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Cold-water corals (CWCs) were found thriving under hypoxic conditions (0.5 – 1.5ml/L) within the oxygen minimum zone (OMZ) off the Angolan margin (Hebbeln et al., 2020). Geological records reveal the consistent occurrence of CWC at least over the last 50 kyr BP, although the beginning of local coral colonization cannot precisely be determined. Here, we reconstructed the bottom-water conditions from marine sediments core GeoB20911 in order to identify the most critical physical-chemical parameters having controlled the development of CWC off the Angola margin. Our results show that the prolific period of corals coincides with enhanced bottom-water hydrodynamics and increased temperature (Fig. 1). Vigorous bottom current, normally associated with internal wave propagations and nepheloid layer, increases the lateral transport of food to corals. The combination of high food supply and warmer thermocline created the ideal condition for regional coral proliferation by increasing their growth rates as documented in cultured experiments (Büscher et al., 2017). Interestingly, our results show a decline of the bottom-water oxygenation in the course of the last 50kyr BP without any negative impact on CWC occurrence. We speculate that local CWC is able to develop under hypoxic conditions as long as an elevated amount of fresh and high-quality food is available to compensate for such deleterious conditions.

Critical environmental parameters affecting deep-sea ecosystems identified, using CWC reef extinctions/recolonisations from the geological record as the examplar and geochemical and kinetic proxies (radiocarbon, U/Th, grain size, foram assemblages) to assess environmental drivers and timings of tipping points.

As “ecosystem engineers,” framework-forming scleractinian cold-water corals (CWC) build reefs that are unique biodiversity hotspots in the deep sea. Studies using common biological techniques such as correlating the spatial occurrence of the most common CWC species with modeled environmental conditions have revealed the ecological requirements and tolerances of these species. However, limited field observations and poorly understood geographical distribution patterns of the CWC restrict the application of existing knowledge toward assessing their fate (e.g., local extinction, newly established populations) under ongoing global change. Hence, the risk to cross ecological tipping points causing the demise (or establishment) of entire CWC reefs remains unclear. A major challenge is to identify the key environmental parameters (or stressors) having the potential to control CWC vitality by providing such tipping points. This is largely hampered by the overall lack of present-day observations of such tipping point crossings. However, evidence for such events is frequently preserved in geological records revealing that entire CWC ecosystems vanished or returned at specific moments in the past. Here, a geological approach is presented that by correlating geological CWC records with paleoceanographic data describing past environmental changes allows to identify a set of key environmental drivers that directly or indirectly control CWC vitality. Thus, by combining such a geological approach with common biological techniques (see above) to describe the ecological tolerance of the most important reefbuilding CWC has a great potential to better assess their future spatial distribution in times of accelerating global change and to improve the sustainable management of the important deep-sea ecosystems formed by CWC.

Cold-water corals (CWCs) were found thriving under hypoxic conditions (0.5 – 1.5ml/L) within the oxygen minimum zone (OMZ) off the Angolan margin (Hebbeln et al., 2020). Geological records reveal the consistent occurrence of CWC at least over the last 50 kyr BP, although the beginning of local coral colonization cannot precisely be determined. Here, we reconstructed the bottom-water conditions from marine sediments core GeoB20911 in order to identify the most critical physical-chemical parameters having controlled the development of CWC off the Angola margin. Our results show that the prolific period of corals coincides with enhanced bottom-water hydrodynamics and increased temperature (Fig. 1). Vigorous bottom current, normally associated with internal wave propagations and nepheloid layer, increases the lateral transport of food to corals. The combination of high food supply and warmer thermocline created the ideal condition for regional coral proliferation by increasing their growth rates as documented in cultured experiments (Büscher et al., 2017). Interestingly, our results show a decline of the bottom-water oxygenation in the course of the last 50kyr BP without any negative impact on CWC occurrence. We speculate that local CWC is able to develop under hypoxic conditions as long as an elevated amount of fresh and high-quality food is available to compensate for such deleterious conditions.

Along the northern Argentine margin numerous cold-water coral mounds have been found. Little is known about the present distribution of Bathelia candida, the major framework-forming coral in this region. However, based on the analysis of five sediment cores taken from coral mounds between 990 m and 1,300 m, some information exists about their past development. Coral age datings with the Uranium/Thorium methods reveal three major coral growth or mound aggradation phases: 17-15 kyr BP, 14-12 kyr BP, and 11-8 kyr BP, which are linked to changes in the Atlantic Meridional Overturning Circulation (AMOC) and in the strength of the Antarctic Intermediate Water (AAIW). Interestingly, in water depths >1,150 m coral growth was restricted to the 17-15 kyr period during the AMOC breakdown. After that, corals only returned to the Argentine margin to waters shallower than 1,150 m. Still, a puzzling question remains to why the corals did not return after ~6.5 kyr BP, when the AAIW got back to normal strength.