Planktonic foraminifera distribution during the mid-Piacenzian warm period in the North Atlantic

The mid-Piacenzian (MP) warm period (3.264-3.025 Ma) has been identified as the most recent time in geologic history during which mean global surface temperatures were considerably warmer than today for a sustained period. This interval has therefore been proposed as a potential (albeit imperfect) analog for future climate change and as such, has received much scientific attention over the past two decades. Central to this research effort is the Pliocene Research, Interpretation, and Synoptic Mapping (PRISM) project, an iterative paleoenvironmental reconstruction of the MP focused on increasing our understanding of warm-period climate forcings, dynamics, and feedbacks by providing three-dimensional data sets for general circulation models. A mainstay of the PRISM project has been the development of a global sea surface temperature (SST) data set based primarily upon quantitative analyses of planktic foraminifer assemblages, supplemented with geochemical SST estimates wherever possible. In order to improve spatial coverage of the PRISM faunal data set in the low and mid-latitude North Atlantic, this study provides a description of the MP planktic foraminifer assemblage from five Ocean Drilling Program sites (951, 958, 1006, 1062, and 1063) in the subtropical gyre, a region critical to Atlantic Ocean circulation and tropical heat advection. Assemblages from each core provide evidence for a temperature- and circulation-driven 5-10° northward displacement of MP faunal provinces, as well as regional shifts in planktic foraminifer populations linked to species ecology and interactions. General biogeographic trends also indicate that, relative to modern conditions, gyre circulation was stronger (particularly the Gulf Stream, North Atlantic Current, and North Equatorial Current) and meridionally broader. A comparison of mid-Piacenzian and modern North Atlantic planktic foraminifer assemblages suggests that low latitude western boundary currents were less than 1 °C warmer while eastern boundary currents were ~1-2 °C warmer, supporting the hypothesis of enhanced northward heat advection along western boundary currents and warming of high latitude Northeast Atlantic source regions for the Canary Current. These findings are consistent with a model of reduced meridional SST gradients, with little-to-no low latitude warming, and more vigorous ocean circulation. Results therefore support the theory that enhanced meridional overturn circulation and associated northward heat advection made an important contribution, in conjunction with elevated atmospheric CO2 concentrations, to the 2-3 °C global surface temperature increase (relative to today) and strong polar amplification of SST warmth during the MP warm period.

(Table 2) Correlation of Mi zones with hiatuses in ODP Site 181-1124

Results from Ocean Drilling Program sites 1121-1124 show the Eastern New Zealand Oceanic Sedimentary System (ENZOSS) evolved in response to: (1) the inception of the circum-Antarctic circulation, (2) orbital and nonorbital regulation of the global thermohaline flow, and (3) development of the New Zealand plate boundary. ENZOSS began in the early Oligocene following opening of the Tasmanian gateway and inception of the ancestral Antarctic Circumpolar Current (ACC) and SW Pacific Deep Western Boundary Current (DWBC). Widespread erosion, marked by the Marshall Paraconformity, was followed by extensive drift formation in the late Oligocene- early Miocene. Alternating nannofossil chalk and nannofossil-rich mud deposited in response to 41-kyr orbital regulation of the abyssal circulation, with the mudstones representing times of increased inflow of corrosive southernsource waters. Drift deposition at the deepest sites was interrupted by bouts of erosion coincident with Mi 1-5 isotopic events signifying expansions of the East Antarctic Ice Sheet and enhanced bottom water formation. By late Miocene times, the basic ENZOSS was established. South of Bounty Trough, the energetic ACC instigated an erosional/low depositional regime. To the north, where the DWBC prevailed, orbitally regulated drift deposition continued. Increased convergence at the New Zealand plate boundary enhanced the terrigenous supply, but little of this sediment reached the deep ENZOSS as the three main sediment conduits - Solander, Bounty and Hikurangi channels - had not fully developed. The Plio-Pleistocene heralded a change from a carbonate- to terrigenous-dominant supply caused by interception of the DWBC by the three channels (~1.6 Ma for Bounty and Hikurangi, time of Solander interception unknown). The Solander and Bounty fans, and Hikurangi Fan-drift systems formed, and drifts downstream of those systems, received terrigenous detritus. Supply increased with accelerating uplift along the plate boundary, but delivery to the DWBC was regulated by eustatic fluctuations of sea level. Times of maximum supply to all three channels was during glacial lowstands whereas the supply either ceased (Bounty, Solander), or reduced (Hikurangi) in highstands. In glacial times, sediment was entrained by a DWBC invigorated by an increased input of Antarctic bottom water. The ACC also accelerated under strengthened glacial winds. Thus, glacials were times of optimum sediment supply to ENZOSS depocentres where depositional rates were 2-3 times more than interglacial rates.

South Pacific dissolved Nd isotope compositions, rare earth element and nutrient concentrations from SONNE cruise SO213

Despite its enormous extent and importance for global climate, the South Pacific has been poorly investigated in comparison to other regions with respect to chemical oceanography. Here we present the first detailed analysis of dissolved radiogenic Nd isotopes (epsilon-Nd) and rare earth elements (REEs) in intermediate and deep waters of the mid-latitude (~40°S) South Pacific along a meridional transect between South America and New Zealand. The goal of our study is to gain better insight into the distribution and mixing of water masses in the South Pacific and to evaluate the validity of Nd isotopes as a water mass tracer in this remote region of the ocean. The results demonstrate that biogeochemical cycling (scavenging processes in the Eastern Equatorial Pacific) and release of LREEs from the sediment clearly influence the distribution of the dissolved REE concentrations at certain locations. Nevertheless, the Nd isotope signatures clearly trace water masses including AAIW (Antarctic Intermediate Water) (average epsilon-Nd = -8.2 ± 0.3), LCDW (Lower Circumpolar Deep Water) (average epsilon-Nd = -8.3 ± 0.3), NPDW (North Pacific Deep Water) (average epsilon-Nd = -5.9 ± 0.3), and the remnants of NADW (North Atlantic Deep Water) (average epsilon-Nd = -9.7 ± 0.3). Filtered water samples taken from the sediment-water interface under the deep western boundary current off New Zealand suggest that boundary exchange processes are limited at this location and highlight the spatial and temporal variability of this process. These data will serve as a basis for the paleoceanographic application of Nd isotopes in the South Pacific.

Calcareous nannofossil abundance and datum events of ODP Holes 181-1123C and 181-1124C, southwest Pacific

Seven sites were drilled off the eastern shore of New Zealand during Ocean Drilling Program Leg 181 to gain knowledge of southwest Pacific ocean history, in particular, the evolution of the Pacific Deep Western Boundary Current (DWBC). Holes 1123C and 1124C penetrated lower Oligocene to middle Eocene sediments containing moderately to poorly preserved calcareous nannofossils. Nannofossil assemblages show signs of dissolution and overgrowth, but key marker species can be identified. Nannofossil abundance ranges from abundant to barren. The lower Oligocene sediments are distinctly separated from the overlying Neogene sequences by the Marshall Paraconformity, a regional marker of environmental and sea level change. An age-depth model for Hole 1123C through this sequence was constructed using nine nannofossil age datums and three magnetostratigraphic datums. There is good agreement between the biostratigraphy and magnetostratigraphy, which indicates that the Marshall Paraconformity spans ~12 m.y. in Hole 1123C. The same sequence in Hole 1124C is disrupted by at least three hiatuses, complicating interpretation of the sedimentation history. The Marshall Paraconformity spans at least 3 m.y. in Hole 1124C. A 4- m.y. gap separates lower Oligocene and middle Eocene sediments, and a ~15 m.y. hiatus separates middle Eocene mudstones from middle Paleocene nannofossil-bearing mudstones. Nannofossil biostratigraphy from Holes 1123C and 1124C indicates that the Eocene-Oligocene transition was a time of fluctuating biota and intensification of the DWBC along the New Zealand margin.

Oxygen isotopic composition of G. ruber and G. sacculifer in ODP Hole 164-994C sediments

The purpose of this study is to establish the oxygen isotopic stratigraphy of the last 0.8 m.y. at Hole 994C, drilled during Ocean Drilling Program (ODP) Leg 164. Hole 994C (31º47.1'N, 75º32.8'W) is on the Blake Ridge off the coast of the southeast United States, at a water depth of 2799 m. The sediments from Hole 994C consist of light pale brown calcareous nannofossil clay containing relatively abundant and well-preserved foraminifers. Because the sediment on the Blake Ridge is composed of contourites derived from north by the Western Boundary Undercurrent beneath the Gulf Stream, the oxygen isotopic record is expected to be a mixture of the indigenous and reworked components. However, normal oxygen isotopic curves since the last 20 k.y. have already been reported from the Blake Ridge (Keigwin and Jones, 1989 doi:10.1016/0198-0149(89)90032-0; Haskell et al., 1991, doi:10.1029/90PA01939). The oxygen isotopic data obtained through this study is also correlatable to the standard oxygen isotopic stratigraphy, although some abnormality exists in certain intervals.

Stable oxygen and carbon isotopes and sediment geochemistry of sediment core GeoB12616-4

The Indian Ocean is an important component of the global thermohaline circulation system, as its western boundary currents feed the Agulhas Current, an integral part of the Atlantic meridional overturning circulation. However, Indian Ocean intermediate to deep-water variability on glacial-interglacial timescales is still a matter of debate. Here we provide stable carbon and oxygen isotopes and sediment elemental compositions of a sediment core from the edge of the Somali Basin. We demonstrate that throughout the past 600 kyr the intermediate western Indian Ocean was primarily bathed by Southern Ocean sourced Upper Circumpolar Deep Water (UCDW). This Southern Ocean sourced water mass enters the Somali Basin via the Amirante Passage or the Mozambique Channel and represents a downstream equivalent of South Atlantic UCDW. We cannot clearly account for the shortterm passage of Red Sea Water (RSW) at 1500 m water depth along the African continental margin, as previously suggested, on glacial-interglacial timescales.

Locations of geochemical samples and tephra from ODP Site 181-1123

Approximately one thousand sediment samples from ODP Site 1123 on the Chatham Rise, east of New Zealand, have been examined for inorganic elemental concentrations. ODP 1123 provides a record of sediment drift deposition under the Deep Western Boundary Current, the main inflow of deep water to the Pacific Ocean since the Early Oligocene, though a major hiatus spans the late Early Oligocene to the Early Miocene. Normalisation of the elemental concentrations by aluminium was used to allow for the effects of variable carbonate dilution. The elemental ratios were used as proxies for sediment composition and as palaeoceanographic indices. The samples were collected at a resolution designed to sample adequately any variation in elemental ratios at the scale of the Milankovitch orbital cycles. The sampled intervals span the Early Oligocene, Early Miocene, mid-Miocene and Late Pleistocene to Recent. Anomalous Si/Al, K/Al, Ti/Al values in the upper Pleistocene section, often associated with horizons of low carbonate, are attributed to tephras derived from North Island. Not all of the tephras detected geochemically had been detected visually in the cores. A total of 37 tephra events between 1.17 Ma BP and the present are recognised based on this and the shipboard investigations. The tephra events cluster at intervals of approximately 326 000 years (326 ka) perhaps due to variations in eruption frequency on North Island and/or to variations in the regional palaeowind intensity and direction. In the Late Pleistocene to Recent P/Al (inferred nutrient availability), percent calcium carbonate (%CaCO3) and Ba/Al (inferred productivity) varied regularly at a period of 40 000 years with these factors lagging minimum global ice volumes (interglacials). During the mid-Miocene CaCO3, Ba/Al, P/Al and Si/Al all gradually increased with %CaCO3 and P/Al showing regular 138 000-yr cyclicity and Ba/Al showing 44-ka cyclicity. Inferred productivity (Ba/Al) may have been rising in association with increasing nutrient availability (P/Al) at the same time as increased vigour of the Deep Western Boundary Current that was connected to a period of rapid ice-sheet growth in Antarctica. In the Early Miocene P/Al and Si/Al were much higher than subsequently and both %CaCO3 and P/Al exhibited 131 000-yr cycles. By far the highest nutrient levels and inferred productivity at this site apparently occurred during the Early Oligocene as revealed by long-term changes in P/Al and Si/Al. A progressive rise in K/Al, but stable Ti/Al from the Early Oligocene to the Recent probably indicates increased proportions of illite in the clay mineral fraction of the drift sediments caused by increased flux of debris from the Southern Alps.

(Appendix) Analysis of benthic foraminifera from ODP Leg 181 and DSDP Leg 90

Canonical correspondence analysis indicates that the distribution of Neogene benthic foraminiferal faunas (>63 µm) in seven DSDP and ODP sites (500-4500 m water depth) east of New Zealand (38-51°S, 170°E-170°W) is most strongly influenced by depth (water mass stratification), and secondly by age (palaeoceanographic changes influencing faunal composition and biotic evolution). Stratigraphic faunal changes are interpretted in terms of the pulsed sequential development of southern, and later northern, polar glaciation and consequent cooling of bottom waters, increased vertical and lateral stratification of ocean water masses, and increased overall and seasonal surface water productivity. Oligocene initiation of the Antarctic Circumpolar Current and Deep Western Boundary Current (DWBC), flowing northwards past New Zealand, resulted in extensive hiatuses throughout the Southwest Pacific, some extending through into the Miocene. Planktic foraminiferal fragmentation index values indicate that carbonate dissolution was significant at abyssal depths throughout most of the Neogene, peaking at upper abyssal depths in the late Miocene (11-7 Ma), with the lysocline progressively deepened thereafter. Miocene abyssal faunas are dominated by Globocassidulina subglobosa and Oridorsalis umbonatus, with increasing Epistominella exigua after 16 Ma at upper abyssal depths. Peak abundances of Epistominella umbonifera indicate increased input of cold Southern Component Water to the DWBC at 7-6 Ma. Faunal association changes imply establishment of the modern Oxygen Minimum Zone (upper Circumpolar Deep Water) in the latest Miocene. Significant latitudinal differences between the benthic foraminiferal faunas at lower bathyal depths indicate the existence of an oceanic front along the Chatham Rise (location of present Subtropical Front), since the early late Miocene at least, with more pulsed productivity (higher E. exigua) along the south side. Modern Antarctic Intermediate Water faunal associations were established north of the Chatham Rise at 10-9 Ma, and south of it at 3-1.5 Ma. Middle-upper bathyal faunas on the Campbell Plateau are dominated by reticulate bolivinids during the early and middle Miocene, indicative of sustained productivity above relatively sluggish, suboxic bottom waters. Faunal changes and hiatuses indicate increased current vigour over the Campbell Plateau from the latest Miocene on. Surface water productivity (food supply) appears to have increased in three steps (at times of enhanced global cooling) marked by substantially increased relative abundance of: (1) Abditodentrix pseudothalmanni, Alabaminella weddellensis, Cassidulina norvangi (16-15 Ma, increased pulsed productivity); (2) Bulimina marginata f. aculeata, Nonionella auris, Trifarina angulosa, Uvigerina peregrina (3-1.5 Ma, increased overall productivity); and (3) Cassidulina carinata (1-0.5 Ma, increased overall productivity). Three intervals of deep-sea benthic foraminiferal taxonomic turnover are recognised (16-15, 11.5-10, 2-0.5 Ma) corresponding to intervals of enhanced global cooling and possible productivity changes. The late Pliocene-middle Pleistocene extinction, associated with increasing Northern Hemisphere glaciation, culminating in the middle Pleistocene climatic transition, was more significant in the study area than the earlier Neogene turnovers.