2016, Vol.36
1 引言
植物钙质根管是陆生植物在生物沉积作用下形成的次生碳酸盐结壳[1, 2, 3]，一般出现于伴随着明显的土壤水分季节性亏损的石灰质土壤中[1, 4, 5]，譬如地中海西部[1, 4, 5, 6]和南非[2]等地区。由于其可以在很短时间内形成并且形成的次生碳酸盐不与外界元素发生交换[1]，使得植物钙质根管在古环境研究中具有巨大潜力。以往的研究侧重植物钙质根管的分类[1]、 形成机制[3, 4, 5]、 同位素组成特征[3, 7]、 形态学特征[4, 5, 8]和年代学[3, 9]等，并尝试重建了区域古环境和古植被[3, 10, 11, 12, 13, 14, 15]。上述研究为探讨植物钙质根管的成因及其环境意义提供了新的证据，也为古环境重建工作提供了可能的新的古环境代用资料。
由于植物钙质根管的形成受到了土壤水分变化的影响[1, 2, 4, 9]，而土壤水分的变化受到了区域有效湿度的影响，因此在理论上，植物钙质根管在形成过程中可能记录了丰富的古环境信号。但如何从中提取可靠的古环境信号，并以此来进行古环境重建研究，则是尚待解决的问题。
阿拉善高原沙漠位于我国中纬度的干旱区[16]，受到季风和西风的共同影响[17]，是古环境研究中重点关注的区域之一[18, 19, 20, 21, 22, 23]。之前的研究主要通过湖相沉积和风成沉积序列重建该区域的古环境变化过程[24, 25]，但由于在沙漠腹地缺乏具有可靠测年结果的沉积地层和具有明确环境指示意义的古环境代用指标，在一定程度上限制了该区域的古环境重建工作。但在该地区，广泛分布着地质历史时期的植物钙质根管[9, 26, 27, 28, 29]，杨小平[29]、 Li等[9, 30]对腾格里沙漠、 巴丹吉林沙漠以及乌兰布和沙漠植物钙质根管进行了年代学和时间分布研究，发现其年代结果在全新世范围的千年尺度上可靠性较高，并且研究认为植物钙质根管的形成可能指示阿拉善高原沙漠地区相对湿润的气候阶段。但能否从植物钙质根管中提取古环境信号并进行古环境重建，尚需进一步探讨。
由于表生环境中元素的迁移和富集受到元素的地球化学性质和环境因素共同支配[31]，因此利用特定地球化学元素的比值可以重建古环境[20, 30, 31, 32, 33, 34, 35, 36, 37]。气候条件会影响土壤中化学元素的含量[31, 38]，而植物钙质根管的形成受到了土壤水分的影响[1, 2, 4, 5]，因此在理论上，植物钙质根管中的化学元素及其含量变化包含了一定的古气候信号。基于上述讨论，本文在腾格里沙漠植物钙质根管的年代学[9]基础上，拟通过X射线荧光光谱法测定腾格里沙漠全新世植物钙质根管中 Sr/Ca 比值变化，探讨其环境指示意义，并以此尝试重建该区域全新世千年尺度的有效湿度变化。
2 区域概况腾格里沙漠（37°26′28″-39°58′59″N，102°48′25″-105°38′13″E） 地处阿拉善高原地区东南部，东接贺兰山、 南邻祁连山山前洪积扇前缘，西北部与雅布赖山相连，西部是民勤盆地的石羊河流域，东西宽180km，南北长240km，总面积约42700km2[39, 40]，是中国的第四大沙漠（图1）。现代气候受到亚洲季风和西风带气流的共同影响[41]，年平均温度7.0-9.7℃，年平均降水量125-160mm，年潜在蒸发量在2400-3000mm之间。冬季盛行西北风，夏季短暂时期盛行东北风，多年平均主风向为W-NNW，次风向为 NE-E，其次为WSW-S[42]。沙漠内海拔由西向东逐渐降低； 沙漠内部沙丘、 湖盆、 山地、 平地交错分布，其中沙丘71％，湖盆草滩7％，山地残丘及平地22％[39]。腾格里沙漠的地带性土壤为灰漠土和棕钙土。植被以沙生灌木、 半灌木为主[43]。
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图1 腾格里沙漠植物钙质根管采样点分布图 Fig.1 Locations of the calcareous root tubes samples from the Tengger Desert |
本文共采集了腾格里沙漠14组植物钙质根管样品，经纬度范围为 37°54′59″-38°55′31″N，104°10′34″-105°22′59″E（图1），植物钙质根管样品多分布于丘间洼地并成片分布，形态结构上呈水平状或直立状，颜色以灰白色为主（图2）。采样点砂层厚度均在1m以上，土壤类型为砂壤土，中值粒径在0.2-0.4mm之间。水平状的植物钙质根管长度从几厘米到十几厘米不等； 直立状植物钙质根管的长度在10-28cm之间，最长为28cm且其底部延伸到地表以下18cm（图2）[9, 30]。在此采集的植物钙质根管样品均为地表样品。
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图2 腾格里沙漠植物钙质根管形态（图片来自文献[9]） Fig.2 Details of the calcareous root tubes samples from the Tengger Desert. The picture is from the reference[9] |
元素分析在兰州大学西部环境教育部重点实验室完成。实验步骤如下: 将已经自然风干的样品磨至200目以下，称取4g样品，加入适量硼酸，在30t/m2的压强下对样品进行压片处理后用荷兰飞利浦公司生产的Magix PW2403型号X-射线荧光光谱仪进行元素测定。
4 结果Li等[9, 30]先前对上述样品的年代学研究结果发现，腾格里沙漠植物钙质根管全部分布于全新世时期，8cal.ka B.P. 之前没有发现植物钙质根管； 8-5cal.ka B.P. 阶段共有植物钙质根管样品9组，占64.29％； 5-2cal.ka B.P. 共有5组，占35.71％。X射线荧光光谱法结果显示（表1）:8-5cal.ka B.P. 阶段，Sr值平均为748.12ppm，最高值1663ppm，最低值350ppm； CaO平均值为37.66％，最高值48.49％，最低值25.66％； Sr的高值基本上对应Ca的高值，并出现了Sr的最高值； Sr/Ca比值的平均值为1215.26μmol/mol，Sr/Ca比值的最高值出现在6829（6670-6988）cal. a B.P.，为2332.02μmol/mol，最低值出现在5462（5326-5597）cal. a B.P.，为845.78μmol/mol（图3a）。5-2cal.ka B.P. 阶段，Sr值平均为473.38ppm，最高值709.5ppm，最低值418.6ppm； CaO平均值为35.28％，最高值63.28％，最低值24.56％； Sr值相对于8-5cal.kaB.P. 有所降低，Ca值则无明显变化； Sr/Ca比值的平均值为891.81μmol/mol，最高值出现在2259（2061-2457）cal. a B.P.，为1035.70μmol/mol，最低值出现在4630（4438-4821）cal. a B.P.，为713.50μmol/mol（图3a）。
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表1 腾格里沙漠植物钙质根管中的Sr、 Ca元素结果 Table 1 Elemental results（Sr and Ca） of calcareous root tubes samples collected from the Tengger Desert |
半干旱地区的石灰质土壤往往缺乏植物生长所需的养分，因此生活在上述地区的植物形成了一种特殊的养分获取机制，即植物根释放质子到根围中[2, 4, 44]，使得根围溶液中的pH值变低，溶解Sr和Ca等元素以供植物生长所需[1, 2, 4]。在植物钙质根管形成过程中产生的次生碳酸盐结壳形成土壤水分运动的导管，并在蒸腾作用影响下，土壤水中溶解的矿物质逐渐在次生碳酸盐结壳周围堆积起来[2, 4]。在上述植物钙质根管形成的过程中，Sr和Ca等元素经过土壤水进入植物钙质根管中，且植物钙质根管形成的次生碳酸盐不与外界发生元素交换[1]，因此植物钙质根管中Sr/Ca比值的高低可以指示植物钙质根管形成时周围土壤水中的Sr/Ca比值的高低。
Sr和Ca都是十分活跃且会发生显著迁移的元素[45, 46]，并且离子半径越小的元素越容易迁移[38]，由于Sr 的离子半径（0.113nm）大于Ca 的离子半径（0.099nm）（Sr>Ca），故而Ca比Sr更容易发生迁移。在降水较多、 土壤水分充足的条件下，离子半径较小的Ca会比Sr更容易迁移，因此Ca会先被土壤水溶解达到饱和，Sr则会继续溶解，故土壤中水分含量越高，土壤水中Sr的含量越高[45]，即Sr/Ca比值也会越高。且植物钙质根管形成的次生碳酸盐不与外界发生元素交换[1]，因此植物钙质根管中Sr/Ca比值可以指示有效湿度的变化，Sr/Ca比值越高，表明土壤水中Sr/Ca比值越高，即有效湿度越高，环境越湿润，反之亦然。
5.2 古环境重建根据腾格里沙漠植物钙质根管Sr/Ca比值变化以及年代分布结果[9, 30]，我们重建了腾格里沙漠地区全新世千年尺度有效湿度的变化并划分为如下几个阶段:
阶段1: 早全新世时期（8cal.ka B.P.之前）: 因植物钙质根管的形成受到有效湿度的控制，在干旱区可以指示相对湿润的气候[30, 47]，在该阶段并未发现植物钙质根管（图3b），表明阶段1为干旱时期。
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图3 腾格里沙漠古环境重建结果对比 （a）植物钙质根管中的Sr/Ca比值，（b）植物钙质根管时间分布频率[30]，（c）全新世猪野泽白碱湖岸堤高程[16, 48]， （d）全新世猪野泽QTH02剖面总孢粉浓度[49]，（e）红水河剖面总孢粉浓度[24]，（f）猪野泽QTL剖面湖相沉积物碳酸盐含量[48]， （g）QTL剖面湖相沉积物C/N比值[48]； 阴影部分表示湿润时期 Fig.3 Comparison of our study and other proxy paleo-climate records. The shaded part indicates the humid period，（a） Sr/Ca ratios of paleo calcareous root tubes； （b） The frequency of 14C dating results[30]； （c） Lake-level record of Baijian Lake[16, 48]； （d） Total pollen content of QTH02 section[49]； （e） Total pollen content of Hongshui River section[24]； （f） The carbonate content of QTL section[48]；（g）C/N ratios from QTL section[48] |
阶段2: 中全新世时期（8-5cal.ka B.P.）: 湿润时期。此阶段的Sr/Ca比值最高（图3a），并且植物钙质根管广泛分布（图3b），表明该阶段有效湿度最高，指示了相对湿润的环境。
阶段3: 晚全新世时期（5-2cal.ka B.P.）: 该阶段Sr/Ca比值明显低于阶段2（图3a），并且根管样品数量较阶段2 减少（图3b），表明阶段3的有效湿度较阶段2降低，气候逐渐干旱。
5.3 区域对比为了验证重建结果的可靠性，本文将腾格里沙漠全新世植物钙质根管中Sr/Ca比值变化与腾格里沙漠猪野泽的QTH02剖面（39°03′N，103°40′E） 、 QTL剖面（39°03′N，103°40′E） 以及毗邻地区石羊河流域红水河剖面（38°10′46″N，102°45′53″E） 全新世气候变化重建结果做了对比（图3）:
早全新世时期（8cal.ka B.P.之前）: 腾格里沙漠猪野泽QTL剖面碳酸盐含量、 C/N比、 QTH02总孢粉浓度和石羊河流域红水河剖面总孢粉浓度均处于低值[24, 48, 49]（图3），指示了干旱环境； 在中全新世时期（8-5cal.ka B.P.），腾格里沙漠猪野泽QTL剖面C/N比值、 碳酸盐含量、 QTH02总孢粉浓度和红水河剖面总孢粉含量都达到全新世时期的最高值，此外地貌证据也表明猪野泽在该时期湖面扩张、 湖泊水位明显升高[16, 25, 48]，综上结果，均表明该时期气候湿润、 有效湿度最高； 中晚全新世时期（5-2cal.ka B.P.），腾格里沙漠猪野泽QTL剖面碳酸盐含量、 C/N比、 QTH02总孢粉浓度以及湖面高度均较前一时期显著降低，红水河剖面总孢粉浓度也呈现类似的变化，指示了中晚全新世时期气候由湿润向干旱转变的特点。上述重建结果的一致性表明，腾格里沙漠植物钙质根管中的Sr/Ca变化可以指示该区域千年尺度的有效湿度变化。
为了进一步探讨该区域千年尺度有效湿度的变化机制，本文对比了本研究与中国西部的季风区、 西风区和季风-西风过渡区全新世千年尺度有效湿度变化的重建结果。在青藏高原季风区全新世有效湿度重建结果显示[50, 51, 52, 53, 54, 55]，早、 中全新世时期气候湿润、 晚全新世气候干旱。在新疆西风区[52, 56, 57, 58, 59]，湖泊沉积记录显示早全新世干旱、 中全新世湿润、 晚全新世干旱但较早全新世湿润。本研究与河西走廊其他的湖泊沉积记录[16, 24, 48, 60, 61, 62, 63]显示，在季风-西风过渡地区呈现了早全新世干旱、 中全新世湿润、 晚全新世干旱的气候变化模式。这种全新世千年尺度有效湿度气候变化模式既有别于典型的季风区又与西风区不同[64, 65]，显示了季风-西风过渡带的气候变化模式。
6 结论本文在腾格里沙漠植物14组植物钙质根管样品 14C测年结果的基础上，通过X射线荧光光谱法测定其Sr/Ca比值。结果显示: 中全新世时期（8-5cal.ka B.P.）9组植物钙质根管Sr/Ca比值的平均值为1215.26μmol/mol，最高值为2332.02μmol/mol，为整个全新世钙质根管中Sr/Ca比值的最高阶段； 晚全新世时期5组植物钙质根管Sr/Ca比值的平均值为891.81μmol/mol，明显低于中全新世时期。早全新世时期（8cal. ka B.P之前）未发现植物钙质根管。
根据腾格里沙漠植物钙质根管中Sr/Ca比值含量的变化及其指示的环境意义，重建了该地区的全新世千年尺度有效湿度变化，结果显示早全新世时期（8cal.ka B.P.之前）气候干旱，中全新世（8-5cal.ka B.P.）时期气候湿润，晚全新世（5-2cal.kaB.P.）气候逐渐干旱。上述重建结果与该区域其他代用指标重建的全新世千年尺度有效湿度变化结果具有较好的一致性，表明了植物钙质根管中的Sr/Ca比值可以指示有效湿度的变化。因此，可以从植物钙质根管中提取可靠的古环境信号，重建区域的有效湿度变化。
腾格里沙漠地区全新世千年尺度有效湿度变化，既与典型的季风区存在一定的差异，又与西风区亦有不同，呈现了一种季风-西风过渡带全新世千年尺度有效湿度变化的模式。
致谢 两位审稿专家对本文提出了宝贵修改意见、 杨美芳老师认真细致的审读了全文、 兰州大学资源环境学院张洵赫同学参与了部分实验，谨致谢忱。
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Abstract
Calcareous root tubes, also called rhizoliths, are defined as secondary carbonate crusts produced by the root of terrestrial plants.Calcareous root tubes frequently occur in calcareous soils with a pronounced seasonal moisture regime.Paleo calcareous root tubes are widely distributed across the Alashan Plateau, Northwest China.Previous studies reveal that calcareous root tubes have potential to reconstruct the paleo-environment and paleo-vegetation.However, it remains unclear whether any environmental signals could be extract from the calcareous root tubes and used for reconstructing paleo-environmental conditions.The Tengger Desert, with area of approximately 42700km2, is the fourth-largest desert in China.The annual average temperature of the Tengger Desert is 7.0~9.7℃, the regional annual average precipitation is 125~160mm and the annual average potential evaporation is 2400~3000mm.The effective moisture level affects the formation of calcareous root tubes, and different geochemical elements has different features of transport and enrichment in various controlling factors.Therefore, influenced by the soil water changes and the special nutrient-acquiring mechanism of terrestrial plants that live in calcareous soils with deficit nutrition, Sr and Ca was enriched in the calcareous root tubes and the Sr/Ca ratios will changes in different effective moisture level.In this study, based on previous conventional 14C dating results of 14 Holocene calcareous root tube samples(37°54'59"~38°55'31"N, 104°10'34"~105°22'59"E) collected from the Tengger Desert(37°26'28"~39°58'59"N, 102°48'25"~105°38'13"E), we used the X-ray fluorescence to analyzed the elemental composition of the calcareous root tubes samples.The results show that the highest value of Sr/Ca ratios(2332.02μmol/mol) was occurred in 8~5cal.ka B.P.with the mean value of 1215.26μmol/mol, and the Sr/Ca ratios of calcareous root tubes was decreased during 5~2cal.ka B.P.with mean value of 891.81μmol/mol, but not in the Early Holocene.Based on the Sr/Ca ratios' variation and temporal distributions of the calcareous root tubes in the Tengger Desert, we reconstructed the effective moisture level changes in this area during the Holocene.The results reveal that the climate was dry in Early-Holocene(ca.8cal.ka B.P.), humidity in Mid-Holocene(8~5cal.ka B.P.), and from humidity to dry in Late-Holocene(5~2cal.ka B.P.).The reconstruction results were consistent with other previous reconstruction results from lake sediments of Zhuyeze terminal lake section and Hongshui River section, and consistent with the Holocene moisture pattern of the Asian monsoon margin of Northwestern China, which demonstrated that the Sr/Ca ratios of paleo calcareous root tubes can reflect millennial-scale effective moisture changes in this area.These studies reveal that the environmental signals can be extract from the calcareous root tubes and can be applied to paleo environment reconstruction.