Massive cryptic microbe-sponge deposits in a Devonian fore-reef slope (Elbingerode Reef Complex, Harz Mts., Germany)

A massive occurrence of microbial carbonates, including abundant sponge remains, within the Devonian Elbingerode Reef Complex was likely deposited in a former cavity of the fore-reef slope during the early Frasnian. It is suggested that the formation of microbial carbonate was to a large part favored by the activity of heterotrophic, i.e., sulfate-reducing bacteria, in analogy to Quaternary coral reef microbialites. The Elbingerode Reef Complex is an example of an oceanic or Darwinian barrier reef system. In modern barrier reef settings, microbialite formation is commonly further facilitated by weathering products from the central volcanic islands. The Devonian microbialites of the Elbingerode Reef Complex occur in the form of reticulate and laminated frameworks. Reticulate framework is rich in hexactinellid glass sponges, the tissue decay of which led to the formation of abundant micrite as well as peloidal and stromatactis textures. Supposed calcimicrobes such as Angusticellularia (formerly Angulocellularia) and Frutexites, also known from cryptic habitats, were part of the microbial association. The microbial degradation of sponge tissue likely also contributed to the laminated framework accretion as evidenced by the occurrence of remains of so-called “keratose” demosponges. Further typical textures in the microbialite of the Elbingerode Reef Complex include zebra limestone, i.e., the more or less regular intercalation of microbial carbonate and cement. Elevated concentrations of magnesium in the microbialite as compared to the surrounding metazoan (stromatoporoid-coral) reef limestone suggests that the microbialite of the Elbingerode Reef Complex was initially rich in high-magnesium calcite, which would be yet another parallel to modern, cryptic coral reef microbial carbonates. Deposition and accretion of the microbialite largely occurred in oxygenated seawater with suboxic episodes as indicated by the trace element (REE + Y) data.

Integrated stratigraphy from a transgressive upper Oligocene section in NW Italy

ABSTRACT: The Oligo-Miocene Transition (OMT) is one of the most important climatic transitions of the last 30 million years. This short period of climate warming coincides with a few biotic turnovers, which are well known in deeper marine settings where stratigraphic successions yield a detailed record; in shallowmarine environments they have been proved difficult to recognize as the occurrence and absence of certain taxa due to ecological preferences hamper the study. This study focuses on the Case Cné section in the late Oligocene of the Tertiary Piedmont Basin (TPB) as it represents a gradual transgressive event, which shows the drowning of a locally developed reef complex and development of a deeper marine sedimentary setting influenced by gravity flow mechanics. Larger foraminifera biostratigraphy was used to date the section to the late Oligocene (SBZ23); preliminary strontium isotope data confirms this result. Using sedimentological, semi-quantitative microfacies and geochemical analysis the sedimentary history of the section was reconstructed and divided into four major phases: the drowning of the reef complex, a short prograding phase of the fluvial system, the onset of gravity flow mechanics and a final transgressive phase with an initial turbiditic influence which continues regionally into the Miocene.

Turbid Coral Reefs: Past, Present and Future—A Review

Increasing evidence suggests that coral reefs exposed to elevated turbidity may be more resilient to climate change impacts and serve as an important conservation hotspot. However, logistical difficulties in studying turbid environments have led to poor representation of these reef types within the scientific literature, with studies using different methods and definitions to characterize turbid reefs. Here we review the geological origins and growth histories of turbid reefs from the Holocene (past), their current ecological and environmental states (present), and their potential responses and resilience to increasing local and global pressures (future). We classify turbid reefs using new descriptors based on their turbidity regime (persistent, fluctuating, transitional) and sources of sediment input (natural versus anthropogenic). Further, by comparing the composition, function and resilience of two of the most studied turbid reefs, Paluma Shoals Reef Complex, Australia (natural turbidity) and Singapore reefs (anthropogenic turbidity), we found them to be two distinct types of turbid reefs with different conservation status. As the geographic range of turbid reefs is expected to increase due to local and global stressors, improving our understanding of their responses to environmental change will be central to global coral reef conservation efforts.

Characterizing Geomorphology of Mesophotic Coral Reef Ecosystems in the Southwestern Gulf of Mexico: Implications for Conservation and Management

Coral reefs are the most biodiverse ecosystems on earth and are presently experiencing severe declines globally. Shallow coral reef ecosystems (<30 m) have been studied extensively while mesophotic coral ecosystems (MCE) are poorly studied. As a result, MCE are rarely included in marine reserve design and management, despite their ecological importance and connectivity to shallow reefs. In this study, we assessed the fine-scale topographic complexity, a proxy for structural complexity, for a group of coastal coral reefs in a marine park in the southwestern Gulf of Mexico, in depths between 2 and 49 m. We conducted hydrographic surveys using a semi-portable multibeam echosounder system to produce 3D bathymetry digital terrain models (DTM) with a 2.5 m spatial resolution for three submerged bank reefs and two emerging reefs. From these models, descriptive terrain parameters were calculated for each reef, including slope, aspect, curvature, rugosity and ruggedness. Results show that all reefs are predominantly northeast-southwest oriented, with well-defined leeward and windward sides. For the three submerged bank reefs, structural complexity increased with depth. Estimated mean ruggedness and rugosity were highest at 20–40 m depth range on windward side slopes. Emerging reefs showed high structural complexity, particularly at the 25–40 m depth range. We identified a spur and groove zone with maximum ruggedness (0.26) and rugosity (3.17) values, and four channels with steep slopes (68°) and dispersed mounds. We found that at mesophotic depths (>30 m), southern reefs basements from two distinct reefs merge to form a continuous complex. This has important management implications since presently, only 28.7% of this reef complex (mostly shallow areas) are within the existing limits of the marine park’s core zone. Considering the newly recognized importance of MCE, we propose expanding and reshaping the core zone to include the entire reef complex which mostly encompasses MCE with high structural complexity. Our study illustrates the value of semi-portable MBES for marine planning in developing countries and remote poorly studied areas.

Late Oligocene Warming Event (LOWE) possibly preserved on top of a reef drowning sequence in NW Italy: insights from an integrated stratigraphic approach.

<p>The Oligo-Miocene Transition (OMT) is one of the most important climatic transitions of younger earth history. This short period of climate warming coincides with a few biotic turnovers. OMT follows the Late Oligocene Warming Event which marks the last warming pulse of a generally cool interval and represents a time frame that could potentially fit well with modern climate change predictions.</p><p>The Case Cné section located within the Tertiary Piedmont Basin (TPB) represents a gradual transgressive event, which shows the drowning of a locally developed reef complex and a development of a deeper marine sedimentary setting influenced by gravity flow mechanics. Larger foraminifera association indicate a late Oligocene (SBZ23) time and this seems confirmed by Sr isotopes data.</p><p>By usage of sedimentological, semi-quantitative microfacies and geochemical analysis the sedimentary history of the section could be reconstructed and divided into four major phases. 1) The growth and establishment of the reef directly on the metamorphic substrate, 2) its development over the basement and the construction of a modest reefal body, 3) the slow drowning of the reef complex due to enhanced prograding fluvial activity and finally 4) the onset of gravity flows passing to turbiditic influence which cap the transgression and that continue regionally throughout into the Miocene. </p><p>The benthic fauna seems to register the warming period by change in biodiversity and abundance. Below the warming event, larger foraminfera are rather sparse over the section and the benthic community seems dominated by suspension feeders. Toward the top of the section, where the LOWE seems to occur, the gravity flows transport a very large amount of operculinid foraminifera that are well adapted to warm and eutrophic conditions as the ones that possibly characterized the LOWE time span in this tectonically active region.</p>