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Late Holocene tephrochronology of the northern Antarctic Peninsula
QUATERNARY
RESEARCH
36, 322-328 (1991)
Late Holocene
Tephrochronology of the Northern Antarctic Peninsula
Department of Quaternary Geology, Lund Universio, Tornav. 13, S-223 63 Lund, Sweden; and SDepartment of Geography, University of Umed, S-901 87 Umed, Sweden Received August 21, 1990 Andesitic and basaltic andesitic tephra layers are abundant in Holocene deposits from the Antarctic Peninsula. Visually discernible tephra horizons occur in three lakes on Livingston Island. Tephra in two other lakes and in a moss bank on Elephant Island, with very low ash concentrations, were detected magnetically. Deception Island is the most likely volcanic source for the tephra. With direct i4C dating, age/depth curves, and cross-correlations at least 14 tephra horizons dating to between ca. 4700 and 250 yr B.P. were identified and now form the basis for a preliminary regional tephrochronology that will be a valuable dating tool for investigating the Holocene climatic 0 1991 University of Washington. history of Antarctica.
INTRODUCTION
Within the South Shetland Islands (Fig. 1) tephra layers have been reported from lake sediment cores on King George Island (Tatur and de1 Valle, 1986; Mausbacher ef al., 1989; Matthies et al., 1990) as well as from marine sediments in the Bransfield Strait (Matthies et al., 1988). Matthies et al. (1990) found a total of 18 tephra layers in five different lakes. They radiocarbondated four eruption episodes (8700, 5200, 4000-4500, and 3100-3900 yr B.P.) and correlated two of these to marine tephras. A very important finding of Matthies et al. (1990) was that all tephra layers found on King George Island came from eruptions on Deception Island (Fig. 1). Here we report occurrences of 23 visible tephra layers from three different lakes on Byers Peninsula, Livingston Island. Another 20 tephra layers were located using magnetic analysis in sediment cores from Hidden Lake, James Ross Island, and Lake Boeckella, Hope Bay, and in a moss bank on Walker Point, Elephant Island (Fig. 1). METHODS
of samples was measured on a spinner magnetometer, first after exposure of the sample to a high magnetic field of 0.7 T resulting in a saturation magnetization (SIRM), and second after exposure to a low reversed field of 0.1 T. The S ratio (S = -0.1,/0.7,) and the mass specific high induced remanent magnetization (HIRM = (1 - S) x SIRM/2) were calculated. The clearly visible tephra layers in the Livingston Island lake sediments have significantly higher concentrations of magnetic minerals, reflected by distinct SIRM and HIRM peaks (Fig. 2) compared to the background values recorded in the sediment itself. This fact was then used to identify tephra layers where they could not be seen by eye (but were visible under the microscope) because of low ash concentrations or the dark color of the sediments. Magnetic analyses of 10 bulk tephra samples from the volcanic Deception Island show S ratios between 0.18 and - 0.11. These S ratios are very similar to the ratios found in the tephra horizons in our sediment cores and are generally much lower than the background S ratios of the sediments. LIVINGSTON
Magnetic analyses were made every 2 cm along each core. The magnetic remanence
Tephra layers were clearly visible in sed322
0033-5894/91 $3.00 Copyright 6 1991 by the University of Washington. AU rights of reproduction in any form reserved.
ISLAND TEPHRAS
323
ANTARCTICTEPHROCHRONOLOGY
Elephant
Drake
Passage I
Deceotlon
island
c
Island
, Istand
Wedell
Sea
FIG. 1. Map of the northern part of the Antarctic Peninsula. Sampled sites are marked with a dot and volcanic islands with a star. On Byers Peninsula, the westernmost part of Livingston Island, three lakes were studied. Two of these, “Lake Asa” and “Chester Cone Lake,” have provisional names.
iments from Midge Lake, “Lake Asa,” and “Chester Cone Lake” on Livingston Island and were described in detail. Typically each tephra layer has a sharp lower boundary and diffuse upper boundary where the tephra grades upward into a mixture of clay and gyttja. This could be an effect of redeposition of tephra from the surrounding catchment following the eruption. The thickest tephra Layers are AP12 (25 mm depth in “Lake Asa” and 15 mm in Midge Lake) and API0 (20 and 15 mm, respectively) . Twenty 14C ages were obtained for sediments from these three lakes. Unfortunately most of the bulk sediment ages were prone to errors causing overestimates in the 14C ages (Bjorck et al., 1991(a)). In Midge Lake, however, five accelerator mass spectrometry (AMS) ages of water mosses provided a reliable 14C chronology (Bjdrck et al., 1991(b)). Only four conventional 14C ages, three of which were for pure moss remains from “Lake Asa” (Fig. 3) and
“Chester Cone Lake,” seem reliable. With these nine ages it is possible to date all tephra horizons found in the three sites on Livingston Island. As anticipated, the same horizons were found in these three nearby lakes. Five main tephra zones (ca. 450,750, 1350, 265&2750, and 4700 yr B.P.) comprising seven different tephra horizons (AP2,3,5, 10, 11, 12, and 14 in Fig. 4) were distinguished. These horizons appear as peaks in the HIRM plots (Fig. 2). Because the sedimentation rate was higher in “Lake Asa,” the resolution was finer. Two of the horizons (AP3 and 5) are complex, consisting of two or three tephra bands separated by thin layers of clayey gyttja. The subdivision of these three horizons into separate events (eruptions) could not be made in Midge Lake or “Chester Cone Lake.” ANTARCTIC PENINSULA AND ELEPHANT ISLAND TEPHRAS
The other three sites along the Antarctic
324
BJijRCK, Lake
SANDGREN, Hidden
ha
AND ZALE Walker
Lake 0.0 0 “““‘~‘1”“““1’
25OLi
Point 0.5
1 .o
16Ou 0
40
80
0
2 HIRM
4
6
mA’kg-’
FIG. 2. HIRM (high induced remanent magnetization) records for “Lake Asa” on Livingston Island, Hidden Lake on James Ross Island, and the moss bank at Walker Point on Elephant Island. Arrows mark the presumed tephra horizons. The magnetically analyzed core from “Lake Asa” was not the same one that was originally described and radiocarbon dated (Fig. 3). Because of the 2-cm sampling interval for the magnetic analyses, the API 1 peak in “Lake Asa” (Fig. 4) is barely evident in the HIRM record because that horizon was less than 1 mm thick; it was 3 mm thick in the original core. The complex character of AP3, with double tephra layers, is clearly illustrated in the Hidden Lake record. Note the order-of-magnitude difference in the absolute values between “Lake Asa” and Hidden Lake, and Hidden Lake and Walker Point.
Peninsula with tephra layers are geographically well separated (Fig. 1). Sediments with high organic content (1530%) were 14C dated in Hidden Lake, James Ross Island (Zale and Karl&, 1989); the dates are regarded as reliable (Fig. 3). The six different tephra horizons in Hidden Lake were detected as peaks in the HIRM magnetic records (Fig. 2). The tephra horizons were dated using an interpolated sedimentation curve. In Lake Boeckella the dating problems were significant, probably because of old carbon in the bedrock and a very large input of penguin guano from an adjacent AdClie penguin rookery. Based on physical analyses and sediment chemistry, these two sources of error probably can be quantified and a correction factor estimated for each dated level (R. Zale, unpublished
data). As in Hidden Lake, the sediments are very dark and only tephra horizon AP 14 (Fig. 4) was originally described in the core. The six younger tephra horizons were identified by the HIRM peaks. Unfortunately, sediments deposited between 4500 and 2800 yr B.P. in Lake Boeckella were unavailable for analysis. Without magnetic analyses (Fig. 2), veritied by microscopy, the tephra horizons in the moss bank at Walker Point on Elephant Island would not have been detected because of their low concentration of tephra. The nine 14C ages (Bjiirck er al., in press) from this moss bank (Fig. 3) have created an excellent basis for a reliable chronology, which in turn has been used to date five tephra horizons (AP6-AP9 and AP13; Fig. 4).
ANTARCTIC
TEPHROCHRONOLOGY
AGE (I’C
325
yr BP.)
FIG. 3. Radiocarbon age vs sediment depth for three of the sites. The Walker Point accumulation curve is marked with a solid line, “Lake Asa” with dashes, and Hidden Lake with short dashes. The curve from “Lake .&a” is dated with three reliable ages in combination with the well-dated tephra horizons in nearby Midge Lake. A rapid accumulation rate between 3000 and 2000 yr B.P. is typical for the Livingston Island lakes (Bjorck et al., 1991(b)). The crosses mark the radiocarbon ages. The vertical bar denotes the sample width and the horizontal bar one standard deviation. All but one of the Walker Point ages are marked with crosses that are drawn larger, for clarity, than the values dictate. Tephra horizons are marked with circles. Note that accumulation on the moss bank at Walker Point ceased ca. 1500 yr B.P. and that “Lake Asa” is too shallow to allow recent sedimentation. The latter is supported by *“Pb and i3’Cs analyses of the surface sediments.
SOURCE
OF THE TEPHRA
The occurrence of different tephra horizons at different sites is a function of both distance to the volcanic source and wind conditions (strength and direction) during and following the eruption. There are a number of possible sources for the tephra. The nearest volcanoes (Fig. 1) with known or suspected Holocene activity are, according to LeMasurier (1990), the following: Deception Island, Bridgman Island, Penguin Island, Paulet Island, and Lindenberg Island-Mt. Christen Christensen on Robertson Island (usually referred to as Seal Nunataks). Of these, only Deception Island has had observed eruptions (e.g., Gonzalez-Ferran et al., 1971; Baker er al., 1975). The petrography of the volcanoes is similar
(Smellie, 1990a) and basalts of different types are reported from all of them. All major tephra horizons, i.e., all horizons found in the lakes on Livingston Island and horizon AP14 from Lake Boeckella, have been analyzed chemically using atomic emission spectroscopy and atomic mass spectroscopy. The analyses show that all the analyzed tephra horizons are basaltic andesitic to andesitic and the distribution pattern in Figure 5 shows that the most likely source for the tephra is Deception Island, a conclusion also reached by Matthies et al. (1990). It has many stillactive volcanoes and the most recent eruptions were in 1967, 1%9, and 1970 (Smellie, 1990b). The current prevailing wind direction in the area, from W to NW (Longton, 1985), could easily transport the tephra to
326
BJbRCK, Tephra horizons
tf;klr
SANDGREN, Chest er cone LIok e
-------1‘-----------
03
AND eke k
?!
ZALE
Hidden !!,?I
Lake -!YC.?!
Walker Point ._...................
E
AP 1 500
AP 2
-
-
-
.
AP 3 loo0
AP 4
______.
q
fi
4000
4500 -
AP14 i
5000 j
t
-------------------------
___............................-...................................------.
....I.
4. The 14 detected tephra horizons from six different sites in the Antarctic Peninsula region (Fig. 1) related to 14C years B.P. At least two, AP3 and APS, seem to consist of more than one tephra layer suggesting multiple eruptions separated by short breaks. The vertical lines show the period analyzed at each site. FIG.
our three remote sites. The similar chemical composition of our tephra layers likewise suggests a rather localized, homogeneous source, which unfortunately makes cross-correlation of the same tephra layers in different lakes ambiguous. DISCUSSION All three lakes from Byers Peninsula on Livingston Island have essentially identical records of tephra deposition (Fig. 4). This is expectable because of their proximity to each other. If Deception Island is the source of these tephra layers, they indicate either anomalous surface wind directions (from S15”E) or large eruptions capable of spreading tephra in many directions by reaching higher atmospheric levels with more northerly wind directions. The Deception Island eruption in 1970 reached the
whole of the South Shetland Islands except for the Byers Peninsula area and islands west of it (Gonzalez-Ferran, 1971). The other three sites are much more distant from Deception Island, but are situated in areas where wind directions from S5O”W (Elephant Island), N75”W (Lake Boeckella), or N45”W (Hidden Lake) are quite common. Three of the tephra horizons (APl, 8, and 13) were found at only a single site (Fig. 4). This possibly indicates short eruptions during periods of rather uniform wind directions. Four of the horizons (AP2, 3,5, and 14) were found on both Byers Peninsula and in at least one of the other three sites, which may indicate large, rather long eruptions and varying wind directions. Three of the horizons (AP6, 7, and 9) were found in all three remote sites, but not on Byers Peninsula. These horizons suggest
ANTARCTIC
Deception
0
] 44
1
I 46
I
I
327
TEPHROCHRONOLOGY
I.
I
I
52
I
56 SiOp
I
60
I
I
64
I
I
66
(%)
FIG. 5. The ratio of TiO,/K,O versus SiO, for the tephra layers in “Lake Asa” (open squares), Midge Lake (open circles), “Chester Cone Lake” (filled circles), and Lake Boeckella (filled square). These ratios are compared with the range of values for ca. 90 pure tephra samples from Deception Island (dotted area), five samples from Bridgman Island, and four samples from Penguin Island, as compiled by Matthies et al. (1990) based on data from Hawkes (1961), Baker et al. (1973, Schultz (1976), Weaver et a/. (1979), Tamey et al. (1982), and Godoy et al. (1987).
slightly smaller eruptions with less-varying wind directions. The four horizons found only on Byers Peninsula (AP5 and APlO12) may represent brief eruptions with odd wind directions or larger eruptions that mainly reached higher altitudes. Tephra from the eruptions on Deception Island in 1967, 1969, and 1970 (Smellie, 199Ob), as well as the probable eruptions in 1842 (Wilkes, 1845) and 1912-1917 (Orheim, 1972), were not found in the sections analyzed, and no convincing correlations could be made to the tephras found and dated by Matthies et al. (1990) on King George Island. Possible reasons for these correlation problems may be (i) errors in some of the dates from King George Island, (ii) that sedimentation more or less ceased in the King George Island lakes after ca. 3000 yr B.P., or (iii) an unusual dispersal pattern of the tephra. The 18 tephra layers, comprising 14 horizons, found at our sites are only a minimum figure for the number
of Deception Island eruptions that have occurred during the last 5000 years. Our work has shown that for an area measuring about 400 x 250 km around the Antarctic Peninsula, tephra horizons from Deception Island eruptions can be identitied by magnetic analyses. By correlation with our preliminary tephrochronology, tephra horizons on the Antarctic Peninsula can be used to date lake sediments, moss banks, and possibly marine sediments and ice cores as much as 5000 yr old. Thus, tephra should be a valuable dating tool in a region where problems with 14C dating seem to be common (Bjorck et al., 1991 (a)), and where palaeoclimatic research requiring accurate chronologies has increased significantly during the last decade. ACKNOWLEDGMENTS We thank A. Wasell, C. Hjort, 6. Ing6lfsson, W. Karlen, and K. Nordstrom for collaboration in field work. W. Karltn submitted the Midge Lake i4C sam-
328
EUbRCK,
SANDGREN,
ples. D. Matthies and two anonymous referees commented on the manuscript, for which we are grateful. We thank the Alfred Wegener Institut fur Polar- und Meeresforschung in Bremerhaven and the crew and helicopter pilots on R/V Polarstem for logistic support during the 1987/1988 field season. The Swedish Polar Research Secretariat and the crew and helicopter pilots on MIS Stena Arctica are acknowledged for their support during the 19880989 expedition. The Swedish Natural Science Research Council (NFR) financed most of the work.
REFERENCES Baker, P. E., McReath, M. A., Harvey, M. R., Roobol, M. J., and Davies, T. G. (1975). The geology of the South Shetland Islands. V. Volcanic evolution of Deception Island. British Antarctic Survey Scientific Reports 78. Bjdrck, S., Hjort, C., Ingolfsson, O., and Skog, G. (1991 (a)). Radiocarbon dates from the Antarctic Peninsula region-Problems and potential. Quaternary Proceedings 1. Bjorck, S., H&ansson, H., Zale, R., Karl&t, W., and Liedbetg Jiinsson, B. (1991(b)). A late Holocene lake sediment sequence from Livingstone Island, South Shetland Islands, with palaeoclimatic implications. Antarctic Science 3, 61-72. Bjiirck, S., Malmer, N., Hjort, C., Sandgren, P., Ing6lfsson, O., Wallen, B., Lewis Smith, R. I., and Liedberg-Jdnsson, B. (in press). Stratigraphic and paleoclimatic studies of a 5500 year old moss bank on Elephant Island, Antarctica, Arctic and Alpine Research. Godoy, E., Harrington, R., and Tidy, E. (1987). On the “anomalous” character of recent volcanism in the South Shetland Islands. Institudo Antcirtico Chileno. Publication, Serie Cientifca 36, 21-32. Gonztiez-Fetran, O., Munizaga, F., and Moreno, H. (1971). Sintesis de la evolution volcanica de Isla Decepcion y la eruption de 1970. Institudo Antrirtico Chileno. Publicaci6n, Serie Cienttfica II (l), l-14. Hawkes, D. D. (1961). The geology of the South Shetland Islands. I. The petrology of King George Island. Falkland Islands Dependencies Survey Scientific Reports 26. LeMasurier, W. E. (1990). Late Cenozoic Volcanism on the Antarctic Plate: An Overview. In “Volcanoes of the Antarctic Plate and Southern Oceans” (W. E. LeMasurier and J. W. Thomson, Eds.), Antarctic Research Series, Vol. 48, pp. 1-17. American Geophysical Union, Washington, DC. Longton, R. E. (1985). Terrestrial habitats-vegetation. In “Antarctica” (J. E. Treheme, Ed.), 381 pp. Pergamon, Oxford. Matthies, D., Storzer, D., and Troll, G. (1988). Volcanic ashes in Bransfield Strait sediments: Geo-
AND
ZALE
chemical and stratigraphical investigations (Antarctica). Proceedings of the Second International Conference on Natural Glasses, 139-147. Prague. Matthies, D., Mausbacher, R., and Storzer, D. (1990). Deception Island tephra: A stratigraphical marker for limnic and marine sediments in Bransfield Strait area, Antarctica. Zentralblatt fir Geologie und Pahiontologie, Teil 1, 153-165. Mausbacher, R., Miiller, J., and Schmidt, R. (1989). Evolution of postglacial sedimentation in Antarctic lakes (King George Island). Zeitschrift fur Geomorphologie N. F. 33, 219-234. Orheim, 0. (1972). Volcanic activity on Deception Island, South Shetland Islands. In “Antarctic Geology and Geophysics” (R. J. Ardie, Ed.), pp. 117120, Universitetsforlaget, Oslo. Schultz, C. H. (1976). Petrology and geochemistry of Deception Island, Antarctica. In “Proceedings of the International Symposium on Andean and Antarctic Volcanology Problems” (0. GonzalezFerran, Ed.), pp. 498-517. Rome. Smellie, J. L. (1990a). D. Graham Land and South Shetland Islands: Summary. In “Volcanoes of the Antarctic Plate and Southern Oceans” (W. E. LeMasurier and J. W. Thomson, Eds.), Antarctic Research Series, Vol. 48, pp. 303-312. American Geophysical Union, Washington, DC. Smellie, J. L. (1990b). D.2 Deception Island. In “Volcanoes of the Antarctic Plate and Southern Oceans” (W. E. LeMasurier and J. W. Thomson, Eds.), Antarctic Research Series, Vol. 48, pp. 316-321. American Geophysical Union, Washington, DC. Tarney, J., Weaver, S. D., Saunders, A. D., Pankhurst, R. J., and Barker, P. F. (1982). Volcanic evolution of the northern Antarctic Peninsula and the Scotia arc. In “Andesites: Orogenic Andesites and Related Rocks” (R. S. Thorpe, Ed.), pp. 371400. Wiley, New York. Tatur, A., and del Valle, R. (1986). Badania paleolimnologiczne i geomorfologiczne na Wyspie Krola Jerzego-Antarktyka zachodnia (1984-1986). (English summary). Preglad Geologiczny 11, 621626. Weaver, S. D., Saunders, A. D., Pankhurst, R. J., and Tamey, J. (1979). A geochemical study of magmatism associated with the initial stages of back-arc spreading: The Quatemary volcanics of Bransfield Strait, from South Shetland Islands. Contributions in Mineralogy and Petrology 68, 151-169. Wilkes, C. (1845). American Exploring Expedition: Narrative of the United States Exploring Expedition during the years 18384842 I-V, Lea and Blanchard, Philadelphia. Zale, R., and Karlen, W. (1989). Lake sediment cores from the Antarctic Peninsula and surrounding islands. Geografiska Annaler 7lA, 21 l-220.
RESEARCH
36, 322-328 (1991)
Late Holocene
Tephrochronology of the Northern Antarctic Peninsula
Department of Quaternary Geology, Lund Universio, Tornav. 13, S-223 63 Lund, Sweden; and SDepartment of Geography, University of Umed, S-901 87 Umed, Sweden Received August 21, 1990 Andesitic and basaltic andesitic tephra layers are abundant in Holocene deposits from the Antarctic Peninsula. Visually discernible tephra horizons occur in three lakes on Livingston Island. Tephra in two other lakes and in a moss bank on Elephant Island, with very low ash concentrations, were detected magnetically. Deception Island is the most likely volcanic source for the tephra. With direct i4C dating, age/depth curves, and cross-correlations at least 14 tephra horizons dating to between ca. 4700 and 250 yr B.P. were identified and now form the basis for a preliminary regional tephrochronology that will be a valuable dating tool for investigating the Holocene climatic 0 1991 University of Washington. history of Antarctica.
INTRODUCTION
Within the South Shetland Islands (Fig. 1) tephra layers have been reported from lake sediment cores on King George Island (Tatur and de1 Valle, 1986; Mausbacher ef al., 1989; Matthies et al., 1990) as well as from marine sediments in the Bransfield Strait (Matthies et al., 1988). Matthies et al. (1990) found a total of 18 tephra layers in five different lakes. They radiocarbondated four eruption episodes (8700, 5200, 4000-4500, and 3100-3900 yr B.P.) and correlated two of these to marine tephras. A very important finding of Matthies et al. (1990) was that all tephra layers found on King George Island came from eruptions on Deception Island (Fig. 1). Here we report occurrences of 23 visible tephra layers from three different lakes on Byers Peninsula, Livingston Island. Another 20 tephra layers were located using magnetic analysis in sediment cores from Hidden Lake, James Ross Island, and Lake Boeckella, Hope Bay, and in a moss bank on Walker Point, Elephant Island (Fig. 1). METHODS
of samples was measured on a spinner magnetometer, first after exposure of the sample to a high magnetic field of 0.7 T resulting in a saturation magnetization (SIRM), and second after exposure to a low reversed field of 0.1 T. The S ratio (S = -0.1,/0.7,) and the mass specific high induced remanent magnetization (HIRM = (1 - S) x SIRM/2) were calculated. The clearly visible tephra layers in the Livingston Island lake sediments have significantly higher concentrations of magnetic minerals, reflected by distinct SIRM and HIRM peaks (Fig. 2) compared to the background values recorded in the sediment itself. This fact was then used to identify tephra layers where they could not be seen by eye (but were visible under the microscope) because of low ash concentrations or the dark color of the sediments. Magnetic analyses of 10 bulk tephra samples from the volcanic Deception Island show S ratios between 0.18 and - 0.11. These S ratios are very similar to the ratios found in the tephra horizons in our sediment cores and are generally much lower than the background S ratios of the sediments. LIVINGSTON
Magnetic analyses were made every 2 cm along each core. The magnetic remanence
Tephra layers were clearly visible in sed322
0033-5894/91 $3.00 Copyright 6 1991 by the University of Washington. AU rights of reproduction in any form reserved.
ISLAND TEPHRAS
323
ANTARCTICTEPHROCHRONOLOGY
Elephant
Drake
Passage I
Deceotlon
island
c
Island
, Istand
Wedell
Sea
FIG. 1. Map of the northern part of the Antarctic Peninsula. Sampled sites are marked with a dot and volcanic islands with a star. On Byers Peninsula, the westernmost part of Livingston Island, three lakes were studied. Two of these, “Lake Asa” and “Chester Cone Lake,” have provisional names.
iments from Midge Lake, “Lake Asa,” and “Chester Cone Lake” on Livingston Island and were described in detail. Typically each tephra layer has a sharp lower boundary and diffuse upper boundary where the tephra grades upward into a mixture of clay and gyttja. This could be an effect of redeposition of tephra from the surrounding catchment following the eruption. The thickest tephra Layers are AP12 (25 mm depth in “Lake Asa” and 15 mm in Midge Lake) and API0 (20 and 15 mm, respectively) . Twenty 14C ages were obtained for sediments from these three lakes. Unfortunately most of the bulk sediment ages were prone to errors causing overestimates in the 14C ages (Bjorck et al., 1991(a)). In Midge Lake, however, five accelerator mass spectrometry (AMS) ages of water mosses provided a reliable 14C chronology (Bjdrck et al., 1991(b)). Only four conventional 14C ages, three of which were for pure moss remains from “Lake Asa” (Fig. 3) and
“Chester Cone Lake,” seem reliable. With these nine ages it is possible to date all tephra horizons found in the three sites on Livingston Island. As anticipated, the same horizons were found in these three nearby lakes. Five main tephra zones (ca. 450,750, 1350, 265&2750, and 4700 yr B.P.) comprising seven different tephra horizons (AP2,3,5, 10, 11, 12, and 14 in Fig. 4) were distinguished. These horizons appear as peaks in the HIRM plots (Fig. 2). Because the sedimentation rate was higher in “Lake Asa,” the resolution was finer. Two of the horizons (AP3 and 5) are complex, consisting of two or three tephra bands separated by thin layers of clayey gyttja. The subdivision of these three horizons into separate events (eruptions) could not be made in Midge Lake or “Chester Cone Lake.” ANTARCTIC PENINSULA AND ELEPHANT ISLAND TEPHRAS
The other three sites along the Antarctic
324
BJijRCK, Lake
SANDGREN, Hidden
ha
AND ZALE Walker
Lake 0.0 0 “““‘~‘1”“““1’
25OLi
Point 0.5
1 .o
16Ou 0
40
80
0
2 HIRM
4
6
mA’kg-’
FIG. 2. HIRM (high induced remanent magnetization) records for “Lake Asa” on Livingston Island, Hidden Lake on James Ross Island, and the moss bank at Walker Point on Elephant Island. Arrows mark the presumed tephra horizons. The magnetically analyzed core from “Lake Asa” was not the same one that was originally described and radiocarbon dated (Fig. 3). Because of the 2-cm sampling interval for the magnetic analyses, the API 1 peak in “Lake Asa” (Fig. 4) is barely evident in the HIRM record because that horizon was less than 1 mm thick; it was 3 mm thick in the original core. The complex character of AP3, with double tephra layers, is clearly illustrated in the Hidden Lake record. Note the order-of-magnitude difference in the absolute values between “Lake Asa” and Hidden Lake, and Hidden Lake and Walker Point.
Peninsula with tephra layers are geographically well separated (Fig. 1). Sediments with high organic content (1530%) were 14C dated in Hidden Lake, James Ross Island (Zale and Karl&, 1989); the dates are regarded as reliable (Fig. 3). The six different tephra horizons in Hidden Lake were detected as peaks in the HIRM magnetic records (Fig. 2). The tephra horizons were dated using an interpolated sedimentation curve. In Lake Boeckella the dating problems were significant, probably because of old carbon in the bedrock and a very large input of penguin guano from an adjacent AdClie penguin rookery. Based on physical analyses and sediment chemistry, these two sources of error probably can be quantified and a correction factor estimated for each dated level (R. Zale, unpublished
data). As in Hidden Lake, the sediments are very dark and only tephra horizon AP 14 (Fig. 4) was originally described in the core. The six younger tephra horizons were identified by the HIRM peaks. Unfortunately, sediments deposited between 4500 and 2800 yr B.P. in Lake Boeckella were unavailable for analysis. Without magnetic analyses (Fig. 2), veritied by microscopy, the tephra horizons in the moss bank at Walker Point on Elephant Island would not have been detected because of their low concentration of tephra. The nine 14C ages (Bjiirck er al., in press) from this moss bank (Fig. 3) have created an excellent basis for a reliable chronology, which in turn has been used to date five tephra horizons (AP6-AP9 and AP13; Fig. 4).
ANTARCTIC
TEPHROCHRONOLOGY
AGE (I’C
325
yr BP.)
FIG. 3. Radiocarbon age vs sediment depth for three of the sites. The Walker Point accumulation curve is marked with a solid line, “Lake Asa” with dashes, and Hidden Lake with short dashes. The curve from “Lake .&a” is dated with three reliable ages in combination with the well-dated tephra horizons in nearby Midge Lake. A rapid accumulation rate between 3000 and 2000 yr B.P. is typical for the Livingston Island lakes (Bjorck et al., 1991(b)). The crosses mark the radiocarbon ages. The vertical bar denotes the sample width and the horizontal bar one standard deviation. All but one of the Walker Point ages are marked with crosses that are drawn larger, for clarity, than the values dictate. Tephra horizons are marked with circles. Note that accumulation on the moss bank at Walker Point ceased ca. 1500 yr B.P. and that “Lake Asa” is too shallow to allow recent sedimentation. The latter is supported by *“Pb and i3’Cs analyses of the surface sediments.
SOURCE
OF THE TEPHRA
The occurrence of different tephra horizons at different sites is a function of both distance to the volcanic source and wind conditions (strength and direction) during and following the eruption. There are a number of possible sources for the tephra. The nearest volcanoes (Fig. 1) with known or suspected Holocene activity are, according to LeMasurier (1990), the following: Deception Island, Bridgman Island, Penguin Island, Paulet Island, and Lindenberg Island-Mt. Christen Christensen on Robertson Island (usually referred to as Seal Nunataks). Of these, only Deception Island has had observed eruptions (e.g., Gonzalez-Ferran et al., 1971; Baker er al., 1975). The petrography of the volcanoes is similar
(Smellie, 1990a) and basalts of different types are reported from all of them. All major tephra horizons, i.e., all horizons found in the lakes on Livingston Island and horizon AP14 from Lake Boeckella, have been analyzed chemically using atomic emission spectroscopy and atomic mass spectroscopy. The analyses show that all the analyzed tephra horizons are basaltic andesitic to andesitic and the distribution pattern in Figure 5 shows that the most likely source for the tephra is Deception Island, a conclusion also reached by Matthies et al. (1990). It has many stillactive volcanoes and the most recent eruptions were in 1967, 1%9, and 1970 (Smellie, 1990b). The current prevailing wind direction in the area, from W to NW (Longton, 1985), could easily transport the tephra to
326
BJbRCK, Tephra horizons
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4. The 14 detected tephra horizons from six different sites in the Antarctic Peninsula region (Fig. 1) related to 14C years B.P. At least two, AP3 and APS, seem to consist of more than one tephra layer suggesting multiple eruptions separated by short breaks. The vertical lines show the period analyzed at each site. FIG.
our three remote sites. The similar chemical composition of our tephra layers likewise suggests a rather localized, homogeneous source, which unfortunately makes cross-correlation of the same tephra layers in different lakes ambiguous. DISCUSSION All three lakes from Byers Peninsula on Livingston Island have essentially identical records of tephra deposition (Fig. 4). This is expectable because of their proximity to each other. If Deception Island is the source of these tephra layers, they indicate either anomalous surface wind directions (from S15”E) or large eruptions capable of spreading tephra in many directions by reaching higher atmospheric levels with more northerly wind directions. The Deception Island eruption in 1970 reached the
whole of the South Shetland Islands except for the Byers Peninsula area and islands west of it (Gonzalez-Ferran, 1971). The other three sites are much more distant from Deception Island, but are situated in areas where wind directions from S5O”W (Elephant Island), N75”W (Lake Boeckella), or N45”W (Hidden Lake) are quite common. Three of the tephra horizons (APl, 8, and 13) were found at only a single site (Fig. 4). This possibly indicates short eruptions during periods of rather uniform wind directions. Four of the horizons (AP2, 3,5, and 14) were found on both Byers Peninsula and in at least one of the other three sites, which may indicate large, rather long eruptions and varying wind directions. Three of the horizons (AP6, 7, and 9) were found in all three remote sites, but not on Byers Peninsula. These horizons suggest
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327
TEPHROCHRONOLOGY
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I
52
I
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FIG. 5. The ratio of TiO,/K,O versus SiO, for the tephra layers in “Lake Asa” (open squares), Midge Lake (open circles), “Chester Cone Lake” (filled circles), and Lake Boeckella (filled square). These ratios are compared with the range of values for ca. 90 pure tephra samples from Deception Island (dotted area), five samples from Bridgman Island, and four samples from Penguin Island, as compiled by Matthies et al. (1990) based on data from Hawkes (1961), Baker et al. (1973, Schultz (1976), Weaver et a/. (1979), Tamey et al. (1982), and Godoy et al. (1987).
slightly smaller eruptions with less-varying wind directions. The four horizons found only on Byers Peninsula (AP5 and APlO12) may represent brief eruptions with odd wind directions or larger eruptions that mainly reached higher altitudes. Tephra from the eruptions on Deception Island in 1967, 1969, and 1970 (Smellie, 199Ob), as well as the probable eruptions in 1842 (Wilkes, 1845) and 1912-1917 (Orheim, 1972), were not found in the sections analyzed, and no convincing correlations could be made to the tephras found and dated by Matthies et al. (1990) on King George Island. Possible reasons for these correlation problems may be (i) errors in some of the dates from King George Island, (ii) that sedimentation more or less ceased in the King George Island lakes after ca. 3000 yr B.P., or (iii) an unusual dispersal pattern of the tephra. The 18 tephra layers, comprising 14 horizons, found at our sites are only a minimum figure for the number
of Deception Island eruptions that have occurred during the last 5000 years. Our work has shown that for an area measuring about 400 x 250 km around the Antarctic Peninsula, tephra horizons from Deception Island eruptions can be identitied by magnetic analyses. By correlation with our preliminary tephrochronology, tephra horizons on the Antarctic Peninsula can be used to date lake sediments, moss banks, and possibly marine sediments and ice cores as much as 5000 yr old. Thus, tephra should be a valuable dating tool in a region where problems with 14C dating seem to be common (Bjorck et al., 1991 (a)), and where palaeoclimatic research requiring accurate chronologies has increased significantly during the last decade. ACKNOWLEDGMENTS We thank A. Wasell, C. Hjort, 6. Ing6lfsson, W. Karlen, and K. Nordstrom for collaboration in field work. W. Karltn submitted the Midge Lake i4C sam-
328
EUbRCK,
SANDGREN,
ples. D. Matthies and two anonymous referees commented on the manuscript, for which we are grateful. We thank the Alfred Wegener Institut fur Polar- und Meeresforschung in Bremerhaven and the crew and helicopter pilots on R/V Polarstem for logistic support during the 1987/1988 field season. The Swedish Polar Research Secretariat and the crew and helicopter pilots on MIS Stena Arctica are acknowledged for their support during the 19880989 expedition. The Swedish Natural Science Research Council (NFR) financed most of the work.
REFERENCES Baker, P. E., McReath, M. A., Harvey, M. R., Roobol, M. J., and Davies, T. G. (1975). The geology of the South Shetland Islands. V. Volcanic evolution of Deception Island. British Antarctic Survey Scientific Reports 78. Bjdrck, S., Hjort, C., Ingolfsson, O., and Skog, G. (1991 (a)). Radiocarbon dates from the Antarctic Peninsula region-Problems and potential. Quaternary Proceedings 1. Bjorck, S., H&ansson, H., Zale, R., Karl&t, W., and Liedbetg Jiinsson, B. (1991(b)). A late Holocene lake sediment sequence from Livingstone Island, South Shetland Islands, with palaeoclimatic implications. Antarctic Science 3, 61-72. Bjiirck, S., Malmer, N., Hjort, C., Sandgren, P., Ing6lfsson, O., Wallen, B., Lewis Smith, R. I., and Liedberg-Jdnsson, B. (in press). Stratigraphic and paleoclimatic studies of a 5500 year old moss bank on Elephant Island, Antarctica, Arctic and Alpine Research. Godoy, E., Harrington, R., and Tidy, E. (1987). On the “anomalous” character of recent volcanism in the South Shetland Islands. Institudo Antcirtico Chileno. Publication, Serie Cientifca 36, 21-32. Gonztiez-Fetran, O., Munizaga, F., and Moreno, H. (1971). Sintesis de la evolution volcanica de Isla Decepcion y la eruption de 1970. Institudo Antrirtico Chileno. Publicaci6n, Serie Cienttfica II (l), l-14. Hawkes, D. D. (1961). The geology of the South Shetland Islands. I. The petrology of King George Island. Falkland Islands Dependencies Survey Scientific Reports 26. LeMasurier, W. E. (1990). Late Cenozoic Volcanism on the Antarctic Plate: An Overview. In “Volcanoes of the Antarctic Plate and Southern Oceans” (W. E. LeMasurier and J. W. Thomson, Eds.), Antarctic Research Series, Vol. 48, pp. 1-17. American Geophysical Union, Washington, DC. Longton, R. E. (1985). Terrestrial habitats-vegetation. In “Antarctica” (J. E. Treheme, Ed.), 381 pp. Pergamon, Oxford. Matthies, D., Storzer, D., and Troll, G. (1988). Volcanic ashes in Bransfield Strait sediments: Geo-
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chemical and stratigraphical investigations (Antarctica). Proceedings of the Second International Conference on Natural Glasses, 139-147. Prague. Matthies, D., Mausbacher, R., and Storzer, D. (1990). Deception Island tephra: A stratigraphical marker for limnic and marine sediments in Bransfield Strait area, Antarctica. Zentralblatt fir Geologie und Pahiontologie, Teil 1, 153-165. Mausbacher, R., Miiller, J., and Schmidt, R. (1989). Evolution of postglacial sedimentation in Antarctic lakes (King George Island). Zeitschrift fur Geomorphologie N. F. 33, 219-234. Orheim, 0. (1972). Volcanic activity on Deception Island, South Shetland Islands. In “Antarctic Geology and Geophysics” (R. J. Ardie, Ed.), pp. 117120, Universitetsforlaget, Oslo. Schultz, C. H. (1976). Petrology and geochemistry of Deception Island, Antarctica. In “Proceedings of the International Symposium on Andean and Antarctic Volcanology Problems” (0. GonzalezFerran, Ed.), pp. 498-517. Rome. Smellie, J. L. (1990a). D. Graham Land and South Shetland Islands: Summary. In “Volcanoes of the Antarctic Plate and Southern Oceans” (W. E. LeMasurier and J. W. Thomson, Eds.), Antarctic Research Series, Vol. 48, pp. 303-312. American Geophysical Union, Washington, DC. Smellie, J. L. (1990b). D.2 Deception Island. In “Volcanoes of the Antarctic Plate and Southern Oceans” (W. E. LeMasurier and J. W. Thomson, Eds.), Antarctic Research Series, Vol. 48, pp. 316-321. American Geophysical Union, Washington, DC. Tarney, J., Weaver, S. D., Saunders, A. D., Pankhurst, R. J., and Barker, P. F. (1982). Volcanic evolution of the northern Antarctic Peninsula and the Scotia arc. In “Andesites: Orogenic Andesites and Related Rocks” (R. S. Thorpe, Ed.), pp. 371400. Wiley, New York. Tatur, A., and del Valle, R. (1986). Badania paleolimnologiczne i geomorfologiczne na Wyspie Krola Jerzego-Antarktyka zachodnia (1984-1986). (English summary). Preglad Geologiczny 11, 621626. Weaver, S. D., Saunders, A. D., Pankhurst, R. J., and Tamey, J. (1979). A geochemical study of magmatism associated with the initial stages of back-arc spreading: The Quatemary volcanics of Bransfield Strait, from South Shetland Islands. Contributions in Mineralogy and Petrology 68, 151-169. Wilkes, C. (1845). American Exploring Expedition: Narrative of the United States Exploring Expedition during the years 18384842 I-V, Lea and Blanchard, Philadelphia. Zale, R., and Karlen, W. (1989). Lake sediment cores from the Antarctic Peninsula and surrounding islands. Geografiska Annaler 7lA, 21 l-220.