Foraminifera in the Arabian Sea oxygen minimum zone and other oxygen-deficient settings: taxonomic composition, diversity, and relation to metazoan faunas

Previous work has shown that some foraminiferal species thrive in organically enriched, oxygen-depleted environments. Here, we compare 'live' (stained) faunas in multicorer samples (0-1 cm layer) obtained at two sites on the Oman margin, one located at 412m within the oxygen minimum zone (OMZ) (O(2) = 0.13 ml l(-1)), the other located at 3350 m, well below the main OMZ (O(2) similar to 3.00 ml l(-1)). While earlier studies have focused on the hard-shelled (predominantly calcareous) foraminifera, we consider complete stained assemblages, including poorly known, soft-shelled, monothalamous forms. Densities at the 412-m site were much higher (16,107 individuals.10 cm(-2) in the > 63-mu m fraction) than at the 3350-m site (625 indiv.10 cm(-2)). Species richness (E(S(100))), diversity (H', Fishers Alpha index) and evenness (J') were much lower, and dominance (R1D) was higher, at 412 m compared with 3350 m. At 412 m, small calcareous foraminifera predominated and soft-shelled allogromiids and sacamminids were a minor faunal element. At 3350 m, calcareous individuals were much less common and allogromiids and saccamminids formed a substantial component of the fauna. There were also strong contrasts between the foraminiferal macrofauna( > 300-mu m fraction) at these two sites; relatively small species of Bathysiphon, Globobulimina and Lagenammina dominated at 412 m, very large, tubular, agglutinated species of Bathysiphon, Hyperammina, Rhabdammina and Saccorhiza were important at 3350 m. Our observations suggest that, because they contain fewer soft-shelled and agglutinated foraminifera, a smaller proportion of bathyal, low-oxygen faunas is lost during fossilization compared to faunas from well-oxygenated environments. Trends among foraminifera (> 63 mu m fraction) in the Santa Barbara Basin (590 and 610m depth; O(2) = 0.05 and 0.15 ml(-1) respectively), and macrofaunal foraminifera(> 300 mu m) on the Peru margin (300-1250 m depth: O(2) = 0.02-1.60 mi l(-1)), matched those observed on the Oman margin. Tn particular, soft-shelled monothalamous taxa were rare and large agglutinated taxa were absent in the most oxygen-depleted ( < 0.20 mi l(-1)) stations. Foraminifera often outnumber metazoans (both meiofaunal and macrofaunal) in bathyal oxygen-depleted settings. However, although phylogenetically distant, foraminifera and metazoans exhibit similar population responses to oxygen depletion; species diversity decreases, dominance increases, and the relative abundance of the major taxa changes. The foraminiferal macrofauna ( > 300 mu m) were 5 times more abundant than the metazoan macrofauna at 412 m on the Oman margin but 16 times more abundant at the 3350 m site. Among the meiofauna (63-300 mu m), the trend was reversed, foraminifera were 17 times more abundant than metazoan taxa at 412 m but only 1.4 times more abundant at 3350 In. An abundance of food combined with oxygen levels which are not depressed sufficiently to eliminate the more tolerant taxa, probably explains why foraminifera and macrofaunal metazoans flourished at the 412-m site, perhaps to the detriment of the metazoan meiofauna. (C) 1999 Elsevier Science Ltd. All rights reserved.