The Gibraltar Archive: a half million year reference record of rainfall isotopes in the western Mediterranean

直布罗陀参考记录将是对研究地球过去气候的重要贡献，这是一个本质上困难的课题，因为关于过去气候的资料必须从间接证据中推断出来。我们将使用直布罗陀洞穴中的洞穴石，主要是方解石、石笋和水滴沉淀形成的流石。它们的化学成分反映了气候，每个标本提供了一个分层的记录，可以覆盖从几十年到数万年的任何时期。为了建立一个更长的记录，必须准确地确定多个标本的年代，以便将重叠的部分放在一起形成一个连续的序列。年代测定依赖于自标本形成以来微量铀对其子元素钍的放射性衰变。对于每一个洞穴，我们都要确定最古老和最年轻的地层以及中间的几个地层的年代，确定任何时间间隔，并建立一个年龄模型，将其与其他标本联系起来。我们已经收集了24个洞穴主题的档案，但还需要200多个日期才能将它们形成完整的合成记录。我们的首要目标是获得这些日期。我们的第二个目标是对每一层进行化学分析，并根据过去50万年来直布罗陀气候的变化来解释结果。因此矿物化学代表了真正的气候。这就提出了两个问题——哪些化学变量是气候的信号，每一个化学变量反映了气候的哪些方面？我们将测量d18O和d13C——氧和碳元素中不同类型原子的比例——以及Mg、Sr、Ba、Y和p的浓度。众所周知，这些都部分受气候控制，但它们也受到当地因素的影响，如土壤和岩石中的水流，或洞穴空气中的二氧化碳水平。我们以前在直布罗陀的工作通过监测现代环境长达10年，将当地和气候的影响分开。我们发现，每年沉积物中的d18O跟踪雨水中的d18O。然而，我们现在要分析的洞穴主题是在与今天不同的气候条件下形成的，所以我们必须利用化学原理和洞穴监测的见解，从每个标本生长过程中化学变量之间的变化关系中推断气候的影响。在冰河时代的时间尺度上，温度对d18O的影响与降雨一样大，为了考虑到这一点，我们将使用海洋表面温度的独立记录，假设洞穴温度跟踪周围的海洋。这样，我们就可以从洞穴中复杂的化学成分中分离出降雨中d18O变化的信号。从直布罗陀及其洞穴开始，降雨d18O在欧洲、地中海和中东的变化模式反映了大气环流和含雨空气的运输。直布罗陀位于地中海和大西洋之间，前者是从北非到中亚的冬季雨水来源，后者是欧洲的主要湿气来源。通过将我们的d18O记录与以色列现有的洞穴记录进行比较，我们将重建从几十年到冰河时代的所有时间尺度上的气候变化对地中海水蒸气的吸收。同样令人感兴趣的是，在上一个冰期反复发生的千年尺度的变化，这些变化被记录在贯穿格陵兰冰盖的冰芯中，作为d18O较高的时期。它们出现在直布罗陀的洞穴中，使我们能够从大西洋上的d18O梯度推断环流的变化。最后，我们打算将直布罗陀档案作为与该地区所有古环境和古海洋学数据进行比较的标准。它将提供两大洋交界处陆地气候变化的高分辨率描述，并支持一个长期记录的新框架，该框架将来可能会用于计算机模拟实验，从而加深我们对冰河时期气候的理解。

The Gibraltar reference record will be an important contribution to the study of the Earth's past climates, an intrinsically difficult topic because information about past conditions must be deduced from indirect evidence. We shall use speleothems from caves in Gibraltar, mainly calcite stalagmites and flowstones built up as precipitates from dripping water. Their chemical composition reflects climate, and each specimen provides a layered record which may cover any period from a few decades to tens of thousands of years. To construct a longer record multiple specimens must be accurately dated, so that overlaps can be put together to form a continuous sequence. Dating relies on the radioactive decay of traces of uranium to its daughter thorium over the time since the specimen was formed. For each speleothem we shall date the oldest and youngest layers and several in between, identifying any time gaps and constructing an age model which will correlate it with other specimens. We have already assembled an archive of 24 speleothems but require 200 more dates to form them into a full composite record. Our first aim is to obtain these dates.Our second aim is to chemically analyse every layer and interpret the results in terms of changing climates in Gibraltar over the last half-million years. Mineral chemistry thus stands proxy for the true climate. This raises two issues - which chemical variables are signals of climate, and what aspects of climate are reflected by each one? We shall measure d18O and d13C - the ratios of different types of atoms in the elements oxygen and carbon - and the concentrations of Mg, Sr, Ba, Y and P. These are all known to be partially controlled by climate, but each is also influenced by local factors such as water flow through soil and rock, or CO2 levels in cave air. Our previous work in Gibraltar separated the local and climatic influences by monitoring the modern environment for 10 years. We found that d18O in each year's deposit tracked the d18O in rainwater. However the speleothems we shall now analyse formed under different climatic conditions from today, so we must deduce the influences of climate from the shifting relations among the chemical variables during each specimen's growth, using chemical principles plus the insights from cave monitoring. On ice age time-scales temperature affects d18O as much as rainfall, and to allow for this we shall use independent records of sea surface temperature, making the assumption that cave temperatures tracked the surrounding sea. In this way we shall isolate the signal of changing d18O in rainfall from the complex chemistry of our speleothems.Stepping up in scale from Gibraltar and its caves, rainfall d18O varies across Europe, the Mediterranean and Middle East in a pattern reflecting atmospheric circulation and the transport of rain-bearing air. Gibraltar stands between the Mediterranean and Atlantic, the former being the source of winter rain from North Africa to central Asia and the latter the main moisture source for Europe. By comparing our d18O record with existing cave records in Israel, we shall reconstruct the uptake of Mediterranean water vapour through climatic shifts on all timescales from decades up to ice ages. Also of interest are millennial-scale shifts that occurred repeatedly during the last ice age and are recorded in cores through the Greenland ice sheet as episodes of higher d18O. They show up in Gibraltar speleothems, allowing us to infer changes in circulation from the gradients of d18O up the Atlantic.Finally, we intend the Gibraltar archive to be a yard-stick for comparison with all paleoenvironmental and paleoceanographic data in the region. It will provide a high-resolution account of climate changes on land, at the junction of two oceans, and support an emerging framework of long records that in future may feed into computer modelling experiments that will deepen our understanding of ice age climates.