Molecular isotopic evidence of environmental and ecological changes across the Cenomanian–Turonian boundary in the Levant Platform of central Jordan

We evaluated the structure of planktonic communities and paleoenvironmental conditions throughout the Cenomanian–Turonian Oceanic Anoxic Event (OAE2) by studying bulk geochemical properties and the molecular isotopic composition of source-specific hydrocarbons from organic-rich sediments deposited in an intrashelf basin at the Levant Platform, central Jordan. High concentrations of desmethyl and 4-methylsteranes as well as dinosteranes indicate that marine algae including dinoflagellates were the main primary producing organisms. The presence of 2-methylhopanes and ¹³C-enriched hopanes, in addition to isotopically enriched aryl isoprenoids, evidenced the contribution of cyanobacteria and green sulfur bacteria, respectively. Additionally, variable but fairly low δ¹⁵N values during OAE2 suggest the occurrence of diazotrophy as a likely important process fueling primary production during OAE2 in this stratified/anoxic continental platform. Variations in the relative contribution of biomarkers revealed changes in planktonic communities associated with sea level change and water column stratification. OAE2 was characterized by strong stratification, anoxic bottom waters and a deep chemocline, as evidenced by high gammacerane and homohopane indices and the absence of photic zone euxinia (PZE) markers, respectively. However, the presence of isorenieratane and its derivatives in post-OAE black shales points to a shoaling of the chemocline and to PZE. This interval was also characterized by an exceptionally high abundance of chlorophyll-derived pristane and phytane (up to 2 mg g^?1 TOC), likely as a result of highly enhanced primary production and organic matter preservation. Remarkably, this high productivity event co-occurs with an exceptionally high abundance of calcispheres reported elsewhere to be part of a global bio-event.     

Glacial to Holocene terrigenous organic matter input to sediments from Orca Basin, Gulf of Mexico — A combined optical and biomarker approach

In this study we assessed changes in the contribution of terrigenous organic matter (OM) to the Gulf of Mexico over the course of the last deglaciation (the last 25 kyr). To this end, we combined optical kerogen analyses with bulk sedimentary, biomarker, and compound-specific carbon isotope analyses. Samples were obtained from core MD02-2550 from Orca Basin (2249 m water depth at 26°56.77N, 91°20.74W) with temporal resolution ranging from multi-decadal to millennial-scale, depending on the proxy. All proxies confirmed larger terrigenous input during glacial times compared to the Holocene. In addition, the kerogen analyses suggest that much of the glacial OM is reworked (at least 50% of spores and pollen grains and 40% of dinoflagellate cysts). The Holocene sediments, in contrast, contain mainly marine OM, which is exceptionally well preserved. During the deglaciation, terrigenous input was generally high due to large meltwater fluxes, whereby discrepancies between different proxies call for additional influences, such as the change in distance to the river mouth, local productivity changes, and hydrodynamic particle sorting. It is possible that kerogen particles and the terrigenous biomarkers studied here represent distinct pools of land-derived OM with inputs varying independently.     

Excess seawater nutrients,enlarged algal symbiont densities and bleaching sensitive reef locations: 2. A regional-scale predictive model for the Great Barrier Reef,Australia

A spatial risk assessment model is developed for the Great Barrier Reef (GBR, Australia) that helps identify reef locations at higher or lower risk of coral bleaching in summer heat-wave conditions. The model confirms the considerable benefit of discriminating nutrient-enriched areas that contain corals with enlarged (suboptimal) symbiont densities for the purpose of identifying bleaching-sensitive reef locations. The benefit of the new system-level understanding is showcased in terms of: (i) improving early-warning forecasts of summer bleaching risk, (ii) explaining historical bleaching patterns, (iii) testing the bleaching-resistant quality of the current marine protected area (MPA) network (iv) identifying routinely monitored coral health attributes, such as the tissue energy reserves and skeletal growth characteristics (viz. density and extension rates) that correlate with bleaching resistant reef locations, and (v) targeting region-specific water quality improvement strategies that may increase reef-scale coral health and bleaching resistance.     

Petroleum degradation and associated microbial signatures at the Chapopote asphalt volcano, Southern Gulf of Mexico

At the Chapopote Knoll in the Southern Gulf of Mexico, deposits of asphalt provide the substrate for a prolific cold seep ecosystem extensively colonized by chemosynthetic communities. This study investigates microbial life and associated biological processes within the asphalts and surrounding oil-impregnated sediments by analysis of intact polar membrane lipids (IPLs), petroleum hydrocarbons and stable carbon isotopic compositions (δ¹³C) of hydrocarbon gases. Asphalt samples are lightly to heavily biodegraded suggesting that petroleum-derived hydrocarbons serve as substrates for the chemosynthetic communities. Accordingly, detection of bacterial diester and diether phospholipids in asphalt samples containing finely dispersed gas hydrate suggests the presence of hydrocarbon-degrading bacteria. Biological methanogenesis contributes a substantial fraction to the methane captured as hydrate in the shallow asphalt deposits evidenced by significant depletion in ¹³C relative to background thermogenic methane. In sediments, petroleum migrating from the subsurface stimulates both methanogenesis and methanotrophy at a sulfate-methane transition zone 6-7 m below the seafloor. In this zone, microbial IPLs are dominated by archaeal phosphohydroxyarchaeols and archaeal diglycosidic diethers and tetraethers. Bacterial IPLs dominate surface sediments that are impregnated by severely biodegraded oil. In the sulfate-reduction zone, diagnostic IPLs indicate that sulfate-reducing bacteria (SRB) play an important role in petroleum degradation. A diverse mixture of phosphohydroxyarchaeols and mixed phospho- and diglycosidic archaeal tetraethers in shallow oil-impregnated sediments point to the presence of anaerobic methane-oxidizing ANME-2 and ANME-1 archaea, respectively, or methanogens. Archaeal IPLs increase in relative abundance with increasing sediment depth and decreasing sulfate concentrations, accompanied by a shift of archaeol-based to tetraether-based archaeal IPLs. The latter shift is suggested to be indicative of a community shift from ANME-2 and/or methanogenic archaea in shallower sediments to ANME-1/methanogenic archaea and possibly benthic archaea in deeper sediments.     

Gas flux and carbonate occurrence at a shallow seep of thermogenic natural gas

The Coal Oil Point seep field located offshore Santa Barbara, CA, consists of dozens of named seeps, including a peripheral ～200 m² area known as Brian Seep, located in 10 m water depth. A single comprehensive survey of gas flux at Brian Seep yielded a methane release rate of ～450 moles of CH₄ per day, originating from 68 persistent gas vents and 23 intermittent vents, with gas flux among persistent vents displaying a log normal frequency distribution. A subsequent series of 33 repeat surveys conducted over a period of 6 months tracked eight persistent vents, and revealed substantial temporal variability in gas venting, with flux from each individual vent varying by more than a factor of 4. During wintertime surveys sediment was largely absent from the site, and carbonate concretions were exposed at the seafloor. The presence of the carbonates was unexpected, as the thermogenic seep gas contains 6.7% CO₂, which should act to dissolve carbonates. The average δ¹³C of the carbonates was ?29.2?±?2.8‰ VPDB, compared to a range of ?1.0 to +7.8‰ for CO₂ in the seep gas, indicating that CO₂ from the seep gas is quantitatively not as important as ¹³C-depleted bicarbonate derived from methane oxidation. Methane, with a δ¹³C of approximately ?43‰, is oxidized and the resulting inorganic carbon precipitates as high-magnesium calcite and other carbonate minerals. This finding is supported by ¹³C-depleted biomarkers typically associated with anaerobic methanotrophic archaea and their bacterial syntrophic partners in the carbonates (lipid biomarker δ¹³C ranged from ?84 to ?25‰). The inconsistency in δ¹³C between the carbonates and the seeping CO₂ was resolved by discovering pockets of gas trapped near the base of the sediment column with δ¹³C-CO₂ values ranging from ?26.9 to ?11.6‰. A mechanism of carbonate formation is proposed in which carbonates form near the sediment–bedrock interface during times of sufficient sediment coverage, in which anaerobic oxidation of methane is favored. Precipitation occurs at a sufficient distance from active venting for the molecular and isotopic composition of seep gas to be masked by the generation of carbonate alkalinity from anaerobic methane oxidation.

Structural diversity and fate of intact polar lipids in marine sediments

Marine sediments harbor an enormous quantity of microorganisms, including a multitude of novel species. The habitable zone of the marine sediment column begins at the sediment-water interface and probably extends to depths of several thousands of meters. Studies of the microbial diversity in this ecosystem have mostly relied on molecular biological techniques. We used a complementary method - analysis of intact polar membrane lipids - to characterize the in-situ microbial community in sediments covering a wide range of environmental conditions from Peru Margin, Equatorial Pacific, Hydrate Ridge, and Juan de Fuca Ridge. Bacterial and eukaryotic phospholipids were only detected in surface sediments from the Peru Margin. In contrast, deeply buried sediments, independent of their geographic location, were dominated by archaeal diether and tetraether lipids with various polar head groups and core lipids. We compared ring distributions of archaeal tetraether lipids derived from polar glycosidic precursors with those that are present as core lipids. The distributions of these related compound pools were distinct, suggestive of different archaeal sources, i.e., the polar compounds derive from sedimentary communities and the core lipids are fossil remnants from planktonic communities with possible admixtures of decayed sedimentary archaea. This in-situ production of distinct archaeal lipid populations potentially affects applications of the TEX86 paleotemperature proxy as demonstrated by offsets in reconstructed temperatures between both pools. We evaluated how varying cell and lipid stabilities will influence the sedimentary pool by using a box-model. The results are consistent with (i) a requirement of continuous inputs of freshly synthesized lipids in subsurface sediments for explaining the observed distribution of intact polar lipids, and (ii) decreasing lipid inputs with increasing burial depth.     

The mid-latitude horizontal and vertical structure of the zonally asymmetric intraseasonal and interannual ozone variability in boreal winters

 A singular value decomposition (SVD) is used to calculate SVD-selected fields of ozone and geopotential, which exhibit maximum covariance, from the observed zonally asymmetric total ozone field and that of the three-dimensional geopotential field thus leaving almost purely dynamical induced variations in the remaining ozone field. This procedure was applied to Total Ozone Mapping Spectrometer data (TOMS) and to geopotential values from the National Centers for Environmental Prediction in the boreal mid-latitudes in the winter months of 1979–1992. Intraseasonal variability (December–February) and trend-eliminated interannual winter mean variability of total ozone and geopotential are analyzed. The first four modes of SVD analysis explain more than 70% of the covariance for the intraseasonal variability and more than 80% of that for the interannual variability. The vertical structure of geopotential regression maps reveals a clear wave-1 pattern for modes one and two and a wave-2 pattern for modes three and four. These patterns show differently but generally westward tilted phases and are more complex at heights below 70 hPa. Further a linear transport model of a conservative tracer was applied to each individual geopotential mode found by the SVD analysis in connection with an observed height and latitude dependent zonal mean ozone distribution. The model results of total ozone reproduce the spatial patterns of the SVD-selected total ozone field quite well whereas their magnitudes are variously underestimated. This method allows us to assess the vertical distribution of the contribution of single modes to the total ozone variability. Maximum contributions are found between 150 and 70 hPa. Smaller amplitude maxima are found around 10 hPa, which result from contributions of horizontal advection of ozone alone. These results reflect an expected dynamical link between the variability of the zonally asymmetric parts of geopotential and ozone. Received: 7 November 1997 / Accepted: 10 June 1998     

Sources,transport, and partitioning of organic matter at a highly dynamic continental margin

Continental shelves play a major role as transition zone during transport of multiply-sourced organic matter into the deep sea. In order to obtain a comprehensive understanding of the origin and fractionation processes of organic matter at the NW Iberian margin, 40 surface sediment samples were analyzed for a structurally diverse range of lipid biomarkers, lignin phenols, grain size distribution, organic carbon content (TOC), its stable carbon isotopic composition (δ¹³CTOC), and the organic carbon to nitrogen ratio (TOC/TN). The biomarker inventory reflected a heterogeneous mixture of organic matter from various marine and terrestrial sources. Soil- and vascular plant-derived continental organic matter, indicated by lignin phenols and plant-derived triterpenoids, was primarily associated with the silt fraction and transported by river run-off. The spatial distribution patterns of higher plant-derived waxes, long-chain n-alkanes, n-alcohols, and n-fatty acids suggested distinct different transport mechanisms and/or sources. The branched tetraether index, a molecular proxy expressing the relative abundance of branched dialkyl tetraethers vs. crenarchaeol and considered to signal soil-derived organic matter, was not as sensitive as the other molecular indicators in detecting continental organic matter. Hydrodynamic sorting processes on the shelf resulted in a separation of different types of terrestrial organic matter; grass and leaf fragments and soil organic matter were preferentially transported offshore and deposited in areas of lower hydrodynamic energy. Algal lipid biomarker distributions indicated a complex community of marine plankton contributing to organic matter. Spatial and seasonal patterns of phytoplankton growth primarily controlled the distribution of algal organic matter components. The interplay of all of these processes controls production, distribution, and deposition of organic matter and results in three distinct provinces at the Galicia–Minho shelf: (I) fresh marine organic matter dominated the inner shelf region; (II) high inputs of terrestrial organic matter and high TOC content characterized the mid-shelf deposited mudbelt; (III) lower concentrations of relatively degraded organic matter with increased proportions of refractory terrestrial components dominated the outer shelf and continental slope.