Rates of erosion and landscape change along the Blue Ridge escarpment, southern Appalachian Mountains, estimated from in situ cosmogenic 10Be

The Blue Ridge escarpment, located within the southern Appalachian Mountains of Virginia and North Carolina, forms a distinct, steep boundary between the lower-elevation Piedmont and higher-elevation Blue Ridge physiographic provinces. To understand better the rate at which this landform and the adjacent landscape are changing, we measured cosmogenic 10Be in quartz separated from sediment samples (n = 50) collected in thirty-two streams and from three exposed bedrock outcrops along four transects normal to the escarpment, allowing us to calculate erosion rates integrated over 104–105 years. These basin-averaged erosion rates (5.4–49 m My-1) are consistent with those measured elsewhere in the southern Appalachians and show a positive relationship between erosion rate and average basin slope. Erosion rates show no relationship with basin size or relative position of the Brevard fault zone, a fundamental structural element of the region. The cosmogenic isotopic data, when considered along with the distribution of average basin slopes in each physiographic province, suggest that the escarpment is eroding on average more rapidly than the Blue Ridge uplands, which are eroding more rapidly than the Piedmont lowlands. This difference in erosion rates by geomorphic setting suggests that the elevation difference between the uplands and lowlands adjacent to the escarpment is being reduced but at extremely slow rates

A loess–paleosol record of climate and glacial history over the past two glacial–interglacial cycles (~150 ka), southern Jackson Hole, Wyoming

Loess accumulated on a Bull Lake outwash terrace of Marine Oxygen Isotope Stage 6 (MIS 6) age in southern Jackson Hole, Wyoming. The 9 m section displays eight intervals of loess deposition (Loess 1 to Loess 8, oldest), each followed by soil development. Our age-depth model is constrained by thermoluminescence, meteoric 10Be accumulation in soils, and cosmogenic 10Be surface exposure ages. We use particle size, geochemical, mineral-magnetic, and clay mineralogical data to interpret loess sources and pedogenesis. Deposition of MIS 6 loess was followed by a tripartite soil/thin loess complex (Soils 8, 7, and 6) apparently re!ecting the large climatic oscillations of MIS 5. Soil 8 (MIS 5e) shows the strongest development. Loess 5 accumulated during a glacial interval (~76–69 ka; MIS 4) followed by soil development under conditions wetter and probably colder than present. Deposition of thick Loess 3 (~43–51 ka, MIS 3) was followed by soil development comparable with that observed in Soil 1. Loess 1 (MIS 2) accumulated during the Pinedale glaciation and was followed by development of Soil 1 under a semiarid climate. This record of alternating loess deposition and soil development is compatible with the history of Yellowstone vegetation and the glacial !our record from the Sierra Nevada

Constraints on the Geologic History of the Karst System in Southern Missouri, U.S.A. provided by Radiogenic, Cosmogenic, and Physical/Chemical Characteristics of Doline Fill

The Ozark Plateaus region of southern Missouri is underlain by dominantly carbonate marine platform rocks of Paleozoic age. The region has been sub-aerially exposed since the late Paleozoic and is characterized by extensive karst. To better understand the geologic history of this regional karst system, we examined the stratigraphic record preserved in the fill of a large doline near the largest spring in the region. Samples of fill from natural exposures and drill core were analyzed using thermoluminescence (TL) and 10Be cosmogenic techniques, and the physical/chemical characteristics of the fill material were determined by visual inspection, X-ray analyses, and grain-size measurements. Drill-hole data indicate that the allochthonous doline fill is 36.3 m thick and rests on at least 15.6 m of cave breakdown and sediment. The doline fill is divisible into 7 zones. Analysis of 10Be concentrations suggest that the entire doline fill was derived from local residuum during the middle (Illinoian) to late Pleistocene (Wisconsinan). X-ray diffraction analyses of clays throughout the doline fill indicate that they consist of nearly equal amounts of kaolinite and illite, consistent with terrestrial weathering

Stratigraphy and palaeoclimatic significance of Late Quaternary loess–palaeosol sequences of the Last Interglacial–Glacial cycle in central Alaska

Loess is one of the most widespread subaerialdeposits in Alaska and adjacent Yukon Territory and may have a history that goes back 3 Ma. Based on mineralogy and major and trace element chemistry, central Alaskan loess has a composition that is distinctive from other loess bodies of the world, although it is quartz-dominated. Central Alaskan loess was probably derived from a variety of rock types, including granites, metabasalts and schists. Detailed stratigraphic data and pedologic criteria indicate that, contrary to early studies, many palaeosols are present in central Alaskan loess sections. The buried soils indicate that loess sedimentation was episodic, or at least rates of deposition decreased to the point where pedogenesis could keep ahead of aeolian input. As in China, loess deposition and pedogenesis are likely competing processes and neither stops completely during either phase of the loess/soil formation cycle. Loess deposition in central Alaska took place before, and probably during the last interglacial period, during stadials of the mid-Wisconsin period, during the last glacial period and during the Holocene. An unexpected result of our geochronological studies is that only moderate loess deposition took place during the last glacial period. Our studies lead us to conclude that vegetation plays a key role in loess accumulation in Alaska. Factors favouring loess production are enhanced during glacial periods but factors that favour loess accumulation are diminished during glacial periods. The most important of these is vegetation; boreal forest serves as an effective loess trap, but sparsely distributed herb tundra does not. Thus, thick accumulations of loess should not be expected where tundra vegetation was dominant and this is borne out by modern studies near the treeline in central Alaska. Much of the stratigraphic diversity of North American loess, including that found in the Central Lowlands, the Great Plains, and Alaska is explained by a new model that emphasizes the relative importance of loess production factors versus loess accumulation factors

Erosion rates and sediment flux within the Potomac River basin quantified over millennial timescales using beryllium isotopes

Beryllium isotopes measured in detrital river sediment are often used to estimate rates of landscape change at a basin scale, but results from different beryllium isotope systems have rarely been compared. Here, we report measurements of in situ and meteoric 10Be (10Bei and 10Bem, respectively) along with measurements of reactive and mineral phases of 9Be (9Bereac and 9Bemin, respectively) to infer long-term rates of landscape change in the Potomac River basin, North America. Using these data, we directly compare results from the two different 10Be isotope systems and contextualize modern sediment flux from the Potomac River basin to Chesapeake Bay. Sixty-two measurements of 10Bei in river sand show that the Potomac River basin is eroding on average at 29.6 ± 14.1 Mg km-2 yr-1 (11 ± 5.2 m m.y.-1 assuming a rock density of 2700 kg m-3)-a rate consistent with other estimates in the mid-Atlantic region. 10Bei erosion rates correlate with basin latitude, suggesting that periglacial weathering increased with proximity to the former Laurentide Ice Sheet margin. Considering the 10Bei-derived erosion rate as a sediment flux over millennia, rates of sediment delivery from the Potomac River to Chesapeake Bay are up to ~5× lower than contemporary sediment yields implying modern land-use practices have accelerated erosion and sediment transport over background rates. However, 10Bei erosion rate data suggest that regulatory benchmark levels used to manage sediment export from the Potomac River basin to Chesapeake Bay are set appropriately to reduce sedimentation and restore the Bay\u27s ecological health. The mean of 56 10Bem/9Bereac-derived denudation rates (40.0 ± 21.7 Mg km-2 yr-1) is higher than, but statistically indistinguishable from, the mean 10Bei erosion rate (29.6 ± 14.1 Mg km-2 yr-1; p = 0.003). However, when considered basin by basin, 10Bem/9Bereac-determined denudation rates are only weakly correlated (R2 = 0.208; p \u3c 0.001) with sediment fluxes determined from the well-established and widely used 10Bei technique. This suggests that the 10Bem/9Bereac technique may not reflect the same geomorphic processes as 10Bei technique, or that the 10Bem/9Bereac technique operates over different time and/or depth scales. Erosion indices (EIs, sensu Brown et al., 1988) derived from 10Bem measurements and contemporary sediment yield data range from 0.07 to 1.24; 75% of basins sampled have EIs that are \u3e 1, suggesting that 10Bem is being retained and sediment is being stored within the Potomac River basin. The Appalachian Plateau is the only physiographic province where sediment export dominates, likely as the result of ongoing relief growth in catchments draining the Appalachian Mountain divide. 10Bem concentrations measured in the 150 k.y. Hybla Valley sediment core, taken from the lower Potomac River basin, suggest that 10Bem and sediment are preferentially stored in the catchment when vegetation proxies for climate suggest warmer conditions prevailed. 10Bem and sediment are exported when vegetation proxies for climate suggest conditions are colder, perhaps a reflection of periglacial activity or changes in storm frequency and/or magnitude over glacial-interglacial cycles