Max Lloyd

The southern Sierra Nevada pediment, central California

The southern Sierra Nevada foothills, central California (USA), expose a fossil pre–40 Ma bedrock pediment which we call the southern Sierra Nevada pediment. We document this landscape with multiple types of data, and also report new apatite ^4He/^3He, (U-Th)/He, and zircon (U-Th)/He data from the pediment that significantly expand the spatial extent of southern Sierra low-temperature thermochronology data westward into the foothills. Applying recently published thermal modeling software for thermochronologic data, which uses a transdimensional Bayesian Monte Carlo Markov chain statistical approach, we tightly constrain the thermal history of the southern Sierra Nevada pediment. Integrating this thermal history with numerous previously published data sets from across the southern Sierra, we present a chronology of tectonic and landscape evolution of the southern Sierra Nevada. For the first time we cover the entire width of the range, integrate the numerous published data sets into a single coherent geologic story, and link each phase of this story to a potential mechanism. Modeling results are consistent with a three-phase cooling history for the southern Sierra Nevada pediment. Rapid exhumation ca. 95–85 Ma resulted in cooling to between 55 °C and 100 °C. Following this, slow cooling to surface conditions occurred from 85 Ma to 40 Ma at rates consistent with those estimated for the axial southern Sierra during the same time period by previous studies. Little if any additional cooling occurred post–40 Ma. We hypothesize that a thin sedimentary cover protected the 40 Ma bedrock landscape through much of the last 40 m.y., and that this cover eroded away post–10 Ma, re-exhuming the southern Sierra Nevada pediment as a fossil pre–40 Ma landscape. Each of these three phases of cooling links to a distinct tectonic or geomorphic regime, including the profound rapid exhumation of the southern Sierra Nevada–Mojave segment of the Cretaceous arc due to subduction of a large oceanic plateau, the formation of the low-relief landscape of the high-elevation areas of the southern Sierra Nevada with more limited tectonic forcing, and Eocene activity on the Western Sierra Fault System.

The isotopic structures of geological organic compounds

Abstract Organic compounds are ubiquitous in the Earths surface, sediments and many rocks, and preserve records of geological, geochemical and biological history; they are also critical natural resources and major environmental pollutants. The naturally occurring stable isotopes of volatile elements (D, 13C, 15N, 17,18O, 33,34,36S) provide one way of studying the origin, evolution and migration of geological organic compounds. The study of bulk stable isotope compositions (i.e. averaged across all possible molecular isotopic forms) is well established and widely practised, but frequently results in non-unique interpretations. Increasingly, researchers are reading the organic isotopic record with greater depth and specificity by characterizing stable isotope ‘structures’ – the proportions of site-specific and multiply substituted isotopologues that contribute to the total rare-isotope inventory of each compound. Most of the technologies for measuring stable isotope structures of organic molecules have been only recently developed and to date have been applied only in an exploratory way. Nevertheless, recent advances have demonstrated that molecular isotopic structures provide distinctive records of biosynthetic origins, conditions and mechanisms of chemical transformation during burial, and forensic fingerprints of exceptional specificity. This paper provides a review of this young field, which is organized to follow the evolution of molecular isotopic structure from biosynthesis, through diagenesis, catagenesis and metamorphism.