Ross Sea Sediment Chronologies Based on Compound-Class (sterol) and Pyrolysis Radiocarbon Dating

Times scales for sediments adjacent to Antarctic ice shelves have posed a continuing challenge. Seeking a new and cost-effective solution, we have determined radiocarbon ages of fractions of bulk organic material released by pyrolysis. These “pyrolysis ages” from core tops in the Ross Sea have been compared to compound-class ages for sterols in the same samples. When pyrolysis techniques yield distinct age plateaus (several consecutive temperature intervals with indistinguishable radiocarbon ages) at low temperatures, the value does not differ significantly (2σ) from the sterol age determinations. Sterols are probably a significant fraction of the youngest, core-top organic material, but their average radiocarbon content suggests that older sterols are also preserved within the homogenous layer of sediment. Thus, sterolic compounds are not only delivered by primary productivity in the overlying water column, but also from pre-aged sources such as adjacent sedimentation sites including crests, slopes, platforms and basins. In a number of samples, including those both from basins and crests, the oldest pyrolysis ages are two or more standard deviations older than sterol ages. Either thermochemical stability is correlated with resistance to in situ degradation or, in the Ross Sea, high productivity ensures that contributions of young components to the sterol pool are dominant. Age plateaus are not found at high temperatures. The ages of the oldest components of the organic mixtures in Ross Sea core tops can thus be estimated only by kinetic modeling. The pyrolysis dating technique offers additional chronologic information about the materials present in sediments, specifically illustrating the mobility of organic material between depositional sites in the Ross Sea.