Living Neogloboquadrina pachyderma sin and its distribution in the sediments from Oman and Somalia upwelling areas

Microfaunal analysis of plankton nets collected offshore Oman/Yemen and sed iment traps offshore Somalia shows that Neogloboquadrina pachyderma sin is abundant during the SW monsoon upwelling. The sediment traps recorded the h ighest flux (136 specimens m(-2) day(-1)) and the highest relative frequenc y (3.55% of the total foraminifera assemblage) of the species during this p eriod. During the intermonsoon it became less abundant and decreased in siz e, and only very few N. pachyderma sin were found in the water column durin g the NE monsoon. Sediment trap and Recent sediment data collected along a downslope transect off Somalia show that the species frequency decreases of fshore. The highest concentrations of N. pachyderma sin in plankton nets of f Oman/Yemen were found at a depth of 300-500 m. However, the oxygen isotop e compositions of N, pachyderma sin at the depth of 300-500 m and from the surface 8 m are identical, and also similar to that of Globigerina bulloide s which reflects properties of the upper 25 m of the water column. The carb on isotope values are relatively consistent within the studied samples from both areas except for the specimens from the surface water samples offshor e Oman/Yemen. The data show that the delta(18)O ratio of N. pachyderma sin is in equilibrium with the ambient water while the delta(13)C values are at least 0.8 parts per thousand lower. We infer that N. pachyderma sin in the Arabian Sea reproduces, growths and calcifies in the upper 25 m of the wat er column, but possibly descends into deeper waters later during its life c ycle. (C) 1999 Elsevier Science B.V. All rights reserved.