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At present, the Basin and Range of the western USA is arid, but geologic studies show evidence of past wetness. The timing of these wetter conditions reveals a close association with glacial conditions. This association has led to the hypothesis of a causal link between glacial climate and regional wetness, but poor age control on the onset of regional wetness thwarts a test of this hypothesis. Here we determine the start of the most recent interval of persistent wetness in the Mono Basin, which is a hydrologically closed depression that sits at the west-central edge of the Basin and Range. The most recent emergence of persistent wetness in the Mono Basin is stratigraphically correlated with the depositional age of Ash 19--a rhyolitic ash bed that represents the oldest tephra of the Wilson Creek Formation and one of the earliest-known products of explosive volcanic activity from the Mono Craters. We constrain the depositional age of Ash 19 by using the U/Th disequilibrium dating method to date carbonates that are younger and older than Ash 19. Our U/Th dating results show that Ash 19 was deposited before the formation of a cross-cutting carbonate bed dated to 69.2 ± 0.3 ka but after an underlying carbonate tufa dated to 67.4 ± 3.5 ka, which suggests that the start of wetness in the Mono Basin was contemporary with the inception of the Last Glaciation--the beginning of Marine Isotope Stage 4--at ca. 70 ka. This finding corroborates the hypothesis of a link between glacial climate and regional wetness. [ABSTRACT FROM AUTHOR]

The Qaidam Basin marks a crucial boundary between the Westerlies and the Asian summer monsoons. Previous studies in the Qaidam Basin have advanced our knowledge of the paleoclimate over glacial to interglacial cycles. However, our understanding of the paleoclimatic sensitivity of the Qaidam Basin to the relative strength of these two climatic driving forces remains limited due to the lack of regional paleoclimatic reconstructions. The Qaidam Basin is proposed as a regional and global eolian dust source during the glacial periods, during which a cold, dry climate is associated with the equatorward shift of the jet stream. On the contrary, paleoshoreline records suggest that a highstand lake stage prevailed in late Marine Isotope Stage 3 (MIS 3) and lasted until 15 ka. To address this conundrum, we have applied an integrated approach to reconstructing the regional paleoclimatic history by combining compound-specific isotope analysis, lake temperature reconstruction, and numerical modeling. Our results show varying paleoclimate associated with the dynamic climate boundary since 45 ka: (1) a wet climate during late MIS 3, when the Asian summer monsoons are strengthened under high summer insolation and penetrate further into Central Asia; (2) a general cold, dry but wetter than at present climate in the Last Glacial Maximum (LGM), when the Asian summer monsoons retreat and the Westerlies become dominant; and (3) three short periods of extreme aridity corresponding to the Younger Dryas and Heinrich 2 and 4 events, when the normal moisture transport via the Westerlies and Asian summer monsoons is interrupted. The numerical modeling supports an increase in the effective precipitation during the LGM due to reduced evaporation under low summer insolation. These results suggest that the Westerlies and Asian summer monsoons alternately controlled the climate in the Qaidam Basin in response to precessional forcing during the late Pleistocene. [ABSTRACT FROM AUTHOR]

The Rio Grande rift hosts a remarkable record of Quaternary river incision preserved in an alluvial terrace sequence that has been studied for more than a century. However, our understanding of Rio Grande incision history in central New Mexico since the end of basin filling ca. 0.78 Ma remains hampered by poor age control. Robust correlations among Rio Grande terrace sequences in central and southern New Mexico are lacking, making it difficult to address important process-related questions about terrace formation in continental-scale river systems. We present new age controls using a combination of 40Ar/39Ar, 36Cl surface-exposure, and 14C dating techniques from alluvial deposits in the central New Mexico Socorro area to document the late Quaternary incision history of the Rio Grande. These new age controls (1) provide constraints to establish a firm foundation for Socorro basin terrace stratigraphy, (2) allow terrace correlations within the rift basin, and (3) enable testing of alternative models of terrace formation. We identified and mapped a high geomorphic surface interpreted to represent the end of basin filling in the Socorro area and five distinct, post-Santa Fe Group (ca. 0.78 Ma) alloformations and associated geomorphic surfaces using photogrammetric methods, soil characterization, and stratigraphic descriptions. Terrace deposits exhibit tread heights up to 70 m above the valley floor and are 5 to >30 m thick. Their fills generally have pebble-to-cobble bases overlain by fine-to-pebbly sand and local thin silt and clay tops. Alluvial-fan terraces and associated geomorphic surfaces grade to former valley levels defined by axial terrace treads. Carbon-14 ages from detrital charcoal above and below a buried tributary terrace tread show that the most recent aggradation event persisted until ca. 3 ka during the transition from glacial to modern climate conditions. Drill-log data show widespread valley fill ~30 m thick that began aggrading after glacial retreat in northern New Mexico and southern Colorado (ca. 14 ka). Aggradation during this transition was likely due to hillslope destabilization, increased sediment yield, decreased runoff, and reduced stream competence. Chlorine-36 ages imply similar controls on earlier terraces that have surface ages of ca. 27-29, 64-70, and 135 ka, and suggest net incision during glacial expansions when increased runoff favored down-cutting and bedload mobilization. Our terrace chronology supports existing climate-response models of arid environments and links tributary responses to the axial Rio Grande system throughout the central Rio Grande rift. The terrace chronology also reflects a transition from modest (60 m/m.y.) to rapid (300 m/m.y.) incision between 610 and 135 ka, similar to patterns observed throughout the Rio Grande rift and the western United States in general. [ABSTRACT FROM AUTHOR]

The snowball Earth hypothesis proposes that if polar ice sheets were to advance equatorward of a mid-latitude threshold, runaway ice-albedo effects would lead to a stable, globally ice-covered climate state that would require extremely high atmospheric pCO2 levels (supplied by volcanic degassing over millions of years) for deglaciation. Geologic evidence, including globally distributed and low-latitude glacial deposits, suggests that two such global glaciations occurred during the Neoproterozoic. We model the coupled carbon and silica cycles through a snowball Earth event, including the extremely high pCO2 and dramatically accelerated chemical weathering of its aftermath. The enhanced delivery of dissolved weathering products to the ocean induces elevated sedimentary burial of CaCO3 (deposited as "cap carbonates") and SiO2. Uncertainty in the relative importance of carbonate versus silicate weathering allows a wide range of possible CaCO3 burial magnitude, potentially dwarfing that of SiO2. However, total SiO2 burial is insensitive to weathering strengths, and is set by the amount of CO2 required for deglaciation (~1019 mol). Chert associated with Marinoan post-glacial cap carbonates in Africa and Mongolia corroborate modeled predictions of elevated SiO2 burial. [ABSTRACT FROM AUTHOR]

Records of post-glacial sea-level change in the Cascadia subduction zone (North American Pacific coast) analyzed using one-dimensional Earth models suggest upper mantle viscosities <~1020 Pa·s, significantly weaker than those based on observations from cratonic regions in North America. We explore this variability by comparing predictions based on a three-dimensional (3-D) model of the glacial isostatic adjustment process with relative sea-level histories from sites along the Oregon and Washington (USA) and Hudson Bay (northeastern Canada) coasts. We demonstrate that a 3-D mantle viscosity field, with a geometry constrained by seismic tomography and that reflects the tectonic setting of the Cascadia subduction zone, can simultaneously reconcile the rapid rate of late Holocene adjustment observed in central Cascadia and the longer decay times of post-glacial uplift curves in Hudson Bay. Specifically, below the Cascadia subduction zone, this field is characterized by a shallow subduction wedge of low viscosity (<1020 Pa·s) and higher viscosity material associated with subduction of the Juan de Fuca plate. [ABSTRACT FROM AUTHOR]

Quantifying the pace of ice-sheet growth is critical to understanding ice-age climate and dynamics. Here, we show that the diversion of the Hudson River (northeastern North America) late in the last glaciation phase (ca. 30 ka), which some previous studies have speculated was due to glacial isostatic adjustment (GIA), can be used to infer the timing of the Laurentide Ice Sheet's growth to its maximum extent. Landscapes in the vicinity of glaciated regions have likely responded to crustal deformation produced by ice-sheet growth and decay through river drainage reorganization, given that rates of uplift and subsidence are on the order of tens of meters per thousand years. We perform global, gravitationally self-consistent simulations of GIA and input the predicted crustal deformation field into a landscape evolution model. Our calculations indicate that the eastward diversion of the Hudson River at 30 ka is consistent with exceptionally rapid growth of the Laurentide Ice Sheet late in the glaciation phase, beginning at 50-35 ka. [ABSTRACT FROM AUTHOR]

Public understanding of climate change is imperative as communities face the consequences of a warming planet. While the vast majority of the scientific community agrees upon the causes and projections of climate change, public and political debates indicate far less public acceptance. This investigation examined whether an earlier proposal of climate change in the history of science could facilitate climate literacy in modern climate discussions. Louis Agassiz's 1837 glacial theory proposal was received with initial skepticism, heavily debated, and eventually accepted by the majority of the scientific community 30-40 yr later. In this research, students (n = 24) in an online Earth history course participated in a six unit climate change discussion, with one unit focused upon Agassiz's proposal of glacial theory. Although pre- and post-Climate Change Survey comparisons (n = 21) documented no significant change in climate change content, the students' incoming knowledge was significantly greater when compared with their colleagues in previous years. Content analysis of students' online discussions documented that students addressed most understandings of the nature of science matrix, while content analysis of students' anonymous survey responses (n = 22) revealed three stable themes, including the benefits of the history of science for providing context for developing theories, facilitating broader understanding of the nature of science, and providing historical episodes that directly relate to modern debates. While students affirmed the importance of the historical climate change episode within their climate discussions, more research is needed to elucidate whether the history can also result in improved climate literacy. [ABSTRACT FROM AUTHOR]

Ice-marginal glaciated landscapes demarcate former boundaries of the continental ice sheets. Throughout circumpolar regions, permafrost has preserved relict ground ice and glacigenic sediments, delaying the sequence of postglacial landscape change that transformed temperate environments millennia earlier. Here we show that within 7 × 106 km2 of glaciated permafrost terrain, extensive landscapes remain poised for major climate-driven change. Across northwestern Canada, 60-100-km-wide concentric swaths of thaw slump-affected terrain delineate the maximum and recessional positions of the Laurentide Ice Sheet. These landscapes comprise ~17% of continuous permafrost terrain in a 1.27 × 106 km2 study area, indicating widespread preservation of late Pleistocene ground ice. These thaw slump, relict ground ice, and glacigenic terrain associations are also evident at the circumpolar scale. Recent intensification of thaw slumping across northwestern Canada has mobilized primary glacial sediments, triggering a cascade of fluvial, lacustrine, and coastal effects. These geologically significant processes, highlighted by the spatial distribution of thaw slumps and patterns of fluvial sediment mobilization, signal the climate-driven renewal of deglaciation and postglacial permafrost landscape evolution. [ABSTRACT FROM AUTHOR]

The eruption of the Campanian Ignimbrite at ca. 40 ka coincided with the final decline of Neanderthals in Europe. Environmental stress associated with the eruption of the Campanian Ignimbrite has been invoked as a potential driver for this extinction as well as broader upheaval in Paleolithic societies. To test the climatic importance of the Campanian eruption, we used a three-dimensional sectional aerosol model to simulate the global aerosol cloud after release of 50 Tg and 200 Tg SO2. We coupled aerosol properties to a comprehensive earth system model under last glacial conditions. We find that peak cooling and acid deposition lasted one to two years and that the most intense cooling sidestepped hominin population centers in Western Europe. We conclude that the environmental effects of the Campanian Ignimbrite eruption alone were insufficient to explain the ultimate demise of Neanderthals in Europe. Nonetheless, significant volcanic cooling during the years immediately following the eruption could have impacted the viability of already precarious populations and influenced many aspects of daily life for Neanderthals and anatomically modern humans. [ABSTRACT FROM AUTHOR]

Time-equivalent Early Permian sedimentary successions from high-latitude Gondwana basins and from equatorial accumulations in North China, covering the glacial to postglacial transition, display correlatable trends in continental weathering intensity based on chemical index of alteration (CIA) values. The successions display a pattern of CIA values that varies with latitude, similar to modern estuarine suspended sediments. Based on the modern-day CIA-temperature correlation, the glacial Early Permian low- to high-latitude land surface temperature gradient is estimated as ~20 °C, slightly higher than the calculated values for the contemporaneous sea-surface temperature gradient. [ABSTRACT FROM AUTHOR]

Past glacials can be thought of as natural experiments in which variations in boundary conditions infl uenced the character of climate change. However, beyond the last glacial, an integrated view of orbital- and millennial-scale changes and their relation to the record of glaciation has been lacking. Here, we present a detailed record of variations in the land-ocean system from the Portuguese margin during the penultimate glacial and place it within the framework of ice-volume changes, with particular reference to European ice-sheet dynamics. The interaction of orbital- and millennial-scale variability divides the glacial into an early part with warmer and wetter overall conditions and prominent climate oscillations, a transitional mid-part, and a late part with more subdued changes as the system entered a maximum glacial state. The most extreme event occurred in the mid-part and was associated with melting of the extensive European ice sheet and maximum discharge from the Fleuve Manche river. This led to disruption of the meridional overturning circulation, but not a major activation of the bipolar seesaw. In addition to stadial duration, magnitude of freshwater forcing, and background climate, the evidence also points to the infl uence of the location of freshwater discharges on the extent of interhemispheric heat transport [ABSTRACT FROM AUTHOR]

Neoproterozoic strata in Death Valley, California, contain eukaryotic microfossils and glacial deposits that have been used to assess the severity of putative snowball Earth events and the biological response to extreme environmental change. These successions also contain evidence for synsedimentary faulting that has been related to the rifting of Rodinia, and in turn the tectonic context of the onset of snowball Earth. These interpretations hinge on local geological relationships and both regional and global stratigraphic correlations. Here, we present new geological mapping, measured stratigraphic sections, carbon and strontium isotope chemostratigraphy, and micropaleontology from the Neoproterozoic glacial deposits and bounding strata in Death Valley. These new data enable us to refine regional correlations, both across Death Valley and throughout Laurentia, and construct a new age model for glacigenic strata and microfossil assemblages. Particularly, our remapping of the Kingston Peak Formation in the Saddle Peak Hills and near the type locality shows for the first time that glacial deposits of both the Marinoan and Sturtian glaciations can be distinguished in southeastern Death Valley, and that beds containing vase-shaped microfossils are slump blocks derived from the underlying strata. These slump blocks are associated with multiple overlapping unconformities that developed during synsedimentary faulting, which is a common feature of Cyrogenian strata along the margin of Laurentia from California to Alaska. With these data, we conclude that all of the microfossils that have been described to date in Neoproterozoic strata of Death Valley predate the glaciations and do not bear on the severity, extent, or duration of Neoproterozoic snowball Earth events. [ABSTRACT FROM AUTHOR]

This study focuses on the ~10 m.y. before the latest Ordovician (Hirnantian) glaciation; we test whether orbital-scale climatic fluctuations controlled the growth and melting of continental glaciers, resulting in glacioeustatic sea-level changes and the development of widespread marine sedimentary cycles. δ18O values of conodont apatite from 14 Late Ordovician (Katian) cycles range from ~17‰ to 21‰. Isotopic values decrease and are lowest in the deepest water facies and increase and are highest in shallow-water facies, supporting the hypothesis that glacioeustasy was the dominant control on water-depth changes. Measured intracycle δ18O changes of 0.7‰-2.5‰ were controlled by changes in ice volume (<60 m sea-level changes), sea-surface temperatures (<5 °C), and potentially local increases in seawater evaporation during drier and/or windier glacial stages. These interpreted orbital-scale climate changes and resultant large glacial ice-volume changes support recent interpretations of a dynamic and prolonged Ordovician greenhouse to icehouse transition. [ABSTRACT FROM AUTHOR]

Little is known about the long-term Quaternary climate evolution of central and south-east Europe, because suitable terrestrial paleoenvironmental records are scarce. We present a multiproxy record of loess paleosol sequences in the Middle and Lower Danube Basins over the past 700 k.y. In these lowlands, the continentality of interglacial climate progressively increased during the Middle Pleistocene. Corresponding trends are seen in other climate proxies in the same region and in the lowlands of the northern Black Sea, but not, or less clearly, in climate archives from outside these lowlands. We conclude that a small-scale increase in paleoelevation of central European mountain ranges (the Alps, Carpathians, and Dinarides) during the Middle Pleistocene might be the cause of the progressive increase in climate continentality of southeast European lowlands. [ABSTRACT FROM AUTHOR]

Past glacial-interglacial climate transitions were accompanied by millennial-scale pulses in atmospheric CO2 that are widely thought to have resulted from the release of CO2 via the Southern Ocean. However, direct proxy evidence for a Southern Ocean role in regulating past ocean-atmosphere CO2 exchange is scarce. Here we use combined radiocarbon and neodymium isotope measurements from the last deglaciation to confirm greatly enhanced overturning and/or air-sea exchange rates relative to today, in particular during the Bølling-Allerød warm interval. We show that this deglacial pulse in ocean ventilation was not driven by the North Atlantic overturning alone, and must have involved an increase in the ventilation of southern-sourced deep waters. Our results thus confirm the removal of a physical and/or dynamical barrier to effective air-sea (CO2) exchange in the Southern Ocean during deglaciation, and highlight the Antarctic region as a key locus for global climate/carbon-cycle feedbacks. [ABSTRACT FROM AUTHOR]

A suite of mechanisms has been proposed to account for natural variations in atmospheric CO2 during the Holocene; all of which have achieved limited success in reproducing the timing, direction, and magnitude of change. Recent modeling studies propose that changes in the latitudinal position and strength of the Southern Hemisphere Westerly Winds (SWW) can greatly influence large-scale ocean circulation and degassing of the deep ocean via changes in wind-driven upwelling in the Southern Ocean. The extent to which the hypothesized SWW-Southern Ocean coupled system could account for changes in atmospheric CO2 is uncertain, because of a lack of observations on the behavior of the SWW in the past, the paucity of appropriate records of productivity changes in the Southern Ocean, and our limited understanding of the sensitivity of the Southern Ocean biological and/or physical system to SWW forcing. Here we report a reconstruction of the behavior of the SWW during the past 14 k.y. based on terrestrial ecosystem proxies from western Patagonia, South America. The reconstructed variations in the intensity of zonal flow correspond to the timing and structure of atmospheric CO2 changes, and are consistent with the modeled magnitude of CO2 changes induced by varying strengths of the SWW. The close match between data and models supports the view that the SWW-Southern Ocean coupled system underpins multimillennial CO2 variations during the current interglacial and, possibly, during glacial cycles over the past 800 k.y. [ABSTRACT FROM AUTHOR]

Rapid warming and consequent ice-shelf collapse have focused attention on the glacial record of the Antarctic Peninsula. Here, we present the fi rst record of terrestrial organic material exposed by recently retreating ice that bears on past glacier extent and climate in this sensitive region. Radiocarbon dates show that ice on Anvers Island was at or behind its present position at 700-970 cal. yr B.P., coincident with ice reduction elsewhere in the Southern Hemisphere. Moreover, the data indicate that present reduced ice extent on the western Antarctic Peninsula is not unprecedented and is similar to that experienced during at least three periods in the last 5600 yr. [ABSTRACT FROM AUTHOR]

We suggest that the Salzachtal-Ennstal-Mariazell-Puchberg (SEMP) fault, a major strikeslip system in the European Alps, is active. It has accommodated lateral extrusion of the central part of the Eastern Alps toward the Pannonian Basin. The main tectonic activity of this fault dates back to Oligocene and Miocene time, but until now it was largely unknown whether the fault was still active. We present here the fi rst fi eld evidence of neotectonic activity from a cave in the Hochschwab karst massif (Styria, Austria) that intersects a segment of the SEMP fault zone. Damaged speleothems in this cave include scratched flowstone (a hitherto undescribed feature), massive flowstone disrupted by a fault, and ruptured fl owstone. The superposition of younger fl owstone layers allows constraining the time frame of the tectonic events using U/Th dating. The youngest fl owstone of the pre-damage generation is ca. 118 ka (end of the Last Interglacial) and the oldest post-event layer is ca. 9 ka (early Holocene). The tectonic event bracketed by these layers coincided with a growth hiatus during the last glacial period, consistent with the high alpine setting of the cave. Geologic evidence precludes deformation mechanisms other than tectonic. These new data are consistent with vectors of continuous global positioning system measurements as well as instrumental seismicity data, and collectively suggest that the SEMP is an active fault and that lateral extrusion of the Eastern Alps is ongoing. [ABSTRACT FROM AUTHOR]

The retreat history of the West Antarctic Ice Sheet (WAIS) since the Last Glacial Maximum is important for understanding the process of rapid deglaciation, constraining models that seek to predict the future trajectory of the ice sheet, and for estimating rates of sea-level change. Here we report new glacial geologic data from the southwestern Weddell Sea embayment that demonstrate that this part of the WAIS was thinner than previously suggested, and that there was progressive thinning of the ice sheet by 230-480 m since ca. 15 ka. We use geomorphological data and a numerical ice sheet model to reconstruct the ice sheet in the Weddell Sea at the Last Glacial Maximum. The volume of this ice would have added between 1.4 and 2.0 m to postglacial sea-level rise and would not have been suffi cient to contribute signifi cantly to meltwater pulse 1A, a rapid rise in sea level ∼14,200 yr ago. [ABSTRACT FROM AUTHOR]

As climate is changing rapidly, there is an increasing need to understand how water and soil resources respond to climate change. Soil and sediment dynamics are sensitive to several external factors such as climate, vegetation type and distribution, human activity, and tectonic activity. However, the relationship between erosion and changes in these factors is diffi cult to constrain with current available approaches. Here we show that uranium isotopes in sediments from river paleochannels can be used to reconstruct variations in the residence time of sediments in a catchment over the past 100 k.y. We fi nd that sediment residence times increase by an order of magnitude during interglacials compared to glacial periods. This is interpreted as a change in sediment stores in the landscape that are tapped by catchment erosion: young, upland soils during glacial periods, reworking of old alluvial sediments during interglacials. A direct correlation is found between the sediment residence time and climatic parameters (sea-surface temperature, atmospheric carbon dioxide content, and paleorainfall estimates), suggesting that during a glacial cycle, sediment dynamics closely follow variations in climate. However, this relationship is not simple because there is no correlation between sediment residence time and paleodischarge estimates. Because sediment residence time variations correlate with changes in vegetation inferred from pollen data, it is hypothesized that the infl uence of climate on erosion over a glacial cycle may be indirect, and operates via the infl uence of climate on the type of plant ecosystems within a catchment. If verifi ed elsewhere, this conclusion would emphasize the important role of biology in the physical evolution of Earth's surface, here observed over a 100 k.y. time scale. [ABSTRACT FROM AUTHOR]

We have analyzed the chemical compositions of 87 samples from the Cariaco Basin and 20 samples from the Orinoco Delta, Venezuela, in order to better characterize the fl uvial sources of material to the Cariaco Basin. We observe distinctive regional patterns in composition; shelf sediments found near the Tuy River, Manzanares River, the Araya-Margarita region, and in the northern portion of the Unare platform record reproducible compositional differences. However, linear mixing models using only the composition from these rivers do not satisfy the variability observed in modern sediment trap samples collected from the basin. Average trap values are best approximated by mixing upper crust (30%-50%) with local river sources (50%-70% of either the Tuy, Unare, or Neverí Rivers) and eolian dust (<2%). The chemical data suggest a glacial-interglacial switching of sources, with the Tuy-Tacarigua, Unare, and Neverí being more important during glacials, and the larger Tuy and Manzanares being more important during interglacials. The K/Al in shelf sediments (0.14-0.27) spans the range of previously documented glacial to interglacial values (0.16-0.26), and the high Ti/Al found during glacials (∼0.052) can perhaps be supplied by some of the local rivers (Ti/Al ∼0.049-0.106) and not exclusively by Saharan dust. The data and statistical models do not offer support for the presence of any signifi cant Orinoco source [ABSTRACT FROM AUTHOR]

Resolving the timing, direction, and magnitude of paleoclimate changes in the southern midlatitudes is a prerequisite for determining the mechanisms underlying abrupt and widespread climate changes between the hemispheres during the Last Glacial-Interglacial transition (LGIT). This issue is still debated, with previous studies producing apparently discordant findings. Here we show evidence for a glacial readvance and a cold episode between ca. 14.8 and 12.6 ka in southwestern Patagonia (50°S), contemporaneous with the Antarctic cold reversal. This was followed by ice recession under cold but relatively milder conditions until ca. 11.5 ka, when paleovegetation records indicate the onset of warm interglacial conditions. These findings differ from those reported in northern Patagonia (∼40°S), where deteriorating conditions before 13.5 ka were followed by the coldest part of the LGIT that lasted until ca. 11.5 ka. We interpret the apparent blend of Greenlandic and Antarctic cold phases as evidence for their co-occurrence in the southern middle latitudes in Patagonia, and hypothesize that the position of the Antarctic Polar Front modulated the strength of these cold events in regions to the north or south of it. [ABSTRACT FROM AUTHOR]

The chemical weathering of rocks with sulfuric acid is usually not considered in reconstructions of the past evolution of the carbon cycle, although this reaction delivers cations and alkalinity to the ocean without involvement of atmospheric CO2. The contribution of sulfuric acid as a weathering agent is still poorly quantified; the identification of riverine sulfate sources is difficult. The use of δ34S and δ18O of dissolved sulfate allows us to demonstrate that most of the sulfate in surface waters of the Mackenzie River system, Canada, derives from pyrite oxidation (85% ± 5%) and not from sedimentary sulfate. The calculated flux of pyrite-derived sulfate is 0.13 × 1012 mol/yr, corresponding to 20%–27% of the estimated global budget. This result suggests that the modern global ocean delivery of sulfide-derived sulfate, and thus chemical weathering with sulfuric acid, may be significantly underestimated. A strong correlation between sulfide oxidation rates and mechanical erosion rates suggests that the exposure of fresh mineral surfaces is the rate-limiting factor of sulfide oxidation in the subbasins investigated. The chemical weathering budget of the Mackenzie River shows that more than half of the dissolved inorganic carbon discharged to the ocean is ancient sedimentary carbon from carbonate (62%) and not atmospheric carbon (38%). The subsequent carbonate precipitation in the ocean will thus release more CO2 in the atmosphere-ocean system than that consumed by continental weathering, typically on glacial-interglacial time scales. [ABSTRACT FROM AUTHOR]

Quantifying past circulation is a vital part of testing our understanding of the modern and future climate system. The isotopic composition of neodymium (Nd) in marine precipitates has considerable promise as a recorder of past circulation patterns, but its robust application requires knowledge of the end-member compositions in order to correctly deconvolute a downstream signal. We show here, using in situ, high temporal resolution analyses of ferromanganese crusts from the North Atlantic, that the Nd isotopic composition of deep water during times of much more extensive Northern Hemisphere ice cover was no different than the modern-day interglacial value. This result is surprising, but greatly simplifies the use of Nd isotopes as tracers of the strength and patterns of circulation in the Atlantic in the past. [ABSTRACT FROM AUTHOR]

Strata in the Paganzo, Tarija, and Paraná Basins of Argentina and Brazil provide evidence for reconstructing the effects of late Paleozoic glacial retreat. The depositional environment of the transgressive and early highstand fine-grained deposits has been controversial, with interpretations ranging from normal-marine shelves to estuaries to lakes. Whereas their counterparts from shallow-marine settings not influenced by glaciation host diverse, fully marine ichnofaunas, these fine-grained postglacial deposits are dominated by nonspecialized grazing trails, simple feeding traces, arthropod trackways, and fish trails. They are typical of freshwater environments and represent examples of the Mermia and Scoyenia ichnofacies. However, the local presence of acritarchs indicates sporadic marine influence. These observations suggest a new interpretation, that freshwater conditions in fjordlike settings across South America were widespread because Gondwanan basins were overwhelmed by strong meltwater discharge issuing from melting of the continental ice masses. [ABSTRACT FROM AUTHOR]

Two distinct Neoproterozoic glacial episodes are known for the Dalradian Supergroup in the British-Irish Caledonides, the Port Askaig Formation and the Inishowen-Loch na Cille ice-rafted debris (IRD) beds. Here we describe a third, intermediate between those two, the Stralinchy-Reelan formations, composed of diamictite and IRD. Developed directly above these rocks is the Cranford Limestone, which consists of a basal, 1–6-m-thick, tan-gray dolostone overlain by a variably developed, but as much as 340-m-thick, sequence of thin-bedded limestone and dolostone. This unit exhibits a C isotopic trend that begins negative in the basal dolostone, reaches a nadir of -7%0, and then rises to 0‰–2‰. These characteristics match strikingly those of Marinoan-style cap carbonates. Consequently, we interpret the Port Askaig Formation, the Stralinchy-Reelan units, and the Inishowan-Loch na Cille beds as equivalents of the ca. 700 Ma Sturtian, the 635 Ma Marinoan, and the ca. 580 Ma Gaskiers glacials, respectively. Two additional observations are noteworthy. Carbonate rocks below the Port Askaig Formation record a δ13C decline to -6‰ that implies that such downturns may occur in both pre-Sturtian and pre-Marinoan strata. In addition, the Bonahaven Dolomite is not a cap carbonate to the Port Askaig Formation, but exhibits a δ13C rise to 12‰, which we correlate with the inferred global Keele peak. These data further document the utility of Neoproterozoic glacial-cap carbonate sequences in global correlations and denote the base of the Cranford Linestone as the Cryogenian-Ediacaran boundary. [ABSTRACT FROM AUTHOR]

Cosmogenic surface-exposure 10Be dating of Last Glacial Maximum (LGM) moraines indicates that glaciers in the southwestern Uinta Mountains remained at their maximum positions until ca. 16.8 ± 0.7 ka, ~2 k.y. after glaciers in the neighboring Wind River Range and Colorado Rockies began to retreat. The timing of the local LGM in the southwestern Uintas overlaps with both the hydrologic maximum of Lake Bonneville and preliminary estimates of the local LGM in the western Wasatch Mountains. This broad synchroneity indicates that Lake Bonneville and glaciers in northern Utah were responding to similar climate forcing. Furthermore, equilibrium line altitudes (ELAs) for reconstructed LGM alpine glaciers increase with distance from the Lake Bonneville shoreline, rising from ~2600 m to ~3200 m over the 120 km length of the glaciated Uintas. This pronounced ELA gradient suggests that the magnitude of the latest Pleistocene glacial advance in the western Uintas was due, at least in part, to enhanced precipitation derived from Lake Bonneville; thus, the lake acted as a local amplifier of regional climate forcing. This relationship underscores the sensitivity of alpine glaciers to moisture availability during the latest Pleistocene, and further demonstrates the importance of local moisture sources on glacier mass balance. [ABSTRACT FROM AUTHOR]

Identifying terrigenous sources in deep-sea sediments may reveal temporal trends in paleocirculation and the relative role of eolian, upwelled, and hemipelagic Fe sources to surface waters. Bulk elemental and isotopic geochemistry of deep-sea sediments recovered during Ocean Drilling Program Leg 177 in the south-eastern Atlantic sector of the Southern Ocean reveal several important aspects of paleocirculation and terrigenous provenance. The sites studied span 43°-53° S and represent different oceanographic settings relative to regional hydrography and sediment type. Bulk sediment geochemistry indicates that terrigenous provenance varied over the past 600 k.y. Site 1089, the northernmost site, exhibits clear glacial-interglacial variability in provenance, while provenance appears to vary regardless of climate state at the more southerly sites (Site 1093 and 1094). Nd and Sr isotopes and Sm/Nd ratios of the terrigenous fraction indicate that study sites have geochemically distinguishable provenance. Nd and Sr isotopes further suggest that Sites 1089 and 1094 both contain detrital components that originated in South America over the past 30 k.y.; however, Site 1089 is also influenced by southern African sources and the strength of the Agulhas Current. The ϵNd data support a more hemipelagic source for the terrigenous material rather than an eolian source based on comparisons with Antarctic ice core data and known sea-ice extent. [ABSTRACT FROM AUTHOR]

The deglacial meltwater pulse in the North Atlantic that induced the Younger Dryas event also prompted climate cooling in East Asian monsoon regions such as Japan and coastal mid-latitude China. However, very little is understood about the mechanism that can transmit changes in the North Atlantic to the Far East. Here we show that the shutdown of the North Atlantic thermohaline circulation brought about significantly lower temperatures and higher precipitation in the Japanese winter, whereas the change in the Japanese summer climate was considerably smaller. The cooling of the Siberian air mass seems to have caused an increased pressure gradient between Siberia and the West Pacific in winter, intensifying the winter monsoon. The Mongolian high-pressure system and the westerly jet stream played an important role in the teleconnection. In contrast, the warming at the onset of the late glacial interstadial (GI-1; B&3x00F8;lling-Aller&3x00F8;d) in the West Pacific did not have season-specific features, implying that the principal driving mechanism of this warming event may lie in a pan-hemispheric or global factor, such as insolation changes. [ABSTRACT FROM AUTHOR]

Well-dated multidecadal- to centennial-scale sediment records from the subarctic northwest Pacific show that the early deglacial 18.5-15.0 ka was marked by 3 pronounced short-term warmings of ∼5 °C. They lasted 500-1500 yr each and were coeval with early to late stages of cold Heinrich event 1 in the North Atlantic. These regional climate windows may have promoted a pre-Clovis emigration of people from the cold-arid monsoon climate in East Asia to the climatically more favorable, then-emerged Beringian and Aleutian shelf regions and the Americas, as suggested by archeological findings. [ABSTRACT FROM AUTHOR]

Marine and rift sediments exert a fundamental control on ice stream flow in the West Antarctic Ice Sheet, and hence on its mass balance and stability. In contrast, most ice streams in the much larger East Antarctic Ice Sheet are thought to be relatively stable features resting on till, perhaps underlain by crystalline rock. Any geological controls on East Antarctic Ice Sheet enhanced flow remain largely unknown. We present aerogeophysical evidence indicating that a region of enhanced ice flow in the interior of the East Antarctic Ice Sheet is underlain by subglacial sediments ∼3 km thick and that these are influencing the flow regime of the overlying ice. We show that subglacial sediments are important in modulating ice dynamics, not just for the West Antarctic Ice Sheet, but also for its much larger neighbor, and suggest that the sedimentary basin identified here may contain information on the Neogene glacial history of this part of the East Antarctic Ice Sheet. [ABSTRACT FROM AUTHOR]

We present marine geological and geophysical data for the Antarctic Peninsula that call for a larger ice-sheet reconstruction during the Last Glacial Maximum (LGM) than suggested by previous studies. Such glacial reconstructions are important for quantifying post-LGM sea-level rise and providing boundary conditions for general circulation models. Megascale glacial lineations on the continental shelf surrounding Antarctica provide documentation for grounded ice that was streaming. Swath bathymetry data (NBP02-01) reveal lineations in each of the major glacial troughs, except for Smith Trough, which features grooves and bedrock drumlins (0.5–3 km spacing, 1:20 elongation ratios) on crystalline bedrock. We place the maximum extent of ice at the seaward limit of the lineations, at or near («10 km landward of) the shelf break. The occurrence of line-sourced gullies at the mouth of each glacial trough (except Vega Trough, Weddell Sea) supports the interpretation of ice grounded at the shelf break. Megascale glacial lineations imaged in this study and other areas surrounding Antarctica (including the Ross Sea, Pine Island Bay) have a consistent morphology, with elongation ratios of »80:1 and spacings of 200–600 m (mode of 300 m). In contrast, lineations (‘bundies’; Canals et al., 2000) in the Gerlache-Boyd Trough exhibit larger spacing (1–5 km), and the upstream portions (∼7 km) are carved directly into bedrock rather than till, as suggested by previously unpublished air gun (∼120 Hz) seismic data. These features have yet to be dated; however, they are believed to have been active during the LGM. Radiocarbon dates in glaciomarine sediments (foraminifera and organic matter) from seven glacial troughs indicate that megascale glacial lineations were formed during the LGM. Other geomorphic features, such as the grounding zone wedge in the northwestern Weddell Sea, formed when the ice sheet was retreating. The initial retreat of the grounded ice from the outer shelf occurred by 18,500 cal yr B.P. The inner shelf was mostly ice free by 13,000 cal yr B.P. This is significantly earlier than numerical models, which suggest Antarctic deglaciation began at 12,000 cal yr B.P. This also suggests the peninsula area contributed to global sea-level rise associated with meltwater pulse (MWP) 1a. [ABSTRACT FROM AUTHOR]

Here we report on mid-Oligocene globally synchronous Arctic dinoflagellate migration events, calibrated against chron C9n. We show that sudden appearances and marked abundance increases of the Arctic taxon Svalbardella at lower and middle latitudes coincide with the Oi-2b benthic δ18O glacial episode, dated as ca. 27.1 Ma. These unprecedented migrations are taken to indicate anomalously strong surface-water cooling during Oi-2b time, in turn associated with strong concomitant Antarctic ice-sheet growth and sea-level lowering. We estimate the duration of these unique Svalbardella migrations and the associated episode of profound cooling as ∼500 k.y. Our records suggest a close link between this distinct Oligocene glaciation episode, strong sea-level fall, and the classic lower-upper Oligocene, or Rupelian-Chattian, boundary, dating this boundary as ca. 27.1 Ma. [ABSTRACT FROM AUTHOR]

Brine springs discharging from Devonian carbonates of the Western Canada Sedimentary Basin have distinct water chemistry from brines in laterally equivalent units deeper in the basin. Stable isotope data suggest that the brine springs originated as Pleistocene meltwater. These waters are interpreted to originate as an influx of subglacial meltwater related to a reversal of the basin-scale flow system, caused by the overriding ice sheet. Esker distribution shows a notable relationship between shield and sedimentary rocks. An integrated sedimentary basin/ice sheet model supports the interpretation that high permeability carbonate units acted as preferential subglacial drains that in turn affected esker development. The fluid flow history of the Western Canada Sedimentary Basin is characterized by back-and-forth movement through geological time in response to changing boundary conditions. Modern-day flow systems may not then be indicative of historic movement of economic fluids through the basin. [ABSTRACT FROM AUTHOR]

Pennsylvanian tropical rain forests flourished during an icehouse climate mode. Although it is well established that Milankovitch-band glacial-interglacial rhythms caused marked synchronous changes in Pennsylvanian tropical climate and sea level, little is known of vegetation response to orbital forcing. This knowledge gap has now been addressed through sequence-stratigraphic analysis of megafloral and palynofloral assemblages within the Westphalian D—Cantabrian Sydney Mines Formation of eastern Canada. This succession was deposited in a low-accommodation setting where sequences can be attributed confidently to glacio-eustasy. Results show that long-lived, low-diversity peat mires dominated by lycopsids were initiated during deglaciation events, but were mostly drowned by rising sea level at maximum interglacial conditions. Only upland coniferopsid forests survived flooding without significant disturbance. Mid- to late interglacial phases witnessed delta-plain progradation and establishment of high-diversity, mineral-substrate rain forests containing lycopsids, sphenopsids, pteridosperms, cordaites, and tree ferns. Renewed glaciation resulted in sea-level fall, paleovalley incision, and the onset of climatic aridity. Glacial vegetation was dominated by cordaites, pteridosperms, and tree ferns; hydrophilic lycopsids and sphenopsids survived in paleovalley refugia. Findings clearly demonstrate the dynamic nature of Pennsylvanian tropical ecosystems and are timely given current debates about the impact of Quaternary glacial-interglacial rhythms on the biogeography of tropical rain forest. [ABSTRACT FROM AUTHOR]

The terrestrial glacial record reflects past snowline variability and atmospheric temperature changes. When combined with secure chronologies, these data can be used to test models of ice-age climate. We present new in situ cosmogenic 10Be, 26Al, and ³He exposure ages, supported by limiting 40Ar/39Ar and 14C ages, for seven of the youngest moraines east of Lago Buenos Aires, Argentina, 46.5°S, that were deposited by a large outlet glacier of the Patagonian Ice Cap. Following a major glaciation that deposited extensive moraines prior to 109 ka, paired 10Be-26Al ages indicate that the next youngest complex of moraines was deposited from 23.0 ± 1.2 to 15.6 ± 1.1 ka (1σ). During the last glaciation, ice was at its maximum extent prior to 22 ka and at least five moraines were deposited in less than 10 k.y. These data are in good agreement with three 14C ages of ca. 16 ka from varved sediment banked on top of the youngest of these five moraines and limiting ³He ages, which range from ca. 33 to 19 ka. The most extensive ice marginal deposits preserved within the last 109 k.y. were formed during marine oxygen isotope stage 2; no moraines dating to stage 4 were found. For stage 2, the distribution of ages at Lago Buenos Aires is similar to cosmogenic nuclide-based glacial chronologies from western North America. In fact, the structure of the last mid-latitude South American ice age—specifically, the overall timing, a maximum ice extent prior to 22 ka, and deglaciation after 16 ka—is indistinguishable from that of the last major glaciation in the Northern Hemisphere, despite a maximum in Southern Hemisphere insolation during this period. The similar midlatitude glacial history in both hemispheres implies that a global climate forcing mechanism, such as atmospheric cooling, as opposed to oceanic redistribution of heat, synchronizes the ice age climate on orbital time scales. [ABSTRACT FROM AUTHOR]

The regional expression of millennial-scale climate variability during the last glacial is examined with particular reference to the vegetation response in Greece. Inspection of three pollen records from contrasting bioclimatic areas suggests that differences in the magnitude of cold events as recognized in the North Atlantic and western Mediterranean are expressed in terms of tree population changes only in areas with a range of favorable habitats. By contrast, records from sites where populations approach their tolerance threshold do not appear to resolve differences in the amplitude of the climate oscillations. Understanding the importance of local factors in modulating the biological response to climate change is critical when attempting to establish the spatial pattern of millennial variability. Keywords: paleoclimatology; paleoecology; last glacial; North Atlantic; Greece. [ABSTRACT FROM AUTHOR]

Reevaluates the timing and extent of late Paleozoic glaciation in Gondwana. Occurrence of glaciotectonite; Sedimentation; Weathering profiles; Soft-sediment and glaciotectonic deformation; Paleocurrent orientations; Changes in the sea level.

The timing of the development of strath terraces with respect to climatic variability remains equivocal. Previous studies attribute strath-terrace formation to glacial or interglacial climates or to variations in sediment and water fluxes that cause lateral erosion followed by vertical incision. A chronology of strath-terrace formation spanning ∼900 k.y. has been generated on the basis of loess-paleosol couplets and paleomagnetic, thermoluminescence, and radiocarbon dating of strath terraces in the Qilian Shan of northeastern Tibet. Repetitive stratigraphic and geomorphic patterns on each terrace indicate that they formed during glacial-interglacial transitions. Long-term bedrock incision rates and inferred rock uplift rates appear steady and unrelated to strath formation over the past 900 k.y. [ABSTRACT FROM AUTHOR]

The response of continental climate to the well-documented climate oscillations during the last glacial period has been a subject of intense interest, yet much less is known about the influence on regional continental climates than in the marine or polar realms of Earth. The detailed lake-level history of the closed Lake Lisan (paleo-Dead Sea) in the Middle East has been reconstructed from shoreline indications and high-resolution U-Th and [sup 14]C chronologies, thus providing data on the response of the lake's catchment area to the climate changes during the corresponding period. We present a correlation between the newly developed Lake Lisan level curve for the past 55 k.y. and the North Atlantic Heinrich events. The correlation indicates a closely connected climate response between these North Atlantic events and the hydrologic conditions that prevailed in the Eastern Mediterranean. Our findings show that although the generally cooler conditions that prevailed during the last glaciation favored high levels of the lake, catastrophic events in the North Atlantic, which are associated with maximum cooling, have been responsible for droughts in the Eastern Mediterranean. We infer that cold-water input to the Mediterranean originating in the collapse of the North Atlantic Deep Water circulation caused the reduction of evaporation and less precipitation in the Eastern Mediterranean. [ABSTRACT FROM AUTHOR]

Climatically sensitive tropical ecosystems provide important information that may help us to fill the gaps in our knowledge concerning the evolution of rainforests during periods of full glaciation. Small changes in precipitation in the Amazon Basin have immediate consequences for the survival of the Andean cloud forest, because its dominant source of moisture today is the Atlantic Ocean. A 40 k.y. lacustrine record from the Eastern Cordillera in Bolivia in an endemic species-rich and ecologically threatened region shows a dry Last Glacial Maximum, indicating a drastic decrease of the Amazonian moisture source. To explain this aridity, we infer steep temperature gradients between the pole and equator in both hemispheres that would have reduced considerably the size and displacement of the Intertropical Convergence Zone and the austral summer precipitation. This major change in water supply induced a dramatic reduction in species diversity and suggests that the Andcan cloud forest did not provide refugia for tropical lowland taxa during full glacial times. [ABSTRACT FROM AUTHOR]

The Indian monsoon system, as recorded by ocean-floor biota (benthic foraminifera) at Ocean Drilling Program Site 758 in the eastern equatorial Indian Ocean, has varied dramatically over the past 5.5 m.y., long after the onset of the monsoons at 10-8 Ma. Benthic foraminifera that thrive with high productivity year-round were common before the formation of Northern Hemisphere continental ice sheets ca. 3.1-2.5 Ma, indicating that the summer (southwest) monsoon had high intensity and long seasonal duration. Ca. 2.8 Ma benthic faunas became dominated by taxa that flourish with a seasonally strongly fluctuating food supply, indicating that the northeast (winter) monsoon, during which primary productivity is relatively low, increased in duration and strength to form a system similar to that of today. The change occurred coeval with the initiation of the Northern Hemisphere glaciation, documenting a close link between the development of the Indian monsoon and Northern Hemisphere glaciation. [ABSTRACT FROM AUTHOR]

Evaluates the Neoproterozoic Fiq glaciation in Oman. Number of units of the Fiq Member; Existence of hydrologic cycle with sediment-routing system; List of facies associations in the Fiq Member.

Investigates the airborne dispersal of silicic tephras during the onset of glaciations in North Atlantic Region. Inception of ice rafting on the Iceland Plateau; Increase in frequency of occurrence of Icelandic tephras; Mechanism of tephra dispersal.

Focuses on the late Quaternary glaciation of the Cordillera de Talamanca in Costa Rica. Production of alpine glacial features; List of several moraine groups; Position of the Intertropical Convergence Zone.

Examines the influence of glacial retreat on the rate of erosion by tidewater glaciers. History of sediment output from retreating glaciers; Seismic profiles of proglacial sediments; Calculation of the sediment flux.