GEOCHEMICAL CLASSIFICATION AND OCCURRENCE OF DRIFT PUMICE IN NEW CALEDONIA

In 2008 and 2010, drift pumice was collected from 42 beaches around the main island of New Caledonia in the S. Pacific. This island is surrounded by a semi-enclosed barrier reef lagoon whose recent designation as a UNESCO World Heritage Site has led to an increased interest in quantifying the lagoon's ongoing health status. Unfortunately the majority of the lagoon is poorly documented in the literature, and the situation is complicated by a long history of strip mining. Mining waste (a fine, unconsolidated sediment) presents a widespread threat to the lagoon. Its effects can be seen in the large volumes of sediment clogging bays and rivers. In addition, high sedimentation rates are known to be harmful to barrier reef corals. This study will attempt to identify the source of the pumice as well as determine the method of distribution.

A total of 34 samples from 25 beaches were analyzed for geochemical composition. This low-K rhyolitic pumice is consistent with intermediate to felsic lavas from Tonga. Trace element data confirms a correlation with basaltic lavas from the Lau Basin, possibly as a continuing fractionation series.

Exotic drift pumice has potential as a visual marker of sediment transport in and around the New Caledonian lagoon since it is somewhat durable, highly transportable in water, and easy to collect, transport, and analyze. Since the permanence of pumice in the depositional environment has not been fully discussed in the literature, this study will analyze and describe the effects of beach characteristics on pumice appearance and abundance. For example, there are loose correlations between sandy (gravelly) beaches and abundant (rare) pumice; and while pumice distribution is not related to N-S or E-W location around the island, there is some correlation to reef proximity. This information could help identify particularly at-risk coastal areas exhibiting elevated fluvial (i.e. mine waste) input in conjunction with reduced marine influence.