Carbonate Dissolution in Nearshore Terrigenous Muds: The Role of Physical and Biological Reworking

Because shallow marine sediments generally underlie waters which are saturated with respect to arago-nite and calcite, physical-chemical dissolution of carbonate in these environments is sometimes discounted. The present investigation, as well as others before it, show such an assumption to be unwarranted. In this case, study of terrigenous mud deposits from Long Island Sound, U.S.A. demonstrates extensive dissolution of molluscan shell debris near the sediment-water interface. Net buildup of carbonate varies significantly between different regions of the Sound despite fairly uniform production of shell debris and similar total sedimentation rates. Aside from this apparent discrepancy between rate of supply and net carbonate sedimentation, evidence for carbonate dissolution comes from the pitting of shells, elevated$Ca^{++}$levels in pore water, and calculated undersaturation of pore waters with respect to carbonate minerals. Models of pore water distributions and carbonate supply imply dissolution rates of at least several mg$CaCO_{3}/cm^{2}/yr$. Dissolution is most extensive in highly bioturbated areas where alkalinity buildup during sulfate reduction is prevented by both burrow irrigation and the oxidation of solid phase sulfides during particle reworking. Irrigation also increases carbonic acid production by providing additional oxygen for aerobic respiration. Maximum shell preservation occurs in regions where physical disturbances cause poorly developed bottom communities, while only minimal preservation occurs where benthic communities are best developed and biogenic reworking is greatest.