n-Alkane analyses from an transect of marine surface sediments off southwest Africa

An isobathic transect of marine surface sediments from 1°N to 28°S off southwest Africa was used to further evaluate the potential of the chain length distribution and carbon stable isotope composition of higher plant n-alkanes as proxies for continental vegetation and climate conditions. We found a strong increase in the n-C29-33 weighted mean average d13C values from -33 per mil near the equator to around -26 per mil further south. Additionally, C25-35n-alkanes reveal a southward trend of increasing average chain length from 30.0 to 30.5. The data reflect the changing contribution of plants employing different photosynthetic pathways (C3 and C4) and/or being differently influenced by the environmental conditions of their habitat. The C4 plant proportions calculated from the data (ca. 20% for rivers draining the rainforest, to ca. 70% at higher latitude) correspond to the C4 plant abundance in continental catchment areas postulated by considering prevailing wind systems and river outflows. Furthermore, the C4 plant contribution to the sediments correlates with the mean annual precipitation and aridity at selected continental locations in the postulated catchment areas, suggesting that the C4 plant fraction in marine sediments can be used to assess these environmental parameters.

Age determination and n-Alkane concentration of Arctic Ocean sediment cores

We have reconstructed the surface water environment of the Arctic Ocean over the last ? 50,000 years using measurements of the organic nitrogen and carbon isotope ratios, carbonate and total organic carbon concentrations (TOC), and terrestrial biomarkers (lignin and long-chain n-alkanes) in four multicores. Variations in nitrogen isotope ratios that are concordant with TOC and carbonate concentrations (representing foraminifera and excluding ice-rafted-debris) reflect differences in relative nutrient utilization of phytoplankton in the surface waters. However, d15N variations also appear to be dependent on the stratification of the water column and therefore potentially track the exchange of nutrients between deep and surface waters. Low Last Glacial Maximum (LGM) d15N values and higher Holocene values are opposite to those recorded in the Southern Ocean. The Arctic Ocean with higher nutrient utilization today compared to the LGM therefore acts as a counterpart to the Southern Ocean, although the global impact on carbon dioxide variations compared to the Southern Ocean is probably low.

(Table 1) n-alkane distribution, and stable carbon and deuterium isotope ratios of DSDP Hole 10-94

Hydrology, source region, and timing of precipitation are important controls on the climate of the Great Plains of North America and the composition of terrestrial ecosystems. Moisture delivered to the Great Plains varies seasonally and predominately derives from the Gulf of Mexico/Atlantic Ocean with minor contributions from the Pacific Ocean and Arctic region. For this work, we evaluate long-term relationships for the past ~ 35 million years between North American hydrology, climate, and floral change, using isotopic records and average carbon chain lengths of higher plant n-alkanes from Gulf of Mexico sediments (DSDP Site 94). We find that carbon isotope values (d13C) of n-alkanes, corrected for variations in the d13C value of atmospheric CO2, provide minor evidence for contributions of C4 plants prior to the Middle Miocene. A sharp spike in C4 input is identified during the Middle Miocene Climatic Optimum, and the influence of C4 plants steadily increased during the Late Miocene into the Pleistocene - consistent with other North American records. Chain-length distributions of n-alkanes, indicative of the composition of higher plant communities, remained remarkably constant from 33 to 4 Ma. However, a trend toward longer chain lengths occurred during the past 4 million years, concurrent with an increase in d13C values, indicating increased C4 plant influence and potentially aridity. The hydrogen isotope values (dD) of n-alkanes are relatively invariant between 33 and 9 Ma, and then become substantially more negative (75 per mil) from 9 to 2 Ma. Changes in the plant community and temperature of precipitation can solely account for the observed variations in dD from 33 to 5 Ma, but cannot account for Plio-Pleistocene dD variations and imply substantial changes in the source region of precipitation and seasonality of moisture delivery. We posit that hydrological changes were linked to tectonic and oceanographic processes including the shoaling and closure of the Panamanian Seaway, amplification of North Atlantic Deep Water Production and an associated increase of meridional winds. The southerly movement of the Intertropical Convergence Zone near 4 Ma allowed for the development of a near-modern pressure/storm track system, driving increased aridity and changes in seasonality within the North American interior.

n-Alkane, fatty acid and hopanoic acid concentrations of ODP Site 190-1178 samples

Ocean Drilling Program (ODP) Leg 190 was programmed to investigate deformational, diagenetic, and hydrologic processes and their interactions in the Nankai Trough accretionary prism. Site 1178 is the northernmost site in the Muroto Transect. Slope sediments and the underlying landward-dipping reflector zone were successfully cored. Temperature measurements and Cl concentrations in pore water indirectly indicate the presence of gas hydrate between 120 and 400 meters below seafloor (mbsf) at Site 1178, with the highest concentrations between 150 and 200 mbsf (Shipboard Scientific Party, 2001, doi:10.2973/odp.proc.ir.190.109.2001). Sedimentary structures show a broad range of deformation structures rich in fractures, suggesting active fluid circulation in the Nankai Trough prism. One of the objectives of Leg 190 was to clarify the interplay of various fundamental processes taking place in the Nankai Trough accretionary prism. Bacteria or prokaryotes in deep subsurface sediment play an important role for material transformation and circulation in an accretionary prism. Significant amounts of bacteria are detected in many of the samples examined (Shipboard Scientific Party, 2001, doi:10.2973/odp.proc.ir.190.109.2001). The type of organic matter in sediments is an important factor related to bacterial activity. To assist investigations on material circulation in deep subsurface sediments, the samples from Site 1178 were analyzed for geolipids (extractable organic matter). The basic data set is preliminarily compiled in the present report to show the types of organic matter and their concentrations in sediments from Site 1178.

Radiocarbon ages and major n-alkane homologues of peat cores LVPS4 and LVPS5B, Russian Arctic

Coupled analyses of n-alkane biomarkers and plant macrofossils from a peat plateau deposit in the northeast European Russian Arctic were carried out to assess the effects of past hydrology on the molecular contributions of plants to the peat. The n-alkane biomarkers accumulated over 9.6 kyr of local paleohydrological changes in this complex peat profile in which a succession of vegetation changes occurred during a transition from a wet fen to a relatively dry peat plateau bog. This study shows that the contribution of the n-C31 alkane from rootlets to peat layers rich in fine and dark roots is important. The results further indicate that the n-alkane Paq and n-C23/n-C29 biomarker proxies that have been useful to reconstruct past water table levels in many peat deposits can be misleading when the contributions of Betula and Sphagnum fuscum to the peat are large. Under these conditions, the C23/(C27 + C31) n-alkane ratio seems to correct for the presence of Betula and S. fuscum and provides a better description for the relative amounts of moisture. The average chain length (ACL) n-alkane proxy also appears to be a good paleohydrology proxy in having larger values during dry and cold conditions in this Arctic bog setting.

Sterol, n-alcohol and n-alkane composition of living fen plants

In groundwater-fed fen peatlands, the surface biomass decays rapidly and, as a result, highly humified peat is formed. A high degree of humification constrains palaeoecological studies because reliable identification of plant remains is hampered. Organic geochemistry techniques as a means of identifying historical plant communities have been successfully applied tobog peat. The method has also been applied to fen peat, but without reference to the composition of fen plants. We have applied selected organic geochemistry methods to determine the composition of the neutral lipid fractions from 12 living fen plants, to investigate the potential for the distributions to characterize and separate different fen plants and plant groups. Our results show correspondence with previous studies, e.g. C23 and C25n-alkanes dominating Sphagnum spp. and C27 to C31 alkanes dominating vascular plants. However, we also found similarities in n-alkane distributions between Sphagnum spp. and the below ground parts of some vascular plants. We tested the efficiency of different n-alkane ratios to separate species and plant groups. The ratios used for bog studies (e.g. n-C23/n-C25 and n-C23/n-C29) did not work as consistently for fen plants. Some differences in sterol distribution were found between vascular plants and mosses; in general vascular plants had a higher concentration of sterols. When distributions of n-alkanes, n-alkane ratios and sterols were all included as variables, redundancy analysis (RDA) separated different plant groups into their own clusters. Our results imply that the pattern for bog biomarkers cannot directly be applied to fen environments. Nevertheless, they encourage further testing to determine whether or not the identification of plant groups, plants or plant parts from highly humified peat is possible by applying fen species-specific biomarker proxies.

Alkane and hydrocarbon concentrations of ODP Leg 104 sites

Sedimentary extractable organic matter was analyzed at three ODP Leg 104 sites in the Norwegian Sea. Organic carbon content ranged from less than 0.1% to a maximum of 1.8%. Extractable organic matter content and unresolved complex mixture concentrations were low and randomly distributed. Low levels of aliphatic (branched and normal) and aromatic hydrocarbons were detected in all of the sediments analyzed. Total aliphatic and aromatic hydrocarbon concentrations ranged from 176 to 3,214 and 6 to 820 ppb, respectively. The concentrations of individual aliphatic (n-C15 to n-C32) and aromatic (two- to five-ring) hydrocarbons were generally less than 50 ppb and less than 10 ppb, respectively. No significant trend with sub-bottom depth was observed in either bulk organic matter or individual hydrocarbon concentrations. The predominant source of Cenozoic sedimentary hydrocarbons is concluded to be ice-rafted debris from the adjacent continent. All sites contain a mixture of recycled, mature petroleum-related and terrestrially derived hydrocarbons.

(Table 3) Relative content of n-alkane groups in samples from hydrothermal fields of the Mid-Atlantic Ridge

Optimum conditions were selected for chromatographic separation of model mixtures of C12-C40 n-alkanes. For one of samples of hydrothermal deposits extraction conditions of hydrocarbons were studied and a sample preparation procedure was selected. The procedure was proposed to determine n-alkanes in samples of hydrothermal deposits by means of gas chromatography - mass spectrometry (GC-MS). Detection limit for n-alkanes was 3x10**-9 to 10**-8% depending on components. On the basis of the proposed procedure composition of n-alkanes was studied in samples of hydrothermal deposits collected at the Mid-Atlantic Ridge (Broken Spur, Lost City, and Rainbow hydrothermal fields). Analyses showed that samples contained C14-C35 n-alkanes. Concentrations of the n-alkanes were rather low and varied from 0.002 to 0.038 µg/g. Hypotheses concerning genesis of identified n-alkanes were offered.