Holocene history of landscape instability in Iceland: Can we deconvolve the impacts of climate, volcanism and human activity?
Holocene history of landscape instability in Iceland: Can we deconvolve the impacts of climate, volcanism and human activity?
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DOI：
10.1016/j.quascirev.2020.106633
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发表时间：
2020-12
影响因子：
4
通讯作者：
Á. Geirsdóttir;David J. Harning;G. Miller;J. Andrews;Y. Zhong;C. Caseldine
中科院分区：
文献类型：
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作者：
Á. Geirsdóttir;David J. Harning;G. Miller;J. Andrews;Y. Zhong;C. Caseldine
来自冰岛湖泊沉积物的生物地球化学代理记录跟踪了北大西洋全新世气候的大规模变化，并突出了北大西洋和大气环流对冰岛气候和环境的影响。在全新世早期的温暖之后，百年尺度的气候变化叠加在千年尺度的变冷上，最终过渡到小冰期（公元1300-1900年）。虽然长期的冷却趋势可能是由地球轨道的变化和随之而来的下降，在北方半球（NH）夏季日照，百年尺度的变化已被链接到大西洋经向翻转环流（AMOC）的强度，火山活动加上海冰/海洋相关的反馈，大气变化的内部模式，并在太阳辐照度的变化。这些区域气候变化对冰岛的一个表现是土壤侵蚀加剧，导致生态系统和景观退化。近千年来，持续和严重的土壤侵蚀也与公元870年人类定居、人口快速增长、牲畜引进和火山灰为主的土壤固结性差等对环境的影响有关。湖泊代理复合记录表明，虽然事件主导的景观不稳定性和土壤侵蚀从早到中全新世可能引发的大型火山爆发，景观是能够恢复。然而，阈值达到1.5万年BP，导致状态变化，冰岛景观不再能完全恢复从冷事件和/或火山灰下降。在1.5kaBP，景观对气候的敏感性进一步增强。因此，广泛的和不可逆转的土壤侵蚀在冰岛定居前几个世纪就开始了，第二次加速是在公元1250年。使用CESM1.1进行的2 ka完全耦合气候瞬变模拟显示，在公元第一个千年，冰岛周围的夏季气温下降了0.5 °C，这与冰岛景观不稳定性和土壤侵蚀的增加一致。模型中持续夏季冷却的第二阶段发生在公元1150年之后，公元1450年之后冷却更强，在公元1850年后不久达到最大值，比模拟开始时低1.21 °C。我们的研究结果表明，区域气候和火山活动的自然变化可能是负责土壤侵蚀之前，人类的影响，这些过程的加剧后，特别是在与小冰期相关的冷却。鉴于本次审查得出的结论偏离冰岛人为土壤侵蚀历史的标准范式，研究应继续关注多学科的这一复杂问题。特别是，新兴的生物地球化学技术（如脂质生物标志物和古DNA）的组合可能是最好的准备测试和量化的自然环境变量和人类住区在冰岛土壤侵蚀的历史中的相对作用。
Biogeochemical proxy records from Icelandic lake sediment track large-scale shifts in North Atlantic Holocene climate and highlight the impact that North Atlantic Ocean- and atmospheric circulation has on Iceland’s climate and environment. Following Early Holocene warmth, centennial-scale climate change is superimposed on millennial-scale cooling, culminating in the transition to the Little Ice Age (∼1300–1900 CE). Although the long-term cooling trend is presumably driven by variations in Earth’s orbit and the concomitant decline in Northern Hemisphere (NH) summer insolation, the centennial-scale variability has been linked to the strength of the Atlantic Meridional Overturning Circulation (AMOC), volcanism coupled with sea ice/ocean related feedbacks, internal modes of atmospheric variability, and plausibly variations in solar irradiance. One manifestation of these regional climate changes on Iceland is the intensification of soil erosion, resulting in the degradation of ecosystems and landscape. In recent millennia, persistent and severe soil erosion has also been linked to human impact on the environment following the settlement ∼870 CE, rapid population growth, introduction of livestock and the poorly consolidated nature of tephra dominated soils. Lake proxy composite records suggest that although event-dominated landscape instability and soil erosion from the Early to Middle Holocene were likely triggered by large volcanic eruptions, the landscape was capable of recovering. However, a threshold was reached ∼5 ka BP, resulting in a state change whereby the Icelandic landscape could no longer fully recover from cold-events and/or tephra fall. Landscape sensitivity to climate further intensified at ∼1.5 ka BP as identified by regime shift analysis. Hence, widespread and irreversible soil erosion began several centuries before the acknowledged settlement of Iceland, with a second acceleration ∼1250 CE. A 2 ka fully coupled climate transient simulation using CESM1.1 shows a ∼0.5 °C reduction in summer temperature around Iceland in the first millennium CE, consistent with increased landscape instability and soil erosion in Iceland. A second phase of persistent summer cooling in the model occurs after 1150 CE, with stronger cooling after 1450 CE, reaching a maximum shortly after 1850 CE, ∼1 °C lower than at the start of the simulation. Our results suggest that natural variations in regional climate and volcanism are likely responsible for soil erosion prior to human impact, with intensification of these processes following settlement particularly during the cooling associated with the Little Ice Age. Given that the conclusions drawn in this review diverge from the standard paradigm of human-induced soil erosion history in Iceland, research should continue to focus on this complex question from multiple disciplines. In particular, a combination of emerging biogeochemical techniques (e.g. lipid biomarkers and ancient DNA) may be best poised to test and quantify the relative roles of natural environmental variables and human settlement in the history of soil erosion on Iceland.