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 ∼37m long sediment cores, drilled with the seafloor drill rig (MeBo200) from RG, provide a continuous sedimentary record of the period between 1.95-0.4 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 GeoB20824-4 and GeoB20846-1, we determined sediment physical properties, oxygen isotope (δ18O) values, and element concentrations based on X-ray fluorescence (XRF) measurements. The element ratios were used as proxies for sediment composition and as paleoenvironmental indices. In sediment physical properties, δ18O values, and geochemical properties, evidence for glacial and interglacial cycles and cyclicities of 405 ka, 100 ka, 41 ka were found. A shift of the cyclicity from 41 to 100 ka took place in sediment cores during (1.4-0.4 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 2.15-2.96 cmka−1 (GeoB20824-4) and 5.49-6.77 cmka−1 (GeoB20846-1), respectively, differ by a factor of two. This may be the reason why two facies-units were identified in core GeoB20824-4, whereas sediments of core GeoB20846-1 all belonged to the same facies. A change of lithofacies in core GeoB20824-4 between Unit I and Unit II in ∼20 mbsf at 1.5-1.4 Ma marks the initiation of the EMPT.