Evaluation of long chain 1,14-alkyl diols in marine sediments as indicators for upwelling and temperature

Long chain alkyl diols form a group of lipids occurring widely in marine environments. Recent studies have suggested several palaeoclimatological applications for proxies based on their distributions, but also revealed uncertainties about their applicability. Here we evaluate the use of long chain 1,14-alkyl diol indices for reconstruction of temperature and upwelling conditions by comparing index values, obtained from a comprehensive set of marine surface sediments, with environmental factors like sea surface temperature (SST), salinity and nutrient concentrations. Previous cultivation efforts indicated a strong effect of temperature on the degree of saturation and the chain length distribution of long chain 1,14-alkyl diols in Proboscia spp., quantified in the diol saturation index (DSI) and diol chain length index (DCI), respectively. However, values of these indices in surface sediments show no relationship with annual mean SST of the overlying water. It remains unknown what determines the DSI, although our data suggests that it may be affected by diagenesis, while the relationship between temperature and DCI may be different for different Proboscia species. In addition, contributions of algae other than Proboscia diatoms may affect both indices, although our data provide no direct evidence for additional long chain 1,14-alkyl diol sources. Two other indices using the abundance of 1,14-diols vs. 1,13-diols and C30 1,15-diols have previously been applied as indicators for upwelling intensity at different locations. The geographical distribution of their values supports the use of 1,14 diols vs. 1,13 diols [C28 + C30 1,14-diols]/[(C28 + C30 1,13-diols) + (C28 + C30 1,14-diols)] as a general indicator for high nutrient or upwelling conditions.

Stable carbon and hydrogen isotope record of n-Alkanoic acids of sediment core SO189/2_144KL

The Indo-Pacific Warm Pool (IPWP) is a key site for the global hydrologic cycle, and modern observations indicate that both the Indian Ocean Zonal Mode (IOZM) and the El Niño Southern Oscillation exert strong influence on its regional hydrologic characteristics. Detailed insight into the natural range of IPWP dynamics and underlying climate mechanisms is, however, limited by the spatial and temporal coverage of climate data. In particular, long-term (multimillennial) precipitation patterns of the western IPWP, a key location for IOZM dynamics, are poorly understood. To help rectify this, we have reconstructed rainfall changes over Northwest Sumatra (western IPWP, Indian Ocean) throughout the past 24,000 y based on the stable hydrogen and carbon isotopic compositions (dD and d13C, respectively) of terrestrial plant waxes. As a general feature of western IPWP hydrology, our data suggest similar rainfall amounts during the Last Glacial Maximum and the Holocene, contradicting previous claims that precipitation increased across the IPWP in response to deglacial changes in sea level and/or the position of the Intertropical Convergence Zone. We attribute this discrepancy to regional differences in topography and different responses to glacioeustatically forced changes in coastline position within the continental IPWP. During the Holocene, our data indicate considerable variations in rainfall amount. Comparison of our isotope time series to paleoclimate records from the Indian Ocean realm reveals previously unrecognized fluctuations of the Indian Ocean precipitation dipole during the Holocene, indicating that oscillations of the IOZM mean state have been a constituent of western IPWP rainfall over the past ten thousand years.

Organic geochemistry of ODP Sites 128-798 and 128-799

Miocene to Quaternary sediments from the Oki Ridge (Site 798) and the Kita-Yamato Trough (Site 799) in the Japan Sea contain organic carbon ranging from about 0.6% in light-colored layers to almost 6% in dark layers. The organic matter consists of a variable mixture of marine and terrigenous contributions, the ratio of which is not correlated to the total organic carbon content. Marine organic particles clearly dominate in the deeper section of Hole 799B. The extractable bitumen is strongly dominated by long-chain alkenones from microalgae in the shallower sediments, whereas bishomohopanoic acid (C32) of eubacterial origin is the single most abundant compound in deeper samples. Normal alkanes and straight-chain carboxylic acids, both of which show a bimodal distribution with odd and even carbon-number predominance, respectively, are two other groups of compounds which are important constituents of the extracts. The deepest samples at Site 799 contain a considerable amount of short-chain components, which probably migrated upward from thermally more altered deeper sediments.

Geochemistry of ODP Sites 123-765 and 123-766

The sediments of the Argo and Gascoyne abyssal plains are generally lean in organic matter, are immature, and contain hydrocarbons trapped during sediment deposition rather than those generated during sediment catagenesis. TOC concentrations in the Argo Abyssal Plain Cenozoic sediments are 0.5 wt%, and organic matter appears to be from mixed marine and reworked, degraded, organic matter sources, with the latter being contributed by turbidity flows from the nearby continental margin. TOC concentrations within the Cenozoic sediments of the Gascoyne Abyssal Plain are mostly undetectable (<0.1 wt%). Biomarker distributions determined by gas chromatography (GC) and gas chromatography-mass spectrometry (GCMS) indicate that organic matter extracted from the Lower Cretaceous sediments from both sites is predominantly marine with varying contributions from terrestrial organic matter. The specific marine biomarker, 24-n-propylcholestane is in relatively high abundance in all samples. In addition, the relatively high abundance of the 4-methylsteranes with the 23,24-dimethyl side chain (in all samples) indicates significant dinoflagellate contributions and marine organic matter. The ratios of n-C27/n-C17 reflect relative contributions of marine vs. terrestrial organic matter. TOC, while generally low at Argo, is relatively high near the Barremian/Aptian boundary (one sample has a TOC of 5.1 wt%) and the Aptian/Albian boundary (up to 1.3 wt% TOC), and two samples from the Barremian and Aptian sections contain relatively high proportions of terrestrial organic carbon. TOC values in the Lower Cretaceous sediments from Gascoyne Abyssal Plain are low (<0.1 wt%) near the Aptian/Barremian boundary. TOC values are higher in older sediments, with maxima in the upper Barremian (1.02 wt%), the Barremian/Hauterivian (0.6 wt%), and Valanginian (1.8 wt%). Sediments from the upper Barremian contain higher amounts of terrestrial organic carbon than older sediments.

Biomarkers, stable carbon isotope record and pollen distribution of the soutwest African continental margin

This is part 2 of a study examining southwest African continental margin sediments from nine sites on a north-south transect from the Congo Fan (4°S) to the Cape Basin (30°S) representing two glacial (MIS 2 and 6a) and two interglacial stages (MIS 1 and 5e). Contents, distribution patterns, and molecular stable carbon isotope signatures of long-chain n-alkanes (C27-C33) and n-alkanols (C22-C32) as indicators of land plant vegetation of different biosynthetic types were correlated with concentrations and distributions of pollen taxa in sediments of the same time horizons. Selected single pollen type data reveal details of vegetation changes, but the overall picture is best illustrated by summing pollen known to predominantly derive from C4 plants or C4 plus CAM plants. The C4 plant signals in the biomarkers are recorded in the delta13C data and in the abundances of C31 and C33 n-alkanes, and the C32 n-alkanol. Calculated clusters of wind trajectories for austral summer and winter situations for the Holocene and the Last Glacial Maximum afford information on the source areas for the lipids and pollen and their transport pathways to the ocean. This multidisciplinary approach provides clear evidence of latitudinal differences in leaf wax lipid and pollen composition, with the Holocene sedimentary data paralleling the current major phytogeographic zonations. The northern sites (Congo Fan area and northern Angola Basin) get most of their terrestrial material from the Congo Basin and the Angolan highlands dominated by C3 plants. Airborne particulates derived from the western and central South African hinterland dominated by deserts, semideserts, and savannah regions are rich in organic matter from C4 plants. As can be expected from the present and glacial positions of the phytogeographic zones, the carbon isotopic signatures of n-alkanes and n-alkanols both become isotopically more enriched in 13C from north to south. In the northern part of the transect the relative importance of C4 plant indicators is higher during the glacials than in the interglacials, indicating a northward extension of arid zones favoring grass vegetation. In the south, where grass-rich vegetation merges into semidesert and desert, the difference in C4 plant indicators is small.

Fatty acids, alcohols and hydrocarbons of microbial mats from Lake 2A on Kiritimati Island

Modern microbial mats are widely recognized as useful analogs for the study of biogeochemical processes relevant to paleoenvironmental reconstruction in the Precambrian. We combined microscopic observations and investigations of biomarker composition to investigate community structure and function in the upper layers of a thick phototrophic microbial mat system from a hypersaline lake on Kiritimati (Christmas Island) in the Northern Line Islands, Republic of Kiribati. In particular, an exploratory incubation experiment with 13C-labeled bicarbonate was conducted to pinpoint biomarkers from organisms actively fixing carbon. A high relative abundance of the cyanobacterial taxa Aphanocapsa and Aphanothece was revealed by microscopic observation, and cyanobacterial fatty acids and hydrocarbons showed 13C-uptake in the labeling experiment. Microscopic observations also revealed purple sulfur bacteria (PSB) in the deeper layers. A cyclic C19:0 fatty acid and farnesol were attributed to this group that was also actively fixing carbon. Background isotopic values indicate Calvin-Benson cycle-based autotrophy for cycC19:0 and farnesol-producing PSBs. Biomarkers from sulfate-reducing bacteria (SRB) in the top layer of the mat and their 13C-uptake patterns indicated a close coupling between SRBs and cyanobacteria. Archaeol, possibly from methanogens, was detected in all layers and was especially abundant near the surface where it contained substantial amounts of 13C-label. Intact glycosidic tetraether lipids detected in the deepest layer indicated other archaea. Large amounts of ornithine and betaine bearing intact polar lipids could be an indicator of a phosphate-limited ecosystem, where organisms that are able to substitute these for phospholipids may have a competitive advantage.

Temporal evolution of organic carbon and lipid content and derived SST of sediment core GIK23258-2, Norwegian Sea

Lipids are used for the evaluation of the different organic matter contributions in the north eastern Norwegian sea (M23258 site; 75ºN, 14ºE) over the last 15,000 years. Development of a mass balance model based on the down core quantification of the C37 alkenones, the odd carbon numbered n-alkanes (Aodd) and the unresolved complex mixture of hydrocarbons (UCM) has allowed three main organic matter inputs involving marine, continental and ancient reworked organic matter to be recognized. The model shows a good agreement between measured and reconstructed TOC values. Similarly, a strong parallelism is observed between predicted components such as marine TOC and carbonate content (CaCO3), which was determined independently. Representation of the model results within a time-scale based on 15 AMS-14C measurements shows that the main changes in organic matter constituents are coincident with the major climatic events of the last 15,000 a. Thus, the predominance of reworked organic matter is characteristic of Termination Ia (up to 70%), continental organic matter was dominant during the Bølling-Allerød (B-A) and Younger Dryas (YD) periods (about 85%) and a strong increase of marine organic matter occurred in the Holocene (between 50 and 75%). This agreement reflects the main hydrographic changes that determined the deposition of sedimentary materials during the period studied: ice-rafted detritus from the Barents continental platform, ice-melting waters from the Arctic fluvial system discharging into the Barents sea and dominance of north Atlantic currents, respectively. In this respect, the high-resolution down core record resulting from the mass balance and lipid measurements allows the identification of millennial-scale events such as the increase of reworked organic matter at the final retreat of the Barents ice sheet at the end of the deglaciation period (Termination Ib).

Sedimentary lipids and palynomorphs of sediment core ABC26 from the Eastern Mediterranean

Selective degradation of organic matter in sediments is important for reconstructing past environments and understanding the carbon cycle. Here, we report on compositional changes between and within lipid classes and kerogen types (represented by palynomorph groups) in relation to the organic matter flux to the sea floor and oxidation state of the sediments since the early Holocene for central Eastern Mediterranean site ABC26. This includes the initially oxic but nowadays anoxic presapropelic interval, the still unoxidised lower part of the organic rich S1 sapropel, its postdepositionally oxidised and nowadays organic-poor upper part as well as the overlying postsapropelic sediments which have always been oxic. A general ~ 2.3 times increase in terrestrial and marine input during sapropel formation is estimated on the basis of the total organic carbon (TOC), pollen, spore, dinoflagellate cyst, n-alkane, n-alkanol and n-alkanoic acid concentration changes in the unoxidised part of the sapropel. The long-chain alkenones, 1,15 diols and keto-ols, loliolides and sterols indicate that some plankton groups, notably dinoflagellates, may have increased much more. Apart from the terrestrial and surface water contributions to the sedimentary organic matter, anomalous distributions and preservation of some C23-C27 alkanes, alkanols and alkanoic acids have been observed, which are interpreted as a contribution by organisms living in situ. Comparison of the unoxidised S1 sapropel with the overlying oxidised sapropel and the organic matter concentration profiles in the oxidised postsapropelic sediments demonstrates strong and highly selective aerobic degradation of lipids and palynomorphs. There seems to be a fundamental difference in degradation kinetics between lipids and pollen which may be possibly related with the absence of sorptive preservation as a protective mechanism for palynomorph degradation. The n-alkanes, Impagidinium, and Nematosphaeropsis are clearly more resistant than TOC. The n-alkanols and n-carboxylic acids are about equally resistant whereas the pollen, all other dinoflagellate cysts and other lipids appear to degrade considerably faster, which questions the practice of normalising to TOC without taking diagenesis into account. Selective degradation also modifies the relative distributions within lipid classes, whereby the longer-chain alkanes, alcohols and fatty acids disappear faster than their shorter-chain equivalents. Accordingly, interpretation of lipid and palynomorph assemblages in terms of pre- or syndepositional environmental change should be done carefully when proper knowledge of the postdepositional preservation history is absent. Two lipid-based preservation proxies are tested the diol-keto-ol oxidation index based on the 1,15C30 diol and keto-ols (DOXI) and the alcohol preservation index (API) whereby the former seems to be the most promising.

Abundances and d13C values of n-alkanes extracted from sediment samples of ODP Hole 116-717C

Siwalik paleosol and Bengal Fan sediment samples were analyzed for the abundance and isotopic composition of n-alkanes in order to test for molecular evidence of the expansion of C4 grasslands on the Indian subcontinent. The carbon isotopic compositions of high-molecular-weight alkanes in both the ancient soils and sediments record a shift from low d13C values (ca. -30 per mil) to higher values (ca. -22 per mil) prior to 6 Ma. This shift is similar in magnitude to that recorded by paleosol carbonate and fossil teeth, and is consistent with a relatively rapid transition from dominantly C3 vegetation to an ecosystem dominated by C4 plants typical of semi-arid grasslands. The n-alkane values from our paleosol samples indicate that the isotopic change began as early as 9 Ma, reflecting either a growing contribution of C4 plants to a dominantly C3 biomass or a decrease in water availability to C3 plants. Molecular and isotopic analyses of other compounds, including n-alcohols and low-molecular weight n-alkanes indicate paleosol organic matter contains contributions from a mixture of sources, including vascular plants, algae and/or cyanobacteria and microorganisms. A range of inputs is likewise reflected in the isotopic composition of the total organic carbon from these samples. In addition, the n-alkanes from two samples show little evidence for pedegenic inputs and we suggest the compounds were derived instead from the paleosol's parent materials. We suggest the record of vegetation in ancient terrestrial ecosystems is better reconstructed using isotopic signatures of molecular markers, rather than bulk organic carbon. This approach provides a means of expanding the spatial and temporal records of C4 plant biomass which will help to resolve possible tectonic, climatic or biological controls on the rise of this important component of the terrestrial biosphere.

Chemical and isotopic analysis of bulk organic matter and hydrocarbon biomarkers from IODP Holes 311-U1327C, 311-U1328B and 311-U1328C

The chemical and isotopic compositions of sedimentary organic matter (SOM) from two mid-slope sites of the northern Cascadia margin were investigated during Integrated Ocean Drilling Program (IODP) Expedition 311 to elucidate the organic matter origins and identify potential microbial contributions to SOM. Gas hydrate is present at both locations (IODP Sites U1327 and U1328), with distinct patterns of near-seafloor structural accumulations at the cold seep Site U1328 and deeper stratigraphic accumulations at the slope-basin Site U1327. Source characterization and evidence that some components of the organic matter have been diagenetically altered are determined from the concentrations and isotopic compositions of hydrocarbon biomarkers, total organic carbon (TOC), total nitrogen (TN) and total sulfur (TS). The carbon isotopic compositions of TOC (d13C TOC = -26 to -22 per mil) and long-chain n-alkanes (C27, C29 and C31, d13C = -34 to -29 per mil) suggest the organic matter at both sites is a mixture of 1) terrestrial plants that employ the C3 photosynthetic pathway and 2) marine algae. In contrast, the d15N TN values of the bulk sediment (+4 to +8 per mil) are consistent with a predominantly marine source, but these values most likely have been modified during microbial organic matter degradation. The d13C values of archaeal biomarker pentamethylicosane (PMI) (-46.4 per mil) and bacterial-sourced hopenes, diploptene and hop-21-ene (-40.9 to -34.7 per mil) indicate a partial contribution from methane carbon or a chemoautotrophic pathway. Our multi-isotope and biomarker-based conclusions are consistent with previous studies, based only on the elemental composition of bulk sediments, that suggested a mixed marine-terrestrial organic matter origin for these mid-slope sites of the northern Cascadia margin.

Highly dealkylated copper and nickel etioporpyfrins in marine sediments

Copper porphyrins have been recognized as natural constituents of marine sediments only within the past 5 years (Palmer and Baker, 1978, Science201, 49-51). In that report it was suggested that these pigments may derive from and be markers for oxidized terrestrial organic matter redeposited in the marine environment. In the present study we describe the distribution of copper porphyrins in sediments from several north Pacific and Gulf of California DSDP/IPQD sites (Legs 56,63,64). These allochthonous pigments have now been found to be accompanied by identical arrays of highly dealkylated nickel etioporphyrins. Evaluation of data from this and past studies clearly reveals that there is a strong carbon-number distribution similarity betweeen coincident Cu and Ni etioporphyrins. This homology match is taken as reflecting a common source for the tetrapyrrole ligands of this population of Cu and Ni chelates. Predepositional generation of these highly dealkylated etioporphyrins is concluded from the occurrence of these pigments in sediments continuing essentially all stages of in situ chlorophyll diagenesis (cf. Baker and Louda, 1983). That is, their presence is not regulated by the in situ diagenetic continuum. Thus, the highly dealkylated Cu and Ni etioporphyrins represent an 'allochthonous' background over which 'autochthonous' (viz. marine produced) chlorophyll derivatives are deposited and are undergoing in situ diagenesis.

Chronology and stable carbon isotopic ratios of sediments from the Western Pacific Warm Pool region

Instrumental data suggest that major shifts in tropical Pacific atmospheric dynamics and hydrology have occurred within the past century, potentially in response to anthropogenic warming. To better understand these trends, we use the hydrogen isotopic ratios of terrestrial higher plant leaf waxes (DDwax) in marine sediments from southwest Sulawesi, Indonesia, to compile a detailed reconstruction of central Indo-Pacific Warm Pool (IPWP) hydrologic variability spanning most of the last two millennia. Our paleodata are highly correlated with a monsoon reconstruction from Southeast Asia, indicating that intervals of strong East Asian summer monsoon (EASM) activity are associated with a weaker Indonesian monsoon (IM). Furthermore, the centennial-scale oscillations in our data follow known changes in Northern Hemisphere climate (e.g., the Little Ice Age and Medieval Warm Period) implying a dynamic link between Northern Hemisphere temperatures and IPWP hydrology. The inverse relationship between the EASM and IM suggests that migrations of the Intertropical Convergence Zone and associated changes in monsoon strength caused synoptic hydrologic shifts in the IPWP throughout most of the past two millennia.