B31C-0506
Diversity And Abundance Of Deep-Water Coral Mounds In The Straits Of Florida: A Result of Adaptability To Local Environments?
To improve the understanding of the Florida-Bahamas deep-water coral mound ecosystem, Autonomous Underwater Vehicle (AUV) surveys were conducted on five coral mound fields throughout the Straits of Florida (three sites at the base of slope of Great Bahama Bank (GBB), one in the middle of the Straits (MS) and one at the base of the Miami Terrace (MT)) in water depths of 590 to 860 m. The AUV provides high-resolution bathymetric maps, sub-bottom profiles and oceanographic data. The AUV survey sites were subsequently groundtruthed via sample collection and video transects, using the Johnson Sealink submersible. Contrary to previous surveys, we found a high diversity in coral mound morphology between sites separated by 15 to 80 km. The MT site is characterized by sinusoidal coral mound ridges, while the MS site contains densely clustered small coral mounds. Meanwhile, mounds of the GBB region are better developed, with some individual mounds reaching up to 90 m in height. Benthic coverage of live corals also differs between sites; the GBB sites are characterized by mounds densely covered by large thickets of live corals, while small thickets of mostly dead corals dominate the MT and MS sites. Several environmental factors may explain these differences. For example, bottom current patterns change between sites. The MT and the MS sites have a unidirectional regime (southward or northward flow, respectively), whereas the GBB sites have a tidal current regime. Sedimentation patterns as depicted by sub-bottom profiles also vary between the sites; coral mounds in the GBB area appear to receive higher sediment input, which can significantly enhance mound growth rates as the reef framework baffles and traps mobile sediments. However, coral mounds that cannot keep-up with the sedimentation rate are buried. Therefore, in the high sedimentation areas of GBB, flourishing live coral mounds are limited to elevated positions (i.e. plateaus, ridges crests) where sediment accumulation is lessened. Corals in these raised locations also benefit from increased exposure to nutrient-rich tidal currents, supporting a denser live coral coverage. Sub-bottom profiles of the MT site show undulating coral ridges developed on top of a relatively flat sub-surface, indicating that antecedent topography is not the only factor determining mound distribution. The integrated AUV data suggest that variable environmental factors, such as sedimentation and current patterns, contribute to the high diversity between coral mound sites of the Straits of Florida. Environmental conditions change over distances of only a few kilometers creating localized and diverse deep-water coral habitats. The deepwater fauna adapts to the local oceanographic and geological conditions. This results in an unexpectedly high abundance of deep-water coral communities with diverse expressions.
B31C-0507
Carbonate mound evolution and coral diagenesis viewed by U-series dating of deep water corals
U-series dating of constructional deep sea corals is a powerful tool to reconstruct the evolution of carbonate mound sediments driven by coral growth, sediment trapping and diagenesis. Here we have investigated in great detail the time framework of constructional corals such as L. pertusa and M. oculata on 5 different mounds of the eastern North Atlantic (on Rockall Bank and in Porcupine Seabight) taken at variable depth and location (610 to 880m water depth). Periods favorable for coral growth are the Holocene and prior interglacials such as marine isotope stage 5 and 7, while glacial coral growth seems inhibited or extremely reduced. Coral development is almost continuous throughout the Holocene since mound re-colonization about 10,500 years ago. Mound accumulation rates vary between 20 and 220 cm/kyr determined from the coral age - depth relationship in each core. Those changes are most likely driven by changes between horizontal and vertical mound accumulation, food supply and ocean circulation. In addition, coral dating allowed to identify an important erosional event recorded in core MD01-2455G from Rockall Bank. Here a 1m thick sediment layer containing ancient corals likely from the start of Holocene re-colonization was displaced (collapsed) from further upslope on top of younger corals of ~2500 to 3000 years age. Prior to the initiation of coral growth diagenesis occurred frequently resulting in (1) the construction of so called carbonate hardgrounds and/or (2) the dissolution of the pre-Holocene coral framework. Solely, the deepest selected core in Porcupine Seabight (MD01-2463G at 880m depth) reveals coral re-colonization on an undisturbed ancient reef structure that dates back to 250,000 years. Diagenesis of earlier coral reef generations leading to coral dissolution leads to a loss of magnetic susceptibility and open system behavior of the coral skeletons with respect to U-series dating. While the processes causing such diagenetic layers are barely understood the disappearance of the magnetic susceptibility can be used to trace such phenomena and a conserved magnetic susceptibility allows sampling of well preserved corals.
B31C-0508
Bamboo Coral Gorgonin: Surface Water Geochemical Records From the Organic Nodes of a Deep Water Coral
Deep-sea bamboo corals (family Isididae) found on seamounts on the California margin (37°22'N to 31°54'N) have the potential to record annual to subannual changes in water mass chemistry over decades to centuries. These corals are composed of calcite internodes precipitated from the surrounding intermediate water (as evidenced by D14C values), and organic nodes formed from the organic matter of the coral's food source. Both organic and inorganic portions of the skeleton are precipitated annually, with growth rates of ~100 microns/year. Geochemical analyses of the organic nodes of living corals collected in 2004 and 2007 (800- 2200m water depth) exhibit post-bomb radiocarbon values of >80 per mil D14C on the outer edge of the organic nodes, suggesting that the organic matter is surface-derived; pre-bomb spike values range from -81.3 per mil to -116.9 per mil in the interior of the coral organic node. Carbon isotopic values of recently precipitated organic nodes range from -16.5 per mil to -18.8 per mil. Nitrogen isotopic values from the organic nodes range between 13.8 per mil and 18.9 per mil, reflecting a zooplankton-derived N source that is strongly affected by water column denitrification processes. Organic node carbon isotopic variability in a 300+ yr old specimen exceeds 3 per mil, suggesting changes in productivity along the California margin during this time period. Organic geochemical analyses of several long-lived corals has the potential to provide century scale, high-resolution records of surface water productivity, nutrient dynamics and biogeochemical cycling along the California margin.
B31C-0509
IODP Expedition 307: A high Resolution Record of Contourite Deposition and Palaeoclimatic Forcing on the Eastern Porcupine Seabight (Irish Continental Margin)
In May 2005, IODP Expedition 307 recovered a continuous sediment succession from a contourite drift sequence from the eastern slope of the Porcupine Seabight. The contourite drift is adjacent to the Belgica Mound Province of which one cold-water coral carbonate mound (Challenger Mound) was also drilled. Grain size analysis of 95m of decalcified Pleistocene succession from IODP Core 1318B with 10cm resolution provides a detailed record of grain size distribution and variability for the last ca. 780kyr. Based on these data, we interpret the climatic history, changing sedimentological, hydrodynamic and palaeoenvironmental conditions within the Belgica Mound province. This information, in conjunction with other analyses provides important regional information on palaeoclimatic forcing of sedimentary systems on the NE Atlantic slope during the Pleistocene. So far, End-Member Modelling and sortible silt analyses have identified significant changes in the sedimentary processes over time. Correlations with the abundance of ice rafted detritus highlight the changes in climate condition as the main factor for changes in the slope sedimentary system. Results indicate periods with a coarse sediment input alternating with prolonged finer grained sediment reflecting sporadic periods of along-slope contourite deposition, down-slope terrigenous components and ice- rafted debris linked to ice mass dynamics. Given the proximity of the study area to the Belgica cold-water coral carbonate mounds, this information provide the regional hydrodynamic and paleoenvironmental context for a segment of the mound story and help elucidate carbonate mound growth vs. sediment input throughout the last c. 700kyr. Although much effort has been invested in the study of the Irish offshore in recent years, this project is unique in the detail at which the Pleistocene sediments will be examined and the depth of borehole recovery promises to provide answers on the interplay of sedimentology, climate and oceanography in this area throughout the Pleistocene.
B31C-0510
Geomicrobiology of Carbonate Mounds in the Gulf of Cadiz off Morocco: Biogeochemistry, Mineralogy and Microbial Community Composition
Carbonate mud mounds, found in marine environments from shallow- to deep-water settings, span from Proterozoic to recent times. Mound building seems to be a fundamental but still enigmatic strategy for life. Various arguments suggest that microorganisms are playing an important role in the reef development, biodiversity and mound formation. Therefore, it is important to evaluate the microbial mediated processes of carbonate precipitation. The Pen Duick Escarpment off Morocco consists of recent carbonate mounds in water depths of 500-600 m, flanked by giant mud volcanoes. Subsequent cruises have confirmed the colonization by dominantly lifeless cold- water corals and have unveiled extensive fields of seep-related carbonates in off-reef regions. Three (from 350 to 640 cm long) piston cores, coming from different sites on these juvenile mounds, were sampled and analyzed for mineralogy, stable isotopes composition, geochemistry, and microbial communities. In order to define the primary microbial community involved in carbonate precipitation, we did direct culturing, DNA isolation and PCR analysis of functional genes, including 16S rRNA gene analysis. Most of the sediment comprises pelagic calcite (coccoliths), detrital quartz and authigenic dolomite, often observed encasing coccoliths. Stable carbon isotope values of the bulk carbonate range from -7 to -15‰ indicating the involvement of microbes in the production of bicarbonate ions. In this paper, we will show and discuss multidisciplinary data obtained after several cruises aimed to elucidate the impact of microorganisms on the construction of these carbonate mounds. The special emphasis in this research will be on the correlation between microbial ecosystems and their metabolic influence on mineral formation and diagenesis.
B31C-0511
The CARBONATE project: Mid-latitude Carbonate Systems – Complete Sequences from Cold-Water Coral Carbonate Mounds in the Northeast Atlantic
Up to now the carbonate stored in carbonate mounds has not been considered in any global carbonate budget or linked to any global carbon budget involving greenhouse gases. A major challenge exists to quantify the amount and flux of carbon stored by these newly discovered areas of enhanced carbonate accumulation in intermediate water depth. Furthermore, investigations so far reveal that all mounds possess different growth histories depending on the environmental setting and the involved faunal associations. Unfortunately, existing cores only penetrated the upper few meters of the mounds thus limiting mound research to the very late stage of mound development. Access to the longer sequences preserved in giant carbonate mounds was overcome in May 2005 when the IODP Expedition 307 (Porcupine Mound Drilling) recovered complete sedimentary records from one 155 m high "Challenger Mound" in the Porcupine Seabight west off Ireland. Furthermore, EU-FP projects have revealed late stage history of giant mounds in different settings showing that different mounds respond in different ways to environmental forcing factors with no one mound being typical of all. CARBONATE will drill complete sequences through a number of mounds in differing environmental settings using the portable drill rig MeBo (University of Bremen). By understanding how biogeochemical processes control the development of these carbonate mounds and their response to climate change, we will make an important step in quantifying their role as mid-latitude carbonate sinks. In the end, a better understanding of the processes involved in mound formation and development may also result in new views on fossil analogues many of which are less accessible hydrocarbon reservoirs.