The carbon cycle and biogeochemical dynamics in lake sediments

The concentrations of organic carbon (OC) and CaCO3 in lake sediments are o ften inversely related. This relation occurs in surface sediments from diff erent locations in the same lake, surface sediments from different lakes, a nd with depth in Holocene sediments. Where data on accumulation rates are a vailable, the relation holds for organic carbon and CaCO3 accumulation rate s as well. An increase of several percent OC is accompanied by a decrease o f several tens of percent CaCO3 indicating that the inverse relation is not due to simple dilution of one component by another. It appears from core d ata that once the OC concentration in the sediments becomes greater than ab out 12%, the CO2 produced by decomposition of that OC and production of org anic acids lowers the pH of anoxic pore waters enough to dissolve any CaCO3 that reaches the sediment-water interface. In a lake with a seasonally ano xic hypolimnion, processes in the water column also can produce an inverse relation between OC and CaCO3 over time. If productivity of the lake increa ses, the rain rate of OC from the epilimnion increases. Biogenic removal of CO2 and accompanying increase in pH also may increase the production of Ca CO3. However, the decomposition of organic matter in the hypolimnion will d ecrease the pH of the hypolimnion causing greater dissolution of CaCO3 and therefore a decrease in the rain rate of CaCO3 to the sediment-water interf ace.