No abstract

Side-scan sonar imaging of the Central Lau Basin (SW Pacific) has revealed a Central Lau Spreading Centre (CLSC) propagating southwards at the expense of an Eastern Lau Spreading Centre (ELSC) with a small Intermediate Lau Spreading Centre (ILSC) forming a ‘relay’ between the two. Volcanic rocks sampled along these spreading centres, and from two adjacent seamounts, are glassy to fine-grained pillow lavas and sheet flows of basalts, ferrobasalts and andesites. The evolved rocks are mostly confined to the propagating tip of the CLSC and can be explained by a high rate of cooling relative to magma supply, as invoked for magma genesis at propagating ridges elsewhere. Compared with equivalent rocks from the eastern Pacific, the most evolved members of the CLSC suite require similarly high degrees (>90%) of fractional crystallization from their basaltic parents. Fractional crystallisation cannot, however, account for the compositional differences between CLSC and ELSC lavas. Whereas the composition of the CLSC lavas lies just within the compositional spectrum of typical N-MORB, the ELSC lavas are distinctly enriched in alkali and alkaline earth elements, reach oxide and apatite saturation at lower Fe, Ti and P concentrations and generally show greater vesicularity despite slightly greater depths of eruption, all indicative of a water-rich subduction component. They also have lower contents of Ni, Sc, Na and Fe and higher contents of Si at a given MgO concentration that indicate a more depleted and more hydrous mantle beneath the ELSC compared with the CLSC. These results provide further evidence that, beneath the Central Lau Basin, the source composition changes progressively from MORB-type to island arc tholeiite type as the subduction zone is approached, both eastwards from the CLSC to ELSC and southwards along the ELSC to the Valu Fa Ridge. They also indicate that the composition of the subduction component may vary systematically away from the arc, with Th, LREE, Ba, Rb and H (as H 2 O) all present close to the arc, only Ba, Rb and H 2 O present at intermediate distances and just H 2 O perceptible at the furthest distances.

During Northern Hemisphere winters, the West Coast of North America is battered by extratropical storms. The impact of these storms is of paramount concern to California, where aging water supply and flood protection infrastructures are challenged by increased standards for urban flood protection, an unusually variable weather regime, and projections of climate change. Additionally, there are inherent conflicts between releasing water to provide flood protection and storing water to meet requirements for the water supply, water quality, hydropower generation, water temperature and flow for at-risk species, and recreation. To improve reservoir management and meet the increasing demands on water, improved forecasts of precipitation, especially during extreme events, are required. Here, the authors describe how California is addressing their most important and costliest environmental issue-water management-in part, by installing a state-of-the-art observing system to better track the area's most severe wintertime storms.