The regional distribution of hydrothermal vent activity in the northeastern (NE) Lau Basin was recently reported by the Ridge 2000 program; however, active venting sources have yet to be located. Here, we report geological and hydrological evidence that indicates the presence of three active hydrothermal venting sources in the NE Lau Basin: the Fonualei Rift and Spreading Center (FRSC), the Northeast Lau Spreading Center (NELSC), and an off-axis caldera (MTJ-1). These examples of hydrothermal activity were recognized by the appearance of hydrothermal plume signals in the water column, including anomalies in light-transmission, methane, adenosine 5'-triphosphate (ATP), and trace metal concentrations (TDMn and TDFe). Three active venting sources were identified by the observation of possible buoyant plumes during conductivity-temperature-depth (CTD) tow-yo surveys and by the recovery of hydrothermal precipitates (chimneys and altered rocks). The strongest light-transmission anomaly, an order of magnitude greater than those at other sites, was observed at the central cone of the MTJ-1 caldera. The recovery of eruption debris at a central volcanic cone, including molten sulfur, volcanic ash, and lapilli, indicates an active volcanic eruption and hydrothermal venting at the MTJ-1 caldera. Our results suggest extensive and various hydrothermal activity in the NE Lau Basin, thereby providing valuable insight into hydrothermal and volcanic processes in back-arc environments.

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To investigate factors affecting the evolution of nitrogen system in rocks during progressive metamorphism, unmetamorphosed to greenschist-facies pelitic samples were collected from both pelitic and (pelitic members of) psammitic rock sequences along two transects of the Taiwan mountain belt. Among all samples, greenschist-facies metapelites from the Tananao metamorphic basement of this mountain belt exhibit the most variable and depleted nitrogen content and δ¹⁵N values. The observations are difficult to interpret and can only be accounted for by complicated tectonic/metamorphic history of the basement complex. On the other hand, irrespective of the different metamorphic grades, samples from the metamorphosed cover strata, which have been subjected to only one stage of metamorphism, show comparable nitrogen and N-isotope composition. The only exception is that greenschist-facies metapelites from the pelitic members of psammitic rock sequence (i.e., the G(Ps) group samples) of the cover strata are enriched in the ¹⁵N isotope by at least 1‰. Taking Al content as a normalizing factor, averaged N/Al and LOI/Al show systematic decrease through progressive metamorphism. Given the non-homogeneous nature of the sedimentary protoliths, the comparable N-isotope composition among the unmetamorphosed to zeolite-facies (i.e., S-Z) group, prehnite-pumpellyite-facies (i.e., PP) group and greenschist-facies (i.e., G) group samples from pelitic rock sequences can be explained by rock devolatilization with limited nitrogen depletion (<10%). High δ¹⁵N composition of the G(Ps) group samples might be mainly due to more extensive nitrogen depletion (∼25%). It is suggested that during metamorphism, the pelitic rock sequence may approximate a closed system, while thick psammitic rock sequence may behave more like an open system, facilitating nitrogen depletion and N-isotope fractionation even for thin pelitic members within a thick psammitic sequence. Lithology, in addition to other factors, therefore plays a role in controlling the evolution of the nitrogen system during low-grade metamorphism. The present study implies that element transport by dehydration fluid in subduction zones may largely take place along fluid channel ways.

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We have detected a family of benzohopanes cyclised at C-20 and two families of diaromatic 8(14)-secohopanoids, in carbonates from the Late Permian Meishan (China) and Bulla (Italy) sections. Furthermore, we have identified a C₃₃ 8(14)-secohopanoid possessing a fluorene moiety. The relative distribution of 8(14)-secohopanoids with a fluorene moiety was similar to that of benzohopanes cyclised at C-20, suggesting that the secohopanoids might be diagenetic products of benzohopanes, or that the benzohopanes and secohopanoids share the same precursor. Furthermore, the series of benzohopanes cyclised at C-16 was not detected from the carbonates of the two sections, which might suggest that the benzohopanes cyclised at C-16 are thermodynamically less stable than those with the aromatic ring attached at C-20.

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The distribution of the rare earths and yttrium (REY) in co-existing hydrous Mn oxides and Fe oxides that form marine hydrogenetic ferromanganese crusts is used to better describe the partitioning and fractionation of the REY between these (hydr)oxides and seawater in the natural marine system. Four fractions (easily exchangeable, Mn-oxide-bound, Fe-oxide-bound, and insoluble-residue-bound REY) were separated by an adjusted sequential leaching procedure from two ferromanganese crusts from the Central Pacific. The distribution of the REY differs significantly between these leaching fractions and gives evidence for decoupling of La, Ce, Gd, Y, and Lu from their respective neighbours in the REY series during partitioning between hydrous Fe oxides, Mn oxides and seawater. Both the Mn oxides and the Fe oxides display pronounced positive Ce anomalies of almost similar size. This suggests that in the natural marine system oxidative scavenging of Ce from seawater is not restricted to Mn oxides but also occurs on hydrous Fe oxides. The distribution of Ce between the Mn oxides and the hydrous Fe oxides follows that of the trivalent REY and contrasts sharply with that of tetravalent Zr, Hf and Th. This suggests that preferential Ce removal from seawater does not result from the oxidation of dissolved Ce(III) within the marine water column, but that Ce(III) is oxidized after its sorption at the metal (hydr)oxide surface. Patterns of apparent hydrous Fe oxide/Mn oxide distribution coefficients show a sigmoidal shape and display negative anomalies for La, Gd, Y, and Lu (and the M-type lanthanide tetrad effect), indicating preferential scavenging of these elements by the Mn oxides as compared to the Fe oxides. The pronounced differences between the REY distribution in the Mn oxides and that in the Fe oxides cannot solely be explained by the REY speciation in seawater, but require additional and mineral-specific REY fractionation during surface-complexation.

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This paper presents stratigraphy, mineralogy, multi-element geochemistry, organic and inorganic carbon contents in the sediments along a 255 cm long depth profile from Lake Tecocomulco in the north-eastern part of the Basin of Mexico. Mineralogical abundance, elemental ratios, chemical index of alteration (CIA), abundance of CO₃ and C_org divide the depth profile into geochemical zones and provide insight to the volcanic eruptions and phases of different paleohydrological events during the last 31,000 ¹⁴C yr BP. The dacite to rhyolite air fall tephra pre-date the LGM (ca. 31,000 ¹⁴C yr BP) and might be from a Plinian eruption event at the Tláloc volcano located at 60 km in the south-west of the lake. The lake sediments are characterized by higher CIA values and metal contents between ca. 30,000-29,000 ¹⁴C yr BP. Between ca. 28,000-25,700 ¹⁴C yr BP, sediments have lower CIA values and contain easily weatherable amphiboles. Abundant calcite and low CIA suggest arid conditions during ca. 25,700 and ca. 16,300 ¹⁴C yr BP, respectively. Between ca. 16,300 and 3,300 ¹⁴C yr BP, the sediments show lower CIA and sediments of the last ca. 3,300 ¹⁴C yr BP show highest CIA, C_org and higher metal contents. Given the regional nature of this record, we interpret the geochemical history at Lake Tecocomulco in terms of variable precipitation, inflow and aeolian activity.

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