ALBERT

Description: Previous studies in Silurian carbonates from Gotland (Sweden) have led to a model for the development of limestone–marl alternations. This model postulates that early diagenesis of precursor sediments without strong primary differences can result in a differentiation by selective dissolution of aragonite in marl beds and reprecipitation of calcite cement in limestone beds. This model is described as a set of mathematical equations that quantify the diagenetic processes (aragonite dissolution and calcite reprecipitation) that occur during the formation of limestone–marl interbeds from a hypothetical homogeneous precursor sediment. The calculations demonstrate that resulting hypothetical limestone–marl alternations show characteristic mathematical relationships between the ratios of the bed thicknesses of limestones and marls on one side, and the carbonate contents, on the other. By reversing this model, the original mineralogical composition of the precursor sediment of real-world rhythmic successions can be determined. In this study, alternations from the Silurian of Gotland, the Cambrian, Devonian, and Mississippian of North America, the Jurassic of France and Germany, and the Cretaceous of France are shown to exhibit mathematical relationships similar to those calculated for hypothetical precursor sediments without primary differences. Therefore, the mineralogical composition of their precursor sediments can be estimated. In contrast, the clear mismatch shown by the Lower Jurassic Belemnite Marls from Dorset indicates that these rhythms did not suffer an early diagenetic overprint. Our model helps to differentiate between rhythmites with strong depositional variations and those without; however, it cannot indicate whether a given alternation is the product of rhythmic diagenesis of a homogeneous precursor sediment or the result of diagenetic enhancement of subtle underlying sedimentary rhythms. For horizontally correlated patterns, such as laterally extensive beds and layers of nodules, an a priori unknown external signal has to be assumed.

Description: Heterozoan or foramol production is typical in extratropical carbonate sedimentary systems. However, under mesotrophic to eutrophic conditions, heterozoan carbonates also form in tropical settings, but such heterozoan tropical sedimentary systems are poorly understood. Nevertheless, distinction between tropical and extratropical heterozoan carbonates in ancient successions is crucial for accurate paleoenvironmental and paleoclimate reconstructions. Here, surficial Holocene and Pleistocene sediments of the northern Mauritanian shelf are studied as an example of a tropical eutrophic carbonate depositional system (11 mg?L21 Chl-a [chlorophyll-a]). Upwelling nutrient-rich waters push onto the wide Mauritanian shelf, where they can warm up to in excess of 25uC. This condition favors production of heterozoan carbonates dominated by bivalves and foraminifers, even in this tropical setting. In addition, sediments are provided by eolian input from the desertic hinterland. The resulting sediments are carbonate and mixed carbonate–siliciclastic facies, in which the carbonates are characterized by a mixture of tropical and cosmopolitan taxa. Benthic photosynthetic biota are absent while suspension-feeding organisms are dominant. This foramol grain association on a shelf scale is reminiscent of cool-water carbonates, therefore recognition of warm-water heterozoan carbonates relies on key taxa related to tropical waters within the biota assemblages associated with a highly productive environment.

Description: Heterotrophe Bakterien sind auf die Versorgung mit organischem Material zur Deckung ihres Kohlenstoff- und Energiebedarfs angewiesen. In Gewässern kann dieses Material durch die Primärproduktion des Phytoplanktons und Phytobenthos autochthon erzeugt werden oder allochthoner Herkunft sein. Auf drei Stationen, zwei in der inneren Schlei (Schleswig-Holstein) und eine im Oderhaff (Mecklenburg-Vorpommern), wurde durch Probenahmen in kurzen Abständen versucht, eine von der Primärproduktion des Phytoplanktons gesteuerte Tagesperiodik der Bakterienaktivität nachzuweisen. Beide Gewässer sind stark eutrophiert und haben eine Jahresprimärproduktion von 600 bis 800 g C m-2. An den Untersuchungstagen (Anfang Juni in der Schlei und Ende August im Oderhaff) lag die Primärproduktion zwischen rd. 2,5 und 3,0 g C m-2 pro Tag. Die Maximalwerte der Thymidin- und Leucin-Aufnahme beliefen sich auf 0,346 bzw. 4,01 nmol l-1 h-1und die höchsten Umsatzraten von Glucose und Acetat als Vertreter der niedermolekularen organischen Verbindungen betrugen 99 bzw. 67 % h-1. Statistisch signifikante Unterschiede zwischen den Mittelwerten der Tages- bzw. Nachtmessungen konnten bei keinem Parameter nachgewiesen werden. Der einzige Hinweis auf eine eventuelle Tagesrhythmik fand sich bei der in der ersten Nachthälfte deutlich geringeren Umsatzrate von Glucose. Ein mehrtägiges Laborexperiment mit einer Vergleichsprobe aus der Schlei, die unter kontrollierten Bedingungen im Lichtinkubator gehalten wurde, bestätigte das Fehlen eines Tag-Nacht-Rhythmus. Aus diesen Befunden wird gefolgert, dass einerseits die Versorgung der Bakterien mit organischem Material allochthoner Herkunft groß ist und dass andererseits tageszeitlich nicht gebundene Prozesse wie Zooplanktonfraß oder Hydrolyse in diesen hypertrophen Systemen eine so bedeutende Rolle spielen, dass die Primärproduktion trotz ihrer absoluten Höhe keine messbaren, an die Tageszeit gebundenen Rhythmen der Bakterien hervorruft.

Description: Alternating cemented and uncemented, fine-grained layers from Pliocene periplatform carbonates of the Great Bahama Bank have fundamentally different diagenetic features. The cemented layers consist predominantly of microspar, interpreted as an early marine, shallow-burial cement. The intercalated, uncemented, softer layers are devoid of microspar cement and exhibit signs of mechanical compac-tion. Precursor sediments of both cemented and uncemented layers apparently consisted largely of aragonite needles. The needles in the compacted layers show signs of dissolution, suggesting that the calcium carbonate required for cementation of the uncompacted layers was provided by aragonite dissolution within the compacted layers. The lack of compaction in the cemented layers shows that cementation, and hence aragonite dissolution in the adjacent compacted layers, took place in the shallow-burial realm. The dissolved carbonate was trans-ported by diffusion to adjacent layers and reprecipitated as calcite cement, thereby preventing significant compaction of these layers. These processes are not yet complete in the material examined, and some aragonite remains in the compacted layers. The sedimentary composition of the two rock types (compacted and uncompacted) is similar, indicating a similar precursor sediment for both. Between 1.5 and 7 times the concentration of palynomorphs oc-curs in the compacted layers, apparently the result of passive diage-netic enrichment. Although the trigger for diagenetic differentiation has not been determined, the pure limestone succession studied here appears to serve as a clay-free analog to limestone–marl alternations.

Description: Ecological and taxonomic study of the mollusk-rich fauna of the Golfe d'Arguin, North Mauritania, investigates the various environmental influences affecting this tropical shelf. The upwelling of nutrient-rich waters leads to a highly productive environment under tropical conditions. The resulting mixed carbonate-siliciclastic sediment contains a large portion of calcareous components produced by heterotrophic organisms-e.g., mollusks, foraminifers, worms, barnacles-that are reworked on the open shelf. On the basis of mollusk assemblages, six taphocoenoses are defined, all being characterized by a mixed fauna of tropical (e.g., Tellina densestriata), subtropical (e.g., Macoma cumana) and temperate (e.g., Spisula subtruncata) species. Differences between the assemblages are related to the medium-grain size ranging from mud to gravel-that results from local hydrodynamic conditions and water depth. Among carbonate grains, Donax burnupi shells are very abundant in the swell-exposed, northern part of the Golfe d'Arguin and reflect the tropical to subtropical, high-energy, and high-nutrient waters. Mollusk assemblages are demonstrated to be a sensitive tool for deciphering complex environmental conditions in sedimentary archives.