D6 Paleoclimate and Reconstruction

Topic

Paleoclimate and reconstruction methods aim to extend the cryosphere's climate record far beyond the period of direct instrumental measurement by accessing natural archives that have preserved information about past climate conditions. Extending the climate record in this way is essential for contextualizing the changes observed today, calibrating climate models with independent data, and understanding the climate system's natural variability across timescales ranging from centuries to millions of years.

Proglacial sediments and varves—laminated sediment layers deposited annually on the beds of glacial lakes—record year-to-year variability in glacial meltwater discharge, providing a resolution of one year per deposited sediment layer.

Glacial geomorphology—specifically the moraines left behind as a glacier retreats, the striations carved into the underlying bedrock, and the cirques excavated at the heads of occupied valleys—documents the maximum extent reached by glaciers during the Last Glacial Maximum, as well as the subsequent retreat they underwent after reaching that peak extent.

Cosmogenic dating measures the accumulation of beryllium-10, aluminum-26, or chlorine-36 within a rock surface since glacial retreat first exposed it to cosmic radiation; this accumulation of cosmogenic isotopes enables the method to assign a direct age to the moment of glacial retreat, with a precision of five to ten percent. A speleothem—whether a stalactite or a stalagmite—growing within a cave in a periglacial region records both the temperature and hydrology of the time of its formation through the oxygen and carbon isotope ratios incorporated during its growth; meanwhile, uranium-thorium dating assigns a highly precise age to each layer of the speleothem.

When combined, these paleoclimate and reconstruction methods are expected to demonstrate multi-proxy coherence: proglacial sediments, glacial geomorphology, cosmogenic dating, and speleothems—being entirely independent natural archives—should converge on the same regional climate history, thereby confirming the ability of these methods to extend the cryosphere's climate record far beyond the instrumental period mentioned at the beginning of this description.

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