Volcanic aerosols are significant drivers of natural climate variability, influencing climate through the reflection of incoming solar radiation, leading to surface cooling. Accurate reconstructions of volcanic aerosol radiative forcing are crucial for climate model simulations. Polar ice cores are primary archives for such reconstructions, preserving sulfuric acid peaks that reflect past volcanic activity. Global-scale synchronization of Greenland and Antarctic ice-core records allows for the generation of global volcanic radiative aerosol forcing estimates, revealing periods of increased and decreased volcanic activity. The Medieval Quiet Period (700-1000 CE) is defined by a scarcity of major tropical volcanic eruptions and minimal solar forcing perturbations. However, Greenland ice-core records show increased background sulfate during this period, while proximal Icelandic geological records indicate increased eruption frequency between 750 and 1000 CE. Existing volcanic detection methods, assuming short-lived volcanic sulfate deposition signals, may not accurately capture the impact of long-lasting eruptions, particularly from Iceland, due to its proximity to Greenland. These long-lasting eruptions might be imprinted in ice-core sulfate records as long-term changes, hindering secure detection and quantification of volcanic contributions. A lack of unique volcanic tracers in polar ice for detection and source attribution further complicates the analysis, making the distinction between short-lived eruptions and longer volcanic episodes difficult. This study uses a multi-proxy approach to resolve the mismatch between Icelandic records and global volcanic forcing estimates by attributing volcanic events in Greenland ice cores and examining climate impacts at the onset of increased background sulfate.

Numerous studies have utilized Greenland and Antarctic ice core records to reconstruct past volcanic activity and its impact on climate. These studies have relied on the detection of sulfate peaks in ice cores as a proxy for volcanic eruptions. The existing literature defines the Medieval Quiet Period (700-1000 CE) as a period of relatively low volcanic activity based on these ice core records. However, other studies have shown inconsistencies with geological evidence of increased volcanic activity in Iceland during this time period. The existing methods for detecting volcanic events in ice cores typically focus on identifying sharp, short-lived sulfate peaks, which may not accurately capture prolonged volcanic episodes that contribute to background sulfate levels. The literature review also indicates a lack of specific tracers for uniquely identifying volcanic eruptions and differentiating between short and long-duration events in ice cores. Studies focusing on sulfur isotopes and other geochemical tracers are emerging as tools for better characterizing volcanic events and their contributions to atmospheric sulfate. This research builds upon this existing literature by employing a multi-proxy approach to improve the detection and characterization of Icelandic volcanic activity and its impact on climate during the Medieval period.

This study utilized seven Greenland ice cores (NEEM-2011-S1, NGRIP1, NGRIP2, EGRIP, RECAP, B19, and TUNU2013) located across the Greenland ice sheet. The ice cores were dated using annual-layer counting, with a dating uncertainty of less than ±2 years during the study period (700–1000 CE). The researchers employed a multi-proxy approach, utilizing four distinct diagnostic tracers: cryptotephra, sulfur isotopes, halogens, and heavy metals. Annual non-sea-salt (nss) sulfur concentrations were analyzed to identify abrupt increases in sulfate levels, indicating volcanic activity. Stacked nss chlorine, fluorine, and non-crustal bismuth and thallium concentration records were also analyzed to further characterize volcanic events. Cryptotephra analysis involved searching for volcanic glass shards in ice core samples to pinpoint eruption sources and verify the timing of events. Geochemical analysis of individual glass shards allowed for correlation with specific Icelandic volcanic systems (Katla, Grímsvötn, Bárðarbunga-Veiðivötn, and Torfajökull). High-resolution sulfur isotope analysis (δ³⁴S and Δ³³S) was performed to determine the origin (marine vs. volcanic) and atmospheric layer (tropospheric vs. stratospheric) of the sulfate deposits. The researchers revised existing reconstructions of volcanic sulfate deposition by attributing long-term background variations of sulfur to volcanic origins. To investigate the climate impacts, particularly at the onset of the identified Icelandic Active Period, the researchers compared forcing reconstructions to palaeoenvironmental records from the North Atlantic region, including speleothem records and historical documentary evidence.

The study identified a prolonged episode of volcanic sulfur dioxide emissions (751–940 CE), dominated by Icelandic volcanism, which they termed the Icelandic Active Period (IAP). This period started with the Hrafnkatla episode (751–763 CE). Analysis of nss-sulfur concentrations revealed a 1.6-fold increase in median sulfur concentrations between 751 and 940 CE compared to the preceding period. The step-change in 751 CE was further supported by elevated concentrations of halogens (Cl, F) and volatile metals (Tl, Bi). Cryptotephra analysis confirmed volcanic events in 753 CE, 757 CE, 763 CE, 766 CE, 781 CE, and 877 CE, with geochemical correlations made to various Icelandic volcanic systems. The 763 CE event was geochemically linked to the Hrafnkatla eruption from the Katla volcanic system. Sulfur isotope analysis (δ³⁴S) during the Hrafnkatla episode showed lower mean values (+6.4‰) compared to background samples (+9.5‰), indicating a largely volcanic origin. Δ³³S analysis indicated a predominantly tropospheric origin for the sulfate fallout during the Hrafnkatla episode, with a brief stratospheric phase in 762/3 CE. The study revised existing estimates of volcanic sulfate deposition in Greenland, attributing more long-term background variations to volcanic sources, resulting in higher deposition rates. The revised estimates challenge the concept of a Medieval Quiet Period. The onset of the IAP, marked by the Hrafnkatla episode, coincided with a cooling signature in European speleothem records and documented harsh winters across Europe. Reduced Nile River flow in the same period suggests a possible link to altered tropical hydroclimate. The study noted a lack of a major summer cooling anomaly in Northern Hemisphere tree-ring reconstructions, possibly due to high background aerosol concentrations. The transition to a Medieval Warm Period coincided with a decrease in Arctic sea ice and a rise in surface air temperatures, possibly facilitating Viking expansion into the Arctic region.

The findings challenge the prevailing notion of a volcanically quiescent Medieval Quiet Period (700–1000 CE). The multi-proxy analysis revealed significant Icelandic volcanic activity, including both prolonged episodes (IAP) and shorter events (Hrafnkatla). The prolonged enrichment of volcanic metals and volatiles suggests continuous activity from multiple Icelandic sources. The dominance of tropospheric sulfate aerosols, indicated by sulfur isotope analysis, highlights the underestimation of tropospheric contributions in past volcanic forcing estimates. The correlation of the Hrafnkatla episode with documented climate anomalies in Europe and reduced Nile flow demonstrates the potential for even relatively high-latitude, prolonged eruptions to have significant, though regionally focused, climate impacts. The absence of a major summer cooling anomaly contrasts with the response to typical large explosive eruptions, suggesting that sustained aerosol emissions may modify climate responses. The study’s revised volcanic sulfate deposition estimates have implications for climate model simulations that currently neglect or underestimate tropospheric aerosol contributions. The significance lies in the improved understanding of pre-industrial volcanic forcing and its role in natural climate variability. Future research should focus on further quantifying the climate effects of sustained tropospheric emissions from prolonged volcanic episodes, particularly their impacts on the hydrological cycle.

This study demonstrates that the period 751–940 CE experienced significant Icelandic volcanic activity, contradicting the established concept of a Medieval Quiet Period. The multi-proxy geochemical approach improved the characterization of volcanic events and highlighted the importance of tropospheric aerosol contributions. The Hrafnkatla episode’s impact on regional climate was evident, underscoring the need for improved inclusion of tropospheric aerosols in climate models. Future work should focus on further investigations into prolonged volcanic episodes and their diverse climate feedbacks during pre-industrial periods to refine climate model simulations and reduce uncertainties in radiative forcing estimates.

While the study utilized a robust multi-proxy approach, some limitations exist. The precise quantification of aerosol emissions from prolonged eruptions remains challenging. The analysis primarily focuses on Icelandic volcanism; contributions from other volcanic regions might not be fully captured. The interpretation of palaeoenvironmental records is inherently complex; other factors than volcanic activity may contribute to the observed climate signals. The spatial resolution of climate impact assessment is limited due to the nature of available proxy data.