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 porewater profiles and fluxes of oxygen (O₂), 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 O₂. Oxygen penetration depth in coarse sands decreased from ~ 1.2 cm during the day 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.