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Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23-14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3-4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2 These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene.

Although much is known of the timing, geographical extent, and character of the late Paleozoic Gondwanan glaciation, its record in eastern Australia is poorly resolved. Some previous investigations have asserted that large parts of Australia were covered by a cold-based continental ice sheet during the Pennsylvanian (late Carboniferous). New data from Queensland in northeast Australia reported herein, however, suggest that the glaciation there was restricted to discrete, short-lived periods, in the Namurian (315 Ma), Westphalian (311 Ma), and Sakmarian (289-293 Ma). Thick successions in three widely separated areas indicate that the glaciation was confined in each case to local (valley or mountain) glaciers and that the mainly proglacial sediments were enclosed in every case by substantial intervals of fluvial, lacustrine, and shallow-marine strata deposited under nonglacial conditions. Accordingly, we submit that the cessation of glacial sediment deposition elsewhere in Gondwana during the Pennsylvanian to earliest Permian was due to the decay of glaciers, rather than to a transition to a cold-based ice sheet or sheets, as has been previously suggested. The interpretations presented herein make a significant contribution toward constraining the size and extent of glaciation in Gondwana, which has major implications for late Paleozoic world climate and sea-level fluctuations. Keywords: Paleozoic, Gondwana, glaciation, Queensland, Australia.

Large-scale, carbonate-hosted magnesite (MgC[O.sub.3]) deposits, although rare, occur mainly in Precambrian strata. Although many occurrences have characteristics consistent with penecontemporaneous formation in an evaporative marine setting, the general absence of CaS[O.sub.4] minerals has precluded the adoption of evaporative marine depositional models. In modern seawater, excess [Ca.sup.2+] and [Mg.sup.2+] relative to C[O.sup.2-.sub.3] and HC[O.sup.-.sub.3-] as well as abundant S[O.sup.2-.sub.4] require that, upon evaporation, MgC[O.sub.3] precipitation is accompanied by substantial deposition of CaS[O.sub.4] minerals. Here we use evidence from a Neoproterozoic magnesite deposit to suggest that differences in Precambrian seawater geochemistry enabled MgC[O.sub.3] to form in isolation under evaporative conditions. During the Precambrian, precipitation of CaS[O.sub.4] evaporites was hindered by (1) elevated dissolved inorganic carbon and enhanced precipitation of CAC[O.sub.3], which limited the availability of [Ca.sup.2+], and (2) a small marine sulfate reservoir. Because sulfate is an inhibitor to dolomitization, low sulfate concentrations increased the potential for penecontemporaneous dolomitization in marine settings. By utilizing [Ca.sup.2+], dolomitization served to increase fluid Mg/Ca ratios. In this HC[O.sup.-.sub.3-]-rich but S[O.sup.2-.sub.4]-poor system, dolomitization coupled with significant evaporative concentration resulted in magnesite formation without coprecipitation of CaS[O.sub.4] minerals. Decreasing carbonate saturation, progressive oxygenation, and a concomitant increase in sulfate availability during the Proterozoic ultimately led to the development of the more familiar conditions of the Phanerozoic, in which dolomitization was restricted to environments where elevated Mg/Ca ratios could overcome the inhibitory effects of sulfate and significant magnesite deposition was restricted to sabkhas and alkaline lakes. Keywords: Neoproterozoic, magnesite deposits, dolomite, marine sedimentation, evaporites.

Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23-14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3-4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (~280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (~500 ppm) atmospheric CO2. These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene. [ABSTRACT FROM AUTHOR]

Facies models that summarize the deposits of fluvial systems are well established for humid climate settings and for desert environments, but the deposits of rivers in the sub-humid and semiarid seasonal tropics have been largely ignored. Our observations and data from modern streams and recent deposits in northeastern Australia show how such rivers, with extremely variable discharge, have distinctive deposit characteristics that are substantially different from conventional fluvial facies models. These properties include (1) erosionally based channel-fill lithosomes that exhibit complex lateral facies changes, with (2) abundant, pedogenically modified mud partings, (3) complex internal architecture that may lack the macroform elements typical of other fluvial sediment bodies, (4) an abundance of sedimentary structures formed under high flow stage, and (5) an abundance of in situ trees that colonize channel floors and are adapted to inundation by fast-flowing water. We illustrate examples of this fluvial style from the rock record, and set out a new facies model. The recognition of such a distinctive fluvial character, and of changes in character through vertical successions, will aid paleoclimate and reservoir analysis.

HIV buds from lipid rafts and requires cholesterol for its egress from and entry into cells. Viral accessory protein Nef plays a major role in this process. In this study, it not only increased the biosynthesis of lipid rafts and viral particles with newly synthesized cholesterol, but also enriched them. Furthermore, via the consensus cholesterol recognition motif at its C terminus, Nef bound cholesterol. When this sequence was mutated, Nef became unable to transport newly synthesized cholesterol into lipid rafts and viral particles. Interestingly, although its levels in lipid rafts were not affected, this mutant Nef protein was poorly incorporated into viral particles, and viral infectivity decreased dramatically. Thus, Nef also transports newly synthesized cholesterol to the site of viral budding. As such, it provides essential building blocks for the formation of viruses that replicate optimally in the host. [ABSTRACT FROM AUTHOR]

A number of near-field (glacial, proglacial, and glacimarine) records of glacial activity in the late Paleozoic constrain the onset of the main phase of glaciation to the late Mississippian (Viséan-Serpukhovian). These events postdate some more isolated occurrences of glacially-influenced deposits in the late Devonian and early Carboniferous. The main onset is, nonetheless, not precisely constrained because of apparent differences in age among widely separated outcrop areas. This suggests that glacial onset was not globally synchronized and/or is inadequately constrained in absolute time. The near-field record holds the advantage that it is a direct reflection of glacial activity, but the disadvantage that such records are notoriously stratigraphically incomplete. On the other hand, far-field records from the paleo-tropical realm preserve the record of an indirect response to glaciation in the form of eustatic sea-level fluctuations, but are more likely to be a complete, time-averaged, global signal. Herein, we review the stratigraphic record of several late Mississippian successions in North America and northwest Europe, all of which show an upward transition from a noncyclic or cryptic stratigraphic stacking pattern to a strongly cyclic motif ("cyclothems") through the late Mississippian. Among these successions are the Big Snowy Group of Montana, Manning Canyon and Doughnut Formations of Utah and Colorado, Windsor and Mabou Groups of Atlantic Canada, Clare Group of western Ireland, and Strathclyde and Clackmannan Groups of eastern Scotland. Cyclothems are m to tens of m thick intervals showing alternation between offshore marine and exposed, nonmarine deposits, and are interpreted as a record of sediment accumulation on low-gradient paleotropical shelves with low rates of sediment supply that were subjected to repeated, large-magnitude sea-level excursions. The transition to a strongly cyclothemic motif is a consistently recognizable change in these successions, and seems to have taken place at about 330-332 Ma (straddling the Viséan-Serpukhovian boundary). This change significantly predates both major, global excursions in carbon and oxygen stable isotope values, and the Mississippian-Pennsylvanian unconformity, and is considered a more reliable indicator of the onset of widespread glaciation. [ABSTRACT FROM AUTHOR]