Distribution of Neogloboquadrina pachyderma in plankton tows, sediment traps and core tops from the Arabian Sea

Understanding the dynamics in faunal composition and species abundances of planktic foraminiferal assemblages in their modern environment is important for a better interpretation of their fossil records. Most studies suggest that the cold-water planktic foraminifer Neogloboquadrina pachyderma (previously called N. pachyderma (sin); Darling et al, 2006), is a reliable indicator for cold, high-latitude environments. However, there is growing evidence that N. pachyderma may also be present in the upwelling cells of the Arabian Sea (e.g. off the Oman coast during the SW monsoon upwelling), consistent to its affinity to cold waters. In order to further document the presence and distribution of N. pachyderma in the Arabian Sea, we have analyzed flux rates, concentrations, and relative abundances in plankton nets, in a sediment trap (CAST, Central Arabian Sea Trap) and in surface sediments off the Oman margin and central Arabian Sea. The plankton multinet transect (mesh size >100μm, from September-October 2007) off the Oman margin and across the Arabian Sea basin records regionally low concentrations both within and outside the upwelling area. Significantly, the highest concentration of the species (7 specimens /m3) was recorded at a depth of 200–300 m, approximately 460 km distance from the coast. In the sediment samples, the highest relative abundance of this species reaches 3.7% of the total planktic foraminiferal assemblage >125 μm within the area off Oman margin, but no N. pachyderma were found in the basin area of the Central Arabian Sea. Out of 16 studied sediment traps samples of CAST 10 (1993-1994) only three contained the species in extremely low numbers with the highest relative abundance (0.52% of the total foraminifera assemblage) and highest flux in spring (2.37 specimens m-² day- ¹) and some flux (1.19 specimens m-² day- ¹) in fall. As we have no data about presence of N. pachyderma during the summer months in that incomplete time series, specimens were found in the trap in both intermonsoon periods. The results indicate that N.pachyderma is present in both coastal upwelling regions of the Arabian Sea and non-upwelling regions further offshore. Since N. pachyderma is also found in potentially advected subsurface waters, a southern linkage to deeper cold Antarctic source water could explain their presence. However, genetic analyses may argue against this since we have identified a new genotype of N. pachyderma within the Arabian Sea, distinct from those found in the Southern Ocean. Further analyses will be required to better understand the observed patterns.