PP43A-0997
North American Moisture Gradients over the Past 15,000 Years Recorded by Lake Levels
Spatial variation in the water levels of lakes provides a useful record of past moisture regimes and their associated synoptic climate patterns. Here, we present updated maps of North American lake levels over the past 15,000 years. Data have been assembled from published lake records in all regions of the continent, and focus on records that provide direct evidence of past shoreline elevations. The results show both the effects of short- term climate variability and the sensitivity of moisture gradients to the boundary conditions of the global climate system. For example, some lakes like Hidden Lake, Colorado, record the infrequent decadal-to-centennial "megadroughts" of the past two millennia that have been captured in tree-ring data, and nearly all lakes in the Rockies and Great Plains record persistently dry conditions for multiple millennia between 8000 and 5000 years BP. Regional climate model simulations show that the documented long-term aridity may have resulted from the direct effects of orbitally-driven changes in seasonal insolation patterns. Consequently, although "megadroughts" may be part of the natural moisture regimes of the late-Holocene, large magnitude shifts in boundary conditions both in the past (e.g., insolation) and future (i.e., greenhouse gases) have the potential to fundamentally alter the availability of water.
PP43A-0998
Speleothem Paleoclimate Records from the Floridian Panhandle
Geochemical time-series constructed from speleothem records provide a high resolution view into paleoclimate conditions temporally unmatched by deep ocean sediment and ice core records. The potential of speleothem records is vast with immense spatial and temporal resolution. One surprise to the speleothem paleoclimate community is the absence of high resolution speleothem isotope and trace element records from Florida, an area of extensive karstic geology, a plethora of caves, and monsoon-like climate. Two stalagmites collected from Brooks Quarry Cave, recently opened in Marianna, FL will provide one of the first Holocene speleothem climate records from the Southeast. Speleothem BC-1 is a 71 cm calcite stalagmite collected in situ. Speleothem BC-2 is a 6.1 cm stalagmite collected inside the quarried entrance to the cave. Both dripstones are banded and laminated at what appear to be 11-year cycles (dating in progress). If these are annual "sparves" (speleothem varves), the growth rates are approximately 100 μm per year. BC-1 would extend through much of the Holocene. RGB scans along the polished length of the growth axis reveal color cyclicity related to bundles of these bands. Isotope records from BC-2 reveal a multi-year record showing a variation of -2.7 ‰ to -6 ‰ in δ18O and -3.7 ‰ to -9.1‰ in δ13C, with no correlation between the two. High resolution multi-element laser ablation scans across 8 "sparves" (2.3 mm length) reveals co-variation in U and Ba with other elements showing weaker banding correlations with "clays" (Si, Fe, Mn, Ce, Rb). Photomicrographs of the laser tracks show correlation of visible "sparves" to chemical variations. Medical CT imaging techniques were applied to sample BC-1using a GE Lightspeed Plus CT scanner at maximum power (140 kV, 565 mA) and medium resolution (0.6 mm per scan slice) to view density differences and pore spaces inside the calcite matrix. CT technology reveals a porous nature of the calcite matrix in the lower 35 centimeters with abundant fluid and air inclusions, while the upper 36 centimeters consist of a denser, non porous calcite matrix with a ring of dissolution features on the outer rind. A biomedical video of the longitudinal stacked scans will be presented during the talk to describe internal features that are otherwise invisible. This tool may routinely guide speleothem selection for paleoclimate studies, and imaging of internal dripstone structures prior to open-rock surgery.
PP43A-0999
Speleoclimatology of a Wild Florida Cave: the Present is Key to the Past
Speleoclimatology is an emerging field of cave paleoclimate research. Understanding carbon dioxide degassing, calcite precipitation, and how chemical and isotopic information transfers from the atmosphere to drip water to cave calcite is critical to interpretations of paleorecords in dripstones. We have instrumented multiple micro- meteorological time-series stations in a wild cave in Northwest Florida (Hollow Ridge) near Marianna and the Chipola River. Each station continuously records temperature, barometric pressure, relative humidity, 2D acoustic airflow, drip rates, air radon and carbon dioxide. Hollow Ridge Cave is a Southeastern Cave Conservancy, Inc. (SSCi) preserve with 4 entrances and 3370 feet of mapped passage located at the contact of the early Oligocene Marianna Limestone and Bumpnose member of the Crystal River formation. In May 2007 carbon dioxide concentrations ranged from 556 ppm 50 feet inside the main entrance (site 1) to 827 ppm 500 feet into the cave (site 2), with δ13C values of -13.2 ‰ to -15.6 ‰, respectively. In August 2007 carbon dioxide ranged from 734 ppm (site 1) to over 13,500 ppm (1.35%) (site 2), with δ13C values of -14.9 ‰ to -21.5 ‰. Higher rainfall in July resulted in elevated CO2 input to the cave from soils and drip waters than in May. Estimates of the endmember δ13C values of the air and soil gas data predict a CO2 soil gas at -22 ‰, as expected from decay of overlying C3 plants. 222Rn and CO2 data from air samples at sites 1 and 2 display a positive correlation. 222Rn activities reach a maximum of 134 dpm/L in May but increased to 1358 dpm/L in August at site 2, presumably due to slower air flushing rates. We plan to seal the cave to allow radon and carbon dioxide to grow into steady-state (while continuously monitoring) in order to estimate exchange rates directly from 222Rn models, and thus estimate CO2 exhalation and calcite precipitation rates. δ13C data reveals air masses throughout the cave are mixtures ranging from 27% soil / 73% atmospheric (site 1) in May, whereas a ratio of 42% / 58% was present in August. At site 2 the composition increased from 47% / 53% in May to soil gas dominated 96% / 4% in August. The mixing ratios appear to be a function of distance from entrance, but data collected during the entire seasonal cycle will detect longitudinal flow-through and exchange with the atmosphere and surrounding soil. These initial data show similar patterns in 222Rn activities and CO2 concentrations in cave air. Further data will reveal the significance of the positive correlation. These speleoclimate studies, when combined with isotope and chemical data from contemporaneous drip water and dripstones, will help connect climate variations to paleoclimate interpretations.
PP43A-1000
Base-level Response to Holocene Climate Change in the Central Appalachian Mountains of North America: Preview of Global Warming?
Global Warming is expected to bring about substantive changes in global precipitation patterns, which will lead to altered stream hydrologies. The directions and magnitudes of streamflow changes can be inferred from climate projections, but changes in stream architecture and base level are open questions. We address base level response to climate change by reconstructing river behavior during the mid- to late Holocene, including the Hypsithermal when peak Holocene temperatures were achieved. We reconstruct the climate of the Greenbrier River watershed of the central Appalachian Mountains of North America using the stable isotope geochemistry of a stalagmite, cave sediments, and published pollen data. Independently, we construct a history of base level position using cave sediments deposited by the river. Stalagmite values of δ18O and δ13C are heavy during the Hypsithermal, which pollen results indicate was warm and dry compared to the rest of the Holocene. Cave sediments deposited by the Greenbrier River record base level as having been below the cave during the early and late Holocene, but above the cave during the Hypsithermal. The mid-Holocene base level rise is attributed to infilling of the channel with as much as 4 m of sediment, presumably a response to changes in storm frequency and stream hydrology. Global Warming will cause temperatures to exceed those of the Hypsithermal. Flood zones will extend to higher elevations and flood risks and vulnerabilities will increase dramatically if Appalachian rivers respond to Global Warming as the Greenbrier River did to the Hypsithermal.
PP43A-1001
North American boreholes and GCM paleotemperatures: New comparison methodology
Subsurface temperatures obtained from boreholes yield a direct, diffused record of past temperatures at local scales. When grouped into ensembles, average subsurface temperature profiles provide robust representations of past climate for large regions. With the availability of more computing power to run state-of-the-art General Circulation Models (GCMs), paleoclimatic simulations spanning the last millennium now exist. By forward-diffusing these paleotemperatures, comparison between the two data sets can be facilitated. The standard method for comparing two temperature profiles, modeled or measured, is via their root mean square (RMS) difference. This method gives equal weight to all depths, despite the fact that heat diffusion is a non-linear process. In this work, two additional criteria are implemented to complement RMS difference: the depth of major trend reversal (τ), and the magnitude of trend seen above (since) this reversal point (δ). Excellent agreement between externally forced ECHO-g integrations and subsurface temperatures is demonstrated in most of the regions studied. The implications of selecting an initial climate reference state are discussed