Introduction
Establishing a depth–age relationship is an important first step in the fuller examination of an ice core. Until the core is dated it is not possible to interpret meaningfully the wealth of important paleo-environmental proxy data contained in the stratigraphie layers of ice sheets. Certain limitations exist in the dating of ice cores from both Greenland and Antarctica. The difficulties are related to the varying environmental conditions which existed at the original snow-deposit sites and to the post-depositional changes occurring there and along the flow-line trajectory leading to the drilling site. In many cases it is less difficult to date Greenland ice cores than, for instance, Antarctic ice cores (Langway and others 1985). The most successful primary methods for dating long ice cores have been the oxygen-isotope and the “acidity” measurement techniques, which are most accurate when performed in parallel (Hammer and others 1986). Both methods rely on unbroken, connected ice cores and continuous measurements, and they have been used to daLe ice cores back 103–104 years B.P.
We present here results of a successful attempt to cross-correlate a single, significant volcanic event in the thirteenth century, as a chemical signal recorded and measured from the same stratigraphie time units in both polar ice sheets.
Samples
Two sets of ice-core samples were used in this study. The first set was from the Greenland locations of Camp Century, Milcent, Crète and Dye 3. The other set was from the Antarctic locations of Byrd Station, South Pole and J-9 (on the Ross Ice Shelf). Table I lists details of samples and collection sites.
The Camp Ceitiury ice core was augered to the 500 m depth using a thermal coring device, and thereafter to bedrock—at about the 1400 m depth—using an electromechanical drill (Reference Ueda and GarfieldUeda and Garfield 1968). Core drilling took place between 1961 and 1966. This was the first continuous ice-core drilling operation to reach bedrock from an inland polar-ice sheet location. Although spot sampling and physical measurements were performed on the ice core in the field, it was not dated by any method until after transport back to the laboratory. The core was augered by two different experimental drilling systems, which caused several sections of the core profile to be fragmented, discontinuous or lost during drilling. The physical condition of the core compounded the difficulties of dating the ice, but it was on this core that the original continuous δ180 dating method for ice cores was developed (Dansgaard and others 1971).
Table 1. Sample and Site Identification for Greenland and Antarctica.
The Holocene ice in this core was dated by reference to seasonal δ180 and dust variations (Hammer and others 1978). At selected depths, d, the mean annual layer thickness λy (m of ice equivalent / year) was determined by counting seasonal variations over an ice-core increment of the length dy. The age t at the depth d = H – y (H is the ice thickness, y is the distance from the bottom) is:
This time-scale for the Camp Century core was later refined and fixed at certain depths, such as the historically recorded large Icelandic volcanic eruptions of Laki (A.D. 1783) and Eldja (A.D. 934), by using a solid electrical-conductivity method, ECM (Reference Hammer, Clausen and DansgaardHammer 1980).
The Milccnt ice core was augered to the 398 m depth during the summer of J973, using a thermal-coring device. The core was dated only by counting annua! variations in the continuously measured series of δ180. The Milcent core exhibits the best example of δ180 seasonals yet measured. This is mainly due to the site location’s relatively high rate of accumulation (53 cm of ice equivalent per year) and relatively low mean annual surface temperature (-22 °C), The high accumulation rate allows the annual δ180 oscillations to survive the active diffusion process which occurs during firnification, and the low surface temperature prevents extensive melting during the summer. The ice at the bottom of the core has been dated as A.D. 1176.
The Crête ice core was augered to the 404 m depth during the summer of 1974, also using a thermal-coring device. The core was dated by counting annual variations in the continuously measured series of δ180 and by a later application of the ECM method. For certain sections of the core, dust measurements and nitrate-concentration levels were used to demarcate limits of annual cycles. The Crête location is almost ideal for obtaining paleo-environmental proxy data and therefore serves as our Greenland reference core for the past 1400 years. The ice at the bottom of the core has been dated as A.D. 553.
Two ice cores were obtained at Dye 3. The first was recovered in 1971, to a depth of 372 m, using a thermal-coring device. The second core was augered to bedrock, at 2037 m, during the Greenland Ice Sheet Program (GISP) 1979–81 field operations, using a newly designed and constructed Danish drill (Gundestrup and others 1984). The 1971 core was dated by continuous δ180 measurements, and the 1979–81 core was dated by continuous δ180 measurements, the ECM method and dust measurements. The annual δ18O cycles are not as regular as those from Milcent because penetration and refreezing of melt water offsets the advantages of the high annual rate of accumulation {53 cm of ice equivalent per year) and the low mean annual surface temperature (-20°C).
The Byrd Station ice core was augered to bedrock, at 2164 m, during the 1967–68 field seasons, using the same electro-mechanical drill as at Camp Century (Reference Ueda and GarfieldUeda and Garfield 1969). This was the first ice-core drilling to reach bedrock in Antarctica.
The South Pole ice core was augered to 100 m in 1978 and the J-9 core to 100 m during the 1975–76 Ross Ice Shelf Project (RISP) field seasons, both cores were augered by electro-mechanical drills. Dating details for these cores are discussed in the next section.
Results and Discussions
The A.D. 1259 acid layer
Large volcanic eruptions are in general connected with the release of great amounts of strong acid gases into the atmosphere. The composition and amount of these gases vary with time and are often deposited as strong acids such as H2S04, HC1 and HF on the polar ice-sheet surfaces in an amount related to the violence and extent of the eruption. Other factors which influence the composition and amount of volcanic acids are the chemical nature of the eruption and the distance to the ice sheet. These acid layers arc identified in ice cores as peaks in the electrical-conductivity records and can be detected by a solid ECM, as discussed by Reference Hammer, Clausen and DansgaardHammer (1980).
Acid signals from major volcanic eruptions in Iceland and from violent eruptions south of 50 °N are clearly present in snow deposits all over Greenland (Reference HammerHammer 1984). In the 400 m deep Crête core, the third-largest acid signal recovered (Fig. 1) was detected in the A.D. 1258–59 layer (Hammer and others 1980). In both Dye 3 cores (Fig. 2), a large acid signal was found in the A.D. 1258-59 layers. This signal is among the largest in the Holocene Dye 3 acidity record. In the Milcent core (Fig. 3), a large acid signal was found in the layer dated A.D. 1259 by counting δ180 oscillations downwards from the top. In the Camp Century core (Fig. 4), a large acid signal was detected in an ice layer dated to about A.D. 1250. This signal is among the largest found for the entire Holocene record of Camp Century.
Figs. 1-9. Title at top refers to the site (Greenland and Antarctica) and the year the core was drilled. Figures 1-6 and 9 show on a depth scale in m below the surface the anion concentrations (shaded curves) of CI”, NO,” and SO4 2” in fi equivalents per kg of ice (jieq/kg) from the A.D. 1259 event in the ice samples. Figures 7 and 8 exhibit the anion concentrations at South Pole for the Krakatoa (A.D. 1883) and Tambora (A.D. 1815) eruptions respectively. Figures 1-3 show the seasonal variation of 6 O in per mille (scale to the right). The top curve in Figures 1-9 (except for Fig.4, see text), shows the electrical current in namp, determined by the ECM at –14 C. This method serves here to identify the exact position of the high volcanic-acid layer in the core.
Figs. 1-9. Title at top refers to the site (Greenland and Antarctica) and the year the core was drilled. Figures 1-6 and 9 show on a depth scale in m below the surface the anion concentrations (shaded curves) of CI”, NO,” and SO4 2” in fi equivalents per kg of ice (jieq/kg) from the A.D. 1259 event in the ice samples. Figures 7 and 8 exhibit the anion concentrations at South Pole for the Krakatoa (A.D. 1883) and Tambora (A.D. 1815) eruptions respectively. Figures 1-3 show the seasonal variation of 6 O in per mille (scale to the right). The top curve in Figures 1-9 (except for Fig.4, see text), shows the electrical current in namp, determined by the ECM at –14 C. This method serves here to identify the exact position of the high volcanic-acid layer in the core.
Figs. 1-9. Title at top refers to the site (Greenland and Antarctica) and the year the core was drilled. Figures 1-6 and 9 show on a depth scale in m below the surface the anion concentrations (shaded curves) of CI”, NO,” and SO4 2” in fi equivalents per kg of ice (jieq/kg) from the A.D. 1259 event in the ice samples. Figures 7 and 8 exhibit the anion concentrations at South Pole for the Krakatoa (A.D. 1883) and Tambora (A.D. 1815) eruptions respectively. Figures 1-3 show the seasonal variation of 6 O in per mille (scale to the right). The top curve in Figures 1-9 (except for Fig.4, see text), shows the electrical current in namp, determined by the ECM at –14 C. This method serves here to identify the exact position of the high volcanic-acid layer in the core.
We find no record of any large volcanic eruption occurring around A.D. 1260 in any Icelandic record or other references (Simkin and others 1981). Therefore we believe that the A.D. 1259 eruption occurred farther south of 50°N. If this event took place in the equatorial zone, it would probably be recorded in the Antarctic ice. In fact, high-acidity signals were found in Antarctic ice cores from Byrd Station, South Pole and J–9 at the A.D. 1250 levels. In the Byrd Station core, one of the largest acid signals in the entire continuous acidity record, which covers about the last 60 ka B.P. (Hammer and others 1985), was detected at the depth of 97.8 m (Fig. 5). In the South Pole core, a large acid signal in the continuous acidity profile was detected at the 87.8 m depth (Fig. 6). Two other distinct acid signals were found at depths of 16.3 m and 26.3 m (Figs.7 and 8).
Between the depth interval 70-95 m in the J-9 core, the largest acidity signal measured was detected at a depth of 85.2 m (Fig. 9). Due to the relatively low rate of accumulation at the Antarctic locations included in this study, seasonal δ180 variations do not survive the diffusion processes during densification of the firn. Table II contains the calculated ages of the acid layers for the Antarctic stations, assuming a uniform vertical strain-rate for all sites. Schwander (unpublished) found similar distinct acid layers at comparable depth in another ice core from the South Pole. Zanolini and others (1985) report that the largest acidity and sulfate peak found in a 200 m ice core from Terre Adéhe could be due to a powerful volcanic eruption some 600 years ago. This powerful eruption is probably the
Table II. Acid-Layer Depths and Ages for Antarctic Sites.
The snow depths in Table II are converted into meters of ice equivalent, using measured density profiles from South Pole (Reference Chiang and LangwayChiang and Langway 1978), from Byrd Station (Reference Bender and GowBender and Gow 1961) and from J-9 (Langway 1976). The annual rate of accumulation in meters of ice equivalent per year at South Pole is 0.09 m (Lambert and others 1977), at Byrd Station it is between 0.12 m (Gow and others 1972) and 0.11 m (Hammer and others, in preparation) and at J–9 it is 0.098 m (Reference Clausen and DansgaardClausen and Dansgaard 1977).
The acidity profile for the Byrd Station core exhibits seasonal variations thousands of years beyond the Holocene/ Wisconsin transition, which is about 11 ka B.P. (Hammer and others 1985). Since the upper 88 m of the Byrd Station core are missing, we have only the calculated age for the acid layer at 97.8 m. As can be seen from Table II, a date of A.D. 1259 for this acid event agrees well with the 1238–99 range for the calculated ages.
The South Pole acidity profile is difficult to interpret in terms of seasonal variations, due to the low rate of accumulation at this site. From accumulation-rate studies of the Ross Ice Shelf (Reference Clausen and DansgaardClausen and Dansgaard 1977), we know it is difficult to apply the method of visible stratigraphy to dating ice cores from regions with a low annual rate of accumulation. Reference Mosley-Thompson and ThompsonMosley-Thompson and Thompson (1982) dated a 104 m South Pole core (drilled in 1974) by counting microparticle oscillations, but concluded that as many as 90 years may be missing from the record, due to wind erosion and lack of precipitation. If we apply their time-scale to the three acid signals we find at the 16.3, 26.2 and 87.8 m depths, we obtain the years 1887, 1812, and 1216 respectively. Although there is a 4 year span between the drillings, and some distance between the cores, the ages we arrived at strongly indicate that these acid signals are the volcanic “footprints” of Krakatoa (A.D. 1883), Tambora (A.D. 1815) and the “A.D. 1259 event” respectively, and only about 40 years of accumulation are missing or poorly represented in the last 700 years of precipitation at the South Pole.
Using 0.098 m of ice equivalent / year for the present annual mean rate of accumulation at J–9, we calculate the 85.2 m depth as being 786 years old, or about 10% older than it should be if it represents A.D. 1259. In other words, the age may have been overestimated by 69 years, or 10%, due to changes in the accumulation rate, which would account for the difference. Since this is the largest acid signal in the 250 year depth interval we measured, we take this peak as representing A.D. 1259.
Anion composition
Increments of the ice cores at depth intervals around the acidity peaks were measured for SO4 2-, NO3 − and C− concentrations by ion chromatography in the Ice Core Laboratory at the State University of New York at Buffalo. The results are shown in Figures 1–9, where the concentrations are given in μ equivalent per kg (μeq/kg). Sulfate is the dominant anion in all acid layers. The nitrate content shows an average of around 1 μeq/kg, with little variation from year to year, even during the deposition of the volcanic acid. The chloride content of these samples shows some well-known trends of higher concentrations at coastal and lower-altitude sites (J–9, Byrd Station and Camp Century) and a decrease inland towards high-altitude sites (South Pole and Crête). These findings are consistent with other studies (Herron and others 1977, Reference BoutronBoutron 1979, Reference Herron, Herrón and LangwayHerron and Langway 1979, Reference Herron, Herrón and LangwayHerron 1982, Rasmussen and others 1984, Finkel and others 1986, Mayewski and others 1986).
In this study, the ECM method (Reference HammerHammer 1980) was merely used to identify high acid signals, and no calibration by liquid pH measurements was performed to convert the electrical currents in μamp into H+ concentration. In Figures 1–9 (but not Fig. 4, Camp Century) the top curve shows measured currents in μamp at –14°C. For Camp Century, a very thin sample that had been stored for over 20 years was measured by ECM. Hence the current level cannot be compared directly to the current values obtained from the other cores; however, a clear increase in the current was observed in the depth interval 263.6–263.8 m. The seasonal variations in δ18O are shown for the three Greenland sites also in Figures 1—3.
Volcanic H2SO4 deposition
On the basis of the measured sulfate-concentration levels, we calculated the deposition rate of volcanic H2SO4, which is defined as the area above the general background in Figures 1–9. Table III lists the mean sulfate concentrations (column 2) over the length of ice (column 5) which represents the period of volcanic deposition, background sulfate concentrations (bg, column 3), volcanic-sulfate concentrations (column 4), deposition rates of volcanic sulfate (column 6) and desposition rates of volcanic sulphuric acid (column 7).
The H2SO4 deposition from the A.D. 1259 eruption was 117 and 90 kg/km2 at Crête and South Pole (SP) respectively, and is of the same order of magnitude as given for the Tambora eruption (69 and 83 kg/km2 respectively). For comparison, the Icelandic eruption of Laki (A.D. 1783) deposited 137 kg/km2 in the Crête region (Reference Clausen and HammerClausen and Hammer 1988, this volume). In the South Pole region, the Tambora eruption deposited 83 kg/km2, some eight times more than that deposited by the Krakatoa eruption (11 kg/km2), which is similar to the natural background of 8 kg/km2 during the time of volcanic deposition.
Table III. Suasjfd Sulfate Deposition Bates
Table IV. Ratio of Depositions in Greenland and Antarctica.
Latitude of 1259 eruption
To estimate the site of the A.D. 1259 eruption, we compare the H2SO4 deposition from known volcanic eruptions with the deposition of the total β activity from atmospheric nuclear-bomb tests. We assume a uniform global deposition pattern of volcanic-acid gases from large volcanic eruptions and of the debris produced by atmospheric nuclear tests if occurring at the same latitude. At present we know of three well-documented global events which are clearly recognized as index horizons for the entire Greenland and Antarctic ice sheets:
the Tambora eruption at 8°S in 1815,
the American bomb tests at 11 °N in 1952 and 1954,
the Russian bomb tests at 75 °N in 1961 and 1962 (UNSCEAR Report 1982). Table IV lists data for these events. Greenland is represented by Dye 3 and Crête, and Antarctica by South Pole, because ice cores from these regions have been analyzed both for anions from the three major volcanic eruptions (“1259”, Laki and Tambora) and for total β activity from the Russian and American atmospheric bomb-test series.
The total β values are given in deposition of mCi/km2, originating from a total β injection of 22.6 and 9.6 MCi (90Sr + 137Cs = “total β”) at 75° and 11°N respectively (Reference Clausen and HammerClausen and Hammer 1988, this volume). R is the ratio between the depositions in Greenland and Antarctica. In Figure 10, R is plotted on a semi-logarithmic scale as a function of the latitude of the site of injection. This figure suggests that the “1259” volcano was most likely to be located close to the Equator, probably in the Northern Hemisphere.
Total volcanic amount of H2SO4.
Combining the deposition of the total β activity originating from the 9.6 MCi injected at low northern latitude in 1952 and 1954 (column 4, Table V) with the deposition of volcanic H2SO4 (column 3, Table V), we calculate in Table V the total amount of HsSO4 injected into the atmosphere (column 6). Column 5 shows that the 9.6 MCi (e.g. at Crête) is equal to 2.86 × 109 times the total β deposition per km2.
Table V. Magnitude of the Volcanic Eruptions.
The total amount of H2SO4 in the 1259 eruption was some 300 × 106 tons, with the mean value (298 ± 60) × 106 and (299 ± 84) × 106 tons for Greenland and Antarctic sites respectively (column 6, Table V). The individual values vary by less than a factor of 2, and the coefficients of variations in the Greenland and Antarctic mean values are 20 and 30% respectively. This scatter is easily explained by drift-snow (sastrugi) noise, which in Greenland typically results in accumulation-series noise of about 40% of the yearly value {Fisher and others 1985). In regions with a low rate of annual accumulation this kind of noise increases. The Tambora and the “1259” event are comparable in magnitude, and Krakatoa is one order of magnitude smaller. The Greenland values of the Tambora eruption are from Reference Clausen and HammerClausen and Hammer 1988 (this volume).
Fig. 10 shows on a semi-logarithmic scale the ratio R from Table IV (between the depositions in Greenland and Antarctica) of injections (total 0 activity from nuclear-bomb tests and strong acids from the Tambora eruption) into the atmosphere as a function of the site of injection (degree latitude). 1961, 1953 and 1815 refer to the 1961-62, 1952 and 1954 atmospheric bomb-test series and to the 1815 Tambora eruption respectively. R of the A.D. 1259 event (dotted line) corresponds to a site of eruption close to the Equator in the Northern Hemisphere.
The calculation of the magnitude of the Tambora eruption at 8°S, based on the Greenland total β deposition from a low northern-latitude test site, will probably underestimate the total H2SO4 production, because the volcano is located south of the Equator and the total β was injected into the atmosphere north of the Equator. In a similar way the calculation based on Antarctic values has probably overestimated the production of the H2SO4, suggesting a total H2SO4 production similar to that of the 1259 event, some 300 × 106 tons.
Conclusion
A major volcanic eruption in A.D. 1259, close to the Equator in the Northern Hemisphere, injected some 300 × 1O6 tons of sulphuric acid into the atmosphere. This volcanic eruption is one of the largest which has occurred in the last 10 000 years, of the same magnitude as the Tambora 1815 eruption, and serves as a distinct inter-hemispheric index horizon for the dating of ice cores.
Acknowledgements
Part of this study was performed while H B Clausen spent 3 months as Visiting Professor in the Ice Core Laboratory at the State University of New York at Buffalo. Thanks are given to K Azuma, who performed the laboratory chemical analyses, and to the Commission for Scientific Research in Greenland, the European Economic Communities (contract CLI-067 DK(G)) and to the U.S. National Science Foundation, Division of Polar Programs, for financial support of the laboratories in Buffalo and Copenhagen.
