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 organical ly 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 stain ed 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)). S pecies richness (E(S-100)), diversity (H', Fishers Alpha index) and evennes s (J') were much lower, and dominance (R1D) was higher, at 412 m compared w ith 3350 m. At 412 m, small calcareous foraminifera predominated and soft-s helled allogromiids and sacamminids were a minor faunal element. At 3350 m, calcareous individuals were much less common and allogromiids and saccammi nids formed a substantial component of the fauna. There were also strong co ntrasts between the foraminiferal macrofauna( > 300-mu m fraction) at these two sites; relatively small species of Bathysiphon, Globobulimina and Lage nammina dominated at 412 m, very large, tubular, agglutinated species of Ba thysiphon, Hyperammina, Rhabdammina and Saccorhiza were important at 3350 m . Our observations suggest that, because they contain fewer soft-shelled an d agglutinated foraminifera, a smaller proportion of bathyal, low-oxygen fa unas is lost during fossilization compared to faunas from well-oxygenated e nvironments. Trends among foraminifera (> 63 mu m fraction) in the Santa Ba rbara 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 dep th: O-2 = 0.02-1.60 mi l(-1)), matched those observed on the Oman margin. T n particular, soft-shelled monothalamous taxa were rare and large agglutina ted taxa were absent in the most oxygen-depleted ( < 0.20 mi l(-1)) station s. Foraminifera often outnumber metazoans (both meiofaunal and macrofaunal) in bathyal oxygen-depleted settings. However, although phylogenetically dista nt, foraminifera and metazoans exhibit similar population responses to oxyg en depletion; species diversity decreases, dominance increases, and the rel ative 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 th e meiofauna (63-300 mu m), the trend was reversed, foraminifera were 17 tim es more abundant than metazoan taxa at 412 m but only 1.4 times more abunda nt at 3350 In. An abundance of food combined with oxygen levels which are n ot depressed sufficiently to eliminate the more tolerant taxa, probably exp lains why foraminifera and macrofaunal metazoans flourished at the 412-m si te, perhaps to the detriment of the metazoan meiofauna. (C) 1999 Elsevier S cience Ltd. All rights reserved.