Evolving landscapes realized through the lens of cosmogenic isotopes : novel applications and new approaches
Keywords: Geological Sciences.

Cosmogenic nuclides have revolutionized the geosciences by serving as a reliable geochronometer and tracer of Earth surface processes. These rare isotopes, occurring in extremely low abundances in surficial rocks and soils, give us a detailed history of the physical setting and duration of exposure of a landform, allowing for the quantification of rates and dates of their movement and stability over time. In this dissertation, I focus on two varieties of the cosmogenic nuclide 10Be (half-life = 1.39 Myr), meteoric and in situ-produced, that differ in their rate of production and delivery mechanism. Both varieties of 10Be are produced mainly through spallation reactions in the atmosphere, in the case of meteoric 10Be (10Bemet), and in quartz crystal lattices in surficial rocks and soils, in the case of in situ-produced 10Be (10Bein situ). These varieties of 10Be primarily differ by their production rate; 10Bemet, which is delivered to Earth's surface through precipitation or as dry deposition, is relatively unconstrained compared to 10Bein situ. Through new approaches and novel applications of these two varieties of 10Be, I present 1) a new method to improve the accuracy of 10Beme delivery rates and 2) assess the effect of precipitation on 10Beme delivery, 3) resolve millennial rates of cliff retreat in Southern California and 4) explore factors that likely influence denudation (the sum of erosion and weathering) rates in the Sierra de las Minas of eastern Guatemala via 10Bein situ. In Chapter 1, I calculate site-specific delivery rates of 10Bemet to eroding areas by leveraging new 10Bemet concentrations and previously published 10Bein situ erosion rates from Pinedale and Bull Lake-aged terminal moraines in the Wind River Range, Wyoming. These loss-corrected, Holocene-averaged calculated delivery rates are compared against two widely used estimation methods: the local, empirical, precipitationbased formula of Graly et al. (2011) and the regional atmospheric model-based estimates of Heikkila & von Blanckenburg (2015), which substantially differ for this site due to differences in how the methods handle the influence of precipitation, atmospheric circulation, and spatial scale on 10Bemet delivery. I found that the calculated delivery rates for both moraines agree within uncertainty with that estimated by Heikkila & von Blanckenburg (2015) and are considerably higher than the Graly et al. (2011) estimate. This provides evidence that model-based estimates agree well with soil-derived estimates, though future workers should estimate the 10Bemet delivery rate using both methods for the most robust calculations of Earth surface processes, particularly in areas of considerable elevation and/or precipitation gradients. In Chapter 2, I also calculate site-specific delivery rates of 10Bemet to eroding areas in the same manner as Chapter 1, utilizing this new technique on soil profiles along a precipitation- and elevation-gradient in the southwestern Sierra Nevada and the rain shadowed White Mountain, California to evaluate the role of precipitation on loss-corrected 10Bemet delivery rates. Recently, increased focus and debate has been drawn toward whether delivery rates increase, or are invariant, with precipitation rate increases with both phenomena occurring around the globe depending on orographic setting and perhaps integration timescale. I find evidence for precipitation-dependent delivery of 10Bemet to the eroding soils of the Sierra Nevada, as calculated loss-corrected delivery rates increase (and decrease) as mean annual precipitation rates increase (and decrease). This effect is also broadly reflected in the measured 10Bemet inventories along this transect. Interestingly, the highest loss- delivery rate among all locations is calculated for rain shadowed White Mountain site , however this is likely a result of different moisture sources and longer advective cloud 10Bemet replenishment compared to the Sierra Nevada transect. These observations indicate that 10Bemet delivery rates likely scale with low to moderate MAP rates increases (i.e., up to 100 cm yr-1), agreeing with observations from other transects, however distance from moisture sources might affect this relationship. In Chapter 3, I utilize 10Bein situ concentrations and measured topography from a > 125 m exposed shore platform in coastal Del Mar, California in conjunction with a coupled model of shore platform evolution and cosmogenic production to estimate a late Holocene coastal cliff retreat rate for this area. Cliff retreat continually threatens coastal communities and infrastructure, but constraints on retreat rates are typically limited to those derived from imagery and maps spanning the last ~100 years. With this relatively new technique and modeling approach, applied for the first time to any coast in North America, I estimated a best-fit cliff retreat rate of 6.2 -- 16.3 cm yr-1 over the last two millennia. These rates are consistent with decadal observed rates of about 5-20 cm yr-1, suggesting that cliff retreat here has likely been relatively stable from the late Holocene to ~100 years before present, representing a time when the rate of relative sea level (RSL) rise was approximately constant in Southern California. Modeled cliff retreat during the earlyand mid-Holocene show that retreat rates decline roughly in tandem with the decline in the rate of RSL rise, suggesting that wave action and sea level rise are primary controls on coastal cliff erosion in Del Mar, though uncertainties are large due to a lack of additional 10Bein situ calibration data further out on the shore platform In Chapter 4, I explore the relationships between vegetation, topography, and climate in modulating denudation rates in a tropical mountain range that exhibits a rain shadow. The Sierra de las Minas, eastern Guatemala, is highly vegetated and characterized by relative tectonic quiescence since ~7 Ma and a strong orographic effect, where northern slopes that intercept Caribbean moisture receive over twice the amount of precipitation, corrected for evapotranspiration (P-ET), as southern slopes.13 new and three previously published catchment-averaged 10Bein situ denudation rates that span the range of climate and lithological variations (with similar average slopes) reveal that denudation rates are approximately twice as fast in the northern, wetter drainage basins compared to the drier, southern drainage basins. Further analysis using Pearson correlation coefficients suggests that denudation rates only have a weak correlation with P-ET, along with root mass fraction (a modeled ratio of roots vs. shoots), slope, area, and local relief, while a fair correlation is observed for mean elevation and mean annual temperature (MAP). No correlation is observed between denudation rate and channel normalized steepness (Ksn), suggesting no tectonic control as expected from the relative quiescence in the Sierra de las Minas over the last few million years, nor the enhanced vegetation index (EVI), suggesting that remotely sensed metrics that only consider aboveground biomass might not accurately reflect the effect vegetation may have in modulation denudation in highly vegetated areas. Correlations between the variables of interest suggest that that topography, primarily in the form of mean elevation, sets the stage for climate (P-ET and MAT) to influence biomass distribution (RMF), all of which contribute to the denudation rate trends along a pronounced orographic gradient in the tropical Sierra de las Minas