the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Jan 2024
Scientific history, sampling approach, and physical characterization of the Camp Century sub-glacial sediment core, a rare archive from beneath the Greenland Ice Sheet
Abstract. Basal materials in ice cores contain information about paleoclimate conditions, glacial processes, and the timing of past ice-free intervals, all of which aid understanding of ice-sheet stability and its contribution to sea-level rise in a warming climate. Only a few ice cores have been drilled through ice sheets to the underlying sediment and bedrock, producing limited material for analysis. The Camp Century ice core, which the US Army drilled in northwest Greenland from 1960–1966 CE, recovered about 3.5 meters of sub-glacial sediment.
Here, we document the scientific history of the Camp Century sub-glacial sediment, and present our recent core-cutting, sub-sampling, and processing methodology and results for what remains of this unique archive. In 1972, curators in the Buffalo Ice Core Laboratory cut the original core segments into 32 segments each about 10-cm long. Since then, two segments are unaccounted for, two were thawed, and two were cut as pilot samples in 2019. With the exception of the two thawed segments, the rest of the extant core remained frozen since collection. In fall 2021, we documented, described, and then cut each of the remaining frozen archived segments (n=26). We saved an archival half and then cut the working half into eight oriented sub-samples under controlled temperature and light conditions for physical, geochemical, isotopic, sedimentological, magnetic, and biological analyses. Our approach maximized sample usage for multi-proxy analysis, minimized contamination, and preserved archive material for future analyses of this legacy sample material.
Grain size, bulk density, sedimentary features, magnetic susceptibility, ice content, as well as pore-ice pH and conductivity, suggest that the basal sediment contains five stratigraphic units. We interpret these stratigraphic units as representing different depositional environments in sub-glacial or ice-free conditions: from bottom to top, a diamicton with sub-horizontal ice lenses (Unit 1); vertically-fractured ice with dispersed fine-grain sediments (< 20 % in mass) (Unit 2); a normally graded bed of pebbles to very fine sand in an icy matrix (Unit 3); bedded very fine to fine sand (Unit 4); and stratified medium to coarse sand (Unit 5). Plant macrofossils are present in all samples and most abundant in Units 3 and 4; insect remains are present in some samples (Units 1, 3, and 5).
Our approach provides a working template for future studies of ice-core basal sediment because it includes intentional planning of core sub-sampling, processing methodologies, and archiving strategies in order to optimize the collection of paleoclimate, glacial process, geochemical, geochronological, and sediment properties from archives of limited size. Our work benefited from a carefully curated and preserved archive, allowing for the application of techniques not available in 1966. Preserving uncontaminated core material for future analytical capabilities is an important consideration for rare archive materials such as these from Camp Century.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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RC1: 'Comment on egusphere-2023-2922', Mike Bentley, 23 Feb 2024
Review of Bierman et al., “Scientific history, sampling approach….Camp Century….”
I enjoyed reading this manuscript. It’s an informative paper and provides some important background and context for the ongoing, and potentially future, analyses of the Camp Century subglacial sediment core. It is a little unusual in that significant parts of it are in a narrative style but I think in this case it is entirely justified. For the samples to be most useful their origin and handling need to be absolutely clear and the authors have done a significant service in doing the ‘detective work’ to track down much of this information. This information will be useful to anyone wanting to work on the material, and I suspect this paper will be their first port of call before starting, and indeed I would expect Figure 8 to be widely cited and reproduced.
The primary analyses reported (mainly summarised Fig 8) are useful in and of themselves for understand the material and its origins and so there is a clear and original research contribution as well as the documenting of process and history.
The paper is original and within scope of The Cryosphere. Methods are appropriate and results are appropriately discussed with excellent use/citation of previous literature. The abstract is a good summary of the paper. The paper makes a contribution to our understanding of sub-glacial sediment form a near-unique physical archive, and is of broad relevance.
I have provided minor edits on the attached pdf but the text is generally clearly structured, well-written and figures and tables are to a high standard.
Other more discursive queries and comments below:
Line 69 – all of the other four sections in this paragraph explain the purpose of the section, except the first one. So could add clause after ‘sub-glacial sediment ‘, along lines: “..in order to provide clear context and documented handling information for ongoing and future analyses”
Table 2:
- Formatting probably needs grid lines to line up entries/rows more clearly. At present some of the relationships between entries in different columns are ambiguous without going to Fig 4. (this also applies to Table 3)
- Line 150 - What subtraction error are you referring to? I can see what look like a number of differences between values in Column 3 and Column 4.
- In the table what is the meaning of the apparent reversal in depth of core (column 4) in core tube 1058? Depths in tube 1057 reach 1373.5 and then a sequence of 1374.69 and 1371.92 (i.e. a reversal) before 1373 in tube 1059
Table 3:
- Please define all the institutional acronyms (final column) in the caption.
- Count 25 – thermal state reads ‘Frozen and’. And what ?
- This table could be improved by ordering rows by the sub-sample a,b etc. This would also have benefit of more closely tying it to text and Fig 6.
Length of core and post-depositional changes
- Line 393 –. The statement on difference of core length between 3.55 and 3.44m is left unexplained or perhaps implicitly might be seen as suggesting an earlier mis-measurement. But I was struck that the difference is only ~3% and visual comparison of the 1972 and 2021 photos suggests that there has been substantial changes to the segments: for example there is much more surface relief, larger surface grooves etc on several of the segments when photographed more recently. I think this raises the possibility that as well as partial thawing noted in the text that there may have been other long-term changes perhaps caused by sublimation-induced drying out and shrinkage. If so this could explain the 3% difference in length without a measurement error.
Figure 8
The missing sections (e.g. 1063-3) are displayed in the stratigraphic log with an assigned brown colour when they should be blank.
Magnetostratigraphy
I note that 6 samples were judged to have been stored upside down. Given the comment on development of a viscous remnance during storage (line 472), it would be helpful to see a comment if there is any difference/anomalies in the magnetic measurements for those samples.
Other analyses
Is there a reason why there is no analytical pathway for microbiology (cf. micro-paleontology). Subglacial biology is of course an area of huge interest and the omission was notable. Was this due to a judgement that the lack of a sterile handling chain over 50 years rules out meaningful or robust analyses on these core samples ? Or another reason? A sentence or two on this might be helpful.
Data from Christ et al., 2021
Line 537 introduces an age constraint that has not been previously discussed. Given the usefulness and likely reproduction of Figure 8 is it worth adding an annotation to figure or comment in caption on likely age ranges of top and bottom (pilot) samples, taken from Christ et al.
Future availability of samples
Given the statement in Line 70 on approach to sample distribution I was expecting something explicit towards end about availability of material. Had the authors considered including something, perhaps in code/data availability, on how other groups might get hold of some of these materials, especially if they can suggest collaborative/novel analyses? Who would they speak to ? Who decides on what happens to the material ? And if not are all these analyses already planned to be undertaken by the institutions noted in final column of Table 3?
Mike Bentley
22/2/24
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AC2: 'Reply on RC1', Paul Bierman, 09 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2023-2922/egusphere-2023-2922-AC2-supplement.pdf
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RC2: 'Comment on egusphere-2023-2922', Anonymous Referee #2, 06 Mar 2024
The manuscript conveys the scientific history and describe the sampling approach of the Camp Century subglacial sediment core, some sixty years after the core was retrieved. The objective is to provide fundamental observations and provide a core stratigraphy for subsequent analytical work performed on subsamples.
The manuscript is well written, well structured, and with a great level of detail to provide an important backbone for any coming analysis. The authors have done an excellent job in setting up a stratigraphy and strategizing the sampling for future studies. The scientific history of coring endeavors is a great addition, that made reading the manuscript more exciting. From these points, I think the manuscript is ready for publication. I have only one minor comment, which regards the readership:
The authors are careful to interpret the stratigraphy to only a certain extent and do not provide these interpretations on the Figure 8 (which by the way could do with larger font sizes). Probably for a reason, although I do find the manuscript a little too much of a ‘’cliffhanger’’ in terms of the next steps with this material. As it is now, the manuscript is seemingly intended for those researchers, who will get to work on the material. I find it a shame, that the authors do not outline some research questions and engage a broader community. Researching ancient Greenland – using tiny bits of sediment – is quite buzzworthy. Therefore, I would propose a resume of what was found in Christ et al. 2024, in the section ‘initial interpretation and paleoenvironment’ along with an outline of the most pending research questions that these two papers have now led to (also in context of what is known from other similar type studies). Summarizing what we think we know now, and what is still uncertain, perhaps aided by conceptual figures, would engage a broader readership and could thereby pave the road for novel approaches (both analytical and modeling) that were not initially considered by the authors.
Citation: https://doi.org/10.5194/egusphere-2023-2922-RC2 -
AC1: 'Reply on RC2', Paul Bierman, 08 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2023-2922/egusphere-2023-2922-AC1-supplement.pdf
-
AC1: 'Reply on RC2', Paul Bierman, 08 Mar 2024
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RC3: 'Comment on egusphere-2023-2922', Anonymous Referee #3, 21 Mar 2024
This paper presents an overview of the scientific history of the Camp Century sub-glacial sediment that was drilled in northwestern Greenland from 1960-1966 CE. It also presents detailed descriptions of the sampling approach, a physical characterization of the sediments, and some scientific data (e.g. paleomagnetic data, pH, and conductivity). In this sense, the paper represents a mix between historical accounts and scientific results. However, I believe this approach is fully justified, and I enjoyed reading the historical accounts of the drilling projects at Camp Century in northwestern Greenland. It is important to document how this rather unique sedimentary archive was retrieved, handled, and stored. The paper is very well written, and the topic is suitable for the journal. Below, I provide a few suggestions and comments that I hope the authors will consider.
General recommendations
The paper provides a detailed description of the sampling approach that was applied to the material available from cores drilled between 1960-1966 CE. Now that the authors have the benefit of hindsight, I wonder if they have any recommendations concerning the processes that preceded their involvement with this material, such as the initial sample treatment, documentation, and storage. This kind of advice could prove useful for future efforts to recover sediment cores beneath ice sheets.
Paleomagnetic data
Upon reading the paper, I felt it would have been appropriate to show the pmag data discussed in section 4.4 on a figure (and not just in table S5). For instance, the NRM (or 20 mT) data could be displayed on a panel in figure 8, but it is also clear that it isn’t straightforward to interpret the data, mainly because the inclinations are scattered (the declinations are bound to scatter at this location). On that note, I think it would be useful if the authors could comment on the high degree of scatter – are there any trends related to the type of material or stratigraphic unit? Although the samples may not be equally susceptible to acquiring a viscous overprint, I would not expect this pattern if the NRM is dominated by a viscous overprint from storage at NBI. It would also be informative to know if the scatter tends to decrease after demagnetization, even though demagnetization at 20 mT may not be sufficient to reveal any patterns. Finally, it would be good to know if the pmag data associated with the six core segments that were stratigraphically inverted during storage stand out, or differ, from the remaining data, as would be expected if the viscous overprint was acquired during storage.
I realize that a detailed discussion of the pmag data may be outside the scope of this paper, so I will let the authors decide on how to deal with this.
Sediment/ice vs. bulk density
At the boundary between Unit 2 and Unit 3, the composition of the core material changes from ice-dominated (~80% ice) to sediment-dominated (~20% ice). This abrupt transition is not clearly reflected in the bulk density, which shows a very gradual change towards higher density. Why this gradual change in bulk density? Is it due to different sample resolutions associated with the sediment/ice versus bulk density measurements? Please comment on this.
Changes in conductivity and pH
Lines 482-490 describe changes in pH and conductivity (Fig. 8H). The details of this plot may not be the focus of this paper, but I’m curious about the variability in these parameters – particularly those that differ from the trend described in the text (e.g. line 485: “pH is a mirror image of the bulk density profile”). In this context, I wonder why the pH is low in the bottom and upper parts of Unit 2 (it is similar to the pH of Units 3-5), which are dominated by ice (~80% ice). Also, I wonder why the conductivity is high in the upper part of Unit 1, which is explained by a high percentage of fine grans (line 484). However, similar amounts of fine grains are found both below (e.g. 1063-5) and above (e.g. 1060-c2), where the conductivity is considerably lower. I guess the conductivity is controlled by the amount of dissolved ions, but does that correlate with grain size (I guess it might)? Any comments on this?
Figure 5
Nice and informative figure. However, it is not clear to me what the authors mean by “Geochronometry” in the table. Luminescence and cosmogenic nuclides are mentioned elsewhere – do you the authors have some specific dating methods in mind?
Future availability of material
It would be good to include a statement towards the end on the future availability of material – and how this process is expected to work. Also, I’m curious about what it will take to make the archive half available for analysis (who will decide this?).
Line 569: Guess “JP Stephenson” should be “JP Steffensen”.
Line 581: “…has declares…”. Perhaps “has declared”?
Citation: https://doi.org/10.5194/egusphere-2023-2922-RC3 -
AC3: 'Reply on RC3', Paul Bierman, 01 Apr 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2023-2922/egusphere-2023-2922-AC3-supplement.pdf
-
AC3: 'Reply on RC3', Paul Bierman, 01 Apr 2024
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-2922', Mike Bentley, 23 Feb 2024
Review of Bierman et al., “Scientific history, sampling approach….Camp Century….”
I enjoyed reading this manuscript. It’s an informative paper and provides some important background and context for the ongoing, and potentially future, analyses of the Camp Century subglacial sediment core. It is a little unusual in that significant parts of it are in a narrative style but I think in this case it is entirely justified. For the samples to be most useful their origin and handling need to be absolutely clear and the authors have done a significant service in doing the ‘detective work’ to track down much of this information. This information will be useful to anyone wanting to work on the material, and I suspect this paper will be their first port of call before starting, and indeed I would expect Figure 8 to be widely cited and reproduced.
The primary analyses reported (mainly summarised Fig 8) are useful in and of themselves for understand the material and its origins and so there is a clear and original research contribution as well as the documenting of process and history.
The paper is original and within scope of The Cryosphere. Methods are appropriate and results are appropriately discussed with excellent use/citation of previous literature. The abstract is a good summary of the paper. The paper makes a contribution to our understanding of sub-glacial sediment form a near-unique physical archive, and is of broad relevance.
I have provided minor edits on the attached pdf but the text is generally clearly structured, well-written and figures and tables are to a high standard.
Other more discursive queries and comments below:
Line 69 – all of the other four sections in this paragraph explain the purpose of the section, except the first one. So could add clause after ‘sub-glacial sediment ‘, along lines: “..in order to provide clear context and documented handling information for ongoing and future analyses”
Table 2:
- Formatting probably needs grid lines to line up entries/rows more clearly. At present some of the relationships between entries in different columns are ambiguous without going to Fig 4. (this also applies to Table 3)
- Line 150 - What subtraction error are you referring to? I can see what look like a number of differences between values in Column 3 and Column 4.
- In the table what is the meaning of the apparent reversal in depth of core (column 4) in core tube 1058? Depths in tube 1057 reach 1373.5 and then a sequence of 1374.69 and 1371.92 (i.e. a reversal) before 1373 in tube 1059
Table 3:
- Please define all the institutional acronyms (final column) in the caption.
- Count 25 – thermal state reads ‘Frozen and’. And what ?
- This table could be improved by ordering rows by the sub-sample a,b etc. This would also have benefit of more closely tying it to text and Fig 6.
Length of core and post-depositional changes
- Line 393 –. The statement on difference of core length between 3.55 and 3.44m is left unexplained or perhaps implicitly might be seen as suggesting an earlier mis-measurement. But I was struck that the difference is only ~3% and visual comparison of the 1972 and 2021 photos suggests that there has been substantial changes to the segments: for example there is much more surface relief, larger surface grooves etc on several of the segments when photographed more recently. I think this raises the possibility that as well as partial thawing noted in the text that there may have been other long-term changes perhaps caused by sublimation-induced drying out and shrinkage. If so this could explain the 3% difference in length without a measurement error.
Figure 8
The missing sections (e.g. 1063-3) are displayed in the stratigraphic log with an assigned brown colour when they should be blank.
Magnetostratigraphy
I note that 6 samples were judged to have been stored upside down. Given the comment on development of a viscous remnance during storage (line 472), it would be helpful to see a comment if there is any difference/anomalies in the magnetic measurements for those samples.
Other analyses
Is there a reason why there is no analytical pathway for microbiology (cf. micro-paleontology). Subglacial biology is of course an area of huge interest and the omission was notable. Was this due to a judgement that the lack of a sterile handling chain over 50 years rules out meaningful or robust analyses on these core samples ? Or another reason? A sentence or two on this might be helpful.
Data from Christ et al., 2021
Line 537 introduces an age constraint that has not been previously discussed. Given the usefulness and likely reproduction of Figure 8 is it worth adding an annotation to figure or comment in caption on likely age ranges of top and bottom (pilot) samples, taken from Christ et al.
Future availability of samples
Given the statement in Line 70 on approach to sample distribution I was expecting something explicit towards end about availability of material. Had the authors considered including something, perhaps in code/data availability, on how other groups might get hold of some of these materials, especially if they can suggest collaborative/novel analyses? Who would they speak to ? Who decides on what happens to the material ? And if not are all these analyses already planned to be undertaken by the institutions noted in final column of Table 3?
Mike Bentley
22/2/24
-
AC2: 'Reply on RC1', Paul Bierman, 09 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2023-2922/egusphere-2023-2922-AC2-supplement.pdf
-
RC2: 'Comment on egusphere-2023-2922', Anonymous Referee #2, 06 Mar 2024
The manuscript conveys the scientific history and describe the sampling approach of the Camp Century subglacial sediment core, some sixty years after the core was retrieved. The objective is to provide fundamental observations and provide a core stratigraphy for subsequent analytical work performed on subsamples.
The manuscript is well written, well structured, and with a great level of detail to provide an important backbone for any coming analysis. The authors have done an excellent job in setting up a stratigraphy and strategizing the sampling for future studies. The scientific history of coring endeavors is a great addition, that made reading the manuscript more exciting. From these points, I think the manuscript is ready for publication. I have only one minor comment, which regards the readership:
The authors are careful to interpret the stratigraphy to only a certain extent and do not provide these interpretations on the Figure 8 (which by the way could do with larger font sizes). Probably for a reason, although I do find the manuscript a little too much of a ‘’cliffhanger’’ in terms of the next steps with this material. As it is now, the manuscript is seemingly intended for those researchers, who will get to work on the material. I find it a shame, that the authors do not outline some research questions and engage a broader community. Researching ancient Greenland – using tiny bits of sediment – is quite buzzworthy. Therefore, I would propose a resume of what was found in Christ et al. 2024, in the section ‘initial interpretation and paleoenvironment’ along with an outline of the most pending research questions that these two papers have now led to (also in context of what is known from other similar type studies). Summarizing what we think we know now, and what is still uncertain, perhaps aided by conceptual figures, would engage a broader readership and could thereby pave the road for novel approaches (both analytical and modeling) that were not initially considered by the authors.
Citation: https://doi.org/10.5194/egusphere-2023-2922-RC2 -
AC1: 'Reply on RC2', Paul Bierman, 08 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2023-2922/egusphere-2023-2922-AC1-supplement.pdf
-
AC1: 'Reply on RC2', Paul Bierman, 08 Mar 2024
-
RC3: 'Comment on egusphere-2023-2922', Anonymous Referee #3, 21 Mar 2024
This paper presents an overview of the scientific history of the Camp Century sub-glacial sediment that was drilled in northwestern Greenland from 1960-1966 CE. It also presents detailed descriptions of the sampling approach, a physical characterization of the sediments, and some scientific data (e.g. paleomagnetic data, pH, and conductivity). In this sense, the paper represents a mix between historical accounts and scientific results. However, I believe this approach is fully justified, and I enjoyed reading the historical accounts of the drilling projects at Camp Century in northwestern Greenland. It is important to document how this rather unique sedimentary archive was retrieved, handled, and stored. The paper is very well written, and the topic is suitable for the journal. Below, I provide a few suggestions and comments that I hope the authors will consider.
General recommendations
The paper provides a detailed description of the sampling approach that was applied to the material available from cores drilled between 1960-1966 CE. Now that the authors have the benefit of hindsight, I wonder if they have any recommendations concerning the processes that preceded their involvement with this material, such as the initial sample treatment, documentation, and storage. This kind of advice could prove useful for future efforts to recover sediment cores beneath ice sheets.
Paleomagnetic data
Upon reading the paper, I felt it would have been appropriate to show the pmag data discussed in section 4.4 on a figure (and not just in table S5). For instance, the NRM (or 20 mT) data could be displayed on a panel in figure 8, but it is also clear that it isn’t straightforward to interpret the data, mainly because the inclinations are scattered (the declinations are bound to scatter at this location). On that note, I think it would be useful if the authors could comment on the high degree of scatter – are there any trends related to the type of material or stratigraphic unit? Although the samples may not be equally susceptible to acquiring a viscous overprint, I would not expect this pattern if the NRM is dominated by a viscous overprint from storage at NBI. It would also be informative to know if the scatter tends to decrease after demagnetization, even though demagnetization at 20 mT may not be sufficient to reveal any patterns. Finally, it would be good to know if the pmag data associated with the six core segments that were stratigraphically inverted during storage stand out, or differ, from the remaining data, as would be expected if the viscous overprint was acquired during storage.
I realize that a detailed discussion of the pmag data may be outside the scope of this paper, so I will let the authors decide on how to deal with this.
Sediment/ice vs. bulk density
At the boundary between Unit 2 and Unit 3, the composition of the core material changes from ice-dominated (~80% ice) to sediment-dominated (~20% ice). This abrupt transition is not clearly reflected in the bulk density, which shows a very gradual change towards higher density. Why this gradual change in bulk density? Is it due to different sample resolutions associated with the sediment/ice versus bulk density measurements? Please comment on this.
Changes in conductivity and pH
Lines 482-490 describe changes in pH and conductivity (Fig. 8H). The details of this plot may not be the focus of this paper, but I’m curious about the variability in these parameters – particularly those that differ from the trend described in the text (e.g. line 485: “pH is a mirror image of the bulk density profile”). In this context, I wonder why the pH is low in the bottom and upper parts of Unit 2 (it is similar to the pH of Units 3-5), which are dominated by ice (~80% ice). Also, I wonder why the conductivity is high in the upper part of Unit 1, which is explained by a high percentage of fine grans (line 484). However, similar amounts of fine grains are found both below (e.g. 1063-5) and above (e.g. 1060-c2), where the conductivity is considerably lower. I guess the conductivity is controlled by the amount of dissolved ions, but does that correlate with grain size (I guess it might)? Any comments on this?
Figure 5
Nice and informative figure. However, it is not clear to me what the authors mean by “Geochronometry” in the table. Luminescence and cosmogenic nuclides are mentioned elsewhere – do you the authors have some specific dating methods in mind?
Future availability of material
It would be good to include a statement towards the end on the future availability of material – and how this process is expected to work. Also, I’m curious about what it will take to make the archive half available for analysis (who will decide this?).
Line 569: Guess “JP Stephenson” should be “JP Steffensen”.
Line 581: “…has declares…”. Perhaps “has declared”?
Citation: https://doi.org/10.5194/egusphere-2023-2922-RC3 -
AC3: 'Reply on RC3', Paul Bierman, 01 Apr 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2023-2922/egusphere-2023-2922-AC3-supplement.pdf
-
AC3: 'Reply on RC3', Paul Bierman, 01 Apr 2024
Peer review completion
Journal article(s) based on this preprint
Data sets
3-d images of core segments Andrew Christ https://www.morphosource.org/concern/cultural_heritage_objects/000583438
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Andrew J. Christ
Catherine M. Collins
Halley M. Mastro
Juliana Souza
Pierre-Henri Blard
Stefanie Brachfeld
Zoe R. Courville
Tammy M. Rittenour
Elizabeth K. Thomas
Jean-Louis Tison
Francois Fripiat
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(15404 KB) - Metadata XML
-
Supplement
(66 KB) - BibTeX
- EndNote
- Final revised paper