Exogenous disturbances and endogenous self-organized processes are not mutually exclusive drivers of spatial patterns in macroalgal assemblages

Complex spatial patterns are common in coastal marine systems, but mechanisms underlying their formation are disputed. Most empirical work has focused on exogenous spatially structured disturbances as the leading cause of pattern formation in species assemblages. However, theoretical and observational studies suggest that complex spatial patterns, such as power laws in gap-size distribution, may result from endogenous self-organized processes involving local-scale interactions. The lack of studies simultaneously assessing the influence of spatially variable disturbances and local-scale interactions has fuelled the idea that exogenous and endogenous processes are mutually exclusive explanations of spatial patterns in marine ecosystems. To assess the relative contribution of endogenous and exogenous processes in the emergence of spatial patterns, an intertidal assemblage of algae was exposed for two years to various combinations of intensity and spatial patterns of disturbance. Localized disturbances impinging at the margins of previously disturbed clearings and homogenous disturbances without any spatial pattern generated heterogeneous distributions of disturbed gaps and macroalgal patches, characterized by a power-law scaling. Spatially varying disturbances produced a spatial gradient in the distribution of algal patches and, to a lesser extent, also a power-law scaling in both patch- and gap-size distributions. These results suggest that exogenous and endogenous processes are not mutually exclusive forces that can lead to the formation of similar spatial patterns in species assemblages

Quantifying frozen melt in crustal rocks: A new melt-o-meter based on zircon rim volumes

International audienceQuantifying the distribution of granitic melt at all scales in mid-to lower crustal migmatitic terranes is critical to understand crustal melting processes, chemical differentiation of the crust and its rheological behavior during deformation. We propose a new method to determine the fraction of frozen granitic melt on a hand specimen scale based on the relative volumes of newly precipitated to total zircon (FPZ = Fraction of newly Precipitated Zircon) as obtained by image analysis on dated zircon cores and rims. Using the calculated Zr-solubility [Zr] sat in the melt at the inferred melting temperature and the Zr concentration in the bulk sample [Zr] bulk , the fraction of melt F melt can be determined through F melt = FPZ × [Zr] bulk / [Zr] sat. The such obtained F melt corresponds to the melt fraction in the hand specimen at the time the system closed for melt mobility. Thermodynamic modelling further allows estimation of H 2 O-contents required to maintain the melt fraction obtained from the melt-o-meter in a molten stage. The applicability of this method has been tested on eight migmatitic samples with peak temperatures between 725 and 925°C. Most of the lower temperature migmatites (800°C) retained F melt of 0.35-0.50 (±0.07-0.10). At these melt fractions, melt extraction and melt migration from and within the source should be efficient. Consequently, these samples are likely open-system migmatites affected by melt accumulation or depletion processes. The melt-o-meter requires that the rock types under consideration produced a granitic melt that remained zircon-saturated and is therefore restricted to migmatitic meta-sediments and meta-granitoids. When applied carefully, this melt-o-meter offers a new and powerful tool to not only quantify melt distribution but also evaluate the extent of melt mobility in migmatites