Amos Bein

Late Cretaceous upwelling system along the Southern Tethys Margin (Israel): Interrelationship between productivity, bottom water environments, and organic matter preservation

Organic-rich Upper Cretaceous sequences in Israel were deposited in an extensive, highly productive upwelling-linked system which prevailed along the southern Tethys margin, and lasted for ∼19 m.y. (Santonian to late Maastrichtian). An understanding of the spatial and temporal characteristics of this system was gained through detailed paleontological and geochemical analyses of subsurface sequences in two basins in Israel, representing an outer (Shefela) and an inner (Zin) facies belt. The nature of the upwelling system, and its effect on the sedimentary record, is related to two basic environmental parameters, namely paleoproductivity intensity and oxygen levels at the bottom. The assessment of these parameters and their interrelationship has been performed through the development of paleontological (foraminiferal) criteria, which are independent of the organic matter content. Following the establishment of these criteria, it is concluded that the productivity reached its maximum intensity during the late Campanian, which was also the time of most notable differentiation between the center of the upwelling system in the inner belt and the less intense conditions in the outer basin. This distribution is expressed in varied lithology (organic-rich carbonates, phosphorites, and siliceous rocks) at the core of upwelling and a uniform lithology (organic-rich carbonates) at the margin of this system. The uniform lithology of the Maastrichtian in both basins, composed of organic-rich carbonates, is ascribed to a gradual weakening of productivity. The bottom conditions in the inner belt during the late Campanian (the time of maximum surface productivity) were near anoxic, changing to more aerated (dysaerobic) conditions during the early Maastrichtian. In the outer belt a more aerated bottom (dysaerobic) prevailed throughout the late Campanian to late Maastrichtian. The elevated organic matter content in both basins reflects the overall environment of high productivity; its actual variations, however, seem to be unrelated to changes in surface productivity and bottom oxygen levels.

Oxygen-isotope composition of diagenetic calcite in organic-rich rocks: Evidence for 18O depletion in marine anaerobic pore water

Upper Cretaceous organic-rich carbonates in Israel contain benthonic foraminifera and varying amounts of early diagenetic infilling of sparry calcite. The sparry calcite has oxygen-isotope values (down to -9.5‰ PDB) that are significantly lower than those of the coexisting skeletal calcite (averaging -2‰ PDB). Low δ18O values are very common in carbonates that are associated with organic matter. These occurrences are conventionally explained as being due to carbonate formation under conditions of low salinity and/or elevated temperature. On the basis of petrographic data and local geologic evidence, however, none of the conventional explanations can be applied in this case. We advance an alternative model that relates the light oxygen compositions of the diagenetic calcite to depletion of 18O in pore water of normal salinity. The proposed operating mechanism is a direct consequence of organic matter decomposition in the sulfate-reducing zone. Being inherent to reducing marine environments, this model can be applied to many organic-rich rock associations.

Dinoflagellate cysts, paleoproductivity and upwelling systems: A Late Cretaceous example from Israel

Abstract Diverse assemblages of dinoflagellate cysts occur in Coniacian-Maastrichtian sedimentary sequences in Israel, which represent the inner (shallower) and outer (deeper, seaward) belt of a Late Cretaceous upwelling system along the margin of the southern Tethys. The assemblages are composed mainly of peridinioid (P) and gonyaulacoid (G) cysts in varying concentrations. It is suggested that the observed fluctuations in the numerical peridinioid/gonyaulacoid (P/G) cyst ratio reflect mainly changes in upwelling intensity and productivity in the studied area: high P/G values represent a higher productivity and vice versa. TheP/G curve indicates a generally high productivity during the Campanian, followed by a decrease during the Maastrichtian. The inner part of the upwelling region is characterized by higherP/G values, representing a higher productivity. The similarity between theP/G ratio curve and the foraminiferal-based paleoproductivity curve from the same sections supports the utility of theP/G ratio as a reliable productivity indicator, applied here for the first time to Upper Cretaceous sediments.

Organic matter in Senonian phosphorites from Israel — origin and diagenesis

Abstract The organic matter associated with Senonian phosphorites from Israel was studied and compared with that in Tertiary phosphorites from Morocco and modern phosphate-rich sediments from South-West Africa/Namibia. Humic acids were found to be the main constituents of the organic matter in all samples. The n-alkane distribution pattern in the C15-C26 range and the lack of odd-over-even predominancy in the higher hydrocarbons indicate microbial degradation of marine-derived organic matter. Significant phytane-over-pristane predominance indicates accumulation under euxinic conditions. The high humic acid content and its relatively light δ 13C composition apparently reflect the two evolution stages suggested for the formation of phosphorites by G.N. Baturin. High decomposition rates of marine organic matter under limited oxygen supply led to the differential accumulation of more resistant fractions such as cellulose and lignin. These compounds are known to be enriched in 12C relative to other marine organic compounds. When exposed to oxidizing conditions during the second stage of non-phosphate mud winnowing, these compounds were easily converted into humic substances. Further conversion into kerogen was prevented due to shallow overburden and low maturation.

Agglutinated Foraminifera in Organic-Rich Neritic Carbonates (Upper Cretaceous, Israel) and their use in Identifying Oxygen Levels in Oxygen-Poor Environments

Two subsurface composite sections of the Upper Cretaceous in Israel were studied, using species of agglutinated foraminifera as indicators for paleoceanographic conditions. The southern section in the Zin Valley includes the Menuha, Mishash and Ghareb formations. It consists of chalks with a high but variable content of organic matter, as well as phosphorites and cherts in the Campanian interval. The synchronous northern section, in the Shefela basin, is more uniformly calcareous, and the equivalent of the Mishash and Ghareb formations is known as the ‘En Zetim Formation. The southern section represents a restricted shelf environment, the northwestern section a somewhat deeper basin of the southeastern Tethys.

Early diagenetic oxidation and maturation trends in organic matter extracted from Eocene chalks and cherts

Abstract Kerogen and humic acid extracted from Eocene chalks and cherts display composition variations which do not follow normal thermal maturation trends. Decrease in the H/C ratio is followed by an increase of the O/C, S/C and N/C ratios; and the 12 C is also enriched. The humic acids are always heavier in their δ 13 C composition than the kerogens extracted from the same samples, their H/C, O/C and S/C ratios are usually higher and their structure is more aliphatic and less aromatic. The compositional trends observed in the humic acids and kerogens apparently result mostly from increased microbial oxidation of aliphatic chains into carboxyl groups by sulfate-reducing bacteria. This process is corroborated by infrared spectra which reveal a negative correlation between aliphatic and oxygen-containing groups, coinciding with the negative H/C-O/C relationship. The change from humic acids to kerogen, on the other hand, is the result of thermal maturation accompanied by microbial oxidation. Since the chert and chalk samples reflect the same evolution trends it appears that the microbial oxidation affected the organic matter prior to chertification.