The Geochemistry of Pāua as a Potential Proxy for Past and Present Environmental Change

Nearshore New Zealand mollusca (shellfish) have the potential to be important archives of environmental conditions and change. Ambient ocean chemistry can be incorporated into the calcium carbonate (CaCO3) shell during the life span of the mollusc providing a high resolution temporal record of the chemical and physical changes of the environments the mollusc lived in. Previous studies on foraminifera and coral have shown that the substitution of magnesium or strontium for calcium (Mg, Sr/Ca) during the formation of the CaCO3 shell is directly correlated with ocean temperatures. Other divalent cations (e.g., Sr2+, Ba2+, Pb2+) can also provide information on ambient salinity, primary productivity or nutrient levels, and local anthropogenic pollution. This study uses new geochemical techniques that have been developed to measure the trace element chemistry of CaCO3 mollusc shells at high temporal resolution, using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in order to calibrate shell chemistry with environmental conditions. This study is the first to explore the use of the geochemistry of Haliotis iris as a potential proxy for (paleo-) environmental conditions. Pāua (Haliotis iris) were collected from six different localities around New Zealand and the Chatham Islands as well as a cultured environment (OceaNZ Blue Ltd). The shells were sectioned following the axis of maximum growth exposing both CaCO3 layers; the prismatic (predominantly calcite) and nacreous (aragonite) layers. The shells were analysed by LA-ICP-MS at 25 μm spot sizes through a high temporal transect of both layers. Observed differences in the element/Ca ratios between the prismatic and nacreous layer reflect the differing crystallinity of each layer. High temporal resolution Mg/Ca ratio data of the prismatic layer of the samples which grew in a cultured environment were compared with temperature and growth data supplied by OceaNZ Blue Ltd. The results showed that temperature was not the primary control on the uptake of Mg within the shells and that influences from biological factors including increased growth rate were also evident. Sr/Ca ratios show a weak inverse relationship with increased growth rate assumed. These results, however, are not reproducible within samples collected from the wild, showing that external factors (high wave energy, diet, predation, lack of food) place metabolic stress on the pāua. The monitoring of other element/Ca including Ba/Ca, Al/Ca, Pb/Ca and Zn/Ca ratios have the potential to provide information into the past frequency of storm events that deliver sediment into the oceans and remobilise other sediments and changing levels of environmental pollution. This is reflected through increased Al/Ca, Pb/Ca and Zn/Ca ratios during the winter season in a number of samples (n = 3) gained from the high resolution analysis of the prismatic layers. Overall, element/Ca ratios are difficult to correlate environmental conditions in samples from the wild as there are many different parameters influencing the uptake of element/Ca ratios with the shells of pāua. Uncertainties lie with a lack of understanding of the biological controls influencing pāua during biomineralisation including the transportation of the elements within organism to the extrapallial fluid to be biomineralised, ontogeny, and the rate and regularity of biomineralisation of shell material.