The results of Leg 181 demonstrate well the synergy that results from placing offshore paleoceanographic research within a well-understood onshore-offshore basinal context. The New Zealand region, set within the mid-latitudes (~35°-55°S) of the Southern Hemisphere, possesses an outstandingly complete and well-studied onland Late Cretaceous-Cenozoic stratigraphic succession. The surrounding marine realm comprises subtropical to polar surface water masses and their separating SAF and STF frontal systems. At depth, the cold, powerful, north-traveling Pacific DWBC passes along the submerged eastern edge of the microcontinent. All major changes in global climate during the last half of the Cenozoic have been reflected in—and in some instances caused, buffered, or reinforced by—concomitant changes in these major Southern Ocean oceanographic features. Of special significance are those changes that were associated with the early Oligocene start of the thermohaline circulation system and the subsequent global climatic deterioration into the late Cenozoic ice ages.
Leg 181 set out (1) to improve our knowledge of the timing of the establishment and evolution of a major segment of the ocean thermohaline circulation system and (2) to provide an understanding of how different parts of the ocean system, especially the DWBC and its overlying mid-latitude frontal systems, respond to climatic cycling at Milankovitch and other periodicities. These goals were achieved, and, relating to them, we draw the following 10 principal conclusions:
In both deep- and shallow-water successions, sediments above the Marshall Paraconformity are ubiquitously affected by vigorous current activity consequent upon a restructured ocean circulation. The Marshall Paraconformity represents the start of a fast, cold current around at least part of Antarctica and the production of increased volumes of cold bottom water. It also marks the start of the ENZOSS, the stratigraphy of which records a dynamic interplay between evolving oceanic hydrography, which today includes two major frontal systems, and sediments provided by biopelagic productivity and by erosion along the evolving New Zealand plate boundary.
Oligocene and Younger ACC/DWBC Erosion (Sites 1121, 1123, and 1124). Erosion and corrosion of the seabed dominated the first 3-10 m.y. of ACC/DWBC activity in the southwest Pacific. At Site 1121, situated beneath the highly energetic combined ACC/DWBC, strong post-Eocene erosion removed the entire late Paleocene-late Eocene top of the Paleogene sediment apron. Sediments above the Marshall Paraconformity at this site comprise only a 15.2-m-thick skin drift of abyssal clay and extensively reworked foraminiferal and quartz sand with episodic layers of small manganese nodules down to 5.40 mbsf. 10Be dating of the nodules and sediment matrix indicates an age of ~18 Ma at ~7 mbsf and identifies probable phases of enhanced DWBC flow at ~15-12, 10-8, and 1.5-0 Ma (Site 1121). At Site 1123, a gap of 13 m.y. (33.6-20.5 Ma) separates late Eocene-early Oligocene nannofossil chalk from the early Miocene DWBC drifts above, whereas a shorter gap of 6 m.y. (33-27 Ma) is present at Site 1124. Earliest Oligocene sediment below the Marshall Paraconformity at these two sites and at DSDP Leg 29 Site 277 suggests that the onset of seafloor erosion generally postdated the Oi-1 isotope event by up to ~2 m.y.
Micropaleontological studies, controlled since 4 Ma against a detailed stable isotope history and prior to that by astrochronologic tuning of physical properties and by paleomagnetic chronology, provide strong evidence for progressive climatic deterioration after ~15 Ma, with a warm period between ~5 and 3.3 Ma in the Pliocene and notable cold punctuations (prior to the 2.5-Ma start of enhanced glacial cycles) at 10.93, 10.61, 9.62, 9.44, 7.00, 6.20, 3.37, 3.03, 2.83, and 2.70 Ma.
Neogene Fluctuations in SAMW/AAIW (Site 1120). At shallower depths across the submerged eastern New Zealand continental rim (Campbell Plateau) but outside the realm of terrigenous influence, slowly accumulating foraminiferal nannofossil chalks are punctuated by hiatuses at ~16.7-15.8, ~5.6-1.9, and ~0.90-0.24 Ma, indicative of strong ancestor SAMW/AAIW flows at these times.
13C signature between 5.6 and 4.8 Ma that gradually returns to more typical levels by ~3 Ma.Deeper sediments at Site 1119 contain a record of SAMW/AAIW activity back to ~3.5 Ma. Because these water masses are formed, respectively, at the SAF and AAPF, their presence indicates the presence also of these two frontal features. The base of Site 1119 lies just within the top of a thick interval of large terrigenous sediment drifts that are inferred to have been deposited from north-flowing AAIW since the early Miocene (~23 Ma). These drifts represent the intermediate-depth equivalents of the deeper-water DWBC drifts located farther east. The thickness and restricted presence of the drifts across the head of the Bounty Trough shows that vigorous northeasterly intermediate flows, sourced from a proto-AAPF, have occurred along the eastern South Island margin since the early Miocene. These flows evolved from the erosive early Oligocene and depositional late Oligocene-early Miocene DWBC-ACC, which deposited the regional Kekenodon Group greensand and carbonate drifts (cf. Fig. F24) that underlie the terrigenous Canterbury Drifts. The further climatic deterioration that occurred during the late middle Miocene (~15-12 Ma) and the late Miocene-Pliocene (~7-2.5 Ma) had the effect of strengthening the STF and AAPF, leading to the development of the SAF, probably sometime during the early-middle Miocene. In turn, the development of these fronts and changes in associated water masses caused (a) intensification of northeasterly flow along the closely spaced density gradients of the STF, which was probably locked onto the eastern South Island and Chatham Rise bathymetry from the early Miocene onward, and (b) intensification of AAIW subduction and SAMW formation at the AAPF and SAF, respectively, thus increasing the supply of intermediate water around and across the Campbell Plateau and into the Bounty gyre.
Glacial-Interglacial Cycling (Sites 1119, 1123, and 1125). The detailed comparison of sediments between late Quaternary glacial and interglacial intervals indicates that glacial periods were characterized by (a) stronger DWBC current flows, as marked by an increased grain size of sortable silt and a higher rate of reworking of microfossils, including Antarctic diatoms; (b) nutrient-enriched 13C values and a smaller cross-Pacific 13C gradient, the latter consistent with enhanced formation of AABW and ventilation of CDW; (c) enhanced inputs of water-borne terrigenous sediment, eolian dust, biopelagic opal, and volcanic ash; (d) reduced biopelagic carbonate entombment, caused both by lower surface carbonate productivity and (in deeper water) by increased dissolution under cold, corrosive flow; (e) freezing cold and highly saline deep water, sourced from CDW rather than from NADW; and (f) enhanced frontal current flows along the South Island sector of the STF, as marked by the deposition of thin beds of sand in the mud-dominated, upper slope environment.
The Site 1119 gamma ray record, which is a proxy for the size of the South Island ice cap, closely matches the deuterium isotope (atmospheric temperature) profile of the Vostok polar ice core back to MIS 11 (~400 ka). This indicates the existence during climate cycling of strong intra-hemispheric coupling between Southern Hemisphere middle and high latitudes.
Sediment Delivery via Submarine Channels (Sites 1122 and 1124). Today, terrigenous sediment is delivered into the AAIW and DWBC sediment drifts by the Solander, Bounty, and Hikurangi Channels, which in glacial times was augmented by eolian input of dust and eruptive ash. "New" terrigenous sediment first appears in both the shallow- and deepwater ENZOSS record in the latest Oligocene, at ~23.4 Ma, closely following the ~25-Ma start of uplift along the South Island alpine plate boundary. This near-coincidence of timing notwithstanding, some of the terrigenous material in the DWBC drifts is undoubtedly derived from seafloor erosion along the southern sector of the ancestral DWBC + ACC system. Outside the area of either a contemporary shore-connected sediment supply or the influence of DWBC-supplied sediment, biopelagic nannofossil ooze accumulated regionally well into the Pliocene. At ~4 Ma, coincident with the start of global 18O enrichment and probable sea level fall, fine-grained terrigenous sediment overspilling from the eastern South Island shelf first reached DSDP Leg 90 Site 594 in the northeastern corner of the Bounty Trough, its transport probably augmented by AAIW and STF advection. At the same time, enhanced DWBC flow is indicated by the presence of a ~5.0- to 2.2-Ma paraconformity at Site 1122 at the abyssal mouth of the Bounty Trough that separates early Pliocene DWBC drifts from latest Pliocene turbidite-augmented drifts. Bounty Channel turbidite activity accelerated at ~2.4 Ma, driven by South Island mountain uplift and by enhanced glacioeustatic sea level lows. Concomitantly, Site 594 exhibits strong terrigenous-carbonate sediment rhythms that are in phase with climatic cycles, and at ~1.7 Ma the main phase of Bounty Fan accumulation commenced.
Within the Hikurangi Channel system, an increase in terrigenous sediment flux at Site 1124 suggests that right bank overspilling turbidites from the channel first reached Rekohu Drift in the earliest Pleistocene, at ~1.65 Ma. This date refines earlier geological estimates of the timing of diversion of the Hikurangi Channel out of the Hikurangi Trough, which became blocked then by the giant Ruatoria submarine landslide off Poverty Bay.
Plate Boundary Volcanism (Sites 1123, 1124, and 1125). It is established in the onland record that arc volcanism in Northland, the earliest predecessor of the Hikurangi subduction margin, commenced in the late Oligocene at ~25 Ma. Through the Neogene, the volcanic alignment migrated successively eastward to the CVZ and TVZ, the latter being Earth's most productive Quaternary rhyolitic volcanic center. The Leg 181 record of 134 tephra, individually up to 92 cm thick, provides a new and detailed history of major explosive eruptions from the CVZ and TVZ since 12 Ma. Through the late Miocene and Pliocene, macroscopic tephra layers occur grouped into intervals with concentrated ash fall separated by intervals of several hundred thousand years of quiescence or lesser activity. From ~1.6 Ma onward, explosive activity was centered in the TVZ and large eruptions were more frequent than previously known. Ash dispersal from North Island volcanoes is overwhelmingly eastward, under the influence of westerly stratospheric winds. Most tephra accumulated during glacial periods, consistent with increased eruption (unloaded magma chambers) and increased windiness (northward wind-belt migration) then.
This stratigraphic treasure trove has already yielded major insights into the evolution and distribution of deepwater benthic foraminifers, including the first detailed delineation of the Stilostomella extinction crisis in the middle Pleistocene; the biochronology and cold climate implications of bolboformids and their late Miocene planktonic foraminifer companions; the phytoplankton stratigraphy of the late Quaternary and of pre-Marshall Paraconformity nannofossil chalks of late Eocene (39-37 Ma) age; short-period climatic fluctuations and water mass changes of late Miocene and younger age, based on census counts of planktonic foraminifers and MAT paleotemperature reconstructions; and the first demonstration from palynological studies of cyclic changes in terrestrial vegetation that occur almost exactly in phase with marine climatic signals.
Finally, the high-resolution stratigraphic studies accomplished using Leg 181 materials and a wide variety of techniques have contributed to a quantum improvement in the accuracy of correlation among southwest Pacific drill sites. An especially important result of this is an improved integration between the regional New Zealand stage classification and global events and timescales. Much of the valuable information contained in publications on New Zealand geology has previously been obscured behind a screen of arcane terminology. Leg 181 results will help greatly to bring this important regional stratigraphic archive within the reach of a wider geologic public and particularly so for information regarding the post-Eocene linkages between the ENZOSS and the evolution of the global ocean circulation system.
