Unique Collections of 14C‐Dated Vegetation Reveal Mid‐Holocene Fluctuations of the Quelccaya Ice Cap, Peru.

Several studies have analyzed the ice margin behavior of the Quelccaya Ice Cap (QIC), Earth's largest tropical ice cap, through the Holocene. However, continuous integration of new information to produce a more cohesive history of the QIC is necessary. Here, the radiocarbon dates of 33 rooted plant specimens collected in situ along the western ice margin between 2002 and 2018 reveal the timing of its past extent as it advanced during the mid‐Holocene. The most recent evaluation of collected specimens indicates that the QIC margin advanced ∼350 m down the Challpacocha Valley between 7.1 and 4.5 ka BP. Past studies of documented ice extent on the western side of the QIC based on a variety of techniques are compiled to create a more comprehensive history of the QIC's behavior throughout the Holocene. Records of documented ice extent, as well as other proxy records, indicate a climate transition ∼5–7 ka BP that created the proper environmental conditions for the expansion of the QIC. Evidence from nearby valleys indicates that the QIC behaved similarly to the documented ice extent in the Challpacocha Valley in response to Holocene climatic fluctuations. The ability to collect the plant specimens and recent analysis of satellite imagery reveals rapid retreat rates of the western outlet glaciers from 1985 to 2020, leaving the western margin of the QIC at its smallest extent since the mid‐Holocene. Plain Language Summary: The area around the Quelccaya Ice Cap (QIC) is unique as wetland plant samples have been found preserved in their growth positions, buried by past advancing ice, whereas the ice typically removes vegetation and soil in its advance down a valley. Using the 14C dates of these plants, we map a 350‐m advance of the ice margin between 7.1 and 4.5 ka BP. Combined with a variety of previous studies involving different methods and records from ice and lacustrine sediment cores, a larger picture of the QIC's behavior comes into view. A largely accepted global climate shift between ∼7 and 5 ka BP occurred in tandem with the QIC's advance. Although the QIC has fluctuated before, at no time has the western margin receded to the current extent since the mid‐Holocene. This is indicated by the exposure of the plants and current margin retreat rates calculated from satellite imagery (1985–2020). Key Points: Mid‐Holocene advance of the Quelccaya Ice Cap (QIC) is documented using 14C dating of in situ vegetation exposed during the recent retreatPlant ages of ∼4.5 to ∼7.1 ka BP provide evidence of a mid‐Holocene climate transition and the QIC ice‐margin expansionAnalysis of satellite imagery from 1985 to 2020 documents a recent rapid retreat (∼20 m/yr) of the QIC margin [ABSTRACT FROM AUTHOR]