海洋地质与环境重点实验室知识库

摘  要

大陆边缘盆地是大陆风化剥蚀产物的主要沉积汇，是揭示风化-碳埋藏过程和机制的天然实验室。本研究基于南海东北部台西南盆地TWS-1和珠江口ZK20岩心沉积物的AMS¹⁴C测年、陆源物质堆积速率、粘土矿物组成和结晶学特征、粒度参数、元素地球化学和有机碳-总氮含量和稳定碳同位素等的分析，揭示了末次冰期23 ka BP以来以来台西南盆地沉积物源、硅酸盐风化和陆源有机碳埋藏的演变历史及驱动机制。

通过对TWS-1站位沉积物Sr-Nd同位素组成和粘土矿物学特征进行分析，并结合浅地层剖面中观测到的埋藏古河道和海底峡谷，明确了台湾是研究站位末次冰期和全新世沉积物的主要物源。基于K/Al、高岭石/伊利石和Rb/Sr等风化指标，重建了23 ka BP以来TWS-1岩心记录的源区化学风化历史，揭示冰期时化学风化强度明显高于全新世，暗示冰期低海平面时期陆架沉积物发生了再次风化。末次冰期期间，海平面与现在相比下降最大约120 m，陆架广泛暴露。南海北部地处热带，其冰期适宜的温度和降水及充分的风化时间，使得暴露陆架上的松散沉积物发生再次风化。因此，证实冰期热带陆架风化是冰期-间冰期碳循环一个被忽略的重要机制。

研究站位的有机碳-总氮含量及有机碳同位素特征与潜在物源对比表明，台湾是研究区陆源有机碳的主要物源，相对海源其贡献比例约为58%，陆源物质可能主要通过海底峡谷水道和低海平面时期陆架河流输入。重建的陆源有机碳通量在末次冰消期早期（19-13 ka BP）和中全新世（7-4 ka BP）期间有两个峰值，分别约0.16 g/cm²/ky和0.09 g/cm²/ky。综合分析表明，二者分别受控于冰期低海平面时期增强的陆架风化剥蚀和全新世季风强盛期降水驱动的台湾大陆剥蚀。我们的工作暗示冰期-间冰期循环中海平面和季风分别驱动的大陆边缘有机碳埋藏可能对全球碳循环和大气CO₂浓度演变有重要影响。

Abstract

The continental margin basin is the main sedimentary sink of the products of continental weathering and erosion, and it is a natural laboratory to reveal the process and mechanism of weathering-carbon burial. This study is based on the analysis of AMS¹⁴C dating, terrigenous material accumulation rate, clay mineral composition and crystallographic characteristics, grain size parameters, elemental geochemical parameters, total organic carbon-total nitrogen content and stable carbon isotopes in TWS-1 core located in Taixinan Basin of the northeastern South China Sea and ZK20 core located in Pearl River Estuary. The sediment source, silicate weathering history and the history and force mechanism of terrigenous organic carbon burial in the Taixinan Basin since 23 ka BP during the last glacial period were revealed.

Based on the comprehensive analysis of the Sr-Nd isotopic composition, clay mineral composition, and the buried ancient river channels and submarine canyons observed in the shallow stratum profile, Taiwan was considered as the main source area of the TWS-1 core during the last glacial period and Holocene. Based on the analysis of the weathering proxy records such as K/Al, kaolinite/illite and Rb/Sr, the chemical weathering history of the sediments in TWS-1 core since 23 ka BP was reconstructed. They all show that the chemical weathering intensity experienced by the sediments in the TWS-1 core during the last glacial period is significantly higher than that in Holocene, which imply that the unconsolidated continental shelf sediments weathered again during glacial sea level low-stands. During the last glacial period, the sea level fell up to 120 m compared to the present, and the continental shelf was widely exposed. The northern South China Sea is located in the tropics. It has favorable silicate weathering conditions such as moderate temperature, suitable precipitation and sufficient duration of silicate chemical weathering during the glacial periods, which makes the unconsolidated sediments on the exposed continental shelf weathered again. Thus, it is proved that the glacial tropical continental shelf weathering is an important neglected mechanism during the glacial-interglacial carbon cycle.

Comparison with potential source end-elements shows that Taiwan Island was the main source of terrigenous organic carbon to the study site, contributing 58% of the total. The terrigenous materials were mainly transported through submarine canyon channels and shelf rivers during the sea level low-stands. The reconstructed terrigenous organic carbon flux shows two peaks of about 0.16 g/cm²/ky during the early last deglaciation (19-13 ka BP) and 0.09 g/cm²/ky during the middle Holocene (7-4 ka BP). The comprehensive analysis showed that the two peaks were controlled by the enhanced weathering and erosion of the continental shelf during the glacial sea level low-stands and the intensified erosion in ancient Taiwan Island driven by precipitation during the Holocene maximum monsoon period, respectively. Therefore, we believe that the burial of organic carbon in continental margin driven by sea level change and monsoon during the glacial-interglacial cycle may have considerable effects on the global carbon cycle and the evolution of atmospheric CO₂ concentration.

目  录

第1章  绪论...................................................................... 1

1.1  选题意义.............................................................................. 1

1.2  国内外研究现状.................................................................. 2

第2章  区域地质概况...................................................... 6

2.1  研究区位置及地质背景...................................................... 6

2.2  研究区水动力特征.............................................................. 8

第3章  材料与方法........................................................ 10

3.1  研究材料............................................................................ 10

3.1.1  台西南盆地TWS-1岩心........................................ 10

3.1.2  珠江口ZK20岩心................................................... 11

3.1.3  台湾浊水溪口JRD-S岩心..................................... 12

3.2  实验方法............................................................................ 12

3.2.1  AMS¹⁴C测年........................................................... 12

3.2.2  粘土矿物分析.......................................................... 13

3.2.3  有机碳-总氮含量和有机碳同位素分析................. 16

3.2.4  陆源通量分析.......................................................... 18

3.2.5  粒度分析.................................................................. 18

3.2.6  常微量元素分析...................................................... 19

3.2.7  Sr-Nd-Pb同位素分析.............................................. 21

第4章  结果.................................................................... 22

4.1  AMS¹⁴C测年..................................................................... 22

4.2  粘土矿物............................................................................ 23

4.2.1  TWS-1孔粘土矿物组成......................................... 23

4.2.2  ZK20孔粘土矿物组成............................................ 25

4.2.3  JRD-S孔粘土矿物组成.......................................... 26

4.3  常微量元素组成................................................................ 27

4.3.1  常量元素组成.......................................................... 27

4.3.2  微量元素组成.......................................................... 29

4.4  Sr-Nd-Pb同位素................................................................ 30

4.5  粒度.................................................................................... 31

4.6  陆源物质堆积速率............................................................ 32

4.7  有机碳-总氮含量和碳同位素........................................... 34

第5章  讨论.................................................................... 35

5.1  末次冰期以来潜在源区物源和风化历史........................ 35

5.1.1  全新世珠江物源和风化历史.................................. 36

5.1.2  末次冰期以来台湾浊水溪风化历史...................... 37

5.2  台西南盆地TWS-1孔沉积物源...................................... 38

5.2.1  粘土矿物约束物源.................................................. 38

5.2.2  Sr-Nd同位素约束物源........................................... 39

5.2.3  台西南盆地沉积物输送方式.................................. 41

5.3  末次冰期以来南海东北部陆架风化记录...................... ..42

5.4  TWS-1孔陆源有机碳埋藏通量演变............................... 45

5.4.1  陆源有机碳来源...................................................... 45

5.4.2  陆源有机碳埋藏通量的演变及驱动机制.............. 51

第6章  结论.................................................................... 55

参考文献............................................................................ 57

致  谢................................................................................ 70

作者简历及攻读学位期间发表的学术论文与研究成果 71