ALBERT

Description: We discuss a recent attempt by Chris Daly and Simon Langford to do away with mathematical explanations of physical phenomena. Daly and Langford suggest that mathematics merely indexes parts of the physical world, and on this understanding of the role of mathematics in science, there is no need to countenance mathematical explanation of physical facts. We argue that their strategy is at best a sketch and only looks plausible in simple cases. We also draw attention to how frequently Daly and Langford find themselves in conflict with mathematical and scientific practice.

Description: Aims The Amazon basin plays an important role in the global carbon budget. Interannual climate variability associated with El Niño can affect the Amazon ecosystem carbon balance. In recent years, studies have suggested that there are two different types of El Ninos: eastern-Pacific (EP) El Niño and central-Pacific (CP) El Niño. The impacts of two types of El Niño on the Amazon climate and Amazon ecosystem are analyzed in the study. Methods A composite method has been applied to highlight the common features for the EP- and CP-El Niño events using observational data, IPCC-AR4 model output. Potential impacts of the two different types of El Niño on ecosystem carbon sequestration over the Amazon have been investigated using a process-based biogeochemical model, the Biome–BioGeochemical Cycles model (Biome–BGC). Important Findings Below-normal rainfall is observed year round in northern, central and eastern Amazonia during EP-El Niño years. During CP-El Niño years, negative rainfall anomalies are observed in most of the Amazon during the austral summer wet season, while there is average or above-average precipitation in other seasons. EP- and CP-El Niño events produce strikingly different precipitation anomaly pattern in the tropical and subtropical Andes during the austral fall season: wetter conditions prevail during EP-El Niño years and drier conditions during CP-El Niño years. Temperatures are above-average year round throughout tropical South America during EP-El Niño events, especially during austral summer. During CP-El Niño events, average or slightly above-average temperatures prevail in the tropics, but these temperatures are less extreme than EP year's temperature except in austral fall. These precipitation and temperature anomalies influence ecosystem productivity and carbon sequestration throughout the Amazon. Using the Biome–BGC model, we find that net ecosystem production (NEP) in the EP-El Niño years is below average, in agreement with most previous studies; such results indicate that the Amazon region acts as a net carbon source to the atmosphere during EP-El Niño years. In the CP-El Niño years, NEP does not differ significantly from its climatological value, suggesting that the Amazon forest remains a carbon sink for the atmosphere. Thus, even if CP-El Niño events increase in frequency or amplitude under global warming climate as predicted in some Global Climate Models, the Amazon rainforest may remain a carbon sink to the atmosphere during El Niño years in the near future.

Description: The tropical conifer Callitris intratropica (Cupressaceae) produces clear annual growth rings, and has been shown to be potentially useful for understanding past climate variability in northern Australia. As climate patterns in this region become less predictable, an understanding of plant responses to different weather patterns is of importance. In this paper, we examine tree water relations using a parameter here called tree water deficit ( D ), determined from de-trended stem size variability in densely grown (‘grove’) and isolated trees. This parameter provides an integrated measure of the trees' response to water supply and demand under constantly changing environmental conditions. The work, conducted over 12 months, found that daily variation in tree water deficit was determined mainly by soil water availability, but temperature and relative humidity contributed more to the variability over some periods. Isolated and grove trees exhibited quite distinct patterns of D development during the year, but particularly during the transition between the dry and wet seasons. The results of this work suggest that the dendrochronological interpretation of tree rings in the context of strongly seasonal water availability should incorporate an understanding of the development of seasonal drought in isolated trees compared with trees experiencing strong intra-specific competition. Different responses based on the ecological situations of the trees will affect their patterns of stem growth, and ultimately the climatic information that is incorporated in ring width variability.

Description: One hypothesis for the origin of alkaline lavas erupted on oceanic islands and in intracontinental settings is that they represent the melts of amphibole-rich veins in the lithosphere (or melts of their dehydrated equivalents if metasomatized lithosphere is recycled into the convecting mantle). Amphibole-rich veins are interpreted as cumulates produced by crystallization of low-degree melts of the underlying asthenosphere as they ascend through the lithosphere. We present the results of trace-element modelling of the formation and melting of veins formed in this way with the goal of testing this hypothesis and for predicting how variability in the formation and subsequent melting of such cumulates (and adjacent cryptically and modally metasomatized lithospheric peridotite) would be manifested in magmas generated by such a process. Because the high-pressure phase equilibria of hydrous near-solidus melts of garnet lherzolite are poorly constrained and given the likely high variability of the hypothesized accumulation and remelting processes, we used Monte Carlo techniques to estimate how uncertainties in the model parameters (e.g. the compositions of the asthenospheric sources, their trace-element contents, and their degree of melting; the modal proportions of crystallizing phases, including accessory phases, as the asthenospheric partial melts ascend and crystallize in the lithosphere; the amount of metasomatism of the peridotitic country rock; the degree of melting of the cumulates and the amount of melt derived from the metasomatized country rock) propagate through the process and manifest themselves as variability in the trace-element contents and radiogenic isotopic ratios of model vein compositions and erupted alkaline magma compositions. We then compare the results of the models with amphibole observed in lithospheric veins and with oceanic and continental alkaline magmas. While the trace-element patterns of the near-solidus peridotite melts, the initial anhydrous cumulate assemblage (clinopyroxene ± garnet ± olivine ± orthopyroxene), and the modelled coexisting liquids do not match the patterns observed in alkaline lavas, our calculations show that with further crystallization and the appearance of amphibole (and accessory minerals such as rutile, ilmenite, apatite, etc.) the calculated cumulate assemblages have trace-element patterns that closely match those observed in the veins and lavas. These calculated hydrous cumulate assemblages are highly enriched in incompatible trace elements and share many similarities with the trace-element patterns of alkaline basalts observed in oceanic or continental setting such as positive Nb/La, negative Ce/Pb, and similiar slopes of the rare earth elements. By varying the proportions of trapped liquid and thus simulating the cryptic and modal metasomatism observed in peridotite that surrounds these veins, we can model the variations in Ba/Nb, Ce/Pb, and Nb/U ratios that are observed in alkaline basalts. If the isotopic compositions of the initial low-degree peridotite melts are similar to the range observed in mid-ocean ridge basalt, our model calculations produce cumulates that would have isotopic compositions similar to those observed in most alkaline ocean island basalt (OIB) and continental magmas after ~0·15 Gyr. However, to produce alkaline basalts with HIMU isotopic compositions requires much longer residence times (i.e. 1–2 Gyr), consistent with subduction and recycling of metasomatized lithosphere through the mantle. EM magmas cannot readily be explained without appealing to other factors such as a heterogeneous asthenosphere. These modelling results support the interpretation proposed by various researchers that amphibole-bearing veins represent cumulates formed during the differentiation of a volatile-bearing low-degree peridotite melt and that these cumulates are significant components of the sources of alkaline OIB and continental magmas. The results of the forward models provide the potential for detailed tests of this class of hypotheses for the origin of alkaline magmas worldwide and for interpreting major and minor aspects of the geochemical variability of these magmas.

Description: We conducted 1 atm experiments on a synthetic Hawaiian picrite at f O 2 values ranging from the quartz–fayalite–magnetite (QFM) buffer to air and temperatures ranging from 1302 to 1600°C. Along the QFM buffer, olivine is the liquidus phase at ~1540°C and small amounts of spinel (〈 0·2 wt %) are present in experiments conducted at and below 1350°C. The olivine becomes progressively more ferrous with decreasing temperature [Fo 92 · 3 to Fo 87 · 3 , where Fo = 100 x Mg/(Mg + Fe), atomic]; compositions of coexisting liquids reflect the mode and composition of the olivine with concentrations of SiO 2 , TiO 2 , Al 2 O 3 , and CaO increasing monotonically with decreasing temperature, those of NiO and MgO decreasing, and FeO* (all Fe as FeO) remaining roughly constant. An empirical relationship based on our data, T (°C) = 19·2 x (MgO in liquid, wt %) + 1048, provides a semi-quantitative geothermometer applicable to a range of Hawaiian magma compositions. The olivine–liquid exchange coefficient, = (FeO/MgO) ol /(FeO/MgO) liq , is 0·345 ± 0·009 (1 ) for our 11 experiments. A literature database of 446 1 atm experiments conducted within 0·25 log units of the QFM buffer (QFM ± 0·25) yields a median of 0·34; values from single experiments range from 0·41 to 0·13 and are correlated with SiO 2 and alkalis in the liquid, as well as the forsterite (Fo) content of the olivine. For 78 experiments with broadly tholeiitic liquid compositions (46–52 wt % SiO 2 and ≤ 3 wt % Na 2 O + K 2 O) coexisting with Fo 92 – 80 olivines, and run near QFM (QFM ± 0·25), is approximately independent of composition with a median value of 0·340 ± 0·012 (error is the mean absolute deviation of the 78 olivine–glass pairs from the database that meet these compositional criteria), a value close to the mean value of 0·343 ± 0·008 from our QFM experiments. Thus, over the composition range encompassed by Hawaiian tholeiitic lavas and their parental melts, ~ 0·34 and, given the redox conditions and a Fo content for the most magnesian olivine phenocrysts, a parental melt composition can be reconstructed. The calculated compositions of the parental melts are sensitive to the input parameters, decreasing by ~1 wt % MgO for every log unit increase in the selected f O 2 , every 0·5 decrease in the Fo-number of the target olivine, and every 0·015 decrease in . For plausible ranges in redox conditions and Fo-number of the most MgO-rich olivine phenocrysts, the parental liquids for Hawaiian tholeiites are highly magnesian, in the range of 19–21 wt % MgO for Kilauea, Mauna Loa and Mauna Kea.