Plant Wax N-alkanes Research Articles

Analytical pyrolysis for determining the molecular composition of contemporary monosulfidic black ooze

On-line flash pyrolysis, micro-scale sealed vessel (MSSV) pyrolysis and catalytic hydropyrolysis (HyPy) were used to characterise the insoluble, macromolecular organic component of monosulfidic black oozes (MBO) which have accumulated within the contemporary eutrophic environment of the Peel–Harvey estuary system (Geographe Bay, Western Australia). Pyrolysates were analysed by gas chromatography–mass spectrometry (GC–MS) and the relative characterisation potential of the three pyrolysis techniques were evaluated with a particular interest in their sensitivity to organic sulfur compounds (OSCs). A similarity of results obtained from three different parts of the ∼1m cores sampled suggests a largely homogenous organic composition throughout the unit. The different pyrolysis techniques did, however, show several notable product differences, particularly the gaseous products detected by the on-line methods of flash- and MSSV-pyrolysis. The very high (i.e., ballistic) heating rate of flash-Py produced very high proportions of gaseous products (e.g., CO2, H2S and SO2). A strong terrestrially sourced product component was reflected in all three pyrolysates profiles: flash-Py showed relatively high concentrations of lignocellulose products, while MSSV-Py and HyPy produced an abundance of plant wax (>n-C20) n-alkanes. Additionally, quantitatively significant levels (7–14% of total GC product signal) of several higher plant derived terpenoids (e.g., cadalene, p-cymene and calamanene) were identified by MSSV-Py. MSSV-Py also produced far greater overall concentrations of GC-amenable products than flash-Py, including a 14-fold increase of OSCs, which comprised nearly 10% of the total sulfur signal (i.e., organic plus ‘inorganic’ sulfur; cf. 4% of flash pyrolysis). An extended series of alkyl (<C9) substituted thiophenes were the main OSCs. HyPy, in contrast, showed no evidence of OSCs – confirmed by GC-flame photometric detection (FPD). HyPy is renowned for its capacity to preserve primary structural units, so its failure to detect thiophenes and other OSCs abundant in the corresponding analysis, suggests these are not indigenous to the macromolecular fraction of the MBO. The detection of these products by MSSV-Py, and to a lesser extent flash-Py, suggests these pyrolysis methods initiate a restructuring of sulfur species towards more thermally stable secondary configurations. One explanation for the limited diagnostic S-speciation data obtained from the present analysis may be the existence of most of the S in the MBO as bridging units between covalently bound organic structures. These relatively weakly bound associations may form on the reaction of reduced sulfur species (from bacterial sulfate reduction) with highly functionalised organic compounds during the early stages of diagenesis.

Plant-wax hydrogen isotopic evidence for postglacial variations in delivery of precipitation in the monsoon domain of China

Research Article| September 01, 2011 Plant-wax hydrogen isotopic evidence for postglacial variations in delivery of precipitation in the monsoon domain of China Osamu Seki; Osamu Seki 1Institute of Low Temperature Science, Hokkaido University, N19W8, Kita-ku, Sapporo 060-0819, Japan Search for other works by this author on: GSW Google Scholar Philip A. Meyers; Philip A. Meyers 2Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA Search for other works by this author on: GSW Google Scholar Shinya Yamamoto; Shinya Yamamoto 1Institute of Low Temperature Science, Hokkaido University, N19W8, Kita-ku, Sapporo 060-0819, Japan Search for other works by this author on: GSW Google Scholar Kimitaka Kawamura; Kimitaka Kawamura 1Institute of Low Temperature Science, Hokkaido University, N19W8, Kita-ku, Sapporo 060-0819, Japan Search for other works by this author on: GSW Google Scholar Takeshi Nakatsuka; Takeshi Nakatsuka 3Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan Search for other works by this author on: GSW Google Scholar Weijian Zhou; Weijian Zhou 4State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710075, China Search for other works by this author on: GSW Google Scholar Yanhong Zheng Yanhong Zheng 5State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China Search for other works by this author on: GSW Google Scholar Geology (2011) 39 (9): 875–878. https://doi.org/10.1130/G32117.1 Article history received: 29 Jan 2011 rev-recd: 13 Apr 2011 accepted: 19 Apr 2011 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Osamu Seki, Philip A. Meyers, Shinya Yamamoto, Kimitaka Kawamura, Takeshi Nakatsuka, Weijian Zhou, Yanhong Zheng; Plant-wax hydrogen isotopic evidence for postglacial variations in delivery of precipitation in the monsoon domain of China. Geology 2011;; 39 (9): 875–878. doi: https://doi.org/10.1130/G32117.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Asian monsoon is a key component of the Earth's climate system that directly affects the livelihood of 60% of the world population. Reliable reconstruction of changes in monsoon precipitation during the Holocene is required to better understand the present climate conditions and to predict future climate processes in the Asian monsoon domain. Reconstruction of changes in Holocene monsoon precipitation has been done in various regions of this domain using a variety of paleoclimate proxies. However, different proxies have yielded different paleoclimate records of long-term monsoon rainfall variability. We apply compound-specific stable hydrogen isotope compositions of the plant wax n-alkanes isolated from the Hongyuan peat sequence in eastern Tibet to reevaluate the regional carbonate oxygen isotopic and lake-level records. The D/H ratios of the n-alkanes resolve the apparent discrepancy among the different paleoclimate proxies. Our result indicates that lake-level fluctuation is a reliable recorder of changes in the amount of summer monsoonal precipitation while long-term isotope records largely reflect large-scale changes in the source of water vapor rather than a local precipitation signal. Combining lake-level, biomarker, and carbonate isotopic records, we conclude that the early Holocene monsoon precipitation in southwest China has been greatly influenced by the Indian monsoon. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Orbital- and millennial-scale changes in the hydrologic cycle and vegetation in the western African Sahel: insights from individual plant wax δD and δ13C

To reconstruct variability of the West African monsoon and associated vegetation changes on precessional and millennial time scales, we analyzed a marine sediment core from the continental slope off Senegal spanning the past 44,000 years (44 ka). We used the stable hydrogen isotopic composition ( δD) of individual terrestrial plant wax n-alkanes as a proxy for past rainfall variability. The abundance and stable carbon isotopic composition (δ 13C) of the same compounds were analyzed to assess changes in vegetation composition (C 3/C 4 plants) and density. The δD record reveals two wet periods that coincide with local maximum summer insolation from 38 to 28 ka and 15 to 4 ka and that are separated by a less wet period during minimum summer insolation. Our data indicate that rainfall intensity during the rainy season throughout both wet humid periods was similar, whereas the length of the rainy season was presumably shorter during the last glacial than during the Holocene. Additional dry intervals are identified that coincide with North Atlantic Heinrich stadials and the Younger Dryas interval, indicating that the West African monsoon over tropical northwest Africa is linked to both insolation forcing and high-latitude climate variability. The δ 13C record indicates that vegetation of the western Sahel was consistently dominated by C 4 plants during the past 44 ka, whereas C 3-type vegetation increased during the Holocene. Moreover, we observe a gradual ending of the Holocene humid period together with unchanging ratio of C 3 to C 4 plants, indicating that an abrupt aridification due to vegetation feedbacks is not a general characteristic of this time interval.

Variation in solvent-extractable lipids and n-alkane compound-specific carbon isotopic compositions with depth in a southern China karst area soil

Because literatures about the lipid compositions of modern soils in karst areas are scarce, we have studied the soil horizons overlying the Heshang (和尚) Cave that has provided paleoclimate records from speleothem lipid contents. Our analysis reveals a series of n-alkanes, free fatty acids, n-alkanols and n-alkan-2-ones distribution and relative abundance changing with the depth, and in which the ratios of the lower molecular weight to higher molecular weight n-alkanes, free fatty acids, n-alkanols and n-alkan-2-ones have a peak at a subsurface depth of 5 to 10 cm. An accompanying peak in 17β(H), 21β(H)-hop-22(29)-ene (diploptene) and a shift to less negative n-alkane carbon isotopic values also identify this layer in the karst soil. This pattern indicates the existence of a subsurface soil layer in which the microorganisms that produce these compounds are especially abundant. The carbon isotopic values of individual plant wax n-alkanes are about 3‰ greater at the base of the 30- to 40-cm soil profile than in the surface layer, probably as a result of selective microbial degradation of n-alkanes from different primary sources. The lipids and carbon isotopic values of individual plant wax n-alkanes study of the overlying soil show a strong microbial activity in this karst soil and help in interpreting the lipid compositions and specific carbon isotopic value of n-alkanes of the stalagmites of the Heshang Cave for paleoenvironmental reconstructions.

Effects of aridity and vegetation on plant-wax δD in modern lake sediments

We analyzed the deuterium composition of individual plant-waxes in lake sediments from 28 watersheds that span a range of precipitation D/H, vegetation types and climates. The apparent isotopic fractionation ( ε a) between plant-wax n-alkanes and precipitation differs with watershed ecosystem type and structure, and decreases with increasing regional aridity as measured by enrichment of 2H and 18O associated with evaporation of lake waters. The most negative ε a values represent signatures least affected by aridity; these values were −125 ± 5‰ for tropical evergreen and dry forests, −130‰ for a temperate broadleaf forest, −120 ± 9‰ for the high-altitude tropical páramo (herbs, shrubs and grasses), and −98 ± 6‰ for North American montane gymnosperm forests. Minimum ε a values reflect ecosystem-dependent differences in leaf water enrichment and soil evaporation. Slopes of lipid/lake water isotopic enrichments differ slightly with ecosystem structure (i.e. open shrublands versus forests) and overall are quite small (slopes = 0–2), indicating low sensitivity of lipid δD variations to aridity compared with coexisting lake waters. This finding provides an approach for reconstructing ancient precipitation signatures based on plant-wax δD measurements and independent proxies for lake water changes with regional aridity. To illustrate this approach, we employ paired plant-wax δD and carbonate-δ 18O measurements on lake sediments to estimate the isotopic composition of Miocene precipitation on the Tibetan plateau.

Hydrogen isotopic ratios of plant wax n-alkanes in a peat bog deposited in northeast China during the last 16 kyr

Based on paleoclimatic reconstructions using various proxies, the Holocene Climate Optimum (10.5–6ka) has been characterized as a warmer and wetter period in most of East Asia. The summer monsoons associated with the East Asian Monsoon evidently intensified and extended further inland from the Pacific Ocean, a source region of moisture. A notable exception to this general pattern exists in northeast China, where less wet conditions are recorded. We determined molecular compositions of individual plant wax hydrocarbons and their hydrogen isotope compositions (δD values) in a radiocarbon-dated peat core recovered from the Hani marsh in Jilin Province (China) and confirmed that the temperature-dependent effective precipitation in northeast China decreased during the Holocene Climate Optimum. A combination of Paq, an indicator of the relative contribution of aquatic to terrestrial plants, and the difference in δD between low (C23, C25 and C27) and high molecular weight (C31) n-alkanes in the Hani peat bog indicates a dramatic change in vegetation from the deglaciation to the Holocene. No significant differences were observed between the δD values of low and high molecular weight n-alkanes with relatively high δD values and low Paq during the early Holocene, indicating that all n-alkanes were produced by evapotranspiration-sensitive terrestrial plants during that time. However, lower δD values of mid-chain n-alkanes (C23, C25 and C27) relative to the long chain n-alkane (C31), together with higher Paq values during the deglaciation (14–11ka), suggest an increase in the contribution of aquatic plants and a higher water level during the period. The study demonstrates that northeast China was under a markedly wetter climate condition during the late deglaciation. For the 16kyr record in the Hani peat sequence, we infer that moisture delivery by the East Asian Monsoon was relatively invariable in northeast China, but increased evaporation during the warmer Holocene Climate Optimum reduced the effective precipitation, defined by the balance between precipitation and evaporation.

Agriculture and environmental change at Qingpu, Yangtze delta region, China: a biomarker, stable isotope and palynological approach

Rice ( Oryza sp.) agriculture sustains vast numbers of people and, despite great advancements made in recent years, questions about its origins and spread throughout Asia remain unanswered. This study uses sedimentary biomarker, stable carbon isotope and palynological analyses to investigate early rice agriculture in the Yangtze delta, a region where rice agriculture emerged at least 7000 years ago. Accelerator mass spectrometer (AMS) 14C dating reveals the age of sedimentary section to be between c. 6000 and 1800 cal. BP. Widespread clearing of forest vegetation c. 2400 cal. BP, is the earliest major human influence detected in the Qingpu record. Following this, rice agriculture probably dominated the Qingpu area. Evidence supporting rice agriculture after c. 2400 cal. BP is provided by increased Poaceae and Cereal-type taxa, which occur with high concentrations of plant wax n-alkanes with a dominant C3 plant origin (C27—C31 with odd/even preference, δ 13C —29.8‰ to —36.3‰). Also, high concentrations of a C20 highly branched isoprenoid (HBI) thought to be from epiphytic algae associated with rice agriculture occur after c. 2400 cal. BP. 13C-depleted diploptene (in high concentrations) and 13C-depleted C31 3b-methyl-hopanes of methanotrophic bacterial origin also occur after c. 2400 cal. BP. The strong methane cycle detected in the trench sediment may have provided an alternative CO2 source for plants and algae associated with rice agriculture.

Chemical characteristics and pollution sources of petroleum hydrocarbons and PAHs in sediments from the Beiluohe River, Northern China

To determine the degree of hydrocarbon contamination and the contribution of local petroleum industries to contaminant loadings in sediments from the Beiluohe River, China, 12 surface sediment samples were collected for geochemical analysis in 2005. Sediment samples were extracted by organic solvents, separated by silica gel column chromatography and the profiles of n-alkanes, biomarkers and polycyclic aromatic hydrocarbons (PAHs) in sediments were analyzed by gas chromatography with flame ionization detector and gas chromatography/mass spectroscopy. Concentrations of total hydrocarbons in the sediments varied from 12.1 to 3,761.5 μg g−1 dry wt, indicating that most sediments in Beiluohe River was only slightly to moderately contaminated by hydrocarbons. Concentrations of PAHs for six samples (sum of 16 isomers) varied from 17.7 to 407.7 ng g−1 dry wt and at present low levels of PAHs did not cause adverse biological effects in Beiluohe River sedimentary environment. PAH compositions, n-alkanes and biomarker profiles all suggested that there were different sources of contaminations in studied areas. n-Alkanes reflect two distinct sources: a fossil n-alkane series from crude oil at sites S40, S43, S87 and plantwax n-alkanes at sites S39 and S45. Judged by their PAH ratios, the sediments at site S15 were pyrolytic, sediments at S17 and S43 were petrogenic, and sediments at S39, S40 and S64 had a mixture source of pyrolytic and petrogenic.

Compound-specific carbon isotope compositions of individual long-chain n-alkanes in severe Asian dust episodes in the North China coast in 2002

The molecular compositions and compound-specific carbon isotope compositions of individual long-chain n-alkanes of atmospheric aerosols collected during two severe Asian dust episodes in Qingdao in spring of 2002 were analyzed using gas chromatography/mass spectrometry (GC/MS) and gas chromatography/isotope ratio mass spectrometry (GC/IRMS). Typical plant wax n-alkanes (C29 and C31) had lower δ13C values than those from anthropogenic (engine exhaust) sources (C21-C23). The average δ13C value of plant wax n-alkane C29 in non-dust episode periods was −30.5‰ (−30.3‰–−31.9‰), while −31.3‰ (−31.1‰–−31.5‰) in dust episode periods; for C31, it was −31.4‰ (−31.1‰–−33.0‰) in non-dust episode periods, and −31.7‰ (−31.3‰–−32.6‰) in dust episode periods. Plant wax in the dust episode samples was mainly from herbaceous plants via long-range transport, while local plant wax was mainly from deciduous plants and woody plants. In North China coast, 83.3% of the plant wax in the severe dust episode samples was from C3 plants while 80.0% for the non-dust samples, indicating that plant wax transported to the northwestern Pacific Ocean by airborne dust from East Asia was mainly from C3 plants. The results suggest that the molecular and molecular-isotopic compositions of individual long-chain n-alkanes can, as an effective indicator, identify the terrestrial organic components in the dust from East Asia and sediments in the northwest Pacific Ocean.

The use of n–alkanes and other plant–wax compounds as markers for studying the feeding and nutrition of large mammalian herbivores

The use of n–alkanes and other plant–wax compounds as markers for studying the feeding and nutrition of large mammalian herbivores

Distribution of terrigenous lipids in marine sediments off northeastern Taiwan

Surface sediments on the continental margin off northeastern Taiwan have been analyzed for terrigenous lipids including n-alkanes, n-fatty alcohols, and sterols. Marine input to the sediments is particularly low based on the average n-C17/n-C29 alkane and n-C16/n-C28 fatty alcohol ratios, 0.15±0.13 and 0.13±0.06, respectively; this may be due to the fact that marine lipids are more prone to degradation than terrestrial ones. The study area has the highest plant wax n-alkane contribution (average carbon preference index 3.9±1.2) among the coastal marine areas surrounding Taiwan; lateral particle transport from the southern East China Sea shelf and river runoff from the east Taiwan coast are considered to be the major contributors. The distributions of plant wax n-alkane and n-alkanol concentrations normalized to total organic carbon (TOC) in the study area generally show maximum values on the upper slope of the southernmost Okinawa Trough, but not for phytosterols. Linear regression of TOC versus plant wax n-alkane concentrations show a weak relationship (r=0.64, p=0.001), and an even weaker relationship (r=0.42, p=0.05) between TOC and plant wax n-fatty alcohol concentrations is found. This could be attributed to several factors: (1) a complex input (not a point source) of terrigenous organic matter to the study area, (2) TOC also including marine organic matter, (3) temporal variations in river flow due to flooding, and (4) different rates of degradation for TOC and individual biomarkers. However, in spite of those factors, TOC and phytosterol concentrations are positively linearly correlated (r=0.85, p<0.001), implying that the dilution of phytosterols in terrigenous organic carbon with marine organic carbon with or without the phytosterols follows a nearly constant ratio, which is remarkable. In addition, the predominant source of diploptene in the sediments does not appear to be of higher plant origin.

Even carbon number predominance of plant wax n-alkanes : a correction

The distributions of n-alkanes of four species of Micromeria have the conventional higher plant pattern of high carbon preference index (CPI) and odd-numbered carbon dominance (maxima at n-C 31 or n-C 33), rather than the even-numbered predominance previously reported (Palić, R., Ristić, N., Simić, N., Kitić, D., Kapetanović, R., 1997. The alkanes from some plants of Micromeria genus. Journal of the Serbian Chemical Society 62, 619–622). The stable carbon isotope ratio values (δ 13C) of the individual n-alkanes (−38 to −34‰) are typical of C 3 plants. Homologous series of odd-numbered predominant iso-alkanes ( i-C 27 to i-C 35) and even-numbered predominant anteiso-alkanes ( a-C 27 to a-C 35) are also present (8 to 18% of the total identified alkanes) and have similar carbon isotopic ratios (−36.8 to−35.1‰).

Plant Wax n-Alkanes Trapped in Soil Humin by Noncovalent Bonds

Soil organic matter is a very complex mixture of compounds derived from the decay of numerous living organisms. Earlier reports suggest that soil organic molecules may occur unaltered in a form, e.g. by encapsulation in the humic substances matrix or by binding involving weak forces such as H bonds and Van der Waals forces. This issue is relevant to the release of pesticides and other toxic compounds in waters during several years, which suggests a particular mechanim of storage of organic compounds in soil organic matter. However, so far, the mechanim of weak binding is poorly known due to possible analytical bias in the isolation of weakly-bound molecules, and to the lack of analytical approaches that can distinguish a compound from its weakly-bound counterpart. Here, we analysed the 13 C isotope composition of plant-derived soil n-alkanes in a soil sample from an experimental field cropped 23 years with maize to label soil organic carbon. Indeed, cultivating maize, a C 4 plant with 13 C-enriched carbon, on a soil previously cropped with C 3 plants, introduces 13 C-enriched carbon in the soil. Thus maize-derived soil carbon can be distinguished from previously-cropped plant soil carbon by 13 C analysis. We used a recently developped analytical technique, gas chromatography - isotope ratio monitoring mass spectrometry (GC-IRMS), that allows to measure the 13 C isotope ratio of individual substances occurring in complex mixtures. We analysed three pools of C 27 -C 33 n-alkanes in the same soil sample: 1) n-alkanes that were extracted with CHCl 3 -MeOH, 2) n-alkanes that were extracted from humin, the macromolecular part of soil organic matter, and 3) n-alkanes that were released by pyrolysis of the pre-extracted humin. Our results show that free n-alkanes are 13 C-enriched by +5.8-7.0‰ versus their bound n-alkanes counterparts. This finding have several implications. First, this clear isotope difference of the the same substance occurring either in free or bound form proves that there is no analytical bias such as a lack of exhautive extraction. Second, the higher 13 C content of free n-alkanes evidences their higher turnover versus bound n-alkanes. Here a first-order kinetic law yields an age difference of 7 years between free and bound n-alkanes. Third, since n-alkanes are apolar compounds our results demonstrate the existence of a mechanism of weak binding involving either encapsulation or weak forces. This mechanism could explain why chemicals such as pesticides are well preserved in soil then released in waters several years after the end of their use in cropping systems. It is also relevant to the storage of organic-N compounds as possible plant nutrient precursors.

A case of post-depositional aerobic degradation of terrestrial organic matter in turbidite deposits from the Madeira Abyssal Plain

A set of oxidized and unoxidized sediment intervals from the f-turbidite identified in two piston cores from the Madeira Abyssal Plain (MAP) was analyzed for calcium carbonate, total organic carbon (TOC), total nitrogen (TN), stable carbon composition of TOC ( δ 13C toc ), total hydrolyzable amino acids (THAA), total neutral sugars (TSUG), lignin phenols (LIG), two different lipid biomarkers of marine phytoplankton origin and vascular plantwax n-alkanes and n-acids. Comparison shows changes for all properties depicting the effect of post-depositional, aerobic oxidation. The oxidation process re-mineralized approximately 80% of the TOC and approximately 60% of the TN in the original deposit and left the residual organic matter depleted in 13C by 1.7–2.9‰ THAA and TSUG account for 10% or less of the TOC in all samples leaving approximately 90% of the TOC chemically unidentified. THAA, TSUG, LIG, marine phytoplankton biomarkers and plantwax n-acids were lost from the oxidized deposit to an extent as great as that for TOC. Two non-protein amino acids (β-alanine and γ-aminobutyric acid) and plant wax n-alkanes displayed much less degradation (9–37%) and concentrated 3.3–4.8 times relative to TOC by the oxidation process. These observations show that both marine and terrestrial components contribute to the re-mineralized TOC fraction. A binary mixing approach was used to model quantitatively the change in δ 13C toc and plantwax n-alkane concentration. Results suggest that terrestrial organic carbon contributes approximately 15% to the TOC content of the original turbidite, its abundance increases approximately 2–4 times as a consequence of the oxidation process and approximately 40 and 90% of the terrestrial and marine component of TOC in the original turbidite, respectively, was destroyed by the oxidation process. Although selective preservation of terrestrial relative to marine organic carbon is a well-documented phenomenon in sedimentary processes, this study represents the first attempt to assess the sensitivity of terrestrial organic matter to oxidative degradation in a sedimentary environment.

Carbon isotope composition of individual n-alkanes in recent sediments

Carbon isotope composition of individual long-chain n-alkanes from Tokyo Bay sediments (a 3 m-core sample and surficial sediment samples) was determined by using gas chromatography/isotope ratio mass spectrometry (GC-IRMS). δ 13C values for C 27–C 33 n-alkanes in shallow (0–40 cm in depth, present—1965 AD) sections range from −31.5 to −28.2‰, showing even carbon number n-alkanes being isotopically heavier than the co-occurring odd n-alkanes. C 27–C 33 n-alkanes in the deep sections (70–220 cm in depth) have the δ 13C values ranging from −29.6 to −32.9‰ (average value: 31.2 ± 0.7‰, with no isotopic differences between odd and even numbered n-alkanes. The results for n-alkanes in shallow sediments are satisfactorily explained by a two end-member mixing model of indigenous higher plant wax n-alkanes from oil pollution. The onset of significant levels of pollution in Tokyo Bay was between 1960 and 1965.

Marine and terrigenous origin of organic matter in modern sediments of the equatorial East Atlantic: the σ 13C and molecular record

The contributions of marine and terrigenous organic carbon in modern organic sediments from the equatorial East Atlantic was quantified, based on the stable carbon isotope composition standardized for sea surface temperature and water depth. In our binary σ 13C mixing model, the marine end member −26 ∞ is redefined at 23°C and 0 m water depth and the terrigenous end member, independent of SST and water depth, at − 26 ∞. Terrigenous carbon fractions account for more than 60% of total organic carbon (TOC) on the shelf off East Liberia and the Ivory Coast and off Gabon. On the upper slope the land-derived fraction decreases in general to less than 20% of the high TOC concentrations, which can reach 3.5 wt%. The distribution of plant wax n-alkanes (C 27, C 29, C 31) and C:N ratios do not parallel those of land-derived organic carbon, but may be controlled largely by carbon degradation and aeolian/aquatic sorting. The ratio of n-alkanols vs n-alkanes (HPA index) varies with water depth in a nonlinear mode. Since both groups of compounds stem from the same source, plant waxes, it is proposed that the HPA index is controlled mainly by degradation and to a lesser extent by sorting prior to degradation. Enhanced n-alkane concentrations (up to 580 μg/gTOC) in the Gambia Basin and in the central Guinea Basin clearly reflect the influx of aeolian organic matter from northeasterly trades near and below the Inter Tropical Convergence Zone (ITCZ). The particle flux from marine plankton is traced by high concentrations of both dinosterol, long-chain unsaturated methyl and ethyl ketones (C 37-C 39), and alkandiols in marine organic matter. Whereas alkenones, synthetized by prymnesiophyte algae generally, reflect upwelling-related productivity off-shore, dinosterol, synthetized by dinoflagellates, is enriched in near-shore areas of high marine productivity linked to fluvial fertilization. All marine biomarker groups show a surprisingly low concentration below the equatorial high productivity belt.

Diploptene: an indicator of terrigenous organic carbon in Washington coastal sediments.

The pentacyclic triterpene 17 beta(H),21 beta(H)-hop-22(29)-ene (diploptene) occurs in sediments throughout the Columbia River drainage basin and off the southern coast of Washington state in concentrations comparable to long-chain plantwax n-alkanes. The same relationship is evident for diploptene and long-chain n-alkanes in soils from the Willamette Valley. Microorganisms indigenous to soils and soil erosion are indicated as the biological source and physical process, respectively, for diploptene in coastal sediments. Similarity between the stable carbon isotopic composition (delta 13CPDB) of diploptene isolated from soil in the Willamette Valley (-31.2 +/- 0.3%) and from sediments deposited throughout the Washington coastal environment (-31.2 +/- 0.5%) supports this argument. Values of delta for diploptene in river sediments are variable and 8-17% lighter, indicating that an additional biological source such as methane-oxidizing bacteria makes a significant contribution to the diploptene record in river sediments. Selective biodegradation resulting from a difference in the physicochemical association within eroded particles can explain the absence of the more-13C-depleted form of diploptene in Washington coastal sediments, but this mechanism remains unproven.

Hydrocarbons in Washington coastal sediments

The sources and distributions of polycyclic aromatic hydrocarbons (PAH) and aliphatic hydrocarbons are characterized in seventeen sediments from a highly river-influenced sedimentary environment off the southwestern coast of Washington. The major hydrocarbons are land-derived, introduced as preformed compounds and display long-term stability in sediment cores. A series of PAH of anthropogenic origin and two naturally derived compounds, retene and perylene, dominate the PAH composition in these sediments. Plantwax n-alkanes are the major aliphatic hydrocarbon constituents. Aliphatic hydrocarbons of marine origin, pristane and a series of four acyclic, multibranched C 25 polyolefins, are also observed in many sediments. The concentrations of these marine-derived hydrocarbons decrease to negligible levels rapidly with sediment depth from the sea-sediment interface, suggesting degradation. In general, the major land-derived hydrocarbons are concentrated in the midshelf silt deposit which extends northwestward along the continental shelf from the Columbia River mouth. A quantitatively more minor, natural series of phenanthrene homologs, also of terrestrial origin, is preferentially advected further offshore and deposited in continental slope sediments. These distributions are consistent with recognized particle associations for these compounds and sediment dispersal processes in this coastal environment. Sediment core records suggest the present pattern of dispersal has persisted for at least the past century and possibly since the Late Pleistocene.

Hydrocarbon geochemistry of the Puget Sound region—II. Sedimentary diterpenoid, steroid and triterpenoid hydrocarbons

Cyclic components of the ‘aliphatic’ hydrocarbon mixtures extracted from Puget Sound sediment cores include a suite of C 19 and C 20 diterpenoid hydrocarbons of which fichtelite. sandaracopimaradiene, and isopimaradiene have been identified. Although apparently also derived from vascular plants, these diterpenoid hydrocarbons have relative abundances distinctly different from the co-existing plant wax n-alkane suite. Five C 27, C 28 and C 29 diasteranes and four C 29, C 30 and C 31 17α(H), 21β(H) hopanes occur in relatively constant proportion as components of a highly weathered fossil hydrocarbon assemblage. These chromatographically resolved cycloalkanes. along with the strongly covarying unresolved complex mixture, have been introduced to Puget Sound sediments from adjacent urban centres at increasing levels over the last 100 yr in the absence of any major oil spill. Naturally-occurring triterpenoid hydrocarbons, including hop-22(29)-ene (diploptene), are also present. A new group of C 30 polyenes has been detected which contains compounds apparently structurally related to a co-existing bicyclic C 25 diene and to C 20 and C 25 acyclic multibranched hydrocarbons described in a previous paper ( Barrick et al., 1980).

Hydrocarbon geochemistry of the Puget Sound region—I. Sedimentary acyclic hydrocarbons

Hydrocarbon compositions have been determined for 210Pb-dated sediment cores collected at 23 sites within the inland marine waters of northwestern Washington State, U.S.A. Concentrations of total aliphatic hydrocarbons (TAH) and an unresolved complex mixture (UCM) are significantly higher in surface sediments near urban areas than at all other locations with a chronology that indicates a predominantly anthropogenic origin. Concentrations of chromatographically resolvable alkanes are comparatively uniform; the major constituents are plant wax n-alkanes and a naturally-occurring suite of fossil isoprenoid and n-alkanes. Pristane concentrations decrease sharply near the sea-sediment interface suggesting rapid degradation of a plankton-derived component. A saturated multibranched, but nonisoprenoid, C 20 hydrocarbon and two novel mono-olefinic analogs have been isolated along with a previously unreported suite for four acyclic multibranched C 25 polyenes. Structural and distributional similarities between the C 20 and C 25 multibranched hydrocarbons suggest that they may be structurally homologous and share a common source.