Abstract
Standard proxies for reconstructing surface mass balance (SMB) in Antarctic ice cores are often inaccurate or coarsely resolved when applied to more complicated environments away from dome summits. Here, we propose an alternative SMB proxy based on photolytic fractionation of nitrogen isotopes in nitrate observed at 114 sites throughout East Antarctica. Applying this proxy approach to nitrate in a shallow core drilled at a moderate SMB site (Aurora Basin North), we reconstruct 700 years of SMB changes that agree well with changes estimated from ice core density and upstream surface topography. For the under-sampled transition zones between dome summits and the coast, we show that this proxy can provide past and present SMB values that reflect the immediate local environment and are derived independently from existing techniques.
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Introduction
Antarctica holds a critical role in the Earth’s hydrosphere, providing long-term storage of 27 million km3 of ice1 and impacting global ocean and atmosphere circulation through its albedo, topography, export of calved glacial ice, and function as an atmospheric heat sink2,3,4,5. Since even small shifts in the surface mass balance (SMB) across Antarctic ice sheets can redistribute huge masses of water between the cryosphere, ocean, and atmosphere, a clear understanding of how its SMB has responded to past climate change is crucial for calibrating forecast models of the global environment and properly interpreting ice cores6,7,8,9,10. Despite this pressing importance, much of Antarctica has insufficient records of both modern and past SMB values, particularly in the transitional zone between the <1000 m elevation wet coastal periphery and the >3000 m elevation ultra-dry dome summits. Although this transitional zone comprises 50% of Antarctica’s surface area11, it hosts few long-term scientific stations and is much less targeted for intensive scientific research and deep (>100 m) ice core studies. Because this zone has a highly dynamic SMB system affected by strong wind-driven transport, rugged small scale surface features, and infrequent but high impact precipitation events, our lack of dedicated studies of the transitional zone impedes a comprehensive understanding of past and present SMB changes in Antarctica.
This lack of data is largely a result of logistical challenges with observing the intermediate SMB values in this transitional zone when using existing techniques. Determining modern SMB for new sites typically requires either installing stake transects that need multiple return visits spanning several years or coring several meters of firn to identify the increasingly buried 1992 Pinatubo volcanic horizon with geochemical analysis. However, the limited time and resources during research expeditions to remote areas usually prevents intensive modern SMB surveys with these methods, and, as a result, existing SMB records in the transition zone are largely restricted to a few frequently traveled supply traverse routes12. This has left vast regions of Antarctica with no ground-verified SMB data.
Although ice cores have been drilled from a few sites in the transitional zone, extracting SMB histories from these cores is often difficult. At interior dome sites, proxy air temperature from water isotopes (δ2H or δ18O) is used to derive snow accumulation rate through water vapor saturation10. However, this approach does not account for wind-driven transport and sublimation of surface snow at warmer and lower elevation sites13,14,15. Additionally, water isotopes reflect many environmental factors other than temperature, such as atmospheric circulation changes, transport pathways, and moisture sources, which can lead to large uncertainty and/or bias in reconstructed SMB16,17. Changes in ice density along the cores may be converted to SMB provided that they are well-dated, but density-based reconstructions become increasingly uncertain with depth due to thinning and deformation of ice layers18 and may be impossible in zones with heavy ice deformation. Cores are also commonly damaged during the drilling and transportation process, and this can make accurate physical measurements of mass and volume very difficult, particularly for the shallow firn segments. There is thus a strong need for alternative independent proxies that record local SMB for modern climatology studies, paleoclimate reconstructions, and ice sheet modeling while avoiding the problems inherent in existing methods.
Here, we present one such SMB proxy based on photolysis-induced changes in the 15N/14N ratio (δ15N, defined as \({\delta }=\frac{{15}_{{{{{{\rm{N}}}}}}}{/{14}_{{{{{{\rm{N}}}}}}}}_{{{{{{\rm{sample}}}}}}}}{{15}_{{{{{{\rm{N}}}}}}}{/{14}_{{{{{{\rm{N}}}}}}}}_{{{{{{\rm{standard}}}}}}}}-1\), relative to the N2-air standard) of nitrate (NO3−) (Fig. 1). Naturally deposited on the Antarctic ice sheet surface as the end product of the atmospheric oxidation of reactive nitrogen19,20,21,22, NO3− within the Antarctic snowpack can be photolytically converted to gaseous nitrogen oxides (NOx = NO + NO2) when exposed to ultraviolet light (λ = 290–350 nm). Because 14NO3− is more readily photolyzed than 15NO3−, the δ15N of NO3− (δ15NNO3) remaining in the snow will increase from its initial depositional value of ≈−20 to +20 ‰ to values as high as +400 ‰21,22,23,24,25,26,27,28 as the isotopically lighter photolytic NOx product is ventilated and lost to the atmosphere. Although NO3− can also be lost through HNO3 volatilization, we interpret δ15NNO3 solely through photolysis since volatilization does not strongly fractionate NO3− and should be a very minor component of NO3− loss outside of the warmest coastal zones24,29,30. Additionally, while the oxygen in NO3− also undergoes isotopic fractionation through photolysis, its interpretation is complicated by isotopic interactions with snow and water vapor24,25,31 and is not further discussed here.
Photolysis is limited to the depth where light penetrates and initiates photochemical reactions, and so the snowpack can be divided into an uppermost photic zone (generally 10–100 cm in East Antarctica) and a deeper archived zone31,32,33,34,35. Photolysis and the resulting isotopic fractionation of NO3− cease once snowfall buries NO3− beneath the photic zone, and the δ15NNO3 value of the NO3− buried in the archived zone (δ15NNO3arc) is assumed to be preserved indefinitely in glacial ice24,25,31,32. The final δ15NNO3arc value reflects the sum total of photolysis inducing radiation experienced by NO3− during the burial process, which, assuming stable insolation and photic zone depth, is itself determined by the rate at which the NO3− is buried and thus inversely related to SMB19,25,28,36. Modeling (Supplementary Discussion 1) and field observations support SMB as the primary driver of spatial variability in δ15NNO3arc values. Based on a new simplified theoretical framework (Methods, Supplementary Discussion 1), this relationship can be expressed as:
where the regression coefficients A and B are parameters that subsume constants and linearly co-varying variables associated with photolytic and fractionation processes. The inverse function of Eq. (1) can then be used as a transfer function to reconstruct SMB from δ15NNO3arc values (SMBδ15N). Calculated and referenced SMB values are given here with units of kg m−2 a−1, which is equal to mm w.e. a−1.
Results and discussion
SMBδ15N relationship and spatial applicability
To obtain parameter estimates for Eq. (1), we sampled NO3− in snow and firn from 92 East Antarctic shallow pits and cores that are reported here. Combined with 43 previously published δ15NNO3arc samples23,24,25,28,31,37, this constitutes a database of 135 total δ15NNO3arc values representing 114 distinct sites across East Antarctica (Fig. 2a). These δ15NNO3arc data were spatially paired with local SMB measurements either observed directly onsite (SMBground) or as an output from the Modèle Atmosphérique Régional (MAR) forced by ERA-interim reanalysis data13 and adjusted for a dry-site bias (SMBadjMAR) (Methods, Supplementary Discussion 2). The sites in our database cover a comprehensive range of East Antarctic SMB, from 20–30 kg m−2 a−1 at dome summits on the high plateau to >300 kg m−2 a−1 for sites on the coastal periphery (Fig. 2b).
The SMB and δ15NNO3arc in our field dataset are correlated with a high degree of confidence, producing a linear regression where ln(δ15NNO3arc + 1) = 6.98 ± 0.19 SMB−1 − 0.02 ± 0.01 (Fig. 2c, r2 = 0.91, p ≪ 0.001, n = 135). Moreover, this relationship is within modeled expectations that use best estimates for photolytic and isotopic fractionation parameters (Supplementary Fig. 1, Supplementary Discussion 1). Although the linear relationship is strong, the spread in regression residuals leads to a relatively large prediction interval of ±0.0085 for each reconstructed SMBδ15N−1 value. This imprecision likely results in part because field sampling techniques varied between studies and best sampling procedures (e.g., well-mixing a > 10 cm layer below the photic zone, taking multiple samples per site) may not always have been followed due to logistical challenges and time constraints. Additionally, the resolution of MAR and other regional climate models cannot capture the impact of small surface features on local SMB, and even hyperlocal SMB variability (i.e., the SMB at scales < 1 m) caused by sastrugi and drifts might be missed by nearby stakes or other ground observations of SMB. Assuming that these factors are not biased toward over- or underestimating SMB, we can expect the SMBδ15N regression to provide accurate modeled values despite these prediction intervals. The precision of the regression and its SMBδ15N modeled outputs should also improve in the future with the addition of data from new sites using best sampling protocols and improved regional climate modeling.
Applying the solved regression to SMB values modeled by MAR across East Antarctica reproduces the spatial variability of δ15NNO3arc observed in samples (Fig. 3, Supplementary Table 3). We find that 74% of Antarctica has δ15NNO3arc values elevated well above the typical range of atmospheric δ15NNO3 (i.e., >20 ‰), illustrating the vast spatial impact of photolytic NO3− loss. The highest modeled values, excluding some small coastal regions with very low or negative modeled SMBs (e.g., the McMurdo Dry Valleys and blue ice zones) where NO3− archiving is not expected, are found on the interior high plateau of East Antarctica between Dome C and Dome Fuji, in agreement with previous global chemical transport models33. Although millennial-scale changes in global NO3− dynamics and atmospheric oxidative capacity are not currently well constrained, the factors parameterized in Eq. (1) (Supplementary Discussion 1) have likely been stable enough during the Holocene for the SMBδ15N proxy’s general use. The large changes in atmospheric chemistry, biogeochemical cycles, and global environment earlier in the Pleistocene possibly changed atmospheric NO3− isotopic values, snow character, and/or insolation values enough that our SMBδ15N proxy based on modern observations will not accurately reconstruct past SMB values in glacial times. However, δ15NNO3arc changes observed between glacial and interglacial periods in Greenland ice cores have been interpreted to partially record SMB changes38, and thus δ15NNO3arc may still offer important insight into relative changes in SMB and into how NO3− dynamics varied during the Pleistocene.
Since the most advanced established technique for NO3− isotopic analysis (see Methods) uses ≈5 nmol of NO3− for δ15NNO3 analysis and ≈100 nmol to include oxygen isotope anomaly (Δ17ONO3) analysis, the potential resolution of the SMBδ15N proxy depends upon the NO3− concentration of the snow or ice sample and upon the mass of snow or ice comprising each sample. For the samples included in our field database, NO3− concentrations ranged between 5 and 131 ng g−1, with lower values at drier sites. To collect 100 nmol of NO3− for maximum isotopic data, these concentrations require between 0.05 to 1.15 kg of snow or ice, with a median requirement of 0.15 kg. For snow pits, sampling at a 2 cm depth interval requires only 0.01–0.16 m2 surface area collected per sample, and thus the storage and transport logistics for large numbers of samples are more restrictive for snow pits than physical sampling limitations. Ice core sampling resolution is dependent upon the core diameter and percent of core available for NO3− recovery. We find that 2–3 samples per ice core meter are typically achievable even when the ice core is only partly partitioned for NO3−, and higher resolution is possible with cores that are drilled solely or primarily for NO3− isotopic analysis.
While our field dataset covers sites with a SMB from 22 to 548 kg m−2 a−1, the SMBδ15N proxy is best suited for sites with SMB values between 40 and 200 kg m−2 a−1. Shallow cores from very dry Dome A and Dome C have lower δ15NNO3arc values at 2–6 m below the surface than at the ~1 m base of the photic zone, possibly because photolytic NOx can be transported downward through firn air convection and re-oxidized into NO3− with low δ15NNO3 values (Supplementary Discussion 3). This phenomenon violates the foundational assumption of “locked-in” NO3− beneath the photic zone, but we observe it only at the ultra-dry interior sites where SMB < 40 kg m−2 a−1. For sites with SMB > 200 kg m−2 a−1, the expected δ15NNO3arc value falls within the general range of atmospheric δ15NNO3 (−20–+20 ‰) because NO3− is buried below the photic zone in less than a year. Despite the short photic zone residence time, more than 80% of NO3− is deposited during sunnier months outside of winter polar night24,39 and some photolytic loss is still likely. As a result, NO3− samples that integrate multiple years of accumulation at high SMB sites might still resolve differences in SMB (Supplementary Fig. 3). Additionally, δ15NNO3arc values are increasingly less sensitive to SMB changes with higher SMB values due to the asymptotic nature of SMB−1 (i.e., the relationship between δ15NNO3arc and SMB is nearly flat where SMB > 200 kg m−2 a−1 as seen in Fig. 2b). Despite these restrictions, over 59 % of Antarctica has a SMB between 40 and 200 kg m−2 a−113 (Fig. 2a), and additional study of NO3− dynamics in wet and dry extremes may reveal regional adjustments that allow for further application of this new SMB proxy (Supplementary Discussion 4, Supplementary Fig. 3).
Aurora Basin North SMB reconstruction
As a proof of concept, we applied the SMBδ15N transfer function to δ15NNO3arc data from the 103 m deep ABN1314-103 ice core. This core was one of three drilled in the Australian Antarctic Program’s 2013–2014 summer campaign at Aurora Basin North (ABN; 71.17 °S 111.37 °E, 2679 m above sea level), a site with moderate modern SMB (≈120 kg m−2 a−1) located midway between coastal Casey Station and the Dome C summit (Fig. 2a). The SMBδ15N history reconstructed from ABN1314-103 covers the period from −47 to 649 years before present (BP, where present = 1950 CE) and has values ranging from 49 to 208 kg m−2 a−1 (Fig. 4a). Each SMBδ15N value integrates an average of 2.4 years of accumulation (total range: 0.7–4.5 years), and thus any impacts from individual precipitation events or seasonal extremes are attenuated. Overall, the SMB values at this site show fairly large variability (coefficient of variation = 0.21). The mean SMBδ15N in the 20th century (126 ± 26.5 kg m−2 a−1) is 34% greater than the mean SMBδ15N before 1900 CE (94 ± 18 kg m−2 a−1) and nearly 52% greater than the driest century that spans the 1600s CE (83 ± 20 kg m−2 a−1) (Fig. 4a).
Since δ15NNO3arc values reflect the snow burial speed of the immediate overlying area, short-term variability in SMBδ15N is likely dominated by small spatial scale factors such as surface roughness (e.g., sastrugi and dune migration)40,41,42,43 and local weather (e.g., snowfall heterogeneity)12,43,44,45. However, the SMBδ15N patterns observed over decadal to centennial scales more likely represent changes to the broader regional environment as the local environmental “noise” has less impact when data is aggregated at longer timescales. Finally, it is important to note that the SMBδ15N values reflect the immediate local snow accumulation, and so some short duration events (e.g., atmospheric rivers) with major region-spanning impacts may not be preserved in an individual ice core due to periods of surface erosion and/or mixing46. This feature should not, however, be viewed as a drawback of the SMBδ15N proxy. Rather, the SMBδ15N record is accurately reflecting the actual SMB experienced at the core site, which is a critical factor to accurately calculating and interpreting other environmental proxies contained in the ice core, such as biogeochemical fluxes.
Validating the SMBδ15N proxy reconstruction
We verified our new proxy’s accuracy by comparing the SMBδ15N values with SMB calculated using the physical density of the ice core and its age-depth relationship (SMBdensity). Because the measurements for SMBdensity are typically performed on each individual ice core segment, it generally has a lower potential resolution than SMBδ15N which, in contrast, can have multiple values per core segment. Still, SMBdensity functions well as an established benchmark for validating newer SMB proxies like SMBδ15N. For each 1-m core segment of ABN1314-103, we calculated a SMBdensity value by dividing the segment’s mass (kg) by both its volume (m3) and the age difference between the top and bottom of the segment (a m−1). The SMBδ15N (aggregated to match the 1-m resolution) and SMBdensity share very similar mean values (100.8 vs. 98.0 kg m−2 a−1, respectively) and total SMB ranges (62.0–157.3 vs. 61.7–153.4 kg m−2 a−1, respectively), and the two SMB reconstructions have a similar pattern of variation with a moderate Pearson correlation (r = +0.46, p < 0.001, n = 90) (Fig. 4b). The correlation increases rapidly when a broader running average is applied to the data, reaching +0.72 with 25 year averaging and +0.82 with 50 year averaging. This agreement in mean value, range, and variability validates our SMBδ15N approach and the potential of δ15NNO3arc as an accurate proxy for paleoenvironmental change.
Interpreting the ABN1314-103 SMB profile is more complicated than for ice cores drilled at dome summits because the ice sheet at the ABN drilling site is flowing horizontally at a rate of 16.2 m a−147. This means that the ice in ABN1314-103 actually accumulated as snow along a continuous 11.5 km transect upstream of the current ABN drilling site, with the oldest and deepest ice originating from the most distant upstream position. Using the horizontal ice flow rate and the ABN1314-103 core’s age-depth model, we can estimate the position along the upstream transect where the snow for each depth in the core originally accumulated48.
Although overall elevation gain is small along the 11.5 km transect (<15 m), the region has abundant 0.5–1 m undulations in surface topography extending over horizontal extents of 3–10 km11 (Fig. 5a). The MAR’s horizontal grid size (35 km) cannot resolve any potential SMB impact from these features, but ground penetrating radar (GPR)49 data collected along the upstream transect reveals that these surface slope and curvature changes correlate with SMB variations of up to 40 kg m−2 a−1 as determined by internal isochronal radar reflection horizons48 (Fig. 5b). These surface features can be identified as buried horizons to depths below the deepest segment of ABN1314-103, which suggests that they have been stable features of the local landscape for at least 700 years.
Because ABN1314-103 is composed of snow that fell along this upstream transect, the ice core SMB record will not only reflect changes due to wetting or drying of the regional climate, but it will also reflect any spatial SMB variability caused by topographic features that existed along the upstream transect. Since the local surface topography has not significantly shifted or changed over the time period covered by ABN1314-103, we take the modern topography-driven SMB changes observed with GPR to be representative of past SMB spatial variability. As each position along the upstream transect is paired to a depth in ABN1314-103, we can transfer the GPR-derived SMB profile along the horizontal transect to ice core depths to produce a SMB reconstruction (SMBGPR) that can be directly compared to the SMBdensity and SMBδ15N reconstructions.
The SMBGPR reconstruction for ABN1314-103 (Fig. 4b) represents the component of the SMB record preserved in the ice core that can be explained by upstream surface topography alone. We find that the general pattern of variability in SMBGPR correlates very well with the patterns recorded in the SMBδ15N (r = +0.74) and SMBdensity (r = +0.63) records (Fig. 4b). Thus, it appears that the primary SMB pattern preserved in ABN1314-103 is driven by upstream changes in surface curvature, which is important for properly interpreting other environmental proxies contained in the ice and for understanding the local ice flow history.
Extracting a climate-driven SMB record
To examine whether a secondary signal related to climate change was also preserved by the δ15NNO3arc, we removed the spatial impact of upstream topography by subtracting the SMBGPR data from the SMBδ15N record. After this “upstream effect detrending” and accounting for a small consistent offset in mean SMB values (3.7 kg m−2 a−1) between SMBGPR and SMBδ15N, we find that the multi-decadal SMB values have been generally stable over the past 700 years (Fig. 4c), with 50-yr running averages of the SMB always within 15 kg m−2 a−1 from the mean of the detrended data. These running averages suggest that drier conditions existed at ABN between 60 and 350 yr BP (1600 and 1890 CE, partially corresponding to the Little Ice Age) and that precipitation has increased in the most recent 100–150 years. This is generally consistent with what has been observed at other East Antarctic sites50,51,52,53 and for Antarctica as a whole18, but we recognize that this pattern is similar to the upstream topographic effect and that it might also arise if the SMBGPR record is excessively smoothed relative to true topographic-driven SMB variability (perhaps by the GPR data processing).
On shorter timescales, SMB values frequently change by ≈50 kg m−2 a−1 around a common mean within 10–20 year periods. This pattern likely reflects the high interannual snowfall variability expected at sites like ABN14. Located at the transition between the coast and the interior East Antarctic Plateau, annual snow accumulation at ABN is sensitive to frequent intrusions of extreme precipitation events and atmospheric rivers44,45, and the observed sub-decadal SMBδ15N variability may represent the frequency of their stochastic occurrence at the site. Additionally, small scale surface roughness features like sastrugi may affect hyperlocal SMB through periods of enhanced accumulation and erosion as they migrate and evolve on the snow surface40,41,42,54. While the temporal evolution and possible life cycle cyclicity of surface roughness features are as yet poorly known, hyperlocal changes in SMB could also explain some of the short-term SMB variability observed in the ABN record if the sampling interval is shorter than the average duration of a surface feature at a given location.
Applied use and potential of the SMBδ15N proxy
With over 8 million km2 of Antarctica having a SMB between 40 and 200 kg m−2 a−113 and over 70% of the ice sheet area modeled to have δ15NNO3 values markedly elevated by photolysis, the SMBδ15N proxy holds great potential for expanding our knowledge of Antarctic SMB variability over time and space and serving as an independent supplemental SMB reconstruction. Currently, regions with moderate SMB have only a handful of sites with SMB records older than 200 years, with the East Antarctic Plateau particularly poorly represented18. For ice coring projects in these regions, the SMBδ15N proxy can perform better at capturing the local effects of strong winds, irregular surface topography, and high interannual snowfall variability than water isotopic techniques while avoiding problems with layer thinning, density modeling, and core damage that affect density-based methods. As regional climate models still struggle to accurately simulate drifting snow and sublimation fluxes in the coast-to-plateau transition13, SMBδ15N can provide critical ground-based data for models predicting future contributions to sea level rise. The SMBδ15N proxy also holds particular value for helping to constrain and validate models of upstream flow effects in research targeting ice streams and broad-scale glacial flow patterns. The SMBδ15N approach may also be useful to estimate relative SMB changes for ice cores that lack robust age-depth models due to severe glacial deformation or discontinuities.
Additionally, sampling for the SMBδ15N proxy can save valuable time and cost compared to existing alternatives to expand current records of modern SMB. Obtaining new ground-based SMB measurements using existing techniques for sites without annually resolved layers requires either coring several meters to the increasingly buried Pinatubo volcanic horizon or repeated visits to newly installed stake transects. However, limited time and resources for research expeditions to remote areas precludes intensive SMB surveys with these methods. With the SMBδ15N proxy, a mean site SMB could be determined with only a series of shallow snow or firn samples extending deep enough into the archived zone to cover only a few seasonal cycles (much shallower than the Pinatubo horizon). After mixing snow well from multiple samples, only 15–75 g (0.3–1.5 kg if Δ17ONO3 results are desired) would need to be kept, transported, and analyzed for each sample, which logistically allows for the rapid collection of robust SMB site means in many locations. On-site melting and NO3− concentration could further reduce logistical requirements.
The SMBδ15N proxy promises to grow and adapt as studies on Antarctic NO3− dynamics continue. More NO3– samples coupled with quality environmental context data from East Antarctic will help us better constrain the uncertainty of SMBδ15N calculations and allow for more confidence in reconstructions. As additional ice cores are analyzed for δ15NNO3arc, we can better understand under which exact conditions δ15NNO3arc most accurately records SMB variability and if we can improve our reconstructions with a more complex model. Differences between calculated SMBδ15N values and well-constrained SMBdensity values may also prove useful in identifying periods of unusual environmental conditions that alter typical photolytic reactivity.
Because the resolution of δ15NNO3arc sampling is limited only by the minimum amount of NO3− needed for analysis, very finely-resolved δ15NNO3arc records can be obtained by increasing the mass of ice collected per depth unit (e.g., by specifically drilling whole cores or replicate cores for NO3− isotopes) and with advances in NO3− isotopic analysis expected in the near future55. This may allow for more precise multi-annual aggregations for SMBδ15N reconstructions and permit a deeper examination of subannual NO3− dynamics that can improve the proxy. Given the potential of the SMBδ15N proxy to advance our understanding of the Antarctic environment and its sensitivity to climate change, we strongly recommend that potential ice coring projects incorporate NO3− analyses into their planning and urge continued studies on Antarctic NO3− dynamics.
Methods
Mathematical framework for δ15NNO3arc and SMB relationships
A linear relationship between δ15NNO3arc and the reciprocal of surface mass balance (SMB−1) has been previously observed and reported in Antarctica19,28,36. Here, we mathematically illustrate how this relationship between δ15NNO3arc and SMB arises through photolysis of NO3−. We focus solely on the characteristics of NO3− contained within a given horizontal plane of snow that is located at the snowpack surface at t = 0. We assume simplified conditions with a constant surface mass balance (SMB), clear sky conditions, no surface roughness, and no significant compaction with burial in the photic zone. Any NO3− that is photolyzed is immediately and permanently removed from the plane of snow, and NO3− recycling31,36 is assumed not to affect NO3− in the plane of snow during the burial process modeled here (i.e., after t = 0).
Defining the relationship between δ 15NNO3arc and SMB
The time that it takes for a given horizontal plane of snow to be buried from the surface to a particular depth z is determined by the SMB (kg m−2 a−1, converted to an equivalent vertical velocity in cm s−1):
The concentration of NO3− within a plane of snow decays through time according to:
where J(z) is the photolytic rate constant at a given depth defined as:
where σ is the absorption cross section for NO3− photolysis (cm2), ɸ is the quantum yield for NO3− photolysis (molec photon−1), and I(z) is the actinic flux of ultraviolet irradiance (photon cm−2 s−1 nm−1) integrated over wavelengths that can induce photolysis of NO3−. However, this photolytic rate “constant” changes with depth because actinic flux exponentially decays with depth as:
where I0 is the initial actinic flux that strikes the snow surface and ze is the e-folding depth (cm) of the snowpack. Note that non-exponential decay of I in the top ~2 cm of snowpack32 is simplified here by assuming the decay to be exponential from the snow surface. Equation (3) can then be expressed as:
Through Eq. (2), we can rewrite Eq. (6) as:
In order to determine the NO3− concentration at a given depth, we use the relationship between depth and time (z = SMB × t) to derive:
And integrate to produce:
Which simplifies to:
At t = 0, [NO3−](t) = [NO3−]0 and therefore:
And thus combining Eqs. (10) and (11):
According to Eq. (12), as time (i.e., burial depth) increases, the NO3− concentration will decrease. However, the rate of decrease will lessen over time as the value of SMB × t approaches 3ze and 95% of the initial irradiance is gone. Here, below the photic zone (i.e., z > 3ze), the NO3− concentration is largely stable and equal to ec.
Therefore, we can calculate the fraction of NO3− archived below the photic zone (fNO3arc) as:
To determine the δ15NNO3arc of this NO3−, Rayleigh fractionation states that δ15NNO3 can be calculated with the fractionation factor ɑ by:
Through our prior calculation of fNO3arc in Eq. (13), we thus produce:
Because (ɑ − 1) is negative for nitrogen during photolysis of NO3−23,24,33,56,57,58 and the other parameters are positive, this means that δ15NNO3arc will vary linearly and positively with SMB−1 when other parameters are held constant or scale linearly with SMB−1. We examine the potential impacts of variability in these other parameters more thoroughly in Supplementary Discussion 1.
Based on modeling and field observations, SMB is the primary driver of change in δ15NNO3arc values. Thus, the non-SMB variables can be subsumed into two parameters A and B to function as linear regression coefficients, producing Eq. (16) of the main text:
The inverse function of Eq. (16) can be used as a transfer function to calculate SMB based on a δ15NNO3arc value:
Finally, since ln(x + 1) ≈ x when x ≈ 0, a simpler relationship of Eq. (15) can be approximated, in a form similar to that previously reported from field observations25,28,36:
Snow sampling techniques
The δ15NNO3arc values in our database are taken from a mix of previously reported values from Antarctic research traverses and values newly reported here (Fig. 2). For all values, snow and ice containing NO3− was sampled in the field in one of three techniques: 1) 1–2 m deep snow pit with continuous sampling at regular intervals from top to bottom, 2) single sample taken of a well-mixed 5–10 cm layer around the 1-m depth layer, and 3) drilled core later cut at desired intervals. For isotopic measurement of NO3– that included Δ17ONO3 analysis, 0.3–1.5 kg of snow or ice per sample were gathered to ensure a sufficient amount of NO3−. Generally, the multiple samples produced by the snow pit technique offered the best and most flexible results, but the 1-m depth layer technique was valuable for quick sampling during limited stops, and cores are necessary to collect samples deeper than ≈5 m.
Laboratory analyses
For δ15NNO3arc results included in our database that have been previously reported, readers are directed to the original papers for specific analytical and sampling techniques. For the δ15NNO3arc data newly reported here, snow and ice samples were collected into clean sealed plastic bags or tubs and stored frozen until melted at room temperature for analysis. The NO3− mass fraction (ω(NO3−)) was determined on aliquots by either a colorimetric method or ion chromatography with detection limits <0.5 ng g−1 and precision of <3 %23,24. The remaining melted samples were passed through an anionic exchange resin (Bio-Rad™ AG 1-X8, chloride form), and the resulting trapped NO3− was eluted with 10 ml of NaCl 1 M solution.
Isotopic analysis occurred at IGE-CNRS, Grenoble, France, where NO3− in these samples was converted to N2O with the denitrifying bacteria Pseudomonas aureofaciens (lacking nitrous oxide reductase), thermally decomposed into O2 and N2 on a 900 °C gold surface, and separated by gas chromatography with a GasBench II™. Oxygen and nitrogen isotopic ratios were then measured on a Thermo Finnigan™ MAT 253 mass spectrometer59,60,61,62. Isotopic effects from this analysis were corrected23,60, using the international reference materials USGS 32, USGS 34, and USGS 35 with ultrapure Dome C water used for standards and samples throughout the analyses to account for potential oxygen isotopic exchanges. Results are reported relative to Vienna Standard Mean Ocean Water (V-SMOW) for oxygen isotopes63 and N2-Air for nitrogen isotopes64.
For snow pits with multiple sequential δ15NNO3arc values, a single δ15NNO3arc value was calculated as the aggregate of samples 30+ cm deep, weighted by the relative mass of NO3− per sample. Although the photic zone boundary can extend lower than 30 cm at some sites31,32, this cutoff was deemed an acceptable compromise to include more data from pits that stopped at 50 cm depth as the great majority of photolysis will have occurred within the top 30 cm due to exponential decay of actinic flux and ω(NO3−) with depth. Exceptions to this were made for three coastal pits from Cap Prud’homme (weighted-means of 3+ cm samples), where high accumulation greatly reduces photolytic impact, higher snow impurities reduce the photic zone depth, and a broader aggregation is necessary to smooth seasonal cycles. Additionally, two pits from Dronning Maud Land were aggregated with 15+ cm samples based on shallow 3ze values (2–5 cm) calculated on site during snow pit sampling31. For cores included in our database, a single δ15NNO3arc value to be considered representative of the site was calculated as the isotopic mean of samples extending from present back to no earlier than 1800 CE.
Noro et al. (2018) reported δ15NNO3 values for 16 pits along the JARE54 and JARE57 transects28, but the sampling methodology for these pits took a single well-mixed sample of the entire pit depth which included the entire photic zone. In order to estimate the δ15NNO3arc values of these sites (i.e., the value as if the photic zone snow had been excluded), we applied a correction factor calculated using data from other pits in our database that were taken on two similar transects spanning from the coast to other interior domes (Dome A and Dome C) of East Antarctica24,25. Because each of the pits on the Dome A and Dome C transects were continuously sampled at discrete intervals from the surface to a point below the photic zone, we calculated different weighted-mean δ15NNO3 values for selected depth spans that matched the three extents of the JARE pits: 0–30 cm, 0–50 cm, and 0–80 cm. Corrective factors were calculated through the linear regression of δ15NNO3arc vs. δ15NNO3.X from Dome A/Dome C transect pits (where δ15NNO3arc is our database’s δ15NNO3 value from the archived zone and δ15NNO3.X is the weighted-mean value of samples from the surface to depth x: 30, 50, or 80 cm) and applied to the JARE pit data through the appropriate depth correction (Supplementary Tables 4, 5). Corrections were not made for JARE samples where δ15NNO3 < 0 ‰, as these low δ15NNO3 values strongly suggest that photolysis was not a significant factor at these coastal sites, and photic zone corrections were thus not warranted.
SMB data
In our database, 74 δ15NNO3arc samples are represented by 51 unique direct ground measurements of SMB (SMBground) values observed at or near the NO3− sampling site, with the numerical discrepancy due to some sites having replicate δ15NNO3arc samples. These previously reported SMBground values were determined by measuring the change in surface height on established stakes or poles, by measuring the mass between known volcanic or radioactivity horizons in an ice core, or by GPR identification of dated horizons10,12,24,25,65,66,67,68,69,70,71.
Regional climate models can be used to estimate modern SMB rates for sites lacking ground observations7,13, and we used the Modèle Atmosphérique Régional (MAR) version 3.6.4 with European Centre for Medium-Range Weather Forecasts “Interim” re-analysis data (ERA-interim) data as applied by Agosta et al. (2019) to model mean annual SMB at all database sites for the period 1979–201713. Because the MAR overestimates SMB at high elevation (>3000 m) interior sites of the East Antarctic plateau72, we calculated a correction factor through linear regressions of SMBground vs. MAR-estimated SMB (SMBMAR) for our 51 sites that have both values (Supplementary Table 6, Supplementary Fig. 4). This correction was applied to all original MAR estimates to produce “adjusted-MAR” SMB (SMBadjMAR) that match more closely with ground observations.
These sites were then split into two overlapping subsets of roughly equal count (SMBMAR < 175 kg m−2 a−1 and SMBMAR is >110 kg m−2 a−1), and a linear regression was calculated for each subset of sites. This regression for sites where SMBMAR < 175 kg m−2 a−1 is tightly constrained (SMBground = 1.0 ± 0.1 × SMBMAR − 5.8 ± 7.1, r2 = 0.84), and it performs well to align the SMBMAR estimates with the SMBground values at low SMB sites. The subset of SMBMAR is >110 kg m−2 a−1 has some samples where the difference between SMBMAR and SMBground are very large, particularly at lower elevation sites where intense aeolian erosion and deposition often produce highly variable local SMB rates that are difficult to accurately model13,14. As a result, this regression is weaker (SMBground = 0.9 ± 0.2 × SMBMAR + 4.2 ± 57.9, r2 = 0.35) than the first regression, but we apply it while acknowledging the possibility of wide deviations. The two regressions intersect at (SMBMAR = 138 kg m−2 a−1, SMBground = 130 kg m−2 a−1), and thus SMBadjMAR values were calculated by applying the first regression to all sites where SMBMAR ≤ 138 kg m−2 a−1 and applying the second regression to all sites where SMBMAR > 138 kg m−2 a−1. We constructed our final primary SMB dataset for the analysis of δ15NNO3arc samples by using the best quality SMB data for each site: SMBground if available and SMBadjMAR otherwise.
Transfer function and SMB reconstruction
We modeled linear relationships between ln(δ15NNO3arc +1) and SMB−1 based on Eq. (15) using previously reported parameter values to compare our theoretical framework to field results and to better understand the sensitivity of the relationships to photolytic and fractionation factors (Supplementary Discussion 1). To determine the coefficients in Eq. (1) from our field data, we performed linear regressions using all database δ15NNO3arc samples and the primary SMB dataset of best available SMB. Additional regressions (Supplementary Discussion 2) were performed for subsets of the database based on SMB type (SMBground vs. SMBadjMAR). With regression coefficients determined for Eq. (1), we modeled the spatial distribution of δ15NNO3arc values across Antarctica using gridded mean SMB (MAR-ERA-interim, 1979–2015) at a 35 km resolution13 that were converted to SMBadjMAR as previously described.
For reconstructing the ABN SMBδ15N history, the ABN1314-103 ice core was cut into 0.33 m samples from 5 to 103 m, and these were processed for NO3− isotopes in 2016 as previously described. We applied an annually resolved age model (ALC01112018) based on seasonal ion and water isotope cycles and constrained by volcanic horizons that was originally developed for a longer core also taken at ABN. Each 1 m ice core segment was individually weighed prior to cutting, and the mass and volume were used to calculate a SMB profile based on dated ice density changes (SMBdensity).
To determine past topographical effects on SMB, a MALA GPR device towing a RTA antenna on the surface (50 MHz out, 100 MHz in) was operated for a 65 km transect upstream of the coring site as part of the 2013–2014 campaign. Radar was triggered every 2 s (i.e., every 6–7 m along the transect) with a recording time window of 3000 nanoseconds that captured returns down to 300 m depth. After postprocessing49, isochronal internal reflecting horizons were identified to 220 m depth, digitized with ReflexW software, and dated by connecting to the ALC01112018 age-depth model. Using a density profile taken from a longer ice core simultaneously drilled at ABN, 2D fields (depth by transect distance) were calculated for age, mean accumulation rate, and local accumulation rate. The mean accumulation rate to the most shallow reflecting horizon was taken as the upstream topographical effect on SMB (i.e., SMBGPR).
Statistical analyses, regressions, SMB reconstructions, visualizations, and other statistical analyses were performed using the R programming language with packages ggplot2, RColorBrewer, gridExtra, cowplot, and tidyverse and with Adobe Illustrator. QGIS was used for spatial analyses and map creation using data produced here or cited in image captions.
Data availability
The data generated in this study have been deposited in the PANGAEA online repository73,74 at https://doi.org/10.1594/PANGAEA.941480 and https://doi.org/10.1594/PANGAEA.941491. All original source and figure data are available in this data or produced using the code linked below.
Code availability
Code for reproducing analyses and figure creation75 is available at https://doi.org/10.5281/zenodo.6806404.
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Acknowledgements
We express thanks to the following individuals for project assistance and data support: Sarah Albertin, Selin Bagci, Albane Barbero, Mathieu Casado, Armelle Crouzet, Vincent Favier, Elsa Gautier, Gaspard Jannot, Alexis Lamothe, Anaïs Orsi, Fred Parrenin, Holly Winton, and the overwintering crews at Concordia Station. We acknowledge the logistical support of IPEV for the French missions in Antarctica, the IPEV and PNRA colleagues and overwintering crews at Concordia Station, and the JARE54 traverse team for fieldwork assistance and access to the S80 site data. We thank the co-investigators for the ABN drilling project (Jérome Chappellaz, Dorthe Dahl-Jensen, David Etheridge, Joe McConnell, Andrew Moy, Steven Phipps, Andrew Smith, Tessa Vance, Meredith Nation) and the French National Center for Coring and Drilling (C2FN, funded by INSU) for critical drilling, logistic, and analytical support at ABN and other sites. Finally, we acknowledge the Glacioclim-SAMBA, ITASE, and IPICS 2kyr Array programs for SMB data, the Air-O-Sol facility at IGE for microbial culturing, and additional support from the MITACS Globalink program and JSPS-CNRS joint research program. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement 889508-SCADI (P.D.A., J.S.). Funding also provided by the Agence Nationale de la Recherche (ANR): ANR-10-LABX56, ANR-11-EQPX-009-CLIMCOR, ANR-16-CE01-0011-01 (J.S.), BNP-Paribas Climate Initiative programs: 1115 (CHICTABA), 1117 (CAPOXI 35-75) (JS), and 1169 (EAIIST) (J.S.), and the Japan Society for the Promotion of Science: MEXT/JSPS KAKENHI 20H0496.
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P.D.A., J.S., N.C., and M.C. conceptualized the project, formal analysis was performed by P.D.A., A.P.M.S., and E.L.M., visuals were created by P.D.A. and E.L.M., funding was acquired by P.D.A., J.S., and M.C., and initial writing was done by P.D.A., A.P.M.S., and P.C. P.D.A., J.S., N.C., A.P.M.S., E.L.M., O.M., J.M., C.A., P.C., K.K., S.H., M.C., T.V.O., L.J., and J.L.R. contributed to the project investigation and to manuscript review and editing.
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Akers, P.D., Savarino, J., Caillon, N. et al. Sunlight-driven nitrate loss records Antarctic surface mass balance. Nat Commun 13, 4274 (2022). https://doi.org/10.1038/s41467-022-31855-7
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DOI: https://doi.org/10.1038/s41467-022-31855-7
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