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Clams offer clues about the Little Ice Age

16 September 2022

A reanalysis of three climate proxy records finds new evidence for the destabilization of the subpolar North Atlantic Ocean.

Hunters in the Snow (Winter), by Pieter Brueghel the Elder. Three human figures on a snow-covered hill overlooking a frozen river with other people on it.
Evidence of the Little Ice Age is recorded in tree-ring data, sediment cores, and documents. The colder-than-usual conditions may even have inspired some artwork, including this 1565 painting, Hunters in the Snow (Winter), by Pieter Brueghel the Elder. Credit: Kunsthistorisches Museum Wien/public domain

Observations of Earth’s climate before the Industrial Revolution predominantly measure natural climate variations. Those changes in temperature and other variables are produced not by human activity but by the interactions among the atmosphere, the oceans, the land surface, and biological organisms. Perhaps the most notable natural climate transition from the past millennium is the cooling known as the Little Ice Age (LIA), which began approximately in the 13th century. It was particularly pronounced in North America and northern Europe, where temperatures dropped by up to 0.5 °C.

One hypothesis for the origin of the LIA is that an anomalous export of sea ice from the Arctic Ocean to the North Atlantic Ocean destabilized the currents there, stalling the transport of heat from tropical regions to northern latitudes. Model simulations have reproduced that slowdown, but observations of sustained weakening of the subpolar ocean’s circulation have been lacking.

Now Beatriz Arellano-Nava and Paul Halloran of the University of Exeter in the UK and their colleagues have found that evidence in three climate records, which were produced from clams collected from the North Icelandic Shelf. The data is punctuated by two periods of decreased resilience of the organisms, which suggest a destabilization of the ocean circulation in the region.

The records, which were published between 2013 and 2017, were developed in part by David Reynolds, a coauthor of the new paper. Although individual clams can live for hundreds of years, no single shell spans the entire past millennium. Reynolds and the other researchers involved with those papers therefore cross-dated each shell so that the overlapping lifespans of the clams constituted a longer and more robust record.

The oxygen-isotope data primarily reflect changes in temperature and salinity brought about by the influence of different water masses. The carbon-isotope time series records various biological and physical processes, and the growth record integrates all the environmental factors that influence the suitability of the clam’s habitat. Arellano-Nava, Halloran, and their colleagues analyzed all the records to assess changes in environmental resilience during the LIA, as measured by trends in variance of the data.

They found two falls in resilience, the first between 1180 and 1260 and the second from 1330 to 1380. The second episode’s timing overlaps with when sea ice was thought to have been transported to the North Atlantic. The first episode, however, occurred before the hypothesized beginning of the LIA. During that time, the North Atlantic was anomalously low in salinity, as inferred from three other proxy records from the region. Melting glaciers, the researchers say, may have freshened the ocean enough to destabilize the subpolar circulation there and explain the loss of resilience they found in the clam records.

The new findings haven’t settled the question of what triggered the LIA. Other research points toward a series of exceptional volcanic eruptions that could have induced a strong cooling effect. Regardless, studying past freshwater influxes and sea-ice transport in the North Atlantic is still relevant. Today’s ice sheets and glaciers could have similarly consequential climate effects as their rapid melting adds fresh water to the North Atlantic. (B. Arellano-Nava et al., Nat. Commun. 13, 5008, 2022.)

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