Northern Scotia Sea Research Articles

Linking predator and prey behaviour: contrasts between Antarctic fur seals and macaroni penguins at South Georgia

Antarctic fur seals Arctocephalus gazella and macaroni penguins Eudyptes chrysolophus are the two main land-based krill Euphausia superba consumers in the northern Scotia Sea. Using a combination of concurrent at-sea (predator observations, net hauls and multi-frequency acoustics), and land-based (animal tracking and diet analysis) techniques, we examined variability in the foraging ecology of these sympatric top predators during the austral summer and autumn of 2004. Krill availability derived from acoustic surveys was low during summer, increasing in autumn. During the breeding season, krill occurred in 80% of fur seal diet samples, with fish remains in 37% of samples. Penguin diets contained the highest proportion of fish in over 20 years of routine monitoring (46% by mass; particularly the myctophid Electrona antarctica), with krill (33%) and amphipods (Themisto gaudichaudii; 21%) also occurring. When constrained by the need to return and feed their offspring both predator species foraged to the northwest of South Georgia, consistent with an area of high macrozooplankton biomass, but fur seals were apparently more successful at exploiting krill. When unconstrained by chick-rearing (during March) penguins foraged close to the Shag Rocks shelf-break, probably exploiting the high daytime biomass of fish in this area. Penguins and seals are able to respond differently to periods of reduced krill abundance (in terms of variability in diet and foraging behaviour), without detriment to the breeding success of either species. This highlights the importance of myctophid fish as an alternative trophic pathway for land-based predators in the Scotia Sea ecosystem.

Feeding ecology of myctophid fishes in the northern Scotia Sea

The diets of 9 species of myctophid fishes, Electrona carlsbergi, E. antarctica, Gymno- scopelus fraseri, G. nicholsi, G. braueri, Protomyctophum bolini, P. choriodon, Krefftichthys ander- ssoni and Nannobrachium achirus, were investigated during austral autumn in the northern Scotia Sea. Based on the percent index of relative importance (%IRI), the data suggest dietary specialisation in some species, which may permit resource partitioning. Hierarchical agglomerative cluster analy- sis of Bray-Curtis similarity (60% threshold) separated the myctophid community into distinct feed- ing guilds. One group (G. braueri and E. antarctica) fed principally on Themisto gaudichaudii, another (P. choriodon and G. fraseri) primarily on copepods (Metridia spp. and Rhincalanus gigas), and a third group (G. nicholsi and P. bolini) consumed copepods and euphausids (mostly Metridia spp. and Euphausia frigida). The diets of E. carlsbergi and K. anderssoni differed from the other spe- cies, with E. carlsbergi being the only species that consumed salps. There was a general switch in diet from copepods to euphausiids and amphipods as the myctophid predator size increased. Dietary spe- cialisation is likely the result of a combination of predator size, gape size, filtering capacity of the gill rakers and the vertical distribution of predators and prey. Antarctic krill were only consumed by the larger myctophids, which represented a numerically minor part of the myctophid community, sup- porting the concept that myctophids can provide a krill-independent link between secondary produc- tion and higher trophic levels. However, the northern Scotia Sea is dominated by adult krill, which are only suitable as prey for larger fish. In the northern Scotia Sea, myctophid predation had a very small impact on copepod production but a higher impact on macrozooplankton, with a best-estimate of 4% of the daily production of Themisto gaudichaudii and 6% of that of Euphausia superba being consumed.

Patterns in the distribution of myctophid fish in the northern Scotia Sea ecosystem

The mesopelagic fish community of the northern Scotia Sea was investigated during the austral autumn using multi-frequency acoustics, opening and closing nets and pelagic trawls fished from the surface to 1,000 m. The Family Myctophidae (15 species in 5 genera) dominated the ichthyofauna, with larval notothenids caught over the South Georgia shelf and bathylagids and stomiids abundant in deeper hauls. The biomass of myctophids was estimated to be 2.93 g wet weight 1,000 m−3, with Electrona carlsbergi, E. antarctica, Protomyctophum bolini, P. choriodon, Gymnoscopelus braueri, G. fraseri, G. nicholsi and Krefftichthys anderssoni, being the most abundant species. Analysis of community structure indicated a high level of depth stratification within the myctophids, with evidence of diurnal vertical migration in some, but not all, species. Length-frequencies of G. braueri, G. nicholsi, E. antarctica and K. anderssoni were multimodal, suggesting that all life stages may be present in the northern Scotia Sea. In contrast, P. choriodon, P. bolini, G. fraseri and E. carlsbergi had unimodal distributions despite having multi-year lifecycles, indicating that they probably migrate into the region from warmer areas to the north.

Dinoflagellate cyst evidence for Quaternary palaeoceanographic change in the northern Scotia Sea, South Atlantic Ocean

A dinoflagellate cyst record has been examined from two cores recovered from the crest and margins of a sediment drift in water depths of 3500–4500 m in the northern Scotia Sea, South Atlantic Ocean. 46 dinoflagellate cyst analyses have been conducted, covering a time-span ranging from the Holocene down to MIS 6, representing about 160 ka. This provides a resolution of approximately 3500 years. The sediments are predominantly fine-grained contourites and diatom-rich hemipelagites, capped by sandy-silty contourites rich in the planktonic foraminifer Neogloboquadrina pachyderma. The cores can be subdivided into four dinoflagellate cyst units, supported by diatom and radiolarian biostratigraphy, biogenic barium geochemistry, oxygen isotopes and magnetic susceptibility curves. The youngest dinoflagellate cyst unit was found only in the core from the drift crest, and has been dated at between 4380 and 12 275 yr BP by radiocarbon dating. The unit is characterised by autotrophic dinoflagellate cysts with similarities to modern cysts from the region, although the assemblages display some marked internal variability that may suggest rather unstable Holocene oceanographic conditions. The next downhole unit was found in both cores and provided radiocarbon ages of 14 580 yr BP and 16 840 yr BP. Recovered cyst assemblages suggest deposition within or near to maximum sea ice limits, corresponding to a northward shift of the Antarctic Convergence during the Last Glacial Maximum within oxygen Marine Isotope Stage (MIS) 2. The final two units lack radiocarbon age control. The next downhole unit is characterised by heterotrophic dinoflagellate cysts such as round, brown Protoperidinium spp. and Selenopemphix antarctica, indicating deposition within maximum sea ice limits. This unit has a wide age range within MIS 5a–d including MIS 6. Toward the base of this unit the assemblage contains autotrophic dinoflagellate cysts such as Impagidium spp., Protoceratium reticulatum and Spiniferites spp., indicative of warmer, interglacial conditions, suggesting a retreat of the Antarctic Convergence and interpreted as MIS 5e, the last interglacial. MIS 3–4 are not resolved by the dinoflagellate cysts. The oldest unit recovered is marked by a return to heterotrophic dinoflagellate cysts with deposition in the presence of seasonal sea ice and open water, suggesting the Antarctic Convergence was now northward of the core site. The dinoflagellate cyst assemblages recovered from the cores illustrate the glacial–interglacial dynamism between two important biogeographical boundaries; the Antarctic Convergence to the north and the maximum sea ice limit toward the south.

Provenance and transport of terrigenous sediment in the south Atlantic Ocean and their relations to glacial and interglacial cycles: Nd and Sr isotopic evidence

Sr and Nd isotopic compositions of Late Quaternary surface sediment and sediment cores from the south Atlantic and southeast Pacific sectors of the Southern Ocean are used to constrain the provenance and transport mechanisms of their terrigenous component. We report isotopic and mineralogical data for core samples from three localities, the Mid-Atlantic Ridge at 41°S and the northern and southern Scotia Sea. In addition, data for surface sediment samples from the south Atlantic and southeast Pacific sectors of the Southern Ocean are presented. The variations of Sr and Nd isotopic compositions of the bulk sediment samples in all cores were correlated with the magnetic susceptibility of the sediment and with the inferred glacial–interglacial stages. The isotopic data indicate that, during glacial periods, sediment was delivered from continental crust with a shorter residence time than that supplying material during interglacial periods. At the core site near the Mid-Atlantic Ridge, Nd isotopic, combined with mineralogical evidence indicates interglacial period deposition of a relatively high amount of kaolinite and silt with low ϵ Nd values < −8. The material was probably supplied by North Atlantic Deep Water from low latitudes. For glacial periods, a high contribution of silt and clay with ϵ Nd > −4.5, probably derived from southern South America, was indicated. The glacial–interglacial shift in sources may be due to either a decreasing influence of North Atlantic Deep Water during glacial times or by a larger contribution of glaciogenic detritus from southern South America. At the core site in the northern Scotia Sea, sediment of interglacial periods is dominated by smectite with ϵ Nd < − 6 and silt with ϵ Nd > −4. We suggest that smectite was derived from the Falkland shelf and silt was derived from the Argentinian shelf. During glacial periods, the Argentinian shelf was an important source for silt and chlorite with ϵ Nd > −4. The contribution from the Falkland shelf seems to have remained similar during glacial and interglacial periods. Hydrographic transport by bottom currents and turbidites could account for the high glacial detrital flux. An evaluation of the significance of an aeolian contribution to deep sea sediment suggests that it plays only a minor role. In the southern Scotia Sea, the Antarctic Peninsula is considered an important source for young material with ϵ Nd > −4, in particular during glacial periods. During interglacial periods, sediment supply from the Antarctic Peninsula was lower than during glacial times, resulting in a relatively high contribution of old material (ϵ Nd < −8) from East Antarctica. Deep water currents and icebergs could account for the transport of the old component to the southern Scotia Sea. The accumulation rates of material from the various source regions for glacial times are in agreement with an increase in the strength of the Antarctic Circumpolar Current. The production rate and the circulation pattern of bottom water in the Weddell Sea appear to have remained similar over most of the last 150 kyr.

Antarctic Circumpolar Deep Water; a Quaternary paleoflow record from the northern Scotia Sea, South Atlantic Ocean

In the northern Scotia Sea, the main pathway of Circumpolar Deep Water (CPDW) flows north to pass through a deep gap in the North Scotia Ridge before turning east into the Falkland Trough. A sediment drift has developed on the seabed since the early-middle Miocene, coincident with the opening of Drake Passage and the inception of deep-water flow. Seismic and acoustic surveys show that the drift covers an area of 10,500 km 2 and forms a broadly asymmetrical mound up to 800 m thick. There is a zone of sediment thinning along the northwestern margin, the result of accentuated CPDW flow around rough ocean floor topography. Small debris flows originating around the margins of the drift suggest localized instability and high sediment supply. Four cores 3-9 m long have been recovered from the crest and margins of the drift in water depths of 3900-4300 m. Biostratigraphy and chemostratigraphy reveal that the longest core extends down to oxygen isotope stage 10 (approx. 370 ka). The sediments are predominantly fine-grained contourites and diatom-rich hemipelagites, capped by sandy-silty contourites rich in the planktonic foraminifer Neogloboquadrina pachyderma. Grain-size analysis of the fine fraction, finer than 4 phi (63 mm), combined with radiocarbon (AMS) dating and magnetic susceptibility, provide an indication of relative CPDW strength over the last 18 ka. Shortly after the last glacial maximum (LGM), at approximately 17 ka, silt modes fluctuated from 5.5 phi to up to 6.25 phi; this increased current winnowing is indicative of an unstable CPDW, with stormier glacial benthic conditions producing sporadic, high-energy currents across the drift crest and flanks. At approximately 12,280 ka, an increase in sediment sorting is noted, indicative of a strong flow of CPDW over the drift crest, suggesting an unstable and fluctuating deep-water flow. During deglaciation and into the Holocene, at approximately 10 ka, CPDW flow stabilized, becoming less vigorous across the drift crest and flanks with silt modes from 6 phi to 5.5 phi accompanied by increased sorting of the sediments. The gross average sedimentation rate from the crest of the drift is 11.2 cm/ky compared to 2.3 cm/ky on the southeastern flank. The unsteadiness of CPDW during glacials compared to interglacial periods may be the result of stronger wind forcing and a northward shift in the Polar Front. Older CPDW flow records from the cores suggest variable and cyclic bottom-current flow corresponding to glacial-interglacial episodes. Modern CPDW flow across the crest of the drift averages 11.6 cm s (super -1) but with intermittent benthic storm activity resuspending the fines.

Antarctic Circumpolar Deep Water: A Quaternary Paleoflow Record from the Northern Scotia Sea, South Atlantic Ocean

Antarctic Circumpolar Deep Water: A Quaternary Paleoflow Record from the Northern Scotia Sea, South Atlantic Ocean

Quaternary history of the Antarctic Circumpolar Current: evidence from the Scotia Sea

Within the Scotia Sea, the axis of the Antarctic Circumpolar Current (ACC) is geographically confined, and sediments therefore contain a record of palaeo-flow speed uncomplicated by ACC axis migration. We outline Holocene and Last Glacial Maximum (LGM) current-controlled sedimentation using data from 3.5-kHz profiles, cores and current meter moorings. Geophysical surveys show areas of erosion and deposition controlled by Neogene basement topography. Deposition occurs in mounded sediment drifts or flatter areas, where 500–1000 m of sediment overlies acoustic basement. 3.5-kHz profiles show parallel, continuous sub-bottom reflectors with highest sedimentation rates in the centre of the drifts, and reflectors converging towards marginal zones of non-deposition. Locally, on the flanks of continental blocks (e.g. South Georgia), downslope processes are dominant. The absence of mudwaves on the sediment drifts may result from the unsteadiness of ACC flow. A core transect from the ACC axis south to the boundary with the Weddell Gyre shows a southward decrease in biogenic content, controlled by the Polar Front and the spring sea-ice edge. Both these features lay farther north at LGM. The cores have been dated by relative abundance of the radiolarian Cycladophora davisiana, and by changes in the biogenic Ba content, a palaeoproductivity indicator. Sedimentation rates range from 3 to 17 cm/ka. The grain size of Holocene sediments shows a coarsening trend from south to north, consistent with strongest bottom-current flow near the ACC axis, though interpretation is complicated by the presence of biogenic grains. Year-long current meter records indicate mean speeds from 7 cm/s in the south to 12 cm/s in the north, with benthic storm frequency increasing northwards. LGM sediments are predominantly terrigenous and show a clearer northward-coarsening trend, with well-sorted silts in the northern Scotia Sea. Assuming a constant terrigenous source, this implies stronger ACC flow at the LGM, contrasting with weaker Weddell Gyre flow deduced from earlier work.

Climatic Control on Variations of Terrigenous Particle Provenance and Transport in the Scotia Sea (Southern Ocean): Evidence from Nd, Pb, and Sr Isotopes

Terrigenous matter constitutes a considerable proportion of total sediment accumulation in Late Quaternary deposits of the Atlantic sector of the Southern Ocean. Input of lithogenic debris from land to sea, mainly to the shelf, is driven by glaciogenic supply from Antarctica, the river systems of South America, and wind transport through the southern westerlies. Deep and bottom water circulation most effectively control the distribution of terrigenous particles in the pelagic realm (Petschick et al., 1996). Variations of terrigenous sediment accumulation rates, grain size properties, and mineralogical composition, as inferred from sediment core studies, support the important role of ocean currents as major particle carriers throughout the late Quaternary (Diekmann et al., 1996; Diekmann and Kuhn, 1997). Clues exist for increased long-distance influxes of air-borne dust from Patagonia to both the Southern Ocean (Kumar et al., 1995) and Antarctica (Petit et al., 1990) during cold climatic stages. A Patagonian provenance of dust particles in ice cores from East Antarctica has been proposed from 143Nd/144Nd isotope data (Basile et al., 1997). This finding documents the importance of an atmospheric pathway during glacial periods, which has also been inferred from deep-sea sediments of the South Atlantic (Kumar et al., 1995). The authors postulated that the increase in primary productivity in the Southern Ocean during glacial periods has resulted from a relief of today's iron limitation by a greater supply of iron-rich aeolian dusts from dry areas of Patagonia. Nevertheless, their interpretation may be challenged because the aeolian dust component of the deep-sea sediments has not been isolated and analysed. Significant dust deposition in deep-sea sediments probably occurred in the southern Scotia Sea (sediment core location PS2319), indicated by down-core fluctuations of grain size parameters and magnetic susceptibility (Fig. 1). The latter may be correlated with dust records in Antarctic ice cores (Hofmann, in press). Maxima of magnetic susceptibility correspond to dust peaks in ice cores. Similar variations of magnetic susceptibility appear in sediment core PS2515 from the northern Scotia Sea (Fig. 1). However, little is known about provenance and proportions of primary aeolian dust components deposited directly on the sea floor. In order to investigate the hypothesis of an increased supply of aeolian dust into the Southern Ocean during glacial times, we have analysed the Nd, Pb, and Sr isotope compositions in bulk samples from sediment cores PS2515 and PS2319 of the Scotia Sea located at 53~ 45~ and 60~ 43~ respectively. Both sediment cores span the climate cycles of the last 140 ka, representing marine isotope stages 1 through 6 (Figs. 1). The isotope data of bulk sediments delineate systematic down-core variations in accordance with signatures of magnetic susceptibility (Fig. 1). In core intervals representing cold stages (marked by maxima of magnetic susceptibility) Nd and Sr isotopes exhibit a higher proportion of a mantle-

Changes in seabird species abundance near South Georgia during a period of rapid change in sea surface temperature

During a three month research cruise near the island of South Georgia, sea surface temperature (SST) increased from c. 2°C to over 4°C. Satellite derived SST show that this corresponded to a rapid southward and eastward shift of isotherms in the northern Scotia Sea, which could have resulted from changes in the wind field. At the same time, observation from the ship of seabirds close to the island indicated changes in the abundance of some non-resident species, whereas resident breeders from South Georgia, such as black-browed albatrosses (Diomedea melanophris) and prions (Pachyptila spp.) which were foraging locally, were present at consistent density in both halves of the survey. Blue petrels (Halobaena caerulea) left the area after breeding, so were associated only with the low water temperatures during the first part of the cruise. In contrast, great shearwaters (Puffinus gravis) and soft-plumaged petrels (Pterodroma mollis) migrated into the area later in the survey. These birds were almost certainly non-breeders which were feeding in the warmer water which had moved towards the island.