Investigating pathways of diagenetic organic matter sulfurization using compound-specific sulfur isotope analysis

We present the results of compound-specific sulfur isotope analyses performed on organic sulfur compounds (OSCs) isolated from sediments deposited in the euxinic Cariaco Basin, Venezuela. Individual OSCs (sulfurized highly branched isoprenoids and malabaricatriene) have sulfur isotope compositions of ca. −15‰, which is ³⁴S enriched by 5-15‰ relative to coeval bulk organic and inorganic sulfur pools. These observed differences in the sulfur isotope composition of bulk organic sulfur in the kerogen and bitumen pools and individual OSCs demonstrate that there are multiple pathways of organic sulfur formation operating simultaneously in marine sediments. Comparison of our measured compound-specific sulfur isotope data with values predicted using simple isotopic mass balance assumptions suggests that the sulfurization process likely involves multiple sources of inorganic sulfur. Further, the isotopic composition of these various precursor inorganic sulfur species and the specific pathway of sulfur incorporation into organic matter (OM) impart distinct isotopic compositions to the resulting organic sulfur compounds. These data represent the first compound-specific sulfur isotope measurements made in marine sediments, and demonstrate the utility of compound-specific sulfur isotope analysis in identification of inorganic sulfur sources for OM sulfurization and tracking pathways of sulfur incorporation, which will lead to a more complete understanding of diagenetic sulfurization of OM.     

Origin of organic sulphur compounds and sulphur-containing high molecular weight substances in sediments and immature crude oils

The distribution patterns of Organic Sulphur Compounds (OSC), occurring in certain sediments and immature crude oils, were compared with those of the corresponding hydrocarbons. Because of the complexity of the OSC mixtures, they were desulphurized to hydrocarbons (n-alkanes, isoprenoid alkanes, steranes, triterpanes and branched alkanes). The hydrocarbons produced by desulphurization of the OSC exhibited distribution patterns different from those of the hydrocarbons originally present. Therefore reaction of elemental sulphur with these hydrocarbons at elevated temperatures must be considered as an unlikely origin for these OSC. Sulphur incorporation reactions on an intramolecular basis with suitable functionalized precursors at the early stages of diagenesis are probably the major origin for these OSc. Desulphurization of high molecular weight fractions also produced hydrocarbons, dominated by n-alkanes up to C₄₀. Therefore it is assumed that these substances contain n-alkanes, 2,5-dialkyl-thiophenes and -thiolanes linked to each other by sulphur briddges. These findings stronly suggest that sulphur-containing high molecular weight substances are formed by the same sulphur incorporation reactions as OSC, but in an intermolecular fashion.     

Application of the TEX86 temperature proxy to the southern North Sea

A novel temperature proxy, the tetraether index of lipids with 86 carbon atoms (TEX₈₆), was applied to the suspended particulate organic matter (POM) and sediment core tops from eight sites in the southern North Sea in different seasons. The TEX₈₆-derived temperatures in many samples did not correlate with mean annual sea surface temperature (SST), but were shifted toward winter SST, apparently because Crenarchaeota are more abundant and metabolically active during periods of low primary production. This indicates that TEX₈₆-derived SST estimates do not necessarily reflect annual mean SST and may provide essential information about seasonal SST palaeoreconstruction. High TEX₈₆-derived SSTs were measured in the water of the river Rhine and in the sediment core tops and seawater from several stations in the southern North Sea. These sites were all characterised by important input of organic matter from soil and peat, as revealed by the relatively high values obtained with the new terrestrial proxy, the branched and isoprenoid tetraether (BIT) index. These data demonstrate that to reconstruct palaeotemperatures it is essential to estimate both TEX₈₆ and BIT indices to check that TEX₈₆ temperatures are not biased as a result of large terrestrial input. Important seasonal variations in TEX₈₆-derived SST were also evident for the surface sediments of several stations characterised by extremely low sedimentation rates, indicating temporary settlement of laterally transported organic matter with a warmer temperature signal. This implies that sediment core top correlations between TEX₈₆ and mean annual SST should not be carried out in areas characterised by transient sediment deposition.     

Bacterial tetraether membrane lipids in peat and coal: Testing the MBT-CBT temperature proxy for climate reconstruction

Peatlands are widespread and important natural archives of environmental change. Here we explore the potential of the recently introduced MBT-CBT proxy (methylation index and cyclisation ratio of branched tetraethers) to estimate past annual mean air temperature (MAT) based on the distribution of bacterially-derived branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in peat and coal. To this end, branched GDGTs in an ombrotrophic peat bog from Switzerland and three coal deposits of increasing maturity were analysed.For the surface of the bog, reconstructed annual MAT is higher than both measured annual MAT and measured in situ pore water temperature. Changes in the CBT ratio, considered a proxy for pH, with depth in the bog do not match with present day in situ pore water pH, but coincide with a peat stratigraphic boundary. This indicates that GDGTs down the bog profile are predominantly fossil and not derived from extant biomass. The MBT-CBT derived annual MAT record also shows a large drop at this stratigraphic boundary, which likely relates to past change in trophic status of the bog. Branched GDGTs are abundant in an immature lignite (vitrinite reflectance, R_o 0.25%), but occur in low amount in a slightly more mature coal (R_o 0.32%). Annual MAT could be reconstructed for the lignite alone and is higher than other proxy-based estimates from approximately the same time and location.Our results indicate potential for the application of the MBT-CBT proxy in peat and immature coals, but improved constraints on the effects of different types of peat on branched GDGT distributions as well as improved calibration of MAT estimates are needed before the method can be confidently applied.     

Lithotype discrimination within a gridded model by inverting gravity data

In this paper we present a method which allows delineation of geologic structures in a bi-modal lithotype setting. We propose to use gravity data in combination with a priori information about the density contrast between the two lithotypes. The iterative method uses an objective function with five tunable parameters which need to be set. Using an efficient parameter search, suitable ranges of these are investigated to determine their optimal values, respectively, which in turn, ensures good inversion results.
The approach produces structural images of the subsurface, without the need of an a priori density model; the depth to the top of the inhomogeneity is also retrieved.
Besides synthetic simulations, the methodology has also been applied to a small gravity data set, acquired by the industry over a basinal structure. A consistent, bi-modal image of the bedrock depression is obtained from the data, which, in this case, was the goal. Other potential areas of application include delineation of salt structures and ore deposits.     

Irish Ice Sheet sector dynamics as indicated by the Tynagh mineral deposit erratic assemblages

Subglacial erratic assemblages derived and dispersed from the Tynagh mineral deposit, SE County Galway, Ireland have been mapped in order to facilitate the reconstruction of Late Midlandian temporal ice flow dynamics over the area. Broadly concomitant Pb, Cu and Zn subsoil sediment anomalies define a ribbon type erratic plume (3D) extending for up to 5.75 km east (095°) of the Tynagh orebody composite source, in-line with other, local subglacial geomorphological evidence of ice flow. The eastern sector of a fragmented surficial fan-shaped erratic clast train (2D) aligns with the plume, the remainder of the assemblage orientated towards the south (095-175°). The alignment of the erratic plume, subglacial streamlined bedforms and eskers indicate that the plume and bedforms formed (pene)contemporaneously followed by esker development, all elements produced during the early deglacial phase of the Late Midlandian Irish Ice Sheet. The SE-S portion of the erratic train is ascribed to a previous period of ice flow southwards. Only multiple phase models of the Late Midlandian Irish Ice Sheet, that include or can accommodate a time transgressive element, can accurately represent the configuration of ice flow indicators both in the Tynagh area and surrounding region. It is possible that such erratic assemblages are far more ephemeral than previously considered.     

A diatomaceous origin for long-chain diols and mid-chain hydroxy methyl alkanoates widely occurring in quaternary marine sediments: indicators for high-nutrient conditions

For the first time a biological source for the long-chain alkyl 1,14-diols and 12-hydroxy methyl alkanoates, lipids widely occurring in the marine water column and sediments, has been identified. Cultures of Proboscia indica and Proboscia alata, rhizosolenoid diatoms belonging to the widespread diatom genus Proboscia, contain C₂₈, C_28:1, C₃₀, and C_30:1 alkyl 1,14-diols, and C₂₇ and C₂₉ 12-hydroxy methyl alkanoates as major neutral lipids. These components form a substantial fraction of lipid fractions from sediment traps or sediments, especially in areas with an elevated primary production such as upwelling regions. Examination of literature data reveals that as much as 20 to 35% of the total lipid flux in the Arabian Sea is derived from Proboscia diatoms during the start of the upwelling season. Their rapid transfer to the water-sediment interface may explain why corresponding 1,14-keto-ols, inferred oxidation products of diols, are hardly formed. These interpretations are supported by compound-specific carbon isotopic analysis of long-chain keto-ols and diols in surface sediments of the Arabian Sea. The data indicate that long-chain alkyl 1,14-diols and 12-hydroxy methyl alkanoates can be applied as indicators for high-nutrient conditions in the photic zone.     

Environmental controls on bacterial tetraether membrane lipid distribution in soils

Over the last years a novel group of branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids has been discovered in peat bogs and soils. They consist of components with 4-6 methyl groups attached to the n-alkyl chains and 0 to 2 cyclopentyl moieties in the alkyl chain. These branched membrane lipids are produced by an as yet unknown group of anaerobic soil bacteria. In this study we analysed the branched membrane lipid content of 134 soil samples from 90 globally distributed locations to study the environmental factors controlling the relative distribution of the different branched GDGT isomers. Our results show that the relative amount of cyclopentyl moieties, expressed in the cyclisation ratio of branched tetraethers (CBT), is primarily related to the pH of the soil (R² = 0.70) and not to temperature (R² = 0.03). The relative amount of methyl branches, expressed in the methylation index of branched tetraethers (MBT), is positively correlated with the annual mean air temperature (MAT) (R² = 0.62) and, to a lesser extent, negatively correlated with the pH of the soil (R² = 0.37). If both parameters are combined, however, it appears that the variation in the MBT is largely explained by both MAT and pH (R² = 0.82). These results suggest that the relative distribution of soil-derived GDGT membrane lipids can be used in palaeoenvironmental studies to estimate past annual MAT and soil pH.     

Seasonal changes in glycerol dialkyl glycerol tetraether concentrations and fluxes in a perialpine lake: Implications for the use of the TEX86 and BIT proxies

To determine where and when glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in lakes are produced, we collected descending particles in Lake Lucerne (Switzerland) using two sediment traps (at 42 and 72 m water depth) with a monthly resolution from January 2008 to late March 2009. Suspended particulate matter (SPM) was monthly filtered from the water column at three different depths. The potential application of GDGTs in palaeoenvironmental and palaeoclimatic reconstructions was investigated by comparing core lipids and their relative GDGT distribution, with lake water temperatures throughout the year. Fluxes of GDGTs and their concentrations in the water column vary according to a seasonal pattern, showing a similar trend in the SPM and sediment traps. Fluxes and concentrations of isoprenoid GDGTs increase with depth, maximum values being observed in the deeper part of the water column, indicating production of isoprenoid GDGTs by Thaumarchaeota in the deep (∼50 m), aphotic zone of Lake Lucerne. The flux-weighted averages of the proxies TEX₈₆ (0.27) and BIT (0.03) based on the total extracted GDGTs are similar at both trap depths. A sediment core from the same location showed that in the first few centimetres of the core TEX₈₆ and BIT values of 0.29 and 0.07, respectively, are similar to those recorded for descending particles and SPM, indicating that the sedimentary TEX₈₆ records the annual mean temperature of deeper waters in Lake Lucerne. TEX₈₆ values are slightly higher below 20 cm in the core. This offset is interpreted to be caused by the present-day trophic state of the lake, which probably resulted in a deeper niche of the Thaumarchaeota. Branched GDGTs represent only a minor fraction of the total GDGTs in the lake and their origin remains unclear. Our data reveal that GDGTs in lakes have a large potential for palaeoclimatic studies but indicate that knowledge of the system is important for accurate interpretation.