Reconstruction of Pliocene-Middle Pleistocene evolution of Siberian permafrost using U-Pb dating of speleothems

北半球2300万平方公里的土地--占土地总面积的四分之一--是永久冻土。这块永久冻结的土地储存的碳是大气中碳含量的两倍，其中很大一部分碳是甲烷。因此，永久冻土的形成和融化是气候系统中一个重要的积极反馈，在地球变冷时消除温室气体，并在诸如今天这样的地球变暖的时期释放温室气体。多年冻土对生态系统和生物多样性也有很强的控制作用，在许多高纬度环境中，它支撑着人类的基础设施(建筑物和交通线路)。对现代环境中活跃的永久冻土过程的大量研究已经存在(并在继续)，但评估永久冻土的长期行为已被证明更加困难。我们还不清楚高纬度大陆地区的温度如何随着时间的推移对全球气候变化做出反应，也不清楚不同气候状态下的永久冻土程度。这些信息对今天永久冻土地区的未来规划以及我们对高纬度碳和气候系统的总体理解都很重要。北半球的主要永久冻土区如何应对全球气候变化，如轨道变化或地球在更新世-更新世期间的逐渐降温？而永久冻土层在通过其对气候的碳反馈放大这些变化方面可能起到什么作用？在这里，我们建议使用在洞穴(洞穴)中形成的碳酸盐来评估世界上最大的永久冻土区域-西伯利亚的永久冻土范围。洞穴需要水才能形成，所以当地面常年结冰时，就不会生长。因此，洞穴的存在或不存在限制了多年冻土的范围。我们一直在研究三个洞穴，这些洞穴从北纬52度的伊尔库茨克附近无冻土区的现代边缘向北延伸，穿过斑驳的永久冻土，一直延伸到北纬60度的连续冻土边缘。这项工作详细地重建了过去450ka期间的永久冻土史，显示了南方每一个温暖的间冰期永久冻土的融化。在北部(60oN)，除了390-430ka前的间冰期外，永久冻土保持稳定，当时全球条件比现在更温暖。我们建议在这些洞穴中继续恢复U-Th测年方法~500ka范围以外的多年冻土史，并在64oN连续多年冻土区的中心增加第四个洞穴。使用新证实的U-Pb测年能力，我们将测定上新世-更新世期间洞穴群落的生长时间，以评估在上新世温暖后地球冷却的时间，即西伯利亚开始形成永久冻土条件的时间。我们将在更新世多变的气候中限制永久冻土的变化程度。通过将这些记录与其他地方的气候信息进行比较，我们将了解高纬度北部大陆如何应对全球气候变化，特别是在比今天更温暖的时期。要了解每个地点的洞穴温度与洞穴上方的年平均温度之间的关系，需要在我们的研究洞穴中开展一项监测活动。我们将与俄罗斯科学院和久负盛名的西伯利亚洞穴社区的俄罗斯同事密切合作，开展这项工作。我们还将利用我们在俄罗斯的关系，确保我们了解到的新信息被提供给未来将受到永久冻土变化影响的地区的利益相关者。

Twenty three million square kilometres of northern-hemisphere land - one quarter of the total land area - is permafrost. This permanently frozen ground stores twice as much carbon as the atmosphere contains, with a significant fraction of this carbon as methane. Formation and thawing of permafrost is therefore a significant positive feedback in the climate system, removing greenhouse gas as Earth cools, and releasing it, in periods such as today, when the planet is warming. Permafrost also exerts a strong control on ecosystems and biodiversity, and it underpins human infrastructure (buildings and transport links) in many high-latitude settings.A significant body of research exists (and continues) into active permafrost processes in the modern environment, but assessing the long-term behaviour of permafrost has proved more difficult. We do not yet have a clear idea of how the temperature of high-latitude continental regions responds to changing of global climates through time, nor of the extent of permafrost in different climate states. Such information is important for future planning in today's permafrost regions, and for our general understanding of high-latitude carbon and climate systems. How do the major permafrost regions of the northern hemisphere respond to global climate change such as orbital variation or the progressive cooling of the planet during the Plio-Pleistocene? And what role might permafrost have in these amplifying these changes through its carbon feedbacks on climate? Here we propose to use carbonates formed in caves (speleothems) to assess the extent of permafrost in the world's largest area of permafrost - Siberia. Speleothems require water to form so, when the ground is frozen year-round, do not grow. The presence or absence of speleothems therefore constrains the extent of permafrost through time. We have been working on a sequence of three caves which stretch from the modern edge of the permafrost-free zone near Irkutsk at 52oN, northwards through patchy permafrost and to the edge of continuous permafrost at 60oN. This work has yielded a detailed reconstruction of the permafrost history during the last 450 ka, showing thawing of the permafrost in each warm interglacial period in the south. In the north (60oN) the permafrost remained stable except during the interglacial period 390-430 ka ago when global conditions were warmer than present. We propose to continue the reconstruction of the permafrost history beyond the ~500 ka limit of the U-Th dating method in these caves, and to add a fourth cave in the centre of the continuous permafrost region at 64oN. Using a newly proven U-Pb dating ability, we will date periods of speleothem growth during the Plio-Pleistocne to assess the time, as the planet cooled after the warmth of the Pliocene, that permafrost conditions initiated in Siberia. And we will constrain the changing extent of permafrost during the variable climates of the Pleistocene. By comparing these records with information about climate elsewhere, we will learn how the high latitude northern continents respond to global climate change, particularly during periods warmer than today.To understand how the cave temperatures in each location related to annual mean temperatures above the caves will require a campaign of monitoring in our study caves. We will conduct this work in close collaboration with Russian colleagues from the Russian Academy of Sciences and the well-established Siberian caving community. We will also use our connections in Russia to ensure that new information we learn is provided to stakeholders in regions that will be impacted by changing permafrost in the future.