discovery of syngenetic and eogenetic karsts in the middle ordovician gypsum bearing dolomites of the eastern ordos basin central china and their heterogeneous impact on reservoir quality abstract the middle ordovician majiagou formation in the eastern ordos basin central china represents a typical case of karst in gypsum bearing carbonate strata it is unique in a long karst history of ca 120 ma since the deposition to the late carboniferous the lithology is complex with the development of both fine grained and granular dolomites thus the karstification and its impact on reservoir quality is challenging to improve the understanding of this critical issue and further provide data for exploration we collected core samples within the representative first and second sub members of the fifth member of the majiagou formation herein referred to as ma 51 2 and conducted comprehensive analyses of their physical and geochemical properties results show that ma 51 2 is generally composed of gypsum bearing micritic dolomites and grain dolomites karst breccias were observed and two periods of karstification were recognized the first period is syngenetic and shows typical features such as gypsum molds and intragranular pores it is predominantly developed within gypsum bearing micritic dolomites due to selective fabric dissolution isotopic data indicate the influence of syngenetic meteoric water and seawater as indicated by slightly depleted 13c values average 0 03 relatively low 18o values average 8 35 and slightly elevated 87sr 86sr values average 0 70999 the second period is interpreted as inland eogenetic karstification and shows typical features such as non selective fabric dissolution including pores and vugs breccias and karst fillings in grain rocks the rocks were subjected to differential dedolomitization grain dolomites were strongly altered by meteoric water and record a terrestrial input as suggested by negative carbon and oxygen isotope excursions average 13c and 18o values are respectively 3 07 and 8 97 as well as high 87sr 86sr values average 0 71093 the two periods of karstification had a profound and heterogeneous influence on the reservoir quality gypsum molds formed due to the alteration of gypsum nodule bearing micritic dolomite by early syngenetic karstification and represent the best reservoir space in ma 51 2 this reflects the key role of syngenetic karstification in the formation of high quality reservoirs in contrast the late inland eogenetic karstification is mainly manifested as strong alteration of grain dolomites with little effect on micritic dolomites due to its relatively poor physical properties the collapse resulting from strong dissolution destroys the continuity of reservoirs in addition relatively high porosity and permeability promote fluid movement resulting in the infilling of the karst cave system by mud and carbonate sand thus a so called inversion of physical property occurs namely the filling of original large pores in the grain dolomites leads to poor reservoir at last and the preservation of initially small pores in the micritic dolomites leads to relatively good reservoir at last this indicates that the evolution of reservoir is complex under the influence of karst

constraining erosional input and deep water formation in the north atlantic using nd isotopes many marine radiogenic isotope records show both spatial and temporal variations reflecting both the degree of mixing of distinct sources in the oceans and changes in the distribution of chemical weathering on the continents however changes in weathering and transport processes may themselves affect the composition of radiogenic isotopes released into seawater the provenance of physically weathered material in the labrador sea constrained through the use of ar ar ages of individual detrital minerals has been used to estimate the relative contributions of chemically weathered terranes releasing radiogenic isotopes into the labrador sea a simple box model approach for balancing observed nd isotope variations has been used to constrain the relative importance of localised input in the labrador sea and the subsequent mixing of labrador sea water into north atlantic deep water the long term pattern of erosion and deep water formation around the north atlantic seems to have been a relatively stable feature since 1 5 ma although there has been a dramatic shift in the nature of physical and chemical weathering affecting the release of hf and pb isotopes the modelled nd isotopes imply a relative decrease in water mass advection into the labrador sea between 2 4 and 1 5 ma accompanied by a decrease in the rate of overturning possibly caused by an increased freshwater input into the labrador sea

an outsider s review of the astronomical theory of the climate is the eccentricity driven insolation the main driver of the ice ages abstract although the astronomical theory of the climate also known as the milankovitch theory explains a number of key features of the paleoclimate time series especially frequency and phase distribution in terms of orbital influence on the waxing and waning of ice sheets it still faces several important unresolved challenges in this review we discuss five main challenges we believe the theory must resolve in order to survive 1 the large amplitude of the 100 ky ice age cycles 2 the switch from 41 ky to 100 ky long glaciations around 0 9 ma bp 3 the absence of significant spectral power at 413 ky over the past 1 2 ma 4 the variation in glacial cycle duration in the last 800 ky and 5 the presence of non orbital spectral peaks in the climate record but in spite of these problems we conclude that the milankovitch theory may still be a viable one at least in the original sense of hays et al science 194 1976 1121 that orbital changes somehow influenced the ice ages many investigators ourselves included appear to see the impact of orbital forcing in the data clear evidence of astronomical forcing as well as evidence which suggests that the climate system responds nonlinearly to all milankovitch orbital frequencies

relation between soil age and silicate weathering rates determined from the chemical evolution of a glacial chronosequence long term weathering rates rlt were determined from base cation depletion in soil profiles developed on six granitic glacial moraines varying in age from 0 4 to 297 ka in the wind river mountains wyoming weathering rates were found to decrease with soil age t according to the power law equation rlt 15 t 0 71 where rlt is in meq m 2 yr 1 and t is in ka comparable data from the literature are consistent with this function and it is similar to power law equations describing changes in laboratory weathering rates with time on the basis of our study we predict that for a catchment unaffected by anthropogenic inputs and in a steady state with respect to the organic and cation exchange pools rlt should be higher than present day weathering rates derived from stream fluxes rpd by a factor of about 3 4

re os isotope systematics and weathering of precambrian crustal rocks implications for the marine osmium isotope record the effects of weathering of freshly exposed precambrian rocks on the mobility of osmium were studied on soils developed on a chronosequence of glacial moraines from the western wind river range in wyoming the os budget of the precambrian granitoid source rocks is dominated by os rich trace phases such as magnetite amongst the major silicates biotite or a trace phase in biotite carries most of the os and re and is also the most radiogenic mineral with 187os 186os of 113 and 187re 186os of 2 000 re os isotope systematics of source rocks and soils indicate that rapid oxidation of magnetite mobilizes os with an isotopic composition similar to the isotopic composition of the bulk soils a very radiogenic fraction of os is mobilized through preferential weathering of biotite radiogenic runoff from precambrian shields inferred from osmium isotope analyses of freshwater fe mn nodules suggests that high latitude precambrian shields are important source areas of radiogenic os in seawater we propose that glacial scouring and weathering of glacial tills exposed after deglaciation of precambrian shields surrounding the north atlantic provides a mechanism for the slightly more radiogenic nature of north atlantic seawater compared to other seawater masses glacial interglacial variations in the osmium isotopic composition of seawater seem plausible and may be triggered by changes in weathering regimes on glacial interglacial time scales in high latitude shield areas surrounding the north atlantic

dolomites and dolomites in mesozoic platform carbonates of the southern apennines geometric distribution petrography and geochemistry abstract a multidisciplinary study of the dolomitized bodies present among the late triassic to late cretaceous carbonate deposits of the monti lattari belt southern apennines of italy compares the different dolomitization styles present within this succession of these mesozoic platform carbonates and furnishes an overview of their formative genetic models the revealing traces of the diverse modes of diagenesis in this region include both surface and subsurface mechanisms utilizing a geometric reconstruction of the dolomitized bodies an integrated facies and petrographic analysis of the components and a geochemical characterization of the sediments including detailed mineralogy and associated stable isotope values the data presented demonstrate that some intervals are characterized by surface related dolomitization processes middle late triassic and cretaceous while others latest triassic to lower middle jurassic and are mainly dominated by later large scale subsurface dolomitization processes all of the middle late triassic norian successions are pervasively dolomitized exhibit fabric preserving textures and have a distinctly heavy stable isotope signature their origin has been related to diagenesis due to particular climatic and palaeoceanographic conditions leading to the formation of extended blankets of shallow warm saline seawater this setting was terminated by climatic and palaeoceanographic changes during the latest triassic which dramatically reduced the significance of surface related dolomitization on a regional scale the overlying latest triassic rhaetian outcrops consist of a widespread coarse fabric destructive massive dolomite with a lighter isotopic signature these dolomites seem genetically related to the ones of the lower middle jurassic which share the same geometrical petrographic and geochemical characteristics caused by large scale fluid circulation this diagenetic pattern apparently started in the early jurassic during burial driven by thermal convection due to the different temperatures between the oceanic waters and the warmer carbonate platform the middle upper jurassic units feature local episodes of early dolomitization that partially replaced the muddy carbonate facies due to tidal pumping and reflux mechanisms dolomitization is not a pervasive feature during this phase of deposition in the cretaceous portion of the succession dolomites have a totally different and distinct character and are generally related to bedding and show variability in textures and mg ca ratios local association with evaporites geometry of dolomitized bodies and stable isotope geochemistry suggest processes comprising evaporative pumping and reflux all related to the surface environment this evidence points to a renewal of a climatic context more favorable for early dolomitization thus this study permits recognition of the presence of three quite different dolomitization styles within the carbonate platform succession of the southern apennines fold thrust belt one ascribed to the norian one to the rhaetian lower jurassic and one to the cretaceous

a low threshold for north atlantic ice rafting from low slung slippery late pliocene ice sheets 1 suborbital variability in late pleistocene records of ice rafted debris and sea surface temperature in the north atlantic ocean appears most extreme during times of enlarged ice sheets with a well constrained benthic oxygen isotope defined ice volume threshold 18ot for the 100 ka inter glacial world information on climate instability for the earlier pleistocene and late pliocene is more fragmentary and or of much lower temporal resolution but the data available suggest similar behavior with 18ot remaining more or less constant over the past 3000 ka this finding is puzzling because it implies that ice rafting is highly sensitive to ice volume on short suborbital glacial interglacial time scales but not to the long term changes in ice sheet composition associated with intensification of northern hemisphere glaciation nhg here we report new high resolution records of stable isotope change and ice rafting in the north atlantic at integrated ocean drilling program site u1308 reoccupation of deep sea drilling project site 609 during two glacials key to intensification of nhg marine isotope stages g4 2640 ka and 100 2520 ka we find a pattern of suborbital ice rafting events showing clear evidence of threshold behavior however contrary to previous reports we find that 18ot for the late pliocene is up to 0 45 vienna peedee belemnite vpdb lower than for the late pleistocene using published plio pleistocene global sea level records we evaluate different potential explanations for this finding we conclude that the observed pliocene pleistocene offset in 18ot is attributable to the existence of low slung pliocene ice sheets that flowed more readily than their late pleistocene counterparts associated with a smaller contemporaneous continental ice volume and less isotopically depleted ice

palaeo mixing zone karst features from palaeocene carbonates of north spain criteria for recognizing a potentially widespread but rarely documented diagenetic system abstract marine meteoric mixing zone dissolution effects are a major feature of present day karst systems in carbonate platforms yet are rarely reported in the geological record an example is described from the upper danian platform limestones of the alava province in the western pyrenees north spain this consists of several narrow zones with sponge like porosity analogous to the swiss cheese features found in present day mixing zones these zones are stained by fe oxides and overlie limestones which are irregularly dolomitized and contain disseminated pyrite these high porosity zones are interpreted as having developed in marine mixing zones where mixing corrosion and microbially mediated processes increased dissolution if collapsed ancient mixing zones could be misinterpreted as terra rossa palaeosols the main criteria to identify them as mixing zone products are their occurrence below a palaeo meteoric phreatic zone their association with stratified oxic and anoxic redox zones and petrographic evidence for highly variable calcite saturation states

structures and mechanisms associated with development of a fold in the cantabrian zone thrust belt nw spain a detailed history of progressive fold development based on mesostructural analysis is provided for a metre scale fold in the cantabrian fold and thrust belt of northwestern spain the well exposed asymmetric syncline displays a wide variety of structures including extensional faults thrusts veins tectonic stylolites bedding plane faults drag folds and a main limb thrust that transects the entire fold important constraints on fold development are provided by the spatial distribution and style of deformation angular relations and cross cutting relations among the structures fold mechanisms varied as a function of structural position while the northwest limb was subjected to pure shear shortening in the form of contractional faults the southeast limb was bent under tangential longitudinal strain resulting in a combination of outer arc extensional faulting in

origin and morphology of limestone caves limestone caves form along ground water paths of greatest discharge and solutional aggressiveness flow routes that acquire increasing discharge accelerate in growth while others languish with negligible growth as discharge increases a maximum rate of wall retreat is approached typically about 0 01 0 1 cm yr determined by chemical kinetics but nearly unaffected by further increase in discharge the time required to reach the maximum rate is nearly independent of kinetics and varies directly with flow distance and temperature and inversely with initial fracture width discharge gradient and pco2 most caves require 104 105 yr to reach traversable size their patterns depend on the mode of ground water recharge sinkhole recharge forms branching caves with tributaries that join downstream as higher order passages maze caves form where 1 steep gradients and great undersaturation allow many alternate paths to enlarge at similar rates or 2 discharge or renewal of undersaturation is uniform along many alternate routes flood water can form angular networks in fractured rock anastomotic mazes along low angle partings or sponge work where intergranular pores are dominant diffuse recharge also forms networks and spongework often aided by mixing of chemically different waters ramiform caves with sequential outward branches are formed mainly by rising thermal or h2s rich water dissolution rates in cooling water increase with discharge co2 content temperature and thermal gradient but only at thermal gradients of more than 0 01 c m can normal ground water co2 form caves without the aid of hypogenic acids or mixing artesian flow has no inherent tendency to form maze caves geologic structure and stratigraphy influence cave orientation and extent but alone they do not determine branch work versus maze character

palaeoenvironments and weathering regime of the neoproterozoic squantum tillite boston basin no evidence of a snowball earth the snowball earth hypothesis describes episodes of neoproterozoic global glaciations when ice sheets reached sea level the ocean froze to great depth and biota were decimated accompanied by a complete shutdown of the hydrological cycle recent studies of sedimentary successions and earth systems modelling however have brought the hypothesis under considerable debate the squantum tillite boston basin usa is one of the best constrained snowball earth successions with respect to age and palaeogeography and it is suitable to test the hypothesis for the gaskiers glaciation the approach used here was to assess the palaeoenvironmental conditions at the type locality of the squantum member through an analysis of sedimentary facies and weathering regime chemical index of alteration the stratigraphic succession with a total thickness of ca 330 m documents both glacial and non glacial depositional environments with a cool temperate glacial to temperate non glacial climate weathering regime the base of the succession is composed of thin diamictites and mudstones that carry evidence of sedimentation from floating glacial ice interbedded with inner shelf sandstones and mudstones thicker diamictites interbedded with thin sandstones mark the onset of gravity flow activity followed by graded sandstones documenting channellized mass gravity flow events an upward decrease in terrigenous supply is evident culminating in deep water mudstones with a non glacial chemical weathering signal renewed terrigenous supply and iceberg sedimentation is evident at the top of the succession beyond which exposure is lost the glacially influenced sedimentary facies at squantum head are more consistent with meltwater dominated alpine glaciation or small local ice caps the chemical index of alteration values of 61 to 75 for the non volcanic rocks requires significant exposure of land surfaces to allow chemical weathering therefore extreme snowball earth conditions with a complete shutdown of the hydrological cycle do not seem to apply to the gaskiers glaciation

pre wisconsin glacial stratigraphy of the central plains region in iowa nebraska kansas and missouri summary recent investigations have documented the presence of several tills and interglacial paleosols that were not recognized in the simple kansan aftonian nebraskan stratigraphic sequence also the single pearlette ash recognized by early workers now is known to be three different pearlette family ash beds of greatly contrasting ages the complexity of the stratigraphy as it is currently understood and the miscorrelations of early work necessitate abandonment of the archaic terms kansan aftonian and nebraskan in stratigraphic terminology continued use of those terms will only promote confusion of stratigraphic nomenclature and erroneous correlations of stratigraphic units the limited chronometric control available within the early and middle pleistocene stratigraphic sequence is provided directly by dating or by correlation of buried soils and volcanic ash beds and it is provided inferentially by interpretation of magnetic polarity data these controls provide at best a general time framework to begin a new synthesis of the quaternary history of the region new methods of dating are needed to facilitate long distance correlation of early and middle pleistocene deposits

middle to late pleistocene loess record in eastern nebraska usa and implications for the unique nature of oxygen isotope stage 2 abstract new subsurface data reveal a nearly continuous stratigraphic record of middle to late pleistocene loess sedimentation preserved beneath upland summits in eastern nebraska usa thickness and grain size trends as well as pedologic evidence indicate significant changes in loess sources accumulation rates and depositional environments the newly defined kennard formation accumulated in the middle pleistocene and may represent multiple thin increments of distal loess from nonglacial sources on the great plains the overlying loveland loess up to 18 m thick and deposited during oxygen isotope stage 6 ois 6 illinoian glaciation probably records the emergence of the missouri river valley as a major glaciogenic loess source the prominent sangamon geosol formed through long term pedogenic alteration of the upper loveland loess during ois 5 and 4 thin loess of the gilman canyon formation records slow loess accumulation and pedogenic alteration in ois 3 the peoria loess ois 2 is similar in thickness to loveland loess but may have accumulated more rapidly in an environment less favorable to bioturbation more importantly comparison of peoria and loveland loess thickness trends indicates much greater influx of nonglaciogenic loess from the great plains during ois 2 than in ois 6 suggesting colder and or drier conditions in the midcontinent during ois 2 than in earlier glacial stages

the imprint of time on canadian soil landscapes abstract the imprint of time on canadian soil landscapes reflects the strong influence of repeated quaternary glaciations with most soils having formed on late pleistocene glacial sediments and to a lesser extent holocene non glacial sediments soil chronosequences in these youthful landscapes seldom span the full holocene and have been documented principally in areas of coastal isostatic uplift and shoreline evolution as well as on recessional moraine sequences in the cordillera these chronosequences have placed quantitative constraints on rates of podzolization in environments ranging from coastal temperate rainforest to subarctic longer sequences involving soils and relict paleosols on pleistocene morainal and glaciofluvial surfaces in the central yukon territory record more complex legacies of pedogenesis and landscape evolution but with much coarser time control and many potentially confounding influences of climatic change and disturbance processes older weathered bedrock and regolith can make significant contributions to soil parent materials in several settings 1 in favoured locations where local conditions protected these materials from glacial erosion in southern canada 2 beyond the maximum limits of glaciation principally in northwestern canada and 3 in the arctic where cold based non erosive ice allowed preservation of these materials closer collaborations between pedologists and other quaternary scientists will be needed to ensure a more complete understanding of these complex pedogenic legacies in canadian landscapes

influence of provenance and transport process on the geochemistry and radiogenic hf nd and sr isotopic composition of pleistocene glacial sediments minnesota usa abstract we develop a broad geochemical dataset from 50 samples of pleistocene glacial till matrix including three lacustrine samples representing four sediment provenances collected from across minnesota usa such a dataset is useful both in the context of regional studies of glaciation and in efforts to understand how provenance and glacial processes impact the geochemistry of sediment archives the principal sediment sources of the four provenances include materials from the archean proterozoic canadian shield the proterozoic midcontinent rift paleozoic carbonates and cenozoic and mesozoic clastic sediments we measured major element geochemistry in all till samples in both

a cosmic trip 25 years of cosmogenic nuclides in geology terrestrial cosmogenic nuclides produced by secondary cosmic ray interactions in the atmosphere and in situ within minerals in the shallow lithosphere are widely used to date surface exposure of rocks and sediments to estimate erosion and weathering rates and to date sediment deposition or burial their use has transformed geomorphology and quaternary geology for the first time allowing landforms to be dated and denudation rates to be measured over soil forming time scales the application of cosmogenic nuclides to geology began soon after the invention of accelerator mass spectrometry ams in 1977 and increased dramatically with the measurement of in situ produced nuclides in mineral grains near earth s surface in the 1980s the past 25 yr have witnessed the development of cosmogenic nuclides from their initial detection to their prevalence today as a standard geochronological and geochemical tool this review covers the major developments of the past 25 yr by comparing the state of the field in 1988 with that of today and by identifying key advances in that period that moved the field forward we emphasize the most commonly used in situ produced nuclides measured by ams for geological applications but we also discuss other nuclides where their applications overlap our review covers ams instrumentation cosmogenic nuclide production rates the methods of surface exposure dating measurement of erosion and weathering and burial dating and meteoric 10 be in memoriam devendra lal 1929 2012 whose vision inspired the field

changes in the silt and clay size mineralogy of sediments at ocean drilling program site 645b baffin bay site 645b ocean drilling program odp leg 105 was cored in 2010 m water depth off the eastern margin of baffin island a 279 m core was dated as extending back to ca 2 ma samples of sediment were analyzed for their silt and clay size mineralogy using x ray diffraction techniques constrained cluster analysis on these data defined three units which indicated that significant changes in mineralogies occurred at approximately 75 m below sea floor bsf ca 0 5 ma and 155 m bsf ca 1 15 ma these boundaries coincide closely with units ia ib and ib ii as designated in the odp shipboard reports and which were based on detrital carbonate and sand contents unit ii 1 2 2 ma is characterized by zero clay size calcite and high amounts of clay size smectite and silt size mica unit ib 0 5 1 15 ma is noteworthy for the first presence of clay size calcite high amounts of detrital dolomite and a peak in clay size kaolinite the uppermost unit ia 0 0 5 ma is characterized by consistently high value

quaternary evolution of glaciated gneiss terrains pre glacial weathering vs glacial erosion vast areas previously covered by pleistocene ice sheets consist of rugged bedrock dominated terrain of innumerable knolls and lake filled rock basins the cnoc and lochan landscape or landscape of areal scour these landscapes typically form on gneissose or granitic lithologies and are interpreted 1 either to be the result of strong and widespread glacial erosion over numerous glacial cycles or 2 formed by stripping of a saprolitic weathering mantle from an older deeply weathered landscape we analyse bedrock structure erosional landforms and weathering remnants and within the cnoc and lochan gneiss terrain of a rough peneplain in nw scotland and compare this with a geomorphologically similar gneiss terrain in a non glacial arid setting namaqualand south africa we find that the topography of the gneiss landscapes in nw scotland and namaqualand closely follows the old bedrock saprolite contact weathering front the roughness of the weathering front is caused by deep fracture zones providing a highly irregular surface area for weathering to proceed the weathering front represents a significant change in bedrock physical properties glacial erosion and aeolian erosion in namaqualand is an efficient way of stripping saprolite but is far less effective in eroding hard unweathered bedrock significant glacial erosion of hard gneiss probably only occurs beneath palaeo ice streams we conclude that the rough topography of glaciated cnoc and lochan gneiss terrains is formed by a multistage process 1 long term pre glacial chemical weathering forming deep saprolite with an irregular weathering front 2 stripping of weak saprolite by glacial erosion during the first glaciation s resulting in a rough land surface broadly conforming to the pre existing weathering front etch surface 3 further modification of exposed hard bedrock by glacial erosion in most areas glacial erosion is limited but can be significant beneath palaeo ice stream the roughness of glaciated gneiss terrains is crucial for modelling of the glacial dynamics of present day ice sheets this roughness is shown here to depend on the intensity of pre glacial weathering as well as glacial erosion during successive glaciations

the evolution of pleistocene climate a time series approach abstract in the later part of the pleistocene variations in global ice volume have been dominated by an approximate 100 000 year cycle analysis of 2 myr long oxygen isotope record from an equatorial pacific core indicates that this is true only for the last 900 000 years prior to this time the amplitude of the 100 000 year cycle is much reduced as is the variance of all oscillations with periods greater than 60 000 years based on results of time series analysis of this 2 myr long record the pleistocene glacial cycles can be divided into three sections 1 the late pleistocene 0 900 kyr b p where the variations in the isotope record are dominated by the 100 000 year cycle 2 the middle pleistocene 900 1450 kyr b p in which low frequency components are not as important as in the later period of the pleistocene and 3 the early pleistocene late pliocene 1450 2000 kyr b p where general reductions of importance at all frequencies is seen as compared to the later intervals recent modeling efforts which describe variations in global ice volume show that the dominant low frequency component observed in the late pleistocene can result from different time constants for the rate of glacial growth and decay in response to variations in the earth s orbital parameters it is hypothesized that during the early pleistocene the rate of growth and decay of glaciers were more similar and that continental erosion by successive glacial advances lowered the land surface in areas of ice cap formation to below sea level when the ice caps became marine based more rapid decay of the ice became possible

the middle pleistocene transition characteristics mechanisms and implications for long term changes in atmospheric pco2 abstract the emergence of low frequency high amplitude quasi periodic 100 kyr glacial variability during the middle pleistocene in the absence of any significant change in orbital forcing indicates a fundamental change internal to the climate system this middle pleistocene transition mpt began 1250 ka and was complete by 700 ka its onset was accompanied by decreases in sea surface temperatures ssts in the north atlantic and tropical ocean upwelling regions and by an increase in african and asian aridity and monsoonal intensity during the mpt long term average ice volume gradually increased by 50 m sea level equivalent whereas low frequency ice volume variability experienced a 100 kyr lull centered on 1000 ka followed by its reappearance 900 ka although as a broad band of power rather than a narrow persistent 100 kyr cycle additional changes at 900 ka indicate this to be an important time during the mpt beginning with an 80 kyr event of extreme sst cooling followed by the partial recovery and subsequent stabilization of long term north atlantic and tropical ocean ssts increasing southern ocean sst variability primarily associated with warmer interglacials the loss of permanent subpolar sea ice cover and the emergence of low frequency variability in pacific ssts and global deep ocean circulation since 900 ka ice sheets have been the only component of the climate system to exhibit consistent low frequency variability with the exception of a near universal organization of low frequency power associated with marine isotope stages 11 and 12 all other components show an inconsistent distribution of power in frequency time space suggesting a highly nonlinear system response to orbital and ice sheet forcing most hypotheses for the origin of the mpt invoke a response to a long term cooling possibly induced by decreasing atmospheric pco2 none of these hypotheses however accounts for the geological constraint that the earliest northern hemisphere ice sheets covered a similar or larger area than those that followed the mpt given that the mpt was associated with an increase in ice volume this constraint requires that post mpt ice sheets were substantially thicker than pre mpt ice sheets indicating a change in subglacial conditions that influence ice dynamics we review evidence in support of the hypothesis that such an increase in ice thickness occurred as crystalline precambrian shield bedrock became exposed by glacial erosion of a thick mantle of regolith this exposure of a high friction substrate caused thicker ice sheets with an attendant change in their response to the orbital forcing marine carbon isotope data indicate a rapid transfer of organic carbon to inorganic carbon in the ocean system during the mpt if this carbon came from terrigenous sources an increase in atmospheric pco2 would be likely which is inconsistent with evidence for widespread cooling apparently rapid carbon transfer from terrestrial sources is difficult to reconcile with gradual erosion of regolith a more likely source of organic carbon and nutrients which would mitigate pco2 rise is from shelf and upper slope marine sediments which were fully exposed for the first time in millions of years in response to thickening ice sheets and falling sealevels during the mpt modeling indicates that regolith erosion and resulting exposure of crystalline bedrock would cause an increase in long term silicate weathering rates in good agreement with marine sr and os isotopic records we use a carbon cycle model to show that a post mpt increase in silicate weathering rates would lower atmospheric pco2 by 7 12 ppm suggesting that the attendant cooling may have been an important feedback in causing the mpt

the laurentide and innuitian ice sheets during the last glacial maximum the late wisconsinan advance of the laurentide ice sheet started from a middle wisconsinan interstadial minimum 27 30 14 c ka bp when the ice margin approximately followed the boundary of the canadian shield ice extent in the cordillera and in the high arctic at that time was probably similar to present ice advanced to its late wisconsinan stage 2 limit in the northwest south and northeast about 23 24 14 c ka bp and in the southwest and far north about 20 21 14 c ka bp in comparison to some previous reconstructions of ice extent our current reconstruction has substantially more late wisconsinan ice in the high arctic where an innuitian ice sheet is generally acknowledged to have existed in the atlantic provinces where ice is now thought to have extended to the continental shelf edge in most places and on eastern baffin island where ice probably extended to the fiord mouths rather than to the fiord heads around most of the ice margin the late wisconsinan maximum ice extent either exceeded the extent of earlier wisconsinan advances or it was similar to the early wisconsinan advance ice marginal recession prior to 14 14 ck a bp occurred mainly in deep water and along the southern terrestrial fringe however heinrich event 1 probably drew down the entire central ice surface at 14 5 14 c ka bp sufficiently to displace the labrador sector outflow centre 900 km eastward from the coast of hudson bay the onset of substantial ice marginal recession occurred about 14 14 c ka bp in the northwest southwest and south but not until about 10 11 14 c ka bp in the northeast and in the high arctic thus the period of maximum ice extent in north america generally encompasses the interval from b24 21 to 14 14 c ka bp or considerably longer than the duration of the lgm defined as occurring during a period of low global sea level as well as during a time of relative climate stability b18 14 c ka bp the interval of advance of much of the laurentide ice sheet to its maximum extent between b27 14 c ka bp and b24 14 c ka bp coincides with a suggested interval of rapid fall in global sea level to near lgmlevels r 2001 elsevier science ltd all rights reserved

ice sheet erosion and the stripping of tertiary regolith from baffin island eastern canadian arctic abstract the erosion of glaciated regions and concomitant changes in ice sheet dynamics through the pleistocene are poorly documented the baffin island landscape which has been shaped by the foxe sector of the laurentide ice sheet lis contains a variety of glacial and proximal glaciomarine sedimentary archives spanning the pleistocene we examine these records to better understand when tertiary regolith was stripped from beneath the foxe sector of the lis till on the interior plateaux of the island in areas with scoured bedrock have low chemical index of alteration cia values low meteoric 10 be 10 be met concentrations and clay mineralogy consistent with erosion from an unweathered bedrock source however in some areas in between fiord onset zones more weathered till is present containing considerably higher cia values high 10 be met concentrations and secondary clay mineral weathering products implying that the till has persisted on the landscape and weathered during successive glaciations using these weathering signatures we analyze the coastal glaciogenic deposits of the clyde foreland formation cff at two sites for evidence of tertiary regolith removal from the interior of baffin island by lis erosion provenance indicators within the cff demonstrate that pleistocene lis ice flow lines across baffin island have remained generally constant the oldest cff glaciogenic unit likely representing one of the first if not the first lis advances across baffin island had high 10 be met concentrations at the time of deposition consistent with extensive regolith erosion evidence of notably weathered sediment is absent in all younger units suggesting that tertiary regolith was likely largely stripped from the interior of the island by 1 6 0 2 ma

north american glacial meltwater history for the past 2 3 m y oxygen isotope evidence from the gulf of mexico a high resolution 18 o record from the northeastern gulf of mexico ocean drilling program leg 100 hole 625b provides a history of surface waters for the past 5 35 m y negative 18 o anomalies between 0 3 and 3 0 in planktic foraminifera beginning at 2 30 ma can only have resulted from the discharge of isotopically light glacial meltwater from the mississippi river meltwater anomalies implicate either the periodic presence of north american mid latitude glacial ice since the late pliocene or a drainage system very different from that of the present

the quaternary is here to stay the proposals to drop the name quaternary to extend the neogene period to the present day and to confirm the base of the pleistocene at 1 8 ma are reviewed it is concluded that there is no scientific case for dropping the name quaternary and that an overwhelming volume of evidence points to 2 6 ma as the most appropriate base for the quaternary pleistocene when extensive northern hemisphere mid latitude glaciation occurred under what were essentially similar physical boundary conditions of continents oceans seaways and mountain ranges that characterise and are unique to the quaternary it is desirable that all stratigraphical and geochronological evidence is tied rigorously into the global stratigraphic timescale copyright 2007 john wiley sons ltd

glacial weathering and the hafnium isotope composition of seawater the hf isotope composition of seawater does not match that expected from dissolution of bulk continental crust this mismatch is generally considered to be due to retention of unradiogenic hf in resistant zircons during incomplete weathering of continental crust during periods of intense glacial weathering zircons should break down more efficiently resulting in the release of highly unradiogenic hf to the oceans we test this hypothesis by comparing nd and hf isotope time series obtained from nw atlantic ferromanganese crusts both isotope systems show a decrease associated with the onset of northern hemisphere glaciation the observed changes display distinct trajectories in nd hf space which differ from previously reported arrays of bulk terrestrial material and seawater such patterns are consistent with the release of highly unradiogenic hf from very old zircons facilitated by enhanced mechanical weathering

glacial stratigraphy and paleomagnetism of late cenozoic deposits of the north central united states the north central united states preserves one of the best continental records of late pliocene and early middle pleistocene glaciations in the northern hemisphere the glaciogenic sequences of this region consist of multiple tills interbedded with paleosols and volcanic ashes here we present results on paleomagnetic measurements and till compositional data from glacial sedimentary sequences in iowa nebraska kansas and missouri periods of normal polarity brunhes chron and reverse polarity matuyama chron were identified in the sedimentary sequences investigated this chronology is further constrained by the presence of three volcanic ashes derived from dated eruptions of the yellowstone caldera which indicate that the oldest till was deposited 2 0 ma based on these results we identify three groups of tills representing at least seven pre illinoian glaciations two older groups of reverse polarity tills containing low and intermediate proportions of crystalline clasts respectively and one younger group of normal polarity tills enriched in crystalline lithologies the clay mineralogy of the reverse polarity tills is enriched in kaolinite and depleted in expandable clays relative to the normal polarity tills which are also characterized by a minor amount of chlorite the silt fraction of tills also shows mineralogical contrasts whereby the normalpolarity tills are characterized by increases in calcite dolomite and feldspar whereas the older tills show depletion in these minerals and a relative enrichment in quartz these petrographic and mineralogic changes are indicative of an increase with time in the areal distribution of unweathered igneous and metamorphic source bedrock

chemical weathering studies in relation to geomorphological research in southeastern canada abstract although chemical weathering provides fundamental information relevant to geomorphology the subject has been overlooked during the 20th century in canada this paper provides an overview of the current state of canadian research on chemical weathering in southeastern canada and takes into account three spatial and temporal contexts 1 the formation of bedrock morphology by chemical weathering 2 occurrences characteristics and age of saprolites and 3 contemporary chemical denudation rates long term geomorphological evolution of southeastern canadian landscapes shows that chemical weathering has played an important role an example is taken from the laurentide region of the canadian shield north of montreal quebec the present topography resulted from the stripping of the former weathering mantle and from the probable subsequent modification of the weathering front first by the action of hillslope processes and rivers and then by glaciers before and during plio pleistocene times the present landscape reflects the timing of the formation of erosion surfaces and of the stripping of the paleozoic cover rocks and exposure of the shield since the late seventies several isolated occurrences of saprolite soil profiles have been discovered in eastern canada and prompted a renewal of the study of these materials about the cenozoic evolution of these regions one of the problems in this field of research is the dating of saprolites and their inclusion in a chronostratigraphic framework along with the other cenozoic surficial deposits and landforms because of the multiple factors involved in the development of secondary minerals in saprolites during the course of weathering it is preferable to distinguish the dating of saprolites from the study of their mineralogical and geochemical evolution fortunately several new techniques are becoming available for the absolute dating of surficial deposits and saprolites including the use of cosmogenic radionuclides saprolites provide a strong potential field of research for our understanding of the geological evolution of eastern canada during the cenozoic contemporary weathering and erosion rates are fields of research that have gained increasing interest recently since modeling landscape geochemical response can be applied to various environmental stress situations such as acidification by rain and forest harvesting rock type may be the main factor explaining the large differences between watersheds in fact variability of cation removal in the temperate zone is probably most closely related to flow paths of water investigations at different scales from entire watersheds to slopes to individual pedons highlight the problem in the catamaran brook watershed new brunswick water chemistry is explained by a mix of groundwater and soil solution from the horizons at the base of the floodplain soils geochemical mass balances based on net outputs give little information on the weathering reactions of primary minerals the weathering products or on the nature of the weathering processes that provide the dissolved load of streams mineralogic and petrographic analysis of selected soil pedons are necessary to determine weathering reactions and their role as sources or sinks for bases silica aluminum and iron in the various compartments through which water percolates before it reaches the stream

oligocene miocene antarctic continental weathering record and paleoclimatic implications cape roberts drilling project ross sea antarctica abstract the bulk chemistry of mudrocks in ross sea drillcores on the antarctic inner shelf is re evaluated to extract records of continental paleotemperatures and ice extent the chemical index of alteration cia of the sedimentary sequence derived from the composite of the cape roberts cores changes in accordance with pco 2 pagani m zachos j c freeman k h tipple b bohaty s 2005 marked change in atmospheric carbon dioxide concentrations during the paleogene science 309 600 603 the k 2 o na 2 o al 2 o 3 molar ratio of the mudrocks is used as a mean summer temperature proxy for continental antarctica and it shows similarities with benthic mg ca temperatures for the southern ocean billups k schrag d p 2002 paleotemperatures and ice volume of the past 27 myr revisited with paired mg ca and stable isotope measurements on benthic foraminifera paleoceanography 17 doi 10 1029 2000pa000567 the ratio is calibrated using intervals with significant pollen assemblages and the temperature proxy enables extrapolation of the paleoclimatic data over a larger portion of the drilled record the cia and k na al ratios support a persistent cooling and drying of climate through the oligocene miocene transition the results reinforce the strong coupling between east antarctic continental temperatures and ice volume with deep sea temperatures and concentrations of atmospheric carbon over long timescales

volcanic ash beds recorders of upper cenozoic silicic pyroclastic volcanism in the western united states stratigraphic relationships and tephrochronologic information for volcanic ash beds 4 0 0 1 m y of the western interior allow the recognition of at least 68 different ash falls the average reoccurrence rate based on the succession of the isotopically dated ash beds is near one eruption every 57 000 years the ash beds were grouped into two chemical types rhyolitic and dacitic the main property that distinguishes dacitic from rhyolitic ashes is the larger ca and mg and smaller k content in dacitic ashes dacitic volcanic ashes were defined to have more than 0 55 wt ca rhyolitic ashes were subdivided further into two types w and g type based on the presence of biotite in the w type and its absence in g type w type rhyolitic ashes are chalky white have less than 0 55 wt fe have colorless glass shards of pumiceous habit g type rhyolitic ashes are light to medium gray have from 0 55 to 2 0 wt fe in the glass shards lack biotite have colorless platy bubble wall and bubble junction glass shards dacitic ashes are light gray to grayish brown have more than 0 55 wt fe and ca and have a complex mixture of colorless to light brown shards including pumiceous fibrous chunky and bubble wall and bubble junction types w type rhyolitic ashes possibly formed from magmas that were cooler 695 to 830 c and thus more viscous than the hotter 870 to 1000 c magma progenitors of g type ashes based on fe ti oxide mineral compositions primary microphenocrysts in the ash beds comprise assemblages of the following quartz sanidine plagioclase biotite amphibole clinopyroxene orthopyroxene fayalite zircon apatite allanite chevkinite sphene magnetite and ilmentite fayalite and chevkinite only have been found in g type biotite and sphene only have been found in w type rhyolitic ashes are highly evolved and contain 74 79 wt sio2 dacitic ashes vary from 67 77 wt sio2 al ranges from 10 5 to 14 5 wt al2o3 mg ranges from 0 06 to 1 0 wt mgo and fe as fe3o4 ranges from 0 50 to 4 1 wt dacitic ash beds have the most 0 80 3 8 wt as fe3o4 and rhyolitic ashes have the least 0 50 1 9 wt as fe3o4 ca varies from an unusually low amount 0 25 wt cao in a few rhyolitic ashes to as much as 2 2 in some dacitic ashes rhyolitic glasses show larger eu deficiencies compared to dacitic glasses on rare earth element diagrams w type rhyolitic glasses have moderately large la to sm ratios and have la contents from about 30 to 100 times chondrite values g type rhyolitic glasses also have moderately large la to sm ratios and contain somewhat more la than do the w type rhyolitic ashes generally from 100 to 300 times chondrites amounts

north atlantic intermediate waters in the late pliocene to early pleistocene we generated benthic isotope records from ocean drilling program odp site 981 on the feni drift 2173 m water depth and from odp site 983 on the gardar drift 1983 m water depth to examine the interaction between north atlantic deep water nadw and glacial north atlantic intermediate water gnaiw formation from 2 0 to 1 4 ma we find nadw at both sites during interglacial periods and a mix of nadw and southern ocean water at the feini drift during most glacial periods prior to 1 7 ma we find no evidence ofr gnaiw at the gardar drift site instead glacial gardar drift 13c values are as low or lower than values for all other sites in the north atlantic and reflect continued glacial overflow from the nordic seas after 1 7 ma gardar drift 13c values increase and suggest that there was gnaiw at the gardar drift site during some glacial intervals overall we find that nadw and gnaiw production changed around 1 7 ma in concert with changes in sea surface temperature and salinity and in the earth s obliquity cycle

long term stability of global erosion rates and weathering during late cenozoic cooling reports that the geological record of erosion indicates a fourfold increase in global sedimentation rates during the past 5 million years merited a global explanation explanations offered include an increased rate of mountain formation and global cooling now jane willenbring and friedhelm von blanckenburg reanalyse original data sets and use ocean records of beryllium isotope ratios to obtain an indirect measure of past erosion rates and rock weathering they find no clear evidence either for an increase in global erosion or for a pulse of weathered material in the oceans during the past 12 million years they suggest that cenozoic global cooling must have some other cause and the mountain building that did occur had little effect on erosion or weathering flux globally accelerated uplift and global cooling have been suggested as possible causes for a fourfold increase in global sedimentation rates and by inference in erosion rates or weathering fluxes during the past 5 million years here proxy evidence is provided that indicates stable weathering fluxes in the late cenozoic era it is proposed that processes different from an increase in denudation caused cenozoic global cooling over geologic timescales co2 is emitted from the earth s interior and is removed from the atmosphere by silicate rock weathering and organic carbon burial this balance is thought to have stabilized greenhouse conditions within a range that ensured habitable conditions1 changes in this balance have been attributed to changes in topographic relief where varying rates of continental rock weathering and erosion1 2 are superimposed on fluctuations in organic carbon burial3 geological strata provide an indirect yet imperfectly preserved record of this change through changing rates of sedimentation1 2 4 widespread observations of a recent 0 5 myr fourfold increase in global sedimentation rates require a global mechanism to explain them4 5 6 accelerated uplift and global cooling have been given as possible causes2 4 6 7 but because of the links between rates of erosion and the correlated rate of weathering8 9 an increase in the drawdown of co2 that is predicted to follow may be the cause of global climate change instead2 however globally rates of uplift cannot increase everywhere in the way that apparent sedimentation rates do4 10 moreover proxy records of past atmospheric co2 provide no evidence for this large reduction in recent co2 concentrations11 12 here we question whether this increase in global weathering and erosion actually occurred and whether the apparent increase in the sedimentation rate is due to observational biases in the sedimentary record13 as evidence we recast the ocean dissolved 10be 9be isotope system as a weathering proxy spanning the past 12 myr ref 14 this proxy indicates stable weathering fluxes during the late cenozoic era the sum of these observations shows neither clear evidence for increased erosion nor clear evidence for a pulse in weathered material to the ocean we conclude that processes different from an increase in denudation caused cenozoic global cooling and that global cooling had no profound effect on spatially and temporally averaged weathering rates

10be analysis of a quaternary weathering profile in the virginia piedmont samples from a residual weathering profile in the virginia piedmont have been analyzed for cosmogenic 10be concentrations are highest in clay rich soil and decrease exponentially to a depth of about 15 m despite uncertainties about the processes by which 10be may be intercepted before entering the solum and eroded after incorporation a minimum age may be calculated for the regolith this calculation is based on the delivery rate of 10be and its decay rate and suggests that this residual profile developed during a period no shorter than 8 105 yr the calculated minimum age may be within a factor of 2 of maximum age estimates based on surface lowering by erosion and on the rate of rock weathering to saprolite the vertical distribution of 10be in the profile could result from a steady state balance of deposition weathering radioactive decay and erosion

improved chronostratigraphic reference curve of late neogene seawater 87sr 86sr we present a reference curve of seawater 87 sr 86 sr variation through the past 7 m y based on 455 samples of planktonic foraminifera from ocean drilling program odp site 758 in the indian ocean this single site curve is superior to extant curves because of several features 1 a continuous sedimentary section free of disturbances 2 a well determined and precise chronostratigraphy that is calibrated to numeric age on the basis of continuous magnetostratigraphy and a refined astronomical time scale 3 a high temporal resolution sample interval averages 15 ka 4 an improved analytical reproducibility and 5 a uniformly high sample quality the gross structure of the site 758 curve is generally similar to that observed in previous work but the increased temporal resolution and decreased analytical scatter of the curve provide improved chronostratigraphic resolution at the 95 confidence level uncertainties on age estimates range from 0 60 m y in the lower pleistocene and upper miocene to 2 03 m y in the middle pliocene moreover this detailed record identifies two periods of steplike increases in seawater 87 sr 86 sr 6 10 to 5 80 ma and 1 46 to 1 13 ma that are characterized by rates of change of 150 10 6 m y these features if verified at other sites indicate abrupt changes in the sr mass balance of the ocean and may be used as precise event markers in late neogene marine sections

a silicate weathering mechanism linking increases in marine 87sr 86sr with global glaciation the 87sr 86sr ratio of sea water is generally regarded as a proxy for the rate of chemical weathering of the continents interpretation of the 87sr 86sr record in marine limestones requires an understanding of how changes in this quantity are linked to climate and tectonic events a connection between the strontium isotope record and the extent of global glaciation has been proposed1 8 but without a well established mechanism glaciation may influence marine 87sr 86sr by changing global riverine sr fluxes that is rock weathering rates and or 87sr 86sr ratios that is relative mineral weathering rates here we investigate how the 87sr 86sr release from silicate weathering is affected by glaciation using the sr isotope systematics of granitoid soils on alpine glacial moraines in the wind river range wyoming we observe a negative correlation between the 87sr 86sr ratio of exchangeable sr in soils and the soil age indicating that the 87sr 86sr ratio of sr released in the early stages of weathering is significantly higher than in later stages we estimate that this mechanism can increase global riverine 87sr 86sr by an average of 0 0002 during periods of glacial interglacial cycling when superimposed on long term trends controlled by tectonic processes2 7 9 10 this shift can account for the correlation between the rate of change of marine 87sr 86sr and the intensity of glaciation over at least the past 10 myr

origin of the middle pleistocene transition by ice sheet erosion of regolith the transition in the middle pleistocene 0 9 ma seen in 18o deep sea core records from relatively low amplitude high frequency 41 kyr to high amplitude low frequency 100 kyr ice volume variations under essentially the same orbital forcing can be attributed to a change from an all soft bedded to a mixed hard soft bedded laurentide ice sheet through glacial erosion of a thick regolith and resulting exposure of unweathered crystalline bedrock a one dimensional ice sheet and bedrock model which includes transport of sediment and ice by subglacial sediment deformation demonstrates that a widespread deforming sediment layer maintains thin ice sheets before the transition which respond linearly to the dominant 23 and 41 kyr orbital forcing progressive removal of the sediment layer eventually causes a transition to thicker ice sheets whose dominant timescale of change 100 kyr reflects nonlinear deglaciation processes in model simulations over the last 3 ma initialized with no ice and a uniform 50 m sediment layer the time series of ice volume and extent agree in several important aspects with the observed records

late cenozoic deep weathering patterns on the fennoscandian shield in northern finland a window on ice sheet bed conditions at the onset of northern hemisphere glaciation abstract the nature of the regolith that existed on the shields of the northern hemisphere at the onset of ice sheet glaciation is poorly constrained in this paper we provide the first detailed account of an exceptionally preserved deeply weathered late neogene landscape in the ice sheet divide zone in northern finland we mine data sets of drilling and pitting records gathered by the geological survey of finland to reconstruct regional preglacial deep weathering patterns within a gis framework using a large geochemical data set we give standardised descriptions of saprolite geochemistry using a variant of the weathering index of parker wip as a proxy to assess the intensity of weathering we also focus on mineral prospects and mines with dense pit and borehole data coverage in order to identify links between geology topography and weathering geology is closely linked to topography on the preglacial shield landscape of northern finland and both factors influence weathering patterns upstanding resistant granulite granite gabbro metabasalt and quartzite rocks were associated with fresh rock outcrops including tors or with thin fines values above 3000 and 4000 beneath valley floors developed along mineralised shear and fracture zones weathering penetrated locally to depths of 50 m and included intensely weathered kaolinitic clays with wip fines values below 1000 late neogene weathering profiles were varied in character tripartite clay gruss saprock profiles occur only in limited areas bipartite gruss saprock profiles were widespread with saprock thicknesses of 10 m weathering profiles included two discontinuities in texture materials and resistance to erosion between saprolite and saprock and between saprock and rock limited core recovery when drilling below the soil base in mixed rocks of the tana belt indicates that weathering locally penetrated deep below upper fresh rock layers such deep seated weathered bands in rock represent a third set of discontinuities incipient weathering and supergene mineralisation also extended to depths of 100 m in mineralised fracture zones the thin weathering crusts found extensively beneath till may represent types of early or middle pleistocene palaeosols we confirm that glacial erosion has been very limited

10be accumulation in a soil chronosequence we have measured the concentration of the cosmogenic isotope10be in soil samples from various horizons at six sites including three independently dated rappahannock river terraces and a previously undated piedmont soil to which we have assigned an age all of the incident10be can be accounted for in one of these soils and a second is within a factor of two in three soils whose concentrations vary widely with depth a significant fraction of the incident10be cannot be accounted for incomplete sampling and enhanced be mobility caused by organic components are the probable reasons for the low inventory of be from these three soils overall the data from these six sites indicate that10be accumulation could be used to assign ages to soils if be is not mobilized and lost from the soil profile

simulating the mid pleistocene transition through regolith removal abstract quaternary 18o ice volume proxy records show a transition from high frequency small amplitude glacial cycles to low frequency large amplitude glacial cycles this reorganization of the climate system termed the mid pleistocene transition mpt is thought to reflect a change in land ice response to orbital forcing despite no significant change in orbital cycles during this period one potential explanation for the mpt proposes that gradual erosion of high latitude northern hemisphere regolith by multiple cycles of glaciation caused a transition in ice sheet response to external forcing here we explore this regolith hypothesis using a complex earth system model we show that simulating a transition from deformable sediment to crystalline bedrock produces an evolution in ice volume response similar to proxy reconstructions of the mpt the simulated change in ice volume response is due to a combination of climate and ice flow changes with crystalline bedrock producing thicker colder ice sheets that accumulate more snowfall and have a smaller ablation zone further experiments that include transient eccentricity amplifying co2 forcing show only small differences in ice response compared to those with orbital forcing only suggesting that cycles of co2 were not the primary cause of the mpt

on the mid pleistocene transition to 100 kyr glacial cycles and the asymmetry between glaciation and deglaciation times 1 a mechanism is proposed for the mid pleistocene transition from a dominant periodicity of 41 kyr to 100 kyr in glacial oscillations the same mechanism is shown to also explain the asymmetry between the long glaciation and short deglaciation phases of each cycle since that transition versus the symmetry of the 41 kyr oscillations prior to the transition these features arise naturally within the framework of the sea ice switch glacial cycle mechanism of gildor and tziperman 2000 as a result of the gradual cooling of the deep ocean during the pleistocene this cooling results in a change of the relation between atmospheric temperature and the rates of accumulation and ablation of continentals ice sheets it is this latter change that leads to the activation of the sea ice switch and therefore to the initiation of the 100 kyr oscillations the gradual glaciation and rapid deglaciations during these oscillations occur because the mean value of the ice sheet ablation is not far from the maximum rate of snow accumulation during warm periods this proximity of mean ablation and maximum accumulation rates is shown to be also a consequence of the mid pleistocene gradual cooling of the deep ocean index terms 3344 meteorology and atmospheric dynamics paleoclimatology 1620 global change climate dynamics 3309 1640 global change remote sensing 4267 oceanography general paleoceanography keywords climate dynamics glacial cycles mid pleistocene transition 100 kyr 41 kyr

insights into the late cenozoic configuration of the laurentide ice sheet from 40ar 39ar dating of glacially transported minerals in midcontinent tills glacial sedimentary sequences in the north central united states record multiple advances of the laurentide ice sheet lis since 2 ma although the tills found in these sequences were deposited by southward flowing glacial lobes little information is available on the geometry of flow lines in the interior of the lis during any one glaciation and the provenance of glacial deposits older than the last ice advance is largely unknown systematic changes in the composition of midcontinent tills and other paleogeographic considerations however raise the possibility of significant shifts in the trajectory of flow lines feeding the lobes of the southwestern lis margin here we constrain till provenance using 40ar 39ar ages of individual hornblende and feldspar grains retrieved from tills representing several glaciations since 2 ma hornblende grains show 40ar 39ar ages that indicate erosion of paleoproterozoic 1 7 2 0 ga and late archean 2 5 ga rock sources whereas feldspar grains show a broad range of paleoproterozoic ages 1 4 2 4 ga dating of hornblende and feldspar minerals in single pebbles suggests that this latter distribution of ages is related to the greater sensitivity of feldspars to thermal resetting during minor tectonic events accordingly the range of 40ar 39ar ages for the predominant population of paleoproterozoic hornblende and feldspar grains in our samples is consistent with a source from terrains forming the churchill province of the canadian shield while the small population of archean age grains likely reflects a source from the southwestern tip of the archean superior province that crops out near the study area these results indicate that midcontinent tills were deposited by ice derived from the northwestern keewatin sector of the lis the nearly identical distribution of hornblende and feldspar ages in the till samples identifies the keewatin ice dome and the related ice flow to the midcontinent as long standing features of the lis throughout the late pliocene pleistocene glaciations

mid pleistocene cosmogenic minimum age limits for pre wisconsinan glacial surfaces in southwestern minnesota and southern baffin island a multiple nuclide approach abstract paired 10 be and 26 al analyses n 14 indicate that pre wisconsinan glaciated bedrock surfaces near the northern baffin island and southern minnesota paleo margins of the laurentide ice sheet have long and complex histories of cosmic ray exposure including significant periods of partial or complete shielding from cosmic rays using the ratio 26 al 10 be we calculate that striated outcrops of sioux quartzite in southwestern minnesota southern margin were last overrun by ice at least 500 000 years ago weathered bedrock tors on the once glaciated uplands of baffin island northern margin are eroding no faster than 1 1 m myr 1 the equivalent of at least 450 000 years of surface and near surface exposure our data demonstrate that exposure ages and erosion rates calculated from single nuclides can underestimate surface stability dramatically because any intermittent burial and the resultant lowering of nuclide production rates and nuclide abundances will remain undetected

anomalous geochemical provenance and weathering history of plio pleistocene glaciomarine fjord strata bardin bluffs formation east antarctica major and trace element chemical analyses of the plio pleistocene bardin bluffs formation on the margin of a major ice stream of the east antarctic ice sheet yield an anomalous chemically altered sediment composition the bardin bluffs formation of the pagodroma group is one of the key deposits on the antarctic continent recording glaciomarine sedimentation under open marine fjord conditions as recently as the plio pleistocene in modern fjords occupied by outlet glaciers of ice sheets the composition of fine grained terrigenous sediments approaches that of unweathered rock types exposed upstream in the bardin bluffs formation average abundances of stable elements ti al zr approach average upper crustal compositions and the element ratios are consistent with sediments with a cratonic source implying glacial dispersal from a large shield area through the lambert glacier drainage system interestingly the chemical index of alteration cia of these sediments has values similar to those of average shales formed under conditions of chemical weathering the sediments are particularly depleted in silicate ca which has been observed elsewhere in glacial muds sourced from pre glacial saprolites the anomalous chemistry of the bardin bluffs formation can be explained by a sequence of events involving chemical weathering prior to glacial expansion and erosion the presence of a remnant 1 5 m deep late neogene weathering profile at the base of the bardin bluffs sequence corroborates this conclusion supply of large quantities of chemically weathered materials to antarctic marginal basins requires at least partial deglaciation of the continent and was previously regarded as uncharacteristic for late neogene antarctica

on the structure and origin of major glaciation cycles 2 the 100 000 year cycle climate over the past million years has been dominated by glaciation cycles with periods near 23 000 41 000 and 100 000 years in a linear version of the milankovitch theory the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity but the 100 000 year radiation cycle arising from eccentricity variation is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing we present phase observations showing that the geographic progression of local responses over the 100 000 year cycle is similar to the progression in the other two cycles implying that a similar set of internal climatic mechanisms operates in all three but the phase sequence in the 100 000 year cycle requires a source of climatic inertia having a time constant similar to 15 000 years much larger than the other cycles similar to 5 000 years our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source when these ice sheets forced by precession and obliquity exceed a critical size they cease responding as linear milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses in our model the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity driven modulations in the 23 000 year sea level cycle during an interval when sea level is forced upward from a major low stand by a milankovitch response acting either alone or in combination with an internally driven higher frequency process ice sheets grounded on continental shelves become unstable mass wasting accelerates and the resulting deglaciation sets the phase of one wave in the train of 100 000 year oscillations whether a glacier or ice sheet influences the climate depends very much on the scale the interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger thereby gradually increasing its radius of influence

geochemical constraints on the regolith hypothesis for the middle pleistocene transition the transition from 41 to 100 kyr glacial cycles and concomitant increase in global ice volume 1 ma remain an enigmatic feature of late cenozoic climate here we examine the petrology mineralogy and geochemistry of the silicate fraction of tills spanning the past 2 ma from the north central united states to evaluate the hypothesis that this so called middle pleistocene transition mpt occurred by erosion of regolith and subsequent exposure of underlying canadian shield bedrock by the laurentide ice sheet these data indicate that late pliocene tills are depleted in crystalline lithologies unstable minerals and major element oxides derived from plagioclase and ferromagnesians and are enriched in kaolinite quartz iron oxides tio2bearing resistates and meteoric 10 be in contrast early and middle pleistocene tills show enrichment in crystalline lithologies stable minerals and major oxides derived from plagioclase and ferromagnesians and depletion in meteoric 10 be whereas late pleistocene tills show major element concentrations that are most similar to that of fresh shield bedrock marine isotope records of sr os and hf show significant changes around the mpt that are consistent with the removal of a regolith and the exhumation of fresh silicate bedrock these results indicate that ice sheets initially expanded on highly weathered bedrock and progressively exhumed a fresher rock source thereby supporting the hypothesis that a change in the composition of the substrate underlying ice sheets best explains the origin of the mpt d 2004 elsevier b v all rights reserved

fluid inclusions in sedimentary and diagenetic systems abstract some of the major problems in sedimentary geology can be solved by using fluid inclusions in sedimentary and diagenetic minerals important fluids in the sedimentary realm include atmospheric gases fresh water of meteoric origin lake water seawater mixed water evaporated water formation waters deep in basins oil and natural gas preserving a record of the distribution and composition of these fluids from the past should contribute significantly to studies of paleoclimate and global change research is essential for improving understanding of diagenetic systems and provides useful information in petroleum geology applications of fluid inclusions to sedimentary systems are not without their complexities some fluid inclusions exposed to natural conditions of increasing temperature may be altered by thermal reequilibration which results in stretching or leakage and refilling of some fluid inclusions similarly overheating in the laboratory can also cause reequilibration of fluid inclusions so fluid inclusions from the sedimentary realm must be handled carefully and protected from overheating natural overheating of fluid inclusions must be evaluated through analysis of the most finely discriminated events of fluid inclusion entrapment fluid inclusion assemblages fia consistency in homogenization temperatures within a fluid inclusion assemblage consisting of variably sized and shaped inclusions is the hallmark of a data set that has not been altered through thermal reequilibration in contrast fluid inclusion assemblages yielding variable data may have been altered through thermal reequilibration if a fluid inclusion assemblage has not been altered by thermal reequilibration its fluid inclusions may be useful as geothermometers for low and high temperature systems or useful as geobarometers applicable throughout the sedimentary realm if a fluid inclusion assemblage has been altered partially by thermal reequilibration techniques for distinguishing between altered and unaltered fluid inclusions may be applied in studies of global change fluid inclusions can be used as sensitive indicators of paleotemperature of surface environments fluid inclusions also preserve microsamples of ancient seawater and atmosphere the analysis of which could figure prominently into discussions of past changes in chemistry of the atmosphere and oceans in petroleum geology fluid inclusions have proven to be useful indicators of migration pathways of hydrocarbons they can delineate the evolution of the chemistry of hydrocarbons and they remain important in understanding the thermal history of basins and relating fluid migration events to evolution of reservoir systems in studies of diagenesis fluid inclusions can be the most definitive record most diagenetic systems are closely linked to temperature and salinity of the fluid thus fluid inclusions are sensitive indicators of diagenetic environments

deformation of till beneath ice stream b west antarctica the behaviour and possible instability of the west antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it model calculations show that most ice stream velocity arises at the bed1 2 and radar sounding has shown the bed to be wet3 but the basal boundary condition is not well understood seismic evidence from the upstream b camp upb on the siple coast of west antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick in which the water pressure is only 50 kpa less than the overburden pressure because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5 we presume that it is glacial till we propose here that deformation within the till is the primary mechanism by which the ice stream moves and we discuss implications of this hypothesis

investigation of an early pleistocene marine osmium isotope record from the eastern equatorial pacific osmium isotope composition 187os 188os and concentrations of os ir and pt are reported for an early pleistocene section from the odp site 849 in the eastern equatorial pacific using the data obtained in this study the contributions from detrital and extraterrestrial particulate matter to os concentration and 187os 188os of sediment are estimated our calculations show that detrital contributions to sedimentary os are too small 2 to significantly shift measured bulk sediment 187os 188os away from seawater values a moderate but significant negative correlation between 187os 188os and 3he 188os indicate that the average particulate extraterrestrial os flux to this site is 1 21 0 47 pg cm 2 kyr 1 which constitutes 3 of total os burial flux the estimates of detrital and extraterrestrial os are used to calculate the seawater 187os 188os in the early pleistocene the most notable features of this early pleistocene 187os 188os record are 1 glacial interglacial 187os 188os differences are insignificant within errors of estimates 2 glacial 187os 188os values are higher compared to those reported for the late pleistocene glacials comparison of 187os 188os values at site 849 to the late pleistocene records suggests that average seawater 187os 188os change has been modest 5 since the early pleistocene assuming that 187os 188os difference between the glacial periods of the late and the early pleistocene results solely from temperature dependence of weathering rates it has been calculated that average surface temperature during the late pleistocene glacials was 0 8 0 2 c lower than glacials in the early pleistocene this inference is consistent with temperature estimates based on a recent study of pco2 reconstruction in the pleistocene this observation based on limited studies of marine 187os 188os records seems to suggest that temperature played an important role in influencing chemical weathering during the pleistocene glacials however more studies are needed to confirm if this temperature weathering feedback was operational throughout the pleistocene a significant down core ir 3he co variation coupled with similar burial fluxes of ir at site 849 and at ll44 gpc 3 in the north pacific point to the utility of ir concentration as a point paleoflux tracer however a twofold difference in ir burial fluxes between the eastern and the western equatorial pacific suggests that calibration in space and time is required to use ir concentration as a robust indicator of paleoflux through time significant co variation of concentrations of os and total alkenone during the glacials coupled with lighter 13c of benthic foraminifera indicates that productivity and carbon burial played a dominant control on scavenging of os at site 849 in a broader context this data set encourages future investigation of response of pge behavior to paleoceanographic processes

pronounced warmth during early middle pleistocene interglacials investigating the mid brunhes event in the british terrestrial sequence the mid brunhes event mbe is a climatic transition occurring between mis 13 and 11 that separates two climatic modes 1 early middle pleistocene interglacials 780 450 ka that are characterised by only moderate warmth and 2 middle and late pleistocene interglacials occurring after 450 ka that are characterised by greater warmth consistent with or warmer than the holocene this event is observable within a variety of long climate records such as specmap and epica but its effect on terrestrial systems is poorly understood the impact of this event is examined in the british terrestrial sequence by comparing the occurrence of a range of thermophilous plant and animal species and climate reconstructions derived from assemblages containing them from interglacials of the early middle pleistocene with those from the late middle and late pleistocene this comparison reveals that interglacial climates during the early middle pleistocene in britain were as warm as and in some cases warmer than those that occurred during the late middle and late pleistocene this interpretation is supported by sea surface temperature records in the north atlantic which show that temperatures during early middle pleistocene interglacials were at least as warm as those experienced during the holocene there is therefore no evidence in climate records of britain and the north atlantic for a mid brunhes event this suggests that the mbe is not a global climatic transition but is restricted to specific regions in particular the higher latitudes of the southern hemisphere further work is needed to understand the regional variability and cause of the mbe

modelling northern hemisphere ice volume over the last 3 ma the northern hemisphere ice sheet volumes over the last 3 ma were simulated using the lln 2 d model the forcings were both insolation and co2 concentration different atmospheric co2 scenarios were used because of a lack of co2 reconstruction over this remote past with constant co2 concentrations the simulated ice volume does not show any gradual increase as recorded in the marine sediments moreover its spectrum changes according to the chosen value of co2 when co2 is fixed at 220 ppmv the simulated ice volume is dominated by the similar to 100 ka period if a pre industrial co2 concentration of 280 ppmv is used the simulation is dominated by the similar to 41 ka period by using a linearly decreasing co2 concentration going from 320 ppmv at 3 ma bp to 200 ppmv at present the simulated changes in the power spectrum are in agreement with those obtained from the sedimentary records roughly before 1 ma bp the dominating period is similar to 41 ka afterwards the similar to 100 ka period becomes dominant this transition is accompanied by a gradual increase of the ice volume potential mechanisms for this transition are discussed using cyclic co2 fluctuations over the last 0 6 ma does not change significantly the spectral characteristics of the simulated ice volume but amplifies the amplitude of its variations c 1998 elsevier science ltd all rights reserved

miocene evolution of atmospheric carbon dioxide changes in pco2 or ocean circulation are generally invoked to explain warm early miocene climates and a rapid east antarctic ice sheet eais expansion in the middle miocene this study reconstructs late oligocene to late miocene pco2 from ep values based on carbon isotopic analyses of diunsaturated alkenones and planktonic foraminifera from deep sea drilling project sites 588 and 608 and ocean drilling program site 730 our results indicate that highest pco2 occurred during the latest oligocene 350 ppmv but decreased rapidly at 25 ma the early and middle miocene was characterized by low pco2 260 190 ppmv lower intervals of pco2 correspond to inferred organic carbon burial events and glacial episodes with the lowest concentrations occurring during the middle miocene there is no evidence for either high pco2 during the late early miocene climatic optimum or a sharp pco2 decrease associated with eais growth paradoxically pco2 increased following eais growth and obtained preindustrial levels by 10 ma although we emphasize an oceanographic control on miocene climate low pco2 could have primed the climate system to respond sensitively to changes in heat and vapor transport

evidence of inception horizons in karst conduit networks this paper outlines the conclusion from an analysis of 18 large cave systems around the world comprising more than 1500 km of conduits the 3d geometry of complex cave systems have been analysed in relation to their geological context as well as their hydrogeological boundary conditions the methodology allowed for the first time statistical evidence of the inception horizon concept thereby it confirms that the development of karst conduits under phreatic conditions is strongly related to a restricted number of so called inception horizons an inception horizon is a part of a rock succession that is particularly susceptible to the development of karst conduits because of physical lithological or chemical deviation from the predominant carbonate facies within the limestone sequence it passively or actively favours the localisation of dissolutional voids

reorganization of ice sheet flow patterns in arctic canada and the mid pleistocene transition 1 evidence for the evolution of laurentide ice sheet lis basal thermal regime patterns during successive glaciations is poorly preserved in the geologic record here we explore a new approach to constrain the distribution of cold based ice across central baffin island in the eastern canadian arctic over many glacial interglacial cycles by combining till geochemistry and cosmogenic radionuclide crn data parts of the landscaped with geomorphic evidence for limited glacial erosion are covered by till characterized by high chemical index of alteration cia values and crn concentrations requiring complicated burial exposure histories till from regions scoured by glacial erosion have cia values indistinguishable from local bedrock and crn concentrations that can be explained by simple exposure following deglaciation crn modeling results based on these constraints suggest that the weathered tills were deposited by 1 9 to 1 2 ma and by that time the fiorded baffin island coastline must have developed close to its modern configuration as piracy of ice flow by the most efficient fiord systems resulted in a major shift in the basal thermal regime across the northeastern lis the resultant concentration of ice flow in fewer outlet systems may help explain the cause of the mid pleistocene transition from 41 to 100 kyr glacial cycles

the null hypothesis globally steady rates of erosion weathering fluxes and shelf sediment accumulation during late cenozoic mountain uplift and glaciation at the largest time and space scales the pace of erosion and chemical weathering is determined by tectonic uplift rates deviations from equilibrium arise from the transient response of landscape denudation to climatic and tectonic perturbations we posit that the constraint of mass balance however makes it unlikely that such disequilibrium persists at the global scale over millions of years as has been proposed for late cenozoic erosion we synthesize weathering fluxes global sedimentation rates sediment yields and tectonic motions to show a remarkable constancy in the pace of earth surface evolution over the last 10 ma and support the null hypothesis that global rates of landscape change have remained constant over this time period despite global climate change and mountain building events this work undermines the hypothesis that increased weathering due to mountain building or climate change was the primary agent for a decrease in global temperatures

evaluating drivers of pleistocene eastern tropical pacific sea surface temperature sea surface temperature sst of the eastern equatorial pacific is a key component of tropical oceanic and atmospheric circulation with global teleconnections forcing factors such as local and high latitude insolation changes ice sheet size and albedo feedbacks and greenhouse gas radiation have been proposed as controls of long term eastern tropical pacific sst though the precise role each mechanism plays is not fully known on glacial interglacial or longer timescales here proposed mechanisms are evaluated by comparing orbital scale records of eastern pacific sst with forcing variability over the past 1 5 ma the primary sst records are a compilation of new and existing data from ocean drilling program site 1239 at the northeastern margin of the modern eastern pacific cold tongue and site 846 sst within the cold tongue using time series analysis we test previously proposed mechanisms for control of long term tropical sst change and sst gradients in the eastern pacific we find that within statistical uncertainties in the precession band eastern pacific sst is consistent with direct forcing by equatorial radiation changes in the tropical cold season summer fall rather than inversely correlated as previously suggested in the obliquity band high latitude solar forcing leads or is in phase with eastern equatorial pacific sst while in the eccentricity band atmospheric greenhouse gas concentrations are closely associated with cold tongue sst pleistocene eastern pacific sst gradients indicate that the gradient on the northern margin of the cold tongue strengthened through the mid pleistocene transition a result compatible with the cold tongue becoming more focused at 900 650 ka