Carbonate sediments in a cool-water macroalgal environment, Kaikoura, New Zealand

Brown and red, and to a lesser extent green, macroalgae are a hallmark of intertidal rocky coasts and adjacent shallow marine environments swept by stormy seas in middle and high latitudes. Such environments produce carbonate sediment but the sediment factory is neither well-documented nor well-understood. This study documents the general marine biology and sedimentology of rocky coastal substrates around Kaikoura Peninsula, a setting that typifies many similar cold-temperate environments with turbid waters and somewhat elevated trophic resources along the eastern coast of South Island, New Zealand. The macroalgal community extends down to 20 m and generally comprises a phaeophyte canopy beneath which is a prolific rhodophyte community and numerous sessile calcareous invertebrates on rocky substrates. The modern biota is strongly depth zoned and controlled by bottom morphology, variable light penetration, hydrodynamic energy and substrate. Most calcareous organisms live on the lithic substrates beneath macroalgae or on algal holdfasts with only a few growing on macroalgal fronds. A live biota of coralline red algae [geniculate, encrusting and nodular (rhodoliths)], bryozoans, barnacles and molluscs (gastropods and epifaunal bivalves), together with spirorbid and serpulid worms, small benthonic foraminifera and echinoids produce sediments that are mixed with terrigenous clastic particles in this overall siliciclastic depositional system. The resultant sediments within macroalgal rocky substrates at Kaikoura contain bioclasts typified by molluscs, corallines and rhodoliths, barnacles and other calcareous invertebrates. In the geological record, however, the occurrence of macroalgal produced sediments is restricted to unconformity-related early transgressive systems tract stratigraphic intervals and temporally constrained to a Cenozoic age owing to the timing of the evolution of large brown macroalgae.