scholarly journals IceChrono1: a probabilistic model to compute a common and optimal chronology for several ice cores

2015 ◽  
Vol 8 (5) ◽  
pp. 1473-1492 ◽  
Author(s):  
F. Parrenin ◽  
L. Bazin ◽  
E. Capron ◽  
A. Landais ◽  
B. Lemieux-Dudon ◽  
...  
Keyword(s):  
Probabilistic Model ◽  
Accumulation Rate ◽  
Ice Core ◽  
Ice Cores ◽  
Complex Problem ◽  
Polar Ice ◽  
Ice Flow ◽  
Marquardt Algorithm ◽  
Lock In

Abstract. Polar ice cores provide exceptional archives of past environmental conditions. The dating of ice cores and the estimation of the age-scale uncertainty are essential to interpret the climate and environmental records that they contain. It is, however, a complex problem which involves different methods. Here, we present IceChrono1, a new probabilistic model integrating various sources of chronological information to produce a common and optimized chronology for several ice cores, as well as its uncertainty. IceChrono1 is based on the inversion of three quantities: the surface accumulation rate, the lock-in depth (LID) of air bubbles and the thinning function. The chronological information integrated into the model are models of the sedimentation process (accumulation of snow, densification of snow into ice and air trapping, ice flow), ice- and air-dated horizons, ice and air depth intervals with known durations, Δdepth observations (depth shift between synchronous events recorded in the ice and in the air) and finally air and ice stratigraphic links in between ice cores. The optimization is formulated as a least squares problem, implying that all densities of probabilities are assumed to be Gaussian. It is numerically solved using the Levenberg–Marquardt algorithm and a numerical evaluation of the model's Jacobian. IceChrono follows an approach similar to that of the Datice model which was recently used to produce the AICC2012 (Antarctic ice core chronology) for four Antarctic ice cores and one Greenland ice core. IceChrono1 provides improvements and simplifications with respect to Datice from the mathematical, numerical and programming point of views. The capabilities of IceChrono1 are demonstrated on a case study similar to the AICC2012 dating experiment. We find results similar to those of Datice, within a few centuries, which is a confirmation of both IceChrono1 and Datice codes. We also test new functionalities with respect to the original version of Datice: observations as ice intervals with known durations, correlated observations, observations as air intervals with known durations and observations as mixed ice–air stratigraphic links. IceChrono1 is freely available under the General Public License v3 open source license.

scholarly journals IceChrono v1: a probabilistic model to compute a common and optimal chronology for several ice cores

2014 ◽  
Vol 7 (5) ◽  
pp. 6811-6844
Author(s):  
F. Parrenin
Keyword(s):  
Probabilistic Model ◽  
Accumulation Rate ◽  
Ice Core ◽  
Ice Cores ◽  
Air Bubbles ◽  
Ice Flow ◽  
Orbital Parameters ◽  
Information Depth ◽  
Dating Methods ◽  
Lock In

Abstract. Polar ice cores provides exceptional archives of past environmental conditions. Dating ice and air bubbles/hydrates in ice cores is complicated since it involves different dating methods: modeling of the sedimentation process (accumulation of snow at surface, densification of snow into ice with air trapping and ice flow), use of dated horizons by comparison to other well dated targets (other dated paleo-archives or calculated variations of Earth's orbital parameters), use of dated depth intervals, use of Δdepth information (depth shift between synchronous events in the ice matrix and its air/hydrate content), use of stratigraphic links in between ice cores (ice-ice, air-air or mix ice-air links). Here I propose IceChrono v1, a new probabilistic model to combine these different kinds of chronological information to obtain a common and optimized chronology for several ice cores, as well as its confidence interval. It is based on the inversion of three quantities: the surface accumulation rate, the Lock-In Depth (LID) of air bubbles and the vertical thinning function. IceChrono is similar in scope to the Datice model, but has differences on the mathematical, numerical and programming point of views. I apply IceChrono on two dating experiments. The first one is similar to the AICC2012 experiment and I find similar results than Datice within a few centuries, which is a confirmation of both IceChrono and Datice codes. The second experiment involves only the Berkner ice core in Antarctica and I produce the first dating of this ice core. IceChrono v1 is freely available under the GPL v3 open source license.

scholarly journals Glacial–interglacial dynamics of Antarctic firn columns: comparison between simulations and ice core air-δ<sup>15</sup>N measurements

2013 ◽  
Vol 9 (3) ◽  
pp. 983-999 ◽  
Author(s):  
E. Capron ◽  
A. Landais ◽  
D. Buiron ◽  
A. Cauquoin ◽  
J. Chappellaz ◽  
...  
Keyword(s):  
Accumulation Rate ◽  
Ice Core ◽  
Ice Cores ◽  
Detailed Comparison ◽  
Last Deglaciation ◽  
Depth Study ◽  
Dronning Maud Land ◽  
The Last Deglaciation ◽  
Correct Estimation ◽  
Lock In

Abstract. Correct estimation of the firn lock-in depth is essential for correctly linking gas and ice chronologies in ice core studies. Here, two approaches to constrain the firn depth evolution in Antarctica are presented over the last deglaciation: outputs of a firn densification model, and measurements of δ15N of N2 in air trapped in ice core, assuming that δ15N is only affected by gravitational fractionation in the firn column. Since the firn densification process is largely governed by surface temperature and accumulation rate, we have investigated four ice cores drilled in coastal (Berkner Island, BI, and James Ross Island, JRI) and semi-coastal (TALDICE and EPICA Dronning Maud Land, EDML) Antarctic regions. Combined with available ice core air-δ15N measurements from the EPICA Dome C (EDC) site, the studied regions encompass a large range of surface accumulation rates and temperature conditions. Our δ15N profiles reveal a heterogeneous response of the firn structure to glacial–interglacial climatic changes. While firn densification simulations correctly predict TALDICE δ15N variations, they systematically fail to capture the large millennial-scale δ15N variations measured at BI and the δ15N glacial levels measured at JRI and EDML – a mismatch previously reported for central East Antarctic ice cores. New constraints of the EDML gas–ice depth offset during the Laschamp event (~41 ka) and the last deglaciation do not favour the hypothesis of a large convective zone within the firn as the explanation of the glacial firn model–δ15N data mismatch for this site. While we could not conduct an in-depth study of the influence of impurities in snow for firnification from the existing datasets, our detailed comparison between the δ15N profiles and firn model simulations under different temperature and accumulation rate scenarios suggests that the role of accumulation rate may have been underestimated in the current description of firnification models.

scholarly journals On the gas-ice depth difference (Δdepth) along the EPICA Dome C ice core

2012 ◽  
Vol 8 (4) ◽  
pp. 1239-1255 ◽  
Author(s):  
F. Parrenin ◽  
S. Barker ◽  
T. Blunier ◽  
J. Chappellaz ◽  
J. Jouzel ◽  
...  
Keyword(s):  
Flow Model ◽  
Ice Core ◽  
Ice Cores ◽  
Convective Zone ◽  
East Antarctica ◽  
Column Height ◽  
Ice Flow ◽  
Depth Difference ◽  
Lock In ◽  
Last Deglacial

Abstract. We compare a variety of methods for estimating the gas/ice depth offset (Δdepth) at EPICA Dome C (EDC, East Antarctica). (1) Purely based on modelling efforts, Δdepth can be estimated combining a firn densification with an ice flow model. (2) The diffusive column height can be estimated from δ15N and converted to Δdepth using an ice flow model and assumptions about past average firn density and thickness of the convective zone. (3) Ice and gas synchronisation of the EDC ice core to the GRIP, EDML and TALDICE ice cores shifts the ice/gas offset problem into higher accumulation ice cores where it can be more accurately evaluated. (4) Finally, the bipolar seesaw hypothesis allows us to synchronise the ice isotopic record with the gas CH4 record, the later being taken as a proxy of Greenland temperature. The general agreement of method 4 with methods 2 and 3 confirms that the bipolar seesaw antiphase happened during the last 140 kyr. Applying method 4 to the deeper section of the EDC core confirms that the ice flow is complex and can help to improve our reconstruction of the thinning function and thus, of the EDC age scale. We confirm that method 1 overestimates the glacial Δdepth at EDC and we suggest that it is due to an overestimation of the glacial lock-in depth (LID) by the firn densification model. In contrast, we find that method 1 very likely underestimates Δdepth during Termination II, due either to an underestimated thinning function or to an underestimated LID. Finally, method 2 gives estimates within a few metres of methods 3 and 4 during the last deglacial warming, suggesting that the convective zone at Dome C cannot have been very large at this time, if it existed at all.

The Paleochrono probabilistic model to derive a consistent chronology for several paleoclimatic sites

2021 ◽  
Author(s):  
Frédéric Parrenin ◽  
Lucie Bazin ◽  
Christo Buizert ◽  
Emilie Capron ◽  
Jai Chowdry Beeman ◽  
...  
Keyword(s):  
Probabilistic Model ◽  
Environmental Changes ◽  
Accumulation Rate ◽  
Ice Core ◽  
Sediment Cores ◽  
Computing Time ◽  
Ice Cores ◽  
Ease Of Use ◽  
Sequence Of Events ◽  
Different Types

&lt;p&gt;Past climatic and environmental changes can be reconstructed thanks to paleoclimatic archives such as ice cores, marine sediment cores, lake sediment cores, speleothems, tree rings, corals, etc. The dating of these natural archives is crucial for deciphering the temporal sequence of events during past climate changes. It is also essential to estimate the absolute and relative errors of such estimated chronologies. This task is, however, complex since it involves the combination of different dating approaches on different paleoclimatic sites and often different types of archives. Here we present Paleochrono, a new probabilistic model to derive a common and probalistically optimal chronology for several paleoclimatic sites with potentially different types of archives. Paleochrono is based on the inversion of an archiving model: a varying deposition rate (also named sedimentation or accumulation rate) and also,&amp;#160;for ice cores, a&amp;#160;lock-in-depth of air bubbles (since air is not trapped at surface) and a thinning function (since ice undergoes flow).&amp;#160;The model integrates several types of chronological information: prior knowledge of the archiving process, independently dated horizons, depth intervals of known duration, undated stratigraphic links between records, and, for ice cores, &amp;#916;depth observations (depth differences between synchronous events recorded in the bubbles and ice, respectively). The optimization is formulated as a least-squares problem, assuming that all densities of probabilities are near-Gaussian and that the model is almost linear in the vicinity of the best solution. Paleochrono is the successor of IceChrono, which was dealing only with ice-core records. Paleochrono&amp;#160;performs better than IceChrono in terms of computational efficiency, ease of use, and accuracy. We demonstrate the ability of Paleochrono in a new AICC2012-Hulu dating experiment, which combines the AICC2012 dating experiment, based on records from five polar ice cores,&amp;#160;with data from two U/Th-dated speleothems from Hulu Cave (China). We analyse the performance of Paleochrono in terms of computing time and memory usage in various dating experiments. Paleochrono is freely available under the MIT open source license.&lt;/p&gt;

scholarly journals How well can we parameterize past accumulation rates in polar ice sheets?

1997 ◽  
Vol 25 ◽  
pp. 418-422 ◽  
Author(s):  
Eric J. Steig
Keyword(s):  
Accumulation Rate ◽  
Order Approximation ◽  
Ice Core ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Ice Cores ◽  
Accumulation Rates ◽  
Polar Ice ◽  
Polar Ice Sheets ◽  
First Order Approximation

An important component of models of the cryosphere is the calculation of accumulation rates over polar ice sheets. As a first-order approximation, many models rely on the assumption that temperature is the main controlling factor for precipitation. However, compilation of available ice-core data, including a new core from Taylor Dome, East Antarctica, suggests that precipitation is significantly decoupled from temperature for a large proportion of both the Greenland and Antarctic ice sheets. While the estimated glacial-to-interglacial change in temperature does not differ greatly among ice cores from each ice sheet, the estimated change in accumulation rate varies by more than a factor of 2. A simple vapor-pressure parameterization gives reasonable estimates of accumulation in the ice-sheet interior, but this is not necessarily the case close to the ice-sheet margin, where synoptic weather systems are important.

scholarly journals Optimisation of glaciological parameters for ice core chronology by implementing counted layers between identified depth levels

2014 ◽  
Vol 10 (4) ◽  
pp. 3585-3616 ◽  
Author(s):  
L. Bazin ◽  
B. Lemieux-Dudon ◽  
A. Landais ◽  
M. Guillevic ◽  
P. Kindler ◽  
...  
Keyword(s):  
Bayesian Approach ◽  
Accumulation Rate ◽  
Ice Core ◽  
Ice Cores ◽  
Age Difference ◽  
Sensitivity Tests ◽  
New Type ◽  
Lock In ◽  
Ice Core Dating

Abstract. A~recent coherent chronology has been built for 4 Antarctic ice cores and the NorthGRIP (NGRIP) Greenland ice core (Antarctic Ice Core Chronology 2012, AICC2012) using a bayesian approach for ice core dating (Datice). When building the AICC2012 chronology, and in order to prevent any confusion with official ice cores chronology, it has been imposed that the AICC2012 chronology for NGRIP should respect exactly the GICC05 chronology based on layer counting. However, such a strong tuning did not satisfy the hypothesis of independence of background parameters and observations for the NGRIP core as required by Datice. We present here the implementation in Datice of a new type of markers that is better suited to constraints deduced from layer counting: the markers of age-difference. Using this type of markers for NGRIP in a 5 cores dating exercise with Datice, we have performed several sensitivity tests and show that the new ice core chronologies obtained with these new markers do not differ by more than 400 years from AICC2012 for Antarctic ice cores and by more than 130 years from GICC05 for NGRIP over the last 60 000 years. With this new parameterization, the accumulation rate and lock-in depth associated with NGRIP are more coherent with independent estimates than those obtained in AICC2012. While these new chronologies should not be used yet as new ice core chronologies, the improved methodology presented here should be considered in the next coherent ice core dating exercise.

scholarly journals Deposition, recycling and archival of nitrate stable isotopes between the air-snow interface: comparison between Dronning Maud Land and Dome C, Antarctica

2019 ◽  
Author(s):  
V. Holly L. Winton ◽  
Alison Ming ◽  
Nicolas Caillon ◽  
Lisa Hauge ◽  
Anna E. Jones ◽  
...  
Keyword(s):  
Stable Isotopes ◽  
Accumulation Rate ◽  
Ice Core ◽  
Ice Cores ◽  
Skin Layer ◽  
Snow Accumulation ◽  
Snow Pack ◽  
Polar Ice ◽  
Depth Profiles ◽  
Dronning Maud Land

Abstract. The nitrate (NO3−) isotopic composition δ15N-NO3− of polar ice cores has the potential to provide constraints on past ultraviolet (UV) radiation and thereby total column ozone (TCO), in addition to the oxidising capacity of the ancient atmosphere. However, understanding the transfer of reactive nitrogen at the air-snow interface in Polar Regions is paramount for the interpretation of ice core records of δ15N-NO3− and NO3− mass concentrations. As NO3− undergoes a number of post-depositional processes before it is archived in ice cores, site-specific observations of δ15N-NO3− and air-snow transfer modelling are necessary in order to understand and quantify the complex photochemical processes at play. As part of the Isotopic Constraints on Past Ozone Layer Thickness in Polar Ice (ISOL-ICE) project, we report new measurements of NO3− concentration and δ15N-NO3− in the atmosphere, skin layer (operationally defined as the top 5 mm of the snow pack), and snow pit depth profiles at Kohnen Station, Dronning Maud Land (DML), Antarctica. We compare the results to previous studies and new data, presented here, from Dome C, East Antarctic Plateau. Additionally, we apply the conceptual one-dimensional model of TRansfer of Atmospheric Nitrate Stable Isotopes To the Snow (TRANSITS) to assess the impact of photochemical processes that drive the archival of δ15N-NO3− and NO3− in the snow pack. We find clear evidence of NO3− photolysis at DML, and confirmation of our hypothesis that UV-photolysis is driving NO3− recycling at DML. Firstly, strong denitrification of the snow pack is observed through the δ15N-NO3− signature which evolves from the enriched snow pack (−3 to 100 ‰), to the skin layer (−20 to 3 ‰), to the depleted atmosphere (−50 to −20 ‰) corresponding to mass loss of NO3− from the snow pack. Secondly, constrained by field measurements of snow accumulation rate, light attenuation (e-folding depth) and atmospheric NO3− mass concentrations, the TRANSITS model is able to reproduce our δ15N-NO3− observations in depth profiles. We find that NO3− is recycled three times before it is archived (i.e., below the photic zone) in the snow pack below 15 cm and within 0.75 years. Archived δ15N-NO3− and NO3− concentration values are 50 ‰ and 60 ng g−1 at DML. NO3− photolysis is weaker at DML than at Dome C, due primarily to the higher DML snow accumulation rate; this results in a more depleted δ15N-NO3− signature at DML than at Dome C. Even at a relatively low snow accumulation rate of 6 cm yr−1 (water equivalent; w.e.), the accumulation rate at DML is great enough to preserve the seasonal cycle of NO3− concentration and δ15N-NO3−, in contrast to Dome C where the profiles are smoothed due to stronger photochemistry. TRANSITS sensitivity analysis of δ15N-NO3− at DML highlights that the dominant factors controlling the archived δ15N-NO3− signature are the snow accumulation rate and e-folding depth, with a smaller role from changes in the snowfall timing and TOC. Here we set the framework for the interpretation of a 1000-year ice core record of δ15N-NO3− from DML. Ice core δ15N-NO3− records at DML will be less sensitive to changes in UV than at Dome C, however the higher snow accumulation rate and more accurate dating at DML allows for higher resolution δ15N-NO3− records.

scholarly journals Glacial-interglacial dynamics of Antarctic firn columns: comparison between simulations and ice core air-δ<sup>15</sup>N measurements

2012 ◽  
Vol 8 (6) ◽  
pp. 6051-6091 ◽  
Author(s):  
E. Capron ◽  
A. Landais ◽  
D. Buiron ◽  
A. Cauquoin ◽  
J. Chappellaz ◽  
...  
Keyword(s):  
Accumulation Rate ◽  
Ice Core ◽  
Ice Cores ◽  
Clear Correlation ◽  
Last Deglaciation ◽  
Main Driver ◽  
Dronning Maud Land ◽  
James Ross Island ◽  
The Last Deglaciation ◽  
Lock In

Abstract. Correct estimate of the firn lock-in depth is essential for correctly linking gas and ice chronologies in ice cores studies. Here, two approaches to constrain the firn depth evolution in Antarctica are presented over the last deglaciation: output of a firn densification model and measurements of δ15N of N2 in air trapped in ice core. Since the firn densification process is largely governed by surface temperature and accumulation rate, we have investigated four ice cores drilled in coastal (Berkner Island, BI, and James Ross Island, JRI) and semi coastal (TALDICE and EPICA Dronning Maud Land, EDML) Antarctic regions. Combined with available δ15N measurements performed from the EPICA Dome C (EDC) site, the studied regions encompass a large range of surface accumulation rate and temperature conditions. While firn densification simulations are able to correctly represent most of the δ15N trends over the last deglaciation measured in the EDC, BI, TALDICE and EDML ice cores, they systematically fail to capture BI and EDML δ15N glacial levels, a mismatch previously seen for Central East Antarctic ice cores. Using empirical constraints of the EDML gas-ice depth offset during the Laschamp event (~ 41 ka), we can rule out the existence of a large convective zone as the explanation of the glacial firn model-δ15N data mismatch for this site. The good match between modelled and measured δ15N at TALDICE as well as the lack of any clear correlation between insoluble dust concentration in snow and δ15N records in the different ice cores suggest that past changes in loads of impurities are not the only main driver of glacial-interglacial changes in firn lock-in depth. We conclude that firn densification dynamics may instead be driven mostly by accumulation rate changes. The mismatch between modelled and measured δ15N may be due to inaccurate reconstruction of past accumulation rate or underestimated influence of accumulation rate in firnification models.

scholarly journals How well can we parameterize past accumulation rates in polar ice sheets?

1997 ◽  
Vol 25 ◽  
pp. 418-422 ◽  
Author(s):  
Eric J. Steig
Keyword(s):  
Accumulation Rate ◽  
Order Approximation ◽  
Ice Core ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Ice Cores ◽  
Accumulation Rates ◽  
Polar Ice ◽  
Polar Ice Sheets ◽  
First Order Approximation

An important component of models of the cryosphere is the calculation of accumulation rates over polar ice sheets. As a first-order approximation, many models rely on the assumption that temperature is the main controlling factor for precipitation. However, compilation of available ice-core data, including a new core from Taylor Dome, East Antarctica, suggests that precipitation is significantly decoupled from temperature for a large proportion of both the Greenland and Antarctic ice sheets. While the estimated glacial-to-interglacial change in temperature does not differ greatly among ice cores from each ice sheet, the estimated change in accumulation rate varies by more than a factor of 2. A simple vapor-pressure parameterization gives reasonable estimates of accumulation in the ice-sheet interior, but this is not necessarily the case close to the ice-sheet margin, where synoptic weather systems are important.

scholarly journals A review of the brittle ice zone in polar ice cores

2014 ◽  
Vol 55 (68) ◽  
pp. 72-82 ◽  
Author(s):  
Peter D. Neff
Keyword(s):  
Atmospheric Pressure ◽  
Accumulation Rate ◽  
Ice Core ◽  
Ice Cores ◽  
Snow Accumulation ◽  
Air Bubbles ◽  
Polar Ice ◽  
Overburden Pressure ◽  
Trapped Air ◽  
Borehole Temperatures

AbstractMaintaining ice-core quality through the brittle ice zone (BIZ) remains challenging for polar ice-core studies. At depth, increasing ice overburden pressurizes trapped air bubbles, causing fracture of cores upon exposure to atmospheric pressure. Fractured ice cores degrade analyses, reducing resolution and causing contamination. BIZ encounters at 18 sites across the Greenland, West and East Antarctic ice sheets are documented. The BIZ begins at a mean depth of 545 ± 162 m (1 standard deviation), extending to depths where ductile clathrate ice is reached: an average of 1132 ± 178 m depth. Ice ages in this zone vary with snow accumulation rate and ice thickness, beginning as young as 2 ka BP at Dye-3, Greenland, affecting ice >160 ka BP in age at Taylor Dome, Antarctica, and compromising up to 90% of retrieved samples at intermediate-depth sites. Effects of pressure and temperature on the BIZ are explored using modeled firn-column overburden pressure and borehole temperatures, revealing complex associations between firn densification and BIZ depth, and qualitatively supporting expected thinning of the BIZ at low ice temperatures due to shallower clathrate stability. Mitigating techniques for drilling, transport, sampling and analysis of brittle ice cores are also discussed.

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