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A calibrated climate record from Gibraltar speleothem: the instrumental era, the Holocene and the last interglacial

直布罗陀洞穴的校准气候记录：器乐时代、全新世和最后一次间冰期

基本信息

批准号：
NE/G007292/1
负责人：
David Mattey
金额：
$ 44.36万
依托单位：
Royal Holloway University of London
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2009
资助国家：
英国
起止时间：
2009 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FG007292%2F1
关键词：
calibrated climate record Gibraltar speleothem

项目摘要

Speleothems (stalagmites and flowstones) are carbonate minerals deposited from dripping water in caves. They represent rich archives of past rainfall and climate change. Speleothem growth layers can be dated at very high precision and the ratio of oxygen isotopes (d18O) in particular can be related to climatic conditions such as sources of moisture, temperature and amount of rainfall above each cave. These oxygen isotope records give much insight into climate change but interpretation still tends to rest on simple assumptions without detailed understanding of which features of local climate the speleothem responds to, and whether the recording is biased or altered during the process of speleothem growth. Unlocking the real climatic information needs understanding of the full system of climate - vegetation - groundwater - cave - speleothem. Thus climate reconstruction from speleothem records depends on two critical steps. The first is close monitoring of the cave environment to identify speleothem sites which record rainwater d18O with good fidelity, and the second is formulating a 'calibration' relating d18O to other aspects of climate. We aim to accomplish both steps and develop a 'fully-calibrated' climate record from Gibraltar covering parts of the last 200,000 years. Our recent work in New St Michaels Cave, located high up in the Rock of Gibraltar, focused on the first step - understanding the climate recording process by monitoring the part of the system between the soil and the speleothem, using comprehensive measurements and novel instruments. Working closely with the Gibraltar Caving Group we tracked monthly and seasonal variations in chemistry and oxygen isotopes in cave drips and soil water, plus the movement and composition of cave air, temperatures, soil and vegetation changes, and related these to amounts of rainfall and its daily isotopic composition. From all this we can deduce the causes of the seasonal isotopic and chemical cycles we have found in a recently deposited stalagmite, and relate relevant parts of the seasonal signal to the isotopic composition of rainfall during the winter months. A key result is that there are specific conditions when speleothems record climate most accurately, and if these are met the isotopic composition of rainfall can be reconstructed. In our new work we shall examine the second step - how the modern part of Gibraltar's historic climate record can be used to derive a calibrated 'transfer function' relating d18O to climate. We intend to test this calibration using stalagmite formed before 1962, when isotope records for rainfall began, by comparing the d18O in speleothem with the value calculated from statistical analysis of the historic climate back to 1792. Furthermore, because Gibraltar has been slowly uplifted by geological forces, caves have been elevated as speleothem grew. We shall investigate how this has affected the d18O-climate 'transfer function' and how the calibration from modern records may have to be adapted for application in more ancient times. To achieve this last objective we must compare the microclimate, air and water chemistry of low and high level caves to understand the controls on speleothem isotopes at different altitude. We shall then date and analyse stalagmites formed since the last interglacial for their isotope ratios, trace elements and water trapped within the mineral, which directly fingerprints ancient cave water. This should allow us to reconstruct the average tracks of rain-bearing systems in the Atlantic. Our overall objective is to produce a well-dated, calibrated record that will be a yardstick for past climates in the western Mediterranean, and can be compared directly with computer climate models. Gibraltar's caves have very high potential for providing a record from a 'natural laboratory' environment running back from modern times to possibly one million years ago.

洞穴(石笋和流石)是洞穴中滴水沉积的碳酸盐矿物。它们代表了过去降雨和气候变化的丰富档案。洞穴生长层的测年精度非常高，氧同位素比率(D18O)尤其与气候条件有关，如每个洞穴上方的湿度来源、温度和降雨量。这些氧同位素记录提供了许多关于气候变化的见解，但解释仍然倾向于建立在简单的假设之上，而不是详细了解洞穴群对当地气候的哪些特征做出反应，以及记录在洞穴群生长过程中是否存在偏见或改变。解开真正的气候信息需要了解气候-植被-地下水-洞穴的完整系统。因此，根据洞穴群记录重建气候取决于两个关键步骤。第一个是对洞穴环境的密切监测，以确定记录雨水d18O的洞穴遗址，并以良好的保真度记录，第二个是制定一个将d18O与气候的其他方面联系起来的“校准”。我们的目标是完成这两个步骤，并开发一份覆盖过去20万年部分时间的直布罗陀气候记录。我们最近在位于直布罗陀岩石高处的新圣迈克尔斯洞进行的工作，重点放在第一步--利用全面的测量和新的仪器，通过监测土壤和洞穴之间的系统部分，了解气候记录过程。与直布罗陀洞穴小组密切合作，我们跟踪了洞穴滴水和土壤水中化学和氧同位素的月度和季节变化，以及洞穴空气的运动和组成、温度、土壤和植被的变化，并将这些与降雨量及其每日同位素组成联系起来。由此，我们可以推断出在最近沉积的石笋中发现的季节性同位素和化学循环的原因，并将季节性信号的相关部分与冬季降雨量的同位素组成联系起来。一个关键的结果是，当洞穴记录最准确地记录气候时，存在特定的条件，如果满足这些条件，就可以重建降雨的同位素组成。在我们的新工作中，我们将研究第二步--如何利用直布罗陀历史气候记录的现代部分来推导出与气候有关的经校准的d18O‘传递函数’。我们打算使用1962年前形成的石笋来测试这种校准，1962年前形成的石笋开始记录降雨的同位素，通过比较洞穴中的d18O与1792年历史气候的统计分析计算出的值。此外，由于直布罗陀被地质力缓慢抬升，洞穴随着洞穴的生长而上升。我们将调查这如何影响d18O气候‘传递函数’，以及来自现代记录的校准可能必须适应更古老的时代的应用。为了实现这最后一个目标，我们必须比较低层洞穴和高层洞穴的小气候、空气和水的化学成分，以了解不同海拔对洞穴同位素的控制。然后，我们将对自上一次间冰期以来形成的石笋进行年代测定和分析，以确定其同位素比率、微量元素和矿物中的水，这些矿物直接确定了古代洞穴水的指纹。这将使我们能够重建大西洋降雨系统的平均轨迹。我们的总体目标是制作一份日期准确、经过校准的记录，作为地中海西部过去气候的标尺，并可以直接与计算机气候模型进行比较。直布罗陀的洞穴具有很高的潜力，可以提供从现代到可能100万年前的自然实验室环境的记录。