2015, Vol.35
1 前言
东亚是一个特殊的气候区，东部和南部湿润区主要受东亚夏季风和西南季风控制，而西北地区主要受东亚冬季风和西风控制。由于中国西南地区为青藏高原，西边为天山和帕米尔高原，西南季风和西风带来的水汽大部分被阻挡，促进了西北内陆的沙漠化。在过去几十年，风尘和深海沉积物提供了研究东亚气候演化的良好材料[1, 2, 3, 4, 5, 6, 7]。大量证据表明，在24~22Ma，亚洲气候由行星风系主控的带状气候模式向季风主控的气候模式转换[8, 9, 10]。构造尺度上，中国内陆的古环境记录都含有青藏高原生长的信号[11]。但是，由于青藏高原的生长历史尚不清楚，使得东亚古气候的研究变得难以捉摸。鉴于此，我们试图分析已发表的气候和构造记录( 表 1和 图 1)，从轨道和构造尺度探讨东亚气候在中中新世以来的时空演化。
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表 1 中中新世以来古气候记录详细信息 Table 1 Detailed information about the paleoclimatic records since the Middle Miocene |
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图 1 研究站位和地形分布图中虚线为等降水量线，实线为山脉，圆点为研究站位分布 Fig. 1 Research sites and geographic features. The dashed lines represent the isohyetal lines； solid lines represent mountains； dots represent the research sites |
全球变冷和青藏高原的生长常常被用于解释东亚中新世以来构造尺度上的气候演化[1, 19, 20, 21, 22]。青藏高原在气候系统的能量、 大气和水循环中扮演着重要角色[11, 23, 24, 25]。一方面，青藏高原作为一个巨大的机械屏障，可以阻断高原南北的热量以及水汽交换，形成热隔绝[24, 26]； 另一方面，青藏高原通过热力作用来增强夏季风[25, 27]。在夏季，高原作为一个热源，形成一个热低压，在高原边缘形成爬升气流，同时在科氏力的作用下，形成一个逆时针气流，增强西南季风，在冬季相反。再者，青藏高原东北缘的隆升剥蚀可能为黄土高原粉尘的堆积提供物源[21]。全球变冷，一方面可以压缩低纬度气候带，在北半球，使降水带南移，或者使热带辐合带收缩； 另一方面，全球变冷也可以通过减弱蒸发，降低空气中的水汽含量。因此，全球变冷和高原隆升对于晚新生代亚洲气候变化起着重要作用。
2.1 中中新世以来全球变冷首先，必须了解中中新世以来全球气候演化的整体趋势。深海综合底栖有孔虫 δ 18 O( 图 2a)、 浮游有孔虫Mg/Ca温度以及有机温度计很好地指示了15Ma以来的全球降温[28, 29, 30, 31]。约13.8Ma，中中新世适宜期(MMCO)结束，随后中中新世气候转型(MMCT)发生，南极冰盖扩张，深海底栖有孔虫 δ 18 O增重约1 ‰ [28, 29, 32]。浮游有孔虫Mg/Ca温度显示南大洋表层水体温度下降约6~7℃[33, 34]； 之后，经过一系列的阶段性降温事件(Mi3~Mi7)[32, 35]； 约在7Ma，北极冰盖开始发育[36]。之后全球持续降温，约2.7Ma，北极冰盖完全形成。从此，地球气候进入以冰期-间冰期旋回主导的气候状态[37]。
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图 2 15Ma以来构造尺度上海陆古气候记录对比 (a)全球深海综合底栖有孔虫氧同位素[28]；(b)ODP 1146站浮游有孔虫深水种百分含量[12]；(c)ODP 885/886 站风尘通量[5]；(d)佳县黄土线性堆积速率[18]；(e)ODP 1148站CRAT[2]；(f)ODP 1143站赤铁矿与针铁矿比值[84]；(g)酒西盆地针叶＋木本百分含量[15]； (h)临夏针叶树孢粉含量[14]；(i)朝那[16]和庄浪[17]磁化率 Fig. 2 Comparison of paleoclimatic records from land and sea since 15Ma on the tectonic timescale. (a)Global synthetic benthic foraminiferal δ 18 O；(b)the relative of abundance( ％ )of the planktonic foraminiferal deep-dwelling species at ODP 1146；(c)eollan flux at ODP 885/886；(d)linear sedimentation rate of Jiaxian section；(e)CRAT records from ODP 1148；(f)Hm/Gt ratio of ODP 1143；(g)conifers and forest percentage of western Jiuquan Basin pollen records；(h)conifer percentage of Linxia Basin pollen records；(i)magnetic susceptibility records of Chaona and Zhuanglang sections |
关于青藏高原的研究，无论是从地质记录还是数值模拟，很少关注青藏高原不同区域在不同时间段隆升所产生的气候效应[38]。中-晚中新世青藏高原向东北方向的快速生长得到了广泛承认[11, 21, 39, 40, 41, 42, 43, 44, 45]。青藏高原在这一时期大范围的扩张，可能确立了现代青藏高原的地貌格局[40]。Ma和Jiang[21]收集了37个青藏高原东北缘和南缘的构造活动记录，认为祁连山东部在15~9Ma构造运动较为活跃； 青藏高原东南部在11~6Ma构造运动较为活跃，8Ma活动范围最广。青藏南部在10Ma时出现了一次强烈的构造活动，而且很多证据表明高原南缘在中新世之前已经达到了3000~5000m[43, 44, 46, 47, 48, 49, 50, 51]。整个青藏高原的构造活动有从北向东发展的趋势，而且10Ma青藏高原的构造活动达到了一个相对较为活跃的时期。
阿尔金山山前粗颗粒、 磨拉石沉积以及较高的沉积速率证明了阿尔金山在13.7~9.0Ma快速隆升[52, 53]，阿尔金断裂的走滑导致塔里木盆地的东北缘表面隆升速率达到100m/Ma[54]。而且对北祁连山沉积盆地综合分析发现，在13Ma以来地壳开始增厚，同时出现磨拉石堆积，祁连山表面开始抬升[55]。古地磁重建的线性堆积速率也显示了这一时期青藏高原北部持续或者间歇性较高的堆积速率[52, 53, 56, 57, 58, 59]，同时大量的热年代学证据也支持这一时期地壳的快速冷却[54, 60, 61, 62]。青藏高原东南部的快速隆升发生在11~6Ma，8Ma构造活动的范围最广，而且研究也最为成熟[21]。11~6Ma，地壳的快速冷却以及堆积速率升高，在青藏高原的东缘普遍存在，例如龙门山[63, 64, 65]、 六盘山[66, 67]、 临夏盆地[68, 69]、 拉脊山[70, 71, 72]、 贵德盆地[73]和贡嘎山[74]。大渡河、 雅砻江以及长江在高原东缘流域在10Ma以来具有较高的河谷下切速率，也支持高原东北缘在这一时期的快速隆升[42, 75]。
3 构造尺度上东亚气候演化的海陆对比 3.1 古气候的替代指标介绍进行古气候的重建，选择合适的指标显得尤为重要。替代指标重建的记录可能包含了古气候信息以外的多种信息，一般需要多指标记录相结合，才能更好地区分和提取古气候信息。我们选择了CRAT(chlorite/chlorite＋hematite＋goethite)、 K/Al比值、 浮游有孔虫深水种丰度、 Hm/Gt(Hematite to goethite ratio)，以及孢粉组合等记录进行对比分析，探索15Ma以来东亚的气候变化。CRAT是绿泥石与绿泥石、 赤铁矿和针铁矿加和的比值，对于湿度、 径流以及风化体系的变化极为敏感[2]。深海沉积物中的K和Al主要来源于粘土矿物，K易于在化学风化中溶于水被带走，而Al不易被带走，深海沉积物K/Al比值主要反映了粘土矿物伊利石和高岭石的丰度比值，可以用来指示风化[2]。ODP 1148站位于南海北部，中新世以来的沉积物主要来源于中国南部[76]，而中国南部的降水主要由夏季风控制，因此ODP 1148站的CRAT和K/Al比值可以作为东亚夏季风或降水指标[2]。同样，来自南海ODP 1143站的Hm/Gt记录也被用来指示东亚夏季风的强度[7, 77, 78]。赤铁矿和针铁矿是土壤和湖海相沉积物中普遍存在的含铁矿物，而且沉积物的颜色也主要由铁化合物浓度体现出来，进而与颜色反射率可以对应[77]。赤铁矿是水合铁矿在暖干的气候条件下脱水形成，而针铁矿是在较湿润的环境中形成[78, 79, 80, 81, 82, 83]。因此，沉积物中这两种矿物的比值可以作为湿度的指标，而且在轨道尺度-千年时间尺度上有较好的应用[7, 78, 84]； 然而在长时间尺度上似乎并不适用。ODP1143站的K/Al[13]和Hm/Gt[7]，在5Ma以来的长趋势上并不一致，这说明了Hm/Gt作为一个夏季风指标，可能不适用于指示长时间尺度上夏季风的演化。同样，黄土的磁化率被证明在轨道尺度上是一个有效的夏季风指标[85, 86, 87, 88]。
不同属种的浮游有孔虫生活在不同的水深，浅水种生活在混合层，而深水种生活在温跃层以下[89, 90, 91, 92, 93]。由于温跃层的变化控制着水体营养的分布，当温跃层加深时，深水种由于生存空间、 营养以及光照受到限制，丰度会减少，反之亦然[92]。现代南海北部温跃层的深度与冬季风的变化联系紧密，当冬季风加强时，由于表层水体的搅混作用，温跃层加深； 冬季风变弱的时候，温跃层变浅。然而在南海北部上升流区，当冬季风增强，冷水上翻，会导致温跃层变浅[93, 94]。在构造尺度上，浮游有孔虫混合层种和深水种的相对丰度变化被用来指示上升流的强弱，进而可以作为构造尺度上冬季风的替代指标[12, 95, 96]。而在轨道尺度上，黄土的粒度是一个优秀的冬季风指标[97, 98]。
3.2 构造尺度上东亚冬季风的演化和东亚夏季风的演化浮游有孔虫深水种相对丰度的增加代表由冬季风驱动的上升流增强和温跃层变浅。ODP 1146站位于南海北部( 图 1)，其深水种丰度变化在15Ma以来具有明显的阶段性，可以将东亚冬季风的演化分为4个阶段( 图 2b)[95]，它们是15~11Ma、 11~8Ma、 8~4Ma和4~0Ma，这4个阶段深水种含量逐渐增加，与全球逐渐降温的步调基本一致( 图 2a)。在约11~12Ma，深水种的突然增加被解释为西太平洋暖池的出现[12, 95]。
ODP 1148站的CRAT( 图 2e)、 K/Al和CIA(Chemical Index of Alteration)显示亚洲夏季风15~10Ma较为强盛，11~4Ma逐渐减弱，同时印度洋的Indus Marine A-1井的K/Al和CIA[2]以及记录中国南部植被黑碳的δ 13 C[99]也支持这一时期逐渐弱化的夏季风。植被草本和木本相对含量的变化在一定程度上可以反应干湿变化。11~4Ma，临夏盆地和酒泉的西部孢粉草本和木本的相对变化[14, 15]与ODP 1148 站的CRAT变化非常相似，也显示了夏季风的逐渐减弱( 图 2g和2h)； 4~0Ma夏季风再次逐渐增强[2, 100]，ODP 1143站的K/Al也显示了夏季风的逐渐增强[13]。然而ODP 1143的Hm/Gt( 图 2f)[84]和黄土高原的磁化率( 图 2i)[16, 17]表现的不尽相同。磁化率从4Ma上升到大约2.7Ma，之后表现出强烈的冰期-间冰期旋回。虽然Hm/Gt也存在冰期-间冰期的旋回[84]，但是在4Ma以来体现出一种升高的趋势，与ODP 1148站和ODP 1143站风化指数重建的夏季风强度表现出相反的趋势。一方面说明了Hm/Gt不适用于构造尺度上夏季风的指标，另一方面也可能说明了海洋沉积物会受到一些潜在物理化学过程，以及南海同时受到高纬和低纬过程的影响。
构造尺度上东亚夏季风可以为西北内陆提供水汽，而冬季风可以为风尘堆积提供动力。因此，我国西北内陆的干旱化过程以及7~8Ma 风尘的大面积堆积与东亚季风之间的关系密不可分。
3.3 西北内陆的干旱化与7~8Ma风尘的大面积堆积在新近纪东亚季风逐渐控制了亚洲气候，中国内陆的干旱化可能主要归因于3个因素:1)东亚夏季风、 西风环流以及周边区域能够提供多少水汽； 2)全球温度变化调控的空气中水汽质量； 3)地貌演化，特别是山脉隆升，对于水汽传输路径和强度的约束。
现代夏季多年平均风场矢量图表明中国南部以及内陆的水汽主要由西风和印度季风提供[101]。西风会受到天山、 青藏高原和帕米尔高原的阻挡。大量证据表明天山在中中新世之前已经隆起[102, 103, 104, 105, 106]。天山区3个孢粉记录显示，中中新世以来在天山出现了相对较为完整的垂直植被图谱，而且不同站位的植被面貌相差较大[20, 107, 108]，可能说明天山至少在中新世之前已经能够在一定程度上阻挡西风带来的水汽形成雨影效应，那么西风为西北内陆提供的水汽有限。最新研究表明，塔克拉玛干沙漠的形成于26.7Ma到22.6Ma，与大范围的干旱以及与周围山体的快速剥蚀联系紧密[109]，可能进一步表明由于帕米尔高原、 天山以及昆仑山等山体隆升导致的雨影效应增强，支持在晚中新世以来西风为西北内陆提供的水汽有限。
值得注意的是，酒泉地处西北内陆，青藏高原的北部( 图 1)，在约11.16Ma，发生了草本向木本的快速转换( 图 2g)[15]。11.16Ma前后东亚夏季风一直处于较强盛的状态( 图 2e)，并无剧烈的增强，而且临夏的植被也紧随亚洲夏季风的变化( 图 2h)[14]。酒泉草本和木本植被相对含量的快速转换可能与区域构造活动控制的地形有关，特别是秦岭和青藏高原东北缘的隆升。
秦岭东西展布约1500km，是中国南北干旱和半干旱区的分界线，几乎与800mm等降水量线重合( 图 1)。现代秦岭南北有明显的植被差异，但是秦岭的平均高度只有2000~3000m。这意味着在现代夏季风的强度下，2000~3000m可以阻挡东亚季风带来的大部分水汽进入中国西北内陆。秦岭是由于华南和华北地块的碰撞汇聚形成，大范围的岩浆活动以及地壳变形主要发生在中生代和新生代[110, 111, 112]，新磷灰石裂变径迹记录表明50~10Ma 北秦岭发生了小幅度的变形和剥蚀，约10Ma之后才加速变形[113]。然而更为精细的多方法热年代学研究表明了西秦岭多相位的剥蚀历史，包括3期快速剥蚀，分别是120~11Ma(约30℃/Ma)，25~22Ma(约5℃/Ma)和最后7Ma[112]。可能表明秦岭在中中新世之前已经达到了较高的高度。
前文2.2节中已经讨论了11~6Ma青藏高原东北缘的快速生长，并向西秦岭靠拢。那么可以推测在11Ma之前，青藏高原东北缘的位置较现在偏西，和西秦岭之间较为开阔，较强的东亚夏季风携带的水汽也很难跋涉较长的距离深入内陆，所以中国内陆大部分区域十分干旱( 图 2g)。由于青藏高原东北缘向东北持续生长，与西秦岭之间的距离不断缩小，可能形成了通道。在大致11.16Ma，通道宽度达到一个阈值，将水汽带到酒泉，酒泉的植被面貌快速转变为森林植被。最终，在约7~8Ma，通道可能全部关闭，同时夏季风的减弱和全球变冷促进了西北内陆全面干旱。ODP 885/886 站位于北太平洋西风带上，该站的风尘通量被解释为亚洲内陆的干旱化过程( 图 2c)[5]，而黄土高原8Ma以来的风尘堆积速率( 图 2d)[18]与ODP 885/886 站风尘通量趋势一致。这可能说明风尘的堆积速率与物源区的干旱程度，范围或者西风有很大关系[21]； 加之，高原东北缘的隆升也提供了粉尘物源，进一步促使风尘在8Ma开始大面积堆积[21]。当然，这只是根据目前已有的构造和气候记录推测，需要高分辨率的模拟和更多的古气候记录验证。
4 5Ma 以来轨道尺度的海陆对比2.75Ma以来，全球冰量和热带海区海水表层温度具有清晰的地球轨道周期( 图 3a，3b和3c)； 约0.9Ma之前，底栖有孔虫 δ 18 O以较小振幅的斜率41ka为主要周期，之后以偏心率100ka的周期为主，这一转换被称为中更新世革命(MPR)( 图 3b)。底栖有孔虫 δ 18 O的功率谱也显示出0.9Ma之后的100ka的功率明显强于0.6~2.5Ma 的41ka周期[13]。ODP 1143站海水表层温度同样记录了MPR( 图 3c)。ODP 1148站的K/Al比值的演化谱显示出较多的噪音( 图 3d)，可能与海洋沉积物会受到一些潜在物理化学过程，以及南海同时受到高纬和低纬过程的影响有关，但仍然有较强的轨道周期，也显示了南海区域的夏季风对于太阳辐射量的响应。
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图 3 5Ma以来不同记录的连续小波分析 (a)ETP，ETP是标准化的偏心率(eccentricity)、 斜率(obliquity)和岁差(precession)的加和[114]； (b)全球综合底栖 δ 18 O记录[37]； (c)ODP 1143站海水表层温度(SST)[115]； (d)ODP 1143 XRF(X-ray Fluorescence)岩芯扫描的K/Al比值(11点滑动平均)[13]； (e)灵台风尘标准化的石英平均粒度(Qtz. GS)[116]； (f)灵台 风尘标准化的磁化率(MS)[116] Fig. 3 Wavelet power spectrum of different paleoclimatic records since 5Ma. (a)EPT(normalized eccentricity＋ normalized obliquity＋ normalized precession)；(b)global synthetic δ 18 O； (c)sea surface temperature of ODP 1143； (d)K/Al ratio from XRF of ODP 1143； (e)normalized mean quartz grain size records of Lingtai loess； (f)normalized magnetic susceptibility records of Lingtai loess |
黄土和古土壤的磁化率和粒度可以用来研究轨道尺度上东亚季风的演化[97, 117, 118]。非轨道调谐的3Ma以来风尘的磁化率和粒度具有地球轨道参数的周期[118]，而后被追溯到7Ma并且以100ka的周期较为强烈( 图 3e和 图 4)[116]。至少表明7Ma以来东亚季风在轨道尺度上对于太阳辐射量有响应，而且东亚季风对于太阳辐射和冰盖的强迫是非瞬时非线性的[97]。黄土粒度记录显示5Ma以来都显示出较强的轨道周期。但是磁化率在2.75Ma以后轨道周期才变得较为明显( 图 3f和 图 4)，表明夏季风对于北半球冰盖的变化较为敏感，可能是由于北极冰盖的进退会对ITCZ的南北摆动产生影响。在2.75Ma之后，粒度和磁化率记录的东亚冬季风和夏季风在岁差23ka的周期上，粒度领先磁化率 30°，在19ka周期上之后粒度滞后磁化率 50°，在斜率40ka周期上，粒度滞后磁化率45°( 图 4)。基本可以认为在东亚冬季风和夏季风在岁差和斜率周期上同相位。在偏心率100ka的周期上，磁化率领先粒度 110°。 也就是在冰期-间冰期旋回上，夏季风的变化领先冬季风的变化约30ka。这可能表明，夏季风即受到低纬度地区太阳辐射量驱动的蒸发强弱的影响，又受到北半球冰盖对夏季风位置的影响，而冬季风则主要响应于北极冰盖的变化，由于北极冰盖的生长滞后于北半球夏季太阳辐射量最大值，因此导致东亚夏季风的变化领先于东亚冬季风。
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图 4 75Ma以来黄土粒度和磁化率的交叉频谱和相位分析 交叉频谱分析采用Tukey窗口，带宽0.0014ka-1，置信区间80 ％ Fig. 4 Cross-spectral coherence and phase wheel for magnetic susceptibility and mean quartz grain size records between 2.75Ma and 0Ma. Cross-spectral analyses use a Tukey window with 80 ％ confidence interval and 0.0014ka-1 bandwidth |
东亚夏季风相对于东亚冬季风要复杂，夏季风不仅需要考虑风速还要考虑水汽多少。在5.00~2.75Ma，氧同位素的偏重主要是由于南极冰盖(现代体积约25.71×106km3)扩张的贡献，北极冰盖(现代体积约2.85×106km3)扩张相对较为缓慢，可能导致全球整个气候带的北移，同时热带辐合带(ITCZ)或者夏季季风降水的前缘也可能整体北移，导致中国内陆逐渐湿润，黄土的磁化率显示出逐渐上升的趋势。2.75Ma之后，北极冰盖逐渐大幅扩张，黄土磁化率在长趋势上显示出逐渐降低的趋势，并呈现出强烈的轨道周期( 图 3f)。此时分两种情况: 冰期的夏季，一方面空气中水汽含量较间冰期少，主要受夏季低纬度太阳辐射量的驱动，另一方面由于北极冰盖的存在，整个气候带南移，水汽进入黄土高原较少显示出较弱的季风； 间冰期的夏季，气候带整个北移，同时空气中水汽含量较高，所以较多的水汽进入黄土高原，显示出较强的夏季风。MPR之后，北极冰盖前缘在冰期-间冰期旋回上表现出经向非常大的进退，从而气候带也表现出较大的南北摆动，所以磁化率也在MPR表现出非常强的冰期-间冰期旋回。粒度反应的东亚冬季风相对于夏季风驱动机制上较为简单，主要受北极冰盖的控制，在冰期-间冰期尺度上振幅较强。
5 结论15Ma以来构造尺度上东亚冬季风的演化分为4个阶段，分别是15~11Ma、 11~8Ma、 8~4Ma和4~0Ma，逐渐增强，与全球降温的步调基本一致。东亚夏季风演化分为3个阶段，15~11Ma强盛期、 11~4Ma逐渐弱化和4~0Ma再次增强。我们推测11.16~7.00Ma青藏高原东北缘和西秦岭之间存在水汽通道，7~8Ma水汽通道可能全部关闭，加上全球变冷和弱化的夏季风，西北内陆全面干旱，同时高原东北缘的隆升也提供了粉尘物源，风尘开始大面积堆积。
轨道尺度上，在2.75~0Ma表现出较强的轨道周期，夏季风领先冬季风约30ka，这可能表明，夏季风即受到低纬度地区太阳辐射量驱动的蒸发强弱的影响，又受到北半球冰盖对夏季风位置的影响，而冬季风则响应于北极冰盖的变化。在MPR(0.6Ma)之后100ka周期的振幅表现的更强烈。在冰期的夏季，一方面空气中水汽含量较间冰期少，另一方面由于北极冰盖的存在，整个气候带南移，水汽进入黄土高原较少显示出较弱的季风，间冰期相反。轨道尺度上热带海洋记录的夏季风较为复杂，可能由于风化指数受到一些潜在的物理化学过程，以及南海同时受到高纬和低纬过程的影响。在未来研究中，还需多指标记录相互印证与数值模拟相结合。
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Abstract
Through compiling and analyzing the paleoclimatic and tectonic records from East Asia since 15Ma, the results suggest the East Asian Winter Monsoon gradually strengthened consisting with global cooling. Asian Summer Monsoon progressively weakened from 10Ma and strengthen again since 4Ma. We speculated there was a water passage between eastern margin of the Tibetan Plateau and western Qinling Mountain during the period of 11.16Ma to 7.00Ma. The Asian Summer Monsoon precipitation could travel a long distance to the northeastern margin of the Tibetan Plateau through the passage. Subsequently, the water passage entirely closed after 8~7Ma resulted from the sustained eastward expansion of the Tibetan Plateau during the Late Miocene. In addition, global cooling and weakening summer monsoon promoted the dry climate in Northwest China and widespread aeolian dust accumulation. In the glacial-interglacial cycles, Asian Summer Monsoon led East Asian Winter Monsoon by 30ka in phase relationship since 2.75Ma. This could suggest that the East Asian Winter Monsoon was dominated by the Northern Hemisphere ice volume, while the Asian Summer Monsoon was forced by the combined effects of the Northern Hemisphere ice volume and low latitude processes.