Vanadium in the massive coral Porites: A potential proxy for historical wood clearing and burning

Abstract

Inshore coral reefs are threatened globally by decreased water quality resulting from anthropogenic land clearing and other drivers. Understanding the timing and rate of past native vegetation clearing and forest firing is critical for improved catchment management to protect reefs and other coastal ecosystems. However, commonly used coral-based sediment proxies (e.g., Ba/Ca and Y/Ca) are controlled partly by variable precipitation and discharge rates. No current marine proxy provides a temporal record of rates and areas of land clearing itself. Here we present a high-resolution temporal record (1957–2010) of Vanadium/Calcium (V/Ca) ratios in a Porites coral colony from the southern Great Barrier Reef (GBR) that correlates uniformly with historical clearance rates of woody vegetation in the local catchment, despite discordant stream discharge. We found that the V/Ca record could not be explained by terrestrial flux, nutrient flux, hydrocarbon pollution or upwelling and propose that the reaction between hot alkaline ash and the upper soil layer under oxidizing conditions during clearing-associated burning may favour formation and desorption of soluble anionic VV species (e.g., HVO42−) from soil-based iron oxyhydroxides containing less mobile VIII. The newly soluble V components can then mobilise quickly into the earliest surface runoff and reach coastal seawater, where they provide a distinct temporal record in corals. That record is temporally decoupled from the sediment-bound elements, such as Y and Ba, which require greater degrees of erosion to mobilise, and thus better reflect particulate sediment flux. The Porites V/Ca record documented here captured the temporal effects of specific legislations aimed at regulating and conserving vegetation in the local catchment and may provide a means for evaluating catchment management practices elsewhere. The new proxy also may provide records of catchment clearing and firing in prehistoric times, with implications for both natural and anthropogenic fire regimes in coastal areas.