Comments on: "Glacial lake evolution and
Atlantic-Pacific drainage reversals during deglaciation of
the Patagonia ice sheet" by Thorndycraft et al. [Quat.
Sci. Rev. 203 (2019), 102-127]

Jacques Bourgois, Maria Eugenia Cisternas, Jose Frutos

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To cite this version:

Jacques Bourgois, Maria Eugenia Cisternas, Jose Frutos. Comments on: "Glacial lake evolution and Atlantic-Pacific drainage reversals during deglaciation of the Patagonia ice sheet" by Thorndycraft et al. [Quat. Sci. Rev. 203 (2019), 102-127]. Quaternary Science Reviews, 2019, 213, pp.167-170. 10.1016/j.quascirev.2019.03.036. insu-03595784

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**Comments on: "Glacial lake evolution and Atlantic-Pacific drainage reversals during deglaciation of the Patagonia Ice Sheet" by Thorndycraft et al. [Quat. Sci. Rev. 203 (2019), 102-127]**

Jacques Bourgois<sup>a</sup>, Maria Eugenia Cisternas<sup>b</sup>, Jose Frutos<sup>c</sup>

<sup>a</sup> Sorbonne Université, CNRS-INSU, Institut des Sciences de la Terre Paris, ISTeP UMR 7193, 4 place Jussieu, 75252 Paris Cedex 05, France

<sup>b</sup> Instituto de Geologia Economica Aplicada, Universidad de Concepcion, Casilla, Concepcion, Chile

<sup>c</sup> San Juan de Luz 4060, Providencia, Santiago, Chile

Keywords: South America, North Patagonia Icefield, Last Glacial Maximum, General Carrera Lake (Chile), Buenos Aires Lake (Argentina), Atlantic-Pacific drainage reversal, Facinal cold events, 8.2 ka cold event.

Thorndycraft et al. (2019, T19 hereafter) present a comprehensive review on paleolake evolution models (Rio Baker catchment, Andean Patagonia, 46°-48°S) for the General Carrera-Buenos Aires (GCBA) and the adjacent Cochrane-Pueyrredon (CP) glacial lakes. Since the Last Glacial Maximum (LGM) at ~19-29 ka (Fleming et al., 1998; Singer et al., 2000; Kaplan et al., 2004; Douglass et al., 2006; Hein et al., 2009; Kaplan et al., 2011; Boex et al., 2013) the GCBA and CP lakes have been connected as a Large Unified Lake (Turner et al., 2005; Hein et al., 2010; Bourgois et al. 2016a; Glasser et al. 2016). T19 have renamed this ephemeral Unified Lake the Lago Chalenko.

Underlying the scientific method developed by T19 for the morphological analysis is the idea that producing a large enough landform dataset treated statistically would facilitate quantification of regional isostatic rebound together with the reconstruction of glacial paleolake history. Also, they associate the landform dataset to a Bayesian modeling of ages. Here after we focus on 5 key points questioning the proposed assumptions.

**1- Dated samples missing**

The Bayesian modeling for age refining used by T19 to reconstruct the age sequence at T19-figure 13 is an important technique in statistics. As any other statistical procedure, the Bayesian method requires taking into account all the experiments and must not eliminate any data. Since the main objective is reconstructing the Lago Chalenko evolution it is paradoxical for T19 to use no GCBA age data from dropstones on terraces that are direct records of lake
level. The C¹⁴, OSL, and cosmogenic missing ages in T19 are: (1) 15.2 ± 0.5 cal ka, C¹⁴ age at Rio Fenix Chico (Douglass et al., 2006); (2) 9.7 ± 0.7 ka, OSL age at Rio Tranquilo-Rio Bayo divide (Glasser et al., 2006); (3) 10.9 ± 1.3 ka and 7.9 ± 1.1 ka, ¹⁰Be ages from moraines at Facinal (Douglass et al., 2005; Bourgois et al., 2016a and b); (4) 18.5 ± 3.7 (sample 57), 9.9 ± 2.5 (sample 73), 18.8 ± 4.0 (sample 71), 15.2 ± 3.7 (sample 61), 13.9 ± 2.7 (sample 13), 16.5 ± 4.1 ka (sample 31), ¹⁰Be ages from dropstones on terraces with elevations from 451 to 528 m (Bourgois et al., 2016a; samples located at Fig. SM1); (5) 15.0 ± 1.8 ka (sample 59), ¹⁰Be age from dropstone on terrace with elevation from 443 to 452 m (Bourgois et al., 2016a; sample located at Fig. SM1); (6) 8.6 ± 2.0 (sample 53) and 14.4 ± 3.0 ka (sample 39), ¹⁰Be ages from dropstones on terraces with elevation from 305 to 349 m (Bourgois et al., 2016a; samples located at Fig. SM1). The Fig. 1 shows the missing ages on the T19-figure 11 used as a background.

## 2- Ho tethra age

The end of the Phase 1 (Bayesian age sequence, T19-figure 13) event for the paleolake evolution is the end date (16.94 ± 0.12 ka BP) documented from the “High-resolution varve chronology” (Bendle et al., 2017). This ~1000 years varve chronology exhibits only one “visible tephra layer”, the Ho tephra. No absolute age for this tephra layer exists. Bendle et al. (2017) claim that the Ho tephra has originated from the Ho oldest eruption of the Hudson volcano (between 17,300 and 17,440 cal years BP).

The detailed 50- and 10- cm isopach mapping for the Ho eruption (Weller et al., 2014 and 2015) shows that no tephra has been ejected to the area studied by Bendle et al. (2017). The 10-cm isopach for the Ho eruption from the Hudson volcano is located 140 km to the north (tephra airfall toward the NE). No confident age for the tephra anchoring the “High-resolution varve chronology” exists.

## 3- Deseado and Bayo levels

T19 define the main lake levels from a paleoshoreline histogram analysis (T19-figure 10a). They document two major shoreline frequency peaks along the GCBA Lake, the Deseado and Bayo levels at 405 and 299 m, respectively. Using altimeter, the Deseado and Bayo levels have been previously identified (Turner et al., 2005; Bell, 2008). Subsequently, GPS measurements (Bourgois et al., 2016a) have documented the Deseado and Bayo terrace elevations ranging from 432 to 468 m and 302 to 347 m, respectively. These level/terrace
elevations obtained through different methods portraying the same paleolake levels reveal discrepancies.

At T19-figure 10b the “shoreline elevation” plotted against “distance along direction of maximum uplift” (across trend of the Andes) shows a high concentration of datum points between ~10 and ~45 km, a long segment with sparsely or no data between ~45 and ~110 km, and a concentration between ~110 and ~140 km. This distribution reflects the first order morphotectonic signatures controlled by the Andean tectonic front located at ~45 km along the GCBA Lake profile. (1) To the east is the low foreland area of the Andes exhibiting the wide-open Buenos Aires Lake (Argentina) with low glacial rebound signature (Ivins and James, 1999 and 2004). (2) As the GCBA Lake crosses the Andes front, the glacial valley exhibits steep walls becoming narrow and deep. Between ~45 and ~110 km the General Carrera Lake (Chile) exhibits little records of paleolake levels. (3) From ~110 to ~140 km, the low region of the western GCBA basin (Chile) is suitable for preserving terraces and strandlines in a wider open area with high glacial rebound signature (Dietrich et al., 2010; Bourgois et al., 2016a).

Mixing the ~10-45 km and the ~110-140 km datum points from areas with contrasting glacial rebound induces a bias producing not only a shift of the histogram peaks towards lower elevations, but also questions the reconstruction of the "isostatic shorelines" for the Deseado and Bayo levels (T19-figures 10 and 12). Also the C14 and OSL ages (a and c Fig.1, and section 1) are constraining the oldest and youngest ages possible for the Deseado and Bayo terraces at 15.7 and 10.3 ka, instead of 15.5 and 7.0 ka, respectively.

## 4- Rio Bayo spillway

As argued by T19, the Deseado spillway (~400 m in elevation) towards the Atlantic has been working from 16.0 to 15.0 ka during stages 3 and 4 (T19-figure 15c, d). At 15.0 ka, the Deseado spillway becomes inactive the GCBA Lake flowing towards the Pacific through the Rio Bayo spillway (Fig. SM2) during stages 5 and 6 (T19-figures 15e, f) from 15.0 to 12.6 ka.

Glasser et al. (2006) have documented that icefield withdrew the divide area (the T19 Bayo spillway) between the Pacific and the GCBA Lake after 12.7 ± 0.9 ka (mean age of samples LTE1 and LTE2, T19-Table SM1) remaining at site until 9.7 ± 0.7 ka (OSL age (b) Fig.1, see section 1). Conflicting to what is proposed by T19, the available data document that the Rio Bayo glacial valley has been ice-dammed during the Stages 5 and 6 as previously proposed by Turner et al. (2005), and Bourgois et al., (2016a and b).
Available data allow inferring that the Bayo valley was ice-dammed not only at the beginning of the Bayo level but also during stages 5 and 6 (T19-figure 15e and f) preventing out-flowing towards the Pacific.

**5- Facinal cold events**

At T19-Table 2, T19 claim: « there is no evidence for ... a glacier readvance at 10.9 ± 1.3 ka and 7.9 ± 1.1 ka (Bourgois et al., 2016 a and b)...because morpho-stratigraphic data from Fenix Chico (Bendle et al., 2017) indicates the maximum lake level of Lago General Carrera/Buenos Aires was at the Deseado level (~400-440 m asl, T19-Fig. 10) ». Two main points weaken the T19 statement:

First, at section 2 we have contemplated that no confident assumption can be constructed from the varve chronology (Bendle et al., 2017). Second, the glacier readvances have been robustly documented. At Facinal (Fig. SM3) Douglass et al. (2005) have described two parallel moraine belts overlying a Bayo level terrace. Recalculated ages for these moraines are 10.9 ± 1.3 and 7.9 ± 1.1 ka (Bourgois et al., 2016b) for the Outer and Inner moraine belts, respectively. Also, the underlying Bayo level terrace (elevation ranging from 293 to 314 m) was dated at 11.0 ± 0.7 ka by OSL (Glasser et al., 2016). These age data substantiate the youngest possible age for the Bayo terrace accumulation at 9.6 ka, which is 2.6 ka older than the 7 ka proposed by T19.

**Conclusions**

The T19 assumptions for the Lago Chalenko evolution and Atlantic-Pacific drainage reversal exhibit the following weaknesses. First, disregarding 13 ages over 37 available along the GCBA Lake allow T19 rebutting the ice damming of the Rio Bayo until 9.7 ± 0.7 ka together with the two major ice readvances at 10.9 ± 1.3 and 7.9 ± 1.1 ka. Second, in mixing datum points from disconnected areas having different geomorphic signatures with contrasted glacial rebound induces an analytical bias. Third, the “High-resolution varve chronology” from Bendle et al. (2017) induces major age uncertainties into the Phase 1 for the T19 proposed Bayesian age sequence model. This together with the disregarded age data and the geomorphic analytical bias propagates inconsistencies throughout the T19 paleolake evolution model.

**Acknowledgements**

We thank the Institut National des Sciences de l' Univers (INSU) program “Syster” and the
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**Figure caption**

**Figure 1.** Age versus sample elevation for the “dated samples missing” (thick black line) listed at section 1 shown on the T19-figure 11 used as background. (a): $C^{14}$ age constrains the oldest age possible for the Menucos moraine at 15.7 ka (Douglass et al., 2006). (b): OSL age documents ice at the Rio Bayo/Rio Tranquilo divide area at 9.7 ± 0.7 ka (Glasser et al., 2006; Fig. SM2 for location). (c): OSL age documents the age of Bayo terrace (333 m) at Facinal at 11 ± 0.7 ka (Glasser et al., 2016). This age together with the ages of the Outer (10.9 ± 1.3 ka) and Inner (7.9 ± 1.1 ka) moraines at Facinal (Douglass et al., 2005; Bourgois et al., 2016a and b) constrain ice readvance at 9.1 to 11.7 ka and 6.8 to 9 ka. CND: 73DS sample records age and elevation of the massive flooding event (Bourgois et al., 2016a). CND-Cosmogenic nuclide dating. Number in bracket following CND refers to samples located at Fig. SM1. DS-Dropstone.

**Supplementary material**

**Figure SM1.** After Bourgois et al (2016a). Location of sampling sites and ages mentioned in the text. It includes samples 13, 31, 39, 53, 57, 59, 61, 71, 73. Note that the GCBA Lake (201m) outflows to the Pacific Ocean through the Rio Baker at present. Location of sites mentioned in the text is also shown that includes the Rio Bayo- Rio Tranquilo divide (350 m), the Rio Fenix Chico, Facinal, and Pto Guadal.

**Figure SM2.** Geomorphological map of the Pacific/GCBA Lake divide area. Two terraces have accumulated along the divide. The Upper terrace (490-510 m) and the Lower terrace (460-479 m) document that the GCBA Lake water level has uplifted at the elevation of these
terraces. This allows inferring that ice damming has occurred along the Rio Bayo during
terrace accumulation preventing the GCBA Lake flowing to the Pacific as the GCBA Lake
level was 150 m higher than the divide at Present. The OSL sample RN1 (Glasser et al., 2006)
location is shown. See figure 1 in Glasser et al. (2006) for more details.

**Figure SM3.** Modified from Bourgois et al. (2016b). Moraines overlying the T19-Bayo level terrace (=T1 terrace, Bourgois et al., 2016a and b) in the area of Facinal (Fig. SM1 for location). A– Aerial photo of the Facinal area. Two moraine ridges (Douglass et al., 2005), the Outer moraine (OM) and the Inner moraine (IM), unconformably overly the T19 Bayo terrace. B– Morphological map of the area shown in A. The bold italic numbers are recalculated ages of the two moraine ridges (see the “Discussion” section of Bourgois et al. 2016a). The area shows the development of terraces, including T0 (201–221 m asl), T1 (299–320 m asl = Bayo level terrace), and T2 (402–412 m asl = Deseado level terrace). The T1 terrace is older than 10.9 ± 1.3 ka, and the T0 terrace is younger than 7.9 ± 1.1 ka. Also shown is the OSL age (11.0 ± 0.7 ka) from Glasser et al. (2016) that constrains the maximum age for the first cold event associated with the OM. The OM and IM were tentatively correlated to the Younger Dryas and the 8.2 ka cold events, respectively (Bourgois et al., 2016b). VS-varved sediment accumulated behind the moraines.