Long term natural lake acidification and heavy metal mobilisation
by weathering of volcanic ashes - A case study of lacustrine lake
sediments of southernmost Patagonia, Chile

Rolf Kilian¹ and Harald Biester²

¹Institute of Geology, University of Freiburg, Albertstr. 23B, 79104 Freiburg, Germany;
Corresponding e-mail: kilianr@uni-freiburg.de

²Institute of Environmental Geochemistry, University of Heidelberg, INF 236, 69120
Heidelberg, Germany

Abstract

Late Glacial to Recent lake sediment cores from the southernmost Andes exhibit systematic variations in their content of heavy metals, e.g. Pb from 30 to 450 mg/kg. The heavy metals are mainly derived from Fe/Mn-hydroxide precipitates in the lakes and stabilised as sulfides in the anoxic sediments. Previously to Fe/Mn-hydroxide precipitation, the heavy metals were leached by acid peat soils from underlying green shists basement. The extent of precipitation was controlled by the lake pH. Periods of high rain fall, large influx of organic matter and without alteration of SO₂-rich tephra were characterized by moderate lake pH and high Fe/Mn-hydroxide precipitation. In contrast, the Late Holocene deposition of coarse grained SO₂-rich tephra from Mt. Burney volcano and its long term alteration produced a lake acidification for a 4300 year lasting period, resulting in a low Fe/Mn-hydroxide precipitation.

Introduction

In the Austral Volcanic Zone of the Andes (AVZ) several plinian volcanic eruptions occurred during the Late Glacial and Holocene (Fig. 1). Tephra deposits of these eruptions covered large areas of a dense rain fall forest and peat land (Markgraf 1993; Stern 1990, 1992). The influences by these ash falls, and rework and alteration of the pyroclastic material on vegetation and different lacustrine ecosystems are unknown. Therefore we have cored two small lakes in the southernmost part of the Andes (53˚S). Both sediment cores document the sedimentation during the least 13.000 years with a high resolution. The sediment profiles show several small tephra layers and two thicker tephra layers (8-15 cm) from two large plinian eruptions of Mt. Burney volcano (Fig. 1). These two volcanic events may have produced significant changes in the terrestrial and lacustrine ecosystems.

Fig.1: Southernmost South America with the area of investigation at the Gran Campo Nevado, the zonation of the vegetation (Markgraf 1993; Heusser 1989) and the distribution of the volcanic eruptions from the Mt. Burney volcano and time intervals in which the major Holocene eruptions occurred (Stern 1990, 1992). Ocean currents and the northern limit of the whole-yearly distribution of the westerlies are also shown.

Sampling and analytical procedures

Tow small lakes, Lago Martillo (350 x 550 m in diameter with 10-12 m water depth) and Lago Chandler (150 x 250 m in diameter with 15-16 m water depth) were selected for drilling. These lakes are characterized by very small tributaries of a small catchment area and are surrounded by evergreen rainforest, peatland and uncovered glacially eroded basement (Fig. 1). The lake sediments were cored stepwise by means of a 2 m long piston corer, each core overlapping 30 cm with the next deeper one. Total core lengths of 8.6 m were obtained from Lago Martillo and 6.3 m from Lago Chandler (without core overlapping). The cores were cutted into half pipes. One half was sampled in 2 cm steps, the other by overlapping 14 cm long pieces for thin sections.

The mineralogical and biogenic components of selected samples were investigated by X-ray diffraction and electron scatter images. The organic carbon and total sulfur content of the samples was determined by photometric (IR) detection of CO₂ and SO₂ after combustion of the homogenized dried and ground sample (0.5 g) in a high frequency induction furnace (CS-225 LECO). The content of the rare earth elements and U, Th and Pb was determined by inductively-coupled plasma mass spectrometry (ICP-MS). Major and some trace elements were investigated by ICP-OES.

Chemical, mineralogical and biogene composition

The lake sediments include variable amounts of organic matter from surrounding peat soils, basement weathering products and volcanic ashes. Concentrations of major and trace elements, sulfur and organic carbon were analyzed in 10 cm sections.

The sediment core of Lago Martillo (lake level is 20 m above see level) is characterized by partially laminated clayey sediments and an interchange from marine to lacustrine sedimentation at about 11 000 to 10 000 years B.P., whereas the sedimentation at Lago Chandler (lake level is 50 m above see level) was lacustrine since the Late Glacial (> 13 000 years). The two sediment cores and especially that of Lago Chandler are characterized by pronounced changes of some chemical components, related to two Holocene eruptions of the Mt. Burney volcano (Fig. 1). These chemical variations are illustrated in Fig. 2 at the case of sediments from Lago Chandler.

The Ti and Al contents which may reflect the terrigenous sediment input remain nearly constant throughout the whole Late glacial and Holocene sedimentation period, whereas S and C as well as such elements which can be transported in water solutions of lacustrine environments show significant variations which are explained in the following.

Late Glacial and Early Holocene sediments have high Pb/Th ratios (~20) and high concentrations of Pb (up to 450 ppm; Fig. 2), Mn and Fe. These elements are likely to be precipitated from the water column as Fe-Mn hydroxides (Aguilar & Nealson-Kenneth 1998). Sediments of this phase have also relatively high contents of organic carbon and sea spray-derived Br (70-150 ppm).

Fig. 2: Chemical variations in the sediment core of Lago Chandler, located near the Gran Campo Nevado in the southern Andes (Fig. 1), during the last 14 000 years. It is obvious that the ash layers of both eruptions of Mt. Burney volcano led to significant long-term changes of geochemical conditions (Lead, sulfur, bromine) and vegetation (organic carbon).

At ~8300 years B.P. the eruption of Mt. Burney volcano produced a 8-15 cm thick tephra layer, causing increased influx of mortified plant material and terrigenous detritus into the lakes. Throughout a >4000 year lasting period after the tephra rework (Fig. 2), the sediments had significantly lower concentrations of Pb (70-82 ppm), Br (~50 ppm), C_org (10-12 %), lower Pb/Th ratios (~10) and increased sulfur contents (from 0.2 to 0.4 % S), compared to the pre-eruption phase. We suggest a low lake pH for this period, due to the formation of sulfuric acid by weathering of SO₂-rich tephra deposits surrounding the lake.

After a second Mt. Burney eruption at ~3600 B.P. (Stern 1990, 1992), the organic carbon content in the sediments increased dramatically from 10 to 30 %, due to increased input of mortified allochtonous plant material, but also by increased biogene production in the lake. The increase of the carbon content is correlated with extremely elevated Br contents (200-500 ppm). The organic matter may have buffered the lake pH (relatively low S content of the lakes). The concentrations of Fe, Mn, Cu and Pb are strongly increased relative to the Ti and Al contents, suggesting that these elements were again precipitated predominantly as hydroxides during this period.

Our results show that plinian volcanic eruptions of the southern Andes could have a long term affect the lacustrine pH conditions and ecosystem. The lake pH was lowered over a long period after the first Mt. Burney eruption, due to long-term weathering of SO₂-rich tephra deposits. On the other hand the lake pH was possibly buffered after the second Mt. Burney eruption through increased organic matter influx. These variable lake pH conditions control the mobility of heavy metals as well as organically bounded bromine.

References

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Heusser, C.J. (1989), Late Quaternary vegetation and climate of Southern Tierra del Fuego.- Quaternary Research, 31: 396-406.

Markgraf, V. (1993), Paleoenvironments and paleoclimates in Tierra del Fuego and southernmost Patagonia, South America. Palaeogeography, Palaeoclimatology, Palaeoecology, 102: 53-68.

Stern, C.R. (1990), Tephrochronology of Southernmost Patagonia. National Geographic Research 6 (1): 110-126.

Stern, C.R. (1992), Tefrochronologia de Magallanes: Nuevos datos e implicaciones. Ans. Inst. Pat. Ser. Cs., Punta Arenas (Chile), 21: 129-141.