Benthic foraminiferal trends in relation to an organic enrichment gradient on the continental slope (850 m water depth) off North Carolina (USA)

Smart, C.W. and Gooday, A.J. (2006) Benthic foraminiferal trends in relation to an organic enrichment gradient on the continental slope (850 m water depth) off North Carolina (USA). Journal of Foraminiferal Research, 36 (1), 34-43. (doi:10.2113/36.1.34).

Abstract

We analyzed benthic foraminiferal assemblages in core samples collected by a submersible at three sites (I, II, III) along an isobathyal gradient of organic enrichment on the North Carolina slope (850 m water depth). The 0–2, 2–5, 5–10, 10–15 cm layers of each core were sieved into 63–125, 125–150 and 150–300 µm size fractions, and each fraction was stained with rose Bengal and dry sorted. Together, the samples yielded 284 benthic foraminiferal species. The vast majority of tests (97%) were unstained; live specimens were scarce at Site I and very rare at Site II, but more common at Site III. The preservation of dead calcareous benthic and planktonic foraminiferal tests varied between sites. Calcareous tests were abundant at Sites I and II, but virtually absent at Site III except in the 2–5 cm layer, where they were more numerous than at the other two sites. Dead benthic tests were also less abundant in the 0–2 cm, 5–10 cm and particularly the 10–15 cm layers at Site III than in the corresponding layers at Sites I and II, and were dominated by species with agglutinated rather than calcareous walls. These patterns suggest that substantial dissolution has occurred at Site III. In striking contrast, the 2–5 cm layer of our Site III core yielded numerous calcareous planktonic and benthic foraminiferal tests. Possibly, this layer consisted of sediment that had been transported by a local mass movement event. In general, diversity trends exhibited by the dead assemblages (63–300 and 63–125 µm fractions) were similar to those reported by Gooday and others (2001) for the live metazoan and foraminiferal macrofauna (>300 µm): diversity and species richness were higher at Sites I and II than at the organically enriched Site III; dominance was higher at Site III than at the other two sites. However, at Site III, diversity was apparently depressed by the destruction of calcareous foraminiferal tests. Some species (Angulogerina angulosa, Bolivina paula, Bulimina aculeata, Cibicidoides pseudoungerianus, Globocassidulina subglobosa, Rosalina floridana and Stetsonia minuta) were common to all three sites. However, the most abundant species (Bolivina spissa, Eggerella advena, Paratrochammina ?bartrami, Veleroninoides jeffreysii) at Site III were different from those at Sites I and II, among both the live and dead assemblages. The common species (e.g., Eggerella advena) at Site III are more typical of shallow-water, inner shelf settings, a result that is consistent with published observations on metazoan macrofauna (>300 µm) at similar depths on the slope off Cape Hatteras.

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