Organic Carbon Isotopic Composition Research Articles

Variations of organic carbon isotopic composition and its environmental significance during the last glacial on western Chinese Loess Plateau

A high-resolution loess section in the western Chinese Loess Plateau, Yuanbao Section, was sampled for organic carbon isotopic analyses. The soil organic carbon isotope (δ 13Corg) varied between −22.6‰ and −27.5‰ during the last glacial at the section. During the last interstadial, the δ 13Corg values were more negative than those in both early and late periods of the last glacial by 4‰ The isotopic composition indicates a coupled response of the pure C3 plants to the temperature, precipitation and the concentration of atmospheric CO2. Decrease in temperature and the atmospheric CO2 concentration from the last interstadial to Last Glaicial Maximum (LGM) caused the organic carbon isotopes to become positive by 1.5‰–2.0‰ The amplitude of 4‰ in the δ 13Corg variation during the last glacial should be mainly caused by the precipitation change. Therefore, the δ 13Corg variations of the Yuanbao Section during the last glacial period documented the large-amplitude fluctuation of the monsoon precipitation, which is estimated to be 250–310 mm more during the last interstadial than that in the LGM, and 100 mm more than that during early last glacial. The rapid changes of the monsoon precipitation on millennial scale during the last glacial have also been recorded in the isotopic variations in Yuanbao loess section. As the isotopic composition varies complicatedly as shown in the loess-paleosol sequence, it cannot be simply attributed to the abundances of C3 and C4 plants or be used as an indicator of the summer monsoon variations.

The land plant δ13C record and plant evolution in the Late Palaeozoic

Based on the evaluation of 1323 carbon isotope values for Silurian to Permian terrestrial organic carbon, measured on plant fossils, cuticules, humic coals and bulk terrestrial organic matter (TOM), we conclude that the temporal trend in δ 13C TOM records variations in the global carbon cycle, notably an increase in the fractional burial of light (terrestrial) organic matter in Late Palaeozoic sediments. δ 13C TOM values suggest that the Late Palaeozoic pO 2 peak could have been restricted to a time frame of ∼ 40 Ma. Carbon isotope data from four taxonomic groups reveal small differences that could be a consequence of habitat conditions. No significant differences in organic carbon isotopic composition in relation to variable climatic conditions are discernible. The carbon isotopic composition solely reflects C 3 plant metabolism.

Distribution and sources of particulate organic matter in the water column and sediments of the Fly River Delta, Gulf of Papua (Papua New Guinea)

Suspended particles from surface and bottom waters and surficial sediments from the seabed were collected throughout the Fly River subaqueous delta region during the monsoon season in January 2003. Because of the unusually low river discharge associated with a strong El Niño, water-column salinities were relatively high (10 to 32) throughout most of the delta, with brackish salinities (<10) only measured within the main distributary channel of the Fly River. The concentration and composition of particulate organic matter (POM) in these samples showed distinct spatial differences and marked contrasts between water column and seabed samples. Overall, relatively low concentrations of total suspended solids (TSS), particulate organic carbon (POC) and particulate nitrogen (PN) were measured in surface (27 ± 50, 0.83 ± 1.2, and 0.05 ± 0.05 mg/L, respectively) and bottom (400 ± 743, 7.1 ± 7.3, and 1.7 ± 7.2 mg/L, respectively) waters throughout the delta. Particles in both surface and bottom waters displayed elevated organic carbon contents (%OC > 4 wt.%), relatively high organic carbon:nitrogen molar ratios (OC:N > 20 mol:mol) and quite depleted stable isotopic compositions of organic carbon (δ 13C OC < −27‰). In contrast, surface sediments in the seabed displayed spatially uniform compositions that were characterized by markedly lower %OC contents (1.1 ± 0.8 wt.%), lower OC:N ratios (17 ± 9 mol:mol) and relatively enriched δ 13C OC compositions (−25.5 ± 1.1‰). The radioisotopic compositions of OC from a selected set of seabed samples (Δ 14C OC of −408 ± 82‰) indicate OM in surface sediments is old ( 14C ages of 2800 to over 6000 years before present). The ratios of organic carbon to mineral surface area exhibited by these sediments are within the typical (mono-layer equivalent) ranges characteristic of shelf sediments and do not reflect the preferential removal of terrigenous OM. Overall, these compositions indicate that while water-column POM appears to be derived mainly from terrigenous vascular plant debris and riverine/estuarine phytoplankton, the source of most of the sedimentary POM is aged, soil OC ultimately derived from C3 vegetation. We speculate that the concentrations and sources of suspended POM in the water column of the Fly River delta region reflect the conditions of low river discharge, low wave energy and neap tides encountered at the time of sampling. In contrast, POM compositions in surface sediments are consistent with the transport and deposition of old, mineral-bound OC most likely eroded from the upland regions of the Fly River watershed, which is characterized by steep slopes, high precipitation and C3 tropical forests and grasslands.

Links of organic carbon cycling and burial to depositional depth gradients and establishment of a snowball Earth at 2.3Ga. Evidence from the Timeball Hill Formation,Transvaal Supergroup, South Africa.

Closely spaced samples of carbonaceous shales and siltstones from two upwards coarsening deltaic parasequences of the early Paleoproterozoic Timeball Hill Formation of the Pretoria Group, Transvaal Supergroup were analysed for their organic carbon content and isotopic composition. We illustrate that trends in the organic carbon isotopic signature are closely linked to changes in the depositional environment and also to variations in climate. Systematic trends displayed by organic carbon isotope values suggest that a biosynthetic chain transporting organic carbon from the oxygen-rich surface water to anoxic deep water was already present in the early Paleoproterozoic Ocean. Just as in modern deltaic environments, aerobic degradation of organic matter occurred in the shallow water and recycling of organic matter in the deeper water led to the preservation of isotopically light organic carbon. Sampling in a sequence stratigraphic framework also reveals that over time, the preserved organic matter became more13C enriched. This trend may be linked to a global decrease in atmospheric CO2 which eventually led to a major ice-age that is recorded by glacial diamictite capping the Timeball Hill Formation. The results confirm that in order to reach a better understanding of organic carbon cycling in the Precambrian, sedimentary facies, depositional environment and post-depositional modifications of carbon isotopic values must be taken into account.

Paleogeography and paleoecology of the upper Miocene Zillingdorf lignite deposit (Austria)

The Zillingdorf deposit formed during the late Miocene filling of Lake Pannon and contains Austria's largest lignite reserves. Two lignite seams are present and developed within frequently flooded, low-lying mires in near shore environments. High sulphur contents are due to the influence of the brackish water body of Lake Pannon. During peat accumulation a transgression forced the NW–SE trending shoreline northeastwards. Differences in soluble organic matter (SOM) yield and hydrocarbon content of borehole samples and woody macrofossils are related to differences in the content and composition of free lipids of microbial origin and/or hydrocarbons derived from the biogeochemical degradation of plant tissue. Variations of pristane/phytane ratios are interpreted to reflect differences in the redox conditions of the mire. Peatification in an acidic and aerobic environment is further reflected by the predominance of aromatic over saturated hydrocarbons, the presence of an intense complex mixture in the GC traces due to biodegradation processes, high ratios for diasterenes relative to sterenes, and high concentration ratios of hopanes to hop-17(21)-ene of the respective samples. Gelification of plant tissue is governed by microbial activity, as indicated by the positive relationship between gelification index and hopanoids concentration. The composition of terpenoid biomarkers indicates the predominance of gymnosperms over angiosperms and increasing proportions of angiosperms in the peat-forming vegetation with decreasing depth in the upper seam. From the sesqui- and diterpenoids present in the lignite and fossil wood remnants, a predominant role of species of the Coniferales families Cupressaceae/Taxodiaceae are concluded. The preservation of plant tissue is governed by the presence/absence of decay-resistant gymnosperms. A general influence of the floral assemblage on the isotopic composition of organic carbon of the lignite (δ13C = − 27.2 to − 24.6‰) is proposed. Decomposition of plant tissue and biogeochemical carbon cycling is assumed to further affect the δ13C values of the lignite. Carbon isotope data of fossil wood remnants are consistent with their chemotaxonomical classification as gymnosperms (δ13C between − 22.5‰ and − 24.9‰) and angiosperms (δ13C between − 25.5‰ and − 26.6‰), respectively. The δ13C data of the extracted cellulose reveal clear differences between fossil wood from gymnosperms (average δ13C = − 20.3‰) and angiosperms (mean δ13C = − 22.9‰). The higher isotopic difference of about 3.5‰ between cellulose and fossil wood, compared to that found in modern trees, is explained by the smaller effect of 13C discrimination for cellulose when compared with wood during decomposition.

Patterns of organic carbon and nitrogen isotopic compositions of latest Pliocene sapropels from six locations across the Mediterranean Sea

Inter-layered in the organic-poor sediments that comprise most of the sedimentary record of the Mediterranean Sea are organic-rich sapropels that represent periods of wetter climate, increased primary productivity, and suboxic to anoxic conditions at depth. In order to explore processes important in the spatial and temporal variability of sapropel-forming conditions, we selected sedimentary sequences containing two latest Pliocene sapropels at ODP Sites 974 and 975 in the western basin, the Vrica Plio-Pleistocene type-section, and ODP Sites 964, 967, and 969 in the eastern basin. Our main tools are nitrogen and organic carbon isotopes, which are diagnostic of changes in nutrient origins and availability and in primary productivity. Extremely low δ 15N values (to − 5.1‰) indicate high rates of nitrogen fixation during sapropel formation stimulated by nearly complete consumption of nitrate in the water column due to exceptionally high rates of denitrification. Interruptions in sapropels represent brief intervals of low productivity and diminished nitrogen fixation. The geographical distribution of the interruptions and the presence of spatial patterns in TOC contents, C org/N tot ratios, and δ 15N values suggest a central role of the eastern basin in initiating sapropel formation. The eastern Mediterranean evidently experienced higher rates of primary productivity during sapropel deposition and probably more intense dysoxic conditions in the water column. Organic carbon isotopes present an intriguing geographic trend across the eastern Mediterranean that is probably related to non-biological processes including variations in water temperature, salinity, and amounts of local fluvial runoff.

タービダイトシークエンスの地球化学的研究による古海洋環境推定：千葉県房総半島安房層群安野層の例

Geochemical and geological studies were performed on the turbidite sequence of Anno Formation, Awa Group, Boso Peninsula, Chiba Prefecture.Based on the correlation of major, trace, and rare earth elements (REEs) contents with SiO2 content, elements are divided into positive correlation group (K, Rb, Zr, Nb, Ba, Hf, Ta, Pb, Th, U) and negative correlation group (Ti, total-Fe, Mn, Mg, Ca, P, Sc, V, Co, Cu, Zn, Sr, Y). This is considered to have been caused by an increase in the acidic component derived from the island-arc during the sea level declining period, and an increase in the basic component from fore-arc basalt during the sea level rising period. An increase in the organic carbon content and a decrease in organic carbon isotopic composition (δ13C) are recognized during the sea level declining period. This is thought to have been caused by the increase in the biogenic productivity related to the upwelling of deep seawater and the settling of organic matter-clay mineral compounds.87Sr/86Sr ratio and other geochemical data (REE pattern, La/Yb ratio) indicate that the hemipelagite is mostly composed of fore-arc basalt with variable amounts of carbonate and acidic component probably derived from the island-arc.These geochemical variations with time are consistent with the variations of δ18O of foraminiferal shell carbonate, the amount of turbidite sedimentation, and the hemipelagite thickness. Therefore, the geochemical data obtained by this study support the conclusion of Ishihara et al. (1997) who indicated that the temporal variation of turbidite in the Anno Formation is caused by the variations of seawater level.

Carbon and nitrogen isotope excursions in mid-Pleistocene sapropels from the Tyrrhenian Basin: Evidence for climate-induced increases in microbial primary production

The modern Mediterranean Sea is oligotrophic, yet its sediment record contains layers of organic-carbon-rich sapropels at 21 ky (precessional) spacing that imply periods of elevated paleoproductivity that approached the high productivities of modern upwelling systems. Resolution to this paradox is provided by lines of evidence suggesting that the mode of primary productivity changed from one dominated by algae to one during times of sapropel deposition in which photosynthetic bacteria were important. We have made a high-resolution comparison of the organic carbon and nitrogen isotopic compositions of three sapropels and their background sediments in a 3-m sequence that corresponds to 1001 to 946 ka. Organic δ 13C values systematically increase from − 26‰ to − 21‰ and δ 15N values systematically decrease from ∼4‰ to < 0‰ as organic carbon mass accumulation rates increase in the sapropel layers. The increase in carbon isotope values mirrors the increases in primary productivity and associated organic matter export indicated by the increased mass accumulation rates. The decrease in nitrogen isotope values implies major contributions of nitrogen-fixing cyanobacteria to the total marine productivity. The precessional minima with which sapropels coincide were times of wetter climate that stratified the surface Mediterranean Sea, increased delivery of soil-derived phosphorus, and evidently amplified microbial primary production. Our high-resolution study reveals several relatively rapid excursions into and out of the high-productivity mode that suggest that sapropel deposition was a climate-sensitive surface-driven phenomenon that was not accompanied by basin-wide stagnation.

Trans-Mediterranean comparison of geochemical paleoproductivity proxies in a mid-Pleistocene interrupted sapropel

Sapropel layers in the sediment record of the Mediterranean Sea reflect periods of climate-induced elevated marine production associated with precessional minima. Some of these layers feature interruptions that represent centennial-to-millennial returns to the predominantly low-productivity conditions of this sea. We have measured multiple geochemical paleoproductivity proxies in the interrupted sapropel layer that corresponds to insolation cycle 90 (955 ka) at 1-cm intervals in a four-site transect from the Balearic Basin to the Levantine Basin to assess similarities and differences in its expression at the four locations. The interrupted sapropels are similarly divided by an organic-carbon-poor interval into an upper layer in which organic carbon concentrations reach 2–7 wt.% and a lower layer in which they peak between 1 and 3 wt.%. The interruption was essentially synchronous in the Balearic Basin, the Tyrrhenian Basin, and the Ionian Basin, but it was delayed and shorter in the Levantine Basin. Lighter nitrogen isotope compositions consistently accompany higher organic carbon accumulation rates in the sapropel layers, which implies a shift from coccolith-dominated organic matter production towards a mode of primary production in which cyanobacteria were important during times of sapropel deposition. Organic carbon isotopic compositions become heavier as organic carbon accumulation increases at three of the locations, which is consistent with elevated productivity. However, they become lighter at the Levantine Basin site, which suggests greater delivery of isotopically light fluvial waters to this part of the Mediterranean Sea. The local paleoclimate conditions conducive to sapropel formation evidently were not completely uniform across the Mediterranean region but their occurrences were synchronous. Moreover, the co-existence of an essentially same-age interruption of the sapropels at the four locations shows that climate could change rapidly and concordantly across the region at much less than precessional intervals.

Characterization of Symbiont Populations in Life-History Stages of Mussels From Chemosynthetic Environments

The densities of chemoautotrophic and methanotrophic symbiont morphotypes were determined in life- history stages (post-larvae, juveniles, adults) of two species of mussels (Bathymodiolus azoricus and B. heckerae) from deep-sea chemosynthetic environments (the Lucky Strike hydrothermal vent and the Blake Ridge cold seep) in the Atlantic Ocean. Both symbiont morphotypes were observed in all specimens and in the same relative proportions, regardless of life-history stage. The relative abundance of symbiont morphotypes, determined by transmission electron microscopy, was different in the two species: chemoautotrophs were dominant (13:1-18:1) in B. azoricus from the vent site; methanotrophs were dominant (2:1-3:1) in B. heckerae from the seep site. The ratio of CH4:H2S is proposed as a determinant of the relative abundance of symbiont types: where CH4:H2S is less than 1, as at the Lucky Strike site, chemoautotrophic symbionts dominate; where CH4:H2S is greater than 2, as at the seep site, methanotrophs dominate. Organic carbon and nitrogen isotopic compositions of B. azoricus (delta 13C = -30 per thousand; delta 15N = -9 per thousand) and B. heckerae (delta 13C = -56 per thousand; delta 15N = -2 per thousand) varied little among life-history stages and provided no record of a larval diet of photosynthetically derived organic material in the post-larval and juvenile stages.

Paleolimnological assessment of human-induced impacts on Walden Pond (Massachusetts, USA) using diatoms and stable isotopes

Multi-proxy analysis of a sediment core spanning 1600 years from Walden Pond, Massachusetts (USA), reveals substantial changes in the nutrient status over the past ∼250 years resulting from anthropogenic impacts on the lake and watershed. Following a period of environmental stability from about 430 AD to 1750 AD, the abundance of the diatom Cyclotella stelligera increased, the chrysophyte cyst to diatom ratio decreased, organic content declined, bulk organic δ13C decreased, and bulk organic δ15N increased. These changes coincided with logging in the watershed, and are mainly attributed to an increase in detrital input of inorganic sediment and delivery of dissolved soil decomposition products from the watershed. With the beginning of intensive recreational development of Walden Pond in the early 20th century, oligotrophic diatom species were largely replaced by disturbance indicators and the diatom-inferred lake pH increased by 0.5 units, while the bulk organic carbon and nitrogen stable isotope composition markedly shifted to lower and higher values, respectively. These changes reflect inorganic inputs from erosion related to trails, beaches, and construction, as well as increased nutrient inputs by wastewater seepage into groundwater and extensive recreational usage. Diatom-inferred total phosphorus increased only slightly, probably because oligotrophic species still persist during spring and autumn, when Walden Pond has lower nutrient concentrations due to reduced recreational activity. During the last 25 years, diatom assemblages stabilized, suggesting that management measures have been effective in reducing the rate of eutrophication. Notably, the changes observed over the past 250 years are well beyond the range of natural variability of the past 1600 years, yet the pre-disturbance record provides a useful target for developing additional restoration and conservation measures to ensure future environmental protection of this historical site.

Palaeoenvironment of the Eocene Eckfeld Maar lake (Germany): implications from geochemical analysis of the oil shale sequence

Drill core samples from the depth interval between 19.4 and 32.0 m of the laminated central lake facies of the Eocene Eckfeld Maar were investigated for biomarker and stable isotope composition. Bulk organic geochemical parameters (C/N, HI) and the molecular composition of the soluble organic matter indicate a dominance of particulate organic matter from land plants and microbially derived lipids in the lower part of the sedimentary succession. An angiosperm-dominated vegetation is indicated from the terpenoid biomarker composition. Abundant 4-methylsteroids in the 25.6–30.8 m section of the oil shale sequence reflect a contribution of algal-derived biomass. Samples with high concentrations of methylsteroids are characterized by low amounts of triterpenoids related to the arborane skeleton, and vice versa. This pattern is interpreted as reflecting differences in autochthonous organic matter production vs. microbial activity. In the lowermost section (32.0–30.6 m), a trend towards heavier δ 18O and δ 13C values indicates the evolution of permanently meromictic conditions in the lake and an increase in methanogenesis. High δ 13C values of siderites (>10‰) throughout most of the sequence are consistent with permanently anoxic conditions at the sediment–water interface. The lighter δ 18O values of siderites from turbidites, relative to siderites from biogenic laminites, are postulated to have been caused by temporary phases of increased precipitation, followed by landslides and improved circulation within the lake. Depletion of the organic matter in 13C (average δ 13C = −29.4‰), in comparison with the fossil wood (ranging from −23.1‰ to −26.6‰), is explained by the dominance of waxy, lipid-rich land plant material (e.g., leaf waxes, resins, bark) over wood supplied to the lake. Carbon cycling during anoxic decomposition of organic matter is assumed to further affect the δ 13C values through the activity of anaerobic (e.g., methanogenic) bacteria, resulting in a depletion of the biomass in 13C. The overall trend in the isotopic composition of organic carbon towards heavier values in the depth interval between 26.0 and 3.2 m is accompanied by decreasing C/N ratios, indicating an increase in aquatic organic matter production. The geochemistry of the oil shale succession points towards a highly productive, eutrophic ecosystem with intense microbial activity under meromictic conditions in the Maar lake. Beside the high input of land plants to the biomass, major contributions from bacteria and phytoplankton are indicated by the biomarker composition.

Response of tropical African and East Atlantic climates to orbital forcing over the last 1.7 Ma

Abstract Records of organic matter accumulation, organic carbon isotopic composition and iron content covering the last 1.7 Ma are presented for the Congo Fan Ocean Drilling Program (ODP) Site 1075, and are compared with their counterparts from ODP Site 663 in the equatorial upwelling region. They are discussed with regard to variations in African precipitation and Congo River discharge and in the context of changes in trade-wind-driven marine productivity for the tropical Atlantic at periodicities typical of Milankovitch forcing. On the Congo Fan, elevated total organic carbon mass accumulation rates (TOC MAR) and Fe intensities occur predominantly during interglacial periods when the African monsoon was most intense. Band-pass filtering applied to TOC MAR shows distinct precessional variations, indicating that the African climate was largely controlled by low-latitude insolation changes. Only for the last 0.6 Ma, an interval of enhanced glacial-interglacial climate changes, is the precessional TOC MAR signal superimposed by a strong 100 ka oscillation. In contrast, variations in terrestrial iron input to the Congo Fan indicate pronounced 100 ka variance already well before global glacial-interglacial cycles increased in amplitude between 0.9 and 0.6 Ma. Obliquity cycles in the Fe signal are strongly expressed for the last 0.9 Ma. The highest amplitudes in the precessional variance of fluvial Fe input occur when amplitudes in the 100 ka oscillation were at intermediate levels and reveal a 800 ka cycle in phase shift with respect to precessional forcing. Together with a pronounced 800 ka signal in the 100 ka amplitude variations during the last 1.7 Ma, the Congo Fan iron record therefore suggests that eccentricity modulation of the low-latitude insolation directly influenced the equatorial African monsoon system and probably the weathering conditions on land. It further suggests that low-latitude precessional forcing and monsoonal response in the tropics might have played an important role for 100 ka cycles in global climate well before huge continental ice sheets existed.

Environmental Change in the Southwestern Coastal Plain of Taiwan since Late Pleistocene: Using Multiple Proxies of Sedimentary Organic Matter

In an attempt to discriminate between organic matter sources and reconstruct further the paleoenvironment of the southwestern coastal plain of Taiwan, organic carbon and nitrogen content (C/N) ratios and isotope composition (δ13C and δ15N) are measured for sedimentary core samples collected from San-liao-wan, Zai-kang and Yihju in the southwestern coastal plain of Taiwan. Heavy δ13C values (up to -14.3‰) and high C/N ratios (average 9.3) with good correlation indicate that this area underwent a period of relative aridity from early Marine Isotopic Stage (MIS) 2 to the early MIS 1. In MIS 1, the δ13C and C/N ratio both vary initially before later steadying, indicating that the environment became progressively more stable, with C3 plants becoming dominant again. The Zai-kang core appears to have come from a shallow lagoon system that potentially preserved the record of δ15N from autochthonous primary producers. Regional organic source variation is revealed through the high δ15N values and constant C/N ratios in MIS 1 of the Zai-kang core. This may suggest that man- groves and macrophytes were the main aquatic organic matter source in the Zai-kang area during MIS 1.

Recent Sediments of Makirina Cove (Northern Dalmatia, Croatia): Their Origin Viewed Through a Multidisciplinary Approach

Makirina Cove was formed by the Holocene sea-level rise whichcaused a marine ingression into the depression formed within Albian–Cenomanian dolomites at approximately 4.5 ka B.P. At present, MakirinaCove represents an restricted, stressed, shallow-marine (&lt;2 m)ecosystem characterized by varying seawater temperatures (0–35°C)as well as fluctuating salinities (up to 41‰) affected by seasonallyenhanced evaporation, continuous freshwater supply through on-shoreand submarine springs associated with the coastal karst area and surface run-off episodes. These environmental conditions have beenconducive to high primary production of organic matter resulting inthe formation of organic-rich deposits which contain up to 5 wt.% oforganic carbon. Up to the present times, 3.5 m of sediments have beendeposited indicating a relatively high sedimentation rate estimated at0.75 m/1.0 ka in the northern central part of the Cove.The sediments are being deposited mostly as poorly sortedclayey–sandy silts. The distribution and concentration of most of thechemical elements is dependant on the mineralogical composition andgranulometric features of the Makirina sediments, which show valuesmore or less similar to those from the Central Adriatic. Accordingly,there is a positive correlation with Al and K concentrations increasingoff-shore and with the depth being associated with increasing concentrations of clay minerals within the clay fraction. The same holdstrue for concentrations of some trace elements, especially Mo and Sewhich is consistent with the distribution pattern of sulphides.Selenium is preferentially enriched in authigenic pyrite and itis probably the major source of Se in the Makirina Cove sediments.The concentrations of Ca, Mg and Sr decrease off-shore and they arelinked to the composition of the surrounding carbonate rocks. Thesaturation indices show that the water is supersaturated with respectto carbonates enabling the precipitation of authigenic amorphous orcrystalline carbonate phases from the pore water in the upper segmentof the sediment column. According to the oxygen isotopic (δ18O) composition,molluscs precipitated their carbonate shells mostly duringwarmer periods (May to November) at or near isotopic equilibriumwith their ambient waters. The carbon isotopic δ13C composition ofmollusc carbonate shells is environmentally affected due to oxidationand decomposition of organic matter as well as influxes of fresh waterinto the Cove, indicating their formation out of the predicted isotopicequilibrium with atmospheric CO2. Palynological and organic carbonisotopic (δ13C) composition shows that the sedimentary organic matter(SOM) is 70–90% lipid- and hydrogen-rich and on average 2/3 marinederived (mainly phytoplankton, bacteria and marine macrophytes) and1/3 terrestrially derived (mainly woody tissue). The variations in composition of SOM have been noted as a function of the distance fromthe shore. The type and the preservation state of SOM and pyrite aswell as the measurements of Eh, pH, total alkalinity, dissolved inorganiccarbon (DIC) and the enrichment of redox-sensitive trace elements,indicate oxygen-depleted depositional conditions and that thesediment is highly reductive even in the uppermost segment at thesediment/water interface. According to the results obtained from theapplied methods, the features of Makirina sediments strongly reflectthe given depositional conditions within this restricted, stressed, shallow-marine environment where these organic-rich sediments originate,and may therefore serve as a calibration standard in further investigations

Stable Isotope Geochemistry of Carbonates and Organic Carbon in Selected Soils from Chaharmahal Bakhtiari Province, Iran

Stable isotope ratios of soil carbonates are useful indicators to establish ecological and environmental changes. The objective of this study was to provide information about the environmental conditions for the formation of calcic horizon in Shahrekord area, in the central west of Iran. The soil moisture and temperature regimes in the study area are xeric and mesic, respectively. The δ13C and δ18O values of pedogenic carbonates and bulk soils were measured in 17 calcic horizons of selected soils derived from calcareous alluvium. In addition, the δ13C and δ18O values of three parent rock samples and the δ13C value of soil organic matter in selected pedons and eight native plants in the region were determined. Results suggest that pedogenic carbonates range from 52 to 96% of total carbonates. Stable isotopic composition of carbonatic pendants shows that the older layers are more depleted in 13C in comparison with younger precipitates. This is likely due to the fact that the older parts precipitated under somewhat different ecological conditions than those of younger parts. Isotopic composition of organic carbon shows that current vegetation consists of C3 plants; whereas, the pedogenic carbonate in the soils was formed in conditions with a lower proportion of C3 plant species. This can be attributed to human induced land degradation and inappropriate land management in the area, which caused rapid changes in the community of plant species. The results show that the stable isotope composition of carbon can be a good indicator for assessing the degree of land degradation.

Stable Carbon and Nitrogen Isotopic

The perennially ice-covered lakes in the McMurdo Dry Valleys, Antarctica, are part of the coldest and driest ecosystem on earth. To understand lacustrine carbon and nitrogen cycling in this end-member ecosystem, and to define paleolimnological proxies for ice-covered lakes, we measured the stable carbon and nitrogen isotopic composition of particulate organic matter (POM) and benthic organic matter (BOM) within the lakes of Taylor Valley. The δ13C compositions of seasonally ice-free edges of the lakes (moats) are enriched relative to under-ice organic matter. Thus, the organic carbon isotopic composition of buried sediments may be a proxy for sample position within the lake. In the moats, δ13C values are governed by limited CO2diffusion across benthic cyanobacterial cell membranes. During a high glacial melt (2001–2002) season, both δ13CPOM and δ13CBOM in the moats were more depleted than during previous low melt years. We propose that this occurred in response to higher [CO2](aq) and/or reduced growth rates resulting from turbidity-induced light limitation. Though moats and under-ice environments are usually poorly connected, during the 2001–2002 season, the enrichment of the δ13CPOM values at 6 m depth in the stream-proximal sites relative to deep-profile sites implies enhanced connectivity between these environments. The δ13C compositions of BOM and POM profiles in Lake Hoare and Lake Fryxell indicate that these lakes are dominated by benthic productivity. In contrast, in Lake Bonney, the similarity of the δ13C values of BOM and POM indicates the pelagic component dominance in the carbon cycle.

The stable carbon isotope composition of soil organic carbon and pedogenic carbonates along a bioclimatic gradient in the Palouse region, Washington State, USA

Isotopic signatures of soil components are commonly used to infer past ecologic and climatic shifts in the soil record. The theory behind the fractionation of isotopes that occurs during ecosystem processes is well understood; however, few isotopic studies have explored ecosystem relationships in modern soils. We discuss relationships of stable carbon isotopic signatures in plant tissue, soil organic carbon (SOC), laboratory-respired CO 2, and modern carbonates at 10 sites (seven containing pedogenic carbonates) along a C 3-dominated climatic gradient (mean annual precipitation (MAP) ranging from 200 to 1000 mm) in the Palouse region of eastern Washington state. A horizon soil organic carbon (SOC) δ 13C values varied from −24.3‰ to −25.9‰ PDB. Values in the arid portion of the gradient (200 to approximately 500 mm MAP) generally decreased and linear regression of SOM 13C vs. MAP was significant ( r 2=0.71, p=0.02). Trends in plant- 13C of two grass species ( Agropyron spicatum and Festuca idahoensis) found throughout this portion of the gradient were similar to that of SOC. Mean pedogenic carbonate δ 13C values varied from −4.1‰ to −10.8‰ PDB. Linear regression was significant for carbonate 13C vs. MAP ( r 2=0.79, p=0.007), estimated above-ground productivity ( r 2=0.88, p=0.002) and soil carbon content ( r 2=0.83, p=0.004). Carbonate δ 13C values at the most arid site exhibited higher variability than other sites (presumably due to greater spatial variation in plant respiration vs. atmospheric diffusion). Our data suggest that carbon isotopic relationships among ecosystem components may prove useful in determining ecosystem level properties in modern systems, and potentially in ancient systems as well.

Sources of nitrogen for the vertical particle flux in the Gotland Sea (Baltic Proper)—results from sediment trap studies

Between summer of 1996 and 1997, a continuous set of samples was collected in a sediment trap at 140-m water depth at a station in the central Gotland Sea. Fluxes of total sedimented matter, particulate organic carbon and nitrogen, biogenic silica and particulate phosphorus as well as the isotopic composition of organic carbon and nitrogen were estimated. The total vertical particles flux was found to be 35 g m −2 year −1 and fluxes of organic carbon, total nitrogen and phosphorus and biogenic silica amounted to 6.0, 0.8, 0.19 and 6.2 g m −2 year −1, respectively. Highest rates of vertical particle-transport for all compounds occurred in September 1996 and lowest fluxes during October–February. Based on the isotopic signature in the particulate nitrogen, fixation of atmospheric nitrogen by pelagic cyanobacteria could be identified as the major source for export production during the summer season. The isotopic composition of the spring sedimentation pulse indicates uptake of a mixture of laterally introduced and locally produced nitrate accumulated in the winter mixed layer during the non-growth season. The contribution of nitrogen fixation to the sedimentary nitrogen inventory in the central Gotland Sea is estimated to be in the range of 45%. This is consistent with nitrogen isotope data of surface sediments and budged calculations. Annual accumulation rates of organic carbon, nitrogen and phosphorus in the underlying sediments (at 241-m water depth) are two- to sevenfold higher than the measured vertical particle flux indicating laterally advected material as the main source for sediment formation in this depositional area. The stable isotope composition of organic carbon and nitrogen of surface sediments was comparable to a weighted annual mean from the sediment trap samples, implying the source of bulk sedimentary organic matter originates from the pelagial of the central Baltic Sea.

Response of an ombrotrophic bog to a regional climate event revealed by macrofossil, molecular and carbon isotopic data

We examined the variations in vegetation, organic carbon isotopic compositions and biomarker distributions spanning a well-constrained climatic shift in a Sub-Boreal Dutch raised bog. The macrofossils record a vegetation shift from a predominance of degraded Ericaceae rootlets to a predominance of well-preserved Sphagnum species, first S. cuspidatum, followed by S. imbricatum. This shift, as well as evidence from other macrofossils and fungal spores, indicates that the local climatic conditions changed from relatively dry to relatively wet, with the wettest local conditions occurring at the base of the transition. At this boundary, a positive shift occurs in the δ13C values of bulk peat (c. 4‰) and compounds derived from specific peat-forming plants (1 to 2‰). This is attributed to a combination of factors, including selective preservation of certain compound classes, changes in precipitation and its effect on the water table, and plant growth rates. The changes in peat preservation also indicate that redox changes occurred across the climatic change event. Indeed, biomarker transformations thought to reflect past redox conditions or bog water acidity – the 1,2,9-trimethyl 1,2,3,4-tetrahydropicene to ursolic acid ratio, diacid to hydroxy acid ratio, and 17β,21β(H)-homohopane to total homohopane ratio – all exhibited shifts spanning the climatic change transition. Although some of these trends indicate the need for more fundamental research on biomarker transformations in peat, some, such as the 17b,21b(H)-homohopane to total homohopane ratio, show promise as indicators of past bog water conditions that could complement macrofossil analyses.