Biodegradation of temperate-water skeletal carbonates by boring sponges on the Scott shelf, British Columbia, Canada

Abstract

Boring sponges are a major agent of shell destruction and, in the process, an important contributor of fine-grained carbonate detritus in temperate skeletal carbonate deposits on Scott shelf, northwestern Vancouver Island. Intensive boring is encouraged by the generally low rates of carbonate production and sedimentation. Skeletal grains become considerably weakened by sponge boring and break preferentially along the directions of sponge-excavated tunnels and galleries within the shell. Boring also facilitates shell dissolution by significantly increasing the surface area—weight ratio of shells and by breaking down the organic matrix of shells. The large aragonitic bivalves Glycymeris and Humilaria appear especially susceptible to sponge biodegradation and have become thoroughly bored and corroded within about 1000 years in ambient shelf waters; thus, their preservation potential is low. Selective loss of aragonitic biota from ancient cool-water shelf skeletal carbonate deposits is anticipated and is evident in many temperate-latitude occurrences of New Zealand Cenozoic limestones.