SHORT COMMUNICATION Radium isotope fingerprinting of permafrost ‐ applications to thawing and intra‐permafrost processes Yishai Weinstein 1 | Dotan Rotem 1,2 | Henk Kooi 3 | Yoseph Yechieli 4 | Jurgen Sültenfuß 5 | Yael Kiro 6 | Yehudit Harlavan 4 | Mor Feldman 1 | Hanne H. Christiansen 2 1 Department of Geography and Environment, Bar‐Ilan University, Ramat‐Gan, Israel 2 Arctic Geology Department, the University Centre in Svalbard, UNIS, Svalbard 3 Unit Subsurface and Groundwater Systems, Deltares, Utrecht, The Netherlands 4 Geological Survey of Israel, Jerusalem, Israel 5 University of Bremen, Bremen, Germany 6 Lamont‐Doherty Earth Observatory, Columbia University, Palisades, New York, USA Correspondence Yishai Weinstein, Department of Geography and Environment, Bar‐Ilan University, Ramat‐ Gan, 52900 Israel. Email: weinsty@biu.ac.il Funding information Israel Science Foundation, Grant/Award Num- ber: Bikura grant 1487/14; Svalbard Science Forum, Grant/Award Number: Arctic Field Grant 2017 Abstract Permafrost in circum‐polar regions has been recently undergoing thawing, with severe environmental consequences, including the release of greenhouse gases and amplification of global warming. Although highly important, direct methods of track- ing thawing hardly exist. In a research study conducted at Adventdalen, Svalbard, we identified a permafrost radioisotope fingerprint, and show that it can be used to track thawing. Ratios of long‐ to the shorter‐lived radium isotopes are higher in ground ice than in active layer water, which we attribute to the permafrost closed system and possibly to the long residence time of ground ice in the permafrost. Also, daughter–parent 224 Ra/ 228 Ra ratios are lower in permafrost than in the active layer. These fingerprints were also identified in a local stream, confirming the applicability of this tool to tracing thawed permafrost in periglacial watersheds. A combination of radium isotope ratios and 3 H allowed the identification of recent intra‐ permafrost segregation processes. The permafrost radium fingerprint should be appli- cable to other permafrost areas, which could assist in regional quantification of the extent of permafrost thawing and carbon emissions to the atmosphere. KEYWORDS active layer, permafrost thawing, radium isotopes 1 | INTRODUCTION More than 20% of the global land surface area is underlain by perma- frost. 1-3 Global warming has already caused extensive permafrost degradation, 4 and some models predict up to >60% reduction in per- mafrost areal extent by the end of the 21 st century. 5 This will probably cause significant changes in the hydrology of these regions (e.g., 6-8 ), and will ultimately result in the oxidation of the permafrost's large inventory of organic carbon and its consequent release to the atmo- sphere, applying additional forcing to the ongoing global warming (e.g., 5,9,10 ). Conventional methods of studying permafrost degrada- tion rely on ground thermal measurements (e.g., 4,11 ) and active layer thickness monitoring (e.g., 12,13 ), as well as modeling. 14-16 While some geochemical characterization of ground ice has been made (e.g., 17-19 ), direct methods of tracking permafrost thawing and of tracing its thawed water in local drainage systems were hardly devel- oped (e.g., 20-23 ). In this study, we identify a distinct radium isotope signature of ground ice, and propose its use as a tracer of permafrost thawing and possibly of intra‐permafrost segregation processes. 1.1 | Radium isotopes Radium has four naturally occurring isotopes, which are all radioactive, part of the U–Th decay chains, and with half‐lives varying between days ( 224 Ra: 3.66 days, 223 Ra: 11.4 days), several years ( 228 Ra: Received: 14 September 2017 Revised: 5 February 2019 Accepted: 5 February 2019 DOI: 10.1002/ppp.1999 104 © 2019 John Wiley & Sons, Ltd. Permafrost and Periglac Process. 2019;30:104–112. wileyonlinelibrary.com/journal/ppp