Onset and demise of coral reefs, relationship with regional ocean circulation on the Wyville Thomson Ridge

Abstract

Results from a vibrocore collected on the northern edge of the Scottish continental shelf at around 300 m water depth, on the Wyville Thomson Ridge, enable to reconstruct the history of cold water coral (CWC) reef growth and demise during the Holocene period. We report on significant age differences between U/Th and 14C dates obtained on pristine well-preserved CWC (Lophelia pertusa), which may reflect an diagenetic process that standard quality tests have failed to highlight. Additional 14C dates derived from bivalve fragments (Venus sp.) and foraminifera (Cibicides refulgens) samples show a gradual ageing with core depth but with significant age inversions in the lower section of the sediment core, which we consider reflects sedimentary mixing. We thus chose to derive an independent age model using planktonic foraminifera Globigerina bulloides stable isotope profiles. The vibrocore record is divided into 3 phases: 1- Mixed sediment deposits of glacial age corresponding to the base of the core with ages older than 13 cal ka BP, 2- The end of the deglacial/early Holocene between 13 and 9 cal ka BP and 3- Finally, the Holocene period from around 9 cal ka BP with abundant Lophelia pertusa fossils. Siliciclastic grain size and clay mineralogical composition show two significant shifts at around 13 and 9 cal ka BP indicating changes in sedimentary sources and transport associated with the dynamics and flow patterns of surface currents during the deglaciation and Holocene. Our results show that the onset of CWC reef growth on the Wyville Thomson Ridge occurred around 9 cal ka BP and was associated with a shift in flow patterns of surface currents in this area. This change of circulation patterns induced favourable sedimentological and hydrological conditions for corals to grow, and is associated with large scale modifications of North Atlantic circulation patterns at the end of the deglaciation.