Rhodoliths as environmental archives in the tropics

Abstract

Rhodoliths are free-living forms of calcareous, coralline red algae that are found worldwide, from the tropics to the poles, in relatively shallow (0->250m) waters. They can live hundreds of years and continuously form a high-Mg calcite skeleton that presents periodical growth bands. Recent coralline red algae studies show that the variations of specific trace elements along these growth bands can reflect secular changes in various environmental parameters. This study aims at assessing and further extending the ability of rhodolith-forms of coralline red algae to reconstruct environmental changes. Modern rhodoliths of the species Sporolithon durum from the Ricaudy Reef in New Caledonia were chosen for this project. The use of laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) permitted to investigate the distribution pattern of Mg across single rhodolith branches. The combination of LA-ICPMS analyses with alizarin red S staining and radiocarbon dating enabled us to determine the seasonal character of the major Mg/Ca cycles recorded along rhodoliths branches, as well as characterise the seasonal pattern of 14 other trace elements over the year 2010-2011. A statistical approach was employed to separate these trace elements into three groups of similar behaviour that respond essentially to either environmental, biologic or anthropogenic influences. A 46-year record of Mg/Ca, Sr/Ca and Li/Ca variations obtained from multiple rhodolith branches display a high level of correlation with the local, instrumental seawater temperature dataset over the last five decades, at both monthly and interannual resolutions. Mg/Ca variations also record larger scale climate pattern such as EI Nino - Southern Oscillation (ENSO) variations, and compare well with the commonly used Sr/Ca-temperature proxy of corals. In addition, interannual-to-decadal variations in specific trace metals concentrations (Mn, Fe and Ni) reflect the intensity of former mining activities that occurred in the studied area from the 1960s to the late 1970s, making S. durum rhodoliths a potentially useful tool for the determination of anthropogenic pollution. Variations of the {u00F0}{u00B9}{u2078}0 composition in the skeleton of S. durum were also investigated using sensitive high-resolution ion microprobe (SHRIMP). After correction for the instrumental mass fractionation (IMF), the {u00F0}{u00B9}{u2078}0 average value is found to be consistent with conventional mass spectrometer measurements but shows a negative offset compared to equilibrium that is, however, consistent with previously-reported vital effects for coralline red algae. The range of {u00F0}{u00B9}{u2078}0 values recorded by the SHRIMP was too large to be explained solely by changes in environmental parameters. Consequently, we proposed that high-resolution SHRIMP measurements of {u00F0}{u00B9}{u2078}0 may alternatively be used to obtain critical insights into metabolic and/or calcification processes in coralline red algae. Show more