Gregory A. Zielinski

The Greenland Ice Sheet Project 2 Depth-age Scale: Methods and Results

The Greenland Ice Sheet Project 2 (GISP2) depth-age scale is presented based on a multiparameter continuous count approach, to a depth of 2800 m, using a systematic combination of parameters that have never been used to this extent before. The ice at 2800 m is dated at 110,000 years B.P. with an estimated error ranging from 1 to 10% in the top 2500 m of the core and averaging 20% between 2500 and 2800 m. Parameters used to date the core include visual stratigraphy, oxygen isotopic ratios of the ice, electrical conductivity measurements, laser-light scattering from dust, volcanic signals, and major ion chemistry. GISP2 ages for major climatic events agree with independent ages based on varve chronologies, calibrated radiocarbon dates, and other techniques within the combined uncertainties. Good agreement also is obtained with Greenland Ice Core Project ice core dates and with the SPECMAP marine timescale after correlation through the δ 18 O of O 2 . Although the core is deformed below 2800 m and the continuity of the record is unclear, we attempted to date this section of the core on the basis of the laser-light scattering of dust in the ice.

Record of Volcanism Since 7000 B.C. from the GISP2 Greenland Ice Core and Implications for the Volcano-Climate System.

Sulfate concentrations from continuous biyearly sampling of the GISP2 Greenland ice core provide a record of potential climate-forcing volcanism since 7000 B.C. Although 85 percent of the events recorded over the last 2000 years were matched to documented volcanic eruptions, only about 30 percent of the events from 1 to 7000 B.C. were matched to such events. Several historic eruptions may have been greater sulfur producers than previously thought. There are three times as many events from 5000 to 7000 B.C. as over the last two millennia with sulfate deposition equal to or up to five times that of the largest known historical eruptions. This increased volcanism in the early Holocene may have contributed to climatic cooling.

Mount Mazama eruption: Calendrical age verified and atmospheric impact assessed

Geochemical identification of Mount Mazama ash in the Greenland Ice Sheet Project 2 (GISP2) ice core gives a calendrical age of 7627 ± 150 cal yr B.P. (5677 ± 150 B.C.) for the eruption, thus providing a more accurate early Holocene stratigraphic time line than previously available. The GISP2 record of volcanically derived sulfate suggests a total stratospheric aerosol loading between 88 and 224 Mt spread over an ∼6 yr period following the eruption of Mount Mazama. Taking into account the likelihood of some tropospheric aerosol transport to Greenland, realistic estimates of the resulting atmospheric optical depth range from 0.6 to 1.5. These values may have produced a temperature depression of ∼0.6 to 0.7 °C at mid to high northern latitudes for 1–3 yr after the eruption. These results indicate that the 5677 B.C. eruption of Mount Mazama was one of the most climatically significant volcanic events of the Holocene in the Northern Hemisphere. We also calculate a maximum stratospheric Cl − release of 8.1 Mt by the eruption, which may have led to substantial stratospheric ozone depletion.

Stratospheric loading and optical depth estimates of explosive volcanism over the last 2100 years derived from the Greenland Ice Sheet Project 2 ice core

The high-resolution and lengthy records of volcanic aerosol deposition in ice cores allow assessment of the atmospheric impact of different styles and magnitudes of past eruptions and the impact of volcanism during periods of varied climatic conditions. The 2100-year long volcanic SO 4 2- time series in the Greenland Ice Sheet Project 2 (GISP2) ice core was used to calculate the mass stratospheric loading (M D ) of H 2 SO 4 and resulting optical depth values (τ D =M D /1.5×10 14 g) for individual, and multiple, closely spaced eruptions. Calibration of the calculated optical depth values with other compilations spanning the last 150 years provides a range of values for each eruption or set of eruptions essential to quantifying the climate forcing capabilities of each of these events. Limitations on the use of the results exist because this is only a single ice core, sampling was biannual and transport, and deposition of aerosols is not consistent among individual eruptions. The record of volcanic optical depth estimates is characterized by distinct trends within three consecutive 700-year time periods. The period from 100 B.C. to A.D. 600 is characterized by the fewest eruptions, and optical depth values are lower than those in the rest of the record. The exception is an extremely large signal of 3 years duration that is probably associated with an unknown Icelandic eruption around 53 B.C., with the possible contribution of another high-latitude eruption. The presence of another signal at 43 B.C. suggests that at least two eruptions impacted climate in the middle decade of the 1st century B.C. The period from A.D. 600 to 1300 has intermediate numbers and magnitudes of volcanic events except for the very large 1259 event. Stratospheric loading and optical depths values for the 1259 event are twice that for Tambora (A.D. 1815). The state of the climate system in the middle of the thirteenth century A.D. may not have been sensitive enough to the atmospheric perturbation of the 1259 eruption, thus the apparent lack of abundant proxy evidence of climatic cooling around A.D. 1260. The most recent 700 years (A.D. 1400-1985) are characterized by the greatest number of eruptions (half of those recorded over the 2100 years of record) and, in general, the highest stratospheric loading and optical depth values for individual and the combined effects of multiple eruptions. The large Kuwae eruption (A.D. 1450s) may have perturbed the atmosphere at least as much as Krakatau and possibly of a magnitude similar to Tambora. Multiple eruptions in the 50- to 60-year periods from A.D. 1580s-1640s and A.D. 1780s-1830s may have had a significant impact on enhancing the already cool climatic conditions in those time periods, particularly around A.D. 1601 and 1641. These findings imply that multiple eruptions closely spaced in time are more likely to have a major impact on a decadal time scale when existing climatic conditions are in a more sensitive or transitional state. The GISP2 ice core record also indicates that several relatively unknown eruptions may have been large sulfur producers during the 17th and 19th centuries A.D., thereby warranting further studies of those particular events.

Volcanic Aerosol Records and Tephrochronology of the Summit, Greenland, Ice Cores

The recently collected Greenland Ice Sheet Project 2 (GISP2) and Greenland Ice Core Project ice cores from Summit, Greenland, provide lengthy and highly resolved records of the deposition of both the aerosol (H2SO4) and silicate (tephra) components of past volcanism. Both types of data are very beneficial in developing the hemispheric to global chronology of explosive volcanism and evaluating the entire volcanism-climate system. The continuous time series of volcanic SO42− for the last 110,000 years show a strong relationship between periods of increased volcanism and periods of climatic change. The greatest number of volcanic SO42− signals, many of very high magnitude, occur during and after the final stages of deglaciation (6000–17,000 years ago), possibly reflecting the increased crustal stresses that occur with changing volumes of continental ice sheets and with the subsequent changes in the volume of water in ocean basins (sea level change). The increase in the number of volcanic SO42− signals at 27,000–36,000 and 79,000–85,000 years ago may be related to initial ice sheet growth prior to the glacial maximum and prior to the beginning of the last period of glaciation, respectively. A comparison of the electrical conductivity of the GISP2 core with that of the volcanic SO42− record for the Holocene indicates that only about half of the larger volcanic signals are coincident in the two records. Other volcanic acids besides H2SO4 and other SO42− sources can complicate the comparisons, although the threshold level picked to make such comparisons is especially critical. Tephra has been found in both cores with a composition similar to that originating from the Vatnaoldur eruption that produced the Settlement Layer in Iceland (mid-A.D. 870s), from the Icelandic eruption that produced the Saksunarvatn ash (∼10,300 years ago), and from the Icelandic eruption(s) that produced the Z2 ash zone in North Atlantic marine cores (∼52,700 years ago). The presence of these layers provides absolute time lines for correlation between the two cores and for correlation with proxy records from marine sediment cores and terrestrial deposits containing these same tephras. The presence of both rhyolitic and basaltic shards in the Z2 ash in theGISP2 core and the composition of the basaltic grains lend support to multiple Icelandic sources (Torfajokull area and Katla) for the Z2 layer. Deposition of the Z2 layer occurs at the beginning of a stadial event, further reflecting the possibility of a volcanic triggering by the effects of changing climatic conditions.

Visual‐stratigraphic dating of the GISP2 ice core: Basis, reproducibility, and application

Annual layers are visible in the Greenland Ice Sheet Project 2 ice core from central Greenland, allowing rapid dating of the core. Changes in bubble and grain structure caused by near-surface, primarily summertime formation of hoar complexes provide the main visible annual marker in the Holocene, and changes in “cloudiness” of the ice correlated with dustiness mark Wisconsinan annual cycles; both markers are evident and have been intercalibrated in early Holocene ice. Layer counts are reproducible between different workers and for one worker at different times, with 1% error over century-length times in the Holocene. Reproducibility is typically 5% in Wisconsinan ice-age ice and decreases with increasing age and depth. Cumulative ages from visible stratigraphy are not significantly different from independent ages of prominent events for ice older than the historical record and younger than approximately 50,000 years. Visible observations are not greatly degraded by “brittle ice” or many other core-quality problems, allowing construction of long, consistently sampled time series. High accuracy requires careful study of the core by dedicated observers.

Hypothesized climate forcing time series for the last 500 years

A new compilation of annually resolved time series of atmospheric trace gas concentrations, solar irradiance, tropospheric aerosol optical depth, and stratospheric (volcanic) aerosol optical depth is presented for use in climate modeling studies of the period 1500 to 1999 A.D. Atmospheric CO 2 , CH 4 , and N 2 O concentrations over this period are well established on the basis of fossil air trapped in ice cores and instrumental measurements over the last few decades. Estimates of solar irradiance, ranging between 1364.2 and 1368.2 W/m 2 , are presented using calibrated historical observations of the Sun back to 1610, along with cosmogenic isotope variations extending back to 1500. Tropospheric aerosol distributions are calculated by scaling the modern distribution of sulfate and carbonaceous aerosol optical depths back to 1860 using reconstructed regional CO 2 emissions; prior to 1860 the anthropogenic tropospheric aerosol optical depths are assumed to be zero. Finally, the first continuous, annually dated record of zonally averaged stratospheric (volcanic) optical depths back to 1500 is constructed using sulfate flux data from multiple ice cores from both Greenland and Antarctica, in conjunction with historical and instrumental (satellite and pyrheliometric) observations. The climate forcings generated here are currently being used as input to a suite of transient (time dependent) paleoclimate model simulations of the past 500 years. These forcings are also available for comparison with instrumental and proxy paleoclimate data of the same period.

Potential atmospheric impact of the Toba Mega‐Eruption ∼71,000 years ago

An {approx}6 year-long period of volcanic sulfate recorded in the GISP2 ice core about 71,000 {+-} 5000 years ago may provide detailed information on the atmospheric and climate impact in the Toba mega-eruption. Deposition of these aerosols occur beginning of an {approx}1000-year long stadial event, but not immediately before the longer glacial period beginning {approx}67,500 years ago. Total stratospheric loading estimates over this {approx}6 year period range from 2200 to 4400 Mt of H{sub 2}SO{sub 4} aerosols. The range in values is given to compensate for uncertainties in aerosol transport. Magnitude and longevity of the atmospheric loading may have led directly to enhanced cooling during the initial two centuries of this {approx}1000-year cooling event. 25 refs., 2 fig., 1 tab.

Global influence of the AD 1600 eruption of Huaynaputina, Peru

It has long been estabished that gas and fine ash from large equatorial explosive eruptions can spread globally, and that the sulphuric acid that is consequently produced in the stratosphere can cause a small, but statistically significant, cooling of global temperatures,. Central to revealing the ancient volcano–climate connection have been studies linking single eruptions to features of climate-proxy records such as found in ice-core and tree-ring chronologies. Such records also suggest that the known inventory of eruptions is incomplete, and that the climatic significance of unreported or poorly understood eruptions remains to be revealed. The AD 1600 eruption of Huaynaputina, in southern Peru, has been speculated to be one of the largest eruptions of the past 500 years; acidity spikes from Greenland and Antarctica ice, tree-ring chronologies, along with records of atmospheric perturbations in early seventeenth-century Europe and China,, implicate an eruption of similar or greater magnitude than that of Krakatau in 1883. Here we use tephra deposits to estimate the volume of the AD 1600 Huaynaputina eruption, revealing that it was indeed one of the largest eruptions in historic times. The chemical characteristics of the glass from juvenile tephra allow a firm cause–effect link to be established with glass from the Antarctic ice, and thus improve on estimates of the stratospheric loading of the eruption.

Inter‐hemispheric Transport of Volcanic Ash from a 1259 A.D. Volcanic Eruption to the Greenland and Antarctic Ice Sheets

A strong volcanic sulfuric acid signal corresponding to an age of 1259 A.D. has been reported in ice cores from Greenland, Antarctica, and Arctic Canada. Tiny (< 5 μm) volcanic glass shards were reported previously in samples from this layer in an ice core from the South Pole. Here we report the discovery of volcanic glass shards from a contemporaneous layer in an ice core from Summit, Greenland. The major element composition of the glass shards in the Greenland sample are identical to those from the South Pole, confirming the assumption that has been made previously that the sulfuric acid signal in the ice cores is an inter-hemispheric time stratigraphic marker. The composition of these glass shards is similar to those produced by a 550–700 yrs. B.P. eruption of El Chichon volcano in Mexico, suggesting that it may be the source of the widely dispersed material.