WHAT CONTROLS ORGANIC MATTER PRESERVATION IN LAKES?

Organic matter preservation in lakes is much like anywhere else, it is facilitated by low residence times of carbon particles in zones of high chemical diffusivity. In other words, the faster a carbon particle is buried in a region where the rate of diffusion of other limiting substances (e.g., sulfate or oxygen) is low, the higher the probability of preservation on geological timescales. This simple relationship explains why lacustrine meromixis so strongly favors organic matter preservation. It is not that anaerobic organisms have intrinsically lower rates of organic matter consumption; it is that the exclusion of obligate aerobes leads to a lack of bioturbation, which in turn quickens the rate at which carbon particles move into zones where the rate of chemical diffusion is reduced by sediment compaction. The net effect is the same as increasing accumulation rate, which is another documented mechanism for enhanced organic preservation. This relationship explains the observed correlation between organic matter preservation and lake depth in moderately to highly productive lakes. As lake depth exceeds the depth of the upper wind-mixed layer, the lower water depths, where molecular rather than eddy diffusion prevails, become oxygen depleted, bioturbating organisms are excluded, and organic matter settles out on the bottom. As a given particle becomes more deeply buried the rate of diffusion of limiting substances becomes lower and the metabolic rate of the sediment community drops, although even upon lithification it still remains biologically active, albeit at very low levels. Thus even in shallow lakes with very high levels of primary production, organic matter preservation can remain very low as long as the accumulation rate remains low, because resuspension and bioturbation keep carbon particles in zones where biological activity is less limited. Because, water depth in any given lake is a function of precipitation over the lake and its watershed, organic matter preservation in lakes tends to be favored by humid rather than arid climates. Geological examples include the Triassic-Jurassic Newark Supergroup and the Eocene Green River Formation.