Carbonate budget of cold-water coral mounds along a latitudinal transect
Final Report Abstract
The focus of the DFG‐ Project Carbonate budget of cold‐water coral mounds along a latitudinal transect was the evaluation of the role of cold‐water coral (CWC) ecosystems as regional to global carbonate sinks and to study local, regional and latitudinal patterns in CWC mound aggradation and carbonate accumulation. Therefore, 21 cores from CWC mounds and 5 cores from the adjacent (off‐ mound) seafloor collected from Atlantic CWC areas ranging from 17° to 70° northern latitude were investigated to reconstruct their mound aggradation and carbonate accumulation rates (CAR). Additionally, CARs of adjacent environments were assembled from the literature, altogether 24 records from the Norwegian Sea and 33 records from the Mediterranean. The outcome of the project provided methodological improvements for the evaluation of mound aggradation and CAR, new insights in spatial‐temporal pattern of CWC mound aggradation and highlighted the importance of CWC ecosystems as regional and potentially global carbon sink. Within the project the methodology of the carbonate content determination within CWC‐bearing cores was improved by (i) the macrofossil determination via computed tomography (CT) analyses and (ii) a calibration study to differentiate reliable high‐ from low‐magnesium calcite within XRD diagrams. Both improvements increased the accuracy of the carbonate determinations and the differentiation of the mound‐derived carbonate from the background‐sediment‐derived carbonate (comparable to the carbonate deposition on the adjacent seafloor) – an important prerequisite for the evaluation of CWC ecosystems as carbonate sinks. Additionally, a new methodological approach for the evaluation of the preservation status of CWC deposits from computed tomography analysis was developed. This appraisal allowed the identification of preserved CWC framework, CWC rubble, condensed intervals and unconformities by their macrofossil clast size and orientation signature. The identified preservation statuses within the CWC reefs suggest that preservation of CWC framework in living position is favoured during phases with high reef aggradation rates, while preservation of coral rubble predominates by intermediate aggradation rates. A high degree of fragmentation, indicated by small clast sizes, suggests condensed intervals or unconformities. This improved CT analysis provides also the opportunity to improve sampling strategies for cost‐intensive radiogenic datings. The obtained maximal CWC mound aggradation rates of nearly 1,500 cm kyr‐1 are close to the annual growth rates of the main reef‐forming CWC Lophelia pertusa and should therefore be very close to the theoretical upper limit of CWC reef aggradation before burial. Furthermore, the maximal aggradation rates and CARs of up to 2,000 g cm‐2 kyr‐1 reach values in the range of tropical reefs, highlighting their rule as regional if not global carbonate sink. In fact, the obtained rates from the Mediterranean Sea suggest that CWCs form one of the most important carbonate factories and regional carbonate sinks in the Mediterranean Sea. In general, it seems that high maximal aggradation and CARs are favored in higher latitudes and shallower water depth. However, high variability occurs within regions, adjacent mound provinces and within mound provinces. Off Norway, a decreasing trend in mound aggradation and carbonate accumulation might be postulated from fjord reefs to shelf margin reefs, and in the Mediterranean Sea from west to east. Within the Belgica Mound Province a strong decline in mound aggradation and carbonate accumulation, accompanied with a postponed onset of mound aggradation, with decreasing water depth was observed, which might be linked to the importance of the water mass boundary between the Mediterranean Outflow Water and the Eastern North Atlantic Water for CWC growth within this region. The obtained results within the project clearly highlight the potential rule of CWC ecosystems as global carbon sink and the urgency of improving our understanding of the variability of mound aggradation and carbonate accumulation within a mound, mound province, ocean basin and global scale for obtaining a reliable carbonate budget for this fascinating carbonate factory.
Publications
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(2011) Mg quantification in calcites [(Ca, Mg)CO3] by Rietveld‐based XRD analysis – Revisiting a well‐established method. American Mineralogist 96: 1028‐1038
Titschack J, Goetz‐Neunhoeffer F, Neubauer J
