PP51D-01 INVITED
Novel Proxies for Continental Palaeo-temperature and Soil pH Reconstruction Based on Tetraether Membrane Lipids of Soil Bacteria
Several proxies based on organic compounds exist of which the Uk'37 and the TEX86 are now well established quantitative temperature proxies applied in the marine realm. Although biomarker proxies exist for the terrestrial realm, none of them can be used to quantitatively estimate continental temperatures. Here we introduce two new proxies based on terrestrial derived branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids to quantitatively estimate continental palaeo-temperatures and palaeo-soil pH. Branched GDGTs are abundant constituents of soils and peat bogs world wide and presumably of bacterial origin based on their stereo configuration. The distribution of individual branched GDGTs, varying in methyl branching and cyclopentanyl content, differs over the 150 soils analyzed. Principle component analysis showed a significant correlation between the degree of cyclisation of branched GDGTs (CBT) and soil pH. Another significant correlation was evident between the degree of methylation (MBT) and both soil pH and annual mean air temperature. This is explained by the fact that microorganisms adjust the composition of their cell membrane as an adaptation to changing environmental conditions. A focused study on soil transects off hot springs confirmed the temperature and pH dependence of the lipid distribution. Upon soil erosion and fluvial transport, branched GDGTs are deposited in near shore marine sediments. Being part of the marine sedimentary archive, branched GDGTs offer the opportunity to reconstruct long term river basin integrated temperature signals for the adjacent continent. A study of branched GDGT distributions in the Congo deep sea fan revealed a gradual temperature rise of 4°C over the last deglaciation in central Africa. Direct comparison with sea surface temperature estimates from the same core pointed to a dual influence of marine and terrestrial temperature development on the central African hydrological cycle. The presence of branched GDGTs in Palaeocene-Eocene sediments from the Arctic Ocean and in a German lignite of Early Cretaceous age, shows the potential of this new proxy on longer time scales.
PP51D-02 INVITED
Marine Archaea lipids: patterns and provenance in the water-column
The paleotemperature proxy TEX86 (TetraEther indeX) is based on a robust correlation between the ratio of marine archaeal lipids (glycerol dialkyl glycerol tetraethers (GDGTs) with different numbers of rings), and temperature. The potential paleoceanographic utility of this proxy is tremendous, and it has already been used successfully in a number of paleoenvironments (e.g. Younger Dryas detection in the southern hemisphere) but poses problems in others (e.g. the southern North Sea with high sediment input). The physiological, phylogenetic, and physical controls on this correlation remain speculative and difficult to test. Different archaeal subgroups live throughout the water column (some with unknown metabolisms), only one representative of marine Crenarchaeota and no marine Euryarchaeota are available in pure culture for experimentation, and lipid ratios observed in enrichment versus in situ samples inexplicably differ. We hypothesize that lipid distribution patterns are affected by ecological changes. We measured archaeal lipids from globally distributed samples of freshwater, marine, and hypersaline suspended particulate matter. Cluster analysis of relative lipid distributions identified four distinct groups, including: 1) marine epipelagic waters, 2) marine mesopelagic/upwelling waters, 3) freshwater/estuarine waters, and 4) hypersaline waters. We propose that community changes regulate the lipid patterns distinguishing these groups, including epipelagic and mesopelagic/upwelling zones. Lipid patterns in mesopelagic/upwelling waters are similar to those expected for nitrifying Group I Crenarchaeota-- predominance of crenarchaeol and abundant cyclic GDGTs. Non-metric multidimensional analysis shows this pattern is associated with high nitrate concentrations. Furthermore, the difference between calculated TEX86 temperature and in situ temperature from surface to deep waters correlates with nitrate concentrations, enforcing a connection between community change (esp. increase/dominance of nitrifying Crenarchaeota) and TEX86 values. The combination of cluster analysis and ordination of lipid patterns, contextualized with environmental factors suggest observed values of TEX86 are subject to changes in archaeal ecology, possibly influenced by nutrient fluctuations or other perturbations can affect both surface and deeper water GDGT production, preservation, and transport. In paleoceanographic applications, TEX86 lipids may not record only temperature, but equally interesting changes in nutrient concentrations, oceanographic conditions, and ecology.
PP51D-03
Towards a paleo-salinity proxy: Decreasing D/H fractionation in algal and bacterial lipids with increasing salinity in Christmas Island saline ponds
We investigated the effect of a wide range of salinities (13 -149 PSU) on the D/H ratio of lipids in microbial mat sediments from hypersaline ponds on Christmas Island. The hydrogen isotope ratios (expressed as δD values) of total lipid extracts, and the individual hydrocarbons heptadecane, heptadecene, octadecane, octadecene, diploptene and phytene from algae and bacteria, became increasingly enriched in deuterium as salinity increased, spanning a range of 100‰ while lake water δD values spanned a range of just 12‰. D/H fractionation between lipids and source water thus decreased as salinity increased. Isotope fractionation factors (αlipid-water) were strongly correlated with salinity and increased in all compound classes studied. The apparent isotope fractionation (εlipid-water) decreased by 0.8 to 1.1‰ per PSU increase in salinity. Differences in the hydrogen isotopic composition of lipids derived from three biosynthetic pathways (acetogenic, MVA and DOXP/MEP) remained similar irrespective of the salinity, suggesting that the mechanism responsible for the observed αlipid-water – salinity relationship originates prior to the last common biosynthetic branching point, the Calvin Cycle. These findings imply that caution must be exercised when attempting to reconstruct source water δD values using lipid δD values from aquatic environments that may have experienced salinity variations of ~3 PSU or more (based on a 1‰ per PSU response of D/H fractionation to salinity changes, and a lipid δD measurement precision of 3‰). On the other hand our results can be used to establish a paleo-salinity proxy based on algal and bacterial lipid δD values if salinity variations exceeded ~3 PSU and/or if additional constraints on source water δD values can be made.
PP51D-04
Alkenone-Based Temperature Reconstruction From Lakes of Southwestern Greenland: Insights from a haptophyte bloom and a new isotopic approach
We have previously reported high concentrations of alkenones from the laminated sediments of a series of brackish lakes in the Sondre Stromfjord region of southwestern Greenland (67°N, 51°W). Down-core analyses from three of these lakes demonstrate that UK37 reconstructed from the lake sediments reflects regional temperature variability during the Holocene. Furthermore, comparison of the Greenland UK37 temperature reconstruction with other high-resolution temperature reconstructions from the North Atlantic reveal spatial patterns which vary over millennial time scales and resemble those associated with the North Atlantic Oscillation. While the Greenland alkenone reconstructions appear robust, the producer organism of lacustrine alkenones (a haptophyte alga) remains unidentified and the growth season and precise habitat of lacustrine haptophytes are unknown. To address these issues, we deployed sediment traps in an alkenone-bearing lake (Braya So) of southwestern Greenland during 2006 and 2007. The automated sediment traps collected discrete seston samples from the lake at ten day intervals. Analysis of the sediment trap material reveals alkenone production in Braya So occurred between mid-June and early July, with more than 50% of alkenone sedimentation occurring over a ten day period in late June. We were present during the haptophyte bloom in June 2007 and filtered water throughout the water column of Braya So to investigate the depth habitat of the alkenone-producing organisms. Alkenone concentration data obtained from the filters elucidate the depth habitat of the haptophytes. UK37 data from the water column filters, along with direct water column temperature measurements, demonstrate a strong temperature dependence on alkenone unsaturation and allow the first lacustrine in situ UK37-temperature calibration. In addition, δ13C measurements made on alkenones from the water column filters and the sediment trap material provide insight into the role of haptophytes in carbon cycling within Braya So. We captured the suspected alkenone-producer and attempts to culture the organism are under way. New down-core isotopic data, which complement the Holocene UK37 reconstruction, will also be presented. These include bulk sediment δ13C and δ15N, as well as δ D reconstructed from individual alkenones; a record generated using a newly developed alkenone separation technique.
PP51D-05
Stable Carbon Isotope Ratios of Individual Pollen Grains as a Proxy for C3- Versus C4-Grass Abundance in Paleorecords: A Validation Study
C3 and C4 grasses have distinct influences on major biogeochemical processes and unique responses to important environmental controls. Difficulties in distinguishing between these two functional groups of grasses have hindered paleoecological studies of grass-dominated ecosystems. We recently developed a technique to analyze the stable carbon isotope composition of individual grass-pollen grains using a spooling- wire microcombustion device interfaced with an isotope-ratio mass spectrometer (Nelson et al. 2007). This technique holds promise for improving C3 and C4 grass reconstructions. It requires ~90% fewer grains than typical methods and avoids assumptions associated with mixing models. However, our previous work was based on known C3 and C4 grasses from herbarium specimens and field collections and the technique had not been test using geological samples. To test the ability of this technique to reproduce the abundance of C3 and C4 grasses on the landscape, we measured δ13C values of >1500 individual grains of grass pollen isolated from the surface sediments of 10 North American lakes that span a large gradient of C3 and C4 grass abundance. Results indicate a strong positive correlation (r=0.94) between % C4-grass pollen (derived from classifying δ13C values from single grains as C3 and C4) and the literature-reported abundance of C4 grasses on the landscape. However, the measured % C4-grass pollen shows some deviation from the actual abundance at sites with high proportions of C4 grasses. This is likely caused by uncertainty in the magnitude, composition, and variability of the analytical blank associated with these measurements. Correcting for this deviation using regression analysis improves the estimation of the abundance of C4 grasses on the landscape. Comparison of the % C4-grass pollen with C/N and δ13C measurements of total organic matter in the same lake-sediment samples illustrates the distinct advantage of grass-pollen δ13C as a proxy for distinguishing C3 and C4 shifts. At 9 of the 10 sites C/N values indicate that surface-sediment organic matter was derived primarily from aquatic production. At the one site where organic matter was produced primarily by vascular plants the δ13C value (-29.3‰) suggests organic matter derived exclusively from C3 plants. However, ~80% of the grasses on the landscape at this site are C4 grasses. The C3- like bulk-sediment δ13C value likely represents woody species, which comprise >90% of the pollen spectra. Thus δ13C analysis of single grains of grass pollen offers a new tool to classify grass pollen into two major functional groups and promises to advance our understanding of grassland ecology and evolution. Reference Nelson, D.M., Hu, F.S., Mikucki, J., Tian, J., and Pearson, A., 2007, Carbon isotopic analysis of individual pollen grains from C3 and C4 grasses using a spooling wire microcombustion interface: Geochimica et Cosmochimica Acta, v. 71, p. 4005-4014.
PP51D-06
Reconstructing paleoaltimetry with D/H isotopic measurements on lipid biomarker
Estimates of the paleoelevation in mountain regions is an important variable in tectonics and climate research. These estimates provide a benchmark to compare with models of mountain formation and records of uplift, erosion and climate. However, past elevation is a difficult variable to quantify. During orographic uplift of an airmass, isotopic distillation of the water vapor results in a predictable isotopic gradient with altitude. Using this gradient, paleoelevations can be calculated from reconstructions of the isotopic composition of precipitation at low- and high-elevations sites. Measurement of the deuterium/hydrogen isotopic ratio (δD) of lipid biomarkers preserved in mountain regions provides a method to reconstruct precipitation δD composition and elevation through time. Isotopic results indicate that the δD of plant leaf-wax n-alkanes preserved in both modern and ancient lake sediments can be used to reconstruct elevation. Lipid δD-based elevation estimates from the sediments of modern alpine lakes successfully predict the actual lake elevation. Elevations inferred from lipid δD analyses of lake basin sediments deposited on the Tibetan plateau over the past 40 million years are in close agreement with carbonate δ18O- and paleoflora-based estimates for the same sediments. Important variables to consider when interpreting n-alkane δD measurements for paleoaltimetry are the type of plants contributing leaf-wax compounds to the sediment, the aridity of the study site relative to the calibration sites for the lipid-water fractionation factor, and the thermal maturity of the organic matter.
PP51D-07
A compound specific nitrogen isotope record utilizing multiple geoporphyrins and chlorins for Oceanic Anoxic Event 2 at Demerara Rise
Compound specific nitrogen isotope analyses on geoporphyrins and chlorins have the unique potential in that they can link nitrogen isotope values directly to chlorophyll and primary producers. Nitrogen isotope values from bulk sediments have proven useful for numerous recent studies investigating the state of the nitrogen cycle in the past, however questions remain as to the fidelity of such records. Bulk sedimentary nitrogen can be composed of both organic and inorganic fractions and organic nitrogen compounds such as proteins are very labile and degraded rapidly in the early diagenetic environment. It is therefore advantageous to utilize, where possible, compounds that resist such degradation, are easily extracted from sedimentary organic matter and recalcitrant on geologic time scales. Several nitrogen isotope records of bulk sediments from Oceanic Anoxic Event 2 have values that are typically less than 0 permil and in some cases show a 2-3 permil decrease through the OAE. These values are interpreted to be the result of primary producers utilizing nitrogen substrates supplied by nitrogen-fixing cyanobacteria that reflect the isotopic composition of their atmospheric nitrogen source. Nitrogen isotope data from geoporphyrins and chlorins from sediments recovered during Ocean Drilling Program Leg 207 to Demerara Rise deposited during OAE 2 will allow us to assess this hypothesis. The geoporphyrin and chlorin moiety is dominated by high concentrations of metallo and free-base C33 bicycloalkanoporphyins (BiCAPs) but also contains small amounts of other porphyrins, the most notable being the C34 and 35 porphyrins that are derived from bacteriochlorophylls. The high concentration of geoporphyrins and chlorins has allowed for analyses of the nitrogen isotopic composition of up to 4 compounds from each sample but only the BiCAP chlorins have been isolated in sufficient concentrations from each sample thus far. Initial nitrogen isotope data through OAE 2 for the BiCAP chlorins and vanadyl BiCAPs reflect the negative excursion in bulk N-isotopic values, however, the range of values is greatly expanded and the negative excursion is much larger. Immediately prior to OAE 2 N-isotopic values for the C33 BiCAP chlorins are +7.1 permil and drop to -3.9 permil during the event and return to 2.7 permil. The vanadyl C33 BiCAPs follow a similar trend with values starting at +1.6 permil and decrease to -6.0 permil. These data support earlier hypotheses suggesting large changes in oceanic nitrogen cycling during OAE 2 on the basis of bulk nitrogen isotope values. However the large range and anomalously positive N-isotopic values outside of the OAE suggest that we have either greatly underestimated the change in nitrogen cycling during OAE II or that diagenetic processes have resulted in fractionation between the geoporphyrin and chlorin fractions.
PP51D-08
Biomass Burning, Long-Range Atmospheric Transport and the Sedimentary Record of Plant Wax Biomarkers
Sedimentary distributions of plant leaf wax molecular and isotopic composition can provide detailed information about past terrestrial ecosystem structure and its variability in response to climatic forcing. However, in many locales (e.g. marine sediments, high elevation lakes), sedimentary plant waxes are derived primarily from atmospheric deposition rather than from local fluvial input or direct runoff. Thus, an understanding of wax atmospheric transport and deposition is essential for accurate interpretation of the sedimentary signal. In this talk we synthesize results from our studies of wax aerosol composition and atmospheric transport at strategically located sites (Northern Alaska, Maine, Florida, Bermuda, Barbados, French Guiana) that sample continental air masses passing over major terrestrial ecosystems (tundra, North American boreal, temperate and southern pine forests, North African desert grasslands, Amazon rain forest). Wax aerosols in boundary layer air masses reflect a large regionally integrated source signal. Over the North Atlantic, the long-range atmospheric transport of plant waxes is essentially uncorrelated with episodes of high African dust transport. Rather, the highest plant wax aerosol concentrations are clearly associated with continental air masses that are laden with smoke from biomass burning, which enhances long-range transport both by the process of steam distillation of wax and other easily volatilized compounds off living (moisture-rich) vegetation in the advancing front of the fire and by deep atmospheric convection, which efficiently injects re- condensed particles into the lower troposphere where they can be most efficiently transported by high altitude winds. The direct linkage between enhanced long-range atmospheric transport of plant waxes and biomass burning suggests that the wax sedimentary record in localities dominated by atmospheric input strongly co-varies with climate-driven changes in fire frequency and is compositionally biased towards the ecosystem structure of the burned source regions.