Patterns and cyclicity of Quaternary sedimentation above a subducting seamount at Rock Garden (Central Hikurangi margin, New Zealand)

Abstract

Rock Garden (RG), located east off the North Island of New Zealand, is part of an accretionary ridge that is influenced by seamount subduction. Two ∼37 m long sediment cores, drilled with the seafloor drill rig (MeBo200) from RG, provide a continuous sedimentary record of the period between 1.95 and 0.4 Ma. This period, the Early-Middle Pleistocene transition (EMPT), was marked by a progressive increase in the amplitude of climate oscillations, and a shift of Milanković cycles from 41 ka towards a quasi-100 ka frequency in the absence of any significant change in orbital forcing. From the recovered core material of cores GeoB 20824–4 and GeoB 20846–1, we determined sediment physical properties, oxygen isotope (δ18O) values, and element concentration based on X-ray fluorescence (XRF) measurements. The element ratios were used as proxies for sediment composition and as paleo-environmental indices. In sediment physical properties, δ18O values, and geochemical properties, evidence for glacial and interglacial cycles and spectral periods of 405, 100, 41 ka were found. A shift of the spectral period from 41 to 100 ka took place in sediment cores during (1.4 to 0.4 Ma). Numerical ages obtained from tephra layers included in the sedimentary record enabled to estimate sedimentation rates from both cores. Although both drill sites are only 1800 m away from each other, sedimentation rates of 2.15 to 2.96 cm ka−1 (GeoB 20824–4) and 5.49 to 6.77 cm ka−1 (GeoB 20846–1), respectively, differ by a factor of two. This may be the reason why two facies-units were identified in core GeoB 20824–4, whereas sediments of core GeoB 20846–1 all belong to the same facies. A change of lithofacies in core GeoB 20824–4 between Unit I and Unit II in ∼20 mbsf at 1.5 to 1.4 Ma marks the initiation of the EMPT.