N-alkane Indices Research Articles

N-Alkanes and stable C, N isotopic compositions as identifiers of organic matter sources in Posidonia oceanica meadows of Alexandroupolis Gulf, NE Greece

We analyzed n-alkane contents and their stable carbon isotope composition, as well as the carbon and nitrogen isotope composition (δ13C, δ15N) of sediment organic matter and different tissues of Posidonia oceanica seagrass sampled in Alexandroupolis Gulf (A.G.), north-eastern Greece, during 2007–2011. n-Alkane contents in P. oceanica and in sediments showed similar temporal trends, but relative to bulk organic carbon content, n-alkanes were much more enriched in sediments compared to seagrass tissue. Individual n-alkanes in sediments had similar values than seagrass roots and rhizomes and were more depleted in 13C compared to seagrass leaves and sheaths, with δ13C values ranging from −35‰ to −28‰ and from −25‰ to −20‰, respectively. n-Alkane indexes such as the Carbon Preference Index, carbon number maximum, and n-alkane proxy 1 (C23+C25/C23+C25+C29+C31) indicate strong inputs of terrestrial organic matter, while the presence of unresolved complex mixtures suggests potential oil pollution in some sampled areas.

Cenozoic Environmental Changes in the Northern Qaidam Basin Inferred fromn-alkane Records

Cenozoic climatic and environmental changes in the arid Asian interior, and their possible relations with global climatic changes and the Tibetan Plateau uplift, have been intensively investigated and debated over past decades. Here we present 40-Myr (million years)-long n-alkane records from a continuous Cenozoic sediment sequence in the Dahonggou Section, Qaidam Basin, northern Tibetan Plateau, to infer environmental changes in the northern basin. A set of n-alkane indexes, including ACL, CPI and Paq, vary substantially and consistently throughout the records, which are interpreted to reflect relative contributions from terrestrial vascular plants vs. aquatic macrophytes, and thus indicate depositional environments. ACL values vary between 21 and 30; CPI values range from 1.0 to 8.0; and Paq values change from 25.8 Ma, 17.5-21.0 Ma, and <13.0 Ma, corresponding to alluvial fan/river deltaic deposits and shallow lacustrine settings, consistent with the observed features in sedimentological facies. The inferred Cenozoic environmental changes in the northern Qaidam Basin appear to correspond to global climatic changes.

High-latitude environmental change during MIS 9 and 11: biogeochemical evidence from Lake El'gygytgyn, Far East Russia

Abstract. Marine isotope stages (MIS) 11 has been proposed as an analog for the present interglacial; however, terrestrial records of this time period are rare. Sediments from Lake El'gygytgyn (67°30' N, 172°5' E) in Far East Russia contain a 3.56 Ma record of climate variability from the Arctic. Here, we present the first terrestrial Arctic reconstruction of environmental and climatic changes from MIS 8 through 12 (289 to 464 ka) using organic geochemical proxies. Terrestrial vegetation changes, as revealed by plant leaf wax (n-alkane) indices and concentrations of arborinol (a biomarker for trees), show increased tree cover around the lake during interglacial periods, with higher concentrations observed during MIS 11 as compared to MIS 9. A similar pattern is also observed in records of aquatic productivity revealed by molecular indicators from dinoflagellates (dinosterol), eustigmatophyte algae (long-chain (C28–C32) 1, 15 n-alkyl diols) in addition to short-chain n-alkanes, where aquatic productivity is highest during MIS 11. Changes recorded in these molecular proxies show a similar structure to relative temperature variability as recorded by the MBT/CBT (Methylation of Branched Tetraether/Cyclization of Branched Tetraether) paleothermometer, based on branched glycerol dialkyl glycerol tetraethers (GDGTs). Additionally, relative MBT/CBT temperature changes generally track pollen and diatom δ18O temperature estimates, compiled by other studies, which suggest glacial-interglacial temperature changes of ~ 9 to 12 °C. These records of environmental and climatic change indicate Arctic sensitivity to external forcings such as orbital variability and atmospheric greenhouse gas concentrations. Overall, this study indicates that organic geochemical analyses of the Lake El'gygytgyn sediment archive can provide critical insight into the response of lake ecosystems and their sensitivity in high latitude regions.

A 220 ka palaeoenvironmental reconstruction of the Fuentillejo maar lake record (Central Spain) using biomarker analysis

The sedimentary record of the volcanic lake known as the Fuentillejo maar (central Spain) offers the opportunity to determine the palaeoenvironmental evolution of the central-southern part of the Iberian Peninsula during the Middle and Upper Pleistocene. A total of 439 samples from the upper 88m of a ca. 142m long core (spanning the last ca. 355ka) were examined. The interval corresponds to the last 220ka of the record. High n-alkane CPI values and the presence of sulfur revealed that little diagenetic signal attenuation or modification had occurred. A number of n-alkane-based indices (e.g. predominant n-alkane, ACL, Paq and TARHC ratios, and the proportion of C27, C29 and C31n-alkanes with respect to the summed C27+C29+C31) alkanes, showed changing conditions in the organic matter input to the maar lake over time, with episodes involving a major input of terrestrial vegetation, alternating with others in which algae or a mixed input of terrestrial plants, aquatic macrophytes and algae occurred. The proportions of C27, C29 and C31 were useful for reconstructing the palaeoenvironmental evolution of the basin as they allowed assignment of several dry intervals during which grasses developed, while during humid phases trees expanded at the expense of grasses.In general, we found good correspondence between these episodes and the climatic cycles observed in δ18O records from ices cores in Greenland and Antarctica, as well as from marine cores. Thus, we conclude that global palaeoclimatological changes were recorded in the maar record. Likewise, Heinrich Events appeared to be reflected in the n-alkane indices. This was confirmed by single spectral analysis and cross spectral analysis with the insolation curve on 21 July at a latitude of 65°N. The approach revealed that the cycles interpreted in the Fuentillejo record (103, 41, 23 and 19ka) showed evidence of orbital influence related to the astronomical cycles of the eccentricity variation in the Earth’s orbit, the Earth’s obliquity oscillation, and the precession variation (Milankovitch theory).

The distribution, sources and toxicity risks of polycyclic aromatic hydrocarbons and n-alkanes in riverine and estuarine core sediments from the Daliao River watershed

The burial characteristics and toxicity risks associated with n-alkanes and polycyclic aromatic hydrocarbons (PAHs) in the riverine and estuarine sediments of the Daliao River watershed were investigated based on three sediment cores. The sum of the n-alkane and PAH concentrations, normalized to organic carbon (OC), ranged from 0.27 to 63.09 μg g−1OC−1 and 6.60 to 366.20 μg g−1OC−1, respectively. The features and the history of industrial activities, such as the oil and chemical industries and port activities near the river and estuary, resulted in different distributions and sources of hydrocarbons. The sources of pollution were identified based on n-alkane indexes and on diagnostic ratios of PAHs. The diagnostic ratios indicated that the n-alkanes were derived from both biogenic and petrogenic sources in different proportions and that the PAHs were derived primarily from petrogenic combustion sources. A hierarchical cluster analysis grouped the core samples into two clusters. The first cluster, river sediments, corresponded to industrial activities; the second cluster, estuarine sediments, corresponded to port shipping activities. The toxic potency of the PAHs in the cores was assessed in terms of toxic equivalents (TEQs) of dibenzo[a,h]anthracene and benzo[a]pyrene. The top layer of the sediment in the cores had a relatively high toxicity. The TEQ values for benzo(a)pyrene (TEQBaP) and dioxins (TEQTCDD) furnished a consistent assessment of the PAHs in the sediment cores.

Climatic and environmental implications from n-alkanes in glacially eroded lake sediments in Tibetan Plateau: An example from Ximen Co

Gas chromatography-mass spectrometry was used to identify a series of n-alkanes in the sediments of a typical glacially eroded lake in the eastern Tibetan Plateau. By comparing the distribution patterns of n-alkanes in lake sediments, surface soils and cow manure, it was shown that n-C27-n-C33 alkanes in the soil ecosystem of Ximen Co are derived from vascular plant species and that the distribution pattern of n-C27-n-C33 alkanes remains unchanged during the feeding and digestion processes of herbivores. The relative percentage of C27, C29 and C31 n-alkanes decreased from the bottom to the top of the sediment core showing a trend of degradation of higher plants in the Ximen Co lake region during the formation of the 44 cm core. 210Pb dating, combined with pre-existing AMS 14C dating results showed that the depositional core reflects climatic and environmental variations since about 900 years before present. The n-alkane indexes (ACL27–33, P aq, P wax) are comparable with regional temperature variation, especially recording the Little Ice Age event (LIA). This study highlights that n-alkanes are valid proxies for paleo-climate and paleo-environment reconstruction, despite the same distribution patterns in n-alkane molecular fossils derived from a typical glacially eroded lake.

Polycyclic aromatic hydrocarbons and n-alkanes in sediments of the Upper Scheldt River Basin: contamination levels and source apportionment

The Scheldt River system is located in northern France, Belgium and the Netherlands and includes a dense network of rivers, which contributed to the urban and industrial development in this area. Three sediment cores, collected in the Upper Scheldt River (Helkijn) and two of its tributaries (the Lys River at Wervik and the Espierre Canal), were analysed for n-alkanes and polycyclic aromatic hydrocarbons (PAHs). Total n-alkane and PAH concentrations in all the sampled cores ranged from 2.8 to 29 mg kg(-1) and from 4.9 to 96 mg kg(-1), respectively. The contributions of biogenic, petrogenic and pyrolytic sources were investigated using n-alkane indexes and PAH diagnostic ratios. n-alkane chromatograms were characterized by the predominance of odd over even long chain n-alkanes (produced by terrestrial plants) and by the occurrence of a broad unresolved complex mixture (UCM) which evidenced biodegraded petroleum residues. For the three studied cores, correlations between the concentrations of UCM and n-alkanes (both expressed on an organic carbon basis) indicated a common origin or similar pathways of these allochtonous compounds to the aquatic environment. Wervik sediments were distinguished by higher n-alkane concentrations and by a major aquatic biogenic source for low molecular weight n-alkanes. The prevalence of combustion-derived PAHs was indicated by the high contribution of four, five and six rings compounds and was confirmed by isomer ratios. Higher levels of low molecular weight PAHs in Helkijn surface sediments suggested modest petrogenic inputs in this navigable canal. High PAH concentrations in Espierre sediments could be explained by a major historical contamination from urban and industrial emissions.

Sea surface temperature and terrestrial biomarker records of the last 260 ka of core MD05-2904 from the northern South China Sea

This paper reports high-resolution biomarker records of the last 260 ka for core MD05-2904 from the northern South China Sea (SCS). The sea surface temperature (SST) record using the U 37 k′ index reveals a minimum of 21.5°C (MIS 2) and a maximum of 28.3°C (MIS 5.5), for a temperature difference of almost 7°C, and provides the longest high-resolution U 37 k′ SST record in northern SCS. The content of odd-number long chain n-alkanes and several n-alkanes indexes such as the CPI, ACL and the C31/C27 ratio, all reveal generally higher values during the glacials and lower values during the interglacials. Terrestrial input as indicated by n-alkane content was mostly controlled by sea-level changes: During the glacials, lower sea-level exposed the continental shelf to enable rivers to transport more terrestrial materials to the slope; and the situation reverses during the interglacials. The n-alkane indexes changes reveal more n-alkanes from contemporary vegetation during glacials as a result of the proximity of the core site to the source region, while the increases in ACL and C31/C27 ratio during glacials indicate a change to more grassy vegetation. However, the highest values for CPI, ACL and the C31/C27 ratio all occurred during late MIS 3, and it was suggested that this period was characterized by a strong summer monsoon-dominated humid climate which resulted in a denser vegetation for the exposed continental shelf region.

Evaluation of biogenic and anthropogenic inputs of aliphatic hydrocarbons to Lake Taihu sediments using biomarkers

Surficial sediments from 13 sites throughout Lake Taihu, one of the largest urbanized freshwater lake systems in China, were analyzed for biomarkers (e.g., n-alkanes and hopanes) to track the origin of organic inputs (biogenic or anthropogenic), and, thus, to identify any ‘hot spots’ of hydrocarbon contamination. A distinct spatial distribution of aliphatic hydrocarbons in sediments was observed in Lake Taihu. At the northern tip of the lake (i.e., Meiliang and Wuli Bays), the highest mean aliphatic hydrocarbon concentration, with a significant contribution of an unresolved complex mixture (UCM), was found, indicating possible anthropogenic petroleum contamination (mainly from untreated and partially treated industrial and domestic sewage from Wuxi, Changzhou and other cities). This was supported by the n-alkane indices (e.g., small Carbon Preference Index and odd-to-even values) and a high degree of maturity of the hopane biomarkers. However, hydrocarbons from East Taihu were mainly biogenic, with the lowest mean concentrations, negligible or no contribution of UCM, abundance of vascular plant C23–C33 n-alkanes with a high odd-to-even predominance, and the presence of biogenic hopanes (e.g., 17β(H), 21β(H)-hopanes and hopenes). In the other areas of the lake, however, the predominance of biogenic in combination with petrogenic hydrocarbons was indicated by the biomarkers.

Paleohydrologic reconstruction based on n-alkane distributions in ombrotrophic peat

Proxies such as plant macrofossils, humification indices and testate amoebae have been developed to reconstruct past hydrological variability in ombrotrophic peatlands. Reconstructions based on these approaches have resulted in records of decadal- to centennial-scale moisture variability in regions of North America and elsewhere. Although the various peatland moisture proxies generally show significant temporal covariance, multi-proxy approaches can refine knowledge of the multivariate nature of climate change and increase confidence in interpretations. Here, we demonstrate that ratios of the abundances of n-alkanes provide a new and efficient way of reconstructing past peatland surface moisture change. We found strong correlations among n-alkane indices, humification indices and testate amoebae-inferred water table depths from a single core. Biogeochemical proxies can be used in paleohydrological reconstructions for ombrotrophic bogs to provide a new and complementary source of data from these under-utilized paleoclimate archives.