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Loess deposits of late Quaternary age in the Channeled Scabland and western Palouse regions of Washington state contain distinctive sequences of calcic buried soils, loess layers, tephra layers, and stratigraphic unconformities caused by cataclysmic flooding. Four new soil-stratigraphic units, the Sand Hills Coulee Soil, Washtucna Soil, Old Maid Coulee Soil, and the Devils Canyon Soil, are recognized across a 100-km transect. The pedostratigraphic correlations are strengthened by two tephra layers found at several sites that are correlative with the Mount St. Helens set S (radiocarbon ages about 13,000 yr B.P.), Mount St. Helens set C (radiocarbon ages about 36,000 yr B.P.), and by a third one that may be from an earlier (pre-set C) Mount St. Helens eruption. The set S tephra layer separates the Sand Hills Coulee and Washtucna Soils; the set C tephra layer separates the Old Maid Coulee and Devils Canyon Soils; and the pre-set C tephra layer underlies the Devils Canyon Soil. The well-known late Wisconsin episode of Scabland flooding and two older episodes of flooding are marked by regional stratigraphic unconformities that lie between pedostratigraphic units. One unconformity that underlies the Washtucna and Old Maid Coulee Soils was caused by a major episode of Scabland flooding that occurred before 35,000 yr B.P., probably during the early Wisconsin (80,000 to 65,000 yr B.P.). A possible second and older episode of flooding, recognized by an unconformity that underlies the Devils Canyon Soil, may be Illinoian (ca. 130,000 to 200,000 yr B.P.) in age.
Pleistocene alluvial terraces of the nonglaciated Ba River drainage basin on the north flank of the Qinling Shan are capped by a succession of loess units and paleosols that correlate with the standard marine isotope chronology and are used to date the subjacent alluvial gravels. Alluvial fills were deposited during isotope stages 2, 6, 8, 12, and 16, whereas terracing occurred during interglacial stages 1, 5, 7, 11, and 15. The apparent absence of terraces dating to stage 14 and stage 4 may be due to the lesser intensity of these glaciations compared to that of stage 2, although disruption of the alluvial regime by local tectonism is a likely alternative for the lack of a stage 4 terrace. A stage 10 terrace was not positively identified from available exposures; its possible absence could be related to post-stage 12 uplift. Aggradational episodes correlate with glaciations and loess deposition, whereas degradational episodes correlate with interglaciations or interstades and soil formation, implying that climate is the primary control on Quaternary paleohydrology. This in turn points to variations in the Earth's orbital geometry as the major factor that modulates both climate and, ultimately, the fluvial system in the Qinling Shan. In this region, glaciations were dominated by a cold, dry winter monsoon climate, whereas during interstades and interglaciations a warmer and wetter climate prevailed, implying strengthening of the summer monsoon. Both the loess/paleosol and the alluvial records are consistent with climate-model simulations spanning the last 18,000 yr that show a change from cold, dry conditions during the last glacial maximum to a climate warmer and wetter than present during the first half of the Holocene.
On western North Island, New Zealand, a record of climatic change during the last glaciation is preserved in a terrestrial coverbed sequence of dominantly andesitic provenance. Here, a succession of five loess-like Andisol units postdates the global high sea-level stand of oxygen isotope substage 5e (<125,000 yr). Tephra erupted from western and central North Island allow precise chronological correlation of the loess-like units. Aerosolic quartz additions determined by quantitative X-ray diffraction (XRD) record two major peaks that correlate with oxygen isotope stages 2 and 4. The most likely source of quartz-rich dust at these times is the surrounding continental shelf, then exposed by low sea level; however, quartz of Australian provenance may also be represented. This study provides the first confirmation from the terrestrial New Zealand record that rates of atmospherically transported particles increase during glacial stages.
This study evaluates obsidian-hydration dating in postglacial fluvial terraces cut into an outwash plain near West Yellowstone, Montana. Fluvial transport fractures obsidian grains. However, some old hydration rinds may be preserved, thus, a grain may record several fracturing events. The most recent fracturing event at West Yellowstone is recorded in surface sediments from all of the terraces, which were cut in a shorter period of time than the technique can discern. They formed about 19,000 ± 1000 yr ago, using published hydration-rate estimates and a mean rind thickness of 6.34 ± 0.14 μm (1 SE). Alternatively, the application of published hydration-rate constants for the Obsidian Cliff flow with an estimated effective hydration temperature of 1.4°C yield an age of 24,400 ± 1100 yr (1 SE). Thicker rinds record fracturing during Bull Lake glaciation and cooling cracks from the emplacement of several source flows. Much of the observed spread in rind thicknesses (6.34 ± 1.69 μm: 1 SD) is probably the result of chemically induced variations in hydration rate. Terrace ages based on a single rind would range from 13,000 to 39,000 yr (±1 SD). Therefore, it is inappropriate to (1) use a set of hydration-rate constants determined from a single sample to calculate ages for multiple artifacts or geological samples, (2) date an archaeological or geological event on the basis of a single artifact, or (3) generate a chronostratigraphy on the basis of individual dates as a function of depth. Multiple evaluations of source chemistry and hydration rates and multiple rind measurements are required to date fracturing events.
The accuracy and precision of 137Cs and 14C for dating post-1950 alluvial deposits were evaluated for deposits from known floods on two rivers in Arizona. The presence of 137Cs reliably indicates that deposition occurred after intensive above-ground nuclear testing was initiated around 1950. There was a positive correlation between the measured level of 137Cs activity and the clay content of the sediments, although 137Cs was detected even in sandy flood sediments with low clay content. 137Cs is a valuable dating tool in arid environments where organic materials for 14C or tree-ring dating are scarce and observational records are limited. The 14C activity measured in different types of fine organic detritus yielded dates within 1 to 8 yr of a 1980 flood deposit, and the accuracy was species-dependent. However, undifferentiated mixtures of fine organic materials from several post-bomb deposits of various ages repeatedly yielded dates between 1958 and 1962, and detrital charcoal yielded a date range of 1676–1939. In semiarid environments, the residence time of most types of organic debris precludes accurate annual resolution of post-bomb 14C dates.
Superimposed phases of stromatolite buildup are observed on the eastern margin of Lake Manyara and depict a paleoshoreline at about 20 m above modern lake level. Radiocarbon and Th/U measurements permitted the dating of the last two phases of stromatolite formation at ca. 90,000 yr and between 27,000 and 23,000 yr B.P., respectively. The Th/U chronology is based on the decay of a strong 230Th-excess (over 234U) inherited with the detrital particles cemented into the stromatolites. The various generations of stromatolites show comparable stable carbon and oxygen isotope contents and are located at the same paleolake stabilization levels. This indicates that stringent hydrological conditions are necessary for the development of the encrusting benthic microbial communities responsible for stromatolite formation. A comparison with similar stromatolitic units from the nearby Lake Natron-Lake Magadi basin shows that such conditions occurred during only a few of the late Quaternary humid episodes known in eastern Africa and that they are different in each basin. Stromatolites do not necessarily represent all high lake levels that Lake Manyara experienced during the late Pleistocene and Holocene.
High-resolution shallow seismic profiles from the Ventura shelf reveal two contrasting, structurally controlled styles of deposition. On the northern shelf, folded Plio-Pleistocene strata comprising the south limb of the Ventura Avenue anticline are unconformably overlain by undeformed late Quaternary strata. In contrast, a thicker section of undeformed, prograding deltaic sequences preserved on the subsiding southern shelf contains a detailed record of at least five middle and late Quaternary glacioeustatic sea-level oscillations. A transgressive surface at −140 m in upper Pleistocene Saugus deposits is overlain by transgressive and highstand systems tracts. A shelf-margin systems tract subsequently developed as the sea level lowered, marking the end of Saugus deposition. Thereafter, over 50 m of post-Saugus upper Pleistocene sediment accumulated during a period of smaller-scale sea-level oscillations, followed by the development of a pair of marine terraces, presently at −50 and −33 m, and a large fluvial channel, now buried at −113 m. Correlations of Saugus and terrace marker horizons with the middle to late Quaternary paleosea-level curve suggest development of the Saugus marker horizons during oxygen isotopic stages 7-5e and formation of the terraces during Wisconsinan high sea levels. Maximum rates of vertical separation on the Oak Ridge and Pitas Point faults are estimated to be 0.13 and 0.30 m/1000 yr, respectively.
Several samples of stalagmitic flowstone (Ensemble E) at Grotte du Lazaret (Nice, France) were dated by U-series isotopes. The results show that this speleothem began to grow about 130,000 yr B.P. and continued to about 70,000 yr B.P., coinciding almost exactly with the last interglaciation (isotope stage 5). Even though Ensemble E is not in direct stratigraphic relation with the cave deposits, this study shows that the Acheulian artifacts industry and fauna within Lazaret are older than the Riss/Wurm interglaciation.
Mollusk aminostratigraphy of Quaternary marine terrace sediments at Coquimbo Bay, Chile, combined with recently available electron spin resonance (ESR) ages, necessitates revision of the northern Chilean relative sea-level and terrace chronology. Protothaca and Mulinia d-alloisoleucine/l-isoleucine values define four aminozones which are consistent with available ESR ages. Terrace reoccupation during successive high sea-level stands is inferred on the basis of litho- and aminostratigraphically defined unconformities in terrace sediments. ESR data and a nonlinear kinetic racemization model give approximate numerical ages for the aminozones and thus yield estimates of net uplift rates. These rates, averaged over intervals of one to several hundred thousand years, have ranged from less than 0.1 m/1000 yr to no more than 0.2 m/1000 yr. Such slow uplift is the cause of terrace reoccupation, as the amount of uplift between successive glacioeustatic high sea-level stands is frequently not sufficient to isolate an earlier-formed abrasion platform from rising sea level during a subsequent high stand.
Precisely dated tree-ring sequences of larch (Larix cajanderi Mayr) extending from 1801–1980 ad from Kamchatka lying in the far east of Asian continent were studied to see the influence of volcanic eruptions on tree growth and to assess the potential of tree rings for dating ancient volcanic events. Nearby volcanic events are clearly reflected in ring-width patterns. Distinctly narrow rings are formed in the year following the year of eruption. Weaker eruptions, however, are marked by narrow rings in the year of eruption only. After strong eruptions, trees usually take 2 to 3 yr to return to normal growth. The results indicate that tree rings are useful for dating ancient volcanic events.