Temperate carbonate cycling and water mass properties from intertidal to bathyal depths (Azores)

[摘要] The rugged submarine topography of the Azores supports a diverse heterozoanassociation resulting in intense biotically-controlled carbonate-productionand accumulation. In order to characterise this cold-water (C) factory a2-year experiment was carried out in the southern Faial Channel to study the biodiversity of hardgroundcommunities and for budgeting carbonate production and degradation along abathymetrical transect from the intertidal to bathyal 500 m depth.

Seasonal temperatures peak in September (above a thermocline) and bottom inMarch (stratification diminishes) with a decrease in amplitude and absolutevalues with depth, and tidal-driven short-term fluctuations. Measuredseawater stable isotope ratios and levels of dissolved nutrients decreasewith depth, as do the calcium carbonate saturation states. Thephotosynthetic active radiation shows a base of the euphotic zone in ~70 m and a dysphotic limit in ~150 m depth.

Bioerosion, being primarily a function of light availability forphototrophic endoliths and grazers feeding upon them, is ~10 timesstronger on the illuminated upside versus the shaded underside of substratesin the photic zone, with maximum rates in the intertidal (−631 g/m²/yr). Rates rapidly decline towards deeper waters where bioerosionand carbonate accretion are slow and epibenthic/endolithic communities takeyears to mature. Accretion rates are highest in the lower euphotic zone(955 g/m²/yr), where the substrate is less prone to hydrodynamic force.Highest rates are found – inversely to bioerosion – on down-facingsubstrates, suggesting that bioerosion may be a key factor governing thepreferential settlement and growth of calcareous epilithobionts ondown-facing substrates.

In context of a latitudinal gradient, the Azores carbonate cycling ratesplot between known values from the cold-temperate Swedish Kosterfjord andthe tropical Bahamas, with a total range of two orders in magnitude.Carbonate budget calculations for the bathymetrical transect yield a mean266.9 kg of epilithic carbonate production, −54.6 kg of bioerosion, and212.3 kg of annual net carbonate production per metre of coastline in theAzores C factory.

[学科分类] 地球化学与岩石