Deep Sea Paleoceanography of the Northern Hikurangi Subduction Margin over the last 1.3 Ma: Insights from IODP Site U1526

Abstract

Deep-sea water masses are critical components in ocean circulation and climate change. Therefore, it is important to understand how they change over long-term periods and the Pleistocene glacialinterglacial cycles. There are few studies in New Zealand focusing on the long-term, deep-sea oceanography during the Pleistocene, with no previous studies at the northern Hikurangi Subduction Margin (NHSM). To understand the NHSM Pleistocene paleoceanography, this study analysed the upper 26.87 m of sediment from the Integrated Ocean Drilling Program core Hole U1526B. Site U1526 is located on the lower flanks of the Tūranganui Knoll seamount at 2888 m below sea level, and ~600 m above the Hikurangi Trough. U1526B sits within the Lower Circumpolar Deep Water (LCDW) today, just below the boundary with the Pacific Deep Water (PDW). In this project, I have used a multiproxy approach to determine the short-term (glacial-interglacial) and long-term (1.3 Ma) changes in the deep-sea oceanography at the NHSM. δ18O of benthic foraminifera was used to identify glacial-interglacial cycles and improve the core’s age model, in combination with the previous biostratigraphic markers, tephrochronology, and new benthic foraminiferal biostratigraphic markers. Changes in bottom water flow velocity and deep-sea sediment depositional processes were determined using sedimentological grain-size analysis (sortable silt) and CT scans. The provenance of the sediments was determined from benthic foraminiferal bathymetric group indicators, planktic foraminiferal percentage, and XRF elemental abundances. Changes in productivity were determined from carbonate percentage, foraminiferal abundance, benthic foraminiferal diversity and evenness, and XRF elemental abundances. Water mass properties were determined from δ18O deep-sea temperature calculations, benthic foraminiferal indicators of carbon and oxygen, and δ13C measured from the benthic foraminifera. This study sheds new light on the paleoceanography of the NHSM over the enigmatic Mid-Pleistocene Transition (MPT) from 1.2 to 0.7 Ma. During this time, the most recent benthic foraminiferal extinction event is evident within U1526B, indicating major changes in oceanic conditions during the MPT. Changes include increased ventilation, production, and velocity of cold deep waters, resulting in more oxygenated bottom waters. Post-MPT, flow velocity and ventilation of deep waters decreased, resulting in less oxygenated and more organic carbon rich waters. These significant oceanographic changes at the NHSM may be linked to global reorganisations of deep water currents seen in Ocean Drilling Program 1123 to the east. Evidence for changes in the deep-sea water mass properties were attributed to changes in the depth of water mass boundaries over glacial-interglacial cycles, which contrasts with previous studies. This study concluded that during interglacial periods, the northwards flowing LCDW dominates to a minimum depth of ~2900 m. During glacial periods, the water mass boundary depth between LCDW- PDW shifts, where the southward flowing PDW dominates at a minimum depth of ~2900 m and maximum depth of ~3000 m. Long term changes in sedimentation were also observed at Hole U1526B, forming a new unit model for background sedimentation processes in the abyssal NHSM. Between 1300-213 ka contourite processes dominate at the core site. Sedimentation processes transition at 213 ka to hemipelagic processes punctuated by turbidity current deposits. A large influx of terrigenous sediment from gravity flows is observed during the last glacial, likely linked to the Tuaheni Landslide Complex to the west and possibly the Ruatoria Mass Transport Deposit to the north. This transition is attributed to the westward movement of the active Hikurangi Subduction Margin. During the last glacial, δ13C is anomalously low, while Mn abundance is high during a period of low oxygenation. This suggests that sediments enriched by gas hydrate seeps may have locally influenced the region.