贵州碳酸盐岩风化壳形成地球化学过程、对比及年代学研究Geochemical Processes, Comparison and Dating for the Weathering Crusts in Karst Regions of Guizhou Province

由于缺乏相应的沉积地层，贵州新生代地质、环境演化恢复一直是学术界的难点问题。夷平面研究的兴起，为陆地地壳抬升、地貌演变等反演提供了理论基础，而风化壳正是夷平面研究的重要替代对象，尤其是对各类沉积地层不发育的地区。因而，贵州高原各级台地上发育有较为广泛的红色风化壳自然地、就成了主要研究载体。然而一与其它岩类不同，碳酸盐岩风化成土作用过程研究并没有得到同等重视，其研究范围和程度均很不充分。显然，要想利用碳酸盐岩风化壳进行夷平面等地质、地貌意义研究，就必需详细了解贵州或中国南方碳酸盐岩风化成土的基本过程。也只有如此，才能明确各种对比指标的使用条件和范围，才能明确各种测年结果的具体意义，才能真正地通过贵州碳酸盐岩风化壳获取全省地质、环境演化信息。本论文通过对贵州省岩溶台地碳酸盐岩石灰土、红色风化壳剖面形成地球化学过程的剖析，开展石灰土与红色风化壳之间的关联和差异性综合对比、红色风化壳之间发育程度的比较，以及风化壳剖面尝试性的侧年等方面研究，取得了以下儿点认识：1、石灰土形成地球化学过程及对比通过矿物组成、粒度分布特征、地球化学方法判别，确定了本文所选石灰土剖面物质来源于下伏碳酸盐岩中的酸不溶物，是碳酸盐岩风化后酸不溶物原地残余堆积的结果。石灰土形成的地球化学过程都遵循碳酸盐岩风化成土之两阶段模式：母岩－酸不溶物，大量可溶性物质Ca、Mg、Mn、P、Na淋失，而Si、K·Fe、Al、Ti则相对富集。该过程的矿物组成变化表现为，伴随碳酸盐矿物（方解石、白云石等）的溶解、迁移，硅酸盐相矿物（长石、2:1型粘土矿物等）残余、累积。酸不溶物，土层或土层的演化，K、Si（长石、伊利石）不断淋失，而Al（伊利石、高岭石、三水铝石）、Fe（针铁矿、赤铁矿等）逐渐富集，体现出脱硅、富铝（铁）过程。我们强调碳酸盐岩风化成土分为两个阶段，但也认识到风化作用是综合的、复杂的：在碳酸盐矿物大量淋失过程中，同时也进行着酸不溶物的风化。石灰土分为原生和次生两种成因类型。原生石灰土的风化程度总体较低，剖面中还残留一定量的CaCO3，剖面从下到上矿物组成、地球化学各种指标具有逐渐演化的过渡特征。原生石灰上土层演化过程显示出正风化序列，从下到上风化程度逐渐增大、可溶性物质逐渐减少、稳定和相对稳定组分（Al、Fe．、REE）含量不断增大。另外，主量元素的质量迁移系数在剖面中的变化与风化程度呈明显的相关性。而次生石灰土不但具有很高的风化程度，而且剖面无过渡特征，显示可能是由遭受强烈风化作用的物质直接转变形成的。剖面中也含有一定量后期加入的碳酸盐，但与高CIA值特征不相匹配；SiO2、Al2O3、Fe2O3组分具有不同的变化特征，与）成化程度无明显关联；REE在剖面岩一上界面出现富集层。2、碳酸盐岩红色风化壳形成地球化学过程及其与石灰土的对比红色风化壳形成的地球化学过程也遵循碳酸盐岩风化成土之两阶段模式：母岩一酸不溶物，大量可溶性物质Ca、Mg、Mn、P、Na淋失，而Si、K、Fe、Al、Ti则相对富集。该过程的矿物组成变化表现为，伴随碳酸盐矿物（方解石、白云石等）的溶解、迁移，硅酸盆相矿物（长石、2:1型粘土矿物等）残余、累积。酸不济物一土层或上层的演化，K、Si（长石、伊利石）不断淋失，而Al（伊利石、高岭石、三水铝石）、Fe（针铁矿、赤铁矿等）逐渐富集，体现出脱硅、富铝（铁）过程。石灰土与红色风化壳之间既存在共性，又有一定的差异性。红色风化壳与原生石灰土一样都具有过渡特征，但后者仅在剖面下部出现一定的过渡层位；原生石灰士的城化程度远低于红色风化壳，仅与后者底部土层相当；原生石灰土不具有红色风化壳之岩-土REE（超常）富集层。总体上，原生石灰土可以作为红色风化壳的前身。次生石灰土与红色风化壳在矿物组成、风化程度等各种地球化学指标等都十分接近（略低），显示与红色风化壳之间存在某种关联：由红色风化壳物质直接转变，或由酸不溶物已强烈风化的（白云岩）岩粉、碎裂岩风化形成。3、碳酸盐岩风化成土母岩差异性碳酸盐岩风化成土存在母岩差异性，石灰土母岩差异性较为明显，而红色风化壳的母岩差异性较弱，体现出红色风化壳的均一化特征。石灰土母岩差异性体现为：母岩为灰岩的石灰土剖面主量元素含量变化特征较为一致，而白云岩则具有不同的变化；灰岩石灰土Fe3＋、Al具有不同的富集、亏损特征，且具有富集层的补偿亏损层，而白云岩石灰上两组分都呈现无补偿层位的相同富集态势；灰岩石灰土剖面出现负Eu异常和具以Gd为中心的倒“V”型MREE稀土富集特征（母岩标准化），而白云岩石灰土无明显Eu异常和具L既E或HREE稀士．富集特征；灰岩和白云岩石灰土的微量元素富集、亏损特征也有所不同，前者从上到下逐渐富集，而后者无明显变化趋势。红色风化壳形成的基本过程、主量元素地球化学行为无明显的母岩相关性，没有如石灰土剖面灰岩和白云岩之间存在的差异性，显示出红土化作用的均一化过程；微量元素中受重矿物影响的元素与母岩类型有一定的关联，但大多数元素的亏损、富集等与具体剖面的微环境关系密切。4、碳酸盐岩红色风化壳发育程度的对比及意义多利指标对比显示所选红色风化壳的风化程度有一定的强弱差别：平坝剖面＞湖潮剖面＞天龙剖面、大兴剖面＞花溪剖面、新蒲剖面，但这种差异性不足以否定各剖面隶属于贵州山盆期广泛夷平面的基本推测。各剖面之间的微弱差异是红色风化壳形成时纬度分带和垂向分带的体现。低纬度的平坝、湖潮、天龙剖面经历的风化作用强于高纬度的新蒲、大兴剖面；高海拔的新蒲剖面风化程度低于海拔低的大兴剖面。5、红色风化壳次生石英裂变径迹测年红色风化壳次生石英裂变径迹方法测年尝试，得到一定的结果和认识：晶形相对较好的石英，是次生的，与母岩中碎屑及成岩阶段没有关联；各剖面上部与中、下部次生石英的诱发裂变径迹的密度有着明显的区别，可能暗示它们的形成环境不同；名一剖面石英的形成或退火年龄分布较为分散，1.2-25.2Ma，同一剖面的石英年龄也不均一，变化较大；从下到上，年龄值呈现出逐渐减小的规律性，与风化作用的正常序列相反。如果能确切其成因，次生石英裂变径迹年代学研究将是确定风化壳形成时代及揭示主要风化作用信，却钩有利武器。

Matching and sorting in online dating

Using data on user attributes and interactions from an online dating site, we estimate mate preferences, and use the Gale-Shapley algorithm to predict stable matches. The predicted matches are similar to the actual matches achieved by the dating site, and the actual matches are approximately efficient. Out-of-sample predictions of offline matches, i.e., marriages, exhibit assortative mating patterns similar to those observed in actual marriages. Thus, mate preferences, without resort to search frictions, can generate sorting in marriages. However, we underpredict some of the correlation patterns; search frictions may play a role in explaining the discrepancy.

Telescoping is not time compression: a model of the dating of autobiographical events.

A model of telescoping is proposed that assumes no systematic errors in dating. Rather, the overestimation of recent occurrences of events is based on the combination of three factors: (1) Retention is greater for recent events; (2) errors in dating, though unbiased, increase linearly with the time since the dated event; and (3) intrusions often occur from events outside the period being asked about, but such intrusions do not come from events that have not yet occurred. In Experiment 1, we found that recall for colloquia fell markedly over a 2-year interval, the magnitude of errors in psychologists' dating of the colloquia increased at a rate of .4 days per day of delay, and the direction of the dating error was toward the middle of the interval. In Experiment 2, the model used the retention function and dating errors from the first study to predict the distribution of the actual dates of colloquia recalled as being within a 5-month period. In Experiment 3, the findings of the first study were replicated with colloquia given by, instead of for, the subjects.

A numerical approach to C-14 wiggle-match dating of organic deposits: best fits and confidence intervals

14C wiggle-match dating (WMD) of peat deposits uses the non-linear relationship between 14C age and calendar age to match the shape of a sequence of closely spaced peat 14C dates with the 14C calibration curve. A numerical approach to WMD enables the quantitative assessment of various possible wiggle-match solutions and of calendar year confidence intervals for sequences of 14C dates. We assess the assumptions, advantages, and limitations of the method. Several case-studies show that WMD results in more precise chronologies than when individual 14C dates are calibrated. WMD is most successful during periods with major excursions in the 14C calibration curve (e.g., in one case WMD could narrow down confidence intervals from 230 to 36 yr).

Radiocarbon wiggle-match dating of proglacial lake sediments - Implications for the 8.2 ka event

The problem of insufficient age-control limits the utilisation of the 8.2 ka BP event for modelling freshwater forcing in climate change studies. High-resolution radiocarbon dates, magnetic susceptibility and lithostratigraphic evidence from a lake sediment core from Nedre Hervavatnet located at Sygnefjell in western Norway provide a record of the early Holocene. We use the method of radiocarbon wiggle-match dating of the lake sediments using the non-linear relationship between the C-14 calibration curve and the consecutive accumulation order of the sample series in order to build a high-resolution age-model. The timing and duration of Holocene environmental changes is estimated using 38 AMS radiocarbon dates on terrestrial macrofossils, insects and chironomids covering the time period from 9750 to 1180 cal BP. Chironomids, Salix and Betula leaves produce the most consistent results. Sedimentological and physical properties of the core suggest that three meltwater events with high sedimentation rates are superimposed on a long-term trend with glacier retreat between 9750 and 8000 cal BP. The lake sediment sequence of Nedre Hervavatnet demonstrates the following: only a reliable high-resolution geochronology based on carefully selected terrestrial macrofossils allows the reconstruction of a more refined and complex environmental change history before and during the 8.2 ka event. (C) 2009 Elsevier Ltd. All rights reserved.

High precision C-14 dating of Holocene peat deposits: A comparison of Bayesian calibration and wiggle-matching approaches

The chronologies of five northern European ombrotrophic peat bogs subjected to a large ANIS C-14 dating effort (32-44 dates/site) are presented here. The results of Bayesian calibration (BCal) of dates with a prior assumption of chronological ordering were compared with a Bayesian wiggle-match approach (Bpeat) which assumes constant linear accumulation over sections of the peat profile. Interpolation of BCal age estimates of dense sequences of C-14 dates showed variable patterns of peat accumulation with time, with changes in accumulation occurring at intervals ranging from 20 to 50 cm. Within these intervals, peat accumulation appeared to be relatively linear. Close analysis suggests that some of the inferred variations in accumulation rate were related to the plant macrofossil composition of the peat. The wiggle-matched age-depth models had relatively high chronological uncertainty within intervals of closely spaced 14 C dates, suggesting that the premise of constant linear accumulation over large sections of the peat profile is unrealistic. Age models based on the assumption of linear accumulation over large parts of a peat core (and therefore only effective over millennial timescales), are not compatible with studies examining environmental change during the Holocene, where variability often occurs at decadal to centennial time-scales. Ideally, future wiggle-match age models should be constrained, with boundaries between sections based on the plant macrofossil composition of the peat and physical-chemical parameters such as the degree of decomposition. Strategies for the selection of material for dating should be designed so that there should be enough C-14 dates to accurately reconstruct the peat accumulation rate of each homogeneous stratigraphic unit. (c) 2006 Elsevier Ltd. All rights reserved.