Comparison of isoleucine epimerization in a model dipeptide and fossil protein

The dipeptide isoleucyl-glycine in aqueous solution has activation energies for isoleucine epimerization and hydrolysis of 23.1 and 20.9 kcal mol^?1, respectively. The activation energy for epimerization of NH₂-terminal isoleucine is 5–8 kcal mol^?1 less than activation energies reported for epimerization of isoleucine in the free state or in various protein systems. Furthermore, the rate of isoleucine epimerization in the NH₂-terminal position exceeds the rate of hydrolysis at temperatures between 0 and 152°C. Consequently, a high rate of epimerization in fossils should only occur when isoleucine is in the terminal position. Partially epimerized isoleucine will be converted through hydrolysis from the terminal form to the slower epimerizing free isoleucine. Over the course of the total isoleucine epimerization reaction, either in dipeptides or proteins, the activation energy will increase and the reaction rate will decrease as terminal isoleucine is converted to free isoleucine. This process may explain the non-linear kinetics observed for isoleucine epimerization in carbonate fossils.The degree of isoleucine epimerization is low in different molecular weight fractions of a fossil protein from Mercenaria in all fractions greater than 500 mol. wt, where most isoleucine is likely to be in the interior position. In the less than 500 mol. wt peptide fraction, where a considerable portion of the isoleucine is likely to be in the terminal position, the degree of epimerization is significantly greater. These analytical results from fossil protein support the interpretation of isoleucine kinetics obtained by study of dipeptides.     

Epimerization of COOH-terminal isoleucine in fossil dipeptides

Using procedures employed for protein sequencing to remove NH₂-terminal amino acids, the degree of epimerization of COOH-terminal isoleucine in dipeptides from fossil mollusc shells has been measured directly. The results show that isoleucine in this position is highly epimerized in fossil dipeptides and that the COOH-terminal isoleucine is more highly epimerized than the free amino acid. Hydrolysis of the highly epimerized terminal isoleucine leads to the high alloisoleucine/isoleucine values found in the free amino acid fraction. The results also provide evidence for the formation of diketopiperazines in fossils as a mechanism to account for the high degree of epimerization of COOH-terminal isoleucine.     

Kinetics and mechanism of the epimerization and decomposition of threonine in fossil foraminifera

Determination of alloThr/Thr ratios in foraminiferal tests ranging in age from Recent to ～ 700,000 yr B.P. has shown that threonine epimerization ratios approach a steady-state value of ～0.2. This is well below the equilibrium ratio of ～0.9 as measured in buffered aqueous solutions. It has also been found that the initial rate of decomposition of threonine in fossil foraminfera is about an order of magnitude faster than would be predicted on the basis of extrapolation from high temperature kinetic studies on free threonine in buffered aqueous solution. These results are apparently a consequence of metal cation catalysis of both the decomposition and the epimerization of free threonine, which is being continually released by the slow hydrolysis of peptide and proteinaceous material present in the fossil shells.     

Comparison of elemental accumulation rates between ferromanganese deposits and sediments in the South Pacific Ocean

Rates of accumulation of Fe and Mn, as well as Cu, Ni, Co, Pb, Zn, Hg, U and Th have been determined for five ferromanganese deposits from four localities in the South Pacific Ocean.Manganese is accumulating in nodules and crusts at a rate roughly equivalent to that found to be accumulating in sediments in the same area. Iron shows a deficiency in accumulation in nodules and crusts with respect to sediments, especially near the continents, but also in the central and south-central Pacific. Copper is accumulating in nodules and crusts at a rate one order of magnitude less than the surrounding sediments.This is interpreted as meaning that most of the Mn is supplied as an authigenic phase to both sediments and nodules while Fe is supplied mostly by ferromanganese micro-nodules and by detrital and adsorbed components of sediments; and Cu is enriched in sediments relative to nodules and crusts most probably through biological activity.     

Estimation of maximum temperature of mudstone by two kinetic parameters; epimerization of sterane and hopane

A geochemical method for estimation of the maximum temperature of mudstones is proposed. The extents of epimerization of the sterane and the hopane are used. The temperature function is:

$\text{T}{}_{\mathrm{max.}}\text{(°C) =}\text{6060}\text{15.0?In(dU}\text{dU}\text{dV)}\text{?273}$

where U = ln (1 ? α/0.54), V = ln(1 ? β/0.61), α = 20S-/20S- + 20R-24-ethyl-5α(H), 14α(H), 17α(H)-cholestane(C₂₉-sterane) and β = 22S-/22S- + 22R-17α(H), 21β(H)-bishomohopane(C₃₂-hopane). The value of dU/dV can be obtained from the tangent to the evolution curve in the UversusV. This temperature function is applicable to the temperature analysis in the range of 50°C–150°C.     

Comparison of soil erosion and deposition rates using radiocesium, RUSLE, and buried soils in dolines in East Tennessee

 Three dolines (sinkholes), each representing different land uses (crop, grass, and forest) in a karst area in East Tennesse, were selected to determine soil erosional and depositional rates. Three methods were used to estimate the rates: fallout radiocesium (¹³⁷Cs) redistribution, buried surface soil horizons (Ab horizon), and the revised universal soil loss equation (RUSLE). When ¹³⁷Cs redistribution was examined, the average soil erosion rates were calculated to be 27 t ha^–1 yr^–1 at the cropland, 3 t ha^–1 yr^–1 at the grassland, and 2 t ha^–1 yr^–1 at the forest. By comparison, cropland erosion rate of 2.6 t ha^–1 yr^–1, a grassland rate of 0.6 t ha^–1 yr^–1, and a forest rate of 0.2 t ha^–1 yr^–1 were estimated by RUSLE. The ¹³⁷Cs method expressed higher rates than RUSLE because RUSLE tends to overestimate low erosion rates and does not account for deposition. The buried surface horizons method resulted in deposition rates that were 8 t ha^–1 yr^–1 (during 480 yr) at the cropland, 12 t ha^–1 yr^–1 (during 980 yr) at the grassland, and 4 t ha^–1 yr^–1 (during 101 yr) at the forest site. By examining ¹³⁷Cs redistribution, soil deposition rates were found to be 23 t ha^–1 yr^–1 at the cropland, 20 t ha^–1 yr^–1 at the grassland, and 16 t ha^–1 yr^–1 at the forest site. The variability in deposition rates was accounted for by temporal differences;¹³⁷Cs expressed deposition during the last 38 yr, whereas Ab horizons represented deposition during hundreds of years. In most cases, land use affected both erosion and deposition rates – the highest rates of soil redistribution usually representing the cropland and the lowest, the forest. When this was not true, differences in the rates were attributed to differences in the size, shape, and closure of the dolines. Received: 10 October 1995 · Accepted: 13 October 1995     

花岗岩风化程度的化学指标及微观特征对比——以香港九龙地区为例

据香港九龙3个不同工程场地取得的149件不同级别的风化花岗岩样品的化学分析结果,用统计学方法,从反映岩石风化程度的18项化学指标中选取9项进行了分析计算.对其结果及其与风化程度分级的关系的研究表明有4种指标比较理想.通过对样品微观特征的观察研究和矿物成分、微孔隙率以及综合显微岩石指标(Ip)进行了描述和定量分析,发现其中一些指标有一定相关性.这些化学指标和微观特征相关的统计值在一定程度上可定量表征花岗岩的风化程度,即:随风化作用进行,碱、碱土金属组分逐渐淋失,脱硅、富铝铁化作用逐渐加强;相应地,先是钠长石风化成高岭石,其次为云母和钾长石的风化.粘土矿物所占体积百分比随风化程度增高而变大,至残积土(Ⅵ级)时已达50%左右;Ip指标值下降,微孔隙率增高.文章综合出各个风化级别的指标变化范围及平均值供花岗岩分布区花岗岩风化野外调查和研究工作参考.     

Amino acids in fossil corals: racemization (epimerization) reactions and their implications for diagenetic models and geochronological studies

Thirty-eight fossil coral samples of known age from late Pleistocene uplifted reef terraces have been analyzed for amino acid composition and for the degree of racemization of selected amino acids. Particular attention has been given to the epimerization (racemization about the α-carbon) of isoleucine. The $\text{D}\text{L}$ ratios observed in many of these samples do not conform to the concept of increasing racemization being associated with increasing fossil age. It is shown that sixteen of the samples demonstrate concordance between their known age and the age estimated from racemization data. Racemization ages are based upon a kinetic model derived from the kinetics observed in foraminifera in marine sediments and published estimates of the temperature dependence of the isoleucine epimerization reaction. Lack of concordance for the remaining samples is explained by either of two separate diagenetic phenomena: extensive leaching of free amino acids from the fossil, or contamination by ‘young’ amino acids. Ambiguities observed in many of the results may be due to the fact that coralline organic matter can have a complex history because of variations in its source and in its relation to the mineral phase. Neither of these factors has been observed to exert such a significant effect on racemization kinetics observed in fossil foraminifera or molluscs. Fossil corals are of only modest value as specimens for amino acid geochronological studies.     

Chronostratigraphic studies of sediments in the Negev desert, using amino acid epimerization analysis of land snail shells

The chronostratigraphy of mostly Holocene sediments in the Northern Negev desert of Israel was studied through amino acid epimerization analysis (alloisoleucine/isoleucine ratio, A/I) of individual land snail shells and ¹⁴C analysis of bulk samples of land snail shells contained within the sediments. Analysis of 31 deposits shows a very strong correlation between A/I and ¹⁴C age. A/I analyses indicate that age mixtures occur within all colluvial and in some fluvial deposits. Consequently, radiocarbon dates of bulk samples, which give an average age, overestimate the time of deposition. Sedimentation rates were generally rapid in both fluvial and colluvial deposits. Colluviation shows a maximum in the early Holocene and decreases progressively thereafter, in accordance with the change in average rainfall. Accumulation of fluvial sediments shows a distinct mid-Holocene maximum and apparently relates to the interaction between variation in rainfall, extent of bedrock exposure, and vegetation density. Late Holocene fluvial deposition may relate to agricultural activity. The understanding of the chronology of sedimentation is dependent on an integrated approach employing amino acid epimerization analysis to study age variation within a layer and ¹⁴C analyses for establishment of an absolute chronology.     

Seismicity and stress evolution in heterogeneous faults with various degrees of roughness

We investigate the effects of the spatial distribution of stress drop ratios (SDRs) on the fault of an earthquake sequence as well as stress evolution. To achieve this, we test four fault models, each with its own SDRs which differ with regard to wavelength components (roughness) but which share first-order similarity and the same mean value. We assume rupture processes are quasi-static and fluid-controlled. The evidence clearly shows that the locations of large earthquakes are more sensitive to distribution than to the values of the SDRs. For this reason, it is expected that reducing fault roughness should increase the number of large earthquakes but decrease the number of small ones. It is also clearly apparent that a clustered distribution of large earthquakes should be more frequently observed on heterogeneous faults, especially in areas with small gradient values of the SDRs. These phenomena are consistent with our finding that long-wavelength components of stress usually develop in areas with low gradients of the SDRs. A fault with a smoother distribution of SDRs should, therefore, exhibit a greater likelihood to generate large earthquakes. By contrast, in that a rough fault covers a smaller area on which long-wavelengths can develop, it should result in fewer large earthquakes but a much more highly clustered distribution.     

Fault scarps and deformation rates in Lazio–Abruzzo, Central Italy: Comparison between geological fault slip-rate and GPS data

We compare new and literature data concerning the integrated deformation rate since 18 ka for the central Apennines with deformation rate data derived from a 6-year GPS campaign. We constructed topographic profiles across the majority of the active faults in the area. We derived deformation rate data from displaced post-glacial sediments and slopes associated with the last major glacial retreat that occurred in the region about 18 kyr ago. Paleoseismic investigations in this region clearly show that offset of these features is the cumulative effect of repeated surface faulting earthquakes with magnitudes in the range 5.5 ≤ Ms ≤ 7.0. A cumulative throw-rate diagram, incorporating both our values and the values extracted from the literature (i.e., previously published trenching studies, scarp profiles and offset terraces), is presented providing a regional picture of the spatial deformation rate distribution in the central Apennines, during the latest Pleistocene and Holocene. By converting cumulative throw-rates into heave rates, we calculate maximum extension rates of ca. 2 mm/year at the location of the same transect where 6 ± 2 mm/year has been measured with GPS over 6 years. This discrepancy between geodetic and geological fault slip-rate data implies that we have to be cautious whenever we use GPS data for seismic hazard assessment purposes or when attempting to reconstruct the tectonic processes in an area, because geodetic data may be unrepresentative of the longer-term deformation rates. On the other hand, this discrepancy may provide insights into the mechanics of the seismic cycle. We discuss various scenarios that may explain the mis-match between long-term and short-term measurements.     

Carbonate minerals in Bulgarian coals with different degrees of coalification

Coals of various rank (ranging from lignite to anthracite), and originating from seven coal basins in Bulgaria, were investigated for the occurrence and types of carbonate minerals. The following carbonates are present in their characteristic forms: Calcite (clastic, organogenic, cryptograined, xenomorphic, massive and infiltrational); siderite (cryptograined, xenomorphic both infilling and replacing, massive, euhedral, framboidal and infiltrational both fracture-filling and cleat-filling); dolomite (cryptograined, xenomorphic, massive and infiltrational) and magnesite.Carbonate mineralization depends on the penetration of ready mineral forms and mineralized solutions during peat genesis and the infiltration of mineralized solutions during coalification. The mineral composition of the carbonates in the coals studied and the occurrences of the various carbonate minerals depend on the combination of a series of factors such as: (1) the presence of carbonate rocks in the province feeding the peat bogs; (2) the way and capacity of feeding; (3) acidity (pH) and reducing conditions (Eh) of the environment, (4) tectonic situation during the peat genesis; and (5) availability of infiltrational mineral solutions during coalifications.No infiltrational carbonate mineralization was found in the lignites studied. Due to the combination of the above five factors, bituminous coals from the Balkan basin and the anthracite from Svoge are characterized by the greatest variety of carbonate minerals and their forms.     

Texture and sedimentation rates in Lake Geneva

Thirty two cores were collected from Lake Geneva sediments along one longitudinal and eight transverse profiles. Rates of sedimentation determined by¹³⁷Cs vary from 0.01 to 1.86 g cm⁻² y⁻¹. The average deposition rates in coastal and slope areas amounts to 0.37 g cm⁻² y⁻¹ in the Upper Lake (Grand Lac) and 0.12 g cm⁻² y⁻¹ in the Lower Lake (Petit Lac). In the deep basins, average rates of 0.13 and 0.05 g cm⁻² y⁻¹ were found for the Grand Lac and Petit Lac, respectively. The estimated mass of sediment deposited yearly outside of the principal deltas and turbidity current depositional areas is about 1.0 million tons (about 13% of the estimated total river load). One turbidite is clearly identified in the deepest, central lake area. There is little variation of surface sediment texture (mean grain size about 8–9μm) with the exception of delta areas. Since the beginning of the twentieth century, both carbonate and organic matter have increased as a result of lake eutrophication.     

降水和开采变化对石家庄地下水流场影响强度

为能精确识别降水和开采对石家庄地下水流场影响强度,应用小波变换和相关分析等研究方法,对区域平均地下水位、地下水降落漏斗面积及中心水位与降水和开采变化之间的互动特征进行了研究。结果表明:① 1961—1973年期间,平均地下水位随降水量增大呈幂函数递减趋势;1974—2010年期间,降水量每减小100 mm,漏斗中心水位下降速率增大7.35 m,平均地下水位下降速率增大2.15 m;② 1961—1973年期间,开采量每增加1亿m3,平均地下水位下降0.28 m,漏斗面积扩大11.74 km2,中心水位下降0.52 m,1974年以来累计超采量每增加1亿m3,漏斗面积增幅1.52 km2,中心水位降幅0.18 m;③ 降水量每减小100 mm,降水贡献度减弱3.0%,人类开采影响强度增大2.76%。     

GPS-derived deformation rates in northwestern Himalaya and Ladakh

Deformation rates derived from GPS measurements made at two continuously operating stations at Leh (34.1°N, 77.6°E) and Hanle (32.7°N, 78.9°E), and eight campaign sites in the trans-Himalayan Ladakh spanning 11 years (1997–2008), provide a clear picture of the kinematics of this region as well as the convergence rate across northwestern Himalaya. All the Ladakh sites move 32–34 mm/year NE in the ITRF2005 reference frame, and their relative velocities are 13–16 mm/year SW in the Indian reference frame and ~19 mm/year W with reference to the Lhasa IGS station in southeastern Tibet. The results indicate that there is no statistically significant deformation in the 200-km stretch between the continuous sites Leh and Hanle as well as between Leh and Nubra valley sites along the Karakoram fault, whereas the sites in and around the splayed Karakoram fault region indicate surface deformation of 2.5 mm/year. Campaign sites along the Karakoram fault zone indicate a fault parallel surface motion of 1.4–2.5 mm/year in the Tangste and western Panamik segment of the Karakoram fault, which quantifies the best possible GPS-derived dextral slip rate of 3 mm/year along this fault during this 11-year period. Baselines of Ladakh sites show convergence rates of 15–18 mm/year with respect to south India and 12–15 mm/year with respect to Delhi in north India and Almora in the Himalaya ~400 km north-northeast of Delhi. These constitute an arc normal convergence of 12–15 mm/year across the western Himalaya, which is consistent with arc normal convergence all along the Himalayan arc from west to east. Baseline extension rates of 14–16 mm/year between Lhasa and Ladakh sites are consistent with the east–west extension rate of Tibetan Plateau.     

Crater taphonomy and bombardment rates in the Phanerozoic

Impact structures can be catalogued according to the age of the rocks in which they are now found ("country rocks"). The observed frequency distribution of craters by age of country rock is shown to be statistically indistinguishable from the predictions of a simple model in which it is assumed that the survival time for craters is the same as that for their target rocks. Other models are considered, but do not match the data. A lower limit of the rate of bombardment through the Phanerozoic, based only on documented craters, is 0.13 +/- 0.09 events/ma/10(8) km2 for craters with diameters > or = 10 km and 0.09 +/- 0.08 events/ma/10(8) km2 for craters with diameters > or = 20 km. The data allow, but do not demand, an increase in meteorite flux over the Phanerozoic but do not allow any significant decrease. We estimate that only about 6% of the existing terrestrial impact structures of diameter greater than 10 km have been discovered to date, and only 16% of those with diameter greater than 20 km.     

Temporal and spatial variation in intertidal sedimentation rates

Sediment deposition and erosion rates are reported for an intertidal zone in the Burry Inlet, South Wales. Measured deposition rates over the salt marsh are compared with deposition rates calculated from observed suspended sediment concentrations. Notably, it is concluded that residual turbulence at slack water should not be discounted when calculating deposition rates. Grain-size distributions of suspended sediments over the marsh surface, during flood and ebb tides, contrast with the grain-size distribution of deposited marsh sediments, the latter being significantly coarser. These data in conjunction with mass budget calculations are used to relate total annual deposition and sediment supply by tidal action during settled meteorological periods. The analysis suggests that episodic storm-induced sediment transport is probably an important mechanism for introducing coarse sediment on to the marsh surface. Finally, it is noted that seasonal reworking of the sandy non-cohesive sediments may be related to variations in the intensity of wave-breaking throughout the year.     

Variations in Melt Productivity and Melting Conditions along SWIR (70{degrees}E-49{degrees}E): Evidence from Olivine-hosted and Plagioclase-hosted Melt Inclusions

Melt inclusion and host glass compositions from the easternend of the Southwest Indian Ridge show a progressive depletionin light rare earth elements (LREE), Na₈ and (La/Sm)_n, but anincrease in Fe₈, from the NE (64°E) towards the SW (49°E).These changes indicate an increase in the degree of mantle meltingtowards the SW and correlate with a shallowing of the ridgeaxial depth and increase in crustal thickness. In addition,LREE enrichment in both melt inclusions and host glasses fromthe NE end of the ridge are compatible with re-fertilizationof a depleted mantle source. The large compositional variations(e.g. P₂O₅ and K₂O) of the melt inclusions from the NE end ofthe ridge (64°E), coupled with low Fe₈ values, suggest thatmelts from the NE correspond to a variety of different batchesof melts generated at shallow levels in the mantle melting column.In contrast, the progressively more depleted compositions andhigher Fe₈ values of the olivine- and plagioclase-hosted meltinclusions at the SW end of the studied region (49°E), suggestthat these melt inclusions represent batches of melt generatedby higher degrees of melting at greater mean depths in the mantlemelting column. Systematic differences in Fe₈ values betweenthe plagioclase- and the olivine-hosted melt inclusions in theSW end (49°E) of the studied ridge area, suggest that theplagioclase-hosted melt inclusions represent final batches ofmelt generated at the top of the mantle melting column, whereasthe olivine-hosted melt inclusions correspond to melts generatedfrom less depleted, more fertile mantle at greater depths. KEY WORDS: basalt; melt inclusions; olivine; plagioclase; Southwest Indian Ridge     

Variations in rates of nucleation and growth of biotite porphyroblasts

Differences in rates of nucleation and diffusion‐limited growth for biotite porphyroblasts in adjacent centimetre‐scale layers of a garnet‐biotite schist from the Picuris Mountains of New Mexico are revealed by variations in crystal size and abundance between two layers with strong compositional similarity. Relationships between fabrics recorded by inclusion patterns in biotite and garnet porphyroblasts are interpreted to reflect garnet growth following biotite growth, without substantial alteration of the biotite sizes. Sizes and locations of biotite crystals, obtained via high‐resolution X‐ray computed tomography, document that of the two adjacent layers, one has a larger mean crystal volume (9.5 × 10^?4v. 2.4 × 10^?4 cm³), fewer biotite crystals per unit volume (232 v. 576 crystals cm^?3), and a higher volume fraction of biotite (23%v. 14%). The two layers have similar mineral assemblages and mineral chemistry. Both layers show evidence for diffusional control of nucleation and growth. Pseudosection analysis suggests that the large‐biotite layer began to crystallize biotite at a temperature ～67 °C greater than the small‐biotite layer. Diffusion rates differed between layers, because of their different temperature ranges of crystallization, but this effect can be quantified. The bulk compositional difference between the layers, manifested in different modal amounts of biotite, has an effect on the biotite sizes that is also quantifiable and insufficient to account for the difference in biotite size. After these other possible causes of variation in crystal sizes have been eliminated, variability in nucleation and diffusion rates remain as the dominant factors responsible for the difference in porphyroblastic textures. Numerical simulations suggest that relative to the small‐biotite layer, the large‐biotite layer experienced elevated diffusion rates because of the higher crystallization temperature, as well as increased nucleation rates in order to achieve the observed size and number density of crystals. The simulations can replicate the observed textures only by invoking unreasonably large values for the thermal dependence of nucleation rates (activation energies), strongly suggesting that the observed textural differences arise from variations between layers in the abundance and energetics of potential nucleation sites.