Knowledge Resource Center for Ecological Environment in Arid Area
| 藏东南然乌冰缘黄土的发育模式及其古气候意义 | |
| 其他题名 | Proglacial loess in Ranwu, southeastern Tibet: Formation model and paleoclimatic implications |
| 张继峰 | |
| 出版年 | 2014 |
| 学位类型 | 博士 |
| 导师 | 冯金良 |
| 学位授予单位 | 中国科学院大学 |
| 中文摘要 | 冰川和风成沉积物在青藏高原分布广泛。虽然在北半球的冰缘区，冰川作用对黄土的形成发挥了重要作用，然而关于青藏高原的黄土及其它风成沉积与冰川变化的关系的研究还较少，靠近冰川的冰缘黄土很少被报道，其发育模式和古气候意义尚不清楚。本文以藏东南然乌地区的来古冰川边缘的全新世黄土为研究对象，通过年代学和多指标研究，探讨该地区冰缘黄土的发育模式，并尝试提取冰川变化，以及区域、局地气候变化信息。 在来古冰川的深切冰川谷中，分布着小规模的冰缘黄土。粒度、微量元素、黄土分布特征及现代沙尘暴观察揭示，黄土的主要物源为冰水沉积物，其搬运动力为局地冰川风。光释光（OSL）和AMS 14C证据揭示，黄土从~6.6 ka开始堆积，并一直持续到现代。对比揭示，然乌黄土的起源可能与“新冰期”的冰川变化有关。6.6 ka以后，该地区的冰川可能进入“新冰期”阶段，来古冰川前进并维持较大的规模，冰川风足够强大，局地沙尘暴开始发生，冰缘黄土开始堆积。 由于冰川规模决定冰川风强弱及黄土沉积区与源区的距离，因此本文用黄土粗颗粒含量指示冰川规模。6.6-5.8 ka，粗颗粒含量高，指示冰川前进。5.8-2.9 ka，粗颗粒含量维持较低值，指示冰川退缩，但冰川规模依然能够维持稳定的冰川风。2.9 ka以后，粗颗粒含量逐渐增加，并伴随3次短期波动，这说明：长尺度上，冰川逐步扩张，并在小冰期（LIA）达到最大规模；百年尺度上，2.5 ka（Iron Age Cold Epoch）、1.3 ka（Dark Ages）和0.5 ka（LIA）左右发生了冰进，3 ka（Bronze Age Optimum）、2 ka（Roman Age Optimum）、1 ka（Medieval Warm Period）左右出现冰退。对比显示，6.6-5.8 ka的冰进可能是温度和冬季降水共同作用的结果。来古冰川2.9 ka以来的冰进与青藏高原及欧洲部分冰川的冰进期一致，同时对应青藏高原、中国、北半球的冷期以及印度季风的减弱期。这说明，至少过去2.9 ka的百年尺度上，青藏高原温度变化与中国其它地区及北半球同步，高原部分冰川对温度变化的敏感性比对季风降水的敏感性更高。全球变暖背景下，即使部分地区季风降水增加，冰川在长尺度上有可能会进一步退缩。 RWS2剖面的TOC、磁化率等成土作用指标揭示，然乌河谷在6.6-6 ka、2.9-0 ka气候干旱，河谷中发育黄土。对比揭示，印度季风强度变化是冰缘河谷干湿变化的重要控制因素之一。C/N、δ15N、δ13C分析揭示，黄土有机质分解，以及C3、C4植被的比例变化，在长尺度上受控于太阳辐射调控的印度季风强度变化，而在百年尺度上，受到温度和干湿度变化共同影响。叶蜡正构烷烃及其δD的分析揭示，该地区陆生叶蜡正构烷烃分布不能反映植被类型变化。长链正构烷烃δD在长尺度上受控于干湿变化，而短尺度上受温度、干湿状况等多因素控制。LIA时期气候冷干，MWP气候暖湿，这两个时期的温度变化对然乌地区陆生正构烷烃δD的影响占主导。 RWS2剖面的孢粉记录揭示，6.6 ka以来，然乌地区经历了剧烈的植被变化。6.6-5.8 ka：区域植被以森林草原为主；乔木花粉含量快速降低，指示降温迅速。5.8-2.9 ka：区域植被以蒿属草原为主，气候温干。2.9-0 ka：类似于现代的区域植被格局开始形成；莎草和藜科逐渐占优势，指示温度继续降低；高海拔地区可能气候湿润，发育高山草甸，而冰缘河谷可能干旱，发育荒漠草原植被。 RWS2剖面的多指标分析揭示，冰缘河谷的局地气候在黄土发育期（6.6-6 ka、2.9-0 ka）干旱，古土壤发育期（6-2.9 ka）湿润。然而，乔木、蒿属和莎草科等花粉的含量变化指示，黄土发育期然乌冰缘区的部分区域湿润，古土壤发育期干旱。季风或温度变化都无法解释这种区域内部气候演化分异现象，本研究认为然乌地区的冰川变化可能是区域内部分异的主控因素。6.6-5.8 ka和2.9-0 ka，区域较湿润，但由于冰川前进，冰川风加强，冰缘河谷的土壤蒸发会加强，同时来自于低海拔河谷的由藏东南水汽通道传输的水汽被冰川风阻挡，这共同导致了冰缘河谷的迅速变干。 总之，本文用冰缘黄土的粒度重建了然乌地区~6.6ka的冰川波动，用成土作用指标、有机质的δ15N及δ13C、叶蜡正构烷烃及δD、孢粉重建了然乌冰缘河谷局地、区域气候演化。研究揭示，然乌地区过去~6.6 ka的冰川波动主要受太阳辐射、北半球温度及冬季降水等因素控制。冰川变化是导致然乌地区区域内部气候演化分异的主控因素。由于地形、季风演化、冰川变化、温度控制的相对湿度变化等复杂因素影响，青藏高原某些区域内部及不同区域的古气候演化特征可能不一致。因此，目前青藏高原还不存在一个统一的全新世气候演化模式。 |
| 英文摘要 | Glaciers and aeolian sediments are widespread on the Tibetan Plateau. Although glaciations have played an important role in the formation of loess in many glaciated regions in the Northern Hemisphere, the proglacial loess on the Tibetan Plateau is rarely reported, and its origin, source and paleoclimatic implications remain unclear.We have studied the proglacial loess deposits in the Ranwu valley, southeastern Tibet, aiming to reconstruct paleoglacier fluctuations and local and regional paleoclimatic changes. A small area of proglacial loess deposits were found in the Ranwu valley, southeastern Tibet. Grain-size, geochemical and geomorphologic evidences, as well as observations of local dust storms, reveal that the outwash sediments of the Laigu glacier (~30 km long) are the primary source for the local loess.We also found that the glacier wind from this glacier is the main transport vector, and thus speculate that the onsetof the Ranwu loess, at ca. 6.6 ka, correlates with the early Neoglacial advance of the Laigu glacier, which generated glacier winds and local dust storms in the deep-incised glacial valley. Because glacial extent determines the strength of glacier wind and the source-to-sink distance, this study attributes the grain size variations of the proglacial loess to the fluctuations of Laigu Glacier. The higher content of coarse grains represents larger glacier extent. Grain size evidences reveal that Laigu Glacier advanced during 6.6-5.8 ka, retreated during 5.8-2.9 ka, and re-advanced after 2.9 ka culminating at the Little Ice Age (LIA). Three cycles of millennial-scale glacier fluctuations were also recorded, with glacier advance events centered at 2.5 ka (Iron Age Cold Epoch), 1.3 ka (Dark Ages) and 0.5 ka (LIA), and with glacier retreat events centered at 3 ka (Bronze Age Optimum), 2 ka (Roman Age Optimum) and 1 ka (Medieval Warm Period). Comparisons with other paleoclimatic records reveal that the glacier advance event during 6.6-5.8 ka may link with low temperature and intensified winter precipitation, and the glacial fluctuations in last 2.9 ka may be modulated by regional temperature changes. The TOC, magnetic susceptibility and other pedogenic proxies indicate that the Ranwu glacial valley is dry during 6.6-6 ka and 2.9-0 ka, when loess rather than paleosol formed in the proglacial valley. Comparisons with other paleocliamtic records show that the intensity changes of the Indian Summer Monsoon were the main driver of the humidity changes in the Ranwu valley. C/N, δ15N and δ13C show that the decomposition of organic matters in Ranwu loess and changes of C3/C4 were controlled by Indian Summer Monsoon intensity changes in orbital scale, and by temperature and humidity in millennial scale. This study also shows that the n-alkanes distribution of the Ranwu loess cannot reflect the vegetation type changes. Comparisons with other proxies reveal that the δD of long-chain n-alkanes was modulated by the humidity of the proglacial valley in long time scale, and by temperature and humidity in short time scale. During the LIA and MWP, the δD of the long-chain n-alkanes mainly reflected temperature signals. The pollen spectra of the RWS2 section reveals that dramatic vegetation changes have occurred in the Ranwu region in past 6.6 ka. Forest steppe dominates the period at 6.6-5.8 ka, suggesting a warm-wet climate in the Ranwu region. During the 5.8-2.9 ka, the forests were replaced by Artemisia steppe, suggesting that the regional climate was temperate and dry. After 2.9 ka, the Artemisia withdrew and the Cyperaceae and Chenopodiaceae dominated this region. This indicates that the vegetation distribution pattern like today in Ranwu has formed since 2.9 ka, when the vegetation of high-elevatonal area in this region has changed into alpine meadow and the desertic steppe dominated the proglacial valley. Muti-proxies of the RWS2 section indicate that the proglacial valley was dry at 6.6-6 ka and 2.9-0 ka, when loess layers |
| 中文关键词 | 青藏高原 ; 冰缘黄土 ; 古土壤 ; 全新世 ; 冰川 ; 印度季风 |
| 英文关键词 | Tibetan Plateau Proglacial loess Paleosol Holocene Glacier Indian Summer Monsoon |
| 语种 | 中文 |
| 国家 | 中国 |
| 来源学科分类 | 自然地理学 |
| 来源机构 | 中国科学院青藏高原研究所 |
| 资源类型 | 学位论文 |
| 条目标识符 | http://119.78.100.177/qdio/handle/2XILL650/287412 |
| 推荐引用方式 GB/T 7714 | 张继峰. 藏东南然乌冰缘黄土的发育模式及其古气候意义[D]. 中国科学院大学,2014. |
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