Holocene tephrochronology around Cochrane (~47° S), southern Chile

Charles R. Stern1, Patricio I. Moreno2, William I. Henríquez2, Rodrigo Villa-Martínez3, Esteban Sagredo4, Juan C. Aravena3, Ricardo de Pol-Holz3

1 Department of Geological Sciences, University of Colorado, Boulder, Colorado, 80309-0399, USA.
jdahlquist@efn.uncor.edu; sverdecchia@gmail.com; ebaldo@efn.uncor.edu; charles.stern@colorado.edu

2 Instituto de Ecología y Biodiversidad, Departamento de Ciencias Ecológicas, Universidad de Chile, Casilla 653, Santiago, Chile.
pimoreno@u.uchile.cl; willybgo@ug.uchile.cl

3 GAIA-Antártica, Universidad de Magallanes, Avda. Bulnes 01855, Punta Arenas, Chile.
rodrigo.villa@umag.cl; juan.aravena@umag.cl; ricardo.depol@umag.cl

4 Instituto de Geografía, Facultad de Historia, Geografía y Ciencia Política, Pontificia Universidad Católica de Chile, Avda. Vicuña Mackenna 4860, Santiago, Chile.
esagredo@uc.cl

Fig. 1. Regional map showing the location of Cochrane relative to the volcanoes at the southern end of the SVZ. Also shown are the locations of some of the monogenetic centers along the Liquiñe-Ofqui Fault System (LOFS) and surrounding Hudson (Gutiérrez et al., 2005; Vargas et al., 2013), and the proposed location of the Arenales volcano (Lliboutry, 1999), the existence of which has not yet been confirmed. Boxed area near Cochrane is the locations of the road-cuts, trenches and cores containing the tephra layers described in the text.

 

 

Fig. 2. Map showing the locations of the road-cut outcrop (Fig. 3), the four pits (Fig. 4) in bogs, the Anónima bog which was cored, and the four lakes (Fig. 5) from which tephras were collected (Table 1). Also shown are the location of two sites (TCHA-10 and TCHA-47) at which H1 tephra was dated by Gardeweg and Sellés (2013).

 

Although Cochrane occurs in a region without active volcanoes, tephra deposits derived from explosive eruptions of volcanoes in the southernmost SVZ have been described in lake sediment cores and outcrops from this area. Villa-Martínez et al. (2012) described two tephra in a sediment core from Augusta Lake located 25 km northeast of Cochrane (Fig. 2), which they identified as being derived from the H1 eruption of Hudson volcano (Naranjo and Stern, 1998) and the MEN1 eruption of Mentolat volcano (Naranjo and Stern, 2004). Gardeweg and Sellés (2013) also described various outcrops of tephra deposits in the area of Cochrane which they attributed to both the H1 and younger H2 eruptions of Hudson. The data of Gardeweg and Sellés (2013) suggest that both these eruptions were significantly larger than previously estimated by Naranjo and Stern (1998). More recently McCulloch et al. (2014) also reported both H1 and MEN1 tephra from a bog sediment core at La Frontera in Argentina, 70 km northeast of Cochrane.

 

Fig. 3. Photo of the ~12 cm thick Hudson H1 tephra from near the road-cut PC14-01-06 (Fig. 2).

 

 

Fig. 4. Photo of 2 cm thick Mentolat MEN1 tephra (sample PC14-01-46) and 10 cm thick H1 tephra (PC14-01-44) from a pit ~30 km southeast of Cochrane (Fig. 2; Table 1). Each blue and black band on the pole are 10 cm in length.

 

Fig. 5. X-ray image of ~10 meter long core PC0902A from Lago Edita (Fig. 2), showing bright white relatively high density H1, H2 and MEN1 tephra layers within darker lower density organic-rich lacustrine sediments. Core is divided into 10 segments each approximately 1 meter long. Bright bands are dense layers which include tephra H2 in segment #4 and MEN1 and H1 in segment #7. H1 tephra is ~14 cm thick, MEN1 ~3 cm thick and H2 only ~1 cm. Higher density (brighter) laminated material in the deeper part of the core (segments #8-10) below H1 and MEN1 is glacial-lacustrine clay, silt and sand. Bright band marked with X in segment #6 is also sand, not tephra.

 

All these recent results are consistent with the previous conclusions of Stern (1991, 2008) and Naranjo and Stern (1998, 2004) that the H1 and H2 eruptions of Hudson and MEN1 of Mentolat, along with the older Ho (Weller et al., 2014) and younger H3 (1991 AD; Scasso et al., 1994) eruptions of Hudson, were among the largest Holocene explosive events generated by the volcanoes in the southern SVZ. The tephra deposits produced by all these eruptions have already and will continue to provide important chronological markers for archaeologic (Prieto et al., 2013) and paleoclimate (Waldmann et al., 2009; Borromei et al., 2010; Unkel et al., 2010; Hermanns and Biester, 2011; Björck et al., 2012; Villa-Martínez et al., 2012; Menounos et al., 2013; McCulloch et al., 2014) studies in southern Patagonia. For these reasons, as well as being significant for evaluating volcanic risks and hazards for the inhabitants of Cochrane and other population centers in southern Chile, further study of tephra deposits produced by these large explosive eruptions are important for refining our understanding of their size, age and the distribution of the pyroclastic products they produce.

Here we describe four different Holocene tephra layers, which occur in road-cut outcrops (Fig. 3), cores and trenches (Fig. 4) in bogs, and lacustrine sediment cores (Fig. 5) collected from four small lakes (Edita, Maldonado, Augusta, and Pepa; Fig. 2) in this area (Table 1). Our new chronologic and petrochemical data indicate that these tephra are derived from the previously identified large eruptions H1, H2 and H3 of Hudson and MEN1 of Mentolat volcanos, and that these tephra provide no evidence for the existence of the Arenales volcano.

2. Methods

Tephra samples were collected from a road-cut exposure (Fig. 3), one core and four hand-dug trenches (Fig. 4) in bogs, and sediment cores (Fig. 5) from four small lakes within spatially limited water-shed basins selected to minimize the amount of inorganic sediment deposited in the lakes (Table 1). The lake sediment cores were obtained using a 5-cm-diameter modified Livingstone piston corer (Wright, 1967). X-ray images of the cores (Fig. 5) were taken to allow for better visual identification of the tephra deposits and to provide a means of stratigraphic correlation of the tephra layers between the cores. The white layers in these images are the denser lithologies, often tephra deposits, but in some cases sand (Fig. 5), and the darker layers are less dense organic-rich lacustrine sediments. The chronology of the tephra in the trenches and cores is controlled by AMS radiocarbon dates of organic material in the overlying and underlying sediments (Tables 2 and 3). In the tables, the ages from the lake deposits are separated from those from surface deposits in bogs. Radiocarbon dates were converted to calendar years before present (cal yrs BP) using the CALIB 7 program and the SHCal13 dataset (Stuiver et al., 1998; Hogg et al., 2013).

The tephra samples were washed to remove any organic matter, and then dried and sieved to remove any coarse fraction material not volcanic in origin. After cleaning, the bulk tephra samples were mounted on petrographic slides to examine under a petrographic microscope in order to identify petrographic characteristics such as tephra glass color and morphology and the identity of mineral phases (Fig. 6). The major element compositions of 2 samples of bulk tephra (Table 4) were determined by Activation Laboratories, Canada. Trace-element data for bulk tephra samples (Tables 5-7) were determined using an ELAN D CR ICP-MS at the University of Colorado. Trace-element compositions are considered accurate to ±5% at the level of concentrations in these samples, based on repeated analysis of standard rock samples of known composition (Saadat and Stern, 2011). The major element compositions of tephra glasses (Table 4) were determined using a Jeol JXA-733 Electron Microprobe operating at 15 KV accelerating potential with a 10 nA probe current and a 5-10 µm diameter beam to minimize volatilization of sodium.

 

Fig. 6. Photomicrographs of  A. Hudson H1 tephra containing brown glass shards, with abundant stretched vesicles, and few crystals; B. Crystal-rich Mentolat MEN1 tephra containing clear glass, plagioclase, orthopyroxene, clinopyroxene and minor amphibole.

 

3. Results

Thickness (Table 1), radiocarbon ages (Tables 2 and 3), and petrochemical information, including tephra glass color and morphology (Figs. 6), as well as mineralogy, bulk tephra major (Table 4) and trace-element chemistry (Tables 5-7), and tephra glass compositions (Table 4), indicate, by comparison with previously described tephra from farther north, that the four different tephra layers were derived from three different large explosive eruptions of the Hudson volcano (H1, H2 and H3; Naranjo and Stern, 1998) and one from the Mentolat volcano (MEN1; Naranjo and Stern, 2004). No late Pleistocene tephra were encountered in the lake cores, some of which, such as the core from Augusta lake, preserve a record that extends back in time to over 19,500 cal yrs BP (Villa-Martínez et al., 2012). The results are described in more detail below in the order from the oldest to the youngest tephra.

3.1. H1 tephra

The oldest and thickest (8-18 cm) tan to orange-brown tephra layers in the road-cut, trenches and lake cores near Cochrane are chronologically constrained by 10 new AMS radiocarbon ages (one from the road-cut outcrop PC14-01-06, four from the two bog trenches PC14-02-06 and PC14-01-14, and five in the cores from Edita, Pepa and Maldonado lakes; Table 2). The minimum AMS ages from these three lakes, and other lakes elsewhere in Patagonia where H1 have been dated, range between 7,525±20 to 7,715±60 ¹⁴C yrs BP, the maximum ages between 7,750±50 and 7,775±60 ¹⁴C yrs BP, and their average is 7,683±33 ¹⁴C yrs BP (median probability age: 8,440 cal yrs BP; Table 2). The AMS ages from these three surface deposits, and other surface deposits in Patagonia, are systematically lower than the ages from the lake cores, with minimum ages ranging from 6,755±40 to 7,176±19 ¹⁴C yrs BP, maximum ages from 6,750±25 to 7,410±25 ¹⁴C yrs BP, and an average age of 7,084±23 ¹⁴C yrs BP (median probability age: 7,891 cal yrs BP; Table 2). Although this average is older than the average obtained in the previous most recent compilation of H1 ages (6,890 ¹⁴C yrs BP; Prieto et al., 2013), this new compilation includes only AMS radiocarbon ages (Table 2) and not any conventional ages. The <200 year difference between the average of these new AMS ages and the previous compilation of conventional ages is nevertheless consistent with these tephra having been generated by the H1 eruption of Hudson.

These tephra layers all contain dominantly brown glass with abundant stretched vesicles (Fig. 6A). Mineral grains include plagioclase, orthopyroxene and clinopyroxene. Bulk tephra major element oxide chemistry (Table 4) indicates that these tephra are andesitic (SiO₂ of 62 wt%), with relatively high TiO₂ (1.4 wt%), Na₂O (5.6 wt%), and K₂O (2.6 wt%), as are tephra glass compositions (Fig. 7). Trace-element analysis (Table 5; Fig. 8) of bulk tephra samples from the lake cores indicate that these tephra, as well as the tephra in three of the bog trenches, the bog core and that collected from the outcrop, all have elevated concentrations of Ti (9,189-10,246 ppm), Rb (38-50 ppm), Zr (288-417 ppm), Ba (604-807 ppm), La (35.0-43.7 ppm) and Yb (3.1-4.8 ppm). These compositions are similar to other samples of lavas and tephra from Hudson volcano (Fig. 8) and distinct from the products of the other large stratovolcanoes (Maca, Cay and Mentolat) in the southernmost SVZ.

Their age, thickness, and all these petrochemical characteristics together indicate that these tephra were produced by the H1 eruption of the Hudson volcano (Stern, 1991, 2008; Naranjo and Stern, 1998).

 

Fig. 7. SiO2 versus K2O for bulk tephra and tephra glass (Table 4) compared to fields for active volcanoes in the southern SVZ from previously published data (Futa and Stern, 1988; López-Escobar et al., 1993; Naranjo and Stern, 1998, 2004; D’Orazio et al., 2003; Gutiérrez et al., 2005; Kratzmann et al., 2009, 2010; Weller et al., 2014; Stern et al., 2015).

 

3.2. MEN1 tephra

The somewhat younger and thinner (1-4 cm) white tephra layers in the trenches (Fig. 4) and cores (Fig. 5) are chronologically constrained by 9 new AMS ages (one from the trench PC14-01-11 and eight from the four lakes Edita, Pepa, Maldonado and Augusta; Table 3). The minimum ages for these tephra from the lake cores range between 6,745±20 to 6,860±20 ¹⁴C yrs BP, the maximum ages between 6,900±20 and 6,910±20 ¹⁴C yrs BP, and their average is 6,872±23 ¹⁴C yrs BP (median probability age: 7,689 cal yrs BP; Table 3). As for H1, the ages from surface (bog) deposits are systematically lower than the ages from the lake cores, with maximum ages from 6,520±40 to 6,690±60 ¹⁴C yrs BP and an average age of 6,572±50 ¹⁴C yrs BP (median probability age: 7,471 cal yrs BP; Table 3). Nevertheless, both sets of ages are consistent with this being the same MEN1 tephra previously described by Naranjo and Stern, 2004)

These tephra are crystal-rich, with plagioclase, orthopyroxene, minor amphibole and clinopyroxene, occasional olivine, and clear glass characterized by round, not stretched, vesicles (Fig. 6B). Bulk tephra major element oxide analysis (Table 4) indicates an andesitic composition (SiO₂ of 61 wt%), with lower TiO₂ (0.9 wt%), Na₂O (4.5 wt%), and K₂O (0.6 wt%) than Hudson tephra, while the tephra glass is rhyolitic (Fig. 7). This tephra has much lower concentrations (Table 6; Fig. 8) of Ti (2,594-4,156 ppm), Rb (17-24 ppm), Zr (122-160 ppm), Ba (233-295 ppm), La (9.6-14.0 ppm) and Yb (1.9-2.9 ppm).

Their age, thickness and all these petrochemical characteristics indicate that they are the same MEN1 tephra identified by Naranjo and Stern (2004) further to the north along both the Simpson and Mañiguales Rivers northwest of Coyhaique (Aisén sample T-36, Table 6), by Weller et al. (2015) in cores near Coyhaique (Mellizas, Table 6), and by Stern et al. (2015) in cores from the upper Río Cisnes valley (Embudo, Table 6).

 

Fig. 8. Ti versus Rb for samples of tephra samples from near Cochrane derived from the Hudson (circles) and Mentolat (squares) volcanoes, compared to previously analyzed samples of lavas and tephra from Hudson, Macá, Cay and Mentolat (Futa and Stern, 1988; López-Escobar et al., 1993; Naranjo and Stern, 1998, 2004; D’Orazio et al., 2003; Gutiérrez et al., 2005; Kratzmann et al., 2009, 2010; Weller et al., 2014; Stern et al., 2015).

 

3.3. H2 and H3 tephra

Thin (<2 cm) Late Holocene tephra layers occur in some of the cores, but not in any of the trenches or outcrops. These are petrologically somewhat similar to H1 tephra, with plagioclase and pyroxene phenocrysts and brown glass with stretched vesicles, although the glass has a lighter color than H1 tephra glass. Bulk tephra also has higher Rb, Ba, Zr, La and Yb, but lower Ti contents than H1 tephra (Table 7; Fig. 8). Their late Holocene age, as indicated by their location relative to H1 tephra in the cores, and their chemistry and thickness is consistent with this tephra being a distal layer of the Hudson H2 eruption at approximately 4000 cal yrs BP (Naranjo and Stern, 1998).

Two separate cores (PC0607A and PC0607B) from Anónima bog contain a thin (1 cm) tephra layer near the very top of the cores, in sections T1 at 5-6 and 7-8 cm, respectively. The bulk tephra trace-element composition of this layer (Table 7; Fig. 8) is similar to other Hudson tephra, and this tephra is interpreted to be derived from the Hudson H3 eruption which occurred in 1991 AD (Scasso et al., 1994).

4. Discussion and Conclusions

4.1. H1 tephra distribution

H1 tephra layers range from 8 to 18 cm in thickness in the outcrop, trenches and cores we studied (Table 1). Gardeweg and Sellés (2013) also described two sections in outcrops from this general area (Fig. 2), one along the Tranquilo River (TCHA-10) and one near the Chacabuco bridge (TCHA-47), that based on their age determinations (Table 2) contain H1 tephra that ranges from 15 to 30 cm in thickness. The overall variability in thickness of all these multiple H1 tephra layers from the spatially closely associated sites near Cochrane suggest some significant degree of redistribution and/or thickening occurred during their deposition. Nevertheless, their widespread presence in this region as >8 cm layers is consistent with the suggestion of Gardeweg and Sellés (2013) that the 10 cm isopach for this eruption is probably larger and more extended in a southern direction than that drawn by Naranjo and Stern (1998). Figure 9 presents a new 10 cm isopachs for the H1 eruption based on these observations.

 

Fig. 9. A revised 10 cm isopach (solid lines) for the Hudson H1 eruption compared to that originally drawn (dashed line) by Naranjo and Stern (1998). The new isopach is based on the thickness of tephra in sections near Cochrane (Table 1), including those described by Gardeweg and Sellés (2013). Also shown is a tentative 10 cm isopach for MEN1 based on the distribution of MEN1 tephra (thickness given in small grey boxes) from cores and trenches near Cochrane (Table 1) and elsewhere, including from outcrops northwest of Coyhaique (Naranjo and Stern, 2004), eight lake cores near Coyhaique (Stern et al., 2013; Weller et al., 2014, 2015), and in a core at La Frontera, Argentina (McCulloch et al., 2014). MEN1 also occurs as diffuse 5 cm layers in cores from the upper Río Cisnes valley east of the volcano (Stern et al., 2015).

 

4.2. H1 eruption age

The seven bulk sediment AMS ages for H1 obtained from lake cores average 7,683±33 ¹⁴C yrs BP (median probability age 8,440 cal yrs BP), and are somewhat older than the 14 AMS ages from surface (bog) deposits, which average 7,084±23 ¹⁴C yrs BP (median probability age 7,891 cal yrs BP; Table 2). This systematic difference between ages of tephra determined from lake compared to bog deposits is statistically significant. Bertrand et al. (2012) demonstrated that bulk radiocarbon AMS age dates from lake sediments were 300 to 1,100 years older than true ages in some northern Patagonian lakes due to variable inputs of terrestrial organic carbon from the Holocene soils that cover the lake watersheds. However, our finding of nearly identical radiocarbon ages for H1 from multiple small, closed-basin lakes located at different elevations, in watersheds having contrasting slopes and degrees of soil development, argues against site-specific processes of soil erosion and redeposition for the striking radiocarbon synchrony among lakes.

Alternatively, dates from lacustrine environments may also be too old because of penetration of dense tephra into the water-saturated nepheloid layer and sediment-water interface in the bottom of lakes (Moreno et al., 2015). On the other hand, surface deposits are may be younger than true ages because humic acids percolate and contaminate these deposits. Finally ¹⁴C ages may potentially be affected by ¹⁴C reservoir effects. Ingram and Southon (1996), Taylor and Berger (1967), and more recently Carel et al. (2011) and Siani et al. (2010, 2013), determined ¹⁴C reservoir effects of 200 to 600 years in the southeast Pacific ocean, but these reflect deep ocean circulation and are not directly applicable to the small, shallow, fresh-water, rain-fed lakes from which the tephra in this study were collected. Elbert et al. (2013) evaluated ¹⁴C reservoir effects in two similar small lakes, Castor and Escondida, near Coyahique and found none. For Castor lake, ¹⁴C reservoir effects were assessed using a paired measurement on bulk organic fraction and on a syndepositional terrestrial leaf macrofossil, which yielded identical ages. For Escondida lake potential reservoir effects were evaluated and discarded by parallel ¹⁴C measurements of syndepositional diagnostic tephra layers which yielded similar ages in both lakes.

The new averages for AMS ages only from lake and surface deposits are both somewhat older than the most recent previously published age estimate for the H1 eruption of 6,890±100 ¹⁴C yrs BP (median probability age 7,698 cal yrs BP; Stern, 2008; Prieto et al., 2013), which was based dominantly on conventional radiocarbon ages for samples from bog deposits. Although the total range of the different AMS ages from lakes and bogs compared to the conventional ages from bogs are overall relatively small (<700 years or <10% of the approximate >8,000 cal year BP age for H1), further refinements of the age of the H1 eruption is of interest, since this was the largest Holocene eruption in the southern Andes during the Holocene (Stern, 1991, 2004, 2008; Naranjo and Stern, 1998), and it produced a widely distributed tephra layer that has been used as a chronologic marker for numerous paleoclimate (Waldmann et al., 2009; Unkel et al., 2010; Borromei et al., 2010; Bjork et al., 2012; Villa-Martínez et al., 2012; Menounos et al., 2013) and archaeological studies (Prieto et al., 2013) in southern Patagonia.

4.3. MEN1 tephra age

The eight new ages for MEN1 from lake cores average 6,872±23 ¹⁴C yrs BP (median probability age 7,689 cal yrs BP; Table 3), and are also, as for H1, somewhat older than the average for surface samples (median probability age 7,471 cal yrs BP), which includes one conventional radiocarbon age determined by Naranjo and Stern (2004). These ages are consistent with the occurrence of MEN1 only a few tens of centimeters above the older Hudson H1 tephra in the cores and trenches (Figs. 4 and 5).

4.4. MEN1 tephra source

Naranjo and Stern (2004) attributed the MEN1 tephra to an eruption of Mentolat based on a poorly constrained 10 cm isopach and geochemical data for one sample of this tephra which had low K₂O content relative to the products with similar SiO₂ erupted from other southernmost SVZ volcanoes (Maca, Cay and Hudson). The new major and trace-element geochemical data for this tephra are consistent with the conclusion that the source of this tephra is the Mentolat volcano. Basically, the few samples of lavas analyzed from Mentolat (López-Escobar et al., 1993), and all the MEN1 tephra samples (Table 6), have relatively low K₂O, Ti, Rb and Sr for samples with similar silica content erupted from other volcanoes in the southernmost SVZ (Figs. 7 and 8; Stern et al., 2015). Figure 9 summarizes the regional distribution in cm thickness of MEN1 tephra and presents a new, but still tentative, 10 cm isopach for this eruption. The area of this isopach is only a bit smaller than that within the 10 cm isopach of the 1932 AD eruption of Quizapu, which erupted 9.5 km³ of pyroclastic material (Hildreth and Drake, 1992), but large enough to suggest an eruptive volume of at least 5 km³ or greater. Although not as large as H1, MEN1 was still a reasonably large eruption, which if it were to happen again today could have a significant impact for local population centers such as Puerto Cisnes, Coyhaique and Cochrane.

4.5. H2 distribution

Gardeweg and Sellés (2013) describe a 45 cm thick orange tephra layer (TCHA-5B) along the south shore of Lago Cochrane, which they attribute to Hudson based on chemical data, and which they date as between 3,620±40 and 4,180±40 ¹⁴C yrs BP. These ages are consistent with this being the late Holocene Hudson H2 tephra, but the thickness far exceeds the thickness of H2 tephra found in the lake cores in this region (~2 cm; Fig. 5), as well as the numerous outcrops previously used to determine isopachs for this eruption (Naranjo and Stern, 1998). We suggest instead that, based on its color and its thickness, this is not H2 tephra, but rather that the ages are incorrect, and that this is H1 tephra greatly thickened as a result of southward directed winds blowing tephra across the surface of Lago Cochrane during and after the H1 eruption. 

4.6. H3 1991 AD tephra distribution

The presence of a 1-2 cm thick layer of H3 tephra at the top of the Anónima bog cores is consistent with the isopach distribution of H3 tephra as determined by Scasso et al. (1994).

4.7. Arenales volcano

No tephra evidence for the existence of the proposed Arenales volcano has been found in the sections studied, and we suggest that the Arenales volcano may not, in fact, exist, or alternatively that it has not had a moderate to large explosive eruption in the last 15,000 years.

Acknowledgements
We thank Á. Amigo for many constructive comments that helped improve the final manuscript. This research was funded by Fondecyt grants #1080485, #1121141 and FONDAP 15110009, Fundación Fondecyt de Iniciación grant #11121280, CEQUA, and ICM grants P05-002 and NC120066.

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