Decoded and precisely dated information encrypted in stalagmites from a cave in China reveal past climatic changes and provide insight into the complex interactions in today's climate system. See Letter p.640
Cave stalagmites represent a meticulous archive of climatic events stretching back over many hundreds of thousands of years and, unlike many geological deposits on land and in the ocean, allow the timing of these events to be precisely determined. On page 640 of this issue, Cheng et al.1 decipher a treasure trove of information concealed in the stalagmites of the Sanbao Cave in the mountains of central China. Their findings extend one of the most valuable benchmark climate records available, which now spans an impressive 640,000 continuous years of Asian monsoon history with unprecedented dating precision. The Asian monsoon is a crucial component of the global climate system, so these data not only provide age constraints for other palaeoclimate time series, but also improve our understanding of the interactions, forcing factors and responses of today's climate systems.
As they grow upwards from the cave floor, stalagmites accumulate layer upon layer of calcium carbonate (Fig. 1). Deposited from oversaturated drip waters, this compound is imprinted with the water's chemical composition, which, in turn, carries the chemical signature of rainwater falling above the cave. The oxygen isotope composition of this water (heavy 18O compared with light 16O, reported as δ18O) is an indicator of the climate conditions at the time the rain fell (Fig. 2). Dissolved uranium leached from the rocks above the cave is also trapped in the forming calcium carbonate, acting as a radiometric clock as it slowly decays. This allows stalagmite age to be accurately determined back to around 650,000 years ago2. The older the material, the less exact this clock becomes, and the more effort is needed to determine precise age constraints. Cheng et al. go to the limit of what can be achieved, and their precisely dated stalagmite record covers more than six glacial-to-interglacial cycles.
Climatic events are often globally interconnected, sometimes to a surprising extent. For example, during glacial periods there is a remarkable synchrony between dry events in the Asian monsoon climate and cold events in the North Atlantic region: this is related to massive discharge of icebergs and changes in ocean and atmospheric circulation3,4. The repercussions extend as far south as Antarctica, which warms during these events5,6, and include alterations in the concentrations of methane and carbon dioxide in the atmosphere. Such connections are well established for periods during which good age control is available for all archives, and can provide crucial lessons about our climate system. Under the (reasonably safe) assumption that the synchrony observed for the more recent past holds further back in history, the extended cave record described by Cheng et al. can be used as an indirect age constraint for other archives3,7 that lack direct radiometric age control. This information therefore greatly extends the time over which we have precisely dated climate data around the globe.
It might be asked why long time series are important, given that it is so much easier to obtain well-dated climate records for the more recent past. The issue is the multitude of contributing factors that coincide at any one time. For example, Earth's climate is strongly influenced by the configuration of its orbit relative to the Sun, which changes cyclically. The time of the year at which Earth is closest to the Sun changes every 20,000 years or so (the precession cycle). Cyclical changes in the tilt of Earth's axis (the obliquity cycle) occur with a periodicity of around 40,000 years, leading to additional variations in the strength of winter and summer seasons at high latitudes8. Which of these two cyclical variations is responsible for the observed duration of glacial cycles of roughly 100,000 years has long been a matter of debate9,10,11.
The answer to this question can come only from a well-dated climate time series that covers a sufficient number of these cycles — like that amassed by Cheng and colleagues. Although this record reflects monsoon strength and not the extent of glaciation, a previously established link between particularly strong dry events and glacial terminations allowed the authors to determine the exact timing of the past seven glacial terminations. These all relate to specific times in the precession cycle and occur every four or five precession cycles, revealing a prominent role for this orbital cycle.
In general, the extended cave record shows that the pattern of monsoon variability seen in the more recent past also occurred in earlier periods, indicating that it is an inherent feature of natural climate variability. Cyclical variation in monsoon strength in relation to orbital cycles is particularly prominent. Furthermore, dry events of shorter duration are evident throughout the record. Cheng et al. investigate these dry periods by removing the orbitally controlled cyclicity from their monsoon record and studying the remaining suborbital signals (termed Δδ18O). This approach could create artefacts, a possibility investigated by Cheng et al. (see Extended Data, Figs 5, 6), but allows the suborbital features to stand out more clearly in this residual time series — and again, these seem to be related to orbital variations in the precession cycle. If it holds up, this could be an invaluable clue about the unknown cause of such short-lived events.
The extended time series that Cheng et al. derive from the Sanbao Cave stalagmites has an array of other interesting features that call for further investigation. This wealth of new information on past climate variability over an exceptionally long time period and with precise age control will bring us a step closer to understanding the drivers and interactions in our climate system.Footnote 1
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Science China Earth Sciences (2019)