PRELIMINARY CHRONOLOGY OF THE STONEMAN LAKE, AZ SEDIMENT CORES: RADIOCARBON, TEPHROCHRONOLOGY AND MAGNETIC STRATIGRAPHY

In Fall 2014, the Stoneman Lake Drilling Project recovered two deep sediment cores to reconstruct the Quaternary paleoclimatic record for central Arizona. Here, we describe the results of our chronological analyses and show that the STL cores record ~1.5 million years of terrestrial climate change. Our preliminary chronology for cores STL-1A (72 m) and STL-1B (36 m) is based on a suite of AMS radiocarbon dates in the upper 6 m of the composite core, tephrochrolonogy of 6 glass bearing ashes and preliminary paleomagnetic data showing magnetic field events including the Bruhnes-Matayuma boundary and the Jaramillo Event. Two AMS dates from the upper 2 m and 7 previously published dates show this to be the Holocene portion of the core. A change from dark peaty clay to light colored laminated clays just below a 10940 cal yr BP date represents the Holocene–Pleistocene transition. Seven additional AMS radiocarbon dates from 3 to 6 m composite depth show a rapid linear increase in age to 46,600 cal yr BP. The deepest AMS samples at 7.1 and 7.6m depth were radiocarbon dead, indicating ages >55ka. Below 27 m depth in the core, 6 tephras were found and microprobe elemental analysis of glass shards allowed us to correlate these with well-dated large volcanic eruptions. A tephra at 27.6 m correlates with the Lava Creek B eruption from the Yellowstone Caldera (631 ka) and a tephra at 32.9m depth correlates with the Bishop Ash from Long Valley, CA (767 ka). A deeper tephra at 41.8 m correlates with the Upper Glass Mountain eruption from Long Valley (1.130 Ma) and a shallower tephra at 29.8 m correlates with a Long Valley post-caldera Early Rhyolite eruption (640-750 ka). Two additional tephras have glass chemistries similar to rhyolitic volcanic centers in the San Francisco Peaks north of Flagstaff, AZ, however, these have not been dated with modern Ar-Ar techniques. The age-depth relationships based on radiocarbon dates, tephrochronology, and magnetostratigraphy show an apparent sedimentation rate of ~4.4 cm/kyr. Projection of this sedimentation rate to the base of the core gives an approximate basal age of ~1.5 Ma. Paleoclimate proxies including pollen, sedimentology, scanning XRF analysis, TOC and carbon isotopes of bulk organics suggest at least 20 glacial-interglacial cycles in the core, consistent with this timeframe.