The Origin of Carbonate Mud

Carbonate mudstones are key geochemical archives for past seawater chemistry, yet the origin of carbonate mud remains a subject of continued debate and uncertainty. Prevailing hypotheses have settled on two mechanisms: (1) direct precipitation in the water column and (2) postmortem dispersal of mud-sized algal skeletal components. However, both mechanisms conflict with geochemical observations in modern systems and are problematic in deep time. We tested the hypothesis that abrasion of carbonate sand during sediment transport might produce carbonate mud using laboratory experiments and a sediment transport model. We documented experimental mud production rates up to two orders f magnitude faster than rates estimated for other mechanisms. Combined with model calculations, these results illustrated that transport and abrasion of carbonate sand is a major source of carbonate mud. Plain Language Summary Carbonate mudstones are widely used as archives of ancient seawater chemistry, under the assumption that the compositions of mud-sized (<62.5m in diameter) carbonate particles that make up these mudstones provide reliable records of seawater at the time the particles were formed and deposited. This assumption relies on understanding how carbonate mud forms-current ideas center on direct mineral precipitation from seawater and the disintegration of algae mineral skeletons-but these mechanisms conflict with some geochemical observations in modern systems. We used experiments to demonstrate that when carbonate sand grains are transported by currents, collisions cause mud-sized carbonate particles to be released from grain surfaces via abrasion. The rapid rates of carbonate mud production observed in our experiments suggest that abrasion has been a significant source of carbonate mud throughout Earth history, which is important for interpreting geochemical records from carbonate mudstones because the material abraded from sand grains may not be instantaneous records of seawater chemistry.