Age and origin of tephras recorded in postglacial lake sediments to the west of the southern Andes,...

Ž .Journal of Volcanology and Geothermal Research 84 1998 239–256

Age and origin of tephras recorded in postglacial lake sedimentsto the west of the southern Andes, 448S to 478S

Simon G. Haberle ), Susie H. LumleyDepartment of Plant Sciences, UniÕersity of Cambridge, Downing St., Cambridge CB2 3EA, UK

Received 5 January 1998; accepted 24 January 1998

Abstract

Tephra deposits preserved in lake sediments to the west of the Andes, between 448 and 478S in southern Chile, provide arecord of local postglacial eruption history. A series of 32 tephra layers in the sediments of eight small lakes in the studyarea have been investigated by mineral magnetic analysis, electron microprobe analysis and radiocarbon age determinations.Comparisons with geochemical data from other volcanic sources in southern Chile indicate that Hudson volcano is the mostlikely source vent for at least seven of the tephra layers found in Chonos–Taitao lake sediments. The seven tephra eruptionsthat have been identified from widespread and identical tephra deposits, date to 14 560, 13 890, 11 060, 7540, 3840, 2740,and 1560 calendar years ago. The frequency of Hudson volcano eruptions in which tephra is dispersed in both a westerly andeasterly direction, similar to that recorded in 1991 from Hudson volcano, may be around 1 per 3800 calendar years duringthe postglacial period. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: tephrochronology; geochemistry; Hudson volcano; radiocarbon age; Chile

1. Introduction

The eruption of Hudson volcano in the australŽ .winter August of 1991 was the second most violent

volcanic event in Chile this century. This eventŽ 3.produce a large volume of tephra )4 km , accom-

panied by SO gas emissions, that had a major2

environmental impact over an area of 150 000 km2

Ž .across Chile and Argentina Naranjo et al., 1993 .Eruptions of this magnitude have occurred in the

) Corresponding author. Department of Archaeology and Natu-ral History, Research School of Pacific and Asian Studies, Aus-tralian National University, Canberra, ACT 0200, Australia.E-mail: [email protected]

past, but the record of volcanic activity and theimpact of these eruptions on the environment ofsouthern South America remains incomplete.

We have developed a postglacial record of tephrafalls occurring to the west of Hudson volcanoŽ X X .45854 S, 72858 W , between 448 and 478S latitudein southwestern Chile, through detailed studies oftephra layers found in lake sediment cores. Small

Ž .lake basins generally -200 m diameter with sim-ple basin morphology and no major stream inputs areideal for reducing to a minimum stratigraphic com-plications of interpretation, such as sediment rework-ing. Tephra layers recovered from lakes fitting thesecriteria were studied to answer the following ques-tions. Firstly, can the volcanic source of each tephra

0377-0273r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0377-0273 98 00037-7

( )S.G. Haberle, S.H. LumleyrJournal of Volcanology and Geothermal Research 84 1998 239–256240

layer be identified using geochemical data? The geo-chemical results of this study are compared with datafrom previous studies in southern South America toidentify potential source volcanoes by means of theweight percent content of major elements of volcanicglass from individual centers. Secondly, are singletephra layers represented in more than one site and

Žhow widespread are these events? Eight lakes Fig..1b that have been studied as part of a wider

palaeoecological investigation into postglacial vege-Žtation change Lumley and Switsur, 1993; Haberle et

.al., 1998 contained tephra layers preserved within

the lake sediments. Finally, how frequent aretephra-producing eruptions during the postglacial pe-riod?

Volcanism in the southern Andes occurs in twoŽ .separate zones Fig. 1a that have been distinguishedŽ .by Stern et al. 1984 on the basis of geographical

distribution and geochemistry as the Southern Vol-Ž .canic Zone SVZ, 338–468S and the Austral Vol-Ž .canic Zone AVZ, 498–548S . Active volcanism in

SVZ results from subduction of the Nazca plateunder the South American plate, with many centersconsisting of basalts and basaltic andesites. In con-

Ž .Fig. 1. a Map showing the locations of some of the major volcanic centers in the southern Andes of South America and the extent of theŽ . Ž .Southern Volcanic Zones SVZ and Austral Volcanic Zones AVZ . NSVZsnorthern part of SVZ, and SSVZssouthern part of SVZ

Ž . Ž .after Stern et al., 1984; Lopez-Escobar et al., 1993 . b Location map of lakes studied in this investigation with major volcanic centers in´the region. Lakes marked by a star: 1, Laguna Facil; 2, Laguna Oprasa; 3, Laguna Lofel; 4, Laguna Lincoln; 5, Laguna Miranda; 6, LagunaStibnite; 7, Laguna Six Minutes; 8, Laguna Marcelo.

( )S.G. Haberle, S.H. LumleyrJournal of Volcanology and Geothermal Research 84 1998 239–256 241

trast, the AVZ results from convergence of theAntarctic plate against the southernmost SouthAmerican plate and centers there are dominantlyandesitic to dasitic. All these volcanic centers havebeen active during the late Quaternary period, withsome resulting in tephra-producing eruptions that

Žhave left buried traces across the landscape Stern,1990; Lopez-Escobar et al., 1993; Naranjo and Stern,´

.1998 .Ž .In 1928, Auer 1974 recognised that tephra lay-

ers in Tierra del Fuego could be used as effectiveisochrons for correlation between postglacial de-posits in the region. Auer extended his correlationsfarther north in southern South America using geo-

Ž .chemical analyses provided by Salmi Salmi, 1941 .Since this pioneering work by Auer and Salmi, littleemphasis has been placed on the use oftephrochronology as an effective relative dating tech-nique or absolute dating technique with radiocarboncontrols. There is, however, a growing body of datarelated to the geochemistry of many SVZ and AVZ

Žvolcanic centers Stern et al., 1984; Futa and Stern,1988; Stern, 1990, 1991; Lopez-Escobar et al., 1993;´

.Naranjo et al., 1993; Stern and Kilian, 1996 , show-ing distinct geochemical signatures, related to differ-ent plate tectonic dynamics, in southern South Amer-ica.

1.1. The study area

Ž .The study area Fig. 1b lies between the northernChonos Archipelago and the southern Taitao Penin-sula, a complex island and channel landscape formedby extensive ice erosion from the Patagonian icefieldthat covered this region during the last glaciationŽ .Clapperton and Sugden, 1988 . The dominantbedrock of the region consists of metamorphic rocksof Paleozoic age and granite of Cretaceous ageŽ .Niemeyer et al., 1984 , with thirteen Quaternarylarge volcanic centers, which form the southern SVZŽ X X .41830 –46800 S, Lopez-Escobar et al., 1993 . Tec-´tonic activity in the region is associated with bothplate subduction and the influence of the Liquine–˜

Ž .Ofqui fault system Cembrano et al., 1996 . Geo-chemically, the volcanic centers of the southern SVZare dominantly high-Al basalts, with andesites anddacites found at Cay, Mentolat, Melimoyu, Yate,Huequi, Michinmahuida and Hudson and rhyolites at

Chaiten volcano. Hudson volcano is characterised by´Žrelatively high TiO and K O contents Futa and2 2

.Stern, 1988 . Volcanic centers in the AVZ are char-acterised by the predominance of andesites anddacites of variable mineralogy.

The region lies within a zone of high precipita-tion, produced by the coupled ocean–atmosphericinfluence of the Southern Polar Front that migrates

Ž .seasonally between 508S summer and 40–458SŽ .winter . The climate is strongly oceanic, with meanannual temperatures around 8–108C and annual pre-cipitation near sea level in the region of 3000 mmŽ .Miller, 1976 . As altitude increases towards theAndes, precipitation is much higher with estimates

Žfor annual rainfall as much as 10 000 mm Fujiyoshi.et al., 1987 . The vegetation is North Patagonian rain

forest dominated by evergreen broadleaf and coniferŽ .taxa Veblen et al., 1983; Gajardo, 1995 . The pre-

Žvailing winds are northerlyrnorthwesterly Miller,.1976 , with only occasional reversals to easterlies

during summer months. Although the Chonos–Taitaoregion lies upwind from the volcanic centers of theAndes, volcanic eruptions of sufficient magnitudeŽwhere tephra must travel between 50 to 120 km

.from these centers could deposit tephra on thepeninsula and islands, depending upon the explosionsize and wind direction at the time of eruption.

2. Methods

Fieldwork was conducted in the southern hemi-sphere summers of 1991, 1992, 1994, 1995 and1996, with the logistic support of Raleigh Interna-

Ž . Žtional UK and CONAF Corporacion Nacional´.Forestal, Chile . Sediment sequences were extracted

Ž .from eight small closed lake basins Fig. 1b using amodified 5-cm diameter Livingstone piston corerŽ .Wright, 1967 . Six full sequences from basal late-glacial clays to the present were recovered from

Ž X X .Laguna Stibnite 46825 S, 74824 W , Laguna SixŽ X X . Ž XMinutes 46825 S, 74820 W , Laguna Lofel 44853 S,

X . Ž X X .74824 W , Laguna Lincoln 45822 S, 74804 W , La-Ž X X .guna Oprasa 44821 S, 73839 W and Laguna Facil

Ž X X .44819 S, 74817 W , and two partial postglacial se-Ž X X .quences from Laguna Marcelo 46828 S, 74810 W

Ž X X .and Laguna Miranda 46808 S, 73826 W . Samplesof the uppermost 10 cm of lake mud sediments were

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Table 1Ž .Chonos–Taitao tephra glass geochemistry total dataset normalised EMPA to 100 wt.%, P O and MnO values, all very low, are not included in table2 5

aSample no. SiO TiO Al O FeO MgO CaO Na O K O Cl N2 2 2 3 2 2

Laguna OprasaOpr-8) 54.01"2.75 2.21"0.50 16.38"2.48 10.56"3.36 3.93"3.57 6.78"0.23 4.31"0.67 1.46"0.36 0.15"0.00 6Opr-7) 56.96"0.48 1.97"0.30 14.55"1.22 10.51"0.95 3.77"0.34 6.88"0.06 3.48"0.18 1.48"0.11 0.14"0.01 22Opr-7)) 62.22 1.87 14.70 8.46 1.76 4.79 3.56 2.33 0.16 1Opr-6) 57.20"1.00 2.06"0.20 15.09"0.42 9.94"0.79 3.35"1.00 6.88"0.14 3.59"0.04 1.58"0.28 0.10"0.02 10Opr-6)) 66.33"0.91 1.15"0.18 14.73"0.95 6.57"0.14 1.04"0.24 3.27"0.50 3.09"0.16 3.55"0.55 0.12"0.03 2Opr-5) 54.77"0.03 1.26"0.01 15.57"0.08 9.36"0.13 5.70"0.06 8.81"0.06 3.15"0.03 1.11"0.06 0.08"0.00 28Opr-4)) 64.47"1.07 1.54"0.15 14.77"1.27 7.30"0.71 1.80"0.40 4.12"0.46 3.83"0.77 1.76"0.16 0.17"0.02 6Opr-3) 55.83".049 2.17".057 14.47"1.03 10.93"0.77 4.08"0.37 7.08"1.00 3.67"0.06 1.48"0.51 0.08"0.02 16Opr-3)) 59.77"0.47 1.76"0.45 16.51"2.27 7.09"2.27 1.96"0.73 5.95"1.39 4.71"0.91 1.98"0.56 0.09"0.03 2Opr-1) 56.24"0.30 1.71"0.15 15.80"0.79 8.46"0.38 3.91"0.28 7.95"0.31 4.12"0.18 1.50"0.17 0.11"0.01 16

Laguna FacilFac-3) 54.75"0.67 1.31"0.03 15.63"0.06 9.45"0.15 5.64"0.13 8.75"0.42 3.28"0.68 0.98"0.06 0.05"0.01 16Fac-2) 55.39"1.11 2.26"0.16 15.55"2.67 9.96"0.92 3.18"1.09 7.75"0.86 3.38"0.30 1.69"0.57 0.08"0.03 10Fac-2)) 60.26"1.40 1.75"0.19 15.81"1.48 7.73"1.07 2.69"0.69 5.67"0.68 3.45"0.15 2.30"0.27 0.11"0.01 6Fac-1)) 68.52"0.18 1.09"0.01 16.78"0.06 3.47"0.01 0.98"0.02 1.88"0.06 3.83"0.12 3.14"0.10 0.12"0.01 17

Laguna LincolnLin-4) 55.42"0.45 1.80"0.16 15.44"2.08 10.55"1.51 3.57"0.52 8.39"0.25 3.61"0.47 0.95"0.12 0.06"0.01 13Lin-3) 56.24"0.62 2.68"0.14 14.48"0.47 11.65"0.90 3.03"0.66 6.81"0.34 2.90"0.71 1.78"0.35 0.14"0.00 17Lin-2)) 64.77"0.06 1.17"0.01 16.99"0.12 5.56"0.31 1.48"0.04 3.56"0.03 3.26"0.09 2.89"0.09 0.19"0.05 26Lin-1) 56.44"0.71 2.24"0.86 15.52"1.57 9.08"0.44 3.34"0.06 7.24"1.43 4.36"0.30 1.47"0.76 0.10"0.02 3

Laguna LofelLof-4) 56.60"1.28 2.04"0.59 14.90"0.41 10.06"0.73 3.42"0.98 6.57"1.56 3.84"0.64 1.80"0.81 0.08"0.06 14Lof-4)) 67.98"2.74 0.85"0.26 15.10"0.11 4.34"0.04 1.17"0.79 3.38"1.45 3.78"0.71 2.80"0.79 0.16"0.04 3Lof-3) 55.44"0.06 2.27"0.00 15.34"0.06 9.88"0.04 3.85"0.04 7.34"0.05 3.62"0.04 1.95"0.07 0.10"0.02 8Lof-3)) 68.62 1.05 16.85 3.43 1.02 2.04 3.51 3.07 0.19 1Lof-2) 54.19 1.90 14.91 11.42 4.24 5.90 4.99 2.15 0.10 1Lof-2)) 66.99"0.44 0.84"0.42 15.11"0.88 4.89"0.88 1.00"0.76 3.17"0.54 5.38"0.96 2.13"0.90 0.04"0.03 2Lof-1) 57.47"0.22 1.88"0.05 16.04"0.98 8.12"0.51 3.26"0.01 5.72"1.42 4.25"0.67 2.72"1.55 0.12"0.01 2

Laguna StibniteStb-4) 54.89 2.18 14.68 11.36 3.35 6.83 4.24 1.59 – 1Stb-4)) 66.28"4.37 1.19"0.57 15.12"0.44 5.18"2.79 1.10"0.78 2.68"1.65 4.43"1.35 3.30"0.42 – 18Stb-3) 56.17"2.70 2.20"0.04 14.20"0.23 10.44"1.70 3.58"1.20 6.55"1.48 3.74"0.13 1.76"0.47 – 15Stb-3)) 61.71 1.85 14.98 7.34 2.27 3.80 4.18 2.68 – 1Stb-2) 54.59"0.27 2.39"0.17 15.70"0.36 9.90"0.13 3.30"0.38 7.86"0.18 3.75"0.06 1.48"0.37 – 2Stb-1) 51.44"2.13 2.12"0.03 15.06"2.36 11.81"2.95 5.25"2.77 7.23"1.32 4.12"0.00 1.38"0.02 – 18

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Laguna MirandaMir-3) 57.41"0.03 2.38"0.05 13.82"0.44 10.58"0.42 3.38"0.08 6.81"0.12 3.41"0.01 1.91"0.01 0.09"0.01 6Mir-3)) 60.98 1.80 16.71 6.84 2.45 4.79 3.92 2.21 0.14 1Mir-2) 55.77"0.17 1.78"0.14 15.88"0.70 8.78"0.57 4.34"0.19 8.06"0.32 3.79"0.28 1.34"0.19 0.10"0.00 10Mir-1)) 66.09"0.13 1.24"0.04 16.60"0.04 5.00"0.07 1.52"0.01 3.52"0.02 3.12"0.14 2.71"0.05 0.10"0.02 22

Laguna Six MinutesSix-4) 57.03"2.96 1.43"0.05 16.34"0.22 7.76"1.00 3.33"1.35 6.64"1.76 4.75"0.38 1.82"0.65 – 13Six-4)) 65.61"1.17 1.15"0.10 16.10"0.07 4.88"0.40 1.25"0.28 2.77"0.93 4.54"0.23 2.88"0.32 – 7Six-3) 54.24"3.93 1.90"0.47 13.18"3.08 11.43"4.36 6.65"5.68 5.81"1.53 3.97"0.89 1.54"0.23 – 17Six-3)) 60.98 1.80 16.71 6.84 2.45 4.79 3.92 2.21 – 1Six-2) 55.33"4.32 1.99"0.30 15.11"0.36 11.05"2.84 3.54"2.50 6.10"0.51 4.17"0.48 1.45"0.59 15Six-1) 52.85"0.79 2.28"0.46 15.98"1.69 9.69"1.12 3.19"0.40 8.27"0.43 5.16"0.38 1.14"0.35 – 13

Laguna MarceloMar-2) 56.01"1.27 1.47"0.07 16.85"0.84 8.03"0.25 3.54"0.20 7.43"1.16 4.26"0.25 1.48"0.32 – 5Mar-2)) 64.36"1.32 1.31"0.30 15.23"0.87 5.58"1.51 1.31"0.10 3.29"0.20 5.95"1.08 2.07"1.20 – 16Mar-1) 54.99"3.02 2.40"0.54 14.53"0.81 10.15"2.45 3.36"0.79 6.97"0.62 4.55"0.28 1.75"0.20 – 17

Surface SamplesSurface) 54.09"0.28 1.52"0.08 16.53"0.18 8.78"0.52 4.59"0.05 8.30"0.01 4.06"0.08 1.31"0.06 – 4Surface)) 64.58"0.05 1.16"0.01 16.11"0.25 5.07"0.39 1.50"0.10 3.24"0.25 4.78"0.71 2.79"0.27 – 18

b( )Hudson Õolcano SSVZHudson) 53.15 2.40 16.31 10.68 3.77 7.01 4.85 1.31 – 1Hudson)) 61.97"2.27 1.52"0.39 18.88"2.96 4.83"0.40 1.68"0.12 4.17"0.29 5.06"1.01 2.54"0.04 – 4

bSSVZCay) 52.84 1.02 18.5 7.63 5.14 10.3 3.52 0.76 – 1Cay)) 66.74 0.6 16.4 4.23 1.29 3.34 5.54 2.1 – 1Maca) 52.02 1.23 18.6 8.92 4.78 9.68 3.35 0.59 – 1

bAVZBurney)) 64.77"0.29 0.38"0.06 17.86"0.58 3.65"0.33 2.19"0.01 5.47"0.09 4.48"0.34 1.01"0.11 – 2Aguilera)) 65.00 0.61 16.71 4.05 2.11 4.98 4.24 2.06 – 1Lautaro)) 66.81 0.59 16.62 3.53 2.05 4.32 3.76 2.20 – 1

–: not determined; N: number of analyses; major elements are normalised to 100%.Ž . Ž .)Basic glass population -59 wt.% SiO . ))Silicic glass populations )59 wt.% SiO .2 2

a Total Fe as FeO.b Ž . Ž .Normalised geochemical data from selected volcanic centres determined from tephra glass only in the SSVZ and AVZ Futa and Stern, 1988; Stern, 1990, 1991 .Analysis was performed at the Department of Earth Sciences, University of Cambridge, using a Cameca SX50 spectrometer microprobe, an electron microscope with three wavedispersive spectrometers and a LINK AN10000 energy dispersive spectrometer running PAP matrix correction software.

Ž .The probe operated at 20 kV, using a 10 nA beam current and a 10 mm defocused beam to minimise loss of Na and K 50 s count time .A mixture of minerals, natural oxides and pure metals were employed as standards that were periodically checked in order to verify internal consistency of the results.


Ž .taken using a Hongve sampler; Wright, 1980 from¨Lagunas Stibnite, Six Minutes and Marcelo. Thetephra found in these lake mud surface samples fromthe Taitao Peninsula is most likely to have beendeposited in 1971, since an explosive eruption of theHudson volcano occurred in August of that yearŽ .Fuenzalida, 1976 . The samples from Laguna Stib-nite and Laguna Six Minutes were collected in Jan-uary 1991, predating the larger more recent eruption

Žof the Hudson volcano in August 1991 Naranjo et.al., 1993 . The samples from Laguna Marcelo were

collected in January 1992 and these may containtephra from this more recent eruption.

Possible tephra layers in the cores were located byvisual inspection, with the aid of the X-ray analysesand sediment magnetic susceptibility data. Each corewas described in detail and scanned by a magneticsusceptibility core scanning loop at exactly 2-cmintervals. This identifies changes in the abundance offerrimagnetic minerals, which is used as an indica-tion of changing erosional input to the lake, and canassist in the location of tephras. Major tephra layerswere located in this way and the glass shards ex-

Ž .tracted H O digestion , mounted and polished for2 2Ž .electron microprobe analysis EMPA , under the

conditions described in the caption of Table 1 andŽ .following the guidelines set down by Froggatt 1992

Ž .and Hunt and Hill 1993 . EMPA was chosen overbulk analytical methods as it is grain specific, avoid-ing the problems associated with detrital contamina-tion and density fractionation during the sedimenta-

Ž .tion process Froggatt and Gosson, 1982 . We at-tempted to obtain more than 10 analysis points foreach sample; in some cases, however, we were ableto obtain less than the recommended sample size inthe thinner tephra layers as only a few shards werepresent.

The similarity between geochemical data fromŽ .different tephra samples was tested by: 1 determin-

ing the mean and standard deviation for each mea-Ž .sured element; 2 constructing plots of the major

elements that have been shown to be useful discrimi-Žnatory variables, namely SiO , K O and TiO Futa2 2 2

. Ž . Žand Stern, 1988 ; and 3 numerical analysis similar-ity coefficients, Borchardt et al., 1972; Beget et al.,´

.1991 of the multivariate geochemical data to evalu-ate and compare the resemblance between selectedsamples. The source volcano for each tephra may be

identified by comparing the geochemistry of tephraglass from each sample with reference geochemicaldata sets from previously published tephra glass

Žgeochemistry Stern et al., 1984; Futa and Stern,1988; Stern, 1990, 1991; Lopez-Escobar et al., 1993;´

.Naranjo et al., 1993 . All major element data havebeen normalised to 100% for valid geochemicalcomparisons. Chronological control is then providedby using a combination of AMS and bulk 14C dating.The dates are reported throughout this paper as

Ž .calibrated ages cal yr BP by using the programŽ .Calib 3.0.3 Stuiver and Reimer, 1993 , though ra-

Ždiocarbon years before present yr BP; present is.1950 AD are also provided.

3. Tephrochronology of the Chonos–Taitao lakesediment cores

3.1. Magnetic susceptibility and Õisibility of tephras

High resolution studies of the mineral magneticproperties of the cores found several narrow layersthat exhibited anomalously high magnetic suscepti-

Ž .bility values Fig. 2 . Similar patterns of susceptibil-ity variation were identified in all the cores and withthe exception of basal fluvioglacial olive-grey clays,are considered to be due to tephra deposition in the

Žlake sediments high magnetite concentration, Old-.field, 1988 . When located, the tephra layers typi-

cally ranged from 0.1 to 0.5 cm in thickness andfrom silt to sand grain size in the more distal sitesŽStibnite, Six Minutes, Marcelo, Lincoln, Lofel,

.Oprasa, and Facil , and up to 8 cm thick and coarseŽ .sand grain size in the more proximal site Miranda .

Susceptibility values were also higher in the proxi-mal site than in the other sites, possibly reflecting agreater concentration of magnetic minerals in thelake sediments. The sediments lying immediatelyabove and below the tephra deposits were investi-gated for evidence of tephra mobilisation, which ineach case suggested that the tephra layers werediscrete and that bioturbation or reworking was not asignificant factor in sedimentation.

3.2. Electron probe analysis of tephras

The geochemical composition of glass shards forŽdifferent tephra layers are summarised as means and


Fig. 2. Stratigraphy, magnetic susceptibility variations and radiocarbon dating locations in a series of cores from the Chonos–Taitao region,Chile. Depth is recorded as meters below water surface.


Fig. 3. Plots of SiO vs. TiO and K O from different volcanic centers in the SSVZ and AVZ. A dashed line marks the limits of the2 2 2

distribution of wt.% values for the lake mud surface samples and reference tephra samples for Hudson volcano.

. Ž . 1standard deviations of the EMPA results Table 1 .The reported means are averages of analyses col-lected on multiple discrete shards from each tephralayer, after normalising to correct for varying de-grees of hydration, which ranged from 1 to 5 wt.%for the tephras analysed during this study. Na O2

percentages are low and very similar throughout,suggesting that Na depletion during analysis was notsignificant. We also include the previously publishednormalised microprobe analysis of glass from vol-canic centers in the SSVZ and AVZ in our analysis.

The reference data from 3 volcanic centers in theŽ .SSVZ Cay, Maca and Hudson , 3 volcanic centers

Ž .in the AVZ Burney, Aguilera and Lautaro andrecent tephra from the 1971 and 1991 Hudson erup-

Ž .tions Table 1 show the range in composition fromŽ . Žbasalt -52 wt.% SiO to rhyolite )70 wt.%2

1 Ž .The entire data set prior to normalisation can be accessedat the WWW page -http:rrwww-palecol.plantsci.cam.ac.ukrchilertephra.html).

.SiO magmas, though there is a predominance of2

basalts and basaltic andesites. Fig. 3 shows that eachvolcanic center exhibits a good correlation betweenSiO –TiO and SiO –K O, but the relationship be-2 2 2 2

tween both oxides varies from center to center.Hudson volcano basalts and basaltic andesites showvariable Al O concentrations between 15 and 192 3

wt.%, intermediate K O, and high TiO and FeO2 2

wt.% values, distinguishing Hudson from other vol-canic centers. The plots of SiO vs. TiO and K O2 2 2Ž .Fig. 3 illustrate this bimodal distribution of theglass geochemistry from the recent eruptions. In thiscase, it is likely that two geochemical populationshave originated from more than one magma sourcewithin the same volcanic center, as was reported

Žduring the 1991 eruption of Hudson volcano Naranjo.et al., 1993 . However, it must be stressed that some

eruptions may be completely basaltic and othersandesiticrdacitic.

The microprobe analysis of glass from 32 tephrasamples show that the tephras range from basalt to

Ž .basaltic andesites and andesites 52–63 wt.% SiO ,2


and more rarely dacitic type magma. All show a highŽ . Ž .TiO )1 wt.% and FeO )5 wt.% content. A2

mixed assemblage of glass is identified in somesamples on the basis of high standard deviations in

Žthe initial averaging analysis Froggatt and Gosson,.1982 . These samples were then divided into basic

Ž . Ž .SiO -59 wt.% and silicic SiO )59 wt.% glass2 2

populations. A bimodal distribution of grains withintephra layers is unlikely to suggest a multiple source,since two eruptions would have to occur simultane-ously, distributing tephra to the same areas in similarproportions.

Ž .Plots of SiO vs. TiO and K O Fig. 4 illustrate2 2 2

the similarity between the distribution of Hudsonvolcano reference data and the Chonos–Taitao laketephra samples. Many samples fall in the field ofHudson composition, indicating that some tephrasare chemically similar and may be derived from thesame volcanic center. There is a good correlationbetween the geochemical composition of tephrasfound to the southwest of Hudson volcano, in theTaitao region, and the Hudson volcano referencedata. Tephra recovered from lakes in the Chonosregion, northwest of Hudson volcano, do not all fallin the field of Hudson composition as well as thosefrom lakes in the Taitao region. Outliers within eachtephra sample may reflect a wider geochemical rangefor Hudson glass element composition than is illus-trated by the limited number of reference samplesavailable to this analysis. Outliers may also be ex-plained by analytical variations, though the low vari-ability in Na O and continued checks against stan-2

dards during analysis suggests that this is unlikely tobe a significant contributing factor. Alternatively,some tephras found in the Chonos region may bederived from other volcanic centers in the southernSVZ.

Ž .Similarity coefficients SC are used to evaluatethe similarity between the multivariate geochemical

Ždata from the Chonos–Taitao tephra layers where.sample size )1 and the Hudson volcano reference

data. A matrix of SC values is presented in Table 2,where values G0.92 are generally taken as indica-tive of correlations between tephra samples and thosebetween 0.6–0.90 are typically obtained for noncor-

Ž .relative, unrelated tephras Froggatt, 1992 . Signifi-cant SC values are found between a number of

Žtephras containing the basic glass population boxed

.values in Table 2a , suggesting that many of thesetephras are identical and may be correlated to thesame eruption event. The silicic glass populationfrom Hudson volcano and the recent surface samples

Ž .are shown to be identical Table 2b .

3.3. 14Crcalibrated age and correlation of tephras

If tephras recovered from the Chonos–Taitao lakesare derived from Hudson eruptions, then there shouldbe at least some with the same age. A series ofradiocarbon dates have been obtained from all lakesŽ .except Laguna Marcelo, Table 3 in order to either

Ždirectly date the tephra deposition events AMS dat-.ing analysis , or to closely bracket the age of the

tephra layers by interpolating bulk-sediment radio-carbon ages, assuming linear age model for sedimen-tation rates in these lake basins.

A total of 7 tephra groups were recognised asbeing correlated on the basis of age determination

Ž .andror geochemical content Fig. 5 . These tephrasŽ .are assigned the codes HW to HW Table 4 ,1 7

denoting a tephra layer that has been correlated andtraced in more than one site, at a distance of morethan 50 km from its source at Hudson volcano.Where more than one age is associated with geo-chemically similar tephra layers, the ages have beenpooled to provide a more precise date for the tephraeruption event. The calibrated ages for correlatedtephra layers found within the Chonos–Taitao lakesare 14 560, 13 890, 11 060, 7540, 3840, 2740, and1560 cal yr BP.

With the exception of the two recently docu-mented eruptions of the Hudson volcano in 1971 and

Ž .1991 Fuenzalida, 1976; Naranjo et al., 1993 , onlytwo postglacial tephras identified in southern SouthAmerican sediments have been previously assigned

Žto this volcano and dated Stern, 1991; Naranjo and.Stern, 1998 . The first, an eruption of significant

magnitude which is dated to around 7530 cal yr BPŽ .H , and a second which is dated at 3885 cal yr BP1Ž .H . Isopleth and isopach maps for these two erup-2

tions shows the tephra dispersal in a predominantlyŽ .southeastern direction Naranjo and Stern, 1998 .

Nevertheless, two tephra layers in this study, Opr-5and Mir-1, that are to the west of Hudson volcanoare possibly correlated with these two eruptions onthe basis of their similar ages. The pooled mean of


Fig. 4. Plots of SiO vs. TiO and K O for the 32 tephra samples from lakes in the Chonos–Taitao region. The dashed line marks the limits2 2 2Ž .of the distribution of wt.% values for the lake mud surface samples and reference tephra samples for Hudson volcano see Fig. 3 .


Ž .Fig. 4 continued .


Table 2Ž . Ž .Similarity coefficient SC calculations after Borchardt et al., 1972 comparing major element analysis of tephras from the Chonos–Taitao

Žregion, including the reference samples for Hudson volcano and other SSVZ and AVZ centers coefficients calculated using SiO , Al O ,2 2 3.TiO , FeO, CaO, Na O and K O2 2 2

Ž .SC values for basic and silicic glass population significant values are boxed .a Ž .Basic glass population -59 wt.% SiO .2b Ž .Silicic glass population )59 wt.% SiO .2

ages for these tephras gives eruption dates at 7540Ž .cal yr BP and 3840 cal yr BP for HW sH and4 1

Ž .HW sH , respectively. The inability to correlate5 2

at least four tephra layers from the Chonos region,Žnamely, Opr-6, Opr-4, Opr-2, and Fac-3 correlations.for Lof-1, Lof-2 and Lof-3 are uncertain with any

other tephra layer may be due to incomplete recov-ery of all tephras in our cores. It cannot be dis-counted that these eruptions deposited tephra in theother Chonos–Taitao lakes, though these depositsmay remain undetected without fine-resolution mi-

Ž .croscopic analyses Rose et al., 1996 . Alternatively,these uncorrelated tephras may be derived from mi-nor postglacial eruptions of other southern SVZ vol-canic centers.

It could be expected that tephra deposited in theChonos–Taitao region from an eruption of the Hud-son volcano would be of similar composition.Tephras HW to HW are all bimodal in composi-1 7

tion, with the exception of HW , though there is5

intersite variation between geochemically identicaltephras. The observed discrepancies could be ex-


Table 3Radiocarbon age determinations for tephras and associated lake

Žsediments from five lakes in the Chonos–Taitao region Q seriesare bulk radiometric dates and the AA and Beta series are AMS

.dates

Sample depth Radiocarbon age Laboratory referenceŽ . Ž .m years BP number

Laguna Stibnite2.91–3.01 1635"40 Q-28352.96 1830"75 AA-102963.25 2600"60 AA-102953.75–3.85 4590"55 Q-28364.32–4.42 6290"60 Q-28374.80 9995"85 AA-102945.04–5.12 11005"75 Q-28385.25 11965"100 AA-102935.53–5.73 13920"80 Q-28396.58–6.78 14335"145 Q-2840

Laguna Six Minutes3.63 1605"55 AA-103003.96 2560"55 AA-102995.05 9930"85 AA-102985.34 11855"120 AA-10297

Laguna Lincoln2.06–2.16 2470"40 Q-29622.82–2.92 4905"30 Q-29613.60–3.70 8380"80 Q-29604.30–4.40 11145"105 Q-29594.94–5.08 13320"120 Q-2958

Laguna Miranda5.05–5.15 1160"30 Q-29776.89–6.99 2710"35 Q-29768.36–8.46 4250"35 Q-2975

Laguna Lofel3.90–4.00 2350"40 Q-29675.90–6.00 4580"45 Q-29667.90–8.00 7275"65 Q-29659.90–10.00 10 720"90 Q-296411.80–11.90 13480"110 Q-2963

Laguna Oprasa3.60–3.70 120"30 Q-29745.52–5.60 3875"35 Q-29737.60–7.68 7750"55 Q-29728.20 9830"90 Beta-1078158.82-8.90 10255"75 Q-2971930 12390"120 Beta-1078169.60–9.70 12165"100 Q-297010.04–10.12 12970"120 Q-296910.30–10.40 13560"125 Q-2968

Ž .Table 3 continued

Sample depth Radiocarbon age Laboratory referenceŽ . Ž .m years BP number

Laguna Facil2.80–2.90 210"30 Q-29824.80–4.90 2690"30 Q-2981

Laguna Facil6.80–6.90 6245"45 Q-2980752 10080"100 Beta-1078127.90–8.00 9050"60 Q-2979830 12480"100 Beta-10781310.12–10.22 13230"140 Q-2978

Depths are given as meters below water surface at time of coring.

plained by within lake processes such as, differentialsettling of grains within the lake system, due tosediment focusing andror differing densities of thebasalticrdacitic grains, relative to other componentsof the sediment. Alternatively, increased distancefrom the source volcano may cause sorting of thebasic and silicic glass shard populations.

3.4. Frequency and distribution of tephra falls fromHudson Õolcano

The sequence of tephra layers recognised in theChonos–Taitao study region constitutes a partialrecord of the actual sequence of tephra eruptionsfrom Hudson volcano during the postglacial period.It is likely that there were many more eruptions fromHudson volcano, and possibly other SSVZ and AVZvolcanic centers, which are not recorded in the lakesdue to small eruption size, a wind direction notfavourable to tephra dispersal in a westerly direction,or methodological procedures that are not sensitiveenough to recognise all tephras in this study. Naranjo

Ž .and Stern 1998 show a series of 9 tephras de-posited in peat over the last 9350 calendar yearsŽ .Junco Alto, 92 km southeast of Hudson volcano ,that they suggest could also have been derived fromthe Hudson volcano. A detailed sedimentologicalanalysis of the Laguna Miranda sequence, the mostproximal lake to Hudson volcano in this study, re-vealed at least 22 presumed tephras deposited over

Žthe last 5 millennia Haberle et al., in press of which

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Ž . Ž .Fig. 5. Stratigraphy and tephra correlations in a series of cores from 8 lakes in a latitudinal transect from south Taitao region to north Chonos region . Correlations betweencores based on electron lake mud–water interface.

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Table 4Ž .Ages standard and calibrated of Hudson volcano tephra deposits, determined from direct AMS dating of organics associated with tephras and geochemical analysis of glass

shards14Tephra Sample Chronological control yr BP Pooled mean of C dates, Dating T Geochem Other possible

2Ž . Ž . Ž .name cal yr BP yr BP cal yr BP test Xi SC mean tephra correlates

Ž . Ž .HW Opr-1 12390"120 14 730–14 280 12440 14560 0.17 – Lin-11Ž . Ž . Ž .Fac-1 12480"100 14 830–14 420 3.84 Lof-1, Lof-2?Ž . Ž .HW Stb-1 11965"100 14 120–13 790 11920 13890 0.29 – Lin-22Ž . Ž . Ž .Six-1 11855"120 14 000–13 650 3.84 Lof-2, Lof-3?

Ž . Ž .HW Opr-3 9830"90 11010–10960 9950 11 060 3.14 0.935 Lin-3, Lof-43Ž . Ž .Fac-2 10080"100 11 960–11 190 7.81

Ž .Stb-2 9995"85 11670–11000Ž .Six-2 9930"85 11 330–11000

a Ž . Ž .HW Opr-5 ;6750 7540 6725 7540 – –4b Ž .H ;6700 75301a Ž . Ž .HW Mir-1 ;3535 3820 3570 3840 – –5b Ž .H ;3600 38852

Ž . Ž .HW Stb-3 2600"60 2760–2720 2580 2740 0.23 0.920 Opr-7, Mar-16Ž . Ž .Six-3 2560"55 2750–2520 3.84 Lin-4, Mir-2Ž . Ž .HW Stb-4 1830"75 1830–1630 1690 1560 – 0.950 Mar-2, Mir-37Ž .Six-4 1605"55 1540–1410 Opr-8

Ž .The significant correlations between each sample are show on the basis of a T test of similarity scores given for samples shown to be the same at 95% confidence level for theAMS dates and mean Similarity coefficients for the geochemical data.Other possible tephra correlates based on high SC scores andror similar ages are also given when shown to be significant.a Inferred ages determined by a linear age model for Laguna Oprasa and Laguna Miranda sedimentation rates.b Ž .Dates of tephra layers H and H found in Patagonia and Tierra del Fuego and attributed to past Hudson eruptions by Naranjo and Stern 1998 .1 2


only the three thickest tephras are studied here.Improved dating control, combined with geochemi-cal analysis, may allow these tephras to be correlatedwith those from the Chonos–Taitao lakes and furtherafield.

This study has identified seven eruption eventsoccurring throughout the postglacial period, distribut-ing tephra from the north to the south of theChonos–Taitao region, and in at least two casesŽ .HW and HW the tephras are found as far south as4 5

ŽPatagonia and Tierra del Fuego Naranjo and Stern,.1998 . The recurrence interval between explosive

eruptions of Hudson volcano is highly irregular andit is difficult to establish a clear frequency for erup-tions from this data. Tephras HW and HW repre-4 5

sent the most widespread tephra deposits in thissequence, suggesting that major eruptions of Hudsonvolcano that distributed tephra in both a westerly andeasterly direction from the Andes, similar to thatwhich occurred in 1991, have occurred in the past ata frequency of between once every 3800 calender

Ž 14years 3350"250 C years, Naranjo and Stern,.1998 . The most proximal site to Hudson volcano is

Laguna Miranda, around 50 km to the southwest ofthe volcano, that records minor eruptions at an aver-age of 1 every 225 calendar years, though thisestimate should be considered a minimum sincetephra preservation at any one site will depend onwind direction at the time of the eruption.

Few studies in southern South America haveŽutilised tephras as relative dating horizons Heusser,

.1964, 1990; Heusser et al., 1989 . The Hudson vol-cano tephrochronology provides the opportunity tostrengthen the chronological interpretations ofpalaeoecological and archaeological records in thisregion. For example, there is currently considerabledebate concerning the evidence in southern SouthAmerica for the cold climate Younger Dryas phase

Žbetween 12 900 and 11 100 cal yr BP Heusser andRabassa, 1987; Ashworth and Markgraf, 1989; Ash-

.worth and Hoganson, 1993 , due primarily to con-flicting interpretations of the chronology of eventsrecorded in the available palaeoecological records.

Ž .Hudson tephras HW 13 890 cal yr BP and HW2 3Ž .11 060 cal yr BP span this period of climaticchange at the transition to the postglacial warmperiod and could be used to strengthen site chrono-logical interpretations and examine environmental

conditions across a number of sites in the region justprior to, and at the end of, the Younger Dryas chronŽ .Haberle et al., 1998 .

4. Summary and conclusions

Many studies have shown that tephra layers canbe used as important stratigraphic marker horizonswhere more than one site is being investigated in aregion. With a tight chronological framework thesetephras can now be utilised as time stratigraphichorizons, reducing the necessity for excessive num-bers of radiocarbon age determinations. Atephrochronology is superior to a radiocarbonchronology in that individual layers are often de-posited in less than one week, enabling one to corre-late cores with precision. In many cases, sedimentsfrom lakes with no inlet streams provide an idealenvironment for the preservation of discrete tephralayers due to their continuous, often undisturbedsediments. The glaciated regions of southern SouthAmerica provide an ideal source for suitable lakes.

The answers given for the three questions posedat the beginning of this study have enabled us toconstruct a preliminary tephrochronology for theChonos–Taitao region. Comparisons with geochemi-cal data from other volcanic sources in southernChile indicate that Hudson volcano is the most likelysource vent for at least seven of the tephra layersfound in Chonos–Taitao lake sediments. The datesfor correlated eruption events are 14 560, 13 890,11 060, 7540, 3840, 2740, and 1560 cal yr BP. Thefrequency of Hudson volcano eruptions in whichtephra is dispersed in both a westerly and easterlydirection, similar to that recorded in 1991 fromHudson volcano, is around 1 per 3800 years duringthe postglacial period.

Acknowledgements

We would like to thank Raleigh International andCONAF for logistic support and permission to carryout this work in Region XI, Chile. Many interna-tional venturers, including some 25 Chilean ventur-ers, contributed outstandingly to the success of thiswork in difficult and remote terrain. Keith Bennett


and Julie Fossitt assisted during fieldwork. StephenRead, Thomas Bleser, Steve Boreham, Jane Buntingand Ray Fella provided assistance with microprobeanalyses. Keith Bennett, Charles Stern and oneanonymous reviewer provided valuable comments onthe results and interpretations of the tephra data set.This project is funded by a Natural EnvironmentResearch Council Studentship and the LeverhulmeTrust Fund.

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