Temporal distribution of cold-water corals in the northwest Atlantic through the Late Quaternary : footprint of intermediate water mass circulation

Abstract

Over the last two decades, extensive geological investigations on cold-water coral (CWC) areas of the northeastern Atlantic revealed that the temporal occurrence of framework-forming CWC presents a climate-dependent pattern. The pattern follows the glacial-interglacial changes of the surface ocean conditions and intermediate-depth water masses. Over geological time scales, CWC can form substantial seabed features, called CWC mounds that may serve as archives for assessing their long-term development. Several CWC mound areas have been described in the northwestern Atlantic, where the presence of living reef-like structures advocates for favorable conditions for CWC at present. A limited number of ages of fossil CWC collected from various sites in the northwestern Atlantic further suggests that conditions were also favorable during the last glacial. However, the less of a handful of available ages emphasizes the need for a thorough investigation of the temporal pattern of CWC occurrence in this area. This study was designed (1) to identify the periods of sustained CWC growth in two coral mound areas of the northwestern Atlantic (the Campeche CWC Province, southern Gulf of Mexico, and the Cape Lookout CWC area, off North Carolina) and (2) to detect the environmental factors influencing their long-term development. The temporal occurrence of CWC was established by using U-series dating, while a multi-proxy approach was followed to reconstruct the present and past environmental conditions (incl. water column analyses; ROV video observation and sampling; foraminifera-based stable isotope analyses, Mg/Ca-thermometry and assemblage analyses; grain size and carbonate content analyses of sediments). The results suggest that for the last 260,000 years, pronounced coral growth at these two coral mound areas was restricted to interglacial periods, which confirms a climate influence on the CWC temporal occurrence also in these northwestern Atlantic areas. The pattern coincides with the glacial-interglacial changes of the intermediate-depth water masses, where periods favorable for CWC growth are marked by enhanced hydrodynamics that increase the food supply to the CWC. At the Campeche CWC area, such conditions result from the density gradient between the Antarctic Intermediate Water and the overlying Tropical Atlantic Central Water, whereas at the Cape Lookout CWC area, coral growth is steered by the Gulf Stream presence. Although similar occurrence patterns were found in the two studied areas, they differ from the few ages previously reported for the NE Gulf of Mexico and the East Florida Strait CWC mound areas, where CWC occur today but obviously also occurred under glacial conditions. Therefore, it is necessary to conduct further research in order to better constrain the general pattern of CWC occurrence in coral mound areas of the northwestern Atlantic and its possible connection to the intermediate-depth circulation.