Lithology and paleontology of the reflective layer horizon a

Cores recovered from horizon A are Late Cretaceous (Maestrichtian) in age and consist o alternating layers of calcareous turbidites and "red clay." The presence of red clay suggests that the water depth in this area during Cretaceous time was at least as great as at present-more than 5100 meters. A middle Cretaceous (Cenomanian) core consisting of interbedded sand and gravel and light-to-dark-gray lutite was taken in the same area from a layer stratigraphically below the horizon; the presence of hydrogen sulfide and iron sulfide may indicate anaerobic conditions that may be attributable to local ponding of sediment in Cenomanian time.     

Hypoxia, global warming, and terrestrial late Permian extinctions

A catastrophic extinction occurred at the end of the Permian Period. However, baseline extinction rates appear to have been elevated even before the final catastrophe, suggesting sustained environmental degradation. For terrestrial vertebrates during the Late Permian, the combination of a drop in atmospheric oxygen plus climate warming would have induced hypoxic stress and consequently compressed altitudinal ranges to near sea level. Our simulations suggest that the magnitude of altitudinal compression would have forced extinctions by reducing habitat diversity, fragmenting and isolating populations, and inducing a species-area effect. It also might have delayed ecosystem recovery after the mass extinction.     

Ecological Controls on the Evolutionary Recovery of Post-Paleozoic Crinoids

Analysis of morphological characters of a global sample of post-Paleozoic crinoid echinoderms shows that this group underwent a rapid diversification after the extinction at the end of the Permian to reach maximal morphological disparity by the Late Triassic, which is essentially the same evolutionary pattern seen during the group's early Paleozoic radiation. The accelerated morphological diversification of a single class implies that, even if clades surviving from the Paleozoic represented ecological incumbents that hindered the origin of new higher taxa, species within individual higher taxa rapidly exploited available ecological opportunities in the Mesozoic.     

Cycads: fossil evidence of late paleozoic origin

Plant fossils from Lower Permian strata of the southwestern United States have been interpreted as cycadalean megasporophylls. They are evidently descended from spermopterid elements of the Pennsylvanian Taeniopteris complex; thus the known fossil history of the cycads is extended from the Late Triassic into the late Paleozoic. Possible implications of the Permian fossils toward evolution of the angiosperm carpel are considered.     

Oceanic Anoxia and the End Permian Mass Extinction

Data on rocks from Spitsbergen and the equatorial sections of Italy and Slovenia indicate that the world's oceans became anoxic at both low and high paleolatitudes in the Late Permian. Such conditions may have been responsible for the mass extinction at this time. This event affected a wide range of shelf depths and extended into shallow water well above the storm wave base.     

Triassic amphibian from Antarctica

A fossil bone fragment-the first record of tetrapod life from Antarctica-was found near Graphite Peak in the upper Beardmore Glacier area (85 degrees 3.3'S; 172 degrees 19'E). The fragment was embedded in a pebbly quartzose sandstone, probably of fluvial origin, in the lower part of the Triassic Fremouw Formation (as yet undefined), which contains Dicroidium in the upper part. The fossil horizon is only 76 meters, stratigraphically, above the Glossopteris-bearing Buckley Formation, a coal-bearing sequence of Permian age. The bone fragment is the back portion of a left mandibular ramus of a labyrinthodont amphibian. This identification is based on the characteristic labyrinthodont external surface sculpturing, with indications of "mucous grooves," as well as on other osteological features.     

Authigenic potassium feldspar in cambrian carbonates: evidence of alleghanian brine migration

The shallow-water limestones and dolostones of the Conococheague Limestone (Upper Cambrian) of western Maryland contain large amounts of authigenic potassium feldspar. The presence of halite daughter crystals in breached fluid inclusions, low whole-rock ratios of chlorine to bromine, and thermochemical data suggest that the potassium feldspar formed at low temperature by the reaction of connate brines with intercalated siliciclastic debris. Analyses of argon age spectra indicate that the authigenic feldspar probably formed during Late Pennsylvanian to Early Permian time. These results may indicate mobilization and migration of connate brines brought about by Alleghanian folding. The widespread occurrence of authigenic potassium feldspar in Cambrian and Ordovician carbonate rocks throughout the Appalachians suggests that this may have occurred throughout the entire basin.     

A double mass extinction at the end of the paleozoic era

Three tests based on fossil data indicate that high rates of extinction recorded in the penultimate (Guadalupian) stage of the Paleozoic era are not artifacts of a poor fossil record. Instead, they represent an abrupt mass extinction that was one of the largest to occur in the past half billion years. The final mass extinction of the era, which took place about 5 million years after the Guadalupian event, remains the most severe biotic crisis of all time. Taxonomic losses in the Late Permian were partitioned among the two crises and the intervening interval, however, and the terminal Permian crisis eliminated only about 80 percent of marine species, not 95 or 96 percent as earlier estimates have suggested.     

A chronology of Paleozoic sea-level changes

Sea levels have been determined for most of the Paleozoic Era (542 to 251 million years ago), but an integrated history of sea levels has remained unrealized. We reconstructed a history of sea-level fluctuations for the entire Paleozoic by using stratigraphic sections from pericratonic and cratonic basins. Evaluation of the timing and amplitude of individual sea-level events reveals that the magnitude of change is the most problematic to estimate accurately. The long-term sea level shows a gradual rise through the Cambrian, reaching a zenith in the Late Ordovician, then a short-lived but prominent withdrawal in response to Hirnantian glaciation. Subsequent but decreasingly substantial eustatic highs occurred in the mid-Silurian, near the Middle/Late Devonian boundary, and in the latest Carboniferous. Eustatic lows are recorded in the early Devonian, near the Mississippian/Pennsylvanian boundary, and in the Late Permian. One hundred and seventy-two eustatic events are documented for the Paleozoic, varying in magnitude from a few tens of meters to approximately 125 meters.     

U/Pb zircon geochronology and tempo of the end-permian mass extinction

The mass extinction at the end of the Permian was the most profound in the history of life. Fundamental to understanding its cause is determining the tempo and duration of the extinction. Uranium/lead zircon data from Late Permian and Early Triassic rocks from south China place the Permian-Triassic boundary at 251.4 +/- 0.3 million years ago. Biostratigraphic controls from strata intercalated with ash beds below the boundary indicate that the Changhsingian pulse of the end-Permian extinction, corresponding to the disappearance of about 85 percent of marine species, lasted less than 1 million years. At Meishan, a negative excursion in delta13C at the boundary had a duration of 165,000 years or less, suggesting a catastrophic addition of light carbon.     

Permian ammonoid cyclolobus from the zewan formation, guryul ravine, kashmir

Ammonoid cephalopods serve significantly as a basis for correlation of strata near the boundary between the Paleozoic and Mesozoic Erathems. This is a report of the diagnostically Late Permian (uppermost Paleozoic) ammonoid Cyclolobus walkeri (Diener, 1903) from the Zewan Formation 20 meters below the Permian-Triassic contact at the classic section in Guryul Ravine, Srinagar region, Kashmir.     

Size of the permo-triassic bottleneck and its evolutionary implications

Rarefaction analysis of extinctions in the Late Permian indicates that as many as 96 percent of all marine species may have died out, thus forcing the marine biosphere to pass through a small bottleneck. With such severity of extinction, chance elimination of certain biologic groups would have been probable. Some of the changes in biologic composition observed at the Permo-Triassic boundary may be explained as an evolutionary founder effect that followed the bottleneck.     

Marine fossils at rancho la brea

Marine assemblages of foraminifera, ostracods, mollusks, and echinoids were recovered from deposits stratigraphically beneath the classic vertebrate assemblages from the tar pits at Rancho La Brea. The marine fossils indicate deposition in quiet, shallow water and suggest that accumulation of the type Rancholabrean material began during Wisconsin time.     

Periodicity of extinctions in the geologic past: deterministic versus stochastic explanations

The temporal spacing and the magnitude of major extinctions over the past 250 and 570 million years, based on the use of different metrics of extinction probability, are analyzed by comparing deterministic and stochastic explanations. The best-fitting time series model is a stochastic autoregressive model that displays a pseudoperiodic behavior with a cycle length of 31 million years for the past 250 million years, regardless of the metric of extinction probability. The periodicity lengthens and weakens when the analysis is extended to the entire Phanerozoic. The history of the probability of extinction for the entire Phanerozoic, based on time series analysis, does not support the reported bipartite distribution of Van Valen. Rather, the probability of extinction has decreased uniformly over Phanerozoic time whereas the inertia or stability of the biotic system after the Late Permian crisis has increased.     

Detrital Zircon Link Between Headwaters and Terminus of the Upper Triassic Chinle-Dockum Paleoriver System

New detrital-zircon geochronologic data reveal that a through-going paleoriver connected Texas with Nevada in Late Triassic time. Sandstone from the Upper Triassic Santa Rosa Sandstone (Dockum Group) from northwestern Texas contains a detrital zircon suite nearly identical to that found in western Nevada in the Upper Triassic Osobb Formation (Auld Lang Syne Group, correlative with the Chinle Formation). The Santa Rosa Sandstone was derived in large part from the eroded Cambrian core of the Amarillo-Wichita uplift, as evidenced by abundant zircons with ages of 515 to 525 million years. Other zircon grains in the sandstone are Permian, Devonian, Proterozoic, and Archean in age and, with the exception of the Archean grain, are also matched by the population in the Nevada strata.     

Spores, pollen, and microplankton from the horizon Beta outcrop

Palynology was used for dating a pre-Pleistocene deep-sea organic lutite layer situated stratigraphically near seismic horizon beta, below horizon A. The spores and pollen are closely identified, quantitatively, with nonmarine and marine Middle Cretaceous assemblages (Albian-Cenomanian) on the continents, an age designation that is confirmed by the occurrence of dinoflagellate cysts, acritarchs, foraminifers, and coccoliths in the investigated cores. The abundance of these well-preserved, land-derived assemblages in an area far removed from a source suggests some tectonic displacement since their deposition.     

The Buttermilk Creek complex and the origins of Clovis at the Debra L. Friedkin site, Texas

Compelling archaeological evidence of an occupation older than Clovis (~12.8 to 13.1 thousand years ago) in North America is present at only a few sites, and the stone tool assemblages from these sites are small and varied. The Debra L. Friedkin site, Texas, contains an assemblage of 15,528 artifacts that define the Buttermilk Creek Complex, which stratigraphically underlies a Clovis assemblage and dates between ~13.2 and 15.5 thousand years ago. The Buttermilk Creek Complex confirms the emerging view that people occupied the Americas before Clovis and provides a large artifact assemblage to explore Clovis origins.     

贵州六盘水地区马铃薯种植地质环境分析

为了探索贵州省六盘水地区马铃薯种植的有利种植区带,为该地区规模化种植马铃薯选区提供科学依据,采用现场调查、区域地质环境分析等方法,对贵州省六盘水地区马铃薯种植区域进行调查研究。结果表明:贵州省六盘水地区马铃薯主要种植区分布在海拔1 700m以上的高寒山区的山坡和谷地,地质环境为石炭系白云岩、二叠系茅口灰岩、二叠系玄武岩、三叠系泥质白云岩分布区域。在调查的基础上,结合区域地层分布和海拔等条件分析指出,六盘水地区优质马铃薯种植区域为二叠系玄武岩分布区域,其次是高海拔的二叠系茅口灰岩分布区域以及石炭系白云岩分布区域。     

Cool glacial temperatures and changes in moisture source recorded in oman groundwaters

Concentrations of atmospheric noble gases (neon, argon, krypton, and xenon) dissolved in groundwaters from northern Oman indicate that the average ground temperature during the Late Pleistocene (15,000 to 24,000 years before present) was 6.5 degrees +/- 0.6 degrees C lower than that of today. Stable oxygen and hydrogen isotopic groundwater data show that the origin of atmospheric water vapor changed from a primarily southern, Indian Ocean source during the Late Pleistocene to a dominantly northern, Mediterranean source today. The reduced northern water vapor source is consistent with a drier Last Glacial Maximum through much of northern Africa and Arabia.     

Equatorial pacific seismic reflectors as indicators of global oceanographic events

The origin of a series of regionally correlatable seismic horizons in the Neogene sediments of the central equatorial Pacific is examined through seismic modeling and the detailed analyses of stratigraphic and physical property relationships in Deep Sea Drilling Project cores. These regionally traceable reflectors are synchronous; the younger reflectors are the direct result of carbonate dissolution events, the older ones of stratigraphically selective diagenetic processes. The changes in ocean chemistry associated with these events appear to be linked to global reorganizations of surface and bottom-water circulation patterns, the most dramatic of which are associated with reorganizations of North Atlantic bottom waters. These deepwater seismic horizons appear to correlate with the major events on the "relative sea-level" curve of Vail et al. for the Neogene.