Issues

COVER EXPLANATION.—Ripples on the inner shelf of Santa Maria island, Azores. The sandy bed here comprises a mixture of carbonate and terrigenous particles. Mitchell et al. (this issue) postulate that, if the terrigenous component is conserved (e.g., it weathers only very slowly), it can act as a tracer, which is progressively diluted away from the island's coasts by carbonate particles. That dilution can be represented by a simple equation based on conservation of mass. Using it, Mitchell et al. find that the rate of production of carbonate particles, mainly from in situ growth of mollusks, increases towards the mid-shelf of the island, where mollusks are less frequently strongly disturbed by water movements.