Late Holocene calcium carbonate accumulation in a cold-water coral mound in the northern Bay of Biscay.

Cold-water coral reefs and communities can be locally important calcium carbonate factories in continental shelf and slope environments.  Cold-water coral mounds dominated by Desmophyllum pertusum (=Lophelia pertusa) occur within Bay of Biscay submarine canyons.  Here we present a late Holocene record of coral carbonate accumulation through a colonial scleractinian coral mound in the 750-850 m depth range in Guilvinec Canyon, northern Bay of Biscay.  Guilvinec Canyon, like most submarine canyons of the Bay of Biscay, is a dominantly siliciclastic sandy environment, with occasional coral gardens.  Maximum live coral cover in the area surveyed in Guilvinec Canyon was about 9%, measured close to the site of the sediment core analyzed.

A 2011 sediment core through the mound recovered 1.18 m of sediment, consisting of mostly siliciclastic silty sand and coral gravel, strongly dominated by Lophelia pertusa fragments.   In addition to standard geophysical, grain size, and mineralogical analyses, the core was analyzed by CT-scan.  The number of coral calices visible per cm³ was counted, and the core was subsampled for coral calice abundance and mass in 5 mm increments, calculating the number of calices and mass of coral carbonate skeletons per cm³ subsample.  An age model from previous ¹⁴C and U/Th ages of coral fragments in the core yielded a long-term average coral carbonate accretion rate of 78 g CaCO₃ m^-2 y^-1 over the past ~2150 y, divided into two phases: 40.8 g CaCO₃ m^-2 y^-1 (from core-bottom to -59 cm, approximately 685 ybp), and 156.2 g CaCO₃ m^-2 y^-1 in the upper half of the core. Coarse coral-dominated gravel in the core-catcher contained coral fragments approximately 7 ka in age, indicating a long hiatus before the renewal of coral growth at this site.  Aragonite % in the fine sediment was not correlated with coral abundance in the core.  A second core recovered nearby was composed of siliciclastic silty sand, but contained almost no coral fragments.  The large variation between the two cores indicates high levels of local heterogeneity in sediment accumulation patterns, apparently much greater than the variation in live coral cover at the surface.

Coral carbonate accumulation rates in Guilvinec Canyon were 1-2 orders of magnitude lower than coral carbonate accumulation estimates from Lophelia pertusa reefs on the Norwegian Margin. Nonetheless, coral carbonate accumulation rates in the core were an order of magnitude higher than Recent coral carbonate production rates based on ROV-video estimates of coral biomass and published growth rates.  This difference may be attributable to time-averaging, local heterogeneity, changes in sedimentation or current regime, or to late-20^th century decline in coral abundance resulting from a variety of anthropogenic pressures.  Recent threats to Bay of Biscay cold-water coral reefs include increased sediment mobilization from bottom trawling near the heads of submarine canyons, rising seawater temperatures, and declining aragonite saturation.  New field and lab experiments on coral growth rates and skeletal degradation may help to disentangle anthropogenic pressures and project the fate of Bay of Biscay cold-water coral reefs in the face of climate change and ocean acidification.