‘Live’ benthic foraminifera at an abyssal site in the equatorial Pacific nodule province: Abundance, diversity and taxonomic composition

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Abstract

Replicate sediment samples were obtained from 3 closely spaced stations in the Kaplan East (KE) area of the abyssal eastern Equatorial Pacific (∼15°N, 119°W; ∼4100 m water depth), just below the carbonate compensation depth. At each site, 2 (Stns 827, 838) or 3 (Stn 824) complete cores (57 mm i.d.) were subsampled using 2–3 cut-off syringes of 6.6 cm3 cross-sectional area. The 0–1 cm sediment layers (>32 μm fraction) of these 20 subsamples together yielded 12,513 small, rose-Bengal stained benthic foraminifera dominated by agglutinated taxa, most of them morphologically simple monothalamous types or komokiaceans. Almost two-thirds (65%) of specimens were either obvious fragments, mainly of komokiaceans and tubular foraminifera, or single chambers or small groups of chambers believed to be fragments of very fragile komokiaceans. The remaining 4438 specimens (35%) were considered to be complete individuals. Most (78%) of these complete tests were indeterminate agglutinated spheres (termed ‘psammosphaerids’) that constituted 27.6% of all specimens (complete plus fragments). Complete individuals that could be assigned to either described or undescribed species accounted for 983 specimens (22% of complete tests=7.6% of all specimens); only 26 specimens (0.59% of complete individuals) were calcareous and these had invariably lost their tests through dissolution. Some groups exhibited considerable spatial heterogeneity. For example, 45% of the 3455 indeterminate psammosphaerids and 45% of the 3087 Komokiacean-like chambers occurred in single subcores. A total of 252 morphospecies was recognised; 168 were represented by complete individuals and 84 by fragments. There are clear differences between these Pacific assemblages and those from other oceans; in particular, psammosphaerids and isolated komokiacean chambers appear to be much more prevalent in the Pacific compared to the Atlantic Ocean. Some morphospecies present in Kaplan samples are known from the Atlantic but many are not. Such species may either (1) be ubiquitous but undersampled because they are rare or (2) have geographically patterned distributions. Without further sampling, there is no way to distinguish between these 2 possibilities. Fossilisable tests represent a very small component of the KE assemblage. Many of the delicate, monothalamous species that have little fossilisation potential, including the komokiaceans, accumulate stercomata (waste pellets) and may consume organic material and bacteria associated with sediment. Because of their enormous abundance at abyssal depths, these poorly known taxa probably play a substantial role in carbon cycling over vast areas of the Pacific seafloor.

Introduction

This study forms part of a larger project (the ‘Kaplan project’), funded by the Kaplan Foundation and International Seabed Authority, to examine the biogeography and biodiversity of selected taxa in an area in the eastern equatorial Pacific that has been designated for possible future manganese nodule mining. Two main questions are being addressed by the Kaplan project: (1) how many species live within areas that may be disturbed by mining operations and (2) what are the geographic ranges of these species within the nodule province. The project is focussed on 3 abyssal sites, Kaplan West, Central and East, which span a distance of ∼1500 km from 151°W to 119°W.

Here, we consider one of the target groups, the foraminifera, in samples collected in the Kaplan East (KE) site. Foraminifera are one of the most important constituents of deep-sea benthic communities. In addition to the hard-shelled species which are familiar from geological studies, they include numerous taxa with ‘soft’ delicate, agglutinated or organic-walled shells that are largely undescribed and contribute substantially to the diversity of the group (e.g. Tendal and Hessler, 1977; Snider et al., 1984; Gooday, 2002). Our main aim in this paper is to characterise the taxonomic composition and biodiversity of the ‘live’ (rose Bengal stained) component of these assemblages as a first step towards examining the geographical ranges of species across the 3 Kaplan sites. In addition to complete foraminiferal tests, we evaluate the importance of the fragmented specimens which are often ignored, despite the fact that they form a large proportion of abyssal Pacific assemblages.

Section snippets

Previous research on abyssal Pacific foraminifera

Although benthic foraminifera from the continental margins around the Pacific Ocean are fairly well known, detailed information about faunas from the abyssal Pacific is fairly sparse (Douglas and Woodruff, 1981; Murray, 1991). Brady (1884) included abyssal samples, and one hadal sample, from the North Pacific (3751–5719, 7228 m water depth) and South Pacific (3340–4758 m) in his classic study of the foraminifera collected during the Challenger Expedition (1872–1876). Later, a large number of

Study site

The KE site is centred on 15°N, 119°W within the Clipperton–Clarion Fracture Zone near the East Pacific Rise. Smith and Demopoulos (2003) summarise the general characteristics of the deep Equatorial North Pacific. This area is physically quiescent and lies near or somewhat below the CCD, which is situated between 3500 and ∼4000 m in this part of the Pacific (Seibold and Berger, 1996, Fig. 8.10 therein). The KE site is overlain by relatively productive surface waters as a result of equatorial

Abundance of complete and fragmented foraminifera

In each multicorer sample, the abundance of foraminiferal specimens that were obviously complete ranged from 264 to 1654 specimens (=133 to 835 specimens 10 cm−2) based on pooled subcores (Table 2). In 6 of the 7 cores, indeterminate psammosphaerids represented more than half of these individuals. Foraminiferal tests that were clearly fragmented (mainly tubular species and komokiaceans), and isolated chambers that we believe to be fragments of very fragile komokiaceans, were more abundant than

Discussion

At the KE site in the eastern Equatorial Pacific, foraminifera are abundant and diverse, predominately agglutinated, and very prone to fragmentation, a tendency that makes quantification difficult.

Acknowledgements

We thank Dr. Craig Smith for inviting us to participate in the Kaplan project and Mrs. Kate Davis for help with the figures. We gratefully acknowledge funding from the Kaplan Foundation and International Seabed Authority. Dr. David Scott and two anonymous reviewers made helpful comments on the manuscript.

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