- Geosciences, Vol. 8, Pages 266: A Geochemical Overview of Mid-Archaean Metavolcanic Rocks from Southwest Greenland
The present contribution reviews bulk-rock geochemical data for mid-Archaean (ca. 3075–2840 Ma) metavolcanic rocks from the North Atlantic Craton of southwest Greenland. The data set includes the most recent high quality major and trace element geochemical analyses for ten different supracrustal/greenstone belts in the region. When distilling the data set to only include the least altered metavolcanic rocks, by filtering out obviously altered samples, mafic/ultramafic cumulate rocks, late-stage intrusive sheets (dolerites) and migmatites, the remaining data (N = 427) reveal two fundamentally distinct geochemical suites. The contrasting trends that emerge from the filtered geochemical data set, which best represents the melt compositions for these mid-Archaean metavolcanic rocks are: (1) tholeiitic (mainly basaltic) versus (2) calc-alkaline (mainly andesitic). These two rock suites are effectively separated by their La/Sm ratios (below or above three, respectively). It is demonstrated by geochemical modelling that the two contrasting suites cannot be related by either fractional crystallization or crustal assimilation processes, despite occurring within the same metavolcanic sequences. The tholeiitic basaltic rocks were directly mantle-derived, whereas the petrogenesis of the calc-alkaline andesitic rocks involve a significant (>50%) felsic component. The felsic contribution in the calc-alkaline suite could either represent slab-melt metasomatism of their mantle source, mafic-felsic magma mixing, or very large degrees of partial melting of mafic lower crust. At face value, the occurrence of andesites, and the negative Nb-Ta-Ti-anomalies of both suites, is consistent with a subduction zone setting for the origin of these metavolcanic rocks. However, the latter geochemical feature is inherent to processes involving crustal partial melts, and therefore independent lines of evidence are needed to substantiate the hypothesis that plate tectonic processes were already operating by the mid-Archaean.
- Geosciences, Vol. 8, Pages 267: Impacts of Material Engineering Properties on Slope Wash and Stability in Fine-Grained Bedrock Slopes at Fossil-Bearing Sites, Badlands National Park, South Dakota, USA
Engineering properties of bedrock materials at Badlands National Park were used to develop models for Park managers to assess slope erosion and stability for fossil resource protection. Six fully instrumented sites were used to document slope conditions. Bedrock consisted of Oligocene White River Group rocks. Bulk erosion rates correlated to grain size with silty-sandy materials producing higher mass erosion rates as a function of the silt-to-clay ratio and plastic index. Data indicated that as grain size decreased, plastic index increased leading to a decrease in erodibility. These parameters were used to construct a grain-size proxy, ψ, that was substituted for grain size, D, in Bagnold’s entrainment equation and provided significant improvement in calculation of critical entrainment velocities for fine-grained materials. Hydraulic analyses of slope and pediment surface processes indicated surface roughness was a controlling factor and materials washed from rough steep slopes were effectively transported across smooth low-angle pediments with slope-to-pediment angle ratios of nearly 6:1. Slope stability modeling of ten slopes produced high factors of safety for all slopes, even under saturated conditions and was attributable to clay cohesion. All results were used to construct models that predicted years until net slope erosion equaled 2.5 cm (1 inch). Using these results, Park managers were advised to visit erosion-prone sites on a 1- to 6-year schedule, based on site geology and slope aspect, to adequately protect critical fossil resources from destruction.
- Geosciences, Vol. 8, Pages 265: Suspended Sediment Variability at the Solimões and Negro Confluence between May 2013 and February 2014
This study focuses on the confluence of two major rivers of the world, the Solimões River (white waters) and Negro River (black waters). Surface suspended sediment samples (SSC) and spectroradiometer taken along transverse profiles at 500 m intervals over a distance of 10 km, as well as satellite images (MODIS) during the hydrological year, were used to follow suspended sediment variability. In January and February, the confluence is dominated by white waters from the Solimões River in the two banks, and in June and July in the right bank by black waters from the Negro River and in the left bank by clear waters from the Solimões River. We found that indirect tools, such as reflectance obtained by spectrometer or MODIS images, can be used to determine surface suspended sediments in a contrasting zone.
- Geosciences, Vol. 8, Pages 264: A Site Amplification Model for Crustal Earthquakes
A global dataset which is composed of more than 20,000 records is used to develop an empirical nonlinear soil amplification model for crustal earthquakes. The model also includes the deep soil effect. The soil nonlinearity is formulated in terms of input rock motion and soil stiffness. The input rock motion is defined by the pseudo-spectral acceleration at rock site condition (PSArock) which is also modified with between-event residual. Application of PSArock simplifies the usage of the site model by diminishing the need of using the period-dependent correlation coefficients in hazard studies. The soil stiffness is expressed by a Gompertz sigmoid function which restricts the nonlinear effects at both of the very soft soil sites and very stiff soil sites. In order to surpass the effect of low magnitude and long-distant recordings on soil nonlinearity, the nonlinear site coefficients are constrained by using a limited dataset. The coefficients of linear site scaling and deep soil effect are obtained with the full database. The period average of site-variability is found to be 0.43. The sigma decreases with decreasing the soil stiffness or increasing input rock motion. After employing residual analysis, the region-dependent correction coefficients for linear site scaling are also obtained.
- Geosciences, Vol. 8, Pages 263: Hf-Nd Isotopes in Archean Marine Chemical Sediments: Implications for the Geodynamical History of Early Earth and Its Impact on Earliest Marine Habitats
The Hf-Nd isotope systems are coupled in magmatic systems, but incongruent Hf weathering (‘zircon effect’) of the continental crust leads to a decoupling of the Hf-Nd isotope systems in low-temperature environments during weathering and erosion processes. The Hf-Nd isotope record was recently dated back from the Cenozoic oceans until the Archean, showing that both isotope systems were already decoupled in seawater 2.7 Ga ago and potentially 3.4 Ga and 3.7 Ga ago. While there might have existed a hydrothermal pathway for Hf into Archean seawater, incongruent Hf weathering of more evolved, zircon-bearing uppermost continental crust that was emerged and available for subaerial weathering accounts for a significant decoupling of Hf-Nd isotopes in the dissolved (<0.2 µm) and suspended (>0.2 µm) fractions of Early Earth’s seawater. These findings contradict the consensus that uppermost Archean continental crust was (ultra)mafic in composition and predominantly submerged. Hence, Hf-Nd isotopes in Archean marine chemical sediments provide the unique potential for future research to trace the emergence of evolved continental crust, which in turn has major implications for the geodynamical evolution of Early Earth and the nutrient flux into the earliest marine habitats on Earth.
- Aluminium and base cation chemistry in dynamic acidification models
– need for a reappraisal?
Aluminium and base cation chemistry in dynamic acidification models – need for a reappraisal? Jon Petter Gustafsson, Salim Belyazid, Eric McGivney, and Stefan Löfgren SOIL Discuss., https//doi.org/10.5194/soil-2018-19,2018 Manuscript under review for SOIL (discussion: open, 0 comments) This paper investigates how different dynamic soil chemistry models describe the processes governing aluminium and base cations in acid soil waters. We find that traditional cation-exchange equations, which are still used in many models, diverge from state-of-the-art complexation sub-models such as WHAM, SHM and NICA-Donnan when large fluctuations in pH or ionic strength occur. In conclusion, the complexation models provide a better basis for the modelling of chemical dynamics in acid soils.
- Assessing the impact of acid rain and forest harvest intensity with the HD-MINTEQ model – Soil chemistry of three Swedish conifer sites from 1880 to 2080
Assessing the impact of acid rain and forest harvest intensity with the HD-MINTEQ model – Soil chemistry of three Swedish conifer sites from 1880 to 2080 Eric McGivney, Salim Belyazid, Therese Zetterberg, Stefan Löfgren, and Jon Petter Gustafsson SOIL Discuss., https//doi.org/10.5194/soil-2018-17,2018 Manuscript under review for SOIL (discussion: open, 0 comments) Forest management may lead to long-term soil acidification due to the removal of base cations during harvest. By means of the HD-MINTEQ model, we compared the acidification effects of harvesting with the effects of historical acid rain at three forested sites in Sweden. The effects of harvesting were predicted to be much smaller than those resulting from acid deposition during the late 20th century. Soil weathering rates were affected differently depending on dissolved Al3+ and pH.
- Comment on "Soil organic stocks are systematically overestimated by misuse of the parameters bulk density and rock fragment content" by Poeplau et al. (2017)
Comment on "Soil organic stocks are systematically overestimated by misuse of the parameters bulk density and rock fragment content" by Poeplau et al. (2017) Eleanor Ursula Hobley, Brian Murphy, and Aaron Simmons SOIL, 4, 169-171, https://doi.org/10.5194/soil-4-169-2018, 2018 This research evaluates equations to calculate soil organic carbon (SOC) stocks. Although various equations exist for SOC stock calculations, we recommend using the simplest equation with THE lowest associated errors. Adjusting SOC stock calculations for rock content is essential. Using the mass proportion of rocks to do so minimizes error.
- Hot regions of labile and stable soil organic carbon in Germany – Spatial variability and driving factors
Hot regions of labile and stable soil organic carbon in Germany – Spatial variability and driving factors Cora Vos, Angélica Jaconi, Anna Jacobs, and Axel Don SOIL, 4, 153-167, https://doi.org/10.5194/soil-4-153-2018, 2018 Soil organic carbon sequestration can be facilitated by agricultural management, but its influence is not the same on all soil carbon pools. We assessed how soil organic carbon is distributed among C pools in Germany, identified factors influencing this distribution and identified regions with high vulnerability to C losses. Explanatory variables were soil texture, C / N ratio, soil C content and pH. For some regions, the drivers were linked to the land-use history as heathlands or peatlands.
- Separation of soil respiration: a site-specific comparison of partition methods
Separation of soil respiration: a site-specific comparison of partition methods Louis-Pierre Comeau, Derrick Y. F. Lai, Jane Jinglan Cui, and Jenny Farmer SOIL, 4, 141-152, https://doi.org/10.5194/soil-4-141-2018, 2018 To date, there are still many uncertainties and unknowns regarding the soil respiration partitioning procedures. This study compared the suitability and accuracy of five different respiration partitioning methods. A qualitative evaluation table of the partition methods with five performance parameters was produced. Overall, no systematically superior or inferior partition method was found and the combination of two or more methods optimizes assessment reliability.