Leg 181 synthesis: fronts, flows, drifts, volcanoes, and the evolution of the southwestern gateway to the Pacific Ocean, Eastern New Zealand

The Late Cretaceous-Cenozoic geology of New Zealand represents the evolution of a post-Gondwana, Pacific-facing passive margin which interacted, first, with the mid-Cenozoic development of the Australian/Antarctic and Australian/Pacific plate boundaries and, second, with the subsequent development of the oceanic thermohaline circulation system. Situated between the Tasmanian and southwest Pacific oceanic current gateways, the stratigraphy of the New Zealand region provides our best record of the evolution of the Pacific Ocean's largest deep cold-water inflow, the Deep Western Boundary Current (DWBC), and also possesses an important record of Antarctic Intermediate Water flow. Prior to Leg 181, our knowledge of southwest Pacific Ocean history and, in particular, the development of the DWBC and its local partner, the Antarctic Circumpolar Current (ACC), was poor. Seven holes were therefore drilled east of New Zealand to determine the stratigraphy, sedimentary systems, and paleoceanography of the DWBC, ACC, and related water masses and fronts. The sites comprised a transect of water depths from 396 to 4488 m and spanned a latitudinal range from 39° to 51°S. Leg 181 drilling provided the data needed to study a wide range of problems in the Southern Ocean Neogene. Driven by rifting and a new cycle of seafloor spreading along the Mid-Pacific Rise, New Zealand's youngest (Kaikoura) stratigraphic cycle begins with Late Cretaceous rift fill followed by subsidence and marine transgression until the late Eocene. Biopelagic oozes accumulated throughout as an abyssal apron around the Pacific perimeter of the New Zealand Plateau, seen as Paleocene siliceous nannofossil chalk, chert, and clay at Site 1121 (water depth = 4488 m) and nannofossil chalk at Site 1124 (water depth = 3967 m). At the Eocene/Oligocene boundary (~33.7 Ma), the spreading ridge between Australia and Antarctica broke through south of the Tasman Rise, linking for the first time the Indian and Pacific Oceans into a continuous Southern Ocean. Powerful wind-forced currents, predecessors to the modern ACC, were funneled through the Tasmanian Gateway and into the Pacific, where their path, combined with that of the thermohaline DWBC, was impeded by the shallowly submergent New Zealand Plateau, centered then at latitude ~55°S. All drill sites within or east of the Tasmanian Gateway and all onland sections in New Zealand record this event as a regional unconformity, the Marshall Paraconformity, across which a there is a time gap of ~3-10 m.y. , a result of a combination of corrosion, erosion, and nondeposition. Above the paraconformity, sedimentation in both shallow and deep water resumed as late Oligocene (~27-29 Ma) sediment drifts (Site 1124; water depth = 3967 m). Younger deepwater drifts at Sites 1123 (water depth = 3290 m) and 1124 comprise alternating nannofossil chalks containing greater or lesser amounts of terrigenous clay. At Site 1123 on the North Chatham Drift, sediment accumulated essentially continuously from ~20.5 Ma onward. Analysis of this record shows that the stratigraphic rhythms there correspond to 41-k.y. Milankovitch climatic cycles, with faster DWBC flow during colder or glacial intervals. Site 1123 is globally unique. It provides an essentially complete, richly microfossiliferous Miocene to Quaternary record of uniform ~4-cm/k.y. sedimentation that has been astronomically tuned. It also contains an almost complete paleomagnetic record since Chron C6r at 20.5 Ma, including the first record of new magnetic subchron C5ADn1r. Shallower-water Sites 1125 (water depth = 1366 m), 1120 (water depth = 546 m), and 1119 (water depth = 396 m) reveal, respectively, a major productivity bloom between 5.6 and 4.8 Ma on the north side of the Subtropical Front (STF) (Site 1125), foraminiferal nannofossil chalk accumulation punctuated by paraconformities at 16.7-15.8, 5.6-1.9, and 0.9-0.24 Ma (Site 1120), and enhanced frontal flows along a seaward-relocated STF during glaciations (Site 1119). The late Quaternary climatic record at Site 1119 also closely matches that of air temperature in the Vostok ice core, indicating close links between climate change in southern middle and polar latitudes. From ~24 Ma onward, abundant terrigenous material was shed into the southwest Pacific from rising mountains along the South Island Alpine Fault plate boundary. Gradually changing clay mineral assemblages in DWBC drifts, with chlorite + illite replacing smectite + kaolinite, reflect the increasing influence of newly unroofed basement (Rangitata) graywackes and schists through the Miocene-Quaternary. From 12 Ma onward, sediments were augmented by an influx of mainly rhyolitic tephra from the North Island volcanic arc. Site 1122 (water depth = 4432 m), on the left bank levee of the abyssal Bounty Fan, records a marked increase in the input of terrigenous turbidites and fan building starting at ~1.7 Ma and peaking at average rates >50 cm/k.y. after 0.7 Ma. Site 1124, on the Rekohu Drift near the Hikurangi Channel, records the start of overbank turbidite deposition, and therefore avulsion of the Hikurangi Channel from the Hikurangi Trough following channel deflection by a large submarine landslide from the North Island continental margin at ~1.65 Ma. Geological and oceanographic events that have occurred in the southwest Pacific since the Eocene/Oligocene boundary (~33.7 Ma) together compose the Eastern New Zealand Sedimentary System (ENZOSS), studies of which are contributing to our understanding of the history of global ocean circulation and climate change.