Holocene ice dynamics and bottom-water formation associated with Cape Darnley polynya activity recorded in Burton Basin,East Antarctica

A multi-proxy study including sedimentological, mineralogical, biogeochemical and micropaleontological methods was conducted on sediment core PS69/849-2 retrieved from Burton Basin, MacRobertson Shelf, East Antarctica. The goal of this study was to depict the deglacial and Holocene environmental history of the MacRobertson Land–Prydz Bay region. A special focus was put on the timing of ice-sheet retreat and the variability of bottom-water formation due to sea ice formation through the Holocene. Results from site PS69/849-2 provide the first paleo-environmental record of Holocene variations in bottom-water production probably associated to the Cape Darnley polynya, which is the second largest polynya in the Antarctic. Methods included end-member modeling of laser-derived high-resolution grain size data to reconstruct the depositional regimes and bottom-water activity. The provenance of current-derived and ice-transported material was reconstructed using clay-mineral and heavy-mineral analysis. Conclusions on biogenic production were drawn by determination of biogenic opal and total organic carbon. It was found that the ice shelf front started to retreat from the site around 12.8 ka BP. This coincides with results from other records in Prydz Bay and suggests warming during the early Holocene optimum next to global sea level rise as the main trigger. Ice-rafted debris was then supplied to the site until 5.5 cal. ka BP, when Holocene global sea level rise stabilized and glacial isostatic rebound on MacRobertson Land commenced. Throughout the Holocene, three episodes of enhanced bottom-water activity probably due to elevated brine rejection in Cape Darnley polynya occured between 11.5 and 9 cal. ka BP, 5.6 and 4.5 cal. ka BP and since 1.5 cal. ka BP. These periods are related to shifts from warmer to cooler conditions at the end of Holocene warm periods, in particular the early Holocene optimum, the mid-Holocene warm period and at the beginning of the neoglacial. In contrast, between 7.7 and 6.7 cal. ka BP, brine rejection shut down, maybe owed to warm conditions and pronounced open-water intervals.     

A geomorphological seabed classification for the Weddell Sea,Antarctica

Sea floor morphology plays an important role in many scientific disciplines such as ecology, hydrology and sedimentology since geomorphic features can act as physical controls for e.g. species distribution, oceanographically flow-path estimations or sedimentation processes. In this study, we provide a terrain analysis of the Weddell Sea based on the 500 m × 500 m resolution bathymetry data provided by the mapping project IBCSO. Seventeen seabed classes are recognized at the sea floor based on a fine and broad scale Benthic Positioning Index calculation highlighting the diversity of the glacially carved shelf. Beside the morphology, slope, aspect, terrain rugosity and hillshade were calculated and supplied to the data archive PANGAEA. Applying zonal statistics to the geomorphic features identified unambiguously the shelf edge of the Weddell Sea with a width of 45–70 km and a mean depth of about 1200 m ranging from 270 m to 4300 m. A complex morphology of troughs, flat ridges, pinnacles, steep slopes, seamounts, outcrops, and narrow ridges, structures with approx. 5–7 km width, build an approx. 40–70 km long swath along the shelf edge. The study shows where scarps and depressions control the connection between shelf and abyssal and where high and low declination within the scarps e.g. occur. For evaluation purpose, 428 grain size samples were added to the seabed class map. The mean values of mud, sand and gravel of those samples falling into a single seabed class was calculated, respectively, and assigned to a sediment texture class according to a common sediment classification scheme.     

南极威德尔海春季海冰物理结构对叶绿素上限含量的影响

2006年冬末春初,在德国POLARSTERN科学考察船执行南极威德尔海西北海域考察期间,调查了考察区海冰物理和海洋生物。本文观测了航线上钻取的27支海冰冰芯的组构和71个冰晶体薄片;分析得到393组冰温数据;348组盐度、密度数据和311组叶绿素a和脱镁叶绿素含量数据;通过302组冰内相同深度孔隙率和叶绿素a含量数据分析,发现海冰物理参数影响冰内叶绿素a含量的新证据;利用收集的雪、冰厚度数据以及环境容量制约生态平衡的规律,建立了雪、冰厚度对冰底叶绿素繁荣的影响以及;确立了南极粒状冰和柱状冰内叶绿素a上限含量同卤水体积的关系。从而表达了冰晶体对卤水排泄的效应和冰物理性质对南极春季冰底和冰-水界面叶绿素a增长的贡献。此外,还得出海冰物理性质影响冰藻,并且是南极冰区水体浮游植物繁荣的关键控制因素。     

Long-term Field Observations on Seasonality in Chlorophyll-<Emphasis Type="Italic">a</Emphasis> Concentrations in a Shallow Coastal Marine Ecosystem,the Wadden Sea

Analyses of long-term field observations (1974–2007) on chlorophyll-a concentrations in the western Wadden Sea showed no long-term trends in the timing of the wax and wane of phytoplankton spring blooms. There is weak evidence, however, that the height of the autumn bloom has decreased since the early 1990s. This fading of the autumn bloom may have had consequences for the carbon transfer to higher trophic levels, currently hampering primary consumer species that mostly rely on food supply during late summer. Current and other findings suggest a shortening of the growing season due to the fading of the autumn bloom in the Wadden Sea and a lengthening of the growing season due to an advancement of the spring bloom in the North Sea. These regionally different changes in seasonality may have contributed to the coinciding decrease in bivalve filtering capacity in the western Wadden Sea and the large-scale offshore shift of juvenile plaice from the Wadden Sea to the adjacent North Sea.     

A pore-throat model based on grain-size distribution to quantify gravity-dominated DNAPL instabilities in a water-saturated homogeneous porous medium

A pore-scale network model based on spherical pore bodies and cylindrical pore throats was developed to describe the displacement of water by DNAPL. The pore body size and the pore throat size were given by statistical distributions with user-specified values for the minimum, mean and maximum sizes. The numerical model was applied to a laboratory experiment conducted on a sand-filled glass column. The parameters relative to pore body size and pore throat size that were used in the construction of the equivalent network were derived from the discrete grain-size distribution of the real porous medium. The calculated arrival times of the DNAPL front were compared with those measured using optic fibre sensors placed at different points on the control section of the experimental device. Furthermore, the model simulated DNAPL pressure measured at the entrance section of the system. In general, the numerical results obtained with the model were in good agreement with the actual measurements.     

用孔隙水中氯离子浓度异常估算天然气水合物含量

水分子与烃类气体沉积物孔隙中结合形成水合物的过程中,只吸取孔隙中的淡水,而盐类物质不能进入水合物的晶体结构,在水合物形成过程中会引起局部孔隙水离子浓度的浓缩,增加周围沉积物孔隙水的盐度;而当水合物分解时,孔隙水就会被稀释,使得钻孔获得的水合物样品比不含水合物的样品具有更低的盐度.因为C1-是浅层海洋没积物孔隙水中一种丰富且保守的离子,C1-离子浓度变化通常被用来监测天然气水合物的形成和分解.……     

Two UBV sequences,north and south of the galactic cluster NGC 5823

Photoelectric UBV observations of 69 stars, distributed in two sequences north and south of the galactic star cluster NGC 5823, are reported.     

The recent capture of the periodic comet P/Scotti (2000 Y3)

Numerical integrations of 99 orbits centered on that of comet P/Scotti (P/2000 Y3), and of the nominal orbit, were made 4000 days backwards in time, and 73000 days into the future. The integrations show that this comet has been transferred into its present orbit as recently as 1998. The future orbital evolution indicates a stable period for almost 150 years, when another close encounter with Jupiter may lead to further drastic changes of the present orbit.