Abstract

A quantitative study of the planktonic foraminiferal fauna and various related parameters in more than 100 surface sediment samples from the Southeast Indian Ocean has identified the major relationships which can be utilized to reconstruct past changes in water-mass boundaries and degree of CaCO3 dissolution. The samples represent Recent to late Holocene sedimentation in water depths from < 1,000 to more than 5,000 m and from 30°S to 65°S latitude. The transitions between the Antarctic fauna (factor 2) and Subantarctic fauna (factor 3), and between the Subantarctic fauna and the transitional-subtropical fauna (factor 1), accurately reflect the water-mass boundaries of the Polar Front and Subtropical Convergence, respectively, and provide useful indicators for paleoceanographic reconstructions. Changes in the species composition, and species diversity identify the foraminiferal lysocline at ∼4,000 m. A dissolution index based on the solution susceptibility ranking of planktonic foraminifera is biased toward high values by the natural increase in Ntogloboquadrina pachyderma with increasing latitude. First-order dissolution effects related to increased water depth produce (a) increases in test fragmentation and benthic foraminiferal abundance and (b) decreases in coarse fraction >63µm and planktonic foraminiferal abundance. A schematic model of dissolution and productivity changes in high latitudes of the Southeast Indian Ocean shows (1) that the percentage of N. pachyderma is the least susceptible to dissolution effects, (2) faunal dissolution effects are most evident in regions of high diversity gradients such as the Polar Front and Subtropical Convergence, and (3) that the numbers of benthic foraminifera and of foraminiferal fragments provide the best indicators of spatial and temporal variations in dissolution intensity in the Southern Ocean.

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