paleomagnetism of the samail ophiolite oman 1 the sheeted dike complex at ibra we have studied the rock magnetism and paleomagnetism of 200 oriented samples from the middle cretaceous sheeted dike sequence of the samail ophiolite oman the samples are from 28 sites spanning a cross strike distance of 30 km most of the dikes have undergone green schist metamorphism the natural remanent magnetization nrm of the samples is about 1 a m and qn is also near 1 thermal and alternating field demagnetization treatment resolves both normal north and reversed south polarities about one third of the samples have low median destructive fields mdf less than 100 oe 7 96 103 a turn m these low mdf samples have scattered directions and are classified as undetermined polarities u hysteresis measurements show pseudo single domain carriers predominate in all samples although the low mdf samples are suggested to have the largest grains in the samples studied thermomagnetic curves show both magnetite and minor maghemite thermal demagnetization and microscopic study reveals that hematite is abundant and is the carrier of the r remanence in addition several dikes exhibit both n and r polarities these facts coupled with the degree of metamorphism indicate that part of the remanence is secondary and could be due to hydrothermal metamorphism below the seafloor we suggest that the n polarity is primary and resides in exsolved magnetite while the r polarity is secondary and resides primarily in metamorphic origin hematite several short r events observed in the albian through cenomanian magnetostratigraphic record are candidates for the r event in the ophiolite if the r polarity originated near the spreading center alternately nonreproducible self reversal is another possible origin for the r polarity regardless of whether the reversal originated at the spreading center secondary magnetization in the dikes considerably increases the directional dispersion another major factor affecting dispersion is viscous remanent magnetization vrm in the larger magnetite grains this extreme dispersion disallows the samail ophiolite dike layer as a significant source for marine magnetic anomalies regardless of the strength of the nrm or the value of q the stable n and r directions yield a paleomagnetic pole which is far sided suggesting northward movement of the ophiolite however the paleomagnetic pole is not as far sided as the contemporaneous africa pole which suggests a component of north south closure between the ophiolite and africa prior to northward movement

chronology of late wisconsinan glaciation in middle north america abstract we propose a chronology of late wisconsinan glacial fluctuations in middle north america from alberta to wisconsin based on radiocarbon dates derived solely from wood previous chronologies of the southwestern margin of the north american continental ice sheet have depended to a considerable degree on radiocarbon dates from fine grained organic sediment this material is commonly contaminated with older carbon resulting in chronologic confusion by using only dates from wood much of the confusion disappears however because of the scarcity of wood dates only four of the sixteen identified fluctuations are accurately dated an advance into iowa about 14 000 to 13 500 bp an advance into south dakota and iowa about 12 300 bp an advance into the lake michigan basin about 11 700 bp and an advance into the lake superior basin about 9900 bp in addition the beginning of late wisconsinan glaciation before 20 000 bp is fairly well documented none of the fluctuations in the western part of the region are accurately dated

prokaryotic and eukaryotic microfossils from a proterozoic phanerozoic transition in china cherts of the doushantuo formation in the yangtze gorges south china contain a superbly preserved heterogeneous assemblage of bacteria cyanobacteria planktonic algae submillimetre sized burrows and problematica the microbiota has a bearing on defining the precambrian cambrian boundary baltisphaeridium and associated spinose microfossils in the chert suggest an early cambrian age whereas other acritarchs vendotaenids and pretrilobite small shelly fossils from the conformably overlying the dengying formation indicate a late proterozoic age for the doushantuo this mixed benthic planktonic assemblage preserved in silicified laminated microbial mat like fabrics provides a unique sample of diverse microbiotic elements existing at the dawn of the phanerozoic and records the earliest evidence of meiofauna

glacial lake agassiz the northwestern outlet and paleoflood valley morphology and sediment in the fort mcmurray region of alberta indicate that a catastrophic flood discharged down the lower clearwater and athabasca river valleys 9900 yr b p geomorphic and chronologic evidence suggests that glacial lake agassiz emerson phase was the probable water source as the flood incised a drainage divide located near the alberta saskatchewan border the level of glacial lake agassiz decreased by 46 m discharged 2 4 x 106 m3 s for at least 78 days and stabilized at 438 m above sea level in the lake wasekamio area at that time water entered the arctic ocean via glacial lake mcconnell and the mackenzie river rather than the gulf of mexico via the mississippi river as previously thought such a large influx of fresh water 8 6 km3 h into the arctic at the close of the last glaciation may have had an abrupt major influence on northern climate

microfossils and precambrian cambrian boundary stratigraphy at maldeota lesser himalaya an assemblage of problematical microfossils of precambrian cambrian boundary age is redescribed from the chert phosphorite member at the base of the lower tal formation of maldeota in the lesser himalaya of india this assemblage has previously been ascribed to various ages from precambrian to cretaceous but is held by us to contain maldeotaia bandalica protohertzina anabarica group trumpet shaped elements acicular elements a b conotheca sp ovalitheca cf multicostata allathecid sp a barbitositheca ansata hexangulaconularia cf formosa coleoloides aff typicalis hyolithellus aff insolitus h cf isiticus h vladimirovae spirellus shankari and olivooides multisulcatus these compare closely with assemblages found above the base of the first anabarites trisulcatus protohertzina anabarica zone in china and in the second pseudorthotheca costata zone of southern kazakhstan the stratigraphic setting of the krol tal succession is reviewed and several similarities are noted between the precambrian cambrian boundary successions of lesser himalaya in india and of yunnan and sichuan in southwest china indicating that correlation between them is possible at several levels

articulated halkieriids from the lower cambrian of north greenland metazoan skeletons appear abruptly in the fossil record near the base of the cambrian 540myrbp before present 1 2 in the initial stages of diversification familiar skeletal remains such as sponge spicules brachiopod valves and echinoderm ossicles are outnumbered by a remarkable array of skeletal types many of millimetric size3 9 particularly conspicuous in these early assemblages are various sclerites some of which are inferred to have formed cataphract coverings of slug like metazoans recognition of articula tory facets and fused sclerites contribute to partial scieritome reconstructions8 11 but discovery of articulated material is a vital ingredient in understanding the palaeobiology of these extinct groups here we report complete halkieriid scleritomes from the lower cambrian of north greenland this finding broadly confirms the predictions of an earlier hypothetical reconstruction11 but reveals entirely unexpected anterior and posterior shells it supports also the proposed phylogenetic link between the halkieriids and wiwoxia corrugata a metazoan with a cataphract scieritome best known from the middle cambrian burgess shale

history of laurentide meltwater flow to the gulf of mexico during the last deglaciation as revealed by reworked calcareous nannofossils the history of meltwater flow from the laurentide ice sheet to the gulf of mexico during the last deglaciation which holds possible implications for the cause of the younger dryas cold episode is not well understood we propose a new chronology based on using the percentage of reworked calcareous nannofossils in orca basin sediments as a proxy for erosion the period of greatest meltwater flow to the gulf was between 12 7 and 12 1 ka during the bolling warm interval and flow remained high until the beginning of the younger dryas cold episode at 11 3 ka this corresponds to meltwater pulse ia a sharp meltwater decrease at 12 2 ka may represent the older dryas glacial readvance little or no meltwater is inferred to have flowed to the gulf from 10 to 9 ka which is the time of the second major meltwater pulse ib therefore meltwater must have been permanently diverted away from the gulf of mexico at the beginning of the younger dryas

a candidate stratotype for the precambrian cambrian boundary fortune head burin peninsula southeastern newfoundland the burin peninsula exhibits an exceptionally thick and essentially continuous succession of marine strata through the precambrian cambrian transition fossils are abundant and include trace fossils small shelly fossils vendotaenid algae soft bodied megafossils and microfossils the burin peninsula is readily accessible and has long been considered a potential area for a precambrian cambrian boundary stratotype a continuous section through the upper part of member 1 and all of member 2 of the chapel island formation is exposed at fortune head and this section is herein proposed as a global stratotype for the precambrian cambrian boundary the boundary horizon is located 2 4 m above the base of member 2 of the chapel island formation this horizon marks the base of the basal cambrian phycodes pedum ichnofossil zone and immediately overlies the top of the late precambrian harlaniella podolica ichnofossil zone shelly fossils sabelliditids first appear a few metres below the proposed boundary sof

geological and biological consequences of giant impacts it seems inescapable that large impacts have been an ongoing phenomenon throughout geological time in the phanerozoic the estimated frequency of impact by bodies of 5 km diameter or more is on the order of 1 0 every 1 00 myr the 5 km lower limit is chosen arbitrarily but it is shown in this review that such a body is certainly large enough to cause very large terrestrial effects a small decrease in this size limit would lead to a large increase in the frequency of occurrence it may be claimed that bodies of a size likely to have drastic and sudden effects on niany physical and biological features of the earth have arrived with a frequency comparable to the average duration of a biostratigraphic stage nothing is said of the interval between arrivals which may be random i e stochastic currently there is not enough geological evidence to allow recognition of other than random spacing between a few positively identified events during the phanerozoic since impacts by extraterrestrial bodies have been clearly established it is reasonable to postulate that they must leave a record in the stratigraphic column why then do we currently have so few horizons that have been claimed to contain evidence of a global impact event since the effects we are considering are unquestionably global in this paper we bring together evidence from several disciplines that

north atlantic thermohaline circulation during the past 20 000 years linked to high latitude surface temperature during a surface cooling event 10 000 to 12 000 years ago higher cd ca and lower 13c 12c ratios are observed in benthic foraminifera shells from rapidly accumulating western north atlantic sediments data from sediment cores show that marked nutrient depletion of intermediate waters occurs in association with reduced glacial north atlantic deep water flux it is proposed that cold high latitude sea surface temperatures enhance intermediate water formation at the expense of deep water formation

sudden changes in north atlantic circulation during the last deglaciation sudden changes in the flow of warm atlantic surface waters into the norwegian sea occurred frequently during the last deglaciation typically involving shifts in sea surface temperature of 5 c in fewer than 40 years these led to equally large and rapid changes in atmospheric temperatures and to shifts in atlantic deep thermo haline circulation and ice sheet melting rates

relationships between anoxia glacial meltwater and microfossil preservation in the orca basin gulf of mexico abstract the isotopic and microfossil records from piston core en32 pc6 from the orca basin gulf of mexico reveal that various factors have moderated the sediment composition of the basin during the last 29 000 yrs 29 ka b p coincident with the introduction of isotopically depleted glacial meltwater into the gulf approximately 16 ka b p anoxic conditions commenced within the intraslope basin as evidenced by sedimentary laminations and improved microfossil pteropod planktonic foraminifera and radiolaria preservation the increased delivery of terrigenous organic matter with the glacial meltwater may have been responsible for inducing the anoxia also at this time the water of the orca basin was most likely somewhat stratified due to elevated salinities and the increased supply of organic matter onto the continental slope from the continent would have depleted the oxygen supply of the orca basin by oxidative consumption about 13 ka b p oxygenated conditions resumed and microfossil abundance preservation decreased when meltwater input diminished at 8 5 ka b p the orca basin again became anoxic due to the development of the orca basin brine

two decades of research on the proterozoic phanerozoic transition 1972 1991 the most radical global change involving the biology chemistry and physics of life occurred near and at the stratigraphical level of the boundary between the neoproterozoic system precambrian and the cambrian system this stratigraphical level is held by consensus to be the proterozoic eonothem phanerozoic eonothem boundary and is regarded as being of fundamental importance research has involved not only archaeocyathans shelly small problematica earliest trilobites stromatolites and algae trace fossils soft bodied metazoans microphytolites and acritarchs but also physical and chemical aspects radio isotope dating carbon strontium and oxygen isotope stratigraphy polar wandering relating to palaeogeography and magnetostratigraphy a selected bibliography 1985 1990 is appended to enable further reading of the extensive research contributions

the younger dryas cool episode in the gulf of mexico data are presented from orca basin piston core en32 pc4 in the gulf of mexico that confirm the existence of surface water cooling during the younger dry as chronozone 11 10 ka late glacial planktonic foraminiferal species made a reappearance between 11 4 and 9 8 ka an episode also marked by distinctly higher oxygen isotopic values derived from the planktonic foraminifer globigerinoides ruber the presence of the younger dryas event in the gulf at 27 n demonstrates that surface water cooling extended to mid latitude regions in the north atlantic the cool surface water interval is bracketed by rapid shifts in 18o related to changes in the influx of meltwater to the gulf of mexico a chronology based upon seven accelerator radiocarbon dates indicates that cooling commenced over a 500 year period and ended in less than 200 years these results are among the first deep sea sediment data documenting the climatic transitions bracketing the younger dryas with a rapidity observed in ice core records a rapid decrease in 18o values measured in the white form of gs ruber at 10 2 ka is explained by significant meltwater influx into the gulf and rapid increase in sea surface temperatures surprisingly a similar decrease is not observed in the pink form of gs ruber a summer surface water dweller in the gulf this discrepancy may be explained by continued meltwater influx throughout the younger dryas during the summers only such that there was no change in the 18o of the pink form at the end of the episode an additional possibility is that warming at the end of the younger dryas raised year average temperatures and summer temperatures remained constant the coincidence of rapid shifts in 18o with the younger dryas strongly suggests a dynamic causal relationship and therefore supports a model for the cause of the younger dryas cooling based on changes in the routing of laurentide glacial meltwater

the chronology of the last deglaciation implications to the cause of the younger dryas event it has long been recognized that the transition from the last glacial to the present interglacial was punctuated by a brief and intense return to cold conditions this extraordinary event referred to by european palynologists as the younger dryas was centered in the northern atlantic basin evidence is accumulating that it may have been initiated and terminated by changes in the mode of operation of the northern atlantic ocean further it appears that these mode changes may have been triggered by diversions of glacial meltwater between the mississippi river and the st lawrence river drainage systems we report here accelerator mass spectrometry ams radiocarbon results on two strategically located deep sea cores one provides a chronology for surface water temperatures in the northern atlantic and the other for the meltwater discharge from the mississippi river our objective in obtaining these results was to strengthen our ability to correlate the air temperature history for the northern atlantic basin with the meltwater history for the laurentian ice sheet

how old is the tommotian shelly faunas and phytoplankton from lower cambrian tommotian strata within and outside the type area in siberia indicate that the tommotian stage is younger than previously believed deposits referred to the tommotian stage in siberia are in part time equivalent to the talsy lukati horizon in the east european platform which has yielded trilobites and is correlated with the schmidtiellus mickwitzi trilobite zone in baltoscandia published micropaleontological data seem to indicate that the yudomian stage preceding the tommotian may also belong to the cambrian system the demonstrated contemporaneity of tommotian small shelly faunas with the trilobites could result in a revision of models dealing with the early radiation of metazoan phyla speciation and skeletonization

palaeoenvironmental trends in the history of trace fossils tracks trails and burrows made by organisms are commonly preserved in phanerozoic sedimentary rocks these biogenic sedimentary structures or trace fossils have been most intensively studied from rocks deposited in marine environments and have proven to be extremely useful in palaeoenvironmental reconstruc tions our systematic analysis of occurrences in the stratigraphic record of two of the most common trace fossils zoophycos and ophiomorpha indicates that occurrences for each show an environmentally linked directional pattern through time zoophycos is commonly reported from palaeozoic strata deposited from nearshore to deep sea environments whereas in post palaeozoic stratait is generally reported only from rocks deposited in deeper water environments ophiomorpha first appeared in permian shallowwater environments and was common in deep sea fan environments by the cretaceous and has been distributed from nearshore to deep sea environments since then these onshore offshore trends which are similar to those previously reported for phanerozoic benthic macroinvertebrate assemblages and individual higher taxa require integration into future palaeoenvironmental reconstructions that use trace fossils

routing of meltwater from the laurentide ice sheet during the younger dryas cold episode rooth1 proposed that the younger dryas cold episode which chilled the north atlantic region from 11 000 to 10 000 yr bp was initiated by a diversion of meltwater from the mississippi drainage to the st lawrence drainage system the link between these events is postulated to be a turnoff during the younger dryas cold episode of the north atlantic s conveyor belt circulation system which currently supplies an enormous amount of heat to the atmosphere over the north atlantic region2 this turnoff is attributed to a reduction in surface water salinity and hence also in density of the waters in the region where north atlantic deep water nadw now forms here we present oxygen isotope and accelerator radiocarbon measurements on planktonic foraminifera from orca basin core en32 pc4 which reveal a significant reduction in meltwater flow through the mississippi river to the gulf of mexico from about 11 200 to 10 000 radiocarbon years ago this finding is consistent with the record for lake agassiz which indicates that the meltwater from the southwestern margin of the laurentide ice sheet was diverted to the northern atlantic ocean through the st lawrence valley during the interval from 11 000 to 10 000 years before present yr bp