Flanks Of Mounds Research Articles

Benthic foraminiferal faunas associated with cold‐water coral environments in the North Atlantic realm

AbstractSurface benthic foraminiferal assemblages associated with cold‐water coral mounds and reefs from the Irish margin and Norwegian shelf (North‐east Atlantic) are for the first time compared quantitatively. Results indicate that the considered sites share a common assemblage, dominated by elevated epibenthic and distinct infaunal species. This surface assemblage is typical of environments that are subject to strong bottom‐water turbulence with enhanced food availability. It provides a benchmark for comparison with fossil benthic foraminiferal assemblages from past cold‐water coral environments. Similar to macrofaunal and megafaunal communities, surface benthic foraminiferal diversity is higher on reefs and mounds than in surrounding off‐mound/off‐reef sediments. Benthic foraminiferal diversity is highest within the living coral macrohabitat, possibly as a result of enhanced availability and variety of food sources, and ecological niche separation. Indeed, living coral generally thrives on the summits or flanks of reefs and mounds where food availability is most important. The second part discusses the use of fossil benthic foraminiferal assemblages as palaeoceanographic proxies from past cold‐water coral environments. The overview of previous observations demonstrates that benthic foraminifera are valuable tools to reconstruct past bottom‐water oxygenation, bottom‐water currents and surface productivity, all of which are key environmental variables controlling cold‐water coral growth. Moreover, the advantages of a detailed investigation of benthic foraminiferal assemblages within cold‐water coral environments are compared to other palaeoceanographic proxies. This study highlights that benthic foraminiferal assemblages are an often overlooked proxy within cold‐water coral environments, despite yielding valuable information.

Evidence from acoustic imaging for submarine volcanic activity in 2012 off the west coast of El Hierro (Canary Islands, Spain)

We report precursory geophysical, geodetic, and geochemical signatures of a new submarine volcanic activity observed off the western coast of El Hierro, Canary Islands. Submarine manifestation of this activity has been revealed through acoustic imaging of submarine plumes detected on the 20-kHz chirp parasound subbottom profiler (TOPAS PS18) mounted aboard the Spanish RV Hesperides on June 28, 2012. Five distinct “filament-shaped” acoustic plumes emanating from the flanks of mounds have been recognized at water depth between 64 and 88 m on a submarine platform located NW El Hierro. These plumes were well imaged on TOPAS profiles as “flares” of high acoustic contrast of impedance within the water column. Moreover, visible plumes composed of white rafts floating on the sea surface and sourcing from the location of the submarine plumes were reported by aerial photographs on July 3, 2012, 5 days after acoustic plumes were recorded. In addition, several geophysical and geochemical data support the fact that these submarine vents were preceded by several precursory signatures: (i) a sharp increase of the seismic energy release and the number of daily earthquakes of magnitude ≥2.5 on June 25, 2012, (ii) significant vertical and horizontal displacements observed at the Canary Islands GPS network (Nagoya University-ITER-GRAFCAN) with uplifts up to 3 cm from June 25 to 26, 2012, (iii) an anomalous increase of the soil gas radon activity, from the end of April until the beginning of June reaching peak values of 2.7 kBq/m3 on June 3, 2012, and (iv) observed positive peak in the air-corrected value of 3He/4He ratio monitored in ground waters (8.5 atmospheric 3He/4He ratio (R A)) at the northwestern El Hierro on June 16, 2012. Combining these submarine and subaerial information, we suggest these plumes are the consequence of submarine vents exhaling volcanic gas mixed with fine ash as consequence of an event of rapid rise of volatile-rich magma beneath the NW submarine ridge of El Hierro. These precursory signals have revealed important to improve and optimize the detection of early warning signals of volcanic unrest episodes at El Hierro.

Danian cool-water coral reefs in southern Scandinavia localised over seafloor highs

Danian (Paleocene) reefs formed by ahermatypic scleractinian corals in relatively deep water are known in a few localities in southern Scandinavia. Reflection and shallow seismic profiles, and samples from drilling and scuba diving in bridge pier excavations in Øresund, the strait between Denmark and Sweden, for the first time allow interpretation of the factors that controlled the localisation of the reefs. After the mass-extinction at the K/T boundary, reef-building scleractinian corals were absent in the Boreal early Danian in the Danish basin. Faunal evolution after the biotic crisis and a rise in relative sea level are interpreted to have favoured formation of deep-water coral reefs in the Øresund region in mid-Danian time. The reefs were 6–20 m high and 20–200 m long on the seafloor and have a patchy distribution within a bryozoan mound-dominated setting in an area of about 10 km 2. They are composed of the framework-building ahermatypic scleractinian coral Dendrophyllia candelabrum with minor occurrences of bryozoans, echinoderms, gastropods and bivalves. It has been suspected that the reefs were located over contemporaneous seafloor highs but this notion was only based on comparison with similar modern deep-water reefs offshore Norway. The data from Øresund indicate that the Danian reef complex was formed over a late Maastrichtian palaeo-seafloor high, the Saltholm-Malmø High, supporting this interpretation and thus adding fundamentally to the understanding of the factors controlling the formation and localisation of these deep and cool-water coral reefs. Individual reefs were initiated and grew on eroded crests and steep south-dipping flanks of bryozoan mounds and were predominantly situated on the southern part of the high and are interpreted as growing towards NW-flowing bottom currents rich in particulate nutrients.

Tertiary Limestone Dikes, Oamaru, New Zealand: ABSTRACT

Indurated calcarenite and calcilutite dikes cut vertically through lower Oligocene volcanic sediments in the Oamaru region, east coast, North Otago, South Island, New Zealand. Individual dikes extend more than 10 m vertically and can be followed for much greater distances along regular or irregular strike. Field relations and paleontologic data suggest the limestone dikes were filled from above; at least 2 younger Oligocene formations contributed debris. Most dikes comprise several sharply defined layers of differing texture, composition, or structure parallel with dike walls. Subhorizontal layering, with either grading or cross-stratification, is also present but less common. Most composite dikes are less than 10 cm thick, but they range up to 30 cm; many of the dikes pi ch and swell along both strike and dip. Flow structures are well developed between 2 vertical layers exposed in strike section along one dike. Limestone filling the interstices of a pillow lava was apparently supplied through channels now preserved as dikes. Some dike layers are well-sorted bioclastic sand, but most layers consist of calcareous mud, with varying End_Page 636------------------------------ proportions of fossils (planktonic and benthonic forams, bryozoans, echinoderm debris, mollusk debris, brachiopod fragments, calcareous algae, sponge spicules), glauconite, tuff fragments, quartz, feldspar, rare pyroboles, clay minerals, intraclasts (probably mostly from older dike layers), and rare pellets. The polyphase dikes are interpreted as infillings of fissures that repeatedly opened on the flanks of volcanic mounds. Infillings appear to be more varied than the possible source rocks and probably record transient sedimentation. End_of_Article - Last_Page 637------------