Mountain building has classically been linked with CO2 drawdown from silicate weathering in the critical zone, although recent views on mountain building recognize the importance of rock-derived CO2 emissions from other organic and inorganic carbon sources. However, the focus on critical zone weathering reactions during mountain building does not consider the emission of metamorphic CO2 from subduction processes in the crust and mantle. Such deep carbon sources could outpace the surficial drawdown and release of carbon, in particular in actively extending mountain ranges that subduct large volumes of carbonate rock. Thus, accounting for weathering processes at depth and in the critical zone in parallel is crucial to fully assess how mountain-range uplift impacts the carbon cycle. Here, we quantify the exchange of CO2 between rock and the atmosphere from subduction-related processes and from critical zone weathering reactions in two major river systems in the central Apennine Mountains of Italy. The catchments straddle a geodynamic gradient across the subduction zone that is expressed as changes in surface heat flow and crustal thickness, whereas climatic boundary conditions are relatively constant.  At the regional scale, we find that metamorphic CO2 sources outpace critical zone inorganic carbon sources and sinks by 2 orders of magnitude above a window in the subducting slab that is characterized by high heat flow and low crustal thickness, and could have driven efficient degassing over the last 2 Ma. In contrast, surficial weathering processes dominate the carbon budget where crustal thickness is greater and heat flow is lower. Importantly, the difference in metamorphic degassing fluxes across the geodynamic gradient is multiple orders of magnitude larger than the difference in critical zone weathering fluxes. Thus, modulations of metamorphic decarbonation reactions are the most efficient process by which tectonics can regulate the inorganic carbon cycle in the Apennines. Both near-surface and deep sources of CO2 must be considered when constructing the carbon budget of orogenic systems that include the subduction of carbonate rock.
Abstract. The Eocene–Oligocene transition (EOT, ca. 40–33 Ma) marks a transformation from a largely ice-free to an icehouse climate mode that is well recorded by oxygen-stable isotopes and sea surface temperature proxies. Opening of the Southern Ocean gateways and decline in atmospheric carbon dioxide levels have been considered as factors in this global environmental transformation and the growth of ice sheets in Antarctica during the Cenozoic. A more comprehensive understanding is still needed of the interplay between forcing versus response, the correlation among environmental changes, and the involved feedback mechanisms. In this study, we investigate the spatio-temporal variation in export productivity using biogenic Ba (bio-Ba) from Ocean Drilling Program (ODP) sites in the Southern Ocean, focusing on possible mechanisms that controlled them as well as the correlation of export productivity changes to changes in the global carbon cycle. We document two high export productivity events in the Southern Ocean during the late Eocene (ca. 37 and 33.5 Ma) that correlate to proposed gateway-driven changes in regional circulation and to changes in global atmospheric pCO2 levels. Our findings suggest that paleoceanographic changes following Southern Ocean gateway openings, along with more variable increases in circulation driven by episodic Antarctic ice sheet expansion, enhanced export production in the Southern Ocean from the late Eocene through early Oligocene. These factors may have played a role in episodic atmospheric carbon dioxide reduction, contributing to Antarctic glaciation during the Eocene–Oligocene transition.
We undertook 87Sr/86Sr analyses for a range of carbonate bearing geological reference materials, and combined these with δ26Mg for a subset of samples. Following chemical purification in a series of chromatographic extractions, isotope ratios were measured by Multi-Collector-ICP-MS using a Plasma II (Nu instruments, Wrexham, UK). To validate efficient sample digestion procedures of carbonate fractions, total samples were treated with either 3 mol l−1 HNO3 and 0.5 mol l−1 HCl, respectively. Results of both leaching procedures are identical within reproducibility. Reference values for SRM 88A (formerly NBS 88A), SRM 1B (formerly NBS 1B), SARM 40, SARM 43, JDo-1, JLs-1, and San Carlos olivine range from 0.70292 to 0.73724 in 87Sr/86Sr and from -2.80 to -0.41 ‰ for δ26Mg, respectively. This set of geological reference materials can be used for sedimentary rock material with different carbonate mineral and matrix composition as quality control measurements of combined stable Mg and radiogenic Sr isotope analyses.•We present a protocol that facilitates the chemical separation of Mg and Sr in carbonate bearing geological reference materials including 87Sr/86Sr and δ26Mg of certified reference materials.
Abstract. Hydrothermal alteration and mineralization processes can affect the physical and chemical properties of volcanic rocks. Aggressive acidic degassing and fluid flow often also lead to changes in the appearance of a rock, such as changes in surface coloration or intense bleaching. Although hydrothermal alteration can have far-reaching consequences for rock stability and permeability, limited knowledge exists on the detailed structures, extent, and dynamic changes that take place near the surface of hydrothermal venting systems. By integrating drone-based photogrammetry with mineralogical and chemical analyses of rock samples and surface gas flux, we investigate the structure of the evolving volcanic degassing and alteration system at the La Fossa cone on the island of Vulcano, Italy. Our image analysis combines principal component analysis (PCA) with image classification and thermal analysis through which we identify an area of approximately 70 000 m2 that outlines the maximum extent of hydrothermal alteration effects at the surface, represented by a shift in rock color from reddish to gray. Within this area, we identify distinct gradients of surface coloration and temperature that indicate a local variability in the degassing and alteration intensity and define several structural units within the fumarole field. At least seven such larger units of increased activity could be constrained. Through mineralogical and geochemical analysis of samples from the different alteration units, we define a relationship between surface appearance in drone imagery and the mineralogical and chemical composition. Gradients in surface color from reddish to gray correlate with a reduction in Fe2O3 from up to 3.2 % in the unaltered regime to 0.3 % in the altered regime, and the latter coincides with the area of increased diffuse acid gas flux. As the pixel brightness increases towards higher alteration gradients, we note a loss of the initial (igneous) mineral fraction and a change in the bulk chemical composition with a concomitant increase in sulfur content from close to 0 % in the unaltered samples to up to 60 % in samples from the altered domains. Using this approach of combined remote-sensing and in situ analyses, we define and spatially constrain several alteration units and compare them to the present-day thermally active surface and degassing pattern over the main crater area. The combined results permit us to present a detailed anatomy of the La Fossa fumarole field, including high-temperature fumaroles and seven larger units of increased alteration intensity, surface temperature, and variably intense surface degassing. Importantly, we also identify apparently sealed surface domains that prevent degassing, likely as a consequence of mineral precipitation from degassing and alteration processes. By assessing the thermal energy release of the identified spatial units quantitatively, we show that thermal radiation of high-temperature fumaroles accounts for < 50 % of the total thermal energy release only and that the larger part is emitted by diffuse degassing units. The integrated use of methods presented here has proven to be a useful combination for a detailed characterization of alteration and activity patterns of volcanic degassing sites and has the potential for application in alteration research and for the monitoring of volcanic degassing systems.
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