Matching carbon pools and fluxes for the Southern Ocean Iron Release Experiment (SOIREE)

The Lagrangian Southern Ocean Iron Release Experiment (SOIREE) allowed study of a gradually evolving iron-mediated phytoplankton bloom in water labelled with the inert tracer sulfur hexafluoride, SF₆. This article describes a pelagic carbon budget for the mixed layer in SOIREE and assesses the extent to which closure of the budget is achieved. Net community production (NCP) converted 837 mmol m⁻² of inorganic carbon to organic carbon in 12.0 d after the first iron addition. A large fraction (41%) of NCP remained as particulate organic carbon in the mixed layer of the iron-enriched patch, while 23% was lost by horizontal dispersion and 0–29% was exported. The closure of the carbon budget is hampered by the lack of measurements of dissolved organic carbon (DOC), by a major uncertainty in carbon export, and by use of empirical conversion factors in estimates of carbon biomass and metabolic rates. Lagrangian carbon-budget studies may be improved by direct measurement of all major carbon parameters and conversion factors. Carbon cycling in the SOIREE bloom resembled that in ‘natural’ algal blooms in the open Southern Ocean in some respects, but not in all. Daily NCP in the SOIREE bloom (70 mmol m⁻² d⁻¹) was higher than in natural blooms, partly because other studies did not account for horizontal dispersion, were for longer periods or included less productive areas. The build-up of POC stock and carbon export as a fraction of NCP in SOIREE were in the lower range of observations elsewhere.     

Pre-Alpine history of the pennine zone in the Tauern Window,Austria: U-Pb and Rb-Sr geochronology

Zircon ages from major lithologies of the Zentralgneis suggest that much of the Variscan magmatism in the Tauern Window is older than previously suggested. In the southeast Tauern Window a tonalite has been dated at 314±7 m.y. and a granodioritic biotite augen gneiss at 313±10 m.y. Two granodiorites from the Granatspitzkern yielded zircon data consistent with a similar age. These zircon data require re-interpretation of some previously published Rb-Sr whole rock ages and raise the possibility that Alpine metamorphism caused more widespread disturbance of Rb-Sr whole rocks than commonly supposed. Rb-Sr data on fabric-forming white micas from two banded gneisses give ages close to 220 m.y., indicating the foliation in these rocks is pre-Alpine and has not been greatly affected by Alpine recrystallisation.     

A regional study of mantle velocity variations beneath eastern Australia and the southwestern Pacific using short-period recordings of P,S, PcP,ScP and ScS waves produced by Tongan deep earthquakes

Deep earthquakes located in the Tonga-Kermadec region produce exceptionally clear and sharp short-period P, S, PcP, ScP, and ScS phases which are recorded at many stations at distances of less than 60°. The data used in this study are produced by short-period stations located in oceanic-type regions (Fiji and New Caledonia), a mobile continental region (eastern Australia) and a shield region (central Australia). Differential travel-time residuals of the above phases at these stations are investigated to determine the contribution to the differential residuals from: (1) the upper part of the mantle (S-P residuals); (2) the core-to-station portion of the mantle (ScS-ScP residuals); and (3) the hypocenter-to core portion of the mantle (ScP-PcP residuals). The use of differential travel-time residuals considerably reduces near-station effects and effects due to inaccurate determination of the source parameters, and hence the results can be interpreted as due to variations along the propagation paths. The results show that (S-P) residuals from phases traveling along event-to-station paths are about 7 s smaller at the shield station than at the oceanic stations. This correlation with surface tectonic environments is equally strong for the (ScS-ScP) residuals, with the shield/oceanic station difference being about 4 s. Moreover, the data suggest that this correlation between differential residuals and surface tectonic environments is caused by variations in shear velocity within the upper part of the mantle. However, the data cannot uniquely resolve the required depth of these variations within the mantle. For example, if the shear velocity variations extend to a depth of 400 km beneath the recording stations, then the average shear velocity difference between shield- and oceanic-type environments is about 4%. However, if the variations extend only to a depth of 200 km, this difference is more than 8%.(ScP-PcP) and (ScS-PcS) residuals vary from about +1 to about +4 s at the different stations, apparently because of compressional velocity variations in the mantle along the Pc path. If the variation in compressional velocity within the mantle below a depth of about 600 km is about 10% and occurs near the source region, these results suggest that, in the vicinity of deep earthquake zones, variations in compressional velocity extend to a depth of about 1000 km. However, these results can equally be explained by a 1% variation in compressional velocity, evenly distributed along the entire Pc path. An estimate of Q determined from the observed predominant frequency of ScS waves, as recorded at the shield station, suggests that the average 〈Q_s〉 of the mantle beneath about 600 km is about 1050 at frequencies of about 1 Hz.     

Lead isotope variations across terrane boundaries of the Tien Shan and Chinese Altay

The Altaid orogen was formed by aggregation of Paleozoic subduction–accretion complexes and Precambrian basement blocks between the Late Proterozoic and the Early Mesozoic. Because the Altaids are the site of abundant granitic plutonism and host some of the largest gold deposits in the world, understanding their formation has important implications on the comprehension of Phanerozoic crustal growth and metallogeny. In this study, we present the first extensive lead isotope data on magmatic and metasedimentary rocks as well as ore deposits of the southern part of the Altaids, including the Tien Shan (Tianshan) and southern Altay (Altai) orogenic belts. Our results show that each terrane investigated within the Tien Shan and southern Altay is characterized by a distinct Pb isotope signature and that there is a SW–NE Pb isotope gradient suggesting a progressive transition from a continental crust environment in the West (the Kyzylkum and Kokshaal segments of the Southern Tien Shan) to an almost 100% juvenile (MORB-type mantle-derived) crust environment in the East (Altay). The Pb isotope signatures of the studied ore deposits follow closely those of magmatic and metasedimentary rocks of the host terranes, thus supporting the validity of lead isotopes to discriminate terranes. Whereas this apparently suggests that no unique reservoir has been responsible for the huge gold concentration in this region, masking of a preferential Pb-poor Au-bearing reservoir by mixing with Pb-rich crustal reservoirs during the mineralizing events cannot be excluded.     

Geochronological challenges posed by continuously developing tectonometamorphic systems: insights from Rb–Sr mica ages from the Cycladic Blueschist Belt,Syros (Greece)

Is metamorphism and its causative tectonics best viewed as a series of punctuated events or as a continuum? This question is addressed through examination of the timing of exhumation of the Cycladic Blueschist Belt (CBB). The cause of scatter beyond analytical error in Rb–Sr geochronology was investigated using a suite of 39 phengite samples. Rb–Sr ages have been measured on phengite microsamples drilled from specific microstructures in thin sections of calcschists and metabasites from the CBB on Syros. The majority are from samples that have well‐preserved blueschist facies mineral assemblages with limited greenschist facies overprint. The peak metamorphic temperatures involved are below the closure temperature for white mica so that crystallization ages are expected to be preserved. This is supported by the coexistence of different ages in microstructures of different relative age; in one sample phengite from the dominant extensional blueschist facies fabric preserves an age of 35 Ma while post‐tectonic mica, millimetres away, has an age of 26 Ma. The results suggest that micro‐sampling techniques linked to detailed microstructural analysis are critical to understanding the timing and duration of deformation in tectonometamorphic systems. North of the Serpentinite Belt in northern Syros, phengite Rb–Sr ages are generally between 53 and 46 Ma, comparable to previous dates from this area. South of the Serpentinite Belt phengite in blueschist facies assemblages associated with extensional fabrics linked to exhumation have ages that range from 42 Ma down to c. 30 Ma indicating that extensional deformation while still under blueschist facies conditions continued until 30 Ma. No age measurements on samples with unambiguous evidence of deformation under greenschist facies conditions were made; two rocks with greenschist facies assemblages gave phengite ages that overlap with the younger blueschist samples, suggesting blueschist facies phengite is preserved in these rocks. Two samples yielded ages below 27 Ma; one is from a post‐tectonic microstructure, the other from a greenschist in which the fabric developed during earlier blueschist facies conditions. These ages are consistent with previous evidence of greenschist facies conditions from c. 25 Ma onwards. The data are consistent with a model of deformation that is continuous on a regional scale.     

Ocean acidification in New Zealand waters: trends and impacts

The threat posed by ocean acidification (OA) to the diversity and productivity of New Zealand marine ecosystems is assessed in a synthesis of published trends and impacts. A 20-year time series in Subantarctic water, and a national coastal monitoring programme, provide insight into pH variability, and context for experimental design, modelling and projections. A review of the potential impact of changes in the carbonate system on the major phyla in New Zealand waters confirms international observations that calcifying organisms, and particularly their early life-history stages, are vulnerable. The synthesis considers ecosystem and socio-economic impacts, and identifies current knowledge gaps and future research directions, including mechanistic studies of OA sensitivity. Advanced ecosystem models of OA, that incorporate the indirect effects of OA and interactions with other climate stressors, are required for robust projection of the future status of New Zealand marine ecosystems.     

Climate change projections for the surface ocean around New Zealand

The future status of the surface ocean around New Zealand was projected using two Earth System Models and four emission scenarios. By 2100 mean changes are largest under Representative Concentration Pathway 8.5 (RCP8.5), with a +2.5°C increase in sea surface temperature, and decreases in surface mixed layer depth (15%), macronutrients (7.5–20%), primary production (4.5%) and particle flux (12%). Largest macronutrient declines occur in the eastern Chatham Rise and subantarctic waters to the south, whereas dissolved iron increases in subtropical waters. Surface pH projections, validated against subantarctic time-series data, indicate a 0.335 decline to ～7.77 by 2100. However, projected pH is sensitive to future CO₂ emissions, remaining within the current range under RCP2.6, but decreasing below it by 2040 with all other scenarios. Sub-regions vulnerable to climate change include the Chatham Rise, polar waters south of 50°S, and subtropical waters north of New Zealand, whereas the central Tasman Sea is least affected.     

Crystallization rates of shock melts in three martian basalts: Experimental simulation with implications for meteoroid dimensions

Dynamic crystallization experiments have been performed on synthetic glasses representative of shock-generated melts observed in Los Angeles, Sayh al Uhaymir 150 and Dar al Gani 476 martian basalts. On the basis of qualitative (texture) and quantitative (fractal analysis) results, we show that melt pockets in Los Angeles cooled at a rate of 1040-1560 °C/h. Sayh al Uhaymir 150 and Dar al Gani 476 melt pockets cooled at 780 °C/h. Conductive cooling models, for a range of meteoroid diameters (10-50 cm), indicate that the minimum meteoroid diameter was small, on the order of 10-15 cm and that melt pockets cooled from post-shock temperatures within minutes. Our results also have bearing on shock implanted martian atmospheric components because it is during cooling that the melt pockets have the potential to lose gases. Modeling of argon diffusion in a spherical melt pocket indicates that during cooling and quench crystallization ∼4-60% of trapped martian atmospheric argon may be lost from the melt pocket through diffusive transport.     

Dissolution of orthopyroxene in basanitic magma between 0.4 and 2 GPa: further implications for the origin of Si-rich alkaline glass inclusions in mantle xenoliths

A large body of recent work has linked the origin of Si-Al-rich alkaline glass inclusions to metasomatic processes in the upper mantle. This study examines one possible origin for these glass inclusions, i.e., the dissolution of orthopyroxene in Si-poor alkaline (basanitic) melt. Equilibrium dissolution experiments between 0.4 and 2 GPa show that secondary glass compositions are only slightly Si enriched and are alkali poor relative to natural glass inclusions. However, disequilibrium experiments designed to examine dissolution of orthopyroxene by a basanitic melt under anhydrous, hydrous and CO₂-bearing conditions show complex reaction zones consisting of olivine, ± clinopyroxene and Si-rich alkaline glass similar in composition to that seen in mantle xenoliths. Dissolution rates are rapid and dependent on volatile content. Experiments using an anhydrous solvent show time dependent dissolution rates that are related to variable diffusion rates caused by the saturation of clinopyroxene in experiments longer than 10 minutes. The reaction zone glass shows a close compositional correspondence with natural Si-rich alkaline glass in mantle-derived xenoliths. The most Si-and alkali-rich melts are restricted to pressures of 1 GPa and below under anhydrous and CO₂-bearing conditions. At 2 GPa glass in hydrous experiments is still Si-␣and alkali-rich whereas glass in the anhydrous and CO₂-bearing experiments is only slightly enriched in SiO₂ and alkalis compared with the original solvent. In the low pressure region, anhydrous and hydrous solvent melts yield glass of similar composition whereas the glass from CO₂-bearing experiments is less SiO₂ rich. The mechanism of dissolution of orthopyroxene is complex involving rapid incongruent breakdown of the orthopyroxene, combined with olivine saturation in the reaction zone forming up to 60% olivine. Inward diffusion of CaO causes clinopyroxene saturation and uphill diffusion of Na and K give the glasses their strongly alkaline characteristics. Addition of Na and K also causes minor SiO₂ enrichment of the reaction glass by increasing the phase volume of olivine. Olivine and clinopyroxene are transiently stable phases within the reaction zone. Clinopyroxene is precipitated from the reaction zone melt near the orthopyroxene crystal and redissolved in the outer part of the reaction zone. Olivine defines the thickness of the reaction zone and is progressively dissolved in the solvent as the orthopyroxene continues to dissolve. Although there are compelling reasons for supporting the hypothesis that Si-rich alkaline melts are produced in the mantle by orthopyroxene – melt reaction in the mantle, there are several complications particularly regarding quenching in of disequilibrium reaction zone compositions and the mobility of highly polymerized melts in the upper mantle. It is considered likely that formation of veins and pools of Si-rich alkaline glass by orthopyroxene – melt reaction is a common process during the ascent of xenoliths. However, reaction in situ within the mantle will lead to equilibration and therefore secondary melts will be only moderately siliceous and alkali poor. Received: 24 August 1998 / Accepted: 2 December 1998