LATE QUATERNARY COCCOLITH PRODUCTIVITY IN THE NORTHERN SOUTH CHINA SEA
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摘要: 通过对南海北部MD05-2904岩心进行有机地球化学分析,以长链不饱和烯酮作为颗石藻生产力的替代性指标,讨论颗石藻生产力的变化及其影响因素。结果显示,260 ka以来,颗石藻生产力有着明显的冰期/间冰期变化:冰期高,间冰期低;冰阶高,间冰阶低;在轨道尺度上岁差周期明显,反映出太阳辐射、东亚季风对颗石藻生产力在长期尺度上起调控作用;而由于特殊的地理位置,河流输送的营养盐对本区海洋初级生产力的影响可能也较大;与前人研究结果一致,同时认为,在地质历史上沉积速率变化大的区域,对生物标记物的含量和堆积速率的对比讨论,更有利于反映生产力的变化。Abstract: We present hereby the C37 long chain alkenones record from the Core MD 05-2904 located in the northern South China Sea. The C37 alkenones was applied to assess the haptophytes productivity variations. The content and Mass Accumulation Rates (MARs) of the C37 alkenones indicate that during the last 260 ka, the productivity of the haptophytes has generally followed a glacial/interglacial pattern with high values in glacials, and low values in interglacials. And three high productivity intervals are identified in MIS 2, 4 and MIS 6. There were precession signals in both records. All these display a close link between East Asian winter monsoon and paleo-productivity, and on the orbital scale, marine productivity was controlled by insolation and monsoon variations, mainly through nutrients mechanism. And for the northern SCS, sea level changes and river transportation may also play an important role. These results are consistent with other reports. We also notice that, with consideration of both contents and MARs of biomarker, we can better decipher biomarker record, especially for those areas with big fluctuations in sedimentation rates.
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采薇平顶海山群位于西太平洋麦哲伦海山链，范围为14.9°~16.1°N、154.4°~156°E。作为富钴结壳勘探合同区，中国已开展了采薇平顶海山群的地形地貌调查工作，实现了多波束数据的全覆盖。前人开展过采薇平顶海山群的地形研究工作，提取了海山的地形分形特征[1]，概括性地统计了包括采薇平顶海山在内的西太平洋多座海山的高度、坡度等地形参数[2-4]，探讨了海山地形地貌作为控矿因素之一与结壳发育的关系[5-7]。
此外，前人也开展过海山斜坡的滑坡形态与成因、海山的星形平面形态特征与成因的研究[8-10]，以及平顶海山平顶成因的研究[11]。ODP144航次在西北太平洋Limalok平顶海山(871站位)、Lo-En平顶海山(872站位)、Wodejebato平顶海山(873-877站位)开展的大洋钻探揭示了西北太平洋平顶海山的沉积层序。沉积层序表明平顶海山曾经位于浅水环境，海山山顶形成平坦的浅水碳酸盐岩台地或者生物礁盘，海山被剥蚀夷平，后来海山下沉，接受远洋沉积[11]。
本文基于多波束数据，报道采薇平顶海山群山顶发育的多级山顶平台的地形特征，并结合最新采集的浅剖数据，探讨采薇平顶海山群多级山顶平台的成因。
1. 区域地质背景
采薇平顶海山群位于西太平洋的麦哲伦海山链。麦哲伦海山链由一系列平顶海山组成，西侧为马里亚纳海沟，东北侧为皮嘉费他海盆，南侧为东马里亚纳海盆，北侧与马尔库斯-威克海山区相邻。
大洋钻探ODP129航次钻探揭示了麦哲伦海山链周围洋盆的年龄。在皮嘉费他海盆实施了两个站位钻探，其中在ODP800站位钻获的碱性辉绿岩40Ar/39Ar测年结果为126.1±0.6Ma，在ODP801站位钻获的玄武岩熔岩40Ar/39Ar测年结果为166.8±4.5Ma。在东马里亚纳海盆实施的ODP802站位钻探获得的玄武岩40Ar/39Ar测年结果为114.6±3.2Ma[12]。
前人统计了麦哲伦海山链不同海山基岩样品的40Ar/39Ar测年数据，结果表明海山链年龄范围为80~100Ma[13](图 1)。一般认为麦哲伦海山链属板内热点成因，于白垩纪期间产生于今南太平洋的法属玻利尼西亚附近位置，随着板块运动慢慢迁移到现今位置，在迁移的过程中发生了多期次的岩浆活动[14-18]。
从区域磁条带发育情况看，麦哲伦海山链被一系列西北走向的转换断层错断[12](图 2)。其中麦哲伦海山链的转换断层有Ogasawara 1号、Ogaswara 2号、Ogasawara 3号和Kashima转换断层等。根据磁条带可推断出Ogasawara转换断层水平错动了约600km[13]。
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2. 数据来源
采薇平顶海山群的全覆盖多波束数据、浅剖资料和碳酸盐岩浅钻样品均来源于大洋航次。多波束数据主要来源于“海洋四号”在1997年执行的DY95-7航次，测量设备为Seabeam2110多波束测深系统。浅剖资料来源于“海洋六号”船在2015年的大洋第36航次，测量设备为ATLAS PARASOUND P70参量阵浅地层剖面仪。碳酸盐岩浅钻样品来源于“海洋六号”船在2011—2015年执行的大洋航次，使用设备为深海浅地层岩芯取样机。
3. 海山群特征
3.1 地形地貌特征
采薇平顶海山群发育三座海山，包括两座平顶海山和一座尖顶海山，分别命名为采薇平顶海山、采杞平顶海山和采菽海山(图 3)。
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图 3 采薇平顶海山群三维地形斜视图(多级山顶平台名称为本文命名，黑色实线为浅剖测线位置)Figure 3. 3D topography of the Caiwei Guyots采薇平顶海山是海山群中规模最大的平顶海山。海山为北东走向，长和宽分别为110km和95km。山顶地形平坦，斜坡地形上陡下缓并发育多个海脊，与麦哲伦海山区其他平顶海山的斜坡地形特征相似。海山山顶发育两级山顶平台。主山顶平台水深约为1350m，坡度小于0.3°，面积约1940km2，占山顶地貌的97%。次山顶平台位于山顶的东北角，水深约为1735m，面积约60km2，占山顶地貌的3%。两级平台高差约385m。相比较而言，次山顶平台的地形没有主山顶平台平坦，往北东向发育微斜坡，坡度可达1.5°。两级平台之间的斜坡较陡，坡度达22°，斜坡走向为北西向，倾向为北东向。
采杞平顶海山规模较小，位于海山群的南部，俯视平面形态呈不规则形；东西向长约80km，南北向宽约54km。山顶平台东西向最长约16.2km，南北向最长约17.8km，面积约为153km2。海山山顶平台水深约为1800m，最浅处水深为1629m，山麓水深为5800m，最大高差为4171m。采杞平顶海山与采薇次山顶平台高差300~450m，与采薇平顶海山次山顶平台水深相当。
3.2 浅剖特征
图 4为穿越采薇平顶海山主山顶平台、次山顶平台和采杞山顶平台的浅地层剖面。根据地形和浅剖资料统计出来的3个山顶平台的特征见表 1。采薇主山顶平台沉积物最大厚度约为100m，采薇次山顶平台沉积物厚度约为37m，厚度相差63m。采杞山顶平台沉积物厚度约为40m，与采薇主山顶平台沉积物厚度相差约60m，与采薇次山顶平台沉积物厚度相当。沉积物基底较为平坦，采杞山顶平台有一明显的突起，根据浅剖显示的特征，推断该生物礁可能为环礁，中间发育瀉湖相沉积。这与ODP144航次揭示的西太平洋平顶海山特征相似。
表 1 采薇平顶海山群的地形和浅剖特征统计Table 1. Topographic features and sediment thickness of Caiwei Guyots名称 水深/m 地形高差/m 面积/km2 坡度/(°) 最大沉积物
厚度/m沉积物
厚度差/m采薇主山顶平台 1350 0 1940 < 0.3 100 0 采薇次山顶平台 1735 385 60 1.5 37 63 采杞山顶平台 1800 450 153 - 40 60 备注：地形高差和沉积物厚度差指与采薇主山顶平台相比较的结果. 4. 讨论
4.1 沉积物厚度不均匀性对山顶平台地形高差的影响
如果海山山顶平台的基底水深一样，沉积物覆盖的厚度不一样，有可能导致多波束数据显示的山顶平台水深不一致。通过浅剖数据我们可以计算海山基底之上的沉积物厚度，分析此种原因的可能性。采薇主山顶平台和次山顶平台沉积物厚度差为63m，而地形高差达385m，因此沉积物不会是造成采薇平顶海山两级山顶平台高差的原因。采杞山顶平台与采薇主山顶平台沉积物厚度相差约60m，而山顶平台地形高差达300m以上，因此沉积物不是造成采薇主山顶平台和采杞山顶平台高差的主要原因。
4.2 相对海平面变化对山顶平台地形高差的影响
全球海平面变化和海山群的整体构造升降引起海平面的相对变化。当海台水深位于海平面附近时，海平面的相对变化会影响海台上珊瑚礁的发育程度。当海平面上升速率比珊瑚礁堆积速率大，但造礁珊瑚仍适合生长时，可能在海台周缘发育水下阶地[20, 21]。大型平顶海山水深在海平面附近时，也可能出现相似的情况。但是，海平面相对变化对海台/平顶海山每一个方位都会产生影响，所形成的水下阶地应该沿着海台/平顶海山的周围均匀性地发育(图 5)。采薇平顶海山群地形特征显示周围未发育相似的多级水下阶地，而本文所讨论的多级山顶平台地形特征独特，并非沿着海山周围均匀发育(图 6)，与南海甘泉海台和永暑礁周围发育的多级水下阶地完全不同。因此海平面的相对变化不是采薇平顶海山群发育多级山顶平台的主要原因。
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图 6 甘泉海台多级水下阶地(左)和采薇平顶海山群多级山顶平台(右)的区别(多级水下阶地在海台周围均有发育，而多级山顶平台将海山分成不对称的几部分)Figure 6. Schematic map of step terrace of Ganquan Plateau and multi-table landform of Caiwei Guyots4.3 浅钻获取的碳酸盐岩样品对多级山顶平台成因的指示意义
浅剖表明采薇平顶海山群可能发育生物礁，在山顶平台的周缘获取了碳酸盐岩样品(图 7)，表明采薇平顶海山群曾经位于海平面附近，海山顶部被剥蚀夷平，接受碳酸盐沉积，局部发育珊瑚礁。由于浅剖数据确定了不同山顶平台沉积物覆盖下的基底面存在200m以上的高差，据此推断，多级山顶平台很可能是在采薇海山群的差异沉降过程中形成。这意味着多级山顶平台之间发育正断层。采薇主山顶平台和次山顶平台之间的斜坡走向为北西向，倾向为东北向，因此推测断层走向为西北向，倾向为东北向。根据地形推测采薇平顶海山和采杞平顶海山之间的正断层的走向为西北向，倾向为西南向(图 7)。
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图 7 推测的采薇平顶海山群多级山顶平台之间的断裂位置Figure 7. Inferred faults between multi-tables of Caiwei Guyots5. 结论
(1) 利用全覆盖的多波束数据对采薇平顶海山群开展微地形地貌研究，发现海山群平坦的顶部发育多级山顶平台。
(2) 根据浅剖数据计算出了山顶平台的沉积物厚度，沉积物厚度差比山顶平台地形高差小，否定了沉积物厚度差是形成多级山顶平台地形的主要原因。
(3) 通过对比海山群多级平台地形特征与南海甘泉海台多级水下阶地特征，否定了海平面相对变化是形成海山群多级平台的主要原因。
(4) 推测采薇平顶海山群形成多级山顶平台可能为断裂构造成因，断裂为西北向正断层。
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