A seasonal cycle in polar atmospheric bromine was first discovered in the 1980s by Sturges and Barrie1 with higher bromine concentrations observed during Arctic spring. Subsequent studies2,3 have established the photochemical nature of bromine atmospheric chemistry and its reaction with tropospheric ozone4,5 to form bromine oxide (BrO)5 (see Supplementary Information for a detailed chemical description). Satellite sensors associate enhanced BrO concentrations with first-year sea ice (FYSI) rather than multi-year sea ice (MYSI)6,7. Enhanced bromine photochemistry at the FYSI surface8 has been related to the presence of highly saline conditions such as those found in brine channels3, frost flowers9 and blowing snow10, which are not generally associated with MYSI, as most of the brine is drained away during the summer melt11,12. As heterogeneous and photochemical recycling of bromine (bromine explosion) is associated with marginal sea ice regions, bromine deposited on land ice is enriched relative to sea salt compositions at coastal locations13 and progressively depleted as airmasses travel further inland14.

An understanding of polar tropospheric halogen chemistry has developed as ground- and ice-core based measurements have established clearer links between FYSI extent and halogen variability recorded in polar ice cores. Interactions between halogens and FYSI are monitored by satellite6 and ground-based7 sensors, while interactions with snowpack are a topic of active investigation8,15,16. Seasonal variability in bromine enrichment has been observed in surface snow samples from Antarctica (Law Dome17) and the Arctic (Northwest Greenland17, Svalbard18, Severnaya Zemlya19). Such enrichment beyond seawater compositions consistently occurs in spring/summer and is consistent with photochemical recycling processes linked to FYSI. A fifty-year record of bromine enrichment and iodine from Severnaya Zemlya is well correlated with observations of Laptev sea FYSI extent19. Bromine enrichment variations observed in the Antarctic Talos Dome ice core agree well with reconstructions of sea ice duration based on fossil diatom assemblages from a marine sediment core over the last two glacial/interglacial cycles14. This study presents the first record of bromine in the Arctic covering the last glacial cycle.

Due to the poor geographical and temporal resolution of sea ice proxies, few quantitative reconstructions of Holocene Arctic total sea ice extent are available. The few reconstructions available are based on dinoflagellate cyst assemblages20, specific highly branched isoprenoid monoenes (IP 25 ) derived from sea ice diatoms21, melt layers in ice cores22 as well as driftwood remains on raised beaches23. Reconstructions of Arctic MYSI describe a broad pattern of minimal MYSI extent during warmer climate phases such as the early Holocene (8–11.7 ky before the year 2000 hereafter referred to as ‘b2k’) and Bølling-Allerød (12.9–14.7 ky b2k)23, contrasting with greater MYSI extent during cooler climate phases such as the late Holocene (0–3 ky b2k) and extensive MYSI extent during the last glacial maximum (14.7–23 ky b2k)24. MYSI variability broadly follows northern latitude summer insolation, with a minimum of Arctic MYSI cover in the Canadian Arctic during the early Holocene (8–11.7 ky ago)13. Frequent, high-amplitude fluctuations in MYSI extent are apparent since 5 ky b2k and particularly over the last 1,450 years, for which well-resolved multi-proxy reconstructions of MYSI extent with good geographical coverage are available25. Rapid fluctuations in iceberg export through the Fram Strait during the Younger Dryas (11.7–12.9 ky BP) and Bølling-Allerød have been reconstructed26,27 from ocean sediment cores recovered from sites featuring high sedimentation rates.

The drivers and feedbacks between polar climate and total sea ice remain a topic of active research. The ice- albedo feedback mechanism has been studied in detail28 and is particularly relevant to decreasing MYSI in the last two decades as well as during abrupt climate changes such as the onset of Greenland interstadial events. Mechanisms controlling Arctic MYSI variability during the glacial are a topic of speculation, with a recent study proposing the formation of warm subsurface Atlantic water as a precursor and driver of rapid MYSI collapse in the Nordic seas29. Possible influences of North Atlantic deepwater formation as well as Atlantic meridional overturning circulation have been also been considered30. Ice core-based reconstructions of precipitation moisture sources, dust transport and sea salt proxies31 indicate large-scale atmospheric and oceanic circulation changes occurring through the Bølling-Allerød and Younger Dryas glacial termination sequence and point to a key role for changes in North Atlantic Ocean surface conditions32.

Here we present bromine and sodium concentrations in the NEEM ice core to investigate Canadian Arctic sea ice variability since the last interglacial period. We define the Canadian Arctic as the area from which aerosols are entrained and transported to the NEEM site, comprising the Canadian archipelago, Baffin Bay and Hudson Bay regions (see Supplementary Information). Bromine is proposed as a FYSI proxy in ice cores14,18,19, supported by halogen chemistry transport modelling which demonstrates the effectiveness of bromine recycling over FYSI and subsequent bromine depletion with transport. Back trajectory modelling indicates the majority of airmasses originate from the west and arrive at NEEM after entraining air primarily from the Baffin Bay, Hudson Bay and Canadian Arctic regions. The study of bromine in polar ice cores offers a new technique to produce quantitative reconstructions of FYSI and MYSI of unprecedented temporal resolution.