South Pole ice core record of explosive volcanic eruptions in the first ...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D17308, doi:10.1029/2011JD015916, 2011

South Pole ice core record of explosive volcanic eruptions in the first and second millennia A.D. and evidence of a large eruption in the tropics around 535 A.D. Dave G. Ferris,1 Jihong Cole‐Dai,1 Angelica R. Reyes,1 and Drew M. Budner2 Received 5 March 2011; revised 21 June 2011; accepted 28 June 2011; published 14 September 2011.

[1] A record of explosive eruptions over the last 1830 years reconstructed from a South Pole, Antarctica, ice core extends the coverage of volcanic history to the start of the first millennium A.D. The ice core dating by annual layer counting carries an uncertainty of ±2% of the number of years from time markers, with the largest dating error of ±20 years at the bottom of the 182 m core. Several aspects of the methodology of detecting and quantifying volcanic sulfate signals in ice cores are examined in developing this record. The new record is remarkably consistent with previous South Pole records. A comparison with records from several Antarctica locations suggests that the South Pole location is among the best for ice core reconstruction of volcanic records, owing to the excellent preservation of volcanic signals at the South Pole, the relatively low and uniform sulfate background, and the moderately high snow accumulation rates which allow for dating by annual layer counting. A prominent volcanic event dated at 531(±15) A.D., along with evidence from other records, indicates that an unusually large eruption took place in the tropics and was probably responsible for the “mystery cloud” climate episode of 536–537 A.D. The date of 536 is suggested for a prominent volcanic signal that appears in the first half of the sixth century A.D. in ice cores, which can in turn be used as a time stratigraphic marker in dating ice cores by annual layer counting or by computing average accumulation rates or layer thicknesses with such markers. Citation: Ferris, D. G., J. Cole‐Dai, A. R. Reyes, and D. M. Budner (2011), South Pole ice core record of explosive volcanic eruptions in the first and second millennia A.D. and evidence of a large eruption in the tropics around 535 A.D., J. Geophys. Res., 116, D17308, doi:10.1029/2011JD015916.

1. Introduction [2] Explosive volcanic eruptions impact climate, for volcanic aerosols increase atmospheric albedo by reflecting and scattering incoming solar radiation [Cole‐Dai, 2010; Robock, 2000]. Volcanic aerosols can be transported over long distances and eventually are deposited on Earth’s surface, including polar ice sheets (Greenland and Antarctica) where they are preserved in the snow strata. The archived history of explosive volcanism can be recovered with polar ice cores [e.g., Hammer, 1977; Hammer et al., 1980]. Information regarding the relationship between past volcanism and climate perturbations is critical to climate change research in which the role and magnitude of natural variations need to be understood and quantified, in order to assess the anthropogenic impact. [3] Ice core volcanic records provide the dates and magnitude of past eruptions [Cole‐Dai, 2010; Gao et al., 2007, 1 Department of Chemistry and Biochemistry, South Dakota State University, Brookings, South Dakota, USA. 2 Department of Chemistry, Whitworth University, Spokane, Washington, USA.

Copyright 2011 by the American Geophysical Union. 0148‐0227/11/2011JD015916

2008]. Volcanic records have been obtained from ice cores from numerous locations in Antarctica [e.g., Castellano et al., 2005; Cole‐Dai et al., 2000; Kurbatov et al., 2006]. In general, existing records are in good agreement with each other on the characteristics of most eruptions that occurred in the last 1000 years [Cole‐Dai, 2010]. However, the agreement is very poor among the few long ice core records when volcanic signals older than 1000 years are considered [Castellano et al., 2005; Cole‐Dai et al., 2000; Kurbatov et al., 2006; Ren et al., 2010; Traufetter et al., 2004]. Ice core records from low snow accumulation sites are typically dated using averaged constant annual snow accumulation rates, derived from surface measurements and/or from time stratigraphic horizons or markers [Castellano et al., 2005; Cole‐Dai et al., 2000; Ren et al., 2010]. The resulting ice core chronology often suffers from large age uncertainty, particularly in time periods when the accumulation rate fluctuates or when time markers become infrequent or absent. Several well‐ documented volcanic eruptions in the last millennium, for example, Tambora (1815), Kuwae (1452–1453), and the 1259 Unknown [Cole‐Dai et al., 2000; Gao et al., 2006], have often been used as time markers. No such marker events have been convincingly established for the previous millennium. [4] We present a record of explosive volcanic eruptions over approximately the last 1830 years reconstructed from

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FERRIS ET AL.: 1830 YEAR SOUTH POLE VOLCANIC RECORD

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Figure 1. Annual winter peaks (denoted by asterisks) in Na+ and Mg2+ concentrations (smoothed with a three‐sample running mean filter) in a 2004–2005 South Pole snow pit. Summer depth hoar layers observed in the visible stratigraphy of the snow pit are represented with dark bands above the first panel. The appearance (first panel) of the volcanic sulfate from the Pinatubo and Cerro Hudson eruptions (1991) during 1992–1993 [Cole‐Dai and Mosley‐Thompson, 1999] supports the accuracy of the annual layer counting. The ambiguous magnesium peak at Year 2000 represents an example of the error in annual layer counting. detailed chemical analysis of a 182 m ice core from South Pole, Antarctica. This new record extends to the beginning of the first millennium A.D., exceeding the coverage of previous South Pole ice core records (1000 years) and of many records from other Antarctica locations. The high accuracy and precision of the chronology of this core, resulting from dating with annual layer counting, allow for the examination of significant volcanic eruptions in the first millennium A.D. Evidence of a significant volcanic eruption at 535 A.D. is presented, which can be used as a time marker in the dating of future ice cores. Several important aspects of the methodology of detecting and quantifying volcanic signals in ice cores are also examined.

2. Ice Cores, Sampling, and Analysis [5] During the 2004–2005 austral summer, two ice cores were drilled at a site (89°57.5′S, 17°40′W) 4.7 km west of the Amundsen‐Scott South Pole Station (90°S). The longer core (SP04C5) was 182 m in depth while the other core (SP04C6) was 102 m. The cores were packaged in approximately 1 m lengths and kept frozen during transport and storage. The two cores were sampled continuously for major ion measurement (Na+, K+, Mg2+, Ca2+, Cl−, NO−3 , SO2− 4 ) in the laboratory using a continuous flow analysis–ion chromatography (CFA‐IC) system which melts an ice core stick

(approximately 3.2 cm square) to produce a continuous stream of uncontaminated meltwater that is sampled and analyzed with a series of ion chromatographs. Details of the CFA‐IC system including limits of detection for all ions have been described previously [Cole‐Dai et al., 2006]. The main results of the analysis of the SP04C5 core are presented here. In all, 10,417 measurements (“samples”) were made along the 182 m core.

3. Results 3.1. Ice Core Dating [6] The relatively high rate of snow accumulation at South Pole, approximately 20 cm (∼7.5 cm water equivalent) per year, combined with high temporal resolution (average sample size 1.6 cm) achieved with the CFA‐IC technique, makes it possible to date the SP04 cores by annual layer counting (ALC). Previous work [Cole‐Dai and Mosley‐Thompson, 1999] has shown that annual South Pole snowfall contains a maximum and a minimum in the concentrations of predominantly single‐source species, such as Na+, Cl−, and Mg2+ in sea salt aerosols. Measurement of 2004–2005 snow pit samples shows that Mg2+ and Na+ concentration cycles (Figure 1) are unambiguously annual. Comparison with the appearance of the annual summer depth hoar layer (Figure 1) confirms that the concentrations generally peak in the austral winter season.

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Annual cycles in Mg2+ and Na+ concentrations were discernable throughout the SP04C5 and C6 cores, and were used to date these cores. In the counting procedure, the Mg2+ cycles served as the primary annual layer indicator, and Na+ cycles were used as a complementary indicator, particularly when Mg2+ cycles appeared ambiguous. Using this ALC technique, the bottom of the core was dated as the year 176 A.D. and, therefore, the 182 m core yielded an annually resolved 1830 year continuous chronology of snow chemical content. 3.2. Detection of Volcanic Events [7] Sulfate in Antarctic snow originates from a variety of aerosol sources such as sea salt from ocean sprays, marine biogenic emissions of sulfur compounds and volcanic eruptions [Cole‐Dai et al., 1997]. Volcanic sulfate is deposited in a brief time period following an eruption and is superimposed over a nonvolcanic background (Figure 2a). The common approach to detecting and quantifying volcanic signals in ice cores is to determine the nonvolcanic background and to establish its upper limit, called detection threshold. Sulfate values above the detection threshold are attributed to volcanic eruptions [Castellano et al., 2005; Cole‐Dai et al., 2000; Delmas et al., 1992; Kurbatov et al., 2006]. The upper limit of the nonvolcanic background can be assessed using the temporal variations of the background. Both the nonvolcanic background and its variation must be determined to establish the detection threshold. As the production of sea salt and marine biogenic compounds is relatively constant under stable climatic conditions, the temporal variations of the background along an ice core are usually very small, probably caused by random fluctuations in source strength, snow preservation processes, and/or in analytical measurement. [8] In this work, several different approaches to establish the detection threshold that have been used for Antarctic ice cores were evaluated and compared. One approach, used in several previous ice core studies [Cole‐Dai et al., 1997, 2000; Delmas et al., 1992], uses sulfate concentrations of individual samples to calculate the background and the threshold. In this approach, the average of the sulfate concentrations of all samples, after the removal of samples with high sulfate values associated with known or apparent volcanic eruptions, is used to represent the nonvolcanic background sulfate concentration; the standard deviation of the average is used to represent the variance of the background. For SP04C5, the background sulfate concentration (X) is found to be 46.4 mg L−1, with a standard deviation (s) of 12.7 mg L−1. Using the definition (X + 2 × s) originally introduced by Cole‐Dai et al. [1997], the SP04C5 volcanic detection threshold is calculated to be 71.8 mg L−1 (Figure 2b). As pointed out in previous work [Cole‐Dai et al., 2000], detecting volcanic signals with this threshold definition carries a significant probability of false positives from spuriously high background concentrations due to random variation. To reduce this probability, a secondary criterion is adopted – that a volcanic event must maintain sulfate concentration above the threshold for a minimum time period. In this work, the minimum duration of a volcanic event is set at 6 months (the length of time in a year is estimated from the proportion of the sample length in the annual layer thickness, assuming accumula-

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tion is constant throughout the year). Using the threshold of 71.8 mg L−1 and the 6 month duration criterion, 76 volcanic events were detected in SP04C5 (List 1, Table 1). [9] It is possible that, even under stable climatic conditions, the nonvolcanic background in Antarctic snow is variable at timescales longer than decades or centuries [Castellano et al., 2004, 2005; Traufetter et al., 2004]. To evaluate the effect of a variable background on the detection of volcanic signals, the concentration data was smoothed with a low‐pass filter resulting in a background of low‐ amplitude fluctuation. The detection threshold was obtained by appending the variations (2 × s) of the original nonvolcanic data set to the variable background (Figure 2c). This approach yielded a total of 74 volcanic signals, all of which were also identified with the constant‐background approach. This demonstrates that, when searching for volcanic signals in South Pole ice cores, similar results can be expected when either a constant or a variable background is assumed. [10] Castellano et al. [2004, 2005] proposed that the distribution of sulfate concentrations in Antarctic snow is better described by a lognormal pattern than a Gaussian pattern. A test of the SP04C5 data with the procedure described by Castellano et al. [2004] yielded a detection threshold (76.6 mg L−1), compared to that (71.8 mg L−1) obtained with the assumed‐Gaussian distribution of background variation. Applying the 76.6 mg L−1 detection threshold, 62 volcanic events were detected in SP04C5. This result is similar to that obtained using the weighted annual average concentration discussed later. [11] Gao et al. [2006] used a slightly different statistical approach to establish the threshold. First, the background and its low‐frequency (>31 years) variations were removed with a high‐pass loess filter; then the short‐term background variations were approximated with a 31 year running median absolute deviation (MAD) of the residuals. The threshold, defined as 2 times MAD (above the removed background), is similar in concept to that used by Cole‐Dai et al. [1997] and Castellano et al. [2005], although the Gao et al. [2006] approach does not require normal distribution of the background sulfate data. [12] Another approach utilizes either annual sulfate concentration or flux, rather than individual sample sulfate concentrations, to calculate the background and the threshold on an annual basis. With high‐resolution chemical analysis, a year containing more than one sample may be represented by the arithmetic mean of all sample concentrations in the year. However, this does not take into account the different lengths of time (depth interval) represented by individual samples. To compute a representative annual concentration, each sample concentration needs to be weighted by its proportional contribution to the length of the year. The weighted average annual concentration (WAA) in a year containing n samples is thus calculated according to ¼

n X i¼1

¼

n X

sampleðiÞsize Xn concentrationðiÞ sample sizeðiÞ i¼1 Weight ðiÞ concentration ðiÞ;

i¼1

in which n is the number of samples in the year.

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ð1Þ

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Figure 2. Sulfate profiles from the SP04C5 core displayed by (a) weighted annual average concentration of sulfate, (b) sample concentration of sulfate with average (solid line) and background threshold (dashed line) values using constant sulfate background approach, and (c) sample concentration of sulfate with smoothed average and background threshold values using variable sulfate background approach. The inset in Figure 2a displays a close‐up of the 531 A.D. volcanic event.

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Table 1. Volcanic Signals in the SP04C5 Core Detected Using the Sample Concentration Approach (List 1) and the Weighted Annual Average Concentration Approach (List 2)a SP04C6

Year A.D.

List 1

List 2

X No No X X X X X No X X X X X X X X X X X X No No X X X X X X X X X No

1992 1987 1979 1963 1887 1836 1816 1808 1754 1687 1668 1635 1616 1600 1594 1473 1453 1448 1443 1373 1334 1326 1287 1282 1274 1269 1260 1235 1196 1176 1133 1116 1080 1052 1044 1001 988 940 908 875 869 848 815 812 771 758 710 662 634 629 627 618 587 585 560 531 459 446 436 418 411 373 368 346 318 288 285 282 269

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

X No No X X X X X X X X X X X X No X X No X X X X X X X X X X X X X X X X X X X X No X No X No X X No X No X X X X No X X X X No X X X No X X X X No No

Table 1. (continued) SP04C6

Magnitude of Signal S S S S S S S S S

S S S S S S S S S S S S S S S S S S S S S

S S S

List 1

List 2

Magnitude of Signal

261 244 241 217 199 194 181

X X X X X X X

X X X No No X X

S S S S S

a

A cross indicates the signal was detected. The sample concentration approach was also used on the 102 m SP04C6 core (1070–2004 A.D.). S, signals with small volcanic sulfate flux ( 15 kg km−2) are not affected by the choice of approach to determine a volcanic threshold. Additionally, the sulfate concentration data of the 102 m core (SP04C6) were examined with the sample concentration approach for volcanic events. The examination showed that 6 of the 33 events in SP04C5 do not appear in SP04C6 in the time period when the two cores overlap (1070–2004 A.D.), suggesting they are possibly false‐positive volcanic signals. 5 of 11

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Table 2. Event Characteristics in the SP04 Volcanic Recorda Event Number

Year A.D.

Duration (Years)

Volcanic Flux ( f )

SP04–01 SP04–02 SP04–03 SP04–04 SP04–05 SP04–06 SP04–07 SP04–08 SP04–09 SP04–10 SP04–11 SP04–12 SP04–13 SP04–14 SP04–15 SP04–16 SP04–17 SP04–18 SP04–19 SP04–20 SP04–21 SP04–22 SP04–23 SP04–24 SP04–25 SP04–26 SP04–27 SP04–28 SP04–29 SP04–30 SP04–31 SP04–32 SP04–33 SP04–34 SP04–35 SP04–36 SP04–37 SP04–38 SP04–39 SP04–40 SP04–41 SP04–42 SP04–43 SP04–44 SP04–45 SP04–46 SP04–47 SP04–48 SP04–49 SP04–50 SP04–51 SP04–52 SP04–53 SP04–54 SP04–55 SP04–56 SP04–57 SP04–58 SP04–59 SP04–60

1992 1963 1887 1836 1816 1808 1754 1687 1668 1635 1616 1600 1594 1453 1448 1373 1334 1326 1287 1282 1274 1269 1260 1235 1196 1176 1133 1116 1080 1052 1044 1001 988 940 908 869 815 771 758 662 629 627 618 587 560 531 459 446 418 411 373 346 318 288 285 261 244 241 194 181

2.2 3.1 1.7 2.2 3.1 4.1 0.9 4.7 3.5 3.3 0.9 2.1 3.6 2.8 1.5 1.1 2.6 1.2 1.0 2.3 2.3 1.4 3.0 2.7 2.9 2.6 1.3 1.6 0.9 1.2 2.5 0.6 2.2 2.3 4.9 0.6 0.9 1.9 3.8 3.6 2.7 1.7 0.8 1.3 0.6 3.2 2.4 1.7 0.8 3.6 2.8 2.5 2.3 3.2 1.3 1.5 3.4 2.2 1.1 2.3

12.2 11.2 12.9 6.9 26.3 23.1 2.5 2.2 1.1 8.1 2.4 16.9 6.7 75.2 4.0 4.8 32.0 ≈0 5.9 14.7 35.3 19.7 99.3 24.4 11.3 10.9 4.5 11.9 4.6 4.6 6.4 4.3 12.9 7.2 26.4 8.6 13.8 9.7 41.8 38.4 27.1 7.7 6.9 1.9 5.9 41.9 5.6 5.4 6.1 14.7 7.5 22.4 21.8 13.5 4.0 1.9 10.9 6.2 0.4 9.8

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ing that time period and found that none of the events absent in List 2 are found in the other ice core records. The result of these examinations suggests that all small events in List 1 that do not appear in List 2 are likely false positives. In other words, the sample concentration approach for detection probably produces many more small, probably false positive events than the WAA approach. Therefore, the list (List 2 in Table 1) by the conservative WAA approach is chosen to represent the SP04 volcanic record. Such a choice may miss a few small events. However, as small volcanic eruptions usually produce small quantities of aerosols and therefore have limited or no impact on climate, volcanic events with the ability to induce climate change have a high likelihood of being identified in ice core records regardless of the approach or method to detect them. 3.3. The SP04 Volcanic Record [15] In Table 2, the volcanic events in SP04C5 detected with the conservative WAA approach are listed and numbered chronologically. The date of an event is the year when the WAA concentration first exceeds the threshold. The duration of an event is the length of time represented by the length of samples containing the event. These data, the dates/ years of the volcanic events, their durations and volcanic sulfate flux (f), represent a new record of explosive volcanism, the SP04 volcanic record covering the time period of 176 to 2004 A.D. (approximately 1830 years), from a South Pole ice core.

4. Discussion

a −2 Volcanic flux is measured in kg sulfate (SO2− 4 ) km . The calculated volcanic flux (total flux minus background flux) is negative for the small event (SP04–18) at 1326 A.D. owing to the unusually short depth interval for the year.

This comparison of events in two parallel cores to reduce the number of false positives is similar to the approach used by Traufetter et al. [2004]. We further examined the 4 events in List 1 in the last 1000 years that do not appear in List 2, against existing Antarctica ice core volcanic records cover-

4.1. Dating Errors [16] The development of an ice core depth‐age profile by ALC entails a measure of uncertainty [Alley et al., 1997; Taylor et al., 2004; Traufetter et al., 2004], owing to errors in determining the number of annual layers counted from the top (or a depth of known age) of the core to any given depth. Error sources include the fact that some concentration maximum‐minimum cycles are ambiguous as indicators of actual annual layers, and that, in some cases, an annual layer may be absent or “missing” in the seasonal concentration cycles. For example, the Mg2+ maximum of the year 2000 (at 100 cm in depth, Figure 1) is not clearly delineated (however, the Na+ data clearly shows a maximum at this depth), and, therefore, simple counting of Mg2+ maxima would have missed one year. On the other hand, the depth interval (85–125 cm) between the two adjacent Mg2+ maxima (labeled 1999 and 2001) is about twice as large as the average annual accumulation (∼20 cm), suggesting that the interval is likely the result of accumulation over 2 years. [17] An error‐counting method was adopted to estimate the magnitude of ALC uncertainty, with the assistance of a few volcanic time stratigraphic markers. An error of positive 1 year is noted if a counted year is not unambiguously represented by an unambiguous Mg2+ cycle (as would be in the case of the year 2000, but for the presence of the Na+ maximum); an error of negative 1 year is assigned to a concentration cycle that is not counted as an annual layer. Using this error‐counting scheme, a cumulative error of +6 and −5 years is estimated between the top of the core (Year 2004) and the age (Year 1816, or 188 years) of the signal of the well‐known Tambora volcanic eruption of 1815 A.D.

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Figure 3. A comparison of annual snow accumulation rate histories obtained from the (a) SP01 ice core and (b) SP04 ice core at the South Pole. The reduction in accumulation rate in SP04 during the period 1500–1900 A.D. is not observed in the SP01 history. This represents an error rate of +3% and −3%, respectively. Similarly, the cumulative errors are +11/−16 years, or +2%/ −3%, for the time period (557 years) between the Tambora signal and that of the well‐known 1259 Unknown Eruption (1259 UE). These data suggest that the maximum relative dating uncertainty for this core by ALC is about 3%, with the largest errors for years furthest away from time markers of known age. Because of the reduced annual layer thickness during the time period of 1500 to 1900 A.D. (see later discussion), the +3%/−3% maximum error rates obtained with the Tambora time marker is probably larger than those after 1900 or prior to 1500 A.D. The oldest time marker is the 1259 UE horizon, beyond which no volcanic signals can be definitively linked to known eruptions. A maximum error of +20/−20 years was obtained when counting from the depth (85 m) of the 1259 UE to the bottom of the 182 m core (176 A.D.). This corresponds to an error rate of +2%/ −2% (20 in 1083 years) and is consistent with those obtained for the upper part (surface to the Tambora marker) of the core. The largest uncertainty (±20 years) is associated with the oldest or the deepest part of the core. The magnitude of the ALC dating uncertainty for this South Pole core is similar to that obtained by Traufetter et al. [2004] using a similar annual layer counting technique. 4.2. Accumulation Variation [18] The annual layer thickness or net accumulation history for the last 1830 years is shown in Figure 3. Although generally stable with short‐term variations, a marked reduction in accumulation rate from the 1830 year

average of 7.5 cm H2O yr−1 is seen in the time period of 1500 to 1900 A.D., when the smoothed average accumulation is less than 7.0 cm H2O yr−1 (Figure 3b). Usually, a significant shift in accumulation rate on a century timescale is a characteristic of climate change [EPICA Community Members, 2004; Li et al., 2009]. However, such a reduction in the accumulation rate for this time period (1500– 1900) has not been previously reported [van der Veen et al., 1999]. In fact, all previous South Pole accumulation measurements using ice cores [Budner and Cole‐Dai, 2003; Cole‐Dai and Mosley‐Thompson, 1999; Delmas et al., 1992; van der Veen et al., 1999] indicate that the average accumulation rate is stable and close to 7.5 cm yr−1. For example, the accumulation history covering the period of 970–2001 in a 2001 ice core obtained at a slightly different location at South Pole (Budner and Cole‐Dai [2003] with updated data) contains no reduction in the 1500–1900 time period (Figure 3a). This suggests that the low accumulation rates for 1500–1900 in SP04C5 (similarly in SP04C6) are likely a result of surface glaciological processes affecting small locales in the South Pole area. [19] The reduced accumulation during 1500–1900, although with no significant effect on the background sulfate concentration level, impacts two characteristics of the SP04 volcanic record. First, the smaller annual layer thicknesses during the time period reduced the average number of sample measurement per year. As a result, the error rates of ALC dating for that period (∼3%) are slightly higher than those (2%) for other parts of the core. Fortunately, the 1259 UE marker used to anchor ALC for the lower portion

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Table 3. Volcanic Events in the Second Millennium A.D. in South Pole Cores and a Comparison of Volcanic Events and Flux in Ice Cores From Several East Antarctica Locations Ice Core Record SP04 SP01 Number of events 1992–1994 1990 1982 1968–1969 1963–1964 1932 1900 1883–1889 1884–1886 1880–1884 1852–1861 1842 1835–1837 1831 1816 1808–1810 1795–1800 1754–1762 1687–1696 1685–1691 1668–1676 1653–1662 1635–1641 1616–1624 1600–1610 1594–1596 1542 1508 1477–1480 1450–1460 1448 1422 1396 1373–1386 1334–1347 1326 1287 1282–1288 1274–1283 1269–1271 1258–1260 1222–1236 1188–1197 1170–1177 1143 1130–1133 1111–1118 1108 1094 1080–1083 1052 1040–1047 1001

32 12.2

34 10.1 3.3

SP84 22

PR84 EDC96 DML DT401 19

21 10.7

6.8

9.1

19

3.4 5.1

6.7 11.3

32 17.3

8.4

5.4 2.3 2.7

12.9

6.9 26.3 23.1 2.5 2.2 1.1 8.1 2.4 16.9 6.7

14.9

6.0–13.5 8.2–9.4 4.4

9.4

3.1 9.3 4.1

9.7 3.5

9.2–12.2 6.3 3.6 64.0 67.6–72.3 22.4 28.0 29.8–32.0 8.3 2.4 6.5 5.9 2.2 10.7 22.2 9.0 5.6 6.1 19.9 19.7 7.1 6.5 10.2 23.0 22.5 4.9 14.8 21.5 7.3

1.5 4.5 9.7 4.8 5.4

39.3 10.2

32.5 15.4

4.5 8.8

4.1 24.0 5.6 7.4

26.9 6.8

5.3

3.0 5.4

15.1 8.0 13.4

11.5 14.2 6.7

18.0

0.9 75.2 4.0

65.6

74.4

133.4

9.5 31.7

47.0

7.3 19.4

2.0 5.8 4.1 25.5

19.0

14.9

10.4

4.3 13.1

8.1 11.0

21.1 55.4 11.9 46.3 31.2 10.6

22.4

24.3 24.9 18.1 74.3 18.4 16.4 16.9

13.1 7.5

4.8 32.0 ≈0 5.9 14.7 24.3 35.3 45.4 19.7 19.9 99.3 179.5 24.4 26.6 11.3 13.2 10.9 15.7 4.5 11.9 4.6 4.6 6.4 4.3

87.3 10.5 135.7 12.1 19.5

20.5 60.4 25.2 18.0 20.8

66.0 7.9 3.9 4.4

11.5

10.2

6.8 5.2

2.2

8.6

8.9 3.5 5.61

of the core lies outside this period; therefore, the higher error rate caused by the change in accumulation does not propagate down the core. Second, since flux is directly affected by accumulation rate, the volcanic fluxes of volcanic events in the 1500–1900 time period are likely altered significantly by the reduction in accumulation rate. For example, the Tambora volcanic flux of 26.3 kg km−2 in SP04 is the smallest of all fluxes reported for South Pole ice cores [Budner and Cole‐Dai, 2003; Cole‐Dai and Mosley‐ Thompson, 1999; Delmas et al., 1992], or approximately 50% of those reported by Budner and Cole‐Dai [2003] and

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Delmas et al. [1992]. The Kuwae flux in SP04, at a time (1453–1455 A.D.) outside the period of the reduced accumulation, is similar to that in the other South Pole volcanic records, indicating that the reduced accumulation only affects the magnitude (flux) of volcanic signals in the period of 1500–1900 A.D. Because of the significantly small Tambora flux as a result of the reduced accumulation, no normalized volcanic flux ( f/fTambora), proposed as a relative scale to measure the magnitude of volcanic signals in ice cores [Cole‐Dai et al., 1997], is presented for the SP04 record. 4.3. Volcanic Record 4.3.1. The Second Millennium (1001–2000 A.D.) [20] Several volcanic records from South Pole ice cores have been previously presented. The two long records are the SP84 record (970–1983 A.D.) by Delmas et al. [1992] and the SP01 record (904–1865 A.D., database updated to 904–2001 A.D. in this work) by Budner and Cole‐Dai [2003]. An examination of volcanic events in the time period of 1001–2000 A.D. common in all three records (the Pinatubo signal, which appears at 1992–1993 in both SP04 and SP01, is added to SP84) shows that SP04 record contains 32 events, in comparison to 34 events in SP01 and 22 events in SP84 (Table 3). Most significant events are common in all three records; the exception is that the events located at approximately 1690 and 1235 A.D. were not detected in the SP84 cores. It is important to note that not all sections of the SP84 cores were analyzed for sulfate; only those sections containing probable volcanic signals, as indicated by ECM data, were analyzed for the presence of volcanic sulfate [Delmas et al., 1992]. [21] An examination of other Antarctic volcanic records shows that fewer volcanic events are recorded in the ice cores from locations in interior East Antarctica, where annual snow accumulation rates are very low. The numbers of volcanic events in this period in Plateau Remote [Cole‐ Dai et al., 2000], Dome C [Castellano et al., 2005], and DT401 [Ren et al., 2010] are 20 (Pinatubo added), 21, and 19, respectively (Table 3). It is likely that the strong postdepositional changes (e.g., partial loss of annual layers and snow resuspension and redistribution) cause the reduction of the amplitude of small volcanic signals to levels below the background noise and therefore below the detection threshold [Cole‐Dai et al., 2000]. On the other hand, Kurbatov et al. [2006] found more than 50 volcanic signals during this period in a Siple Dome, West Antarctica core, where annual accumulation is measured at about 13 cm H2O yr−1 [Taylor et al., 2004]. This much larger number of detected volcanic signals in the Siple Dome core could be a result of a low volcanic detection threshold against a highly variable nonvolcanic background [Kurbatov et al., 2006]. Another factor is the presence of volcanic deposits from local volcanic centers, such as those in Marie Byrd Land and the Pleiades [Dunbar et al., 2003; Kurbatov et al., 2006]. [22] A total of nine large signals (volcanic sulfate flux >15 kg km−2) are found in the 1001–2000 time period. The two largest signals in the SP04 record are those of the 1259 UE (f ≈ 100 kg km−2) and 1453 Kuwae eruption (75.2 kg km−2) [Gao et al., 2006]. Three other events (1334, 1274, and 1235) are comparable to Tambora (Table 2) in volcanic flux magnitude. However, the actual atmospheric

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aerosol mass loadings by these volcanic events are probably not as large as that by Tambora, for, as discussed earlier, the Tambora flux in SP04 is much smaller than expected as a result of the significantly reduced accumulation rate in the period of 1500–1900 A.D. The Kuwae eruption and 1259 UE are the largest volcanic events in the second millennium A.D. in most Antarctica ice core volcanic records, except those from Dome C [Castellano et al., 2005], Siple Dome [Kurbatov et al., 2006], and DT401 [Ren et al., 2010], in which the Kuwae signal is smaller than that of Tambora. The magnitude of volcanic signals in low‐accumulation areas (e.g., Dome C and DT401) is highly variable, compared with signals in areas of moderate or high accumulation [Cole‐Dai et al., 2000]. Signals in the Siple Dome record may be significantly affected by the unusually high and variable nonvolcanic background at the coastal West Antarctica location [Kurbatov et al., 2006]. [23] The volcanic records from the South Pole ice cores are remarkably consistent with each other (Table 3). The results of the comparison with ice core volcanic records from several Antarctica locations suggest that the South Pole location is highly advantageous over other Antarctic locations for constructing volcanic records owing to the excellent preservation of volcanic signals, the relatively low and uniform sulfate background levels, and the moderately high snow accumulation rates allowing ALC dating. 4.3.2. The First Millennium (176–1000 A.D.) [24] The three largest of the 28 volcanic signals in this period are similar in magnitude ( f ≈ 40 kg km−2) and are dated at 758, 662, and 531 A.D., with dating errors of ±10, ±12, and ±15 years (±2% of the number of years from 1259 A.D.), respectively. Moderately large signals (30 kg km−2 > f > 15 kg km−2) are found at 988(±5), 908(±7), 629(±13), 346(±18), and 318(±19). None of the signals are of the Kuwae and 1259 UE magnitude. The total number of very large and moderately large events is seven, compared with nine in the second millennium A.D. Other available Antarctic volcanic records [Castellano et al., 2005; Cole‐Dai et al., 2000; Kurbatov et al., 2006; Ren et al., 2010] contain no moderate or large volcanic signals in the first and second centuries A.D., other than the Taupo event (discussed below) and a moderate signal in the PR84 record [Cole‐Dai et al., 2000]. The SP04 record, along with these other records, suggests that there are more large and potentially climate‐impacting volcanic eruptions in the second millennium than in the first millennium A.D. [25] The Taupo, New Zealand, volcanic eruption, believed to have occurred in the second century A.D. [Simkin and Siebert, 1994; Wilson et al., 1980], is one of the largest known eruptions during that period. The large signal in the PR84 record [Cole‐Dai et al., 2000] was assigned the date of 186 A.D., on the basis of the estimate by Wilson et al. [1980]. However, the Taupo event, if present, appears as either a moderate or small signal in the Dome C and Siple Dome volcanic records [Castellano et al., 2005; Kurbatov et al., 2006]. The only moderate or large signal in the second century A.D. in the SP04 record is dated at 181(±21) A.D., which possibly corresponds to the Taupo eruption. [26] The large event dated at 531(±15) A.D. is of much interest and warrants discussion. The volcanic sulfate flux ( f = 42 kg km−2) of this event is comparable to that (44– 65 kg km−2) of Tambora measured in previous South Pole

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ice cores [Budner and Cole‐Dai, 2003; Cole‐Dai and Mosley‐Thompson, 1999; Delmas et al., 1992]. A similarly large sulfate event was ALC dated at 542(±17) in two Dronning Maud Land (DML), East Antarctica cores by Traufetter et al. [2004]. The overlap of the date ranges and the lack of other significant volcanic signals in the period of 500–578 in both the DML and SP04 records (Figure 2a, inset) suggest that these are likely from the same, probably very large, volcanic eruption. The lack of a corresponding signal in the Plateau Remote [Cole‐Dai et al., 2000] (the only noticeable volcanic signal in this time window in the PR84 record is found at 550 A.D.), Dome C [Castellano et al., 2005], and DT401 [Ren et al., 2010] records may be accounted for by the possible loss of volcanic horizons in the snow strata due to postdepositional changes in the extremely low accumulation environment, while the lack of such a signal in the Siple Dome record could be explained by the unusually high nonvolcanic background and large background variability. [27] Several volcanic events in the sixth century A.D. have been found in Greenland ice cores [Larsen et al., 2008]. Two prominent events separated by 5 years are proposed by Larsen et al. [2008] to result from two volcanic eruptions in 529 and 534(±2), respectively. The later event was suggested to correspond to a large explosive eruption in the tropics, while the earlier event was believed to be an eruption in the midlatitudes of the Northern Hemisphere (Japan). A close examination by Baillie [2008] of both tree ring and ice core records suggests that the dates of these two events should be revised to 535 and 542, respectively. It is believed [Baillie, 2008] that the earlier eruption occurred in mid northern latitudes, and the later eruption occurred in the tropics. One critical piece of evidence for this suggestion is that a large volcanic signal was found at 542(±17) in the ALC‐dated DML core [Traufetter et al., 2004]. The date and uncertainty (±15) of the 531 A.D. signal in the SP04 record support the conclusion by Traufetter et al. [2004] that a large eruption occurred in Southern Hemisphere or the tropics sometime in the period of 520–550 and no other large volcanic signals are found in the period of 500–560 A.D. [28] The debate about the timing of these ice core volcanic signals is important, for a clear and convincing result may have significant implications on the origin of the “mystery cloud” of 536–537 A.D. [Stothers and Rampino, 1983; Stothers, 1984]. According to contemporary reports (see references of Stothers [1984]), a widespread stratospheric haze/cloud (“dry fog”) covered most of the northern midlatitudes throughout the year of 536 and part of 537. The dimmed sun behind the prolonged haze probably resulted in cooler temperatures for more than 1 year and may have negatively affected crops and food production. A likely cause of the haze is a large volcanic eruption, either in the Northern Hemisphere or in the tropics, although the possibility of impact by an extraterrestrial object has been raised [Baillie, 1994; Rigby et al., 2004]. Stothers [1984] suggested that, because the haze appeared to be thicker in the southern part of the affected latitude zone than in the northern part, the volcano was likely located in the tropics (20°N–20°S). Such a large eruption would probably result in an outstanding sulfate signal during 536–538 in Antarctica ice cores. This date is within the range of the possible

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dates of the prominent sixth century event in the DML and SP04 records. [29] The SP04 and DML records [Traufetter et al., 2004], from two ALC‐dated Antarctica ice cores, support the proposal that in the year 535, a large volcanic eruption occurred in the low latitudes. This eruption significantly elevated stratospheric aerosol loading and atmospheric optical depth, reduced solar heating of the Earth surface, and caused significant and widespread human impact from the Mediterranean, to Mesopotamia and East Asia [Stothers and Rampino, 1983; Stothers, 1984], and possibly to Mesoamerica [Dull et al., 2001; Mehringer et al., 2005; R. A. Dull, personal communication, 2011]. We suggest that the date of 536 be assigned to the prominent volcanic signal in the time period of 520–550 A.D. in Antarctica ice cores, which can be used as a time stratigraphic marker in dating ice cores by annual layer counting or by computing average accumulation rates or layer thicknesses with such markers.

5. Conclusions [30] A new 1830 year (176–2004 A.D.) volcanic record (the SP04 record) has been obtained from the sulfate measurement of a 182 m South Pole ice core. The high temporal resolution of the chemical analysis captured the annual Mg2+ and Na+ concentration cycles and enabled the dating of the ice core through annual layer counting. The dating uncertainty is estimated to be about ±2% of the number of years from specific time markers including the top of the core, the well‐documented signals of the Tambora (1815), Kuwae (1453), and 1259 Unknown eruptions. [31] Several approaches of establishing the volcanic detection threshold over the nonvolcanic sulfate background have been used by investigators in previous studies of polar ice cores. These approaches have been compared and evaluated using data presented in this work. We found that the use of annually averaged sulfate concentration reduces the number of potentially false positive volcanic events, compared with using concentrations of individual samples. It can be concluded that, for Antarctica ice cores, the choice of the detection threshold affects only very small signals likely representing small volcanic eruptions with minimal climate impact. [32] Previous South Pole ice cores provided volcanic records covering the last 1000 years. The SP04 record extends the coverage to most of the first millennium A.D. It is consistent with previously published records from South Pole ice cores covering the second millennium A.D. During this time period, more volcanic events are found in South Pole ice cores than in ice cores from other East Antarctica locations where snow accumulation rates are much lower than that at South Pole. We suggest that the South Pole location provides several significant advantages (excellent signal preservation, low background noise, and the opportunity for annual layer counting) over other East Antarctica locations, when ice cores are used to reconstruct high‐ quality paleovolcanic records. [33] A prominent volcanic event is dated at 531(±15) A.D. in the SP04 record, corroborating the evidence of a large volcanic signal in a Dronning Maud Land, East Antarctica ice core [Traufetter et al., 2004]. No other large or moderate signals are found in the period of 420(±17)–620(±13) A.D.

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The unusual magnitude of this signal and the dating uncertainty, along with evidence from Greenland ice cores, suggest that a large volcanic eruption took place in the tropics in the first half of the sixth century A.D. This eruption may well be the cause of the “mystery cloud” of 536–537, which significantly reduced solar receipt, depressed surface temperatures, and may have resulted in worldwide catastrophic impact on human societies. [34] Acknowledgments. We thank Ice Drilling Design and Operations (formerly Ice Coring and Drilling Services), University of Wisconsin, for field assistance in drilling the South Pole ice cores. Financial support for this work has been provided by the U.S. National Science Foundation (NSF, Office of Polar Programs) via awards 0337933, 0538553, and 0087151. We acknowledge constructive and helpful comments by two anonymous reviewers.

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