Carbonate Cementation in Coralline Algal Nodules in the Skagerrak, North Sea: Biochemical Precipitation in Undersaturated Waters

North Sea sublittoral carbonate sediments in the Skagerrak are affected by submarine dissolution which attacks skeletal grains of aragonite, Mg-calcite, and low-Mg calcite, and coarse detrital calcite in limestone clasts. All post-mortem changes in skeletal carbonate grains are destructive and intragranular space in hollow grains and microborings remains empty of secondary carbonate after the disappearance of the cavity-forming organisms. As a consequence, the boring--filling micritization mechanism does not operate in these waters. Evidently, the sublittoral marine environment is undersaturated with calcium carbonate. Yet, in this environment, carbonate cement is forming in various kinds of openings within living coralline algal nodules (rhodoliths) consisting mostly of Lithothamnium . The internal cement is composed of aragonite needle cement, aragonite spherulitic cement, Mg-calcite fringe cement, Mg-calcite micrite, and Mg-calcite intraskeletal micro-druses, the latter exclusively in the algal skeleton proper. Cement growth is related to metabolic activities in the living carbonate-secreting red algae. After death of the algae, the rhodoliths and their cement undergo dissolution. Theoretically, precipitation may take place in several steps, involving accessory organisms in the rhodoliths, for example, unicellular green algae, or bacteria. However, all observations indicate that the primary cause for the cementation lies in the life processes of the frame-building algae. The thin films of encrusting algae are exceptionally well suited as instruments for a microenvironmental control. Few passages lead to the interior of rhodoliths, except through the sieve of algal filaments. Furthermore, the red algae may secrete as much as 30-40% of their total photosynthetic product in the form of extracellular metabolites. The data suggest that biochemical influence of frame-building algae may be an important cause for internal cementation of reefs and algal crusts in the marine environment.