The Carbon Cycle and Biogeochemical Dynamics in Lake Sediments

The concentrations of organic carbon (OC) and CaCO 3 in lake sediments are often inversely related. This relation occurs in surface sediments from different locations in the same lake, surface sediments from different lakes, and with depth in Holocene sediments. Where data on accumulation rates are available, the relation holds for organic carbon and CaCO 3 accumulation rates as well. An increase of several percent OC is accompanied by a decrease of several tens of percent CaCO 3 indicating that the inverse relation is not due to simple dilution of one component by another. It appears from core data that once the OC concentration in the sediments becomes greater than about 12%, the CO 2 produced by decomposition of that OC and production of organic acids lowers the pH of anoxic pore waters enough to dissolve any CaCO 3 that reaches the sediment-water interface. In a lake with a seasonally anoxic hypolimnion, processes in the water column also can produce an inverse relation between OC and CaCO 3 over time. If productivity of the lake increases, the rain rate of OC from the epilimnion increases. Biogenic removal of CO 2 and accompanying increase in pH also may increase the production of CaCO 3 . However, the decomposition of organic matter in the hypolimnion will decrease the pH of the hypolimnion causing greater dissolution of CaCO 3 and therefore a decrease in the rain rate of CaCO 3 to the sediment-water interface.