Constraints on the role of tectonic and climate on erosion revealed by two time series analysis of marine cores around New Zealand

Abstract:

Physical and chemical weathering govern rocks and chemical elements cycles at the Earth surface. The importance of climate and tectonics has been identified but their relative influence still needs to be investigated. We address this question by studying neodymium (Nd) isotopic composition time series in two sediment cores located on the eastern and western sides of New Zealand. The current and past climatic and oceanographic settings of this region have been already well studied, providing the underlying framework. Both detrital and authigenic phases of the sediment can be measured to decouple oceanic from continental contributions. The results show glacial-interglacial variations of the authigenic signal, with larger amplitudes for the eastern core. We propose that continental erosion on the South Island controls Nd isotopes records in both cores. Mixing calculations show that Nd discharge was 2 to 10 times higher during glacial times than during interglacials on the eastern side, whereas these variations are almost negligible on the western side. We suggest that higher physical erosion during glacial on the East, with sediments characterized by higher specific area that can be easily weathered, are due to larger ice volumes and fluxes during glacial periods. The steady rates on the western side imply that they are rather controlled by rock uplift rates. The temporal relationship between climatic indicators like oxygen isotopes and the Nd isotopic composition shows non linear response of the erosion-weathering to climate forcing. Those results contrast with previous studies using the same tracer, that showed stronger erosion and weathering over Himalaya during interglacial periods. This study illustrates the complexity of erosion and weathering responses to climatic and tectonic forcings.