Darryl E. Granger

Spatially Averaged Long-Term Erosion Rates Measured from in Situ-Produced Cosmogenic Nuclides in Alluvial Sediment

Spatially averaged erosion rates of small catchments can be accurately inferred from the concentrations of cosmogenic nuclides in stream sediment. Here we test this technique at two catchments by comparing erosion rates inferred from cosmogenic nuclides with rates of alluvial fan deposition over the past 16,000 years. These two independent estimates agree within one standard error, demonstrating that cosmogenic nuclide signatures of stream sediment can be used to measure spatially averaged long-term erosion rates. Using this technique, we show that long-term erosion rates are an exponential function of average hillslope gradient at these sites.

The first hominin of Europe

The earliest hominin occupation of Europe is one of the most debated topics in palaeoanthropology. However, the purportedly oldest of the Early Pleistocene sites in Eurasia lack precise age control and contain stone tools rather than human fossil remains. Here we report the discovery of a human mandible associated with an assemblage of Mode 1 lithic tools and faunal remains bearing traces of hominin processing, in stratigraphic level TE9 at the site of the Sima del Elefante, Atapuerca, Spain. Level TE9 has been dated to the Early Pleistocene (approximately 1.2–1.1 Myr), based on a combination of palaeomagnetism, cosmogenic nuclides and biostratigraphy. The Sima del Elefante site thus emerges as the oldest, most accurately dated record of human occupation in Europe, to our knowledge. The study of the human mandible suggests that the first settlement of Western Europe could be related to an early demographic expansion out of Africa. The new evidence, with previous findings in other Atapuerca sites (level TD6 from Gran Dolina), also suggests that a speciation event occurred in this extreme area of the Eurasian continent during the Early Pleistocene, initiating the hominin lineage represented by the TE9 and TD6 hominins.

Mountain erosion over 10 yr, 10 k.y., and 10 m.y. time scales

We used cosmogenic 10 Be to measure erosion rates over 10 k.y. time scales at 32 Idaho mountain catchments, ranging from small experimental watersheds (0.2 km 2 )t o large river basins (35 000 km 2 ). These long-term sediment yields are, on average, 17 times higher than stream sediment fluxes measured over 10‐84 yr, but are consistent with 10 m.y. erosion rates measured by apatite fission tracks. Our results imply that conventional sediment-yield measurements—even those made over decades—can greatly underestimate long-term average rates of sediment delivery and thus overestimate the life spans of engineered reservoirs. Our observations also suggest that sediment delivery from mountainous terrain is extremely episodic, sporadically subjecting mountain stream ecosystems to extensive disturbance.

Beyond threshold hillslopes: Channel adjustment to base-level fall in tectonically active mountain ranges

Numerous empirical and model-based studies argue that, in general, hillslopes and river channels increase their gradients to accommodate high rates of base-level fall. To date, however, few data sets show the dynamic range of both these relationships needed to test theoretical models of hillslope evolution and river incision. Here, we utilize concentrations of 10 Be in quartz extracted from river sand on the eastern margin of the Tibetan Plateau to explore relationships among short-term (10 2 –10 5 a) erosion rate, hillslope gradient, and channel steepness. Our data illustrate nonlinear behavior and a threshold in the relationship between erosion rate and mean hillslope gradient, confi rming the generalization that hillslopes around the world are limited by slope stability and cease to provide a metric for erosion at high rates (>~0.2 mm/a). The relationship between channel steepness index and erosion rate is also nonlinear, but channels continue to steepen beyond the point where threshold hillslopes emerge up to at least 0.6 mm/a, demonstrating that channel steepness is a more reliable topographic metric than mean hillslope gradient for erosion rate and that channels ultimately drive landscape adjustment to increasing rates of base-level fall in tectonically active settings.

Dating sediment burial with in situ-produced cosmogenic nuclides: theory, techniques, and limitations

Abstract Dating sediment burial over million-year time scales is crucial in many areas of the Earth sciences and archeology, but is often difficult using traditional techniques. Sediment burial can be dated by the radioactive decay of cosmogenic nuclides, provided that the sediment was exposed to cosmic rays prior to burial. Dating calculations are straightforward if sediment is buried deeply and rapidly enough to prevent cosmogenic nuclide production after burial. However, the analysis can be complicated by postburial production if sediment is insufficiently shielded from secondary cosmic-ray nucleons and muons. This paper discusses how buried sediments can be dated over timescales up to 5 Myr using 26 Al and 10 Be in quartz.

Strong tectonic and weak climatic control of long-term chemical weathering rates

The relationships among climate, physical erosion, and chemical weathering have remained uncertain, because long-term chemical weathering rates have been difficult to measure. Here we show that long-term chemical weathering rates can be measured by combining physical erosion rates, inferred from cosmogenic nuclides, with dissolution losses, inferred from the rock-to-soil enrichment of insoluble elements. We used this method to measure chemical weathering rates across 22 mountainous granitic catchments that span a wide range of erosion rates and climates. Chemical weathering rates correlate strongly with physical erosion rates but only weakly with climate, implying that, by regulating erosion rates, tectonic uplift may significantly accelerate chemical weathering rates in granitic landscapes.

Pliocene−Pleistocene incision of the Green River, Kentucky, determined from radioactive decay of cosmogenic 26Al and 10Be in Mammoth Cave sediments

Cosmogenic 26Al and 10Be in sediments washed into Mammoth Cave, Kentucky, record the history of 3.5 m.y. of water-table position, governed by incision and aggradation of the Green River, a tributary of the Ohio River. Upper levels of the cave formed during a period of slow river incision and were later filled with sediment due to river aggradation at 2.3–2.4 Ma. A brief surge of river incision ca. 2 Ma was followed by river stability and cave-passage formation at a lower level. Rapid incision through 15 m of bedrock ca. 1.5 Ma was prompted by repositioning of the Ohio River to its present course along an ice-sheet margin. Renewed incision ca. 1.2 Ma and aggradation at 0.7–0.8 Ma correlate with major ice advances in the Ohio River basin. Measurements of 26Al and 10Be also indicate that sandstone-capped uplands have maintained slow erosion rates of 2–7 m/m.y. for the past 3.5 m.y., despite accelerated Pleistocene river incision rates of ∼30 m/m.y.

Minimal climatic control on erosion rates in the Sierra Nevada, California

Climate is widely thought to regulate erosion rates, but the relationships among precipitation, temperature, and erosion rate have remained speculative, because long-term erosion rates have been difficult to measure. We used cosmogenic nuclides to measure long-term erosion rates at climatically diverse sites in the Sierra Nevada, California, spanning 20‐180 cm/yr in annual precipitation and 4‐15 8C in mean annual temperature. Average erosion rates vary by only 2.5 fold across these sites and are not correlated with climate, indicating that climate only weakly regulates nonglacial erosion rates in mountainous granitic terrain.

Dating buried sediments using radioactive decay and muogenic production of 26Al and 10Be

Abstract Cosmogenic [26Al] and [10Be] depth profiles can be used to date river terrace sediments over million-year timescales, provided that production rates are well constrained with depth. Production by nucleon spallation, negative muon capture, and fast muon reactions can be approximated as exponential functions of depth, up to ∼5000 g cm−2. These production rates are used to date a ∼10 m profile of eroded terrace sediments to 1.50+0.32/−0.25 Ma. The measured [10Be] profile and the terrace age agree well with expectations, demonstrating feasibility of the technique.

Abrupt glacial valley incision at 0.8 Ma dated from cave deposits in Switzerland

Glacial erosion dramatically alters mountain landscapes, but the pace at which glaciers carve a previously fluvial landscape remains poorly defined because long-term valley incision rates are difficult to measure. Here we reconstruct the lowering history of the Aare Valley, Switzerland, over the past 4 m.y. by dating cave sediments with cosmogenic 26Al and 10Be. Incision accelerated from ∼120 m/m.y. to ∼1200 m/m.y. at 0.8–1.0 Ma, at least 1 m.y. after the onset of local glaciation. Rapid incision may have been triggered by lowering of the equilibrium line altitude at the mid-Pleistocene climate transition.