Fine Mineral Fractions Research Articles

Dynamics of 13C‐labeled mustard litter (Sinapis alba) in particle‐size and aggregate fractions in an agricultural cropland with high‐ and low‐yield areas

AbstractThe application of 13C‐labeled litter enables to study decomposition processes as well as the allocation of litter‐derived carbon to different soil C pools. 13Carbon‐labeled mustard litter was used in order to compare decomposition processes in an agricultural cropland with high‐yield (HY) and low‐yield (LY) areas, the latter being characterized by a finer texture and a lower organic‐C (OC) content. After tracer application, 13C concentrations were monitored in topsoil samples in particulate organic matter (POM) and in fine mineral fractions (silt‐ and clay‐sized fractions).After 568 d, approximately 5% and 10% of the initial 13C amount were found in POM fractions of LY and HY areas, respectively. Higher amounts were found in POM occluded in aggregates than in free POM. Medium‐term (0.5–2 y) storage of the initial 13C in fine silt‐ and clay‐sized fractions amounts to 10% in HY and LY soils, with faster enrichment but also faster disappearance of the 13C signal from LY soils. Amounts of 80%–90% of the added 13C were mineralized or leached in the observed period. Decomposition of free POM was faster in HY than in LY areas during the first year, but the remaining 13C amounts in occluded‐POM fractions were higher in HY soils after 568 d. High‐yield and low‐yield areas showed different 13C dynamics in fine mineral fractions. In LY soils, 13C amounts and concentrations in mineral‐associated fractions increased within 160 d after application and decreased in the following time period. In HY areas, a significant increase in 13C amounts did not occur until after 568 d. The results indicate initially faster decomposition processes in HY than in LY areas due to different soil conditions, such as soil texture and water regime. The higher silt and clay contents of LY areas seem to promote a faster aggregate formation and turnover, leading to a closer contact between POM and mineral surfaces in this area. This favors the OC storage in fine mineral fractions in the medium term. Lower aggregate formation and turnover in the coarser textured HY soil leads to a delayed C stabilization in silt‐ and clay‐sized fractions.

Decomposition and distribution of 15N labelled mustard litter ( Sinapis alba) in physical soil fractions of a cropland with high- and low-yield field areas

High-yield (HY) areas of an agricultural cropland were characterized by different positions on a slope and lower silt and clay contents, compared to low-yield (LY) areas, and this was associated with differences in water regime and C and N turnover. To understand differences in N flows of HY and LY areas, a combination of 15N tracer techniques and physical fractionation procedures was applied. Within 570 d after application of 15N labelled mustard litter to an agricultural cropland, the distribution of 15N was measured in particulate organic matter (POM) fractions and in fine mineral fractions (fine silt- and clay-sized fractions). After 570 d, only 2.5% of the initial 15N amount was found in POM fractions, with higher amounts in POM occluded in aggregates than in free POM. After this period, stabilization of the initial 15N in fine silt- and clay-sized fractions amounts to 10% in HY, but 20% in LY soils. 70% to 85% of the added 15N were lost. Initial decomposition of labelled material was faster in HY than in LY areas during the first year, but the remaining 15N amounts in POM fractions of the different areas were similar after 570 d. 15N amounts and concentrations in mineral-associated fractions increased within 160 d after application. From 160 to 570 d, HY and LY areas showed different 15N dynamics, resulting in a decline of 15N amounts in HY, but constant 15N amounts in LY soils. The results indicate faster decomposition processes in HY than in LY areas, due to different soil conditions, such as soil texture and water regime. The higher silt and clay contents of LY areas seem to promote N stabilization in fine mineral fractions. As a whole, N flows were higher in HY compared to LY areas, thus supporting higher yields and accelerated organic matter degradation due to higher N supply.

Acid hydrolysis of easily dispersed and microaggregate‐derived silt‐ and clay‐sized fractions to isolate resistant soil organic matter

SummaryThe current paradigm in soil organic matter (SOM) dynamics is that the proportion of biologically resistant SOM will increase when total SOM decreases. Recently, several studies have focused on identifying functional pools of resistant SOM consistent with expected behaviours. Our objective was to combine physical and chemical approaches to isolate and quantify biologically resistant SOM by applying acid hydrolysis treatments to physically isolated silt‐ and clay‐sized soil fractions. Microaggegrate‐derived and easily dispersed silt‐ and clay‐sized fractions were isolated from surface soil samples collected from six long‐term agricultural experiment sites across North America. These fractions were hydrolysed to quantify the non‐hydrolysable fraction, which was hypothesized to represent a functional pool of resistant SOM. Organic C and total N concentrations in the four isolated fractions decreased in the order: native > no‐till > conventional‐till at all sites. Concentrations of non‐hydrolysable C (NHC) and N (NHN) were strongly correlated with initial concentrations, and C hydrolysability was found to be invariant with management treatment. Organic C was less hydrolysable than N, and overall, resistance to acid hydrolysis was greater in the silt‐sized fractions compared with the clay‐sized fractions. The acid hydrolysis results are inconsistent with the current behaviour of increasing recalcitrance with decreasing SOM content: while %NHN was greater in cultivated soils compared with their native analogues, %NHC did not increase with decreasing total organic C concentrations. The analyses revealed an interaction between biochemical and physical protection mechanisms that acts to preserve SOM in fine mineral fractions, but the inconsistency of the pool size with expected behaviour remains to be fully explained.

Factors affecting trace element concentrations in soils developed on recent marine deposits from northern France

Total concentrations of 18 trace and 2 major elements (Al, Fe) as well as physico-chemical characteristics (texture, organic C, pH, CaCO 3, CEC) were measured in cultivated soil samples (fraction <2 mm) from 23 surface and 38 deeper horizons developed in recent marine deposits. Correlations between trace element contents and other soil parameters were compared in order to reveal those soil factors affecting the trace element distributions. Whatever the horizon type, Co, Cr, In, Ni and V are almost exclusively associated with the fine (<20 μm) mineral fraction and do not show a direct association with organic matter. Bismuth, Sn, Tl and Zn also show a close association with the fine mineral fraction of the deep horizons, but seem to be partly associated with organic matter in the ploughed horizons. In the deep horizons, the association of Cd, Cu and Pb with the fine mineral fraction is clearly less close, while these elements appear to be associated with organic matter in the ploughed horizons. The Mn content increases with that of the fine mineral fraction up to a certain point. Arsenic, Mo and Sb contents are poorly correlated with the fine solid phase of the deep horizons. The distribution of Hg does not appear to be associated with any specific soil component. The ploughed horizons are highly enriched with Pb, Cu, Cd, Hg and Se (+84% to +225%) and moderately enriched with Tl, Mn, Sb, Bi, Sn and Zn (+7% to +48%). There is no surface enrichment of As, Co, Cr, In, Mo, Ni and V. It is possible to model most of the trace element distributions with pedotransfer functions of the physico-chemical characteristics.

Comparative K–Ar and Rb–Sr age determinations of retrograde processes on rocks from the KTB deep drilling project

Conventional K–Ar and Rb–Sr age determinations were carried out on fine mineral fractions of the KTB rocks and of rocks from the surrounding area in order to get an idea of the timing of retrograde processes which the rocks had suffered in the Late Variscan and afterwards. Coarse-grained mica minerals were used as reference minerals. It can be demonstrated that conventional K–Ar dating on fine mineral fractions is a useful tool to date retrograde processes in crystalline rocks. For methodical reasons the Rb–Sr method is less suitable. A complex age pattern was found which could be divided into different age groups representing different periods of either penetrative retrograde overprint or formation and reactivation of distinct cataclastic fault zones. Whereas the rocks of the KTB have undergone a penetrative retrograde overprint during the Permo-Triassic without any visible deformation, the Jurassic brought a first reactivation of cataclastic fault zones. In Cretaceous time an overprint led to a pronounced second reactivation of cataclastic fault zones. This latter process turned out to be the important one for the stacking and a repetition of the drilled profile in the KTB. The data and interpretations obtained on the fine mineral fractions coincide excellently with fission track data on sphene from the drill site, with sedimentological records concerning a larger regional scale and with hydrothermal alterations and ore-forming events in mid-Europe.

Age of low-grade regional metamorphism in the North Greenland fold belt: mineralogical and Rb–Sr-isotope evidence from pelitic metasediments

Established isotopic sediment-dating techniques have been used to date the time of deformation and metamorphism of the lower Palaeozoic rocks within the North Greenland fold belt. Fine mineral fractions &lt; 2 μm have been separated from pelitic sediments at widely distributed localities and analysed using X-ray diffraction, isotope, and geochemical techniques.The mineralogical study identified two slightly different mineral parageneses of low greenschist facies. One assemblage without the sodium mica paragonite is confined to calcareous lithologies of the Amundsen Land and Vølvedal groups. The second assemblage includes paragonite and is found in noncalcareous lithologies of the Polkorridoren Group.Rb–Sr isotope dating of the mineral fractions yielded isochron ages in the range 318–346 Ma, which shows that final uplift and cooling associated with Ellesmerian tectonism in North Greenland occurred in the Mississippian. The results suggest that, although the metamorphism was of regional scale, recrystallization and isotopic homogenization occurred in closed systems of local extent.

Geochronological cross section through northern Thailand

Isotopic age determinations were carried out on seven different localities along a cross section through northern Thailand. First results of UPb investigations on zircons of rocks from the crystalline basement west of Tak indicates a maximum age of some 200 Ma for the last amphibolite facies overprint instead of Precambrian. Tertiary KAr ages of around 30 Ma on mica separates of these crystalline rocks must be interpreted as cooling ages requiring tectonic movements with strong uplift in Tertiary time. Additional information of the tectono-metamorphic evolution of northern Thailand is given by KAr ages on fine mineral fractions of slates which date the peak of a regional metamorphism and the time of deformation. The low grade metamorphic overprinting in the eastern parts of northern Thailand covers a range from 250 Ma to 60 Ma indicating a long lasting deformational history. The obtained data are consistent with the idea of a continuous transpressive regime since Permian/Triassic time resulting in thin-skinned tectonics in the eastern part and ramping, shearing and strong uplifting especially in Tertiary time in the western parts.

Aux Vases Sandstone Diagenesis: Implications for Hydrocarbon Recovery from Southern Illinois Reservoirs

The Aux Vases Sandstone (Mississippian) is a problematic yet productive reservoir in the Illinois basin. The Aux Vases Formation was deposited in a mixed siliciclastic-carbonate offshore environment. Hydrocarbon reservoirs are dominantly elongate sandstone bodies interpreted as subtidal facies within a prograding tidally influenced deposystem. Oil saturated zones, in most cases, are composed of fine-grained, cross-bedded, friable, feldspathic quartz sandstone. Resistivity logs, even from productive wells, are typically characterized by unusually low resistivities (2-4 ohms) that lead to high calculated water saturations. X-ray diffraction, SEM/EDS, and thin section studies were used to analyze diagenetic processes that may affect hydrocarbon production in several Aux Vases reservoirs. Diagenetic processes common to the field studied include: (1) early calcite cementation occluding primary porosity and inhibiting compaction, (2) dissolution of feldspar grains to form authigenic clay, (3) dissolution of early calcite cement, (4) diagenesis of detrital clay minerals into new authigenic clay minerals, and (5) multiple stages of quartz overgrowths that reduce porosity. Commonly used drilling and completion practices may adversely affect reservoir quality by reacting with diagenetic products. Loosening of the fine mineral fraction that causes migration of fines and swelling of expandable clay minerals are examples of this type of damage. Knowledge ofmore » reservoir mineralogy and diagenesis may influence drilling and completion practices and affect selection of secondary and tertiary recovery methods.« less

Isotopic dating of low-grade metamorphic shales in northern Namibia (South West Africa) and implications for the orogenic evolution of the Pan-African Damara-Belt

The fine mineral fractions (< 2 μm) from pelitic sediments of the molasse-type Mulden Group in the Pan-African Damara belt of Namibia were dated by the Rb-Sr and K-Ar methods. The sediments contain two major parageneses of metamorphic origin which can be related to two separate low-grade regional tectono-thermal events of anchizonal intensity at about 535 (545) Ma∗ and 455 (465) Ma respectively. A depositional model age for the top of the Mulden Group is about 550–560 (565–570) Ma and it is possible that the Precambrian-Cambrian boundary in northern Namibia may be placed at the base of the Mulden sequence. The above metamorphic events are reflected by mineral and whole-rock ages throughout the Damara belt and are therefore of regional significance. They are part of Clifford's (1967) Damaran Episode which is now more complex than previously suggested and which postdates an earlier tectonic event of preMulden age, provisionally dated at about 650 (665) Ma. We propose to abandon the use of the terms Damaran and Katangan episodes in their original connotation for two chronologically and regionally distinct Pan African events as defined by Clifford (1967) and suggest to restrict these names to the tectono-thermal domains of the Damara and Katanga belts whose orogenic development was broadly coeval.

Strontium and argon isotopic homogenization of pelitic sediments during low-grade regional metamorphism: The pan-African upper Damara sequence of Northern Namibia (South West Africa)

The fine mineral fractions < 2 μm extracted from pelitic sediments of the Eocambrian upper Damaran Mulden Group were subjected to a detailed isotopic study by the Rb-Sr and K-Ar methods in combination with a mineralogical analysis by X-ray diffractometry. The sediments contain two major parageneses of metamorphic origin which can be related to two separate low-grade regional metamorphic events of anchizonal intensity with temperatures between 250 and 300°C and pressures up to 2 kbar. These events are dated at about 535 m.y. and 455 m.y. respectively (with λ( 87Rb) = 1.42 × 10 −11 yr −1). Anomalously high K-Ar ages on samples from specific stratigraphic horizons can be related to open system behaviour and K migration during the formation of stilpnomelane from ferromagnesian illites. This study shows that age dating of sedimentary rocks subjected to low-grade regional metamorphism can only yield meaningful results if the interpretation of isotopic analyses is based on a clear understanding of the mineralogical processes that led to the formation of those parageneses which characterize the rock unit dated.