Riukojietna, a small low-altitude ice cap that may have persisted through the Holocene: Evidence from combining cosmogenic multi-nuclide dating and lacustrine sediment records

Abstract. Riukojietna, a small, low-altitude, low-gradient plateau ice cap in northern Sweden, has been retreating rapidly over at least the last century. Its low surface gradient implies that it should be quite sensitive to, and therefore a potentially valuable indicator of, climate change since regional deglaciation at 9.8 ka. Here, we assess its former extent and activity by combining cosmogenic nuclide measurements in bedrock (in situ ¹⁴C, ¹⁰Be, and ²⁶Al) that constrain ice-free and ice-buried conditions with indirect evidence of glacial activity from proglacial lake sediment records, complemented by historical ice thickness reconstructions. These data are the basis for subsequent forward modeling of measured cosmogenic nuclide concentrations to constrain the Holocene history of Riukojietna.

The ice cap has an outlet glacier tongue that drains to the northeast, with a bouldery moraine deposit further down valley constraining its extent at the end of the Little Ice Age (LIA, ca. 1910 CE). Five cosmogenic nuclide samples were collected: two from bedrock on the plateau adjacent to the ice cap, two from a bedrock knob protruding from the outlet glacier tongue (exposed in 2011), and one from an outcrop adjacent to the LIA moraine at the outlet of the most proximal of a series of four proglacial lakes. The latter sample yielded concentrations of ¹⁴C, ¹⁰Be, and ²⁶Al consistent with continuous exposure since 8.1 ± 0.1 ka (weighted mean). Nuclide measurements in the other four samples indicate complex exposure/burial histories. Lake cores from Pajep Luoktejaure, the third of the four down-valley proglacial lakes, indicate up to three periods of glacigenic sediment deposition since deglaciation, separated by gyttja, with radiocarbon age constraints from bulk sediment and plant macrofossils.

Modeled in situ ¹⁴C inventories for the two plateau samples are consistent with early Holocene ice cover, followed by exposure between 8.1 ± 0.1 ka and the start of Riukojietna neoglacial expansion 1.8 ± 0.1 cal ka BP, yet ¹⁰Be and ²⁶Al concentrations are underestimated, indicating significant pre-Last Glacial Maximum exposure not considered in the modeling. Modeled in situ ¹⁴C concentrations of the samples from the emerging bedrock knob with this ice-cover history and a subglacial erosion rate of 0.05 mm yr^-1 are consistent with the measured values, while ¹⁰Be and ²⁶Al concentrations again underestimate the measured values. Observed glacigenic laminated sediments in Pajep Luoktejaure between ca. 5.4–5.0 ka may indicate a brief readvance over the sampled cosmogenic nuclide sites, but agreement between modeled and measured in situ ¹⁴C values deteriorates slightly with that ice-cover interval. We use these results to infer that Riukojietna persisted during the Holocene Thermal Maximum (ca. 8–5 ka), in contrast to earlier suggestions that Scandinavian glaciers vanished during the Holocene, as a result of increased precipitation due to atmospheric circulation changes. The glacier has been in a retracted state similar or smaller than today during the late Holocene, as climate grew colder and drier. This approach combining short- and long-lived cosmogenic nuclides with lake sediments can thus provide new constraints on high-latitude Holocene glacial and paleoclimate history.