Jihong Cole-Dai

Late Glacial Stage and Holocene Tropical Ice Core Records from Huascarán, Peru

Two ice cores from the col of Huascar�n in the north-central Andes of Peru contain a paleoclimatic history extending well into the Wisconsinan (W�rm) Glacial Stage and include evidence of the Younger Dryas cool phase. Glacial stage conditions at high elevations in the tropics appear to have been as much as 8� to 12�C cooler than today, the atmosphere contained about 200 times as much dust, and the Amazon Basin forest cover may have been much less extensive. Differences in both the oxygen isotope ratio ζ18O (8 per mil) and the deuterium excess (4.5 per mil) from the Late Glacial Stage to the Holocene are comparable with polar ice core records. These data imply that the tropical Atlantic was possibly 5� to 6�C cooler during the Late Glacial Stage, that the climate was warmest from 8400 to 5200 years before present, and that it cooled gradually, culminating with the Little Ice Age (200 to 500 years before present). A strong warming has dominated the last two centuries.

Onset of deglacial warming in West Antarctica driven by local orbital forcing

The cause of warming in the Southern Hemisphere during the most recent deglaciation remains a matter of debate. Hypotheses for a Northern Hemisphere trigger, through oceanic redistributions of heat, are based in part on the abrupt onset of warming seen in East Antarctic ice cores and dated to 18,000 years ago, which is several thousand years after high-latitude Northern Hemisphere summer insolation intensity began increasing from its minimum, approximately 24,000 years ago. An alternative explanation is that local solar insolation changes cause the Southern Hemisphere to warm independently. Here we present results from a new, annually resolved ice-core record from West Antarctica that reconciles these two views. The records show that 18,000 years ago snow accumulation in West Antarctica began increasing, coincident with increasing carbon dioxide concentrations, warming in East Antarctica and cooling in the Northern Hemisphere associated with an abrupt decrease in Atlantic meridional overturning circulation. However, significant warming in West Antarctica began at least 2,000 years earlier. Circum-Antarctic sea-ice decline, driven by increasing local insolation, is the likely cause of this warming. The marine-influenced West Antarctic records suggest a more active role for the Southern Ocean in the onset of deglaciation than is inferred from ice cores in the East Antarctic interior, which are largely isolated from sea-ice changes.

Annually resolved southern hemisphere volcanic history from two Antarctic ice cores

The continuous sulfate analysis of two Antarctic ice cores, one from the Antarctic Peninsula region and one from West Antarctica, provides an annually resolved proxy history of southern semisphere volcanism since early in the 15th century. The dating is accurate within ±3 years due to the high rate of snow accumulation at both core sites and the small sample sizes used for analysis. The two sulfate records are consistent with each other. A systematic and objective method of separating outstanding sulfate events from the background sulfate flux is proposed and used to identify all volcanic signals. The resulting volcanic chronology covering 1417–1989 A.D. resolves temporal ambiguities about several recently discovered events. A number of previously unknown, moderate eruptions during late 1600s are uncovered in this chronology. The eruption of Tambora (1815) and the recently discovered eruption of Kuwae (1453) in the tropical South Pacific injected the greatest amount of sulfur dioxide into the southern hemisphere stratosphere during the last half millennium. A technique for comparing the magnitude of volcanic events preserved within different ice cores is developed using normalized sulfate flux. For the same eruptions the variability of the volcanic sulfate flux between the cores is within ±20% of the sulfate flux from the Tambora eruption.

A 4100‐year record of explosive volcanism from an East Antarctica ice core

Extensive archives of volcanic history are available from ice cores recovered from the Antarctic and Greenland ice sheets that receive and preserve sulfuric acid fallout from explosive volcanic eruptions. The continuous, detailed (average 1.2 samples per year) sulfate measurements of a 200-m ice core from a remote East Antarctica site (Plateau Remote) provide a record of Southern Hemisphere volcanism over the last 4100 years. This extends the volcanic record beyond the last 1000 years covered by previous Antarctic ice cores. An average of 1.3 eruptions per century is recorded in East Antarctic snow during the last 4100 years. The record shows that on average eruptions have been more frequent and more explosive during the most recent 2000 years than in the previous 2100 years. Intervals up to 500 years are observed in which few explosive volcanic signals are detected. These periods include 2000–1500 B.C. (no eruptions), 500–1 B.C. (two eruptions), and 700–1200 A.D. (two eruptions). This new Plateau Remote volcanic record is compared with those from previous Antarctic ice cores covering the last 1000 years. In terms of dates for volcanic events, the new record is in excellent agreement with the earlier records. However, significant discrepancies are found between these records in relative signal magnitude (volcanic flux) of several well-known events. The discrepancies among the records may be explained by the differences in the glaciology at the ice core sites, analytical techniques used for sulfate and sulfuric acid measurement, and the selection of detection thresholds for volcanic signals. Comparison with Greenland ice core volcanic records indicates that during the last millennium, nine large, low-latitude eruptions contributed significant amounts of volcanic aerosols to the atmosphere of both hemispheres, potentially affecting global climate. In contrast, only one or possibly two such eruptions are found in the first millennium A.D.

Precise interpolar phasing of abrupt climate change during the last ice age

The last glacial period exhibited abrupt Dansgaard–Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard–Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard–Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard–Oeschger dynamics.

Cold decade (AD 1810-1819) caused by Tambora (1815) and another (1809) stratospheric volcanic eruption.

Climate records indicate that the decade of AD 1810–1819 including “the year without a summer” (1816) is probably the coldest during the past 500 years or longer, and the cause of the climatic extreme has been attributed primarily to the 1815 cataclysmic Tambora eruption in Indonesia. But the cold temperatures in the early part of the decade and the timing of the Tambora eruption call into question the real climatic impact of volcanic eruptions. Here we present new evidence, based on sulfur isotope anomaly (Δ33S), a unique indicator of volcanic sulfuric acid produced in the stratosphere and preserved in polar snow, and on the precise timing of the volcanic deposition in both polar regions, that another large eruption in 1809 of a volcano is also stratospheric and occurred in the tropics. The Tambora eruption and the undocumented 1809 eruption are together responsible for the unusually cold decade.

Nitrogen isotopes in ice core nitrate linked to anthropogenic atmospheric acidity change.

Significance The specific cause of the long-term decrease in stable nitrogen isotope ratio (15N/14N) of ice core nitrate beginning ∼1850 is a subject of debate, hindering the efforts to understand changes in the global nitrogen cycle. Our high-resolution record of ice core 15N/14N combined with model calculations suggests that the decrease is mainly caused by equilibrium shift in gas−particle partitioning of atmospheric nitrate due to increasing atmospheric acidity resulting from anthropogenic emissions of nitrogen and sulfur oxides. Our high-resolution record also reveals a leveling off of 15N/14N ∼1970, synchronous with changes in acidity and sulfate and nitrate concentrations. This leveling off suggests a measurable reduction in air pollution following the implementation of the US Clean Air Act of 1970. Nitrogen stable isotope ratio (δ15N) in Greenland snow nitrate and in North American remote lake sediments has decreased gradually beginning as early as ∼1850 Christian Era. This decrease was attributed to increasing atmospheric deposition of anthropogenic nitrate, reflecting an anthropogenic impact on the global nitrogen cycle, and the impact was thought to be amplified ∼1970. However, our subannually resolved ice core records of δ15N and major ions (e.g., , ) over the last ∼200 y show that the decrease in δ15N is not always associated with increasing concentrations, and the decreasing trend actually leveled off ∼1970. Correlation of δ15N with H+, , and HNO3 concentrations, combined with nitrogen isotope fractionation models, suggests that the δ15N decrease from ∼1850–1970 was mainly caused by an anthropogenic-driven increase in atmospheric acidity through alteration of the gas−particle partitioning of atmospheric nitrate. The concentrations of and also leveled off ∼1970, reflecting the effect of air pollution mitigation strategies in North America on anthropogenic NOx and SO2 emissions. The consequent atmospheric acidity change, as reflected in the ice core record of H+ concentrations, is likely responsible for the leveling off of δ15N ∼1970, which, together with the leveling off of concentrations, suggests a regional mitigation of anthropogenic impact on the nitrogen cycle. Our results highlight the importance of atmospheric processes in controlling δ15N of nitrate and should be considered when using δ15N as a source indicator to study atmospheric flux of nitrate to land surface/ecosystems.

Quantifying the Pinatubo volcanic signal in south polar snow

Recent snow and firn core samples from South Pole contain increased sulfate (SO42−) concentrations during 1992–1994 as a result of the June 1991 Pinatubo eruption and the August 1991 Cerro Hudson eruption in Chile. Traces of Pinatubo tephra (volcanic ash) were identified in the 1993 and 1994 snow layers, supporting the conclusion that increased SO42− in 1993–1994 is from the Pinatubo eruption. Although the Pinatubo eruption preceded Hudson, its SO42− signal in south polar snow follows and is resolved from that of Hudson. The deposition of the Pinatubo SO42− aerosol was delayed due to the long transport to the high southern latitudes and its initial existence at high altitudes in the Antarctic atmosphere. Multi-year, multi-site sampling demonstrates that the volcanic signals are well preserved and spatially consistent. Measurements on 2 firn cores show that the South Pole SO SO42− flux from Pinatubo is 10.9±1.2 kg km−2 over 2.2 years, while the Hudson flux is 3.2±0.6 kg km−2 in 1.1 years. These results, when combined with satellite-determined Pinatubo sulfur dioxide (SO2) emission, make it possible to link quantitatively the atmospheric aerosol mass loading from a low-latitude volcanic eruption to its signal in polar ice cores.

The number and magnitude of large explosive volcanic eruptions between 904 and 1865 A.D.: Quantitative evidence from a new South Pole ice core

A new volcanic record covering the period of 904 to 1865 A.D. was produced from continuous chemical analysis of a 2001 South Pole ice core. This new record is consistent with previous records in the number and dates of large volcanic events. The relative magnitudes of several prominent events in this new record were compared to the same events in previous records from South Pole and Plateau Remote (East Antarctica) ice cores. The comparison demonstrates that the discrepancies in reported magnitudes of these events are probably a result of the glaciological complications at Plateau Remote, and that volcanic deposit or flux measurements from South Pole ice cores are therefore more reliable parameters of the atmospheric mass loadings of volcanic aerosols. The new record also confirms the previous finding that five large or moderately large volcanic eruptions occurred in the 13 th century. The total atmospheric aerosol mass loadings, inferred from volcanic sulfate flux in this new ice core, from these five eruptions appear to be 3 to 20 times those in other centuries during the last millennium, suggesting a significant role by explosive volcanism in the climatic transition from the Medieval Warm Period to the Little Ice Age.

Evidence of the 1991 Pinatubo volcanic eruption in South Polar snow

Traces of tephra and increased sulfate (SO42−) concentrations were identified in the 1992–1994 snow layers in 2 firn cores from South Pole. The deposition of the Pinatubo SO42− aerosol was delayed due to the long transport to the high south latitudes and its initial existence at high altitudes in the Antarctic atmosphere. Electron microscopic analyses show that the element composition of the tephra is identical to that of volcanic ash found near the Pinatubo volcano in Philippines. Detailed stratigraphic snow sampling resolved the Pinatubo signal from that of Cerro Hudson eruption during August 1991 in Chile. The South Pole sulfate flux from Pinatubo is calculated to be (10.9±1.1) kg·km−2, while the Hudson sulfate flux is (3.2±1.1) kg·km−2. This information will be useful to estimating the magnitudes of the past volcanic eruptions recorded in Antarctic ice core.