origin of the neoproterozoic fulu iron formation south china insights from iron isotopes and rare earth element patterns abstract in the neoproterozoic era there was widespread deposition of iron formations in close association with global or near glaciations these snowball earth glaciations likely played a key role in iron formation distribution and deposition however the environmental conditions fe sources and formation mechanisms remain debated here we present the rare earth element geochemistry and fe isotope composition of the synglacial iron formation within the neoproterozoic fulu formation south china the fulu iron formation consists of layers of authigenic minerals mainly hematite and detrital components quartz feldspars fe chlorite and minor biotite positive eu anomalies in one of the fulu localities indicate a hydrothermal influence suggesting that fe was mainly sourced from distal hydrothermal systems the bulk rock fe isotope composition of the fulu iron formation shows a large range with 56fe from 0 23 to 1 78 correlation between bulk rock 56fe values and al fe ratios demonstrates that 56fe variability reflects in part varying proportions of authigenic versus detrital components the fe isotope composition of authigenic hematite is calculated by a linear regression and shows 56fe between 0 83 and 2 21 with an average at 1 54 0 50 2 n 41 using a dispersion reaction model the high 56fe values of hematite constrain local dissolved o2 concentrations of the ocean to less than 0 4 nmol l even in the shallow part of the water column this relationship is consistent with highly reducing conditions in the neoproterozoic oceans favored by isolation from the atmosphere by a sea ice we attribute the extremely positive values to partial iron oxidation in waters that were cold relative to modern surface oceans the dominant occurrence of hematite supports an abiotic precipitation pathway given that biological activity would have introduced organic matter to the sediments and led to partial reduction of fe iii oxides and subsequent formation of magnetite and or siderite as is observed in archean and paleoproterozoic iron formations oxic glacial meltwater and or o2 vertical transfer from the atmosphere to the upper ocean linked to ice dynamics is likely to have mediated the abiotic oxidation we propose that vertical transfer of o2 resulted from the deposition of snow that trapped air bubbles at the top of the glacier coupled to a melting of the bottom of the glacier which in combination delivered a limited but continuous amount of o2 to the ocean

organic geochemical proxies of paleoceanographic paleolimnologic and paleoclimatic processes abstract organic matter constitutes a minor fraction of marine and freshwater sediments yet its important contribution to the sedimentary record can be used to reconstruct marine and continental paleoenvironments the organic matter content of sediments is the residue of past biota the amounts and types of organic matter present in sediments consequently reflect environmental conditions that impacted ecosystems at different past times general sources of the organic matter are inferred from bulk properties such as elemental compositions carbon and nitrogen stable isotope ratios rock eval pyrolysis data and organic petrography details of organic matter origins are refined by analyses of biomarker molecular compositions source changes are proxies for fluctuations in sea level oceanic surface currents and continental climates algal paleoproductivity rates are indicated by organic carbon mass accumulation rates and carbon and nitrogen stable isotopic compositions these parameters record past availability of nutrients and therefore are proxies of surface mixing in the oceans and amount of land runoff to lakes sea surface paleotemperatures are recorded by the number of carbon carbon double bonds in lipid biomarkers produced by marine algae larger proportions of the double bonds are proxies for the cooler surface waters that accompanied periods of global glaciation and intervals of enhanced upwelling the 13c and d values of plant organic matter record past concentrations of carbon dioxide in the atmosphere and changes in delivery of atmospheric moisture respectively diagenesis which causes the concentration and composition of organic matter in sedimentary settings to differ from those of the original biologically synthesized materials can bias organic geochemical paleoenvironmental records the magnitude of this potential source of misinformation must always be considered and evaluated comparison of multiple organic geochemical proxies of past conditions helps to compensate for the effects of diagenetic alterations and thereby to improve interpretations of paleoenvironmental change

lack of enhanced preservation of organic matter in sediments under the oxygen minimum on the oman margin the impingement of oxygen minima on continental margins is widely thought to promote the accumulation of sedimentary facies enriched in well preserved organic matter it is shown here however that such a relationship does not clearly apply to the productive oman margin in the arabian sea which hosts one of the most severe oxygen minima in the oceans measurements made on the 0 1 cm depth interval from fourteen box cores collected from the outer shelf upper continental slope area off oman show that 1 1 deposited organic matter is overwhelmingly of marine origin 2 2 there is no significant correlation between the abundance of sedimentary organic carbon corg and the bottom water o2 concentration 3 3 there is no relation between the sedimentary corg n ratio and bottom water o2 4 4 there is no correlation between the hydrogen index hi of the organic matter and bottom water oxygen there are however significant correlations between the corg n ratio and the i corg cr al and zr al ratios as well as between the corg n ratio and the hydrogen index these relationships are interpreted as being the result of winnowing and attendant reworking which produce a progressive increase in the corg n ratio and a decrease in the hi and the i corg ratio as organic matter becomes progressively degraded commensurate rises in the cr al and zr al ratios result from the increased proportion of heavy minerals associated with winnowed lag deposits overall these data suggest that the bottom water oxygen concentration has little effect in governing either the distribution or the degree of preservation of organic matter on this margin thus the generally high but spatially variable corg content of the sediments on the oman margin may not reflect the occurrence of an oxygen minimum but instead be the result of a high settling flux of organic matter supported by monsoon driven upwelling and post depositional redistribution of the organic material by hydrodynamic influences

last glacial interglacial productivity and associated changes in the eastern arabian sea abstract we reconstruct paleo productivity and bottom water oxygenation changes during the past 32 ka from the southeastern arabian sea using absolute abundance of planktic foraminifera relative abundance of globigerina bulloides angular asymmetrical benthic foraminifera aabf measurements of total organic carbon c org caco 3 and globigerinoides ruber 18 o and 13 c the faunal and geochemical proxies suggest that productivity in the southeastern arabian sea was high during mis 3 a distinct decrease in productivity is inferred during the last glacial maximum 19 23 ka lgm bottom water was well oxygenated during mis3 only to become oxygen depleted during the lgm productivity decreased abruptly during heinrich stadial 1 hs 1 but the response to heinrich stadial 2 hs 2 was different low productivity during the early deglaciation is also synchronous with an increase in ice volume corrected 18 o 18 o sw ivc a salinity proxy between 18 9 18 3 18 9 ka bp and 15 9 15 0 16 3 ka bp and a concomitant decrease in 13 c between 18 2 17 5 18 7 ka bp and 16 4 15 3 16 7 ka bp these patterns suggest that the global 13 c minimum event during the last deglaciation was associated with the weak monsoon interval during hs 1 and a drop in productivity in tropical regions a progressive increase in productivity is observed throughout the holocene with a distinct jump at 5 4 3 8 6 3 ka bp we infer that although productivity was higher in the eastern arabian sea during most of the last glacial interval the overall resultant carbon sequestration was confined only to a restricted zone and not large enough to substantially alter atmospheric co 2

hydrohalite in cold sea ice laboratory observations of single crystals surface accumulations and migration rates under a temperature gradient with application to snowball earth 1 when nacl precipitates out of a saturated solution it forms anhydrous crystals of halite at temperatures above 0 11 c but at temperatures below this threshold it instead precipitates as the dihydrate hydrohalite nacl 2h2o when sea ice is cooled hydrohalite begins to precipitate within brine inclusions at about 23 c in this work hydrohalite crystals are examined in laboratory experiments their formation their shape and their response to warming and desiccation sublimation of a sea ice surface at low temperature leaves a lag deposit of hydrohalite which has the character of a fine powder the precipitation of hydrohalite in brine inclusions raises the albedo of sea ice and the subsequent formation of a surface accumulation further raises the albedo although these processes have limited climatic importance on the modern earth they would have been important in determining the surface types present in regions of net sublimation on the tropical ocean in the cold phase of a snowball earth event however brine inclusions in sea ice migrate downward to warmer ice so whether salt can accumulate on the surface depends on the relative rates of sublimation and migration the migration rates are measured in a laboratory experiment at temperatures from 2 c to 32 c the migration appears to be too slow to prevent formation of a salt crust on snowball earth

formation of patterned ground and sublimation till over miocene glacier ice in beacon valley southern victoria land antarctica a thin glacial diamicton informally termed granite drift occupies the floor of central beacon valley in southern victoria land antarctica this drift is 40 ar 39 ar analyses of presumed in situ ash fall deposits that occur within granite drift at odds with the great age of this ice are high centered polygons that cut granite drift if polygon development has reworked and retransported ash fall deposits then they are untenable as chronostratigraphic markers and cannot be used to place a minimum age on the underlying glacier ice our results show that the surface of granite drift is stable at polygon centers and that enclosed ash fall deposits can be used to define the age of underlying glacier ice in our model for patterned ground development active regions lie only above polygon troughs where enhanced sublimation of underlying ice outlines high centered polygons the rate of sublimation is influenced by the development of porous gravel and cobble lag deposits that form above thermal contraction cracks in the underlying ice a negative feedback associated with the development of secondary ice lenses at the base of polygon troughs prevents runaway ice loss secondary ice lenses contrast markedly with glacial ice by lying on a d versus 18 o slope of 5 rather than a precipitation slope of 8 and by possessing a strongly negative deuterium excess the latter indicates that secondary ice lenses likely formed by melting downward percolation and subsequent refreezing of snow trapped preferentially in deep polygon troughs the internal stratigraphy of granite drift is related to the formation of surface polygons and surrounding troughs the drift is composed of two facies a nonweathered matrix supported diamicton that contains 25 striated clasts in the 16 mm fraction and a weathered clast supported diamicton with varnished and wind faceted gravels and cobbles the weathered facies is a coarse grained lag of granite drift that occurs at the base of polygon troughs and in lenses within the nonweathered facies the concentration of cosmogenic 3 he in dolerite cobbles from two profiles through the nonweathered drift facies exhibits steadily decreasing values and shows the drift to have formed by sublimation of underlying ice these profile patterns and the 3 he surface exposure ages of 1 18 0 08 ma and 0 18 0 01 ma atop these profiles indicate that churning of clasts by cryoturbation has not occurred at these sites in at least the past 10 5 and 10 6 yr although granite drift is stable at polygon centers low frequency slump events occur at the margin of active polygons slumping together with weathering of surface clasts creates the large range of cosmogenic nuclide surface exposure ages observed for granite drift maximum rates of sublimation near active thermal contraction cracks calculated by using the two 3 he depth profiles range from 5 m m y to 90 m m y sublimation rates are likely highest immediately following major slump events and decrease thereafter to values well below our maximum estimates nevertheless these rates are orders of magnitude lower than those computed on theoretical grounds during eruptions of the nearby mcmurdo group volcanic centers ash fall debris collects at the surface of granite drift either in open thermal contraction cracks or in deep troughs that lie above contraction cracks these deposits subsequently lower passively as the underlying glacier ice sublimes the fact that some regions of granite drift have escaped modification by patterned ground for at least 8 1 ma indicates long term geomorphic stability of individual polygons once established polygon toughs likely persist for as long as 10 5 10 6 yr our model of patterned ground formation which applies to the hyperarid cold desert polar climate of antarctica may also apply to similar sized polygons on mars that occur over buried ice in utopia planitia

variations in δ13c values of sedimentary organic matter since late miocene time in the indus fan iodp site 1457 of the eastern arabian sea

preservation of miocene glacier ice in east antarctica antarctic climate during the pliocene has been the subject of considerable debate one view holds that during part of the pliocene east antarctica was largely free of glacier ice and that vegetation survived on the coastal mountains1a 4 an alternative viewpoint argues for the development of a stable polar ice sheet by the middle miocene which has persisted since then5a 10 here we report the discovery of buried glacier ice in beacon valley east antarctica which appears to have survived for at least 8 1 million years we have dated the ice by 40ar 39ar analysis of volcanic ash in the thin overlying glacial till which we argue has undergone little if any reworking isotope and crystal fabric analyses of the ice show that it was derived from an ice sheet we suggest that stable polar conditions must have persisted in this region for at least 8 1 million years for this ice to have avoided sublimation

glacial flow of floating marine ice in snowball earth 1 simulations of frigid neoproterozoic climates have not considered the tendency of thick layers of floating marine ice to deform and spread laterally we have constructed a simple model of the production and flow of marine ice on a planetary scale and determined ice thickness and flow in two situations when the ocean is globally ice covered hard snowball and when the tropical waters remain open soft snowball in both cases ice flow strongly affects the distribution of marine ice flowing ice probably carries enough latent heat and freshwater to significantly affect the transition into a snowball earth climate we speculate that flowing marine ice rather than continental ice sheets may be the erosive agent that created some neoproterozoic glacial deposits

glacial to holocene fluctuations in hydrography and productivity along the southwestern continental margin of india oxygen isotope and sedimentological records of a sediment core from the southwestern continental margin of india are used to reconstruct the fluctuations in sea surface hydrography and productivity during the last deglaciation the delta o 18 records of globigerinoides ruber and globigerinoides sacculifer exhibit high amplitude oscillations during the early deglaciation and holocene significant variations in delta o 18 during the holocene and a high glacial interglacial amplitude in delta o 18 values delta delta o 18 similar to 2 1 o suggest large fluctuations in sea surface hydrography related to monsoonal precipitation on land our results suggest that although the last deglacial events are coherent in time with the regional and global climatic events fluctuations in sea surface salinity during holocene are unique to this region maximum monsoonal precipitation appears to have occurred after similar to9 8 ka bp unlike the records from the western arabian sea biological productivity was maximum between similar to 18 and 15 ka bp and substantially diminished between similar to 13 and 6 ka bp the increased strength in the winter monsoon during the glacial period could have enhanced convective mixing and nutrient injection leading to high primary production reduced surface productivity during the early holocene may be attributed to the complex dynamics of the circulation system and hydrological cycle along the southwest coast of india c 2001 elsevier science b v all rights reserved

early to late holocene contrast in productivity omz intensity and calcite dissolution in the eastern arabian sea the oxygen minimum zone omz is a potential source of co2 to the atmosphere and the extensive omz spread to 1 37 106 km2 in the northern arabian sea would be an important contributor to the co2 budget in this perspective we report here that the eastern arabian sea experienced coeval increase in productivity and denitrification from 7 to 0 kyr which coincides with minimum dissolved oxygen levels in bottom waters in addition during the same period an increased calcium carbonate dissolution occurred as evidenced by low caco3 content decreasing shell weights of planktonic foraminifer species globigerinoides ruber with well marked dissolution features on their shells therefore this study has implications on the role of omz in governing caco3 dissolution and contributing to an increase in atmospheric co2

mudball surface dust and snowball earth deglaciation 1 recent modeling results have raised doubts about the ability to deglaciate from a global glaciation at atmospheric carbon dioxide levels that are realistic for a neoproterozoic snowball earth here we argue that over the lifetime of a snowball event ice dynamics should lead to the development of a layer of continental and volcanic dust at the ice surface in the tropics that would significantly lower the tropical surface albedo and encourage deglaciation this idea leads to the prediction that clay drapes found on top of neoproterozoic glaciations should be thicker in tropical than extratropical regions we test this idea by running the foam general circulation model gcm with an added tropical dust layer of different sizes and albedos and find that the tropical dust layer causes snowball deglaciation at pco2 0 01 0 1 bar in a reasonable regime of these parameters we find similar though more nuanced results from a limited number of test cases using national center for atmospheric research s cam gcm

analysis of local ice crystal growth in snow the structural evolution of snow under metamorphism is one of the key challenges in snow modeling the main driving forces for metamorphism are curvature differences and temperature gradients inducing water vapor transport and corresponding crystal growth which is detectable by the motion of the ice air interface to provide quantitative means for a microscopic validation of metamorphism models a vtk based image analysis method is developed to track the ice air interface in time lapse ct experiments to measure local interface velocities under both isothermal and temperature gradient conditions using estimates of local temperatures from microstructure based finite element simulations a quantitative comparison of measured interface velocities with theoretical expressions is facilitated for isothermal metamorphism the data are compared with a kinetics and a diffusion limited growth law in both cases the data are largely scattered but consistently show a mean curvature dependency of the interface velocity for temperature gradient metamorphism we confirm that the main contribution stems from the temperature gradient induced vapor flux accompanied by effects of mean curvature as a secondary process the scatter and uncertainties are discussed in view of the present theoretical understanding the experimental setup and complications such as mechanical deformations

reduced himalayan sediment production 8 myr ago despite an intensified monsoon uplift of the tibetan plateau about 7 8 myr ago1 2 may have been responsible3 5 for the apparent intensification of the asian monsoon 6 10 around that time increases in the oceanic 87sr 86sr ratio during the neogene period have been attributed11 12 to increased erosion from the himalayan orogen it has been suggested that the monsoonal intensification may have enhanced overall erosion rates in the region5 13 if this were the case sediment accumulation rates would have increased in the surrounding basins at this time here we present a reanalysis of stratigraphic data from the indo gangetic foreland and the bengal fan which demonstrates that both of these basins experienced a decline in sediment accumulation rates 8 myr ago thus it seems that monsoonal intensification was accompanied by a decrease in mechanical weathering this decrease could be due to reduced tectonic activity decreased himalayan glaciation or slope stabilization from dense plant cover

migration of air bubbles in ice under a temperature gradient with application to snowball earth 1 to help characterize the albedo of sea glaciers on snowball earth a study of the migration rates of air bubbles in freshwater ice under a temperature gradient was carried out in the laboratory the migration rates of air bubbles in both natural glacier ice and laboratory grown ice were measured for temperatures between 36 c and 4 c and for bubble diameters of 23 2000 m the glacier ice was sampled from a depth near close off 74 m in the jems2 ice core from summit greenland migration rates were measured by positioning thick sections of ice on a temperature gradient stage mounted on a microscope inside a freezer laboratory the maximum and minimum migration rates were 5 45 m h 1 k cm 1 1 at 4 c and 0 03 m h 1 k cm 1 1 at 36 c besides a strong dependence on temperature migration rates were found to be proportional to bubble size we think that this is due to the internal air pressure within the bubbles which may correlate with time since close off and therefore with bubble size migration rates show no significant dependence on bubble shape estimates of migration rates computed as a function of bubble depth within sea glaciers indicate that the rates would be low relative to the predicted sublimation rates such that the ice surface would not lose its air bubbles to net downward migration it is therefore unlikely that air bubble migration could outrun the advancing sublimation front transforming glacial ice to a nearly bubble free ice type analogous to low albedo marine ice

studies of snow surface characteristics by landsat tm in dronning maud land antarctica large scale melting phenomena like meltwater drainage channels and meltwater accumulation basins or frozen lakes have been surveyed on the land ice mass in dronning maud land antarctica these melting features were also detected in a landsat thematic mapper tm image recorded on 12 february 1990 image processing techniques such as principal component pc analysis band ratioing and histogram equalizing are carried out to emphasize the melting phenomena interestingly a histogram equalized single tm band 5 image appears to be a good discriminator leaving the melting phenomena as bright areas in the tm scene the largest frozen lake is close to 1km wide while some of the drainage channels stretch more than 5 km there is also a comparison of satellite derived and in situ reflectance and temperature the atmospheric corrected satellite data underestimate surface snow albedo by about 16 while tm measurements of surface temperature are 2 to 6 c lower than that measured on the ground

productivity fluctuations in the southeastern arabian sea during the last 140 ka abstract detailed analyses of multiple geochemical productivity proxies such as calcium carbonate biogenic opal biogenic ba organic carbon and total nitrogen have been carried out on a sediment core from the southeastern arabian sea to trace the palaeoproductivity fluctuations over the last 140 ka high calcium carbonate biogenic opal biogenic ba contents wt and their mass accumulation rates g cm2 ka suggest higher productivity during major interglacials than in the glacials organic carbon content

surficial glaciology of jakobshavns isbrae west greenland part i surface morphology jakobshavns isbrae is a large fast moving ice stream outlet glacier in west greenland which ends at a floating calving front it drains about 6 5 of the area of the greenland ice sheet studies of its surface morphology are described in this paper the surface is relatively steep 0 01 0 03 and the thickness is large up to 2600 m along the center line clarke and echelmeyer 1989 indicating very high driving stresses 200 300 kpa the ice stream is about 6 km wide and 85 90 km long all of which is in an area of surface melting the base of the ice stream and of much of the drainage area is below sea level marginal crevasse zones have a width on the order of the width of the ice stream itself unique surficial features are ice blisters and lakes the latter have a sequence of ogive like features on their floating ice cover which can be used to determine velocity there is a pinning point near the terminus which may act as a stabilizing influence possibly playing a role in halting at least temporarily a recent retreat of the terminus ice thickness estimates at the terminus lead to a flux which is less than previously assumed by others e g bindschadler 1984 pelto and others 1989 when estimating jakobshavns isbrae s drainage basin to be nearly in balance the driving stresses on jakobshavns isbrae are an order of magnitude higher than those of the ice streams of west antarctica its crevasse patterns are much less localized its relatively unconfined terminus is more comparable to that of relatively unbuttressed ice streams such as pine island and thwaites glaciers than it is to other west antarctic ice streams which terminate in large confined ice shelves

global variations of surface ocean productivity in low and mid latitudes influence on co2 reservoirs of the deep ocean and atmosphere during the last 21 000 years based on detailed reconstructions of global distribution patterns both paleoproductivity and the benthic 13c record of co2 which is dissolved in the deep ocean strongly differed between the last glacial maximum and the holocene with the onset of termination i about 15 000 years ago the new export production of low and mid latitude upwelling cells started to decline by more than 2 4 gt carbon year this reduction is regarded as a main factor leading to both the simultaneous rise in atmospheric co2 as recorded in ice cores and with a slight delay of more than 1000 years to a large scale gradual co2 depletion of the deep ocean by about 650 gt c this estimate is based on an average increase in benthic 13c by 0 4 0 5 the decrease in new production also matches a clear 13c depletion of organic matter possibly recording an end of extreme nutrient utilization in upwelling cells as shown by sarnthein et al 1987 the productivity reversal appears to be triggered by a rapid reduction in the strength of meridional trades which in turn was linked via a shrinking extent of sea ice to a massive increase in high latitude insolation i e to orbital forcing as primary cause

distribution and sources of organic matter in surface sediments of the eastern continental margin of india the sources and distribution of organic matter om in surface sediments of the eastern continental margin of india including the region influenced by river discharge were investigated using content molar c n ratios and stable isotopes of carbon and nitrogen despite relatively high water column integrated chlorophyll a concentrations were found in the continental shelf than the slope however the lower sediment organic carbon soc was found in the former than the latter region suggesting that in situ production did not play significant role on preservation of soc in the coastal bay of bengal the broad range of 13 c of soc 23 2 to 16 7 suggests that om is a broad mixture of terrestrial and marine om relative contributions from terrestrial c3 and c4 plants and marine sources are quantified as 34 23 and 43 respectively indicating that dominant source of allochthonous om 57 in the coastal bay of bengal relatively higher contribution of om from c4 plants were found in the sediments at off river krishna indicating that this region received detritus of agricultural crops such as jowar bajra and sugar cane which are dominant in its drainage basin during sw monsoon this study revealed that relatively high om preserved in the slope than shelf region along the coastal bay of bengal and the composition of om is primarily controlled by the type of agricultural crops and vegetation in the drainage basin of the river

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

organic carbon preservation in southeastern arabian sea sediments since mid holocene implications to south asian summer monsoon variability the earlier studies show a contrasting long term trend of the south asian summer monsoon sasm after attaining the precessional forcing induced mid holocene maximum the increasing total organic carbon toc concentration of marine sediments in the southeastern arabian sea seas has been interpreted to imply strengthening sasm since mid holocene by a few studies however toc concentration is also influenced by redox conditions sedimentation rate and an influx of terrigenous matter depending on the regional settings so it needs to be ascertained whether the toc concentration of the sediments in the seas is a signal of productivity related to the sasm strength or preservation therefore we studied multiple proxies toc total nitrogen atomic c n 13corg caco3 and major and trace elements concentration for determining the productivity redox conditions detrital supply and provenance in a sediment core from the upper continental slope of the seas spanning the past 4700 years at centennial scale resolution the present study shows that the observed toc increase is not a result of enhanced productivity but is because of better preservation due to the increased sedimentation rate along with increasingly reducing conditions since mid holocene we further show that the sasm has been declining since mid holocene after attaining a precession forced maximum which corroborates the earlier model ensemble studies

preservation of elemental and isotopic source identification of sedimentary organic matter the amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments organic matter alterations can continue to sub bottom depths of hundreds of meters corresponding to millions of years source and paleoenvironmental information nonetheless remains preserved in the molecular elemental and isotopic compositions of organic matter c n and 13c values of total organic matter in particular appear to retain paleoenvironmental information for multi myr time periods

large scale hydrological change drove the late miocene c4 plant expansion in the himalayan foreland and arabian peninsula carbon isotope changes in paleosols from siwalik pakistan and marine sediments from the bengal fan indicate a major c 4 plant expansion in the himalayan foreland during the late miocene however the timing and mechanisms behind the c 4 plant expansion remain enigmatic here we present high resolution 60 k y biomarker and compound specific isotope data spanning the past 11 m y from ocean drilling program site 722 in the arabian sea an 5 6 increase in leaf wax 13 c values indicates a marked rise of c 4 plants from 10 to 5 5 ma with accelerated expansion from 7 9 to 5 5 ma a concurrent 50 rise in leaf wax d values is attributed to a combined effect of changes in precipitation amount and evaporation indicating that source regions for the plant waxes became progressively drier from 10 to 5 5 ma in contrast to earlier reports our isotope records biomarker abundances alkenone u k 37 and globigerina bulloides abundance data do not suggest enhanced summer monsoon circulation during this time interval rather our results suggest that large scale hydrological changes drove the late miocene expansion of c 4 plants in the himalayan foreland and arabian peninsula

snowball earth a thin ice solution with flowing sea glaciers 1 the late neoproterozoic era 600 800 myr ago was marked by at least two intervals of widespread cold that left glacial deposits at low paleolatitudes both snowball solutions with global ice cover and slushball solutions with ice free tropical oceans have been proposed to explain the paleomagnetic data the snowball model is best able to explain the auxiliary geological evidence particularly the existence of cap carbonates but it implies drastic survival pressure on the photosynthetic biota if the ice cover was everywhere thick enough to prevent sunlight from reaching the underlying ocean a thin ice solution that avoids this problem has been proposed but is thought to be inconsistent with realistic optical properties of sea ice and with the equatorward flow of sea glaciers here we use a coupled energy balance climate sea glacier model to argue that these apparent difficulties can be overcome and that thin tropical ice may have prevailed during these remarkable glacial episodes we also suggest that 1 some processes beyond the scope of zonal mean energy balance models may significantly affect the solutions and 2 thin ice could have prevailed on low latitude seas or large lakes protected from large scale sea glacier flow by surrounding land even if most of the earth was in a hard snowball state

monsoonal upwelling in the western arabian sea since the middle miocene the asian monsoon has long been argued to be a product of the himalaya tibetan plateau and simulation experiments have confirmed the key role of the himalaya tibetan plateau in transforming regional atmospheric and oceanic circulations however temporal constraints on the strengthening of the asian monsoon inferred from foraminifer isotopic and faunal data and terrestrial climatic and ecological records are inconsistent with each other which has obscured the tectonic climatic linkage in particular discriminating the post middle miocene global cooling from the monsoon upwelling cooling is critical but poorly understood due to the lack of adequate constraints for monsoonal upwelling here we present new middle to late miocene biomarker based reconstructions of sea surface temperature sst for the western arabian sea our new ssts capture a long term ocean cooling since ca 14 8 ma and a major drop in sst in the period 11 10 ma after which the ssts reached similar values as the holocene the new sst record is consistent with planktonic foraminifer siliceous biota and geochemical tracer studies suggestive of ocean cooling and high productivity associated with monsoonal upwelling the 11 10 ma ocean cooling is not clearly expressed in other tropical oceans indicating that the ocean cooling in the western arabian sea is not a simple reflection of global cooling we interpret the 11 10 ma ocean cooling as representing the establishment of monsoonal upwelling in the western arabian sea triggered by strong cyclonic activities as a result of the neogene outward expansion of the himalaya tibetan plateau

koci drill for drilling in ice sand and rock drill requirements design performance difficulties a new man portable drill designed to perform in ice with entrained sand and rock has been built designated the koci drill it was first used on rock covered glaciers in the mcmurdo dry valleys antarctica during the 2006 07 field season this drill is designed to be disassembled into component parts no heavier than 30 kg not including the generator for stability the drill platform weight can be augmented with available rock the field performance of the drill was generally good and both ice and rock were successfully penetrated and acceptable quality ice cores up to 1 m long were collected during the 2007 08 season the maximum depth achieved was 10 25 m given a concentrated effort at one or two sites per season a depth of 40 m is achievable problems encountered in the field will be addressed in the next generation design

variations of the monsoon regime during the upper quaternary evidence from carbon isotopic record of organic matter in north indian ocean sediment cores abstract the 13 c 12 c ratios and contents of organic carbon in deep sea sediment cores of the north indian ocean have been compared for the different climatic stages as determined by oxygen isotope stratigraphy using the differences in the 13 c values between marine and terrestrial organic matter the terrestrial contribution to the eastern gulf of bengal and the andaman sea has been determined for modern and glacial times the geographical distribution of the terrestrial organic carbon linked to sea surface circulation shows that during the last glacial period the ne monsoon circulation was the dominant feature the organic carbon abundance within the sediment which is related to local productivity was greater during glacial times in the areas where production depends on the ne monsoon strength coast of north india in the arabian sea and andaman sea suggesting its reinforcement

physical and structural properties of the greenland ice sheet project 2 ice core a review substantial data sets have been collected on the relaxation characteristics density grain size c axis fabrics and ultrasonic velocities of the greenland ice sheet project 2 gisp2 core to its contact with bedrock at 3053 4 m changes in all these properties paralleled closely those found in cores from byrd station antarctica and dye 3 greenland density increased progressively with depth to a maximum of 0 921 mg m3 at about 1400 m at which depth the ice became bubble free below about 2000 m in situ densities began to decrease in response to increasing ice sheet temperatures since drilling much of the ice core has undergone significant volume expansion relaxation due to microcracking and the exsolving of enclathratized gases especially in the brittle ice zone between 650 and 1400 m grain size increased linearly to about 1000 m thereafter remaining fairly constant until the younger dryas event at 1678 m where a twofold to threefold decrease in grain size occurred these grain size changes were accompanied by a progressive clustering of crystal c axes toward the vertical including a small increase in c axis concentration across the younger dryas holocene boundary increased dust levels in the wisconsin ice have contributed to the maintenance of a fine grained texture which with its strong vertical c axis fabric persisted to nearly 3000 m however beginning at about 2800 m layers of coarse grained ice intermixed with the much finer grained matrix ice are observed below 3000 m the ice became very coarse grained this change attributed to annealing recrystallization at elevated temperatures in the ice sheet was accompanied by a dispersed or ring like redistribution of the c axes about the vertical ultrasonic measurements of vertical and horizontal p wave velocities made at 10 m intervals along the entire length of the gisp2 core fully confirmed the results of the crystallo optical observations a return to fine grained ice coincided with the first appearance of brown silty ice 13 m above bedrock bedrock material consisted of 48 cm of till including boulders and cobbles overlying gray biotite granite comprising the true bedrock there is evidence that disturbed structure in the gisp2 cores begins little more than 70 of the way through the ice sheet this disturbance increases with depth until it becomes large enough to cast suspicion on features lasting centuries or more in the bottom 10 of the ice sheet

comment on snowball earth a thin ice solution with flowing sea glaciers by david pollard and james f kasting 1 pollard and kasting 2005 hereinafter referred to as pk have coupled an energy balance climate model to an ice shelf flow model to investigate the snowball earth episodes of the neoproterozoic 600 800 million years ago when the ocean apparently froze all the way to the equator hoffman and schrag 2002 pk s particular concern was to investigate the possibility that over a wide equatorial band where sublimation exceeded snowfall the bare ice may have been thin enough to permit transmission of sunlight to the water below providing an extensive refugium for the photosynthetic eukaryotes that survived the snowball events this possibility was first proposed by mckay 2000 whose model of radiative transfer and heat conduction predicted tropical ice only a few meters thick for ice albedo up to 0 7 under assumed conditions of sunlight and temperature at the snowball equator however when spectral resolution was incorporated into the radiative transfer model warren et al 2002 the ice albedo had to be reduced below 0 4 to obtain the thin ice solution under otherwise identical conditions it seemed unlikely that such dark sea ice could avoid melting under the equatorial sun but even if it could avoid melting the thin ice would risk being crushed by the inflow of kilometer thick sea glaciers from higher latitudes goodman and pierrehumbert 2003 to further investigate the feasibility of tropical thin ice it was therefore necessary to couple a climate model to models of sea ice thermodynamics and sea glacier flow this is what pk have done surprisingly they conclude that thin ice 3 m thick may have prevailed in a 20 degree latitude band centered on the equator we argue here that this conclusion is too optimistic because pk s thin ice solution apparently required that several controlling variables be set outside their measured ranges the albedo of cold glacier ice the depth of transition from snow to ice and the thermal conductivity of ice we then raise the more general question of how wide is the thin ice domain in the parameter space of model variables 2 choices of model variables that favor thin ice 2 1 albedo of cold glacier ice 2 as sea glaciers flowed equatorward into the tropical region of net sublimation their surface snow and subsurface firn would sublimate away exposing bare glacier ice to the atmosphere and solar radiation this ice would be freshwater meteoric ice which originated from compression of snow so it would contain numerous bubbles giving a high albedo the albedo of cold nonmelting glacier ice exposed by sublimation antarctic blue ice has been measured as 0 55 0 65 in four experiments in the atlantic sector of antarctica bintanja and van den broeke 1995 bintanja et al 1997 liston et al 1999 reijmer et al 2001 0 63 in the transantarctic mountains warren et al 2002 and 0 66 near the coast of east antarctica weller 1968 weller s blue ice albedo is often quoted as 0 69 e g weller 1980 king and turner 1997 but that is an average value which included a few times with patchy snow cover on the ice not all glacier ice is bubbly under several hundred meters of ice thickness the pressure becomes so great that each air bubble dissolves in the ice to form a clathrate crystal and the ice becomes relatively clear price 1995 however when this ice becomes depressurized as its cover sublimates away as in the antarctic blue ice areas where the albedos were measured the bubbles reform lipenkov 2000 so that the albedo of 0 55 0 66 is observed at the surface 3 in contrast to glacier ice which forms by compression of snow sea ice forms by freezing of liquid water in their model pk tried two values of scattering coefficient each of which they applied uniformly to both sea ice and glacier ice with albedo 0 47 a reasonable value for meter thick snowfree sea ice brandt et al 2005 figure 1 the sea glaciers terminated at 9 degrees latitude leaving an equatorial strip of thin ice but with albedo 0 64 i e within the measured range for glacier ice the sea glaciers advanced to the equator with a thickness of 1 km to add realism to the model it would be good to assign different values of scattering coefficient to these two very different types of ice

cold based debris covered glaciers evaluating their potential as climate archives through studies of ground penetrating radar and surface morphology we describe the morphology and internal structure of mullins and friedman glaciers two cold based debris covered alpine glaciers that occur in neighboring valleys in the mcmurdo dry valleys antarctica both glaciers are overlain by a single dry supraglacial debris layer 8 75 cm thick each mantling debris layer is marked with near identical patterns of arcuate ridges and steps as well as corresponding changes in bulk grain size meter scale surface topography and thermal contraction crack polygons results from 24 km of ground penetrating radar data show that the ice within the uppermost 1 2 km of mullins and friedman glaciers is essentially free of englacial debris 1 by volume but thereafter is interspersed with bands of englacial debris each 3 m thick and dipping up glacier that intersect the ice surface at all major surface ridges and steps the similarity in number and pattern of englacial debris bands and corresponding surface ridges and steps across both glaciers along with model results and observations that call for negligible basal entrainment is best explained by episodic environmental change at valley headwalls our working hypothesis is that layers of englacial debris originate as supraglacial lags that form in ice accumulation areas during times of reduced net ice accumulation following renewed net ice accumulation these lags are subsequently buried by snow and ice flow englacially and intersect the ice surface to impart distinctive changes in the texture of supraglacial debris and topographic relief the implication is that the englacial structure and surface morphology of these cold based debris covered glaciers preserves a consistent record of climate and environmental change

improved landslide tsunami prediction effects of block model parameters and slide model 1 subaerial landslide tsunamis and impulse waves are caused by mass movements impacting into a water body and the hazards they pose have to be reliably assessed empirical equations developed with physical froude model studies can be an efficient method for such predictions the present study improves this methodology and addresses two significant shortcomings in detail for the first time these are the effect of three commonly ignored block model parameters and whether the slide is represented by a rigid block or a deformable granular material a total of 144 block slide tests were conducted in a wave flume under systematic variation of three important block model parameters the slide froude number the relative slide thickness and the relative slide mass empirical equations for the maximum wave amplitude height and period as well as their evolution with propagation distance are derived for most wave parameters remarkably small data scatter is achieved the combined influence of the three block model parameters affects the wave amplitude and wave height by up to a factor of two the newly derived equations for block slides are then related to published equations for granular slides this comparison reveals that block slides do not necessarily generate larger waves than granular slides as often argued in the technical literature in fact it is shown that they may also generate significant smaller waves the new findings can readily be integrated in existing hazard assessment methodologies and they explain a large part of the discrepancy between previously published data

changes in surface productivity and subsurface denitrification during the holocene a multiproxy study from the eastern arabian sea spatial and temporal variations in the intensity of subsurface denitrification in the water column of the eastem arabian sea have been investigated using chemical and isotopic proxies the data show a steady increase in the denitrification intensity from 10 ka to 2 ka bp probably caused by changes in surface productivity fuelled by strengthening of upwelling monsoonal intensity in addition changes in redox conditions and detrital material input are assessed using abundances of a suite of major and trace elements the temporal variations in some of the monsoonal proxies are characterized by periodicities of 1400 and 700 years similar to those reported in the south asian chinese monsoon and high latitude climate north atlantic region

variations of oxygen minimum and primary productivity recorded in sediments of the arabian sea abstract two deep sea sediment cores from the northeastern and the southeastern arabian sea were studied in order to reconstruct the palaeoenvironments of the past glacial cycles core 136kl was recovered from the high productivity area off pakistan within the modern oxygen minimum zone omz by contrast modern primary productivity at the site of md900963 close to maldives is moderate and bottom waters are today well oxygenated for both cores we reconstructed the changes in palaeoproductivity using a set of biomarkers alkenones dinosterol and brassicasterol the main result is that primary productivity is enhanced during glacial stages and lowered during interstadials the proxies associated with productivity show a 23 kyr cyclicity corresponding to the precession related insolation cycle palaeoredox conditions were studied in both cores using a new organic geochemical parameter c35 c31 n alkane ratio developed by analysing surface sediments from a transect across the omz off pakistan the value of this ratio in core 136kl shows many variations during the last 65 kyr indicating that the omz was not stable during this time it disappeared completely during heinrich and the younger dryas events pointing to a connection between global oceanic circulation and the stability of the omz the c35 c31 ratio determined in sediments of core md900963 shows that bottom waters remained rather well oxygenated over the last 330 kyr which is confirmed by comparison with authigenic metal concentrations in the same sediments a zonally averaged circulation biogeochemical ocean model was used to explore how the intermediate indian ocean responds to a freshwater flux anomaly at the surface of the north atlantic as suggested by the geochemical time series both the abundance of southern ocean water and the oxygen concentration are significantly increased in response to this freshwater perturbation

monsoon related changes in sea surface productivity and water column denitrification in the eastern arabian sea during the last glacial cycle sea surface salinity reconstruction for the holocene and the last glacial maximum lgm suggests that the lgm eastern arabian sea was more saline by 1 5 psu than the holocene due to weakened freshwater flux to the region and intensified winter monsoons time series of sedimentary organic carbon corg p alkenone and y 13 corg for the last 100 kyr together indicate increased glacial productivity significantly reduced sedimentary y 15 n during the lgm indicates however diminished water column denitrification in spite of increased productivity the distinct decoupling of denitrification from productivity during the lgm can be explained by vigorous ventilation of the thermocline as a result of simultaneously intensified formation of high salinity water and deep winter mixing in northern arabian sea related to the extreme cold climate the closely comparable time series y 15 n records across the arabian sea suggest basin wide homogenisation of the isotopic signal this probably resulted from combined monsoon dependent surface water mixing due to changes in the relative strength of reversing surface circulations and intermediate water mixing due to changes in north south salinity gradient d 2005 elsevier b v all rights reserved

development of asian monsoon revealed by marked ecological shift during the latest miocene in northern pakistan carbon isotopes from soil carbonate1 4 and soil organic matter5 6 yield palaeoecological information because the carbon in the soil carbonate forms in isotopic equilibrium with local soil co2 refs 1 7 the isotopic composition of which is in turn determined by local plant cover siwalik group sediments in northern pakistan contain a well exposed palaeosol record spanning the past 18 myr here we report on stable carbon isotope results from associated pedogenic carbonate which indicate a dramatic ecological shift from to dominated floodplain biomass beginning 7 4 7 0 myr ago the earlier c3 floodplain biomasses were probably mainly composed of trees and shrubs whereas c4 grasslands dominated in the plio pleistocene oxygen isotopes also exhibit a shift in the latest miocene probably corresponding to a major climate change which may have induced the forest to grassland transition this dramatic ecological shift in the latest miocene may mark the inception or a marked strengthening of the asian monsoon system

δ13c of organic carbon in the bengal fan source evolution and transport of c3 and c4 plant carbon to marine sediments carbon isotopic measurements on organic carbon oc in sediment cores from the bengal fan odp leg 116 show a dramatic 10 increase beginning ca 7 ma ago and a rapid decrease after 0 9 ma these shifts reflect changes in the mixing ratio of terrigenous carbon derived from c3 and c4 plants the rapid increase in 13c of bengal fan oc at 7 ma shows that late miocene expansion of c4 plants already documented in the siwaliks was widespread over all the himalayan foreland after 7 ma relations of 13c with sedimentological parameters show that c4 plants dominate in the foreland whereas c3 plants remain abundant in the mountain range variations in the source of the sediments and of the oc appear to be sensitive to climate hydrologic conditions in the basin major changes in the isotopic composition of the carbon flux in one of the worlds largest river systems modify the isotopic budgets of both marine dissolved carbon and the sedimentary carbon mass

snowball earth ice thickness on the tropical ocean 1 on the tropical oceans of a neo proterozoic snowball earth snow free ice would have existed in regions of net sublimation photosynthesis could have continued beneath this bare ice if it was sufficiently thin and sufficiently clear the steady state ice thickness is determined by the necessity to balance the upward conduction of heat with three subsurface heating rates the heat flux from the ocean to the ice base the latent heat of freezing to the ice base and the solar energy absorbed within the ice a preliminary study using a broadband model for solar radiation and assuming a large freezing rate had indicated that tropical ice might be only a few meters thick here we show that the vertical throughput of ice by surface sublimation and basal freezing would be too slow to keep the ice thin and that the broadband model had exaggerated the absorption depth of sunlight we use a spectral model for solar absorption computing radiative transfer at 60 wavelengths considering absorption by the ice and scattering by bubbles with the spectral model the computed ice thickness is much greater for a solar flux of 320 w m 2 at the equatorial surface and expected albedo of 0 5 for bare sea ice we find that surface temperatures below 12 c generate ice layers too thick for photosynthesis 100 m if the albedo were as low as 0 4 thick ice would occur only for surface temperatures below 25 c but such low temperatures would be difficult to maintain with such low albedo for surface temperatures warmer than these limits the ice becomes thin 1 m and is unlikely to represent a coherent ice layer however glacial deformation of thick floating ice from nearby oceanic regions may preclude the existence of thin or ice free patches

a 225 kyr record of dust supply paleoproductivity and the oxygen minimum zone from the murray ridge northern arabian sea abstract analysis of a 15 m long sediment core recovered from the murray ridge northern arabian sea reveals a 225 000 year record of climate induced variations in surface water productivity mid water ventilation and dust input in the northern arabian sea productivity and dust input records closely resemble those from the owen ridge and oman margin also the phase lag of 10 kyr between maximum productivity and precession minimum is in phase with the summer monsoon upwelling record from the owen ridge and the oman margin this implies that the productivity variations in the northern arabian sea during the last 225 kyr are primarily controlled by changes in the indian ocean summer monsoon the winter monsoon obviously played a minor role in the productivity history of the northern arabian sea another result from this study is that the mg al record from the murray ridge nicely documents the post glacial flooding of the persian gulf which is the main glacial source for dolomite dust in this region finally it is concluded that the intensity of the omz co varies with surface water productivity being weak during periods of reduced summer monsoon especially during glacial periods

effects of bubbles cracks and volcanic tephra on the spectral albedo of bare ice near the transantarctic mountains implications for sea glaciers on snowball earth 1 spectral albedo was measured along a 6 km transect near the allan hills in east antarctica the transect traversed the sequence from new snow through old snow firn and white ice to blue ice showing a systematic progression of decreasing albedo at all wavelengths as well as decreasing specific surface area ssa and increasing density broadband albedos under clear sky range from 0 80 for snow to 0 57 for blue ice and from 0 87 to 0 65 under cloud both air bubbles and cracks scatter sunlight their contributions to ssa were determined by microcomputed tomography on core samples of the ice although albedo is governed primarily by the ssa and secondarily by the shape of bubbles or snow grains albedo also correlates highly with porosity which as a proxy variable would be easier for ice sheet models to predict than bubble sizes albedo parameterizations are therefore developed as a function of density for three broad wavelength bands commonly used in general circulation models visible near infrared and total solar relevance to snowball earth events derives from the likelihood that sublimation of equatorward flowing sea glaciers during those events progressively exposed the same sequence of surface materials that we measured at allan hills with our short 6 km transect representing a transect across many degrees of latitude on the snowball ocean at the equator of snowball earth climate models predict thick ice or thin ice or open water depending largely on their albedo parameterizations our measured albedos appear to be within the range that favors ice hundreds of meters thick

late miocene global cooling and the rise of modern ecosystems a period of continental aridification and ecosystem change occurred about seven million years ago a global sea surface temperature reconstruction identifies cooling temperatures and a strengthened meridional temperature gradient at this time

monsoon influenced variations in productivity and lithogenic flux along offshore saurashtra ne arabian sea during the holocene and younger dryas a multi proxy approach abstract a sediment core from ne arabian sea was studied to reveal the variations in shelf environment of western continental margin of india a multi proxy analysis based on planktonic foraminifera abundance pfa benthic foraminifera abundance bfa planktonic benthic foraminiferal ratio p b caco 3 percentage wt oxygen 18 o g ruber and carbon 18 o g ruber isotopes of foraminifera as well as sediment x ray studies were carried out on 3 4 m long core sk 240 485 recovered from offshore saurashtra ne arabian sea the study aims to understand the monsoon variability in the ne arabian sea offshore saurashtra since the late pleistocene holocene transition and how monsoon climate coupled with sea level change influenced paleo productivity pattern six ams radiocarbon dates place the core in the time interval 12 1 ka our study reveals that there have been three periods of major changes in the monsoonal climate productivity and sea level viz 12 8 ka younger dryas and early holocene 8 4 ka middle holocene and 4 1 ka late holocene the studied time period was also marked by weakened south west monsoon swm during the cold events such as younger dryas yd abrupt event ae centring around 9 8 ka as well as 8 2 and 4 ka cold events the younger dryas and early holocene shows the low sea level stand and less indus river discharge thereafter during middle and late holocene the monsoon strengthened which resulted in raised sea level and increased indus river discharge the proxy records broadly suggest relatively low foraminiferal productivity at the core location during the younger dryas and early holocene we record a moderate foraminiferal productivity during the middle holocene between 8 and 4 ka which suggest the mesotrophic condition the foraminiferal productivity however was relatively high since 4 ka suggesting the eutrophic conditions at the core site

through thick and thin marine and meteoric ice in a snowball earth climate 1 marine ice cover in the frigid neoproterozoic climate system tends to isolate the ocean the thickness of this ice and thus the degree of isolation is an important unknown in geochemical and biological arguments regarding the fully glaciated snowball earth hypothesis the pollard and kasting 2005 coupled atmosphere ice model has been modified to track ice of marine and atmospheric origin separately and model their different optical properties in contrast to pollard and kasting s results a tropical region of thin ice is not stable in this model ice is hundreds of meters thick everywhere the overall pattern of the hydrological cycle in a snowball climate is also discussed