transfer of basal sliding variations to the surface of a linearly viscous glacier on etudie pour une plaque a faces paralleles d epaisseur h et de viscosite lineaire l effet en surface des fluctuations de la vitesse de derapage

new age constraints for the proterozoic aravalli delhi successions of india and their implications abstract proterozoic sedimentary successions of india are important archives of both the tectonic history of the indian subcontinent and the geochemical evolution of earth surface processes however the lack of firm age constraints on many of these stratigraphic units limits their current utility here we present new detrital zircon age data from strata of the southern aravalli delhi orogenic belt adob and the rajasthan vindhyan successions the alwar group of the southern delhi supergroup yielded a large population of 1 2 ga detrital zircon grains which refutes the 1 9 1 7 ga age assertion for this unit detrital zircon age distributions from the southern alwar group differ strongly from the alwar group of the north delhi belt demonstrating miscorrelation between these two regions the jhamarkotra formation of the lower aravalli group contains a large population of 1 9 1 7 ga detrital zircon grains therefore the unit cannot be 2 1 ga as traditionally assumed age distributions of the aravalli and delhi supergroups are similar to those of the lower and upper vindhyan successions and we postulate contiguous sediment sources for both regions with strata of the tectonically deformed adob representing the distal margin equivalents of the vindhyan successions additionally a late paleoproterozoic age for the jhamarkotra formation nullifies the hypothesis that the markedly positive carbonate 13c values in this unit are linked to the 2 3 2 0 ga lomagundi jatuli positive isotope excursion the potential of a large late paleoproterozoic ca 1 7 ga positive 13c excursion contrasts with the long held view of a prolonged period of carbon isotope stasis during the so called boring billion

extreme 13ccarb enrichment in ca 2 0 ga magnesite stromatolite dolomite red beds association in a global context a case for the world wide signal enhanced by a local environment abstract the palaeoproterozoic positive excursion of 13 c carb is now considered as three positive shifts of 13 c carb separated by returns to 0 which all occurred between 2 40 and 2 06 ma this isotopic event is unique in terms of both duration 300 ma and 13 c enrichment up to 18 the mechanism responsible for one of the most significant carbon isotopic shifts in earth history remains highly debatable to date 13 c of 10 to 15 cannot be balanced by organic carbon burial forg as there is no geological evidence for an enhanced corg accumulation prior to or synchronous with the excursion instead termination of these excursions is followed by formation of a vast reservoir of 13 c depleted organic material 45 at shunga and by one of the earliest known oil generation episodes at 2 0 ga none of the three positive excursions of 13 c carb is followed by a negative isotopic shift significantly below 0 as has always been observed in younger isotopic events reflecting an overturn of a major marine carbon reservoirs this may indicate that forg was constant implying that the mechanism involved in the production of corg was different onset of intensive methane cycling resulting in c change is another possibility the majority of sampled 13 c carb rich localities represents shallow water stromatolitic dolostones red beds and evaporites formed in restricted intracratonic basins and may not reflect global 13 c carb values closely spaced drill core samples n 73 of stromatolitic dolostones from the 1980 27 ma tulomozerskaya formation in the onega palaeobasin russian karelia have been analysed for 13 c carb and 18 o carb in order to demonstrate that different processes were involved in the formation of 13 c carb rich carbonates the 800 m thick magnesite stromatolite dolomite red beds succession formed in a complex combination of environments on the karelian craton peritidal shallow marine low energy protected bights barred basins evaporative ephemeral ponds coastal sabkhas and playa lakes the carbonate rocks exhibit extreme 13 c enrichment with 13 c values ranging from 5 7 to 17 2 vs v pdb mean 9 9 2 3 and 18 o from 18 6 to 26 0 vs v smow mean 22 0 1 6 the tulomozerskaya isotopic excursion is characteristic of the global 2 4 2 06 ga positive shifts of carbonate 13 c 12 c although it reveals the greatest enrichment in 13 c known from this interval an external basin s is considered to have provided an enhanced corg burial and global seawater enrichment in 13 c the global background value for the isotopic shift at tulomozero time ca 2 0 ga is roughly estimated at around 5 an explosion of stromatolite forming microbial communities in shallow water basins evaporative and partly restricted environments high bioproductivity enhanced uptake of 12 c and pene contemporaneous recycling of organic material in cyanobacterial mats with the production and consequent loss of co2 and ch4 are believed to be additional local factors which may have enhanced 13 c from 5 up to 17 such factors should be taken into account when interpreting carbon isotopic data and attempting to discriminate between the local enrichment in 13 c and globally enhanced 13 c values we propose that many previously reported 13 c values from other localities where environmental interpretations are not available or have not been taken into account may not represent the global 13 c values

spatial patterns of glacier motion during a high velocity event haut glacier d arolla switzerland the surface motion of haut glacier d arolla switzerland was monitored at a high spatial and temporal resolution data are analyzed to calculate surface velocities surface strain rates and the components of the glacier force budget before during and after an early melt season speed up or spring event we investigate the extent to which variations in glacier motion can be attributed to hydrologically induced local forcing or to non local forcing transmitted via horizontal stress gradients enhanced glacier motion is dependent on a change in the spatial distribution of areas of high drag across the glacier

a positive δ 13 c carb anomaly in paleoproterozoic carbonates of the aravalli craton western india support for a global isotopic excursion abstract carbon isotope measurements carried out on carbonate samples from different localities of the early proterozoic aravalli craton western india yield positive 13 c values up to 11 20 pdb this widespread isotopically anomalous carbonate province reported from this subcontinent coincides with the appearance of stromatolites and the development of red beds these observations support a global paleoenvironment change during the paleoproterozoic as reported from other parts of gondwanaland including the kapvaal craton africa and the sao francisco craton brazil

late paleoproterozoic magmatism in delhi fold belt nw india and its implication evidence from epma chemical ages of zircons abstract zircons from six granitoid plutons occurring in the northern part of the delhi fold belt nw india have been chemically dated using an electron microprobe most of the zircons occurring in the granites are chronologically unzoned and gave ages around 1 78 1 71 ga but younger ages of 1 1 0 9 ga have also been obtained from the rim of some grains monazite and zircons from the metasediments of delhi supergroup adjacent to the granitoids were also analyzed the older age of 1 9 1 7 ga obtained for monazites could possibly indicate their provenance age detrital zircon grains mostly rounded in the delhi metasediments which gave older ages of 1 9 ga could possibly have been derived from the archaean basement rocks exposed in this aravalli delhi fold belt the geochemical signatures of these granitoids are consistent with an extensional tectonic regime it is quite likely that contrary to the prevailing assumption of the granitoids being intrusive our geochronologic data suggests them to be the basement for sedimentation the new geochronologic information on these granitoids in the northern part of the delhi fold belt and their comparison with other correlative events establishes a widespread thermal event during late paleoproterozoic

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

violent volcanism stagnant oceans and some inferences regarding petroleum strata bound ores and mass extinctions past changes from a well mixed aerated ocean to a stratified stagnant ocean are indicated by decreasing 13c of limestone opposite to prior conclusions as well as by increasing 34s of marine sulfates the proposed stagnant ocean model includes a warm co2 enriched atmosphere and a bathyal to abyssal marine system dominated by bacterial components in the food web and by consequent light carbon in organic material and biogenic carbonates culminations of prolonged stagnant episodes correspond with marine faunal extinctions of late permian and late cretaceous time and the protracted changes and correlations provide evidence against any hypothesis of mass extinction by sudden worldwide catastrophe including the asteroid impact hypothesis the cretaceous stagnant ocean taken as the prime example is attributed to climatic warming triggered by volcanic co2 greenhouse effect and several feedbacks including decreased earth albedo and increased sinking of warm evaporite brines instead of aerated polar waters marine extinctions are attributed to upward expansion of the oxygen minimum zone and to catastrophic mixing of surface waters with poisonous sulfidic waters of the deeps the stagnant ocean provided a counterbalance between deep reduction and shallow oxidation conditions that favored maximal formation of black sulfidic shales protopetroleum and sedimentary sulfide ores and shallow to mid depth barite phosphorite iron ore cherty iron formation and sulfate bearing red beds trace elements concentrated in stagnant ocean sediments include chalcophile and pt group metals negating claims that ir provides a unique fingerprint of meteorite impact and cosmic accretion

strontium and carbon isotope geochemistry applied to dating of carbonate sedimentation an example from high grade rocks of the norwegian caledonides isotope stratigraphy has been applied to an undated high grade carbonate sequence from the norwegian caledonides in order to constrain the depositional age sokumfjell group marbles of the beiarn nappe complex in the uppermost allochthon of the nordland caledonides were subjected to several phases of deformation culminating in amphibolite facies metamorphism the ca 1000 m thick marble sequence studied is composed of five different lithofacies lithofacies 1 4 are calcite marbles and lithofacies 5 is dolomite marble all the calcite marble lithofacies exhibit throughout rhythmic bedding and textural grading on different scales suggesting that carbonate material was deposited in a pelagic turbidite system 123 samples were analysed for major and trace elements and acid soluble mg and ca of which 42 samples of calcite marbles and four samples of dolomite marbles were selected for 13 c 18 oa nd 87 sr 86 sr analyses all selected calcite marbles have high sr contents averaging 2000 ppm they are characterised by 87 sr 86 sr ratios ranging between 0 70642 and 0 70999 and 13 c values ranging between 1 3 and 8 4 v pdb 18 o values are highly variable 14 1 24 8 v smow the best preserved samples selected on the basis of mn sr and rb concentration and mn sr and mg ca ratios yield average 87 sr 86 sr values of 0 70669 0 70697 0 70727 0 70749 and 0 70737 for lithofacies 1 2 3 4 and 5 respectively post sedimentary alterations including amphibolite facies metamorphism resulted in only a slight increase of 87 sr 86 sr values in the marbles studied apparently due to aragonite precursor high sr concentration and a very early and fast diagenetic alteration caused by fluids isotopically comparable with that of seawater projection of these values along with 13 c measurements onto 87 sr 86 sr and 13 c calibration curves for the secular variations of seawater suggests that the lithofacies studied were apparently deposited between 635 and 600 ma 2001 elsevier science b v all rights reserved

links between short term velocity variations and the subglacial hydrology of a predominantly cold polythermal glacier the surface velocity of a predominantly cold polythermal glacier john evans glacier ellesmere island canada varies significantly on both seasonal and shorter time scales seasonal variations reflect the penetration of supraglacial water to the glacier bed through significant thicknesses of cold ice shorter term events are associated with periods of rapidly increasing water inputs to the subglacial drainage system earlyseason short term events immediately follow the establishment of a drainage connection between glacier surface and glacier bed and coincide with the onset of subglacial outflow at the terminus a mid season short term event occurred as surface melting resumed following cold weather and may have been facilitated by partial closure of subglacial channels during this cold period there is a close association between the timing and spatial distribution of horizontal and vertical velocity anomalies the temporal pattern of surface water input to the glacier and the formation seasonal evolution and distribution of subglacial drainage pathways these factors presumably control the occurrence of highwater pressure events and water storage at the glacier bed the observed coupling between surface water inputs and glacier velocity may allow predominantly cold polythermal glaciers to respond rapidly to climate induced changes in surface melting

water pressure and basal sliding on storglaciaren northern sweden the subglacial hydrology of the ablation area of storglaciaren a small valley glacier in northern sweden is dramatically affected by a subglacial ridge or riegel water pressures above this riegel are relatively constant while down glacier from it they vary significantly the lower part of the glacier accelerates in response to peaks in basal water pressure the upper part may be weakly coupled to the lower part during these peaks a power law fit of observed basal water pressures and measured surface velocities yields where u s is the surface velocity and p e is the effective water pressure ice overburden pressure minus subglacial water pressure data from findelengletscher reported by iken and bindschadler 1986 yield an identical exponent and a coefficient one order of magnitude larger the similar exponent implies that the process producing the velocity variations on both glaciers is similar the variations in velocity are inferred to be due to hydraulic jacking on both glaciers

influence of subglacial drainage conditions on the velocity distribution within a glacier cross section substrate properties and hydrological conditions at the base of haut glacier d arolla switzerland are nonuniform the thickness and grain size of subglacial sediment vary on length scales less than one ice thickness whereas hydrological conditions vary seasonally and on length scales of about one ice thickness transverse to ice motion there is a close relationship between the annually averaged velocity field and this nonuniformity of bed conditions basal motion dominates in an area with large water level variations in boreholes whereas ice deformation contributes significantly to total movement elsewhere localized enhanced basal motion occurs primarily in summer especially during a july spring event but this motion is barely discernible in annually averaged surface velocity measurements because transverse coupling suppresses surface expression of the basal motion discontinuity these results highlight the need to include representations of bed nonuniformity in models of glacier flow and to consider ice deformation and basal motion as interdependent processes

physical and chemical signatures of a developing anticyclonic eddy in the leeuwin current eastern indian ocean a multidisciplinary cruise aboard the r v southern surveyor was conducted in may 2006 to sample a developing anticyclonic eddy of the leeuwin current off western australia the eddy formed from a meander of the leeuwin current in mid april 2006 and remained attached to the current until mid august in this study a combination of satellite data altimeter sea surface temperature and chlorophyll a and shipboard measurements acoustic doppler current profiler and conductivity temperature depth were used to characterize the physical and chemical signatures of the eddy the temperature salinity properties of the mixed layer waters within the anticyclonic eddy and on the shelf were both connected to that of the leeuwin current indicating the water mass in the eddy is mainly derived from the leeuwin current and the modified leeuwin current water on the shelf above the salinity maximum near the eddy center there was a regionally significant concentration of nitrate 0 9 mol l 1 and the maximum 2 mol l 1 was at 150 in depth below the photic zone nitrification within the eddy and or local upwelling due to the forming eddy could be responsible for this high concentration of nitrate near the eddy center which potentially makes the eddy a relatively productive feature in the leeuwin current

genesis of upper proterozoic cambrian phosphorite deposits of india isotopic inferences from carbonate fluorapatite carbonate and organic carbon carbon and oxygen isotope analyses performed on sedimentary carbonates organic carbon kerogen and structurally bound co2 3 groups of carbonate fluorapatite francolite from four widely separated indian phosphorite provinces have shown that each of the deposits is characterized by specific basically facies dependent distribution patterns of 13ccarb 13ocarb 13corg and 13capat comparison of the carbon isotope composition of apatite co2 with that of the coexisting sedimentary carbonate has served to further constrain the genetic type of the individual phosphorite deposit investigated augmenting relevant geological evidence by an additional geochemical discriminator as a whole the results confirm the importance notably of carbon isotope work for elucidating the processes involved in sedimentary phosphorite formation

stabilization of the aravalli craton of northwestern india at 2 5 ga an ion microprobe zircon study abstract in order to establish the stabilization age of the aravalli craton of northwestern india we have used an ion microprobe to measure 207 pb 206 pb ages of zircons from six samples collected from the southern aravalli mountains our analyses gave precise results for four granitoids whose minimum crystallization ages ranged between 2562 6 and 2440 8 ma 1 a single sample of gneiss gave an age of 2502 4 ma which is unexpectedly younger than an age of 3281 ma which we previously obtained from another gneissic sample we interpret the 2502 ma age as the time of emplacement of the sample s igneous protolith suggesting that the mewar gneiss contains multiple metamorphic components of differing ages based on our data we conclude that the southern segment of the aravalli craton had broadly stabilized by 2 5 ga this stabilized landmass formed the basement on which the younger aravalli supergroup was unconformably deposited this study represents the first application of a small geometry ion microprobe for routine geochronology

the importance of pressurized subglacial water in separation and sliding at the glacier bed l effet de l eau sous glaciaire sous pression sur le processus de glissement est examine a partir d un parametre appele indice de decollement du lit qui exprime les contraintes perpendiculaires au lit et la pression d eau les donnees proviennent de trois glaciers de formes differentes une loi puissance du type de weertman avec un exposant voisin de 3 modifiee pour tenir compte de l effet de la pression des eaux sous glaciaires apparait comme etant la meilleure

misinterpreting proxy data for paleoclimate signals a reply to srivastava and jovane 2020 srivastava and jovane 2020 have made several comments on our assessment of proxy data and challenged the outcome of shukla et al 2020 based mainly on interpretation of environmental magnetic p

high δ13c paleoproterozoic carbonates from the transvaal supergroup south africa marbles from the transvaal supergroup kaapvaal craton south africa record two positive carbon isotope excursions to average 13 c carbonate values of 6 to 8 relative to v pdb separated by carbonates with normal marine 13 c values 0 2 these elevated values have been preserved through metamorphic devolatilization together with an already documented high 13 c excursion in overlying rocks three separate positive carbon isotope excursions occurred on the kaapvaal craton in the interval 2 43 to 1 93 ga it is not yet clear how many of these excursions reflect global rather than local processes

carbon isotopes and the rise of atmospheric oxygen new data for the isotopic composition of carbon in carbonate sediments deposited between 2 6 and 1 6 ga indicate that the value of 13c in these sediments underwent a very large positive excursion between 2 22 and 2 06 ga a reassessment of the earlier 13c data for carbonate sediments shows that this excursion was probably worldwide and that it was preceded and followed by several hundred million years during which the 13c of carbonate sediments differed little from that of modern carbonates the large 13c excursion between 2 22 and 2 06 ga was probably related to an abnormally high rate of organic carbon deposition which generated an abnormally high rate of o2 production we estimate that the total excess o2 produced during the excursion was between 12 and 22 times the present atmospheric o2 inventory the 13c data therefore suggest that the o2 content of the atmosphere increased very significantly between 2 22 and 2 06 ga this inference is supported strongly by several other lines of evidence

b_na rich palaeoproterozoic aravalli metasediments of evaporitic association nw india a new repository of gold mineralization we report field observations and preliminary petrochemical and stable isotopic compositions of the host rock and sulfide minerals for a recently discovered sulfide hosted gold deposit from the palaeoproterozoic aravalli rocks in northwestern india gold occurs associated with feascu co sulfides in lithofacies of diverse mineralogical composition dominated by amphibole bearing dolomitic marble and albite rich quartzo feldspathic rocks qfr the presence of vugs believed to be after dissolution of halite patches coupled with the development of scapolite bearing assemblages in dolomite marble occurrence of tourmaline rich mineralized rocks and very high na content of carbonates and qfr indicate hypersaline depositional environment for the host rock a narrow spread of 34 scdt values from 10 5 to 12 8 for stratiform arsenopyrite and remobilized pyrrhotite corroborates hypersaline to evaporitic depositional environment the 13 c values for gold bearing carbonates show bimodal distribution one population mean 0 4 v pdb is close to the sea water carbonate values while the other 3 l indicates probable mantle affinity the 18 o values for carbonates mean 18 4 v smow are not discriminative as they appear to have been modified by subsequent metamorphism and hydrothermal activity interpretation of mantellic source for carbon is borne out from the absence of biogenic activity and incompatibility of significantly negative 13 c values in the evaporitic setting of deposition of bhukia rocks we suggest that nab rich fluids channeled through rift related faults were responsible for creating hypersaline conditions that eventually evolved into an evaporitic environment mantle carbon through some still deeper faults was probably responsible for the gold transport and its subsequent deposition in an otherwise uncommon evaporitic repository of carbonates and qfr 2002 elsevier science b v all rights reserved

the flow of a polythermal glacier mccall glacier alaska u s a we have analyzed the flow of polythermal mccall glacier in arctic alaska using measurements of surface velocity from the 1970s and 1990s together with measurements of ice thickness and surface slope we have investigated both the present flow and seasonal and long term flow variations our analysis of the present flow reveals that i longitudinal stress coupling is important along the entire length of the glacier and ii there is significant basal sliding beneath a 2 km long section of the lower glacier this sliding exists year round and it accounts for more than 70 of the total motion there we have developed a numerical model which shows that such a sliding anomaly causes an asymmetric decrease in ice thickness accompanying this decrease in thickness is a decrease in surface slope at the center of the anomaly and an increase in slope up glacier from it both effects are reflected in the observed surface profile of mccall glacier the longitudinal stress coupling length of mccall glacier is three times the ice thickness almost twice that typical of temperate glaciers this is a direct effect of lower strain rates which themselves are associated with the smaller mass balance gradients of arctic and continental glaciers long term variations in surface velocity between the 1970s and 1990s are explained solely by the effects of changes in glacier geometry on the deformational flow contribution this means that long term variations in the spatial patterns of longitudinal stresses and basal sliding must have been small seasonally velocities reach their annual minimum in spring and increase during the short summer melt season by up to 75 above mean winter values however the extra motion associated with the period of elevated velocities is only about 5 of the total annual motion the speed up is due to an increase in basal sliding this implies that most of the glacier bed is at the melting point the zone affected by the melt season speed up extends well up glacier of any moulins or other obvious sources for meltwater at the bed

nutrients plankton and productivity in a warm core ring in the western gulf of mexico argos drift buoy trajectory data showed that a region of anticyclonic circulation about 100 km in diameter was present over the upper continental slope of the nw corner of the gulf of mexico in september october 1988 guided by these data texas am however a maximum of only 36 54 psu at t 25 5 in contrast to as much as 36 88 psu at this density surface in a fresh ring indicated that this feature had spent many months in the western gulf since its separation from the loop current biologically the warm core ring was oligotrophic its surface waters were generally depleted in nitrate to depths of more than 100 m and chlorophyll standing stocks 20 mg m 2 primary productivity 0 4 mg c m 3 h 1 and zooplankton biomass only 4 ml 100 m 3 day and 6 ml 100 m 3 night in the upper 200 m were all extremely low deployments of floating sediment traps within the feature on two consecutive days intercepted fluxes of only 10 20 mg dry weight m 2 d 1 at 50 m and at 100 m by comparison at ring periphery where there was measurable nitrate at 100 m 0 2 1 g at l 1 chlorophyll standing stocks and primary production in the surface mixed layer were 1 5 2 times higher

intra annual and intra seasonal flow dynamics of a high arctic polythermal valley glacier measurements of surface dynamics onpolythermaljohn evans glacier nunavut canada over two winter periods and every 7 10 days throughout two melt seasons june july 2000 2001 provide new insight into spatio temporal patterns of high arctic glacier dynamics in the lower ablation zone mean annual surface velocities are 10 21ma 1 but peak velocities up to 50 higher are attained during latejune earlyjuly in the upper ablation zone and lower accumulation zone mean annual surface velocities are typically10 18ma 1 and peak velocities up to 40 higher occur during latejuly in the upper accumulation zone mean annual surface velocities are 2 9ma 1 and motion in mid to latejuly exceeds this by up to 10 rapid drainage of ponded supraglacial water in the upper ablation zone to an initially distributed subglacial drainage system in mid june may force excess surface motion in the warm based lower glacier the data indicate thatthe durationof the velocity response maybe relatedto the rate of channeliz ation of the basal drainage and the velocity response may be transmitted up glacier by longitudinal coupling an increase in surface velocities in the middle glacier in latejuly occurs in conjunction with the opening of two further moulins in the accumulation zone

on the thermal regime of an arctic valley glacier a study of white glacier axel heiberg island n w t canada from 1974 to 1981 a total of 32 bore holes was drilled on white glacier and vertical ice temperature profiles measured the data obtained allowed the construction of three longitudinal and four transverse profiles of the two dimensional temperature distributions thus an extensive layer of temperate or near temperate ice was discovered close to the bedrock in the lowest part of the glacier tongue it was also found that the temperature distribution cannot be in a steady state since there is a temperature minimum 100 150 m below the glacier surface in the accumulation area a simple numerical model calculation shows that this minimum can be mostly explained by the general climatic warming since 1880 the 10 m temperatures show diffuse relations to climatic conditions and balance zones a simple scheme for extrapolating surface temperatures is discussed

hydrological discharges and motion of fels and black rapids glaciers alaska u s a implications for the structure of their drainage systems characteristics of the hydrology and motion of black rapids and fels glaciers alaska were observed from 1986 to 1989 hydrological measurements included stage electrical conductivity and suspended sediment concentration in the discharge stream of each glacier and were made at 0 5 1 h intervals continuously through most of the melt seasons variations in the glacier speed were monitored through the full year at a number of locations along the length of each glacier using time lapse photography 1 d time resolution strain meters 0 5 1 h resolution and seismometers set up to count acoustic emissions both glaciers show similar seasonal diurnal and short term event changes in hydrological discharges and ice speed the hydrological behavior is analyzed in terms of a fast sub system composed of surface streams moulins and large tunnels with discharge that responds rapidly and a slow sub system composed of heterogeneous small passageways through the ice and distributed over the bed that maintain approximately uniform discharge over a day the timing and amplitude of water discharge in the diurnal cycle indicate that roughly 10 40 of the water is routed directly into the fast system the remaining 90 60 of the water enters the slow system dilution of the solute discharged from the slow system by the variable discharge in the fast system results in changes in water discharge and solute concentration that are approximately equal in relative amplitude and inversely related a small time lag from discharge maximum minimum to solute minimum maximum suggests that the fast system is confined to roughly the lowermost 30 40 of the full glacier length the residence time of water in the fast system is short compared to 1 d the slow system contains both short and long residence time passages characteristics of the diurnal cycles are somewhat variable through the melt season but no systematic evolutionary patterns were discerned even though large changes in the mean discharges of water and solutes occur which suggests parallel evolution of the variables that control the response of the fast system events were characterized by contemporaneous increases in suspended sediment concentration in the discharge water and distinct changes in straining on the glaciers events caused by increases in melt or precipitation related to weather and events related to release from reservoirs internal to the glaciers could be distinguished based on the changes in electrical conductivity of the discharge water the correlated changes in sediment discharge and motion of the glaciers indicate that the events were associated with temporary modifications of the slow passages distributed over the bed that allowed enhanced sliding and access of basal water flow to erosion products hydrological differences between black rapids and fels glaciers can be explained by differences in the size of the glaciers if there is a difference in bed structure that explains the difference in dynamics surge black rapids glacier versus non surge fels glacier it does not affect the hydrological parameters that were observed

systematic decrease of high δ13c values with burial in late archaean 2 8 ga diagenetic dolomite evidence for methanogenesis from the crixas greenstone belt brazil abstract remnants of fe poor 2700 4400 ppm dolomite crystals crystal aggregates and microconcretions are cryptically distributed within stratiform bodies tens of metres across of metamorphic fe rich 3 6 wt fe dolomite that occur in graphitic schists of the 2 8 ga ribeira o das antas formation in the crixas greenstone belt brazil fe poor dolomite is a diagenetic carbonate formed during burial of organic bearing muddy sediments using cathodoluminescence microscopy a consistent sequence of dolomite growth stages is well defined within individual crystals crystal aggregates and microconcretions from comparison with similar textures in phanerozoic carbonates we suggest that the early dolomite formed during progressive burial aggregates having developed from mergent growth of individual crystals during mechanical and then chemical compaction the 13cdolomite values of crystals and concretion cores are very high 12 14 pdb and a negative shift of about 7 occurs over millimetres in subsequent dolomite generations early diagenetic fe poor dolomite and late stage fe rich dolomite show overlapping ranges of initial 87sr86sr ratios between 0 7080 and 0 7164 and 18o values 14 to 16 pdb these similarities may indicate isotopic exchange between the early dolomite and subsequent metamorphic fluids yet the well preserved growth zonation of the fe poor dolomite suggests that such exchange did not follow a process of recrystallization the decrease in the 13c of dolomite is correlated with crystal and concretion growth during burial this behaviour is interpreted to indicate a change in the 13c of pore water bicarbonate resulting from bacterially mediated organic diagenesis within the lower part of the zone of methanogenesis the crixas dolomite represents direct evidence that methanogenic bacteria were important contributors in shallow subsurface geochemical systems in the early precambrian the pattern of 13c values found for the crixas dolomite is also documented in other early to middle precambrian dolomites and limestones previous interpretations have considered positive 13c values to be the result of primary oceanographic controls whereas variability in 13c is commonly linked to metamorphic alteration we suggest that the role played by organic diagenesis in controlling the 13c values of diagenetic carbonates has been underestimated

three long lived anticyclonic eddies in the northern south china sea 1 in august 2007 three long lived anticyclonic eddies ace1 ace2 and ace3 were detected by both satellite sea level anomaly sla map and in situ measurements in the northern south china sea scs ace3 had a two core ace3 1 and ace3 2 structure in situ stations along 18 degrees n almost cut through the centers of ace2 and ace3 2 near the centers of ace2 and ace3 2 mean temperature and sound velocity are similar to 0 65 degrees c and similar to 2 m s 1 larger than those in their surrounding areas respectively while mean salinity and density are similar to 0 02 psu and similar to 0 15 m 3 s 1 smaller than those in their surrounding areas due to downwelling near the eddy cores the depths of maximum and minimum salinity near the eddy cores are similar to 65 m and similar to 35 m larger than those in their surrounding areas the vertical depth with current speed larger than 0 05 m s 1 can reach similar to 900 m their detailed evolutionary processes were depicted based on the variation of geostrophic currents and the trajectories of five drifting buoys ace1 lasted 147 days while ace2 and ace3 lasted 168 days and 210 days respectively ace1 had a smaller mean sla 18 8 cm in its lifetime than ace2 21 8 cm and ace3 25 3 cm but had a larger negative mean vorticity 7 7 x 10 6 s 1 than ace2 7 0 x 10 6 s 1 and ace3 7 0 x 10 6 s 1 one short lived anticyclonic eddy that split from ace2 and another one that merged with ace3 both had a smaller sla negative vorticity and diameter than ace2 and ace3 respectively

did global tectonics drive early biosphere evolution carbon isotope record from 2 6 to 1 9 ga carbonates of western australian basins the 13 ccarb record of well preserved carbonates in outcrop and core is here examined from the 2 6 to 1 9 ga old basins of western australia these data which are constrained by a well defined stratigraphic and tectonic framework and by u pb zircon ages provide an insight into the variables coincident with the evolution of an oxidative atmosphere and the evolution of the early biosphere in the latest archaean ca 2 6 ga the secular 13 ccarb curve is flat much like that seen in the later palaeoproterozoic basins of northern australia 1 8 ga this implies that photosynthesis was a major component of the biosphere at that time and that the carbon mass balance was stable in the early palaeoproterozoic beginning after 2 5 ga and continuing until at least 1 9 ga the 13 ccarb the curve is much more dynamic with significant positive and negative excursions including a major positive excursion 9 pdb close to 2 2 ga these excursions can be correlated with the lomagundi event identified in africa europe and north america previously published studies of the overlying meso to palaeoproterozoic bangemall basin and of 1 8 1 5 ga old basins in northern australia suggest that the 13 ccarb curve became relatively monotonic again after ca 1 8 ga and remained so for most of the following mesoproterozoic comparisons with data from other ancient cratons especially africa suggest that the secular carbon curve may be even more complex than presently understood and probably comparable to the major excursions seen in the neoproterozoic when the western australian data are placed in their stratigraphic and tectonic framework we find that the monotonic latest archaean curve coincides with a tectonically quiescent period in which carbonates formed in an basinal setting on a craton surrounded by passive margins the data are consistent with an earth in which the carbon mass balance was in equilibrium the 13 ccarb curve began to oscillate following the onset of glaciation as the pilbara and yilgarn cratons began to converge during the capricorn orogeny suggesting periods of rapid carbon burial during continental dispersal however the major positive excursion is preserved in carbonates from back arc basins formed as the ocean closed and subduction began since similar tectonic processes can be recognised not only in northern australia but also on other early cratons it can be argued that the carbon excursions relate to supercontinent cycles and to major periods of mantle overturn and superplume development we explain this coincidence of carbon isotopic excursions

seasonal surface velocity variations on a sub polar glacier in west greenland huit balises implantees dans les zones d accumulation et d ablation d un glacier de groenland ont ete relevees tous les dix jours pendant les etes 1982 et 1983 l analyse des vitesses laisse penser qu un reseau sous glaciaire permet a l eau de fusion produite dans la zone d ablation de remonter vers l amont

paleoproterozoic positive δ13ccarb excursion in the northeastern sino korean craton evidence of the lomagundi event abstract the 2 33 2 06 ga positive 13 c carb excursion associated with environmental change and the breakup of the kenorland or superia supercontinent is called the lomagundi or jatulian event or great oxidation event and has been reported in many early precambrian cratons but not yet in the sino korean craton the guanmenshan formation of the liaohe group occurs in the northeastern part of the sino korean craton 13 c carb and 18 o values in 42 samples from this formation range from 3 5 5 9 v pdb and 15 4 24 8 v smow respectively showing a clear positive 13 c carb excursion that characterizes the lomagundi event thirty five of the 42 samples with less hydrothermal alteration have higher 13 c carb and 18 o carb values than the other 7 samples obviously affected by fluid flow confirming that it was fluid flow that reduced the 13 c carb and 18 o carb values this positive 13 c carb excursion places deposition of the guanmenshan formation within the age range of 2 33 2 06 ga

present status of the geochronology of the precambrian rocks of rajasthan abstract age determinations mostly by rb sr analyses of the precambrian rocks of rajasthan by us and by others are summarized and discussed broad periods of acid magmatism at 1 3000 2900 m y 2 2600 2500 m y 3 2000 1900 m y 4 1700 1500 m y and 5 850 750 m y were identified the oldest rocks in the area are the yet undated banded gneisses bgc east of udaipur intruded by the untala granite dated at 2950 m y and hence of mid archean age the basal status hitherto attributed to the berach granite dated at 2600 m y is no longer tenable the radiometric control on the beginning and duration of the overlying aravalli supergroup is not yet satisfactory though a lower limit at 2000 m y is indicated heron s original delhi rocks have recorded two magmatic events widely separate in space and time while the earliest granitic activity at 1600 m y is recorded only in the alwar basin in the northeast the younger activity between 850 750 m y is widespread as shown by the nearly concordant ages of erinpura type granites along the aravalli mountain range and the malani rhyolites in the western plains of the aravalli range

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes

paleosol occurrences along the archean proterozoic contact in the aravalli craton nw india abstract the archean proterozoic contact apc comprises a large scale folded surface around udaipur in nw india the apc is exposed on both eastern and western flanks of a synclinorium formed during thick skinned thrusting of the archean basement and the paleoproterozoic cover units paleosol occurrences on both the limbs of this structure imply an originally continuous paleoweathering horizon and not individual pockets of paleosol as seen today preservation of paleosol sections along the archean proterozoic interface in the aravalli craton is strongly controlled by the structural grain complete paleosol sections are recognized in the eastern flank of the udaipur synclinorium in low strain regions with respect to the regional f1 and f2 folding in this sector the tulsi namla paleosol section represents an ancient lateritic weathering profile with an iron depleted pallid lower zone lithomarge and an iron enriched upper zone mottled zone to ferricrete original kaolinite in the saprolite and pallid zone has been transformed into kyanite during metamorphism and subsequently to white mica during retrograde overprint while fe rich clay and goethite were probably the primary minerals for the chloritoid and hematite which now constitute the upper zone at the western flank of the synclinorium the contact is heavily sheared and archean basement and proterozoic units show interfingering on a small scale here the paleosol sections are incomplete the weathering interval can be inferred by the stabilization of the craton at 2 5 ga and deposition of rift related sequences of the aravalli basin delwara formation at 2 15 ga the udaipur paleosols especially in the eastern flank represent the most complete archean paleoproterozoic paleosol sections in india

carbon and oxygen isotopic variations in peritidal stromatolite cycles paleoproterozoic kajrahat limestone vindhyan basin of central india abstract variations of carbon and oxygen isotopic ratios in response to cyclical sea level fluctuations have been documented from a paleoproterozoic peritidal stromatolite succession the upper division of the kajrahat limestone vindhyan supergroup of central india consists of several shallowing upward stromatolite cycles identified by regular and systematic changes in stromatolite size normally larger stromatolites are followed upward in the succession by smaller stromatolites and microbial laminites that occupy the top of the cycle desiccation cracks are found in all the facies indicating subaerial exposure we investigated the stable isotope compositional variations across nine complete stromatolite cycles showing frequent subaerial emergence carbon and oxygen isotopic values of the limestones in general are comparable to contemporary marine values available from earlier studies but show regular depletion in response to shallowing of the water level the 13 c and 18 o values of the limestones vary within an individual stromatolite cycle depleted values characterize the topmost part of the cycles the isotope pattern is explained by micritic carbonate deposition in different sub environments of the shallow marine domain having different salinity and variable duration of exposure these variations also probably caused the observed scatter in 13 c and 18 o values of supratidal microbial laminites

circulations and eddies over the south china sea derived from topex poseidon altimetry topex poseidon t p altimeter data were used to derive time varying circulation and eddies over the south china sea scs for 1993 1999 large variabilities of sea surface heights and dense distribution of eddies were found to occur along a band across the scs basin a cross over method is introduced to compute pointwise velocity components the t p derived circula tions and eddies are consistent with the drifter results from the world ocean circulation experi ment data center 1 because of monsoons the scs circulation is largely cyclonic in winter is largely anticyclonic in summer and is modulated interannually by el nifio southern oscillation 2 both cold and warm core eddies were found in the waters east of vietnam west of luzon and west of the luzon strait but their times of occurrence and the way they evolved vary interannually 3 the reversal of the alongshore currents east of vietnam was in complete accordance with wind stress the locations and kinematic properties of eddies over scs were estimated by the least squares method the averaged vorticities of the cold and warm core eddies are 1 684x 10 6 and 1 738x l0 6 tad s respectively and the shearing and stretching deformations are one thousandth of the vorticities the angular velocity of eddy decreases with increasing radius coherence analy sis suggests that the interannual and seasonal variations of circulation are largely due to wind stress and the semi annual variation is largely due to wind stress curl the angular velocities of eddies over the central scs basin are coherent with the magnitudes and signs of wind stress curl the seasonal steric anomaly was also computed and its spatial pattern and scale are quite different from those of circulation and eddy

evolution of lower proterozoic epicontinental deposits stromatolite bearing aravalli rocks of udaipur rajasthan india abstract the aravalli rocks 2060 ma old which crop out around udaipur western india comprise a thick sequence of metasediments with stromatolites and basal volcanics resting unconformably over a peneplained basement known as the banded gneissic complex ca 2585 ma old the rocks have undergone a very low grade of metamorphism and display a complex structure resulting from two major and several minor episodes of folding there are two distinctly different facies sequences in the aravalli rocks indicating deep sea and nearshore shelf environments the stratigraphic sequence of the rocks deposited under the shelf environment starts with basic volcanics and tuffs altered to greenschists and quartizites with arkosic conglomerate in the next sequence carbonates predominate in association with orthoquartzites carbonaceous phyllites phyllites and stromatolitic rock phosphate the carbonate sequence passes upward into greywacke phyllite lithic arenite in the distal parts and conglomerate arkose orthoquartzite in proximal areas the youngest sequence comprises orthoquartzite with silty arenite the distribution of different facies particularly that of dolomite with stromatolitic rock phosphate is controlled by sea floor topography suggesting the presence of an epicontinental sea bounded by a landmass to the west and a series of islands and shoals sedimentation in the shelf and epicontinental sea was presumably triggered by development of fault controlled troughs along craton margins terrigenous debris was deposited in newly developed troughs with contemporaneous volcanicity along trough margins with the erasing of the ephemeral relief in the provenance carbonate deposition was initiated the environment encouraged algal growth and formation of stromatolitic rock phosphate carbonaceous phyllites developed in areas of restricted circulation rapid influx of terrigenous detritus with renewed tectonism in the next phase resulted in the deposition of a turbidite sequence of greywacke phyllite and lithic arenite in the deeper parts of the epicontinental sea and conglomerate arkose orthoquartizite in the marginal areas the final phase of sedimentation was presumably under fluvial conditions which marked the completion of epicontinental trough filling the nature of the terrigenous clasts indicates a predominantly granitic source of sediments supply of sediment was mainly from the continent to the east and partly from a landmass to the west the cycle of sedimentation noted in the epicontinental aravalli sea is broadly similar to the model of tectonic stages suggested by krynine 1942

enhanced primary production in the oligotrophic south china sea by eddy injection in spring concentration typically at 0 05 0 08 mgm 3 it is intriguing to explore this unusual happening based on six different remote sensing data and numerical modelling the results suggest that the injection of an ocean eddy is the most likely cause of the bloom due to long range transport of a large 700 500 km anti cyclonic ocean eddy coastal nutrients and plankton could be brought across hundreds of kilometres to the centre of northern scs and impact the biogeochemistry the open ocean part of the northern scs basin has long been considered generally free from coastal influences this work provides new evidence that proves otherwise moreover from the perspective of physical oceanography it is interesting to observe that outside the monsoon seasons there can be well defined anti cyclonic ocean circulation existing in the scs without the prevailing monsoonal wind citation lin i i c c lien c r wu g t f wong c w huang and t l chiang 2010 enhanced primary production in the oligotrophic south china sea by eddy injection in spring geophys res lett 37 l16602 doi 10 1029 2010gl043872

comment on new age constraints for the proterozoic aravalli delhi successions of india and their implications by mckenzie et al precambrian res 238 2013 120 128 the newly discovered 1 9 1 7 ga positive anomaly of carbonate carbon remains to be proven

nutrients and plankton spatial distributions induced by a coastal eddy in the gulf of lion insights from a numerical model a plankton functional types model forced by a hydrodynamic model is used in the present work to study the impacts of a coastal eddy on the distribution of nutrients and plankton in the western part of gulf of lion nw mediterranean sea this study based on a realistic simulation of the year 2001 focuses on a long life anticyclonic eddy detected during summer the studied anticyclonic eddy has some biogeochemical characteristics of those observed in the open ocean as for example the low productivity at their core and rising up of nutricline on their edges however the functioning and consequences of such coastal eddy on nutrients and plankton distributions are complicated by potential interactions with topography wind induced upwelling along the languedoc coast and nearby northern current nc especially the proximity of the southern edge of the eddy with the nc makes possible the exchanges of organic matter plankton at times during the eddy s life the coastal eddy thus transports organic matter from the coastal zone to the offshore domain the coupled model also suggests the importance of offshore to coastal transport by nc through a seeding process of the eddy in plankton at the beginning of its life a detailed biogeochemical functioning of the studied eddy all along its lifetime is proposed from the model results at the beginning the eddy mainly acts as a transporter of the plankton wind induced bloom occurring along the languedoc coast several days before the eddy set up at the same time the eddy shows upwards vertical velocities on its edges creating upwelling of nutrient enriched waters the process of eddy induced upwelling is notably enhanced on the eddy western side along the roussillon coast due to the shelf topography the around eddy advection and relative lower temperatures prevent the large development of phytoplankton along the roussillon coast and the eddy induced bloom is predicted to occur near the northwestern side of the eddy the eddy induced and wind induced blooms of phytoplankton then merge on the eastern side by advection and this process of aggregation may explain the persistent occurrence of a filament of high chlorophyll observed on the offshore edge of the eddy throughout the study period the changes in plankton composition due to a combination of top down and bottom up processes during the coast to offshore transport may explain the disappearance of the filament south of 43 n at the surface

13c enrichment in the palaeoproterozoic carbonate rocks of the aravalli supergroup northwest india influence of depositional environment abstract the dolomitic carbonates from the jhamarkotra formation of the palaeoproterozoic aravalli supergroup are characterized by widely variable carbon isotope ratios 13 c ranging from near zero to as high as 12 v pdb the estimated maximum age ca 2150 ma of the aravalli carbonates help bracketing these with the coeval carbonate bodies of the world that show high positive carbon isotope values the intriguing existence of normal marine 13 c values in some pockets suggest influence of local scale depositional conditions prevailing in different sub basins amongst these two sub basins which showed high 13 c values a hypersaline evaporative condition is considered responsible for the necessary enrichment in one while methanogenesis possibly in conjunction with sulphate reduction processes might have caused such enrichment in the other from the empirical association of the sub basins with profuse stromatolitic phosphorite we infer that the depositional setting that favoured cyanobacterial growth leading to formation of stromatolites prevented growth of methanogenetic archaea in such anoxic environments our study therefore highlights the fact that the early palaeoproterozoic 13 c excursion in the aravalli supergroup is not essentially a time specific event but is greatly dependent on the variation in the depositional palaeoenvironment prevailing in different sub basins

influences of the juan de fuca eddy on circulation nutrients and phytoplankton production in the northern california current system 1 a diagnostic circulation model and water mass analyses are used to examine variability in the structure and circulation of the juan de fuca eddy a highly productive region at the northern end of the california current results from three years of field studies demonstrate that the eddy increases in spatial extent from early to late summer as the vertically averaged contribution of california undercurrent source water grows from 60 in june to 80 in september typical near surface eddy radii range from 15 km in the early summer to 30 km in september and increase with depth below 100 m eddy radii are 40 km fresher water associated with the estuarine outflow from the juan de fuca strait is advected around the eddy margin during southward wind conditions the combination of cyclonic geostrophic flow and wind driven currents in the surface ekman layer cause the eddy to be leaky on its southern perimeter eddy surface circulation becomes more retentive up to 32 d observed during periods of weak winds or frequent northward reversals the presence of the eddy facilitates large inputs of dissolved inorganic nutrients into the region through two mechanisms doming of california undercurrent water within the eddy and enhanced cross shelf advection of juan de fuca strait outflow the combination of these sources results in a persistent broad 100 km offshore region of elevated macronutrients the retentive circulation patterns combined with persistent nutrient supply may favor the development of toxigenic diatom blooms of pseudo nitzschia species in this region

geochemistry of precambrian carbonates iv early paleoproterozoic 2 25 0 25 ga seawater in an effort to narrow down the first and second order variations in isotopic composition of the early paleoproterozoic 2 25 0 25 ga seawater we report a comprehensive study of mineralogy chemistry and isotopic composition of the malmani dolomite transvaal supergroup south africa duck creek dolomite wyloo group australia and bruce limestone member of the espanola formation huronian supergroup canada based on trace element data their diagenetic rank increases in the order duck creek bruce malmani the interpolation of alteration trends to best values yields an estimate of 0 70550 for 87sr86sr comparable to our previously published value of 0 70496 for 13c the measured range of 0 1 5 pdb is similar to that observed for phanerozoic marine carbonates in contrast the best 18o value for dolostones is 5 pdb depleted in 18o relative to phanerozoic counterparts but comparable to the estimates obtained by similar approach for archean facies the isotope geochemistry and mineralogy of bruce limestone member is consistent with the proposition that the sequence may have been deposited in a lacustrine environment possibly related to the recession of the brace glaciers if such interpretation proves unacceptable from a sedimentological standpoint the origin of the unusual isotopic signatures must be sought despite the above trace element interpretation in as yet unspecified post depositional alteration phenomena in contrast to phanerozoic carbonates the early paleoproterozoic as well as the previously described archean counterparts are enriched in mn and fe

single zircon evaporation ages constraining the growth of the archaean aravalli craton northwestern indian shield single grain 207 pb 206 pb zircon ages were determined for granitoids and gneisses which constitute the archaean basement rocks of the aravalli craton of rajasthan northwestern indian shield the protolith ages for two gneisses collected from east of udaipur are 3230 ma and 2887 ma respectively the granitoids display an intrusive relationship with the gneisses and yielded ages ranging between 2666 ma and 2620 ma these ages provide the basis for a geochronological model of evolution of the oldest basement of the aravalli craton

a mesoscale eddy driving spatial and temporal heterogeneity in the productivity of the euphotic zone of the northeast atlantic in this paper we show how different water masses from a similar geographic region provide an explanation for perturbations in the signal of declining productivity at the porcupine abyssal plain pap study site in the northeast atlantic furthermore we show that the passage of these different water masses is affected by the filamentary instabilities of a cyclonic eddy just southwest of the pap site we describe a high resolution spatial hydrographic survey conducted with a towed instrument package complemented by biogeochemical sampling maximum rates of primary production of 110 mmol c m 2 d 1 seen at the centre of the survey area were associated with the passage of an eddy filament and were enhanced 3 fold relative to far field conditions 36 mmol c m 2 d 1 the rotation and stirring influence of the eddy resulted in the sequential passage of 3 distinct water masses past the observation point this understanding of the lateral stirring around the site enabled us to explain the sharp changes observed in daily primary production rates and other biogeochemical parameters the spatial survey also revealed a fluorescence maxima associated with the cyclonic eddy that was laterally displaced northwards away from the core an observation supportive of recent modelling studies

mesoscale eddies dominate surface phytoplankton in northern gulf of alaska abstract the hnlc waters of the gulf of alaska normally receive too little iron for primary productivity to draw down silicate and nitrate in surface waters even in spring and summer our observations of chlorophyll sensed by seawifs north of 54 n in pelagic waters 500 m depth of the gulf found that on average more than half of all surface chlorophyll was inside the 4 cm contours of anticyclonic mesoscale eddies the ratio approaches 80 in spring months yet these contours enclosed only 10 of the total surface area of pelagic waters in the gulf therefore eddies dominate the chlorophyll and phytoplankton distribution in surface pelagic waters we outline several eddy processes that enhance primary productivity eddies near the continental margin entrain nutrient and fe rich and chlorophyll rich coastal waters into their outer rings advecting these waters into the basin interior to directly increase phytoplankton populations there in addition eddies carry excess nutrients and iron in their core waters into pelagic regions as they propagate away from the continental margin as these anticyclonic eddies decay their depressed isopycnals relax upward injecting nutrients up toward the surface layer we propose that this transport brings iron and macro nutrients toward the surface mixed layer where they are available for wind forced mixing to bring them to surface these mesoscale eddies decay slowly but steadily perhaps providing a relatively regular upward supply of macro nutrients and iron toward euphotic layers they might behave as isolated oases of enhanced marine productivity in an otherwise iron poor basin we note that much of this productivity might be near or just below the base of the surface mixed layer and therefore poorly sampled by colour sensing satellites it is possible then that eddies enrich phytoplankton populations to a greater extent than noted from satellite surface observations only

eddy induced mixed layer shallowing and mixed layer thermocline exchange the effects of eddies on the mixed layer and on the transfer of properties between the mixed layer and thermocline are studied in a high resolution model of an unstable front a spring scenario is considered with a mixed layer maintained against surface warming by wind mixing tilting of the buoyancy gradients by ageostrophic flow is found to shallow the mixed layer over the front giving a mixed layer as shallow as 10 m in some places compared with the equilibrium monin obukhov depth of 30 m away from the front variations of mixed layer depth with time and lateral advection lead to a subduction field that is substantially different from the vertical velocity field vertical velocities associated with the growing instabilities push tongues of a ventilation tracer initially only present within the surface layer down to depths of up to 100 m after 24 days a quarter of this ventilation tracer has been deposited in the thermocline conversely the vertical motions pull nutrients from the thermocline up into the mixed layer giving enhanced surface values along the front in the basin average the geostrophic eddies give vertical eddy diffusivities for a ventilation tracer of o 3 10 10 4m2s 1

stromatolites in precambrian carbonates evolutionary mileposts or environmental dipsticks stromatolites are attached lithified sedimentary growth structures accretionary away from a point or limited surface of initiation though the accretion process is commonly regarded to result from the sediment trapping or precipitation inducing activities of microbial mats little evidence of this process is preserved in most precambrian stromatolites the successful study and interpretation of stromatolites requires a process based approach oriented toward deconvolving the replacement textures of ancient stromatolites the effects of diagenetic recrystallization first must be accounted for followed by analysis of lamination textures and deduction of possible accretion mechanisms accretion hypotheses can be tested using numerical simulations based on modern stromatolite growth processes application of this approach has shown that stromatolites were originally formed largely through in situ precipitation of laminae during archean and older proterozoic times but that younger proterozoic stromatolites grew largely through the accretion of carbonate sediments most likely through the physical process of microbial trapping and binding this trend most likely reflects long term evolution of the earth s environment rather than microbial communities

late holocene climate response and glacial fluctuations revealed by the sediment record of the monsoon dominated chorabari lake central himalaya we studied a periglacial lake situated in the monsoon dominated central himalaya where an interplay of monsoonal precipitation and glacial fluctuations during the late holocene is well preserved a

a review of temporal constraints for the palaeoproterozoic large positive carbonate carbon isotope excursion the lomagundi jatuli event abstract the palaeoproterozoic lomagundi jatuli event is one of the largest magnitude and earliest known positive carbonate carbon isotope excursions preserving 13 c values between 5 and 16 and even higher it is recorded in sedimentary rocks on all continents bar antarctica and spans stratigraphic thicknesses ranging from several to many tens of metres this unique positive 13 c interval signals fundamental changes in the global carbon cycle and is a key event in earth system evolution following oxygenation of earth s atmosphere here we present a comprehensive review of the age constraints on the lomagundi jatuli event the first such effort in two decades this new chronology compilation focuses on the u pb and re os chronometers and demonstrates that global synchronicity of the lomagundi jatuli event is permissible an interpretation supported by the apparent wide dispersion of lomagundi jatuli event bearing successions in the most recent palaeoproterozoic plate reconstructions assuming the lomagundi jatuli event is synchronous worldwide then the bounds on its duration range from a maximum of 249 9 myr 2306 9 ma to 2057 1 ma to a minimum of 128 9 4 myr 2221 5 ma to 2106 8 ma

chemostratigraphy of paleoproterozoic carbonate successions of the wyoming craton tectonic forcing of biogeochemical change the archean wyoming craton is flanked on the south and east by belts of paleoproterozoic supracrustal successions whose correlation is complicated by lack of geochronologic constraints and continuous outcrop however carbonate units in these successions may be correlated by integrating carbon isotope stratigraphy with lithostratigraphy the 10 km thick paleoproterozoic snowy pass supergroup in the medicine bow mountains was deposited on the present day southern flank of the wyoming craton it contains three discrete levels of glacial diamictite correlative with those in the huronian supergroup on the southern margin of the superior craton the nash fork formation of the upper snowy pass supergroup is significantly younger than the uppermost diamictite and was deposited after the end of the paleoproterozoic glacial epoch carbonates at the base of the nash fork formation record remarkable 13 cenrichment up to 28 v pdb whereas those from overlying members of the lower nash fork formation have d 13 c values between 6 and 8 carbonates from the upper nash fork formation above the carbonaceous shale have carbon isotope values ranging between 0 and 2 5 the transition from high carbon isotope values to those near 0 in the nash fork formation is similar to that at the end of the ca 2 2 2 1 ga carbon isotope excursion in fennoscandia this chemostratigraphic trend and deposition of bifs mn rich lithologies carbonaceous shales and phosphorites at the end of the global ca 2 2 2 1 ga carbon isotope excursion are likely related to ocean overturn associated with the final breakup of the kenorland supercontinent correlative carbonates from the slaughterhouse formation in the sierra madre wy and from the whalen group in the rawhide creek area in the hartville uplift wy have highly positive carbon isotope values in contrast carbonates from other exposures of the whalen group in the hartville uplift and all carbonate units in the black hills sd have carbon isotope values close to 0 combined with existing geochronologic and stratigraphic constraints these data suggest that the slaughterhouse formation and the succession exposed in the rawhide creek area of the hartville uplift are correlative with the lower and middle nash fork formation and were deposited during the ca 2 2 2 1 ga carbon isotope excursion the estes and roberts draw formations in the black hills and carbonates from other exposures in the hartville uplift postdate the ca 2 2 2 1 ga

misinterpreting proxy data for paleoclimate signals a comment on shukla et al 2020 shukla et al explored paleoclimatic signals from a 8 m thick profile of a moraine dammed lake in the central himalaya exposed due to lake burst from a flash flood in 2013 the main objective of t

links between palaeoproterozoic palaeogeography and rise and decline of stromatolites fennoscandian shield through a review of literature and new data we have documented two major events in the palaeoproterozoic history of stromatolites as indicated by palaeontological and palaeoenvironmental studies with a time resolution of between 40 and 200 ma we confirm semikhatov and raaben s and awramik s albeit approximately maximum in diversity and abundance of stromatolites between 2330 and 2060 ma ago jatulian diversification we suggest that this taxonomic diversity was driven by a major phase of cratonisation formation of the karelian carbonate platform and numerous rift related shallow water carbonate basins supersaturated with ca 2 mg 2 and co2 the jatulian stromatolite explosion is synchronised with a positive 13ccarb shift of jatulian age carbonates we also document stromatolite decline which occurred on the fennoscandian shield somewhere between 2060 and 1900 ma ago this decline both in abundance and in taxonomic diversity is interpreted as having been caused by the first phase of oceanisation the oceanisation led to the considerable reduction in ecological niches that could be utilised by cyanobacteria the post jatulian decline of stromatolites coincides with an abrupt downward 13corg shift from 19 to 38 and is roughly coeval with the appearance of the first eukaryotic algae documented elsewhere the systematics of the fennoscandian diversity of palaeoproterozoic stromatolites is identical to that reported from india and china and reveals a dissimilarity with abundance and diversity patterns in australia and northern america

chlorophyll dispersal by eddy eddy interactions in the gulf of mexico 1 a lagrangian analysis of the transport and dispersal of plumes observed in satellite derived ocean color images was conducted using a data assimilating model of the gulf of mexico the interaction between pervasive cyclonic and anticyclonic eddies in the gulf generated advective paths that connect remote shelf regions these paths aligned remarkably well with the plume events recorded with the chlorophyll a ocean color product from seawifs two such events were studied in one event material was transported in a thin strip between the northern wall of the loop current and an adjacent cyclone connecting the eastern campheche shelf off the yucatan peninsula and south florida shelves the other event began off the louisiana shelf break as a small plume traced by chlorophyll and then developed into a long and thin feature which meandered to the shelf break off the northern yucatan peninsula moving between a large anticyclone and several adjacent cyclones these results indicate that inter eddy advection plays a crucial role in developing the ocean color patterns observed in the satellite ocean color data

positive δ13c excursion in carbonate and organic fractions from the paleoproterozoic aravalli supergroup northwestern india abstract carbon and oxygen isotopic investigations have been carried out on the archean and paleoproterozoic carbonate rocks of the udaipur region in the aravalli mountain belt northwestern india the study has led to the interesting finding of 13 c enrichment in the carbonate rocks of the jhamarkotra formation 13 c carb up to 11 1 v pdb belonging to the 2200 1900 ma paleoproterozoic aravalli supergroup further it is observed that the organic carbon from phosphorite bearing stromatolitic dolomites of the same formation are also enriched in 13 c 13 c org up to 11 1 however it must be emphasized that the 13 c enriched carbonates and the organic fractions are from different stratigraphic levels although in the same formation the former being at a lower stratigraphic level a critical analysis of the field geological petrological as well as the isotopic data indicates that the observed 13 c excursion in the carbonate carbon is not due to local causes such as methanogenesis or evaporitic conditions the shallow water environment of deposition of these carbonates as well as the immediately overlying formations rules out the stratified ocean model for the observed excursion the model involving high sedimentation rates and organic carbon burial has been preferred to explain the 13 c excursion in the carbonates of the jhamarkotra formation the 13 c enrichment in organic fraction occurred in localized regions of the aravalli sedimentary basin where high productivity was supported by phosphorous supply indicating that the diffusion limited pathways of organic carbon fixation were operative in the paleoproterozoic itself the 13 c enriched carbonate rocks of the jhamarkotra formation serve as the indian example for the paleoproterozoic global 13 c excursion

mapping thermal and hydrological conditions beneath a polythermal glacier with radio echo sounding spatial patterns in residual bed reflection power brp r derived from ground based radio echo sounding were mapped and interpreted in terms of the thermal and hydrological conditions at the base of a high arctic polythermal glacier john evans glacier ellesmere island canada brp r is the residual from a statistical relationship between measured bed reflection power and ice thickness that describes the rate of dielectric loss with depth in the glacier we identified three types of thermal structure a positive brp r and an internal reflecting horizon occur over the glacier terminus the reflecting horizon is interpreted as the boundary between warm and cold ice and suggests the presence of a warm basal layer b positive brp r occurs without an internal reflector in the upper part of the ablation zone this suggests that ice is at the pressure melting point only at the bed c negative brp r without an internal reflector occurs in all other regions suggesting cold ice at the bed where brp r is positive its pattern is similar to the pattern of subglacial water flow predicted from the form of the subglacial hydraulic equipotential surface this suggests that hydrological conditions at the glacier bed are a major control on brp r probably because the dielectric contrast between ice and water is higher than that between ice and other subglacial materials

the wineglass effect shapes particle export to the deep ocean in mesoscale eddies mesoscale eddies in the ocean strongly impact the distribution of planktonic particles mediating carbon fluxes over 1 3 of the world ocean however mechanisms controlling particle transport through eddies are complex and challenging to measure in situ here we show the subsurface distribution of eddy particles funneled into a wineglass shape down to 1000 m leading to a sevenfold increase of vertical carbon flux in the eddy center versus the eddy flanks the wineglass effect we show that the slope of the wineglass r is the ratio of particle sinking velocity to the radially inward velocity such that r represents a tool to predict radial particle movement here 0 05ms 1 a simple model of eddy spindown predicts such an ageostrophic flow concentrating particles in the eddy center we explore how size specific particle flux toward the eddy center impacts eddies biogeochemistry and export fluxes

mechanisms for vertical nutrient transport within a north atlantic mesoscale eddy abstract prompted by observational evidence for an enhanced source of surface nutrients within an anticyclonic eddy in the ne atlantic we investigate vertical transport processes that may produce such a phenomenon for the eddy investigated the dominant mechanism is found to be ageostrophic circulation resulting from a perturbation of the circular flow of the eddy this can produce upwelling velocities of order 10 m d 1 ekman pumping due to wind stress on the eddy also produces upwelling within but on a smaller scale of 0 5 m d 1 there is no evidence that self propagation of the eddy leads to an enhanced nutrient flux by displacement upwards of nutrient laden deeper isopycnals over the core of the eddy deeper winter mixing within the eddy relative to the surroundings however may contribute to the elevated nutrient levels

provenance of proterozoic basal aravalli mafic volcanic rocks from rajasthan northwestern india nd isotopes evidence for enriched mantle reservoirs abstract komatiites komatiitic basalts and tholeiites from nathdwara rajasthan have been investigated for their nd isotopic characteristics 143nd 144nd vary broadly in the komatiites 0 51131 0 51252 and show a restricted range in komatiitic basalts and tholeiites 0 51120 0 51179 and 0 51141 0 51202 respectively the corresponding epsilon nd nd values indicate an enriched source for these rocks the model ages of komatiitic basalts and tholeiites cluster between 2 2 and 3 0 ga the komatiites show older model ages between 3 5 and 3 8 ga if the correlations found between nd and sm isotopic ratios can be interpreted as isochrones then the komatiites point to the age of 2300 ma and the komatiitic basalts and tholeiites constrain the age of 1800 2000 ma the age of 2300 ma for the komatiite samples is similar to the range estimated for the komatiitic basalt and tholeiite samples 1800 2000 ma considering the errors involved collectively the data suggest an age of 2300 1800 ma for the basal aravalli volcanism the variation of nd isotopic ratios between the komatiites and the associated basalts is too large to reflect analytical bias or open system behavior for sm and nd it may be due to significant mantle heterogeneity in the source region isotopically distinct sources for the komatiites and komatiitic basalts are possible but simple mixing between a depleted mantle component and a crustal component in the manner of magmatic assimilation of wall rock material is inconsistent with the observed isotopic ratios the inference of an enriched mantle component appears inevitable as the alternative model of mixing would have generated more siliceous high magnesian boninitic rocks with much lower fe and ti than are observed this enriched material may have been part of the sub continental lithospheric mantle sclm under the aravalli craton which could have generated the basal aravalli magmatic rocks we suggest that during early middle proterozoic the sclm under cratonic regions consisted of relatively fertile mantle capable of generating komatiites tholeiites and alkalic magmas

high phosphate availability as a possible cause for massive cyanobacterial production of oxygen in the paleoproterozoic atmosphere abstract the deposition of major precambrian phosphorites was restricted to times of global change and atmospheric oxygenation at both ends of the proterozoic phosphorites formed after highly positive carbon isotope excursions in carbonates deposited during the paleoproterozoic lomagundi jatuli event and the neoproterozoic cryogenian and ediacaran periods the correlative step wise rise in atmospheric oxygen over the proterozoic has been linked to changes in the carbon cycle however the postulated relations between carbon isotope events phosphorites and atmospheric oxygenation remain unexplained paleoproterozoic carbonates of the aravalli supergroup india preserve evidence for cyanobacterial blooms in the form of tightly packed stromatolitic columns in the world s oldest significant sedimentary phosphate deposit restricted basins of the lower aravalli group with stromatolitic phosphorites in jhamarkotra udaipur jhabua and sallopat exhibit near zero 13 c carb values and large ranges of 13 c org values between 33 3 and 10 1 indicative of a complex carbon cycle because phosphate accumulates primarily in oxic sediments these eutrophic microbial ecosystems likely developed within the photic zone of the shallow oxygenated marine realm this is consistent with deposition during the time of increasingly more oxidizing conditions after the great oxidation event goe approximately contemporaneous basins without phosphate deposits from ghasiar karouli negadia umra and babarmal exhibit a range of positive 13 c carb excursions some with values up to 11 2 that suggest high rates of organic carbon burial and others with moderately high 13 c carb values around 6 or 3 that suggest smaller carbon cycle perturbations the 15 n values of all these rocks vary between 0 7 and 3 4 and are consistent with the predominance of nitrogen fixation during cyanobacterial blooms in all basin types such low nitrogen isotope values are interpreted to have arisen from the biological response to high phosphate availability we conclude that increased phosphate availability during and after the paleoproterozoic lomagundi jatuli event likely caused cyanobacterial blooms and was a key factor in the oxygenation of earth s atmosphere increasing oxygenation of the shallow ocean seafloor favored the removal of excess phosphate as authigenic apatite thus dampening effects of weathering increases on organic burial and marine 13 c carb after about 2 0 ga

carbon isotopes as biogeochemical recorders of life over 3 8 ga of earth history evolution of a concept ever since pioneering studies in the late 1930s had shown that the conversion of inorganic carbon into biogenic substances entails sizeable redistributions of the stable carbon isotopes biologically mediated 13c 12c fractionations have come to be recognized as a common corollary of biochemical reactions meanwhile it is firmly established that the universal bias in favour of 12c characterizing biological materials primarily derives from a kinetic isotope effect that is imposed on the first carbon fixing enzymatic carboxylation reaction in the primary metabolism of co2 fixing autotrophic organisms this preference for 12c has turned out to be one of the most enduring relics of the ordered state of the biological precursor substances that may be preserved in fossil organics over billions of years with the currently known sedimentary record at hand it can be stated with confidence that biological carbon isotope fractionations have persisted throughout 3 8 ga of recorded earth history indicating that microbial prokaryotic and archaeoprokaryotic ecosystems had been prolific already on the archean earth while for the time span 3 5 ga the isotopic evidence is unequivocal the information encoded in the preceding record is commonly blurred by a metamorphic overprint this holds particularly for the metasediments of the 3 8 ga old isua supracrustal belt of west greenland which apart from widespread metasomatism have suffered amphibolite grade metamorphism it is known that 13c 12c exchange occurs in organic kerogenous rock constituents during both amphibolite and granulite facies metamorphism if a second carbon partner is available as either fluids or carbonate with isotopic re equilibration often only partially achieved due to the sluggish kinetics of the exchange reaction thermodynamic equilibria predict however that 13c 12c ratios in kerogen and graphite increase during this process hence high t exchange equilibria are always bound to drive 13c values in positive direction the lowermost values encountered being consequently the least exchanged and most pristine with the lowest values of reduced graphitic carbon obtained in early isua studies falling into the range 22 to 28 pdb we had straightforward evidence since the late 1970s that carbon constituents with the isotopic composition of biogenic matter were indeed present in the pre metamorphic isua suite it was therefore by no means surprising that the results of recent isotope work performed on apatite hosted carbonaceous microdomains in isua banded iron formation utilizing advanced techniques of instrumental microanalysis had prompted similar conclusions hence the mainstream of the sedimentary carbon isotope record can be best interpreted as the geochemical manifestation of the isotope discriminating properties of the principal co2 fixing reactions s in biological carbon assimilation suggesting an extreme degree of evolutionary conservatism in the biochemistry of autotrophic carbon fixation as a consequence biological modulation of the geochemical carbon cycle had been established at least 3 8 ga ago having been fully operative by the time of formation of the earth s oldest sediments

apparent enhancement of 234th based particle export associated with anticyclonic eddies national basic research program of china 973 program 2009cb421200 national natural science foundation of china nsfc 41121091 41130857

geochemistry of precambrian carbonates v late paleoproterozoic seawater in an effort to obtain better constrained data for the first and second order variations in the isotopic composition of late paleoproterozoic 1 8 0 2 ga seawater we report a study of mineralogy chemistry and isotopic composition of the coronation supergroup 1 9 ga nwt canada and the mcarthur group 1 65 ga nt australia petrologically both carbonate sequences are mostly dolostones the mcarthur population however contains more abundant textural features that attest to the former presence of sulfates and halite and the facies investigated herein probably represent ancient equivalents of modern evaporitic sabkhas and lacustrine playa lakes this is reflected in 18 o enrichment up to 2 9 pdb and high mn fe and sr concentrations nonetheless the available 18 o data indicate that the best estimate for the archean to late paleoproterozoic limestones is about 7 pdb with dolostones some 3 2 heavier the observed 18 o doiomitc calcite is thus within the range of the proposed fractionation factor for a cogenetic mineral pair this together with the consistency of isotopic patterns for sedimentological facies over almost 2 billion years from the late paleoproterozoic to the present suggest that dolomitization was an early diagenetic event and that the 18 o depletion of the archean to late paleoproterozoic carbonates is not an artefact ofpost depositional alteration the 13 c although within the range of other precambrian and phanerozoic carbonates indicates a possible secular evolution from 0 2 pdb at 1 9 ga ago to 1 1 5 at 1 65 ga ago the secular evolution for reflects the 87 sr increase from 0 7031 0 0008 at 3 5 0 1 ga ago through 0 7025 0 0015 at 2 8 0 2 ga 0 7033 at 2 5 ga 0 7050 at 2 25 0 25 ga 0 7047 at 1 91 to 0 706 at 1 65 ga ago