Tropical vegetational change in the late Pleistocene of New Caledonia

The south eastern plateau of New Caledonia (22°S) preserves remnants of a deep regolith of ultramafic soils which is extensively gullied or eroded to form large alluvial fans. Dated sediments from four closed basins on the Plaine des Lacs (ca. 220 m altitude) show that organic muds were accumulating prior to 30,000 yr B.P. Over the next 5000–14,000 years limonitic clays infilled two basins as a result of slope instability in their catchment. Resistant laterite and bog iron surfaces became exposed during the period 30,000–15,000 yr B.P. by erosional events that seem to have been of greater magnitude than any in the earlier Pleistocene. Two sites, Lake Emeric and Lake Suprin, were chosen for pollen analysis. During the phases of organic deposition of these lakes, Nothofagus forests collapsed several times and were replaced by Gymnostoma maquis, apparently as a result of fires. These Pleistocene fires were associated with some inorganic deposition, presumably due to the substantial loss of primary forest cover. Regeneration of forest usually followed after an estimated gap of several centuries. Thus, fire was able to cause forest retreats of the same order of magnitude as that caused by massive fires today. The low nutritional status of the soils or lateritic crusts means that, as now, forest had difficulty reclaiming sites. Human activity is unknown prior to 3000 years ago, so the major erosion is presumed to have had natural causes. Thus, many of the wide-ranging changes are the result of natural instability in the sub-tropics, and some may be much older than hitherto thought. Climate change towards periods of dry, fire-prone, conditions may have been an underlying cause. In the Holocene, some climatic factor, possibly cyclonic storms, seems to prevent organic sedimentation. Switching from cyclone-free to cyclone-prone at the end of the Pleistocene may reflect temperature change in sea surfaces in the region.