[show abstract][hide abstract] ABSTRACT: As in SiO2 which, at high pressures, undergoes the alpha-quartz-->stishovite transition, GaAsO4 transforms into a dirutile structure at 9 GPa and 1173 K. In 2002, a new GaAsO4 polymorph was found by quenching the compound from 6 GPa and 1273 K to ambient conditions. The powder diagram was indexed on the basis of a hexagonal cell (a=8.2033, c=4.3941 A, V=256.08 A3), but the structure did not correspond to any known structure of other AXO4 compounds. We report here the ab initio crystal structure determination of this hexagonal polymorph from powder data. The new phase is isostructural to beta-MnSb2O6 and it can be described as a lacunary derivative of NiAs with half the octahedral sites being vacant, but it also contains fragments of the rutile-like structure.
Acta Crystallographica Section B Structural Science 12/2006; 62(Pt 6):1019-24.
[show abstract][hide abstract] ABSTRACT: We present benthic foraminiferal stable isotope and assemblage data from
the Bering Sea continental slope (U1343, ~2000m water depth), in order
to elucidate changes in productivity and deep water ventilation over the
last ~2 Ma. The Bering Sea is the third largest marginal sea in the
world, connecting the Pacific and Arctic Oceans, but there is still very
little known of its palaeoceanographic past. Its open connections to the
North Pacific make it an important location to monitor subarctic North
Pacific palaeoceanography. Site U1343 is situated near the continental
slope, and its high latitude location makes it sensitive to sea ice and
glacial meltwater input, which caused large fluctuations in
stratification, primary productivity and deep water properties through
time. Although there is very little deep water forming in the Bering Sea
today, potential intmediate and/or deep water formation in the past may
also have affected water properties. High productivity in surface water
adds to the nutrient content of the aged waters entering the Bering Sea
at depth from the Pacific, causing oxygen levels in some locations to be
significantly depleted and benthic foraminifera tolerant to low oxygen
levels and high primary productivity to thrive. Changes in the
proportions of the low oxygen and high productivity species (e.g.
Bulimina, Globobulimina, Globocassidulina) show large fluctuations
through time, with an overall increase from the beginning of the
Mid-Pleistocene Transition (MPT) onwards (~1.2 Ma) indicating more
prevalent episodes of low oxygen conditions persisted after this time.
Bottom water δ13C(Uvigerina) exhibit more negative values before
the MPT compared with eastern equatorial Pacific Site 849, suggesting
the presence of aged deep water in the Bering Sea for at least the last
2 Ma. During the MPT bottom water δ13C becomes more negatively
offset from the Pacific which, coupled with the presence of lower oxygen
benthic foraminifera, suggests a lower oxygen and higher nutrient bottom
water mass was present in the Bering Sea from the MPT onwards. This may
have been caused by an increase in primary productivity since the MPT,
but proxies for productivity and sea ice presence suggest a more likely
explanation would be better ventilation of the Bering Sea before the
[show abstract][hide abstract] ABSTRACT: Today, biodiversity decreases from equatorial to polar regions. This is a fundamental pattern governing the distribution of extant organisms, the understanding of which is critical to predicting climatically driven biodiversity loss. However, its causes remain unresolved. The fossil record offers a unique perspective on the evolution of this latitudinal biodiversity gradient (LBG), providing a dynamic system in which to explore spatiotemporal diversity fluctuations. Deep-time studies indicate that a tropical peak and poleward decline in species diversity has not been a persistent pattern throughout the Phanerozoic, but is restricted to intervals of the Palaeozoic and the past 30 million years. A tropical peak might characterise cold icehouse climatic regimes, whereas warmer greenhouse regimes display temperate diversity peaks or flattened gradients.