Variability of Stable Isotope Fingerprints of the Serpulid Ficopomatus enigmaticus Within a Permanently Stratified Estuary: Implications for (Palaeo)environmental Interpretations

This paper examines how the mixing of freshwater and seawater, and related mixing of freshwater and marine particulate organic matter (POM) in the permanently stratified estuary of the River Krka, Croatia, are reflected in the stable isotope fingerprints of soft tissues and tubes of the serpulid Ficopomatus enigmaticus. The carbon stable isotope composition (δ¹³C values) of the river-borne POM is retained over long distances, causing a depletion in ¹³C of POM in brackish waters. A trophic depletion in ¹³C was recorded in serpulid soft tissues. The serpulid carbonate tubes were depleted in ¹³C even at locations with salinity close to that of the sea and were subject to large isotope fractionation between dissolved inorganic C (DIC) and carbonate caused by vital effects, making carbonate depleted in ¹³C by several per mil compared with DIC. These effects, though large in the freshwater zone, fade towards the sea. The carbonate δ¹⁸O values of tubes reflect the δ¹⁸O values of the water. The temperature-related differences in δ¹⁸O values of tubes from different sites are masked by source-related differences in the δ¹⁸O values of water arising from mixing of freshwater and seawater in the estuary. Therefore, in serpulide tubes, the terrestrial component can easily be overestimated because of vital effects during biomineralisation and trophic depletion in ¹³C in freshwater and brackish environments.     

Inferred geothermal inflows to Lake Rotorua

Chemical data on samples from tributaries to Lake Rotorua are used to show that a large proportion of the chloride load on the lake comes from a source other than the surface tributaries. This source is probably a flow of c. 0.6 m³ s^‐1 of geothermal water with a chloride concentration of c. 500 g m^‐3.     

Contemporary fiddling in human geography while Rome burns: has quantitative analysis been largely abandoned--and should it be?

Chris Hamnett [Geoforum 34 (2003) 1] has certainly been provocative in his claim that the nature of much contemporary human geography means that the discipline ‘will cease to be taken seriously in the world beyond the narrow confines of academe’ (p. 1). Much of his critique focuses on what he terms ‘post-modernism, new cultural geography and the interpretative turn’ (p. 2) practiced by those he identifies as ‘the new intellectual dilettanti’ (p. 3). In responding, however, our concern is not with that aspect of his critique, but rather with Hamnett’s misrepresentation of one section of contemporary human geography.     

Zircon trace element chemistry at sub-micrometer resolution for Tarawera volcano,New Zealand,and implications for rhyolite magma evolution

Zoned crystals can be important recorders of magmatic processes in space and time. However, in most situations, the temporal dimension is difficult to quantify. Here, we have employed secondary ion mass spectrometry depth profiling to excavate parallel pits into non-polished crystal faces of zircon to obtain ~5 μm resolution U–Th disequilibrium ages (one pit) that can be correlated with trace element zoning at sub-μm resolution derived from a second pit. Data from 17 crystals representing each of the four rhyolite eruptions of Tarawera volcano, an intra-caldera edifice within the Okataina Volcanic Centre, reveal diverse zircon growth conditions over time. Most crystals display rimward depletions in Zr/Hf and Ti, broadly consistent with cooling and crystallization. However, a significant fraction of crystals lacks these patterns and displays rimward trace element variations consistent with isothermal or prograde crystallization. Oscillatory zonation patterns in Y, Th, and U are superimposed on the Zr/Hf and Ti trends. Despite the limited number of crystals analyzed in this way, the striking lack of ubiquitous trace element zoning patterns in crystals from the same hand sample implies that fractional crystallization upon cooling was punctuated by magma recharge and crystal mixing affecting different parts of the magma reservoir. By combining data from all crystals, a systematic change to more heterogeneous trace element abundances is revealed by zircon crystal domains <45 ka following the Rotoiti caldera-forming eruption. This contrasts with the more uniform conditions of zircon crystallization lasting >100 ka prior to caldera formation and is best explained by the post-caldera system consisting of small, isolated melt pockets that evolved independently. An important conclusion is that the zircon ‘cargo’ in volcanic rocks reflects thermally and compositionally divergent processes that act near simultaneously in a magma storage region and not exclusively the conditions in the eruptible magma.