[Show abstract][Hide abstract] ABSTRACT: Whereas ice cores from high-accumulation sites in coastal Antarctica clearly demonstrate annual layering, it is debated whether a seasonal signal is also preserved in ice cores from lower-accumulation sites further inland and particularly on the East Antarctic Plateau. In this study, we examine 5 m of early Holocene ice from the Dome Fuji (DF) ice core at a high temporal resolution by continuous flow analysis. The ice was continuously analysed for concentrations of dust, sodium, ammonium, liquid conductivity, and water isotopic composition. Furthermore, a dielectric profiling was performed on the solid ice. In most of the analysed ice, the multi-parameter impurity data set appears to resolve the seasonal variability although the identification of annual layers is not always unambiguous. The study thus provides information on the snow accumulation process in central East Antarctica. A layer counting based on the same principles as those previously applied to the NGRIP (North Greenland Ice core Project) and the Antarctic EPICA (European Project for Ice Coring in Antarctica) Dronning Maud Land (EDML) ice cores leads to a mean annual layer thickness for the DF ice of 3.0 ± 0.3 cm that compares well to existing estimates. The measured DF section is linked to the EDML ice core through a characteristic pattern of three significant acidity peaks that are present in both cores. The corresponding section of the EDML ice core has recently been dated by annual layer counting and the number of years identified independently in the two cores agree within error estimates. We therefore conclude that, to first order, the annual signal is preserved in this section of the DF core. This case study demonstrates the feasibility of determining annually deposited strata on the central East Antarctic Plateau. It also opens the possibility of resolving annual layers in the Eemian section of Antarctic ice cores where the accumulation is estimated to have been greater than in the Holocene.
Climate of the Past 09/2015; 11:1127-1137. DOI:10.5194/cp-11-1127-2015 · 3.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Climate changes in the North Atlantic region during the last glacial cycle were dominated by the slow waxing and waning of the North American ice sheet as well as by intermittent, millennial-scale Dansgaard–Oeschger climate oscillations. However, prior to the last deglaciation, the responses of North American vegetation and biomass burning to these climate variations are uncertain. Ammonium in Greenland ice cores, a product from North American soil emissions and biomass burning events, can help to fill this gap. Here we use continuous, high-resolution measurements of ammonium concentrations between 110,000 to 10,000 years ago from the Greenland NGRIP and GRIP ice cores to reconstruct North American wildfire activity and soil ammonium emissions. We find that on orbital timescales soil emissions increased under warmer climate conditions when vegetation expanded northwards into previously ice-covered areas. For millennial-scale interstadial warm periods during Marine Isotope Stage 3, the fire recurrence rate increased in parallel to the rapid warmings, whereas soil emissions rose more slowly, reflecting slow ice shrinkage and delayed ecosystem changes. We conclude that sudden warming events had little impact on soil ammonium emissions and ammonium transport to Greenland, but did result in a substantial increase in the frequency of North American wildfires.
[Show abstract][Hide abstract] ABSTRACT: An important share of paleoclimatic information is buried within the lowermost layers of deep ice cores. Because improving our records further back in time is one of the main challenges in the near future, it is essential to judge how deep these records remain unaltered, since the proximity of the bedrock is likely to interfere both with the recorded temporal sequence and the ice properties. In this paper, we present a multiparametric study (δD-δ18Oice, δ18Oatm, total air content, CO2, CH4, N2O, dust, high resolution chemistry, ice texture) of the bottom 60 m of the EPICA Dome C ice core from central Antarctica. These bottom layers have been subdivided in two sections: the lower 12 m showing visible solid inclusions (basal ice) and the 48 m above which we refer to as "deep ice". Some of the data are consistent with a pristine paleoclimatic signal, others show clear anomalies. It is demonstrated that neither large scale bottom refreezing of subglacial water, nor mixing (be it internal or with a local basal end-term from a previous/initial ice sheet configuration) can explain the observed bottom ice properties. We focus on the high-resolution chemical profiles and on the available remote sensing data on the subglacial topography of the site to propose a mechanism by which relative stretching of the bottom ice sheet layers is made possible, due to the progressively confining effect of subglacial valley sides. This stress field change, combined with bottom ice temperature close to the pressure melting point, induces accelerated migration recrystallization, which results in spatial chemical sorting of the impurities, depending on their state (dissolved vs. solid) and if they are involved or not in salt formation. This chemical sorting effect is responsible for the progressive build-up of the visible solid aggregates that therefore mainly originate "from within", and not from incorporation processes of allochtone material at the ice–bedrock interface. We also discuss how the proposed mechanism is compatible with the other variables described. We conclude that the paleoclimatic signal is only marginally affected in terms of global ice properties at the bottom of EPICA Dome C, but that the time scale has been considerably distorted by mechanical stretching of MIS20 due to the increasing influence of the subglacial topography, a process that might have started well above the bottom ice.
The Cryosphere Discussions 01/2015; 9(1):567-608. DOI:10.5194/tcd-9-567-2015
[Show abstract][Hide abstract] ABSTRACT: We present a synchronization of the NGRIP, GRIP and GISP2 ice cores onto a master chronology extending back to 104 ka before present, providing a consistent chronological framework for these three Greenland records. The synchronization aligns distinct peaks in volcanic proxy records and other impurity records (chemo-stratigraphic matching) and assumes that these layers of elevated impurity content represent the same, instantaneous event in the past at all three sites. More than 900 marker horizons between the three cores have been identified and our matching is independently confirmed by 24 new and previously identified volcanic ash (tephra) tie-points. Using the reference horizons, we transfer the widely used Greenland ice-core chronology, GICC05modelext, to the two Summit cores, GRIP and GISP2. Furthermore, we provide gas chronologies for the Summit cores that are consistent with the GICC05modelext timescale by utilizing both existing and new gas data (CH4 concentration and δ15N of N2). We infer that the accumulation contrast between the stadial and interstadial phases of the glacial period was ∼10% greater at Summit compared to at NGRIP. The δ18O temperature-proxy records from NGRIP, GRIP, and GISP2 are generally very similar and display synchronous behaviour at climate transitions. The δ18O differences between Summit and NGRIP, however, changed slowly over the Last Glacial-Interglacial cycle and also underwent abrupt millennial-to-centennial-scale variations. We suggest that this observed latitudinal δ18O gradient in Greenland during the glacial period is the result of 1) relatively higher degree of precipitation with a Pacific signature at NGRIP, 2) increased summer bias in precipitation at Summit, and 3) enhanced Rayleigh distillation due to an increased source-to-site distance and a potentially larger source-to-site temperature gradient. We propose that these processes are governed by changes in the North American Ice Sheet (NAIS) volume and North Atlantic sea-ice extent and/or sea-surface temperatures (SST) on orbital timescales, and that changing sea-ice extent and SSTs are the driving mechanisms on shorter timescales. Finally, we observe that maxima in the Summit-NGRIP δ18O difference are roughly coincident with prominent Heinrich events. This suggests that the climatic reorganization that takes place during stadials with Heinrich events, possibly driven by a southward expansion of sea ice and low SSTs in the North Atlantic, are recorded in the ice-core records.
[Show abstract][Hide abstract] ABSTRACT: Due to their outstanding resolution and well-constrained chronologies, Greenland ice-core records provide a master record of past climatic changes throughout the Last Interglacial–Glacial cycle in the North Atlantic region. As part of the INTIMATE (INTegration of Ice-core, MArine and TErrestrial records) project, protocols have been proposed to ensure consistent and robust correlation between different records of past climate. A key element of these protocols has been the formal definition and ordinal numbering of the sequence of Greenland Stadials (GS) and Greenland Interstadials (GI) within the most recent glacial period. The GS and GI periods are the Greenland expressions of the characteristic Dansgaard–Oeschger events that represent cold and warm phases of the North Atlantic region, respectively. We present here a more detailed and extended GS/GI template for the whole of the Last Glacial period. It is based on a synchronization of the NGRIP, GRIP, and GISP2 ice-core records that allows the parallel analysis of all three records on a common time scale. The boundaries of the GS and GI periods are defined based on a combination of stable-oxygen isotope ratios of the ice (δ18O, reflecting mainly local temperature) and calcium ion concentrations (reflecting mainly atmospheric dust loading) measured in the ice. The data not only resolve the well-known sequence of Dansgaard–Oeschger events that were first defined and numbered in the ice-core records more than two decades ago, but also better resolve a number of short-lived climatic oscillations, some defined here for the first time. Using this revised scheme, we propose a consistent approach for discriminating and naming all the significant abrupt climatic events of the Last Glacial period that are represented in the Greenland ice records. The final product constitutes an extended and better resolved Greenland stratotype sequence, against which other proxy records can be compared and correlated. It also provides a more secure basis for investigating the dynamics and fundamental causes of these climatic perturbations.
[Show abstract][Hide abstract] ABSTRACT: The seasonal and annual representativeness of ionic aerosol proxies (among others, calcium, sodium, ammonium and nitrate) in various firn cores in the vicinity of the NEEM drill site in northwest Greenland have been assessed. Seasonal representativeness is very high as one core explains more than 60% of the variability within the area. The inter-annual representativeness, however, can be substantially lower (depending on the species) making replicate coring indispensable to derive the atmospheric variability of aerosol species. A single core at the NEEM site records only 30% of the inter-annual atmospheric variability in some species, while five replicate cores are already needed to cover approximately 70% of the inter-annual atmospheric variability in all species. The spatial representativeness is very high within 60 cm, rapidly decorrelates within 10 m but does not diminish further within 3 km. We attribute this to wind reworking of the snow pack leading to sastrugi formation. Due to the high resolution and seasonal representativeness of the records we can derive accurate seasonalities of the measured species for modern (AD 1990-2010) times as well as for pre-industrial (AD 1623-1750) times. Sodium and calcium show similar seasonality (peaking in February and March respectively) for modern and pre-industrial times, whereas ammonium and nitrate are influenced by anthropogenic activities. Nitrate and ammonium both peak in May during modern times, whereas during pre-industrial times ammonium peaked during July-August and nitrate during June-July.
The Cryosphere 10/2014; 8(5):1855-1870. DOI:10.5194/tc-8-1855-2014 · 5.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Building chronological frameworks for proxy sequences spanning 130–60 ka b2k is plagued by difficulties and uncertainties. Recent developments in the North Atlantic region, however, affirm the potential offered by tephrochronology and specifically the search for cryptotephra. Here we review the potential offered by tephrostratigraphy for sequences spanning 130–60 ka b2k. We combine newly identified cryptotephra deposits from the NGRIP ice-core and a marine core from the Iceland Basin with previously published data from the ice and marine realms to construct the first tephrostratigraphical framework for this time-interval. Forty-three tephra or cryptotephra deposits are incorporated into this framework; twenty three tephra deposits are found in the Greenland ice-cores, including nine new NGRIP tephras, and twenty separate deposits are preserved in various North Atlantic marine sequences. Major, minor and trace element results are presented for the new NGRIP horizons together with age estimates based on their position within the ice-core record. Basaltic tephras of Icelandic origin dominate the framework with only eight tephras of rhyolitic composition found. New results from marine core MD99-2253 also illustrate some of the complexities and challenges of assessing the depositional integrity of marine cryptotephra deposits. Tephra-based correlations in the marine environment provide independent tie-points for this time-interval and highlight the potential of widening the application of tephrochronology. Further investigations, however, are required, that combine robust geochemical fingerprinting and a rigorous assessment of tephra depositional processes, in order to trace coeval events between the two depositional realms.
[Show abstract][Hide abstract] ABSTRACT: Phosphorus (P) is an essential macronutrient for all living organisms. Phosphorus is often present in nature as the soluble phosphate ion PO4(3-) and has biological, terrestrial, and marine emission sources. Thus PO4(3-) detected in ice cores has the potential to be an important tracer for biological activity in the past. In this study a continuous and highly sensitive absorption method for detection of dissolved reactive phosphorus (DRP) in ice cores has been developed using a molybdate reagent and a 2-m liquid waveguide capillary cell (LWCC). DRP is the soluble form of the nutrient phosphorus, which reacts with molybdate. The method was optimized to meet the low concentrations of DRP in Greenland ice, with a depth resolution of approximately 2 cm and an analytical uncertainty of 1.1 nM (0.1 ppb) PO4(3-). The method has been applied to segments of a shallow firn core from Northeast Greenland, indicating a mean concentration level of 2.74 nM (0.26 ppb) PO4(3-) for the period 1930-2005 with a standard deviation of 1.37 nM (0.13 ppb) PO4(3-) and values reaching as high as 10.52 nM (1 ppb) PO4(3-). Similar levels were detected for the period 1771-1823. Based on impurity abundances, dust and biogenic particles were found to be the most likely sources of DRP deposited in Northeast Greenland.
[Show abstract][Hide abstract] ABSTRACT: When drilling ice cores deeper than ∼100 m, drill liquid is required to maintain ice-core quality and to limit borehole closure. Due to high-pressure air bubbles in the ice, the ice core can crack during drilling and core retrieval, typically at 600–1200 m depth in Greenland. Ice from this 'brittle zone' can be contaminated by drill liquid as it seeps through cracks into the core. Continuous flow analysis (CFA) systems are routinely used to analyse ice for chemical impurities, so the detection of drill liquid is important for validating accurate measurements and avoiding potential instrument damage. An optical detector was constructed to identify drill liquid in CFA tubing by ultraviolet absorption spectroscopy at a wavelength of 290 nm. The set-up was successfully field-tested in the frame of the NEEM ice-core drilling project in Greenland. A total of 27 cases of drill liquid contamination were identified during the analysis of 175 m of brittle zone ice. The analyses most strongly affected by drill liquid contamination include insoluble dust particles, electrolytic conductivity, ammonium, hydrogen peroxide and sulphate. This method may also be applied to other types of drill liquid used at other drill sites.
Journal of Glaciology 07/2013; 59(215):503-506. DOI:10.3189/2013JoG12J124 · 3.24 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The important active and passive role of mineral dust aerosol in the climate and the global carbon cycle over the last glacial/interglacial cycles has been recognized. However, little data on the most important aeolian dust-derived biological micronutrient, iron (Fe), has so far been available from ice-cores from Greenland or Antarctica. Furthermore, Fe deposition reconstructions derived from the palaeoproxies particulate dust and calcium differ significantly from the Fe flux data available. The ability to measure high temporal resolution Fe data in polar ice-cores is crucial for the study of the timing and magnitude of relationships between geochemical events and biological responses in the open ocean. This work adapts an existing flow injection analysis (FIA) methodology for low-level trace Fe determinations with an existing glaciochemical analysis system, continuous flow analysis (CFA) of ice-cores. Fe-induced oxidation of N,N'-dimethyl-p-pheylenediamine (DPD) is used to quantify the biologically more important and easily leachable Fe fraction released in a controlled digestion step at pH ∼1.0. The developed method was successfully applied to the determination of labile Fe in ice-core samples collected from the Antarctic Byrd ice-core and the Greenland Ice-Core Project (GRIP) ice-core.
[Show abstract][Hide abstract] ABSTRACT: The last glacial period (110 - 15 ka) has been marked by millennial
scale climate variations, the trigger of which is still under debate.
Such variations have been recorded in marine, ice and continental
records over most of the world, but especially in the Northern
Hemisphere. We first investigate the high-resolution δ18O and dust
records from Greenland ice, indicating important variations in the
respective moisture and dust source areas. We show that the dust
concentration decrease associated with the Dansgaard-Oeschger (DO)
warming events 17 to 2 happened on average within about 50 years, and
that δ18O reached peak DO interstadial values faster than dust,
suggesting a lag in the continental response to the abrupt warming. The
individual analyzed interstadial phases lasted between 200 and 4200
years. In European eolian sequences, the different duration of the
interstadials is expressed by different types of paleosols observed
along a west-east transect at 50° latitude North. Discussing the
paleodust cycle variations during the last climate cycle, we propose a
link between European loess sequences, Chinese ones, dust records in
Greenland and the variations of the North Atlantic sea ice extent and
surface temperature. Changes in the dust sources are discussed
(present-day deserts, but also emerged continental shelves due to
sea-level lowering, dried river beds, glaciogenic dust sources along the
ice-sheet edges, areas exposed to eolian erosion due to a scarce
vegetation in cold climate conditions), as well as in the transport
pathways in the stadial versus interstadial phases.
[Show abstract][Hide abstract] ABSTRACT: During the field season in summer 2009, the first 600 m (corresponding
to 3 kyr b2k (3000 years before A.D. 2000) on the GICC05 timescale) of
the Greenland NEEM ice core have been analysed for a variety of aerosol
constituents using Continuous Flow Analysis (CFA). Here, the records of
electric conductivity, sodium (Na+), calcium (Ca2+), particle numbers of
insoluble dust, ammonium (NH4+), nitrate (NO3-) and hydrogen peroxide
(H2O2) are presented with an average effective resolution of 1-2 cm,
depending on the component. Since the annual layer thickness ? amounts
to 15cm at minimum sub-annual signals are resolved in all components
over the Holocene period. We achieved to extend the aerosol record over
the early Holocene period except for a large gap over the brittle zone
from 5-9 kyr b2k. Seasonal variations and extreme events are
preserved in great detail and all components. H2O2 is a reliable proxy
for the strength of photochemical processes in the lower atmosphere and
thus shows its minima and maxima at the summer and winter solstice,
respectively. Dust-derived species (insoluble dust, Ca2+) show peak
concentrations in early spring and minima in mid-summer. The
marine-derived Na+peaks in mid-winter and is lowest during early summer.
The mean annual variability in concentrations is about 20 ppbw for both
Ca2+andNa+. Moreover, it is of the same order of magnitude in NH4+,
butconsiderably larger in NO3- (100 ppbw), both representing continental
biogenic sources peaking in spring and showing minima in autumn. The
interpretation itsclimatic signal is restricted by NO3- undergoing
post-depositional redistribution processes. Not only is the analysis
of impurities in sub-annual resolution crucial for the accurate dating
of the ice core, but also for establishing a detailed chronology of the
occurrence of extreme events such as volcanic eruptions and wildfires.
Furthermore, possible changes in the seasonal variability of aerosol
concentrations can be investigated. First results are presented here.
[Show abstract][Hide abstract] ABSTRACT: The Toba eruption that occurred some 74 kyr ago in Sumatra, Indonesia,
is among the largest volcanic events on Earth over the last 2 million
years. Tephra from this eruption has been spread over vast areas in Asia
where it constitutes a major time marker close to the Marine Isotope
Stage 4/5 boundary. As yet, no tephra associated with Toba has been
identified in Greenland or Antarctic ice cores. Based on new accurate
dating of Toba tephra from Malaysia and on accurately dated European
stalagmites the Toba event is known to occur between the onsets of
Greenland Interstadials (GI) 19 and 20. Furthermore, the existing
linking of Greenland and Antarctic ice cores by gas records and by the
bipolar seesaw hypothesis suggests that the Antarctic counterpart is
situated between Antarctic Isotope Maxima (AIM) 19 and 20.
In this work we suggest a direct synchronization of Greenland (NGRIP)
and Antarctic (EDML) ice cores at the Toba eruption based on matching of
a pattern of bipolar volcanic spikes. Annual layer counting between
volcanic spikes in both cores allows for a unique match. We first
demonstrate this bipolar matching technique at the already synchronized
Laschamp geomagnetic excursion (41 kyr BP) before we apply it to the
suggested Toba interval. The Toba synchronization pattern covers some
2000 yr in GI-20 and AIM 19/20 and includes nine acidity peaks that are
recognized in both ice cores. The suggested bipolar Toba
synchronization has decadal precision. It thus allows a determination of
the exact phasing of inter-hemispheric climate in a time interval of
poorly constrained ice core records, and it allows for a discussion of
the climatic impact of the Toba eruption in a global perspective.
Furthermore, our bipolar match provides a way to place
paleo-environmental records other than ice cores into a precise climatic
Climate of the Past 03/2013; 9:749-766. DOI:10.5194/cp-9-749-2013 · 3.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Efforts to extract a Greenland ice core with a complete record of the Eemian interglacial (130,000 to 115,000 years ago) have until now been unsuccessful. The response of the Greenland ice sheet to the warmer-than-present climate of the Eemian has thus remained unclear. Here we present the new North Greenland Eemian Ice Drilling (‘NEEM’) ice core and show only a modest ice-sheet response to the strong warming in the early Eemian. We reconstructed the Eemian record from folded ice using globally homogeneous parameters known from dated Greenland and Antarctic ice-core records. On the basis of water stable isotopes, NEEM surface temperatures after the onset of the Eemian (126,000 years ago) peaked at 8 ± 4 degrees Celsius above the mean of the past millennium, followed by a gradual cooling that was probably driven by the decreasing summer insolation. Between 128,000 and 122,000 years ago, the thickness of the northwest Greenland ice sheet decreased by 400 ± 250 metres, reaching surface elevations 122,000 years ago of 130 ± 300 metres lower than the present. Extensive surface melt occurred at the NEEM site during the Eemian, a phenomenon witnessed when melt layers formed again at NEEM during the exceptional heat of July 2012. With additional warming, surface melt might become more common in the future.
[Show abstract][Hide abstract] ABSTRACT: High-resolution measurements of chemical im-purities and methane concentrations in Greenland ice core samples from the early glacial period allow the extension of annual-layer counted chronologies and the improvement of gas age-ice age difference (essential to the syn-chronization of ice core records. We report high-resolution measurements of a 50 m section of the NorthGRIP ice core and corresponding annual layer thicknesses in order to con-strain the duration of the Greenland Stadial 22 (GS-22) be-tween Greenland Interstadials (GIs) 21 and 22, for which inconsistent durations and ages have been reported from Greenland and Antarctic ice core records as well as Euro-pean speleothems. Depending on the chronology used, GS-22 occurred between approximately 89 (end of GI-22) and 83 kyr b2k (onset of GI-21). From annual layer counting, we find that GS-22 lasted between 2696 and 3092 years and was followed by a GI-21 pre-cursor event lasting be-tween 331 and 369 yr. Our layer-based counting agrees with the duration of stadial 22 as determined from the NALPS speleothem record (3250 ± 526 yr) but not with that of the GICC05modelext chronology (2620 yr) or an alternative chronology based on gas-marker synchronization to EPICA Dronning Maud Land ice core. These results show that GICC05modelext overestimates accumulation and/or under-estimates thinning in this early part of the last glacial period. We also revise the possible ranges of NorthGRIP (5.49 to 5.85 m) and (498 to 601 yr) at the warming onset of GI-21 as well as the range at the onset of the GI-21 precursor warming (523 to 654 yr), observing that temperature (represented by the δ 15 N proxy) increases before CH 4 concentration by no more than a few decades.
Climate of the Past 11/2012; 8(6):1839-1847. DOI:10.5194/cp-8-1839-2012 · 3.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This work presents a new, field-deployable technique for continuous, high-resolution measurements of methane mixing ratios from ice cores. The technique is based on a continuous flow analysis system, where ice core samples cut along the long axis of an ice core are melted continuously. The past atmospheric air contained in the ice is separated from the melt water stream via a system for continuous gas extraction. The extracted gas is dehumidified and then analyzed by a Wavelength Scanned-Cavity Ring Down Spectrometer for methane mixing ratios. We assess the performance of the new measurement technique in terms of precision (±0.8 ppbv,), accuracy (±8 ppbv), temporal (ca. 100 s), and spatial resolution (ca. 5 cm). Using a firn air transport model, we compare the resolution of the measurement technique to the resolution of the atmospheric methane signal as preserved in ice cores in Greenland. We conclude that our measurement technique can resolve all climatically relevant variations as preserved in the ice down to an ice depth of at least 1980 m (66 000 yr before present) in the North Greenland Eemian Ice Drilling ice core. Furthermore, we describe the modifications, which are necessary to make a commercially available spectrometer suitable for continuous methane mixing ratio measurements from ice cores.