Clear Lake sediments: Anthropogenic changes in physical sedimentology and magnetic response

We analyzed the sedimentological characteristics and magnetic properties of cores from the three basins of Clear Lake, California, USA, to assess the depositional response to a series of land use changes that occurred in the watershed over the 20th century. Results indicate that distinct and abrupt shifts in particle size, magnetic concentration/mineralogy, and redox conditions occur concurrently with a variety of ecological and chemical changes in lake bed sediments. This coincidence of events occurred around 1927, a datum determined by an abrupt increase in total mercury (Hg) in Clear Lake cores and the known initiation of open-pit Hg mining at the Sulphur Bank Mercury Mine, confirmed by ²¹⁰Pb dating. Ages below the 1927 horizon were determined by accelerator mass spectrometry on ¹⁴C of coarse organic debris. Calculated sedimentation rates below the 1927 datum are ~1 mm/yr, whereas rates from 1927 to 2000 are up to an order of magnitude higher, with averages of ~3.5–19 mm/yr. In both the Oaks and Upper Arms, the post-1927 co-occurrence of abrupt shifts in magnetic signatures with color differences indicative of changing redox conditions is interpreted to reflect a more oxygenated diagenetic regime and rapid burial of sediment below the depth of sulfate diffusion. Post-1927 in the Oaks Arm, grain size exhibits a gradual coarsening-upward pattern that we attribute to the input of mechanically deposited waste rock related to open-pit mining activities at the mine. In contrast, grain size in the Upper Arm exhibits a gradational fining-upward after 1927 that we interpret as human-induced erosion of fine-grained soils and chemically weathered rocks of the Franciscan Assemblage by heavy earthmoving equipment associated with a road- and home-building boom, exacerbated by stream channel mining and wetlands destruction. The flux of fine-grained sediment into the Upper Arm increased the nutrient load to the lake, and that in turn catalyzed profuse cyanobacterial blooms through the 20th century. The resulting organic biomass, in combination with the increased inorganic sediment supply, contributed to the abrupt increase in sedimentation rate after 1927.KEYWORDS: Clear Lake, California, USA, particle size trends, ¹⁴C, paleolimnology, late Holocene, land use patterns, Sulphur Bank Mercury Mine, mercury, magnetic dissolutio

Integrated Sr isotope variations and sea-level history of Middle to Upper Cambrian platform carbonates: Implications for the evolution of Cambrian seawater 87 Sr/ 86 Sr

ABSTRACT A high-resolution Sr isotope study of Middle to Upper Cambrian platform carbonates of the southern Great Basin significantly refines the structure of the existing seawater Sr isotope curve. Samples were selected using rigorous stratigraphic, petrographic, and geochemical criteria in order to minimize the effects of diagenetic alteration and contamination from noncarbonate components

Lehman06ai.eps

ABSTRACT The Cupido and Coahuila platforms of northeastern Mexico are part of the extensive carbonate platform system that rimmed the ancestral Gulf of Mexico during Barremian to Albian time. Exposures of Cupido and Coahuila lithofacies in several mountain ranges spanning an ~80 000 km 2 area reveal information about platform morphology and composition, paleoenvironmental relations, and the chronology of platform evolution. New biostratigraphic data, integrated with carbon and strontium isotope stratigraphy, significantly improve chronostratigraphic relations across the region. These data substantially change previous age assignments of several formations and force a revision of the longstanding stratigraphy in the region. The revised stratigraphy and enhanced time control, combined with regional facies associations, allow the construction of cross sections, isopach maps, and timeslice paleogeographic maps that collectively document platform morphology and evolution. The orientation of the Cupido (BarremianAptian) shelf margin was controlled by the emergent Coahuila basement block to the northwest. The south-facing margin is a highenergy grainstone shoal, whereas the margin facing the ancestral Gulf of Mexico to the eas

(Appendix A1) Paleomagnetic of DSDP Site 47-398

(Appendix A1) Paleomagnetic of DSDP Site 47-39

(Appendix A2) Calcium carbonate and TOC concentrations and isotope ratios of Santa Rossa Canyon section

The early Aptian Oceanic Anoxic Event (OAE1a, 120\ua0Ma) represents a geologically brief time interval in the mid-Cretaceous greenhouse world that is characterized by increased organic carbon accumulation in marine sediments, sudden biotic changes, and abrupt carbon-isotope excursions indicative of significant perturbations to global carbon cycling. The brevity of these drastic environmental changes (< 106\ua0year) and the typically 106\ua0year temporal resolution of the available chronologies, however, represent a critical gap in our knowledge of OAE1a. We have conducted a high-resolution investigation of three widely distributed sections, including the Cismon APTICORE in Italy, Santa Rosa Canyon in northeastern Mexico, and Deep Sea Drilling Project (DSDP) Site 398 off the Iberian margin in the North Atlantic Ocean, which represent a range of depositional environments where condensed and moderately expanded OAE1a intervals are recorded. The objectives of this study are to establish orbital chronologies for these sections and to construct a common, high-resolution timescale for OAE1a. Spectral analyses of the closely-spaced (corresponding to ~ 5 to 10\ua0kyr) measurements of calcium carbonate content of the APTICORE, magnetic susceptibility (MS) and anhysteretic remanent magnetization (ARM) of the Santa Rosa samples, and MS, ARM and ARM/IRM, where IRM is isothermal remanent magnetization, of Site 398 samples reveal statistically significant cycles. These cycles exhibit periodicity ratios and modulation patterns similar to those of the mid-Cretaceous orbital cycles, suggesting that orbital variations may have modulated depositional processes. Orbital control allows us to estimate the duration of unique, globally identifiable stages of OAE1a. Although OAE1a had a duration of ~ 1.0 to 1.3\ua0Myr, the initial perturbation represented by the negative carbon-isotope excursion was rapid, lasting for ~ 27-44\ua0kyr. This estimate could serve as a basis for constraining triggering mechanisms for OAE1a