Latitudinal Contrasts in the Net Preservation of Skeletal Carbonate

Skeletal carbonate is our primary source of paleo-biological information, and the primary means by which carbon is transferred from the atmosphere-ocean to the lithosphere. As such, processes which control its preservation and fidelity (i.e. taphonomy) are of primary importance to interpreting the fossil record and the carbon cycle.

In order to test for megabiases among depositional systems, shell degradation experiments were deployed in 27 marine and freshwater study sites (0-150m water depth) in 8 geographic areas. These included both carbonate and siliciclastic environments and tropical, temperate, and polar latitudinal zones. Experimental arrays used fresh Mytilus edulis and Mercenaria mercenaria valves (8-16/treatment), deploying them above, at, and below the sediment/water interface for periods ranging from 8 days to over 5 years. Data on net weight change is presented here for ~2000 valves.

Average net weight changes range across 2 orders of magnitude among sites and treatments, with maximum gain in tropical exposed shells (91% in one year) and maximum loss in freshwater exposed shells (-46% in one year). The range is significantly dampened by burial in all environments. The rates of weight change slow after 1 year in most environments. Skeletal weight changes result from, in decreasing order, encrustation, breakage, bioerosion, dissolution, and authigenic mineral precipitation. Weight gain, due to encrustation and authigenic mineral precipitation, is more significant in the tropics, while shell weight loss, due to breakage, bioerosion, and dissolution, is more significant in temperate and polar latitudes. Net microstructural shell loss shows decreasing rank order across environments: temperate freshwater > > polar marine >? temperate marine ≈ tropical marine sea-grass > tropical carbonate marine > tropical siliciclastic marine. Significant differences across latitude and depositional environment indicate potential for significant bias in the preservation of skeletal carbonate.