Spatial and temporal development of the Namibian cold-water coral mounds

Abstract

Over the last 25 years, extensive geological investigations on coral mounds, seafloor structures in the deep-sea built by ecosystem-engineering cold-water corals (CWC), have revealed that the spatial and temporal occurrences of CWC depend on environmental and climate conditions. Coral mounds in the NE Atlantic were the first and have been the most studied coral mound provinces (CMPs), providing the fundamental evidence of the environmental forcing supporting CWC and on the development of coral mounds over glacial-interglacial cycles. However, the continuous discovery of coral mounds under the extremely variable oceanographic conditions along the Atlantic margins have brought new evidence to re-discuss our previous knowledge on CWC and coral mounds. This study was designed for one of the latest CMP discovered: the Namibian coral mounds. More than 2000 mounds are sitting on the Namibian inner shelf (~20°S) between 160-270 m water depths. The Namibian CMP occurs under an oceanographic setting controlled by the Benguela Upwelling System (BUS). The BUS is one of the large eastern-boundary upwelling systems with the highest primary production in the world ocean. Due to these highly productive conditions, shelf-bottom waters become severely oxygen-depleted resulting from the consumption of oxygen through the decomposition of organic matter. The Namibian coral mounds occur exactly at the core of the local oxygen minimum zone (OMZ, 160–270 m water depth), where dissolved oxygen concentrations (DO) are 0-0.5 mL L−1 and accompanied by relatively high potential temperatures of 12.4 to 13.4 °C which can already be critical for CWC physiology. These present-day conditions prevent any CWC proliferation, and indeed no living colonies have been observed during the dives of a remotely operated vehicle. After the analysis and discussion of the present-day environmental condition (Manuscript I), this thesis aimed to identify how oceanographic conditions controlled the spatial-morphological variability (Manuscript II) and temporal development (Manuscript III) of the Namibian coral mounds. The results show that spatial distribution and morphometric appearance of the coral mounds is highly controlled by the interplay between the underlying topography (erosional features) and the hydrodynamic regime (internal tides). Moreover, U-series dating on CWC skeletons combined with local paleoceanographic reconstructions link the temporal development (4.5-9.5 kyr BP) of the Namibian coral mounds to a decrease of the BUS activity, with a consequent relative relaxation of the local OMZ. This likely brought DO values comparable to the present-day hypoxic conditions on the Angola CMP, where living CWC colonies have been observed. The findings of this thesis show how a comprehensive geological investigation on coral mounds can reveal important insights on the life and demise of CWC. The methods applied and the results obtained might serve as framework for investigating new CMPs, as well stimulating re-analyses of older datasets acquired from well-known CMPs. Moreover, improving our knowledge on CWC, to which this study on the Namibian coral mounds contributed, will provide insights on the fate of CWC under the changing ocean and climate conditions in the future.