Paleoceanograpic changes on the Farallon Escarpment off central California during the last 16,000 years
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The Uranium-trend dating method: Principles and application for southern California marine terrace deposits
Uranium-trend dating is an open-system method for age estimation of Quaternary sediments, using disequilibrium in the 238U234U230Th decay series. The technique has been applied to alluvium, colluvium, loess, till, and marine sediments. In this study we tested the U-trend dating method on calcareous marine terrace deposits from the Palos Verdes Hills and San Nicolas Island, California. Independent age estimates indicate that terraces in these areas range from ???80 ka to greater than 1.0 Ma. Two low terraces on San Nicolas Island yielded U-trend plots that have a clustered array of points and the ages of these deposits are indeterminate or highly suspect. Middle Pleistocene terraces and one early Pleistocene terrace on San Nicolas Island and all terraces on the Palos Verdes Hills gave reasonably linear U-trend plots and estimated ages that are stratigraphically consistent and in agreement with independent age estimates. We conclude that many marine terrace deposits are suitable for U-trend dating, but U-trend plots must be carefully evaluated and U-trend ages should be consistent with independent geologic control. ?? 1989.
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Report of 1st discussion group: The last interglacial in high latitudes of the Northern Hemisphere: Terrestrial and marine evidence
The paleoclimatic informations preserved at some selected sites from high northern latitudes correlative with the last interglacial maximum of Isotopic Substage 5e have been compiled on a hemisphere scale. The mapped compilation reveals much warmer climate conditions on average than present in both terrestrial and marine environments, and indicates a northward shift of subpolar and boreal bioclimatic zones. The data also suggest that temperature and precipitation gradients were significantly different than those of the Holocene, with particularly efficient latitudinal and eastward transport of warm air masses. The paleoenvironmental and paleoclimate trends through the last interglacial sensu lato (Isotopic Stage 5) are more difficult to assess because the chronostratigraphical framework is poorly constrained. Nevertheless, the data suggest important regionalism in the climatostratigraphical trends. In particular, the large amplitude climatic cyclicity that characterizes the last interglacial of Europe is barely observed elsewhere. Moreover, by the end of the last interglacial (Isotopic Substage 5a), conditions slightly cooler than present are recorded in western Europe, while relatively warm conditions, similar to today, apparently prevailed in circumpolar regions adjacent to the North Atlantic despite the onset of ice growth. The last interglacial provides paleosynoptic situations, unlike those of the present, that may contribute to understanding interactions within the environmental system on a hemispheric scale. There is, however, an imperative need for uniformization of data sets and for more accurate chronological control on regional scales.
Quaternary International↗
Report of 2nd discussion group: Inter-relationships and linkages between the land, atmosphere and oceans during the last interglacial
No abstract available.
Quaternary International↗
The last interglaciation in Alaska: Stratigraphy and paleoecology of potential sites
At least 20 localities in Alaska contain deposits that may provide information on the last interglaciation (Oxygen-Isotope Substage 5e). These widely dispersed localities include river bluffs, coastal bluffs and terraces, elevated marine shorelines, lake basins, and artificial excavations. Most of the inferred interglacial deposits contain macrofossils or pollen that are older than the range of radiocarbon dating and commonly indicate climate as warm as or warmer than the present. At a few localities, evidence for deep thaw of permafrost also indicates a warm paleoclimate. At eight localities, the Old Crow tephra occurs at or below organic deposits that may represent Substage 5e. The tephra occurs beneath conspicuous organic deposits at Fairbanks, the Yukon Palisades, and Holitna lowland, and directly above a peat bed at Hogatza Mine. At Birch Creek, Halfway House, Ky-11, and Imuruk Lake, the tephra occurs within a paleosol or organic deposit, but other organic horizons that more likely indicate interglacial conditions occur at higher stratigraphic levels. The varied stratigraphic relations of the Old Crow tephra suggest that it may have been deposited close to the boundary between Isotope Substages 6 and 5, which is dated at about 130 ka in the marine record and between 132 and 140 ka on land. These age relations suggests that the tephra may have been deposited about 135 ?? 5 ka, validating the recent fission-track age determination of 140 ?? 10 ka for this deposit. Six coastal localities contain deposits of probable interglacial age, and these commonly are associated with evidence for eustatic sea levels higher than those of the present. Beach and sublittoral sediments of the Pelukian transgression occur up to 12 m asl along the northwest coast of Alaska, and are correlative with barrier island and lagoonal sediments on the Alaskan Arctic Coastal Plain. Both sets of deposits commonly contain extralimital mollusks and microfauna that indicate marine water slightly warmer than present and suggest that seasonal sea ice did not extend south of Bering Strait during the last interglacial as it does today. Farther south, elevated marine-terrace deposits on Amchitka Island contain marine invertebrates that indicate a climate warmer than at present. Peat horizons in coastal exposure at Goose Bay and coastal terraces at Lituya Bay contain pollen spectra that suggest forests like those of the present day, and spruce macrofossils exposed on Baldwin Peninsula indicate boreal forest more extensive than at present. Sediments from several lakes in northwestern Alaska may contain continuous records of the last interglaciation. A major warm interval, possibly Isotope Substage 5e, has been identified in a core from Squirrel Lake by a peak in Picea pollen that indicates forest extension beyond present limits. Similar pollen records are potentially available from two maars which formed in the Cape Espenberg area more than 125 ka. Terrestrial organic deposits thought to record the last interglaciation occur interstratified with marine and glaciogenic sediments in the Nushagak Lowland of southwest Alaska and on Baldwin Peninsula in Kotzebue Sound. Extensive exposures along the Copper and Nenana Rivers may also contain organic deposits that record the last interglaciation. ?? 1992.
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The Pearlette family ash beds in the Great Plains: Finding their identities and their roots in the Yellowstone country
For many years the numerous deposits of so-called 'Pearlette volcanic ash' in the Great Plains region of the United States were considered to be the remnants of the same volcanic event, and were used as a time-stratigraphic marker of probable Middle Pleistocene age. Although a few early workers had suggested that more than one air-fall event might be represented among the Pearlette occurrences, it was not until the latter half of the present century, after identification of volcanic ash beds by detailed chemical and mineralogical methods had been developed, that it could be established that the 'Pearlette family' of volcanic ashes included three ash beds of subtly differing characteristics. Development of isotopic methods of age determination has established that the ages of the three are significantly different (2.09, 1.29, and 0.60 Ma). The area of distribution of the Pearlette family ash beds was found to include not only the Great Plains, but also to extend across the Rocky Mountain and the Basin and Range provinces to the Pacific Ocean. The search for the sources of these three similar appearing ash beds, facilitated greatly by information gained from concurrent mapping projects underway in areas of major Late Cenozoic volcanic activity in western United States, ultimately led to the sites of the caldera-forming eruptions in the Yellowstone National Park region. ?? 1992.
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Fluvial deposits of Yellowstone tephras: Implications for late Cenozoic history of the Bighorn basin area, Wyoming and Montana
Several deposits of tephra derived from eruptions in Yellowstone National Park occur in the northern Bighorn basin area of Wyoming and Montana. These tephra deposits are mixed and interbedded with fluvial gravel and sand deposited by several different rivers. The fluvial tephra deposits are used to calculate stream incision rates, to provide insight into drainage histories and Quaternary tectonics, to infer the timing of alluvial erosion-deposition cycles, and to calibrate rates of soil development. ?? 1992.
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Correlation of Pliocene and Pleistocene tephra layers between the Turkana Basin of East Africa and the Gulf of Aden
Electron-microprobe analyses of glass shards from volcanic ash in Pliocene and Pleistocene deep-sea sediments in the Gulf of Aden and the Somali Basin demonstrate that most of the tephra layers correlate with tephra layers known on land in the Turkana Basin of northern Kenya and southern Ethiopia. Previous correlations are reviewed, and new correlations proposed. Together these data provide correlations between the deep-sea cores, and to the land-based sections at eight levels ranging in age from about 4 to 0.7 Ma. Specifically, we correlate the Moiti Tuff (???4.1 Ma) with a tephra layer at 188.6 m depth in DSDP hole 231 and with a tephra layer at 150 m depth in DSDP hole 241, the Wargolo Tuff with a tephra layer at 179.7 m in DSDP Hole 231 and with a tephra layer at 155.3 m depth in DSDP Hole 232, the Lomogol Tuff (defined here) with a tephra layer at 165 m in DSDP Hole 232A, the Lokochot Tuff with a tephra layer at 140.1 m depth in DSDP Hole 232, the Tulu Bor Tuff with a tephra layer at 160.8 m depth in DSDP Hole 231, the Kokiselei Tuff with a tephra layer at 120 m depth in DSDP Hole 231 and with a tephra layer at 90.3 m depth in DSDP Hole 232, the Silbo Tuff (0.74 Ma) with a tephra layer at 35.5 m depth in DSDP Hole 231 and possibly with a tephra layer at 10.9 m depth in DSDP Hole 241. We also present analyses of other tephra from the deep sea cores for which correlative units on land are not yet known. The correlated tephra layers provide eight chronostratigraphic horizons that make it possible to temporally correlate paleoecological and paleoclimatic data between the terrestrial and deep-sea sites. Such correlations may make it possible to interpret faunal evolution in the Lake Turkana basin and other sites in East Africa within a broader regional or global paleoclimatic context. ?? 1992.
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Modeling of ancient climate from deuterium content of water in volcanic glass
The explosive nature of the eruptions that produced rhyolitic tephras resulted in the ash being distributed over large areas. This ash, within a few thousand years after deposition, incorporated relatively large amounts of environmental water (up to 3.5%) into the glass structure. This hydration water is shown to retain its original deuterium concentration through time, and because the deuterium content of precipitation has been used for climate characterization, the hydration water, which is related to ancient precipitation, can be used to investigate ancient climates. Based on the analysis of ash samples dated at 13,700, 11,200 and 8500 BP, the climate in the states of Washington and Montana may have been about 3-6??C cooler at the end of the Pleistocene or Early Holocene than the present. We observe no change in the deuterium concentration of surface waters, and hence climate, in that region post-8500 BP. ?? 1992.
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Holocene coseismic and aseismic uplift of Isla Mocha, south-central Chile
During the past 6000 years Isla Mocha, a 12 km-long island 30 km off the coast of south-central Chile, experienced a 38 m fall of relative sea level caused primarily by rapid tectonic uplift of the island. As many as 18 raised shorelines (strandlines) record this uplift. Historic accounts of uplift during the great earthquakes (M > 8) of 1835 and 1960 suggest some of the more prominent prehistoric strandlines also emerged during great earthquakes on the interface between the Nazca and South America plates. But the close elevational spacing of strandlines, subdued morphology of strandline beaches, scarcity of exposed bedrock wave-cut platforms, and the extremely high rates of aseismic uplift (ca. 70 mm/yr) of the island since the last great earthquake suggest that many strandlines were raised by aseismic rather than coseismic uplift. Strandline heights and 14 new radiocarbon ages on marine shells show that the present-day uplift rate is more than three times the net rate (ca. 20 mm/yr) of the past 1000 years. The recent high rate probably reflects increased aseismic slip on an inferred thrust fault in the overriding South America plate. Isla Mocha overlies an area of high stress concentration between two major segments of the Chilean subduction zone. The inferred high rate of slip on the thrust fault may be a response to stress changes on the plate interface near the boundary between the segments. ?? 1992.
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Net Late Holocene emergence despite earthquake-induced submergence, south-central Chile
Intertidal deposits show net Late Holocene emergence at three sites along the Pacific coast near Maulli??n and Carelmapu, Chile (latitude 41.6-41.7??S.). The maximum amount of net emergence is ca. 1 m in the past 1500 years and ca. 2 1 2 m in the past 4000 years. Emergence probably would have prevailed at a fourth site near Maulli??n were the site not underlain by easily compacted deposits; this site shows slight (< 1 m) net submergence in the past 3000 years. Despite net emergence, all four sites underwent 1-2 m of submergence from tectonic subsidence during a magnitude-9.5 earthquake in 1960, and two of the sites show evidence for earlier submergence events of Late Holocene age. The net emergence is probably due to some combination of cyclic uplift from elastic strain accumulation between earthquakes and monotonic uplift from postglacial loading of the Pacific Ocean floor. The small amount of the net emergence shows that neotectonics has had little net impact on Holocene relative sea level in part of the focal region of the largest earthquake recorded by seismograph. ?? 1992.
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Late quaternary uplift rates of marine terraces on the Pacific coast of North America, southern Oregon to Baja California sur
Marine terraces are common landforms along the Pacific coast of North America from southern Oregon to southern Baja California. We report 60 new uranium-series ages of marine terrace corals, hydrocorals, and other fossils. Terraces representing the ca. 80 ka and ca. 125 ka high stands of sea are the most common along the Pacific coast and we have used these ages, along with terrace shoreline angle elevations and paleo-sea-level estimates, to calculate Late Quaternary uplift rates. Uplift rates in the forearc region of the Cascadia subduction zone range from 0.45-1.08 m/kyr. Farther south, along most of the coast west of the San Andreas fault zone, uplift rates range from 0.15-0.35 m/kyr, but some areas have significantly higher rates of uplift and others have apparently experienced little or no uplift in the last 125 ka. In Baja California Sur, west of the East Pacific Rise, little or no uplift has occurred during the Late Quaternary. On average, the magnitude of rates of uplift for the tectonic regimes is as follows: Cascadia subduction zone > San Andreas fault zone region > East Pacific Rise region. ?? 1992.
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Deposition of the late Wisconsin Johnstown moraine, south-central Wisconsin
The Johnstown moraine is made up of three different materials. The lower part of the sequence consists of closely spaced bodies of boulder gravel, which are cylindrical and plunge upglacier several degrees. They are interpreted to be esker bodies. They grade distally into proglacial outwash fans. The esker bodies are overlain by sandy till, some of which is uniform and some of which has colour layers. The colour layers are a few millimetres to a metre thick, drape over the eskers and dip upglacier several degrees. Both the uniform and layered till have pebbles plunging upglacier several degrees steeper than the dip of the layers. Both the uniform and layered till are interpreted to have been deposited subglacially, probably both by melting out and by lodgement. The till is overlain by a thin layer of boulder gravel interpreted to be washed supraglacial sediment. ?? 1993.
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Stratigraphy and origin of an area of hummocky glacial topography, northern Wisconsin, U.S.A.
The Winegar Phase of the Wisconsin Glaciation produced a band of hummocky glacial topography in northern Wisconsin. Some hummocks are composed of flow till and somewhat sorted till-like debris-flow sediment. These hummocks formed when ice melted beneath a cover of debris that flowed to its present position. Other hummocks contain interlayered lodgement and meltout till, flow till, debris-flow and slopewash sediment, lake, and meltwater-stream sediment. These hummocks formed where sediment was stacked near the ice margin. In other hummocks till and supraglacial debris-flow and slopewash sediment deposited during the Winegar Phase form a thin veneer over older meltwater-stream sediment. These hummocks formed when ice buried in pre-Winegar Phase stream sediment melted, and the overlying sediment collapsed. ?? 1993.
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Miocene and Pliocene lacustrine and fluvial sequences, Upper Ramparts and Canyon village, Porcupine river, east-central Alaska
Cenozoic strata exposed along the Porcupine River between the Upper Ramparts and Canyon Village, Alaska, can be divided into five unconformity-bounded units (sequences) which are: lower and middle Miocene unit A, the white sandy fluvial sequence with peat beds; middle Miocene unit B, the basalt sequence-part B1 is basalt, and part B2 is organic-rich sedimentary beds; upper Miocene unit C, mudrock-dominated lake sequence; late Miocene or Pliocene to Pleistocene unit D, terrace gravels, detrital organic matter and associated sediments, and Holocene unit E, mixed sand and gravel-rich sediment and other sedimentary material including peat and eolian silt. The sequence (unit A) of lower and middle Miocene fluvial deposits formed in streams and on flood plains, just before the inception of local volanism. Fossil pollen from unit A suggests conifer-dominated regional forests and cool temperate climates. Peat beds and lake deposits from unit B contain pollen that indicates a warmer temperate climate coinciding with the middle Miocene thermal maximum. The lake deposits (unit C) downstream from the basalts accumulated in a small basin which resulted from a hydrologic system that was dammed in the late Miocene but breached soon thereafter. The lower part of the terrace gravels (unit D) expresses breaching of the dammed hydrologic system (of unit C). The Porcupine River became a major tributary of the Yukon River in late Pleistocene time when Laurentide ice blocked drainage from the Yukon interior basins causing meltwater to spill over the low divide separating it from the Porcupine River drainage initiating erosion and capture of the Yukon interior basins. ?? 1994.
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40Ar 39Ar age constraints on neogene sedimentary beds, Upper Ramparts, half-way Pillar and Canyon village sites, Porcupine river, east-central Alaska
40 Ar/ 39 Ar ages of volcanic rocks are used to provide numerical constraints on the age of middle and upper Miocene sedimentary strata collected along the Porcupine River. Intercalated sedimentary rocks north of latitude 67°10′N in the Porcupine terrane of east-central Alaska contain a rich record of plant fossils. The fossils are valuable indicators of this interior region's paleoclimate during the time of their deposition. Integration of the 40 Ar/ 39 Ar 40 Ar 39 Ar "> results with paleomagnetic and sedimentological data allows for refinements in estimating the timing of deposition and duration of selected sedimentary intervals. 40 Ar 39 Ar "> 40Ar39Ar plateau age spectra, from whole rock basalt samples, collected along the Upper Ramparts and near Half-way Pillar on the Porcupine River, range from 15.7 ± 0.1 Ma at site 90-6 to 14.4 ± 0.1 Ma at site 90-2. With exception of the youngest basalt flow at site 90-2, all of the samples are of reversed magnetic polarity, and all 40 Ar/ 39 Ar age spectrum results are consistent with the deposition of the entire stratigraphic section during a single interval of reversed magnetic polarity. The youngest flow at site 90-2 was emplaced during an interval of normal polarity. With age, paleomagnetic and sedimentological data, the ages of the Middle Miocene sedimentary rocks between the basalt flows at sites 90-1 and 90-2 can be assigned to an interval within the limits of analytical precision of 15.2 ± 0.1 Ma; thus, the sediments were deposited during the peak of the Middle Miocene thermal maximum. Sediments in the upper parts of sites 90-1 and 90-2 were probably deposited during cooling from the Middle Miocene thermal maximum. 40 Ar/ 39 Ar results of plagioclase and biotite from a single tephra, collected at sites 90-7 and 90-8 along the Canyon Village section of the Porcupine River, indicate an age of 6.57 ± 0.02 Ma for its time of eruption and deposition. These results, together with sedimentological and paleomagnetic data, suggest that all of the Upper Miocene lacustrine sedimentary rocks at these sites were deposited during a single interval of reversed magnetic polarity and may represent a duration of only about 40,000 years. The age of this tephra corresponds with a late late Miocene warm climatic interval. The results from the Upper Ramparts and Half-way Pillar sites are used to estimate a minimum interval of continental flood basalt activity of 1.1–1.5 million years, and to set limits for the timing and duration of Tertiary extensional tectonic activity in the Porcupine terrane. Our data indicate that the oroclinal flexure that formed before the deposition of the basalts at the eastern end of the Brooks Range was created prior to 15.7 ± 0.1 Ma.
Alaska↗
Palynology, paleoclimatology and correlation of middle Miocene beds from Porcupine River (locality 90-1), Alaska
Beds in the Upper Ramparts Canyon of the Porcupine River, Alaska (67?? 20' N, 141?? 20' W), yielded a flora rich in pollen of hardwood genera now found in the temperate climates of North America and Asia. The beds are overlain or enclosed by two basalt flows which were dated to 15.2 ?? 0.1 Ma by the 40Ar 39Ar method, fixing the period of the greatest abundance of warm-loving genera to the early part of the middle Miocene. The assemblage is the most northern middle Miocene flora known in Alaska. Organic bed 1 underlies the basalt and is older than 15.2 Ma, but is of early to middle Miocene age. The pollen assemblage from organic bed 1 is dominated by conifer pollen from the pine and redwood-cypress-yew families with rare occurrences of temperate hardwoods. Organic bed 2 is a forest floor containing redwood trees in life position, engulfed by the lowest basalt flow. A pine log has growth rings up to 1 cm thick. Organic beds 3 and 4 comprise lacustrine sediment and peat between the two basalt flows. Their palynoflora contain conifers and hardwood genera, of which about 40% have modern temperate climatic affinities. Hickory, katsura, walnut, sweet gum, wingnut, basswood and elm pollen are consistently present, and beech and oak alone make up about 20% of the pollen assemblage. A warm high latitude climate is indicated for all of the organic beds, but organic bed 3 was deposited under a time of peak warmth. Climate data derived by comparison with modern east Asian vegetation suggest that, at the time of deposition of organic bed 3, the Mean Annual Temperature (MAT) was ca. 9??C, the Warm Month Mean Temperature (WMMT) was ??? 20??C and the Cold Month Mean Temperature (CMMT) was ca. -2??C. In contrast, the modern MAT for the region is -8.6??C, WMMT is 12.6??C and CMMT is -28??C. Organic beds 3 and 4 correlate to rocks of the middle Miocene-late Seldovian Stage of Cook Inlet and also probably correlate to, and more precisely date, the lower third of the Suntrana Formation in the Alaska Range, beds at Unalaklect, part of the upper Mackenzie Bay sequence in the Beaufort-Mackenzie basin, and the Mary Sachs gravel of Banks Island. This suggests that forests with significant percentages of temperate deciduous angiosperms existed between latitudes 60?? and 72??N during the early middle Miocene. ?? 1994.
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Pliocene terrace gravels of the ancestral Yukon River near Circle, Alaska: Palynology, paleobotany, paleoenvironmental reconstruction and regional correlation
Gravels deposited by the ancestral Yukon River are preserved in terrace remnants on the margins of the Yukon River valley near the village of Circle in east-central Alaska. Plant fossils recovered from sandy silt lenses within these gravels include cones and needles of Picea and Larix and a variety of seeds. Seed types include several taxa which no longer grow in Alaska, such as Epipremnum, Prunus and Weigela. Pollen types recovered from these deposits represent tree and shrub taxa that grow in interior Alaska today, such as Picea, Larix, Betula and Alnus, as well as several taxa that no longer grow in interior Alaska today, such as Pinus, Tsuga, Abies and Corylus. Pollen of herb taxa identified include Gramineae, Cyperaceae, Caryophyllaceae, Compositae, Polemonium and Epilobium. The fossil flora from the gravels near Circle are similar and probably age-equivalent to the flora recovered from the Nenana Gravel in the Alaska Range 250 km to the south. Palynological and tectonic evidence summarized in this paper now suggests that the Nenana Gravel was deposited during the early and middle Pliocene. The presence of plant fossils of Tsuga, Abies, Pinus, Weigela and Prunus suggests that the mean annual temperature (MAT) of eastern interior Alaska during the early and middle Pliocene was perhaps 7-9??C warmer and less continental than today's MAT of -6.4??C. ?? 1994.
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