bluuebunny/crossref_metadata_2025_split huggingface dataset:
Lake Magadi, Kenya: a model for rift valley hydrochemistry and sedimentation?
Authors: Eugster H. P. 
Published: April 2008

Summary

Previous studies on Lake Magadi, Kenya, and the Green River Formation of Wyoming, are reviewed in order to delineate the differences between these two settings, to determine whether Lake Magadi can serve as a reference for rift valley environments and to define a set of diagnostic criteria for recognizing saline alkaline lake deposits in the geologic column. Magadi inflow waters are influenced strongly by weathering of volcanic rocks and are characterized by Na≈Ca and (Ca + Mg)/HCO
3
≪1. Compared to the Green River Formation, calcite production is low to moderate and occurs largely in the soil zone. Sediments are dominated by volcaniclastic material transported during flood events. In contrast, Eocene Lake Gosiute and its mudflats had more gentle gradients, and inflow chemistry was such that vast amounts of calcite and dolomite were produced before brines became alkaline. Sedimentation in both cases is represented by the ephemeral floodplain-saline lake complex; a preliminary subfacies map for Magadi has been constructed. Magadi possesses the typical attributes of a rift valley basin in terms of hydrology, hydrochemistry and sedimentation, as well as the indices characterizing saline alkaline lakes: sodium carbonate evaporites or their pseudomorphs, Magadi-type chert, and tuffaceous rocks altered to zeolites and K-feldspar. Many similar environments have been recognized in the Pleistocene lake beds of North America. Magadi-type cherts were found from the Pleistocene to the Jurassic, and are also present in the Cambrian Officer Basin of Australia. Proterozoic Magadi-Green River-type environments have been suggested for the Duruchaus Formation of Namibia and HYC shale of the McArthur River area of North Australia. Many more Proterozoic examples remain to be discovered.
Palaeoenvironment of Lake Abijata, Ethiopia, during the past 2000 years
Authors: Bonnefille R. , Robert C. , Lezine A. M. , Perinet G. , Delibrias G. , Elenga C. , Herbin J. P. , Tiercelin J. J. 
Published: April 2008

Summary
Multidisciplinary studies, including the examination of sedimentology, palynology, diatoms and organic matter have been undertaken on a 6 m core from Lake Abijata, Ethiopia. The bottom of this core has been dated at 1720 ± BP. Analyses have shown four distinct stages in lake evolution. Two transgressive phases that have been recognized can be correlated with other lake level evidence. The transgressions result from changing inputs from lowland and highland regions within the catchment. They are most probably related to changes in seasonal rainfall patterns.
Palaeolimnology of Lake Bogoria (Kenya) during the 4500 BP high lacustrine phase
Authors: Vincens A. , Casanova J. , Tiercelin J. J. 
Published: April 2008

Summary
Lake Bogoria (altitude 990 m) lies within the Kenya Rift Valley (0°15′N, 36°10′E). A high lake stage is marked by a double stromatolite belt: the first, occurring at 999 m, is radiocarbon dated 4140 ± 60 BP and the second, at 995 m, is dated 3880 ± 60 and 3750 ± 180 BP. Palynological data from shoreline stromatolites and lacustrine sediments recovered in three 15 m cores permit a reconstruction of former limnological conditions. They show that the lacustrine extension resulted from a positive water balance brought about by runoff under climatic conditions more humid than today but drier than those prevailing during the early and mid-Holocene.
Role of tephra in the preservation of fossils in Cenozoic deposits of East Africa
Authors: Hay R. L. 
Published: April 2008

Summary
Volcanism has been particularly important in preservation of fossils at some East African sites. It has supplied sediment for burial, and carbonatite ash which has chemically aided fossilization. Moreover, subaerial tuffs have preserved the remains of faunas unlike those of riverine and lacustrine environments. Examples include faunal sites at Napak, Uganda; Songhor and Koru, Kenya; and Laetoli, Tanzania. Some, and perhaps most, of the carbonatite ash erupted from East African volcanoes was natrocarbonatite, alkali-rich and broadly similar to the modern natrocarbonatite. The pre-modern natrocarbonatites have been leached of their alkalies by meteoric water. In some examples the original natrocarbonatite composition can be reliably inferred from textural and chemical evidence; in other examples only the possibility or probability of former natrocarbonatite ash can be demonstrated. Fossiliferous tuffs at Napak, Koru, Songhor, and the Footprint Tuff at Laetoli contain good evidence of natrocarbonatite ash; evidence is suggestive for Rusinga, and for the fossiliferous aeolian tuffs of the Laetolil Beds at Laetoli.
The genesis and palynofacies characteristics of marine petroleum source rocks
Authors: Tyson R. V. 
Published: April 2008

SummaryThis paper attempts to demonstrate that the multi-disciplinary philosophy behind palynofacies investigations is of great value to the meaningful interpretation of the origins and palaeoenvironment of marine petroleum source rocks (MPSRs).The key control on the formation of MPSRs is identified as bottom water oxygenation (correlated with the location of the Eh interface and the intensity of macrobenthic activity). Particular emphasis is placed on the necessity for greater accuracy in the terminology used to describe levels of oxygenation. Aerobic environments are characterized by organically lean sediments (0–3.0% TOC) with Type III kerogen assemblages composed of relatively refractory land plant debris or highly degraded, marine-derived, amorphous organic matter (AOM). Their organic richness is largely dependent on sediment accumulation rate and proximity to sources of terrestrial organic matter supply. They produce mainly gas at maturation. Dysaerobic to anoxic environments are characterized by MPSR facies with high TOC values and Type II kerogen assemblages dominated by relatively lipid-rich AOM. These represent the classic ‘black shale’ source rocks and are the main source of petroleum in marine basins.Except where they are redeposited, dinocysts are characteristically absent or rare in marine ‘black shales’. They are mainly produced in unstable, seasonally mixed water masses and may consequently be regarded as indices of hydrographic stability. Prasinophycean phycomas, which differ from dinocysts in their function, are often the dominant, or most conspicuous, marine palynomorphs of pelagic sediments and stably stratified ‘black shale’ basins.
Pyrite formation in Jurassic shales of contrasting biofacies
Authors: Fisher I. St. J. , Hudson J. D. 
Published: April 2008

Summary

The quantity and composition of organic matter passing into the hydrocarbon window is dependent on the extent of organic degradation occurring during early diagenesis. An important early diagenetic reaction in which organic matter is consumed is sulphate reduction. Relationships exist between pyrite sulphur (a product of sulphate reduction), organic carbon, and degree of pyritization of available iron, which can be used to help characterize the source-rock potential. In Jurassic, marine, organic-rich shales, these parameters are related to the environment of deposition, as indicated by biofacies analysis, and to variations in bottom water oxygenation and supplies of organic carbon, sulphate and iron. The best preservation of organic matter, for reaction in subsequent diagenetic zones, does not necessarily occur in the most anoxic sediments, but in those in which the organic matter buried in the sediment is more than sufficient to satisfy the supply of sulphate by diffusion. In the shales studied this is indicated by the most positive (> −28‰) δ
34
S values for fine grained pyrite.
Palaeoenvironmental assessment of marine organic-rich sediments using molecular organic geochemistry
Authors: Brassell S. C. , Eglinton G. , Howell V. J. 
Published: April 2008

Summary
The solvent extracts of contemporary aquatic sediments contain a complex assemblage of compounds derived from living organisms. Among such biological debris many lipid components occur which possess structural and stereochemical characteristics that attest their origin from terrigenous higher plants, unicellular algae such as diatoms and dinoflagellates, or bacteria, including methanogens. Such compounds are markers of the organisms contributing to a given environment, and can provide information not available from micropalaeontological studies, namely evidence of bacterial activity, contributions from calcareous algae to sediments deposited below the calcite compensation depth and possibly sediment inputs from coccolithophorids in their non-coccolith bearing growth stage. In certain circumstances, especially in sedimentary sequences not subjected to elevated temperatures, many source-specific compounds can survive unaltered and thus are found in Mesozoic marine sediments deposited over 165 million years ago. In such immature ancient sediments, unaltered lipids occur together with their early stage diagenetic products, many of which retain molecular features that reflect their biological origins and are therefore also of value as source indicators. As diagenesis proceeds to catagenesis and kerogen breakdown increases, part of the molecular information is gradually lost as defunctionalization occurs and aliphatic and aromatic hydrocarbons come to dominate the sedimentary lipid distributions. Hence, attempts to identify the detailed sources of sedimentary organic matter from geolipid distributions become more difficult and rely more heavily on the occurrence of particular biologically-specific skeletons (e.g. head-to-head isoprenoids). The recognition of the diagenetic products of biolipids does, however, provide the basis for understanding lipid diagenetic pathways and offers the possibility of extending the scope of molecular assessment of sediment inputs to older and more thermally mature sediments. Despite these limitations imposed by the processes of sediment diagenesis and catagenesis, molecular organic geochemistry provides a unique record of past and present environments, with the potential of evaluating both biological inputs and subsequent sedimentary processes.
The use of lipids as facies indicators
Authors: Comet P. A. , Eglinton G. 
Published: April 2008

Summary
An approach to palaeoenvironmental interpretation of sediments using lipid data, is developed. This approach initially attempts to summarize the major lipid components of plants, bacteria and invertebrate animals and to relate these data to sedimentary lipid distributions. Most of the major lipid groups found in sediments are used in this study. The very large amount of lipid data generated, necessitated the development of a simple system for data processing. The method of data analysis used involved the construction of a simple taxonomic hierarchy based on ‘Ruzicka’s Rules’ i.e. the biosynthetic origins of the major lipid types. By bringing in the concept of taxonomic hierarchy based on the biosynthetic pathways, a palaeontological approach towards lipid geochemistry is being evolved.
Secondly, DSDP lipid data (derived from the O.G.U., Bristol University) are collated and grouped using matrices of lipid data and histograms of the lipid groups, i.e. all compounds of a common structure are ‘summed’. These data are compared with the geological data and the results briefly discussed for each sediment.
In conclusion, the diagenetically stable lipid patterns of three representative environments, i.e. marine planktonic, bacterially-dominated marine, and a lacustrine facies, are presented and discussed. From these data the rules by which environments are described are extended; the parameters so derived have good potential for source correlation.
The carbon cycle—tracking the path of organic particles from sea to sediment
Authors: Degens E. T. , Ittekkot V. 
Published: April 2008

Summary
Sedimentation of organic matter in the marine habitat represents a major link in the transfer of carbon from the hydrosphere to the geosphere. Evidence is presented that zooplankton and phytoplankton communities are actively involved in the fast removal of small-sized mineral and organic particles from the euphotic zone to the deep sea. For instance, copepods package detritus into faecal pellets which are jetted to the sea floor at velocities of about 500 m per day. Attention is also drawn to coccoliths. They excrete mucus which is instrumental in scavenging detritus and in forming macroflocs which rapidly sink. Time series sediment traps deployed in various shallow and deep-sea environments permit collection of such particles in mid-water. Variations in the distribution pattern of sugars, amino sugars, and amino acids indicate seasonality in the fluxes of ‘fresh’ materials. These data suggest that the bulk of small-sized particles presently suspended in surface waters of the open ocean is carried off by planktonic organisms to the sea floor. Prior to the advent of copepods or mucus-excreting nannoplankton in the stratigraphic record, the ancient sea must have looked different with respect to the content of suspended solids since settling velocities—according to Stoke’s law—are only a few centimetres a day for clay-sized particles. Even at later geological times, environmental events that lead to unbalanced plankton communities should have found their echo in the sediment facies.
Oceanographic controls on the accumulation of organic matter in marine sediments
Authors: Calvert S. E. 
Published: April 2008

Summary
The concentration of organic matter in marine deposits depends on the relative rates of accumulation of the various sedimentary components and the ease with which these components are preserved after burial. The supply of organic matter to any area of sea floor is controlled by the primary production rate in the surface ocean and the depth through which particulate material must settle. The accumulation of organic matter in sediments depends on the primary setting flux and on the bulk sedimentation rate, more carbon surviving decomposition where it is buried rapidly.
The preferential preservation of organic matter under anoxic conditions, a widely-accepted explanation for the formation of sapropels, black shales and petroleum source beds, is considered to be of secondary importance in governing the accumulation of carbon in marine deposits. The modern sediments of the Black Sea, a widely-used analogue of the environment of black shale formation, do not have exceptionally high carbon contents, although an earlier sapropel is known to have formed during the change from an oxic lake to an anoxic marine basin. Likewise, the available information on modern anoxic fjords shows that they do not have sediments containing more carbon than their oxic counterparts. And finally, the inverse correlation between the carbon content of slope sediments and the oxygen content of the near-bottom waters in areas where the oxygen minimum intersects the sea floor is shown to be one of coincidence and understandable on the basis of the relationship between sedimentary carbon levels and other sediment properties.
The supply of organic matter to marine deposits, controlled by the primary production rate, and the bulk sedimentation rate appear to be the most important factors which determine the organic content of sediments. Recent work on the location of former upwelling centres suggests that variations in palaeoproductivity may be responsible for the formation of many organic-rich rocks and may provide a better basis for predicting the location of petroleum source beds in the geological record.
Coastal upwelling and a history of organic-rich mudstone deposition off Peru
Authors: Suess E. , Kulm L. D. , Killingley J. S. 
Published: April 2008

SummaryThe present-day upwelling circulation off Peru, the regional pattern of organic matter in surface sediments and the stable carbon isotope characteristics of Neogene and Quaternary carbonate lithologies suggest a unique feedback mechanism in continental margin deposition and subsequent alteration after burial. In such a scenario, the high bioproductivity, the position of a poleward flowing undercurrent and the rate of subsidence of margin basins appear to be the principal variables controlling this mechanism.Transfer of organic matter from the sea surface to the sea floor is particularly efficient in the upwelling ecosystem off Peru. Preservation and burial are enhanced by high bulk sedimentation rates along the upper continental slope (between 11°–15°S) at depths where the subsurface current velocities decrease below those normally associated with the poleward flow. Burial and preservation are diminished, however, where shallow water depths promote continuous reworking of the bottom sediments by onshore flows and alongshore water movement (between 6°–10°S). The resulting sedimentary facies are distinctly different from each other in that the former process yields an organic-rich (> 5 wt % Corg) and the latter process yields a calcareous (> 15 wt % CaCo3) mud facies. The bulk sediment accumulation and individual component fluxes are estimated for both portions of the margin situated between 6° and 15°S latitude and lying in < 500 m of water depth. Furthermore, the chemical environment of organic-matter decomposition in the rapidly accumulating carbonate-poor facies is dominated by microbial fermentation and methanogenesis, whereas, the muds containing lesser amounts of organic matter are dominated by microbial sulphate reduction. These differences in facies composition persist throughout the subsequent stages of compaction and diagenesis. Most prominent among these is the formation of ‘organic’ dolomites with distinctly different isotopic signatures and mineral assemblages. The original upwelling facies (i.e., organic-rich muds or calcareous muds), the extent of reworking by subsurface currents, and the subsidence history of the margin basins may be inferred from these sedimentary signatures.
Palaeo-upwelling and the distribution of organic-rich rocks
Authors: Parrish J. T. 
Published: April 2008

Summary
As many as half the world’s organic-rich rocks may have been deposited in upwelling zones.
This paper alludes to the way in which ancient upwelling zones may be predicted and the criteria by which such areas may be recognized from the sedimentary record. The predicted distribution of upwelling sites is compared with the distribution of organic-rich rocks, and the other probable indicators of upwelling, notably phosphatic rocks, glauconite and biogenic siliceous sediments. A correspondence is shown. This correspondence is consistent with not all upwelling zones being underlain by organic-rich sediments and with not all organic-rich sediments originating beneath areas of upwelling.
The depositional environment and petroleum geochemistry of the Marl Slate-Kupferschiefer
Authors: Gibbons M. J. 
Published: April 2008

Summary
The Marl Slate is the basal unit of the Upper Permian (Zechstein) in northeast England. A finely laminated, organic-rich, silty dolomite/dolomitic siltstone, it is equivalent to the Kupferschiefer of the Southern North Sea basin and northern Germany and Poland. Several lines of evidence indicate the Marl Slate/Kupferschiefer accumulated in the anoxic bottom waters of a shallow (< 200 m), stratified, epicontinental sea. The vertical succession of the first Zechstein cycle is explicable in terms of the effect on sedimentation of a stratified water column undergoing progressive evaporitic concentration.
Total organic contents of the Marl Slate generally fall in the range 5–15%, decreasing upwards. The kerogen is largely amorphous and oil-prone. Original pyrolysis yields approach 100 kg hydrocarbon/tonne rock. It is immature in northeast England but even where mature in the Southern North Sea basin and Northern Europe the unit is unlikely to have sourced commercially significant amounts of oil because of its limited thickness (generally <2 m).
Carbonaceous sediments and palaeoenvironment of the Cretaceous South Atlantic Ocean
Authors: Zimmerman H. B. , Boersma A. , McCoy F. W. 
Published: April 2008

Summary
The distribution of sapropelic sediments in the early South Atlantic Ocean is used to test palaeocirculation models related to oxygen-deficient conditions. The data presented here demonstrate that preservation of organic matter was widespread and occurred in many phsyiographic settings. Given the silled-basin physiography of the relatively isolated South Atlantic and the benign Cretaceous climate, a preservational model of sapropel accumulation and its corollary of restricted bottom circulation successfully satisfy the constraints of the geological record. Under the environmental conditions outlined for the early and mid-Cretaceous, a generally dysoxic condition for the warm saline bottom-water is appropriate. The prevailing dysoxia, however, was punctuated by episodic bottom anoxia, which often expanded into the upper reaches of the water column. In the Cretaceous South Atlantic, the only excursion from the dysoxic condition occurred during a Cenomanian ‘ventilation event’ (represented by a widespread hiatus in the stratigraphic record) which is tied to an interval of climatic cooling and the establishment of effective circulation between the North and South Atlantic Oceans. The geological record for epicontinental regions, although modified by local physiography and other environmental parameters, suggests a relationship between sea-level positions and sapropel accumulation. No relationship, however, can be established between sea-level and sapropel preservation in the oceanic basins of the South Atlantic.
South Atlantic organic-rich sediments: facies, processes and environments of deposition
Authors: Stow D. A. V. 
Published: April 2008

Summary
Organic-rich sediments (black shales) occur in parts of the South Atlantic ocean within Jurassic, Cretaceous and Neogene sections. These black shales are represented by various lithologies, including mudstones, marlstones, limestones, sandstones, pebbly mudstones and diatomites. They range from thin to very thick bedded, laminated, fissile or structureless, and show varying degrees and types of bioturbation. Texturally and compositionally they are equally varied. Organic carbon may be of principally marine or terrigenous source and forms from 1% to over 20% of the sediment. In many cases, the black shales are interbedded with organic-poor lithologies in cycles with irregular periodicities that average 20,000 to 140,000 years. The range of depositional processes of these black shales includes sliding, debris flows, high- and low-concentration turbidity currents, pelagic/hemipelagic sedimentation, and winnowing by shelf bottom currents. They occur in various palaeogeographical settings from nearshore shelf to ocean basin, and have been differently influenced by variations in bottom water oxygenation, organic matter supply and rates of sedimentation.