The impact of spatially varying ice sheet basal conditions on sliding at glacial time scales

Spatially variable bed conditions govern how ice sheets behave at glacial time scales (>1000 years). The presence or lack of complete sediment cover is responsible for changes in dynamics between the core and peripheral regions of the Laurentide and Fennoscandian ice sheets. A key component of this change is because sliding is promoted when unconsolidated sediments below the ice become water saturated, and become weaker than the overlying ice. We present an ice sheet sliding module for the Parallel Ice Sheet Model (PISM) that takes into account changes in sediment cover. This model routes meltwater, derived from the surface and base of the ice sheet, towards the margin of the ice sheet. The sliding is accomplished through water saturated sediments, or through hard-bedded sliding induced by changes in the effective pressure in the water drainage system. In areas with continuous, water saturated sediments, sliding is almost always accomplished through sediment deformation, except during times of high discharge. In areas with even a small portion of bare rock, sliding is dependent on the seasonally changing supply of water. Our model causes a more rapid buildup of ice sheets compared to a sediment-deformation only model, especially into areas with complete sediment cover.

Ранний - средний голоцен северного Сахалина

New palynological and radiocarbon data obtained from sections of two ancient peatbogs permitted to specify the landscape and climatic changes in the nature of the first half of the Holocene on the southern outskirts of North Sakhalin. The Early Holocene was generally colder and drier in comparison with modern climatic conditions. The vegetation cover was dominated by larch/birch forests with the shrub birch undergrowth. In the middle of the Boreal Period (9000-8400 yrs. BP / 10,100-9300 Cal. yrs. BP), the climate became significantly warmer and slightly wetter, yet remaining drier than the modern one. Birch forests with the presence of broad-leaved and dark coniferous tree species were widespread. In the initial phase of the Atlantic Period (probably 7800-7500 yrs. BP / 8800-8300 Cal. yrs. BP), in the climatic conditions warmer and much more humid than modern ones, the first expansion of spruce/fir forests occurred. The second half of the early and first half of the middle phase of the Atlantic Period (7500-6500 yrs. BP / 8300-7400 Cal. yrs. BP) were distinguished by decreased humidity and further warming of the climate, which resulted in dominating birch forests, subordinate spruce/fir taiga, and a lot of deciduous forests. There were two cold episodes at the beginning and in the middle of this stage. The first of them, about 7300 yrs. BP / 8100 Cal. yrs. BP, was dry and provided the return of birch/larch forests. The second, about 7100 yrs. BP / 7900 Cal. yrs. BP, had a more humid climate, which caused an increase in the role of dark coniferous taiga and a weakening of birch forests. In the second half of the middle and, possibly, the beginning of the late phase of the period (probably 6500-5400 yrs. BP / 7400-6200 Cal. yrs. BP), spruce/fir forests with participation of broad-leaved tree spread. They developed in a warmer and more humid climate, similar to the current climate in the south of the Sakhalin Island and close to the optimal climatic conditions of the post-glacial time. The middle of the Late Atlantic Period, about 5300 yrs. BP / 6100 Cal. yrs. BP, was marked by the second, maximum expansion of spruce/ fir forests, due to the onset of the cool and, possibly, the wettest episode in the Middle Holocene.

The Origin of Terraces in Buried Valleys in the Northwest of the East European Plain

—The paper considers the origin of terraces that often complicate the slopes of abnormally deep buried valleys in the northwest of the East European Plain. The Izborsk–Maly valley (Pskov Region, Russia), which is half filled with Quaternary sediments, was chosen as the object of study. Since the upper part of the valley remains unfilled, it is possible to study in detail both geologic and geomorphologic structures of its terraced slopes being of particular interest in the context of the origin of buried valleys and the role of substrate in it. The four denudation-terrace levels that have been identified on the slopes of the Izborsk–Maly valley at absolute elevations of 53, 56–58, 70–72, and 75–77 m formed during the destruction of the preglacial karst landscape under the influence of Pleistocene glaciations. The formation of the terraces was probably linked with several stages of overdeepening of the valley and its expansion by the glacial erosion processes, which corresponded to at least two stages of the ice sheet advance in this area. The last stage may have been the beginning of the Late Valdai (Weichselian) glaciation or the activation of the edge of an ice sheet during its Luga phase (~15.7 cal kyr BP). Results of the study suggest that the Izborsk–Maly valley formed in subglacial conditions. The degree of glacial erosion was determined primarily by the coherence of Devonian carbonate-terrigenous rocks hosting the valley and the degree of the area transformation by karst processes. The cirque shape of slopes is the evidence of karst processes expression in pre-glacial time. Analysis of literature data shows that the origin of denudation terraces in the buried valleys in the northwest of the East European Plain has been paid little attention. The obtained data contribute to the study of this problem and provide insights into the origin of buried valleys.

LATE PLEISTOCENE SHASTA GROUND SLOTH (XENARTHRA) DUNG, DIET, AND ENVIRONMENT FROM THE SIERRA VIEJA, PRESIDIO COUNTY, TEXAS

We present new information about the Late Pleistocene Shasta ground sloth (Nothrotheriops shastensis). Spirit Eye Cave in the Sierra Vieja along the Rio Grande provides the newest evidence that the Shasta ground sloth inhabited further south in the mountains of the southwestern Trans-Pecos, Texas, than has been previously documented. The cave is one of only twelve known Nothrotheriops dung localities. During excavation of the cave, packrat middens and sloth dung were discovered. Two areas within the cave provide radiocarbon dated ground sloth dung and packrat midden macrobotanical remains which permit the reconstruction of the sloth diet and local biotic habitat at 30,800 and 12,900 calibrated YBP. The local community at 30,800 calibrated years ago was a pinyon-juniper woodland with yucca, sandpaper bush, globemallow, cactus, and barberry in the understory based on the packrat midden from the cave. The dung contents indicate that the diet of the sloth included C3 and C4 grasses along with Agave. Data for the local vegetation community and sloth diet from 12,900 years ago indicate that during this late glacial time, the region was still a pinyon-juniper woodland but also contained Celtis, Quercus, and Larrea, among other taxa.

PISM paleo simulations of the Antarctic Ice Sheet over the last two glacial cycles

<p>Simulations of the glacial-interglacial history of the Antarctic Ice Sheet provide insights into dynamic threshold behavior and estimates of the ice sheet's contributions to global sea-level changes, for the past, present and future. However, boundary conditions are weakly constrained, in particular at the interface of the ice-sheet and the bedrock. We use the Parallel Ice Sheet Model (PISM) to investigate the dynamic effects of different choices of input data and of various parameterizations on the sea-level relevant ice volume. We evaluate the model's transient sensitivity to corresponding parameter choices and to different boundary conditions over the last two glacial cycles and provide estimates of involved uncertainties. We also present isolated and combined effects of climate and sea-level forcing on glacial time scales. </p>

Storage/Release of Geologic Carbon Influenced Pleistocene Glacial/Interglacial Atmospheric pCO2 Cycles

<p>For over 100 years scientists have puzzled over the mechanisms responsible for the repeated climate changes known as Ice Ages. A breakthrough was achieved when ice cores and marine archives revealed that the Ice Ages were paced at 100kyr intervals in alignment with Earth’s eccentricity cycle for the past million years. A second breakthrough was achieved when ice core records revealed that the Ice Ages were accompanied by ~80-90ppm variations in atmospheric pCO<sub>2</sub>. But after decades of research the mechanisms responsible for those atmospheric pCO<sub>2</sub> variations remains an open and unresolved puzzle.</p><p>Here we present new findings that challenge the long-standing paradigm that geologic processes that regulate carbon exchange between the Earth’s interior and exterior act too slowly to have influenced the ocean and atmosphere carbon budgets on glacial time scales. The evidence includes large Δ<sup>14</sup>C excursions found in biogenic sediments in each of the Ocean basins at the last glacial termination. These excursions point to a sustained release of <sup>14</sup>C-dead carbon spanning several thousand years.  In the Atlantic, Pacific and Indian Ocean the excursions are found near seafloor deformation features, including pockmarks that are indicative of gas-rich fluid release from sub-surface reservoirs. In the eastern equatorial Pacific, the Δ<sup>14</sup>C excursions are associated with enhanced hydrothermal metal concentrations including Fe, and Z that point to a hydrothermal source. Our ongoing research seeks to identify the storage and release mechanisms that operate on these carbon reservoirs on glacial time scales and to put constraints on the amount of carbon released at the last glacial termination. While the amount of carbon released from these geologic sources remains an open question for now, it is clear that geologic processes have affected changes in the global carbon budget on glacial time scales.</p>

A unique ~12 ka subaerial record of rift-transform triple-junction tectonics, NE Iceland

<p>In northern Iceland the European-North American plate boundary is broad and complex but includes a remarkable subaerial triple-junction intersection between the Husavik-Flatey Fault (HFF) dextral transform and rifting in the Northern Volcanic Zone. Fortuitously, the triple junction occurs in a sheet of ~12 ka pahoehoe lavas; a tabula rasa recording innumerable fault features displayed in exquisite detail. High-resolution drone imagery, coupled with 120 field measurements of fault slip directions and opening amounts, made possible the mapping and analysis of this detail and, importantly, enabled recognition and exclusion of potentially misleading primary deformation features associated with emplacement of the lavas. Rift-transform interactions in this natural laboratory have remained spatially stable throughout post-glacial time, although with transform-affinity faults reactivated to accommodate rift extension and transform ‘encroachment’ into the rift domain. First-order en-echelon Riedel fault complexes are recognised, linked by transpressional faulting and compressional strike-slip relay ramps, as well as second-order R shears, R’ and P shears, and previously undescribed R’ Riedel-in- Riedel relationships. A pahoehoe flow front offset along a first-order Riedel fault complex records slip at ~3.8 mm a<sup>−1</sup>, which may be consistent with the published GPS-based current slip-rate estimate of ~6.8 mm a<sup>−1</sup> across the HFF as a whole.</p>

African climate response to orbital and glacial forcing in 140,000-y simulation with implications for early modern human environments

A climate/vegetation model simulates episodic wetter and drier periods at the 21,000-y precession period in eastern North Africa, the Arabian Peninsula, and the Levant over the past 140,000 y. Large orbitally forced wet/dry extremes occur during interglacial time, ∼130 to 80 ka, and conditions between these two extremes prevail during glacial time, ∼70 to 15 ka. Orbital precession causes high seasonality in Northern Hemisphere (NH) insolation at ∼125, 105, and 83 ka, with stronger and northward extended summer monsoon rains in North Africa and the Arabian Peninsula and increased winter rains in the Mediterranean Basin. The combined effects of these two seasonally distinct rainfall regimes increase vegetation and narrow the width of the Saharan–Arabian desert and semidesert zones. During the opposite phase of the precession cycle (∼115, 95, and 73 ka), NH seasonality is low, and decreased summer insolation and increased winter insolation cause monsoon and storm track rains to decrease and the width of the desert zone to increase. During glacial time (∼70 to 15 ka), forcing from large ice sheets and lowered greenhouse gas concentrations combine to increase winter Mediterranean storm track precipitation; the southward retreat of the northern limit of summer monsoon rains is relatively small, thereby limiting the expansion of deserts. The lowered greenhouse gas concentrations cause the near-equatorial zone to cool and reduce convection, causing drier climate with reduced forest cover. At most locations and times, the simulations agree with environmental observations. These changing regional patterns of climate/vegetation could have influenced the dispersal of early humans through expansions and contractions of well-watered corridors.

Enamel hypoplasia and dental wear of North American late Pleistocene horses and bison: an assessment of nutritionally based extinction models

Approximately 50,000–11,000 years ago many species around the world became extinct or were extirpated at a continental scale. The causes of the late Pleistocene extinctions have been extensively debated and continue to be poorly understood. Several extinction models have been proposed, including two nutritionally based extinction models: the coevolutionary disequilibrium and mosaic-nutrient models. These models draw upon the individualistic response of plant species to climate change to present a plausible scenario in which nutritional stress is considered one of the primary causes for the late Pleistocene extinctions.In this study, we tested predictions of the coevolutionary disequilibrium and mosaic-nutrient extinction models through the study of dental wear and enamel hypoplasia of Equus and Bison from various North American localities. The analysis of the dental wear (microwear and mesowear) of the samples yielded results that are consistent with predictions established for the coevolutionary disequilibrium model, but not for the mosaic-nutrient model. These ungulate species show statistically different dental wear patterns (suggesting dietary resource partitioning) during preglacial and full-glacial time intervals, but not during the postglacial in accordance with predictions of the coevolutionary disequilibrium model. In addition to changes in diet, these ungulates, specifically the equid species, show increased levels of enamel hypoplasia during the postglacial, indicating higher levels of systemic stress, a result that is consistent with the models tested and with other climate-based extinction models. The extent to which the increase in systemic stress was detrimental to equid populations remains to be further investigated, but suggests that environmental changes during the late Pleistocene significantly impacted North American equids.

Tectonic and climatic controls on Quaternary fluvial processes and river terrace formation in a Mediterranean setting, the Göksu River, southern Turkey

Climate and tectonics effect the fluvial evolution of the Mediterranean Mut basin. The basin contains a river terrace staircase of 16 levels (T16–T1) ranging from 365 to 10 m above the current Göksu River in its middle and lower sections. These river terraces records tectonic uplift in the Mut basin. Optically stimulated luminescence (OSL) dating of the fluvial sediments of the youngest terrace (T16) provides a chronology for the assessment of the important impacts of climatic changes. The ages from the youngest river terrace deposits in T16 may be subdivided into two intervals: (1) 239–194.7 ka during the later part of Marine Oxygen Isotope Stage (MIS) 7, implying that the aggradation of T16 started in (the final phase of) this warm period; and (2) 187.9–171 ka during much of MIS 6. Thus, it appears that the Göksu River continued depositing sediment from an interglacial into a glacial time. The differences in climate-driven fluvial evolution between this Mediterranean fluvial system and the classical, well-studied temperate–periglacial river systems in Europe may be the result of different vegetation cover and greater thaw of more intense snowfalls.