Factors influencing the distribution of Subantarctic deep-sea benthic foraminifera, Campbell and Bounty Plateaux, New Zealand

Abstract

We investigate the combination of environmental factors that influence the distribution patterns of benthic foraminiferal tests (> 63 μm) in a topographically varied region crossed by both the Subtropical and Subantarctic Fronts, south-east of New Zealand. Seafloor sample sites, extending from outer shelf (50 m) to abyssal (5000 m) depths, are bathed by five different water masses, and receive phytodetritus from Subtropical, Subantarctic and Circumpolar surface water masses. Eight mappable associations are recognised by Q-mode cluster analysis of the benthic foraminiferal census data. Similar associations are identified using cluster analysis based solely on the presence or absence of species. Canonical correspondence analysis and a correlation coefficient matrix were used to relate the faunal data to a set of environmental proxies. These show that factors related to water depth (especially decreasing food supply with increasing depth) are the most significant in determining the overall foraminiferal distribution. Other contributing factors include surface water productivity and its seasonality; bottom water ventilation; energetic state of the benthic boundary layer and resulting substrate texture; and bottom water carbonate corrosiveness. Three shallow-water associations (50–700 m), dominated by Cassidulina carinata, Trifarina angulosa, Globocassidulina canalisuturata, Gavelinopsis praegeri, and Bolivina robusta, occur in coarse substrates on the continental shelf, and on the crests and upper slopes of four seamounts under well-oxygenated, high energy regimes, and high food input. Three mid bathyal to upper abyssal associations (500–3300 m), dominated by Alabaminella weddellensis, C. carinata, and Epistominella exigua, occur in biopelagic sandy mud, beneath a region of strongly seasonal food supply, with their composition influenced by total food flux, ventilation (Oxygen Minimum Zone), and bottom current strength. An unusual lower bathyal association (1200–2100 m), dominated by T. angulosa and Ehrenbergina glabra, occurs in a belt of coarser sandy substrate that runs along the crest of the submarine plateaux slopes beneath the strongly-flowing Subantarctic Front-related currents. A deep abyssal association (3500–5000 m), dominated by Nuttallides umbonifer and Globocassidulina subglobosa, occurs on the abyssal plain beneath oligotrophic lower Circumpolar Water south-east of the Subantarctic Front and is strongly influenced by the cold, carbonate-corrosive conditions. The faunal composition of this subantarctic region is most similar to adjacent southern New Zealand, but with the addition of a few significant Antarctic elements (e.g., E. glabra, Globocassidulina crassa).