Cosmogenic nuclide measurements in southernmost South Americaand implications for landscape change
M.R. Kaplan a,⁎, A. Coronato b, N.R.J. Hulton a, J.O. Rabassa b,
P.W. Kubik c, S.P.H.T. Freeman d
a School of GeoSciences, University of Edinburgh, Edinburgh, EH8 9XP, Scotland, UK
b Laboratorio de Geología del Cuaternario, CADIC-CONICET, Ushuaia 9410, Tierra del Fuego, Argentina
c Paul Scherrer Institut, c/o Institute of Particle Physics, HPK H30, ETH Hoenggerberg, CH-8093 Zurich, Switzerland
d Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, UK
Received 7 September 2005; received in revised form 28 September 2006; accepted 1 October 2006
Available online 28 November 2006
We measured in situ 10Be, 26Al and 36Cl on glacial deposits as old as 1.1 Myr in the southernmost part of Patagonia and on northern Tierra del Fuego to understand boulder and moraine and, by inference, landscape changes. Nuclide concentrations indicate that surface boulders have been exposed for far less time than the ages of moraines they sit upon. The moraine ages are themselves constrained by previously obtained 40Ar/39Ar ages on interbedded lava flows or U-series and amino acid measurements on related
(non-glacial) marine deposits. We suggest that a combination of boulder erosion and their exhumation from the moraine matrix could cause the erratics to have a large age variance and often short exposure histories, despite the fact that some moraine landforms are demonstrably 1 Myr old. We hypothesize that fast or episodic rates of landscape change occurred during glacial times or near the sea during interglacials. Comparison with boulder erosion rates and exhumation histories derived for the middle latitudes of semi-arid Patagonia imply different geomorphic processes operating in southernmost South America. We infer a faster rate of landscape degradation towards the higher latitudes where conditions have been colder and wetter.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Cosmogenic; Geomorphology; Quaternary; Glacial geology; South America; Argentina
Surface exposure measurements using in situ cosmogenic
nuclides in southern South America allow study of
the timing and rates of Earth surface processes well
beyond the limits of the radiocarbon method. We investigated
the exposure history of boulders on moraines
older than the last glacial maximum (LGM, ∼25 to
17 ka; Kaplan et al., 2004; McCulloch et al., 2005) in the
southernmost part of Patagonia and on northern Tierra
del Fuego. Our investigation indicates that in these areas
cosmogenic nuclide data from erratics located on glacial
drift deposits cannot be used reliably to constrain glacial
chronologies of landforms older than ∼25 ka. This
finding differs from that in central Patagonia (Kaplan
et al., 2005), despite the semi-arid climate and
preservation of original morphology in the area (Ercolano
et al., 2004), but is similar to other studies on ‘old’
moraines (cf., Putkonen and O'Neal, 2006). On the other
hand, the data provide quasi-quantitative information on
the rates of moraine degradation, boulder exhumation
and erosion, and thus geomorphic processes in this region.
Such knowledge has potentially a wide range of
implications, from understanding South American landscape
development to offshore studies of the products of
continental erosion.
www.elsevier.com/locate/geomorph
⁎ Corresponding author. Present address: Geochemistry, LDEO,
Columbia University, Palisades, NY 10964, USA. Tel.: +1 8453658652.
E-mail address: mkaplan@ldeo.ed.ac.uk (M.R. Kaplan).
0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
Southern South America has the longest, most
complete record of Quaternary glaciations in the world
outside Antarctica (Mercer, 1983; Clapperton, 1993;
Coronato et al., 2004a; Rabassa et al., 2005). Figs. 1 and 2
show the boundaries of former glaciations in Patagonia
and Tierra del Fuego, including the maximum extent of
Fig. 2. Maps of Fuego–Southernmost Patagonia with major glacial limits (Meglioli, 1992; Rabassa et al., 2000; Bentley et al., 2005), locations of all
samples measured in this study, and locations of independent age constraints, given on Shuttle Radar Topography Mission (STRM) 3 arc sec shaded
relief elevation data (http://gisdata.usgs.net/Website/Seamless/). A. Overview of three former ice lobes and 5 major glacial units correlated across the
area. The names of the units are for the Magellan lobe (see Table 1). Area of B, C, and D outlined. B. Detailed map of north side of outer Magellan
Strait showing limits of glacial drifts and age data. Also shown in inset is histogram of ages. Samples from moraines constrained in age with limiting
40Ar/39Ar dated lava flows have filled bars. BV = Bella Vista; RC = Rio Ciaike; MA = Monte Aymond; CP = Cerro la Pirca; SA = Segunda
Angostura; PD = Punta Delgada; CV = Cabo Vírgenes; and SF = Sierra de Los Frailes. C. Detailed map showing details of glacial limits and age data
obtained on Tierra del Fuego. Inset shows ages for the Sierras de San Sebastián and Río Cullen drifts—note that the age Y-scale changes and there are
two ages N50 ka. D. Focus on the Bahía Inútil–San Sebastián ancient ice lobe, its associated drift units, and related interglacial materials (see text).
The boulder field near Punta Sinaí with 10Be dates (Fig. 2C) is located in the hummocky Río Cullen moraine, at present forming an active sea-cliff on
its eastern edge. Southwestward of Punta Sinaí the two glaciofluvial fans (solid outline) were formed by paraglacial (e.g., glaciofluvial) processes
during the Río Cullen and San Sebastián glaciations, which provided gravels for the later marine depositional processes. The area was occupied by the
sea during MIS 11, 7–9 and 5e interglacials and the Middle Holocene, forming – from the present coast to inland – gravel ridges, beaches and
estuarine environments. During MIS 5e the best preserved marine raised terrace (La Sara Fm.) was deposited (after Meglioli, 1992; Coronato et al.,
1999; Bujalesky et al., 2001).
former ice, the Greatest Patagonian Glaciation (GPG).
Interbedded lava flows and glacial deposits have allowed
application of 40Ar/39Ar, K-Ar and cosmogenic dating
techniques, which provide, or at least constrain, the ages
of the different glacial units and events that have occurred
in southern South America over the Late Tertiary and
Quaternary Periods (Mercer, 1983; Singer et al., 1997,
2004; Kaplan et al., 2005). The focus of this investigation
is the glacial record in Fuego–southernmost Patagonia.
Three former ice lobes existed in southernmost
Patagonia/northern Tierra del Fuego, which are discussed
throughout this paper. From north to south, these are the
Rio Gallegos, Magallen and Bahía Inútil–San Sebastián
ice lobes (Fig. 2; Table 1). Five distinct glacigenic
(stratigraphic) deposits are associated with each of the
three lobes, which were defined by Meglioli (1992) who
built upon the work of Caldenius (1932). Mapping
presented here (Table 1; Fig. 2) is based on their work.
Meglioli (1992) and others (e.g., Rabassa et al., 2000;
Coronato et al., 2004b) correlated the five different glacial
deposits based on a suite of relative weathering indexes
and chronological constraints. These include relationship
of outwash deposits to moraines, moraine morphology,
physical characteristics of the till (color, oxidation, depth
of weathering, grussfication of clasts, weathering of
pebbles), till boulder frequency and weathering, cryogenic
features (presence and size of frost wedge casts), soil
thickness, and logical reconstruction of ice lobe geometry
(oldest extents have little or no obvious relation to frontal
topography). Themajor glaciations represented by the five
units are ‘nested’ within each other (Fig. 2), the younger
ones at inner and/or lower positions in the landscape. The
chronology used to constrain the age of the deposits
includes radioisotopic dates on interbedded lava flows, and
amino acid racimization (AAR), U-series, and infinite
radiocarbon dates on fossil material in marine sediments
morpho-stratigraphically related to the drift units.
The oldest glacial drift is associated with the GPG,
which is well-constrained in age in the area studied
(Mercer, 1983; Meglioli, 1992; Ton-That et al., 1999;
Rabassa et al., 2000; Singer et al., 2004). 40Ar/39Ar ages
of 1168±14 and 1070±20 (Fig. 2B; Bella Vista (BV)
and Río Cullen (RC) flows) on basalts above and below
the Bella Vista drift (which was deposited by the Rio
Gallegos lobe during the GPG) provide maximum and
minimum ages, respectively (Meglioli, 1992; Singer
et al., 2004). Singer et al. (2004) provided an additional
minimum 40Ar/39Ar age of 1016±14 ka on a lava flow
overlying GPG drift at Lago Buenos Aires (LBA)
∼1000 km to the north (Fig. 1), indicating an age of the
GPG of ∼1.1 Myr considering all the sites. Other local
minimum ages for the GPG drift are provided (Meglioli,
1992) by three dated basalt flows, at 320±20, 310±30,
360±40, and 450±100 ka, which stratigraphically
overlie the Sierra de Los Frailes Drift, the GPG
equivalent for the Magellen lobe (i.e., a correlative to
the Bella Vista drift of the Rio Gallegos lobe, Table 1);
the two ‘oldest minimum’ 40Ar/39A ages for the GPG
Sierra de Los Frailes Drift, on the Monte Aymond (MA)
and Cerro la Pirca (CP) lava flows, are shown on
Fig. 2B.
The first major drift unit younger than GPG drift in
Fuego–southernmost Patagonia includes the Cabo Vírgenes
drift for the Magellan lobe (and its correlatives;
Table 1). This drift is also constrained in age with
minimum 40Ar/39Ar ages of 450±100 and 360±40 ka on
the Monte Aymond and Cerro la Pirca lava flows (Fig. 2)
north of the Strait of Magellan (Meglioli, 1992). The next
two younger drift units, e.g., the Punta Delgada and
Primera Angostura drifts for the Magellan lobe, are only
indirectly dated with two 14C ages of 47 and 45 14CkaBP
in shelly till just north of Punta Arenas. These radiocarbon
dates are assumed to be infinite and therefore minimum
ages for these two drift units and the glaciations they
represent (Porter, 1990; Clapperton et al., 1995). AAR
data also strongly support a pre-LGMage for the Primera
Angostura and, thus, older moraine material (Meglioli,
1992; Clapperton et al., 1995; McCulloch et al., 2005). In
addition, a suite of relative dating methods, including soil
development and weathering rind thickness, and topographic
setting, suggest that at least a glacial cycle
separates the Primera Angostura (and equivalents) from
the younger Segunda Angostura glacial record deposited
during marine oxygen isotope stage (MIS) 2 (Porter,
1990; Meglioli, 1992). Based on the 14C, AAR,
topographic setting, and relative weathering data the
Primera Angostura has been assumed to be 150 ka (i.e.,
MIS 6) and the Punta Delgada drift at least one glacial
cycle older (e.g., Rabassa et al., 2000). The youngest
major drift unit in Fuego–southernmost Patagonia, e.g.,
Segunda Angostura Drift in the Magellan area, corresponds
to the LGM, ca. 25 to 17.6 ka (Clapperton et al.,
1995; Sugden, 2005; McCulloch et al., 2005; Kaplan
et al., in preparation).
The deposits and ages described above were for the
Rio Gallegos and Strait of Magellan lobes. South of the
Magellan Straits, the third lobe of interest, the Bahía
Inútil–San Sebastián lobe, deposited four major drift
units on both sides of this depression; namely, from
older to younger, Río Cullen, San Sebastián, Lagunas
Secas and Bahía Inútil drifts. All these units are
morainic systems and younger than the GPG, represented
locally by the Pampa de Beta Drift, which is
preserved only as till remnants on the higher tablelands
(Meglioli, 1992; Rabassa et al., 2000; Coronato et al.,
2004b). Areas focused on in this study include near
Punta Sinaí and Chorrillos, which are on the Río Cullen
and San Sebastián drifts (Fig. 2D), respectively. Given
the lack of lava flows south of the Strait of Magellan
there are no 40Ar/39Ar age constraints on these drifts.
However, age constraints on the glacial deposits are
provided by AAR and U-series data on (interglacial)
raised marine terraces and geomorphological analysis of
glaciofluvial fans, as suggested by Bujalesky et al.
(2001), in addition to the correlations between the three
lobes described above. The Río Cullen Drift is
considered to be younger than the GPG (ca. 1 Ma)
and older than an interglacial marine terrace dated at ca.
400–600 ka (U-series on marine shells). The San
Sebastián Drift is assumed to be N300 ka and b400–
600 ka, based also on U-series ages of the related
interglacial marine terraces. During the last interglacial,
a marine terrace composed of La Sara Formation
(Fig 2C), of identified Sangamon age (MIS 5e;
Codignoto and Malumian, 1981; Rutter et al., 1989;
Bujalesky et al., 2001), developed when the sea eroded
the margins of the Río Cullen and San Sebastián
moraines and extensive marine beaches were deposited
south of Punta Sinaí. The two other, younger drifts
(Lagunas Secas and Bahía Inútil Drifts) are found much
farther west of the Punta Sinaí area and away from the
interglacial terraces. They are correlated with the
penultimate and last glaciations, respectively (Rabassa
et al., 2000; McCulloch et al., 2005).
To summarize, across Fuego–southernmost Patagonia
five different units, for three different lobes, shown
in Table 1 and on Fig. 2, have been correlated to each
other based on a suite of techniques and dates. These
include, for example, position from the mountains,
relation to topography (e.g., post Cabo Vírgenes there
were ‘distinct’ ice lobes along easternmost area), relative
weathering features, absolute ages on interbedded lava
flows, and infinite 14C, AAR, and U-series data on
shells from related interglacial or marine sediments.
3. Sampling and methods
Sampling for surface exposure measurements was
targeted at moraine crests or drift units older than the
LGM (Table 2). A multi-nuclide approach allowed
‘checks’ on the different isotopic chronometers, specifically
10Be which is used mainly. All the erratics sampled
were quartzites, granites or granodiorites originating from
within the Andes. 10Be concentrations were measured on
a quartzite and granite sample (BV-04-01 and 03) from
two different moraine crests on the Bella Vista drift,
deposited by the Río Gallegos ice lobe (Fig. 2). 10Be was
measured on one erratic from the Cabo Vírgenes Drift
(TF-04-08), and 10Be, 36Cl and 26Al on ten erratics from
the equivalent Río Cullen drift (RC-04-01 to -07 and TF-
04-01 to -03) associated with the adjacent ice lobe,
including seven samples from near Punta Sinaí (Fig. 2A).
The Punta Sinaí samples are all granodiorites (D.
Acevedo, personal communication). Two samples were
collected fromthe San Sierras de San Sebastián Drift; one
sample was measured from Punta Delgada drift (SM-02-
31); and four from the younger Primera Angostura drift
(TF-04-07, TF-04-09 to -11). 10Be, 36Cl, and 26Al were
also measured in three erratics from a kame terrace just
southeast of Bahía Inútil (TF-04-04 to -06), which we
assumed from field evidence to be part of pre-LGM
Laguna Seca drift (Bentley et al., 2005).
We preferentially selected the largest boulders
available on moraine crests. Samples were collected
with hammer and chisel from the upper few cm of
moraine boulders, at least 5 cm from edges and sharp
facets. Elevations were recorded using a GPS; based on
comparisons (when possible) to elevations recorded with
a barometric-based altimeter standardized to sea level,
GPS elevation accuracy is estimated to be within 10–
20 m. Snow cover is not corrected for, but it is assumed to
have had an insignificant effect on ages (b1%; presently,
mean annual precipitation b 500mmwith no appreciable
snow accumulation) at least during interglacial times.
For 10Be and 26Al, quartz was separated and purified at
the University of Edinburgh following the method outlined
in Bierman et al. (2002) and Ivy-Ochs (1996). 10Be/Be and
26Al/Al isotope ratios were measured at theAMS facility in
Zurich operated jointly by the Paul Scherrer Institut and
ETH Zurich, or the Scottish Universities Environmental
Research Centre (Tables 3 and 4).
For 36Cl analyses, the four measurements in Table 2
are on the same erratics as for the 10Be and 26Al analyses.
Initial processing was at the University of Edinburgh and
the samples were prepared so that the measurement was
done on either the quartz or quartz–feldspar fraction
(Table 6). The pure quartz sample (TF-04-04), which
was measured to try to isolate the neutron activation
pathway (cf., Liu et al., 1994), barely produced a measurement
(1σ is N50%), perhaps due to a lack of target
35Cl. Three other quartz-rich samples were also prepared
(for RC-04-04, RC-04-03, and TF-04-06), but these
produced no measurable 36Cl, and are not presented. The
three other samples in Table 2 (TF-04-05, RC-04-01 and
TF-04-04f) were mainly quartz, but they did contain
some K-bearing grains and these provided measurable
36Cl/Cl (Tables 5 and 6) due to spallation production.
Non-quartz grains remained in the two samples TF-04-
05 and RC-04-01 because they were not leached long
enough in Hydrofluoric/Nitric acids. For TF-04-04f,
some feldspar grains were purposely left in the sample to
ensure a 36Cl AMS measurement, given the uncertainty
of ‘pure’ quartz having enough target 35Cl. Final preparation
of samples for 36Cl/Cl, including dissolution
and extraction, followed methods outlined in Stone et al.
(1996a) at the Cosmogenic Isotope Laboratory at the
University of Washington. Before dissolution, a ∼1 g
aliquot was taken for ICP analyses (Table 6), and 37Cl
added to the remaining sample. 36Cl/35Cl and 35Cl/37Cl
were measured at the Center for Accelerator Mass
Spectrometry at Lawrence Livermore National Laboratory.
Target element concentrations and the neutron
production and capture properties of samples analysed
for 36Cl are based on XRF analyses of major elements
and ICP–MS analyses of trace Gd and Sm (Table 5). B
was measured by ICP–OES. U and Th were not measured
and were assumed to be 3±1 and 15±3, respectively
(Table 5).
For age calculations, we use the 10Be–26Al exposure
age calculator at http://hess.ess.washington.edu-/math/
al_be_stable/al_be_multiple.php. This program uses a
global production rate of 4.98 ± 0.34 atoms/g/yr for 10Be
(±2σ; sea level and high latitude and standard atmosphere;
Gosse and Stone, 2001). Production by neutron
spallation and muons are 4.87 ± 0.33 and 0.11 ±
0.01 atoms/g/yr, respectively. The 26Al production rate
used is based on the production ratio 26Al:10Be of 6.5±
0.4 (Kubik et al., 1998) with a muogenic component of
0.022% at sea level (Stone, 2000). Production rates for
all cosmogenic nuclides are corrected for a local air
pressure of 1002 mbar and annual temperature of 281 K
at sea level (Taljaard et al., 1969; Gosse and Stone,
2001). For ages N100 ka (only 10Be was measured on
samples exposed for this length of time), we increase the
spallation production rate by 11% given the evidence for
a higher long-term rate in southern Patagonia (Ackert
et al., 2003), although we emphasize that using the
different production rate decreases the ages by ∼10%
and has no effect on our interpretations and main
conclusions. 36Cl production rates are calculated using
sea-level, high latitude values of 48.8 and 4.8 atoms/gCa/
yr from spallation and muon capture reactions on Ca
respectively, and 161 and 10.2 atoms/gK/yr from
spallation and muon capture reactions on K respectively
(Stone et al., 1996a,b, 1998). Thermal and epithermal
neutron capture rates are treated according to the method
of Phillips et al. (2001) and Stone et al. (1998). Scaling to
altitude and geographic latitude is based on Stone's
(2000) reformulation of Lal (1991). Because of the
latitude, i.e. 53°S, changes in geomagnetic field strength
are assumed to cause b1% change in ages for the time
period studied (Gosse and Phillips, 2001).
Individual exposure ages are shown and discussed
with analytical errors only (±1σ) (Tables 3, 4, and 6),
and generally without erosion. Ages are also shown with
an erosion rate of 1.4 mm/kyr, derived for granite-like
rocks at Lago Buenos Aires, mid-latitude Patagonia,
∼1000 km to the north (Kaplan et al., 2005). An erosion
rate of 1.4 mm/kyr is used to illustrate the effect on
apparent cosmogenic ages in southernmost South
America if a value from a different setting in semi-arid
Patagonia is applied.
4. Results
4.1. 10Be and 26Al
On the oldest, GPG Bella Vista drift, two 10Be ages of
106,000 and 47,400 yr (erosion rate [E]=0 mm/kyr), or
124 and 50 kyr (E=1.4 mm/kyr; Table 3) were obtained.
On the younger Cabo Vírgenes Drift, one sample's
exposure age is 51 ka (E=0 mm/kyr) or 54 ka
(E=1.4 mm/kyr), and on the equivalent Río Cullen
Drift (Tables 1 and 3) ten ages range from 50 to 13 ka
with a mean and median of 25–26 ka and a standard
deviation of ∼9 ka (E=0 mm/kyr). A major subset of
these Río Cullen ages (8 of 10) cluster between 26 and
19 kyr (1σ). For six of the seven Río Cullen samples near
Punta Sinaí, which are all granodiorite erratics, there is
no apparent relation between height and age for these
boulders (7th column in Table 2), with the exception that
the shortest boulder (RC-04-06) is also the youngest.
Another notable point for the RC samples is that RC-04-
07, which has the longest exposure history (50 ka), is an
erratic that is from a different part of the moraine than the
other six RC samples, being slightly further inland by
∼1 km(Fig. 2C). On the next younger San Sierras de San
Sebastián Drift, two samples in close proximity provide
considerably different ages of 174 and 21 ka (E=0 mm/
kyr) and the higher boulder (close to 5 m, compared to
2.75 m) has the older exposure age. On its correlative
around Magellan, the Punta Delgada drift produced an
age of 133 ka. The samples on the second to youngest
Primera Angostura Drift produce exposure ages from ca.
32 to 22 ka (E=0 mm/kyr), or 33 to 22 ka (E=1.4 mm/
kyr). The three samples from the kame terrace southeast
of Bahía Inútil (TF-04-04 to 06) are ∼18–15 ka.
Within 1σ, all 26Al ages overlap with the respective
boulder 10Be ages. Thus, the multi-nuclide approach
indicates that (at least) these five boulders have not had a
complex history of exposure and prolonged burial
(Gosse and Phillips, 2001).
4.2. 36Cl
Four samples had measurable 36Cl/Cl (Table 6),
including the two samples from the kame terrace south
of Bahía Inútil and two samples from the Río Cullen
Drift near Punta Sinaí. TF-04-05 and RC-04-04f are
∼22 ka and 25.2 ka, respectively, in good agreement
with the respective 10Be and 26Al ages (Fig. 2). One
sample, RC-04-01, produced a 36Cl age of ∼50 kyr,
whereas the 10Be ages on the same sample are around
20 ka (Tables 3 and 6). The mismatch can't be explained
by erosion. The 36Cl errors (∼10%) are less than the age
discrepancies and there is no solution of time and
erosion that allows concordance of the nuclide data (cf.,
Gosse and Phillips, 2003). Although it is not clear why
there is a difference in age between 10Be and 36Cl for
this sample (and perhaps TF-04-04), possibly Kfeldspar
grains were not uniformly distributed among
the quartz grains, i.e., there were compositional
differences. For ICP analyses, we took a ∼1 g aliquot
from the sample before dissolution. Even b5%
difference in K-feldspar between the aliquot and the
rest of the sample that was subsequently dissolved and
measured on the AMS would have led to discordant
ages.
We attempted to solve for erosion rate and age using
10Be (or 26Al)/36Cl paired analyses (cf., Liu et al.,
1994). For TF-04-04, the only pure quartz sample with
measurable 36Cl/Cl, the uncertainty is too large, N50%.
The other three samples have ‘non-overlapping’ age/
erosion rate solutions, in part because the measured
nuclides are produced mainly by spallation, the errors
are too large and the samples exposed too recently (cf.,
Gosse and Phillips, 2003). Nonetheless, for the boulders
the multiple nuclide data provide general information on
time and erosion. Overall, the 36Cl ages support 10Be—
(the most common isotope used here) based inferences
for the exposure history (see below). In addition,
assuming constant exposure, the ages derived with
either nuclide do not change significantly for rates up to
∼10 mm/kyr.
5. Discussion
The apparent exposure ages for erratics on old (i.e.,
pre-LGM) moraines are far less than the limiting
40Ar/39Ar ages provided by interbedded lava flows,
and many are younger than minimum ages provided by
14C dates (and supported by AAR analyses) from shelly
till in the Strait of Magellan. Albeit, there are only a few
boulders found and measured on the pre-LGM drift
north of Magellan Strait, but they are all much younger
than the 40Ar/39Ar limiting ages. For example, on the
1.1 Myr GPG deposit cosmogenic exposure ages are 47
and 106 ka, and on a deposit 40Ar/39Ar dated N450 ka a
10Be exposure age is 51 ka. In addition, notable age
inversions exist in two places. Along the north side of
the Strait of Magellan, a boulder age of 133 ka on the
Punta Delgada drift is older than the boulder TF-04-08
at 51 ka on the adjacent and older Cabo Vírgenes drift
(Fig. 2). A second major age inversion is in northern
Tierra del Fuego, where an age of 174 ka (SS-04-01) on
San Sebastián Drift is much older than any age on the
older Río Cullen drift. These are all minimum ages
assuming an erosion rate N 0 mm/kyr. Taken at face
value, the 133 and 174 ka ages could indicate the Punta
Delgada and San Sebastián drifts were deposited during
MIS 6. However, it must be emphasized, particularly in
light of processes discussed below, that these are clearly
minimum ages, especially for the latter sample, a short
∼25 cm high boulder (Fig. 3B). On Tierra del Fuego,
glacial drift N400 ka in age contains erratics with
relatively short exposure histories, b50 ka, with the
important exception of 174 ka. The cosmogenic ages for
the San Sebastián and Río Cullen drifts are also far
younger than the inferred ages based on U-series and
AAR data (Bujalesky et al., 2001) from nearby
equivalent marine deposits (Fig. 2D).
Collectively, the cosmogenic data support pre-LGM
ages for (at least) the three oldest drift units shown in
Table 1. The range and variance of the cosmogenic age
distributions are far higher than any LGM deposit
studied in middle and southern Patagonia (Kaplan et al.,
2004; McCulloch et al., 2005; Douglass et al., 2006),
which is outside the high latitude regions where
inheritance is a widespread problem (e.g., Briner et al.,
in press). For example, in three different localities two
boulders on the same morainic deposit within 1 km of
each other (or even a few 100 m) provide completely
different ages, 106 and 50 ka (Bella Vista drift), 50 and
13 ka (Río Cullen drift), and 170 and 21 ka (San
Sebastián drift). For the 7 samples on the Río Cullen
moraine at Punta Sinaí the standard deviation is ∼9 ka,
almost the entire age range of the recorded LGM in the
nearby Strait of Magellan (McCulloch et al., 2005) and
southern Tierra del Fuego (Rabassa et al., 2000).
Furthermore, Zreda and Phillips (1995) and Hallet and
Putkonen (1994) made a case that on ‘old’ moraines the
oldest age(s) assuming no erosion will be the one closest
to the ‘true’ moraine deposition age, given that the
relevant geomorphic processes will almost all lead to
minimum ages. On the oldest four drift units in the
region (Table 1), the oldest age is greater than the time of
LGM (i.e., N25 ka). This assumption is also supported at
Lago Buenos Aires ∼1000 km to the north (Kaplan
et al., 2005).
On the kame terrace southeast of Bahía Inútil all three
isotopes provide ages overlapping with LGM time
(Fig. 2C), including the TF-04-04 36Cl age within 1σ.
Originally, when sampled in the field, it was assumed
that this terrace was just beyond the mapped limit of
LGM drift (e.g., see Fig. 2 in Bentley et al., 2005).
Based on the analyses presented here, and its position so
close to mapped deposits of the last glacial period, we
now conclude that the kame terrace was formed during
the LGM (Fig. 2A and C).
5.1. Implications for landscape change in Fuego–
southernmost Patagonia
For the pre-LGM deposits in Fuego–southernmost
Patagonia, we infer that boulder erosion and exhumation
from the moraine matrix explains the discrepancy
between the cosmogenic ages and the constraining
40Ar/39Ar ages of the interbedded lava flows (and AAR
and U-series ages from the Strait of Magellan and Tierra
del Fuego). Differences in lithology most likely cannot
explain the findings as they are not fundamentally
different between samples measured, which are almost
all granites or granodiorites with similar composition
(e.g., the four boulders shown in Table 5). At Punta
Sinaí (and other Río Cullen sites), the erratics are coarse,
medium grained, megascopically foliated granodiorites
with quartz, plagioclase and K-feldspar (orthoclase) as
essential components (D. Acevedo, personal communication).
Advanced weathering and erosion of erratics is
clearly evident in the form of boulder surface potholes
and weathered ‘micro-gulleys’ 5–10 cm deep, especially
on Tierra del Fuego deposits (Fig. 3D, E, and F). As
an example, TF-04-01 shown in Fig. 3D provides a
young age of 23 ka, yet intense erosion (and
exhumation) is obvious on the sides and top of the
erratic. Interestingly, the boulders on the Río Cullen drift
exhibit relatively more weathering compared to those to
the north (e.g., Bella Vista and Cabo Vírgenes drifts)
and in general provide younger ages (Fig. 2), despite a
roughly similar lithology (granite or granodiorite).
There appears to be no obvious general relation between
boulder height and age, except for the two San Sebastián
erratics measured. On the Río Cullen drift near Punta
Sinaí, the only distinct difference for the boulder with
the oldest and statistically higher age is that it is from a
different part of the moraine, being further inland by
∼1 km, than the other 6 samples.
There is a non-unique combination of erosion,
exhumation and reburial that could have produced the
apparent ages, all operating in an episodic or gradual
manner. Any burial was brief (less than the last glacial
cycle) as indicated by the 26Al/10Be (and 36Cl) ratios.
For the sake of discussion, if no exhumation and a
steadily eroding surface are assumed, then boulder
maximum ‘model’ erosion rates can be estimated for the
oldest apparent samples and by assuming ages N400 ka
for the Cabo Vírgenes Drift, 1.1 Myr for the Bella Vista
drift, and N400 ka for the San Sebastián and Río Cullen
Drifts (Bujalesky et al., 2001). It is emphasized that
these are only ‘apparent’ rates and they are presented to
infer geomorphic processes that may have caused given
isotope concentrations in southernmost South America.
This simple exercise produces erosion rates for boulder
material of N5 and 11 mm/kyr (Bella Vista drift),
N3 mm/kyr for San Sebastián Drift, and N11 mm/kyr for
the Cabo Vírgenes Drift (Fig. 4A). Almost identical
erosion rates are calculated assuming that isotope
concentrations are in secular equilibrium (Lal, 1991).
Available evidence indicates that, in general, boulders
erode slower than their surrounding landscape, thus the
erosion rates presented above may be less than that for
the moraine matrix, provided there is no desert pavement
or matrix cement (Granger et al., 2001). For the sake of
discussion, if the apparent ages are due entirely to
exhumation (i.e., no erosion), then a stripping of up to at
least ∼2–3 m (i.e., amount needed to have the ‘clock’ at ∼0 yr prior to exposure) of moraine matrix during the last glacial cycle could explain the nuclide concentrations (except for SS-06-01), assuming the boulders had no inherited cosmogenic nuclides prior to being exposed at the moraine surface; considering erosion decreases the amount of stripping (e.g., b2–3 m) needed to explain the apparent ages. Also, the boulders could have had a complex history of exposure and burial (e.g., due to loess) since moraine formation, which has kept isotope concentrations relatively low. For sample SS-06-01, given its height at ∼5moff the ground, a combination of
erosion and exhumation seems likely.
A key point is that long-term erosion and/or exhumation rates must be low enough that the old deposits still exist, and in some areas with original morphologic features. The 40Ar/39Ar dated Bella Vista and Cabo Vírgenes drifts, and correlatives such as the Río Cullen drift, still contain (subdued) moraine crests and icemolded landforms (Meglioli, 1992; Rabassa et al., 2000;
Coronato et al., 2004a,b; Ercolano et al., 2004). In addition, meltwater systems linked to the original glaciation (or deglaciation) that deposited the given moraine drift, including the Cabo Vírgenes and BellaVista units (which are overlain by the dated lava flows),are still clearly visible and easily mapped (see Fig. 2). As shown in Fig. 2, subsequent glaciations are ‘nested’ within each other and the younger ones are at inner and/or lower positions in the landscape. Thus, it is unlikely that landforms on the surface of the drift have been reworked or reburied by younger, major glaciations (e.g., forming meltwater channels fundamentally different in age).
Given that the ‘old’ glacial deposits, with original characteristics including meltwater channels, contain erratics with relatively low cosmogenic nuclide concentrations, an explanation is that short but intense periods of erosion, exhumation (e.g., loess removal) or reburial during glaciations or interglaciations must cause 10Be, 26Al and 36Cl ages to provide comparatively recent minimum ages. Boulder exposure ages for any given dated moraine are up to an order of magnitude less than the limiting 40Ar/39Ar ages. Taken at face value, these nuclide concentrations and the 40Ar/39Ar data imply erosion of N10msince moraine formation (or exhumation ∼10–20 m). If such erosion or exhumation rates have been constant, we infer that moraine crests, meltwater channels, and other features should not be expected, at
least not without repeated reburial (e.g., by loess). Thus, over the history of the moraine, boulder erosion or exhumation rates could not have been constant.
The cosmogenic data may indicate that boulders located near Punta Sinaí, in general, appear to be eroding more quickly (or have been exhumed more recently) than those farther to the north on the Bella Vista and Punta Deglada Drifts (Figs. 2 and 4), except perhaps for SS-04-01. This finding is consistent with surface erosional features on boulders (Fig. 3), as Tierra
del Fuego erratics typically contain deep relief (N5 cm)
and potholes, indicating advanced stages of erosion, which were not observed on southern Patagonian erratics (Fig. 3). A possibility is that during the LGM and pre-LGM glaciations, Río Cullen boulders were eroded more rapidly by frost action. Periglacial or permafrost conditions existed at the present Atlantic coast during the LGM, as demonstrated by the nearby presence of ice-wedges in the area around the town of Río Grande (Coronato et al., 2004c; Perez Alberti et al., 2005). Also, several marine transgressions could have facilitated salt weathering and high erosion rates at Puna Sinaí, which is near the coast. Perhaps, the climate has been harsher (i.e., colder or wetter) on Tierra del Fuego than in Southernmost Patagonia, due to the slightly
more southerly location and higher humidity, (Fig. 4), and close proximity of the Darwin Cordillera ice mass, the largest part of the southernmost Patagonia ice sheet at the LGM. It should be noted that for northern Tierra del Fuego, in general, the landscape development has
been different from that of the ‘Patagonian tablelands’ due to regional climate and marine influence (Clapperton, 1993; Coronato et al., in press).
5.2. Implications for landscape change in Southern
Patagonia Exposure histories of pre-LGM boulders in southernmost Patagonia–Fuego are quite unlike those in the middle latitudes of Patagonia. In semi-arid mid-latitude Patagonia at Lago Buenos Aires (LBA) (Fig. 1), moraines that are at least two and perhaps three glacial cycles old can be constrained in age with cosmogenic nuclide concentrations in erratics (Kaplan et al., 2005). ‘Apparent’ rates of boulder erosion or exhumation (or reburial) at LBA were estimated by measuring the concentrations of 10Be and 26Al in glacial erratics on the oldest moraines, which are constrained in age independently with 40Ar/39Ar data to between 760 ka and 1100 ka (Singer et al., 2004; Kaplan et al., 2005). The LBA ‘apparent’ rates are almost all b2.5 mm/kyr, with a variability of ∼1 mm/kyr, much less than ‘apparent model rates’ anywhere in this investigation on Fuego–southernmost Patagonia. Most recent 36Cl/10Be pairing at LBA indicates rates (Douglass, 2005) even slightly lower than those presented in Kaplan et al. (2005). For comparison, boulder surface erosional features such as potholes, relief N5 cm, and deep rills evident on Tierra del Fuego boulders (Fig. 3), are rare at LBA where mainly
evidence of wind ventifaction is commonly observed (e.g., see Fig. 3 in Kaplan et al., 2005). Applying erosion rates derived at LBA, ∼1.4 mm/kyr, to boulder ages in Fuego–southern Patagonia (Table 3) does not increase the ages even close to the 40Ar/39Ar minimumage constraints for these moraines. Given the different findings for pre- LGM moraines at LBA compared with the area of this study, the erosion rates derived for the former obviously
are not applicable (or comparable?) to the latter, in part because it is unclear how the nuclide concentrations were produced in the different geomorphic settings.
At the very least, the evidence suggests that geomorphic processes in Fuego–southernmost Patagonia are quite different compared to that in the LBA area and the middle latitudes (Fig. 4). Important differences between Fuego–southernmost South America and LBA could
include the following.
1) Temperatures have been lower or precipitation higher southward, especially during glacial times. Presently, the LBA climate is classified (Fig. 5) as arid (aridity index=0.2), the area around BellaVista as cold sub-arid (aridity index=0.5), and around Punta Sinaí as cold, sub-humid and oceanic (aridity index=0.75) (Aridity index=mean annual rainfall/potential evapotranspiration [UNESCO, 1977]). Currently, Tierra del Fuego is only 3–5° north of the Antarctic Frontal Zone, which may have maintained a more northerly position during
the LGM (Ackert et al., 2003; Sugden, 2005; Kaplan et al., in preparation). In addition, compared to at LBA and the mid-latitudes, the Southern Hemisphere westerlies transport more humid air to Tierra del Fuego, and especially near Punta Sinaí at the coast, for most of the
year the temperature range is less and soils are more humid (i.e., more moisture in the air) (Fig. 5; Coronato et al., in press). An observed record of LGM permafrost features on Tierra del Fuego but not at LBA may attest to colder more humid conditions further south during
glacial times (Clapperton, 1993; Coronato et al., 2004c; Perez Alberti et al., 2005).
2) Wind erosion has been more intense in Fuego– southernmost Patagonia, especially during the LGM when the sea was ∼250 km distant and a broad continental shelf was exposed (Fig. 4). Presently, southernmost South America lies in the core of the westerlies (Taljaard et al., 1969). High aeolian dust in southern Patagonia during ‘glacial times’ is welldocumented by local terrestrial (Rabassa et al., 2000), nearby marine (Kolla et al., 1979; Fig. 4B) and Antarctic ice core studies (Delmonte et al., 2004). Although the exact source area of dust in Patagonia is still debated (Delmonte et al., 2004), possibly, more aeolian material exists or there are stronger winds during glacial maxima (i.e., causing more intense wind abrasion and erosion) towards
southernmost South America.
3) Proximity to the sea during interglacial periods has led to differences in weathering (i.e., salt weathering).
Throughout much of the last glacial cycle sea levelwas far from the present coastline on the broad Argentine continental shelf (Fig. 4B), e.g., during the LGM the coastline was N250 km away. In contrast, (at least) since MIS 11, during interglacial periods, the sea has been close to its present position at Punta Sinaí (Bujalesky et al., 2001). The oldest exposure age near
Punta Sinaí is also farthest from the present coastline.
In summary, more intense aeolian abrasion, colder or more humid conditions, or proximity to the sea could explain the observation of ‘apparent’ higher boulder and moraine erosion rates towards the southern tip of South America.
6. Summary and conclusions
Cosmogenic nuclide data in southernmost South America indicate that glacial erratics have much shorter exposure durations than the ages of the landforms they sit upon. In southern Patagonia and on northern Tierra del Fuego, the range and variability of the cosmogenic ages are in agreement with the moraines being old, i.e., pre-last glacial maximum (LGM) or older than MIS 2 (Mercer, 1983; Meglioli, 1992; Rabassa et al., 2000; Coronato et al., 2004a,b; Singer et al., 2004). Geomorphic processes are causing profound differences in exposure ages on pre-LGM deposits, even within 1 km. These processes do not seem to be as pervasive on LGM
deposits to the west, within the Strait of Magellan and around Bahía Inútil, where interestingly cosmogenic dating has excellent potential for elucidating the chronology of LGM glacial deposits, as evident by good agreement with a 14C-based chronology (McCulloch et al., 2005; Kaplan et al., in preparation).
An explanation that accommodates various lines of evidence is that fast episodic erosion rates occurred only during peak glacial times, perhaps aided by periglacial processes (Coronato et al., 2004c; Perez Alberti et al., 2005). Such a history takes into account observations of
original morphologic features (e.g., Ercolano et al., 2004) on drift up to 1 Myr, or at least N450 ka, and the relative youthfulness’ of boulder exposure data. This could also explain why many of the ages on the Punta Delgada and Río Cullen deposits are in the 30–13 ka range (i.e., the last
glacial period). The nuclide concentration data indicate that, essentially, most of the surface boulders have total exposure histories of about 100 ka or less, which is the approximate length of the last glacial cycle. The evidence may imply that the erratics are eroding relatively quickly
or they all have been exhumed from the moraine matrix within the last glacial cycle, whereas the geomorphic processes that limit the utility of the cosmogenic chronometers on pre-LGM deposits have not had such a pervasive effect over the last ∼17 ka.
The age distributions thus indicate that cosmogenic nuclide dating cannot be used reliably as a chronometer on glacial deposits older than the LGM in Fuego– southernmost Patagonia. For comparison, in the middle latitudes of Patagonia at Lago Buenos Aires, cosmogenic dating holds excellent potential to date pre-LGM moraines (Kaplan et al., 2005; Douglass et al., 2006).
Thus, appreciation of present and past climate regimes can serve as a useful guide to the limit of the probable utility of the technique on old glacial surfaces.
The ‘short’ exposure history of boulders on old pre- LGM moraines in Fuego–southernmost Patagonia, compared to the findings from Lago Buenos Aires in central Patagonia, leads us to hypothesize that towards the southern tip of South America geomorphic processes (e.g., boulder
erosion or exhumation rates) have been operating differently over at least the last few glacial cycles. Higher rates of landscape change between middle and high latitude South America is compared with offshore data (Fig. 4B).
Although there was a limited amount of data, the work of Kolla et al. (1979) did show increased percent quartz by weight of sediment, i.e., increased aeolian input, in the past (and present) towards the south in the adjacent southwest Atlantic Ocean. The marine data may reflect higher
production and thus availability of quartz-bearing sediments due to glacial or non-glacial erosion. The terrestrial, and perhaps marine, evidence may suggest that towards southern South America overall landscape denudation has been faster during the Quaternary Period.
Acknowledgments
We thank Charlie Mann at Estancia Sara (where Punta Sinaí is located), Steven Binnie, Rob Ellam, Dodie James, Willie Gibson,AnthonyNewton and ScottDreher. Special thanks to John Stone and Robert Finkel for 36Cl instruction, data and analyses. Comments from Steve
Binnie, David Fink, and reviews by Florian Kober and an anonymous reviewer greatly clarified and strengthened the manuscript. This research was supported by a Royal Society of London Postdoctoral Fellowship (Kaplan) a NERC Small Grant NER/B/S/2003/00286 (Hulton and
Kaplan), and SECYT-CONICET (Coronato andRabassa).
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