Andrew J. Cyr

Quantifying rock uplift rates using channel steepness and cosmogenic nuclide–determined erosion rates: Examples from northern and southern Italy

Rock uplift rates can be difficult to measure over 10 3 –10 5 yr time scales. If, however, a landscape approaches steady state, where hillslope erosion and rock uplift rates are steady and locally similar, then it should be possible to quantify rock uplift rates from hillslope erosion rates. Here, we test this prediction by comparing channel steepness index values and 10 Be catchment-averaged erosion rates to well-constrained rock uplift rates in two landscapes in Italy. The first field area is the Romagna Apennines, northern Italy, where rock uplift rates are relatively uniform, between 0.2 and 0.5 mm/yr (regional mean 0.40 ± 0.15 [SE] mm/yr), and have been steady since 0.9 Ma. The second area is the region around northeastern Sicily and the southernmost Italian peninsula, where rock uplift rates are higher and exhibit a strong spatial gradient, from ∼0.7 to ∼1.6 mm/yr (regional mean 1.09 ± 0.13 [SE] mm/yr). In both regions, channel steepness indices and 10 Be erosion rates vary directly with rock uplift rates. Although there is considerable variability in erosion rates, regionally averaged rates in both the northern (0.46 ± 0.04 [SE] mm/yr) and southern (1.21 ± 0.24 [SE] mm/yr) areas accurately measure rock uplift rates. Although channel steepness indices do not quantify rock uplift rates, they are useful for (1) identifying regional patterns of rock uplift, (2) identifying areas where uplift rates might be expected to be uniform, and (3) informing 10 Be sampling strategies. This study demonstrates that, together, channel steepness and hillslope erosion rates can provide a powerful tool for determining rock uplift rates.

Geochemical evaluation of fenghuoshan group lacustrine carbonates, north-central Tibet : Implications for the paleoaltimetry of the eocene Tibetan plateau

A sedimentologic, petrologic, and geochemical evaluation of lacustrine carbonates from the Eocene‐Oligocene Fenghuoshan Group of north‐central Tibet provides insight into the paleoenvironmental and paleolimnological setting of the Hoh Xil basin during the mid‐Tertiary. Fenghuoshan lacustrine rocks consist primarily of carbonate mudstones and wackestones. These carbonates are generally less than 3 m in thickness and are intercalated with siliciclastic lacustrine and fluvial/alluvial plain deposits. Individual limestone beds coarsen and contain increasing amounts of siliciclastic material upward. Fenghuoshan carbonates also contain abundant ostracode, bivalve, and charaphyte fossils. Sedimentologic evidence suggests that carbonate deposition occurred in shallow, relatively short‐lived lacustrine systems that were subsequently filled by alluvial deposits of coeval fluvial systems. Stable C and O isotopic analysis of Fenghuoshan carbonates show δ18O values ranging from −11.7‰ to −10.3‰ (Vienna Pee Dee Belemnite [VPDB]) and δ13C values between −7.1‰ and −2.2‰ (VPDB). The lack of correlation between the two isotopic systems supports the sedimentological interpretation of a shallow, hydrologically open lacustrine setting during the time of carbonate deposition. This interpretation is reinforced by the mineralogy and Mg/Ca molar ratios from Fenghuoshan lacustrine carbonates that indicate fresh water conditions. In addition, petrographic analysis and the δ18O composition of samples suggest that Fenghuoshan lacustrine rocks have not undergone significant diagenetic alteration. Collectively, these data indicate that the isotopic composition of Fenghuoshan Group carbonates may be used to infer the oxygen isotopic composition of meteoric water feeding these lacustrine basins at the time of deposition, from which we derive estimates of the paleoaltimetry of north‐central Tibet during Middle Eocene time. Model results using the isotopic data from Fenghuoshan carbonates indicate that the hypsometric mean elevation of the drainage basins feeding Hoh Xil lakes was ≲2 km. Together with estimates of the Eocene paleoaltimetry from the Lunpola basin to the south, these results provide the first direct evidence for the differential uplift of the northern margin of the Tibetan Plateau.

Dynamic equilibrium among erosion, river incision, and coastal uplift in the northern and central Apennines, Italy

Erosion, river incision, and uplift rates in the northern and central Apennines, Italy, since 0.9 Ma, are determined from new cosmogenic nuclide data. Beryllium-10 concentrations in modern and middle Pleistocene sediments indicate erosion rates from 0.20 to 0.58 mm/yr. These rates are similar to estimates of sediment yield (0.12–0.44 mm/yr), river incision (0.35 mm/yr), and uplift (0.01–1.0 mm/yr) rates inferred from other methods that integrate landscape process rates since the early Pleistocene. These rates of landscape change are significantly lower than long-term exhumation rates of ∼1.2 mm/yr since ca. 4.5 Ma, inferred from thermochronometry. Collectively, these data suggest that hillslope erosion and river incision rates in the northern and central Apennines have balanced local uplift rates for ∼1 My, but that exhumation rates have slowed significantly since emergence of the mountain chain in the Pliocene. This condition of dynamic equilibrium was potentially achieved within ca. 3 Ma, similar to some model predictions of hillslope and fluvial system adjustment.

Paleodischarge of the Mojave River, southwestern United States, investigated with single-pebble measurements of 10Be

The paleohydrology of ephemeral stream systems is an important constraint on paleoclimatic conditions in arid environments, but remains difficult to measure quantitatively. For example, sedimentary records of the size and extent of pluvial lakes in the Mojave Desert (southwestern USA) have been used as a proxy for Quaternary climate variability. Although the delivery mechanisms of this additional water are still being debated, it is generally agreed that the discharge of the Mojave River, which supplied water for several Pleistocene pluvial lakes along its course, must have been significantly greater during lake highstands. We used the 10 Be concentrations of 10 individual quartzite pebbles sourced from the San Bernardino Mountains and collected from a ca. 25 ka strath terrace of the Mojave River near Barstow, California, to test whether pebble ages record the timing of large paleodischarge of the Mojave River. Our exposure ages indicate that periods of discharge large enough to transport pebble-sized sediment occurred at least 4 times over the past ∼240 k.y.; individual pebble ages cluster into 4 groups with exposure ages of 24.82 ± 4.36 ka (n = 3), 55.79 ± 3.67 ka (n = 2), 99.14 ± 12.07 ka (n = 4) and 239.9 ± 52.16 ka (n = 1). These inferred large discharge events occurred during both glacial and interglacial conditions. We demonstrate that bedload materials provide information about the frequency and duration of transport events in river systems. This approach could be further improved with additional measurements of one or more cosmogenic nuclides coupled with models of river discharge and pebble transport.