RADIOCARBON STUDIES OF ORGANIC CARBON IN ANTARCTIC ICE-SHELF SEDIMENTS

Accurate radiocarbon ages for Antarctic marine sediments, particularly those beneath ice shelves, are essential to understanding the dynamics of the Antarctic Ice Sheet and its relations to global climate and sea level. In most cases these sediments lack calcium carbonate. Total organic carbon yields finite radiocarbon ages faithfully reflecting the mass-averaged age of all components present. To whatever extent these include transported materials that are older than the depositional horizon in which they are found, the age will be in error. Experience shows that this is often the case. To reduce the effects of this problem and, especially, to separate those samples in which it does occur from those in which it does not, we are releasing organic carbon from sediments using programmed-temperature techniques. Either combustion (sample exposed directly to O2) or pyrolysis (no O2, volatile products combusted downstream) is possible. The evolution of C is monitored. Temperature intervals are chosen to optimize separation of components with differing ages. In earlier examinations of Antarctic coastal sediments, this technique has yielded age corrections approaching 10,000 years less than bulk ages for a single sedimentary horizon. The system now in use incorporates a nondispersive infrared spectrophotometric system to quantify CO2 and thus provides a superior record of the evolution of C from each sample. We describe new results from the Larsen and Ross Ice Shelves. Sediments from the Ross Sea are analyzed in order to resolve two prevalent issues. The first concerns obtaining ages for sub-ice-shelf sediments that directly overlie glacial-till horizons and underlie open-marine sediments. The second pertains to establising a chronology using a set of finite radiocarbon ages derived from the till horizon itself. These ages provide no stratigraphic order within the till (uniform ages with depth) but are well within the resolution capabilities of AMS dating (i.e. 26 to 32 ka). We compare these results to similarly processed ages from Larsen Ice Shelf sediments (NW Weddell Sea), where duplicate calcium carbonate ages are available from foraminifera. These results significantly improve the accuracy of radiocarbon ages derived from Antarctic Ice Shelf sediments and other facies that are plagued by pervasive reworking of detrital organic matter.