517 research outputs found
Alkenone concentration and sea surface temperature in surface sediments
No full textAlkenone concentration and radiocarbon age in 7 gobally-distributed surface sediment samples and their associated grain-size fractions. Total organic carbon (TOC) and fractional abundance of grain-size fractions from bulk sediments (Bulk%) are taken from Ausín et al. (2021). Analytical precision of Uk'37 is 0.003 units. Uk'37-SST propagated error is ±0.51℃. These parameters were measured to explore the influence of alkenone-mineral associations and hydrodynamic mineral sorting processes on alkenone proxy signals. Bulk sediment samples were fractionated into four grain-size fractions (sand (>300-63 µm); coarse silt (63-10 µm); fine silt (10-2 µm); and clay (< 2 µm) prior to lipid extraction and manual column chromatography to obtain a ketone fraction containing the alkenones. The concentration and distribution of C37 alkenones was analyzed using gas chromatography with flame ionization detection (GC-FID) at the Biogeoscience Group Laboratories, ETH Zürich in 2018. The ketone fractions used for determination of alkenone concentration and unsaturation were further purified for compound specific radiocarbon analysis following Ohkouchi et al. (2005). Samples, were measured as CO2 using an Elemental-Analyzer system interface coupled to a gas ion source (GIS)-equipped Minicarbon Dating System (MICADAS) at the Laboratory of Ion Beam Physics, ETH Zürich in 2018. References Ausín, B., Bruni, E., Haghipour, N., Welte, C., Bernasconi, S. M., & Eglinton, T. I. Controls on the abundance, provenance and age of organic carbon buried in continental margin sediments. Earth and Planetary Science Letters, 558, 116759, doi:10.1016/j.epsl.2021.116759, 2021. Ohkouchi, N., Xu, L., Reddy, C. M., Montluon, D., & linton, T. I. Radiocarbon dating of alkenones from marine sediments: I. Isolation Protocol Radiocarbon, 47, 401-412, doi:10.1017/S0033822200035189, 2005
Bacterial Tetraether lipids in lacustrine environments – implications for their use as Paleoclimate proxies
Get PDFAnthropogenic global warming poses a long-term threat to human society. The reliable prediction of future climate evolution, however, not only builds upon our understanding of present-day processes, but also requires profound knowledge of Earth system responses on timescales exceeding the range of instrumental observations. Current numerical simulations steadily grow in complexity, increasing the need for high-quality paleoclimate data as a means of model evaluation, especially from the spatially heterogeneous terrestrial realm. One promising approach for the reconstruction of past continental temperatures is based on organic-geochemical analyses of bacterial tetraether lipids (i.e., branched GDGTs), which are prevalent in the environment, and show systematic compositional changes with ambient temperature. Several studies established quantitative relationships between atmospheric T and brGDGT distribution in modern lake sediments, yet, application of these transfer functions to different limnological settings does not always yield robust paleotemperature estimates. Attempts for further improvement of the brGDGT proxy are currently hindered by the fact that the vast majority of brGDGT-producing microbes are still unidentified, and the ecophysiological mechanisms behind the apparent T response are unknown. In order to shed light on the ecology and lifestyle of these microbes in lacustrine systems, we investigated a variety of recent lake sediments from the Swiss and Italian Alps, as well as suspended- and sinking biomass from the permanently stratified north basin of Lake Lugano (Southern Switzerland), by a combination of stable isotope-, geochemical-, and molecular biological methods.
Almost 50 % of the investigated lake deposits contained an as yet unknown brGDGT isomer that was not detectable in soils collected from the catchments, which provided unprecedented molecular evidence for brGDGT biosynthesis within lacustrine systems. This compound was also abundantly present in hypoxic and anoxic deep water of Lake Lugano. Strikingly, however, it was completely absent from the overlying oxic waters, implying an exclusive origin from microbes adapted to low dissolved oxygen concentrations. In contrast, concentration profiles and stable carbon isotope data show that another structural brGDGT isomer is dominantly synthesized by bacteria inhabiting oxygenated waters, attesting to a vertical differentiation of the brGDGT-producing microbial community. In order to constrain the identity of the source organisms, we investigated prokaryotic diversity by sequencing of ribosomal DNA, with special emphasis on members of the phylum Acidobacteria — the only group known thus far to produce brGDGTs. Indeed, members of different acidobacteria subdivisions showed a depth differentiation that is reminiscent of the concentration gradients found for individual brGDGT lipids, with a number of uncultured representatives exclusively occurring in the anoxic water column.
In Lake Lugano, we further studied the vertical transport of brGDGTs by deploying three sediment traps, (i) at the base of the thermocline, (ii) at the redox transition zone, and (iii) 90 m within the anoxic water body. The brGDGT content of settling organic particles increased successively from the upper towards the lower trap, attesting to pronounced hypolimnetic in-situ production under both, oxic and hypoxic/anoxic conditions. Subsequent laboratory experiments showed quick uptake of isotopically labeled organic C into brGDGTs from surface water, and a ~100-fold abundance increase within six weeks, demonstrating a high potential for aerobic brGDGT production by heterotrophic bacteria. Surprisingly, however, a corresponding anoxic incubation with biomass from the bottom waters of Lake Lugano did not result in measurable label uptake, suggesting that anaerobic brGDGT production may be limited to the redox transition zone (RTZ) where alternative electron acceptors such as nitrate are still present.
Our work in Lake Lugano also revealed a steep decline in the natural 13C content of brGDGTs (δ13CbrGDGT) at the RTZ, most likely related to the cycling of 13C-depleted organic compounds derived from the oxidation of methane. This is consistent with the fact that in the majority of the investigated lakes, brGDGTs were substantially depleted in 13C (up to 10 ‰) compared to the bulk of sedimentary organic C (δ13CTOC). Indeed, we found significantly lower δ13CbrGDGT values in meso-eutrophic lakes that exhibit bottom water anoxia, than in oligotrophic and fully-oxygenated settings, which is best explained by the co-occurrence of methane-oxidizing- and brGDGT-producing bacteria in the vicinity of the RTZ.
The 36 lakes investigated in this study span an altitudinal air temperature difference of ~10 °C. However, the correlation between mean annual air temperature (MAT) at the lake sites and commonly applied brGDGT proxy indices in the surface sediments was poor (R2=0.1), contradicting previously reported data from lakes of other regions. In light of the important role of deep water-derived brGDGTs in lake sediments we show here, we reason that the brGDGT–MAT relationship in our data set is likely compromised by the variable contribution of brGDGTs from terrestrial, shallow-, and deep water pools, each of which carry distinct temperature imprints.
Specialized lipid extraction techniques are required for the recovery of GDGTs from living biomass. We tested three commonly used protocols on suspended particulate matter from Lake Lugano, and found substantial extraction biases affecting the intact polar lipid (IPL) fraction of brGDGTs. Up to 90 % of the IPL pool were not accessible by means of normal ultrasonic solvent extraction, nor were they recovered with the widely applied, phosphate-buffered ‘modified Bligh-Dyer’ protocol. Near complete IPL recovery was, however, only achieved after substitution of the P-buffer with a solution of 5% trichloroacetic acid. Our results therefore suggest that previous studies may have substantially underestimated the ‘living fraction’ of brGDGTs and its contribution to the total lipid pool
GDGT concentration and stable carbon isotope from Mackenzie Delta lakes
No full textGlycerol Dialkyl Glycerol Tetraethers (GDGTs) have been analyzed from lacustrine sediment cores (3 lakes) in the Mackenzie Delta (Northwestern Territories, Canada). The cores are covering the last 200 years. GDGT have been extracted using EDGE and separated over aluminium oxide column using dichloromethane:methanol (1:1), then filtered. They have been quantified using an internal standard on a HPLC (high precision liquid chromatograph). Their compound-specific 13C ratios have been measured at low resolution (~3 samples per cores) on a SWIM-IRMS (spooling-wire microcombustion device interfaced with an isotope-ratio mass spectrometer). Their values have been compared to 13C of the bulk and 13C of diplopten to assess their sources and the metabolism of their producers
Bulk characteristics, lipid quantification and compound-specific carbon isotope ratios of Mackenzie Delta lake sediments
No full textThe Arctic is undergoing accelerated changes in response to ongoing modifications to the climate system, and there is a need for local to regional scale records of past climate variability in order to put these changes into context. The Mackenzie delta region in northern Canada is populated by numerous small shallow lakes. They are classified as no-, low- and high-closure (NC, LC, HC, respectively) lakes, reflecting varying degrees of connection to the river main stem, and have different sedimentation characteristics. This study examines sedimentological (mineral surface area, grain size), carbon isotopic (bulk and molecular-level) and inorganic isotopic (neodymium) characteristics of sediment cores from three lakes representing each class. We find that HC lake sediments exhibit strikingly different properties from the other lake sediments. Specifically, they are characterized by higher organic carbon loadings per unit mineral surface area and with relatively minor influence from allochthonous, petrogenic (rock-derived) organic carbon. In contrast, LC and NC lakes, have the potential to record basin-scale climatic changes at a high resolution by virtue of enhanced detrital sedimentation. Overall the delta have the capacity to bury about 2 MtC yr-1, with little changes since 200 years
Sea Surface Temperature estimates and benthic foraminifera stable isotopes from IODP 1406 over the Oligocene Miocene time interval
No full textHere, we present a new record of SST for the time interval 30 to 17 Ma derived from the long‐chain alkenone unsaturation ratio uk'37 and TEX 86 at Integrated Ocean Drilling Program Site 1406A in the midlatitude North Atlantic. Results confirm that warm temperatures from 24°C to over 30°C prevailed in midlatitudes in this time and suggest a transition from colder early‐middle Oligocene to warmer average conditions after 24.5 Ma. Complex and temporally varying relationships are observed between North Atlantic SST and benthic δ18O in paired samples. Significant covariation is only observed around the Oligocene‐Miocene transition, coincident with a lower average marine ice extent. These North Atlantic temperature records provide a new context in which to examine the stability of climate and the Antarctic ice sheet during the Oligocene and early Miocene
Compositions of sedimentary organic matter of surface sediments in the northern South China Sea
No full textThis dataset includes total organic carbon (TOC), stable carbon isotope (δ13C), and radiocarbon activity (expressed as Fm) of TOC, as well as mineral surface area (SA), mean grain size, and calcium carbonate (CaCO3) of 89 surface sediment samples from the northern South China Sea. The samples were collected through the German R/V SONNE cruises SO50, SO72A, SO95, and SO132 during July–September 1987, October–November 1990, April–June 1994, and June–July 1998, respectively. TOC and stable carbon isotope (δ13C) of the decarbonated samples were measured by an elemental analyzer interfaced with an isotope ratio mass spectrometer (EA-IRMS; Flash EA 1112 HT-Delta V Advantage). The standard deviations of TOC content and δ13C were 0.03% and 0.08‰, respectively, by replicate measurements of the same acetanilide standard samples. Radiocarbon activity of carbonate free sediment were measured using a Mini Carbon Dating System (MICADAS) accelerated mass spectrometry at the Laboratory of Ion Beam Physics, ETH Zürich. Radiocarbon data are reported as "fraction modern" (Fm), representing the deviations of the 14C/12C ratio from 95% modern oxalic acid standard. The absolute error of Fm is better than ±0.01. The conventional radiocarbon ages are calculated based on the equation of t = –8033 ln (Fm), in years Before Present (yr BP, where present is 1950 AD). Organic-free mineral surface area (SA) was measured using a 5-point Brunauer-Emmett-Teller (BET) nitrogen adsorption method on NOVA 4000 SA analyzer (Quantachrome Instrument). The standard deviations of two reference minerals are 0.03 and 0.14 m2 g–1, respectively. The sample was then performed for mean grain size analysis using a laser Particle Size Analyzer (Malvern Mastersizer 2000). The calcium carbonate content of the samples was determined by a vacuum-gasometric system with a relative accuracy of ±0.7%
REIT capital structure : an examination of the use of unsecured debt over traditional equity and changes in dividend policy
Get PDFThesis (M.S.)--Massachusetts Institute of Technology, Dept. of Urban Studies and Planning, 1997 (first author), and Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Architecture, 1997 (second author).Includes bibliographical references (leaves 97-98).by Joshua T. Anderson and Ian R. Ponniah.M.S
(Table 3a) Carbon isotopic composition of n-alkanes and n-alkanoic acid biomarkers of DSDP Hole 24-231
No full text(Table 3a) Carbon isotopic composition of n-alkanes and n-alkanoic acid biomarkers of DSDP Hole 24-23
- …
