PP42A-01 INVITED 10:20h
Molecular Proxy Approaches for Paleohydrology
There is a rich assembly of isotopic and mineral indicators for paleohydrologic properties of ancient environments. Commonly employed examples include mineral abundance ratios and the isotopic signatures of minerals and macromolecular organic phases such as cellulose. Preservation of these materials can be influenced strongly by natural processes in the environment, most notably resulting in the alteration or loss of carbonate mineral isotopic signatures. In order to expand our ability to document paleoclimatic conditions in continental environments, additional tools for both aquatic and terrestrial settings are in development based on the hydrogen isotopic signatures of individual lipids from microbes, algae and vascular plants. Plant leaf waxes (long-chain n-alkanes) preserve well in soils and aquatic sediments. Deuterium signatures in ancient leaf lipids potentially record isotopic properties of ancient plant water, reflecting isotopic signatures of rainfall and soil waters as well as the level of relative humidity. We have studied grasses, trees and other plant types from both greenhouse and field localities in order to understand the relative influences of plant physiology, physiognomy,and growth conditions (humidity) on lipids as recorders of plant water isotopic signatures. Submerged aquatic algae are not directly influenced by humidity, and recent work has shown their biomarkers to be promising paleolimnological proxies. We will discuss the potential for algal compounds as recorders of waters in modern high altitude sites and for ancient paleoaltimetry applications. Recent theoretical considerations in conjunction with analyses of lipids from ancient sediments point to the limitation of the preservation of paleohydrologic signatures set by thermal maturation approaching oil-generating conditions.
PP42A-02 10:35h
Hydrogen Isotope Ratios of Leaf Waxes in C3 and C4 Grasses Record Meteoric Water and Aridity Signatures
Hydrogen isotope ratios of sedimentary n-alkanes (C27-C33) from vascular plants potentially provide a valuable record of past hydrologic conditions. To explore this, we analyzed grasses grown in a greenhouse and calculated fractionation factors (epsilon) between source water and n-alkane for each sample. An average difference of 21 permil is observed between C3 and C4 grasses, which is comparable to that determined for grasses collected from the Great Plains. The more positive isotope values in C4 grasses likely reflects smaller interveinal distance compared to C3 grass leaves, allowing greater back-diffusion of transpirationally enriched water from stomata, as documented with the oxygen isotope ratios of grass leaf water and cellulose by Helliker and Ehleringer (2000). The oxygen isotope difference is magnified at low relative humidity, when transpiration rates are higher. A similar effect is expected in hydrogen isotope ratios of leaf water and plant compounds. However, preliminary results from grasses grown hydroponically at different relative humidities suggest that there may be a decoupling of the hydrogen isotope ratio of leaf-wax n-alkanes and the oxygen isotope ratio of leaf water and cellulose. To examine the effects of source water delta D and climate on n-alkane delta D values, we analyzed grasses collected from the Great Plains. We use river water delta D values as a proxy for source water and the epsilon values determined in the greenhouse experiments, to predict expected values for C3 and C4 grass lipids. Measured values compare well to predicted values, with the exception of two semi-arid sites where evapotranspiration may have led to leaf-waters that are enriched in deuterium. Residual delta D values (measured-expected) correspond strongly with measures of aridity, such as annual precipitation and recipitation/evaporation ratios.
PP42A-03 10:50h
Hydrogen Isotopic Ratios of Lacustrine Algal and Terrestrial Organic Matter as a Quantitative Proxy for the Reconstruction of Relative Humidity and Source Water Composition in Continental Settings
Sedimentary studies have indicated that hydrologic conditions in low latitude continental environments have varied significantly during the late Quaternary and throughout the mid to late Holocene in association with sea level variations and major climatic phenomena such as the migration and mean position of the ITCZ. However, a quantitative approach to understanding hydrologic variability (i.e. variations in source water composition and relative humidity) is needed to accurately reconstruct changes in moisture balance and paleoclimatic conditions. Lake Tulane, located in Central Florida, is a subtropical, groundwater-fed acidic lake with a high sedimentation rate and well-preserved organic matter. This study utilizes the hydrogen isotopic composition (dD) of organic molecules (fatty acids) associated with algal (C16) and terrestrial (C28) materials in a lacustrine system to provide a modern calibration that quantifies the isotopic behavior associated with 1) changes in the chemistry of source waters and 2) variability in relative humidity. Over an annual cycle, the dDC16 of modern algal material shows little variability reflecting a constant dD of lake water coincident with the groundwater-fed nature of the lake and relatively constant dD of precipitation. Terrestrial biomass shows a seasonal variability of ~15% with more enriched values occurring in winter when higher rates of evapotranspiration lead to isotopic enrichment. Seasonal variations in the magnitude of isotopic offset between the algae and terrestrial markers, (DdD(C16-C28) ), removes the variations in the source waters and, is quantitatively related to seasonal changes (~6%) in the relative humidity. Together, dDC16 and dDCc16-c28) from lacustrine archives can serve as proxies for the quantitative reconstruction of source water composition and relative humidity from sedimentary records preserved in continental settings. These newly developed dD molecular-hydrologic proxies are applied to the sedimentary record in Lake Tulane spanning the past 80 Kyr, including the glacial-interglacial interval and a high-resolution study of the late Holocene (Little Ice Age (LIA) and Midieval Warm Period (MWP)). During the glacial-interglacial transition, dD of lake water showed a gradual +15% trend whereas relative humidity (DdD(C16-C28)) showed an abrupt +20% change at the deglaciation suggesting that sea level fluctuation was the dominant control on regional relative humidity. At times of relatively constant sea level, such as during the LIA and MWP, variations in the dD of lake water of 8% and significant changes in regional relative humidity of 15% are correlated with long-term migration and mean position of the ITCZ and the Bermuda High, two climatic phenomena largely responsible for supplying moisture to subtropical North America.
PP42A-04 11:05h
Alkenone and Isotopic Records of Holocene Climatic and Environmental Change From Laminated West Greenland Lakes
Long chain alkenones (LCAs) are a key class of biomarkers for paleotemperature reconstructions. These compounds are ubiquitous in ocean sediments, but rare in lake sediments. Here we report the first discovery of LCAs in a downcore profile and surface sediments of five Greenland lakes. The concentrations of LCAs in surface sediments of these lakes are one to two orders of magnitude higher than those reported previously in other lake surface sediments around the world. Alkenones are present in five Greenland lakes with elevated salinity, but absent from five freshwater lakes. The alkenones have exceptionally low \delta$^{13}$C values ranging from -40 to -43\permil, and are depleted by 10 to 15\permil relative to short-chain fatty acids and sterols within the same samples. These \delta$^{13}$C values are the lowest ever reported for alkenones in a natural setting and have important implications for tracing the alkenone producers in lakes. Using the published calibration for lake sediments, the alkenone unsaturation indices in the surface sediments of the Greenland lakes record late spring/early summer temperature when algal blooms occur, suggesting the applicability of lacustrine alkenones as a paleotemperature proxy. LCA unsaturation indices and \deltaD from sediment cores taken from these Greenland lakes will help elucidate the environmental controls on these sedimentary parameters, and will aid the reconstruction of Holocene climate variability in West Greenland. Ongoing work on the saline lakes includes determining high resolution alkenone unsaturation ratios/abundances and bulk/compound-specific isotopic values from sediment cores, algal culturing, and establishing microbial community structure in the saline lakes using DNA/RNA fingerprinting. Up-to-date results will be presented in the meeting.
PP42A-05 11:20h
The Development of TEX$_{86}$ for Continental Paleotemperature Construction
We have developed a new calibration for the TEX$_{86}$ paleotemperature proxy from a climatically diverse suite of globally distributed lacustrine systems (N=10). The results of this calibration show a strong linear relationship (r$^{2}$= 0.96) between TEX$_{86}$ values and published mean annual lake surface temperatures. The TEX$_{86}$ index as it currently stands appears to work only in large volume lakes, which are typically the best integrators of regional climate variability. The "marine" crenarchaeota responsible for producing the tetraether membrane lipids used in the TEX$_{86}$ index do not appear to be ubiquitous in lakes as previously thought, or are not in great enough abundance to be detected in the sediments of some, especially small, lakes. In contrast to the aquatically derived tetraether lipids, we have found terrestrial tetraether lipids in all lacustrine sediments analyzed thus far. The terrestrial tetraethers are primarily produced by soil bacteria and some methanogenic archaea. In very few cases the presence of terrestrial tetraethers, co-occurring with the aquatic tetraethers, confuse the TEX$_{86}$ signal and predict a colder than normal temperature. Here we explore the relationship between surface temperature and tetraether abundance, attempting to minimize the influence of the terrestrial tetraethers on the temperature signal. We can use the presence and abundance of these terrestrial tetraethers to examine climatic and landscape changes within the watershed. Here we present down-core tetraether data for temperature and landscape changes within the watershed of Elk Lake, MN, through the Holocene.
PP42A-06 11:35h
$\delta$$^{13}$C and $\delta$$^{15}$N Values of Soil Organic Matter Over Drought and Non-drought Affected Elevation Gradients in Ethiopia: Calibrating for Environmental Reconstruction
Portions of Ethiopia today are experiencing increasing temperatures and drought frequencies. The longest known hominid record is in Ethiopia's Awash Basin. Reconstructing past environments in Ethiopia may, therefore, contribute both to understanding present day and past consequences of climate change. Studies suggest that at least 7000 years of environmental reconstruction may be possible from isotopic analyses of organic matter in some Ethiopian paleosols. We have measured $\delta$$^{13}$C and $\delta$$^{15}$N of organic matter from modern soils in Ethiopia to explore the climatic dependence of these signals and thus to determine the maximum resolution of climatic reconstruction possible by bulk isotopic analyses of soil organic matter (SOM). Surface soil samples were taken at elevations from 350 - 3500 m in drought affected regions and from 1050 - 3100 m in regions with no history of drought. Collections were made at an altitude resolution of better than 150 m. Deeper soil samples (max. 27 m) were also obtained at 22 elevations in sites of the Awash Basin that had already been studied using other paleoenvironmental proxies. Soils were sampled in grassland, shrubland, forest, and grass/sedge wetland. The $\delta$$^{15}$N values of SOM decreased significantly with increase in elevation and were sensitive to both overlaying vegetation type and drought proclivity. Our results support hypotheses that $\delta$$^{15}$N values vary with total nitrogen pools in soils which, in turn vary with humidity and associated microbial influences. The $\delta$$^{13}$C values of SOM had a quadratic relationship to elevation that most likely reflected the relative compositions of C3 and C4 biomass in overlying vegetation. Exposure to drought could not be detected by $\delta$$^{13}$C values. At sites in the Hadar region where depth profile measurements were made to 27 m, $\delta$$^{13}$C values decreased with depth. This result conforms to inferences from other proxy that a cooler, wetter climate previously existed. Nitrogen contents of soils below 3 m were often too low to be reliably analyzed for $\delta$$^{15}$N by on-line elemental analysis mass spectrometry. Our measurements of Ethiopian SOM $\delta$$^{13}$C and $\delta$$^{15}$N values correlate well with modern climate and vegetation. The mechanisms by which elevation and aridity strongly influence SOM $\delta$$^{15}$N values merit specific investigation.
PP42A-07 11:50h
Two new methods for high-resolution Micro-XRF analyses of trace elements in speleothems
Trace elements (TE) variability in speleothems encodes climatic signals mediated by the geomorphic and hydrologic environment at annual scale. Current research involves modelling by appropriate transfer functions to recover aspects of the original signal. This requires knowledge of the dependency of element partitioning on growth rate and of the effects of sector zoning. Until recently, there was a lack of analytical methods that would allow high temporal resolution and the simultaneous scanning and mapping of multiple TE concentrations through time (along the growth axis of stalagmites) and space (along single growth layers). The XRF microscope/scan yields rapid, non-destructive, multi-elemental analysis (80 TEs) on the 50$\mu$m scale along the growth axis of whole specimens. Because of the relatively rapid analytical time, several parallel scans can be performed to assess the reproducibility of the data and recognize patterns of lateral distribution of the tracers. The beam penetrates into the sample material to a few ?m, giving a local average and a decadal resolution for slow-growing specimens (<50$\mu$m/year). Synchrotron radiation micro-XRF yields spot sizes at sample surface in the 1$\mu$m-range, allowing resolution at monthly to annual changes in speleothems. Scans and maps of the elemental distribution (on 1 cm-long sections) provide immediate insight on simultaneous concentration changes for several TEs across and along single growth layers, and recognition of processes that are unrelated to environmental changes. Oxidation states are obtained by tuning the energy of the incident beam across the absorption K-edge. The method is not applicable to whole stalagmites, requires long analytical times, and provides semi-quantitative concentration distributions. We applied these methods to two Holocene stalagmites. Both examples show simultaneous spatial and temporal concentration changes for some TEs that can be unequivocally ascribed to climate-related processes. Coupling these two methods has the potential to become an extremely powerful tool in palaeoclimate research.
PP42A-08 12:05h
A Climate Driven Speleothem Stable Isotope Model
We have constructed a climate driven stalagmite growth model that faithfully reproduces the major annual growth trends of temperate climate stalagmites. Model results indicate that speleothem growth rate in temperate regions, although depending primarily on precipitation amount, is a complex function of the timing of precipitation relative to seasonal temperature changes as well as other non-climatic parameters. We have incorporated into this climate driven growth model the capability to simulate climate driven carbon and oxygen stable isotope changes and their incorporation in speleothem calcite. The model allows us to investigate the relationship between isotopic changes in soil CO2 and seepage fluids, and the isotopic composition of the growing stalagmite. We also explore the impact of sampling resolution on the extracted speleothem isotope record. We calibrated the model to replicate the growth and isotopic record of a stalagmite collected in 1982 from Mystery Cave State Park, in Southeastern Minnesota and using temperature and precipitation records spanning 1935-1982 from a nearby weather station. The model generally replicates the \delta13C and \delta18O record for this case. Model ouput indicates that that large deviations of temperature or precipitation from average conditions in a single year can be recorded in speleothems. Increases in temperature have a clear postive correlation with \delta13C values, and a less direct negative correlation with \delta18O values. Increases in precipitation have an inconsistent positive correlation with \delta13C values and a clear positive correlation with \delta18O values.