Ecological, Oceanographic and Temperature Controls on the Incorporation of Trace Elements into Globigerina Bulloides and Globoconella Inflata in the Southwest Pacific Ocean

Abstract

Trace element ratios (Mg/Ca, Al/Ca, Mn/Ca, Zn/Ca, Sr/Ca, Ba/Ca) measured by laser ablation inductively coupled plasma mass spectrometry plus and test weight and size data are presented for two planktonic foraminiferal species, Globigerina bulloides and Globoconella inflata. These data will be used to investigate the potential of Mg/Ca ocean thermometry and other trace element proxies of past ocean chemistry using these species. Foraminifera were sampled from core-top sediments from 10 sites in the Southwest Pacific Ocean, east of New Zealand, spanning latitudes of c.33' to 54' S and temperatures of 6-19' C at 75-300 m water depth. Mg/Ca in G. bulloides correlates strongly with observed water temperatures at 200 m depth and yields a new calibration of Mg/Ca = 0.941 exp 0.0693*T (r2 = 0.95). When G. bulloides Mg/Ca data from this study are combined with previously published data for this species, a calibration of Mg/Ca = 0.998 exp 0.066*T (r2 = 0.97) is defined. Significant variability of Mg/Ca values (20-30%) was found for the four largest chambers of G. bulloides with the final chamber consistently recording the lowest Mg/Ca values. This is interpreted to reflect changes in the depth habitat towards the end of the life cycle of G. bulloides. Levels of A1 and the micronutrients Mn and Zn in G. bulloides were found to differ significantly between Subtropical and Subantarctic Water masses, suggesting these elements can potentially be used as water mass tracers. No clear relationship between Mg/Ca and temperature was observed for G.inflata. This is interpreted, in part, to reflect the ecological niche that G. inflata occupies at the base of the thermocline, coupled with the impact of heavy secondary calcite which lowers Mg/Ca values. A correlation between size normalized test weight, water temperature and seawater carbonate ion concentration is observed for G. bulloides suggesting a modern calibration that could be potentially applied for paleoceanographic reconstructions of ocean water temperature and carbonate ion concentrations. No correlation between temperature or carbonate ion was found with size normalized G. inflata test weights. However, a bimodal population of G. inflata test weights indicates a possible link between high levels of chlorophyll-a in surface waters and light G. inflata tests. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and solution-based techniques for measuring Mg/Ca in G. bulloides yield compatible results. However, this is possible only when minimal dissolution of test calcite has occurred during the reductive and dilute acid leaching stages of cleaning prior to solution analysis, or, if only the older three visible chambers are used for LA-ICP-MS analysis. LA-ICP-MS analysis is an effective method for measuring trace element/Ca values in foraminifera, especially for small sample sizes, and enables the test to be used for further geochemical analysis (e.g. boron or carbon/oxygen stable isotope analysis).