M.J. Whitehouse

Spatial and temporal operation of the Scotia Sea ecosystem: a review of large-scale links in a krill centred food web

The Scotia Sea ecosystem is a major component of the circumpolar Southern Ocean system, where productivity and predator demand for prey are high. The eastward-flowing Antarctic Circumpolar Current (ACC) and waters from the Weddell–Scotia Confluence dominate the physics of the Scotia Sea, leading to a strong advective flow, intense eddy activity and mixing. There is also strong seasonality, manifest by the changing irradiance and sea ice cover, which leads to shorter summers in the south. Summer phytoplankton blooms, which at times can cover an area of more than 0.5 million km2, probably result from the mixing of micronutrients into surface waters through the flow of the ACC over the Scotia Arc. This production is consumed by a range of species including Antarctic krill, which are the major prey item of large seabird and marine mammal populations. The flow of the ACC is steered north by the Scotia Arc, pushing polar water to lower latitudes, carrying with it krill during spring and summer, which subsidize food webs around South Georgia and the northern Scotia Arc. There is also marked interannual variability in winter sea ice distribution and sea surface temperatures that is linked to southern hemisphere-scale climate processes such as the El Niño–Southern Oscillation. This variation affects regional primary and secondary production and influences biogeochemical cycles. It also affects krill population dynamics and dispersal, which in turn impacts higher trophic level predator foraging, breeding performance and population dynamics. The ecosystem has also been highly perturbed as a result of harvesting over the last two centuries and significant ecological changes have also occurred in response to rapid regional warming during the second half of the twentieth century. This combination of historical perturbation and rapid regional change highlights that the Scotia Sea ecosystem is likely to show significant change over the next two to three decades, which may result in major ecological shifts.

Ion microprobe UPb zircon geochronology and isotopic evidence for a trans-crustal suture in the Lapland–Kola Orogen, northern Fennoscandian Shield

The Lapland–Kola Orogen (LKO; former Kola craton) in the northern Fennoscandian Shield comprises a collage of partially reworked late Archaean terranes with intervening belts of Palaeoproterozoic juvenile crust including the classic Lapland Granulite Terrane. Rifting of Archaean crust began at c 2.5–2.4 Ga as attested by layered mafic and anorthositic intrusions developed throughout the northernmost Fennoscandian Shield at this time. Oceanic separation was centred on the Lapland Granulite, Umba Granulite (UGT) and Tersk terranes within the core zone of the orogen. Importantly, SmNd data show that Palaeoproterozoic metasedimentary and metaigneous rocks within these terranes contain an important, generally dominant, juvenile component over a strike length of at least 600 km. Evidently, adjacent Archaean terranes, with negative eNd signatures, contributed relatively little detritus, suggesting a basin of considerable extent. Subduction of the resulting Lapland–Kola ocean led to arc magmatism dated by the NORDSIM ion probe at c 1.96 Ga in the Tersk Terrane in the southern Kola Peninsula. Accretion of the Tersk arc took place before c 1.91 Ga as shown by ion probe UPb zircon dating of post-D1, pre-D2 pegmatites cutting the Tersk arc rocks, juvenile metasediments as well as Archaean gneisses in the footwall of the orogen. Deep burial during collision under high-pressure granulite-facies conditions was followed by exhumation and cooling between 1.90 and 1.87 Ga based on SmNd, UPb and ArAr data. Lateral variations in deep crustal velocity and Vp/Vs ratio, together with reflections traversing the entire crust observed in reprocessed seismic data from the Polar Profile, may be interpreted to image a trans-crustal structure — possibly a fossilised subduction zone — supporting an arc origin for the protoliths of the Lapland Granulite, UGT and Tersk terranes and the location of a major lithospheric suture — the Lapland–Kola suture.

Zircon Geochronology of the Ollo de Sapo Formation and the Age of the Cambro-Ordovician Rifting in Iberia

The 600-km-long Ollo de Sapo Formation was generated during the Cambro-Ordovician rifting that affected the pre-Variscan basement of Europe and represents the largest accumulation of pre-Variscan igneous rocks in Iberia. It consists of variably metamorphosed felsic volcanic rocks and granites characterized by an abnormally elevated zircon inheritance (70%–80% or more of the zircon grains contain premagmatic cores), the age of which has long been a matter of debate. This article presents the results of dating the northwestern area of the Ollo de Sapo Formation via U-Pb spot (ion-microprobe and laser ablation–inductively coupled plasma mass spectrometry) and Pb-Pb single-grain zircon analysis, complementing our previous results from the southeastern area of the formation, and the results of a survey of the major- and trace-element and Sr-Nd isotope composition of these rocks, which unequivocally indicate that they are crustal melts. The age span recorded in the northwestern Ollo de Sapo Formation is 492–486 Ma, with the only exception being the San Sebastián metagranite (470 ± 3 Ma), a small body with notably less zircon inheritance and a Sr-Nd isotope composition more primitive than the rest of the Ollo de Sapo rocks. When these results are considered jointly with other reliable zircon ages of similar rocks from all over Iberia, it may be deduced that in the Central Iberian Zone the Cambro-Ordovician rifting started at ∼495 Ma, reached a maximum between 492 and 483 Ma, and ceased at ∼470 Ma. The age pattern of inherited zircons of the Ollo de Sapo Formation reveals that the source of the Cambro-Ordovician magmas was mostly igneous rocks of Ediacaran age, with minor Orosirian and Archean components. A detailed study of these components might be key for revealing which part of the north was connected to Iberia before the early Paleozoic breakup of Gondwana.

Zircon thermometry and U–Pb ion-microprobe dating of the gabbros and associated migmatites of the Variscan Toledo Anatectic Complex, Central Iberia

In the Central Iberian Zone there are several large thermal domes in which small bodies of ultramafic, mafic and intermediate rocks appear intimately associated with crustal granites and migmatites. The closest spatial association between the ultramafic, mafic and intermediate rocks and migmatites is in the Toledo Anatectic Complex, where field relationships suggest that these rocks are coeval and have an age close to 340 Ma. This, and the recent discovery in the neighbouring Ossa Morena Zone of a large mid-crustal seismic reflector interpreted as a 335–350 Ma mafic sill, reinforce the hypothesis that heat for crustal melting was supplied from early Variscan mantle magmas emplaced in the middle crust. However, precise ion-microprobe U–Pb zircon dating and Ti-in-zircon thermometry in Toledo do not support this idea. Whereas the mean age of four mafic bodies is 307 ± 2 Ma, the migmatites are c. 25 Ma older. The migmatites hosting ultramafic, mafic and intermediate bodies have the same age and Ti-in-zircon temperatures as migmatites far from any mafic intrusion. These data reveal that ultramafic, mafic and intermediate magmas are late Variscan; they were emplaced in already cooling anatectic zones once the extensional collapse was initiated, and their thermal impact on the mid-crustal Variscan anatexis of Central Iberia was negligible.

The tectonothermal evolution and provenance of the Tyrone Central Inlier, Ireland: Grampian imbrication of an outboard Laurentian microcontinent?

The Tyrone Central Inlier is a metamorphic terrane of uncertain affinity situated outboard of the main Dalradian outcrop (south of the Fair Head–Clew Bay Line) and could represent sub-arc basement to part of the enigmatic Midland Valley Terrane. Using a combination of isotopic, structural and petrographic evidence, the tectonothermal evolution of the Tyrone Central Inlier was investigated. Sillimanite-bearing metamorphic assemblages (c. 670 °C, 6.8 kbar) and leucosomes in paragneisses are cut by granite pegmatites, which post-date two deformation fabrics. The leucosomes yield a weighted average 207Pb/206Pb zircon age of 467 ± 12 Ma whereas the main fabric yields a 40Ar–39Ar biotite cooling age of 468 ± 1.4 Ma. The pegmatites yield 457 ± 7 Ma and 458 ± 7 Ma Rb–Sr muscovite–feldspar ages and 40Ar–39Ar step-heating plateaux of 466 ± 1 Ma and 468 ± 1 Ma, respectively. The metasedimentary rocks yield Palaeoproterozoic Sm–Nd model ages and laser ablation inductively coupled plasma mass spectrometry detrital zircon U–Pb analyses from a psammitic gneiss yield age populations at 1.05–1.2, 1.5, 1.8, 2.7 and 3.1 Ga. Combined, these data permit correlation of the Tyrone Central Inlier with either the Argyll or the Southern Highland Group of the Dalradian Supergroup. The inlier was thus part of Laurentia onto which the Tyrone ophiolite was obducted.

The shelf break front to the east of the sub-Antarctic island of South Georgia

The sub-Antarctic island of South Georgia lies in polar waters to the south of the Polar Front and is influenced by the Weddell Scotia Confluence whose waters reach the island’s south east coast. In January 1996 a detailed hydrographic survey to the east of the island showed that on-shelf waters were both warmer and fresher than those off-shelf. These differences were not confined to surface waters. The inherent density differences resulted in a shelf break front, the position of which was defined as the point where the 26.96 kg m-3 isopycnal crossed 30 dbar as it rose towards the surface. The slope of the shelf break front between the on- and off-shelf waters was found to be approximately geostrophic. Although the position of the shelf break front was not linked to any particular bottom depth, its position was strongly influenced by the bottom topography in the region. Evidence is presented for upwelling in one specific region along the front. Differences between the on- and off-shelf waters are also observed in nutrient and chlorophyll a data: These data implied greater productivity on-shelf. Both physical and chemical data suggest that on- and off-shelf waters communicate at depth. This study also shows that shelf water can cross the shelf break front and remain in the vicinity of the island.

Early Silurian magmatism and the Scandian evolution of the Kalak Nappe Complex, Finnmark, Arctic Norway

Three orogenic events, Porsanger (pre-800 Ma), Finnmarkian (490–540 Ma) and Scandian (400–425 Ma) have been proposed to explain the evolution of the Kalak Nappe Complex, a major component of the Caledonian orogen in Norway. The Kalak Nappe Complex has been considered as Baltoscandian passive margin metasediments telescoped eastwards onto the Baltic Shield. Two granitoid bodies with a weighted mean U–Pb zircon age of 438 ± 2 Ma cut the Hellefjord Schist, regarded as the highest stratigraphic unit in the Kalak Nappe Complex. A volcaniclastic unit within the Hellefjord Schist yields an identical U–Pb zircon age of 438 ± 4 Ma. Magmatism and sedimentation are synchronous within analytical error. The Hellefjord Schist cannot have been affected by Finnmarkian or Porsanger orogenesis and cannot be part of a continuous stratigraphic package. A Laurentian affinity for these sediments is suggested and radical revision of the tectonostratigraphy is required. The Hellefjord Schist is reassigned to the Magerøy Nappe of the Upper–Uppermost Allochthon. As the entire stratigraphy is infolded with the Hellefjord Schist a major deformation phase of Scandian age affected the Kalak Nappe Complex. The Hellefjord Schist represents a Laurentian flysch sequence that was juxtaposed with Baltica during the Scandian Orogeny.

Chlorophyll/nutrient characteristics in the water masses to the north of South Georgia, Southern Ocean

Abstract Chlorophyll a and nutrient concentrations along with temperature and salinity values were measured at 22 CTD stations along a 735-km transect running to the northwest of the island of South Georgia, Southern Ocean. Measurements were repeated during five summer surveys (January and February 1994, January 1996, December 1996, January 1998) and one spring survey (October 1997). The transect sampled Sub-Antarctic Zone water in the north, Polar Frontal Zone water and Antarctic Zone water in the south. Chlorophyll a concentrations were lowest to the north of the transect and frequently high (up to 17 mg m−3) in the deep open ocean of the Antarctic Zone. Sub-surface peaks were measured in all zones and chlorophyll a was detectable to a depth of 150 m. There was a clear latitudinal temperature gradient in the near-surface waters (0–50 m), the warmest water occurring in the north (∼12 °C), and the coolest in the Antarctic Zone (∼2 °C). There was also a well-defined latitudinal gradient in summer near-surface silicate concentrations (∼2, 4, and 10 mmol m−3 in the Sub-Antarctic Zone, the Polar Frontal Zone and the Antarctic Zone, respectively), increasing to >20 mmol m−3 near South Georgia. Distinct differences in silicate concentrations were also evident in all three zones to a depth of 500 m. Near-surface nitrate and phosphate concentrations were relatively low to the north of the transect (∼14 and 1 mmol m−3, respectively) and higher in the Polar Frontal Zone and Antarctic Zone (∼18 and 1.4 mmol m−3, respectively). Ammonium and nitrite were restricted to the upper 200 m of the water column, and exhibited sub-surface concentration peaks, the lowest being in the Sub-Antarctic Zone (0.68 and 0.25 mmol m−3, respectively) and the highest in the Antarctic Zone (1.72 and 0.29 mmol m−3, respectively). Surface (∼6 m) spring nutrient measurements provided an indication of pre-bloom conditions; ammonium and nitrite concentrations were low (∼0.27 and 0.28 mmol m−3, respectively), while silicate, nitrate and phosphate concentrations were high and similar to previously measured winter values (e.g. ∼26, 23, 2 mmol m−3, respectively in the Antarctic Zone). Although the values measured were very variable, and there was some evidence of a seasonal growth progression, the chlorophyll a and nutrient distribution patterns were dominated by intercruise (interannual) factors. Approximate nutrient depletions (spring minus summer) appeared similar in the Polar Frontal Zone and Antarctic Zone for nitrate and phosphate, while silicate showed a marked latitudinal increase from north to south throughout the transect. Highest chlorophyll a concentrations coincided with the highest apparent silicate depletions over the deep ocean of the Antarctic Zone. In this area, relatively warm, easterly flowing Antarctic Circumpolar Current water meets cooler, westerly flowing water that is influenced by the Weddell-Scotia Confluence and is rich in nutrients, especially silicate.

U–Pb ion-microprobe zircon geochronology from the basement inliers of eastern Graham Land, Antarctic Peninsula

New geological mapping combined with U–Pb ion microprobe zircon geochronology on the isolated but locally extensive exposures of crystalline basement inliers of eastern Graham Land has greatly improved our understanding of the region’s early crustal evolution and has allowed a more thorough evaluation of Patagonia–Antarctic Peninsula connections prior to Gondwana break-up. At Eden Glacier, diorite gneisses yield Early Ordovician protolith ages of 487 ± 3 and 485 ± 3 Ma and represent the oldest in situ rocks recorded on the Antarctic Peninsula, and indicate a significant spatial extension of Famatinian-age magmatism of Patagonia. Zircon overgrowths in the Early Ordovician protoliths and granitic leucosomes developed within them record two phases of Permian metamorphism at c. 275 and c. 257 Ma, coincident in part with diorite plutonism of the area at 272 ± 2 Ma. At Adie Inlet, granitic leucosomes from paragneiss have been dated at 276 ± 3 Ma, and these are in turn cut by 257 ± 3 Ma xenolith-rich diorite gneiss. The diorite intruded during a second phase of deformation, which folded the paragneiss leucosomes into tight folds. This whole assembly is cut by intensely brecciated megacrystic granodiorite, which yielded a 259 ± 3 Ma age. South of Cabinet Inlet a very different sequence of events is evident, with Triassic magmatism at c. 236 Ma extensive along the Joerg Peninsula. Migmatitic leucosomes are dated at c. 224 Ma and magmatism and deformation events apparently continued to c. 209 Ma at Cape Casey. Our data indicate that the Devonian and Carboniferous magmatism at Target Hill, considered to represent the ‘classic’ basement complex of the Antarctic Peninsula, is not representative regionally. The Target Hill crustal block contains a major break along Cabinet Inlet; to the north, Ordovician and Permian protoliths were variably migmatized during two episodes of Permian deformation and metamorphism, whereas to the south, Triassic protoliths and Triassic metamorphism are encountered. Supplementary material: U–Pb isotopic data are available at http://www.geolsoc.org.uk/SUP18526.

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.