high resolution stalagmite climate record from the yucatan peninsula spanning the maya terminal classic period the decline of the classic maya civilization was complex and geographically variable and occurred over a 150 year interval known as the terminal classic period tcp c e 800 950 paleoclimate studies based on lake sediments from the yucatan peninsula lowlands suggested that drought prevailed during the tcp and was likely an important factor in the disintegration of the classic maya civilization the lacustrine evidence for decades of severe drought in the yucatan peninsula however does not readily explain the long 150 year socio political decline of the classic maya civilization here we present a new absolute dated highresolution stalagmite 18o record from the northwest yucatan peninsula that provides a much more detailed picture of climate variability during the last 1500 years direct calibration between stalagmite 18o and rainfall amount offers the first quantitative estimation of rainfall variability during the terminal classic period our results show that eight severe droughts lasting from 3 to 18 years occurred during major depopulation events of classic maya city states during these droughts rainfall was reduced by 52 to 36 the number and short duration of the dry intervals help explain why the tcp collapse of the mayan civilization occurred over 150 years

uranium series dating of great artesian basin travertine deposits implications for palaeohydrogeology and palaeoclimate abstract travertine deposits precipitated by groundwater discharging from the great artesian basin gab are widespread in central australia and have the potential to provide a record of palaeohydrogeology and palaeoclimate the gab is one of the largest artesian basins in the world and a relationship between travertine deposits and recharge sites has potential importance regarding the time and position of past climate events given that the travertines growth forms from precipitation discharge we sampled numerous travertine sites in the southwest section of the gab as a first approach to test this relationship u series dating of the travertine deposits reveal that spring discharge has likely been episodic for the last several hundred thousand years spring travertine deposition occurred episodically around 465 50 ka 370 20 ka 335 15 ka 285 240 ka 185 10 ka 160 150 ka 110 100 ka and during the past 30 ka the periodicity of travertine ages observed with simultaneous deposition at multiple locations argues for regional palaeohydrologic controls comparison of the travertine deposit ages with climate proxies in australia shows that elevated travertine deposition rates are synchronous with wet periods in both central and southern australia due to the large size of the gab and that the recharge zones extend over multiple climatic regions of australia the times of travertine deposition are interpreted to represent times of high rainfall regionally this study shows that the travertine deposits of central australia provide a datable archive of past climate and hydrogeology of importance for understanding the groundwater evolution of the great artesian basin and that further more comprehensive studies are warranted

a speleothem record of holocene paleoclimate from the northern wasatch mountains southeast idaho usa abstract a speleothem stable isotope record from minnetonka cave located in the northern wasatch mountains is shown to be a winter season biased record of holocene paleoclimate the season specific sensitivity of the record provides an opportunity to more robustly evaluate long term variability in large scale pacific teleconnection patterns such as enso and pdo as precipitation anomalies associated with these teleconnection patterns occur primarily in winter seasonal climate composite maps of high and low snowpack years at the study site demonstrate that winter precipitation is strongly linked to the relative strength of the aleutian low and the associated changes from to more meridional or more zonal steering level wind patterns minnetonka cave data suggest wetter cooler than modern early holocene winters and a dry middle holocene with a consistent warming trend from 7400 to 3800 bp after 4000 bp data suggest a return to wetter and or colder than modern winters for much of the late holocene prolonged winter dry intervals are identified from 7400 to 6000 bp and 4200 bp

global nature of the paleoproterozoic lomagundi carbon isotope excursion a review of occurrences in brazil india and uruguay abstract positive carbon isotope excursion is reported from paleoproterozoic carbonates of the aravalli supergroup northwestern india the minas supergroup brazil and new sections of the paso severino formation uruguay the 2 42 ga gandarela formation minas gerais brazil contains red carbonate facies bif grading into dolostones and limestones and yielding 13 c values ranging from 1 6 to 0 4 v pdb the positive c isotope excursion up to 11 v pdb in marine shallow water carbonates in india and brazil jhamarkotra formation in northwestern india and cercadinho and fecho do funil formations in minas gerais state brazil is comparable to that observed in 2 22 2 1 ga carbonate successions worldwide that were deposited during the lomagundi excursion in uruguay 13 c values up to 11 6 v pdb in the deep water paso severino formation of the piedra alta terrane are compatible with deposition at ca 2 15 ga as indicated by the 2146 7 ma u pb age of dacites occurring at the top of the unit negative 13 c values are also present in carbonates of the paso severino formation but an origin related to organic matter remineralization cannot be ruled out thin carbonate beds in the rio itapicuru greenstone belt bahia state brazil are associated as in the paso severino formation with deep water black shales and have carbon isotope values up to 9 v pdb high metamorphic grade carbonates of the jacurici terrane in the medrado ipueira area bahia brazil have carbon isotope values up to 6 9 v pdb consistent with their minimum age of 2085 5 ma inferred from the intrusive contact with and the age of the medrado norite no evidence was found in india brazil or uruguay for paleoproterozoic glacial events recognized in the 2 45 2 22 ga sedimentary successions worldwide unconformities between the gandarela and cercadinho formations in brazil and the banded gneissic complex and the lower aravalli supergroup in india might explain the absence of glacial record compositional and isotopic data presented here for studied paleoproterozoic carbonate successions allow their integration into the global record of the paleoproterozoic evolution as well as correlation with other successions of similar age the study highlights the global nature of the lomagundi excursion furthermore it indicates that the lomagundi excursion is recorded in both shallow water aravalli and minas supergroups and deep water carbonates paso severino formation and rio itapicuru greenstone belt negating a significant impact of stromatolite productivity and hypersaline conditions on carbon isotope values of carbonates deposited in shallow water open marine and isolated basins

isotope stratigraphy of neoproterozoic cap carbonates in the araras group brazil abstract the neoproterozoic carbonate sequence on the southeastern border of the amazon craton is divided into three lithostratigraphic units a basal cap dolomite an intermediate limestone limestone mudstone unit and an upper dolarenite dolorudite unit sections of the cap carbonate were measured from the inner shelf to the outer shelf carbon isotope ratios relative to pdb vary between 10 5 and 1 7 in cap dolomite and between 5 4 and 0 1 in laminated limestone and mud limestone limestones and mud limestones exhibit 87sr 86sr ratios ranging from 0 70740 to 0 70780 a comparative isotope stratigraphy between the inner shelf and the middle shelf basin shows differences in carbon isotope ratios the cap dolomite and limestones have lower 13c ratios on the border of the basin inner shelf than in the middle shelf of the basin these lower values can be related to shallower environmental conditions and to a stronger influence of the continental border the 87sr 86sr ratios are the same in both areas and are consistent with seawater composition at around 600 ma

carbon and oxygen isotope profiles across meso neoproterozoic limestones from central brazil bambui and paranoa groups we present carbon and oxygen isotopic data on carbonates along three profiles of the neoproterozoic bambui group in central brazil this unit covers an area of more than 300 000 km2 and comprises carbonate silicliclastic sediments at the base that grade into siliciclastic sediments towards the top the bambui group overlies by unconformity the paranoa group which consists mostly of siltstone quartzite and minor limestone the data presented here improve the stratigraphic correlation within the bambui basin and show that it evolved in an environment significantly different from that of the paranoa basin our data show large fluctuations of 13cpdb in limestones from the bambui group from 0 8 to 13 5 in all the three studied areas some of these fluctuations represent stratigraphic markers that can be used as a chronostratigraphic tool within a basin scale this observation is relevant considering the lack of fossil record and other stratigraphic markers in neoproterozoic sequences we also present the first isotopic profiles along the paranoa bambui transition which shows that the 13cpdb values grade from 1 0 in the paranoa group to 2 6 in the lower portion of the bambui group increasing up to 12 in the upper part of this unit based on our carbon isotope data as well as other geological mineralogical and nd isotope studies we argue that the sediments of the paranoa group were deposited on an open platform that was fully connected to the ocean on the other hand the sediments of the bambui group were deposited in an epicontinental sea and during a tectonic inversion in a foreland basin at about 790 600 ma this unit displays an increased amount of clastic sediments upwards we argue that the high carbon isotope values observed in limestones and marlstones from the bambui group are correlated to worldwide high carbon isotope values reported for the neoproterozoic however we also point out that novel marine conditions induced by the tectonic inversion of the basin may also have contributed to increase the carbon isotopic composition of the bambui carbonates

climatic backdrop to the terminal pleistocene extinction of north american mammals north american terminal pleistocene mammal extinctions are the subject of a long running scientifi c debate although the role of climate has fi gured centrally we lack clear knowledge of the timing and nature of terminal pleistocene climate variability herein we document lengthy terminal pleistocene drought in the southwestern united states usa using 13 c and 234 u effective moisture proxy data in speleothem calcite stalagmite fs2 from fort stanton cave new mexico supplemented with age data from pool basin shelfstone speleothems from the big room in carlsbad cavern this terminal pleistocene drought defi ned by a sharp rise in both 13 c and 234 u values began just before 14 5 k y ago and lasted at least until 12 9 k y ago when it was briefl y and only mildly interrupted by the younger dryas the timing and length of this drought 1500 yr match the northern hemisphere bolling allerod oscillation preserved in greenland ice cores and exhibited in the 18 o record of stalagmite fs2 rapid transition from cool moist late glacial to warm dry holocenelike climatic conditions was likely unfavorable to many species of pleistocene mammals in the southwestern usa a climate induced extinction implies that this last glacial cycle and its termination were more extreme than previous glacial cycles and or glacial terminations

extreme winds and waves in the aftermath of a neoproterozoic glaciation the most severe excursions in the earth s climatic history are thought to be associated with proterozoic glaciations according to the snowball earth hypothesis the marinoan glaciation which ended about 635 million years ago involved global or nearly global ice cover at the termination of this glacial period rapid melting of continental ice sheets must have caused a large rise in sea level here we show that sediments deposited during this sea level rise contain remarkable structures that we interpret as giant wave ripples these structures occur at homologous stratigraphic levels in australia brazil canada namibia and svalbard our hydrodynamic analysis of these structures suggests maximum wave periods of 21 to 30 seconds significantly longer than those typical for today s oceans the reconstructed wave conditions could only have been generated under sustained high wind velocities exceeding 20 metres per second in fetch unlimited ocean basins we propose that these extraordinary wind and wave conditions were characteristic of the climatic transit and provide observational targets for atmospheric circulation models

neoproterozoic glacial deposits from the aracuai orogen brazil age provenance and correlations with the sao francisco craton and west congo belt article i nfo glacigenic diamictite successions of the macaubas group are widespread in the western domain of the aracuai orogen east of the sao francisco craton brazil diamictites also occur on this craton and in the african counterpart of the aracuai orogen the west congo belt detrital zircon grains from the matrix of diamictites and sandstones from the macaubas group were dated by the u pb shrimp technique the geochronological study sets the maximum depositional age of the glacial diamictites at 900 ma and indicates multiple sources for the macaubas basin with ages ranging from 900 to 2800 ma sm nd tdm model ages determined on whole rock samples range from 1 8 ga to 2 5 ga and get older up section comparison of our data with those from the cratonic area suggest that these glacial deposits can be correlated to the jequitai and carrancas diamictites in the sao francisco craton and to the lower mixtite formation of the west congolian group exposed in africa the 900 1000 ma source is most probably represented by the zadinian mayumbian volcanic rocks and related granites from the west congo belt however one of the most voluminous sources with ages in the 1 1 1 3 ga interval has not been detected in the sao francisco congo craton possible sources for these grains could occur elsewhere in africa or possibly from within the brasilia belt in western central brazil

pb pb dating and pb isotope geochemistry of neoproterozoic carbonate rocks from the sao francisco basin brazil implications for the mobility of pb isotopes during tectonism and metamorphism abstract a u pb study was carried out on carbonate rocks from the neoproterozoic bambui group in the southern part of the sao francisco basin brazil pb isotopic compositions and u and pb concentrations were determined on more than 90 samples from different parts of the basin these samples were found to contain four distinct types of pb here called types i ii iii and iv type i pb was found in samples with low pb concentrations and relatively high u concentrations it represents in situ growth of radiogenic pb and has a large enough variation to define pb pb isochron ages type ii pb is present in samples with relatively high pb concentrations and low u concentrations it is nonradiogenic crustal pb that could either represent average crustal pb at the time of deposition or as interpreted here at the time of deformation of these rocks type iii pb is also found in samples with high pb concentrations and low u concentrations but it apparently is radiogenic crustal pb from the archean or paleoproterozoic basement that was incorporated into the carbonates during the 600 ma brasiliano orogeny type iv pb is intermediate in composition between type iii and type i pb it represents a mixture of these two types pb pb isochron ages obtained from mesoscopically undeformed carbonates containing type i pb range from 686 69 to 520 53 ma the older age is the minimum depositional age for carbonate rocks from the sete lagoas formation during the interval from 690 to 500 ma the pb isotope system of carbonates from the sao francisco basin was disturbed and in some areas it was totally reset type iii pb compositions define a straight line type ii pb falls on the lower end of this line which intercepts the stacey and kramers s k pb growth curve at about 520 and 2100 ma this line is interpreted as representing a stage of crustal pb evolution beginning about 2100 ma and ending about 550 to 500 ma when variably radiogenic pb from the basement was incorporated into these carbonates the ages determined in this study are in agreement with most published ages for brasiliano fold belts marginal to underlying sao francisco craton showing that isotopic systems of sao francisco basin rocks were largely affected by brasiliano tectonism

u pb geochronology of the southern brasilia belt se brazil sedimentary provenance neoproterozoic orogeny and assembly of west gondwana abstract the brasilia belt borders the western margin of the sao francisco craton and records the history of ocean opening and closing related to the formation of west gondwana this study reports new u pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the sao francisco craton and date the orogenic events leading to the amalgamation of west gondwana ages of detrital zircons by id tims and la mc icpms were obtained from metasedimentary units of the passive margin of the sao francisco craton from the main tectonic domains of the belt the internal allochthons araxa group in the araxa and passos nappes the external allochthons canastra group serra da boa esperanca metasedimentary sequence and andrel ndia group and the autochthonous or cratonic domain andrel ndia group the patterns of provenance ages for these units are uniform and are characterised as follows archean paleoproterozoic ages 3 4 3 3 3 1 2 7 and 2 5 2 4 ga paleoproterozoic ages attributed to the transamazonian event 2 3 1 9 ga with a peak at ca 2 15 ga and to the ca 1 75 ga espinhaco rifting of the sao francisco craton ages between 1 6 and 1 2 ga with a peak at 1 3 ga revealing an unexpected variety of mesoproterozoic sources still undetected in the sao francisco craton and ages between 0 9 and 1 0 ga related to the rifting event that led to the individualisation of the sao francisco paleo continent and formation of its passive margins an amphibolite intercalation in the araxa group yields a rutile age of ca 0 9 ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin detrital zircons from the autochthonous and parautochthonous andrel ndia group deposited on the southern margin of the sao francisco craton yielded a provenance pattern similar to that of the allochthonous units this result implies that 1 6 1 2 ga source rocks must be present in the sao francisco craton they could be located either in the cratonic area which is mostly covered by the neoproterozoic epicontinental deposits of the bambui group or in the outer paleo continental margin buried under the allochthonous units of the brasilia belt crustal melting and generation of syntectonic crustal granites and migmatisation at ca 630 ma mark the orogenic event that started with westward subduction of the sao francisco plate and ended with continental collision against the parana block and goias terrane continuing collision led to the exhumation and cooling of the araxa and passos metamorphic nappes as indicated by monazite ages of ca 605 ma and mark the final stages of tectonometamorphic activity in the southern brasilia belt whilst continent continent collision was proceeding on the western margin of the sao francisco craton along the southern brasilia belt eastward subduction in the east was generating the 634 599 ma rio negro magmatic arc which collided with the eastern sao francisco margin at 595 560 ma much later than in the brasilia belt thus the tectonic effects of the ribeira belt reached the southernmost sector of the brasilia belt creating a zone of superposition the thermal front of this event affected the proximal andrel ndia group at ca 588 ma as indicated by monazite age the participation of the amazonian craton in the assembly of western gondwana occurred at 545 500 ma in the paraguay belt and ca 500 ma in the araguaia belt this together with the results presented in this work lead to the conclusion that the collision between the parana block and goias terrane with the sao francisco craton along the brasilia belt preceded the accretion of the amazonian craton by 50 100 million years

a 2400 yr mesoamerican rainfall reconstruction links climate and cultural change droughts are a recurring feature of mexican climate but few high resolution data are available to test for climate change forcing of mesoamerican civilizations we present a quantitative 2400 yr rainfall reconstruction for the basin of mexico from a precisely dated and highly resolved speleothem that documents highly variable rainfall over the past 2400 yr dry conditions peaked during a 150 yr long late classic ca 600 900 ce common era mega drought that culminated at 770 ce which followed centuries of climatic drying that spanned the fall of the city of teotihuacan ca 550 ce the wettest conditions in the 1450s ce were associated with fl ooding in the basin of mexico our data suggest that rainfall variability was likely forced by the el nino southern oscillation and impacts on spring fed irrigation agriculture may have been a stressor on mesoamerican civilizations

late proterozoic aragonitic cement crusts bambui group minas gerais brazil the late proterozoic pedro leopoldo facies bambui group in the vicinity of belo horizonte brazil comprises alternating laminated microsparitic limestones 10 35 mm thick beds and fibrous limestones 10 55 mm thick the latter are composed of a mosaic of sparry calcite crystals these irregularly crosscut rays and fans are composed of feathery precursor crystal bundles with squared off growth zones ghosts of an original fibrous mineral hexagonal in cross section are visible the petrographic characteristics very high strontium content and low magnesium content of the fibrous beds as well as microspar beds strongly argue for an original aragonitic mineralogy the rays are interpreted as having formed by precipitation at the sediment water interface whereas the micrite was precipitated from the water column prior to deposition on the sea floor the lack of emergence features suggests widespread aragonite precipitation under persistently subtidal conditions

marinoan glaciation in east central brazil abstract remnants of a neoproterozoic glaciation in east central brazil are represented by thin diamictite layers jequitai formation and correlative units locally overlying striated pavements on the sao francisco craton the diamictites are covered by the sete lagoas formation of the basal bambui group which is generally accepted to be a typical cap carbonate sequence although most authors have preferred a mid cryogenian post sturtian age for it based mainly on pb pb whole rock data the sete lagoas formation bears lithostratigraphic and isotopic characteristics that are identical to early ediacaran cap carbonates worldwide including a basal thin 0 10 m pale and flinty cap dolostone preserving a drop in 13c values from around 3 2 to 4 5 with associated 18o around 5 and crystal fan facies interpreted as aragonite pseudomorphs ediacaran zircons have been recovered from the middle of the sete lagoas formation constraining the deposition of its upper half to be younger than 610 ma rodrigues 2008 although there is an unconformity below the point where the zircons were collected it is short lived as suggested by the identical typically ediacaran 87sr 86sr values above and below 0 7074 0 7076 carbonate clasts from the jequitai formation and correlative diamictite bearing units in the fold belts that surround the sao francisco craton canabravinha and serra do catuni formations display similar ranges in 13c 6 7 to 2 6 suggesting the erosion of a pre glacial carbonate platform with negative 13c values i e the islay and or trezona anomalies the cratonic carrancas formation on the other hand yielded pale dolostone clasts with 13c in a small range between 4 2 and 3 4 and 18o values around 6 5 these clasts could be derived from the cap dolostone unit itself in which case the carrancas formation would represent resedimented basal sete lagoas formation and imply that sections of the sete lagoas formation sitting atop the carrancas formation are incomplete the base truncated sections have confused previous attempts to correlate the sete lagoas formation with other cap carbonate successions in light of the available lithostratigraphic isotopic and u pb zircon data we propose that the sete lagoas formation represents a basal ediacaran cap carbonate sequence 635 610 ma deposited after the marinoan glaciation in east central brazil

ice thrust features and a possible intertillite pavement in the proterozoic macaubas group jequitai area minas gerais brazil in north central minas gerais large bodies of quartzite up to 25 m long are found in tillite of the proterozoic macaubas group although one body is intact others are disrupted and intermixed with sheared siltstone and tillite two tillites separated by a thin conglomerate are found in the same area striations found on the top of the lower tillite display two directions of ice movement east west and northeast southeast the orientation of the quartzite bodies shear planes in the tillite and the striations on the lower tillite combine to suggest that the ice was moving in an east to northeast direction

chemostratigraphy of carbonates from the minas supergroup quadrilatero ferryifero iron quadrangle brazil a stratigraphic record of early proterozoic atmospheric biogeochemical and climactic change the ca 2 42 ga old gandarela formation overlies the finely laminated caue banded iron formation bif of the itabira group minas supergroup brazil and consists of red carbonate facies bif grading upsection into buff dolomites and limestones which are locally stromatolitic carbonates of the gandarela formation have 1 3 c values ranging narrowly from 1 6 to 0 4 permil versus pdb n 93 the cercadinho formation at the base of the overlying piracicaba group is a thick arenaceous succession with rare thin carbonates that have 1 3 c values ranging from 3 3 to 5 4 permil n 12 stromatolitic dolomites of the overlying fecho do funil formation have pb pb carbonate age 2 11 0 11 ga babinski and others 1995 that provide the minimum age for this unit and 1 3 c values ranging from 5 6 to 7 4 permil n 69 at present we find no evidence in brazil for the any of the three early paleoproterozoic glacial events recognized in 2 45 to 2 22 ga sedimentary successions in north america given the radiometric constraints however it is possible that geological and geochemical traces of these ice ages are missing along the unconformity between the gandarela and cercadinho formations if correct carbonates of the gandarela formation may provide a record of the carbon isotope composition of the seawater prior to any of the paleoproterozoic ice ages combined with other available 1 3 c data results from the gandarela formation suggest that seawater preceding the ice ages was not significantly enriched in 1 3 c on the other hand the carbon isotopic signature of the cercadinho carbonates and chemical composition of the cercadinho arenites are consistent with deposition after the end of the glacial epoch and likely during the early stage of ca 2 22 to 2 1 ga carbon isotope excursion the 1 3 c values of marine carbonates of the fecho do funil formation are consistent with carbon isotope record of contemporaneous carbonate successions worldwide providing additional evidence for a global biogeochemical anomaly at ca 2 22 to 2 1 ga these compositional and isotopic changes allow integration of the minas supergroup into the global record of the paleoproterozoic evolution as well as correlation with other successions of similar age

isotope stratigraphy of the lapa formation sao francisco basin brazil implications for late neoproterozoic glacial events in south america the lapa formation is a thick carbonate sequence 900 m that constitutes the upper part of the vazante group on the s francisco craton brazil it conformably overlies a previously unrecognized glacial diamictite unit of poorly constrained age the sequence above the glacial unconformity consists predominantly of organic rich shale subtidal rhythmic dolomicrites and microbialaminites and intertidal stromatolites four boreholes spanning different depositional settings were sampled at highresolution and investigated for their petrographic and chemical criteria to evaluate their degree of preservation the 13 c and 18 o values of well preserved lapa carbonate microsamples range from 8 2 to 3 3 vpdb and from 13 6 to 0 9 vpdb respectively each of the 13 c profiles of the investigated cores reveals two strong negative excursions of up to 8 an event in post glacial dolomicrites immediately above the glaciogenic unit and a 10 m interval of organic rich shale and a second near the top of the sequence associated with a shale interval based on the observation of dropstones and sedimentary iron formation in the underlying diamictite as well as the distinguishable carbon isotope trends the lapa formation is considered as a cap carbonate lithofacies the age of the lapa formation is presently unknown but the least radiogenic 87 sr 86 sr value 0 7068 associated with a negative carbon isotope excursion matches that from the rasthof formation in namiba on the congo craton which is radiometrically constrained to be younger than ca 750 ma

imprint of long term solar signal in groundwater recharge fluctuation rates from northwest china multiple spectral and statistical analyses of a 700 yearlong temporal record of groundwater recharge from the dry lands badain jaran desert inner mongolia of northwest china reveal a stationary harmonic cycle at 200 20 year interestingly the underlying periodicity in groundwater recharge fluctuations is similar to those of solar induced climate cycle suess wiggles and appears to be coherent with phases of the climate fluctuations and solar cycles matching periodicity of groundwater recharge rates and solar and climate cycles renders a strong impression that solar induced climate signals may act as a critical amplifier for driving the underlying hydrographic cycle through the common coupling of long term sun climate groundwater linkages

latest proterozoic stratigraphy and earth history the end of the proterozoic eon was a time of pronounced biological biogeochemical climatic and tectonic change new bio and chemostratigraphic data provide an improved framework for stratigraphic correlation making possible a deeper understanding of latest proterozoic earth history and providing tools for a chronostratigraphic division of late proterozoic time

late neoproterozoic cap carbonates mackenzie mountains northwestern canada precipitation and global glacial meltdown the 3 27 m thick cap carbonate overlying marinoan ice brook formation glacigene sediments and keele formation carbonate and terrigenous clastic rocks consists of two distinctive stratigraphic units a lower splintery buff weathering microcrystalline dolostone of extensive lateral uniformity comprises mm laminated peloidal sediment with local low angle hummocky like cross stratification micro ridges and synsedimentary tepees all elongated per pendicular to depositional strike this dolostone is unconformably overlain by an upper limestone that exhibits pro nounced facies variation from inboard peloidal lime grainstone and mudstone to shelf edge cementstone to outboard lime wackestone and mudstone calcite cementstones range from isolated crystal fans in laminated limestone to huge decimetre scale crystal arrays to hemispherical and elongate crystal stromatolites wholly composed of acicular crystals that form decametre scale reeflike structures crystal stromatolites are precipitates and replaced microbiolites that con structed biostromes and bioherms like those on many flat topped reef rimmed platforms the calcite crystals have all the physical and chemical attributes of neomorphosed aragonite this aragonite extensively replaced sediment and microbiolite just below the sea floor and grew upward into the overlying water column such interpreted massive synsedimentary replacement is rare in geological history and attests to the highly saturated state of the immediate postglacial ocean all sediment is interpreted to have been caco3 originally low and constant 18 o values reflect diagenetic modification of these carbonates although chemical attributes such as sr and c isotopes in some lithologies are near pristine lower dolostones virtually identical to most other coeval marinoan caps worldwide were part of a global precipitation event of remarkable similarity upper limestones are a more local phenomenon deposited during sea level rise in an aragonitic sea returning to equilibrium after global glaciation low 87 sr 86 sr ratios and varying 13 c values with carbonate sedimentary facies indicate that both units must have formed relatively rapidly prior to significant flu vioglacial runoff or that the influence of this runoff on the chemistry of seawater along continental shelves was mini mal the cap carbonate is thus interpreted to have formed in two steps 1 during initial marine ice melting accompanied by oceanic overturn and upwelling preceding continental margin rebound and 2 during initial stages of sea level rise accompanying continental deglaciation while confirming brief but extensive carbonate precipitation from an ocean highly perturbed by global glaciation the rocks also suggest that this event did not permanently affect either late neoproterozoic ocean chemistry or the contained marine biosphere 1262

variable winter moisture in the southwestern united states linked to rapid glacial climate shifts the last glacial period was characterized by large rapid climate fluctuations an analysis of a speleothem from new mexico shows that the coldest conditions over greenland coincide with increased winter precipitation in the southwestern united states which can be attributed to a southward displacement of the polar jet stream and the north american storm track

deglacial paleoclimate in the southwestern united states an abrupt 18 6 ka cold event and evidence for a north atlantic forcing of termination i we present a new u series dated speleothem record pc 1 from the great basin that documents deglacial climate variability between ca 20 1 and 15 6 ka our data show an abrupt 18 6 ka cold event preceding heinrich event 1 that is consistent with expansion of the laurentide ice sheet during the binge phase of ice growth this event coincided with dessication of pluvial lake mojave suggesting cold and dry conditions in the southern great basin at this time pc 1 d 18 o values before and during heinrich event 1 are similar but an increase in stalagmite growth rates suggests wetter conditions that coincided with deposition of spring deposits in southern nevada the time interval of our record is consistent with the timing of pluvial conditions in the great basin as evident from a comparison to regional wetness proxies our new speleothem record recovered from the recharge area for devils hole does not show a d 18 o increase coincident with the abrupt increase in devils hole d 18 o at c 18 ka challenging the view

north atlantic forcing of millennial scale indo australian monsoon dynamics during the last glacial period recent studies of the last glacial period indo australian summer monsoon iasm have revealed links to both northern and southern hemisphere high latitude climate as well as to regional ocean conditions particular interest has been paid to the monsoon response to heinrich events with variability explained by meridional shifts in positioning of the intertropical convergence zone itcz but this model has not been adequately tested in addition the shorter lived dansgaard oeschger d o events have not been detected beyond d o 1 the bolling allerod in land based records from the indo pacific despite their prominent expression in stalagmites from southern asia raising questions about the sensitivity of the iasm to these events here we present a southern hemisphere stalagmite oxygen isotopic time series from ball gown cave bgc tropical northern australia located on the margins of the modern austral summer itcz that spans 40e31 and 27e8 ka elevated iasm rainfall coincides with heinrich stadials and the younger dryas while decreased rainfall characterizes d o interstadials a response that is antiphased with sites spanning the indo pacific warm pool and with chinese records of the east asian summer monsoon the bgc time series thus reveals a precipitation dipole consistent with a southward northward migration of the itcz during periods of high northern latitude cooling warming as the primary driver of millennial scale iasm variability during the last glacial period our record indicates a strengthening of the iasm after the younger dryas period likely as a result of rising sea level and sea surface temperatures breaking the link with the high latitudes

c sr isotope and hg chemostratigraphy of neoproterozoic cap carbonates of the sergipano belt northeastern brazil abstract two cap carbonates overlying glaciogenic diamictites crop out extensively in the eastern vaza barris domain of the sergipano belt northeastern brazil they are represented by carbonates of the jacoca formation resting on top of diamictite of the ribeiropolis formation and by the olhos d agua formation carbonates organic rich towards the top which overlies diamictite of the palestina formation these two sequences were deformed and metamorphosed at sub greenschist facies conditions during the brasiliano cycle 650 600 ma in the western vaza barris domain dolostone of the acaua formation rest in sharp contact on diamictites of the juete formation and are much less deformed than the olhos d agua formation values of 13c for the jacoca and acaua formations cluster around 4 to 5 in the olhos d agua formation however negative values around 5 in the shallow marine base of the sequence are replaced up section by values close to zero and abruptly by positive values between 8 and 10 at the top strontium isotope ratios for carbonates of these three formations are within the range of late neoproterozoic seawater 0 7060 0 7090 the acaua formation displays 87sr 86sr ratios from 0 7072 to 0 7073 while the jacoca and olhos d agua formations display values within the range from 0 7077 to 0 7081 strontium isotopes coupled with available c isotopes detrital zircon u pb ages and structural data suggest that the jacoca formation and acaua are probably correlative and mid cryogenian in age and that the olhos d agua formation is lowermost ediacaran marinoan in age mercury concentrations in carbonates of the jacoca olhos d agua and acaua formations are usually much higher than those observed in carbonates deposited not concomitantly with volcanic activities

late proterozoic glaciation in central eastern brazil synthesis and model abstract in the surroundings of the middle proterozoic sao francisco craton of central eastern brazil numerous traces of an ancient glaciation can be found almost all criterua necessary to prove an ancient glaciation occur including scratched and facetted pebbles a striated surface quarrying outwash plains eskers varvites dropstones decollement structures etc poorly sorted tillites with varying pebble composition are widely distributed continental tillites and glaciomarine sequences interfinger in the east and southeast of the craton traces of ice movement suggest flow directions of the ice towards the east and the southeast of the craton at the coast the wet base glaciers passed over to floating shelf ice in the northeast glaciers also came from the serrinha craton in northeastern bahia the glaciation covered an area of at least 1300 700 km the sequences containing traces of glaciation discordantly overlie archean to middle proterozoic rocks they in turn are overlain by late proterozoic calcareous and psammopelitic sediments of the bambui group present data indicate that glaciation took place around 1000 ma ago

simultaneous remagnetization and u pb isotope resetting in neoproterozoic carbonates of the sao francisco craton brazil abstract the southern part of the neoproterozoic sao francisco basin in minas gerais state brazil can be divided into three structural domains a the central part of the basin where the rocks are undeformed b the western domain where the rocks have been deformed by the 600 550 ma brasilia fold belt and c the eastern domain where the rocks have been affected by the 600 550 ma aracuai fold belt u pb and pb isotopic data rock magnetism data and paleomagnetism data from the carbonates from different domains support a close connection between a pervasive remagnetization and a large scale fluid percolation event that strongly affected the isotopic system of these rocks at 530 500 ma during the last stage of the brasiliano pan african orogeny a pb pb isochron age of 686 69 ma has been determined from undeformed carbonates in the center of the basin and it is interpreted as the minimum depositional age however most of the pb pb and the u pb ages obtained from deformed as well as other undeformed carbonates fall in an interval of 550 500 ma carbonates containing radiogenic crustal pb with an isotopic signature of the archean paleoproterozoic basement were found in the central portion of the basin which was not affected by deformation suggesting that this pb was incorporated into the carbonates through fluids which promoted the resetting of the isotope system and severe remagnetization in the carbonates the post depositional character of the characteristic magnetizations is strengthened by the following a the disclosed rock magnetic properties such as wasp waisted hysteresis loops anomalously high hysteresis ratios and contradictory lowrie fuller and cisowski tests are typical of remagnetized carbonates b thermomagnetic analysis and scanning electron microscopy suggest authigenic magnetite as the main magnetic carrier c moderate to high paleolatitudes inferred from paleomagnetic data for the study area would require a different climate pattern during the sedimentation of the wide carbonate platforms d magnetization directions with a single polarity were found along the whole sedimentary sequence e mean magnetization components identified in the carbonates show lower dispersion than would be expected if the secular variation of the geomagnetic field was fully averaged out f paleomagnetic poles from carbonate sequences and adjacent brasiliano metamorphic rocks are similar and coincide with high quality gondwanan paleomagnetic poles for the 530 500 ma interval the similarity between paleomagnetic and isotopic results from the bambui and the salitre carbonates ca 1000 km to the northeast implies a large scale fluid percolation event that simultaneously affected the whole basin the paths of these fluids may have been along old basement faults reactivated during the last stage of the brasiliano pan african orogeny

neoproterozoic glacial dynamics revealed by provenance of diamictites of the bebedouro formation sao francisco craton central eastern brazil one of the main questions on neoproterozoic geology regards the extent and dynamics of the glacial systems that are recorded in all continents we present evidence for short transport distances and localized sediment sources for the bebedouro formation which records neoproterozoic glaciomarine sedimentation in the central eastern sao francisco craton sfc brazil new data are presented on clast composition based on point counting in thin section and shrimp dating of pebbles and detrital zircon cluster analysis of clast compositional data revealed a pronounced spatial variability of clast composition on diamictite indicating the presence of individual glaciers or ice streams feeding the basin detrital zircon ages reveal distinct populations of archean and palaeoproterozoic age the youngest detrital zircon dated at 874 9 ma constrains the maximum depositional age of these diamictites we interpret the provenance of the glacial diamictites to be restricted to sources inside the sfc suggesting deposition in an environment similar to ice streams from modern high latitude glaciers

dissolution methods for strontium isotope stratigraphy whole rock analysis the preparation of samples for strontium isotope stratigraphy sis by whole rock dissolution results in contributions of sr from contaminant phases as well as from the target carbonate phase here we highlight and quantify the errors in 87sr 86sr values that might result when calcium carbonate is dissolved using an inappropriate method we report the variation in 87sr 86sr value of sr released from four samples of differing calcite content and degrees of cementation when subject to different methods of dissolution we show that errors of up to 0 000076 in 87sr 86sr which correspond to errors of 3 68 ma in age can be avoided by careful preleaching of the samples

steady incision of grand canyon at the million year timeframe a case for mantle driven differential uplift abstract the grand canyon region provides an excellent laboratory to examine the interplay between river incision magmatism and the geomorphic and tectonic processes that shape landscapes here we apply u series ar ar and cosmogenic burial dating of river terraces to examine spatial variations in incision rates along the 445 km length of the colorado river through grand canyon we also analyze strath terrace sequences that extend to heights of several hundred meters above the river and integrate these with speleothem constrained maximum incision rates in several reaches to examine any temporal incision variations at the million year time frame this new high resolution geochronology shows temporally steady long term incision in any given reach of grand canyon but significant variations along its length from 160 m ma in the east to 101 m ma in the west spatial and temporal patterns of incision and the long timescale of steady incision rule out models where geomorphic controls such as climate oscillations bedrock strength sediment load effects or isostatic response to differential denudation are the first order drivers of canyon incision the incision pattern is best explained by a model of neogene and ongoing epeirogenic uplift due to an eastward propagating zone of increased upper mantle buoyancy that we infer from propagation of neogene basaltic volcanism and a strong lateral gradient in modern upper mantle seismic structure

granites and the geodynamic history of the neoproterozoic brasilia belt central brazil a review abstract recent field and geochronological studies have demonstrated the importance of granitic magmatism in the evolution of the neoproterozoic brasilia belt in central brazil this is an orogenic belt developed in response to the convergence between the amazon sao francisco congo and parana continental blocks the presence of neoproterozoic juvenile arc rocks and syn collisional peraluminous granites challenged previous intracontinental evolution models for the belt the granitoid intrusions reviewed in this paper record the different stages of evolution of the orogen and their field and isotopic characteristics can be used to reconstruct the tectonic history of the belt the main field and isotopic characteristics of four granite suites associated with the brasilia belt are reviewed i 1 77 1 58 ga old rift related a type granite intrusions ii ca 0 8 0 7 syn collisional granitoids iii arc metatonalites and metagranodiorites ca 0 9 to 0 63 ga and iv bimodal post orogenic suite ranging in age from ca 0 59 to 0 48 ga these rocks suggest that during most of the neoproterozoic the western margin of the sao francisco continent faced a large oceanic basin where subduction and oceanic lithosphere consumption started at ca 0 9 ga roughly coeval with the initial stages of the break up of rodinia final ocean closure happened at ca 0 63 0 60 ga with crustal thickening uplift and erosion post orogenic extension related magmatism took place between ca 0 6 and 0 5 ga and was partially contemporaneous with the deposition of the paraguay and tucavaca sedimentary successions resulting from the rifting event related to the break up of laurentia from southwestern gondwana