A Sedimentological and Micropaleontological Study of Quaternary Sections of Cores from the New Zealand Sector of the Southern Ocean

• Main Author: Mackie, Jon
• Other Authors: Nelson, Campbell S.
• Format: Thesis
• Language: English
• Published: The University of Waikato 2019
• Subjects: Antarctic; Campbell Plateau; Huxley; New Zealand; Pacific; Southern Ocean; The Antarctic; Antarc*; Antarctica; Neogloboquadrina pachyderma
• Online Access: https://hdl.handle.net/10289/12868

The Southern Ocean lies between the Subtropical Front (STF) at about 45⁰S latitude and the continental landmass of Antarctica. It plays an important role in global climatic and oceanic circulation patterns, and is responsible for production of most water characteristics below the thermocline. The New Zealand sector of the Southern Ocean (NZSSO) is situated in a unique position to study past changes in oceanic circulation and climate from the record preserved in deep-sea sediment cores. Using piston cores collected from water depths of 461-5100 m by USNS Eltanin during 1968-1972, a sedimentological, micropaleontological, and stable oxygen and carbon isotope study, including some radiocarbon dating, has been conducted on the upper 1 m of sediment in 12 cores spanning a latitudinal transect from 52°S to 61°S south of New Zealand. The transect crosses the modem position of the Subantarctic Front (SAF) at ∼56°S and extends as far as the Antarctic Polar Front (AAPF) at ∼63°S. The study has shown the NZSSO cores must be viewed as two subgroups. The cores are divided at the SAF into a northern subgroup (north of the SAF) and a southern subgroup (south of the SAF) on the basis of sedimentary lithologies, textures, composition and depositional histories. The northern cores are located in the vicinity of the southern margin of Campbell Plateau and are all highly calcareous ( ∼95% ), being composed of planktic and benthic foraminifera and nannofossils with minimal terrigenous material content. The northern cores include two textural facies controlled by the dominant biogenic sediment. Facies A, a muddy sand, is dominated by the Subantarctic foraminiferal assemblage of Globigerina bulloides, Neogloboquadrina pachyderma (sinstral and dextral), Globigerinita spp., Globorotalia injlata and Globorotalia truncatulinoides. Facies B, a sandy mud, is dominated by calcareous nannofossils, especially Emiliania huxley, Coccolithus spp., and Calcidiscus spp. The southern subgroup of cores lies in the Southwest Pacific Basin and are dominated by biogenic siliceous material and common (11-27%) terrigenous material. They belong to Facies C, a sandy silt, composed mainly of silt-sized frustules of diatomsand nassellaria radiolaria and terrigenous material (quartz, plagioclase feldspars, biotite and muscovite). Where calcareous material exists south of the SAF it is composed of an Antarctic foraminiferal assemblage involving Neogloboquadrina pachyderma (sinstral),Globigerina bulloides and Globorotalia injlata. All foraminifera of the Antarctic assemblage show evidence of dissolution. The northern subgroup of cores lies in an average water depth of 1200 m, while south of the SAF the average water depth of the cores is 4700 m. The lysocline for the NZSSO is determined at ∼3200-4100 m, with a carbonate compensation depth (CCD) of 4800 m. Any calcareous material south of the SAF is actively being affected by dissolution which biases the sedimentary record of the southern subgroup of cores to dissolution resistant organisms (diatoms and radiolaria) and terrigenous material. The stable isotope records for the northern NZSSO cores show only a 0.8% shift between inferred glacial and interglacial conditions. Smoothing of the record may be due to bioturbation and/or to extensive sediment reworking by bottom currents, forming rather condensed sedimentary records, especially during glacial periods. Oxygen isotope stages back to stage 5 have been suggested in one core (36-42); this places the Last Glacial-Holocene transition at ∼7 cm, stage 2/3 boundary at ∼21 cm, stage 3/4 boundary at ∼65cm, and a possible stage 4/5 boundary at ∼95 cm downcore. The position of the Last Glacial and stage 2/3 boundary are confirmed by radiometric dates obtained from the same northern core: 19,120 +/- 110 yrs BP at 10 cm downcore and 23,900 yrs +/- 200 yrs BP at 20 cm downcore. Faunal changes in the benthic foraminifera occur near the glacialinterglacial transitions from Globocassidulina subglobosa-dominated during glacial periods to common Epistominella exigua during interglacials. Textural facies also show changes near these transitions, with Facies A characterising glacial periods and Facies B during interglacial periods. Associated with changes in texture are colour changes which allow the correlation of the glacial-interglacial transitions across many of the northern cores. The Last Glacial situation cannot be determined uniquely for the southern subgroup of cores as they have, to varying degrees, experienced extensive erosion and reworking during the Late Quaternary or earlier. Collectively, the data suggest there has not been significant change in oceanic paleoenvironmental conditions north of the SAF, immediately south of New Zealand, between glacial and interglacial periods. This could be because the topographic high marking the southern edge of the Campbell Plateau has essentially fixed the SAF, irrespective of oceanic climatic conditions. The topographic lock would be further enhanced by sea level drops of ∼120 m during glacial periods. In contrast, east and west of New Zealand in open water conditions evidence from other studies indicates major northward shifts in the position of both the SAF and the AAPF. The limited variation downcore in the abundances of sinistrally coiled Neogloboquadrina pachydenna, the small shifts in the stable oxygen isotope record, and the lack of change in faunal assemblages together suggest that ranges in the sea surface temperature change between glacial and interglacial periods at higher latitudes were probably not so severe as at intermediate latitudes. Due to the relatively small temperature changes between glacial and interglacial conditions at higher latitudes the faunal and isotopic indicators may only record extreme events.