PP53A-01
Clumped-isotope thermometry of modernpedogenic carbonates
The carbonate "clumped-isotope" thermometer (Ghosh et al., 2006a) constrains the crystallization temperatures of carbonate minerals by comparing the abundance of 13C18O bonds in these minerals to a stochastic abundance derived from the bulk isotopic composition of C and O atoms. Unlike previous 18O-based carbonate paleo-thermometers, this method does not require knowledge of the parent water's composition. It was calibrated using synthetic carbonates, and has subsequently been applied to paleo- pedogenic rhizoliths of Miocene age (Ghosh et al., 2006b). In order to test the technique's applicability to the study of soils and paleosols, we measured temperatures recorded by "modern" (Late Holocene) pedogenic carbonates from several sites in Arizona. At each site, clasts exhibiting stage-I carbonate coating were sampled at different depths. The carbonate was then processed using the protocol described by Ghosh et al.\ (2006a). Coating extracts from one or more clasts sampled at the same depth exhibit the same "clumped-isotope" temperature within instrumental precision limits. These temperatures vary as a function of depth in a pattern which fits the profile predicted by a simple heat diffusion model for the months of July and August, which together account for most of the yearly rainfall at these sites. Our results thus support using "clumped-isotope" measurements as an indicator of precipitation temperatures in soils and paleosols, and highlight the technique's sensitivity to the seasonality of precipitation. Many potential applications in pedologic, climatic and tectonic studies stand to benefit from further investigation of the signal recorded by modern soils. \ Ghosh, P. et al., 2006a. C-13-O-18 bonds in carbonate minerals: a new kind of paleothermometer. Geochimica et Cosmochimica Acta 70(6):1439--1456. Ghosh, P. et al., 2006b. Rapid uplift of the Altiplano revealed through C-13-O-18 bonds in paleosol carbonates. Science, 311(5760):511--515.
PP53A-02
Glacial/interglacial temperature variations in Soreq cave speleothems as recorded by 'clumped isotope' thermometry
Isotopic composition in speleothem carbonates provide high resolution, well dated climatic records. However, deducing paleotemperature from δ18O suffers a major disadvantage as it reflects a combination of variations in both temperatures and isotopic composition of the precipitating solution. This is particularly difficult in continental records, due to the complex nature of δ18O variations in rainwater. In contrast, the carbonate 'clumped isotopes' thermometry (based on abundance of 13C-18O bonds) independently determines carbonate growth temperatures and can constrain the δ18O values of past precipitation. 'Clumped isotopes' thermometry is based on analyzing mass 47 in CO2 extracted from carbonates. It is reported using the Δ47 value, which is defined as the deviation of R47 from that at a random distribution of isotopologues. At thermodynamic equilibrium, CO2 approaches the random distribution (Δ47=0) at high temperatures and preferentially clumps 13C and 18O into bonds with each other to produce positive Δ47 values as temperature decreases. This approach was used successfully in paleosol nodules, brachiopods, mollusks and meteoritic carbonates. We applied this approach to Holocene and Last Glacial Maximum (LGM) speleothem samples from Soreq cave, Israel. The observed Δ47 values of LGM carbonates were 0.668±0.004 and 0.673±0.008‰ for samples dated 19 and 20ka, respectively. Late Holocene samples (1-2ka) were 0.641±0.012 and 0.660±0.013‰. 8-11ka BP samples varied between 0.628±0.011 and 0.647±0.009‰. A 56ka BP sample was 0.657±0.011‰. For calibration purposes a sample of a modern speleothem was analyzed, resulting in 0.642±0.006‰. Using the Δ47-temperature calibration of Ghosh et al (2006) results in a growth temperature of 26°C for the modern speleothem, higher than the estimated cave temperature over the growth period (18°C). This difference is consistent with kinetic isotope effects associated with CO2 degassing, possibly related to surface processes in speleothem production. We therefore corrected all the temperatures obtained by 8°C. This resulted in a temperature of 11-12°C for the LGM samples and 15°C at 56ka BP, similar to the temperatures estimated before for Soreq Cave, using fluid inclusions, and temperature estimates for the Eastern Mediterranean using alkenones. 14-18°C were obtained at 1ka. Temperatures of 17-21°C were obtained for the 8-11ka samples, slightly higher than the modern day temperatures. These temperatures were then combined with the δ18O values in measured carbonates to estimate the δ18O of cave water, resulting in LGM water being 1.1‰ more enriched than the modern, and Holocene values being more depleted than the modern (by 0.6‰ at 1ka and 1.0‰ at 8-11ka). Combining this with previously measured δD of fluid inclusions places the 1ka sample within the range of modern rain, close to the Mediterranean meteoric water line. An ~11ka sample is on the global meteoric water line, suggesting higher humidity then currently in the region, consistent with its time correlation with sapropel layers in Mediterranean sediments that are interpreted as reflecting high amounts of rain. The LGM samples are close to the global meteoric water line, and the 56ka sample is between the two lines, consistent with its intermediate temperature.
PP53A-03 INVITED
Fossil DNA Stratigraphy revealed Multiple Sources of Alkenones in the Holocene Black Sea at the Strain Level: Implications for UK37 Paleothermometry
The fossil distribution of long-chain alkenones is now a widely accepted tool to reconstruct past sea surface temperatures (SST) (i.e. UK37-index). In most studies, the UK37 index is calibrated for the main source of alkenones, the coccolithophorid haptophyte Emiliania huxleyi. Besides temperature, additional factors such as salinity, growth conditions, or different or multiple biological sources seem to influence the level of unsaturation of alkenones and the reliability of the UK37-inferred SST. The Black Sea is an interesting setting to study such factors since unreliable SST were reconstructed from the Holocene sapropel with high concentrations of an unusual "Black Sea" alkenone (C36:2 ethyl ketone) whereas calcium-bearing microfossils (coccoliths) of haptophytes are lacking. To identify Holocene sources for alkenones in the Black Sea at the unprecedented strain-level and to refine paleoenvironmental conditions, we searched for multiple fossil genetic signatures of haptophytes. This revealed that the slow increase in salinity as a result of post-glacial introduction of Mediterranean waters in the paleo lacustrine Black Sea, caused a succession between alkenone-biosynthesizing haptophytes from Isochrysis spp. (which do not produce coccoliths), to a mixture of Isochrysis and E huxleyi strains, then only E. huxleyi strains, and when the salinity reached a threshold of 18 per mille at 3000 years BP, the fossilized calcium-bearing E. huxleyi strain was introduced. At least 11 E. huxleyi strains were identified and the first non-fossilizing strains already colonized the Black Sea 4000 years before the fossilized calcium-bearing strain appeared. Most E. huxleyi strains were likely sources of C36:2 eK but the presence of one fossil "phylotype" coincided with the highest levels of this unusual alkenone ( more than 80 percent of the total alkenone content) and unreliable past SST (varying between 5 and 30 degrees C; 7500-5500 years BP). C36:2 eK was not biosynthesized by the Isochrysis spp. and the co-occurrence of Isochrysis did not play a significant role in the unreliable SST. Perhaps the past trophic status rather than a low salinity caused the single E. huxleyi strain to biosynthesize high levels of C36:2 eK since this unusual alkenone was recently also found in the oligotrophic DYFAMED station in the Ligurian Sea but its source remains to be identified.
PP53A-04
Growth-Phase Dependence of Hydrogen Isotope Fractionation in Long-Chain Alkenones from Emiliani huxleyi
Emiliani huxleyi CCMP1742, a paleoceanographic benchmark strain, was cultured for the purpose of isolating the possible effects of growth phase on water-alkenone hydrogen isotope fractionation. Given molecular evidence that much of the geologic alkenone record derives from stressed, senescent cells (Prahl et al. 2006), cultures were sampled during both the mid-log and senescent phases to bolster previous work investigating fractionation in log-phase samples. Analysis of the δD of extracted C37 alkenones via GC-TC-MS and the δD of the growth media via TCEA-MS is currently taking place. Preliminary results for the fractionation factor of senescent-phase cells (~0.772) agree well with those determined for different strains of E. huxleyi harvested during exponential growth (0.775, Englebrecht and Sachs, 2005) and at a two- to three-fold greater light saturated growth rate (0.779, Schouten et al., 2006). These results are suggestive when considered in light of recent work (D`Andrea et al., 2007) reporting significant isotopic differences between di- and tri-unsaturated C37 alkenones. Given the large changes in UK`37 that occurred in our isothermal (15°C) experiment between log- (~0.50) and senescent-phase (~0.30) growth, the similarity of our results to previous log-phase work may bolster D`Andrea's hypothesis that the isotopic fractionation of combined C37 alkenones represents an unchanging "weighted average". D`Andrea's preferred hypothesis, that di- and tri-unsaturated C37 alkenones exhibit unchanging, distinct water-alkenone fractionations, would lead us to predict a fractionation factor of 0.783 for the log-phase samples based on UK`37 measurements. This corresponds to a difference, if δDwater was calculated using alkenone values and the fractionation factor, of approximately ~14‰. The results necessary to compare log- and senescent-phase samples from the same cultures and determine which case appears to be true are now being generated and will be presented.
PP53A-05
The Influence of Salinity, Growth Rate and Temperature on D/H Fractionation in Algal Lipids from Culture and Field Studies
The use of compound-specific D/H ratios to decipher biochemical, geochemical, oceanographic, and climatic processes is expanding rapidly. The relative success of these efforts depends on an understanding of the environmental conditions that influence the deuterium depletion relative to environmental water observed in all plant, algal and bacterial lipids, and the sensitivity of D/H fractionation responses to changes in those environmental conditions. Presently very little is known about this interplay between the environment and D/H fraction in algal lipids. Here we present results from field studies (in the Chesapeake Bay, Christmas Island, the Great Salt Lake, and saline basins in Alberta and Saskatchewan) and culture studies (both continuous and batch) that indicate that salinity, growth rate and temperature each influence D/H fractionation in algal lipids to varying degrees, depending on the algae and the lipid. Our initial results indicate that D/H fractionation (1) decreases with increasing salinity, (2) increases with increasing growth rate in isoprenoid lipids, (3) is insensitive to growth rate in acetogenic lipids, and (4) increases with increasing temperature.
PP53A-06
Climatic and physiological controls on the stable isotope composition of modern and ancient Cupressaceae
Unique and abundant secondary metabolites found in waxes and resins of the Callitroid, Cupressoid, and Taxodioid clades of the Cupressaceae family can be identified and quantified in complex mixtures of sedimentary organic compounds. This unusual feature makes it possible to study relatively simple (taxon-specific) isotope systems back in time across the broad array of environments in which these conifers are found. Work on these systems can potentially provide both robust paleoenvironmental proxies (i.e. for source water δD and growing season relative humidity) and quantitative probes into the ecophysiology of these plants in modern and ancient environments. Our research focuses on three genera representing environmental end-members of Cupressaceae - Juniperus, Thuja, and Chamaecyparis - (1) across geographic and environmental gradients in the field, and (2) in specific Holocene and late Pleistocene environmental records. The latter research focuses on peat cores from New England and Oregon and fossil packrat middens from the southwestern United States. Modern transects highlight the sensitivity of Cupressaceae to climatic variables. These include both variables during growth (relative humidity, soil moisture, etc.) and variables affecting seasonal and diurnal growth rates (temperature, winter precipitation, insolation, microhabitat, etc.). Work on ancient records has demonstrated the sensitivity of these unique taxon-specific archives to both subtle and dramatic climate shifts during the Pleistocene and Holocene. This work will result in an improved understanding of climatic and physiological controls on the stable isotopic composition of modern and ancient Cupressaceae - and by extension, other arborescent gymnosperms and C3 plants - providing a framework for understanding more complexly sourced organic inputs to sediments, coals, and petroleum prior to the advent of C4 plants. This research also has direct implications for stratigraphic stable isotope studies of gymnosperm markers across the last millenium, the Pleistocene, and important climatic events in the Mesozoic and Tertiary.
PP53A-07
Exploring the Source of Sedimentary Archaeal Lipids by Comparative Radiocarbon Analysis of Alkenones and GDGTs.
The utility of biomarker-based paleotemperature proxies depends critically on the assumption that the alkenones of haptophyte algae or glycerol dialkyl glycerol tetraethers (GDGTs) of marine archaea are exported to the sediment and that they are buried quickly. However, lessons from compound-specific radiocarbon analysis of sedimentary organic matter have shown that sediments contain a heterogeneous mixture of organic compounds with different radiocarbon ages. More recent work has shown that even individual-compound radiocarbon values can represent contributions from differently-aged sources. In the case of the alkenones extracted from Bermuda Rise sediment cores, radiocarbon ages are offset from foraminiferal ages by up to 7000 years (1). This discrepancy indicates that the total sedimentary alkenone pool has a significant contribution from a pre-aged source in addition to export of surface production. This source of aged alkenones is believed to be their transport in association with fine-grained sediments. Presumably it is enabled in part by the relatively high resistance of alkenones to biodegradation. GDGTs also are highly resistant to degradation and are further complicated by the fact that they may be produced throughout the water column, not just at the surface. In this study, we present measurements of archaeal GDGTs from Santa Monica Basin, an area of pronounced sediment redistribution and focusing, and Bermuda Rise, a location known to accumulate drifting sediment. We account for the expected reservoir effect associated with archaeal autotrophy and use a mass balance model to constrain the amount of sedimentary GDGTs that may come from exported surface production rather than deep water column production or sediment re-distribution. The results suggest that paleotemperature reconstructions based on GDGTs and alkenones are affected similarly by sediment transport and that site selection is critical for accurate paleotemperature reconstructions. Reference: (1) Ohkouchi N, Eglinton TI, Keigwin LD, Hayes JM (2002) Science 298:1224-1227.
PP53A-08
Late Quaternary Environmental Changes Inferred from the stable Oxygen Isotope Composition of Aquatic Insects (Chironomidae: Diptera) and Stable Hydrogen Isotope Composition of bulk sediments from Idavain Lake, Southwest Alaska
Several techniques are available to examine the isotopic composition of historic lake waters, providing data that can subsequently be used to examine environmental changes. Recently-developed techniques are the stable oxygen isotope analysis of subfossil chironomid (Diptera: Chironomidae) head capsules (mostly chitin) preserved in lake sediments and stable hydrogen isotope analyses directly on bulk sediments. An advantage of using δ18O of chironomids is that the chitinous chironomid headcapsules preserve well in lake sediments, retaining the stable oxygen isotope signature of the lake in which they lived. An advantage of δD analyses of bulk sediments is that a sediment core can be analyzed relatively easily and when the %C (total organic carbon) and %H profiles correlate the data can be used to infer past δD changes of the organics in the sediments. We present results from these analyses of a lake sediment core from Idavain Lake (58°46'N, 155°57'W, 223m above sea level) in southwest Alaska in concert with other paleolimnological proxies, including δ15N, δ13C, LOI, magnetic susceptibility, organic content and opal concentrations for a better understanding of paleolimnological changes since deglaciation for the region. Our preliminilary result shows that downcore shifts of δ18O analyzed from chironomid head capsules coincide well with LOI and pollen changes. The δD of sediments and TOM showed large magnitude changes and reflected the relative lake level changes during the record. This study aim to test the correlation between stable isotope analyese on chiornomid head capsules, lake water, and bulk sediments. In the addition, our study will add to the relatively small database of paleoenvironmental reconstructions from terrestrial sites in Southwest Alaska.