A late Holocene onset of Aboriginal burning in southeastern Australia

The extent to which Aboriginal Australians used fire to modify their environment has been debated for decades and is generally based on charcoal and pollen records rather than landscape responses to land-use change. Here we investigate the sensitivity of in-situ–produced 10Be, an isotope commonly used in geomorphological contexts, to anthropogenic perturbations in the southeastern Australian Tablelands. Comparing 10Be-derived erosion rates from fluvial sediment (8.7 ± 0.9 mm k.y.–1; 1 standard error, SE; n = 11) and rock outcrops (5.3 ± 1.4 mm k.y.–1; 1 SE; n = 6) confirms that landscape lowering rates integrating over 104–105 yr are consistent with rates previously derived from studies integrating over 104 to >107 yr. We then model an expected 10Be inventory in fluvial sediment if background erosion rates were perturbed by a low-intensity, high-frequency Aboriginal burning regime. When we run the model using the average erosion rate derived from 10Be in fluvial sediment (8.7 mm k.y.–1), measured and modeled 10Be concentrations overlap between ca. 3 ka and 1 ka. Our modeling is consistent with intensified Aboriginal use of fire in the late Holocene, a time when Aboriginal population growth is widely recognized

Directly dating postglacial Greenlandic land-surface emergence at high resolution using in situ 10Be

Postglacial emergence curves are used to infer mantle rheology, delimit ice extent, and test models of the solid Earth response to changing ice and water loads. Such curves are rarely produced by direct dating of land emergence; rather, most rely on the presence of radiocarbon-datable organic material and inferences made between the age of sedimentary deposits and landforms indicative of former sea level. Here, we demonstrate a new approach, 10Be dating, to determine rates of postglacial land emergence in two different settings. In southern Greenland (Narsarsuaq/Igaliku), we date directly the exposure, as relative sea level fell, of gravel beaches and rocky outcrops allowing determination of rapid, post–Younger Dryas emergence. In western Greenland (Kangerlussuaq), we constrain Holocene isostatic response by dating the sequential stripping of terrace sediment driven by land-surface uplift, relative sea-level fall, and resulting fluvial incision. The technique we employ provides high temporal and elevation resolution important for quantifying rapid emergence immediately after deglaciation and less rapid uplift during the middle Holocene. 10Be-constrained emergence curves can improve knowledge of relative sea-level change by dating land emergence along rocky coasts, at elevations and locations where radiocarbon-datable sediments are not present, and without the lag time needed for organic material to accumulate

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’s 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 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. GeoRef Subject alluvium Appalachians Central Appalachians Eastern U.S. alkaline earth metals clastic sediments Be-10/Be-9 beryllium North America Potomac River basin geomorphology isotopes isotope ratios metals sediments radioactive isotopes sand stable isotopes United State