Along the South Fork of the Eel River in northern California, paleoerosion rates derived from 10 Be concentrations in late Pleistocene strath terrace sediment are a factor of 2 greater than erosion rates derived from modern stream sediment and 3.5 times greater than paleoerosion rates from the Pleistocene-Holocene transition. Using these results as a proxy for sediment supply, we provide quantitative fi eld-based evidence that extensive strath planation is linked to elevated sediment supply conditions. We have used optically stimulated luminescence (OSL) to date strath terrace sediment and fi nd that the highest erosion rates and most extensive period of strath planation occurred during a period of increased precipitation in the late Pleistocene. Based on our OSL data, we estimate that bedrock channel lowering rates have outpaced basin-averaged erosion rates by a factor of three since abandonment of the extensive late Pleistocene strath surface. Thus, our data indicate that hillslope relief has been increasing for the past ~20 ka.
Glacial Lake Agassiz has been implicated as the trigger for numerous episodes of abrupt climate change at the close of the last ice age, yet the beginning age of the lake has never been determined. Here we report the fi rst numerical age data on the Big Stone Moraine and the oldest beaches of glacial Lake Agassiz. Organic remains from lakes, bogs, and channels distal to, and inset to, the Big Stone Moraine require that glacial activity at this moraine ceased prior to 12,000 14 C yr B.P. (13,950 cal [calendar] yr). A site near New Effi ng ton, South Dakota (United States), implies full glacial recession north of the topographic divide prior to 11,810 14 C yr B.P. (13,670 cal yr), synchronous with the beginning of glacial Lake Agassiz. Lake Agassiz shorelines inset to the moraine yield optically stimulated luminescence (OSL) ages from 14,200-12,600 yr cal. Lower strandlines are younger, but the similarity of ages suggests that initial lake lowering was faster than OSL ages can currently resolve. Nevertheless, the OSL ages represent the fi rst numerical age assignments for the Herman, Norcross, and Upham beach ridges, setting the stage for future numerical age assignments within the Lake Agassiz basin. These two dating methods yield strongly consistent results within stated uncertainties. The age of the Big Stone Moraine implies an interval of rapid retreat for the Des Moines lobe of the Laurentide Ice Sheet during the Bölling-Alleröd warm interval. The overlapping ages for the uppermost beach levels and abandonment of the highest Lake Agassiz spillway indicate a rapidly evolving lake until at least 13,500 yr cal.
The advent of single aliquot optically stimulated luminescence (OSL) techniques along with parallel equipment advances have made it possible and practical to obtain statistically meaningful quantities of equivalent dose (De) data from individual samples. Now that hundreds of dose determinations can be made for one sample De distributions may be scrutinised, leading to the problem of how to make unbiased comparisons among distributions, and how to decide objectively which dose is representative of the age of the deposit. In this paper, an objective analytical method is presented for treating dose distributions, including a mathematically rigorous means of determining a representative equivalent dose and a statistical definition of its corresponding uncertainty. This analytical method has been applied to Holocene aeolian and fluvial quartz sands from Central Oklahoma. The results are compared to equivalent doses determined via other proposed analytical procedures.
Using optically stimulated luminescence (OSL) analysis we obtained depositional ages ranging from 25 ± ± ± ± ± 10 to 928 ± ± ± ± ± 144 years before present for sediments deposited in oxbow lakes along three lowland river systems. The dated sediments were collected from the banks of tie channels along the Lower Mississippi River, the Fly River in Papua New Guinea, and Birch Creek along the Yukon River in Alaska. Tie channels connect the oxbow lakes to the main stem river and allow the exchange of water and suspended sediment between the two. The banks consist of fine sand and sandy silt beds interlayered with silt and clay. OSL samples were collected both horizontally from exposed banks and vertically by coring through levee crests; sample collection was targeted at beds containing appreciable quantities of fine sand. OSL ages were determined using single-grain or in some cases single-aliquot techniques and dose distribution analysis. Samples were first collected along the Lower Mississippi tie channel to compare OSL dates with historical data sources and test the applicability of OSL in these settings; the OSL dates agreed closely with historical data. In all three river systems, OSL dating allowed the determination of vertical accretion rates, tie channel advancement rates, and oxbow lake ages. In Papua New Guinea, OSL sampling also provides an estimate of lateral migration rates of the Fly River and allows a comparison of modern mineinfluenced deposition rates with natural background rates over the last 1000 years. Results from Papua New Guinea and the Mississippi River suggest that the advancement rate of tie channels responds directly to changes in the sediment load of the main stem river.
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