Late Quaternary oceanic fronts along Chatham Rise indicated by phytoplankton assemblages, and refined calcareous nannofossil stratigraphy for the mid-latitude SW Pacific

• Published in: Marine geology 2004-04, Vol.205 (1), p.59-86
• Main Authors: Fenner, Juliane, Di Stefano, Agata
• Format: Article
• Language: English
• Subjects: calcareous nannoplankton; Calcidiscus leptoporus; Coccolithus pelagicus; diatoms; endemic subantarctic species; Fragilariopsis doliolus; Gephyrocapsa; Gephyrocapsa muellerae; Joides Resolution; Leg 181; Nitzschia; Ocean Drilling Program; quantitative biostratigraphy; Rhizosolenia bergonii; Subtropical Convergence; Thalassiosira lentiginosa; volcanic ash
• ISSN: 0025-3227; 1872-6151
• DOI: 10.1016/S0025-3227(04)00018-0

A quantitative analysis was carried out of planktonic diatoms (biogenic opal) and calcareous nannofossils (biogenic calcite) in late Quaternary sediments (MIS 1–6) from four cores along a N–S transect east of New Zealand from 39°50′S to 50°04′S across the E–W-trending submarine ridge, the Chatham Rise. This was done to trace movements of oceanic fronts and to improve calcareous nannofossil stratigraphy for the last 130 000 yr in the SW Pacific. Sites ODP 1123 and Q 858 are below present day subtropical surface waters north of Chatham Rise. Site DSDP 594 is below present-day mixed temperate–subantarctic surface water south of the rise, and site ODP 1120 is below subantarctic surface water. The more diverse and opportunistic planktonic diatoms provided marker species for subtropical surface waters ( Alveus marina, Fragilariopsis doliolus, Rhizosolenia bergonii and Azpeitia nodulifer) and others for subantarctic surface waters ( Nitzschia kerguelensis, Thalassiosira lentiginosa). Application of these tracers permits the following conclusions: (1) subtropical conditions persisted north of Chatham Rise throughout the past 130 000 yr, in spite of the cooling of surface waters during colder periods; (2) during warm times (MIS 5 and MIS 3, and in MIS 1), the sporadic occurrence of subtropical species south of Chatham Rise indicates occasional admixture of subtropical surface waters that far south; (3) subantarctic waters extended to the southern slopes of the Chatham Rise during MIS 5b, late MIS 5a to early MIS 4, during the warmer time intervals in early MIS 3, and during latest MIS 3 to early MIS 2; (4) subantarctic frontal conditions existed over southern Chatham Rise during early MIS 4 and late MIS 3 to early MIS 2; and (5) it is probable that during cooler times, MIS 6, MIS 5b, and in MIS 2, intensified particle transport from the Bounty Trough to the northern flank of Chatham Rise occurred by intensified boundary currents. The larger abundance fluctuations in both microfossil groups at the sites south of Chatham Rise than north of Chatham Rise reflect northward shifts of the Circumpolar Subantarctic Water (CSW) and a contemporaneous disappearance of Australasian Subantarctic Water (ASW), implying an elevated temperature gradient between the surface water masses north and south of the Chatham Rise at the times of such northward shifts of CSW. Calcareous nannofossils are less diverse than diatoms, and are less specialised. Some calcareous nannofossil species s