Antarctic glaciation recorded in Early Miocene New Zealand foraminifera

New Zealand sedimentary sequences are important repositories of southern temperate palaeoenvironmental data, as may be interpreted from biogenic chemical signals preserved in marine microfossils. Calibration curves for Mg/Ca ratios versus water temperatures were established using modern benthic foraminifera, Notorotalia and Cibicides. Notorotalia is a long-ranged endemic benthic genus with a good record in shelf sediments, while Cibicides allows comparisons with similar studies elsewhere. The resulting correlations were T (°C) = ln(Mg/Ca [mmol/mol] / 1.64) × 10.89 for Cibicides spp., and T (°C) = ln(Mg/Ca [mmol/mol] / 0.44) × 5.71 for Notorotalia spp. Well-preserved Early Miocene Notorotalia and Cibicides were collected for paired Mg/Ca and δ18O analysis from a 3.6 m section of the Mount Harris Formation spanning an estimated 60 ka and dating from about 17.7 Ma (Globoconella zealandica zone, roughly middle Burdigalian, Early Miocene) within the local Altonian Stage (15.9–18.7 Ma). Mg/Ca bottom-water palaeotemperature estimates from Cibicides and Notorotalia gave concordant results: 13.3 ± 1.0 °C for Notorotalia spinosa, 15.5 ± 3.0 °C for Cibicides spp. Estimates of oxygen isotopic composition for Altonian sea water (δ18Opalaeo-sw) were − 0.4 ± 0.4‰, suggesting the presence of small ice sheets on Antarctica. The method used to generate such results has far reaching implications for reconstructing δ18Opalaeo-sw, and should allow Antarctic ice volume history to be finely resolved from New Zealand sequences.

► We measured δ18O and Mg/Ca from Early Miocene benthic foraminifera (17.7 Ma).
► Bottom water palaeotemperature was approximately 14 °C at palaeolatitude 43.3°S.
► Palaeo-seawater δ18O averaged − 0.4 ± 0.4‰ (VSMOW) throughout the studied sequence.
► δ18O and Mg/Ca in New Zealand strata reveal changes in Antarctic ice volume.