GISP2 Ice Core Research Articles

Magnetic properties of sediments in the Bay of Bengal and the Andaman Sea: impact of rapid North Atlantic Ocean climatic events on the strength of the Indian monsoon

The results of a high-resolution mineral magnetic study combined with major element geochemistry analysis, oxygen isotopes and 14C AMS stratigraphy are reported for deep-sea gravity cores MD77-169 and MD77-180 located in the Andaman Sea and the Bay of Bengal, respectively. Core MD77-169 covers the last 280 kyr and core MD77-180 covers the last 160 kyr. In both cores, rock magnetic parameters indicate that the magnetic assemblage is dominated by pseudo-single domain titanomagnetite grains, with grain-size variations following a strong 23 kyr periodicity. Smaller magnetic grain sizes are observed during periods characterized by a strong summer monsoon. In addition, in core MD77-180, we observe a correlation between magnetic grain size and a chemical index of alteration. This suggests that these magnetic grain-size changes are related to chemical weathering driven by summer monsoon rainfall. A comparison of the GISP2 ice core isotopic record and the magnetic grain-size record of the Bay of Bengal shows that rapid temperature variations documented in the ice core (Dansgaard–Oeschger cycles and Heinrich events), during the last glacial period are also present in the magnetic grain-size record. Heinrich events and cold stadial events are characterized by relatively large magnetic grain sizes. Furthermore, Heinrich events are characterized by lower values of the chemical index of alteration implying a lower degree of chemical weathering related to significantly drier conditions on the continent. We suggest that rapid cold events of the North Atlantic (Heinrich events) during the last glacial stages are characterized by a weaker summer monsoon rainfall over the Himalaya via an atmospheric teleconnection.

New Ice-Core Evidence Challenges the 1620sBCage for the Santorini (Minoan) Eruption

Determining a reliable calendrical age of the Santorini (Minoan) eruption is necessary to place the impact of the eruption into its proper context within Bronze Age society in the Aegean region. The high-resolution record of the deposition of volcanically produced acids on polar ice sheets, as available in the SO42−time series from ice cores (a direct signal), and the high-resolution record of the climatic impact of past volcanism inferred in tree rings (a secondary signal) have been widely used to assign a 1628/1627BCage to the eruption. The layer of ice in the GISP2 (Greenland) ice core corresponding to 1623±36BC, which is probably correlative to the 1628/1627BCevent, not only contains a large volcanic-SO42−spike, but it contains volcanic glass. Composition of this glass does not match the composition of glass from the Santorini eruption, thus severely challenging the 1620sBCage for the eruption. Similarly, the GISP2 glass does not match the composition of glass from other eruptions (Aniakchak, Mt. St. Helens, Vesuvius) thought to have occurred in the 17th centurybcnor does it match potential Icelandic sources. These findings suggest that an eruption not documented in the geological record is responsible for the many climate-proxy signals in the late 1620sBC. Although these findings do not unequivocally discount the 1620sBCage, we recommend that 1628/1627BCno longer be held as the “definitive” age for the Santorini eruption.

Penny ice cap cores, baffin island, canada, and the wisconsinan foxe dome connection: two states of hudson bay ice cover

Ice cores from Penny Ice Cap, Baffin Island, Canada, provide continuous Holocene records of oxygen isotopic composition (delta18O, proxy for temperature) and atmospheric impurities. A time scale was established with the use of altered seasonal variations, some volcanic horizons, and the age for the end of the Wisconsin ice age determined from the GRIP and GISP2 ice cores. There is pre-Holocene ice near the bed. The change in delta18O since the last glacial maximum (LGM) is at least 12.5 per mil, compared with an expected value of 7 per mil, suggesting that LGM ice originated at the much higher elevations of the then existing Foxe Dome and Foxe Ridge of the Laurentide Ice Sheet. The LGM delta18O values suggest thick ice frozen to the bed of Hudson Bay.

Tele-connections between East Asian Monsoon and the High-latitude Climate: A Comparison between the GISP2 Ice Core Record and the High Resolution Marine Records from the Japan and the South China Seas.

Based on the high resolution marine sediment records from the Japan and the South China Seas, a comparison to the GISP 2 ice core record suggests a climatic tele-connections between the low-to-mid latitude East Asian Monsoon climate and that over the high latitude Greenland during the last glacial period. Episodic warm periods of Dansgaard-Oeschger events are correlated to the periods when increased monsoon precipitation caused excess of rainfall in South and East China, hence the decrease in sea surface salinity in the South China Sea, and to the development of the dark laminated sediment layers due to the reduced vertical ventilation by a fresh water lid in the Japan Sea. The possible link in this tele-connection is believed to be a counterbalance between the westerly and the southwest-to-southeast summer monsoon wind. Whenever the high latitude polar Greenland was warmed up, the westerly would have reduced in its strength and/or extension. Consequently, the monsoon circulation culminated in the East Asia due to the increased land-sea pressure contrast during summer, when the low pressure cell over mid-high latitude land areas was intensified due to the high-latitude warming.

Grain‐scale processes, folding, and stratigraphic disturbance in the GISP2 ice core

Flow disturbances have been shown to alter stratigraphic order in the lower part of the ice sheet in central Greenland. Vertical thin sections of the Greenland Ice Sheet Project 2 ice core show that in the lower 30%, the expected c axis‐vertical fabric is interrupted by planes of grains (“stripes”) with c axes oriented approximately in the dip directions of the planes. Stripe‐parallel shear produces small‐scale folds. The stripes can be explained qualitatively by a simple nucleation‐and‐growth model based on the strong anisotropy of ice‐crystal deformation. Nucleation probably is sensitive to spatially variable rates of polygonization, producing spatially variable stripe densities. Stripes are modeled to affect the ice viscosity, so variations in stripe density may contribute to viscosity contrasts that might produce larger deformational features and loss of stratigraphic order.

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.

Eleven year cycle of dust concentration variability observed in the dust profile of the GISP2 Ice core from central Greenland: Possible solar cycle connection

We have re‐examined the dust record from the GISP2 ice core retrieved from central Greenland and found that the background dust concentration is modulated with a period of 11 years all the way back to ice from at least 100,000 yrs BP. The only known, relevant, naturally occurring phenomenon with such a period is the well‐known solar cycle. We believe that the background modulation we observe is related to the solar cycle and discuss this hypothesis.

The Younger Dryas Termination and North Atlantic Deep Water Formation: Insights from climate model simulations and Greenland Ice Cores

Results from the GISP2 and GRIP ice cores show that the termination of the Younger Dryas (YD) climate event in Greenland was a large and extremely fast climate change. A reinitiation of North Atlantic Deep Water formation following a shutdown, and its associated winter release of heat to the atmosphere, has been suggested as the most likely cause of this climate transition. To test this idea, two general circulation model experiments using GENESIS have been completed for YD time (12,000 calendar years ago): one with low heat flux in the Nordic Seas (10 W/m², deep water shutdown) and one with high Nordic Sea heat flux (300 W/m², active deep water formation). Comparison of Greenland climate differences between these experiments with the ice core records shows that when deep water is turned on, much of the YD termination warming is achieved. The increase in precipitation is underestimated because of a model tendency to overestimate summertime precipitation, which obscures the dominantly wintertime response to the specified forcing. The winter storm track shift toward Greenland contributes much of the climate change at the YD termination.

A New 14C Calibration Data Set for the Last Deglaciation Based on Marine Varves

Varved sediments of the tropical Cariaco Basin provide a new 14C calibration data set for the period of deglaciation (10,000 to 14,500 years before present: 10–14.5 cal ka bp). Independent evaluations of the Cariaco Basin calendar and 14C chronologies were based on the agreement of varve ages with the GISP2 ice core layer chronology for similar high-resolution paleoclimate records, in addition to 14C age agreement with terrestrial 14C dates, even during large climatic changes. These assessments indicate that the Cariaco Basin 14C reservoir age remained stable throughout the Younger Dryas and late Allerød climatic events and that the varve and 14C chronologies provide an accurate alternative to existing calibrations based on coral U/Th dates. The Cariaco Basin calibration generally agrees with coral-derived calibrations but is more continuous and resolves century-scale details of 14C change not seen in the coral records. 14C plateaus can be identified at 9.6, 11.4, and 11.7 14C ka bp, in addition to a large, sloping “plateau” during the Younger Dryas (∼10 to 11 14C ka bp). Accounting for features such as these is crucial to determining the relative timing and rates of change during abrupt global climate changes of the last deglaciation.

Correlation of Marine 14C Ages from the Nordic Seas with the GISP2 Isotope Record: Implications for 14C Calibration Beyond 25 ka BP

We present two new high-resolution sediment records from the southwestern Iceland and Norwegian Seas that were dated by numerous 14C ages up to 54 14C ka bp. Based on various lines of evidence, the local 14C reservoir effect was restricted to 400–1600 yr. The planktic stable isotope records reveal several meltwater spikes that were sampled with an average time resolution of 50 yr in PS2644 and 130 yr in core 23071 during isotope stage 3. Most of the δ18O spikes correlate peak-by-peak to the stadials and cold rebounds of the Dansgaard-Oeschger cycles in the annual-layer counted GISP2 ice core, with the major spikes reflecting the Heinrich events 1–6. This correlation indicates large fluctuations in the calibration of 14C ages between 20 and 54 14C ka bp. Generally the results confirm the 14C age shifts as predicted by the geomagnetic model of Laj, Mazaud and Duplessy (1996). However, the amplitude and speed of the abrupt decrease and subsequent major increase of our 14C shifts after 45 14C ka bp clearly exceed the geomagnetic prediction near 40–43 and 32–34 calendar (cal) ka bp. At these times, the geomagnetic field intensity minima linked to the Laschamp and the Mono Lake excursions and confirmed by a local geomagnetic record, probably led to a sudden increase in cosmogenic 14C and 10Be production, giving rise to excess 14C in the atmosphere of up to 1200%.

Volcanic ash from Icelandic ∼57,300 Yr BP eruption found in GISP2 (Greenland) Ice Core

We have found volcanic ash in Wisconsinan ice from the GISP2 ice core retrieved from the Summit location in Central Greenland. The ash consists, predominantly of large rhyolitic glass shards similar to the ones found in Wisconsinan Dye 3 ice from Southern Greenland. We give the results of our analysis of the ash and present evidence that correlates this ash with the Z2 ash layer found in Atlantic sediment cores, with the Dye 3 ash, with ash recently found in the GRIP ice core from Central Greenland and with ash associated with the Icelandic Thorsmörk ignimbrite. The mineralogy of the ash is very similar to that of ash from the Icelandic Tindfjallajökull volcano that is also present in the Thorsmörk ignimbrite and we suggest that this is the source volcano. Tindfjallajökull has also been suggested as the source of the Z2 ash. Based on the excellent dating of the GISP2 ice core, we determined that the eruptions that produced the GISP2 ash occured 57,300±1,700 yr BP which agrees extremely well with the most recent dating of 57,500±1,300 yr BP of the Z2 ash based on the SPECMAP chronology. This provides us with a reliable revision of the Dye 3 ice core dating at a depth of ∼1950 m. This ice was originally dated at ∼70,000 yr BP.

Depletion of atmospheric nitrate and chloride as a consequence of the Toba Volcanic Eruption

Continuous measurements of SO42− and electrical conductivity (ECM) along the GISP2 ice core record the Toba mega‐eruption at a depth 2590.95 to 2091.25 m (71,000±5000 years ago). Major chemical species were analyzed at a resolution of 1 cm per sample for this section. An ∼6‐year long period with extremely high volcanic SO42− coincident with a 94% depletion of nitrate and 63% depletion of chloride is observed at the depth of the Toba horizon. Such a reduction of chloride in a volcanic layer preserved in an ice core has not been observed in any previous studies. The nearly complete depletion of nitrate (to 5 ppb) encountered at the Toba level is the lowest value in the entire ∼250,000 years of the GISP2 ice core record. We propose possible mechanisms to explain the depletion of nitrate and chloride resulting from this mega‐eruption.

Rapid Variations in Atmospheric Methane Concentration During the Past 110,000 Years

A methane record from the GISP2 ice core reveals that millennial-scale variations in atmospheric methane concentration characterized much of the past 110,00 years. As previously observed in a shorter record from central Greenland, abrupt concentration shifts of about 50 to 300 parts per billion by volume were coeval with most of the interstadial warming events (better known as Dansgaard-Oeschger events) recorded in the GISP2 ice core throughout the last glacial period. The magnitude of the rapid concentration shifts varied on a longer time scale in a manner consistent with variations in Northern Hemisphere summer insolation, which suggests that insolation may have modulated the effects of interstadial climate change on the terrestrial biosphere.

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

An ∼6‐year long period of volcanic sulfate recorded in the GISP2 ice core about 71,100 ± 5000 years ago may provide detailed information on the atmospheric and climatic impact of the Toba mega‐eruption. Deposition of these aerosols occur at the beginning of an ∼1000‐year long stadial event, but not immediately before the longer glacial period beginning ∼67,500 years ago. Total stratospheric loading estimates over this ∼6‐year period range from 2200 to 4400 Mt of H2SO4 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 ∼1000‐year cooling event.

A 110,000-Yr Record of Explosive Volcanism from the GISP2 (Greenland) Ice Core

The time series of volcanically produced sulfate from the GISP2 ice core is used to develop a continuous record of explosive volcanism over the past 110,000 yr. We identified ∼850 volcanic signals (700 of these from 110,000 to 9000 yr ago) with sulfate concentrations greater than that associated with historical eruptions from either equatorial or mid-latitude regions that are known to have perturbed global or Northern Hemisphere climate, respectively. This number is a minimum because decreasing sampling resolution with depth, source volcano location, variable circulation patterns at the time of the eruption, and post-depositional modification of the signal can result in an incomplete record. The largest and most abundant volcanic signals over the past 110,000 yr, even after accounting for lower sampling resolution in the earlier part of the record, occur between 17,000 and 6000 yr ago, during and following the last deglaciation. A second period of enhanced volcanism occurs 35,000–22,000 yr ago, leading up to and during the last glacial maximum. These findings further support a possible climate-forcing component in volcanism. Increased volcanism often occurs during stadial/interstadial transitions within the last glaciation, but this is not consistent over the entire cycle. Ages for some of the largest known eruptions 100,000–9000 yr ago closely correspond to individual sulfate peaks or groups of peaks in our record.

Biomass burning recorded in the GISP2 ice core: a record from eastern Canada?

Combustion byproducts from biomass burning are preserved in annually stratified layers in the Greenland ice sheet. Electrical conductivity (ECM) and ammonium records from the GISP2 ice core in central Greenland have been used to develop a 6000-year record of biomass burning. Although there is some uncer tainty regarding the source area for the combustion byproducts, it is likely that the majority of the material originates in eastern Canada. Time periods of high biomass-burning activity occurred in 0-150 BP, 350-750 BP, and 5000 to >6000 BP. The period 1150-3250 BP is characterized by moderately elevated levels of biomass-burning activity. These periods are associated with drier conditions. The 5000 to >6000 BP period of high biomass-burning activity may also be related to forest successional sequences following the removal of the Laurentide ice sheet. This paper explains how ice cores can be used to develop records of biomass burning and offers suggestions for future studies.

Impurity influence on normal grain growth in the GISP2 ice core, Greenland

AbstractIntercept analysis of approximately bi-yearly vertical thin sections from the upper part of the GISP2 ice Core, central Greenland, shows that grain-size ranges increase with increasing age. This demonstrates that something in the ice affects grain-growth rates, and that grain-size cannot be used directly in paleothermometry as has been proposed. Correlation of grain-growth rates to chemical and isotopic data indicates slower growth in ice with higher impurity concentrations, and especially slow growth in “forest-fire” layers containing abundant ammonium; however, the impurity/grain-growth relations are quite noisy. Little correlation is found between growth rate and isotopic composition of ice.

Impurity influence on normal grain growth in the GISP2 ice core, Greenland

AbstractIntercept analysis of approximately bi-yearly vertical thin sections from the upper part of the GISP2 ice Core, central Greenland, shows that grain-size ranges increase with increasing age. This demonstrates that something in the ice affects grain-growth rates, and that grain-size cannot be used directly in paleothermometry as has been proposed. Correlation of grain-growth rates to chemical and isotopic data indicates slower growth in ice with higher impurity concentrations, and especially slow growth in “forest-fire” layers containing abundant ammonium; however, the impurity/grain-growth relations are quite noisy. Little correlation is found between growth rate and isotopic composition of ice.

Nature of basal debris in the GISP2 and Byrd ice cores and its relevance to bed processes

Successful core-drilling to bedrock of both the Greenland and Antarctic ice sheets offers unique opportunities for examining processes acting at the bed. At Byrd Station, Antarctica, penetration of the bed was accompanied by upwelling of glacial meltwater into the drillhole. The nature and disposition of sediment in the 4.83 m thick debris-rich basal ice, together with stable-isotope and gas analyses of the enclosing ice, confirm that incorporation of the debris occurred simultaneously with periodic “freeze-on” of basal meltwater. Currently, the presence of substantial meltwater at the ice/rock interface likely precludes any erosive activity at the bed. At GISP2, Greenland, basal silly ice, 13.1 m thick, is currently frozen to the bed at −9° C. Limited studies of the silty ice at GISP 2, together with results of more comprehensive investigations obtained by GRIP researchers on basal ice at a companion site at Summit indicate that the sediment-bearing basal ice likely formed in the absence of an ice sheet and was therefore unrelated to direct interaction of the present ice sheet with its bed. The fact that the basal ice at Summit is frozen to the bottom also precludes any likelihood of erosive activity at the bed.

Nature of basal debris in the GISP2 and Byrd ice cores and its relevance to bed processes

Successful core-drilling to bedrock of both the Greenland and Antarctic ice sheets offers unique opportunities for examining processes acting at the bed. At Byrd Station, Antarctica, penetration of the bed was accompanied by upwelling of glacial meltwater into the drillhole. The nature and disposition of sediment in the 4.83 m thick debris-rich basal ice, together with stable-isotope and gas analyses of the enclosing ice, confirm that incorporation of the debris occurred simultaneously with periodic “freeze-on” of basal meltwater. Currently, the presence of substantial meltwater at the ice/rock interface likely precludes any erosive activity at the bed. At GISP2, Greenland, basal silly ice, 13.1 m thick, is currently frozen to the bed at −9° C. Limited studies of the silty ice at GISP 2, together with results of more comprehensive investigations obtained by GRIP researchers on basal ice at a companion site at Summit indicate that the sediment-bearing basal ice likely formed in the absence of an ice sheet and was therefore unrelated to direct interaction of the present ice sheet with its bed. The fact that the basal ice at Summit is frozen to the bottom also precludes any likelihood of erosive activity at the bed.