Abstract:
Li, B, Mg, Al, Mn, Cu, Zn, As, Sr, Ba and U/Ca ratios were measured by laser ablation inductively coupled plasma mass spectrometry for 11 modern Austrovenus stutchburyi clams to assess the potential of this molluscan species as a proxy for paleo-ocean temperature and environmental change. A. stutchburyi is an intertidal, infaunal, bivalve, widespread in New Zealand coastal regions and throughout the Quaternary-Pliocene sedimentary rock record. Five individuals from Ligar Bay and Estuary (South Island, New Zealand) were analysed to evaluate the variability between individuals calcifying in similar environmental conditions. A further six individuals were sampled from a range of latitudes (38˚ to 40˚) in the North Island, New Zealand to evaluate variability between individuals from different environments.
A strong positive correlation between growth rate and Mg, Al, Mn, Sr, Ba and U/Ca ratios was observed, and a marked negative correlation was found between the same trace element/Ca ratios and ontogenetic age as growth rates slow during the molluscs' life. Thus, biological effects are the primary influence on trace element incorporation in A. stutchburyi. No clear seasonal variations were observed in the Mg and Sr/Ca ratio profiles through A. stutchburyi shells representing time periods of several years. Furthermore, for two shells for which chronologies could be reliably constructed, there were no significant correlations between Mg and Sr/Ca ratios and sea surface temperature. When Mg/Ca ratios were normalised to Sr/Ca ratios in order to eliminate the growth rate effect on trace element incorporation into the mollusc shells, some of the remaining variations appeared to visually correlate positively with sea surface temperature in several sections of a shell. However, a quantitative correlation did not confirm this (r² = 0.012). It is likely that neither Mg nor Sr incorporation into A. stutchburyi shell are primarily thermodynamically controlled.
Several coincident Ba/Ca peaks in two of the Ligar Bay shells are most likely caused by environmental processes such as short periods of phytoplankton blooms or elevated seawater Ba/Ca from river flooding. Mn/Ca and U/Ca variations in A. stutchburyi from different coastal sites with different sediment characteristics appeared to be linked to the redox conditions prevailing at an open ocean sand-dominated environment (Ligar Bay) versus tidal mud flat environments (e.g. Miranda). Thus, while A. stutchburyi is unlikely to be a useful archive for past coastal ocean temperatures, it holds considerable promise for tracking past changes in coastal ocean productivity and river run-off, as well as sediment redox conditions.
