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New geochemical and [.sup.40]Ar/[.sup.39]Ar hornblende and biotite data from the Grenvillian Trenton Prong inlier provide the first constraints for the identification of lithotectonic units, their tectonic setting, and their metamorphic to post-metamorphic history. Gneissic tonalite, diorite, and gabbro compose the Colonial Lake Suite magmatic arc that developed along eastern Laurentia prior to 1.2 Ga. Spatially associated low-and high-Ti[O.sub.2] amphibolites were formed from island-arc basalt proximal to the arc front and mid-ocean ridge basalt-like basalt in a back-arc setting, respectively. Supracrustal paragneisses include meta-arkose derived from a continental sediment source of Laurentian affinity and metagraywacke and metapelite from an arc-like sediment source deposited in a back-arc basin, inboard of the Colonial Lake arc. The Assunpink Creek Granite was emplaced post-tectonically as small bodies of peraluminous syenogranite produced through partial melting of a subduction-modified felsic crustal source. Prograde mineral assemblages reached granulite-to amphibolite-facies metamorphic conditions during the Ottawan phase of the Grenvillian Orogeny. Hornblende [.sup.40]Ar/[.sup.39]Ar ages of 935-923 Ma and a biotite age of 868 Ma record slow cooling in the northern part of the inlier following the metamorphic peak. Elsewhere in the inlier, biotite [.sup.40]Ar/[.sup.39]Ar ages of 440 Ma and 377-341 Ma record partial to complete thermal resetting or new growth during the Taconian and Acadian orogens. The results of this study are consistent with the Trenton Prong being the down-dropped continuation of the Grenvillian New Jersey Highlands on the hanging wall of a major detachment fault. The Trenton Prong therefore correlates to other central and northern Appalachian Grenvillian inliers and to parts of the Grenville Province proper. Key words: Mesoproterozoic, Appalachian Grenvillian inliers, Ottawan Orogeny, Trenton Prong, [.sup.40]Ar/[.sup.39]Ar thermochronology. De nouvelles donnees geochimiques et de [.sup.40]Ar/[.sup.39]Ar dans la hornblende et la biotite de l'enclave de Trenton (<< Trenton Prong>>) d'age grenvillien fournissent les premieres contraintes pour l'identification d'unites lithotectoniques, de leur milieu tectonique et de leur histoire metamorphique et post-metamorphique. Des tonalites, diorites et gabbros gneissiques forment l'arc magmatique de la serie de Colonial Lake qui s'est developpe le long de la Laurentie orientale avant 1,2 Ga. Des amphibolites pauvres et riches en Ti[O.sub.2] qui leur sont associees dans l'espace se sont formees a partir de basalte d'arc magmatique proximal par rapport au front de l'arc et de basalte s'apparentant a du basalte des dorsales medio-oceaniques dans un milieu d'arriere-arc, respectivement. Des paragneiss supracrustaux comprennent des meta-arkoses issues d'une source de sediments continentale d'affinite laurentienne et des metagrauwackes et metapelites issus d'une source de sediments semblable a un arc deposes dans un bassin d'arriere-arc du cote continental de l'arc de Colonial Lake. Le granite d'Assunpink Creek, dont la mise en place est post-tectonique, se presente sous la forme de petits corps de syenogranite hyperalumineuse produite par la fusion partielle d'une source crustale felsique modifiee par la subduction. Les assemblages de mineraux progrades ont atteint des conditions metamorphiques du facies des granulites a celui des amphibolites durant la phase ottavienne de l'orogenese Grenvillienne. Des ages [.sup.40]Ar/[.sup.39]Ar sur hornblende de 935-923 Ma et un age sur biotite de 868 Ma temoignent du refroidissement lent de la partie nord de l'enclave apres le pic du metamorphisme. Ailleurs dans l'enclave, des ages [.sup.40]Ar/[.sup.39]Ar sur biotite de 440 Ma et 377-341 Ma temoignent d'une reinitialisation thermique complete ou d'une nouvelle croissance durant les orogeneses taconique et acadienne. Les resultats de la presente etude concordent avec l'interpretation voulant que l'enclave de Trenton soit le prolongement affaisse des hautes terres du New Jersey, d'age grenvillien, dans le toit d'un important decollement. L'enclave de Trenton est donc correlee a d'autres enclaves du centre et du nord de l'orogenese grenvillienne et a des portions de la province du Grenville proprement dit. [Traduit par la Redaction] Mots-cles: Mesoproterozoique, enclaves grenvilliennes dans les Appalaches, orogenese ottavienne, enclave de Trenton, thermochronologie [.sup.40]Ar/[.sup.39]Ar., Introduction Mesoproterozoic rocks are preserved in a series of inliers that form parallel western and eastern belts in the central Appalachians (Fig. 1). Inliers along the western belt are remnants [...]

Revision of palynochronologic and radiocarbon age estimates for the termination of glacial Lake Iroquois, mainly based on a currently accepted younger determination of the key Picea--Pinus pollen transition, shows agreement with recently established constraints for this late glacial event in the Lake Ontario basin at 13 000 cal years BP. The date of emergence or isolation of small lake basins reflects the termination of inundation by glacial lake waters. The increasing upward presence of plant detritus and the onset of organic sedimentation marks the isolation level in the sediments of a small lake basin. The upward relative decline and cessation of pollen from trees such as Pinus, Quercus, and other thermophilous hardwoods that were wind transported long distances from southern areas also mark the isolation of inundated small lake basins by the declining water level of Lake Iroquois as local vegetation grew and local pollen overwhelmed long-distance-transported pollen. Reexamination of data in small lake basins north of Lake Ontario using the above criteria shows that the age range for the termination of Lake Iroquois derived from these data overlaps other age constraints. These constraints are based on a varve-estimated duration of post-Iroquois phases before incursion of the Champlain Sea, a newly discovered late ice advance into northern New York State, and the age of a mastodon at Cohoes, New York. The new age (13 000 cal years BP) for Lake Iroquois termination is significantly younger than the previous estimate of 11 800 14C (13 600 cal) years BP. Key words: glacial Lake Iroquois terminal age, Lake Ontario basin, small lake basin emergence, pollen stratigraphy, Late Wisconsinan. La revision d'estimations palynochronologiques et au carbone radioactif d'ages pour la fin du lac glaciaire Iroquois, basee principalement sur l'etablissement accepte actuellement d'un age plus jeune pour la transition cle Picea--Pinus dans les pollens, revele une concordance avec des contraintes temporelles recemment etablies pour cet episode glaciaire tardif dans le bassin du lac Ontario a 13 000 a etal. BP. Le moment de l'emergence ou de l'isolement de bassins de petits lacs reflete la fin de l'inondation par les eaux du lac glaciaire. La presence croissante vers le haut de detritus de plantes et le debut de la sedimentation de matiere organique marquent le niveau de l'isolement dans les sediments d'un bassin de petit lac. La diminution relative vers le haut et la disparition de pollen d'arbres tels que Pinus, Quercus et d'autres feuillus thermophiles transporte par le vent sur de longues distances en provenance de regions au sud marquent aussi l'isolement, par la baisse du niveau d'eau du lac Iroquois, de bassins de petits lacs inondes, alors que la vegetation locale se developpait et que le pollen local supplantait le pollen transporte sur de longues distances. Le reexamen de donnees sur des bassins de petits lacs au nord du lac Ontario a la lumiere des criteres precedents montre que la fourchette d'ages pour la fin du lac Iroquois obtenue de ces donnees chevauche d'autres contraintes temporelles. Ces contraintes reposent sur une estimation obtenue de varves de la duree des phases post-Iroquois avant l'incursion de la mer de Champlain, la decouverte recente d'une avancee tardive de la glace dans le nord de l'Etat de New York et l'age d'un mastodonte a Cohoes, dans ce meme Etat. Le nouvel age (13 000 a etal. BP) pour la fin du lac Iroquois est significativement plus jeune que l'estimation anterieure de 11 800 a 14C (13 600 a etal.) BP. [Traduit par la Redaction] Mots-cles : age terminal du lac glaciaire Iroquois, bassin du lac Ontario, emergence de bassins de petits lacs, stratigraphie du pollen, Wisconsinien tardif., Introduction Terasmae (1980) estimated that regional glacial Lake Iroquois in the Lake Ontario basin existed from 12 500 to 11 800 14C years (14 700-13 600 cal years) BP. This [...]

A large amount of bedded chert widely deposited during the Ordovician and Silurian transition in South China. In this study, analyses of the petrographic characteristics, the major elements and rare earth elements (REEs), were conducted on 31 bedded chert samples obtained from the Wufeng and Longmaxi Formations at the Qiliao section in the Shizhu area of the Sichuan Basin to determine the sedimentary environment and the origin of the bedded chert during the Ordovician and Silurian transition. The following conclusions were obtained: (i) the bedded chert in the Wufeng and Longmaxi Formations at the Qiliao Section originated mainly from terrigenous input and siliceous organisms and was slightly influenced by hydrothermal fluid, (ii) siliceous organisms were a key factor controlling the differences in the Si[O.sub.2] content of the bedded chert in the Wufeng and Longmaxi Formations at the Qiliao Section, and (iii) the bedded chert in the Wufeng and Longmaxi Formations deposited in a continental margin environment during the Ordovician and Silurian transition. Key words: bedded chert, geochemistry, Wufeng Formation, Longmaxi Formation, shale gas, Sichuan Basin. Resume : Une importante quantite de chert lite a ete deposee a grande echelle durant le passage Ordovicien-Silurien en Chine meridionale. Des analyses des caracteristiques petrographiques, des elements majeurs et des terres rares ont ete realisees sur 31 echantillons de chert lite des Formations de Wufeng et Longmaxi de la coupe de Qiliao de la region de Shizhu du bassin du Sichuan, afin de determiner le milieu sedimentaire et l'origine des cherts lites durant le passage Ordovicien-Silurien. Les conclusions suivantes en sont tirees : (i) les cherts lites dans les Formations de Wufeng et Longmaxi de la coupe de Qiliao proviennent principalement d'apports terrigenes et d'organismes siliceux et ont subi une legere influence de fluides hy-drothermaux, (ii) les organismes siliceux sont des facteurs cles expliquant les differences des teneurs en Si[O.sub.2] entre les Formations de Wufeng et de Longmaxi dans la coupe de Qiliao et (iii) les cherts lites des Formations de Wufeng et Longmaxi ont ete deposes dans un milieu de marge continentale durant le passage Ordovicien-Silurien. [Traduit par la Redaction] Mots-cles : chert lite, geochimie, Formation de Wufeng, Formation de Longmaxi, gaz de schiste, bassin du Sichuan., 1. Introduction Chert is predominantly composed of Si[O.sub.2] and is widely present throughout the geologic record. Numerous studies on chert have been carried out to reconstruct the evolution of paleoenvironment, [...]

Understanding ice sheet behaviour in the geological past is essential for evaluating the role of the cryosphere in the climate system and for projecting rates and magnitudes of sea level rise in future warming scenarios.sup.1-4. Although both geological data.sup.5-7 and ice sheet models.sup.3,8 indicate that marine-based sectors of the East Antarctic Ice Sheet were unstable during Pliocene warm intervals, the ice sheet dynamics during late Pleistocene interglacial intervals are highly uncertain.sup.3,9,10. Here we provide evidence from marine sedimentological and geochemical records for ice margin retreat or thinning in the vicinity of the Wilkes Subglacial Basin of East Antarctica during warm late Pleistocene interglacial intervals. The most extreme changes in sediment provenance, recording changes in the locus of glacial erosion, occurred during marine isotope stages 5, 9, and 11, when Antarctic air temperatures.sup.11 were at least two degrees Celsius warmer than pre-industrial temperatures for 2,500 years or more. Hence, our study indicates a close link between extended Antarctic warmth and ice loss from the Wilkes Subglacial Basin, providing ice-proximal data to support a contribution to sea level from a reduced East Antarctic Ice Sheet during warm interglacial intervals. While the behaviour of other regions of the East Antarctic Ice Sheet remains to be assessed, it appears that modest future warming may be sufficient to cause ice loss from the Wilkes Subglacial Basin.Studies of an Antarctic marine sediment core suggest that the East Antarctic Ice Sheet retreated in the vicinity of the Wilkes Subglacial Basin during extended warm periods of the late Pleistocene, when temperatures were similar to those predicted to occur within this century., Author(s): David J. Wilson [sup.1] [sup.2] , Rachel A. Bertram [sup.1] [sup.2] , Emma F. Needham [sup.1] , Tina van de Flierdt [sup.1] [sup.2] , Kevin J. Welsh [sup.3] , [...]

The global biosphere is commonly assumed to have been less productive before the rise of complex eukaryotic ecosystems than it is today.sup.1. However, direct evidence for this assertion is lacking. Here we present triple oxygen isotope measurements ([increment].sup.17O) from sedimentary sulfates from the Sibley basin (Ontario, Canada) dated to about 1.4 billion years ago, which provide evidence for a less productive biosphere in the middle of the Proterozoic eon. We report what are, to our knowledge, the most-negative [increment].sup.17O values (down to -0.880/00) observed in sulfates, except for those from the terminal Cryogenian period.sup.2. This observation demonstrates that the mid-Proterozoic atmosphere was distinct from what persisted over approximately the past 0.5 billion years, directly reflecting a unique interplay among the atmospheric partial pressures of CO.sub.2 and O.sub.2 and the photosynthetic O.sub.2 flux at this time.sup.3. Oxygenic gross primary productivity is stoichiometrically related to the photosynthetic O.sub.2 flux to the atmosphere. Under current estimates of mid-Proterozoic atmospheric partial pressure of CO.sub.2 (2-30 times that of pre-anthropogenic levels), our modelling indicates that gross primary productivity was between about 6% and 41% of pre-anthropogenic levels if atmospheric O.sub.2 was between 0.1-1% or 1-10% of pre-anthropogenic levels, respectively. When compared to estimates of Archaean.sup.4-6 and Phanerozoic primary production.sup.7, these model solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O.sub.2 over geologic time.sup.8.Triple oxygen isotope measurements of 1.4-billion-year-old sedimentary sulfates reveal a unique mid-Proterozoic atmosphere and demonstrate that gross primary productivity in the mid-Proterozoic was between 6% and 41% of pre-anthropogenic levels., Author(s): Peter W. Crockford [sup.1] [sup.2] [sup.3] , Justin A. Hayles [sup.4] [sup.5] , Huiming Bao [sup.5] [sup.6] , Noah J. Planavsky [sup.7] , Andrey Bekker [sup.8] , Philip W. [...]