early marine diagenesis of biogenic aragonite and mg calcite new constraints from high resolution stem and aem analyses of modern platform carbonates prior studies of porewater compositions from modern shallow marine carbonates have established that dissolution and recrystallization of metastable aragonite and mg calcites occur during the earliest stages of sedimentation yet significant microstructural and mineralogical changes in sediment allochems have not been observed in part this is due to the high resolution required to identify diagenetic changes in these finely crystalline materials herein we present ultrastructural and microgeochemical evidence for early marine diagenesis using scanning electron microscopy sem and high resolution scanning transmission analytical electron microscopy stem aem methods to characterize original biogenic materials and sediment boxcores from modern deposits on the floridan bahamas platform we selected the two most common sediment producers a mg calcite foraminifer peneropolid and an aragonitic green algae halimeda for study both are composed of submicron sized crystallites whose excess free energy could provide a thermodynamic driving force for recrystallization stem observations of grain mounts containing dispersed mg calcite and aragonite crystallites from gently disaggregated individual peneropolids and halimeda segments indicate that living specimens contain euhedral prismatic crystals with blunt terminations and smooth faces specimens from the sediment column show abundant evidence for alteration the most dramatic changes involve crystallite dissolution and precipitation of thin overgrowths causing the overall aspect ratio of crystallites to shorten and widen direct measurements of several hundred individual crystallites confirm that crystallite length and width distributions differ significantly between original and altered materials these changes are consistent with crystal growth processes that tend to minimize surface free energy by increasing overall crystal size and by producing more equant shape the precipitated material has a composition and structure which is grossly similar to the host crystallite as determined by aem and electron diffraction analyses aem analyses of hundreds of mg calcite crystallites from living specimens of peneropolid foraminifera indicate a composition of 13 14 mol mgco3 whereas crystallites from altered tests are 1 3 mole lower taken together these data suggest that early diagenesis in shallow marine carbonates is indeed manifested in the solids as well as the sediment pore waters producing significant changes in biogenic mg calcite and aragonite crystallite size and shape this alteration would not produce changes in the mineral assemblage because dissolution is accompanied by precipitation of compositionally similar overgrowths under major element conditions not drastically dissimilar from overlying seawater however crystallite recrystallization likely has implications for incorporation of minor elements which undergo cycling via organic matter decomposition or redox changes e g uranium cadmium manganese rare earth elements

sedimentary challenge to snowball earth evidence from the magnetic field fossilized in sedimentary rocks suggests that more than 600 million years ago ice occupied tropical latitudes a popular explanation for these findings the snowball earth concept envisages a fully frozen earth for millions of years caused by a runaway ice albedo feedback a rapid catastrophic meltback at very high levels of atmospheric carbon dioxide is thought to have ended this extreme climatic state however sedimentary rocks deposited during these cold intervals indicate that dynamic glaciers and ice streams continued to deliver large amounts of sediment to open oceans throughout the glacial cycle the sedimentary evidence therefore indicates that despite the severity of glaciation some oceans must have remained ice free significant areas of open ocean have important implications for the survival and diversification of life and for the workings of the global carbon cycle the snowball earth concept envisages a fully frozen earth for millions of years several times during the neoproterzoic era between 1 000 and 542 million years ago however the sedimentary evidence suggests that despite the severity of glaciation some oceans must have remained ice free

identification and diagenesis of a phylloid alga archaeolithophyllum from the pennsylvanian providence limestone western kentucky abstract phylloid algae in the pennsylvanian providence limestone desmoinesian in western kentucky are typical in appearance thin arcuate leaflike forms with a calcite mosaic interior cathodoluminescence and backscattered electron imaging reveal well preserved skeletal structure and their identity as archaeolithophyllum sp johnson 1956 and a missouriensum johnson 1956 thin skeletal walls are composed of nonluminescent neomorphic calcite polygonal hypothallial and perithallial cells in the skeletons are filled by luminescent calcite cement chemical staining also distinguishes cell morphology when ferroan calcite cement is present in skeletal pores in contrast to the nonferroan calcite forming skeletal walls hypothallial cells in archaeolithophyllum are 25 m 50 m and are commonly arranged in arcuate layers skeletal walls 5 m thick are composed of calcite and lack distinctive relict features although some are microporous and contain fine disseminated pyrite some thalli contain large irregularly shaped hypothallial cells 50 100 m partitioned by skeletal walls these are interpreted as poorly calcified thallus fragments neomorphic calcite of the skeletons contains high concentrations of sr average 3702 ppm range 339 10 549 ppm strongly suggesting that the original archaeolithophyllum skeleton was aragonitic archaeolithophyllum shares morphological structural and compositional characteristics with holocene peyssonnelid algae this evidence substantiates earlier hypotheses that archaeolithophyllum may be an ancestral peyssonnelid alga calcite cements filling skeletal cells include a bright yellow cl phase and two subsequent phases of dull red and dull yellow luminescence mg mn and fe are enriched in these calcites reflecting reducing diagenetic fluids particularly enriched in fe cementation of skeletons by ferroan calcite during early diagenesis appears to have protected skeletons from dissolution and promoted slow rates of neomorphism conditions of a nearly closed system are constrained by the high retention of sr in the skeletal calcite

an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

calcification of encrusting aragonitic algae peyssonneliaceae implications for the origin of late paleozoic reefs and cements abstract encrusting aragonitic calcareous algae of the family peyssonneliaceae are the largest group of calcified red algae outside the well known mg calcite corallinaceae they are distributed worldwide are most heavily calcified in temperate and tropical waters and grow as prone but arched sheets on soft mud substrates on hard rock surfaces as extensive bridge like networks between corals in reefs and as concentric layers in nodules calcification is entirely aragonitic species specific and both intracellular and extracellular below the thallus as a hypobasal layer the hypobasal layer develops outside the tissue as an encrustation of small aragonite botryoids attached to the lower surface between rhyzoids and on living plants it may exceed the thickness of the thallus calcification ranges from nonexistent in some cold water forms to thallus calcification only to both thallus and hypobasal calcification to species in which the thallus is noncalcified but there is a hypobasal layer of aragonite botryoids although the confirmed fossil range of this family extends only to the early cretaceous striking similarities between these aragonitic forms and some late paleozoic phylloid algae suggest that they may be closely related their ability to grow on soft mud substrates and form structures composed of irregular arched sheets with extensive pore space as well as their brittle nature so susceptible to fragmentation are all characteristics of mound forming fossil phylloid algae the hypobasal layer of botryoidal aragonite developed while the plant is still growing could easily act as a nucleation site for further epitaxial submarine precipitation thus explaining the common association of paleozoic phylloid algae and extensive fossil reef cements that resemble botryoidal aragonite the presence of hypobasal botryoidal aragonite on plants with a noncalcified thallus raises the possibility that some fossil reef cements may be related to now vanished algae

aeolian creeping mass of different grain sizes over sand beds of varying length creep is an important mode of aeolian sand transport but it has received little attention in previous studies due to experimental difficulties and insufficient theory in this study we conducted 116 groups of experiments with three repeats for each group in a wind tunnel to measure the creeping mass of four different mean grain sizes 152 257 321 and 382 m over six bed lengths 2 0 3 5 5 0 6 5 8 0 and 10 0 m at six different friction velocities 0 23 0 35 0 41 0 47 0 55 and 0 61 m s we attempted to develop a comprehensive model of the aeolian creeping mass by analyzing the effect of wind velocity the particle size and the sand bed length based on the experimental data the primary conclusions are as follows 1 the complex relationship among the wind velocity the grain size the length of the bed and the surface shape determines sand creep there was no unified formula to express the effect of particle sizes and the sand bed length on aeolian creeping masses and their effects appeared to depend on each other and wind velocity whereas the creeping mass increases with increasing wind velocity for any particle size with any length of sand bed 2 this paper presented a predicting model to determine the aeolian creeping mass whose calculating results can match to experimental data with correlation coefficients r2 of 0 94 or higher 3 the effect of grain size on creeping mass can be classified into three categories the creeping mass increases with increasing grain size the creeping mass initially decreases and subsequently increases with increasing grain size and the creeping mass fluctuates with the grain size 4 the effect of increasing bed length appears to depend on the grain size for mean grain sizes of 152 257 and 321 m creep initially increases with increasing bed length before decreasing above a certain value while for a mean grain size of 382 m the creeping mass gradually increased with increasing bed length the results help to elucidate aeolian creep and provide an intense foundation for advanced study

palaeoclimate snowball climate conundrum evidence for a neoproterozoic snowball earth in the sedimentary record has been controversial a weathered horizon preserved in sedimentary rocks from svalbard may provide a rare signature of prolonged global glaciation

persistence of a freshwater surface ocean after a snowball earth geochemical data from cap carbonates deposited above cryogenian glacial deposits have been widely used to infer the conditions that prevailed in the aftermath of snowball earth however the time scale over which these carbonates were deposited and the degree to which they record the chemistry of a globally well mixed ocean have remained poorly constrained during deglaciation a large volume of meltwater entered the ocean creating two distinct layers the fresh hot and light upper layer and the salty cold and dense lower layer here we estimate the ocean mixing time scale based on energetic constraints we find that the mixing time scale is 10 4 10 5 yr with a best estimate of 5 10 4 yr or up to 100 times longer than that of the modern ocean mixing of the surface temperature anomaly implies a delayed sea level rise of 40 50 m associated with pure thermal expansion this result reconciles geological geochemical and paleomagnetic data from basal ediacaran cap carbonates with physical oceanographic theory in particular our model suggests that 1 the cap dolostones formed predominantly in a freshwater environment 2 the waters in which the dolostones formed were not well mixed with saline deep water allowing for large geochemical differences between the cap dolostones and the deep ocean and 3 the cap carbonate sequences formed in a two phase transgression that lasted 10 4 yr which is consistent with both local sea level records and the preservation of magnetic excursions and reversals

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

seafloor precipitated carbonate fans in the neoproterozoic rainstorm member johnnie formation death valley region usa cm sized carbonate seafloor fans occur in the neoproterozoic rainstorm member of the johnnie formation death valley usa the fans formed in a mixed carbonate clastic succession near storm wave base at the base of parasequences on a storm dominated ramp petrographic observations indicate that the fans were originally precipitated as aragonite and later inverted to calcite during diagenesis although not directly dated the rainstorm member preserves a large magnitude negative carbon isotopic anomaly down to 11 pdb tentatively correlated to the largest known carbon isotope excursion found in many stratigraphic successions around the world between 585 ma and 550 ma thus the age distribution of seafloor aragonite fans in neoproterozoic strata appears more widespread than previously thought occurring in strata significantly younger than the last widespread neoproterozoic glaciation rainstorm member carbonate fans and oolitic units representing time correlative shallower environments record similar carbon isotope ratios during the negative carbon isotopic anomaly the carbon isotopic homogeneity displayed between fans and other carbonate sediments implies that waters across the shelf were well mixed rather than stratified during the late neoproterozoic isotopic anomaly in addition the similarity of carbon isotope ratios shared among fans along a stratigraphic horizon on a m to cm scales suggests that the local source of alkalinity required for fan growth was derived from a well mixed reservoir likely seawater rather than local diagenetic fluids increased alkalinity and the presence of inhibitors to carbonate nucleation perhaps fe2 under anoxic conditions likely fostered precipitation of aragonite crystal fans on the seafloor

an application of l band synthetic aperture radar to tide height measurement a method for measuring the tide height near the coast from l band synthetic aperture radar sar data is presented twenty one coherent interferograms have been successfully constructed from japanese earth resources satellite 1 jers 1 sar data obtained over oyster sea farming structures a coherence analysis of the 21 interferometric pairs showed that a perpendicular baseline of less than 3 km with a temporal baseline within 500 days are required to obtain a coherent pair with a coherence higher than 0 25 in the study area the coherent phases preserved in the interferograms showed a close relation with the sea level the problem of phase unwrapping to restore an absolute tide height was overcome by introducing normalized image intensities the radar measurements estimated by the proposed method were verified using tide gauge data and comparison of the two datasets yielded a correlation coefficient r sup 2 of 0 91 with a root mean square error of 5 76 cm the results demonstrate that radar interferometry can be applied for a tide height measurement near the coast given sufficient structures that return off nadir radar pulses to the antenna the multipolarized l band sar system will provide better results using only double bounced signals in the future

recovery of marine primary producers after the cretaceous tertiary mass extinction paleocene calcareous red algae from the iberian peninsula abstract calcareous red algae corallines solenoporaceans and peyssonneliaceans are major contributors to paleocene platform deposits in the pyrenean basin n spain and prebetic zone betic cordillera se spain the earliest danian middle thanetian time interval in the pyrenean basin is represented by four depositional sequences the ma da earliest danian da 1 early danian da 2 late danian and th 1 selandian mid thanetian sequences from bottom to top this precise chronostratigraphic framework allows an accurate assessment of the recovery of these calcareous algal groups after the cretaceous tertiary k t mass extinction calcareous algae are recorded shortly after the k t boundary 0 2 0 3 ma after the k t although the richness of identifiable species remains low during the danian 9 species in the earliest danian 9 species in the early danian and 11 species in the late danian in addition no change in taxonomic composition is observed in this interval since approximately the same algal species occur in all three danian sequences the same low species richness 11 species and species composition is observed in the danian algal assemblages of the betic cordillera however this low level of diversification changes radically in the selandian mid thanetian interval when 29 species are recorded together with the increase in species richness the taxonomic composition also changes since 19 species of corallines are recorded for the first time in the th 1 sequence therefore it can be deduced that species diversity recovery due to a significant radiation of corallines was delayed 6 ma after the end cretaceous event the recovery of corallines during the paleocene shows a characteristic depth dependent pattern throughout the danian middle thanetian interval representatives of the family hapalidiaceae subfamily melobesioideae and plants of the family sporolithaceae largely dominate the algal assemblages in the deep water outer platform settings as they do in present day seas members of these two groups are also the most abundant coralline algae in the paleocene reef and back reef paleoenvironments indicating that the shallowest and best illuminated areas were occupied by algal groups that in modern tropical seas mainly occur in relatively deep habitats this suggests that all platform habitats were re occupied after the end cretaceous event by the groups that selectively survived the extinction only after a time interval of about 6 ma did the habitat distribution of coralline groups change to its modern patterns the subfamilies most abundant in present day shallow tropical waters that is mastophoroideae and lithophylloideae for the first time account for more than 1 of the algal assemblages in the thanetian and even then they remain very minor components these shallow water coralline groups were affected more intensively by the k t extinction event accounting for the delay in their recovery in their preferred shallow water habitats shallow water lithophylloids and geniculate corallines have not been found in danian sequences

reservoir characterization of the pennsylvanian caddo limestone in stephens county texas a case study of komia dominated algal mounds abstract the caddo limestone forms economic carbonate reservoirs in stephens county northern texas this study demonstrates that in the caddo limestone of the eliasville and breckenridge fields porosity and permeability are best developed in phylloid algal wackestones and packstones as well as komia wackestones and packstones prevalent within the uppermost interval i e cycle a of the formation in the study area the main reservoirs formed in the upper and middle intervals of the caddo algal mounds because of meteoric dissolution related to subaerial exposure which created a large volume of secondary pores and early cementation that prevented mechanical compaction a great portion of the secondary pores remain open providing the principal pore spaces of the reservoir interval vugs including moldic voids are abundant and dissolution enhanced intragranular pores are very common within widespread komia intercrystalline pores are prevalent in dolomitized and neomorphised lithofacies where micrite was converted to microsparitic and sparitic calcite micropores are abundant in the matrix and within grains especially komia fragments the lower or basal interval of cycle a is commonly much less porous owing to the substantial loss of primary pores by physical compaction and lack or rare presence of secondary pores laterally wells in the areas with thicker cycle a interpreted as algal mounds have higher porosity and thicker net reservoir than those in intermound areas this work provides a case study of carbonate reservoirs in which komia wackestones and packstones are the major reservoir rocks

chronostratigraphic significance of cathodoluminescence zoning in syntaxial cement mississippian lake valley formation new mexico abstract echinoderm syntax ial cement crystals have been collected at several stratigraphic levels within the lake valley formation which is overlain by a major pre pennsylvanian subaerial exposure surface the crystals were microsampled along growth bands and yielded high resolution elemental and isotopic information that record fluid evolution during their growth although cement crystals show little variation in cathodoluminescence character and bulk chemistry throughout the regional extent of the formation intracrystalline patterns in minor element and stable isotope chemistry allow for the identification of several settings of cementation including the marine phreatic marine meteoric mixing and meteoric phreatic zones when placed in a regional stratigraphic context crystal growth records enable reconstruction of the temporal and spatial extent of these diagenetic environments isotopic petrographic and stratigraphic constraints indicate that cementation was related to two temporally distinct meteoric systems most lake valley formation syntaxial cement formed in the marine phreatic and marine meteoric mixing zones during the earliest phase of cementation of this cement in excess of 60 formed in the marine phreatic zone and lowermost marine meteoric mixing zone smaller volumes precipitated from progressively fresher mixtures of marine and meteoric fluid during the gradual expansion of a freshwater lens that developed in response to pre pennsylvanian lowering of sea level meteoric phreatic fluids were relatively unimportant during this episode of cementation in contrast cement of entirely meteoric phreatic origin is associated with a later meteoric system but is found only in the northern part of the study area and at stratigraphic levels immediately adjacent to the pre pennsylvanian unconforrnity our results indicate that cements in the upper and lower parts of the lake valley formation are genetically and temporally unrelated thus the cement cathodoluminescence stratigraphy which has been previously applied to the lake valley can not be valid over the full extent of the formation this has wider implications our demonstration that diverse and temporally distinct diagenetic settings can produce identical cl characteristics shows that caution should be employed when using cement cl stratigraphy alone to extend diagenetic interpretations over large geographic regions or through substantial stratigraphic successions

snowball earth asynchronous coupling of sea glacier flow with a global climate model during snowball earth episodes of the neoproterozoic and paleoproterozoic limited amounts of tropical open ocean jormungand or tropical ocean with thin ice cover would help to explain 1 vigorous glacial activity in low latitudes 2 survival of photosynthetic life and 3 deglacial recovery without excessive buildup of atmospheric co2 some previous models have suggested that tropical open ocean or thin ice cover is possible however its viability in the presence of kilometer thick sea glaciers flowing from higher latitudes has not been demonstrated conclusively here we describe a new method of asynchronously coupling a zonal sea glacier model with a 3 d global climate model and apply it to snowball earth equilibrium curves of ice line vs co2 are mapped out as well as their dependence on ocean heat transport efficiency sea glacier flow and other model parameters no climate states with limited tropical open ocean or thin ice are found in any of our model runs including those with sea glaciers if this result is correct then other refugia such as cryoconite pans would have been required for life to survive however the reasons for the differences between our results and others should first be resolved it is suggested that small scale convective dynamics affecting fractional snow cover in low latitudes may be a critical factor accounting for these differences

discrimination of multiple episodes of meteoric diagenesis in a kimmeridgian reefal complex north iberian range spain abstract stratigraphic relations detailed petrography and fine scale geochemical and isotopic analysis of diagenetic phases formed within kimmeridgian age reefal carbonates of the torrecilla en cameros formation of northern iberian ranges in spain indicate a complex history of alteration in marine meteoric burial and uplift related settings the reefal succession is separated from the overlying tithonian berriasian fluvial and lacustrine continental deposits by a single unconformity which is marked by brecciation and karstification nevertheless a record of three distinct stages of alteration associated with this unconformity is preserved in the succession of calcite cements present within the reefal carbonates recognition of temporally distinct episodes of meteoric diagenesis associated with a single unconformity is atypical in carbonate rocks but this study illustrates how such complex systems can be deconvoluted the first episode of subaerial exposure and meteoric alteration resulted in neomorphism of marine allochems and precipitation of a first generation of nonferroan clear calcite nfc cement following local faulting and brecciation a second nfc precipitated throughout the reefal unit although similar in petrographic character this phase of cementation is distinct in its isotopic composition reflecting changes in the regional climate of the iberian peninsula during the early tithonian another phase of meteoric alteration of the reefal unit is recorded by renewed corrosion including dissolution of preexisting calcite cements and precipitation of prismatic calcite prior to the deposition of tithonian berriasian lacustrine and continental sediments these continental units in turn record yet another episode of alteration by meteoric waters as nfc cements formed within intragranular and dissolution porosity in the lacustrine limestones notably these cements are also distinct on the basis of their petrographic and geochemical character following subsidence and burial during aptian to late cretaceous times migration of regionally derived fluids perhaps in response to the onset of tectonic deformation to the north led to cementation of saddle ankerite and ferroan calcite throughout the sedimentary sequence alteration of these burial related diagenetic phases has subsequently taken place in response to the regional uplift during the tertiary this final episode of meteoric alteration is indicated by replacement of the ferroan calcite and ankerite by a cloudy nonferroan calcite

original mineralogy of carboniferous worm tubes evidence for changing marine chemistry and biomineralization petrographic and geochemical evidence indicates that the original mineralogy of calcareous worm tubes in carboniferous limestones changed from calcitic in the mississippian to aragonitic in the pennsylvanian this change in mineralogy of caco 3 produced by polychaete worms which are relatively simple biomineralizers parallels changes observed in nonbiological carboniferous marine carbonates suggesting that skeletal secretion of caco 3 by polychaetes responded to a global change in seawater chemistry this is the first documentation of long term change in the mineralogy of biologically induced biomineralization it raises the possibility that other bioinduced biomineralization changed in response to changing marine chemistry in contrast to biocontrolled biomineralization by more sophisticated organisms that persisted with no mineralogical change

secular variation in abiotic marine carbonates constraints on phanerozoic atmospheric carbon dioxide contents and oceanic mg ca ratios abiotic marine carbonates have been precipitated as ooids and synsedimentary cement within shallow marine carbonate sequences throughout the phanerozoic these components exhibit variable abundances and compositions that temporally correlate with estimates of global eustasy with rising sea level aragonite and acicular magnesium calcite precipitation gave way to the formation of equant calcite and possibly dolomite similarly the abundance of ooids was limited by positions of global sea level and associated changes in ocean chemistry modern abiotic marine carbonates also exhibit variable abundances and compositions as a function of changing fluid chemistry with latitude and depth chemical data on ambient marine waters suggest that carbonate ion concentrations and rates of precipitation presently control the nature of marine cement and ooids using data from modern oceans to calculate the requisite atmospheric pco 2 levels necessary to form this range of phases in ancient seas suggests that atmos

the sea level fingerprint of a snowball earth deglaciation cap dolostones are thought to represent deposition from seas transgressing over formerly glaciated continental margins during marinoan snowball deglaciation nevertheless facies associations within some cap dolostones indicate that an episode of regional regression punctuated these transgressive sequence tracts to date inferences of sea level change during and after the marinoan snowball deglaciation have been interpreted using simple qualitative arguments in the present study we explore the full spatio temporal variability of sea level change during snowball deglaciation and its aftermath using a gravitationally self consistent theory that accounts for the deformational gravitational and rotational perturbations to sea level on a viscoelastic earth model the theory is applied to model marinoan snowball deglaciation on a generalized ediacaran paleogeography with a synthetic continental ice sheet distribution we find that sea level change following a synchronous rapid 2 kyr collapse of snowball ice cover will exhibit significant geographic variability including site specific histories that are characterized by syn deglacial sea level fall or stillstand moreover some sites that experience syn deglacial transgression will continue to experience transgression in the post deglacial phase taken together these results suggest that sea level change recorded by strata capping snowball glaciogenic units may reflect a more complicated trajectory than previously thought including deposition that was not limited to the deglaciation phase these simulations as well as others that explore the response to asynchronous melting and deglaciation phases of longer duration demonstrate that an episode of regional regression interrupting a cap dolostone transgressive sequence tract may reflect one of several processes or their combination 1 near field adjustment associated with rapid local melting during an otherwise global hiatus in deglaciation 2 post glacial uplift of sites during a period of slowing deglaciation and 3 a transition at some sites from a sea level fall dominated by post glacial uplift to a phase of sea level rise due to eustasy and peripheral bulge subsidence throughout an extended order 50 kyr or greater snowball deglaciation

is high obliquity a plausible cause for neoproterozoic glaciations 1 the main features of the low latitude neoproterozoic glaciations remain the subject of controversial debates concerning possible triggers here we use an agcm coupled with a slab ocean to test one of the earliest and simplest explanation for tropical glaciations a higher obliquity of the earth s rotation axis we show that high obliquity may result in an extensive glaciation during the sturtian episode 750 ma due to the location of continental masses in the tropical areas but cannot produce a large glaciation in the case of mid to high latitude paleogeographies such as those thought to typify the varangian vendian episodes 620 580 ma

influences of temperature and mg ca ratio on caco3 precipitates from seawater the mg ca ratio at which the calcium carbonate mineral that has pseudohomogeneously precipitated from seawater changes from calcite to aragonite was experimentally determined as a function of temperature results indicate a dramatic change in the critical mg ca ratio over a relatively small temperature range for example in normal seawater with a mg ca of 5 1 the critical temperature is 6 3 c at a temperature of 25 c however mg ca must be less than 1 4 0 1 for calcite to form these data indicate that paleoclimatic conditions at the time of carbonate deposition must be considered when attempting to make inferences about variation in other factors such as paleo p co 2 p pressure and paleo mg ca ratios that may control carbonate mineral formation by abiotic processes however the inverse relation between inferred global paleoclimatic conditions and the dominant type of caco 3 polymorph being abiotically deposited i e warm conditions and low mg calcite versus cool conditions and aragonite suggest that substantial changes must have taken place in ocean chemistry over phanerozoic time

unsupervised classification of scattering behavior using radar polarimetry data the use of an imaging radar polarimeter data for unsupervised classification of scattering behavior is described by comparing the polarization properties of each pixel in an image to that of simple classes of scattering such as even number of reflections odd number of reflections and diffuse scattering for example when this algorithm is applied to data acquired over the san francisco bay area in california it classifies scattering by the ocean as being similar to that predicted by the class of odd number of reflections scattering by the urban area as being similar to that predicted by the class of even number of reflections and scattering by the golden gate park as being similar to that predicted by the diffuse scattering class it also classifies the scattering by a lighthouse in the ocean and boats on the ocean surface as being similar to that predicted by the even number of reflections class making it easy to identify these objects against the background of the surrounding ocean

polarimetric features of oyster farm observed by airsar and jers 1 the polarimetric features of an oyster farm in a coastal area are analyzed to verify the applicability of radar polarimetry and interferometry l band airborne synthetic aperture radar airsar data and japan earth resources satellite jers 1 data are used to examine the unique structure of an oyster farm located in south korea a specific feature of the oyster farm is the presence of numerous arrays of structures of various orientations that consist of exercise bar shaped poles protruding above sea level this paper demonstrates that tide level is strongly correlated with the double bounce scattering power from the vertical pole structures this phenomenon is also verified by laboratory measurements using a network analyzer in the laboratory experiment double bounce scattering and total power showed increasing trends with increased height of the vertical poles single bounce scattering is sensitive to the orientation of horizontal poles relative to antenna orientation hh polarization is the most effective technique for imaging oyster farms from l band polarimetric airsar data the authors were able to use a three component decomposition of the airsar data to distinguish an exposed tidal flat from a submerged tidal flat the characteristics of the exposed tidal flat are similar to those of the carbon sponge in the laboratory test except that the double bounce scattering power is slightly greater in the real world example the single bounce scattering component in airsar data is generally greater than that in laboratory measurements because of sea surface conditions and oyster growth when the horizontal pole was aligned normal to the radar look direction single bounce scattering was greater than the double bounce scattering even under water covered conditions while a difference in tide height of 10 cm contributed approximately 3 0 db in the laboratory experiment a difference in tide height of 20 cm contributed to only approximately 1 7 db in the jers 1 sar image intensity jers 1 sar image intensity for areas dominated by double and single bounce scattering was 0 78 and 0 56 respectively results confirm that polarimetric sar data are useful in selecting areas dominated by double bounce scattering in oyster farms

detection of oyster habitat in tidal flats using multi frequency polarimetric sar data abstract exposed oyster reefs in tidal flats have complex and rough surfaces because of their unique surface texture which are quite distinct from the surrounding mud or sand flats here we investigate the microwave signatures backscattered from naturally distributed oyster reefs in tidal flats utilizing the polarimetric analysis techniques to fully polarimetric radarsat 2 c band and alos palsar l band data the study areas include the tidal flats around jebu island and hampyung bay on the west coast of the korean peninsula we analyzed the microwave scattering mechanisms associated with oyster reefs and surrounding areas using the polarimetric target decomposition theorem and quantitatively measu hhvv red target depolarization effects the cross polarized ratio hv vv the co polarized correlation coefficient hhvv and the co polarized phase difference between hh and vv on the basis of a large increase in the cross polarized backscattering hv in the c band sar data one can observe strong volume or multiple scattering and depolarization effects over oyster reefs areas whereas only surface scattering was dominant in most parts of the background tidal areas in oyster reefs the proportion of volume scattering and the cross polarized ratio were greater than 0 7 and 8 db respectively these scattering characteristics were also verified from in situ measurements in the field using a ground based polarimetric scatterometer system however almost no difference was observed between the scattering signatures of oyster reefs and background mudflat areas from l band data which have a considerably longer wavelength than c band the study clearly suggests that multi frequency c and l band polarimetric sar systems can be used to detect the naturally distributed oyster reefs in tidal flats

ocean chemistry neoproterozoic glass bleeding volcanic eruptions at ocean ridges produce large volumes of glass that is rapidly leached by seawater geochemical calculations suggest that this process helps to explain the deposition of carbonates at the end of extreme ice ages

echinoderm skeletal preservation calcite aragonite seas and the mg ca ratio of phanerozoic oceans abstract the alternation between abiotic calcite and aragonite precipitation in shallow seawater and the oscillation of seawater mg ca ratio throughout the phanerozoic are both popular hypotheses echinoderms with well preserved stereom provide new empirical evidence that supports both these hypotheses analyses are reported from 29 specimens cambrian to eocene age containing ossicles preserved either as mg calcite 3 3 to 8 mole mgco3 or as mixed calcite and dolomite 5 to 12 5 mole mgco3 echinoderms with high mole mgco3 occur in the early cambrian and late carboniferous to triassic low values come from the silurian and jurassic to cretaceous the average composition of echinoderms with preserved stereom does not fall below 3 5 mol mgco3 during the phanerozoic and reaches its highest mean value of 16 0 mol mgco3 today an empirical partition coefficient of 0 03182 for modern tropical echinoids is used to indicate the mg ca ratio of ancient seawater from the mg ca ratio of fossil stereom some assumptions are involved and a likely error is calculated from the mg2 variation of modern echinoids high mean seawater mg ca ratios are calculated for early cambrian 3 3 and the late carboniferous to triassic 2 3 but never reached today s value of 5 2 low mg ca ratios 1 1 are indicated from jurassic to cretaceous echinoderms the 29 echinoderm samples plot close to first order mg ca seawater oscillations derived from geochemical models and mg ca ratios determined from fluid inclusions but considerable discrepancies exist when shorter 106 myr time intervals are considered further data and improved understanding of mg partitioning is required before accurate secular variation in the mg ca of seawater can be determined from echinoderms however they are an underused resource in this context and provide an excellent seawater archive

paleoreef maps evaluation of a comprehensive database on phanerozoic reefs to get a better understanding of controls on reef development through time we created a comprehensive database on phanerozoic reefs the database currently comprises 2470 reefs and contains information about geographic position paleoposition age reef type dimensions environmental setting paleontological and petrographical features and reservoir quality of each buildup reef data were analyzed in two qualitatively different ways the first type of analysis was by visualization of paleogeographic reef distribution maps five examples late devonian early permian late triassic late jurassic middle miocene are presented to show the potential of paleoreef maps for paleogeographic and paleoclimatological reconstructions the second type of analysis was a numerical processing of coded reef characteristics to realize major trends in reef evolution and properties of reef carbonates the analysis of paleolatitudinal reef distributions through time shows pronounced asymmetries in some time slices probably related to climatic asymmetries rather than controlled by plate tectonic evolution alone the dominance of particular reef builders through time suggests that there are seven cycles of phanerozoic reef development first curves for the phanerozoic distribution of bioerosion in reefs bathymetric setting and debris potential of reefs are presented the observed pattern in the temporal and spatial distribution of reefs with reservoir quality may assist in hydrocarbon exploration statistical tests on the dependencies of reefal reservoir quality suggest that large size high debris potential low paleolatitude high amount of marine aragonite cement and a platform shelf edge setting favor reservoir quality reefal reservoirs are significantly enhanced in times of high evaporite sedimentation elevated burial of organic carbon low oceanic crust production low atmospheric co2 content and cool paleoclimate as well as when they are present in aragonite oceans

climatic cycles during a neoproterozoic snowball glacial epoch the profound glaciations of the neoproterozoic cryogenian period ca 850 544 ma represent an extreme climatic mode when it is claimed earth was fully or almost completely covered with ice for millions of years we show that the geochemistry and mineralogy of fine grained neoproterozoic sedimentary rocks in oman are best explained by climatic oscillations that drove variations in the intensity of chemical weathering on contemporary land surfaces the cold climate modes of the cryogenian were therefore cyclical punctuated with well defined warm humid interglacial periods the hydrological cycle and the routing of sediment were active throughout the glacial epoch which requires substantial open ocean water this reconstruction represents a significantly different target for numerical climate models at this critical time in the evolution of earth s biosphere

mg fractionation in crustose coralline algae geochemical biological and sedimentological implications of secular variation in the mg ca ratio of seawater the mg ca ratio of seawater has varied significantly throughout the phanerozoic eon primarily as a function of the rate of ocean crust production specimens of the crustose coralline alga neogoniolithon sp were grown in artificial seawaters encompassing the range of mg ca ratios shown to have existed throughout the phanerozoic significantly the coralline algae s skeletal mg ca ratio varied in lockstep with the mg ca ratio of the artificial seawater specimens grown in seawater treatments formulated with identical mg ca ratios but differing absolute concentrations of mg and ca exhibited no significant differences in skeletal mg ca ratios thereby emphasizing the importance of the ambient mg ca ratio and not the absolute concentration of mg in determining the mg ca ratio of coralline algal calcite specimens grown in seawater of the lowest molar mg ca ratio mmg ca 1 0 actually changed their skeletal mineralogy from high mg skeletal mmg ca 0 04 to low mg calcite skeletal mmg ca 0 04 suggesting that ancient calcitic red algae which exhibit morphologies and modes of calcification comparable to neogoniolithon sp would have produced low mg calcite from the middle cambrian to middle mississippian and during the middle to late cretaceous when oceanic mmg ca approached unity by influencing the original mg content of carbonate facies in which these algae have been ubiquitous this condition has significant implications for the geochemistry and diagenesis of algal limestones throughout most of the phanerozoic the crustose coralline algae s precipitation of high mg calcite from seawater that favors the abiotic precipitation of aragonite indicates that these algae dictate the precipitation of the calcitic polymorph of caco3 however the algae s nearly abiotic pattern of mg fractionation in their skeletal calcite suggests that their biomineralogical control is limited to polymorph specification and is generally ineffectual in the regulation of skeletal mg incorporation therefore the mg ca ratio of well preserved fossils of crustose coralline algae when corrected for the effect of seawater temperature may be an archive of oceanic mg ca throughout the phanerozoic magnesium fractionation algorithms that model algal skeletal mg ca as a function of seawater mg ca and temperature are presented herein the results of this study support the empirical fossil evidence that secular variation of oceanic mg ca has caused the mineralogy and skeletal chemistry of many calcifying marine organisms to change significantly over geologic time

evaluation of terrasar x observations for wetland insar application this paper assesses the potential of using spaceborne x band synthetic aperture radar sar data for monitoring water level changes over wetlands our analysis is based on three sets of terrasar x tsx observations acquired over south florida s everglades wetlands during an eight month period in 2008 the first set was acquired in single hh polarization stripmap mode over our northern study area consisting of managed wetlands and urban environments the second set was acquired in dual polarization stripmap mode over the western half of the same area consisting mostly of managed wetlands the third set was also acquired with dual polarization stripmap mode over our southern study area consisting of natural flow freshand salt water wetlands in the southern everglades the first data set was used for a proof of concept study to verify that x band data can generate coherent interferograms in wetland areas interferometric processing of this data set shows a high level of coherence 0 35 over both wetland and urban regions maintaining interferometric phase in all three interferograms spanning 11 days surprisingly phase is maintained over some of the wetlands even for interferograms spanning 33 days the other two data sets were used to evaluate interferometric coherence of all four polarization modes and to determine dominant scattering mechanism in each wetland environment our results show high coherence values 0 4 in all polarization modes with highest values in hh then vv and lowest in hv or vh interferograms calculated from multipolarization data show very similar fringe patterns regardless of the polarization type suggesting that the phase information in all polarization data reflects water level changes in wetlands and that volume scattering may be less important than commonly believed we also used the two multipolarization data sets to conduct the pauli decomposition finding a strong dependence of scattering mechanism on vegetation type the high interferometric coherence level of all polarization data suggests that a significant part of the x band scattered signal interacts with lower sections of the vegetation trunks and branches because scattering from wind affected canopies cannot support such a high coherence level the high spatial resolution of tsx combined with its 11 day repeat orbit makes this x band sensor surprisingly suitable for wetland interferometric sar applications

diagenesis of fibrous magnesian calcite marine cement implications for the interpretation of δ18o and δ13c values of ancient equivalents fibrous high mg calcite hmc marine cement from the pliocene hope gate formation jamaica occurs as 1 2 mm thick isopachous coatings on skeletal grains in local occurrences of halimeda grainstone and is commonly syntaxially overgrown by clear prismatic low mg calcite lmc interpreted as a mixing zone cement the presence of inclusions at discrete levels in the fibrous marine cement produces internal growth banding wherein clear inclusion free zones alternate with cloudy inclusion rich zones clear zones have uniform mg contents 13 7 0 2 mol mgco3 and stable isotopic compositions 18o 0 8 0 1 13c 3 2 0 1 consistent with a marine origin in contrast inclusion rich zones contain fine scale intergrowths of lmc dolomite and hmc and have carbon and oxygen isotopic compositions that define a linear mixing trend between compositions of inclusion free hmc and prismatic lmc cement overgrowths petrographic observations indicate that the mixing trend defined by isotopic data from inclusion rich zones is related to the addition of large volumes of secondary lmc as cement in primary pore space as intergrowths among fibers of primary hmc marine cement and or as replacements of primary hmc crystallites dolomite is present in inclusion rich zones in volumes generally compatible with amounts predicted to result from the closed system stabilization of hmc to lmc and dolomite in addition isotopic and geochemical data from some areas of inclusion rich zones are consistent with mass balance constraints assuming a closed system which require that bulk isotopic and geochemical compositions are retained these relations suggest that small remnants of hmc in inclusion rich zones of hope gate marine cement have altered to lmc and dolomite while isolated from system pore fluids such that primary marine isotopic signatures were imparted on diagenetic products similar petrographic geochemical and isotopic relations have been noted in ancient occurrences suggesting that 18o and 13c values from some former marine cements should be useful for the determination of isotopic compositions of original marine cements and ultimately for the determination of ocean chemistry in the geologic past

an interglacial on snowball earth dynamic ice behaviour revealed in the chuos formation namibia the sturtian is the oldest ca 716 ma of three pan global glaciations in the cryogenian at omutirapo in northern namibia a 2 km wide 400 m deep palaeovalley is filled by glaciogenic strata of the chuos formation which represents the sturtian glacial record sedimentary logging of an exceptionally high quality exposure permits detailed stratigraphic descriptions and interpretations allowing two glacial cycles to be identified at the base of the exposed succession strong evidence supporting glaciation includes diamictites ice rafted dropstones and intensely sheared zones of interpreted subglacial origin these facies collectively represent ice proximal to ice rafted deposits upsection dropstone free mudstones in the middle of the succession and the absence of diamictites imply sedimentation free from glacial influence however the reappearance of glacial deposits above indicates a phase of sturtian glacial re advance comparison with age equivalent strata in south australia where evidence for sea ice free sedimentation has been established previously suggests that a sturtian interglacial may have been extensive implying global scale waxing and waning of ice sheets during a cryogenian glacial event

low magnesium calcite produced by coralline algae in seawater of late cretaceous composition abstract shifts in the mg ca ratio of seawater driven by changes in midocean ridge spreading rates have produced oscillations in the mineralogy of nonskeletal carbonate precipitates from seawater on time scales of 108 years since cambrian time skeletal mineralogies of anatomically simple organisms functioning as major reef builders or producers of shallow marine limestones have generally corresponded in mineral composition to nonskeletal precipitates here we report on experiments showing that the ambient mg ca ratio actually governs the skeletal mineralogy of some simple organisms in modern seas coralline algae produce skeletons of high mg calcite 4 mol mgco3 we grew three species of these algae in artificial seawaters having three different mg ca ratios all of the species incorporated amounts of mg into their skeletons in proportion to the ambient mg ca ratio mimicking the pattern for nonskeletal precipitation thus the algae calcified as if they were simply inducing precipitation from seawater through their consumption of co2 for photosynthesis presumably organic templates specify the calcite crystal structure of their skeletons in artificial seawater with the low mg ca ratio of late cretaceous seas the algae in our experiments produced low mg calcite 4 mol mgco3 the carbonate mineral formed by nonskeletal precipitation in those ancient seas our results suggest that many taxa that produce high mg calcite today produced low mg calcite in late cretaceous seas

the snowball earth aftermath exploring the limits of continental weathering processes article i nfo carbonates capping neoproterozoic glacial deposits contain peculiar sedimentological features and geochemical anomalies ascribed to extraordinary environmental conditions in the snowball earth aftermath it is commonly assumed that post snowball climate dominated by co2 partial pressures several hundred times greater than modern levels would be characterized by extreme temperatures a vigorous hydrological cycle and associated high continental weathering rates however the climate in the aftermath of a global glaciation has never been rigorously modelled here we use a hierarchy of numerical models from an atmospheric general circulation model to a mechanistic model describing continental weathering processes to explore characteristics of the earth system during the supergreenhouse climate following a snowball glaciation these models suggest that the hydrological cycle intensifies only moderately in response to the elevated greenhouse indeed constraints imposed by the surface energy budget sharply limit global mean evaporation once the temperature has warmed sufficiently that the evaporation approaches the total absorbed solar radiation even at 400 times the present day pressure of atmospheric co2 continental runoff is only 1 2 times the modern runoff under these conditions and accounting for the grinding of the continental surface by the ice sheet during the snowball event the simulated maximum discharge of dissolved elements from continental weathering into the ocean is approximately 10 times greater than the modern flux consequently it takes millions of years for the silicate weathering cycle to reduce post snowball co2 levels to background neoproterozoic levels regarding the origin of the cap dolostones we show that continental weathering alone does not supply enough cations during the snowball melting phase to account for their observed volume

basinal setting and origin of thick 1 8 km mass flow dominated grand conglomerat diamictites kamoa democratic republic of congo resolving climate and tectonic controls during neoproterozoic glaciations

extensive and rapid changes in seawater chemistry during the phanerozoic evidence from br contents in basal halite abstract variation of seawater chemical composition during the phanerozoic is still a contentious subject br concentration in primary marine basal halites was used to indicate variations in seawater composition during the past 550 ma evaporation experiments and thermodynamic modelling suggests that the partition coefficient of br in halite depends on the composition of the seawater a combination of this correlation with the time dependent models for ocean chemistry of hardie leads to a prediction of br in basal halite during the phanerozoic this prediction is in good agreement with data from different ancient evaporite basins the br data give evidence for rapid and massive changes in seawater chemistry during the phanerozoic and are thus a first independent test for the hardie model

secular oscillations in the carbonate mineralogy of reef building and sediment producing organisms driven by tectonically forced shifts in seawater chemistry the primary mineralogy of oolites and early marine carbonate cements led sandberg nature 305 1983 19 22 to divide the phanerozoic eon into three intervals of aragonite seas and two intervals of calcite seas hardie geology 24 1996 279 283 has shown that these oscillations together with synchronous oscillations in the mineralogy of marine potash evaporites can be explained by secular shifts in the mg ca ratio of seawater driven by changes in spreading rates along mid ocean ridges the hardie model also predicts that high mg calcite should precipitate along with aragonite as it does in today s aragonite sea we have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms ones that have produced massive skeletons large reefs or voluminous bodies of sediment that correspond to sandberg s aragonite seas and calcite seas and that are predicted by the hardie model particular groups of corals sponges and algae appear to have been dominant reef builders only when favored by an appropriate mg ca ratio in seawater in early and middle paleozoic calcite seas calcite i reefs were dominated by calcitic tabulate heliolitid and rugose corals and calcitic stromatoporoids in contrast during the period of late paleozoic early mesozoic aragonite seas aragonite ii aragonitic groups of sponges scleractinian corals and phylloid algae as well as high mg calcitic red algae were principal reef builders during late cretaceous time at the acme of calcite ii massive rudists displaced aragonitic hermatypic corals in today s aragonite sea aragonite iii scleractinian corals are again dominant reef builders along with high mg calcitic coralline algae major sediment producing algae exhibit temporal patterns similar to those of reef builders calcitic receptaculitids flourished during calcite i whereas aragonitic dasycladaceans did not become dominant rock formers until aragonite ii during calcite ii calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the late cretaceous after the mg ca ratio of seawater had reached a very low value and calcium concentration a very high value as the mg ca ratio of seawater rose and calcium concentration fell during the cenozoic era individual coccoliths on average became less massive and encrusted cells less thickly by pliocene time during aragonite iii the prominent genus discoaster secreted only narrow rayed coccoliths that covered less than 25 of the cell surface also during aragonite iii the aragonitic green alga halimeda emerged as the dominant skeletal sediment producer in reef tracts the influence of seawater chemistry on skeletal secretion appears to have been especially strong for morphologically simple taxa that exert relatively weak control over their own calcification such groups include algae sponges corals and bryozoans morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs this linkage in addition to the basic chemical demands of hypercalcification has given the mg ca ratio of seawater strong control over the success of individual reef building taxa more generally this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of sponges and cheilostome bryozoans throughout their history we conclude that throughout phanerozoic time a chain of causation has extended from mid ocean ridge processes via seawater chemistry to the mineralogical and biological composition of reef communities and bioclastic carbonate deposits

orbitally forced ice sheet fluctuations during the marinoan snowball earth glaciation the presence of dynamic ice sheets during snowball earth glaciations is controversial geological evidence and ice sheet modelling suggest that ice sheets may have responded to orbital forcing when pco2 reached a certain threshold

stable waterbelt climates controlled by tropical ocean heat transport a nonlinear coupled climate mechanism of relevance to snowball earth ongoing controversy about neoproterozoic snowball earth events motivates a theoretical study of stability and hysteresis properties of very cold climates a coupled atmosphere ocean sea ice general circulation model gcm has four stable equilibria ranging from 0 to 100 ice cover including a waterbelt state with tropical sea ice all four states are found at present day insolation and greenhouse gas levels and with two idealized ocean basin configurations the waterbelt is stabilized against albedo feedback by intense but narrow wind driven ocean overturning cells that deliver roughly 100 w m 2 heating to the ice edges this requires three way feedback between winds ocean circulation and ice extent in which circulation is shifted equatorward following the baroclinicity at the ice margins the thermocline is much shallower and outcrops in the tropics sea ice is snow covered everywhere and has a minuscule seasonal cycle the waterbelt state spans a 46 w m 2 range in solar constant has a significant hysteresis and permits near freezing equatorial surface temperatures additional context is provided by a slab ocean gcm and a diffusive energy balance model both with prescribed ocean heat transport oht unlike the fully coupled model these support no more than one stable ice margin the position of which is slaved to regions of rapid poleward decrease in oht convergence wide ranges of different climates including the stable waterbelt are found by varying the magnitude and spatial structure of oht in both models some thermodynamic arguments for the sensitivity of climate and ice extent to oht are presented

slow net sediment accumulation sets snowball earth apart from all younger glacial episodes during the sturtian and marinoan snowball earth episodes ice cover is thought to have extended from polar to tropical latitudes we test the supposition that such an extreme glacial climate not repeated in the subsequent 635 m y of earth history would have reduced the vigor of the hydrologic cycle and thus diminished sediment flux to the oceans with 500 sediment accumulation rates to characterize sturtian and marinoan deposits we find median accumulation rates at least four to 15 times slower than expected for phanerozoic glaciomarine deposits as characterized by 10 000 rates our comparison is conservative with respect to time span latitude and distance from the ice margin phanerozoic accumulation rates decrease systematically when averaged over longer time spans comparisons were drawn therefore at 5 and 57 m y time spans to match minimum marinoan and sturtian durations respectively cenozoic glaciomarine accumulation also slows with increasing latitude from temperate to polar climates and with increasing distance from the ice margin after accounting for time span snowball earth deposits at low latitude are found to be thinner than would be expected either for high latitude cenozoic glacial deposits or for very distal glaciomarine abyssal muds with ice rafted debris the rate discrepancy is not readily attributed to overestimates of the total marinoan or sturtian durations if sediment fluxes during warm melt intervals did approach phanerozoic rates these intervals must have occupied a much smaller proportion of snowball earth episodes than in younger glacial climates

the jormungand global climate state and implications for neoproterozoic glaciations 1 geological and geochemical evidence can be interpreted as indicating strong hysteresis in global climate during the neoproterozoic glacial events 630 ma and 715 ma standard climate theory only allows such strong hysteresis if global climate enters a fully glaciated snowball state however the survival of photosynthetic eukaryotic marine species through these glaciations may indicate that there were large areas of open ocean a previously proposed slushball model for neoproterozoic glaciations could easily explain the survival of these organisms because it has open ocean throughout the tropics but there is only a small amount of hysteresis associated with the slushball state in this paper a new state of global climate the jormungand state is proposed in this state the ocean is very nearly globally ice covered but a very small strip of the tropical ocean remains ice free the low ice latitude of the jormungand state is a consequence of the low albedo of snow free bare sea ice if the ice latitude propagates into the subtropical desert zone it can stabilize without collapsing to the equator because subtropical ice covered regions have a relatively low top of atmosphere albedo as a result of the exposure of bare sea ice and relatively lower cloud cover moreover there is strong hysteresis associated with the jormungand state as greenhouse gas levels are varied because of the high albedo contrast between regions of bare and snow covered sea ice the jormungand state is illustrated here in two different atmospheric global climate models and in the budyko sellers model by offering a scenario that could explain both strong hysteresis in global climate and the survival of life the jormungand state represents a potential model for neoproterozoic glaciations although further study of this issue is needed

quaternary glaciations from observations to theories abstract ice ages are known since the mid nineteenth century from the beginning they have been at the center of theories of climate and climate change still the mechanisms behind these large amplitude oscillations remain poorly understood in order to position our current knowledge of glacial interglacial cycles it is useful to present how the notion of climate change appeared in the xixth century with the discovery of glacial periods and how the two main theories the astronomical one and the geochemical one emerged progressively both from sound physical principles but also from extravagant ideas major progresses in geochemistry in the xxth century led first to the firm evidence of an astronomical pacemaker of these cycles thanks to the accumulation of paleoceanographic data still the milankovitch s theory predicts an ice age cyclicity of about 41 000 years while the major periodicity found in the records is 100 000 yr besides ice cores from antarctica proved unambiguously that the atmospheric carbon dioxide was lower during glacial periods even more importantly during the last termination the atmospheric pco 2 increases significantly by about 50 ppm several millenia before any important change in continental ice volume this fact together with many other pieces of information strongly suggests an active role of greenhouse gases in the ice age problem at least during deglaciations since terminations are precisely at the heart of the 100 ka problem we need to formulate a new synthesis of the astronomical and geochemical theories in order to unravel this almost two century old question of ice ages the foundations of such a theory have already been put forward and its predictions appear in surprisingly good agreement with many recent observations

dynamic model constraints on oxygen 17 depletion in atmospheric o2 after a snowball earth a large perturbation in atmospheric co2 and o2 or bioproductivity will result in a drastic pulse of 17o change in atmospheric o2 as seen in the marinoan oxygen 17 depletion mosd event in the immediate aftermath of a global deglaciation 635 mya the exact nature of the perturbation however is debated here we constructed a coupled four box and quick response biosphere atmosphere model to examine both the steady state and dynamics of the mosd event our model shows that the ultra high co2 concentrations proposed by the snowball earth hypothesis produce a typical mosd duration of less than 106 y and a magnitude of 17o depletion reaching approximately 35 both numbers are in remarkable agreement with geological constraints from south china and svalbard moderate co2 and low o2 concentration e g 3 200 parts per million by volume and 0 01 bar respectively could produce distinct sulfate 17o depletion only if postglacial marine bioproductivity was impossibly low our dynamic model also suggests that a snowball in which the ocean is isolated from the atmosphere by a continuous ice cover may be distinguished from one in which cracks in the ice permit ocean atmosphere exchange only if partial pressure of atmospheric o2 is larger than 0 02 bar during the snowball period and records of weathering derived sulfate are available for the very first few tens of thousands of years after the onset of the meltdown in any case a snowball earth is a precondition for the observed mosd event

a three component scattering model for polarimetric sar data an approach has been developed that involves the fit of a combination of three simple scattering mechanisms to polarimetric sar observations the mechanisms are canopy scatter from a cloud of randomly oriented dipoles evenor double bounce scatter from a pair of orthogonal surfaces with different dielectric constants and bragg scatter from a moderately rough surface this composite scattering model is used to describe the polarimetric backscatter from naturally occurring scatterers the model is shown to describe the behavior of polarimetric backscatter from tropical rain forests quite well by applying it to data from nasa jet propulsion laboratory s jpls airborne polarimetric synthetic aperture radar airsar system the model fit allows clear discrimination between flooded and nonflooded forest and between forested and deforested areas for example the model is also shown to be usable as a predictive tool to estimate the effects of forest inundation and disturbance on the fully polarimetric radar signature an advantage of this model fit approach is that the scattering contributions from the three basic scattering mechanisms can be estimated for clusters of pixels in polarimetric sar images furthermore it is shown that the contributions of the three scattering mechanisms to the hh hv and vv backscatter can be calculated from the model fit finally this model fit approach is justified as a simplification of more complicated scattering models which require many inputs to solve the forward scattering problem

glacitectonism subglacial and glacilacustrine processes during a neoproterozoic panglaciation north east svalbard

aragonitic algae in calcite seas effect of seawater mg ca ratio on algal sediment production abstract secular variation in seawater mg ca would have subjected the aragonite producing codiacean algae to two intervals of low mg calcite seas mmg ca mmg ca 2 seas since their origin in the ordovician it is asserted that the aragonite high mg calcite affinity of modern and recent tropical seas mmg ca 2 has enabled the aragonitic codiaceae to act as major carbonate sediment producers in these seas to test this assertion specimens of the sediment producing codiacean alga udotea flabellum were grown in artificial seawaters formulated at mmg ca ratios 1 0 2 5 5 2 that have occurred in the oceans throughout the geologic history of the codiaceae while the u flabellum algae produced exclusively aragonite in both the artificial boundary seawater mmg ca 2 5 and the artificial aragonite high mg calcite seawater modern seawater mmg ca 5 2 the algae produced a portion 25 3 of their caco3 as low mg calcite 4 mol mgco3 in the artificial calcite seawater mmg ca 1 0 suggesting that their caco3 polymorph control can be partially overridden by ambient seawater chemistry primary productivity calcification and linear growth of the algae decreased with reductions in ambient mg ca despite increases in absolute ca concentration of the seawater reduced calcification in the lower mg ca seawaters is probably due to kinetic difficulties of producing aragonite from seawater that favors the nucleation of calcite the decreased linear growth and primary production are likely associated with reductions in the ambient co2 available for photosynthesis resulting from the reduced calcification these findings support the assertion that the dominant ecological and sedimentological roles of codiacean algae in recent tropical seas is facilitated by a seawater mg ca ratio that favors the nucleation of aragonite over calcite furthermore producing aragonite in seawater that favors the nucleation of low mg calcite mmg ca

effect of ambient mg ca ratio on mg fractionation in calcareous marine invertebrates a record of the oceanic mg ca ratio over the phanerozoic the mg ca ratio of seawater has changed significantly over the phanerozoic primarily as a function of the rate of ocean crust production echinoids crabs shrimps and calcareous serpulid worms grown in artificial seawaters encompassing the range of mg ca ratios that existed throughout the phanerozoic exhibit a direct nonlinear relationship between skeletal and ambient mg ca specimens grown in seawater with the lowest mg ca 1 changed their mineralogy to low mg calcite 4 mol mgco3 suggesting that these high mg calcareous organisms would have produced low mg calcite in the cretaceous when oceanic mg ca was lowest 1 these results support the empirical evidence that the skeletal chemistry of calcareous organisms has varied significantly over the phanerozoic as a function of the mg ca of seawater and that the mg ca of unaltered fossils of such organisms may be a record of oceanic mg ca throughout the phanerozoic mg fractionation algorithms which relate skeletal mg ca seawater mg ca and temperature were derived from these and other experiments they can be used to estimate paleoceanic mg ca ratios and temperatures from fossil skeletal mg ca of the organisms evaluated paleoceanic mg ca ratios recalculated by using the echinoderm mg fractionation algorithm from published fossil echinoid mg ca crinoid mg ca and paleotemperature data are consistent with other estimates and models of oceanic mg ca over the phanerozoic

increased production of calcite and slower growth for the major sediment producing alga halimeda as the mg ca ratio of seawater is lowered to a calcite sea level abstract laboratory experiments were conducted to subject the calcareous alga halimeda a prolific producer of aragonite in the modern ocean to a variety of ambient mg ca ratios these experiments were based on three predictions 1 that halimeda would calcify at a lower rate when the ambient mg ca ratio was reduced from its high level in modern seawater 5 2 2 that this alga would grow more slowly under these conditions because its calcification yields as a byproduct co2 which is available for photosynthesis and 3 that the calcite content of halimeda s skeleton would increase as a percentage of total caco3 as predicted reduction of the mg ca ratio of ambient artificial seawater from its modern level to estimated early oligocene and early eocene levels 2 5 and 1 5 respectively resulted in progressively lower rates of calcification production of organic matter and linear growth for halimeda incrassata colonies the ambient mg ca ratio exerted a stronger influence over the rate of calcification than ca2 which in the geologic past has risen in seawater whenever the ambient mg ca ratio has declined thus although elevation of ca2 at such times would have promoted the calcification and growth of halimeda the concomitant decline of the mg ca ratio would have had a stronger negative effect in addition the percentage of skeletal material consisting of calcite rose from 8 in modern day seawater to 16 in artificial early oligocene seawater and 46 in artificial early eocene seawater magnesium was incorporated into this calcite as a function of the ambient mg ca ratio with a fractionation pattern differing only slightly from that of nonskeletal calcite halimeda exerts less biological control over its biomineralization than the related genera penicillus and udotea which in previous experiments produced only 22 25 of skeletal caco3 as calcite in seawater having a very low mg ca ratio 1 0 nonetheless it is likely that many taxa of green algae that produce little or no calcite today would have produced a substantial proportion of skeletal low mg caco3 as calcite in ancient seas such as those of the cretaceous whose mg ca ratios were close to unity

late proterozoic glacial carbonates in northeast spitsbergen new insights into the carbonate tillite association carbonate rich glacial deposits from two discrete vendian glacial periods are described the older is represented by the 24 40 m thick petrovbreen member e2 of the elbobreen formation which contains abundant detrital dolomite clasts in e2 and their possible source rocks have positive 13 c and negative 18 o pdb values in contrast the carbonate mud fraction of e2 sediments has different cathodoluminescence characteristics from clasts slightly negative 13 c values and higher fe and mn concentrations than clasts oxygen isotopes vary from 2 5 to 4 5 pdb thought to be related to various seawater meltwater mixtures in the depositional environment preservation of information about glacial sedimentary environments is attributed to early diagenetic recrystallization forced by excess surface free energy ostwald s ripening and coinciding with sulphate reduction the younger glaciation is represented by the wilsonbreen formation 170 m which has a distinctive glaciolacustrine middle carbonate member w2 w2 contains precipitated periglacial carbonates with high mn fe ratio both limestone rhythmitic and stromatolitic and dolostone rhythmites stromatolites and dolomite rich sandstones which are compared with carbonates in modern antarctic lakes evaporitic environments for dolomitic sandstones and stromatolitic dolostones are indicated by heavy oxygen isotope values up to 10 5 pdb high na concentration and evidence for dissolved evaporites high mn concentration in detrital dolostones in w2 is suggestive of syn sedimentary dolomite recrystallization in freshwater diamictites and haematitic siltstones the carbonate tillite association ultimately arises from the erosion of underlying carbonates which originated in this case under radically different climatic conditions glacial depositional waters then became carbonate saturated as a result of dissolution of detrital carbonate massive recrystallization of glacially transported carbonate is proposed as a geologically significant process with considerable potential for palaeoenvironmental analysis in glacial lakes carbonate precipitated in response to evaporation or photosynthesis carbonate precipitation as the result of seawater freezing or in warm interglacial conditions is not yet established oxygen isotope value are inconsistent too heavy with the presence of high latitude meltwaters implying that glaciation extended to low latitudes as proposed by harland

stretching the envelope of past surface environments neoproterozoic glacial lakes from svalbard the oxygen isotope composition of terrestrial sulfate is affected measurably by many earth surface processes during the neoproterozoic severe snowball glaciations would have had an extreme impact on the biosphere and the atmosphere here we report that sulfate extracted from carbonate lenses within a neoproterozoic glacial diamictite suite from svalbard with an age of similar to 635 million years ago falls well outside the currently known natural range of triple oxygen isotope compositions and indicates that the atmosphere had either an exceptionally high atmospheric carbon dioxide concentration or an utterly unfamiliar oxygen cycle during deposition of the diamictites

paleozoic mg calcite preserved implications for the carboniferous ocean modern mg calcite echinoderm skeletons are thought to lose mg so rapidly that no pre pleistocene ossicles retain their original mg concentrations however carboniferous echinoderm skeletons from the imo formation of arkansas and the holder formation of new mexico are reported herein that are still composed of mg calcite 7 8 to 14 5 mol mgco 3 their sr concentration is also high 620 to 3710 ppm the pore system in these echinoderm skeletons is filled with ferroan calcite or siderite that has not grown in optical continuity with the skeleton the discovery of carboniferous mg calcite confirms an interpretation already made from their diagenetic replacements marine mg calcite has been associated with proposed icehouse conditions and although the carboniferous echinoderm skeletons are compatible with this as they formed during gondwana glaciation they are too few to be of significance also they are organic secretions and so can override secular mineral trends based on inorganic precipitates the carboniferous echinoderm skeletons produced in tropical waters have a concentration of mg similar to values for modern echinoderms the simplest explanation for this similarity is that both precipitated by similar processes from oceans of similar composition this does not support predictions based on mass balance modeling that the mg ca ratio of the carboniferous ocean was half that of today9s the sr content of carboniferous echinoderms is less than that for modern echinoderms leading to a prediction of 6 ppm sr concentration for the carboniferous ocean further examples of mg calcite are required to substantiate these interpretations

study of the polarimetric characteristics of mud flats in an intertidal zone using c and x band spaceborne sar data abstract the polarimetric characteristics of a mud flat area in the intertidal zone near jiangsu china are analyzed under different environmental conditions using x band and c band spaceborne synthetic aperture radar sar data from the terrasar x tsx and radarsat 2 r2 satellites the dominant scattering mechanisms of different objects including mud flats and aquaculture facilities in the intertidal zone are studied based on the polarimetric target decomposition of fully polarimetric r2 data the polarimetric characteristics of different objects in the intertidal zone are quantitatively compared using the depolarization parameters derived from the r2 data which indicate that tides have different effects on the dominant scattering mechanisms of the mud flats and aquaculture farms furthermore comparisons of the c band and x band polarimetric characteristics of the mud flats and aquaculture farms in the intertidal zone are analyzed by combining the tsx and r2 data the results show that surface scattering is the dominant mechanism of the mud flats whereas different dominant mechanisms occur in the aquaculture areas present during different tides interestingly strikingly similar polarimetric characteristics of mud flats and aquaculture in x band and c band sar are observed and these novel results were achieved through a depolarization analysis based on the tsx and r2 data in synergy

a cryogenian chronology two long lasting synchronous neoproterozoic glaciations the snowball earth hypothesis predicts globally synchronous glaciations that persisted on a multimillion year time scale geochronological tests of this hypothesis have been limited by a dearth of reliable age constraints bracketing these events on multiple cratons here we present four new re os geochronology age constraints on sturtian 717 660 ma and marinoan 635 ma termination glacial deposits from three different paleocontinents a 752 7 5 5 ma age from the base of the callison lake formation in yukon canada confirms nonglacial sedimentation on the western margin of laurentia between ca 753 and 717 ma coupled with a new 727 3 4 9 ma age directly below the glacigenic deposits of the grand conglomerate on the congo craton africa these data refute the notion of a global ca 740 ma kaigas glaciation a 659 0 4 5 ma age directly above the maikhan uul diamictite in mongolia confirms previous constraints on a long duration for the 717 660 ma sturtian glacial epoch and a relatively short nonglacial interlude in addition we provide the first direct radiometric age constraint for the termination of the marinoan glaciation in laurentia with an age of 632 3 5 9 ma from the basal sheepbed formation of northwest canada which is identical within uncertainty to u pb zircon ages from china australia and namibia together these data unite re os and u pb geochronological constraints and provide a refined temporal framework for cryogenian earth history

cenozoic deep sea temperatures and global ice volumes from mg ca in benthic foraminiferal calcite a deep sea temperature record for the past 50 million years has been produced from the magnesium calcium ratio mg ca in benthic foraminiferal calcite the record is strikingly similar in form to the corresponding benthic oxygen isotope 18o record and defines an overall cooling of about 12 c in the deep oceans with four main cooling periods used in conjunction with the benthic 18o record the magnesium temperature record indicates that the first major accumulation of antarctic ice occurred rapidly in the earliest oligocene 34 million years ago and was not accompanied by a decrease in deep sea temperatures

sturtian glaciation in siberia evidence of glacial origin and u pb dating of the diamictites of the chivida formation in the north of the yenisei ridge abstract knowledge on the age and distribution of glacial deposits is essential in order to solve issues surrounding the geological periodization of the neoproterozoic and the large scale changes in the environment and biosphere that took place at that time the glacial origin of diamictites of the precambrian chivida formation on the yenisei ridge on the western periphery of the siberian platform was proposed almost a century ago but has since been disputed the present study confirms the glacial origin of the chivida diamictites based on the findings of striated and faceted clasts and other characteristic features of glacial deposits results from u pb dating of zircon grains from diamictites matrix with and without volcanogenic admixture tuffaceous sandstones and volcanic breccias interbedding the diamictites of the chivida formation are presented the youngest population of zircon grains revealed in the volcanogenic and tuffogenic rocks of the chivida formation is assumed to characterize syn depositional volcanic activity thus its mean average age 696 4 ma is considered to characterize the glacial sediments time of deposition these results prove for the first time the presence of an ice sheet on the siberian craton during the sturtian glaciation