In situ Mg isotope measurements of biogenic carbonates using laser ablation multi-collector inductively coupled plasma mass spectrometry: A new tool to understand biomineralisation.

RATIONALE: Magnesium is one of the most abundant elements in the earth's crust and in seawater. Fractionation of its stable isotopes has been shown to be a useful indicator of many geological, chemical, and biological processes. For example, biogenic carbonates display an ~5‰ range of δ26 Mg values, which is attributed to variable degrees of biological control on Mg ions during biomineralisation. Understanding this biological control is essential for developing proxies based on biogenic carbonates. METHODS: In this work, we present a new approach of measuring Mg isotopes in biogenic carbonates using Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS). RESULTS: Our results show that this microanalytical approach provides relatively fast, high spatial resolution (<0.2 µm) measurements with high precision and accuracy down to 0.2‰ (2SE). To achieve high levels of precision and accuracy, baseline interferences need to be monitored and a carbonate standard with a relatively low trace metal composition similar to biogenic carbonates should be used. We also demonstrate that the matrix effect on Mg isotopes in carbonates with low Fe and Mn is limited to less than 0.2‰ fractionation under different laser parameters and low oxide condition (<0.3% ThO/Th). CONCLUSIONS: Our newly developed LA-MC-ICPMS method and its applications to biogenic carbonates show significant advantages provided by the microanalytical approach in understanding complex processes of biomineralisation in marine calcifiers.

Impact of European settlement and land use changes on Great Barrier Reef river catchments reconstructed from long-term coral Ba/Ca records.

The increase in sediment and nutrient loads entering the coastal waters of the Great Barrier Reef (GBR) and the associated degradation of water quality represents a major threat to coral reefs. Although the strengthening of preventative management strategies remains a priority, there is a general lack of terrestrial runoff baseline information with respect to the spatial and temporal severity of disturbances associated with ongoing European-style land use practices. Here we use new and existing high-resolution coral Ba/Ca and luminescence records from the central Cairns region to the southern GBR shelf to reconstruct sediment fluxes discharged into the GBR from before European settlement in the 1860s to the present-day. Since the commencement of European settlement in the 1860s we document a tripling of flood-plume suspended sediment loads delivered by the Burdekin River to the GBR lagoon relative to 'natural' pre-European baseline levels. We show that this is indicative of a much more extreme degradation of the river catchments than hitherto appreciated with intensified discharge events particularly from the central and southern catchments carrying higher sediment loads. More-over from the 1930s onwards the Burdekin River, the largest source of both sediment and freshwater to the GBR, has also exhibited a progressive northwards expansion of its flood plume. This, together with increased variability of freshwater inputs indicated by coral luminescence records, now shows that the inner GBR not only continues to be impacted by increasing sediment/nutrient loads but is also subject to higher intensity river discharge events due to the loss of ground cover causing increased overland runoff and erosion.

Coral calcification in a changing World and the interactive dynamics of pH and DIC upregulation.

Coral calcification is dependent on the mutualistic partnership between endosymbiotic zooxanthellae and the coral host. Here, using newly developed geochemical proxies (δ11B and B/Ca), we show that Porites corals from natural reef environments exhibit a close (r2 ∼0.9) antithetic relationship between dissolved inorganic carbon (DIC) and pH of the corals' calcifying fluid (cf). The highest DICcf (∼ × 3.2 seawater) is found during summer, consistent with thermal/light enhancement of metabolically (zooxanthellae) derived carbon, while the highest pHcf (∼8.5) occurs in winter during periods of low DICcf (∼ × 2 seawater). These opposing changes in DICcf and pHcf are shown to maintain oversaturated but stable levels of carbonate saturation (Ωcf ∼ × 5 seawater), the key parameter controlling coral calcification. These findings are in marked contrast to artificial experiments and show that pHcf upregulation occurs largely independent of changes in seawater carbonate chemistry, and hence ocean acidification, but is highly vulnerable to thermally induced stress from global warming.