# Unravelling the high-altitude Nansen blue ice field meteorite trap (East Antarctica) and implications for regional palaeo-conditions

Harry Zekollari, Steven Goderis, Vinciane Debaille, Matthias van Ginneken, Jérôme Gattacceca, A.J. Timothy Jull, Jan T.M. Lenaerts, Akira Yamaguchi, Philippe Huybrechts, Philippe Claeys

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Harry Zekollari, Steven Goderis, Vinciane Debaille, Matthias van Ginneken, Jérôme Gattacceca, et al.. Unravelling the high-altitude Nansen blue ice field meteorite trap (East Antarctica) and implications for regional palaeo-conditions. Geochimica et Cosmochimica Acta, 2019, 248 (2), pp.289-310. 10.1016/j.gca.2018.12.035. hal-02048181

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# Unravelling the high-altitude Nansen blue ice field meteorite trap (East Antarctica) and implications for regional palaeo-conditions

Zekollari, Harry; Goderis, Steven; Debaille, Vinciane; van Ginneken, Matthias; Gattacceca, Jérôme; The ASTER team; Timothy Jull, A.J.; Lenaerts, Jan T.M.; Yamaguchi, Akira; Huybrechts, Philippe; Claeys, Philippe

Published in:
Geochimica et Cosmochimica Acta

DOI:
10.1016/j.gca.2018.12.035

Publication date:
2019

License:
CC BY-NC-ND

Document Version:
Accepted author manuscript

Link to publication

## Citation for published version (APA):

Zekollari, H., Goderis, S., Debaille, V., van Ginneken, M., Gattacceca, J., The ASTER team, Timothy Jull, A. J., Lenaerts, J. T. M., Yamaguchi, A., Huybrechts, P., & Claeys, P. (2019). Unravelling the high-altitude Nansen blue ice field meteorite trap (East Antarctica) and implications for regional palaeo-conditions. *Geochimica et Cosmochimica Acta*, 248, 289-310. https://doi.org/10.1016/j.gca.2018.12.035

## Copyright

No part of this publication may be reproduced or transmitted in any form, without the prior written permission of the author(s) or other rights holders to whom publication rights have been transferred, unless permitted by a license attached to the publication (a Creative Commons license or other), or unless exceptions to copyright law apply.
1 Unravelling the high-altitude Nansen blue ice field meteorite trap (East

2 Antarctica) and implications for regional palaeo-conditions

3

4 Harry Zekollari<sup>a,b,c</sup>, Steven Goderis<sup>a</sup>, Matthias van Ginneken<sup>d,a</sup>, Jérôme Gattacceca<sup>e</sup>, ASTER Team<sup>1</sup>,

5 A.J. Timothy Jull<sup>f</sup>, Jan Lenaerts<sup>g</sup>, Akira Yamaguchi<sup>h,i</sup>, Philippe Huybrechts<sup>a</sup>, Vinciane Debaille<sup>d</sup> and

6 Philippe Claeys<sup>a</sup>

7

8 <sup>a</sup> Earth System Science, Vrije Universiteit Brussel, Brussels, Belgium

9 <sup>b</sup> Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zürich, Zürich, Switzerland

10 <sup>c</sup> Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Birmensdorf, Switzerland

11 <sup>d</sup> Laboratoire G-Time, Université Libre de Bruxelles, Brussels, Belgium

12 <sup>e</sup> Aix Marseille Univ, CNRS, IRD, Coll France, CEREGE, Aix-en-Provence, France

13 <sup>f</sup> Department of Geosciences, University of Arizona, Arizona, United States of America

14 <sup>g</sup> Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, United States of America

15 <sup>h</sup> National Institute of Polar Research, Tokyo, Japan

16 <sup>i</sup> The Graduate University for Advanced Studies (SOKENDAI), Tokyo, Japan

17 <sup>1</sup> ASTER Team: Georges Aumaître, Didier L. Bourlès, Karim Kedadouche

18

19 Correspondence to: Harry Zekollari (zharry@ethz.ch)

20

21 Manuscript (GCA-D-18-00234) resubmission for *Geochimica et Cosmochimica Acta*

22

23 **Keywords:** Antarctic meteorite trap; Nansen ice field; terrestrial age; stable isotope

24 geochemistry; satellite derived velocities; palaeoclimate
ABSTRACT

Antarctic blue ice zones, the most productive locations for meteorite retrieval on Earth, contain old ice that is easily accessible and available in large quantities. However, the mechanisms behind these meteorite traps remain a topic of ongoing debate. Here, we propose an interdisciplinary approach to improve our understanding of a meteorite trap in Dronning Maud Land (East Antarctica) on the Nansen blue ice field meteorite trap (2600-3100 m above sea level), where more than half of the Asuka meteorites were collected. Based on 185 surface blue ice samples, one of the largest observed spatial patterns in oxygen isotopic variation to date is found. Relying on meteorites for which the terrestrial ages are determined using ¹⁴C and ³⁶Cl, this surface ice is interpreted to date from the Last Interglacial up to the present-day. By combining state-of-the-art satellite derived surface velocities, surface mass balance modelling and ice flow modelling, we estimate that about 75% to 85% of the meteorites found on the ice field were supplied by ice flow after entering the ice sheet in an accumulation area of a few hundred square kilometres located south (upstream) of the ice field. Less than 0.4 new meteorites per year are supplied to the ice field through ice flow, suggesting that the hundreds of meteorites found 25 years after the first visit to this ice field mostly represent meteorites that were previously not found, rather than newly supplied meteorites. By combining these findings, the infall rate of meteorites from space is estimated, which is in line with values from the literature, but situated at the higher end of the range. A comparison of the oxygen isotopic variation of the surface blue ice to that of the European Project for Ice Coring in Antarctica (EPICA), Dronning Maud Land (EDML) ice core (located 750 km to the west, at the same elevation), suggests that the regional changes in topography have been relatively limited since the Last Interglacial, supporting theories of an overall stable Eastern Antarctic Ice Sheet (EAIS) over this time period.
# 1. INTRODUCTION

The Antarctic Ice Sheet is the most productive region for collecting meteorites on Earth (Cassidy et al., 1992; Harvey, 2003). Of all meteorites collected to date, more than 35000 (ca. 65%) were retrieved on the Antarctic Ice Sheet (Meteoritical Bulletin Database, 2018). The high Antarctic meteorite retrieval probability is mainly related to three factors. First, given the contrast between the typically dark, fusion crusted meteorites and the blue/white surroundings, they are visually easily detectable (Harvey et al., 2015). Secondly, due to the cold and dry conditions the meteorites are generally well-preserved and less susceptible to chemical and physical weathering compared to warmer environments (Shimizu et al., 1983; Koeberl and Cassidy, 1991; Scherer et al., 1997; Welten et al., 1997, 2008; Pourkhorsandi et al., 2017). Thirdly, and most importantly, blue ice areas on the Antarctic Ice Sheet can act as meteorite traps, or so called ‘meteorite stranding zones’ (e.g. Righter et al., 2015).

In these meteorite stranding zones, meteorites that fell inland, in the ice sheet’s accumulation area, are brought to the surface (Figure 1a). This contrasts with the ordinary transport mechanism of meteorites, where they are moved to the edge of the ice sheet, before eventually being discharged into the ocean (Yoshida et al., 1971; Yanai, 1978; Whillans and Cassidy, 1983). In meteorite stranding zones, the upward-flowing ice, combined with surface ablation, leads to high concentrations of meteorites at the surface (Bintanja, 1999; Corti et al., 2003). Furthermore, low ice surface velocities can extend the residence time of meteorites at the surface and increase their local concentration. As a consequence, most meteorite recovery missions in the Antarctic focus on meteorite stranding zones (Delisle et al., 1993; Eppler, 2011; Herzog et al., 2015). Some meteorite stranding zones were subject to various studies and are relatively well understood (Corti et al., 2003; Folco et al., 2006; Welten et al., 2008). Other meteorite stranding zones, especially those located at high elevation, in terrain that is difficult to access, remain poorly understood and are topic of ongoing debate (Sinisalo and Moore, 2010).
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Blue ice areas are also interesting from a palaeoclimatic perspective (Bintanja, 1999; Corti et al., 2008; Sinisalo and Moore, 2010), as old ice is directly accessible at the surface (e.g. Spaulding et al., 2013). As a consequence, the sampling of ice is logistically and financially far less demanding compared to the classic deep drilling projects (e.g. Masson-Delmotte et al., 2011). Furthermore, blue ice areas have the potential to contain surface or shallow ice that is older than that recovered from deep drilling projects and can in some cases provide a higher temporal resolution (e.g. Sinisalo et al., 2007; Higgins et al., 2015; Bibby et al., 2016; Kehrl et al., 2018). Equally important, ice is available in large quantities, which is necessary for certain types of measurements, such as the characterization of trapped gasses and their isotopic composition (e.g. Sinisalo and Moore, 2010; Buizert et al., 2014). The main difficulty associated to using blue ice area records for palaeoclimatic purposes resides in the dating of the ice. This is more complex than in vertical drilling projects, where the ice age increases with depth. For surface blue ice dating, different techniques have been used and combined, including ¹⁴C dating of trapped air (e.g. Uglietti et al., 2016), ⁸¹Kr dating (e.g. Buizert et al., 2014), ⁴⁰Ar dating (e.g. Higgins et al., 2015), radiometric dating of tephra layers found at the surface (e.g. Muhs et al., 1999; Curzio et al., 2008), cosmogenic dating of glacial tills (e.g. Bibby et al., 2016), dating by comparison to isotopic variations from ice cores through wiggle-matching (e.g. Moore et al., 2006) and constraining the terrestrial age from surface meteorites (e.g. Folco et al., 2006; Welten et al., 2006; Herzog et al., 2015).

In this study, through an interdisciplinary approach, we improve the current understanding of an Antarctic meteorite stranding zone located on the Nansen blue ice field (Dronning Maud Land, East Antarctica). By combining the oxygen isotopic composition (δ¹⁸O) of the collected surface blue ice samples with the terrestrial age of selected meteorites, through ice flow and surface mass balance modelling, and by using a newly developed approach in which state-of-the-art satellite-derived surface velocities are combined with surface topographical information, we refine the mechanism behind the meteorite trap. This work also introduces
106 new concepts that can be used to better characterize and understand other meteorite traps.
107 Finally, we also discuss the implications of our findings for the past glaciological and
108 climatological conditions of this region.

109

110 **2. FIELD SITE AND CAMPAIGNS**

111

112 The Nansen blue ice field (72°S 24°E, Dronning Maud Land) is located on the Antarctic
113 plateau, about 100-120 km south of the Belgian Princess Elisabeth Station (see **Figure 2**).

114 Based on the spatial discontinuity of the ice patches, the blue ice field has been subdivided
115 into Nansen A, Nansen B and Nansen C (Imae et al., 2015) (see **Figure 2**). Nansen A is the
116 lowest part of the ice field, with its western side at an elevation of about 2600 m above sea
117 level (a.s.l.). Nansen C is the highest part of the ice field, with a surface elevation up to 3100
118 m in the southeast (see **Figure 3**). The ablation of this ice field is thought to occur almost
119 entirely as sublimation and is driven by strong katabatic winds, rather than by surface
120 melting, in line with most Antarctic meteorite traps (Sinisalo and Moore, 2010).

121

122 The ice field was first explored by the 29th Japanese Antarctic Research Expedition (JARE-
123 29) (1987-1989), during which a total of 1582 meteorites were retrieved, of which 573 on
124 Nansen A, 698 on Nansen B and 311 on Nansen C (Naraoka et al., 1990; Yanai et al., 1993).

125 Nansen A was subsequently revisited during the austral summer of 2010-11 (Belgian
126 Antarctic Research Expedition, BELARE 2010-2011), resulting in 218 newly collected
127 meteorites (Goderis et al., 2011). Two years later, during the austral summer of 2012-13, a
128 meteorite search expedition (JARE-54, BELARE 2012-2013) was organized on Nansen B
129 and on the northern part of Nansen C (Debaille et al., 2013; Imae et al., 2013, 2015). During
130 this expedition, 424 meteorites with a total weight of about 70 kg were collected, of which
131 368 on Nansen B, 55 on Nansen C and 1 on Nansen A (Figure 4) (Imae et al., 2015) (Figure
132 1b). With a total of more than 2200 collected meteorites, the Nansen ice field is one of the
133 most productive meteorite collection sites in Antarctica: to date, 6% of all Antarctic
134 meteorites (ca. 60% of all Asuka meteorites) were collected on this ice field, (Naraoka et al., 1990; Yanai et al., 1993; Goderis et al., 2011; Imae et al., 2015; Meteoritical Bulletin Database, 2018). Notice that given the relatively low pairing for this ice field compared to other meteorite retrieval locations in Antarctica (see section 6.2.2.), and the fact that Antarctic meteorites are not corrected for pairing, the relative contribution from Nansen meteorites to the total number is probably higher.

140
141 During the 2012-13 expedition, in addition to the meteorites, 185 surface blue ice samples were collected (Figure 4a and Figure 5). The analyses and interpretations reported in this study are largely based on the meteorites and surface blue ice samples collected during this 143 144 2012-2013 campaign. Two moraines were also visited on Nansen B (see Figure 5b for 145 location), but only terrestrial rocks, which are entrained from the bedrock to the surface (e.g. 146 Licht and Hemming, 2017; Graly et al., 2018), were found here (Jacobs et al., 2017). No 147 samples were collected in large areas of Nansen B (in the south and in the northeast; Figure 148 4a) because these zones were not (or only briefly) visited due to terrain conditions (crevasses, 149 topography not accessible using snowmobiles) or were covered by a snow layer, impeding the 150 retrieval of meteorites and collection of blue ice surface samples (Imae et al., 2015).

151
152 3. SURFACE BLUE ICE SAMPLES ISOTOPIC SIGNAL

153
154 Surface blue ice samples were crushed and transported in sealed containers to the lab, where 155 their isotopic composition was subsequently determined (at the AMGC stable isotope lab; see 156 Appendix A. for details). A clear spatial pattern is observed in the oxygen isotopic 157 composition (δ¹⁸O) of the surface samples from Nansen B, with δ¹⁸O = 158 [($^{18}$O/$^{16}$O)sample/($^{18}$O/$^{16}$O)ref - 1] x 10³ (in ‰), relative to the Vienna Standard Mean Ocean 159 Water (V-SMOW) (Figure 5). To our knowledge, this is one of the largest observed spatial 160 patterns in δ¹⁸O found to date in a blue ice area, with an extent of about 20 km (see e.g. Faure 161 et al., 1993; Spaulding et al., 2013; Winter et al., 2016; with patterns along a horizontal scale
of respectively ca. 2km, 5 km and 9 km). In the southern part of Nansen B, which will be referred to as Nansen B1 (Figure 5a), the δ¹⁸O of the analysed samples typically varies between -48‰ and -44‰ (n = 46) (see Appendix A. for details). The Central part of the ice field (Nansen B2, n = 82) is characterized by higher δ¹⁸O values, fluctuating between -42‰ and -38‰. The northwestern part of the ice field (Nansen B3, n = 32) has δ¹⁸O values in the same range as Nansen B1, varying between -48‰ and -44‰. The surface samples collected in the northern part of Nansen C (n = 25) range from -42‰ to -36‰ in δ¹⁸O (Figure 5a).

## 4. TERRESTRIAL AGE OF METEORITES

The terrestrial age of a meteorite, defined as the time since its fall on Earth, can be used as a minimum estimate to constrain the age of the ice at the meteorite collection site (Folco et al., 2006) (see Appendix B. for elaborate explanation). A common way to determine the terrestrial age of meteorites is through radiocarbon (¹⁴C) dating (Jull, 2006; Herzog et al., 2015). Given the short half-life time of ¹⁴C (5730 years), the method can generally only be applied to meteorites with a terrestrial age up to 40 ka (Jull, 2006), depending on the ¹⁴C blank correction. Eight meteorites from the ASUKA-88 / JARE-29 expeditions on Nansen B were dated this way (Ninagawa et al., 2006) and found to have terrestrial ages between 9.7 and 24.4 ka old (Table 1; measurements at Arizona AMS Facility; see Appendix B. for details). Unfortunately, these meteorite ages are of limited use to constrain the age of the ice, as the retrieval location of the meteorites is unknown. Furthermore, these eight meteorites weigh only between 19.3 and 119.3 grams (Table 1), which is below the meteorite wind transport threshold of approximately 200 grams (Folco et al., 2002). These meteorites have therefore potentially been transported by the strong katabatic winds (between t3 and t4 on Figure 1a) and are as such likely not representative for their collection location.

Meteorites collected during the JARE-54 – BELARE 2012-2013 expedition (Imae et al., 2015), for which the retrieval positions are precisely known (GPS coordinates), were
therefore selected for terrestrial age dating. Given the occurrence of strong year-round katabatic winds (Thiery et al., 2012; Imae et al., 2015), an initial selection of six meteorites weighing more than 1 kg was made. Four of these meteorites are well-spread across Nansen B (Figure 5b), while the two other are from Nansen C. The terrestrial ages were determined through radiocarbon dating and ranged between a recent fall (<2.4 ka) and >45 ka (measurements at Arizona AMS Facility; see Table 1 and Appendix B.). Subsequently, the two meteorites with the oldest terrestrial ages (A 12324 and A 12326) were re-dated using the ³⁶Cl method, together with two additional meteorites that were not radiocarbon dated (A 12127 and A 12216, measurements at ASTER AMS national facility at CEREGE; see Table 1 and Appendix B.). Given the longer half-life time of ³⁶Cl (301 ka), meteorites with older terrestrial ages can be dated (up to about 2 Ma), although the method is characterized by a larger uncertainty (Jull, 2006; Herzog et al., 2015). The ³⁶Cl terrestrial age measurements resulted in ages between <51 and 129± 28 ka (see Table 1).

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## 5. REGIONAL ICE FLOW

205

Today, 3-D ice flow models constitute the most comprehensive method to determine the flow of an ice body (e.g. Huybrechts et al., 2007; Goelzer et al., 2012; Pattyn et al., 2017; Zekollari et al., 2017b). However, the use of a 3-D ice flow model is not justified here, as various boundary conditions have large uncertainties. The best estimates on the regional ice thickness are from the BEDMAP2 dataset (Fretwell et al., 2013), and recently performed ice thickness airborne radar measurements over parts of the ice field and its surroundings (Eagles et al., 2018). These datasets suggest that the local ice thickness is typically in the order of 1000 m around the ice field (see e.g. Figure 4), but their low resolution (5 km for BEDMAP2, 10 km between flight lines for Eagles et al. (2018)) do not justify a setup with a detailed ice flow model over this region. Also, other boundary conditions, such as the surface mass balance (SMB) and the englacial ice temperatures, which determine the stiffness and thereby the
217 velocity of the ice, are relatively poorly constrained and therefore hamper the use of a 3-D ice
218 flow model.

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220 Therefore, to obtain an insight in the regional ice flow, satellite derived surface velocities are
221 utilized. We rely on a recently released surface velocity dataset by Mouginot et al. (2017) and
222 Rignot et al. (2017), in which measurements from a variety of sources (ALOS, COSMO-
223 SKYMED, ERS-1, RADARSAT-1, RADARSAT-2 and SENTINEL-1A) are combined to
224 derive the mean surface velocity over the Antarctic Ice Sheet for the period 1996-2016 at a
225 450 m spatial resolution (Figure 3). Combining different sources results in errors on the
226 horizontal surface velocity components that are generally lower compared to earlier versions
227 of the dataset (Rignot et al., 2011) and products relying on a single platform (Callens et al.,
228 2014; Fahnestock et al., 2016; Gardner et al., 2018). Despite this increased accuracy, for areas
229 with low surface velocities, such as the Nansen ice field (Figure 3), the errors on the
230 velocities (1.4 to 2.3 m a⁻¹ in Nansen B and C) remain of the same order of magnitude as the
231 velocities (typically 0.5-2.0 m a⁻¹ for Nansen B, 0.5-3.0 m a⁻¹ for Nansen C). Consequently, in
232 certain regions unrealistic ice flow is obtained, such as an upstream ice flow over large spatial
233 scales. To correct for this unrealistic ice flow, the direction of the satellite derived surface
234 velocity vectors (Mouginot et al., 2017; Rignot et al., 2017) is adapted by making use of the
235 surface topography data. The surface velocity vectors are forced to follow the steepest
236 negative surface gradient (as derived from the BEDMAP2 dataset; Fretwell et al., 2013)
237 within the error bars for both the x- and y-direction (Figure 6). At each location, the surface
238 gradient is determined over a distance of several kilometres, corresponding to several ice
239 thicknesses. This is used to mimic the effect of longitudinal stress gradients, causing the ice
240 flow to depend on the surrounding topography, and not only the local geometry (Kamb and
241 Echelmayer, 1986; Callens et al., 2014). The order of magnitude of the satellite derived
242 surface velocities (0.5-3.0 m a⁻¹) is confirmed by ice flow modelling, utilizing typical ice
243 thicknesses and englacial ice temperatures that can be expected for the Nansen ice field
244 (details in Appendix C.).
245 **6. DISCUSSION**

246

247 **6.1. Dating and distribution of surface ice**

248

249 The δ¹⁸O signal observed in the surface samples on Nansen B and the occurrence of three
250 pronounced clusters (Figure 5a), suggest that ice from both glacial and interglacial periods is
251 present at the surface. The difference in δ¹⁸O between the ‘low δ¹⁸O areas’ (Nansen B1 and
252 Nansen B3) and the ‘high δ¹⁸O area’ (Nansen B2) is around 5-7‰, which exactly
253 corresponds to the offset observed between glacial and interglacial periods in Antarctic deep
254 drilling cores (e.g. Masson-Delmotte et al., 2011). However, solely relying on the δ¹⁸O values
255 does not allow determining from which (inter)glacial period the surface ice originates, as the
256 clusters fit various glacial-interglacial transitions, as illustrated for the current and previous
257 interglacial-glacial transition in Figure 7 (discussed further in section 6.5.).

258

259 **6.1.1. Nansen B**

260

261 As the terrestrial age of the meteorites are interpreted to reflect minimum age estimates for
262 the underlying ice (see Appendix B. and e.g. Folco et al., 2006), combining those ages with
263 the δ¹⁸O measurements indicates that the ice on Nansen B formed during the last glacial
264 period (Nansen B1 and Nansen B3) and during the Last Interglacial (LIG, ca. 130-115 ka
265 ago) (Nansen B2). The differences in terrestrial ages between meteorites collected in each
266 other's vicinity is explained by a different timing of appearance at the surface: e.g. for Nansen
267 B2, meteorite A12216 (129±28 ka) likely appeared recently at the surface, while meteorite
268 A12324 (49±30 ka) must have appeared higher upstream and travelled over a substantial
269 distance, increasing the difference between the terrestrial age and the underlying ice (see e.g.
270 Folco et al., 2006).

271
272 In combination with the corrected surface velocities, this suggests that Nansen B1 and B2 are
273 stratigraphically connected, with Nansen B2 being the prolongation of Nansen B1 (Nansen
274 B1 is upstream from Nansen B2 in the schematic representation in Figure 1a). Although the
275 area south of Nansen B1 was not visited, the sequence of isotopic clusters and the regional ice
276 flow imply that here ice from the present-day interglacial and the end of the previous glacial
277 period is present.

278

279 For Nansen B3, the distribution of ice is possibly more complex, as two potential supply
280 mechanisms are identified:

281 (i) A fraction of the meteorites/ice on Nansen B3 may originate from a (relatively
282 small) separate source area, which is independent from that of Nansen B1 and
283 B2. This is supported by the observation that the southernmost ice samples
284 collected on Nansen B3 show higher δ¹⁸O values (see Figure 5), which may
285 suggest that this ice is from the present interglacial. The relatively young
286 terrestrial ages (< 40 ka) for the meteorites recovered on Nansen B3 (Figure 5b)
287 also support this mechanism.

288 (ii) Another supply mechanism may consist of meteorites that exited Nansen B1
289 through ice flow (cf. t4 on Figure 1a), re-entered the ice sheet (at shallow depth)
290 below the snowfield between Nansen B1 and B3 (t5 on Figure 1a), and
291 subsequently reappeared at the surface of Nansen B3 (follow trajectory on Figure
292 1a after t5 and forced back to the surface on Nansen B3). Such mechanism could
293 also explain that the collected meteorites/ice on Nansen B3 are younger than
294 those found on Nansen B2.

295

296 **6.1.2. Nansen C**

297

298 For the northern part of Nansen C, the high δ¹⁸O values (-42 to -36‰), combined with the
299 young terrestrial age of meteorites, suggest that the ice is younger than the Last Glacial
Maximum (LGM). Here, additional constraints on the age of the ice can be derived from volcanic ashes that were extracted from local tephra-bearing ice samples during the 2012-2013 expedition (see Figure 5a for location). By comparing the chemical composition of this volcanic ash with that of other ashes retrieved from major Antarctic ice cores, Oda et al. (2016) proposed that the volcanic ashes in the northern part of Nansen C originate from the South Sandwich Islands. The chemical composition of the ashes correlates particularly well with that of ashes from two post-LGM eruptions:

(i) at about 10.7 ka BP. These ashes are found at 340 m depth in EPICA-Dome C (Narcisi et al., 2005).

(ii) at about 3.5 ka BP. These ashes are found at 133 m depth in EPICA-Dome C (Narcisi et al., 2005) and at 100 m depth in Vostok (Palais et al., 1989; Basile et al., 2001).

It is thus likely that the ashes retrieved on Nansen C correspond to one of these two major late/post-LGM events. Although the chemical composition of some older ash layers from Antarctic ice cores also correlates relatively well with that of the Nansen C ash layer (e.g. ash layer at 505 m depth in Dome Fuji, dated at ca. 20 ka BP (Kohno et al., 2004; Fujita et al., 2015)), these are all from previous glacial periods, in disagreement with the high $\delta^{18}$O values for the ice at the surface of Nansen C. For the LIG, when $\delta^{18}$O values were also high (cf. Figure 7b), the chemical composition of the ashes retrieved from major ice cores does not resemble that of the ash layer retrieved at the surface on Nansen C. The compositional analysis of the ashes, combined with the isotopic signatures of the surface samples and the terrestrial ages of the meteorites, strongly suggest that the surface ice on Nansen C originates from after the LGM.

### 6.1.3. Nansen A

Nansen A was not visited during the 2012-2013 field campaign. Isotopic analyses of surface blue ice samples and terrestrial dating of meteorites can therefore not be used to constrain the
age of the local ice. Most likely the distribution of meteorites (and surface ice) is relatively complex here because most meteorites that exit Nansen B and Nansen C (cf. t4 on Figure 1a), re-enter the ice sheet (at shallow depth, t5) and subsequently reappear at the surface in Nansen A (cf. the potential mechanism ii described for Nansen B3) (see Figure 6). Therefore, both relatively young (from Nansen C) and older (from Nansen B) ice and meteorites are anticipated to mix at Nansen A.

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## 6.2. Meteorite weight, weathering, pairing and surface residence time

336

### 6.2.1. *Meteorite weight and weathering*

338

A clear pattern in the weight of the collected meteorites (fragments) is observed (Figure 6). On Nansen B1, Nansen B2 and Nansen C, meteorites show little to no weathering, and here meteorites of all sizes are collected, with many (47%) weighing more than 40 grams (Figure 4b). However, for Nansen B3 the meteorites are small, mostly weighing less than 40 grams (in 85% of cases), and field observations suggest (Imae et al., 2015) that these have been subjected to more substantial weathering in general (vs. Nansen B1 and B2). As weathering mainly occurs at the surface (e.g. Koeberl and Cassidy, 1991; Scherer et al., 1997; Welten et al., 1997, 2008; Pourkhorsandi et al., 2017), this likely indicates that a substantial fraction of the meteorites on Nansen B3 has been transported at the surface for prolonged periods of time. This may result from particularly low surface velocities at Nansen B3 (which are lower than over Nansen B1 and B2, cf. Figure 6). However, it could also support the earlier proposed mechanism in which most meteorites on Nansen B3 were previously transported at the surface of Nansen B1, after which they re-entered the ice sheet and subsequently re-appeared at the surface. Furthermore, the preferential transport of light meteorites by wind may also play a role here, and a part of these meteorites may have been transported from Nansen B2 to Nansen B3 (cf. Zwinger et al., 2015). The meteorites on Nansen B2 are on average slightly smaller (40% of meteorites weighing more than 40 g) and slightly more
356 weathered in general (Imae et al., 2015) than those on Nansen B1 (60% of meteorites weighing more than 40 g) (Figure 6), which may be explained as the effect of a long travel time at the surface for some of these, i.e. if they first appeared at the surface on the southern part of Nansen B1. These are complemented by 'fresh meteorites', which appear at the surface of Nansen B2 for the first time, explaining also why many large meteorites, with limited weathering, are found here.

362
363 The meteorites collected during the JARE-54/BELARE 2012-2013 expedition were on average much heavier (ca. 175 gram average weight) and less weathered than those collected during the BELARE 2010-2011 mission on Nansen A (ca. 30 grams average weight) (Goderis et al., 2011; Imae et al., 2015). Ninagawa et al. (2005) also reported older terrestrial ages (based on thermoluminescence studies) and higher degree of weathering for meteorites on Nansen A compared to those on Nansen B. This supports the proposed mechanism, in which most of the meteorites collected on Nansen A are meteorites that indeed reached the surface on Nansen B and C, where they were transported at the surface (i.e. undergoing weathering), before re-entering the ice sheet (at shallow depth) to reappear on Nansen A.

372
373 6.2.2. *Meteorite falls and pairing*

374
375 Some of the meteorites collected on the Nansen ice field may be fragments from an individual fall, as a fragmentation of meteorites may occur during (i) atmospheric passage and/or (ii) during their terrestrial history (Benoit et al., 2000). For Antarctic meteorites the number of meteorite fragments per fall is typically in the order of 2 to 6, i.e. with a pairing ratio varying between 1:2 and 1:6 (Lindstrom and Score, 1995; Benoit et al., 2000). Given the relatively limited weathering of meteorites on the Nansen ice field compared to other Antarctic sites at lower elevation (where higher temperatures and potential presence of meltwater favour weathering), the pairing is expected to be at the lower end of this range. This is confirmed by previous pairing studies on Nansen meteorites, which indicate that the ordinary chondrites
(which make up for most meteorites) have less than 2 fragment per fall (Ikeda and Kimura, 1992). An assessment of the meteorite weight distribution of the collected meteorites when compared to a mass-dependent function for meteorite falls (Halliday et al., 1989) also supports a relatively low pairing. We thus estimate the number of fragments per fall at the surface of Nansen to be somewhere between 2 and 4.

### 6.2.3. Meteorite mean surface residence time

Many of the meteorites collected on the Nansen ice field may have been present at the surface since a long time. Here, we describe this process through the meteorite's mean surface residence time (MSRT), which is used further on to assess the characteristics of the Nansen blue ice field (e.g. infall rate, supply rate of fresh meteorites, estimation of source area size). The MSRT is a similar concept to the 'surface exposure age' introduced by Benoit (1995) based on natural thermoluminescence (TL), and the 'on-ice travel time' described by Folco et al. (2006). It corresponds to the average time between the appearance of a meteorite at the surface of an ice field (t4 on Figure 1a) and its disappearance from this particular ice field (as it flows out of the field, at t5). Here, we derive the MSRT from the corrected surface velocities (section 5; Figure 6); two distinct approaches are considered:

(a) The MSRT is obtained by assuming a spatially evenly distributed appearance rate of meteorites at the ice field surface and by subsequently forcing the meteorites to move at the surface, along with the ice flow, until they exit the ice field.

(b) Starting from their retrieval location, the meteorites are projected at the surface both forward and backward (i.e. following the opposite direction of the ice flow) with time, until they exit the ice field at both ends. Here, the surface residence time of each meteorite corresponds to half the time it remains on the ice field (i.e. half the time needed for the hypothetical transport from the upper to the lower ice field boundary).

Through the application of these two approaches, MSRTs of 5600 years (approach a) and 5900 years (approach b) are obtained for Nansen B. As the ice flow and the katabatic winds
412 are largely in the same direction (Imae et al., 2015), the MSRT may be reduced, as light 413 meteorites (<200 g) are prone to transport by wind. Accounting for this effect and considering 414 the uncertainties and possible changes in the surface velocity field over time, the MSRT on 415 Nansen B is estimated to be between 4000 and 6000 years. Applying the same principles to 416 the northern part of Nansen C leads to a MSRT time of 3300 years (approach a) and 3000 417 years (approach b), respectively. Considering the effect of the katabatic winds and 418 uncertainties associated with the surface velocity field, it is estimated that the MSRT for 419 meteorites on the northern part of Nansen C is between 2500 and 3500 years. 420

420

421 **6.3. Source area size, location and supply mechanism**

422

423 **6.3.1. Source area size**

424

425 In a stable glacial system, consisting of an accumulation area and an ablation area, the 426 integrated surface mass balance is zero. For the Nansen meteorite trap, by estimating the 427 surface mass balance in the ablation area (i.e. the Nansen ice field) and the accumulation area 428 (i.e. the source area of the Nansen ice field), the ratio between these areas can be determined. 429

429

430 Based on simulations with an energy-balance model, which is forced with measurements 431 from two nearby automatic weather stations (AWSs), the annual surface mass balance over 432 the Nansen ice field is estimated to be between -150 and -75 mm w.e. $a^{-1}$ (all details are 433 provided in Appendix D.). For the accumulation area, the SMB is derived from a high- 434 resolution simulation with the RACMO2 regional climate model and estimated to be between 435 25 and 50 mm w.e. $a^{-1}$ (details in Appendix D.). These surface mass balance estimates thus 436 suggest that the ratio between the source area and the Nansen ice field varies between 75:50 437 (1.5:1) and 150:25 (6:1). With a visited area of about 100 km$^2$ for Nansen B; the source area 438 is thus estimated to be between 150 and 600 km$^2$. 439

439
440 6.3.2. *Source area location and supply mechanism*

441

442 As the estimation of the source area size varies by a factor of four and the regional
443 topography may have changed over time, it is difficult to address the exact location of the
444 source area. However, a rough estimate of the source area location can be obtained by
445 projecting the meteorites back in time. For this, we assume that the surface velocities are
446 constant in time and that the meteorites within the ice sheet are transported at the same
447 velocity as at the surface. As sliding acts at the base and most deformation occurs in the
448 lowest part of the ice sheet (e.g. Pattyn et al., 2017), this is a reasonable assumption. For a
449 more realistic insight on the transport of meteorites within the ice sheet, a 3-D ice flow model
450 would be needed in which the particles can be tracked back in time (e.g. Huybrechts et al.,
451 2007), but as argued before, given the large uncertainties in boundary conditions, this is not
452 justified here. The duration of the projection is determined by the estimated age of the oldest
453 retrieved ice, which is approximately 140 ka for Nansen B and about 20 ka for the northern
454 part of Nansen C. Following this method, the source area of Nansen B could reach as far as
455 150 km to the south and reach up to 3350 m a.s.l. (palaeoclimatic conditions of the source
456 region are addressed in section 6.5.). This indicates that the source area is located south of the
457 ice field and that it likely has a rather elongated form (i.e. a large north-south to east-west
458 ratio). Notice that some meteorites that reached the ice sheet surface further south likely also
459 cross the Nansen ice field through their englacial trajectory, but that given the long trajectory
460 towards the Nansen ice field, these meteorites reach deeper parts of the ice sheet and are
461 therefore believed to be underneath the surface on the Nansen blue ice field (cf. lowest flow
462 lines on Figure 1a).

463

464 For Nansen C, the surface velocities indicate that the source area is likely located to the east
465 of the ice field. Here, the source area is constrained by a surface ridge (Figure 3), which
466 explains the lack glacial ice at the surface of Nansen C. For the central and southern parts of
467 Nansen C, which were not revisited since the 1980s, the source area is presumably situated
468 more to the south, but it is most likely also confined by the ridge situated east of the ice field.

469

470 Combining the available information on surface and subglacial topography with the estimated
471 source areas, thus suggest that the Sør Rondane Mountains do not directly act as the natural
472 barrier causing the meteorites to reach the surface. The Nansen ice field is not a so-called
473 'closed type' blue ice area, but should be interpreted as an 'open type' blue ice area, in which
474 the ice flow is potentially slowed by subglacial bedrock ridges and forced upwards in
475 combination with surface ablation (Grinsted et al., 2003; Sinisalo and Moore, 2010). For
476 Nansen B, the 'open' character of the blue ice area explains the limitation in the age of the ice
477 (until the previous interglacial), as the oldest ice and meteorites are thought not to reach the
478 surface on the ice field (Grinsted et al., 2003 cf. lowest flow lines on Figure 1a).

479

480 **6.4. Implications for infall rate, direct fall and supply rate**

481

482 **6.4.1. Infall rate**

483

484 The infall rate of meteorites is defined as the number of meteorites entering the atmosphere
485 above a certain weight threshold for a certain area and per time unit. In the literature, infall
486 rates are typically estimated based on recovery statistics and weathering rates from arid areas,
487 and vary broadly by a factor three, from 36 to 116 meteorites (>10 g) / 10⁶ km² / a (Bland,
488 2001). Another widely used estimate is the 83 meteorites (>10 g) / 10⁶ km² / a derived by
489 Halliday et al. (1989) based on meteoroid observations. Recovery statistics from Antarctic
490 meteorites are an interesting alternative to derive infall rates, but their use is generally
491 hampered by various uncertainties (Zolensky et al., 2006). However, for the Nansen ice field
492 the large and diverse dataset of collected data can be used to get an insight in the infall rate by
493 relying on an alternative estimate of the source area (vs. estimate based on SMB; see section
494 6.3.1.).
495

The source area (A, in km²), can be related to the mean surface residence time (MSRT) of a meteorite at the surface (Tres, in a), the number of (pairing-corrected) meteorites (M, dimensionless), and the meteorite infall rate (I):

$$A=M/(I*T_{res}) \qquad (1)$$

Given that the supply of meteorites to the ice field (S, in meteorites a⁻¹) corresponds to:

$$S=M/T_{res} \qquad (2)$$

Eq.1 can be rewritten as:

$$A=S/I \qquad (3)$$

501

For Nansen B, a plausible range of values for the supply of meteorites to the ice field (S) can be obtained by combining a lower and upper boundary estimate of (i) Tres (between 4 and 6 ka for Nansen B, see section 6.2.3.) and (ii) M, which is estimated to be somewhere between:

* 325 pairing-corrected meteorites: this estimate assumes that the almost the entirety of meteorites on Nansen B was found (1300 meteorites), and that the pairing is at the higher end of the range (1:4). Given the weight distribution in the meteorites found on Nansen B (Figure 4a,b), a total of 325 pairing-corrected meteorites translates into about 270 pairing-corrected meteorites with a weight of 10 grams or more.

* 750 pairing-corrected meteorites: this approximation assumes that not all meteorites were found yet (1500 meteorites) and that the pairing is at the lower end of the range (1:2). This results in about 625 pairing-corrected meteorites weighing 10 grams or more.

The supply of meteorites (> 10 g) to Nansen B is thus somewhere between 270/6000 = 0.045 meteorites a⁻¹ and 625/4000 = 0.156 meteorites a⁻¹ (eq. 2). This estimate for S, combined with the range of plausible values for I from the literature, allows us to calculate the source area size (A) based on eq. 3 (Figure 8). A realistic ratio between the accumulation and the ablation area size as determined from SMB modelling (i.e. between 1.5:1 and 6:1, see section 6.3.1. and Appendix D.) can only be obtained for a relatively large accumulation area (ratio of
520 accumulation to ablation area above 3:1) with a supply ratio lower than <0.1 pairing-corrected
521 meteorites > 10 g a⁻¹, combined with an infall rate above 60 meteorites (>10 g) / 10⁶ km² / a
522 (Figure 8a). Similar results are obtained for the northern part of Nansen C (M = 33-104
523 pairing-corrected meteorites >10 g), where based on eq. 3 the entire range of infall rates from
524 the literature is realistic, but where higher values for I are deemed to be more likely (Figure
525 8b). Besides the implications for the infall rate, the low supply ratio may indicate that the
526 number of meteorites is relatively low, i.e. most meteorites were found and a substantial
527 amount of them represent fragments of a more limited number of falls, and/or that the MSRT
528 of meteorites is at the higher of the estimated range.

529

530 6.4.2. Supply rate

531

532 Applying a MSRT of 4 ka to Nansen B and assuming there are 1500 meteorites at the surface
533 of this ice field, an upper estimated supply rate of 0.4 meteorites a⁻¹ is obtained. Taking into
534 account the infall rates from the literature and the source area size estimates, eq. 3 and **Figure
535 8**, even suggests that the upper boundary is lower than this. During the 25-year period
536 between the JARE-29 and JARE-54 / BELARE 2012-2013 expeditions, less than 10 new
537 meteorites have thus reached the surface of Nansen B. From this, we can conclude that the
538 retrieval of 368 meteorites during the JARE-54 / BELARE 2012-2013 expedition on Nansen
539 B is therefore not related to a supply of new ('fresh') meteorites, but rather indicates that
540 many meteorites were not found during the previous expeditions. Some meteorites may have
541 been found in sites that were previously not visited on Nansen B, while other meteorites may
542 have been overlooked during the JARE-29 expedition. Migrating snow patches and ice field
543 boundaries may also have exposed new meteorites in the 2012-2013 field season, which were
544 snow-covered during the JARE-29 expedition.
545 6.4.3. *Direct fall*

546

547 The fraction between the meteorites that fell on the surface of the ice field directly and those
548 that reached the ice field through an englacial transport can be determined from the ratio
549 between the accumulation area and ablation area (assuming a spatially evenly distributed
550 appearance of meteorites at the surface after englacial transport). Based on the likely
551 accumulation to ablation area ratio between 6:1 to 3:1 (see section 6.4.1.), this suggests that
552 between 16.7 % (1:6) and 33.3% (1:3) of meteorites on the surface of the ice field are from
553 direct fall.

554

555 An alternative and independent estimate for the role of direct fall can be obtained by
556 estimating the number of meteorites that fall on the surface during the MSRT of meteorites on
557 a particular ice field. Within this respect, it should be noted that light meteorites may be
558 transported by katabatic winds, but that the resulting effect on the total number of meteorites
559 on the field is considered to be relatively limited. For Nansen B, where an area of about 100
560 km² was explored (see Figure 4a and Figure 5), assuming an upper limit for the infall rate of
561 116 falls over 10 g / 10⁶ km² / a (Bland, 2001) and a long MSRT of 6 ka, results in a total of
562 69.6 direct falls (>10 g). By relying on a lower limit for the infall rate of 36 falls over >10 g /
563 10⁶ km² / year (Bland, 2001) and a MSRT of 4 ka, a lower bound estimate of 14.4 direct falls
564 (>10 g) is obtained. The relative contribution of direct infall relies on the total number of
565 meteorites present at the surface of Nansen B before the meteorite recovery expeditions (see
566 section 6.4.1. for estimates). From this, it is estimated that the amount of meteorites on
567 Nansen B deriving from direct falls is somewhere between 2.3% (14.4/625) and 25.8%
568 (69.6/270). Given the assumed relatively high infall rate (see section 6.4.1.), the higher end of
569 this range is more likely, which also agrees with the 16.7-33.3% estimate from the
570 accumulation to ablation area ratio. Combining both estimates therefore suggests that about
571 15 to 25% of all collected meteorites fell on the Nansen ice field directly. It should be noted
572 that this estimate is higher than suggested by the very limited sample of twenty meteorites for
which the terrestrial age was determined (Table 1). From these twenty meteorites, only one (A 12430) results from a direct fall, as its terrestrial age (<2.4 ka) is younger than the time needed to transport it from the snow field to the collection site, and one other (A12084) may be a direct fall which fell on Nansen B1, re-entered the ice sheet, and subsequently reappeared on Nansen B3 (cf. section 6.1.1.).

578

## 6.5. Implications for palaeoclimate and palaeoglaciology

580

Given the absence of fixed age estimates and the presence of large uncertainties in boundary conditions, which inhibit the use of a complex ice flow model, it is not possible to derive a continuous palaeoclimatic record from the surface blue ice samples collected on the Nansen ice field (e.g. Spaulding et al., 2013). The magnitude of the isotopic signal and the difference between isotopic clusters can however be used to shed a light on the palaeo-conditions that reigned over this region.

587

The δ¹⁸O values at Nansen B are consistently less negative than those measured at the EPICA Dronning Maud Land (EDML) drill site (see Figure 7), which is located 750 km to the west, at about the same elevation (2800-2950 m a.s.l. for Nansen B vs. 2882 m a.s.l. for EDML).

For both the EDML drill site and the Nansen B ice field, the elevation at the formation site is higher than the present-day elevation. For Nansen B, this elevation difference is estimated to be up to 350-400 m between the LIG formation location and present-day elevation (see section 6.3.2.), while for EDML the LIG ice was formed about 300 m higher (Huybrechts et al., 2007). The distance to the ocean differs somewhat for both sites, as the Nansen blue ice field is located about 300 to 400 km from the coast (depending on direction), while for the EDML drill the coast is 550 to 600 km away. This may explain a part of the difference in δ¹⁸O between both sites, as air masses preferentially precipitate heavy oxygen isotopes while advancing over the continent (e.g. Jouzel et al., 2003). The δ¹⁸O difference is however larger than would be expected from the preferential precipitation of heavy oxygen isotopes only. We
therefore suggest that the δ¹⁸O difference between both sites may also reflect differences in surface temperatures. The constantly lower δ¹⁸O values for EDML suggest that temperatures at the time of formation were lower for EDML than for the Nansen ice field. The δ¹⁸O difference between both records varies approximately from 3 to 5 ‰ (see Figure 7), which, after correcting for the difference in distance to the ocean for both sites (correction of about 1 ‰), is somewhere between 2 to 4 ‰. Assuming the Antarctic wide isotope-temperature relationship of 0.8 ‰ / °C (Masson-Delmotte et al., 2008, fig. 6b), this corresponds to a temperature difference of about 2.5 to 5°C. It should be noted that this estimation is dependent on the isotope-temperature relationship, which is known to vary spatially and over time (Masson-Delmotte et al., 2008; Touzeau et al., 2016). However, Atmospheric General Circulation Models (AGCMs) suggest that the modern temperature-isotope relationship for central Antarctica is a good approximation for glacial to present-day changes (Jouzel et al., 2003). Note that the difference in δ¹⁸O (and thus temperature) will even be slightly more pronounced when considering that the formation site is likely higher for Nansen B compared to EDML. The δ¹⁸O difference should be interpreted with care, but in general, it indicates that it is now, and consistently was, warmer on the (accumulation area of the) Nansen ice field than at the (accumulation area of the) EDML drill site (Kohnen station) since the Last Interglacial.

The relative changes in surface elevation over time, which affect the δ¹⁸O signal, are believed to have been roughly similar for both sites, as the δ¹⁸O differences between both records remain largely the same over time. The local elevation at both sites may have evolved in a similar way over time, but the similar signal more likely indicates that surface elevation changes at both sites have been relatively limited. This suggests the topography over Dronning Maud Land was relatively stable during the last glacial and the present interglacial, providing additional support for studies suggesting limited changes in elevation at Kohnen (up to ca. 100 m between glacial-interglacial cycles; Huybrechts et al., 2007) and around the
Sør Rondane (evidence from morainic material and exposure dating; Mackintosh et al., 2014; 628
Suganuma et al., 2014), in line with a stable interior East Antarctic ice sheet over this time 629
period (e.g. Bentley et al., 2014; Mackintosh et al., 2014; Bader et al., 2017; Kaplan et al., 630
2017). 631

632

633 7. CONCLUSIONS

634

635
Combining different types of field, laboratory and remote sensing data allowed us to better
636
constraint the meteorite concentration mechanism at the Nansen ice field. On Nansen B, one
637
of the largest spatial patterns in contrasting isotopic oxygen values to date was found,
638
indicating that ice from both glacial and interglacial periods is present at the surface. By
639
combining this data with terrestrial age of large meteorites, we suggest that this ice is from
640
the previous glacial (ca. 115-15 ka BP) and interglacial period (ca. 130-115 ka BP). By using
641
information from previously collected ash layers, the ice on the northern part of Nansen C
642
was found to be from after the Last Glacial Maximum. 643

643

644
The size of the source areas of Nansen B and C was estimated based on the ratio between the
645
modelled SMB in the ablation area (the ice field) and the accumulation area (the source area).
646
The relatively limited source area size of a few hundred square kilometers suggests that the
647
Sør Rondane Mountains do not act as a barrier (i.e. ‘closed type’ blue ice area), but rather that
648
the meteorite trap acts on a smaller spatial scale and is therefore an ‘open type’ blue ice area.
649

649

650
In this study, we also used and further elaborated the concept of the mean surface residence
651
time (MSRT) of a meteorite, which we derived from corrected state-of-the-art satellite-
652
derived velocity data. We showed that the MSRT can be used, in combination with other field
653
measurements, to get an insight in:

(i) The infall rate of meteorites. Our analyses does not allow making a detailed
654
statement on the debated infall rate of meteorites, but suggests that the values at
655
656 the lower end of the literature are not compatible with the various datasets and
657 quantities (including the MSRT), therefore suggesting that the infall rate is at the
658 higher end of the range found in the literature.

659 (ii) The role of direct infall. It was estimated that between 15 and 25% of all
660 collected meteorites (>10 g) directly fell on the ice field, indicating that the
661 importance of direct infall may be relatively large and not negligible.

662 (iii) The supply rate of new meteorites. For Nansen B, it was estimated that less than
663 0.4 new meteorites reach the surface every year. This implies that meteorites
664 found during the 2012/2013 expedition are mainly those that were not found
665 during expeditions conducted 25 years earlier, rather than a recent resupply. By
666 now, it is likely that most of the meteorites on Nansen A, B and the northern part
667 of Nansen C have been found. New meteorites may still be collected in the
668 future, in part due to the migration of the ice field boundary and local snow
669 patches, but given the limited supply of fresh meteorites, the numbers are
670 expected to be substantially lower than for the 2012/2013 expedition. Future
671 meteorite search expeditions on this ice field should focus on new, not well-
672 explored regions, such as the southern part of Nansen B (which is difficult to
673 access with snowmobiles) and the southern part of Nansen C (which was only
674 visited once), rather than a revisit of Nansen A or B.

675
676 In summary, the interdisciplinary approach employed in this study leads to a better
677 understanding of the mechanism behind the Nansen meteorite trap. Some of the findings are
678 specific for the Nansen meteorite trap, but the developed methodology, and the use of the
679 mean surface residence time (MSRT) concept based on satellite derived surface velocities,
680 have the potential to improve our understanding of other meteorite traps. As such, this
681 methodology could also be used to plan future expeditions and may be applied to identify
682 new meteorite collection sites (e.g. by identifying blue ice areas with long MSRTs).

683
Finally, the oxygen isotopic signal of the surface blue ice samples could not be used to derive continuous series, but the absolute difference between the clusters allowed for certain interpretations about past climatic and environmental conditions. These suggest that temperatures over the Nansen ice field were likely warmer than at the Kohnen station (EDML drill site) since the Last Interglacial. Furthermore, the relatively constant difference between the two records suggests that regionally the topography did not change since the Last Interglacial. In future works, more palaeoclimatic information could be obtained by dating the surface blue ice samples directly (e.g. through isotopic analyses of gases). This will be crucial in the 'quest for the oldest ice' and allow to re-collect older ice than that from deep drilling projects.

## ACKNOWLEDGMENTS

We thank everyone who participated in the JARE-54 & BELARE 2012/2013 joint expedition. BELARE 2012/2013 was funded through the Belgian Science Policy Office (BELSPO) projects SAMBA and BELAM. HZ thanks Sophie Berger for helping with the retrieval of surface velocity data and Kevin De Bondt for introducing him to the Picarro Cavity Ring-Down Spectrometer. HZ was funded through a fellowship of the Research Foundation Flanders (FWO) (grant 11T0114N/11T0116N) and performed a substantial part of this research at ETH Zürich and WSL Birmensdorf, thanks to a FWO travel grant for a long stay abroad (V427216N), and a WSL innovative project grant. VD thanks the FRS-FNRS and ERC StG "ISoSyC" for present funding. PC acknowledges the support of the VUB Strategic Research Program. JG acknowledges funding from the French Agence Nationale de la Recherche (grant ANR-13-BS05-0009). We are grateful for the very thorough and thoughtful comments by two anonymous reviewers, which greatly helped us improve the manuscript.
709

# APPENDIX A. SURFACE BLUE ICE SAMPLES COLLECTION AND ANALYSES

710

711

All surface blue ice samples were collected during the 2012-2013 meteorite retrieval operations. When a meteorite was found by an expedition member, and when time and meteorological conditions allowed sampling, a surface blue ice sample was collected by another expedition member. The distance between the meteorite collection site and surface blue ice collection site varied between a few metres up to about 100 m, depending on the search party formation and the respective positioning of the meteorite collector and the ice sampler. Consequently, for almost the entirety of the regions where a meteorite was found, a correlated surface blue ice sample was collected (Figure 4a and Figure 5b).

712

713

714

715

716

717

718

719

720

At every collection site the upper 3 to 5 cm of surface ice were removed (cf. Spaulding et al., 2013), after which the underlying ice was crushed and collected in 60 ml containers. The containers were subsequently transported back in liquid phase to the Analytical, Environmental and Geo-Chemistry (AMGC) stable isotope laboratory at the Vrije Universiteit Brussel (VUB). As the containers were sealed, the isotopic fractionation during the time between original collection and analysis was limited, and the original isotopic signal was therefore maintained. The isotopic $\delta^{18}$O and $\delta$D compositions of the 185 samples were simultaneously determined with a Picarro L2130-i Cavity Ring-Down Spectrometer ($2\sigma$ = $\pm$0.1‰ for $\delta^{18}$O and $\pm$0.5‰ for $\delta$D). Replicates were used, and all values were corrected with laboratory standards. These standards span a wide range of isotopic values and were previously calibrated against standard samples from the International Atomic Energy Agency (IAEA). The errors on the isotopic measurements ($\sigma$) are on average 0.1041‰ for $\delta^{18}$O and 0.8346‰ for $\delta$D, which is about one to two order of magnitudes lower than the variations considered in our study (typically a few ‰ for $\delta^{18}$O). Note that in the text only the $\delta^{18}$O values are described (these values and their errors are also added as supplementary material; see Appendix E.); but that the $\delta$D values have a similar spatial pattern (Figure A.1).

721

722

723

724

725

726

727

728

729

730

731

732

733

734

735
736 APPENDIX B. TERRESTRIAL AGE DETERMINATION OF SELECTED

737 METEORITES

738

739 If a meteorite is assumed to reach the surface and enter the ice sheet in the accumulation area
740 (the source area, t1 on Figure 1a), to be transported within the ice (t2), to reappear at the
741 surface (in the blue ice area, i.e. the meteorite stranding zone, at t3) and to subsequently move
742 along with the surface flow, then the terrestrial age of the meteorite can be used as a younger
743 limit estimate for the age of the local ice. The age difference between the ice and the
744 terrestrial age of the meteorite results from the surface ablation that occurs between the time
745 of appearance at the surface (t3) and the collection time (somewhere between t3 and t4),
746 which exposes older ice (see Folco et al., 2006 for a detailed account). In this study, the
747 terrestrial age of meteorites was determined by radiocarbon (¹⁴C) dating and ³⁶Cl dating.

748

749 All ¹⁴C terrestrial age datings of meteorites were performed at the University of Arizona AMS
750 Laboratory, as described by Jull et al. (1993, 2010). Samples were crushed, pretreated with
751 85% phosphoric acid to remove weathering products, washed in distilled water and dried. The
752 meteorite powder (~0.2-0.4g) was mixed with ~3g of iron chips, which are a combustion
753 accelerator. The mixture was placed in a ceramic crucible and preheated to 500°C to remove
754 organic contaminants. The crucible was then placed in an RF furnace (Jull et al., 1993) and
755 the sample heated to melting for 2 minutes in a flow of oxygen. The recovered gases were
756 passed through a CuO-CeO furnace to ensure conversion to CO₂ and recovered from the
757 oxygen flow cryogenically. CO₂ was converted to graphite using standard methods at the
758 Arizona AMS Laboratory and measured by accelerator mass spectrometry. The error on the
759 terrestrial ages accounts for the error in the saturated activity and an estimated 3 sigma error
760 in the blank values.

761

762 The ³⁶Cl terrestrial ages were measured at the ASTER AMS national facility at CEREGE.

763 Results are compiled in Table B.2. The metal was dissolved in HNO₃. After addition of ³⁵Cl
carrier solution ($^{36}$Cl/$^{35}$Cl ratio of 2.96 $10^{-15}$ ± 5.5 $10^{-16}$, and a total Cl concentration of 6.92 mg/g), Cl was separated as AgCl. All $^{36}$Cl concentrations were normalized to KNSTD1600 standard with an assigned $^{36}$Cl/$^{35}$Cl ratio of 2.112 ± 0.038 $10^{-12}$ (Fifield et al., 1990; Sharma et al., 1990). Errors (1 sigma) include the uncertainties in the AMS measurements of samples and standards, as well as the $^{36}$Cl content measured in an associated chemistry blank sample. Terrestrial ages were computed using a saturation $^{36}$Cl concentration of 23.18±1.35 dpm/kg for H chondrites, 22.65±0.95 dpm/kg for L chondrites. These saturation values were computed using $^{36}$Cl concentration measured in the metal of 18 H falls (Graf et al., 2001; Dalcher et al., 2013) and 10 L falls (Dalcher et al., 2013). For the LL chondrite, we used the L chondrite saturation value (22.65±0.95 dpm/kg).

774

## APPENDIX C. ICE DYNAMIC 1-D FLOWLINE MODEL

776

Ice sheets are characterized by a very small ratio between the ice thickness and the horizontal scales. To describe the ice flow in their interior regions (i.e. away from the coast), the shallow-ice approximation (SIA) is commonly used (Hutter, 1983). In the SIA, the ice flow is assumed to be entirely driven by vertical gradients in shearing. For ice flow along a flowline (1-D), the depth-averaged velocity is given by:

781

$$ \bar{u} = AH\tau^3 \qquad (C.1) $$

Where $H$ is the ice thickness (m), $A$ is the rate factor ($Pa^{-3} a^{-1}$), and $\tau$ is the driving stress (Pa):

782

$$ \tau = -\rho g H \frac{\partial h}{\partial x} \qquad (C.2) $$

Here, $\rho$ is the density of ice ($910 \text{ kg m}^{-3}$), $g$ is the gravitational acceleration ($9.81 \text{ m s}^{-2}$) and

$\frac{\partial h}{\partial x}$ is the surface slope (unitless).

784

785

These equations can be used to estimate the magnitude of the ice flow surface velocities on the Nansen ice field. The ice thickness ($H$) is estimated to be around 1000 m (see section 5),

786

787
while the surface slope ($\partial h / \partial x$) along the ice flow direction (i.e. perpendicular to the elevation contours) is close to 1% (130 m over distance of 12 km for Nansen B). The main unknown is the rate factor (A), which is strongly dependent on the ice temperature. In ice flow modelling studies, this relationship is typically described through an Arrhenius-type law (e.g. Huybrechts et al., 2011; Pollard and Deconto, 2012; Seddik et al., 2012; Zekollari et al., 2017a; Hindmarsh, 2018):

$$A = A_0 e^{-Q/RT} \qquad (C.3)$$

Where $A_0$ is a constant ($1.14 \times 10^{-5} \text{ Pa}^{-3} \text{ a}^{-1}$ for $T<-10^{\circ}\text{C}$; $5.47 \times 10^{10} \text{ Pa}^{-3} \text{ a}^{-1}$ for $T>-10^{\circ}\text{C}$), Q is the activation energy for creep ($60 \text{ kJ mol}^{-1}$ for $T<-10^{\circ}\text{C}$; $139 \text{ kJ mol}^{-1}$ for $T>-10^{\circ}\text{C}$), R is the universal gas constant ($8.314 \text{ J mol}^{-1} \text{ K}^{-1}$) and T is the temperature relative to the melting point (K).

For the Nansen ice field, the ice temperatures at the base of the ice sheet are expected to be close to the pressure melting point (Van Liefferinge and Pattyn, 2013; Van Liefferinge et al., 2018). Close to the surface the ice temperatures are around the mean annual air temperature, which is somewhere around -40°C. Most of the deformation occurs in the lower parts of the ice column, where ice temperatures are the highest, and we therefore consider depth-averaged rates factors between $2.7 \times 10^{-17}$ (corresponding to -25°C) and $1.47 \times 10^{-18}$ (corresponding to -10°C) as realistic. By relying on this spread in rate factors and by accounting for a possible spread in ice thicknesses around the ca. 1000 m thickness obtained from a few airborne flight lines (Eagles et al., 2018) (see section 5), depth-averaged velocities in the order of 0.4 to 12 m a$^{-1}$ are typically expected over the Nansen ice field (Figure C.1).

## APPENDIX D. SURFACE MASS BALANCE MODELLING

Ablation: measurements from two AWS.
814 Accumulation: RACMO high resolution. Van Wessem and others (2014)? 27 km, driven by
815 ERA-Interim reanalysis? Was shown to reproduce measured SMBs well in and around Sør
816 Rondane mountains (Callens et al., 2015).

817

818 APPENDIX E. SUPPLEMENTARY MATERIAL

819

820 The δ¹⁸O values of the surface blue ice samples and the errors (1σ) are available as
821 supplementary material. The supplementary material also includes a picture of every newly
822 dated meteorite (from the 2012/13 expedition) at the collection site.
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  </tr>
</table>
<table><thead><tr><th>Expedition</th><th>Meteorite</th><th>Weight (kg)</th><th>Classification</th><th>Longitude (°)</th><th>Latitude (°)</th><th>Dated terrestrial age (ka) and dating technique</th><th>Comment/reference</th></tr></thead><tbody><tr><td rowspan="4">JARE-29 (Nansen A)</td><td>A 881371</td><td>0.01</td><td>Angrite</td><td rowspan="4">Nansen A<br>(exact retrieval location unknown)</td><td></td><td>&lt;60 (⁸¹Kr)</td><td>⁸¹Kr dating by Weigel et al. (1997); collected from western side of Nansen A</td></tr><tr><td>A 881400</td><td>0.70</td><td>H3.8</td><td>21.2±1.4 (¹⁴C)</td><td></td><td>¹⁴C dating at Arizona AMS Lab (Ninagawa et al., 2006); collected from western side of Nansen A</td></tr><tr><td>A 881387</td><td>0.02</td><td>H3.8</td><td>16.7±1.3 (¹⁴C)</td><td></td><td>¹⁴C dating at Arizona AMS Lab (Ninagawa et al., 2006); collected from western side of Nansen A</td></tr><tr><td>A 881757</td><td>0.44</td><td>Lunar (gabbro)</td><td>60±40 (⁸¹Kr)<br>&lt;50 (³⁶Cl)</td><td></td><td>⁸¹Kr dating by Nagao and Miura (1993); ³⁶Cl dating by Nishiizumi et al. (1992); collected from eastern side of Nansen A</td></tr><tr><td rowspan="10">JARE-29 (Nansen B)</td><td>A 881069</td><td>0.03</td><td>H3.7</td><td rowspan="10">Nansen B<br>(exact retrieval location unknown)</td><td></td><td>20±1.4 (¹⁴C)</td><td>¹⁴C dating at Arizona AMS Lab (Ninagawa et al., 2006)</td></tr><tr><td>A 880793</td><td>0.07</td><td>H3.6</td><td>24.4±1.5 (¹⁴C)</td><td></td><td>¹⁴C dating at Arizona AMS Lab (Ninagawa et al., 2006)</td></tr><tr><td>A 880676</td><td>0.11</td><td>H3.6</td><td>9.7±1.3 (¹⁴C)</td><td></td><td>¹⁴C dating at Arizona AMS Lab (Ninagawa et al., 2006)</td></tr><tr><td>A 880620</td><td>0.10</td><td>H3.7</td><td>9.9±1.3 (¹⁴C)</td><td></td><td>¹⁴C dating at Arizona AMS Lab (Ninagawa et al., 2006)</td></tr><tr><td>A 881216</td><td>0.12</td><td>H3.7</td><td>18.0±1.3 (¹⁴C)</td><td></td><td>¹⁴C dating at Arizona AMS Lab (Ninagawa et al., 2006)</td></tr><tr><td>A 880624</td><td>0.09</td><td>H3.8</td><td>9.9±1.3 (¹⁴C)</td><td></td><td>¹⁴C dating at Arizona AMS Lab (Ninagawa et al., 2006)</td></tr><tr><td>A 880684</td><td>0.06</td><td>H3.9</td><td>12.4±1.3 (¹⁴C)</td><td></td><td>¹⁴C dating at Arizona AMS Lab (Ninagawa et al., 2006)</td></tr><tr><td>A 880863</td><td>0.02</td><td>H3.7</td><td>14.8±1.3 (¹⁴C)</td><td></td><td>¹⁴C dating at Arizona AMS Lab (Ninagawa et al., 2006)</td></tr><tr><td>A 12084</td><td>1.51</td><td>H4</td><td>23.92056 -72.85741</td><td>10.7±2.2 (¹⁴C)</td><td></td><td>¹⁴C dating at Arizona AMS Lab</td></tr><tr><td>A 12105</td><td>1.13</td><td>H4</td><td>24.11539 -72.86960</td><td>37.8±2.6 (¹⁴C)</td><td></td><td>¹⁴C dating at Arizona AMS Lab</td></tr><tr><td rowspan="5">JARE-54 BELARE 2012-2013 (Nansen B)</td><td>A 12127</td><td>2.51</td><td>LL</td><td>24.29333 -72.92125</td><td>&lt;51 (³⁶Cl)</td><td></td><td>³⁶Cl dating at the ASTER AMS national facility at CEREGE</td></tr><tr><td>A 12216</td><td>1.04</td><td>H6</td><td>24.34909 -72.86281</td><td>129±28 (³⁶Cl)</td><td></td><td>³⁶Cl dating at the ASTER AMS national facility at CEREGE</td></tr><tr><td>A 12324</td><td>2.17</td><td>L4</td><td>24.23070 -72.88072</td><td>&gt;43 (¹⁴C)<br>49±30 (³⁶Cl)</td><td></td><td>¹⁴C dating at Arizona AMS Lab<br>³⁶Cl dating at the ASTER AMS national facility at CEREGE</td></tr><tr><td>A 12326</td><td>6.42</td><td>L4</td><td>24.34971 -72.91552</td><td>&gt;46 (¹⁴C)<br>65±30 (³⁶Cl)</td><td></td><td>¹⁴C dating at Arizona AMS Lab<br>³⁶Cl dating at the ASTER AMS national facility at CEREGE</td></tr><tr><td>A 12389</td><td>18.07</td><td>LL3-6</td><td>24.85403 -72.66674</td><td>15.8±2.4 (¹⁴C)</td><td></td><td>¹⁴C dating at Arizona AMS Lab</td></tr><tr><td>JARE-54 BELARE 2012-2013 (Nansen C)</td><td>A 12430</td><td>4.00</td><td>LL4</td><td>24.84283 -72.65079</td><td>&lt;2.4 (¹⁴C)</td><td></td><td>¹⁴C dating at Arizona AMS Lab</td></tr></tbody></table>

Table 1: Terrestrial age of selected meteorites collected on the Nansen blue ice field. For all new measurements, the technical details are summarized in
<figure><img src="image_3.png" /><figcaption>**Figure 1:** (a) Schematic 2-D representation of mechanism behind meteorite trap. Transect view, in which the light blue area represents a part of the ice sheet and in which the vertical dimension is exaggerated. The black lines are flow lines (arrows represent flow direction), while the red lines show a possible meteorite trajectory over time: t1: location where the meteorite reaches the surface and where it immediately enters the ice sheet (due to downward flow), t2: meteorite location while being transported within the ice sheet, t3: point where the meteorite reaches the surface (in blue ice area), t4: point until which the meteorite is transported at the surface of the blue ice area (through ice flow, potentially enhanced by katabatic winds); here, at the boundary of the blue ice area, the meteorites enters the ice sheet again (due to downward flow); t5: meteorite location while being transported further, until being forced back to the surface in a new ablation area, or until being discharged into the ocean. (b) Example of a meteorite collected on the Nansen blue ice field during the JARE-54 / BELARE 2012-2013 expedition. This meteorite (A 12389), with its weight of 18 kg, is the largest meteorite retrieved on this ice field since its revisit in the 2010s. For more photographic material and expedition details, refer to Imae et al. (2015).</figcaption></figure>
<figure><img src="image_4.png" /></figure>

**Figure 2:** Location of the Nansen Blue Ice Field and subdivision into Nansen A, Nansen B and Nansen C. The mountains in the north are the Sør Rondane Mountains. The location of the Princess Elisabeth Research Station (PE) is indicated with a red dot. Satellite images from the Landsat Image Mosaic of Antarctica (LIMA) (Bindschadler et al., 2008). The inset shows the position of the study area (black dot) in Antarctica.
<figure><img src="image_5.png" /><figcaption>**Figure 3:** Surface elevation (Fretwell et al., 2013) and satellite derived surface velocities (Mouginot et al., 2017; Rignot et al., 2017) around and south of the Sør Rondane Mountains (outlined in red). The white lines represent the surface elevation contours (in m a.s.l.), while the contours of the Nansen ice field are outlined in black.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 4:** (a) Sample locations of meteorites and blue ice samples collected on the Nansen ice field during the JARE-54 / BELARE 2012-2013 expedition. The red dots represent the retrieval locations of the meteorites, while the blue dots represent the locations of the surface blue ice samples. The transparent surface is the surface elevation from the BEDMAP2 dataset (Fretwell et al., 2013), for which the 100-m contours are outlined. The bedrock elevation (brown surface) is also derived from the BEDMAP2 dataset. The thick black lines outline the Nansen ice field. Figure partly created with TopoZeko toolbox (Zekollari, 2017). (b & c) Weight distribution of all meteorites collected during the JARE-54 / BELARE 2012-2013 expedition, with a weight between 0 and 620 g (10 g bins) (panel b) and a weight between 1 and 19 kg (1 kg bins) (panel c). Note that no meteorites were collected with a weight between 620 g and 1 kg. The vertical dotted line in panel b is the median weight of 31.3 g.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 5:** (a) δ<sup>18</sup>O values of ice samples collected on Nansen blue ice field, (b) δ<sup>18</sup>O values of blue ice samples on the Nansen B ice field, together with the position and age of the dated meteorites from the JARE-54/BELARE2012-2013 expedition (Table 1). The small coloured dots represent the δ<sup>18</sup>O values of the surface blue ice samples, while the small white dots represent the retrieval location of undated meteorites. The large black dots represent dated meteorites. The corresponding light grey boxes denote the terrestrial ages as determined by <sup>14</sup>C dating, while the dark grey boxes correspond to the <sup>36</sup>Cl dated terrestrial ages. The black lines are the outlines of the ice fields. The black triangles (panel a) represent the location of the ash samples on Nansen C, while the black stars (panel b) represent the position of the two moraines on Nansen B2.</figcaption></figure>

1179

1180

1181

1182

1183

1184

1185

1186

1187

1188

1189
1190

<figure><img src="image_8.png" /></figure>

1191

1192

Figure 6: (a) Surface ice flow over Nansen B and C ice fields, based on a hybrid method in which satellite derived surface velocity direction (Mouginot et al., 2017; Rignot et al., 2017) is corrected based on surface topography (Fretwell et al., 2013) (see text for details). The thick black lines represent surface elevation contours (Fretwell et al., 2013), while the coloured dots represent the weight of the meteorites collected in 2012-2013. The shaded areas correspond to the blue ice fields.

1193

1194

1195

1196

1197
<figure><img src="image_9.png" /><figcaption>**Figure 7:** δ<sup>18</sup>O variations (in ‰, original records) measured from selected major ice core drillings (TALDICE = Talos Dome; EDML = EPICA Dronning Maud Land; EDC = EPICA Dome C; DF = Dome Fuji; VK = Vostok), displayed on the EDC3 age scale (in ka), together with measured δ<sup>18</sup>O ranges for Nansen B1 and B3 (-48‰ to -44‰) and Nansen B2 (-42‰ to -38‰) for (a) the current interglacial-glacial transition and (b) the previous interglacial-glacial transition. δ<sup>18</sup>O data from Masson-Delmotte et al. (2011). For every major ice core, the two related numbers correspond to the surface elevation and the distance to the Nansen ice field.</figcaption></figure>

1199

1200

1201

1202

1203

1204

1205

1206

1207
<figure><img src="image_10.png" /><figcaption>**Figure 8:** Ratio between the accumulation and source area for the Nansen B ice field (panel a) and the northern part of the Nansen C ice field (panel b), as calculated from eq. 3. The ratios that fall out of the range determined from SMB modelling (1.5:1 to 6:1, see section 6.3.1.) are shown in transparent colours.</figcaption></figure>
APPENDIX TABLES AND FIGURES

<table><caption>Table B.1: Radiocarbon (¹⁴C) dated terrestrial age of meteorites and technical details from measurements</caption><thead><tr><th>Meteorite sample</th><th>Sample Weight (g)</th><th>Fm</th><th>CO<sub>2</sub> (cc)</th><th>Diluted (cc)</th><th><sup>14</sup>C atoms</th><th><sup>14</sup>C/g</th><th>dpm/kg</th><th>Saturated Activity</th><th>Age (ka)</th></tr></thead><tbody><tr><td>A 12084</td><td>0.413</td><td>2.347±0.012</td><td>0.355</td><td>0.355</td><td>2.279E+07</td><td>5.519E+07</td><td>12.70±0.09</td><td>46.4</td><td>10.7 ±2.2</td></tr><tr><td>A 12105</td><td>0.269</td><td>0.445±0.016</td><td>0.138</td><td>0.138</td><td>5.591E+05</td><td>2.078E+06</td><td>0.48±0.09</td><td>46.4</td><td>37.8 ±2.6</td></tr><tr><td>A 12324</td><td>0.241</td><td>0.334±0.009</td><td>0.087</td><td>0.087</td><td>-3.729E+05</td><td>-1.547E+06</td><td>-0.36±0.1</td><td>51.1</td><td>>43</td></tr><tr><td>A 12326</td><td>0.376</td><td>0.407 ± 0.011</td><td>0.082</td><td>0.082</td><td>-2.486E+05</td><td>-6.611E+05</td><td>-0.15±0.06</td><td>51.1</td><td>>46</td></tr><tr><td>A 12389</td><td>0.18297</td><td>1.659 ±0.01</td><td>0.162</td><td>0.162</td><td>6.532E+06</td><td>3.570E+07</td><td>8.22±0.14</td><td>55.2</td><td>15.8±2.4</td></tr><tr><td>A 12430</td><td>0.31801</td><td>35.15 ±0.33</td><td>0.096</td><td>0.096</td><td>9.600E+07</td><td>3.019E+08</td><td>69.46±0.66</td><td>55.2</td><td>&lt;2.4</td></tr></tbody></table>

<table><caption>Table B.2: Concentration of <sup>36</sup>Cl in the metal fraction and derived terrestrial ages.</caption><thead><tr><th>Meteorite sample</th><th>Dissolved metal (mg)</th><th><sup>36</sup>Cl concentration (dpm/kg)</th><th>Age (ka)</th></tr></thead><tbody><tr><td>A 12127</td><td>12.7</td><td>21.71±0.73</td><td>&lt;51.2</td></tr><tr><td>A 12216</td><td>184.9</td><td>17.21±0.40</td><td>129.3 ± 27.9</td></tr><tr><td>A 12324</td><td>27.2</td><td>20.24±0.56</td><td>48.9 ± 30.2</td></tr><tr><td>A 12326</td><td>83.3</td><td>19.48±0.55</td><td>65.5 ± 30.5</td></tr></tbody></table>
<figure><img src="image_11.png" /><figcaption>**Figure A.1:** δD values of ice samples collected on Nansen blue ice field. The black lines are the outlines of the ice fields</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Figure C.1:** Depth-averaged velocity estimation for the Nansen blue ice field ($\frac{\partial h}{\partial x}=0.011$), as a function of varying rate factors (A) and ice thicknesses (H). The black box delineates the plausible range for the rate factor and the ice thickness.</figcaption></figure>