An Eocene/Oligocene blueschist‐/greenschist facies P–T loop from the Cycladic Blueschist Unit on Naxos Island,Greece: Deformation‐related re‐equilibration vs. thermal relaxation

Geothermobarometric and geochronological work indicates a complete Eocene/early Oligocene blueschist/greenschist facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure–temperature (P–T) method of thermocalc coupled with detailed textural work, we separate an early blueschist facies event at 576 ± 16 to 619 ± 32°C and 15.5 ± 0.5 to 16.3 ± 0.9 kbar from a subsequent greenschist facies overprint at 384 ± 30°C and 3.8 ± 1.1 kbar. Multi‐mineral Rb–Sr isochron dating yields crystallization ages for near peak‐pressure blueschist facies assemblages between 40.5 ± 1.0 and 38.3 ± 0.5 Ma. The greenschist facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of greenschist facies reworking, obtained from disequilibrium patterns, cluster near c. 32 Ma, with one sample showing rejuvenation at c. 27 Ma. We conclude that the high‐P rocks from south Naxos were exhumed to upper mid‐crustal levels in the late Eocene and early Oligocene at rates of 7.4 ± 4.6 km/Ma, completing a full blueschist‐/greenschist facies metamorphic cycle soon after subduction within c. 8 Ma. The greenschist facies overprint of the blueschist facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid‐controlled metamorphic re‐equilibration, and is unrelated to the strong high‐T metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged.     

A GLUE‐based uncertainty assessment framework for tritium‐inferred transit time estimations under baseflow conditions

The last decade has seen major technical and scientific improvements in the study of water transfer time through catchments. Nevertheless, it has been argued that most of these developments used conservative tracers that may disregard the oldest component of water transfer, which often has transit times greater than 5 years. Indeed, although the analytical reproducibility of tracers limits the detection of the older flow components associated with the most dampened seasonal fluctuations, this is very rarely taken into account in modelling applications. Tritium is the only environmental tracer at hand to investigate transfer times in the 5‐ to 50‐year range in surface waters, as dissolved gases are not suitable due to the degassing process. Water dating with tritium has often been difficult because of the complex history of its atmospheric concentration, but its current stabilization together with recent analytical improvements open promising perspectives. In this context, the innovative contribution of this study lies in the development of a generalized likelihood uncertainty estimation‐based approach for analysing the uncertainties associated with the modelling of transit time due to both parameter identification and tracer analytical precision issues. A coupled resampling procedure allows assessment of the statistical significance of the transfer time differences found in diverse waters. This approach was developed for tritium and the exponential‐piston model but can be implemented for virtually any tracer and model. Stream baseflow, spring and shallow aquifer waters from the Vallcebre research catchments, analysed for tritium in different years with different analytical precisions, were investigated by using this approach and taking into account other sources of uncertainty. The results showed three groups of waters of different mean transit times, with all the stream baseflow and spring waters older than the 5‐year threshold needing tritium. Low sensitivity of the results to the model structure was also demonstrated. Dual solutions were found for the waters sampled in 2013, but these results may be disambiguated when additional analyses will be made in a few years. Copyright © 2016 John Wiley & Sons, Ltd.     

Spatial and temporal structure of the Denmark Strait Overflow revealed by acoustic observations

In spite of the fundamental role the Atlantic Meridional Overturning Circulation (AMOC) plays for global climate stability, no direct current measurement of the Denmark Strait Overflow, which is the densest part of the AMOC, has been available until recently that resolve the cross-stream structure at the sill for long periods. Since 1999, an array of bottom-mounted acoustic instruments measuring current velocity and bottom-to-surface acoustic travel times was deployed at the sill. Here, the optimization of the array configuration based on a numerical overflow model is discussed. The simulation proves that more than 80% of the dense water transport variability is captured by two to three acoustic current profilers (ADCPs). The results are compared with time series from ADCPs and Inverted Echo Sounders deployed from 1999 to 2003, confirming that the dense overflow plume can be reliably measured by bottom-mounted instruments and that the overflow is largely geostrophically balanced at the sill.     

Future ocean uptake of CO2: interaction between ocean circulation and biology

We discuss the potential variations of the biological pump that can be expected from a change in the oceanic circulation in the ongoing global warming. The biogeochemical model is based on the assumption of a perfect stoichiometric composition (Redfield ratios) of organic material. Upwelling nutrients are transformed into organic particles that sink to the deep ocean according to observed profiles. The physical circulation model is driven by the warming pattern as derived from scenario computations of a fully coupled ocean-atmosphere model. The amplitude of the warming is determined from the varying concentration of atmospheric CO₂. The model predicts a pronounced weakening of the thermohaline overturning. This is connected with a reduction of the transient uptake capacity of the ocean. It yields also a more effective removal of organic material from the surface which partly compensates the physical effects of solubility. Both effects are rather marginal for the evolution of atmospheric pCO₂. Running climate models and carbon cycle models separately seems to be justified. Received: 9 August 1995 / Accepted: 22 April 1996     

Discussion of the new kinematic mantle convection theory

Summary First of all it is shown that the nine fundamental suppositions of the kinematic theory of the mantle convection fit well into the image which is obtained from the mantle by other geophysical results. The question of episodicity of the orogenesis is dealt with in connection with the questions of steadiness and unsteadiness of the convection. Moreover it is shown that the ocean-spreading theory and the kinematic mantle convection theory are compatible with each other in their main aspects and that they complement each other.     

The infrared spectrum of asteroid 433 Eros

The mineralogical composition of asteroid Eros has been determined from its infrared spectrum (0.9–2.7μm; 28cm^?1 resolution). Major minerals include metallic NiFe and pyroxene; no spectroscopic evidence for olivine or plagioclase feldspar was found. The IR spectrum of Eros is most consistent with a stony-iron composition.     

Lithium isotope fractionation in the southern Cascadia subduction zone

We present lithium (Li) abundances and isotope compositions for a suite of anhydrous olivine tholeiites (HAOTs) and hydrous basalt-andesitic (BA) lavas from the Mt. Shasta and Medicine Lake regions, California. The values of δ⁷Li vary from + 0.9‰ to + 6.4‰ and correlate inversely with distance from the trench. These data are consistent with continuous isotope fractionation of Li during dehydration of the subducted oceanic lithosphere, an interpretation corroborated by uniformly high pre-eruptive H₂O contents in basaltic andesites accompanied by high Li, Rb, Sr, Ba and Pb abundances. The subduction-derived component that was added to these hydrous magmas is shown to be very similar beneath both Mt. Shasta and Medicine Lake volcanoes despite characteristically distinct Li isotope compositions in the magmas themselves. More evolved andesites and dacites from Mt. Shasta have δ⁷Li from + 2.8 to + 6.9‰ which is identical with the range obtained for HAOTs and BA lavas from Mt. Shasta. Therefore, Li isotopes do not provide evidence for any other crustal component admixed to Mt. Shasta andesites or dacites during magmatic differentiation and magma mixing in the crust.