ALBERT

Description: We have compared 14 different sediment incubation chambers, most of them were used on bottom landers. Measurements of mixing time, pressure gradients at the bottom and Diffusive Boundary Layer thickness (DBL) were used to describe the hydrodynamic properties of the chambers and sediment–water solute fluxes of silicate (34 replicates) and oxygen (23 replicates) during three subsequently repeated incubation experiments on a homogenized, macrofauna-free sediment. The silicate fluxes ranged from 0.24 to 1.01 mmol m−2 day−1 and the oxygen fluxes from 9.3 to 22.6 mmol m−2 day−1. There was no statistically significant correlation between measured fluxes and the chamber design or between measured fluxes and hydrodynamic settings suggesting that type of chamber was not important in these flux measurements. For verification of sediment homogeneity, 61 samples of meiofauna were taken and identified to major taxa. In addition, 13 sediment cores were collected, sectioned into 5–10-mm slices and separated into pore water and solid phase. The pore water profiles of dissolved silicate were used to calculate diffusive fluxes of silicate. These fluxes ranged from 0.63 to 0.87 mmol m−2 day−1. All of the collected sediment parameters indicated that the sediment homogenization process had been satisfactorily accomplished. Hydrodynamic variations inside and between chambers are a reflection of the chamber design and the stirring device. In general, pump stirrers with diffusers give a more even distribution of bottom currents and DBL thicknesses than paddle wheel-type stirrers. Most chambers display no or low static differential pressures when the water is mixed at rates of normal use. Consequently, there is a low risk of creating stirrer induced pressure effects on the measured fluxes. Centrally placed stirrers are preferable to off-center placed stirrers which are more difficult to map and do not seem to give any hydrodynamic advantages. A vertically rotating stirrer gives about five times lower static differential pressures at the same stirring speed as the same stirrer mounted horizontally. If the aim is to simulate or mimic resuspension at high flow velocities, it cannot be satisfactorily done in a chamber using a horizontal (standing) rotating impeller (as is the case for most chambers in use) due to the creation of unnatural conditions, i.e. large static differential pressures and pre-mature resuspension at certain locations in the chamber.

Description: The Narryer Terrane within the northwestern Yilgarn Craton contains the oldest crust in Australia. The Jack Hills greenstone belt is located within the southern part of the Narryer Terrane, and structures cutting it and surrounding rocks have been dated using the 40Ar/39Ar technique. The results show that east-trending, dextral, transpressive shearing was related to the 1830–1780 Ma Capricorn Orogeny, followed by further deformation and/or cooling between c. 1760 and 1740 Ma. These results confirm that major deformation has affected the northwestern part of the Yilgarn Craton in an intracratonic setting during the Proterozoic. Proterozoic structures have been interpreted to extend south beyond the Narryer Terrane into the northern part of the Youanmi Terrane (Murchison Domain), and include the Yalgar Fault, previously interpreted as the boundary between the Narryer and Youanmi Terranes. Terrane amalgamation pre-dated the emplacement of c. 2660 Ma granites in both terranes, and the current expression of the Yalgar Fault must represent a younger, reworked, post-amalgamation structure, possibly controlled by the tectonic boundary. However, new aeromagnetic and gravity imagery does not show the eastern part of the Yalgar Fault as a major structure. Its signature is more akin to a series of east- to east-northeast trending faults that are interpreted to be Proterozoic in age. This suggests that this part of the Yalgar Fault may not be a terrane boundary, and is possibly no older than Proterozoic. The 40Ar/39Ar dating also shows a younger, less intense deformation and/or cooling event at c. 1172 Ma.