Deep crustal assimilation during the 2021 Fagradalsfjall Fires, Iceland.

Active basaltic eruptions enable time-series analysis of geochemical and geophysical properties, providing constraints on mantle composition and eruption processes1-4. The continuing Fagradalsfjall and Sundhnúkur fires on Iceland's Reykjanes Peninsula, beginning in 2021, enable such an approach5,6. Earliest lavas of this volcanic episode have been interpreted to exclusively reflect a change from shallow to deeper mantle source processes7. Here we show using osmium (Os) isotopes that the 2021 Fagradalsfjall lavas are both fractionally crystallized and strongly crustally contaminated, probably by mid-ocean-ridge gabbros and older basalts underlying the Reykjanes Peninsula. Earliest eruptive products (187Os/188Os ≤ 0.188, platinum (Pt)/iridium (Ir) ≤ 76) are highly anomalous for Icelandic lavas or global oceanic basalts and Os isotope ratios remain elevated throughout the 2021 eruption, indicating a continued but diluted presence of contaminants. The 2022 lavas show no evidence for contamination (187Os/188Os = 0.131, Pt/Ir = 30), being typical of Icelandic basalts (0.132 ± 0.007). Initiation of the Fagradalsfjall Fires in 2021 involved pre-eruptive stalling, fractional crystallization and crustal assimilation of earliest lavas. An established magmatic conduit system in 2022 enabled efficient magma transit to the surface without crustal assimilation.

Delayed postglacial colonization of Betula in Iceland and the circum North Atlantic.

As the Arctic continues to warm, woody shrubs are expected to expand northward. This process, known as 'shrubification,' has important implications for regional biodiversity, food web structure, and high-latitude temperature amplification. While the future rate of shrubification remains poorly constrained, past records of plant immigration to newly deglaciated landscapes in the Arctic may serve as useful analogs. We provide one new postglacial Holocene sedimentary ancient DNA (sedaDNA) record of vascular plants from Iceland and place a second Iceland postglacial sedaDNA record on an improved geochronology; both show Salicaceae present shortly after deglaciation, whereas Betulaceae first appears more than 1000 y later. We find a similar pattern of delayed Betulaceae colonization in eight previously published postglacial sedaDNA records from across the glaciated circum North Atlantic. In nearly all cases, we find that Salicaceae colonizes earlier than Betulaceae and that Betulaceae colonization is increasingly delayed for locations farther from glacial-age woody plant refugia. These trends in Salicaceae and Betulaceae colonization are consistent with the plant families' environmental tolerances, species diversity, reproductive strategies, seed sizes, and soil preferences. As these reconstructions capture the efficiency of postglacial vascular plant migration during a past period of high-latitude warming, a similarly slow response of some woody shrubs to current warming in glaciated regions, and possibly non-glaciated tundra, may delay Arctic shrubification and future changes in the structure of tundra ecosystems and temperature amplification.