Deep-water Coral Mounds Research Articles

Adaptive brachiopod morphologies in four key environments of the Late Cretaceous–Danian Chalk Sea of northern Europe: A comparative study

Rich brachiopod faunas are known from the four main, but highly different, palaeoenvironments in the Late Cretaceous–Danian Chalk Sea of northern Europe: Maastrichtian soft-bottom chalk (Møns Klint, Stevns Klint), lower Danian bryozoan mounds (Stevns Klint), middle Danian coral mounds (Faxe), all in the Danish Basin, and a late Campanian rocky shore (Ivö Klack) in the Kristianstad Basin, southern Sweden. There is almost no genus or species overlap between the rocky shore fauna and the three other faunas; the former is interpreted as endemic, having evolved in the small basin which is situated adjacent to the eastern end of the Danish Basin. The Maastrichtian and Danian faunas differ significantly at the species level, reflecting the mass extinction at the Cretaceous–Paleogene (K/Pg) boundary. In spite of having inhabited very different environments, the brachiopod faunas are remarkably similar in terms of modes of life and population structure, and by a marked predominance of micromorphic species. The main difference is the total absence in the Danian faunas of highly specialised, secondarily free-living hemispherical species, which are a characteristic element of the Maastrichtian chalk fauna. This adaptation was developed in several unrelated taxonomic groups during the c. 40-Myr-long existence of the Late Cretaceous interval of the Chalk Sea. No species of this group survived the K/Pg mass extinction. Identification of brachiopod morphogroups reported as characterising different environments is thus successful for the rocky shore environment, but tends to break down between the soft chalk sea floor and the bryozoan or coral mounds. This is interpreted to reflect the availability of a multitude of small, hard substrates, notably slender fragments of bryozoans, in all the three of the last-named environments and by the fact that the survivors of the mass extinction were represented mainly by micromorphic species in addition to a few generalists from which the new, Danian fauna rapidly evolved. Three new major environments, each characterised by distinct brachiopod morphotypes, are recognised in addition to the seven described previously in the literature, namely the sublittoral chalk sea floor, the bryozoan mounds, and the cool and deep-water coral mounds. The rocky shore may be considered as representing a fourth new distinct environment.

Autonomous Underwater Vehicles (AUVs): Their past, present and future contributions to the advancement of marine geoscience

Autonomous Underwater Vehicles (AUVs) have a wide range of applications in marine geoscience, and are increasingly being used in the scientific, military, commercial, and policy sectors. Their ability to operate autonomously of a host vessel makes them well suited to exploration of extreme environments, from the World’s deepest hydrothermal vents to beneath polar ice sheets. They have revolutionized our ability to image the seafloor, providing higher resolution seafloor mapping data than can be achieved from surface vessels, particularly in deep water. This contribution focuses on the major advances in marine geoscience that have resulted from AUV data. The primary applications are i) submarine volcanism and hydrothermal vent studies, ii) mapping and monitoring of low-temperature fluid escape features and chemosynthetic ecosystems, iii) benthic habitat mapping in shallow- and deep-water environments, and iv) mapping of seafloor morphological features (e.g. bedforms generated beneath ice or sediment-gravity flows). A series of new datasets are presented that highlight the growing versatility of AUVs for marine geoscience studies, including i) multi-frequency acoustic imaging of trawling impacts on deep-water coral mounds, iii) collection of high-resolution seafloor photomosaics at abyssal depths, and iii) velocity measurements of active submarine density flows. Future developments in AUV technology of potential relevance to marine geoscience include new vehicles with enhanced hovering, long endurance, extreme depth, or rapid response capabilities, while development of new sensors will further expand the range of geochemical parameters that can be measured.

Habitat Characterization, Distribution, and Areal Extent of Deep-sea Coral Ecosystems off Florida, Southeastern U.S.A.

Abstract. The deep-sea (200–1000 m) seafloor off the southeastern U.S. has a variety of extensive deep-sea coral ecosystem (DSCE) habitats including: deep-water coral mounds; various hard-bottom habitats off Florida including the Miami Terrace, Pourtales Terrace, and deep-water canyons (Agassiz and Tortugas Valleys); and deep island slopes off western Bahamas and northern Cuba. The dominant structure-forming scleractinian corals are Lophelia pertusa and Enallopsammia profunda; other structure-forming taxa include stylasterid corals, gorgonians, black corals, and sponges. This biota is associated with hard-bottom seafloor of variable high-relief topography which can be remotely identified from bathymetric data. NOAA bathymetric contour maps and digital elevation models were used to identify and delineate the areal extent of potential DSCE habitat in the region from northeastern Florida through the Straits of Florida. These were ground-truthed with 241 dives with submersibles and remotely operated vehicles ...

The Pleistocene Cooling Built Challenger Mound, a Deep-water Coral Mound in the NE Atlantic: Synthesis from IODP Expedition 307

IODP Expedition 307 revealed the interior of a deep-water coral mound in NE Atlantic for the first time and improved our understanding of these intriguing structures. From the summit of our drilling target, Challenger Mound at ~800 m deep in the Porcupine Seabight, south west of Ireland, we recovered the entire mound section of 155 m long, which almost entirely consists of coral-bearing sediments and rests on the Miocene siliciclastics. The mound initiation is temporally correlated to the global cooling at the beginning of Pleistocene, when modern circulation was established in Atlantic.A key oceanographic feature of the mound provinces is the density gradient that developed above the saline Mediterranean Outflow Water where organic particles persist for a longer time and fuel the coral communities. Growth of the deep-water mounds reflected the glacial/interglacial change. Our age model recognized two growth stages separated by a substantial hiatus; the depositionally continuous lower mound (2.61.7 Ma) accumulated under the low-amplitude relative sea-level change, and the discontinuous upper mound (1.0 Ma to midHolocene) developed under the high-amplitude relative sea-level change. Low cellular abundances in the geochemical features of the mound sediments did not support the hypothesis that hydrocarbon seepage and associated microbial activity significantly enhanced mound initiation and development.

Hard- and soft-bottom thanatofacies from the Santa Maria di Leuca deep-water coral province, Mediterranean

Thanatofacies and the skeletonized components of the living facies, from which they originate have been studied from the Santa Maria di Leuca (SML) deep-water coral mound province. Faunal analysis was carried out by means of bottom sampling and underwater video observations, taking into account all benthic taxonomic groups, mostly corals, molluscs, serpulids, bryozoans, ostracods, foraminifers and barnacles, which permitted recognition of six different facies. These thanatofacies are easily distinguishable and appear to be largely corresponding and overlapping with related living facies. Some occur in mound areas, others in the intermound ones. They are as follows: the Framework-building Coral facies (FC), characterised by colonial corals, mostly Madrepora oculata; the Coral Rubble facies (CR), with proximal and distal aspects, characterised by large- to small-sized and densely to loosely packed coral fragments; the Solitary Coral facies (SC), dominated by different species depending on the availability and dimensions of hard exploitable surfaces; the Gryphus and Isidella facies (GI) in relatively coarse-grained bottoms; the Mollusc Mud facies (MM) and the Foraminifer Mud facies (FM) in comparably homogeneous silty bottoms. Facies distribution and spatial variability are discussed, in relation to hydrology and sea-floor topography. Furthermore, the SML facies are compared with living facies from the present-day Mediterranean and Pleistocene sediments of the same area. Data on bioclastic assemblages can serve for comparison with other recent aphotic, non-tropical carbonates.

Age constraints on the origin and growth history of a deep-water coral mound in the northeast Atlantic drilled during Integrated Ocean Drilling Program Expedition 307

Sr isotope stratigraphy provides a new age model for the first complete section drilled through a deep-water coral mound. The 155-m-long section from Challenger Mound in the Porcupine Sea-bight, southwest of Ireland, is on Miocene siliciclastics and consists entirely of sediments bearing well-preserved cold-water coral Lophelia pertusa. The 87Sr/86Sr values of 28 coral specimens from the mound show an upward-increasing trend, correspond to ages from 2.6 to 0.5 Ma, and identify a significant hiatus from ca. 1.7 to 1.0 Ma at 23.6 m below seafloor. The age of the basal mound sediments coincides with the intensification of Northern Hemisphere glaciations that set up the modern stratification of the northeast Atlantic and enabled coral growth. Mound growth persisted throughout glacial-interglacial fluctuations, reached a maximum rate (24 cm/k.y.) ca. 2.0 Ma, and ceased at 1.7 Ma. Unlike other buried mounds in Porcupine Seabight, Challenger Mound was only partly covered during its growth interruption, and growth restarted ca. 1.0 Ma.

Seabed pockmarks associated with deepwater corals off SE Brazilian continental slope, Santos Basin

Several factors bear evidence of chemosynthetic ecosystem occurrence in the Brazilian coast; these motivated our investigation to sample three pockmarks located on the continental slope off SE Brazil (26°S and 46°W) at ∼700 m depth. The crater-like features were ∼1 km in diameter and 100 m in depth, and were identified with an echo-sounder (EK-500). They were sampled along two transects using a rectangular dredge. Our main objective was to identify possible evidence of a seep-related community. The surrounding sediments of the pockmarks seem to be filled with coral rubble harboring a rich fauna of metazoans including epilithic and chasmolithic organisms. A total of 9565 organisms were sampled. The most abundant group was the Ophiuroidea, followed by Cirripedia and Polychaeta. A number of new occurrences and new species were registered. The polychaete worm Amelinna was the first record for the Atlantic Ocean, which has been found in reducing habitats such as wood islands. The echo-sounder records also showed the presence of big deepwater coral mounds associated with the edge of the pockmarks. These features are ∼ 20 m in height and from 180 to 360 m wide. Records of demersal fishes were also registered right above coral mounds. The presence of deepwater coral mounds at the edge of pockmarks may be related to the flow of the cold, oxygen-rich Antarctic Intermediate Water over the bottom, or else may benefit from the fertilization of the water by the microseepage of hydrocarbons from the sediment. We suggest a more thorough survey in order to map and recognize these environments. Furthermore, our findings may shed some light on new research perspectives concerning the ecology, biogeography and evolution of reducing habitats off the Brazilian coast.

The Modern Carbonate Ramp Slope of Central West Florida

ABSTRACT A holistic investigation of the carbonate ramp slope of central west Florida has been completed via analysis of approximately 1,500 km of air-gun seismic reflection profiles tied to two drillcores and bottom samples recovered at 92 stations. Our primary goal has been to characterize depositional processes and products; to compare and contrast results with rimmed-platform slopes; and to develop a model for application to the rock record. The modern depositional environment of the west Florida ramp slope was initiated 12-15 Ma when depositional style changed from seaward-prograding clinoforms to a pelagic, slope-front-fill system bound along the shelf margin by contour-parallel flow of the Loop Current. A seaward-thickening wedge (0-250 m) of late Miocene to Recent pelagic carbonate ooze has been deposited across antecedent mid-Miocene topography. In general, the sea floor dips 1-2° seaward from water depths of about 200-2,000 m, and slopes are relatively smooth. However, contemporary submarine geomorphology is controlled, in part, by earlier slope failures. The primary control of sedimentation along the west Florida slope is Loop Current flow, which acts as a dynamic oceanographic boundary between outer shelf and upper slope, winnows bottom sediments, catalyzes early submarine cementation, and stimulates pelagic production of calcium carbonate. The axis of Loop Current circulation has also migrated laterally in response to climatically controlled sea-level fluctuations, which has resulted in a relatively wide ( 25 km) band of winnowed sands and hardgrounds along the shelf margin. Climatic oscillations have also resulted in cyclic deposition of pelagically derived aragonite (pteropods) on the ramp slope that is out of phase with bank-derived aragonite cycles of rimmed platform slope . There are three major facies belts that parallel isobaths: 1) a hardground facies of intraclastic grainstones and rhodolith rudstones, as well as algal ridges, along the outer shelf at water depths of approximately 200-400 m; 2) a winnowed-sand facies of planktonic forams, intraclasts, glauconite, and reworked phosphorite grains as well as deep-water coral mounds along the shelf margin at depths of approximately 400-600 m; and 3) a bioturbated pelagic-ooze facies along the ramp slope at depths of approximately 600-2,000 m. These facies patterns, geometries, and grain types, along with a general paucity of sediment-gravity-flow deposits, readily distinguish this carbonate ramp slope from better-known rimmed-platform slopes of the Bahamas.

Coral Mounds on the West Florida Slope: Unanswered Questions regarding the Development of Deep-Water Banks

Late Pleistocene deep-water coral mounds of 10-15 m relief occur in a20 km linearzone parallel to the 500 m isobath along the West Florida carbonate-ramp slope. These relict mounds were constructed by the densely calcified, ahermatypic framework builder, Lophelia prolifera, and provided habitats for a host of associated invertebrates, including epizoans, epifaunal commensal organisms, nestlers and crevice dwellers, and macroendoliths. Scleractinian diversity and taxonomic composition are congruent with those of other Lophelia buildups in the NorthAtlantic, particularly buildups in the eastern Atlantic. The scleractinians also retain primaty mineralogic, isotopic, and trace-elementgeochemical signatures, indicating relatively little diagenetic alteration, despite dead (>40,000 years b.p.) radiocarbon ages. The small but rapidly expanding global data base on deepwater coral mounds has magnifted two key questions concerning the ecologic and environmental controls on mound nucleation, growth, and death. First, what are the principal ecologic controls on dominance within communities of deep-water framework builders? Second, why are there so many relict and so few living deep-water mounds in the modern ocean? Ecological and paleoecological investigation of these questions would elucidate much about the dynamics of deep-water mound growth.

Faunal Succession Within Deep-Water Coral Mounds North of Little Bahama Bank: ABSTRACT

Deep-water coral mounds of 5 to 40 m relief occur at depths of 1,000 to 1,300 m over a 2,500 sq km area of the lower slope north of Little Bahama Bank. These coral/gorgonian buildups, apparently unlithified, have yielded radiocarbon ages of 860 ± 50 and 940 ± 40 years for the best preserved corals and gorgonians, and preliminary dates of 22,100 years for the most intensively bored corals, the youngest deep-water coral mounds ever reported. Eight genera of deep-water coral represent the highest diversity recorded from a single locality. These ahermatypes are predominantly solitary, although branching and weakly branched forms are also present. The colonial ahermatypes from the mounds possess large-diameter corallites and relatively few corallites per specimen. Se eral of the coral general, most notably Thecopsammia, have significant stereomal deposits in the skeleton, a feature common among deep-water corals. The scleractinians are associated with a diverse fauna. The primary framework builders of the mounds, however, appear to be branching corals and gorgonians. Based on the relative amounts of boring and Mn-oxide coating on coral specimens recovered from dredge hauls, there appears to be a crude faunal succession within the mounds. Branching colonial corals and gorgonians seem to be the pioneer forms, colonizing hardgrounds. These initial coral thickets form a baffle for sediment as well as substrates for later stages of attached and free-lying ahermatypes such as Desmophyllum, Stephanocyathus, and Deltocyathus. Thus the mounds grow through a combination of sediment trapping and colonization by a greater diversity of coral and other invertebrates. The coarse nature of intermound sediments and the presence of scour and ripple marks in underwater photographs indicate that bottom currents are vital to the development of these deep-water coral s ructures. End_of_Article - Last_Page 965------------

Anatomy of Modern Open-Ocean Windward Carbonate Slope: Northern Little Bahama Bank: ABSTRACT

The modern carbonate slopes in the northern Bahamas are classified as windward or leeward types along open oceans, open seaways, or closed seaways. Morphologically the open-ocean windward carbonate slope, End_Page 962------------------------------ north of Little Bahama Bank, can be divided into upper and lower slopes. The upper slope is relatively steep (2 to 4°) and heavily dissected by numerous, roughly evenly spaced submarine canyons or gullies that display 50 to 100 m of relief. The seismic facies of this upper slope consists of parallel to subparallel reflectors. Core data indicate that most sediments on this upper slope, including those immediately adjacent to shallow-water reefs, are fine-grained periplatform oozes. Core data combined with 3.5 kHz PDR profiles reveal a gradual downslope decrease in the amount of submarine cementation. Where cementation is not an important factor, submarine sliding begins, probably due to overloading. In contrast, the lower slope is a broad, smooth, relatively gentle (~ 1°) region with few canyons. The seismic facies here consists of chaotic/discordant to wavy, subparallel reflectors. Core data indicate a dominance of coarse-grained debris flow and turbidity current deposits, interlayered with minor pelagic sediments. Most of the coarse detritus in these sediment gravity flows was derived from the upper slope. Pelagic sedimentation, incipient submarine cementation, and mass movements interact to produce very coarse-grained material on the lower slope from initially fine-grained upper slope sediments. Numerous unlithified deep-water coral mounds are also present on the lower slope. The morphology and sedimentary/seismic facies relations of this carbonate slope indicate sedimentation from a line source rather than a point source. In contrast to submarine fan development from a point source, base of slope sedimentation along a line source produces a broad apron of coarse sediment, gravity flow deposits on the lower slope paralleling the shelf/slope break. In terms of hydrocarbon exploration the lower slope sediment gravity flow facies appears to have the greatest potential as a reservoir. End_of_Article - Last_Page 963------------

Modern Deep-Water Coral Mounds North of Little Bahama Bank: Criteria for Recognition of Deep-Water Coral Bioherms in the Rock Record

ABSTRACT Deep-water, ahermatypic coral mounds are present at water depths of 1,000-1,300 m on the lower slope north of Little Bahama Bank. The mounds are patchily distributed over a minimum area of 2,500 km2 and typically display 5-40 m of relief above the surrounding sea bottom. A diverse benthic community exists on these apparently unlithified mounds, including l 1 genera and 16 species of ahermatypic coral (Bathypsammia, Caryophyllia, Deltocyathus, Desmophyllum, Enallopsammia, Javania, Madrepora, Polymyces, Solenosmilia, Stephanocyathus, and a previously undescribed genus), alcyonaceans, gorgonians, antipatharians, hydroids, ophiuroids, crinoids, barnacles, galatheid crabs, polychaetes, gastropods, bivalves, and sponges. Conspicuously absent from the coral fauna are Lop elia and Dendrophyllia, common deep-water corals in other parts of the Atlantic. Radiocarbon dates on fresh coral and gorgonian fragments of 940 ± 40 and 860 ± 50 years indicate the mounds are at least in part Recent and are probably actively forming today. Bored and stained corals date at around 22 10314C years B. P., which establishes a minimum age for these mounds. We speculate that the mounds develop on sea-floor perturbations in areas where strong bottom currents provide needed oxygen and nutrients to the fauna. The mounds may have undergone multi-stage evolution from colony to thicket to coppice to bank. This evolution may be accomplished through the in situ contribution of skeletal material along with the baffling and trapping of fine-grained carbonate sediment winnowed from adjacent areas by bottom currents. If found in the rock record, the coral mounds north of Little Bahama Bank would probably be considered bioherms. Geologists should thus be aware that scleractinian bioherms are not uniquely shallow-water in origin. The distinction between ancient deep- and shallow-water coral buildups involves using multiple recognition criteria, including: 1) presence or absence of algae; 2) diversity of corals; 3) coral morphology and microstructure; 4) abundance of planktonic/benthonic components; 5) microborings; 6) surrounding litho- and biofacies; 7) stable isotopes of carbon and oxygen; and 8) trace element geochemistry, particularly Sr and U concentrations. The distinction between deep- and shallow-water bioherms is crucial to regional paleoenvironmental and stratigraphic interpretations.