Orbital forcing of the late Pleistocene boreal summer monsoon: Links to North Atlantic cold events and El Nino; Southern Oscillation. Geologica Ultraiectina (313)
Orbital forcing of the late Pleistocene boreal summer monsoon: Links to North Atlantic cold events and El Nino; Southern Oscillation. Geologica Ultraiectina (313)
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发表时间：
2005-07
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通讯作者：
M. Ziegler
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作者：
M. Ziegler
本论文围绕北方夏季风中与进动相关的变化的时间以及北大西洋寒冷事件和厄尔尼诺南方涛动对此时间的影响展开。瞬态气候模拟实验表明，北半球夏季季风的强度与轨道时间尺度上的夏季日照峰值同相变化。相比之下，海洋代理记录表明季风最大值的响应滞后于日照强迫几千年。在阿拉伯海的记录中，这种相位滞后特别大。本论文表明，源自地中海腐泥记录的非洲季风约 3,000 年的滞后不仅与海洋同位素年代学一致，而且与地中海地区和中国的辐射测年洞穴记录以及南极洲的大气甲烷记录一致。有人认为，北大西洋寒冷事件系统地推迟了强季风强度的到来。这一解释意味着北大西洋冰漂流存在明显的岁差强迫，也可以解释过去 300 万年非洲季风的非平稳行为。过去两个冰川周期中北大西洋有孔虫底栖氧同位素堆积记录的轨道调谐独立年代学支持了这一理论，并表明北大西洋冰涌事件对外部气候触发机制作出反应，这与导致大冰盖增减的过程不同。与轨道时间尺度上的厄尔尼诺南方涛动气候模型研究的比较表明，冰涌事件与厄尔尼诺事件频繁发生的时期相一致。厄尔尼诺事件期间北美北部变暖可能导致融水填充地表裂缝而导致边缘冰架迅速崩解，从而引发大规模的冰涌事件。此外，有人提出，阿拉伯海生产力记录产生的长相位滞后与亚洲夏季风和阿拉伯海生产力在进动频段的脱钩有关。大西洋经向翻转的强度可能主导阿拉伯海的进动信号。根据海洋模型，大西洋翻转的增加导致向北印度洋的富光层输送的养分增加，从而引发高生产力和氧气最小区最大值。建议采用相同的过程来解释海洋同位素阶段 13 期间异常高的生产力条件。在论文的最后，介绍了过去 30 万年墨西哥湾的海面温度记录。该记录的变化受到大西洋暖池北边界迁移的控制，并且对冬季条件相对不敏感。由于记录中缺乏快速气候变化事件（例如新仙女木期寒冷事件）的明确特征，因此得出的结论是，高纬度寒冷事件仅影响加勒比地区的冬季气候条件，这支持了突然降温事件期间北半球极端季节性的假设。
This thesis revolves about the timing of precession-related variations in the boreal summer monsoon and the impact of North Atlantic cold events and the El Nino Southern Oscillation on this timing. Transient climate modelling experiments indicate that the intensity of the Northern Hemisphere summer monsoon varies in-phase with peak summer insolation on orbital timescales. In contrast, marine proxy records suggest a lagged response of the monsoon maximum of a few thousand years behind the insolation forcing. This phase lag is particularly large in records from the Arabian Sea. In this thesis it is shown that a ~3,000 year lag of the African monsoon, which is derived from the Mediterranean sapropel record, is consistent not only with the marine isotope chronology but also with radiometrically dated speleothem records from the Mediterranean region and China, and with the atmospheric methane record from Antarctica. It is suggested that North Atlantic cold events systematically delay the onset of strong monsoon intensity. This explanation implies a distinct precession forcing of North Atlantic ice rafting and could also explain a non-stationary behaviour of the African Monsoon over the past 3 million years. An orbital-tuning independent chronology for a North Atlantic foraminiferal benthic oxygen isotope stacked record over the last two glacial cycles supports this theory and suggests that North Atlantic ice surge events respond to an external, climatic triggering mechanism, which is distinct from the process that causes the waxing and waning of the large ice sheets. A comparison with a climate modelling study of the El Nino Southern Oscillation on orbital time scales shows that the ice surge events coincide with periods of frequent El Nino events. Warming of northern North America during El Nino events could have led to rapid disintegrations of fringing ice shelves induced by meltwater infilling of surface crevasses, thereby triggering large-scale ice surge events. It is moreover suggested, that the long phase-lag that is derived from Arabian Sea productivity records is related to a decoupling of the Asian summer monsoon and Arabian Sea productivity at the precession frequency band. The intensity of the meridional overturning in the Atlantic potentially dominates the precession signal in the Arabian Sea. According to ocean modelling, increased Atlantic overturning results in an increased nutrient delivery into the euphotic layer of the northern Indian Ocean and triggers thereby high productivity and oxygen minimum zone maxima. The same process is suggested as explanation for anomalously high productivity conditions during marine isotope stage 13. To the end of the thesis a sea surface temperature record from the Gulf of Mexico over the last 300,000 years is presented. Variations in this record are controlled by the migration of the northern boundary of the Atlantic Warm Pool and are relatively insensitive to winter conditions. Because a clear signature of rapid climate-change events, such as the Younger Dryas cold event, is lacking in the record, it is concluded that high-latitude cold events influence only the winter Caribbean climate conditions, supporting the hypothesis of extreme northern-hemisphere seasonality during abrupt cooling events.