Extended vegetation histories from ultramafic karst depressions

Abstract

Solutional landforms (karst) can form on old surfaces on ultramafic rocks in the tropics because of the solubility of some magnesium-rich minerals under warmth and high CO2. The radiocarbon dating of organic pond deposits in several tropical ultramafic karst hollows demonstrates that very slow sediment accumulation has occurred, relative to other tropical shallow lakes. Some sites have gaps in their records, whereas others appear continuous. Sections of organic lake muds from the Indonesian sites Wanda and Hordorli provide sequences of ages from modern back to >35 000 years ago at depths of 3–4 m. In New Caledonia, no Holocene record has been obtained, and dates of 17 000–30 000 years ago are found near the top of deep organic layers that, in some cases, are buried by inorganic muds derived from an erosion event. These ages, pollen analyses and the increasingly compressed organic sediments with depth mean that deeper levels should be well beyond radiocarbon dating limits. Only at one New Caledonian lake were deeper sediments beyond detectable 14C measurement. Other sites returned finite dates at all levels tested, suggesting that some mechanism is moving small amounts of younger organics down profile. The slow sediment-accumulation rates provide an explanation why high concentrations of pollen relative to tropical peats and limnic sediments derived from high nutrition substrates are preserved in the sediments. This makes them attractive targets for studying the palaeoecology and forest stability of the surrounding vegetation. The sites are sensitive to disturbance because the poor nutrition impedes successional recovery after disturbances such as fire and landslips.