Jörn Geister

Nature and origin of organic matter in carbonates from speleothems, marine cements and coral skeletons

Organic matter in speleothem calcite, marine carbonate cements and aragonitic coral skeletons was studied to determine its location, molecular structure, functionality and effect on mineral growth. SEM analyses showed that inorganically precipitated carbonates incorporate, during growth, adsorbed organic matter between submicroscopic subunits of the crystals whereas biologically secreted carbonates incorporate biogenic tissue between the crystals. Molecular fluorescence spectroscopy indicated that low molecular weight fulvic acids are the most important constituents of the organic matter. The fulvic acids are probably derived from soils (speleothem calcite), dissolved organic matter (marine carbonates) and biological decay products (aragonitic coral skeleton).

Holozäne westindische Korallenriffe: Geomorphologie, Ökologie und Fazies

ZusammenfassungDie holozänen westindischen Korallenriffe treten uns in einer Vielfalt von Ausbildungsformen entgegen, die sich vor allem in ihrem Umriß, ihrem Relief und ihrer Größe, ihrem inneren Aufbau, dem Sedimentationsgeschehen sowie in der Verbreitung der riffbildenden Organismen voneinander unterscheiden.Die Verschiedenartigkeit der Ausbildung der Riffe erklärt sich vor allem durch:a)die Geomorphologie der überlieferten pleistozänen Landoberfläche, welche während der holozänen Transgression überflutet wurde, Sie hat im Einzelfall eine sehr unterschiedliche Entstehungsgeschichte durchgemacht, wurde während der holozänen Transgression überprägt und erst danach von Riffbildnern besiedelt. Durch die vorgegebene Topographie zur Zeit der Besiedlung wurden die Lage und die äußeren Umrisse der meisten Riffe bestimmt. Die Ausbildung der geomorphologischen Grundtypen von Korallenriffen wie Saumriff, Wallriff, Atoll, Korallenbank und Fleckenriff geht letztlich auf sie zurück.b)die geographisch-geomorphologische Situation der Riffe. Sie ist gegeben durch den Typ des Meeresbodens (Ozeanboden oder Kontinentalschelf), auf dem der Riffkomplex abgelagert wurde, durch die Position des einzelnen Riffes auf dem Kontinentaloder Insularschelf, durch die Lage in Bezug auf die Küste, zum offenen Meer, zum Meeresspiegel, zur vorherrschenden Windrichtung und Wellenbewegung. Zusätzlich unterscheidet man bei bestimmten Rifftypen eine einfache von einer doppelten oder mehrfachen Ausbildung der Riffzüge.c)das Wirken physikalischer und biologischer Ökofaktoren wie Art des Substrates, Licht, Wellenbewegung, Sedimentation und Abweiden, welche vor allem die Verteilung der Riffbildner kontrollieren.d)duch die unterschiedlichenBedingungen von Erosion, Transport und Ablagerung der Sedimente in den verschiedenen Riffabschnitten. Sie bestimmen letztlich die Faziesverteilung, beeinflussen den Typ des Riffgerüstes und die Verbeitung der riffbildenden organismen.e)dierelativen Veränderungen des Meeresspiegels (Ansteigen, Stillstand oder Absinken), bei denen sich die Riffe gebildet haben, Sie wirkten sich von allem auf die Anordnung der Fazieszonen innerhalb des Riffkörpers aus.Es wird versucht, die holozänen Flachwasserriffe mit den sehr ähnlich gebauten pleistozänen Riffterrassen in einen gemeinsamen Rahmen zu stellen und die Ursachen für die unterschiede im Bau zu solchen älteren Korallenriffen aufzuzeigen, die sich nicht in perioden mit schnellen Meeresspiegelschwankungen gebildet haben. Dabei ergeben sich wichtige Argumente für die ökologische Deutung von präquartären Riffen.SummaryThe great diversity of Recent coral reefs encountered in the West Indian seas is clearly demonstrated by their diverging size, outline, topography, internal structure, sedimentary and erosional environments and by the distribution of their reef building biota. The reasons for this diversity are multiple and complex. They are mostly physical and biological in nature and there is also a strong historinature and there is also a strong historical component in the formation of the sea floor topography, colonized by the reef builders.Most aspects of reef diversity can be explained in the following terms:a)The morphology of the Pleistocene land surface was flooded and modified during the Holocene transgression and was subsequently colonized by the reef builders. It determined the position and outline of most reefs as well as the development of the basic geomorphological reef types such as fringing reef, barrier reef, atoll, coral bank and patch reef.b)The type of underlying sea floor (oceanic or shelf), the location of the reef on the shelf, its position in relation to sea, coast, prevailing wind and waves, sea level and eventually to one or more parallel reef tracts.c)The effects of physical and biological environmental factors such as type of substrate, light intensity, wave exposure, sedimentation and grazing pressure, all of which control the distribution of the framebuilding biota.d)The local conditions of erosion, transport and deposition of sediments which determine or influence the facies distribution and the development of reef biota and framework.e)The relative changes of sea level during the growth of the reefs which determine the geometry of internal facies zonation within the reef bodies.Both Holocene shallow-water reefs and Pleistocene reef terraces were formed towards the end of glacioeustatic transgressions. This is reflected by a comparable internal structure and reef topography. Based on observations in these Quaternary reefs an attempt has been made to explain structural and ecological features encountered in older reefs. As a result, a number of arguments are presented that are important for the ecological interpretation of pre-Quaternary reefs.

Use of fractal dimensions to quantify coral shape

A morphometrical method to quantify and characterize coral corallites using Richardson Plots and Kaye’s notion of fractal dimensions is presented. A Jurassic coral species (Aplosmilia spinosa) and five Recent coral species were compared using the Box-Counting Method. This method enables the characterization of their morphologies at calicular and septal levels by their fractal dimensions (structural and textural). Moreover, it is possible to determine differences between species of Montastraea and to tackle the high phenotypic plasticity of Montastraea annularis. The use of fractal dimensions versus conventional methods (e.g., measurements of linear dimensions with a calliper, landmarks, Fourier analyses) to explore a rugged boundary object is discussed. It appears that fractal methods have the potential to considerably simplify the morphometrical and statistical approaches, and be a valuable addition to methods based on Euclidian geometry.

Modern reef development and cenozoic evolution of an oceanic island/reef complex: Isla de Providencia (Western Caribbean sea, Colombia)

SummaryProvidencia Island in the SW Carbbean is 4.5 to 8.5 km across (including Sta. Catalina Island). In contrast to nearby San Andrés, which is an elevated Tertiary atoll, Providencia is formed by an extinct Miocene volcano. This lies far off the Middle American mainland, and therefore its geological history is somewhat unique among other western Caribbean islands. The submarine basement of Providencia rises with steep to vertical slopes from an ocean sea floor of approximately 2,000 m depth. The island itself is rugged with peaks reaching up to more than 360 m above present sea-level. It is surrounded by a wide carbonate insular shelf protected towards the N,E and SE by the second largest barrier reef (after that of Belize) of the Caribbean Sea. The entire reef complex forms a carbonate shelf, which consists of a 32 km long windward bank-barrier reef with lagoonal environments in its lee, dotted with patch reefs and minor fringing reefs. Seaward of the barrier there is a wide fore-reef terrace dropping off to the upper island slope. In contrast, the leeward shelf, lagoon and coastal areas are unprotected to the open sea by major coral reefs and submarine showls, though minor reef structures resembling relics of a former barrier reef are present. Hence, the leeward environments are exposed against storm waves approaching from the West.The submarine topography of the insular shelf is characterized by several lagoonal basins, up to 14 m deep, which may be partly of karstic origin. At water depths of 2–6m wide areas are occupied by extensive shallow lagoonal terraces. At least, two stream gullies continue from the island as submarine channels onto the insular shelf. A submerged elongate ridge in shelf-margin position is situated at more than 25 m of depth and may be a drowned shelf-edge barrier reef. These observations and the presence of submerged terraces indicate that the contemporary submarine topography of the modern reef complex has geomorphologic features inherited during lowered Pleistocene sea-level stands.A major part of the barrier reef is formed by a wide belt consisting of numerous patch reefs, mostly of the pinnacle type, which rise from the sea floor at −6 to −8 m reaching to the low-tide level. Such discontinuous parts of the barrier reef may be from 100 up to 1,000 m wide. The continuous segments of the barrier reef are around 100 m wide and display well-developed groove-and-spur systems. Locally, segments of a continuous barrier are also present in front of the discontinuous reef belt. The reef crests and upper forereef are overgrown by luxuriantMillepora alcicornis with local patches ofAcropora palmata. Otherwise, the latter species is found mainly in front or behind the crest. Near the NW end of the barrier the outer margin of the reef flat is marked by a true algal ridge. The lagoonal patch reefs vary in shape, size and outline, and their crests are normally below 1 or 2 m water depth. They are characterized by thickets ofAcropora cervicornis in the E and by dense growths ofPorites furcata in the SE of the island. Calm-water associations withMontastraea annularis prevail at deeper and/or calmer lagoonal sites. Crests of unprotected shallow leeward reefs (‘Lawrance Reef’, ‘Pearstick Bar’) show dense growth ofAcropora palmata.Providencia itself is a volcanic island formed by pre-Miocene(?) to Miocene lava flows, pyroclastics and epiclastic deposits. All of its pre-Miocene(?) to possibly very early Miocene effusives are of the rhyolitic type and seem to be submarine. Subsequent subaerial eruptions in Early to Upper Miocene time yielded large masses of basaltic to andesitic lavas and pyroclastics. As calculated from the dips of their volcanic bedding planes, the former rim of the central crater area may have emerged more than 1,000 m above the present sea-level in Upper Miocene time. Since then, the relief of the island gradually diminished by subaerial erosion aquiring its present aspect of a deeply eroded volcanic cone. The crater diametre was about 1 km. Intercalations of carbonates within the younger series of volcanic deposits reveal lagoonal deposits near its base and carbonate slope deposits interfingering with the higher volcanic strata. The lagoonal deposits are horizontally bedded tuffaceous carbonates, which yielded a soft-bottom coral fauna, burrowing echinoids and pelecypods signalling the first direct evidence of an early carbonate island shelf. The seaward-sloping higher strata contain massive and branching reef corals and large isolated oyster valves. The fossils suggest an Early to possibly Middle Miocene age of the carbonates.The regional tectonic pattern of the Western Caribbean deep-sea floor is notable for its conspicuous fracture zones. It appears that the primitive submarine volcano of Providencia initiated in early Tertiary time along a tectonic fracture line paralleling the NNE trending San Andrés Trough to its E. It is assumed that the outflows of submarine(?) lavas along a fissure formed an elongated submarine volcanic ridge in fairly shallow water. By Miocene time, subsidence compensated by shallow water carbonate sedimentation and upgrowth of coral reefs lead to the formation of a carbonate platform, most likely of the coral bank or atoll type. It probably showed already the approximate outline and size as the present insular shelf. A second period of volcanic activity in Early to Late Miocene yielded basaltic to andesitic lavas and pyroclastics during several major subaerial eruptions. These formed five conspicuous volcanic tongues, which radiate to the sea from a crater area in the centre of the island.The early eruptive phase was probably contemporaneous with the formation of several additional shallow submarine volcanos in the Western Caribbean. They appear equally bound to fracture lines on the sea-floor. These volcanic structures are deeply submerged today and capped by thick limestone deposits forming the remaining atolls and islands of the archipelago. Of these, only nearby San Andrés was uplifted in latest Tertiary times thus revealing today its Miocene reef and lagoonal deposits. But, in contrast to Providencia, in none of these was there a second period of eruptive activity in late Tertiary times to form a long-living emergent volcanic build-up.Quaternary sea-level oscillations are indicated by subaerial and submarine terraces cut into coastal limestone by advancing sea cliffs. There is a relic of an erosional terrace at +50–60 m in the Miocene limestone, probably of Early or Middle Pleistocene age. The wide fore-reef terrace with its outer margin at depths around −20 to −40 m indicates a prolonged low sea-level stand of pre-Sangamonian age. A fossil fringing reef terrace of Sangamonian age, reaches a maximum elevation of about +3 m above present sea-level. The fossil coral associations of this reef indicate an environment fairly protected from major waves. Thus, it may be assumed that the contemporary outer reef barrier protecting the island coast reached more than 3 m above the present sea-level. In addition there is evidence that the coral associations of the fossil reef lived in water depths possibly near 10 m. The very shallow terraces situated in front of the active limestone cliffs and around certain patch reefs were formed by planation towards the end of the Holocene transgression.Size and shape of the island changed periodically during Pleistocene sea-level fluctuations. Due to the high and relatively steep island relief, the Pleistocene high sea-level recorded by the ’+50–60 m—Terrace’ would not have submerged more than about 35% of the present land surface area. With the exception of the flooding of the coastal low-lands and some deeper valleys and the formation of smaller satellite islands by temporarily isolating some of the higher headlands, the configuration of Providencia did not undergo any essential change. By contrast, during low stands (−25 to −120 m) that followed the great Sangamonian transgression until the Wisconsinian stage, the total of the extensive island shelf was almost permanently emergent for a period of more than 100,000 years. Geomorphologically, the reef complex appeared as an elongated limestone table mountain bounded by sheer cliffs which rose more than 100 m above sea-level. During this period the emergent insular shelf formed an extended northern prolongation of the original volcanic island. The entire island measured some 30 km in length in Wisconsinian times, and its surface area totalled roughly 12 times that of the present island.From the evidence above we may draw the preliminary conclusion that the existence of an insular shelf can be traced back at least to Miocene time. The contemporary shelf morphology is the product of a complex history of sea-level oscillations accompanied by terracing at different levels, renewed reef growth and erosion. Of this history, at present, only a few evolutionary stages may be recognized. Volcanic activity did not contribute to the geomorphologic evolution of the island and shelf in post-Miocene time. The shelf was last exposed to subaerial weathering during the sea-level lowering that accompanied the late Wisconsin glaciation. It appears that since reflooding in the early Holocene some 5,000 years ago, renewed reef growth and sedimentation have only partly concealed or modified the pre-existing shelf topography.ZusammenfassungDie Insel Providencia im südwestlichen Karibischen Meer erreicht einen Durchmesser von 4,5 bis 8,5 km (einschließlich Sta. Catalina). Im Gegensatz zur nahegelegenen Insel San Andrés, einem herausgehobenen tertiären Atoll, wird sie von einem erloschen miozänen Vulkan gebildet. Dieser liegt fern vom Festland und ist deshalb etwas ungewöhnlich unter den karibischen Inseln. Das untermeerische Fundament der Insel Providencia steigt steil bis senkrecht von einem ozeanischen Meeresboden in rund 2 000 m Tiefe auf. Die Insel selbst erhebt sich mit zackigen Spitzen

Oxygen isotopes and climatic control of Oxfordian coral reefs (Jurassic, Tethys)

Abstract Stable isotope studies were carried out on shells of reef-dwelling brachiopods and oysters to evaluate the impact of climate changes on coral communities during the Oxfordian (Late Jurassic) in western Europe and northwestern Africa. Low to medium diversities observed in coral associations in the pioneering and terminal reef phases correlate well with average seawater paleotemperatures of <20.3 °C. The reef climax coincides with optimum environmental conditions, reflected by a high coral diversity and an average seawater temperature between 22 and 30 °C. The results of this study show that water temperatures set the physiological limits for the distribution of corals and coral reefs in Oxfordian time.

Lebensspuren made by Marine Reptiles and their Prey in the Middle Jurassic (Callovian) of Liesberg, Switzerland

SummaryNumerous gutter-like furrows, up to 60 cm wide and up to 9 m long are preserved at the interface “Macrocephalus Beds”/“Callovian Marl” over a surface of 20 by 200 m. They are interpreted as feeding traces made by large marine vertebrates, most likely plesiosaurs and ichthyosaurs searching for food in the lime mud of the shallow Middle Jurassic sea floor. Possible prey animals were infaunal invertebrates (crustaceans) which produced an intricate meshwork of burrows (mainlyRhizocorallium irregulare andThalassinoides) in the bottom sediments, as well as infaunal bivalves.Evidence from cololites of predatory pelagic reptiles (ichthyosaurs, plesiosaurs) as well as reptile regurgitalites indicate that these animals fed not only on fast-swimming vertebrates and cephalopods but also on epi- and endobenthic invertebrates. In addition, the cololites show that the predators ingested considerable amounts of bottom sediment.Different sizes and shapes of the traces suggest that the gutters were produced by different reptiles or age groups. Candidates for the widest gutters are pliosaurs. Of the marine vertebrates known from Jurassic time, only the snout of adult pliosaurs of the genusLiopleurodon was broad enough to produce gutters more than 40 cm wide. Smaller, less than 15 cm wide gutters, could have been made by plesiosauroids or by the narrow pointed snouts of ichthyosaurs.Almost identical traces described from the Oxfordian of Spain and similar but smaller traces from the Lower Devonian of Prague are equally interpreted as feeding traces on the sea floor. Feeding traces of vertebrates in bottom sediments may give detailed information on the hunting behaviour of the predators. However, the attribution of the traces to definite vertebrate taxa remains uncertain.