Ralf Littke

The molecularly-uncharacterized component of nonliving organic matter in natural environments.

Molecularly-uncharacterized organic matter comprises most reduced carbon in soils, sediments and natural waters. The origins, reactions and fates of these ubiquitous materials are relatively obscure, in large part because the rich vein of geochemical information that typically derives from detailed structural and stereochemical analysis is yet to be tapped. This discussion highlights current knowledge about the origins and characteristics of molecularly uncharacterized organic matter in the environment and outlines possible means by which this structurally uncharted frontier might best be explored.

Evolution patterns of radiolaria and organic matter variations: A new approach to identify sea-level changes in mid-Cretaceous pelagic environments

The relation between sea-level changes, plankton productivity, and evolution, as well as the occurrence of anoxic sediments, provides an interesting avenue of paleooceanographic research. In this context, we examined mid-Cretaceous radiolarian faunas, carbonate isotopic and organic matter type data of Deep Sea Drilling Project and Ocean Drilling Program cores from the North Atlantic, and samples from outcrops of the western Tethys from central and northern Italy. Former studies indicate that an expansion of the oxygen minimum zone caused plankton extinctions at the Cenomanian-Turonian boundary. An expanded oxygen minimum zone would destroy deeper habitats of planktic foraminifera, causing the extinction of deeper dwelling forms. Although this model is well established for the Cenomanian-Turonian boundary, not much is known about the causes of extinctions and radiations during the entire mid-Cretaceous (Aptian-Turonian). We demonstrate that the dimension of the oxygen minimum zone, which depends on the relative sea level and the corresponding nutrient supply, causes the complex pattern of evolution and radiation of planktic protozoa and the sedimentation of black shales in the mid-Cretaceous. This new depositional model allows correlation of micropaleontologic data and different types of black shales in the pelagic realm within a sequence stratigraphic framework.

Thermal History of Sedimentary Basins

Geoscientists have always been interested in temperature values, as the crucial role of temperature in various geological, geochemical, and geophysical phenomena was recognized relatively early. However, this interest was long limited to aspects such as temperature distribution in the earth’s crust and mantle, melting temperatures of different rocks, equilibrium temperatures of various metamorphic mineral assemblages, and temperature dependency of some geophysical parameters. Almost all of these are related to thermal conditions in the deeper parts of the earths crust. Low temperature fields at shallow depth have been of less concern. It is interesting to recognize that all these studies sought to define either the present distribution of temperature or the maximum temperature reached at some time in the geological past. In other words, temperature distribution as a steady state case was the subject of interest.

Water column anoxia, enhanced productivity and concomitant changes in δ13C and δ34S across the Frasnian–Famennian boundary (Kowala — Holy Cross Mountains/Poland)

The investigation of the trace element and organic geochemistry of the Frasnian–Famennian boundary section at Kowala (Holy Cross Mountains/Poland) shows that the lower water column was oxygen-deficient during late Frasnian and early Famennian times. The abundance and carbon isotopic composition of diaryl isoprenoids, biomarkers indicative for green sulfur bacteria, prove that euxinic waters reached into the photic zone, at least episodically. Total organic carbon (TOC) contents show two maxima that are time-equivalent to the Kellwasser horizons deposited in shallower water settings. Enhanced TOC concentrations are explained by a higher primary productivity, presumably as a consequence of an enhanced nutrient supply from the continent. The increase in the abundance of hopanes and bituminite suggests that the bacterial contribution to TOC increased at the Frasnian–Famennian transition. The sulfur isotopic composition of pyritic- and organically bound sulfur shows a +27‰ excursion across the boundary. The observation that the δ34S values of organic-bound sulfur closely resemble that of pyrite sulfur indicates a common sulfur source, likely early diagenetic sulfide. A change in the δ13C of total dissolved inorganic carbon as a consequence of an enhanced burial of 12C-enriched organic carbon is indicated by a +3‰ excursion measured for TOC as well as for individual n-alkanes and isoprenoids. The burial of large amounts of organic carbon is expected to result in a decrease in pCO2 and should affect the photosynthetic carbon isotope fractionation (ep). The fact that we observe no change in ep can be explained by the circumstance that ep was most probably at maximum values, as a consequence of high atmospheric and oceanic-dissolved CO2 concentrations during the Devonian.

Occurrence and alteration of organic contaminants in seepage and leakage water from a waste deposit landfill

Organic-geochemical analyses have been applied to seepage water and leakage water samples of a waste deposit landfill in order to give a comprehensive view on the composition of the organic contaminants. Based on intense GC/MS screening analyses a wide variety of organic substances were identified and attributed to natural or xenobiotic waste components. Apart from plant material-derived compounds and degradation products of peptides, carbohydrates and lignin, numerous xenobiotic substances were identified and attributed to the groups of pharmaceuticals, plasticizers, pesticides or chlorinated aromatics. Not all of the substances identified in the seepage water samples were recovered in the leakage water sample due to degradation processes or dilution by uncontaminated water. Quantitative analysis of selected contaminants was used to discriminate substances affected by degradation processes and persistent compounds.

Quantification of loss of calcite, pyrite, and organic matter due to weathering of Toarcian black shales and effects on kerogen and bitumen characteristics

Abstract Comparison of geochemical data on Posidonia Shale (Early Toarcian) from a shallow unweathered core and from adjacent weathered exposures of the same fades was performed. Results revealed that quantitatively the most affected petrographic constituent is pyrite and that organic matter and carbonate were also altered by weathering. Due to this process of weathering, the bulk composition of organic matter, especially the molecular composition of soluble organic matter and the fluorescence colour of organic particles, are changed. Estimates of weathering rates reveal that the annual release of sulphur and organic carbon from black shales may significantly add to anthropogenic pollution.

Generation of nitrogen and methane from sedimentary organic matter: implications on the dynamics of natural gas accumulations

Nitrogen (N2) contents of natural gases in Rotliegend and Buntsandstein reservoirs of the North German basin regionally approach 100%. A review is given of the various hypotheses (primordial origin, volcanic or magmatic origin, radiogenic origin, atmospheric origin, organic origin, inorganic nitrogen in sedimentary rocks) presented to account for nitrogen anomalies in this area and other parts of the world. The objective of the present study was to investigate sedimentary organic matter, in particular coals, as a potential source of molecular nitrogen in the subsurface. Comparison of reservoir sizes and gas generation potentials indicates that Carboniferous coal measures, which are considered as the source of the natural gas in the North German basin, can readily account for the nitrogen quantities found in present-day reservoirs. Laboratory pyrolysis experiments were carried out to investigate the kinetics of generation of methane and molecular nitrogen from coals of different type and rank. Under experimental conditions nitrogen is formed at higher temperatures than methane supporting the concept of a ‘fractional generation’ of methane and nitrogen in natural systems. Based on the kinetic parameters derived from laboratory experiments methane and nitrogen generation rates from coals were calculated for geologic heating rates. Gas containing more than 50% nitrogen is generated under these conditions at temperatures in excess of 300°C. Nitrogen-rich gases are thus formed only in the final stage of gas generation after methane generation has practically ceased. It is concluded that the amounts of gas encountered in nitrogen-rich gas accumulations represent only a small fraction (possibly < 1 %) of the total gas generation potential of this area while the bulk of the generated gas has escaped to the atmosphere. The present-day composition of the reservoir gases reflects the composition of only the most recently generated gas (on a geologic time scale).

Organic matter maturation under the influence of a deep intrusive heat source: A natural experiment for quantitation of hydrocarbon generation and expulsion from a petroleum source rock (Toarcian shale, northern Germany)

Four shallow boreholes were drilled in the Hils syncline, northern Germany, in order to determine quantitatively the amount of hydrocarbons generated and expelled during maturation of a typical kerogen-type-II-bearing source rock. The holes penetrated the carbonceous Lias ϵ shales (Posidonia shale, Lower Toarcian) and part of the adjacent Dogger α and Lias δ mudstones. The maturity of the organic matter in the cores recovered ranges from immature (0.48% R0) to overmature 1.45% R0) due to location of the Hils syncline in the vicinity of the Vlotho Massif, which is deep-seated intrusive body. Facies variations of the Lias ϵ within the short geographical distances in the study area are negligible. Organic matter mass balance calculations were based on detailed organic geochemical analyses of residual material in the Lias ϵ shales (kerogen, bitumen etc.) and on the evidence of a uniform initial composition of these sediments in the study area. Dead carbon determinations supported this latter criterion but were not used as a parameter in the calculations. About 50% of the initial kerogen was transformed into oil, gas and inorganic compounds during the vitrinite reflectance increase from 0.48 to 0.88% Ro and only marginally more during the maturity increase from 0.88 to 1.45% Ro. Only a small portion of the generated material remained in the source rock even at a relatively early stage of generation (0.68% Ro). Expulsion efficiency of oil plus gas reached a value of 86% at the end of the main generation stage (0.88% Ro).

Molecular Nitrogen in Natural Gas Accumulations: Generation from Sedimentary Organic Matter at High Temperatures

The occurrence of natural gas accumulations with high percentages (up to 100%) of molecular nitrogen in various hydrocarbon provinces represents a largely unresolved problem and a serious exploration risk. In this context, a geochemical and basin modeling study was performed to evaluate the potential of sedimentary organic matter to generate molecular nitrogen. The masses of nitrogen present in coals--if converted into molecular nitrogen--are sufficient to fill commercial gas reservoirs. A calculation for gas accumulations in northern Germany, where percentages of molecular nitrogen range from less than 5 to greater than 90%, reveals that the molecular nitrogen generated in underlying coal-bearing strata is sufficient to account for the nitrogen gas even in the largest fi lds. In addition, much of the total nitrogen in clay-rich rock types, such as shales and mudstones, is fixed in sedimentary organic matter and may add to the nitrogen generation capacity of the coals. A kinetic interpretation of the nitrogen and methane generation characteristics of humic coals during laboratory pyrolysis indicates that nitrogen is generated from organic matter in sedimentary basins at higher temperatures than methane. Minimum temperatures for preferential nitrogen generation are in excess of 300°C for extremely low heating rates or isothermal conditions lasting over 300 m.y. Thus, nitrogen-rich gases are mainly formed in the final stage of gas generation, when sedimentary rocks grade into metamorphic rocks. Applications of the kinetic parameters for the generation of nitrogen and methane from coaly organic matter in Carboniferous sedimentary rocks are presented for part of the Northwest German basin. In this area, immense volumes of the methane-rich gas gener ted during the late Paleozoic and Mesozoic were lost to the atmosphere. The present gas composition mainly reflects the latest Cenozoic gas generation. This Cenozoic gas is nitrogen rich where the Carboniferous source rock sequences are highly mature and deeply buried.

Identification of specific organic contaminants for estimating the contribution of the Elbe river to the pollution of the German Bight

Abstract GC/MS analyses have been applied to sediment samples of the German Bight in order to document the state of organic contamination as well as to identify specific molecular markers that are appropriate to estimate the discharge of anthropogenic compounds derived from the Elbe river. Detailed screening analyses revealed a wide variety of organic lipophilic compounds of biogenic, petrogenic as well as anthropogenic origin. Potential marker compounds indicating the contribution of the Elbe river could be attributed mainly to the chlorinated aromatic contaminants. Specifically, these include tetra- to hexachlorobenzenes, mono- to dichloronaphthalenes, hexachlorobutadiene, tetrabutyl tin, alkylsulfonic acid phenylesters, 1,2,3,6,7,8-hexahydro-1,1,6,6-tetramethyl-4-isopropyl- as -indacene and 4,4′-dichlorodiphenylsulfide. These compounds are suitable to indicate the spatial distribution of Elbe river derived organic matter.