Environmental constraints on Holocene cold-water coral reef growth off Norway: Insights from a multiproxy approach

摘要

High-latitude cold-water coral (CWC) reefs are particularly susceptible due to enhanced CO2 uptake in these regions. Using precisely dated (U/Th) CWCs (Lophelia pertusa) retrieved during research cruise POS 391 (Lopphavet 70.6 degrees N, Oslofjord 59 degrees N) we applied boron isotopes (B-11), Ba/Ca, Li/Mg, and U/Ca ratios to reconstruct the environmental boundary conditions of CWC reef growth. The sedimentary record from these CWC reefs reveals a lack of corals between similar to 6.4 and 4.8ka. The question remains if this phenomenon is related to changes in the carbonate system or other causes. The initial postglacial setting had elevated Ba/Ca ratios, indicative of meltwater fluxes showing a decreasing trend toward cessation at 6.4ka with an oscillation pattern similar to continental glacier fluctuations. Downcore U/Ca ratios reveal an increasing trend, which is outside the range of modern U/Ca variability in L. pertusa, suggesting changes of seawater pH near 6.4ka. The reconstructed bottom water temperature at Lopphavet reveals a striking similarity to Barent sea surface and subsea surface temperature records. We infer that meltwater pulses weakened the North Atlantic Current system, resulting in southward advances of cold and CO2-rich Arctic waters. A corresponding shift in the B-11 record from similar to 25.0 to similar to 27.0 probably implies enhanced pH up-regulation of the CWCs due to the higher pCO(2) concentrations of ambient seawater, which hastened mid-Holocene CWC reef decline on the Norwegian margin.