the diversity of tectonic modes and thoughts about transitions between them plate tectonics is a particular mode of tectonic activity that characterizes the present day earth it is directly linked to not only tectonic deformation but also magmatic volcanic activity and all aspects of the rock cycle other terrestrial planets in our solar system do not operate in a plate tectonic mode but do have volcanic constructs and signs of tectonic deformation this indicates the existence of tectonic modes different from plate tectonics this article discusses the defining features of plate tectonics and reviews the range of tectonic modes that have been proposed for terrestrial planets to date a categorization of tectonic modes relates to the issue of when plate tectonics initiated on earth as it provides insights into possible pre plate tectonic behaviour the final focus of this contribution relates to transitions between tectonic modes different transition scenarios are discussed one follows classic ideas of regime transitions in which boundaries between tectonic modes are determined by the physical and chemical properties of a planet the other considers the potential that variations in temporal evolution can introduce contingencies that have a significant effect on tectonic transitions the latter scenario allows for the existence of multiple stable tectonic modes under the same physical chemical conditions the different transition potentials imply different interpretations regarding the type of variable that the tectonic mode of a planet represents under the classic regime transition view the tectonic mode of a planet is a state variable akin to temperature under the multiple stable modes view the tectonic mode of a planet is a process variable that is something that flows through the system akin to heat the different implications that follow are discussed as they relate to the questions of when did plate tectonics initiate on earth and why does earth have plate tectonics this article is part of a discussion meeting issue earth dynamics and the development of plate tectonics

decoupled fresh saline groundwater circulation of a coastal carbonate aquifer spatial patterns of temperature and specific electrical conductivity summary the coastal carbonate aquifer of the eastern yucatan peninsula discharges groundwaters to the caribbean sea temperature and specific electrical conductance sec are used as natural tracers to gain new insight into the fresh and saline groundwater circulation along an 80 km section of the caribbean coastline the aquifer is density stratified with a cooler freshwater lens overlying a warmer saline water zone non conduit sites generally have lower temperatures and sec in the freshwater lens than conduit sites in conduits 4 km inland within the shallow saline water zone temperature decreases exponentially with distance from the coast with near equilibrium with the fresh waters reached at 10 km inland a distance coincident with the known limit of conduit development this pattern is indicative of the progressive cooling of warm seawater moving inland from the coast a flow direction opposite to that of both the conventional freshwater flow entrainment and geothermal convection models of coastal aquifer circulation

groundwater geochemistry of the yucatan peninsula mexico constraints on stratigraphy and hydrogeology summary we report 87sr 86sr and ion concentrations of sulfate chloride and strontium in the groundwater of the northern and central yucatan peninsula mexico correlation between these data indicates that ejecta from the 65 95 m y old chicxulub impact crater have an important effect on hydrogeology geomorphology and soil development of the region ejecta are present at relatively shallow subsurface depths in north central yucatan and at the surface along the rio hondo escarpment in southeast quintana roo where they are referred to as the albion formation anhydrite gypsum and by inference celestite are common in impact ejecta clasts and in beds and cements of overlying paleocene and lower eocene rocks cored around the margin of the crater the sulfate rich minerals that are found in rocks immediately overlying the impact ejecta blanket may either be partially mobilized from the ejecta layer itself or may have been deposited after the k t impact event in an extensive pre oligocene shallow sea these deposits form a distinctive sedimentary package that can be easily traced by the eocene cretaceous 87sr 86sr signal a distinct sr isotopic signature and high so4 cl ratios are observed in groundwater of northwestern and north central yucatan that interacts with these rocks moreover the distribution of the gypsum rich stratigraphic unit provides a solution enhanced subsurface drainage pathway for a broad region characterized by dissolution features poljes extending from chetumal quintana roo to campeche campeche the presence of gypsum quarries in the area is also consistent with a sulfate rich stratigraphic package that includes ejecta the distinctive chemistry of groundwater that has been in contact with evaporite ejecta can be used to trace flow directions and confirms a groundwater divide in the northern peninsula information about groundwater flow directions and about deep subsurface zones of high permeability is useful for groundwater and liquid waste management in the area where it discharges at the coast the unique chemistry of the groundwater that has interacted with the evaporite ejecta strata may also have significant geomorphologic implications while groundwater seawater mixing at the coast has been shown to dissolve and erode limestone phreeqc modeling shows that mixing of water nearly saturated in caso4 with seawater has a less vigorous dissolution effect due to its high ca content

cathodoluminescence petrography and isotope geochemistry of kt impact ejecta deposited 360 km from the chicxulub crater at albion island belize the depositional and diagenetic history of cretaceous tertiary kt impact ejecta deposited 360 km from the chicxulub crater at albion island belize has been investigated using integrated cathodoluminescence and isotopic analyses a quarry exposes 26 m of upper cretaceous barton creek formation dolomitized marine limestone overlain by 16 m of dolomitized albion formation impact ejecta the albion formation consists of a lower finegrained 1 m thick spheroid bed and an upper 15 m thick coarse conglomeratic diamictite bed a 14 event paragenetic sequence has been documented and used as a temporal framework to interpret chemostratigraphic trends in bulk rock d 18 o d 13 c and 87 sr 86 sr the uppermost surface of the barton creek formation was subaerially exposed before the kt impact as indicated by a brecciated palaeosol that caps upsection decreases in d 13 ca nd d 18 o small 1 cm diameter spheroids in the spheroid bed exhibit vermicular crystalline textures but lack the concentric zonations common to accretionary lapilli these spheroids are hypothesized originally to have been impact glass or reactive ca and mg oxide dusts that adhered to water vapour particles condensing from the cooling impact vapour cloud the spheroids were dolomitized soon after deposition the earliest dolomitization in the matrix sediments of the albion formation was also post depositional replacing clays formed by devitrification of impact glass dolomite and clay 87 sr 86 sr exhibit a distinct symmetrical distribution in the spheroid bed ranging from 0ae707745 to 0ae707872 although unproven this may represent primary changes in the chemical composition of the impact glass the limestone clasts in the diamictite bed were dolomitized before the kt impact and exhibit upsection decreases in bulk rock 87 sr 86 sr this suggests that the clasts were excavated from strata equivalent in age or older than the barton creek formation at locations closer to or in the chicxulub crater

geological and archaeological implications of strontium isotope analysis of exposed bedrock in the chicxulub crater basin northwestern yucatan mexico the surface geology of the site of the chicxulub impact crater in northwestern yucatan mexico has not been studied extensively since the discovery of the crater almost two decades ago strontium isotope 87sr 86sr measurements in carbonate rock outcrops reveal near uniform strontium signatures of 0 70905 inside the ring of cenotes water filled sinkholes which represents the rim of the crater basin measured strontium isotope ratios were used to infer rock ages employing the marine sr isotope curve we estimate the age of the exposed limestone within the chicxulub crater basin to be late miocene to early pliocene representing the age of the youngest sediment fill discovery of a large terrain of near uniform strontium isotope ratios in northwestern yucatan offers new geoarchaeological opportunities to track ancient maya migration and determine sources of manufactured goods our results have implications for applying the sr isotope method to maya archaeological sites such as mayapan the last maya capital and chichen itza

crustal thickness and support of topography on venus united states national aeronautics and space administration nasa planetary geology and geophysics program grant

water chemistry and caco3 dissolution in the saline part of an open flow mixing zone coastal yucatan peninsula mexico along the caribbean coast of the yucatan peninsula brackish ground water mixed fresh water and sea water is channeled through upper pleistocene limestone via fracture controlled caverns in caves cenotes and caletas at xcaret yalku and tancah this open flow coastal mixing zone comprises three major layers 1 an upper dilute zone of gradually increasing salinity with depth slow mixing 2 a thin intermediate zone of rapidly increasing salinity with depth rapid mixing and 3 a lower saline zone of gradually increasing salinity with depth the intermediate layer occurs at different absolute salinities at different localities and it generally corresponds to the level of a notch apparently related to dissolution in the wall rock of the caverns chemical composition of the mixed ground water is consistent with that predicted from theoretical conservative mixing of fresh water and sea water end members sampled in the yucatan coastal region inorganic c alkalinity mg ca and sr ca and 18o and 13c of the mixed water give no indication of dissolution or precipitation of cac03 as the ground water flows rapidly seaward field and petrographic observations however show that aragonite is preferentially dissolving throughout the modern mixing zone and in particular in the saline portion comparison of saturation indices to calculated mixing curves shows that the coastal mixing zone of the eastern yucatan peninsula is undersaturated with respect to cac03 over a wide range of salinity including mixtures of as much as 95 sea water for aragonite and 90 sea water for calcite supersaturation with respect to stoichiometric dolomite is maintained over the whole range of salinities but super saturation with respect to calcian dolomite ca57mg43 can be confined to high salinity mixtures depending on the thermodynamic data base used in calculations these data suggest that 1 dissolution of coastal limestone can take place in the sea water dominated portion of the mixing zone and 2 the mixing zone model for dolomitization can be extended to much higher salinities than is indicated by the widely accepted dorag model

tectonics and evolution of venus the global tectonics of venus differs significantly from that of earth most markedly in that the surface is covered predominately by gently rolling terrain there apparently are no features like ocean rises the gravity is positively correlated with topography at all wavelengths and the few highlands are estimated to be supported or compensated at a depth of approximately 100 kilometers the surface of venus appears to be covered mainly by an ancient crust the high surface temperature making subduction difficult it seems likely that well over 1 billion years ago water was destabilized at the surface and soon after plate tectonics ceased the highlands appear to be actively supported presumably as manifestations of long enduring hot spots

spatial variation of strontium isotopes 87sr 86sr in the maya region a tool for tracking ancient human migration strontium isotope ratios 87sr 86sr in human bones and teeth have become a useful tool to study migration and sedentism of individuals from archaeological contexts here we analyzed 87sr 86sr of water bedrock soils and plants across a broad geographic region to test the potential of this method in the ancient maya area our aims were two fold first to test if the sources of dietary strontium i e plants and water in humans reflect the 87sr 86sr ratio of exposed bedrock and second to determine whether the ranges of 87sr 86sr values were sufficiently distinct among the principal maya geocultural areas to infer past migration we identified five distinct subregions on the basis of geologic maps and evaluated the variability of 87sr 86sr values values given as mean 87sr 86sr 2 standard deviations number of samples 1 northern lowlands 0 70888 0 00066 n 16 2 southern lowlands 0 70770 0 00052 n 86 3 volcanic highlands and pacific coast 0 70415 0 00023 n 34 4 metamorphic province 0 70743 0 00572 n 50 and 5 the maya mountains of belize 0 71327 0 00167 n 3 although the sample size is small and overlap exists in 87sr 86sr values among some subregions most areas can be readily distinguished from one another on the basis of strontium isotopes these subregional 87sr 86sr differences provide archaeologists with a powerful tool to recognize geographic outliers in ancient maya burials and thereby test hypotheses concerning the origin of specific individuals inferred population migration patterns and the possibility of outside cultural influences in the maya region

ries and chicxulub impact craters on earth provide insights for martian ejecta blankets terrestrial impact structures provide field evidence for cratering processes on planetary bodies that have an atmosphere and volatiles in the target rocks here we discuss two examples that may yield implications for martian craters 1 recent field analysis of the ries crater has revealed the existence of subhorizontal shear planes detachments in the periphery of the crater beneath the ejecta blanket at 0 9 1 8 crater radii distance their formation and associated radial outward shearing was caused by weak spallation and subsequent dragging during deposition of the ejecta curtain both processes are enhanced in rheologically layered targets and in the presence of fluids detachment faulting may also occur in the periphery of martian impacts and could be responsible for the formation of lobe parallel ridges and furrows in the inner layer of double layer and multiple layer ejecta craters 2 the ejecta blanket of the chicxulub crater was identified on the southeastern yucatn peninsula at distances of 3 0 5 0 crater radii from the impact center abundance of glide planes within the ejecta and particle abrasion both rise with crater distance which implies a ground hugging erosive and cohesive secondary ejecta flow systematic measurement of motion indicators revealed that the flow was deviated by a preexisting karst relief in analogy with martian fluidized ejecta blankets it is suggested that the large runout was related to subsurface volatiles and the presence of basal glide planes and was influenced by eroded bedrock lithologies it is proposed that ramparts may result from enhanced shear localization and a stacking of ejecta material along internal glide planes at decreasing flow rates when the flow begins to freeze below a certain yield stress

calcite raft geochemistry as a hydrological proxy for holocene aquifer conditions in hoyo negro and ich balam sac actun cave system quintana roo mexico abstract two cores from calcite rafts deposits located in cenote ich balam and hoyo negro were dated and analyzed for 87sr 86sr 18o 13c sr ca and cl ca the geochemical records show changing aquifer salinity spanning the last 8 5 cal kyrs bp and interrelationships with holocene climate trends wet and dry periods during the wet mid holocene the salinity of the meteoric water mass wm at 7 8 8 3 cal kyrs bp was relatively high at 1 5 2 7 ppt and then became less saline 1 0 1 5 ppt during the last 7000 yrs as climate became progressively drier high salinity of the meteoric wm during the wet mid holocene is attributed to increased turbulent mixing between the meteoric and underlying marine wm increased precipitation in terms of amount frequency and intensity e g hurricanes causes higher flow of meteoric water towards the coast and mixing at the halocline a phenomenon recorded with recent instrumental monitoring of the aquifer conversely during dry periods reduced precipitation and flow in the meteoric wm would result in lower salinity karst properties and holocene sea level rise also seem to have an effect on the aquifer when the regionally extensive network of shallow cave passages 10 12 m water depth are flooded at 8000 cal yrs bp there is a rapid shift in salinity this study demonstrates that calcite raft deposits can be used as paleo environmental recorders documenting the effects of sea level and climate change on aquifer condition

topographic comparisons of uplift features on venus and earth implications for venus tectonics abstract venus and earth display different hypsography we use topographic profiles to search for well understood terrestrial analogs to venusian features specifically by using cross correlation we correlate average profiles for terrestrial rifts slow and fast ultra slow incipient and inactive and also hotspots oceanic and continental with those for venusian chasmata and regiones to draw inferences as to the processes responsible for shaping venus surface correlations tend to improve with faster spreading rates venus correlations rank considerably lower than terrestrial ones suggesting that if chasmata are analogous to terrestrial spreading centers then spreading on venus barely attains ultra slow rates individual features normalized average profiles are correlated with profiles of other such features to establish the degree of similarity which in turn allows for the construction of a covariance matrix principal component analysis of this covariance matrix shows that yellowstone more strongly resembles atla beta and w eistla regiones than it does the terrestrial oceanic hotspots and that venusian chasmata especially ganis most closely resemble the ultra slow spreading arctic ridge

seasonal trends in calcite raft precipitation from cenotes rainbow feno and monkey dust quintana roo mexico implications for paleoenvironmental studies abstract calcite raft precipitation was monitored in three yucatan cenotes rainbow feno and monkey dust over a 2 year period site specific variables including water temperature relative humidity water level and salinity were recorded as well as rainfall cozumel calcite raft surface area was monitored through trail cameras that collected photographs every 60 min accumulation rates were recorded using sediment traps that were collected in may and december of each year calcite raft surface area was calculated using an image segmentation procedure that identified the boundary of objects edge detection that share certain pixel characteristics removed non points of interest and measured the sum of the area covered by the raft material results show that large rainfall events have a regional effect on the meteoric water mass i e salinity as well as the precipitation of calcite rafts the large rainfalls and increased inflows cause dilution of the caco3 supersaturated meteoric water mass and cause increased flow hindering calcite raft precipitation for days to weeks after the rainfall raft precipitation gradually returns with deceasing flow after the event and stagnation allows the recurrence of caco3 supersaturation and accumulation of nucleation particles small rainfalls did not seem to have the same effect as slower percolation of rainwater through the epikarst would be more saturated and micro topographic relief may isolate the water pools from the lower flow the sediment trap data shows that calcite rafts accumulate relatively constantly throughout the year in all three sites sedimentation rates from feno and monkey dust responded to seasonal changes in surface water disturbance associated with rainfall wet and dry although rainbow didn t show any seasonality likely due to other site specific factors affecting water disturbance e g bats wind

a review of human and natural changes in maya lowland wetlands over the holocene abstract in the maya lowlands of mexico belize and guatemala two main types of wetlands have played important roles in human history bajos or intermittently wet environments of the upland interior yucatan and perennial wetlands of the coastal plains many of the most important maya sites encircle the bajos though our growing evidence for human wetland interactions is still sparse the deposits of these wetlands record two main eras of slope instability and wetland aggradation the pleistocene holocene transition as rainfall increased and forests eclipsed savannas and the maya preclassic to classic as deforestation land use intensity and drying increased the ancient maya adapted with terraces around these bajo margins but retracted in the late preclassic in some areas the perennial wetlands of the coastal plains have different histories and the first conceptual model of human wetland interaction described intensive wetland agriculture in the preclassic through classic based on raised fields and canals but a second model arose that interpreted the wetland stratigraphy and canals as more indicative of natural aggradation by accelerated erosion and gypsum precipitation that buried archaic and preclassic fields and there was little classic era use we present new data on a third and fourth model in this study the third is a hybrid of the models one and two including the archaic to preclassic aggradation of the second model and the first model s classic period fields and canals as piecemeal attempts by the maya to adapt to these and other environmental changes the fourth conceptual model describes a very late terminal classic preplanned project on a floodplain these wetland fields were short lived aggraded rapidly but with some reoccupation in the postclassic all of these new models display the burgeoning evidence for intricate maya interactions with wetlands and the diversity of evidence from the relatively few studies underscores the infancy of our understanding of maya interaction with tropical wetlands

isotopic and geochemical evolution of ground and surface waters in a karst dominated geological setting a case study from belize central america abstract analysis of stable isotopes and major ions in groundwater and surface waters in belize central america was carried out to identify processes that may affect drinking water quality belize has a subtropical rainforest savannah climate with a varied landscape composed predominantly of carbonate rocks and clastic sediments stable oxygen 18 o and hydrogen d isotope ratios for surface and groundwater have a similar range and show high d excess 10 40 8 the high d excess in water samples suggest secondary continental vapor flux mixing with incoming vapor from the caribbean sea model calculations indicate that moisture derived from continental evaporation contributes 13 to overhead vapor load in surface and groundwater concentrations of dissolved inorganic carbon dic ranged from 5 4 to 112 9 mg c l and 13 c dic ranged from 7 4 to 17 4 so 4 2 ca 2 and mg 2 in the water samples ranged from 2 163 2 6593 and 2 90 mg l respectively the dic and 13 c dic indicate both open and closed system carbonate evolution combined 13 c dic and ca 2 mg 2 and so 4 2 suggest additional groundwater evolution by gypsum dissolution and calcite precipitation the high so 4 2 content of some water samples indicates regional geologic control on water quality similarity in the range of 18 o d and 13 c dic for surface waters and groundwater used for drinking water supply is probably due to high hydraulic conductivities of the karstic aquifers the results of this study indicate rapid recharge of groundwater aquifers groundwater influence on surface water chemistry and the potential of surface water to impact groundwater quality and vise versa

importance of pre impact crustal structure for the asymmetry of the chicxulub impact crater the final geometry of an impact crater can be significantly influenced by the geological and geomorphological structure of the impact site therefore it may not on its own provide accurate information regarding the direction and angle of impact

formation of phreatic caves in an eogenetic karst aquifer by co2 enrichment at lower water tables and subsequent flooding by sea level rise formation of extensive phreatic caves in eogenetic karst aquifers is widely believed to require mixing of fresh and saltwater extensive phreatic caves also occur however in eogenetic karst aquifers where fresh and saltwater do not mix for example in the upper floridan aquifer these caves are thought to have formed in their modern settings by dissolution from sinking streams or by convergence of groundwater flow paths on springs alternatively these caves have been hypothesized to have formed at lower water tables during sea level low stands these hypotheses have not previously been tested against one another analyzing morphological data and water chemistry from caves in the suwannee river basin in north central florida and water chemistry from wells in the central florida carbonate platform indicates that phreatic caves within the suwannee river basin most likely formed at lower water tables during lower sea levels consideration of the hydrological and geochemical constraints posed by the upper floridan aquifer leads to the conclusion that cave formation was most likely driven by dissolution of vadose co2 gas into the groundwater sea level rise and a wetter climate during the mid holocene lifted the water table above the elevation of the caves and placed the caves tens of meters below the modern water table when rising water tables reached the land surface surface streams formed incision of surface streams breached the pre existing caves to form modern springs which provide access to the phreatic caves phreatic caves in the suwannee river basin are thus relict and have no causal relationship with modern surficial drainage systems neither mixing dissolution nor sinking streams are necessary to form laterally extensive phreatic caves in eogenetic karst aquifers dissolution at water tables potentially driven by vadose co2 gas offers an underappreciated mechanism to form cavernous porosity in eogenetic carbonate rocks copyright 2012 john wiley sons ltd

limits on modes of lithospheric heat transport on venus from impact crater density we present a formalism by which the size frequency distribution of impact craters on venus may be used to estimate upper bounds on the mean global rates of volcanic resurfacing and lithospheric recycling on that planet over the past several hundred million years the principal assumptions are that an upper bound on the rate of crater production on venus may be estimated and that craters are volcanically obliterated only when flows completely cover their rims the impact crater density reported from venera 15 16 observations if valid for the entire venus surface then indicates a mean volcanic flux no greater than 2 km y corresponding to a maximum average rate of resurfacing of about 4 km b y the fraction of global heat loss due to such a rate of volcanic resurfacing is negligible for the lowest estimated crater retention age of the surface imaged by venera 15 16 if globally representative the rate of lithospheric recycling on venus does not exceed 1 5 km y corresponding to 25 of the global heat loss more likely estimates of the mean surface age limit the contribution to global heat loss of any lithospheric recycling to be less than 10 these results support the hypothesis that simple conduction dominates heat transport at lithospheric levels in the venus interior

the floating gardens of chan cahal soils water and human interactions abstract ancient maya wetland agriculture has been studied since the 1960s but there are still too few multiproxy excavations to adequately portray this phenomenon this paper builds on multidisciplinary and multiproxy research near the ancient maya site of chan cahal near blue creek belize with two new excavations four new cores 22 new ams dates and a new review of the site s water chemistry and archeology a long term study of the site s water chemistry has shown its pervasive impacts on wetland formation and limits to agricultural uses we argue that a reservoir in the midst of the wetlands became inundated above a paleosol during the earliest maya occupation in the middle preclassic was dredged out during a time of population expansion sometime in the late preclassic to early classic and was filled again with maya clay during and after the classic period vibracores through two wetland fields and one canal corroborated our earlier model of preclassic through classic inundation of a paleosol by peat formation flood sands clay deposition and gypsum precipitation these cores produced new evidence for field and canal construction starting in the late preclassic a later addition was a platform added at the very end or after the main occupation ended in the terminal classic a new excavation at sayap ha on the coastal plain wetlands 7 m lower than chan cahal experienced a similar history of inundation and aggradation here maya farmers reclaimed these wetlands with canals and raised fields late in the late or terminal classic and abandoned them soon thereafter the sayap hah fields thus parallel the bop fields 9 km south since both were built and abandoned at a late stage as a comparison with other wetland field studies the stable c isotopes of soil humin through soil profiles showed a strong classic period to present influence of c 4 species at sayap hah and an increase in c 4 inputs in the upper 25 cm at both sayap hah and the chan cahal platform perhaps due to tropical pasture grasses prevalent here for the last 50 years

quantitative tests for plate tectonics on venus if plate tectonics exists on venus the rate of topographic dropoff from spreading centers should be about half that on earth due to greater rock fluid density contrast and lower temperature differential between the surface and interior using 1 1 mean elevations 21 000 km of ridge were identified on venus and 33 000 km on earth heights h at distances up to 1000 km to either side of the ridges were analyzed to make least squares determinations of the parameters ho h and n in where s is off crest distance the venus ridges have a much less well defined mode in crest heights ho than the earth ridges and a somewhat smaller exponent n on the average indicating greater concavity fixing n to the theoretical value of 0 5 plate velocities were inferred from the h s using h t from north atlantic and north pacific data on earth these velocity solutions were compared to those from plate rotations which indicate the h t is probably about 40 too high for a global mean the resulting plate creation rates on venus assuming comparable total heat generation and taking into account the density and temperature differences are 0 7 km yr or less and indicate that not more than 15 of venus s energy is delivered to the surface by plate tectonics in contrast to 2 9 km yr and 70 energy delivery on earth

petrological evolution of the crust and mantle in its simplest form crust mantle evolution can be thought of as a study of the processes by which material is lost from the mantle and forms new crust the overall goal is achievement of mass balance between the average composition of material known to have been added to the crust and the estimated average composition of the entire crust attainment of this goal for the earth has been elusive primarily because the materials available for study have generally been involved in several stages of evolution including fractionation within both the crust and mantle and recycling of material from crust to mantle thereby complicating the interpretation of available data although the overall composition of the continental crust generally has been modelled as andesitic taylor and mclennan 1981a the present upper continental crust is granodioritic and requires a more mafic lower crust it has been proposed taylor and mclennan 1981a that originally the archean crust had an andesitic composition throughout its volume and that partial melting in the lower crust developed more felsic melts that rose to enrich the upper crust leaving a more mafic residuum of lower crust this concept has been challenged by weaver and tarney 1981a who argue that the granulites of presumed lower crustal origin do not have the correct compositions for this model if one considers more recent additions to the crust from the mantle over the past several hundred million years it appears that these latest additions are basaltic with very little if any granitic additions in fact barker 1981 in his introduction to a special issue of jgr on granites and rhyolites states to date no one has demonstrated that such magmas may form in the mantle indeed much experimental and geochemical work indicates that such siliceous liquids cannot originate there therefore such magmas must be generated in the crust in island arc environments where the average composition of added material normally is thought to be andesitic a anderson 1982 has shown that the parent melt is basaltic not andesitic in addition kay 1980 has shown that the k content of the newly added material is less than the average k content of the previously existing crust in the oceanic crust the additions from the mantle are basaltic and according to o hara 1982 probably are derived from very mafic picritic melts as parents continental flood basalts which have also added material from mantle to crust also are clearly derived from basaltic parental melts basaltic volcanism study project 1981 thus the evidence for the nature of material subtracted from the mantle and added to the crust for the past several hundred million or perhaps a billion years points to very mafic melts this contrasts significantly with the overall andesitic composition of continental crust it implies furthermore that if the overall archean crustal composition was andesitic as estimated by taylor and mclennan 1981a then additions to the crust have become more mafic with time a concept in direct conflict with most models in which secular trends are from mafic to felsic with time veizer and jansen 1979 and mclennan et al 198o a question which is beginning to appear either explicitly or implicitly is whether crustal volcanic rocks or the crust as a whole really represents the parent melt that formed in the mantle a anderson 1982 o hara 1982 and nisbet and walker 1982

the effect of seasonal rainfall on nutrient input and biological productivity in the yax chen cave system ox bel ha mexico and implications for μxrf core studies of paleohydrology abstract lakes and speleothems from mexico s yucatan peninsula have been used extensively over the past decades for paleoclimate studies however aquifer condition and its response to climate change has received little attention cenotes sinkholes and coastal caves have been shown to record the paleohydrology of the aquifer but there is little information on sedimentation in these cave systems and its response to climate change newly developed xrf instrumentation for the analysis of cores can achieve subannual resolution due to small measurement increments but short period studies examining weathering inputs and rainfall have not yet been undertaken hindering paleoenvironmental interpretations of lake and cave sediment records this study examines the spatial and temporal relationship of cave sediment geochemistry in the anchialine cave system of yax chen quintana roo mexico sediment traps n 51 were placed at seventeen stations along the 2 7 km flooded cave system which transitions from mangrove to upland forest terrain with cenotes of variable size and frequency along its length sediment traps were collected every 6 months from may 2013 may 2017 along with rainfall and groundwater level data there are distinct responses of lithogenic fe ti sr limestone weathering and biogenic influenced si k s phytoplankton and mangrove sediment elements in the sediment samples associated with seasonal rainfall and hurricanes while lithogenic elements ti k show a direct relationship seasonal rainfall the sedimentation of biologically influenced si ti elements exhibit a 6 12 month lagged response with large rainfalls such as hurricane ingrid in 2013 and other tropical storms throughout the study period

margin for error anthropogenic geomorphology of bajo edges in the maya lowlands abstract many early maya cities developed along the edges of large structural or karst depressions bajos this topographic position aided growing populations to more effectively capture and store rainwater a necessity for year round occupation of interior portions of the maya lowlands of mexico and central america ancient maya forest clearance on sloping terrain led to accelerated soil loss and the aggradation of the bajo margins these newly created margins of colluvial lands became a focus of subsequent intensive agriculture and helped underwrite further urban expansion we document this long term landscape transformation with data derived principally from field investigations at tikal guatemala and yaxnohcah mexico but with reference to other maya centers in the elevated interior region eir data are derived from field investigations interpretation of lidar imagery and laboratory analyses we present a model of three variants of bajo margin landscape change with differences attributable to topography lithology hydrology and cultural processes we present preliminary data on crops that were cultivated on bajo margin soils we further describe how agriculture was adapted to evolving bajo margins as evidenced by systems of field walls terraces and ditches

venus limited extension and volcanism along zones of lithospheric weakness three global scale zones of possible tectonic origin are described as occurring along broad low rises within the equatorial highlands on venus lat 50 n to 50 s long 60 to 310 the two longest of these tectonic zones the aphrodite beta and themis atla zones extend for 21 000 and 14 000 km respectively several lines of evidence indicate that beta and atla regiones located at the only two intersections of the three major tectonic zones are dynamically supported volcanic terranes associated with currently active volcanism rift valleys south of aphrodite terra and between beta and phoebe regiones are characterized by 75 to 100 km widths raised rims and extensions of only a few tens of kilometers about the same magnitudes as in continental rifts on the earth horizontal extension on venus was probably restricted by an early choking off of plate motion by high crustal and upper mantle temperatures and the subsequent loss of water and an asthenosphere

mantle dynamics in mars and venus influence of an immobile lithosphere on three dimensional mantle convection numerical calculations of fully three dimensional convection in constant viscosity compressible spherical shells are interpreted in terms of possible convective motions in the mantles of venus and mars the shells are heated both internally and from below to account for radiogenic heating secular cooling and heat flow from the core the lower boundary of each of the shells is isothermal and shear stress free as appropriate to the interface between a mantle and a liquid outer core the upper boundary of each of the shells is rigid and isothermal as appropriate to the base of a thick immobile lithosphere calculations with shear stress free upper boundaries are also carried out to assess the role of the rigid surface condition the ratio of the inner radius of each shell to its outer radius is in accordance with possible core sizes in both venus and mars a calculation is also carried out for a mars model with a small core to simulate mantle convection during early core formation different relative proportions of internal and bottom heating are investigated ranging from nearly complete heating from within to almost all heating from below the rayleigh numbers of all the cases are approximately 100 times the critical rayleigh numbers for the onset of convection cylindrical plumes are the prominent form of upwelling in the models independent of the surface boundary condition so long as sufficient heat derives from the core thus major volcanic centers on mars such as tharsis and elysium and the coronae and some equatorial highlands on venus may be the surface expressions of cylindrical mantle plumes the form of the downwelling sheets is significantly affected by the rigid boundary in that the sheets are more irregular in their horizontal structure than when the top boundary is shear stress free in the mainly heated from within models the downwelling sheets are also shorter and less temporally durable when the top boundary is rigid than when it is stress free thus the free motion of plates on the earth facilitates extensive durable convective currents that drive the plates while the stiffening of the lithospheres on mars and venus promotes a style of convection that is not particularly effective in breaking the lithosphere into plates in the rigid top cases the upper boundary layer surrounding the plumes appears to be interspersed with downwelling currents emanating radially from the plumes axes these currents may establish a stress field at the base of hotspot swells that could lead to radial fractures such as those on tharsis models with rigid upper boundaries have higher interior temperatures than do similar models with shear stress free upper boundaries on this basis venus not only has a higher surface temperature than earth but it would have a hotter mantle as well upwelling plumes are more numerous when the outer boundary is rigid as compared with shear stress free flows dominated by a few stong plumes occur when the proportion of basal heating is large thus if the martian crustal dichotomy were caused by a convective system dominated by spherical harmonic degree l 1 then the convection may have been driven strongly from below by a heating pulse accompanying core formation or from the flow of heat from an early hot core if a convective mechanism is responsible for the crustal dichotomy then the dichotomy is likely a very ancient feature however the small core models we consider consistently produce a convective pattern with a dominant l 2 signature which does not correlate with the martian crustal dichotomy a yet smaller core may be necessary to yield the l 1 pattern the small core convective pattern does correlate with the geoid and topography signatures of tharsis which have strong l 2 components and the model produces dynamic uplift comparable to the total topography of the tharsis rise models with larger cores i e with the probable inner to outer radius ratio of mars present mantle generate 4 km of uncompensated topography similar to estimates of the uncompensated tharsis topography thus the tharsis rise could have achieved its full height early in the evolution of mars by mantle plume dynamic uplift at present the uncompensated portion of the tharsis rise topography can be accounted for by dynamic uplift obviating the need for elastic support the present compensated portion of the tharsis topography could be attributed to volcanic or magmatic crustal thickening or depletion of the underlying mantle large core models appropriate to the present mars produce too many plumes to account for just two major volcanic centers i e tharsis and elysium mantle plume activity could be focussed beneath regions like tharsis if fracturing or thinning of the lithosphere in these regions has facilitated magma and heat transport across the lithosphere a similar mechanism could be responsible for clustering of coronae on venus there are no deep seated active linear upwellings in the venus models that could be associated with linear spreading centers in aphrodite if linear spreading is actually occurring in aphrodite the phenomenon is probably a shallow passive one similar to mid ocean ridges on earth

features on venus generated by plate boundary processes various observations suggest that there are processes on venus that produce features similar to those associated with plate boundaries on earth synthetic aperture radar images of venus taken with a radar whose wavelength is 12 6 cm are compared with gloria images of active plate boundaries obtained with a sound source whose wavelength is 23 cm features similar to transform faults and to abyssal hills on slow and fast spreading ridges can be recognized within the artemis region of venus but are not clearly visible elsewhere the composition of the basalts measured by the venera 13 and 14 and the vega 2 spacecraft corresponds to that expected from adiabatic decompression like that which occurs beneath spreading ridges on earth structures that resemble trenches are widespread on venus and show the same curvature and asymmetry as they do on earth these observations suggest that the same simple geophysical models that have been so successfully used to understand the tectonics of earth can also be applied to venus

bilateral topographic symmetry patterns across aphrodite terra venus western aphrodite terra venus is characterized by a series of parallel linear structural discontinuities 2000 4000 km in length and 100 200 km wide which strike at high angles to the general topographic trend of the aphrodite terra highlands the broad characteristics of the cross strike discontinuities csds are similar to both strike slip fault zones and terrestrial oceanic fracture zones in an effort to distinguish between these two hypotheses topographic profiles were taken across aphrodite terra to test for bilateral symmetry of the type associated with thermal boundary layer topography at divergent plate boundaries on earth in addition to a broad bilateral symmetry at a range of angles across aphrodite terra detailed bilateral symmetry is observed within domains between linear discontinuities in directions generally parallel to the strike of the discontinuities in addition within a domain the centers of symmetry of several profiles define a linear rise crest that is oriented normal to the bounding csds and terminates against them furthermore linear rise crests are offset in a right lateral sense along the csds between domains by 200 1200 km if the discontinuities represent strike slip movement then retrodeformation of the axes of symmetry along the csds to form a straight line should form a broadly symmetrical topographic high instead when the topography is retrodeformed the edges of the highlands are characterized by distinctive misfits of topography at the edge of the rise where it intersects the csds the nature and configuration of the csds the characteristics of the topography and topographic patterns including bilateral topographic symmetry generally parallel to csds within domains the existence of linear rise crests defined by the axes of bilateral symmetry within domains and the offsets of these linear rise crests between domains all suggest that western aphrodite terra on venus is similar to terrestrial oceanic divergent plate boundary environments and that the cross strike discontinuities are analogous to oceanic fracture zones rather than strike slip faults

ring of cenotes sinkholes northwest yucatan mexico its hydrogeologic characteristics and possible association with the chicxulub impact crater a 180 km diameter semicircular band of abundant karst sinkholes ring of cenotes in northwest yucatan mexico coincides approximately with a concentric ring of the buried chicxulub structure a circular feature manifested in cretaceous and older rocks that has been identified as the product of the impact of a bolide the ring expressed in tertiary rocks marks a zone of high permeability as shown by 1 the sinkholes themselves 2 breaks in the coastal dune system and high density of springs where the ring intersects the coast and 3 water level transects characterized by a decline in water level toward the ring any direct relation that exists between the ring of cenotes and the chicxulub structure bears on regional hydrogeology if the layer or zone responsible for the ring is deeply buried it may act as a barrier to the movement of ground water across the main flow direction shallower zones of horizontal permeability could result in less complete diversion of ground water through its influence on yucatan aquifer characteristics the ring may provide a link between modern environmental problems and astrogeology possible origins for the ring of cenotes are 1 faulting perhaps reactivated by post eocene mid miocene basin loading 2 permeability in a buried reef complex developed in the shallow paleocene sea around the crater rim or 3 breccia collapse occasioned by consolidation or by solution of evaporite components if the ring developed on ancient faults it may outline hydrothermal systems and mineral deposits produced during paleocene cooling of the chicxulub melt sheet

post chicxulub depositional and diagenetic history of the northwestern yucatan peninsula mexico abstract the chicxulub sedimentary basin of the northwestern yucatan peninsula mexico which was formed because of the largest identified phanerozoic bolide impact on earth became a site of deposition of dominantly marine carbonate sediments during most of the cenozoic era this is a study of the filling and diagenetic history of this basin and surrounding areas the study makes use of lithologic biostratigraphic petrographic and geochemical data obtained on core samples from boreholes drilled throughout the northwestern yucatan peninsula the core sample data indicate that 1 the chicxulub sedimentary basin concentrated the deposition of pelagic and outer platform sediments during the paleocene and eocene and in places during the early oligocene as well and filled during the middle miocene 2 deeper water limestone also is present within the paleocene and lower eocene of the proposed santa elena depression which is located immediately south of the basin 3 shallow water deposits are relatively more abundant outside the basin and depression than inside 4 the autigenic and allogenic silicates from the paleogene formations are the most abundant inside the depression 5 sediment deposition and diagenesis within the basin also were controlled by impact crater topography 6 the abundance of the possible features of subaerial exposure increases upward and outward from the center of the basin and 7 the formation of replacive low magnesium calcite and dolomite dedolomitization dissolution and precipitation of vug filling calcite and dolomite cement have been more common outside the basin than inside 18 o in whole rock excluding vug filling calcite from core samples ranges from 7 14 to 0 85 pdb 13 c varies from 6 92 to 3 30 pdb both stable isotopes correlate inversely with the abundance of subaerial exposure features indicating that freshwater diagenesis has been extensive especially outside and at the edge of the chicxulub sedimentary basin 18 o and 13 c in whole rock excluding vug filling dolomite ranges from 5 54 to 0 87 pdb and 4 63 to 3 38 pdb respectively most dolomite samples have negative 18 o and positive 13 c suggesting that replacive dolomitization involved the presence of a fluid dominated by freshwater and or an anomalously high geothermal gradient most dolomite xrd determined mole percent caco 3 varies between 51 and 56 replacive dolomite is larger more euhedral and less stoichiometric inside the chicxulub sedimentary basin than outside

geoid anomalies and dynamic topography from convection in cylindrical geometry applications to mantle plumes on earth and venus summary a variety of evidence suggests that at least some hotspots are formed by quasi cylindrical mantle plumes upwelling from deep in the mantle we model such plumes in cylindrical axisymmetric geometry with depth dependent newtonian viscosity cylindrical and sheet like cartesian upwellings have significantly different geoid and topography signatures however rayleigh number nusselt number systematics in the two geometries are quite similar the geoid anomaly and topographic uplift over a plume are insensitive to the viscosity of the surface layer provided that it is at least 1000 times the interior viscosity increasing the rayleigh number or including a low viscosity asthenosphere decreases the geoid anomaly and the topographic uplift associated with an upwelling plume increasing the aspect ratio increases both the geoid anomaly and the topographic uplift of a plume the nusselt number is a weak function of the aspect ratio with its maximum value occurring at an aspect ratio of slightly less than 1

determination of flow characteristics in the aquifer of the northwestern peninsula of yucatan mexico abstract flow characteristics were studied in the northwestern peninsula of yucatan mexico the yucatan aquifer is a mature karst system influenced by the ring of cenotes sinkholes this zone of aligned sinkholes is a high permeability zone with respect to its surroundings the aquifer is unconfined within the study area water level measurements at 48 locations were made in june and october 1994 they were measured up to four times in october water levels in a limited group of wells showed variations up to 60 during october 1994 whereas others had variations of the order of 5 in the same period by studying the resulting equipotential regimes a highly variable zone hvz was identified drain characteristics are significantly different from the rest of the study area which leads to the hypothesis that the hvz represents an independent subsystem of interconnected fractures to determine directions of high permeability within the study area azimuthal resistivity surveys were conducted at 22 locations resistivity curves particularly in the hvz show two or more peaks each indicating a direction of high permeability directions of high permeability are interpreted to be preferential directions of aligned fractures comparison with the two different equipotential regimes in october 1994 shows that in the majority of the cases the direction of the hydraulic gradient coincides with the direction of one fracture system the hydraulic gradient can be considered to be the major control in the dissolution process at least in the hvz resulting in the selective opening of the fractures peaks in the resistivity curves are of the same order of magnitude this shows that both regimes of equipotential lines and the corresponding changes of the hydraulic gradients are sufficiently frequent to be able to create different fracture systems with a comparable fracture density the variations of the equipotential regime in time may have severe consequences in terms of the risk of aquifer contamination in the highly variable zone contaminants generated mainly in the city of merida are generally believed to flow towards the north coast of the peninsula however reversals in the hydraulic gradient may allow contaminants to flow towards the southeast of the city at least for limited periods in the year

rayleigh taylor instability as a mechanism for corona formation on venus abstract in this study we explore the idea that coronae have formed on venus as a result of gravitational rayleigh taylor instability of the lithosphere the lithosphere is represented by a system of stratified homogeneous viscous layers low density crust over high density mantle over lower density layer beneath the lithosphere a small harmonic perturbation imposed on the base of the lithosphere is observed to result in gravitational instability under the constraint of assumed axisymmetry topography develops with time under the influence of dynamic stress associated with downwelling or upwelling and spatially variable crustal thickening or thinning topography may therefore be elevated or depressed above a mantle downwelling but the computed gravity anomaly is always negative above a mantle downwelling in a homogeneous asthenosphere the ratio of peak gravity to topography anomaly depends primarily on the ratio of crust to lithospheric viscosity average observed ratios are well resolved for two groups of coronae 40 mgal km 1 consistent with models in which the crust is perhaps 5 times stronger than the lithosphere group 3a rim surrounding elevated central region coronae are inferred to arise from a central upwelling model whereas group 8 depression coronae are inferred to arise from central downwelling observed average coronae radii are consistent with a lithospheric thickness of only 50 km an upper low density crustal layer is 10 20 km thick as inferred from the amplitude of gravity and topography anomalies

stagnant lid convection on venus the effect of strongly temperature dependent viscosity on convection in the interior of venus is studied systematically with the help of finite element numerical models for viscosity contrasts satisfying experimental constraints on the rheology of rocks venus is likely to be in the regime of stagnant lid convection this regime is characterized by the formation of a slowly creeping very viscous lid on top of the mantle venusian lithosphere and is in agreement with the tectonic style observed on venus stagnant lid convection explains large geoid to topography ratios on venus by the thermal thinning of a thick lithosphere the thickness of the lithosphere can be as large as 400 550 km for beta regio and 200 400 km on average geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle 10 20 10 21 pa s the convective stresses in the interior 0 2 0 5 mpa the stresses in the lid 100 200 mpa the velocity in the interior 0 5 3 cm yr 1 and the heat flux beneath the lithosphere 8 16 mw m 2 parameterized convection calculations of thermal history of venus are difficult to reconcile with a thick present day lithosphere however a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0 5 b y ago one of the possible explanations for the cessation of venusian plate tectonics is that during the evolution of venus stresses in the lid dropped below the yield strength of the lithosphere this model predicts a drastic drop in the heat flux thickening of the lithosphere and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on venus around 0 5 b y ago

the isotopic record of ocean chemistry and diagenesis preserved in non luminescent brachiopods from mississippian carbonate rocks illinois and missouri fossil brachiopod shells are commonly used as a faithful record of the chemical and isotopic composition of the ancient ocean water in which they grew owing to their stable mineralogy and their presence in marine strata throughout phanerozoic time the utility of these fossils for preserving ocean water compositions is evaluated through analyses of multiple specimens that grew in contemporaneous sea water at individual stratigraphic horizons in the mississippian age burlington keokuk formation of mid continent north america significant variations in sr c and o isotopic compositions are observed in contemporaneous non luminescent shells indicating that apparently pristine brachiopod shell material has been altered by post depositional processes of diagenesis the least altered compositions at each horizon are used to estimate secular variations of 87 sr 86 sr in early mississippian ocean water the estimated rates of change of ocean water 87 sr 86 sr are comparable in magnitude to the rapid sr isotopic changes documented for cenozoic ocean water for which high frequency glacial cycles are inferred as a driving mechanism the rigorous assessment of diagenetic effects on the isotopic signatures of marine phases is requisite for the advancement of high resolution reconstructions of paleo ocean chemistry

ancient maya impacts on the earth s surface an early anthropocene analog abstract the measure of the mayacene a microcosm of the early anthropocene that occurred from c 3000 to 1000 bp comes from multiple late quaternary paleoenvironmental records we synthesized the evidence for maya impacts on climate vegetation hydrology and the lithosphere from studies of soils lakes floodplains wetlands and other ecosystems maya civilization had likely altered local to regional ecosystems and hydrology by the preclassic period 3000 1700 bp but these impacts waned by 1000 bp they altered ecosystems with vast urban and rural infrastructure that included thousands of reservoirs wetland fields and canals terraces field ridges and temples although there is abundant evidence that indicates the maya altered their forests even at the large urban complex of tikal as much as 40 of the forest remained intact through the classic period existing forests are still influenced by ancient maya forest gardening particularly by the large expanses of ancient stone structures terraces and wetland fields that form their substrates a few studies suggest deforestation and other land uses probably also warmed and dried regional climate by the classic period 1700 1100 bp a much larger body of research documents the maya impacts on hydrology in the form of dams reservoirs canals eroded soils and urban design for runoff another metric of the mayacene are paleosols which contain chemical evidence for human occupation revealed by high phosphorus concentrations and carbon isotope ratios of c 4 species like maize in the c 3 dominated tropical forest ecosystem paleosol sequences exhibit maya clays a facies that reflects a glut of rapidly eroded sediments that overlie pre maya paleosols this stratigraphy is conspicuous in many dated soil profiles and marks the large scale maya transformation of the landscape in the preclassic and classic periods some of these also have increased phosphorous and carbon isotope evidence of c 4 species we synthesize and provide new evidence of maya period soil strata that show elevated carbon isotope ratios 13 c indicating the presence of c 4 species in typical agricultural sites this is often the case in ancient maya wetland systems which also have abundant evidence for the presence of several other economic plant species the mayacene of c 3000 to 1000 bp was thus a patchwork of cities villages roads urban heat islands intensive and extensive farmsteads forests and orchards today forests and wetlands cover much of the maya area but like so many places these are now under the onslaught of the deforestation draining and plowing of the present anthropocene

plate tectonics on venus the high surface temperature of venus implies a permanently buoyant lithosphere and a thick basaltic crust terrestrial style tectonics with deep subduction and crustal recycling is not possible overthickened basaltic crust partially melts instead of converting to eclogite because mantle magmas do not have convenient access to the surface the 40 ar abundance in the atmosphere should be low venus may provide an analog to archean tectonics on the earth

hydrogeology of the south eastern yucatan peninsula new insights from water level measurements geochemistry geophysics and remote sensing summary the yucatan peninsula is one of the world s largest karstic aquifer systems it is the sole freshwater source for human users and ecosystems the region hosts internationally important groundwater dependent ecosystems gdes in the 5280 km2 sian ka an biosphere reserve the gdes are threatened by increasing groundwater abstractions and risks of pollution hydrogeological exploration work is needed as basis for sound groundwater management a multidisciplinary approach was used to study this data scarce region geochemical data and phreatic surface measurements showed distinct hydrogeological units in the groundwater catchment of sian ka an the hilly southwestern areas had a low hydraulic permeability likely caused by a geology containing gypsum whereas the transition zone and flat areas in the east and north had a high permeability in the latter areas the fresh groundwater could be described by a dupuit ghyben herzberg lens geophysical borehole logging and time domain electromagnetic soundings identified a shallow low resistive and high gamma radiation layer present throughout the hilly area and transition zone its thickness was 3 8 m apparent conductivity was 200 800 ms m and natural gamma radiation about 80 counts pr second the layer is proposed to be ejecta from the chicxulub impact cretaceous paleogene boundary spatial estimates of recharge were calculated from modis imagery using the triangle method average recharge constituted 17 of mean annual precipitation in the study area recharge was greatest in the hilly area and towards valladolid near the coast average actual evapotranspiration exceeded annual precipitation the multidisciplinary approach used in this study is applicable to other catchment scale studies

on the reliability of lithospheric constraints derived from models of outer rise flexure summary we evaluate the common practice of utilizing elastic models of outer rise flexure to constrain lithospheric parameters such as mechanical thickness and ambient in plane force we numerically compute synthetic flexural profiles consistent with empirically determined constraints on lithospheric rheology and representative trench type boundary conditions and misfit minimization is utilized to determine the analytical solution to elastic plate flexure that most closely resembles each synthetic profile we then determine if it is possible to use the best fitting elastic solutions to recover the lithospheric mechanical thickness and level of in plane force that were assumed during the numerical computation of the synthetic profiles this methodology is analogous to the common practice of estimating lithospheric parameters by modelling bathymetric profiles with analytical descriptions of elastic plate flexure our results unequivocally indicate that in plane force cannot be reliably constrained in this manner such an approach does not even allow the qualitative nature of in plane force to be distinguished i e compressional versus tensional although in principle elastic plate models may provide reliable constraints on the mechanical thickness of oceanic lithosphere in practice uncertainties associated with bathymetric noise and the level of in plane force may in some instances render such constraints unreliable

seasonal cycles in radium and barium within a subterranean estuary implications for groundwater derived chemical fluxes to surface waters abstract there is increasing evidence that submarine groundwater discharge sgd is an important source of water and dissolved materials to the ocean one of the primary tracers of this process is the quartet of radium isotopes 223 ra 224 ra 226 ra and 228 ra whereby excess activities in surface waters can often be attributed to an input supplied via sgd this approach requires the radium end member activity to be well constrained however natural variability in groundwater radium may span several orders of magnitude therefore this variability is usually the main driver of uncertainties in volumetric sgd estimates to investigate the physical and biogeochemical controls on groundwater radium activities we conducted a three year time series of radium and barium a chemical analogue for radium within the subterranean estuary of a coastal aquifer waquoit bay ma usa gonneea et al 2013 demonstrated that movement of the salinity interface within the subterranean estuary is driven by changes in the hydraulic gradient between groundwater level and sea level height for waquoit bay seasonal scale sea level change not groundwater level was the main driver in hydraulic gradient fluctuations seasonal changes in groundwater chemistry can be attributed to the resulting movement of the salinity transition zone between terrestrial and marine groundwater landward movement of the interface results in a large release of radium isotopes 226 ra 1400 dpm 100 l 1 and barium 3000 nmol kg 1 associated with an increase in groundwater salinity the magnitude of these releases cannot be explained by in situ production or weathering alone but is likely due to salinity driven desorption from surface bound sediment inventory the timing of these peak concentrations is not always in phase with model derived estimates of sgd as a result the groundwater concentration rather than the water flux is the main driver of ra and ba inputs to waquoit bay surface waters the behavior of the subterranean estuary as an ion exchange reservoir has important implications for the timing and flux of various nutrients and pollutants that transit this region prior to discharge in addition to modulating chemical fluxes via submarine groundwater discharge on seasonal time scales transgression of the subterranean estuary may alter the input of chemicals to the ocean on decadal and longer time scales during this study the observed excess flux of 226 ra and ba from the subterranean estuary can be accounted for with sorbed sediment pools and accelerating rates of sea level rise in this region

tests of crustal divergence models for aphrodite terra venus aphrodite terra the largest highland region of venus is a likely site of mantle upwelling active volcanism and extensional tectonics we examine three alternative kinematic models for the interaction of mantle convection and the surface one in which little horizontal surface displacement results from mantle flow vertical tectonics one in which shear strain from large horizontal displacements is accommodated only in narrow zones of deformation plate divergence and one in which strain from large horizontal motions is broadly accommodated distributed deformation in support of a divergent plate model for aphrodite head and crumpler have cited the presence of an organized system of lineaments held to be analogues to oceanic fracture zones small scale bilateral symmetry of topographic elements hypothesized to be rifted and separated relief analogous to some terrestrial oceanic plateaus and subsidence of topography proportional to the square root of distance consistent with that expected for a divergent thermal boundary layer we undertake quantitative tests of each of these assertions and we compare the results with those from similar tests of the mid atlantic ridge we find that apart from the long wavelength symmetry of broadly elevated regions there is no convincing evidence for regional bilateral symmetry of features several hundred kilometers in size on either planet the fit of the topography of aphrodite to that of a thermal boundary layer is in general much poorer than for the earth and so other mechanisms such as dynamic uplift or crustal thickness variations must dominate the topography the broad saddle shaped region between thetis and atla regiones shows the best fit to the root distance relation yielding apparent spreading half rates of a few centimeters per year but the goodness of fit and apparent spreading rates are quite sensitive to the distance range analyzed calculation of a single pole of relative motion for the entire postulated system of transform faults shows that the inferred fracture zone traces are not consistent with a simple two plate model regardless of past pole motions a multiple plate geometry is therefore called for and one or more of the lineaments must act as a plate boundary along nearly its full length if the plate divergence hypothesis is correct such a boundary would be distinguishable from other lineaments by geological evidence for recent nontransform motion in forthcoming magellan radar images in the absence of such evidence as long as the existence and distribution of the lineaments are verified then broad disruption of a thin lithosphere is favored in such a model lineaments may be surface manifestations of mantle convective flow a model dominated by primarily vertical tectonics is tenable only if the lineaments are not confirmed by magellan

gravitational spreading of high terrain in ishtar terra venus magellan altimetry measurements indicate that the mountain belts and plateau margins of ishtar terra have topographic slopes in the range 2 30 magellan radar images reveal that numerous sets of narrow closely spaced troughs and lineations which we interpret as graben and normal faults are located in many of these regions of high slope the orientation of many of these graben sets perpendicular to the downslope direction suggests that they formed as a result of gravitational spreading in several of the mountain belts the extensional structures are parallel to the apparent direction of shortening consistent with the interpretation that gravitational spreading occurred while mountain building was still active to explore the implications of this spreading for the tectonic evolution of ishtar terra we model the process by means of a finite element algorithm for viscoelastic deformation in a vertical section of the crust at the margin of a plateau or broad mountain range flow in the crust is assumed to be governed by a nonlinear depth dependent rheology we predict brittle failure and relaxation of topographic relief as functions of time for ranges of crustal thickness 10 30 km and thermal gradient 5 25 k km and for observed ranges in topographic elevation 1 6 km and slope 1 30 this parameter range predicts a large spread in the time scales for initial failure and significant relaxation of relief for a crustal thickness greater than 10 km a thermal gradient of 15 k km or more and average values of relief 3 km and slope 3 the topography relaxes to 25 of its original height in less than 10 m y although initial failure in the form of normal faulting on the highlands and margin and occasional shallow thrusting in the lowlands is predicted to occur much earlier values of horizontal surface strain that are likely to be observable 1 do not accumulate until significant relaxation of relief begins given the predicted rate of topographic relaxation either the crust of ishtar terra must be anomalous in some way either much stronger than predicted by standard flow laws or very thin or cold or the topographic relief and high slopes have been actively built and maintained until times significantly younger than the average crater retention age for venus of 500 ma the large apparent depth of compensation the abundance of volcanism and the uncertainty in the local age favor the view that mountain building in ishtar terra has occurred until times at least as recent as 10 ma

sea level rise sedimentary record and organic carbon fluxes in a low lying tropical coastal ecosystem abstract sea level rise one of the most evident effects of recent climate change is already impacting coastal ecosystems because of its low relief population increase and economic development during the last century the celestun lagoon is highly vulnerable to global change including sea level rise here we study a sediment core from a tropical saltmarsh to evaluate the impact of sea level rise on sediment accretion and carbon fluxes some geochemical indicators in a 210pb dated sediment core showed clear signals of marine influence this was reflected on increased accretion rates from 0 3 0 1 mm yr 1 in 1941 to 2 9 1 2 mm yr 1 in 2012 these accretion rates were similar to eustatic sea level rise and mean acceleration ranged from 0 037 0 22 mm yr 2 implying a slr by 2100 ranging from 14 to 86 cm the one century 1917 2013 organic carbon stock in sediments was calculated to be 10 1 0 2 mg c ha 1 during the same period the organic carbon flux corrected for organic carbon degradation increased from 7 3 g c m 2 yr 1 to 73 26 g c m 2 yr 1 attributed to the impact of sea level rise in this ecosystem this work shows that tropical saltmarshes may provide useful information on sea level rise and the presented methodology may be used where instrumental records do not exist or they are too short

effects of lateral resolution on the identification of volcanotectonic provinces on earth and venus topographic data sets for the continental united states and the north pacific have been degraded and displayed in the same manner as the pioneer venus altimeter data for venus results for the u s data show that the cordillera orogenic zone would be seen as a broad swell with a height of 2 km and a wavelength of about 2500 km individual structural elements within the cordillera are reduced to mild height about 700 m wavelength about 500 km swells that are superimposed on the broad cordillera rise for the pacific data the hawaiian swell the e w trending fractures in the eastern pacific the increase in depth away from the east pacific rise and most trenches can still be discerned the venusian data do not show the integrated trench transform and ridge system evident in the degraded pacific data these features would be discernible in the degraded pacific data even after correcting depths for a the higher surface temperature on venus and b the lack of loading due to ocean water thus a plate tectonic regime similar to earth s does not appear to currently exist on venus unfortunately as shown by the degraded cordillera data the pv altimetry data are not of sufficient quality to discern whether or not the highlands of venus preserve evidence for orogenic events related to an ancient epoch of plate tectonics

pedogenic carbonate crusts calcretes in karstic landscapes as archives for paleoenvironmental reconstructions a case study from yucatan peninsula mexico abstract in the northeastern margin of the yucatan peninsula which is located in southeastern mexico three levels of compact carbonate crusts calcretes interlayered with indurated coastal calcareous sediments calcarenites are discernible since both the calcretes and calcarenites form a continuous carbonate sequence a combined methodology macro and micromorphology geochemistry and mineralogy is used to differentiate them additionally the calcrete layers are dated using the uranium thorium method in order to establish a chronological framework the carbonate differentiation and chronology are then used to establish the paleoenvironmental conditions that prevailed during calcrete formation as well as to reconstruct the coastal landscape evolution and its relation to pleistocene sea level changes during marine isotope stages mis 6 and 5 samples of calcrete layers and calcarenites were taken from two sections in the cancun tulum corridor the lowest level of calcrete underlies a sequence of calcarenites and the two younger levels cap the sequence in calcretes the micromorphology provides evidence of the presence of a relict soil with subangular blocky microstructure root traces and needle fiber calcite associated with soil fungi interactions in contrast the micromorphology of the calcarenites shows ooids and bioclasts typical of a coastal environment geochemically calcretes are depleted in 13c compared to calcarenite with values ranging between 2 and 10 and they have a lower proportion of rare earth elements calcrete mineralogy consists mainly of calcite over aragonite with a low proportion of mg calcite u th dating indicates that the oldest calcrete was formed during the penultimate glacial maximum pgm in mis 6 and the two younger calcretes correspond to the last interglacial period in mis 5c the calcarenite formation occurred earlier also during mis 5 soil formation strong karstification and soil erosion have occurred on younger calcretes this study suggests that these processes developed between mis 4 to mis 1

a mantle plume model for the equatorial highlands of venus the equatorial highlands of venus consist of four main structures atla beta ovda and thetis regiones each has a circular to oval shaped planform and rises 4 5 km above the mean planetary radius these highlands are associated with long wavelength geoid highs with amplitudes ranging from 35 m at ovda to 120 m at atla they also contain topographic valleys interpreted as extensional rift zones and beta is known to contain shield volcanoes these characteristics are all consistent with the equatorial highlands being formed by mantle plumes an alternative model in which ovda and thetis are interpreted as spreading centers analogous to terrestrial mid ocean ridges fails to explain most of the observed geoid anomalies and topography in these regions some smaller highlands such as bell regio eistla regio and the hathor innini ushas region may also be plume related but most coronae are unlikely to be the direct result of plume activity we have modeled plumes using a cylindrical axisymmetric finite element code and a depth dependent newtonian rheology we compare our model results with profiles of geoid and topography across atla beta ovda and thetis our best model fits are for beta and atla assuming whole mantle convection and that earth and venus have similar mantle heat flows venus must lack an earth like low viscosity zone in its upper mantle in order satisfy the observed geoid and topography for these features this conclusion is consistent with the long wavelength admittance spectrum of venus and with the observed differences in the slopes of the geoid spectra for the two planets one explanation for the different viscosity structures of the two planets could be that the mantle of venus is drier than earth s mantle

trace element geochemistry of groundwater in a karst subterranean estuary yucatan peninsula mexico ndseg nsf chemical oceanography program oce 0751525 coastal ocean institute ocean and climate change institute at woods hole oceanographic institution

geologic and environmental aspects of surface cementation north coast yucatan mexico the northernmost part of the fresh water aquifer of yucatan mexico is confined near the coast by a thin flat nearly impermeable calcareous layer the authors think it probable that this confining layer is now developing by a process of sedimentation and precipitation of pore filling cement on and near the surface of older limestone at the landward margin of the swamp that extends virtually continuously along the north yucatan coast it is their interpretation that this caco sub 3 cemented layer which is 0 5 to 1 4 m thick developed in the zone of discharge of the fresh water aquifer the presence of sup 14 c 6 0 plus minus 0 4 modern carbon in a sample of aquitard supports the hypothesis that cementation is an ongoing process further support comes from the remarkable regularity between modern mean sea level and the elevation of the landward boundary between the confining zone and normal karst along 250 km of coast confinement of the aquifer produces an elevation of the piezometric surface to about 0 5 m above mean sea level and a concurrent depression of the fresh water salt water interface to an estimated depth of about 18 m below mean sea level at the coast breaching of the confining layer implicit inmore some development schemes for the region could dramatically decrease the thickness of the fresh water lens a valuable water resource the mixing zone beneath the confined part of the aquifer is a chemically active volume that may be vigorously pumped by tides as evidenced by increased salinity of ground water near the coast thus making this zone a likely place for rock water interaction including perhaps dolomite formation less

human and natural impacts on fluvial and karst depressions of the maya lowlands abstract this paper begins to differentiate the major drivers and chronology of erosion and aggradation in the fluvial and fluviokarst landscapes of the southern and central maya lowlands we synthesize past research on erosion and aggradation and add new data from water soils radiocarbon dating and archaeology to study the quantity timing and causes of aggradation in regional landscape depressions geomorphic findings come from many excavations across a landscape gradient from upland valleys karst sinks and fans into the coastal plain floodplains and depressions findings from water chemistry show that sources in the uplands have low quantities of dissolved ions but water in the coastal plains has high amounts of dissolved ions often nearly saturated in calcium and sulfate we found significant geomorphic complexity in the general trends in upland karst sinks in a few instances sediments preserve late pleistocene paleosols buried 2 3 m though many more have distinct middle to late holocene paleosols buried 1 2 m after c 2300 bp maya early to late preclassic from 2300 1100 bp late preclassic to classic periods the landscape aggraded from five main mechanisms river flooding climatic instability accelerated erosion ancient maya landscape manipulation and gypsum precipitation from a rise in a water table nearly saturated in calcium and sulfate ions evidence exists for two or three high magnitude floods possibly driven by hurricanes moreover lake core and geophysical studies from the peten lakes region have shown high rates of deposition of silicate clays maya clays starting and peaking during the maya preclassic and continuing to be high through the late classic the main driver on upland karst depressions the peten lakes upland valleys and fans was accelerated soil erosion but water table rise probably driven by sea level rise was the main driver on the wetlands of the coastal plain because the aggraded sediments here are dominantly composed of gypsum precipitated from the groundwater this latter mechanism represents a little recognized mechanism of aggradation over a large region these large scale environmental changes occurred during periods of intensive ancient maya land use and climatic instability both of which may have contributed to erosion by increasing runoff despite these geomorphic changes ancient maya farmers adapted in several key cases