Lucas J. Reusser

Quantifying human impacts on rates of erosion and sediment transport at a landscape scale

Establishing background (geologic) rates of erosion is prerequisite to quantifying the impact of human activities on Earth’s surface. Here, we present 10 Be estimates of background erosion rates for ten large (10,000–100,000 km 2 ) river basins in the southeastern United States, an area that was cleared of native forest and used intensively for agriculture. These 10 Be-based rates are indicative of the pace at which the North American passive-margin landscape eroded before European settlement (∼8 m/m.y.). Comparing these background rates to both rates of post-settlement hillslope erosion and to river sediment yields for the same basins, we find that following peak disturbance (late 1800s and early 1900s), rates of hillslope erosion (∼950 m/m.y.) exceeded 10 Be-determined background rates more than one-hundred fold. Although large-basin sediment yields during peak disturbance increased 5–10× above pre-settlement norms, rivers at the time were transporting only ∼6% of the eroded material; work by others suggests that the bulk of historically eroded material remained and still remains as legacy sediment stored at the base of hillslopes and along valley bottoms. Because background erosion rates, such as we present here, reflect the rate at which soil is generated over millennial time scales, they can inform and enhance landscape-management strategies.

Using meteoric 10Be to track fluvial sand through the Waipaoa River basin, New Zealand

We use meteoric 10 Be measured in 24 fluvial sand samples collected along the mainstem and from prominent tributaries within the tectonically active Waipaoa River basin, New Zealand, to identify sediment sources and monitor the mixing of sediment as it travels from headwater basins to the sea. Deforestation for agriculture beginning in the early 1900s resulted in severe, but nonuniformly distributed erosion. Tributaries in the northern headwaters, where large amphitheater gullies that continually feed prodigious amounts of deeply sourced sediment to the mainstem are prevalent, yield exceptionally low concentrations of meteoric 10 Be (∼1.5 × 10 6 atoms g −1 ). In the more stable eastern and western tributaries, concentrations of meteoric 10 Be are nearly an order of magnitude greater (∼14 × 10 6 atoms g −1 ). Meteoric 10 Be concentrations in samples collected along the mainstem above and below tributary confluences steadily and regularly increase downstream (R 2 = 0.92) as large amounts of low-concentration gully-derived sediments are augmented with higher concentration sediment from more stable tributaries, providing strong evidence that meteoric 10 Be concentrations reflect sediment sourcing in this fluvial network. A two-component mixing model indicates that the gullied northern region of the Waipaoa basin produces sediment at a rate ∼20 times that of the eastern and western regions. These results suggest that meteoric 10 Be, in contrast to the widely applied in situ technique that is limited by the availability and distribution of quartz, is an effective tool for the rapid assessment of sediment dynamics and movement within a wide range of fluvial networks.

Calibrating a long‐term meteoric 10Be accumulation rate in soil

[1] Using 13 samples collected from a 4.1 meter profile in a well-dated and stable New Zealand fluvial terrace, we present the first long-term accumulation rate for meteoric 10 Be in soil (1.68 to 1.72 × 10 6 at/(cM 2 ·yr)) integrated over the past ~18 ka. Site-specific accumulation data, such as these, are prerequisite to the application of meteoric 10 Be in surface process studies. Our data begin the process of calibrating long-term meteoric 10 Be delivery rates across latitude and precipitation gradients. Our integrated rate is lower than contemporary meteoric 10 Be fluxes measured in New Zealand rainfall, suggesting that long-term average precipitation, dust flux, or both, at this site were less than modern values. With accurately calibrated long-term delivery rates, such as this, meteoric 10 Be will be a powerful tool for studying rates of landscape change in environments where other cosmogenic nuclides, such as in situ 10 Be, cannot be used.

Incorporating Concept Sketching Into Teaching Undergraduate Geomorphology

ABSTRACT Constructing concept sketches (diagrams annotated with short captions in which students demonstrate their understanding of form, process, and interactions) provides a new and different way to teach Earth surface processes and assess the depth of student learning. During a semester-long course in Geomorphology, we used concept sketches as an icebreaker, as a means to help students place field observations in a spatial context, and as a catalyst for understanding complex graphical presentations of data. For the mid-term and final assessment components of the course, we required students to consider a historic aerial photograph of a local site they had not visited previously in order to strengthen their abilities in landscape interpretation based upon imagery alone; a task many of them will be required to undertake in their future endeavors. Anecdotal student response to the use of concept sketches in Geomorphology was uniformly positive with students self-reporting that the sketches helped them to synthesize large amounts of seemingly disparate information. As instructors, we found concept sketches particularly useful for motivating students and for identifying misconceptions and knowledge gaps.