Search

International audience; Through the interplay of a stabilising cold-water coral framework and a dynamic sedimentary environment, cold-water coral carbonate mounds create distinctive centres of bio-geological accumulation in often complex (continental margin) settings. The IODP Expedition 307 drilling of the Challenger Mound (eastern Porcupine Seabight; NE Atlantic) not only retrieved the first complete developmental history of a coral carbonate mound, it also exposed a unique, Early-Pleistocene sedimentary sequence of exceptional resolution along the mid-latitudinal NE Atlantic margin. In this study, a comprehensive assessment of the Challenger Mound as an archive of Quaternary palaeo-environmental change and long-term coral carbonate mound development is presented. New and existing environmental proxy records, including clay mineralogy, planktonic foraminifer and calcareous nannofossil biostratigraphy and assemblage counts, planktonic foraminifer oxygen isotopes and siliciclastic particle-size, are thereby discussed within a refined chronostratigraphic and climatic context. Overall, the development of the Challenger Mound shows a strong affinity to the Plio-Pleistocene evolution of the Northern Hemisphere climate system, albeit not being completely in phase with it. The two major oceanographic and climatic transitions of the Plio-Pleistocene - the Late Pliocene/Early Pleistocene intensification of continental ice-sheet development and the mid-Pleistocene transition to the more extremely variable and more extensively glaciated late Quaternary - mark two major thresholds in Challenger Mound development: its Late Pliocene (>2.74 Ma) origin and its Middle-Late Pleistocene to recent decline. Distinct surface-water perturbations (i.e. water-mass/polar front migrations, productivity changes, melt-water pulses) are identified throughout the sequence, which can be linked to the intensity and extent of ice development on the nearby British-Irish Isles since the earliest Pleistocene. Glaciation-induced shifts in surface-water primary productivity are thereby proposed to fundamentally control cold-water coral growth, which in turn influences on-mound sediment accumulation and, hence, coral carbonate mound development throughout the Pleistocene. As local factors, such as proximal ice-sheet dynamics and on-mound changes in cold-water coral density, significantly affected the development of the Challenger Mound, they can potentially explain the nature of its palaeo-record and its offsets with the periodicities of global climate variability. On the other hand, owing to this unique setting, a regionally exceptional, high-resolution palaeo-record of Early Pleistocene (ca 2.6 to 2.1 Ma) environmental change (including early British-Irish ice-sheet development), broadly in phase with the 41 ka-paced global climate system, is preserved in the lower Challenger Mound. All in all, the Challenger Mound record highlights the wider relevance of coral carbonate mound archives and their potential to capture unique records from dynamic (continental margin) environments.

International audience

Sub-recent cold-water carbonate mounds localized in deeper slope settings on the Atlantic continental margins cannot be any longer neglected in the study of carbonate systems. They clearly play a major role in the dynamics of mixed siliciclastic-carbonate and/or carbonate-dominated continental slopes. Carbonate accumulation rates of cold-water carbonate mounds are about 4 to 12 % of the carbonate accumulation rates of tropical shallow-water reefs but exceed the carbonate accumulation rates of their slope settings by a factor of 4 to 12 (Titschack et al.,2009). These findings emphasize the importance of these carbonate factories as carbonate niches on the continental margins. The primary environmental architecture of such carbonate bodies is well-characterized. However, despite proven evidences of early diagenesis overprinting the primary environmental record (e.g. aragonite dissolution) (Foubert & Henriet, 2009), the extent of early diagenetic and biogeochemical processes shaping the petrophysical nature of mounds is until now not yet fully understood.Understanding (1) the functioning of a carbonate mound as biogeochemical reactor triggering early diagenetic processes and (2) the impact of early diagenesis on the petrophysical behaviour of a carbonate mound in space and through time are necessary (vital) for the reliable prediction of potential late diagenetic processes. Approaching the fossil carbonate mound record, through a profound study of recent carbonate bodies is innovative and will help to better understand processes observed in the fossil mound world (such as cementation, brecciation, fracturing, etc. . . ).In this study, the 155-m high Challenger mound (Porcupine Seabight, SW of Ireland), drilled during IODP Expedition 307 aboard the R/V Joides Resolution (Foubert & Henriet, 2009), and mounds from the Gulf of Cadiz (Moroccan margin) will be discussed in terms of early diagenetic processes and petrophysical behaviour. Early differential diagenesis overprints the primary environmental signals in Challenger mound, with extensive coral dissolution and the genesis of small-scaled semi-lithified layers in the Ca-rich intervals. The low cementation rates compared to the extensive dissolution patterns can be explained by an open-system diagenetic model. Moreover, Pirlet et al. (2009) emphasizes the occurrence of gypsum and dolomite in another mound system (Mound Perseverance) in Porcupine Seabight, which might be also related with fluid oxidation events in a semi-open diagenetic system. Along the Moroccan margins, fluid seepage and fluxes in pore water transport affect the development of mound structures, enhancing extensive cold-water coral dissolution and precipitation of diagenetic minerals such as dolomite, calcite, pyrite, etc. (Foubert et al., 2008). Recent carbonate mounds provide indeed an excellent opportunity to study early diagenetic processes in carbonate systems without the complications of burial and/or later meteoric diagenesis.

Cold-water coral ecosystems represent a worldwide distributed carbonate factory, which is so far not considered in global carbonate budget estimations. The herein presented study evaluates the carbonate budget of a complete cold-water coral mound sequence from Challenger Mound (Belgica Mound Province, Porcupine Seabight, SW offshore Ireland; drilled during IODP Expedition 307) reaching back to the Late Pliocene (< 2.7 Ma) especially focusing on the different carbonate sources (coral-derived aragonite versus background-sediment-derived calcite-dominated carbonate). Cores from adjacent drift deposits were examined for comparison studies.Challenger Mound, which developed in a fast growth phase (unit M1; ~ 2.7 to ~ 1.6 Ma) and a slower growth phase or mound-decline (unit M2; ~ 0.8 Ma until today), separated by a so called ‘mound crisis’, displays increased carbonate accumulation rates for both phases (17.3 g/(cm2 × ka) for unit M1 and 5.7 g/(cm2 × ka) for unit M2) compared to adjacent drift deposits. But only about 33 to 40 wt.% of the carbonate is derived from corals. Bulk sedimentation rates of on- and off-mound deposits suggest a decline of Challenger Mound since the onset of drift deposition in the area about 1.2 Ma ago. Yet total carbonate accumulation rates as well as hemipelagic Background-Sediment-Derived Carbonate accumulation rates from Challenger Mound (unit M2) still exceed the carbonate accumulation rates of the adjacent drift deposits. The selective enrichment of Background-Sediment-Derived Carbonate (by a factor of 2.0) in a CWC Mound is herein observed for the first time. This might be caused by the different hydrodynamic behaviour and slower settlement of marine fluff rich in calcareous detritus, compared to siliciclastic sediments, and its effective baffling by the coral framework and/or the active catch of the calcareous plankton by the corals.The carbonate accumulation rates of Challenger Mound are about 4 to 12% of the carbonate accumulation rates of tropical shallow-water reefs but exceeds the carbonate accumulation rates of continental slopes by a factor of 3.9 to 11.8. The carbonate production for the entire Belgica Mound Province is estimated with 3.2 × 103 t/a for unit M1 and 1.0 × 103 t/a for unit M2.