Decadal-to-centennial increases of volcanic aerosols from Iceland challenge the concept of a Medieval Quiet Period

Volcanic aerosols are major drivers of natural climate variability by reflecting incoming solar radiation and cooling Earth’s surface. Polar ice-core records, synchronized across Greenland and Antarctica, are the primary archives for reconstructing volcanic radiative forcing over millennia via preserved sulfuric acid peaks. Based on these reconstructions, the period 700–1000 CE has been termed the Medieval Quiet Period due to a scarcity of major tropical eruptions and minimal solar forcing perturbations. However, this interpretation conflicts with Greenland ice cores that show elevated background sulfate during this interval and with proximal Icelandic geological records indicating increased eruption frequency (particularly from Katla, Grímsvötn, and Bárðarbunga-Veiðivötn). Traditional volcanic detection methods assume short-lived (≤3 years) sharp sulfate peaks consistent with sulfate’s short atmospheric lifetime, potentially missing long-lasting eruptive episodes and quiescent degassing. Clear diagnostic tracers for securely detecting and attributing volcanic sulfate in polar ice have been lacking. The study aims to resolve mismatches between proximal Icelandic records and global forcing reconstructions by applying a multi-proxy toolkit to Greenland ice cores for 700–1000 CE, characterizing source, plume height, and the climatic relevance of prolonged aerosol emissions.

Previous reconstructions relied on short-duration sulfate peaks and global bipolar synchronization to estimate volcanic forcing, leading to the concept of a Medieval Quiet Period (700–1000 CE). Yet, Greenland ice cores show elevated background non-sea-salt sulfate during this time, and Icelandic geological records indicate increased eruptive frequency and cumulative tephra volumes for multiple volcanic systems (Katla, Grímsvötn, Bárðarbunga-Veiðivötn). Prior ice-core studies lacked diagnostic tracers to differentiate short-lived stratospheric eruptions from prolonged tropospheric emissions and quiescent degassing. The assumption that most background variations are non-volcanic has likely led to underestimation of volcanic sulfate deposition and forcing. Sulfur triple isotopes (Δ33S) have been used to distinguish stratospheric from tropospheric sulfate formation, but such high-resolution applications in the North Atlantic high-latitude context have been limited. Heavy metals and halogens have been proposed as indicators of proximal volcanic emissions due to their volatility and rapid decay with distance, but their systematic use in Greenland records for the first millennium CE has been minimal.

Seven Greenland ice cores (NEEM-2011-S1, NGRIP1, NGRIP2, EGRIP, RECAP, B19, TUNU2013) covering 700–1000 CE with annual-layer-counted chronologies (NS1-2011 for most, DRI_N-GRIP2 for NGRIP2) and dating uncertainties of ±2 years were analyzed. Analytical methods: continuous flow analysis (CFA) with ICP-MS at DRI for elemental species (S, Cl, Bi, Tl, Na, Ce, etc.) on NEEM-2011-S1, NGRIP2, TUNU2013, and B19; ion chromatography at AWI and PICE for SO4²−, Cl−, F− in EGRIP and NGRIP1; electrospray MS-MS for MSA and CRDS for water isotopes on TUNU2013. Non-sea-salt S and Cl were estimated using Na as a sea-salt tracer and standard seawater ratios; non-crustal Tl and Bi were computed using Ce as a mineral-dust tracer and mean crustal Tl/Ce and Bi/Ce ratios, with Ce recovery correction. Targeted cryptotephra sampling was guided by co-registered insoluble particle counts (Abakus counter) and sulfur peaks; large cross sections of archived ice were processed, and shards analyzed by EPMA (major elements) and LA-ICP-MS (trace elements). Tephra were correlated with proximal Icelandic volcanic materials sampled in 2021–2022 using major and trace element fields for source attribution (Katla, Grímsvötn, Bárðarbunga-Veiðivötn, Torfajökull). Sulfur isotope measurements (δ34S, δ33S, Δ33S) were conducted at high temporal resolution on TUNU2013 between 740–765 CE using column extraction and MC-ICP-MS; background correction and isotope mass balance separated volcanic and background sulfate contributions. Volcano detection and sulfate deposition reconstructions used adjusted statistical methods: a 91-year running median (RM) and median absolute deviation (MAD) derived from background periods (300–700 CE; for B19, 950–1250 CE) defined thresholds (upper K=3 for peak detection; lower K=1 for event duration). Non-volcanic background (SRRMi) was recomputed after removing detected peaks; volcanic sulfate deposition was obtained by subtracting SRRMi from annual nss-S and multiplying by site accumulation, and cumulative deposition was summed over each event. Climate context: Greenland δ18O stack for surface air temperature, TUNU2013 MSA for sea-ice proxy, two central European speleothem temperature reconstructions (Milandre, Spannagel), and Nile low-stand data were used to explore climatic impacts.

- Identification of a prolonged Icelandic Active Period (IAP) from 751–940 CE dominated by Icelandic volcanism, commencing with a 12-year Hrafnkatla episode (751–763 CE). - Stacked Greenland ice-core nss-S shows an abrupt increase at 751 CE; median sulfur concentrations during 751–940 CE were 1.6× higher than 700–750 CE. Superimposed frequent sulfate peaks persisted 1–12 years. - Co-registered halogens and volatile metals exhibit a pronounced step-change at 751 CE and sustained elevations: • 751–763 CE: F up to 8×, Cl 4×, Tl 12×, Bi 5× relative to pre-event; elevated above pre-event for 12 years. After 765 CE: F ~3× and Cl, Tl, Bi ~2× above early 8th century baseline. - Cryptotephra identifications and geochemical correlations to Icelandic sources: • 753 CE (Grímsvötn), 757 CE (Katla), 763±1 CE (Katla, Hrafnkatla eruption), 766±1 CE (source uncertain), 781±2 CE (Grímsvötn, correlated to Lake Lögurinn), 877±2 CE Settlement Layer (Torfajökull silicic and Bárðarbunga-Veiðivötn basaltic components). - Sulfur isotopes (TUNU2013) across 751–763 CE indicate predominantly tropospheric sulfate over ~12 years (Δ33S ≈ 0 within ±0.2‰), with a brief stratospheric signal at 762/763 CE (Δ33S to −0.5‰) coincident with Hrafnkatla. Mean δ34S during the episode: +6.4‰ (lower than background +9.5±0.93‰), background-corrected volcanic δ34S ≈ +2.6±2.5‰, with anti-correlation to S peaks. - Revised volcanic sulfate deposition: previous reconstructions underestimated volcanic sulfate by attributing background to non-volcanic sources. Updated annual mean volcanic sulfate deposition increased from 1.0 to 2.4 kg km−2 yr−1 (NGRIP1) and from 2.1 to 4.1 kg km−2 yr−1 (NEEM-2011-S1). New Greenland stack (N=6) reconstructs cumulative volcanic sulfate of 658 kg km−2 for 750–1000 CE (vs. 305 kg km−2 NGRIP1 and 608 kg km−2 NEEM-S1). - Tabled eruptive attributions within 700–1000 CE include: 753 CE (explosive; VSSI 6.79 Tg S), 757 CE (explosive; 3.07 Tg S), 763±1 CE Hrafnkatla (explosive; 4.77 Tg S), 781±2 CE (explosive; 1.72 Tg S), 822 CE Katla K-822 (explosive; 0.4 km3; 3.93 Tg S), 853 CE Mt Churchill WRAe/AD860B (Plinian; 39.4 km3; 2.48 Tg S), 877 CE Settlement Layer (Plinian, phreatomagmatic; 5 km3; 3.05 Tg S), 940 CE Eldgjá (effusive/Strombolian/phreatomagmatic; 1.3 km3; 16.23 Tg S), 946 CE Millennium eruption (Plinian; 40 km3; 1.72 Tg S). - Climatic impacts: The Hrafnkatla end-phase (762/763 CE) coincides with strong winter cooling anomalies across Europe documented historically (762–764 CE among harshest winters), a cluster of extreme Nile low-stand years (20 of lowest 5th percentile between 750–790 CE), but no major NH summer cooling in tree-ring reconstructions in 764 CE. TUNU2013 MSA indicates no strong sea-ice expansion in the 760s but a general Arctic sea-ice reduction through the IAP toward the late 10th century.

The study overturns the notion of a volcanically quiescent Medieval Quiet Period by demonstrating sustained Icelandic volcanic activity between 751 and 940 CE that substantially elevated the tropospheric aerosol burden. The multi-proxy approach—combining cryptotephra, halogens, volatile metals, and sulfur isotopes—enabled secure detection and source attribution of long-lasting, predominantly tropospheric volcanic emissions that traditional methods missed. The Hrafnkatla episode establishes a 12-year period of elevated aerosols with a brief stratospheric culmination, corresponding to significant winter cooling in Europe and hydrological perturbations (e.g., Nile low stands), consistent with hemispheric confinement and limited spatial reach of high-latitude eruptions. The lack of a pronounced summer cooling signal following the 762/763 CE stratospheric phase suggests that sustained background aerosols may have modulated the typical climatic response and that sea-ice feedbacks differed during this interval. The revised volcanic sulfate deposition (approximately doubling prior estimates) highlights that existing forcing reconstructions underrepresent pre-industrial tropospheric volcanic aerosol contributions. This has implications for climate simulations (PMIP3/PMIP4) that exclude tropospheric volcanic aerosols for the pre-industrial millennium, potentially biasing modeled climate sensitivity and responses to volcanic forcing.

Applying a new multi-proxy geochemical toolkit to Greenland ice cores, the study identifies an Icelandic Active Period (751–940 CE) of heightened volcanic activity, beginning with the 751–763 CE Hrafnkatla episode. Elevated halogens, volatile metals, and sulfur, together with cryptotephra correlations and sulfur isotope constraints, demonstrate that much of the aerosol load was tropospheric, with a short-lived stratospheric phase in 762/763 CE. Revised volcanic sulfate deposition rates across 700–1000 CE are roughly doubled relative to previous reconstructions, challenging the concept of a Medieval Quiet Period and emphasizing the climatic relevance of tropospheric volcanic aerosols. The findings suggest substantial implications for volcanic forcing reconstructions used in past-millennium climate simulations and underscore the need to incorporate tropospheric volcanic aerosol effects. Future research should quantify prolonged volcanic emission dynamics, disentangle aerosol–cloud–sea-ice feedbacks, refine source attribution where cryptotephra are sparse, and build pre-industrial aerosol baselines to reduce forcing uncertainties.

- Chronological uncertainty of ±2 years remains for 700–1000 CE despite multiple constraints (e.g., 774 and 993 CE cosmogenic events). - High-resolution sulfur isotope analyses were conducted only for 740–765 CE (Hrafnkatla episode); the rest of the IAP lacks Δ33S constraints, limiting plume-height inference outside this interval. - Detection differences among ice cores are influenced by site elevation and location, affecting sensitivity to particular source regions and event detectability. - Attribution of some cryptotephra (e.g., 766±1 CE) is limited by sparse geochemical data; secondary processes may influence shard deposition. - Volatile metal and halogen behavior during transport and deposition is incompletely understood, introducing uncertainty in using them as universal tracers. - Traditional detection methods tailored to short-lived stratospheric signals may still bias event identification and quantification; assumptions in background estimation and accumulation rates affect sulfate deposition reconstructions. - The absence of a clear summer cooling signal after 762/763 CE is unexplained; feedbacks (e.g., sea-ice dynamics) under prolonged aerosol loading remain uncertain.