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

<p>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 &#8764;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 Milankovi&#263; 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 (&#948;¹⁸O) 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, &#948;¹⁸O 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^&#8722;1 (GeoB20824-4) and 5.49-6.77 cmka^&#8722;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 &#8764;20 mbsf at 1.5-1.4 Ma marks the initiation of the EMPT.</p>