Helen Neil

Response of surface water masses and circulation to Late Quaternary climate change east of New Zealand

A series of cores from east of New Zealand have been examined to determine the paleoceanographic history of the late Quaternary in the SW Pacific using planktonic foraminiferal data. Distinct shifts of species can be seen between glacial and interglacial times especially south of Chatham Rise east of South Island. Foraminiferal fragmentation ratios and benthic/planktonic foraminiferal ratios both show increased dissolution during glacials, especially isotope stage 2 to the south of Chatham Rise. The present-day Subtropical Convergence appears to be tied to the Chatham Rise at 44°S, but during glacial times this rise separated cold water to the south from much warmer water to the north, with an associated strong thermal gradient across the rise. We estimate that this gradient could have presented as much as an 8°C temperature change across 4° of latitude during the maximum of the last ice age. There is only weak evidence of the Younger Dryas cool event, but there is a clear climatic optimum between 8 and 6.4 ka with temperatures 1°–2°C higher than the present day. The marine changes compare well with vegetational changes on both South and North Island.

Subantarctic oceanography around New Zealand: Preliminary results from an ongoing survey

Abstract An ongoing observational program focused on variability in the subantarctic currents and water masses around southern New Zealand was initiated in May 1998. This paper describes the preliminary results deduced from CTD (conductivity, temperature, depth) data collected during three hydrographic surveys, and moored current and temperature records between 1998 and 1999. An extensive archived data set has also been analysed to provide a hydrographic climatology of the region. The low‐frequency circulation within the subantarctic zone is described, revealing previously unreported flow features: persistent but weak anticyclonic and cyclonic circulations over the Campbell Plateau and a strong cyclonic flow around the western edge of the Bounty Trough. Flow within the Subantarctic Front (SAF) is strongly constrained by the New Zealand bathymetry; diverting to follow the south‐eastern flanks of the Campbell Plateau and crossing to the Bounty Plateau, before separating to join the basin‐scale circulation. The flow features deduced from recent data are consistent with the hydrographic climatology as well as mid‐depth float trajectories. Subantarctic Mode Water (SAMW) is observed year‐round over an extensive fraction of the subantarctic zone: along the equatorward side of the SAF, over the Campbell Plateau, and flowing within the cyclonic circulation around the Bounty Trough. There is a marked cooling and freshening of SAMW between the deep water along the western flanks of Campbell Plateau and waters further east, consistent with a blocking of the eastward flow carrying warm and salty SAMW by the plateau. Earlier ideas that a substantial volume of SAMW is formed over the Campbell Plateau by deep vertical mixing (Heath 1981) are not substantiated by our data, which include seasonal observations of the upper water column.

Glacial-interglacial sea surface temperature changes across the subtropical front east of New Zealand based on alkenone unsaturation ratios and foraminiferal assemblages

0 are compared to those derived from foraminiferal assemblages (using the modern analog technique) in two of these cores. Reconstructions of SST in core tops and Holocene sediments agree well with modern average summer temperatures of � 18� C in subtropical waters and � 14� C in subpolar waters, with a 4� -5� C gradient across the front. Down core U37 K 0 SST estimates indicate that the regional summer SST was 4� -5� C cooler during the last glaciation with an SST of � 10� C in subpolar waters and an SST of � 14� C in subtropical waters. Temperature reconstructions from foraminiferal assemblages agree with those derived from alkenones for the Holocene. In subtropical waters, reconstructions also agree with a glacial cooling of 4� to � 14� C. In contrast, reconstructions for subantarctic pre- Holocene waters indicate a cooling of 8� C with glacial age warm season water temperatures of � 6� C. Thus the alkenones suggest the glacial temperature gradient across the front was the same or reduced slightly to 3.5� -4� C, whereas foraminiferal reconstructions suggest it doubled to 8� C. Our results support previous work indicating that the STF remained fixed over the Chatham Rise during the Last Glacial Maximum. However, the differing results from the two techniques require additional explanation. A change in euphotic zone temperature profiles, seasonality of growth, or preferred growth depth must have affected the temperatures recorded by these biologically based proxies. Regardless of the specific reason, a differential response to the environmental changes between the two climate regimes by the organisms on which the estimates are based suggests increased upwelling associated with increased winds and/or a shallowing of the thermocline associated with increased stratification of the surface layer in the last glaciation. INDEX TERMS: 4267 Oceanography: General: Paleoceanography; 4850 Oceanography: Biological and Chemical: Organic marine chemistry; 1050 Geochemistry: Marine geochemistry (4835, 4850); 1055 Geochemistry: Organic geochemistry; KEYWORDS: paleoceanography, sea surface temperature, alkenones, Southern Ocean, Last Glacial Maximum

Factors influencing the distribution patterns of Recent deep-sea benthic foraminifera, east of New Zealand, Southwest Pacific Ocean

Abstract This study investigates which combination of environmental factors most strongly influences the distribution patterns of benthic foraminferal tests (>0.63 μm) in a region bisected by the Subtropical Front, east of New Zealand. Seafloor sample sites extend from outer shelf (90 m) to abyssal (4700 m) depths, across substrates ranging from biogenic/terrigenous gravelly sand to hemipelagic mud, and occur under the influence of Antarctic intermediate water (AAIW) and circumpolar deep waters as well as receiving detritus from both Subtropical and Subantarctic surface water masses. Elevated values of the planktic foraminiferal fragmentation index and reworked small Paleogene planktic foraminifera at outer shelf and bathyal depths coincide with areas of strong bottom currents. Q-mode cluster analysis on the census counts of 398 benthic species clusters the 66 samples into three large groups (shallow, bathyal, abyssal), and at a lower level 10 mappable associations are recognised. A combination of canonical correspondence analysis and a correlation coefficient matrix was used to relate the faunal data to a set of measured environmental proxies. These analyses show that factors that have a relationship with depth are the most significant in determining foraminiferal distribution. The principal environmental factors which appear to most strongly influence this benthic foraminiferal distribution are: dissolved oxygen content in bottom waters; sustainability of organic carbon flux rates; seasonality of food supply; lateral advection of water masses; bottom water carbonate corrosiveness; energetic state at the benthic boundary layer; grain-size composition of substrate; salinity and temperature of the bottom waters. Shallow water associations (90–1250 m), dominated by Cassidulina carinata and Trifarina angulosa, occur within coarse substrates under well-oxygenated, high energy regimes and sustained food input. The occurrence of the bathyal associations (230–2840 m), dominated by C. carinata, Alabaminella weddellensis and Abditodentrix pseudothalmanni, closely mirrors the distribution of AAIW within a region of variable food supply. The sustainability of food supply combined with bottom water type and associated ventilation and dissolution strongly influence the composition of abyssal associations (1200–4700 m), mostly dominated by Epistominella exigua and A. weddellensis.

Sea surface temperature estimates from the Southwest Pacific based on planktonic foraminifera and oxygen isotopes

Planktonic foraminiferal census data were collected on 34 surficial sediment samples from east of New Zealand. Sea surface temperature estimates were derived from the data using the modern analogue technique (MAT) and compared to data from CLIMAPs FA-20 equation. They were also computed from oxygen isotope analyses of Globigerina bulloides. The temperature estimates compared well between the three methods and were found to correlate closely with the appropriate seasonal sea-surface temperature as measured by satellite. The foraminiferal assemblages and derivative temperatures clearly identify the major watermasses and oceanic fronts in the region, and thus can be applied to palaeoceanographic analysis of Southwest Pacific sediments.

Late Quaternary ice-rafting events in the SW Pacific Ocean, off eastern New Zealand

New records of ice-rafted debris (IRD) in eight cores from Campbell Plateau reveal periodic incursions of icebergs over the last 200 kyr. Observations of modern iceberg trajectories and provenance studies show the icebergs originated mainly in Antarctica and traveled east and north to the plateau margins via the Antarctic Circumpolar Current. There, local currents and winds dispersed icebergs across the plateau and adjacent regions as far north as Chatham Rise at 43°S. The shallow rise crest, along-rise circulation and strong thermal gradient associated with the nearby Subtropical Front inhibited further northward transport. Although IRD concentrations are very low (≤26 grains/g), downcore profiles and a δ18O chronology reveal distinct peaks during (i) the transition from marine isotope stages (MIS) 7 to 6, (ii) late interglacial MIS 5 and (iii) glacial MIS 2. In addition, smaller peaks occur in MIS 4 and 3. This generalised pattern is also evident off Antarctica, indicating that periodic destabilisation of ice shelves was the main driving force behind the Campbell Plateau IRD events. However, there are significant differences between Antarctic and New Zealand IRD records that probably reflect palaeoceanographic influences on iceberg dispersal. The MIS 5 IRD event is better represented off Antarctica, confirming that last interglacial changes in sea level and temperatures encouraged iceberg discharge and proximal melting. Conversely, the MIS 2 event is relatively better shown on Campbell Plateau, implying that last glacial temperatures inhibited proximal iceberg melting and, together with a more vigorous oceanic circulation, allowed more icebergs to reach the distant plateau. A tentative correlation with records from the Southeast Atlantic and Southwest Indian oceans suggests that the IRD events were felt over much of the Southern Ocean for at least the last 70 kyr.

Age frequency distribution and revised stable isotope curves for New Zealand speleothems: palaeoclimatic implications.

The occurrence of speleothems in New Zealand with reversed magnetism indicates that secondary calcite deposition in caves has occurred for more than 780 thousand years (ka). 394 uranium-series dates on 148 speleothems show that such deposition has taken place somewhere in the country with little interruption for more than 500 ka. A relative probability distribution of speleothem ages indicates that most growth occurred in mild, moist interglacial and interstadial intervals, a conclusion reinforced by comparing peaks and troughs in the distribution with time series curves of speleothem δO and δC values. The stable isotope time series were constructed using data from 15 speleothems from two different regions of the country. The greater the number of overlapping speleothem series (i.e. the greater the sample depth) for any one region, the more confidence is justified in considering the stacked record to be representative of the region. Revising and extending earlier work, composite records are produced for central-west North Island (CWNI) and north-west South Island (NWSI). Both demonstrate that over the last 15 ka the regions responded similarly to global climatic events, but that the North Island site was also influenced by the waxing and waning of regional subtropical marine influences that penetrated from the north but did not reach the higher latitudes of the South Island. Cooling marking the commencement of the last glacial maximum (LGM) was evident from about 28 ka. There was a mid-LGM interstadial at 23-21.7 ka and Termination 1 occurred around 18.1 ka. The glacial-interglacial transition was marked by a series of negative excursions in δO that coincide with dated recessional moraines in South Island glaciers. A late glacial cooling event, the NZ Late Glacial Reversal, occurred from 13.4-11.2 ka and this was followed by an early Holocene optimum at 10.8 ka. Comparison of δO records from NWSI and EPICA DML ice-core shows climatic events in New Zealand to lag those in Antarctica by several centuries to a thousand years. Waxing and waning of subantarctic and subtropical oceanic influences in the Tasman Sea are considered the immediate drivers of palaeoclimatic change.

A 2300‐Year Paleoearthquake Record of the Southern Alpine Fault and Fiordland Subduction Zone, New Zealand, Based on Stacked Turbidites

Turbidites from three sedimentary basins data, and probably the A.D. 1826 Fiordland earthquake and the well‐dated A.D. 1717 Alpine fault earthquake. The recurrence intervals are shorter than recently published recurrence data from the Alpine fault on land, reflecting mixed fault‐source earthquake records and potentially increased Alpine fault segmentation offshore. Online Material: Details of the laboratory methods and analytical techniques applied to sedimentrary cores, photos of seafloor substrates and fan core, OxCal codes and results, and sediment transport analysis. [1]: /embed/inline-graphic-1.gif

Changes in the position of the Subtropical Front south of New Zealand since the last glacial period

This study fills an important gap in our understanding of past changes in the Southern Subtropical Front (S-STF) in the southwest Pacific Ocean. Paleo-sea surface temperatures (SST) were estimated from planktic foraminiferal census counts from cores straddling the modern S-STF in the Solander Trough, south of New Zealand. The estimated SST were compared for 6 time slices; glacial period (25-21 ka), Last Glacial Maximum (LGM; 21-18 ka), early deglaciation (18-16 ka), late deglacial/early Holocene period (14-8 ka), mid-Holocene period (8-4 ka), and late Holocene period (4-0 ka). The position of the S-STF was determined by two methods: (1) the location of the 10 degrees C isotherm and (2) the location of the highest SST gradients. These new results suggest that the S-STF was not continuous between east and west of New Zealand during the glacial period. Steep SST gradients indicate that a strong S-STF rapidly shifted south during the LGM and early deglaciation. During the late deglacial and Holocene periods the position of the S-STF differs between the two methods with reduced SST gradients, suggesting amore diffuse S-STF in the Solander Trough at this time. The glacial SST data suggest that the S-STF shifted north to the west of New Zealand, while to the east there was a stronger SST gradient across the front. This was possibly the result of an increased wind stress curl, which could have been caused by stronger, or more northerly Southern Hemisphere westerly winds (SHWW), or a merging of the SHWW split jet in this region.

Sedimentary record of radiolarian biogeography, offshore eastern New Zealand

Abstract Examination of 38 surface sediment samples from offshore eastern New Zealand, between 33°S and 54°S, yielded 100 radiolarian taxa, which are common to abundant in sediments deposited at > 1000 m water depth but rare at shallower depths. In general, radiolarians are most abundant, most diverse, and best preserved in assemblages north of the Subtropical Front (STF). Multivariate analysis of census data for 29 radiolarian‐rich samples identifies six sample groups and eight species groups. The STF forms a major biogeographic barrier, separating three transitional zone (TR) and three subantarctic zone (SA) sample groups. The three primary sample groups (TR1, TR2, SA1) record a southward latitudinal trend of decreasing abundance of subtropical‐tropical or warm‐water taxa (species group 4) and increasing abundance of subantarctic ‐Antarctic or cool‐water taxa (species groups 7 and 8). Two secondary sample groups (TR3 and SA2) may record the influence of shallow‐water processes or strong surface currents on either side of the STF. A distinctive sample group (SA3), characterised by low abundance and diversity, records relatively shallow, stratified, and silica‐limited conditions of the central Campbell Plateau.