Abstract:
Ocean Drilling Program Site 1119 is located east of South Island, New Zealand, in 394 m water depth on the upper continental slope. Though 93 km offshore today, during recent glaciations the site lay near the lowstand shoreline, directly beneath the path of seasonal riverine meltwater plumes. The core comprises an upper portion (0-86.19 metres composite depth (mcd)) deposited as upper slope clinoforms and a lower portion (to 513.5 mcd) deposited as mid-slope, intermediate water depth sediment drifts, the two intervals being separated across a short c. 25 k.y. long unconformity within Marine Isotope Stage (MIS) 8. Almost throughout, the core comprises alternating micaceous muds and silts (glacials) and muddy, sometimes calcareous, sands (interglacials). Because of the enhanced potassium content of the terrigenous muds, the lithologies that characterise the two different climatic states possess markedly different natural gamma radiation signals, high and low, respectively. The natural gamma profile of Site 1119 therefore provides a high quality climatic time series with a 1-2 k.y. resolution (as sampled) back to the mid-Pliocene. The cyclic gamma ray pattern provides a proxy measure of ice volume in the Southern Alps, and is therefore an atmospheric record. This record matches closely that of oceanic oxygen isotope curves back to MIS 100 (2.4 Ma) and less closely beyond to MIS Gi-11 (3.91 Ma) at the base of the core. Marked, high gamma ray intervals at 3.68-3.63, 3.38, 3.12, and 2.80-2.67 Ma may reflect sharp mid-latitude atmospheric coolings at these times, as supported by marine faunal and isotopic evidence elsewhere in the New Zealand region. Alternatively, they may in part reflect changes in clay provenance consequent upon tectonic uplift in the hinterland. The natural gamma measurements are consistent with an overall decline of 6 degrees C in average atmospheric temperature over South Island, New Zealand, since 2.45 Ma. Milankovitch 41 k.y. cyclicity is also prominent in the natural gamma record over this period, and its 60 API unit magnitude implies temperature swings up to 12 degrees C between glacials and interglacials (G/I). Similarly large G/I temperature changes occur also in nearby sea surface temperatures east of South Island and in atmospheric temperatures in Antarctica (Vostok), suggesting that Pleistocene climate signals were closely synchronised across wide areas of Southern Hemisphere middle and high latitudes. Together with changing tectonic and related palaeogeographic patterns, these temperature changes must have played a significant role in influencing the local time ranges of taxa used in biostratigraphy. Very few, if any, traditional New Zealand Pliocene-Pleistocene index fossils have synchronous ranges across the region, and other methods of correlation (magnetostratigraphy, cyclostratigraphy, tephrochronology, isotope stratigraphy, and numeric dating) have therefore come to be of particular importance. The Site 1119 natural gamma record provides a unique reference template for climate variation in New Zealand mid-latitudes, with which are compared the available correlation markers for both onshore and offshore Pliocene-Pleistocene strata in the New Zealand region.