Recent benthonic foraminifera from the Western Antarctic Ocean

Approximately 60 species of benthonic foraminifera are recorded from 14 samples collected by the RRS Discovery from water depths of 200–4200 m on the Antarctic Peninsula Shelf and Slope and adjacent areas of the Western Antarctic Ocean. Discussion focuses firstly on benthonic foraminiferal abundance, diversity and assemblage composition and the effects of calcite dissolution and, secondly, on the characteristic assemblages of the study area and their relationships to water-masses.


INTRODUCTION Previous studies
In contrast to the Arctic, the Antarctic or Southern Ocean (i.e. the area between Antarctica and the Subtropical Convergence at about 40-50" S) received scant attention from foraminiferologists in the last century.
The earliest account of the Recent benthonic foraminifera from the area is that of Brady (1884). who described the material collected from around Heard Island and Kerguelen Island during the cruise of HMS Challenger .
The high-resolution diatom stratigraphy of the Quarternary of the Scotia Sea is discussed by Jordan & Pudsey (1992).
The geology of Antarctica is discussed by, among others, St John (1990). from the Scotia Sea to the Southwest Atlantic at about eastwards in the northern limb of the cyclonic Weddell Gyre in the Northern Weddell Sea (Deacon, 1979). Some AABW flows into the southern Scotia Sea (Gordon, 1971: Jordan & Pudsey, 1992). Indeed, cold deep water of Weddell Sea type has been detected as far west as the South Shetlands (Nowlin & Zenk, 1988). CPDW and AABW meet at the so-called Weddell-Scotia Confluence (Patterson & Sievers, 1980), which widens and becomes more diffuse east of the South Orkneys (Foster & Middleton, 1984). Comparatively little is known of the shallow water-masses of the study area. However, unpublished expendable bathythermograph (XBT) measurements show bottom temperatures to be around 1.3"C. This suggests that the Antarctic Peninsula Shelf, unlike other shelf areas around the Antarctic, is not a reservoir of cold, dense water (Kilworth, 1983). Material. Fourteen surface samples were made available for foraminifera1 analysis through the good offices of the British Antarctic Survey, together with another ten shallow subsurface samples (which were ignored other than for systematic purposes as they were too few in Recent Benthonic Foraminifera number to provide any meaningful regional stratigraphic or palaeoenvironmental information). Two of the samples (878TC030 0-5 cm and 878TC050 CC) were from trigger cores. The remaining twelve (prefixed D) were obtained opportunistically using a pipe dredge rigged beneath a rock dredge being dragged up slope in order to sample hard rocks (basalts, metasediments, etc.) in connection with various tectonic studies. A pipe dredge deployed like this (see also Kidd et al. (1990)) can collect a large quantity (up to 30-401) of soft sediment, of which 1-2 1 i:i normally retained for study. It probably fills up almost immediately on striking the sea-floor rather than gradually over a range of depths. However, it can sink into soft sdeiment under its own weight and retrieve sediment from the shallow subsurface rather than from the surface. As noted below, the sample from Station D11SP could be of Latest Pliocene-Early Pleistocene, rather than Holocene, age. Methods. 10cm' splits of the study samples, stained for live protoplasm (using Rose Bengal), were disaggregated using standard micropalaeontological techniques. The dried 'disaggregated material was then passed through a nest of sieves (the finest one of which has a 75 p m mesh), and the benthonic foraminifera from each sieve fraction were picked, sorted and identified.

RESULTS
The semiquantitative distribution of benthonic foraminiferal species in the studied samples is shown in Table 2. Note that all of the species were only recorded 'dead' (unstained by Rose Bengal).

Antarctic Peninsula Shelf
The seven samples from the Antarctic Peninsula Shelf are from water depths of 200-575 m (Table 1). At the sampling stations, lithologies vary from oozes and silty and sandy muds to ashy muds and crystal and/or vitric volcanic ashes (some are situated on the flanks of volcanoes) (Table 1). At D140F', the bottom temperature is 1.3"C, and the current velocity up to 25 cm s-I.
The Antarctic Peninsula Shelf samples are generally characterized by low benthonic foraminiferal abundance and diversity. One (D139P) is barren. Agglutinating and calcareous benthonic foraminifera occur in equal proportions. Rare planktonic foraminifera (Neoglohoquadrina padiytlerma) and locally abundant siliceous plankton (diatoms) also occur.
Abundance and diversity are highest in D140P (480 m) and D13SP (550 m), which are situated on topographic 'highs' on the deeper parts of the shelf. Conspicuous benthonic foraminiferal taxa, common or abundant in one or other of these samples, include Hormosininae div. spp., Qiiinqiiel~~culina spp. and Trifarinu ungulosu.

Drake Passage
The two samples from the Drake Passage are from water depth!; of 2'700-3000 m. At both the sampling stations. the lithologies are 'sandy muds', the bottom temperature 0 to -O.S"C, arid the current 'strong'. The samples are characterized by moderate benthonic foraminiferal abun-dance and diversity. Agglutinating and calcareous bcnthonic foraminifera occur in equal proportions. The most conspicuous species are Martinottiella communis, Pyrgo murrhina, Uvigerinu sp. and Cihicides ex gr. lohatulus. Abundant planktonic foraminifera (Globigerina hulloides, Glohorotulia spp., Neogloboquadrina pachyderma) also occur.

South Sandwich
The two samples from South Sandwich are from water depths of 2700--3200111. At both the sampling stations, lithologies are 'diatom-bearing ashy muds'. At D108P, the bottom temperature is 0 to -0.5"C, and the current velocity 4-5 cm s I.
The sample from D108P is characterized by low to moderate benthonic foraminiferal abundance and diversity. Agglutinating foraminifera predominate, and indeterminate astrorhizids are common.
The sample from D115P is also characterized by low to moderate benthonic foraminiferal abundance and diversity. with agglutinating foraminifera predominant. Planktonic foraminifera (Neoglohoquadrina pachyderma) and diatoms also occur. The diatoms include Actinocyclus ingens, Coscinodiscus elliptopora and ?reworked Denticulopsis sp.. indicating that this sample could be of Latest Pliocene-Early Pleistocene, rather than Holocene, age.

Scotia Sea
The two samples from the Scotia Sea are from water depths of 2730-2900m. At D123P, the lithology is a 'silty ooze', and the bottom temperature is 0 to -0.3"C. At 878TC030, the lithology is a 'mud-bearing ooze'. The samples are characterized by low benthonic foraminiferal abundance and diversity.

Antarctic Peninsula Slope
The sample from the Antarctic Peninsula Slope is from a water depth of 4200 m. The lithology at the sampling station is a 'silty mud'. The bottom temperature is 0.2-0.4"C. and the current velocity up to 1 0 c m s I. The sample is characterized by low benthonic foraminiferal abundance and diversity. Agglutinating foraminifera predominate. The only calcareous foraminifera found are a single indeterminate benthonic species, and a single reworked Cretaceous planktonic species (Glohigerinelloides sp.). Indeterminate astrorhizids, Crihrostomoides suhglohosus and Martinottiella communis are all common. Diatoms are again abundant.

DISCUSSION
Abundance, diversity and assemblage composition, and the effects of calcite dissolution To summarize the results given above, the study samples as a whole are characterized by low to moderate bcnthonic foraminiferal abundance and diversity (which may be at least in part attributable to the small sample size). The highest values are associated with mid-shelf topographic  Foraminifera1 Association (Murray, 1991) ,  (480-550 m), and in the Drake Passage (2700-3000m). Values are low by comparison with those obtained from bathyal and abyssal samples from low to moderate latitudes, but closely comparable with those obtained from bathyal and abyssal samples from high latitudes in the northern hemisphere (Jones, 1084).
Agglutinating and calcareous benthonic species occur in equal proportions in the study samples on the Antarctic Peninsula Shelf (200-575 m), while agglutinating species generally predominate on the Antarctic Peninsula Slope and in South Sandwich, the Drake Passage and the Scotia Sea (2700-4200 m). This is probably at least partly explicable in terms of local inhibition of secretion (andlor preservation) of calcareous tests in corrosive, carbon dioxide-enriched bottom water. Note, however, that the relation between the preservation and partial or total dissolution of calcareous foraminifera is dependent less on depth than on water-mass association (see Murray, 1991; see also below).
Plankl-onic foraminifera (calcareous) and diatoms (siliceous) are widespread and locally abundant in the study samples. Planktonic foraminifera are most common in the Drake Passage (2700-2900 m), where they occur to the total exclusion of diatoms. Diatoms are most common in the Scotia Sea (2730-2900 m), and on the Antarctic Peninsula Slope (4200m), where they occur to the total exclusion of planktonic foraminifera (possibly because of dissolution effects at or near the local calcite compensation depth (CCD).
The Antarctic Peninsula Shelf assemblages locally characterized by Hormosininae div. spp., Quinqueloculina spp., and Trifarina angulosa (200-575m) are similar to those of the 'Fresh Shelf Water Facies' of Anderson (1975), the 'Trifarina angulosa Assemblage' of Mackensen et al. (1990), and the 'Trifarina angulosalearlandi Association' of Murray (1991) (which are typically similarly dominated by calcareous foraminifera, though locally affected by partial dissolution). Mackensen et al. (1990) recorded their Trifarina angulosa Assemblage 'live' and 'dead' at the shelf break and on the uppermost continental slope, and in predominantly sandy sediments (in areas of strong bottom currents) (in the Eastern Weddell Sea). Murray (1991) recor'ded his Trifurina angulosalearlandi Association in water depths of 0-2100 m (348-713 m in the Weddell Sea), in muddy sandy lithologies, and in bottom temperatures of -1.9 to 0.6"C. He inferred a correlation with Fresh Shelf Water, a variant of Antarctic Surface Water occurring in ice-free areas in the Weddell Sea.
The Drake Passage assemblages characterized by Pyrgo rnurrhina, Uuigerina sp. and Cibicides ex gr. lobatulus (2700-3000m) are similar to those of the 'Deep Water Calcareous-Arenaceous Facies' of Anderson (1975) and the 'Cassidulina crassa Association of Murray (1991) (which are dominated by calcareous foraminifera). Murray (1991) recorded his Cassidulina crassa Association in water depths of 50-4008 m (104-4008 m in the Drake Passage and Scotia Sea), in fine sandy lithologies, and in bottom temperatures of -1.9 to 0.6"C and salinities of 33. 96-35.19~~1. These temperature and salinity ranges embrace those of both CPDW and AABW.
The South Sandwich assemblage, characterized by indeterminate astrorhizids (3200 m), and the Antarctic Peninsula Slope assemblage, characterized by indeterminate astrorhizids, Cribrostomoides suhglobosus and Martinottiella communis (4200m), are similar to the 'Abyssal Facies' of Anderson (1973, the 'Cribrostomoides subglobosus Assemblage' of Mackensen et al. (1990 Murray (1991) inferred no correlation between foraminiferal associations and water-masses in the Scotia Arc area, commenting that there '. . . the distribution of water masses is complex and poorly understood. . .'.

CONCLUSIONS
The study samples, from water depths of 200-4200 m on the Antarctic Peninsula Shelf and Slope and adjacent areas of the Western Antarctic Ocean, are characterized by generally low to moderate benthonic foraminifera1 abundance and diversity (which may be at least in part attributable to the small sample size). Agglutinating and calcareous benthonic species occur in equal proportions in the study samples from 200-575 m, while agglutinating species generally predominate in the samples from 2700-4200m. The low abundance and diversity values and shallow threshold t o agglutinatedominated assemblages may both be related to inhibition of secretion (and/or preservation) of calcareous tests in corrosive bottom waters.
Antarctic Peninsula Shelf assemblages, locally characterized by Hormosininae div. spp., Quinqueloculina spp., and Trifarina angitlosa (200-575 m), are similar to those of the 'Fresh Shelf Water Facies' of Anderson (1975), the 'Trifarina angulosa Assemblage' of Mackensen et al. (1990), and the ' Trifarina angulosalearlandi Association' of Murray (1991), which may be correlated with Fresh Shelf Water (a variant of Antarctic Surface Water occurring in ice-free areas in the Weddell Sea).
Drake Passage assemblages, characterized by Pyrgo murrhina, Uuigerina sp. and Cibicides ex gr. lohatulus (2700-3000m), are similar to those of the 'Deep Water Calcareous-Arenaceous Facies' of Anderson (1975) and the 'Cassidulina c r a n a Association' of Murray (1991), which can be correlated with either CPDW or AABW.