Ice Core Record Research Articles

Increasing fire activity in a warming climate? Ice core record insights from the present and the last interglacials

Increasing fire activity in a warming climate? Ice core record insights from the present and the last interglacials

A new bipolar ice core record of volcanism from WAIS Divide and NEEM and implications for climate forcing of the last 2000 years

Volcanism is a natural climate forcing causing short‐term variations in temperatures. Histories of volcanic eruptions are needed to quantify their role in climate variability and assess human impacts. We present two new seasonally resolved, annually dated non‐sea‐salt sulfur records from polar ice cores—WAIS Divide (WDC06A) from West Antarctica spanning 408 B.C.E. to 2003 C.E. and NEEM (NEEM‐2011‐S1) from Greenland spanning 78 to 1997 C.E.—both analyzed using high‐resolution continuous flow analysis coupled to two mass spectrometers. The high dating accuracy allowed placing the large bi‐hemispheric deposition event ascribed to the eruption of Kuwae in Vanuatu (previously thought to be 1452/1453 C.E. and used as a tie‐point in ice core dating) into the year 1458/1459 C.E. This new age is consistent with an independent ice core timescale from Law Dome and explains an apparent delayed response in tree rings to this volcanic event. A second volcanic event is detected in 1453 C.E. in both ice cores. We show for the first time ice core signals in Greenland and Antarctica from the strong eruption of Taupo in New Zealand in 232 C.E. In total, 133 volcanic events were extracted from WDC06A and 138 from NEEM‐2011‐S1, with 50 ice core signals—predominantly from tropical source volcanoes—identified simultaneously in both records. We assess the effect of large bipolar events on temperature‐sensitive tree ring proxies. These two new volcanic records, synchronized with available ice core records to account for spatial variability in sulfate deposition, provide a basis for improving existing time series of volcanic forcing.

Assessment of diffusive isotopic fractionation in polar firn, and application to ice core trace gas records

During rapid variations of the atmospheric mixing ratio of a trace gas, diffusive transport in the porous firn layer atop ice sheets and glaciers alters the isotopic composition of that gas relative to the overlying atmosphere. Records of past atmospheric trace gas isotopic composition from ice cores and firn need to be corrected for this diffusive fractionation artifact.We present a novel, semi-empirical method to accurately estimate the magnitude of the diffusive fractionation in the ice core record. Our method (1) consists of a relatively simple analytical calculation; (2) requires only commonly available ice core data; (3) is not subject to the uncertainties inherent to estimating the accumulation rate, temperature, close-off depth and depth–diffusivity relationship back in time; (4) does not require knowledge of the true atmospheric variations, but uses the smoothed records obtained from ice cores; (5) arguably gives more accurate results than a combined firn densification–firn air transport modeling study would.We apply the method to records of CH4, CO2 and N2O mixing ratios, and we find that the correction is particularly important for δ13C–CH4. We apply the correction to δ13C–CH4 records over the last glacial termination and the 8.2ka event. In both cases the diffusive signal exceeds the analytical precision of the data, and has a significant impact on the observed isotopic trends. For the 8.2ka event the corrected data show an isotopic enrichment in δ13C–CH4 for the duration of the event, consistent with reduced wetland emissions.

Amplitude and timing of the Laschamp geomagnetic dipole low from the global atmospheric 10Be overproduction: Contribution of authigenic 10Be/9Be ratios in west equatorial Pacific sediments

Authigenic 10Be/9Be ratios were measured along a sediment core collected in the west equatorial Pacific in order to reconstruct cosmogenic 10Be production variations near the equator, where the geomagnetic modulation is maximum. From 60 to 20 ka, the single significant 10Be production impulse recorded at 41 ka results from the geomagnetic dipole low that triggered the Laschamp excursion. No significant 10Be overproduction signature is recorded at the age of the Mono Lake excursion (∼34 ka). A compilation of authigenic 10Be/9Be records obtained from sediments was averaged over a 1 kyr window and compared with the 1 kyr averaged 10Be flux record of Greenland ice cores. Their remarkable similarity demonstrates that 10Be production is globally modulated by geomagnetic dipole variations and redistributed by atmosphere dynamics. After calibration using absolute values of the virtual dipole moment drawn from paleomagnetic database, the authigenic 10Be/9Be stack allows reconstructing the geomagnetic dipole moment variations over the 20–50 ka time interval. Between 48 and 41 ka, the dipole moment collapsed at a rate of −1.5 × 1022 A m2 kyr−1, which will be an interesting criterion for the assessment of the loss rate of the historical field and the comparison of dipole moment loss prior to excursions and reversals. After a 2 kyr duration of the minimum dipole moment (∼1 × 1022 A m2), a slow increase started at 39 ka, progressively reaching 5 × 1022 A m2 at 20 ka. The absence of a significant dipole moment drop at 34 ka, the age of the Mono lake excursion, suggests that the duration and amplitude of the dipole weakening cannot be compared with that of the Laschamp. This study provides a reliable basis to model the production of radiocarbon and in situ cosmogenic nuclides and to improve the calibration of these dating methods.

Delayed build-up of Arctic ice sheets during 400,000-year minima in insolation variability

Knowledge of the past variability of climate at high northern latitudes during astronomical analogues of the present interglacial may help to inform our understanding of future climate change. Unfortunately, long-term continuous records of ice-sheet variability in the Northern Hemisphere only are scarce because records of benthic (18)O content represent an integrated signal of changes in ice volume in both polar regions. However, variations in Northern Hemisphere ice sheets influence the Siberian High (an atmospheric pressure system), so variations in the East Asian winter monsoon (EAWM)--as recorded in the aeolian dust deposits on the Chinese Loess Plateau--can serve as a useful proxy of Arctic climate variability before the ice-core record begins. Here we present an EAWM proxy record using grain-size variations in two parallel loess sections representative of sequences across the whole of the Chinese Loess Plateau over the past 900,000 years. The results show that during periods of low eccentricity and precessional variability at approximately 400,000-year intervals, the grain-size-inferred intensity of the EAWM remains weak for up to 20,000 years after the end of the interglacial episode of high summer monsoon activity and strong pedogenesis. In contrast, there is a rapid increase in the EAWM after the end of most other interglacials. We conclude that, for both the 400,000-year interglacials, the weak EAWM winds maintain a mild, non-glacial climate at high northern latitudes for much longer than expected from the conventional loess and marine oxygen isotope records. During these times, the less-severe summer insolation minima at 65° N (ref. 4) would have suppressed ice and snow accumulation, leading to a weak Siberian High and, consequently, weak EAWM winds.

Sulphate–climate coupling over the past 300,000 years in inland Antarctica

Sulphate aerosols, particularly micrometre-sized particles of sulphate salt and sulphate-adhered dust, can act as cloud condensation nuclei, leading to increased solar scattering that cools Earth's climate. Evidence for such a coupling may lie in the sulphate record from polar ice cores, but previous analyses of melted ice-core samples have provided only sulphate ion concentrations, which may be due to sulphuric acid. Here we present profiles of sulphate salt and sulphate-adhered dust fluxes over the past 300,000 years from the Dome Fuji ice core in inland Antarctica. Our results show a nearly constant flux of sulphate-adhered dust through glacial and interglacial periods despite the large increases in total dust flux during glacial maxima. The sulphate salt flux, however, correlates inversely with temperature, suggesting a climatic coupling between particulate sulphur and temperature. For example, the total sulphate salt flux during the Last Glacial Maximum averages 5.78 mg m(-2) yr(-1), which is almost twice the Holocene value. Although it is based on a modern analogue with considerable uncertainties when applied to the ice-core record, this analysis indicates that the glacial-to-interglacial decrease in sulphate would lessen the aerosol indirect effects on cloud lifetime and albedo, leading to an Antarctic warming of 0.1 to 5 kelvin.

Estimation of noise parameters in dynamical system identification with Kalman filters

A method is proposed for determining dynamical and observational noise parameters in state and parameter identification from time series using Kalman filters. The noise covariances are estimated in a secondary optimization by maximizing the predictive likelihood of the data. The approach is based on internal consistency; for the correct noise parameters, the uncertainty projected by the Kalman filter matches the actual predictive uncertainty. The method is able to disentangle dynamical and observational noise. The algorithm is demonstrated for the linear, extended, and unscented Kalman filters using an Ornstein-Uhlenbeck process, the noise-driven Lorenz system, and van der Pol oscillator as well as a paleoclimatic ice-core record as examples. The approach is also applicable to the ensemble Kalman filter and can be readily extended to non-Gaussian estimation frameworks such as Gaussian-sum filters and particle filters.

Constraining recent lead pollution sources in the North Pacific using ice core stable lead isotopes

Trends and sources of lead (Pb) aerosol pollution in the North Pacific rim of North America from 1850 to 2001 are investigated using a high‐resolution (subannual to annual) ice core record recovered from Eclipse Icefield (3017 masl; St. Elias Mountains, Canada). Beginning in the early 1940s, increasing Pb concentration at Eclipse Icefield occurs coevally with anthropogenic Pb deposition in central Greenland, suggesting that North American Pb pollution may have been in part or wholly responsible in both regions. Isotopic ratios (208Pb/207Pb and 206Pb/207Pb) from 1970 to 2001 confirm that a portion of the Pb deposited at Eclipse Icefield is anthropogenic, and that it represents a variable mixture of East Asian (Chinese and Japanese) emissions transported eastward across the Pacific Ocean and a North American component resulting from transient meridional atmospheric flow. Based on comparison with source material Pb isotope ratios, Chinese and North American coal combustion have likely been the primary sources of Eclipse Icefield Pb over the 1970–2001 time period. The Eclipse Icefield Pb isotope composition also implies that the North Pacific mid‐troposphere is not directly impacted by transpolar atmospheric flow from Europe. Annually averaged Pb concentrations in the Eclipse Icefield ice core record show no long‐term trend during 1970–2001; however, increasing208Pb/207Pb and decreasing 206Pb/207Pb ratios reflect the progressive East Asian industrialization and increase in Asian pollutant outflow. The post‐1970 decrease in North American Pb emissions is likely necessary to explain the Eclipse Icefield Pb concentration time series. When compared with low (lichen) and high (Mt. Logan ice core) elevation Pb data, the Eclipse ice core record suggests a gradual increase in pollutant deposition and stronger trans‐Pacific Asian contribution with rising elevation in the mountains of the North Pacific rim.

Recent Antarctic Peninsula warming relative to Holocene climate and ice-shelf history

Rapid warming over the past 50 years on the Antarctic Peninsula is associated with the collapse of a number of ice shelves and accelerating glacier mass loss. In contrast, warming has been comparatively modest over West Antarctica and significant changes have not been observed over most of East Antarctica, suggesting that the ice-core palaeoclimate records available from these areas may not be representative of the climate history of the Antarctic Peninsula. Here we show that the Antarctic Peninsula experienced an early-Holocene warm period followed by stable temperatures, from about 9,200 to 2,500 years ago, that were similar to modern-day levels. Our temperature estimates are based on an ice-core record of deuterium variations from James Ross Island, off the northeastern tip of the Antarctic Peninsula. We find that the late-Holocene development of ice shelves near James Ross Island was coincident with pronounced cooling from 2,500 to 600 years ago. This cooling was part of a millennial-scale climate excursion with opposing anomalies on the eastern and western sides of the Antarctic Peninsula. Although warming of the northeastern Antarctic Peninsula began around 600 years ago, the high rate of warming over the past century is unusual (but not unprecedented) in the context of natural climate variability over the past two millennia. The connection shown here between past temperature and ice-shelf stability suggests that warming for several centuries rendered ice shelves on the northeastern Antarctic Peninsula vulnerable to collapse. Continued warming to temperatures that now exceed the stable conditions of most of the Holocene epoch is likely to cause ice-shelf instability to encroach farther southward along the Antarctic Peninsula.

Ice core records of biomass burning tracers (levoglucosan and dehydroabietic, vanillic and p-hydroxybenzoic acids) and total organic carbon for past 300 years in the Kamchatka Peninsula, Northeast Asia

We successfully detected biomass burning tracers including levoglucosan and vanillic, p-hydroxybenzoic and dehydroabietic acids in an ice core (153m long, ca. 300years old) taken from Ushkovsky ice cap (altitude, 3903m), the Kamchatka Peninsula, Northeast Asia. Concentrations of total organic carbon (TOC) were also determined in the ice core. Levoglucosan, which is produced by pyrolysis of cellulose and hemicellulose and thus is a general tracer of biomass burning, showed sporadic peaks in the years of 1705, 1759, 1883, 1915, 1949 and 1972, with the largest peak in 1949. However, its concentrations did not show a systematic increase in the last century although the concentration peaks seemingly corresponded to the higher ambient temperatures in the northern high latitudes. In contrast, dehydroabietic acid, a specific tracer of the pyrolysis of conifer resin, showed a gradual increase from the early 1900s to 1990s with a significant peak in 1970. Contributions of dehydroabietic acid to TOC also showed an increasing trend for the 20th century. Similarly, vanillic and p-hydroxybenzoic acids presented higher concentrations in the last half-century with sporadic peaks in 1705, 1759 and 1949. This study showed that general biomass burning tracers such as levoglucosan have been sporadically transported over the glacier of the Kamchatka Peninsula. In contrast, the ice core record of dehydroabietic acid indicated that fires of boreal conifer forest have more frequently and increasingly occurred in Far East and Siberia during the last century and transported to the Northwestern Pacific. The present study demonstrates that organic tracers of biomass burning preserved in ice core could provide historical records of biomass burning and boreal forest fires.

Dome Fuji Station in East Antarctica and the Japanese Antarctic Research Expedition

AbstractJapanese Antarctic Research Expedition (JARE) commenced on the occasion of International Geophysical Year in 1957–1958. Syowa Station, the primary station for JARE operations, is located along the northeastern coastal region of Lützow-Holm Bay, East Antarctica (69° 00′S, 39° 35′E), and was opened on 29 January 1957. Since then, JARE have been carrying out research in various fields of earth and planetary sciences and life science. Astronomical science, however, has not been popular in Antarctica. In 1995, JARE established a new inland station, Dome Fuji Station (77° 19′S, 39° 42′E), which, at 3,810 m a.s.l., is located on one of major domes of the Antarctic ice sheet, some 1,000 km south of Syowa. The climatic conditions at Dome Fuji are harsh, with an annual average air temperature of −54°C, and a recorded minimum of −79°C. In 2007, JARE completed scientific drilling to obtain ice core samples of the Antarctic ice sheet reaching 3,050 m in depth. These ice cores record environmental conditions of the earth extending back some 720,000 B.P. In recent years, it is widely known that the high-altitude environment of inland Antarctica is suitable for astronomical observations and the Japanese astronomy community identified Dome Fuji Station as a potential candidate for a future astronomical observatory. In this article, the history of Japanese Antarctic activities are described in terms of access to the inland plateau of the Antarctic continent. The general scheme and future plans of science objectives and logistics of JARE will also be introduced.

Little Ice Age climate and oceanic conditions of the Ross Sea, Antarctica from a coastal ice core record

Abstract. Increasing paleoclimatic evidence suggests that the Little Ice Age (LIA) was a global climate change event. Understanding the forcings and associated climate system feedbacks of the LIA is made difficult by the scarcity of Southern Hemisphere paleoclimate records. We use a new glaciochemical record of a coastal ice core from Mt. Erebus Saddle, Antarctica, to reconstruct atmospheric and oceanic conditions in the Ross Sea sector of Antarctica over the past five centuries. The LIA is identified in stable isotope (δD) and lithophile element records, which respectively demonstrate that the region experienced 1.6 ± 1.4 °C cooler average temperatures prior to 1850 AD than during the last 150 yr and strong (>57 m s−1) prevailing katabatic winds between 1500 and 1800 AD. Al and Ti concentration increases of an order of magnitude (>120 ppb Al) are linked to enhanced aeolian transport of complex silicate minerals and represent the strongest katabatic wind events of the LIA. These events are associated with three 12–30 yr intervals of cooler temperatures at ca. 1690 AD, 1770 AD and 1840 AD. Furthermore, ice core concentrations of the biogenic sulphur species MS− suggest that biological productivity in the Ross Sea polynya was ~80% higher prior to 1875 AD than at any subsequent time. We propose that cooler Antarctic temperatures promoted stronger katabatic winds across the Ross Ice Shelf, resulting in an enlarged Ross Sea polynya during the LIA.

Controls on the tropospheric oxidizing capacity during an idealized Dansgaard‐Oeschger event, and their implications for the rapid rises in atmospheric methane during the last glacial period

The ice core record reveals large variations in the concentration of atmospheric methane, [CH4], over the last 800 kyr. Amongst the most striking natural features are the large, rapid rises in [CH4], of 100–200 ppbv, on timescales of less than 100 years, at the beginning of Dansgaard‐Oeschger (D‐O) events during the last glacial period (21–110 kyr before present). Despite the potential insight they could offer into the likelihood of future rapid rises in [CH4], the relative roles of changes in methane sources and sinks during D‐O events have been little explored. Here, we use a global atmospheric chemistry‐transport model to explore—for the first time, in a process‐based fashion—controls on the oxidizing capacity during an idealized D‐O event that features a characteristically rapid rise in [CH4]. We find that the two controls previously identified in the literature as having had significant (though opposing) influences on the oxidizing capacity between glacial and interglacial periods—changes in air temperature and emissions of non‐methane volatile organic compounds from vegetation—offset one another between idealized Heinrich stadial and Greenland interstadial states. The result is, the net change in oxidizing capacity is very small, implying the rapid rises in [CH4] at the beginning of D‐O events were almost entirely source‐driven. This poses a challenge to earth‐system models—to generate a sufficiently large increase in methane emissions in response to a simulated D‐O event, via a more realistic freshwater forcing impacting the strength of the Atlantic meridional overturning circulation or, possibly, other climate‐change mechanisms.

The Holocene Indian monsoon variability over the southern Tibetan Plateau and its teleconnections

A high-resolution stalagmite oxygen isotope record from Tianmen Cave, located in the south central Tibetan Plateau, characterizes detailed climatic variations between 8.7 and 4.3ka BP on centennial and decadal time scales, with a temporal resolution of 3 to 7 years. The Tianmen record is in good agreement with speleothem records from Asian monsoon regions (i.e., Dongge Cave, Dykoski et al., 2005; Wang et al., 2005; Qunf Cave, Fleitmann et al., 2003), upwelling records from the Arabian Sea (Gupta et al., 2003, 2005) and peat bog records from southeast Tibetan Plateau (Hong et al., 2003), indicating that the Indian summer monsoon (ISM) gradually weakened as Northern Hemisphere summer insolation declined during the early-mid Holocene and that ISM intensity, rather than temperature, dominates the precipitation δ18O on centennial to decadal time scales in the southern TP. On centennial to decadal time scales, the detrended Tianmen record correlates well with the Greenland ice core record, further confirming the mechanistic connection between the ISM and high northern latitude temperature changes. Meanwhile, the Tianmen record bears significant solar activity cycles, suggesting that the solar output changes may affect the variability of the ISM and likely the position of the Intertropical Convergence Zone (ITCZ) as well. Moreover, due to joint effects of changes in monsoon precipitation, moisture source and temperature, the Tianmen δ18O record shows much larger amplitude changes than speleothem δ18O records from low-elevation Asian Monsoon regions, which is similar to the previously reported Marine Isotope Stage (MIS) 5 record (Cai et al., 2010), suggesting heightened sensitivity of precipitation isotope composition to climate changes over the high-elevation regions and further demonstrating that the stable isotope lapse rate may change under different climatic conditions.

Twentieth-Century Surface Temperature Trends in the Western Ross Sea, Antarctica: Evidence from a High-Resolution Ice Core

Abstract A 125-yr ice core record of climate from the Whitehall Glacier ice divide provides exceptionally high-resolution stable isotope data from the northwest margin of the Ross Sea, Antarctica. This is the only proxy data available to extend the instrumental record of temperature in this region, where little is known about climate variability over the past two centuries. Using ECMWF Interim Re-Analysis (ERA-Interim) data, this study develops a precipitation-weighted δ18O-temperature transfer function of 0.62‰ °C−1, which is comparable to other proximal ice cores, such as Taylor, Talos, and Law Domes. Reconstructed mean annual temperatures show no significant change between 1882 and 2006. However, a decrease in cold season [April–September (AMJJAS)] temperatures of −1.59° ± 0.84°C decade−1 (at 90% confidence) is observed since 1979. This cooling trend is in contrast to a surface temperature record from Ross Island (Scott Base) where significant spring warming is observed. It is also coincident with a positive trend in the southern annular mode, which is linked to stronger southerly winds and increased sea ice extent and duration in the western Ross Sea.

Greenland ice core evidence for spatial and temporal variability of the Atlantic Multidecadal Oscillation

The Greenland δ18O ice core record is used as a proxy for Greenland surface air temperatures and to interpret Atlantic Multidecadal Oscillation (AMO) variability. An analysis of annual δ18O data from six Arctic ice cores (five from Greenland and one from Canada's Ellesmere Island) suggests a significant AMO spatial and temporal variability within a recent period of 660 years. A dominant AMO periodicity near 20 years is clearly observed in the southern (Dye3 site) and the central (GISP2, Crete and Milcent) regions of Greenland. This 20‐year variability is, however, significantly reduced in the northern (Camp Century and Agassiz Ice Cap) region, likely due to a larger distance from the Atlantic Ocean, and a much lower snow accumulation. A longer time scale AMO component of 45–65 years, which has been seen clearly in the 20th century SST data, is detected only in central Greenland ice cores. We find a significant difference between the AMO cycles during the Little Ice Age (LIA) and the Medieval Warm Period (MWP). The LIA was dominated by a ∼20 year AMO cycle with no other decadal or multidecadal scale variability above the noise level. However, during the preceding MWP the 20 year cycle was replaced by a longer scale cycle centered near a period of 43 years with a further 11.5 year periodicity. An analysis of two coupled atmosphere‐ocean general circulation models control runs (UK Met Office HadCM3 and NOAA GFDL CM2.1) agree with the shorter and longer time‐scales of Atlantic Meridional Overturning Circulation (AMOC) and temperature fluctuations with periodicities close to those observed. However, the geographic variability of these periodicities indicated by ice core data is not captured in model simulations.

Insights from a synthesis of old and new climate-proxy data from the Pyramid and Winnemucca lake basins for the period 48 to 11.5 cal ka

A synthesis of old and new paleoclimatic data from the Pyramid and Winnemucca lake basins indicates that, between 48.0 and 11.5·103 calibrated years BP (hereafter ka), the climate of the western Great Basin was, to a degree, linked with the climate of the North Atlantic. Paleomagnetic secular variation (PSV) records from Pyramid Lake core PLC08-1 were tied to the GISP2 ice-core record via PSV matches to North Atlantic sediment cores whose isotopic and(or) carbonate records could be linked to the GISP2 δ18O record. Relatively dry intervals in the western Great Basin were associated with cold Heinrich events and relatively wet intervals were associated with warm Dansgaard-Oeschger (DO) oscillations. The association of western Great Basin dry events with North Atlantic cold events (and vice versa) switched sometime after the Laurentide Ice Sheet (LIS) reached its maximum extent. For example, the Lahontan highstand, which culminated at 15.5 ka, and a period of elevated lake level between 13.1 and 11.7 ka were associated with cold North Atlantic conditions, the latter period with the Youngest Dryas event. Relatively dry periods were associated with the Bølling and Allerød warm events. A large percentage of the LIS may have been lost to the North Atlantic during Heinrich events 1 and 2 and may have resulted in the repositioning of the Polar Jet Stream over North America. The Trego Hot Springs, Wono, Carson Sink, and Marble Bluff tephras found in core PLC08-1 have been assigned GISP2 calendar ages of respectively, 29.9, 33.7, 34.1, and 43.2 ka. Given its unique trace-element chemistry, the Carson Sink Bed is the same as Wilson Creek Ash 15 in the Mono Lake Basin. This implies that the Mono Lake magnetic excursion occurred at approximately 34 ka and it is not the Laschamp magnetic excursion.The entrance of the First Americans into the northern Great Basin is dated to approximately 14.4 ka, a time when the climate was relatively dry. Evidence for human occupation of the Great Basin is lacking for the next 1100 years (y); i.e., the oldest western stemmed point site in the Great Basin dates to 13.3 ka. Two hypotheses are suggested for this cultural hiatus: (1) the climate had deteriorated to the point that people vacated the Great Basin, or (2) people moved to basin-bottom wetlands that persisted during the dry period, and then the subsequent Younger Dryas wet event erased the archaeological evidence deposited around the low-elevation wetland sites.

Ice-core net snow accumulation and seasonal snow chemistry at a temperate-glacier site: Mount Waddington, southwest British Columbia, Canada

AbstractA 141m ice core was recovered from Combatant Col (51.385° N, 125.258° W; 3000ma.s.l.), Mount Waddington, Coast Mountains, British Columbia, Canada. Records of black carbon, dust, lead and water stable isotopes demonstrate that unambiguous seasonality is preserved throughout the core, despite summer surface snowmelt and temperate ice. High accumulation rates at the site (>4 m ice eq. a-1) limit modification of annual stratigraphy by percolation of surface meltwater. The ice-core record spans the period 1973–2010. An annually averaged time series of lead concentrations from the core correlates well with historical records of lead emission from North America, and with ice-core records of lead from the Greenland ice sheet. The depth-age scale for the ice core provides sufficient constraint on the vertical strain to allow estimation of the age of the ice at bedrock. Total ice thickness at Combatant Col is ~250 m; an ice core to bedrock would likely contain ice in excess of 200 years in age. Accumulation at Combatant Col is significantly correlated with both regional precipitation and large-scale geopotential height anomalies.

Evidence for phase-locked changes in climate between Scotland and Greenland during GS-1 (Younger Dryas) using micromorphology of glaciolacustrine varves from Glen Roy

There is a current need to develop annual/decadal chronologies from periods of rapid climate change in order to understand the rate and timing of climate events and identify how other proxies either lead or lag this climate forcing. Annually-laminated or varved sediments are key proxies for understanding environmental change at such a scale. This paper presents the results of microscale analysis of glaciolacustrine varve sediments from Scotland and develops a new index called varve group, which ranks the number of pulses of sediment observed in the summer component of the varve. The varve group index is compared with data from the Greenland ice-core record and correspondence can be observed between the oxygen isotope records from Greenland during the latter part of GS-1 and the 515-year Lochaber varve group index. The processes that control the number of sediment pulses into the lake basin are discussed and we examine how these factors might control both the isotopic composition of the ice cores and varve group formation. It is proposed that the position of the sea ice in the North Atlantic may provide such a mechanism.

Reconstruction of a continuous high-resolution CO<sub>2</sub> record over the past 20 million years

Abstract. The gradual cooling of the climate during the Cenozoic has generally been attributed to a decrease in CO2 concentration in the atmosphere. The lack of transient climate models and, in particular, the lack of high-resolution proxy records of CO2, beyond the ice-core record prohibit, however, a full understanding of, for example, the inception of the Northern Hemisphere glaciation and mid-Pleistocene transition. Here we elaborate on an inverse modelling technique to reconstruct a continuous CO2 series over the past 20 million year (Myr), by decomposing the global deep-sea benthic δ18O record into a mutually consistent temperature and sea level record, using a set of 1-D models of the major Northern and Southern Hemisphere ice sheets. We subsequently compared the modelled temperature record with ice core and proxy-derived CO2 data to create a continuous CO2 reconstruction over the past 20 Myr. Results show a gradual decline from 450 ppmv around 15 Myr ago to 225 ppmv for mean conditions of the glacial-interglacial cycles of the last 1 Myr, coinciding with a gradual cooling of the global surface temperature of 10 K. Between 13 to 3 Myr ago there is no long-term sea level variation caused by ice-volume changes. We find no evidence of change in the long-term relation between temperature change and CO2, other than the effect following the saturation of the absorption bands for CO2. The reconstructed CO2 record shows that the Northern Hemisphere glaciation starts once the long-term average CO2 concentration drops below 265 ppmv after a period of strong decrease in CO2. Finally, only a small long-term decline of 23 ppmv is found during the mid-Pleistocene transition, constraining theories on this major transition in the climate system. The approach is not accurate enough to revise current ideas about climate sensitivity.