neoproterozoic snowball earth simulations with a coupled climate ice sheet model ice sheets may have reached the equator in the late proterozoic era 600 800 myr ago according to geological and palaeomagnetic studies possibly resulting in a snowball earth but this period was a critical time in the evolution of multicellular animals posing the question of how early life survived under such environmental stress here we present computer simulations of this unusual climate stage with a coupled climate ice sheet model to simulate a snowball earth we use only a reduction in the solar constant compared to present day conditions and we keep atmospheric co2 concentrations near present levels we find rapid transitions into and out of full glaciation that are consistent with the geological evidence when we combine these results with a general circulation model some of the simulations result in an equatorial belt of open water that may have provided a refugium for multicellular animals

influence of surface topography on the critical carbon dioxide level required for the formation of a modern snowball earth abstractthe influence of continental topography on the initiation of a global glaciation i e snowball earth is studied with both a fully coupled atmosphere ocean general circulation model aogc

labrador shelf benthic foraminifera and stable oxygen isotopes of cibicides lobatulus related to the labrador current foraminifera from 38 samples of surface sediments contained 22 calcareous and 17 agglutinated species eight stenotopic species relate to different waters of the labrador current cassidulina reniforme reophax arctica and cribrostomoides crassimargo are associated with the inner shelf estuarine environment islandiella helenae and haynessina orbiculare are associated with the cold and diluted inner shelf waters buccella frigida is related to the slope waters and melonis zaandamae is related to the deep waters of the basins and the outer labrador shelf the distribution of these species down core in core 13 collected from one of the basins on the labrador shelf indicates the presence of inshore waters in the bottom of the basin about 15 000 years bp the ratios of the two stable oxygen isotopes 18o and 16o were determined in the tests of cibicides lobatulus from surface sediment samples collected at 28 localities on the labrador shelf the range of bottom salinities at station locations was 28 6 to 34

clouds and snowball earth deglaciation neoproterozoic and possibly paleoproterozoic glaciations represent the most extreme climate events in post hadean earth and may link closely with the evolution of the atmosphere and life according to the snowball earth hypothesis the entire ocean was covered with ice during these events for a few million years during which time volcanic co 2 increased enough to cause deglaciation geochemical proxy data and model calculations suggest that the maximum co 2 was 0 01 0 1 by volume but early climate modeling suggested that deglaciation was not possible at co 2 0 2 we use results from six different general circulation models gcms to show that clouds could warm a snowball enough to reduce the co 2 required for deglaciation by a factor of 10 100 although more work is required to rigorously validate cloud schemes in snowball like conditions our results suggest that snowball deglaciation is consistent with observations

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

late pleistocene holocene paleoceanographic changes on the eastern canadian margin stable isotopic evidence abstract earlier studies of this area show profound holocene pleistocene paleoceanographic changes along the eastern continental margin of canada based on benthic foraminiferal assemblages here we demonstrate that stable isotopic measurements from benthonic foraminifera substantiate and place numerical limits on physical parameters from some of the glacial postglacial environments in holocene intervals where modern analogues of present day benthonic foraminiferal faunas can be used to delimit the physical environment the isotopic results closely parallel the faunal results in the glacial marine intervals where we have no modern analogues the isotopic results provide us with the first independent evidence that the salinities were not lower than 30 in the bottom waters along this former ice margin at the time of peak glaciation salinities may actually have been above normal we see no indication of a large meltwater event in the 10 11 k y b p time period that has been suggested to occur with the younger dryas events these results demonstrate that stable isotopes can be useful outside the context of relatively stable deep sea environments

meltwater and deglaciation se baffin shelf ne margin laurentide ice sheet between 13 5 and 7 ka from o and c stable isotopic data stable isotopic values on planktonic foraminifera in a suite of cores from basins across the se baffin shelf are used to extract a record of meltwater events during termination i deglaciation resolution and hatton basins lie on the se baffin shelf at water depths 500 m seaward of major conduits for ice drainage from the eastern sector of the laurentide ice sheet lis accelerator mass spectrometry 14c dates are used to constrain our chronology of events in ten cores in resolution basin three cores have 14c ams dates on foraminifera of 20 ka at their bases whereas hatton basin cores terminate in sediments 13 kyr sedimentation rates varied between 0 1 to 4 5 m ka stable oxygen and carbon isotopic ratios were obtained on 146 samples of the planktonic foraminifera neogloboquadrina pachyderma ehrenberg sinistral from seven of the ten cores no evidence was found to indicate that test morphology or size affected 18o between 7 and 13 5 ka the surface water on the shelf was on average 1 lower than the open ocean signal significant temporal variations were found in both 18o and 13c evidence for significant low 18o events occurred between 13 and 8 ka the 13c record from the planktonic foraminifera suggests a threefold division of events between 13 and 7 ka with positive values between 10 8 and 13 0 ka negative values between 9 and 10 8 ka and positive values from 7 to 9 ka the 18o data suggest the presence of meltwater on the shelf some 3 000 years prior to the first late glacial dates on terrestrial deglaciation at circa 10 4 ka hudson strait must be the real key to the importance of the calving process during deglaciation because it is potentially the largest marine outlet for the laurentide ice sheet and because it leads into the very center of the ice sheet the rates of calving through hudson strait during the period of initial 18o rise unfortunately are unknown w f ruddiman 1987 p 151

robust elements of snowball earth atmospheric circulation and oases for life 1 atmospheric circulation in a snowball earth is critical for determining cloud behavior heat export from the tropics regions of bare ice and sea glacier flow these processes strongly affect snowball earth deglaciation and the ability of oases to support photosynthetic marine life throughout a snowball earth here we establish robust aspects of the snowball earth atmospheric circulation by running six general circulation models with consistent snowball earth boundary conditions the models produce qualitatively similar patterns of atmospheric circulation and precipitation minus evaporation the strength of the snowball hadley circulation is roughly double modern at low co2 and greatly increases as co2 is increased we force a 1 d axisymmetric sea glacier model with general circulation model gcm output and show that neglecting zonal asymmetry sea glaciers would limit ice thickness variations to o 10 global mean ice thickness in the 1 d sea glacier model is well approximated by a 0 d ice thickness model with global mean surface temperature as the upper boundary condition we then show that a thin ice snowball solution is possible in the axysymmetric sea glacier model when forced by output from all the gcms if we use ice optical properties that favor the thin ice solution finally we examine snowball oases for life using analytical models forced by the gcm output and find that conditions become more favorable for oases as the snowball warms so that the most critical time for the survival of life would be near the beginning of a snowball earth episode

cretaceous sea surface temperature evolution constraints from tex86 and planktonic foraminiferal oxygen isotopes it is well established that greenhouse conditions prevailed during the cretaceous period 145 66 ma determining the exact nature of the greenhouse gas forcing climatic warming and climate sensitivity remains however an active topic of research quantitative and qualitative geochemical and palaeontological proxies provide valuable observational constraints on cretaceous climate in particular reconstructions of cretaceous sea surface temperatures ssts have been revolutionised firstly by the recognition that clay rich sequences can host exceptionally preserved planktonic foraminifera allowing for reliable oxygen isotope analyses and secondly by the development of the organic palaeothermometer tex86 based on the distribution of marine archaeal membrane lipids here we provide a new compilation and synthesis of available planktonic foraminiferal 18o 18opl and tex86 sst proxy data for almost the entire cretaceous period the compilation uses ssts recalculated from published raw data allowing examination of the sensitivity of each proxy to the calculation method e g choice of calibration and places all data on a common timescale overall the compilation shows many similarities with trends present in individual records of cretaceous climate change for example both sst proxies and benthic foraminiferal 18o records indicate maximum warmth in the cenomanian turonian interval our reconstruction of the evolution of latitudinal temperature gradients low 48 palaeolatitudes reveals temporal changes in the valanginian aptian the low to higher mid latitudinal temperature gradient was weak decreasing from 10 17 c in the valanginian to 3 5 c in the aptian based on tex86 ssts in the cenomanian santonian reconstructed latitudinal temperature contrasts are also small relative to modern 14 c based on low latitude tex86 and 18opl ssts minus higher latitude 18opl ssts compared with 20 c for the modern in the mid campanian to end maastrichtian latitudinal temperature gradients strengthened 19 21 c based on low latitude tex86 and 18opl ssts minus higher latitude 18opl ssts with cooling occurring at low middle and higher palaeolatitude sites implying global surface ocean cooling and or changes in ocean heat transport in the late cretaceous these reconstructed long term trends are resilient regardless of the choice of proxy tex86 or 18opl or calibration this new cretaceous sst synthesis provides an up to date target for modelling studies investigating the mechanics of extreme climates

zircon u pb and lu hf isotopic and whole rock geochemical constraints on the lanhe and heichashan groups implications for the paleoproterozoic tectonic basin evolution of the luliang complex abstract the luliang complex is located at the western margin of the middle segment of the trans north china orogen along which the western and eastern blocks collided to form the north china craton the complex mainly consists of metamorphosed granitic plutons and supracrustal rocks of which the latter are subdivided into the jiehekou luliang yejishan lanhe and heichashan groups the lanhe group is composed of meta conglomerates quartzites and phyllites with minor meta basalts whereas the heichashan group consists of molasse like meta conglomerates and coarse grained quartzites geochemistry of the yejishan meta sedimentary rocks indicates weak source weathering and dominantly chemical immature features whereas the lanhe and heichashan samples display opposite features u pb ages of detrital zircons from the lanhe groups yield four age peaks at 2180 ma 2370 ma 2520 ma and 2700 ma the former three peaks coincide with ages of the chijianling guandishan ttg gneisses 2199 2151 ma and meta volcanic rocks from the luliang and yejishan groups 2213 2156 ma age of the gaijiazhuang porphyritic gneisses 2375 2364 ma and age of the yunzhongshan ttg gneisses 2499 ma respectively whereas detrital zircons forming the oldest age peak were most likely derived from the early neoarchean crust of the eastern block for the heichashan group the dominant 2 2 2 0 ga detrital zircons were probably recycled from the underlying jiehekou group and the minority is directly derived from the early paleoproterozoic granitoids in the luliang complex the youngest detrital zircon age peaks of 2 17 ga and 1 82 ga place maximum depositional ages on the lanhe and heichashan groups respectively whereas the local 1 81 1 79 ga massive granites place constraint on their minimum depositional ages taking into account the lithostratigraphic features provenance and formation ages we suggest that the lanhe group formed in a shrinked remnant back arc basin and the heichashan group was deposited in a foreland basin conversion of the tectonic basin from the middle paleoproterozoic back arc basin to the late paleoproterozoic foreland basin is well consistent with the model that the single collision to form the basement of the north china craton happened at 1 85 ga

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

temperature reconstructions for sw and n iceland waters over the last 10 cal ka based on δ18o records from planktic and benthic foraminifera abstract we have obtained 18 o carb data from planktic n pachyderma s and benthic foraminifera c lobatulu s m barleeanu s from eight cores between 40 and 500 m water depth along the sw core b997 347 and n iceland margins cores b997 321 324 327 328 329 330 and 332 that span the last 10 cal ka sampling resolution varies between centuries to millennial scales over this time range changes in 18 o carb in foraminifera are the product of changes in the global ice volume and the temperature and salinity of the water at the level in which the foraminifera live in order to evaluate the relative roles of these variables we investigate 1 the salinity s and 18 o water values for present day waters off iceland and 2 the present day relationships between 18 o carb or calcite values of foraminifera and water column temperature and salinity data we estimate the average error in the temperature estimate to be 0 3 c based on a monto carlo simulation of the various error terms we argue that around iceland the variations in 18 o carb of both benthic and planktic species over the last 10 cal ka are principally controlled by temperature variations while salinity changes prove less important spatial and temporal changes are examined for both bottom water and 50 m water temperature reconstructions based on six and four cores respectively reconstructed temperatures for sites around sw n iceland indicate that surface and seafloor water temperatures were warmest in the early holocene 10 6 cal ka beginning at 6 cal ka surface and seafloor temperatures began to cool in n iceland and to become less stratified in sw iceland over the last 5000 yrs sea surface temperature variations have been 1 c for 50 of the observations with a maximum range of 2 5 c a profound decrease in temperature of 1 5 c is recorded in our fjord site b997 328 during the little ice age an abrupt earlier cold event is well expressed in at least three n iceland cores around 5 cal ka

paleoceanographical development off sisimiut west greenland during the mid and late holocene a multiproxy study abstract a multiproxy study of paleoceanographic and climatic changes in holsteinsborg dyb on the west greenland shelf shows that the area is sensitive to variations in the mixing between the two components of the west greenland current i e the atlantic water of the irminger current and the polar water from the east greenland current a seismic profile illustrates the general development in the area since the deglaciation and analyses of benthic foraminifera stable isotopes 18 o and 13 c and x ray fluorescence elemental counts in two cores covering ca 6650 and 1100 years respectively provide details on the oceanographic changes during the mid and late holocene the chronology is based on 210 pb and ams 14 c age determinations during the time interval of 6650 1600 cal yr bp the bottom waters are characterized by the presence of modified atlantic water with some influence of meltwater either local or regional depleted 18 o values and relatively high si fe ratio suggest warmer conditions from 6650 to 3800 cal yr bp corresponding in time to part of the holocene thermal maximum followed by a general slight cooling with some fluctuation after 3800 cal yr bp at 1600 cal yr bp there is an abrupt change in almost all investigated proxies the foraminiferal assemblage and the 18 o values suggest cold low salinity bottom water conditions and there is indication of seasonal sea ice cover between 1400 and 1200 cal yr bp corresponding in time to the dark ages cold period and to a minimum in solar irradiance from ca 1150 to 580 cal yr bp a time interval equivalent to the historical medieval warm period mwp the foraminiferal fauna remains indicative of cold low salinity bottom waters but without any indication of sea ice cover the influence of polar water meltwater at the sea floor during the mwp is thought to be a result of enhanced mixing of the two components of the west greenland current caused by an increase in wind activity during generally positive north atlantic oscillation conditions maybe combined with a displacement of the irminger current pathway to the outer shelf during the little ice age lia there is a strong influence of modified atlantic water at the sea floor related to sea ice formation and stratification of the water column after ad 1900 the benthic foraminiferal assemblage suggests a slight increase in polar water influence and thereby mixing even though the environment is not much different from that during the lia

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

sediments sedimentation rates and environments southeast baffin shelf and northwest labrador sea 8 26 ka ten 14c dated cores are described from the labrador sea continental slope and hatton and resolution basins on the southeast baffin shelf based on sharply defined detrital carbonate layers in the labrador sea cores we propose that the laurentide ice sheet reached the shelf break at both 20 and 15 14c ka 24 3 and 18 2 sidereal ka and contributed significant sediment to the northwest labrador sea both times the flux was 720 kg m2 ka during these periods of maximum ice extent the laurentide ice sheet retreated from its 15 ka position at the shelf break to the inner shelf between 14 and 12 14c ka ice proximal conditions recognized by relatively light 18o on near surface planktonic foraminifera high detrital carbonate and benthic foraminiferal faunas dominated by elphidium excavatum forma clavata cassidulina reniforme prevailed until at least 10 14c ka the ice readvanced toward resolution basin and across part of hatton basin at 11 14c ka accelerator mass spectrometry dates on core tops i

stable isotopes in benthic foraminiferal calcite from a river influenced arctic marine environment kara and pechora seas oxygen and carbon stable isotope compositions of modern benthic foraminifera in the kara and pechora estuarine regions of the arctic continental shelf were compared with water 1 8 o 1 3 c d i c temperature and salinity the foraminiferal 1 8 o distribution is mostly similar to that of equilibrium calcite primarily controlled by mixing of runoff and seawater in near estuarine areas depth 15 20 m with additional temperature control seaward the 1 3 c composition is controlled by remineralization of organic matter mostly imported by rivers and by water mixing most analyses used the common arctic foraminifer elphidium excavatum forma clavata which yields depleted 1 3 c as expected for an infaunal species isotope records for sediment cores off the pechora and yenisey rivers show a decelerating rise in isotope values of up to 10 in 1 8 o over the last 10 kyr in a pattern similar to that of rising sea level and concomitant retreat of river mouths with local complexities probably resulting from glacio isostatic rebound and changes in sedimentation

the faint young sun climate paradox volcanic influences it has been suggested that the early earth may have frozen over as a result of a fainter early sun see ulrich 1975 if this had happened climate models suggest the earth would have remained frozen through the present epoch and into the distant future we suggest that volcanic influences could allow a passage from the frozen branch into the unfrozen branch of climate models should conditions on earth be suitable for the latter climate change a broad equatorial belt of volcanic ash is one scenario which would allow a transfer from the frozen earth state into the unfrozen one

a carbon isotope challenge to the snowball earth the snowball earth hypothesis contends that most of the planet was covered with thick ice for millions of years about three quarters of a billion years ago a dramatic warming event thought to have been fuelled by high atmospheric carbon dioxide levels eventually defrosted earth a new isotopic analysis of rocks that were laid down after the initial glaciation challenges the idea of a hard snowball earth the data are consistent with a rather low partial pressure of atmospheric co2 not the orders of magnitude above current levels that would provide the extreme warming necessary for escape from snowball earth this supports the idea that the initial glaciation was much less severe than has been proposed the snowball earth hypothesis postulates that the planet was entirely covered by ice for millions of years in the neoproterozoic era in a self enhanced glaciation caused by the high albedo of the ice covered planet in a hard snowball picture the subsequent rapid unfreezing resulted from an ultra greenhouse event attributed to the buildup of volcanic carbon dioxide co2 during glaciation1 high partial pressures of atmospheric co2 from 20 000 to 90 000 p p m v in the aftermath of the marinoan glaciation 635 myr ago have been inferred from both boron and triple oxygen isotopes2 3 these values are 50 to 225 times higher than present day levels here we re evaluate these estimates using paired carbon isotopic data for carbonate layers that cap neoproterozoic glacial deposits and are considered to record post glacial sea level rise1 the new data reported here for brazilian cap carbonates together with previous ones for time equivalent units4 5 6 7 8 provide estimates lower than 3 200 p p m v and possibly as low as the current value of 400 p p m v our new constraint and our re interpretation of the boron and triple oxygen isotope data provide a completely different picture of the late neoproterozoic environment with low atmospheric concentrations of carbon dioxide and oxygen that are inconsistent with a hard snowball earth

abrupt deglaciation events and holocene palaeoceanography from high resolution cores cartwright saddle labrador shelf canada the uppermost quaternary sediments in cartwright saddle labrador shelf are acoustically laminated with reflectors that can be traced over long distances two piston cores from the saddle record changes in sediment and meltwater delivery from the northeast margin of the laurentide ice sheet lis during deglaciation variations in sediment properties indicate a similar history of sediment accumulation over the last 12 kyr the temporal sampling interval reaches decadal resolution in the last deglacial period 7 9 ka analyses of total carbonate content sediment magnetic variables foraminiferal species and stable isotope measurements on planktic foraminifers show that abrupt changes occurred ca 10 9 9 2 8 8 7 9 and 7 ka with 450 yr correction there was no distinct change in sediment character during much of the younger dryas chronozone in the 18o record the 8 8 ka event is a dramatic 1 shift toward lower values which we associate with the noble inlet glacial event within hudson strait we do not see the pronounced low 18o event at 7 1 ka reported off nova scotia but surprisingly neither the nova scotia records nor other records in the labrador sea capture the impressive 8 8 ka change serious consideration must be given to the final collapse of the lis as the cause of the 8 2 cal ka cold event recorded in greenland and northwest europe copyright 1999 john wiley sons ltd

toward the snowball earth deglaciation the current state of knowledge suggests that the neoproterozoic snowball earth is far from deglaciation even at 0 2 bars of co2 since understanding the termination of the fully ice covered state is essential to sustain or not the snowball earth theory we used an atmospheric general climate model agcm to explore some key factors which could induce deglaciation after testing the models sensitivity to their parameterizations of clouds co2 and snow we investigated the warming effect caused by a dusty surface associated with ash release during a mega volcanic eruption we found that the snow aging process its dirtiness and the ash deposition on the snow free ice are key factors for deglaciation our modelling study suggests that under a co2 enriched atmosphere a dusty snowball earth could reach the deglaciation threshold

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

abrupt climate transition of icy worlds from snowball to moist or runaway greenhouse ongoing and future space missions aim to identify potentially habitable planets in our solar system and beyond planetary habitability is determined not only by a planet s current stellar insolation and atmospheric properties but also by the evolutionary history of its climate it has been suggested that icy planets and moons become habitable after their initial ice shield melts as their host stars brighten here we show from global climate model simulations that a habitable state is not achieved in the climatic evolution of those icy planets and moons that possess an inactive carbonate silicate cycle and low concentrations of greenhouse gases examples for such planetary bodies are the icy moons europa and enceladus and certain icy exoplanets orbiting g and f stars we find that the stellar fluxes that are required to overcome a planet s initial snowball state are so large that they lead to significant water loss and preclude a habitable planet specifically they exceed the moist greenhouse limit at which water vapour accumulates at high altitudes where it can readily escape or the runaway greenhouse limit at which the strength of the greenhouse increases until the oceans boil away we suggest that some icy planetary bodies may transition directly to a moist or runaway greenhouse without passing through a habitable earth like state icy planets and moons could become habitable as their host stars brighten and their ice melts climate simulations instead show a rapid transition from a snowball to an inhospitable greenhouse climate with significant water loss

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

boron and calcium isotope composition in neoproterozoic carbonate rocks from namibia evidence for extreme environmental change the level and evolution of atmospheric carbon dioxide throughout earth s history are key issues for palaeoclimate reconstructions especially during times of extreme climate change such as those that marked the neoproterozoic the carbon isotope ratios of marine carbonates are crucial in the correlation and identification of neoproterozoic glacial deposits and they are also used as a record for biogeochemical cycling and potential proxy for atmospheric pco2 likewise the boron and calcium isotope compositions of marine carbonates are potential proxies for palaeo seawater ph and the ratio of calcium fluxes into and out of seawater respectively and together they may be used to estimate atmospheric carbon dioxide here we use b and ca isotopes to estimate palaeoenvironmental conditions in the aftermath of a major neoproterozoic glaciation in namibia the validity of the b and ca isotope variation in the ancient marine carbonates is evaluated using the oxygen isotope composition of the carbonates and its correlation to the carbon isotope variation a negative 2 7 to 6 2 11b excursion occurs in the postglacial carbonates and is interpreted to reflect a temporary decrease in seawater ph associated variations in 44ca values ranging between 0 35 and 1 14 are linearly coupled with the carbon isotope ratios and imply enhanced postglacial weathering rates the reconstructed seawater ph and weathering profiles indicates that high atmospheric co2 concentrations were likely during the melt back of neoproterozoic glaciations and precipitation of cap carbonates however the b isotope trend suggests that these concentrations rapidly ameliorated and they do not co vary with 13c thus models attempting to link long lived negative 13c excursions to elevated pco2 need to be reconsidered

climate forcings and the initiation of low latitude ice sheets during the neoproterozoic varanger glacial interval the giss gcm was used to determine if a diverse set of climate forcings alone or in combination could have initiated the low latitude ice sheets of the varanger 600 ma glacial interval the simulations use a realistic reconstruction of the paleocontinental distribution and test the following forcings alone and in combination 6 solar luminosity decrease four atmospheric co2 scenarios 1260 315 140 and 40 ppm a 50 increase and a 50 decrease in ocean heat transports and a change in obliquity to 60 none of the forcings individually produced year round snow accumulation on low latitude continents although the solar insolation decrease and 40 ppm co2 scenarios allowed snow and ice to accumulate at high and middle latitudes combining forcings further cools the climate when solar luminosity co2 and ocean heat transports were all decreased annual mean freezing and snow accumulation extended across tropical continents no simulation would have initiated low latitude glaciation without contemporaneous glaciation at higher latitudes a finding that matches the distribution of glacial deposits but which argues against high obliquity as a cause of the varanger ice age low level clouds increased in most scenarios as did surface albedo while atmospheric water vapor amounts declined all are positive feedbacks that drive temperatures lower in the most severe scenario global snow and ice cover increased to 68 compared to 12 under modern conditions and water vapor dropped by 90 these results do not necessarily preclude a snowball earth climate scenario for the varanger glacial interval however either more severe forcings existed or radical changes occurred in the ocean atmosphere system which are unaccounted for by the gcm also as sea ice extent increased in these experiments snow accumulation began to decline because of an increasingly dry atmosphere under snowball earth conditions glaciation would be impossible since the hydrological cycle would all but cease if the atmosphere s primary moisture source were cut off

mobility of nutrients and trace metals during weathering in the late archean abstract the evolution of the geosphere and biosphere depends on the availability of bio essential nutrients and trace metals consequently the chemical and isotopic variability of trace elements in the sedimentary record have been widely used to infer the existence of early life and fluctuations in the near surface environment on the early earth particularly fluctuations in the redox state of the atmosphere in this study we applied late archean weathering models hao et al 2017 developed to estimate the behavior of major elements and the composition of late archean world average river water to explore the behavior of nutrient and trace metals and their potential for riverine transport we focused on p mn cr and cu during the weathering of olivine basalt in our standard late archean weathering model pco2 g 10 1 5 bars ph2 g 10 5 0 bars crustal apatite was totally dissolved by the acidic rainwater during weathering our model quantitatively links the pco2 g of the atmosphere to phosphate levels transported by rivers the development of late archean river water ph 6 4 resulted in riverine phosphate of at least 1 7 molar much higher than at the present day at the end of the early proterozoic snowball earth event when pco2 g could be 0 01 0 10 bars river water may have transported up to 70 molar phosphate depending on the availability of apatite thereby stimulating high levels of oxygenic photosynthesis in the marine environment crustal levels of mn in olivine dissolved completely during weathering except at large extents of weathering where mn was stored as a component of a secondary carbonate mineral the corresponding mn content of river water about 1 2 molar is higher than in modern river water whiffs of 10 5 mole o2 gas or hno3 kg 1 h2o resulted in the formation of pyrolusite mno2 and abundant hematite and simultaneous dramatic decreases in the concentration of mn ii in the river water chromite dissolution resulted in negligible dissolved cr in archean river water however amorphous cr oh 3 representing easily weatherable cr bearing minerals dissolved totally during the weathering simulations resulting in concentrations of cr iii in the river water of up to 0 14 molar higher than at the present day late archean weathering of accessory chalcopyrite produced chalcocite and bornite and extremely low concentrations of cu 10 15 molar because of the low solubilities of the copper sulfides however pulses of either o2 g or hno3 produced native copper chalcocite and bornite much more hematite and river water containing levels of dissolved cu comparable to the present day copper mineralogy predicted by weathering models might provide a new correlation with evidence from studies of copper mineral evolution overall our results implied that the redox state of the atmosphere the ph of surface waters and the availability of easily weatherable minerals are all important factors controlling the dissolution of trace elements in river water interpretation of the sedimentary signatures of trace elements should consider not only the redox state but also the ph and availability of accessory minerals

resolved snowball earth clouds abstractrecent general circulation model gcm simulations have challenged the idea that a snowball earth would be nearly entirely cloudless this is important because clouds would provide a strong warming to a high albedo snowball earth gcm results suggest that clouds could lower the threshold co2 needed to deglaciate a snowball by a factor of 10 100 enough to allow consistency with geochemical data here a cloud resolving model is used to investigate cloud and convection behavior in a snowball earth climate the model produces convection that extends vertically to a similar temperature as modern tropical convection this convection produces clouds that resemble stratocumulus clouds under an inversion on modern earth which slowly dissipate by sedimentation of cloud ice there is enough cloud ice for the clouds to be optically thick in the longwave and the resulting cloud radiative forcing is similar to that produced in gcms run in snowball conditions this result is robust to large changes in the clo