Benthonic ostracods and deep watermasses: A qualitative comparison of Southwest Pacific, Southern and Atlantic Oceans

Abstract

Abstract The upper depth limits of the dominant cosmopolitan deep-water benthonic ostracod taxa present in five localities in the Southern Ocean (Kerguelen Plateau, Southeast Australian Continental Slope) and Southwest Pacific (Chatham Rise, Tasman Sea, Coral Sea) are compared to investigate the degree to which the position of intermediate and deep watermasses: Antarctic Intermediate Water (AAIW), Deep Water (DW) and Antarctic Bottom Water (AABW) control the vertical faunal succession. Although none of the taxa studied is restricted to one watermass, their upper depth limits appear to be closely related to the position of Antarctic Intermediate water (AAIW). Taxa showing the most consistent pattern include: Krithe (upper depth limit above AAIW), Poseidonamicus (upper depth limit in lower AAIW, but in underlying Deep Water on the Kerguelen Plateau) and Pterygocythere mucronalatum (DW, but in AABW on the Kerguelen Plateau). Bathyal (above approximately 1000 m) upper depth levels of taxa, species diversity, species turnover, and changes in the proportion of the dominant deep-sea taxa Krithe and Poseidonamicus match closely that of the Atlantic Ocean (Dingle and Lord, 1990). However, the upper depth limits of the abyssal taxa Poseidonamicus and Dutoitella appear to be very different in the Cape Basin, SE Atlantic, and the SW Pacific with respect to watermass characteristics. For example, in the Atlantic Poseidonamicus has its upper limit within Lower North Atlantic Deep Water (NADW) whereas in the SW Pacific it occurs consistently within the AAIW. These discrepancies in the upper depth level of abyssal taxa between the Pacific and Atlantic Oceans suggest that different watermasses or other environmental variables control their upper range in different oceanic basins. It follows that these differences must be taken into account if deep-sea ostracods are to be used as a reliable means to trace modern or reconstruct ancient watermass pathways between ocean basins.