Organic and inorganic geochemical aspects of Mediterranean Late Quaternary sapropels and Messinian evaporitic deposits

This thesis describes the results of organic and inorganic geochemical investigations obtained for samples collected from the eastern Mediterranean sea floor and from Italy. The samples vary in age from the Late Miocene up to the Holocene. The environmental conditions prevailing during deposition of the samples differ greatly. Sediment samples deposited under both marine anoxic and oxic conditions were studied as well as samples from hypersaline depositional environments. Late Quaternary eastern Mediterranean sediments are characterized by the occurrence of organic rich layers, so-called sapropels. Detailed investigations of the soluble organic matter of these sapropels indicate that the organic matter is of a mixed marine, terrigenous and bacterial origin (Chapters 2 and 3). A study of the lateral variation in concentration of organic compounds in the youngest sapropel (SI) revealed a trend of relatively increasing amounts of continentally derived organic matter going from coastal towards more seaward and deeper realms (Chapter 3). Variations in the sea-surface water temperatures during deposition of the sapropels could be inferred from the relative abundance of di- and triunsaturated C37 ketones and from the relative abundance of 27-nor-24-methylcholesta-5,22E-dien-3p-ol and cholesta-5,22E-dien-3p-ol (Chapter 4). Three samples of Holocene age from the anoxic brine-filled Tyro basin were investigated (Chapter 5). Most compounds identified in these samples were also present in the sapropels, but some compounds, such as trisnorhopan-21-one and bishomohopanoic acid, are ascribed to an input from microbiota living at the brine-seawater interface. The brine is thought to be formed by leaching of tectonically exposed Messinian evaporitic deposits, consisting of gypsum, halite and some soluble salts (Chapter 6). The interstitial water chemistry of sediments from the Tyro basin suggests that selective leaching has taken place, namely halite dissolution followed later by gypsum dissolution (Chapter 7). The intersti tial water chemistry of a core collected in the adjacent Kretheus basin, where at present the bottom salinity and oxygen content is at a normal level, is characterized by a strong downward increase of Na and Cl, indicating that conditions similar to the Tyro basin must once have prevailed in the Kretheus basin. The interstitial water chemistry of "normal" Mediterranean sediments is also discussed in chapter 7. Chapter 8 describes the occurrence and significance of hydrocarbons present in three samples from a Messinian evaporitic basin (northern Apennines). Several new compounds were detected and are discussed. A series of extended hop-17(2l)-enes maximizing at the C35 members, a very low pristane/phytane ratio, and an even over odd predominance of long chain alkanes seem characteristic for hypersaline conditions prevailing in the past. Using the biological markers described in chapter 8 it is suggested that the source rock of the Rozel Point Oil was deposited under hypersaline conditions (Chapter 9). The investigations of the samples from ancient hypersaline environments also revealed some anomalies in hopanoid and steroid maturi ty indices (Chapters 8 and 9). Extended 17a(H),21~(H)-hopanesand extended hop-17(21)-enes occur fully isomerized at C-22; 20R- and 20S-5a(H),14~(H),17~(H)steranes are relatively abundant whereas 20S-5a(H),14a(H),17a(H) steranes are virtually absent. Possible diagenetic pathways explaining these phenomena are proposed in chapter 1

Organic and inorganic geochemical aspects of Mediterranean Late Quaternary sapropels and Messinian evaporitic deposits

This thesis describes the results of organic and inorganic geochemical investigations obtained for samples collected from the eastern Mediterranean sea floor and from Italy. The samples vary in age from the Late Miocene up to the Holocene. The environmental conditions prevailing during deposition of the samples differ greatly. Sediment samples deposited under both marine anoxic and oxic conditions were studied as well as samples from hypersaline depositional environments. Late Quaternary eastern Mediterranean sediments are characterized by the occurrence of organic rich layers, so-called sapropels. Detailed investigations of the soluble organic matter of these sapropels indicate that the organic matter is of a mixed marine, terrigenous and bacterial origin (Chapters 2 and 3). A study of the lateral variation in concentration of organic compounds in the youngest sapropel (SI) revealed a trend of relatively increasing amounts of continentally derived organic matter going from coastal towards more seaward and deeper realms (Chapter 3). Variations in the sea-surface water temperatures during deposition of the sapropels could be inferred from the relative abundance of di- and triunsaturated C37 ketones and from the relative abundance of 27-nor-24-methylcholesta-5,22E-dien-3p-ol and cholesta-5,22E-dien-3p-ol (Chapter 4). Three samples of Holocene age from the anoxic brine-filled Tyro basin were investigated (Chapter 5). Most compounds identified in these samples were also present in the sapropels, but some compounds, such as trisnorhopan-21-one and bishomohopanoic acid, are ascribed to an input from microbiota living at the brine-seawater interface. The brine is thought to be formed by leaching of tectonically exposed Messinian evaporitic deposits, consisting of gypsum, halite and some soluble salts (Chapter 6). The interstitial water chemistry of sediments from the Tyro basin suggests that selective leaching has taken place, namely halite dissolution followed later by gypsum dissolution (Chapter 7). The intersti tial water chemistry of a core collected in the adjacent Kretheus basin, where at present the bottom salinity and oxygen content is at a normal level, is characterized by a strong downward increase of Na and Cl, indicating that conditions similar to the Tyro basin must once have prevailed in the Kretheus basin. The interstitial water chemistry of "normal" Mediterranean sediments is also discussed in chapter 7. Chapter 8 describes the occurrence and significance of hydrocarbons present in three samples from a Messinian evaporitic basin (northern Apennines). Several new compounds were detected and are discussed. A series of extended hop-17(2l)-enes maximizing at the C35 members, a very low pristane/phytane ratio, and an even over odd predominance of long chain alkanes seem characteristic for hypersaline conditions prevailing in the past. Using the biological markers described in chapter 8 it is suggested that the source rock of the Rozel Point Oil was deposited under hypersaline conditions (Chapter 9). The investigations of the samples from ancient hypersaline environments also revealed some anomalies in hopanoid and steroid maturi ty indices (Chapters 8 and 9). Extended 17a(H),21~(H)-hopanesand extended hop-17(21)-enes occur fully isomerized at C-22; 20R- and 20S-5a(H),14~(H),17~(H)steranes are relatively abundant whereas 20S-5a(H),14a(H),17a(H) steranes are virtually absent. Possible diagenetic pathways explaining these phenomena are proposed in chapter 1

Interstitial water studies of Late Quaternary Eastern Mediterranean sediments with emphasis on early diagenetic reactions and evaporitic salt influences

Interstitial waters of five piston cores from the Eastern Mediterranean were recovered by shipboard squeezing and analyzed for their major elements. Additionally, organic carbon, CaCO3 and HCl extractable Mg were determined in the sediments from these cores. Three cores contain sapropels, which are considered to be the sedimentary expression of anoxic periods during the Quaternary. The pH in the interstitial waters extracted from the sapropels is significantly lower than the pH of the interstitial waters from the surrounding sediment. It is suggested that the low pH in sapropel interstitial waters is induced by bacterial activity. Other dissolved elements are not influenced by these sapropel layers. Core T83-46, taken in the hypersaline anoxic Tyro Basin, shows a strong downward SO4 decrease, accompanied by a strong NH4 increase. The interstitial water chemistry of the Tyro Basin sediment is attributed to selective leaching of evaporitic salt; first halite dissolution, later followed by gypsum dissolution. The interstitial water chemistry of core T83-45, taken in the adjacent Kretheus Basin, is dominated by diffusive processes, which became important after oxic conditions returned in this formerly anoxic hypersaline basin

Distribution of organic sulphur compounds in Mesozoic and Cenozoic sediments from the Atlantic and Pacific Oceans and the Gulf of California

Gas chromatography-mass spectrometry data of the "aromatic hydrocarbon" fractions of nearly 100 Deep Sea Drilling Project and Ocean Drilling Program sediment samples have been re-examined for the occurrence of organic sulfur compounds. Approximately 70% of the samples contain OSC with varying distribution patterns, although C₂₀ isoprenoid thiopenes are invariably present

Origin and diagenetic transformations of C25 and C30 highly branched isoprenoid sulphur compounds : further evidence for the formation of organic sulphur compounds during early diagenesis

A number of C₂₅ and C₃₀ highly branched isoprenoid (HBI) sulphur compounds (e. g. , thiolanes, l-oxo-thiolanes, thiophenes, and benzo[b]thiophenes) with 2, 6, 10, 14-tetramethyl-7-(3-methylpentyl) pentadecane and 2, 6, 10, 14, 18-pentamethyl-7-(3-methylpentyl)nonadecane carbon skeletons were identified in sediments, ranging from Holocene to Upper Cretaceous. These identifications are based on mass spectral characterisation, desulphurisation, and, in some cases, by comparison of mass spectral and relative retention time data with those of authentic standards. The presence of unsaturated C₂₅ and C₃₀ HBI thiolanes in a Recent sediment from the Black Sea (age 3 — 6 X 10³ a) strongly supports their formation during early diagenesis. The co-occurrence of HBI polyenes (C₂₅ and C₃₀) and unsaturated HBI thiolanes (C₂₅ and C₃₀) possessing two double bonds less than the corresponding HBI polyenes, in this Recent sediment, testifies to the formation of unsaturated HBI thiolanes by a reaction of inorganic sulphur species with double bonds of the HBI polyenes. Furthermore, a diagenetic scheme for HBI sulphur compounds is proposed based on the identification of HBI sulphur compounds in sediment samples with different maturity levels

Organic geochemical studies of a Messinian evaporitic basin, Northern Apennines (Italy) II. Isoprenoid and n-alkyl thiophenes and thiolanes

Series of n-alkyl and isoprenoid thiophenes and thiolanes, most of which have not been previously reported, have been identified in an extract from a Messinian (Upper Miocene)layer deposited under hypersaline, euxinic conditions. The identifications were based on mass spectra and chromatographic data of synthesized reference compounds and on comparison of mass spectra, relative retention times and response on the FPD. Their specific structures and their distribution patterns show similarities with those of the alkanes. Inorganic sulphur is therefore considered to be incorporated into specific lipid moieties from (archae)bacterial and/or algal input during diagenesis, A biosynthetic origin of these compounds is also possible, however. The organic sulphur compounds encountered are thought to be indicators of a hypersaline depositional environment

Restricted utility of the pristane/phytane ratio as a palaeoenvironmental indicator?

The acyclic C₁₉ and C₂₀ isoprenoid hydrocarbons, pristane (Pr) and phytane (Ph), respectively, have been widely assumed to be diagenetic products of the phytyl side chain of chlorophyll¹⁻³, although alternative sources of precursors have been suggested. The ratio of these two compounds is usually interpreted to be an indicator of the oxicity of the environment of deposition. Recent advances in organic geochemistry in combination with geological constraints lead us to suggest that the Pr/Ph ratio cannot be used as an indicator for oxygen levels. However, in hypersaline environments of deposition the rationale behind a low Pr/Ph ratio is easier to understand, and in these environments application of the Pr/Ph ratio can be expected to be successful

Pristane/phytane ratio as environmental indicator - Reply

We agree with Powell that pristane (Pr) to phytane (Ph) ratios close to unity should be interpreted with great caution and that fewer difficulties arise with extremely high (as in coals) or extremely low (as in sediment from hypersaline environments) Pr/Ph ratios. In these latter cases, the Pr/Ph ratios are probably influenced by specific sources of the organic matter and specific depositional environments, where the redox condition is only one of the factors. The main point of our paper concerns the generalizations and oversimplifications implied in the use of the Pr/Ph ratio as an indicator of the level of oxygen at the site of organic matter deposition into sediments. We made several new arguments based on novel geochemical findings to emphasize the restricted use of the Pr/Ph ratio as a palaeoenvironmental indicator. Of course, we do not object to using the Pr/Ph ratio as a correlative tool when samples of the same origin and maturation level are compared with one another (as in oil/source-rock correlation)

Late Quaternary Mediterranean sapropels II. Organic geochemistry and palynology of S1 sapropels and associated sediments

Six S1 sapropel samples from the eastern Mediterranean were investigated organic geochemically to study the lateral variation within this organic-rich layer. Five of these samples were also analyzed palynologically. In addition, some sediment samples from below and overlying the sapropel intervals were investigated for their organic geochemical characteristics. Organic matter of S1 sapropels is of a mixed marine, terrigenous and bacterial origin. A trend of relatively increasing amounts of continentally derived organic matter towards more seaward and deeper realms can be observed from both palynological and organic geochemical data. This trend is supported to some extent by δ13C-values of the organic matter. The sapropelic intervals deposited on the Nile Cone are characterized by expanded thicknesses and a “diluted” organic carbon content due to a higher sedimentation rate. The environmental conditions (in terms of preservation) during sapropel formation over the eastern Mediterranean were probably not uniform. At site 29 the conditions were favourable for the deposition of sapropel with a higher organic carbon content than at the other locations. This might have been caused by better preservation conditions. An increasing discharge from the river Nile seems to be the driving force for formation of the S1 sapropels. Based on this assumption a model for sapropel formation is proposed. Although not all the data could be explained properly by this model, most phenomena can be explained