Northern Rockall Trough Research Articles

The reproductive biology of the deep-sea asteroids Benthopecten simplex (Perrier), Pectinaster filholi Perrier, and Pontaster tenuispinus Düben & Koren (phanerozonia : benthopectinidae) from the rockall trough

Samples of three species of benthopectinid starfish were collected in a time-series from stations in the northern Rockall Trough. A detailed study was made of the reproductive biology of Benthopecten simplex (Perrier) from a fixed station at 2200 m. Studies of the gametogenic cycles of Pectinaster fllholi Perrier and Pontaster tenuispinus Düben & Koren from less regular and more scattered samplings are also presented. The oogenic cycles of the three species are similar. Growing oocytes line the ovary wall and come to lie in the lumen only when well developed. Accessory cells surrounding young oocytes become more numerous as the oocytes develop. Vitellogenesis begins when oocyte diameter reaches ≈ 250–300 μm and oocytes attain a maximum diameter of 950 μm in Benthopecten simplex, 850 μm in Pectinaster filholi, and 800 μm in Pontaster tenuispinus. Unspent oocytes are broken down by phagocytes and degenerate ovaries become filled with the breakdown products. Internal degeneration of large oocytes is not common. Size frequency data show a dominance of previtellogenic oocytes and a pattern of continuous growth of vitellogenic oocytes. There is no evidence of synchrony of growth between individuals and no suggestion of seasonal reproduction is evident from summated sample data. Spermatogenesis follows the pattern previously established for deep-sea asteroids. A mature male remains in a state of constant ripeness, ready to shed sperm at any chance encounter with a female releasing ripe eggs. There is no evidence of brooding in any of the species and from the large size and yolk content of the egg, we can infer direct lecithotrophic development demersally.

Volcanic ash deposits of early Eocene age from the Rockall Trough

The stratigraphy of the Rockall Trough (Fig. 1) has proved difficult to elucidate because a mantle of Recent sediments usually prevents piston corers and dredges from sampling older accumulations. Although a seismic stratigraphy has been erected1,2, considerable uncertainty surrounds the history of sedimentation and, consequently, the petroleum potential of the region. The upper 500–1,000 m of the succession are clearly Cenozoic deposits whose distribution has been strongly influenced by movements of Norwegian Sea overflow water1. Deeper in the section, major current-controlled accumulations are absent, a feature observed elsewhere in the Atlantic1. In view of the paucity of samples from these older deposits3, we report here the recovery of sediments that were laid down in the northern Rockall Trough before polar waters radically changed the depositional regime. The sediments record a period of explosive volcanicity during the early Eocene in the vicinity of the Wyville–Thomson Ridge.

Magnetic fabric of bioturbated sediment from the northern Rockall Trough: Comparison with modern currents

The properties of five short sediment cores from the northern Rockall Trough have been examined. Although all show bioturbation a substantial part of the depositional magnetic properties remains. Consideration of the composition of the sediments, the consistency of the magnetic properties from core to core and comparison with the magnetism of sediments deposited under known conditions suggest that the results indicate trends of depositing currents. The relationship of the trends to current directions observed day-to-day is now being investigated.

The reproductive biology of Hymenaster membranaceus from the Rockall Trough, North-East Atlantic Ocean, with notes on H. gennaeus

Samples of the deep-sea spinulosan asteroid Hymenaster membranaceus Wyville Thomson were collected in a timeseries of 19 bottom trawls spanning the period April 1978 to October 1981 from a 2200 m-deep station in the northern Rockall Trough. The reproductive biology of this species was studied from histological sections of the gonad, and compared with that of H. gennaeus H. L. Clark which was collected in the same hauls. At first sexual development, oogonia develop in nests surrounded by small accessory cells. Previtellogenic oocytes remain in the periphery but, at maturity, oocytes ranging up to 1 100 μm fill the ovary. A variety of accessory cells pack the lumen and may be nutritive or degenerative. It appears that a small number of oocytes are spawned intermittently, but there is no evidence of overproduction and break-down of superfluous oocytes. A few large oocytes become senescent and undergo internal break-down, releasing periodic acid Schiff-positive material into the lumen. Size-frequencies of oocytes indicate that eggs may be spawned as a continuous slow release, and there is no evidence of reproductive synchrony between or within samples. On reaching maturity, males appear always to be ready to release spermatozoa. Spawning is probably stimulated by egg release during chance encounters with mature females. There is no evidence for brooding, and from the large size and yolky nature of the egg direct lecithotrophic development at or near the seabed is inferred. A limited histological study of H. gennaeus indicates that egg production is very similar, but the two species differ in the nature of the accessory cells and amorphous material filling the lumen.

The magnetisation of Rosemary Bank Seamount, Rockall Trough, northeast Atlantic

Rosemary Bank is a non-uniformly magnetised seamount in the northern Rockall Trough. The reversely magnetised major component of the anomaly field was simulated by a numerical method and modelled using the Talwani three-dimensional magnetics program. The results suggest a higher Koenigsberger ratio than earlier reported for Rosemary Bank and a remanent magnetisation vector compatible with post-Jurassic formation and probably of a Late Cretaceous to Tertiary age. The limited depth to the base of the model implies that Rosemary Bank post-dates the underlying basement in agreement with a volcanic origin. The residual of the observed anomaly field is interpreted as being caused by normally magnetised bodies within and on top of the bank. This suggests subsequent volcanic activity during an interval of normal polarity.

The overflow of Norwegian sea deep water across the wyville-thomson Ridge

Accumulating evidence has pointed to a source of Northeast Atlantic Deep Water in the northern Rockall Trough and prompted a re-examination of geological and hydrographic data. The first showed that the sedimentary Feni Ridge could be traced along the length of the westernRockall Through to the foot of the Wyville-Thomson Ridge, and the second that dense water frequently existed at the lowest point of the ridge crest. Collected soundings and new data suggested a path by which this might enter the Rockall Trough, and overflow of Norweigian Sea Deep Water was observed taking place from the ridge crest in a narrow layer in July 1972. The flow is probably intermittent and appears to entrain Atlantic Water from intermediate depths.