Periodic Reporting for period 1 - SCADI (Snow Core Accumulation from Delta-15N Isotopes)
Période du rapport: 2021-01-01 au 2022-12-31
The East Antarctic ice sheet (EAIS) is the Earth’s largest reserve of glacial ice and could potentially contribute to >1 m of sea level rise by 2100 and >15 m by 2500. Accurate predictions of East Antarctic ice loss are critical for societal adaptation strategies related to future climate change, but models disagree on how the EAIS will respond to future warming. Increased snow accumulation on the EAIS due to warmer air temperatures is expected to partly compensate for greater ice loss to the ocean, but we do not currently know how much compensation is occurring and how it will change with an increasingly warmer planet.
More observations of snow accumulation rates on the EAIS could improve modeled projections by providing hard evidence of how the EAIS has and is responding to past and present climate variability. However, the most common methods of determining snow accumulation in Antarctica are logistically intensive and/or fail to work well for the vast regions of the EAIS between the coast and ice sheet dome summits. We sought to develop and apply a new technique for observing accumulation rates based on the nitrogen isotopic ratios of nitrate present in snow and ice. This approach of our project SCADI (Snow Core Accumulation from Delta-15N Isotopes) is based on the phenomenon where nitrate deposited on the Antarctic snow surface has a distinct nitrogen isotopic change after reacting under sunlight. If the snow accumulation rate is lower, the nitrate is exposed to sunlight for a longer time before being buried by later snow and the isotopic change is greater. As a result, analyzing nitrate from many Antarctic sites that cover a wide range of modern accumulation rates would let us mathematically define the relationship between snow accumulation and nitrogen isotopes. Once this relationship is defined, we can calculate a site’s accumulation rate solely by knowing the nitrogen isotopic ratio of the local nitrate.
To fully realize this concept, we aimed to (a) create a standardized dataset of nitrate isotopes and accumulation rates for sites spanning the full wet-to-dry range of the EAIS, (b) develop a theoretical model for the relationship between nitrogen isotopes and accumulation rate and then use our standardized dataset to quantify this as an empirical model, and (c) validate this empirical model’s accuracy by applying it to nitrate isotopic data in East Antarctic ice cores. With the conclusion of SCADI, we have successfully achieved these objectives and substantially expanded the understanding of nitrate dynamics in East Antarctica.
More observations of snow accumulation rates on the EAIS could improve modeled projections by providing hard evidence of how the EAIS has and is responding to past and present climate variability. However, the most common methods of determining snow accumulation in Antarctica are logistically intensive and/or fail to work well for the vast regions of the EAIS between the coast and ice sheet dome summits. We sought to develop and apply a new technique for observing accumulation rates based on the nitrogen isotopic ratios of nitrate present in snow and ice. This approach of our project SCADI (Snow Core Accumulation from Delta-15N Isotopes) is based on the phenomenon where nitrate deposited on the Antarctic snow surface has a distinct nitrogen isotopic change after reacting under sunlight. If the snow accumulation rate is lower, the nitrate is exposed to sunlight for a longer time before being buried by later snow and the isotopic change is greater. As a result, analyzing nitrate from many Antarctic sites that cover a wide range of modern accumulation rates would let us mathematically define the relationship between snow accumulation and nitrogen isotopes. Once this relationship is defined, we can calculate a site’s accumulation rate solely by knowing the nitrogen isotopic ratio of the local nitrate.
To fully realize this concept, we aimed to (a) create a standardized dataset of nitrate isotopes and accumulation rates for sites spanning the full wet-to-dry range of the EAIS, (b) develop a theoretical model for the relationship between nitrogen isotopes and accumulation rate and then use our standardized dataset to quantify this as an empirical model, and (c) validate this empirical model’s accuracy by applying it to nitrate isotopic data in East Antarctic ice cores. With the conclusion of SCADI, we have successfully achieved these objectives and substantially expanded the understanding of nitrate dynamics in East Antarctica.
The overall objectives of this project listed above were divided into three work packages (WP). We accomplished the primary goals of each WP, although the final details and analyses sometimes differed somewhat from the original proposed WP due to COVID-19 and a new understanding of nitrate dynamics revealed by our early results. For WP1, we completed isotopic analysis on 1578 individual nitrate samples collected in East Antarctic snow. These include over 500 samples collected by the fellow in Antarctica and over 650 samples archived from 10 years of Antarctic snow monitoring programs. After (1) aggregating our results to produce individual site averages, (2) combining these data with previously reported isotopic data, and (3) identifying snow accumulation rates for each site, we created a standardized dataset of 135 sites. These results greatly increase the number and spatial coverage of nitrate isotope data reported for Antarctica. These data are archived through PANGAEA, and all future data will be similarly archived following FAIR rules. Two subsets of this database are each the subject of a manuscript currently in production with planned submissions in open access journals in the next three months.
We then used this database to complete WP2 by quantifying an empirical model that relates the nitrogen isotopes of nitrate to the local snow accumulation rate. This quantified relationship is the focus of the article “Sunlight-driven nitrate loss records Antarctic surface mass balance” which is currently under revised review at Nature Communications. In this article, we present the findings of WP1 and WP2 along with a newly-derived theoretical framework. We apply our new model to reconstruct 700 years of accumulation history from an ice core taken at Aurora Basin North, Antarctica, and validate our findings with comparative results from ice core density and ground penetrating radar. This paper serves as a summary of the primary accomplishments of SCADI.
WP3 focused on applying our new empirical model to nitrate data from Vostok and Dome C ice cores located on the ultra-dry interior Antarctic Plateau. Our work in WP3 revealed that the archiving process for nitrate in the driest regions of Antarctica has a previously undocumented vertical transportation component that prevents our empirical model from accurately reproducing accumulation rates. Despite the inability to directly convert nitrate isotopes into accumulation histories at these sites, our identification of this undocumented component is critical to properly interpreting the variability of nitrate and other chemical species in ice cores sampled from the ultra-dry regions. This discovery and its ice core science impacts are the focus of a manuscript currently in progress. At wetter sites such as the West Antarctic Ice Sheet Divide, our model accurately reconstructs accumulation variability going back tens of thousands of years, and continuing research aims to further refine the environmental bounds of our model’s application.
We then used this database to complete WP2 by quantifying an empirical model that relates the nitrogen isotopes of nitrate to the local snow accumulation rate. This quantified relationship is the focus of the article “Sunlight-driven nitrate loss records Antarctic surface mass balance” which is currently under revised review at Nature Communications. In this article, we present the findings of WP1 and WP2 along with a newly-derived theoretical framework. We apply our new model to reconstruct 700 years of accumulation history from an ice core taken at Aurora Basin North, Antarctica, and validate our findings with comparative results from ice core density and ground penetrating radar. This paper serves as a summary of the primary accomplishments of SCADI.
WP3 focused on applying our new empirical model to nitrate data from Vostok and Dome C ice cores located on the ultra-dry interior Antarctic Plateau. Our work in WP3 revealed that the archiving process for nitrate in the driest regions of Antarctica has a previously undocumented vertical transportation component that prevents our empirical model from accurately reproducing accumulation rates. Despite the inability to directly convert nitrate isotopes into accumulation histories at these sites, our identification of this undocumented component is critical to properly interpreting the variability of nitrate and other chemical species in ice cores sampled from the ultra-dry regions. This discovery and its ice core science impacts are the focus of a manuscript currently in progress. At wetter sites such as the West Antarctic Ice Sheet Divide, our model accurately reconstructs accumulation variability going back tens of thousands of years, and continuing research aims to further refine the environmental bounds of our model’s application.
Our empirical model offers the cryosphere community a way to reconstruct past changes in snow accumulation that is independent from existing techniques and, unlike water isotopes, accounts for local processes like wind driven erosion and deposition. Nitrate isotopes can serve as a primary accumulation proxy for newly drilled ice cores or as a supplemental technique for sections where ice deformation makes other proxies less reliable. Our model can also help expand the spatial coverage of modern accumulation observations since taking a snow sample for nitrate isotopic analysis at a new sites is much quicker and logistically simpler than installing and repeatedly revisiting stakes or than collecting and dating a shallow ice core. These accumulation data are critical input and validation for global and regional climate models predicting future changes to Antarctic ice storage and sea level rise.
The 1578 nitrate isotope samples collected and analyzed through SCADI greatly increase the number of such data reported for Antarctica. These include sites not previously sampled as well as multi-year monitoring at individual sites, which greatly improves our ability to investigate the spatial and temporal variability of nitrate in the Antarctic environment. The sheer number of these samples, all produced within the past decade, provide a solid baseline to compare with potential future changes to polar atmospheric chemistry and nitrate dynamics in a globally warmer world.
Finally, this MSCA facilitated the training of four student interns who gained skills in advanced scientific analysis in an internationally-diverse research setting. The fellow also helped to plan and enact a summer science outreach program to underprivileged children in Grenoble that focused on environmental science and sustainability.
The 1578 nitrate isotope samples collected and analyzed through SCADI greatly increase the number of such data reported for Antarctica. These include sites not previously sampled as well as multi-year monitoring at individual sites, which greatly improves our ability to investigate the spatial and temporal variability of nitrate in the Antarctic environment. The sheer number of these samples, all produced within the past decade, provide a solid baseline to compare with potential future changes to polar atmospheric chemistry and nitrate dynamics in a globally warmer world.
Finally, this MSCA facilitated the training of four student interns who gained skills in advanced scientific analysis in an internationally-diverse research setting. The fellow also helped to plan and enact a summer science outreach program to underprivileged children in Grenoble that focused on environmental science and sustainability.