- NSF-PAR ID:
- Date Published:
- Journal Name:
- Page Range / eLocation ID:
- 435 to 453
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The quantification of rates for the competing forces of tectonic uplift and erosion has important implications for understanding topographic evolution. Here, we quantify the complex interplay between tectonic uplift, topographic development, and erosion recorded in the hanging walls of several active reverse faults in the Ventura basin, southern California, USA. We use cosmogenic 26Al/10Be isochron burial dating and 10Be surface exposure dating to construct a basin-wide geochronology, which includes burial dating of the Saugus Formation: an important, but poorly dated, regional Quaternary strain marker. Our ages for the top of the exposed Saugus Formation range from 0.36 +0.18/-0.22 Ma to 1.06 +0.23/-0.26 Ma, and our burial ages near the base of shallow marine deposits, which underlie the Saugus Formation, increase eastward from 0.60 +0.05/-0.06 Ma to 3.30 +0.30/-0.41 Ma. Our geochronology is used to calculate rapid long-term reverse fault slip rates of 8.6–12.6 mm yr–1 since ca. 1.0 Ma for the San Cayetano fault and 1.3–3.0 mm yr–1 since ca. 1.0 Ma for the Oak Ridge fault, which are both broadly consistent with contemporary reverse slip rates derived from mechanical models driven by global positioning system (GPS) data. We also calculate terrestrial cosmogenic nuclide (TCN)-derived, catchment-averaged erosion rates that range from 0.05–1.14 mm yr–1 and discuss the applicability of TCN-derived, catchment-averaged erosion rates in rapidly uplifting, landslide-prone landscapes. We compare patterns in erosion rates and tectonic rates to fluvial response times and geomorphic landscape parameters to show that in young, rapidly uplifting mountain belts, catchments may attain a quasi-steady-state on timescales of <105 years even if catchment-averaged erosion rates are still adjusting to tectonic forcing.more » « less
Abstract. We collected a debris-rich ice core from a buried icemass in Ong Valley, located in the Transantarctic Mountains in Antarctica. Wemeasured cosmogenic nuclide concentrations in quartz obtained from the icecore to determine the age of the buried ice mass and infer the processesresponsible for the emplacement of the debris currently overlaying the ice.Such ice masses are valuable archives of paleoclimate proxies; however, thepreservation of ice beyond 800 kyr is rare, and therefore much effort hasbeen recently focused on finding ice that is older than 1 Myr. In Ong Valley,the large, buried ice mass has been previously dated at > 1.1 Ma.Here we provide a forward model that predicts the accumulation of thecosmic-ray-produced nuclides 10Be, 21Ne, and 26Al in quartzin the englacial and supraglacial debris and compare the model predictionsto measured nuclide concentrations in order to further constrain the age.Large downcore variation in measured cosmogenic nuclide concentrationssuggests that the englacial debris is sourced both from subglacially derivedmaterial and recycled paleo-surface debris that has experienced surfaceexposure prior to entrainment. We find that the upper section of the icecore is 2.95 + 0.18 / −0.22 Myr old. The average ice sublimation rate duringthis time period is 22.86 + 0.10 / −0.09 m Myr−1, and the surfaceerosion rate of the debris is 0.206 + 0.013 / −0.017 m Myr−1. Burialdating of the recycled paleo-surface debris suggests that the lower sectionof the ice core belongs to a separate, older ice mass which we estimate tobe 4.3–5.1 Myr old. The ages of these two stacked, separate ice masses canbe directly related to glacial advances of the Antarctic ice sheet andpotentially coincide with two major global glaciations during the early andlate Pliocene epoch when global temperatures and CO2 were higher thanpresent. These ancient ice masses represent new opportunities for gatheringancient climate information.more » « less
Paired in situ cosmogenic nuclides14C and10Be present an opportunity to explore erosion rate disequilibria over Holocene to latest Pleistocene timescales and are a new avenue in surface processes research.14C and10Be concentrations in quartz from river sand collected at the outlets of five mountainous catchments in the Argentine Andes are compared in this study. River gauge and10Be‐derived erosion rates are in good agreement; however,14C concentrations are approximately 2.7–4 times lower than expected relative to10Be under steady‐state erosion. Low14C to10Be ratios imply that sediment eroded from the high mountains was shielded for at least 7–15 ky. Neoglacial advances and storage in terraces may account for some of the reduced14C concentrations but are insufficient alone. Transient storage in dynamic talus slopes in the steep topography of the High Andes provides the best explanation for the observed14C concentrations.
Tropical islands, including many in island arcs, are subjected to recurring disturbances from extreme storms such as tropical cyclones. To test whether such storms influence cosmogenic nuclide concentrations such that they do not reflect long‐term rates of erosion, we measured meteoric and
in situ10Be in river sediment samples from Dominica, an andesitic island in the Caribbean, before and after category five Hurricane Maria (in 2017). Populations of before‐ and after‐storm concentrations are statistically indistinguishable ( n= 7 pairs for in‐situ10Be, n= 11 pairs for meteoric10Be).10Be concentrations vary from −138% to +73% within before–after sample pairs relative to the mean of the pair. These new data suggest that the effects of extreme storms on the depth and amount of near‐surface erosion on Dominica vary spatially. Our data support the calculations of Niemi et al. (2005) and Yanites et al. (2009) suggesting that basin‐by‐basin comparisons of erosion rates based on cosmogenic nuclides should be approached with caution in small (<~100 km2) watersheds affected by mass movements and extreme storms. Erosion rates determined from in‐situ10Be on Dominica (geometric mean = 0.102 mm y−1, n= 12) are low compared to similarly steep and wet areas globally and correlate positively with the spatial density of mass movements.
This study examines dissolved rhenium (Re) concentrations as a function of water runoff using river samples from two contrasting mountainous watersheds, the Eel and Umpqua Rivers in the Pacific Northwest, USA. These watersheds share many key characteristics in terms of size, discharge, climate, and vegetation, but they have a 15-fold difference in sediment yield due to differences in their tectonic setting and uplift and erosion rates. We evaluate concentration-runoff (C-R) relationships and ratios of coefficients of variation (CVC/CVR) for major cations, anions, dissolved inorganic carbon, selected trace elements including Re, and 87Sr/86Sr ratios. Recent research outlines the potential of Re to serve as a tracer for the oxidation of ancient/fossil organic matter because of its close association with petrogenic carbon (OCpetro) in rocks. In both the Eel and Umpqua Rivers, our measurements show that Re behaves similarly to major weathering derived-solutes corrected for atmospheric input, such as Ca2+*, Mg2+*, and Na+* with modest dilution across all tributaries with increasing runoff. Rhenium behaves dissimilarly from other trace elements, such as Mo and U, and is also dissimilar to biologically-cycled nutrients, such as NO3 – , PO4 3 , and K+*, suggesting differences in sources, solute generation mechanisms, and flowpaths. Rhenium behavior is also distinct from that of colloids, which have increasing concentrations with increasing runoff. We find that Re and sulfate corrected for atmospheric input (SO4 2 *) have distinct CR relationships, in which SO4 2 * undergoes greater dilution with increasing runoff. This implies that Re is not dominantly sourced from sulfide weathering, which leaves primary bedrock minerals and OCpetro hosted in bedrock of these watersheds as the likely dominant sources of dissolved Re release. At mean discharge, Re concentration in the Eel river (3.5 pmol L-1) is more than two times greater than Re concentrations in the Umpqua River (1.5 pmol L-1). Furthermore, comparison of two tributary watersheds with similar bedrock but marked differences in erosion rates show higher Re concentrations in Bull Creek (erosion rate of 0.5 mm yr 1) relative to Elder Creek (erosion rate of 0.2 mm yr 1). The results of this study suggest that dissolved Re in the Eel and Umpqua River basins is likely derived from primary mineral dissolution or OCpetro oxidation, and Re fluxes are higher in areas with higher erosion rates, suggesting that tectonic setting is one factor that controls Re release and therefore OCpetro oxidation.more » « less