The influence of pore-water advection, benthic photosynthesis, and respiration on calcium carbonate dynamics in reef sands

To investigate diel calcium carbonate (CaCO3) dynamics in permeable coral reef sands, we measured pore-water profiles and fluxes of oxygen (O-2), nutrients, pH, calcium (Ca2+), and alkalinity (TA) across the sediment-water interface in sands of different permeability at Heron Reef, Australia. Background flushing rates were high, most likely as a result of infaunal burrow irrigation, but flux chamber stirring enhanced pore-water exchange. Light and pore-water advection fueled high rates of benthic primary production and calcification in sunlit surface sediments. In the light, benthic photosynthesis and calcification induced surface minima in Ca2+ and TA and peaks in pH and O-2. Oxygen penetration depth in coarse sands decreased from similar to 1.2 cm during the day to similar to 0.6 cm at night. Total oxygen uptake (TOU) in dark chambers was three to fourteen times greater than diffusive uptake and showed a direct effect of pore-water advection. Greater sediment oxygen consumption rates were observed in higher permeability sands. In the dark, TA release was not stimulated by increasing TOU because of a damping effect of pore-water advection on metabolic CaCO3 dissolution efficiency. On a daily basis, CaCO3 undergoes net dissolution in Heron Reef sands. However, pore-water advection can reverse the CaCO3 budget and promote CaCO3 preservation under the most energetic conditions.