstract an objective classification of paleoclimatic proxies from 75 lake and peat records is used to re evaluate data quality and inferred climatic patterns in beringia during the younger dryas mapped data reveal coherent but spatially complex regional patterns suggesting the younger dryas was characterized by 1 cooling in southern alaska eastern siberia and portions of northeastern siberia and 2 uniform to warmer than present conditions through most of central alaska northeastern siberia and possibly the russian far east and northern alaska the beringian patterns correspond to distinctive large scale climatic forcings although in some locations further modified by more local influences such as topography general circulation models and modern synoptic climatology provide a conceptual framework for exploring possible mechanisms responsible for the observed changes forcings and associated climatic responses consistent with the proxy data include 1 lowered sea surface temperatures in the north atlantic and pacific reducing temperature and precipitation in eastern siberia 2 intensified aleutian low and lowered sea surface temperatures causing cooler summers and higher winter precipitation in southern alaska 3 stronger pacific subtropical high and an eastward shift of the east asian trough reducing summer temperatures in southern alaska and causing relative warmth in northeastern siberia and 4 strong high pressure system producing warm dry conditions in interior alaska

a simplified analysis of parameters controlling dewatering in accretionary prisms abstract one of the many dynamic geologic processes taking place at subduction zones is the compaction driven dewatering of seafloor sediments as they are accreted to the overriding plate the rate of dewatering is equal to the divergence of the sediment matrix velocity field this rate can be estimated analytically if simplifying assumptions are made about the geometry of the prism and motion of the sediments the analytical expression depends only on the sediment accretion velocity thickness of the accreted section prism taper angle and sediment porosity distribution a sensitivity analysis of the solution shows that the fluid production distribution is relatively insensitive to the sediment porosity distribution however the solution is very sensitive to the taper angle of the wedge high angle wedges expel almost all of the incoming water within 20 km while low angle wedges may retain a significant fraction of the incoming water for 50 or more kilometers thickness of the incoming section is also important dewatering of thin accreted sections is more concentrated near the toe of the wedge analyses of transects through the northern barbados makran vancouver and nankai accretionary prisms illustrate a range of dewatering rates and spatial distributions of dewatering

moisture source temperatures and precipitation δ18o temperature relationships across the united states 1 understanding and modeling precipitation isotope 18 o and 6d patterns for large regions of the globe requires quantifying processes governing continental scale climatology and hydrology in this study we have evaluated the extent to which inclusion of monthly moisture source temperature and moisture source locations in the rayleigh distillation model aid in reproducing the relationships between monthly time series of precipitation isotope 18 o values and temperature across the contiguous united states the steepest isotope temperature slopes 0 5 0 6 18 o c and greatest 18 o value correlations with temperature r 2 0 5 0 8 derived from 5 continuous years of the data from the united states network for isotopes in precipitation occurred in the high altitudes of the rocky mountains and the upper great lakes region the isotope temperature slopes derived from the site specific time series were consistently lower along the coastal regions of the united states as were the coefficients of determination the low coastal isotope temperature slopes are not easily explained by the simple rayleigh model that uses condensation temperatures as the primary driver of precipitation isotopes however the raleigh model equipped with moisture source temperatures defined by seasonal temperature oscillations and migrating moisture source locations provides a robust replication of the 18 o temperature slopes along coastal regions these findings emphasize the importance of moisture sources conditions when interpreting palaeoclimate proxies i e tree rings ice cores etc and argue that moisture source dynamics should always be included in models that use isotopes as diagnostic tools in testing hydrologic models

postglacial vegetation and climate change in the northern bristol bay region southwestern alaska pollen analysis of sediment cores from grandfather and ongivinuk lakes reveals a record of postglacial vegetation and climate change in the northern bristol bay region the chronology is based on six conventional 14c dates of bulk organic matter from the grandfather core a mesic herb tundra dominated the landscape 13 000 9800 yr b p betula shrubs probably first appeared in the region 11 300 yr b p but were restricted to favorable microhabitats until 9800 yr b p the later establishment of betula shrubs and relatively low betula pollen abundance in these records compared to other areas of eastern beringia suggest that postglacial warming in southwestern alaska was dampened by regional climatic controls possibly low sea surface temperatures of the north pacific ocean between 10 800 and 9800 yr b p diminished betula shrub cover along with decreased aquatic productivity as recorded by pediastrum cell nets and biogenic silica suggest a brief reversion to colder and drier climatic conditions possibly associated with the younger dryas event around 9800 yr b p betula shrub tundra and meadow communities expanded probably in response to increased temperature and precipitation alnus arrived and formed extensive thickets within the region ca 7400 yr b p the establishment of the modern boreal forest tundra ecotone is marked by the arrival of picea glauca at grandfather lake ca 4000 yr b p and the subsequent increase to present population densities ca 2000 yr b p the unique features of these pollen records emphasize the spatial complexity of late quaternary vegetation and climate history in eastern beringia

thermal annealing of fission tracks in fluorapatite chlorapatite manganoanapatite and durango apatite experimental results it is well known that the optically measured lengths of fission tracks in apatite crystals are a function of etching conditions crystallographic orientation of the track composition of the crystal and the state of thermal annealing in this study we standardize etching conditions and optimize track length measurability by etching until etch pits formed at the surface of each apatite crystal reached widths of about 0 74 m etching times using 5m hno3 at 21 c were 31 s for otter lake quebec fluorapatite 47 s for durango mexico apatite 33 s for portland connecticut manganoanapatite and 11 s for bamle norway chlorapatite an etching experiment using two etchant strengths 5m and 1 6m hno3 revealed that despite significant differences in etch pit shape fission track length anisotropy with respect to crystallographic orientation of the tracks is not a chemical etching effect a series of 227 constant temperature annealing experiments were carried out on nuclear reactor induced tracks in orient

inversion of apatite fission track data for thermal history information apatite fission track length distributions and ages provide an integrated record of the low temperature 20 150 c thermal history of the host rock over geologic time scales the information content of these data is quantified by applying a stochastic optimization technique simulated annealing to determine the range of thermal histories compatible with synthetic apatite fission track data given a mathematical description of fission track annealing the goodness of fit between the observed and predicted data parameters and a slowly decreasing control parameter are treated as analogous to the free energy and temperature respectively of a thermodynamic system on this basis a boltzmann type probability distribution is defined and used to accept or reject random thermal history perturbations a priori information used to constrain the model space may include the temperature range in which the solutions are sought present day temperature the complexity of the thermal history and the maximum duration of the data sensitive thermal history this approach allows determination of the range of thermal histories compatible with an observed confined track length distribution and associated fission track age and it provides an estimate of the probability distribution of temperatures at a specific time results for synthetic data sets for diverse model thermal histories suggest that the resolution of thermal history is generally poor parts of the thermal history to which the data are most sensitive can be resolved to no better than about 10 c for ideal data bounds on acceptable time temperature space are sensitive to the assumed complexity of the thermal history but are relatively insensitive to typical uncertainties associated with the data application of this approach to real data is fundamentally limited by the uncertainty associated with extrapolating annealing models to geologic conditions

oxygen isotope geothermometers in lacustrine sediments new insights through combined δ18o analyses of aquatic cellulose authigenic calcite and biogenic silica in lake gościąz central poland abstract two water based oxygen isotope geothermometers calcite cellulose and silica cellulose were tested using well dated intervals of laminated sediment retrieved from lake go ci z a small hardwater lake in central poland five sections spanning ca 90 200 years of deposition were selected and subsampled at approximately decadal resolution to characterize relatively stable climatic and hydrologic conditions during the middle of the younger dryas cold period the rapid environmental changes that occurred at the younger dryas preboreal yd pb boundary and three subsequent periods of relative stability during the early middle and late holocene the two geothermometers are based on the premise that aquatic cellulose 18 o serves as a direct proxy for lakewater 18 o thereby allowing resolution of temperature signals in 18 o records obtained from co existing authigenic calcite or diatom silica qualitatively consistent results from the calcite cellulose geothermometer were obtained for all five intervals clearly showing the expected low epilimnion water temperatures in the mid yd rapid warming during the yd bp transition and maximum temperatures during the early holocene followed by slightly lower temperatures during the middle and late holocene results from the silica cellulose geothermometer also showed lower temperatures during the yd than the late holocene although insufficient diatom silica for analysis was present in the intervening intervals the calcite cellulose geothermometer yielded consistently higher estimates of epilimnion temperatures than the silica cellulose thermometer in the yd and late holocene intervals and unrealistically high values throughout the holocene we speculate that the calcite cellulose geothermometer is influenced by kinetic effects during rapid carbonate precipitation which offsets temperature dependent fractionation and thus leads to high temperature estimates in contrast the silica cellulose geothermometer may be affected by the production of diatom silica early in the spring prior to seasonal warming and isotopic enrichment of the epilimnion thus generating low temperature estimates

kinematic behavior of southern alaska constrained by westward decreasing postglacial slip rates on the denali fault alaska long term slip rates for the denali fault in southern alaska are derived using 10 be cosmogenic radionuclide crn dating of offset glacial moraines at two sites correction of 10 be crn model ages for the effect of snow shielding uses historical regional snow cover data scaled to the site altitudes to integrate the time variation of snow cover we included the relative changes in effective wetness over the last 11 ka derived from lake level records and 18 o variations from alaskan lakes the moraine crn model ages are normally distributed around an average of 12 1 1 0 ka n 22 1 the slip rate decreases westward from 13 mm a at 144 49 w to about 7 mm a at 149 26 w the data are consistent with a kinematic model in which southern alaska translates northwestward at a rate of 14 mm a relative to a stable northern alaska with no rotation this suggests progressive slip partitioning between the denali fault and the active fold and thrust belt at the northern front of the alaska range with convergence rates increasing westward from 4 mm a to 11 mm a between 149 w and 145 w as the two moraines sampled for this study were emplaced synchronously our suggestion of a westward decrease in the slip rate of the denali fault relies largely upon the measured offsets at both sites regardless of any potential systematic uncertainty in the crn model ages

chemoherms on hydrate ridge unique microbially mediated carbonate build ups growing into the water column two active chemoherm build ups growing freely up into the oceanic water column the pinnacle and the south east knoll chemoherms have been discovered at hydrate ridge on the cascadia continental margin these microbially mediated carbonate formations rise above the seafloor by several tens of meters and display a pinnacle shaped morphology with steep flanks the recovered rocks are pure carbonates dominated by aragonite based on fabric and mineralogic composition different varieties of authigenic aragonite can be distinguished detailed visual and petrographic investigations unambiguously reveal the involvement of microbes during the formation of the carbonates the fabric of the cryptocrystalline and fibrous aragonite can be described as thrombolitic fossilized microbial filaments in the microcrystalline aragonite indicate the intimate relationship between microbes and carbonates the strongly 13c depleted carbon isotope values of the samples as low as 48 1 pdb are characteristic of methane as the major carbon source for the carbonate formation the methane rich fluids from which the carbonates are precipitated originate most probably from a gas reservoir below the bottom simulating reflector bsr and rise through fault systems the 18o values of the aragonitic chemoherm carbonates are substantially higher as high as 5 0 pdb than the expected equilibrium value for an aragonite forming from ambient seawater 3 5 pdb as a first approximation this indicates formation from glacial ocean water but other factors are considered as well a conceptual model is presented for the precipitation of these chemoherm carbonates based on in situ observations and the detailed petrographic investigation of the carbonates this model explains the function of the consortium of archaea and sulfate reducing bacteria that grows on the carbonates performing anaerobic oxidation of methane aom and enabling the precipitation of the chemoherms above the seafloor surrounded by oxic seawater beggiatoa mats growing on the surface of the chemoherms oxidize the sulfide provided by sulfate dependent anaerobic oxidation of methane within an oxic environment the contact between beggiatoa and the underlying microbial consortium represents the interface between the overlying oxic water column and an anoxic micro environment where carbonate formation takes place