Discovery of sulfur mass-independent fractionation (MIF) anomaly of stratospheric volcanic eruptions in Greenland ice cores

Volcanic eruptions emit sulfur dioxide which is oxidized to sulfuric acid aerosols in the atmosphere. Sulfuric acid aerosols affect global climate by altering the atmospheric radiative properties. Stratospheric eruptions can impact global climate due to the widespread distribution and relatively long atmospheric residence times (months to years) of the sulfate aerosols. Volcanic sulfate acid may be deposited on the polar ice sheets of Antarctica and Greenland and preserved by continuous accumulation of snow. Therefore, records of volcanic eruptions can be found in polar ice cores. The preserved sulfuric acid from large volcanic eruptions may serve as a valuable tracer of the oxidation process, particularly in the stratosphere.Ice core volcanic sulfate records are often used to assess the volcanic contribution to climate change. However, numerous sulfate signals of tropospheric eruptions in the ice core records make it difficult to evaluate the climatic impact of stratospheric eruptions in a glaciological record. Recent research suggests that the isotope composition of the volcanic sulfate can be used to distinguish between sulfate from stratospheric eruptions and that of tropospheric eruptions and, therefore, measurement of volcanic sulfate isotope composition can significantly improve the ice core records.Sulfuric acid generated in the stratosphere from sulfur dioxide contains a unique sulfur isotope signature through mass-independent fractionation (MIF). The sulfur MIF signature can be used to identify stratospheric volcanic events in the glaciological record. The stable isotope composition of sulfur is expressed by Equation 1, where delta represents the ratio of the isotope of interest (x = 33, 34, 36) to the most abundant isotope (32 S). dxS&percnt0 =&fll0Sx/S 32sample/ Sx/S32 std.-1&flr0*1000 Equation 1. Sulfur isotope ratios between heavy isotopes and 32S.Normally, a quantitative, mass-dependent relationship exists between the delta of one less abundant isotope and that of another. An MIF anomaly describes the extent of deviation from the relationship, as measured by "Delta", and calculated using Equations 2 and 3. DS33=dS 33-1000x&sqbl0&parl01+dS3 4/1000&parr00.515-1&sqbr0 DS36=dS 36-1000x&sqbl0&parl01+dS3 4/1000&parr01.91-1&sqbr0 Equations 2 and 3. Values of Sulfur MIF: Delta 33S and Delta36S deviations from mass-dependence.Previously, S-33 MIF anomaly has been found in volcanic sulfate in Antarctic ice cores. This is the first time the sulfur MIF anomaly has been measured in Greenland ice cores. This study hoped to find homogeneity in global stratospheric oxidation chemistry by finding similar MIF anomaly in the Northern Hemisphere. This study reports the results of sulfur MIF measurement of sulfate from seven volcanic events preserved in ice cores from Greenland between 1815 and 1259 A.D. Five of the seven eruptions exhibited significant Delta33S MIF values and have been determined to be stratospheric: Tambora (1815), the unknown 1809 event, two as yet undetermined eruptions in the mid 1400s (1461 and 1454), and the 1259 unknown event. The other two events (Laki, 1783 and an event in 1478 A.D.) seem to be tropospheric in nature.The 1259 A.D. eruption has the largest Delta33S values ever reported, beginning at 1.60&permil and ending with -1.34&permil. Tambora, which has been estimated to be 2-3 times smaller in magnitude than 1259 U.E., had measured Delta33S values of 0.23&permil to -0.27&permil along the beginning and end of the eruption, respectively. One of the two events in the mid-1400s is the Kuwae eruption, which was another stratospheric event with an estimated eruptive date of 1452 or 1453. Because the sulfate of both events contained MIF anomaly, this study is unable to conclude which of the two in Greenland ice cores is Kuwae. Finally, the 1809 unknown event was tested for the Delta33S anomaly and found to be stratospheric in nature (0.18&permil and -0.11&permil).Two of the measured events did not exhibit any Delta33S MIF anomaly: the 1783 Laki volcanic eruption (average 0.02&permil) and an event occurring in the year 1478 A.D. (average 0.05&permil), suggesting that neither eruption was stratospheric. Although some climatic variations in the 1780s have been attributed to the Laki eruption, the results presented here indicate that no significant amount of Laki SO2 was directly injected into the stratosphere and, therefore, that the Laki climatic impact was probably very limited.