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

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.