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How the weathering zone propagates into un-weathered rock at depth is subject of increasing research. Some models suggest that the depth of the ground water table controls weathering zone thickness. Others predict that regolith thickness is pre-conditioned by fracture density. Still others invoke weathering-induced fracturing by iron oxidation. To explore the influence of climate versus tectonic processes on weathering intensity and regolith thickness we drilled 5 cores in granitic bedrock along a climatic gradient within the EarthShape project (from N to S dry dessert, semi-arid, Mediterranean, humid) along the Chilean Coastal Cordillera. We combined borehole logging with geochemical and geophysical analyses of core and soil samples to evaluate the controls on weathering. Based on this geochemical data set we are able to show that the studied weathering profiles consist of three main weathering parts characterized by distinct geochemical gradients. The first part comprises the soil layer and is characterized by high weathering intensities, the second part comprises the saprolite down to the bedrock and includes the weathering front, the third part is found below the weathering front and is associated with fractures. Thus multiple weathering zones developed at major fractures. We found that the depth of the weathering front is mainly controlled by the characteristics of the fracture network: fracture aperture and dip, whereas weathering intensity is controlled by a combination of fracture network and the influence of vegetation through soil CO2 and organic acids. Generally, our data show that for a given fracture density fracture apertures control regolith thickness.

During the past decades, cosmogenic nuclides, such as in-situ produced 10Be, evolved as state-of-the-art tool to quantify millennial-scale denudation in quartz-rich landscapes. However, applications of cosmogenic nuclides to carbonate-dominated lithologies are still rare, even though carbonate dissolution is a major weathering process that may compensate anthropogenically elevated CO2 levels over centennial-millennial timescales. Recent advances in quantifying carbonate erosion have been made using cosmogenic 36Cl and of carbonate weathering using dissolved loads (e.g.[1],[2]). As these integrate over two distinct time-scales we developed a method that records such rates simultaneously: the cosmogenic meteoric 10Be over 9Be ratio (10Be/9Be). We adapted a framework[3] combining a known atmospheric flux tracer, meteoric 10Be (T1/2=1.4 My), with stable 9Be, a trace released from rocks by weathering, to the limestone-dominated French Jura Mountains. We analyzed water, soil, sediment, and bedrock for 10Be/9Be, major/trace elements, and Sr and C isotopes, to quantify i) Be contribution from carbonate vs. silicate minerals and ii) from primary vs. secondary carbonate phases, iii) solid-solute load partitioning, and iv) deep (sediment) vs. surficial (soils) weathering and erosion. Our results indicate average denudation rates of 300 t/km2/yr, denudation being dominated by weathering flux (W/D ratios of 0.7-0.97), and a consistently higher contribution from deep weathering. These rates agree to decadal-scale denudation rates from combined suspended and dissolved fluxes within < 2x which highlights the great potential of this method for future Earth’s surface studies. [1]Ott et al., JGR-ES, 2019. [2]Ben-Asher et al., GSA-Bull., 2021. [3]von Blanckenburg, F, Bouchez, J. and Wittmann, H., EPSL, 2012.

The geochemical composition of oceanic basalts provides us with a window into the distribution of geochemical elements within the Earth’s mantle in space and time. In conjunction with a throughout knowledge on how the different elements behave e.g. during melt formation and evolution or on their partition behaviour between e.g. minerals and melts this information has been transformed into various models on how oceanic crust is formed along plume influenced or normal mid-ocean ridge segments, how oceanic crust evolves in response to seawater, on subduction recycling of oceanic crust and so forth. The work presented in this habilitation was aimed at refining existing models, putting further constraints on some of the major open questions in this field of research while at the same time trying to increase our knowledge on the behaviour of noble gases as a tracer for melt formation and evolution processes. In the line of this work the author and her co-workers were able to answer one of the major questions concerning the formation of oceanic crust along plume-influenced ridges – in which physical state does the plume material enter the ridge? Based on submarine volcanic glass He, Ne and Ar data, the author and her co-workers have shown that the interaction of mantle plumes with mid-ocean ridges occurs in the physical form of melts. In addition, the author and her co-workers have also put further constraints on one of the major questions concerning the formation of oceanic crust along normal mid-ocean ridges – namely how is the mid-ocean ridge system effectively cooled to form the lower oceanic crust? Based on Ne and Ar data in combination with Cl/K ratios of basaltic glass from the Mid-Atlantic ridge and estimates of crystallisation pressures they have shown, that seawater penetration reaches lower crustal levels close to the Moho, indicating that hydrothermal circulation might be an effective cooling mechanism even for the deep parts of the oceanic crust. Considering subduction recycling, the heterogeneity of the Earth’s mantle and mantle dynamic processes the key question is on which temporal and spatial scales is the Earth’s mantle geochemically heterogeneous? In the line of this work the author along with her co-workers have shown based on Cl/K ratios in conjunction with the Sr, Nd, and Pb isotopes of the OIBs representing the type localities for the different mantle endmembers that the quantity of Cl recycled into the mantle via subduction is not uniform and that neither the HIMU nor the EM1 and EM2 mantle components can be considered as distinct mantle endmembers. In addition, we have shown, based on He, Ne and Ar isotope and trace-element data from the Foundation hotspot that the near ridge seamounts of the Foundation seamount chain formed by the Foundation hotspot erupt lavas with a trace-element signature clearly characteristic of oceanic gabbro which indicates the existence of recycled, virtually unchanged lower oceanic crust in the plume source. This is a clear sign of the inefficiency of the stirring mechanism existing at mantle depth. Similar features are seen in other near-axis hotspot magmas around the world. Based on He, Sr, Nd, Pb and O isotopes and trace elements in primitive mafic dykes from the Etendeka flood basalts, NW Namibia the author along with her co-workers have shown that deep, less degassed mantle material carried up by a mantle plume contributed significantly to the flood basalt magmatism. The Etendeka flood basalts are part of the South Atlantic LIP, which is associated with the breakup of Gondwana, the formation of the Paraná-Etendeka flood basalts and the Walvis Ridge - Tristan da Cunha hotspot track. Thus reinforcing the lately often-challenged concept of mantle plumes and the role of mantle plumes in the formation of large igneous provinces. Studying the behaviour of noble gases during melt formation and evolution the author along with her co-workers has shown that He can be considerable more susceptible to changes during melt formation and evolution resulting not only in a complete decoupling of He isotopes from e.g. Ne or Pb isotopes but also in a complete loss of the primary mantle isotope signal. They have also shown that this decoupling occurs mainly during the melt formation processes requiring He to be more compatible during mantle melting than Ne. In addition, the author along with her co workers were able to show that incorporation of atmospheric noble gases into igneous rocks is in general a two-step process: (1) magma contamination by assimilation of altered oceanic crust results in the entrainment of air-equilibrated seawater noble gases; (2) atmospheric noble gases are adsorbed onto grain surfaces during sample preparation. This implies, considering the ubiquitous presence of the contamination signal, that magma contamination by assimilation of a seawater-sourced component is an integral part of mid-ocean ridge basalt evolution.

The effects of climate and topography on soil physico-chemical and microbial parameters were studied along an extensive latitudinal climate gradient in the Coastal Cordillera of Chile (26 degrees-38 degrees S). The study sites encompass arid (Pan de Azucar), semiarid (Santa Gracia), mediterranean (La Campana) and humid (Nahuelbuta) climates and vegetation, ranging from arid desert, dominated by biological soil crusts (biocrusts), semiarid shrubland and mediterranean sclerophyllous forest, where biocrusts are present but do have a seasonal pattern to temperate-mixed forest, where biocrusts only occur as an early pioneering development stage after disturbance. All soils originate from granitic parent materials and show very strong differences in pedogenesis intensity and soil depth. Most of the investigated physical, chemical and microbiological soil properties showed distinct trends along the climate gradient. Further, abrupt changes between the arid northernmost study site and the other semi-arid to humid sites can be shown, which indicate non-linearity and thresholds along the climate gradient. Clay and total organic carbon contents (TOC) as well as Ah horizons and solum depths increased from arid to humid climates, whereas bulk density (BD), pH values and base saturation (BS) decreased. These properties demonstrate the accumulation of organic matter, clay formation and element leaching as key-pedogenic processes with increasing humidity. However, the soils in the northern arid climate do not follow this overall latitudinal trend, because texture and BD are largely controlled by aeolian input of dust and sea salts spray followed by the formation of secondary evaporate minerals. Total soil DNA concentrations and TOC increased from arid to humid sites, while areal coverage by biocrusts exhibited an opposite trend. Relative bacterial and archaeal abundances were lower in the arid site, but for the other sites the local variability exceeds the variability along the climate gradient. Differences in soil properties between topographic positions were most pronounced at the study sites with the mediterranean and humid climate, whereas microbial abundances were independent on topography across all study sites. In general, the regional climate is the strongest controlling factor for pedogenesis and microbial parameters in soils developed from the same parent material. Topographic position along individual slopes of limited length augmented this effect only under humid conditions, where water erosion likely relocated particles and elements downward. The change from alkaline to neutral soil pH between the arid and the semi-arid site coincided with qualitative differences in soil formation as well as microbial habitats. This also reflects non-linear relationships of pedogenic and microbial processes in soils depending on climate with a sharp threshold between arid and semi-arid conditions. Therefore, the soils on the transition between arid and semi-arid conditions are especially sensitive and may be well used as indicators of long and medium-term climate changes. Concluding, the unique latitudinal precipitation gradient in the Coastal Cordillera of Chile is predestined to investigate the effects of the main soil forming factor - climate - on pedogenic processes.

System Erde ; 7, One of the major goals of the Geosciences is to identify and comprehend material cycling in our planet in order to understand mantle geodynamic processes. These processes are fundamental to the formation of the continental crust and the atmosphere, mantle-crustatmosphere interactions, and planetary formation as a whole. With our systematic, integrated studies of He, Ne, Sr, Nd and Pb isotopes of globally distributed mid-ocean ridge basalts and ocean island basalts we have contributed significantly to refining existing models on the behaviour of He and Ne during melt formation and evolution as well as models of the geochemical structure of the entire mantle. We have e.g. shown that He and Ne are more compatible during melt formation than their mother isotopes 235U, 238U and 232Th. Therefore, it is no longer necessary to postulate the existence of a virtually non-degassed, isolated lower mantle reservoir. We have also shown that the upper mantle is chemically heterogeneous at different spatial and temporal scales, indicating that the mixing efficiency of the upper mantle is not as good as previously thought. In addition, based on He, Ne and Ar isotope and trace-element data from the Foundation hotspot we have shown that the associated near ridge seamounts erupt lavas with a trace-element signature characteristic of oceanic gabbro, which indicates the existence of recycled, virtually unchanged oceanic lithosphere in the plume source.

International audience; Using temperature gradients measured in 10 holes at 6 sites, we generate the first high fidelity heat flow measurements from Integrated Ocean Drilling Program drill holes across the northern and central Lesser Antilles arc and back arc Grenada basin. The implied heat flow, after correcting for bathymetry and sedimen- tation effects, ranges from about 0.1 W/m2 on the crest of the arc, midway between the volcanic islands of Montserrat and Guadeloupe, to 15 km from the crest in the back arc direction. Combined with previous measurements, we find that the magnitude and spatial pattern of heat flow are sim- ilar to those at continental arcs. The heat flow in the Grenada basin to the west of the active arc is 0.06 W/m2, a factor of 2 lower than that found in the previous and most recent study. There is no thermal evidence for significant shallow fluid advection at any of these sites. Present-day volcanism is confined to the region with the highest heat flow.

[1] The structural and chemical evolution of palagonite was studied as a function of glass composition, alteration environment, and time by applying a range of analytical methods (electron microprobe, infrared photometry, atomic force microscopy, X-ray fluorescence, and X-ray diffraction). Palagonitization of volcanic glass is a continuous process of glass dissolution, palagonite formation, and palagonite evolution, which can be subdivided into two different reaction stages with changing element mobilities. The first stage is characterized by congruent dissolution of glass and contemporaneous precipitation of “fresh,” gel-like, amorphous, optically isotropic, mainly yellowish palagonite. This stage is accompanied by loss of Si, Al, Mg, Ca, Na, and K, active enrichment of H2O, and the passive enrichment of Ti and Fe. The second stage is an aging process during which the thermodynamically unstable palagonite reacts with the surrounding fluid and crystallizes to smectite. This stage is accompanied by uptake of Si, Al, Mg, and K from solution and the loss of Ti and H2O. Ca and Na are still showing losses, whereas Fe reacts less consistently, remaining either unchanged or showing losses. The degree and direction of element mobility during palagonitization was found to vary mainly with palagonite aging, as soon as the first precipitation of palagonite occurs. This is indicated by the contrasting major element signatures of palagonites of different aging steps, by the changes in the direction of element mobility with palagonite aging, and by the general decrease of element loss with increasing formation of crystalline substances in the palagonite. Considering the overall element budget of a water-rock system, the conversion of glass to palagonite is accompanied by much larger element losses than the overall alteration process, which includes the formation of secondary phases and palagonite aging. The least evolved palagonitized mafic glass studied has undergone as much as 65 wt% loss of elements during palagonite formation, compared to ∼28 wt% element loss during bulk alteration. ABout 33 wt% element loss was calculated for one of the more evolved, in terms of the aging degree, rocks studied, compared to almost no loss for bulk alteration.

The cruise SO 160 mapped and sampled a region of the seafloor of the Galapagos Rise, a fossil spreading centre in the SE Pacific Ocean. The objective of the cruise was to determine the exact location of the fossil axis as well as its structure, age, and composition. Previous work in the 1970ies had suggested the fossil axis further in the east of the structure that we studied. However, satellite altimetric data clearly indicated the exact position of the fossil spreading centre. This feature was mapped in detail and we also conducted gravimetry and magnetic measurements during the mapping. The mapping revealed an e10ngated structure which is bounded by a large fracture zone in the north and which can be divided into three segments, each of about 50 km length and a NNE strike. The northernmost segment consists of a deep rift which shows clear similarities to a slow-spreading centre, e.g. the Mid-Atlantic Ridge. The southern two segments are volcanic ridges with numerous volcanic flank cones which reach water depths up to 490 m. We interpret this volcanic ridge as a continuation of the fossil spreading axis, which is magmatically starved in the north but apparently erupted large volumes of magmas after cessation of spreading in the south. The rock samples we recovered are mainly plagioc1ase-phyric lavas with abundant glass rims and manganese crusts on the rift flanks in the north. On the two volcanic ridge segments in the south we recovered plagioclase- and olivine-phyric lavas which are highly vesicular in contrast to the lavas from the northern rift zone. The apparent difference in volatile content between the northern rift and the southern two ridge segments indicated a significant compositional variation. The southern lavas were classified as alkaline basalts to trachytic lavas while the northern samples resembled typical tholeiitic Mid-ocean Ridge Basalts. The trachytic samples were recovered at the peak of the volcanic ridge where round domes with heights of several hundred metres occurred.