Landfast sea-ice and platelet-layer thickness and conductivity of Atka Bay, Antarctica, December 2012

Ice shelves strongly impact coastal Antarctic sea-ice and the associated ecosystem through the formation of a sub-sea-ice platelet layer. Although progress has been made in determining and understanding its spatio-temporal variability based on point measurements, an investigation of this phenomenon on a larger scale remains a challenge due to logistical constraints and a lack of suitable methodology. In this study, we applied a laterally-constrained Marquardt-Levenberg inversion to a unique multi-frequency electromagnetic (EM) induction sounding dataset obtained on the landfast sea ice of Atka Bay, eastern Weddell Sea, in 2012. In addition to consistent fast-ice thickness and -conductivities along > 100 km transects; we present the first comprehensive, high resolution platelet-layer thickness and -conductivity dataset recorded on Antarctic sea ice. The reliability of the algorithm was confirmed by using synthetic data, and the inverted platelet-layer thicknesses agreed within the data uncertainty to drill-hole measurements. Ice-volume fractions were calculated from platelet-layer conductivities, revealing that an older and thicker platelet layer is denser and more compacted than a loosely attached, young platelet layer. The overall platelet-layer volume below Atka Bay fast ice suggests that the contribution of ocean/ice-shelf interaction to sea-ice volume in this region is even higher than previously thought. This study also implies that multi-frequency EM induction sounding is an effective approach in determining platelet layer volume on a larger scale than previously feasible. When applied to airborne multi-frequency EM, this method could provide a step towards an Antarctic-wide quantification of ocean/ice-shelf interaction.

Isotopic composition of diagenetic silica and associated pore waters of Japan Sea sediments

Over a broad region of the eastern Japan Sea, Neogene opaline diatomaceous sediments alter with depth to hard porcellanites and cherts composed of opal-CT and quartz. We examined the oxygen isotopic compositions of these diagenetic silica minerals at four widely spaced sites occupied during ODP Leg 127 in order to investigate the thermal history of the region. Formation temperatures computed from these isotopic data range from 22° to 68°C for opal-CT and from 44° to 92°C for diagenetic quartz, quite similar to temperature ranges estimated from the extrapolated modern gradients, 36°-43°C and 49°-64°C, respectively. At each site the isotopic temperature values cluster near the extrapolated ambient sediment temperatures. As a first approximation, the similarities suggest that the positions of the silica transformations in the basin are controlled by the present thermal regime. In detail, isotopic and ambient temperatures differ. If these differences are real, then they reflect variations in the thermal histories at these sites. At Sites 794 and 797 in the Yamato Basin, isotopic temperatures and gradients computed from these data are lower than or comparable to ambient temperatures and gradients. We suggest that the silica zones have roughly equilibrated with the modern gradients at these localities. At Site 795 in the Japan Basin, isotopic temperatures are also lower than ambient sediment temperatures at comparable depths, but the gradient computed from the isotopic temperatures is higher than the present measured gradient. For both scenarios to hold, the silica zones must have formed under initially high gradients during the early post-rift period at this locality. These zones were then rapidly buried and have yet to equilibrate with the modern lower gradient. At Site 796 on Okushiri Ridge, isotopic temperatures exceed present temperatures as expected for an area of recent uplift. The gradient computed from our isotopic data and the thickness of the opal-CT zone indicate a higher gradient than at present at this site, apparently reflecting higher heat fluxes during the early post-rift period or recent frictional heating from nearby reverse fault activity.

Crabeater seals (Lobodon carcinophaga) in the Weddell Sea during DRE1998 campaign

The crabeater seal (Lobodon carcinophaga) is the most abundant Antarctic seal and inhabits the circumpolar pack ice zone of the Southern Ocean. Until now, information on important environmental factors affecting its distribution as well as on foraging behaviour is limited. In austral summer 1998, 12 crabeater seals of both sexes and different age classes were equipped with satellitelinked dive recorders at Drescher Inlet (72.85°S, 19.26°E), eastern Weddell Sea. To identify suitable habitat conditions within the Weddell Sea, a maximum entropy (Maxent) modelling approach was implemented. The model revealed that the eastern and southern Weddell Sea is especially suitable for crabeater seals. Distance to the continental shelf break and sea ice concentration were the two most important parameters in modelling species distribution throughout the study period. Model predictions demonstrated that crabeater seals showed a dynamic response to their seasonally changing environment emphasized by the favoured sea ice conditions. Crabeater seals utilized ice-free waters substantially, which is potentially explained by the comparatively low sea ice cover of the Weddell Sea during summer 1998. Diving behaviour was characterized by short (>90 % = 0-4 min) and shallow (>90 % = 0-51 m) dives. This pattern reflects the typical summer and autumn foraging behaviour of crabeater seals. Both the distribution and foraging behaviour corresponded well with the life history of the Antarctic krill (Euphausia superba), the preferred prey of crabeater seals. In general, predicted suitable habitat conditions were congruent with probable habitats of krill, which emphasizes the strong dependence on their primary prey.

Composition of bottom sediments from the Mediterranean Sea off Syria

An investigation of recent bottom sediments between the Cyprus Island and the Syrian seacoast during Cruise 27 of R/V Vityaz-2 (1993) gave comprehensive field data significantly complementing our understanding of the sedimentation process in this part of the Mediterranean Sea. Mineralogical and geochemical indicators testify to different input into sedimentation of the Syrian and Nile River sources. The Nile River plays a leading role in terrigenous sedimentation in the southeastern Mediterranean Sea, especially in deep-sea areas. In contrast, contribution of weathering products of basalts and ophiolites from the Syrian drainage area (hornblende, monoclinic and rhombic pyroxenes, olivine, spinel, palagonite, and epidote) are particularly detectable in sediments of the near-coast zone. During Late Quaternary contribution of terrigenous material both from the Syrian and Nile sources was irregular in time.