Glacial Geology, Soil Development and Paleoclimate Reconstruction For Mid-Latitude South America, 1 Ma to Recent.
Contents
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Exceptionally well-preserved Pleistocene deposits near Lago Buenos Aires, Argentina, including moraines, outwash terraces, and the soils that developed on these landforms, provide a geologic archive of past climate fluctuations in southern South America for the last one million years. I integrate multiple geochronometers, glacial geology, and soil geomorphology to determine the timing and nature of climatic change in this area.
The main goals of this dissertation are as follows:
The Quaternary geology of the Lago Buenos Aires area was first mapped by Caldenius (1932), and has been described as "one of the most complete and intact sequences of Quaternary moraines in the world" (Clapperton, 1993; p. 358). The moraines have been revisited by several workers (see Singer et al., 2004 for a summary of mapping efforts), and while the resulting maps are all quite similar, the chronologic interpretations have been quite different. We adapt the chronology of Singer and others (2004), who identify four moraine groups on the basis of moraine geomorphology and stratigraphic relationships to three 40Ar/39Ar dated basalt flows. The moraine groups are informally named, from youngest to oldest, the Fenix, Moreno, Deseado, and Telken moraines. The 109 ± 3 ka (this and all uncertainties reported at the 95% confidence level) Cerro Volcán flow separates the younger Fenix moraines from the older Moreno and Deseado moraines, the 760 ± 14 ka Arroyo Page flow separates the younger Moreno and Deseado moraines from the older Telken moraines, and the 1016 ± 10 ka Arroyo Telken flow is the upper limit for the Telken moraines. In addition to the four moraine groups described above, my research has also focused on two Holocene moraines at Fachinal, Chile.
Despite their antiquity, the landforms in this environment are exceptionally
well preserved. The Telken moraines are over 760 ka, and are subdued, but identifiable
landforms; braiding patterns are still preserved on outwash associated with
these moraines. The mean annual air temperature is 8°C and the mean annual
precipitation is 200 mm/yr, based a decade of local climate observations and
longer regional records. Average summer (Dec.-Feb.) and winter (June-Aug.) temperatures
are 14°C and 3°C, respectively. Approximately 75% of the annual precipitation
falls in April through September (late fall through early spring). The study
area is part of the Southern Patagonian Steppe Eco-region and contains semi-desert,
shrub-steppe, and grass-steppe vegetation types (Soriano, 1983); plant cover
generally varies from 30-60%.
Preliminary Results
A landform map has been compiled for moraines east of Lago Buenos Aires from satellite images and airphoto analysis, as well as field investigation. This map is currently undergoing final revisions.
Cosmogenic Surface Exposure Ages
The arid climate in which these moraines were deposited renders 14C dating ineffective, due to lack of organic material, but very low erosion rates make the environment ideally suited to surface exposure dating using in situ cosmogenic nuclides. I use 10Be and 26Al surface exposure dating methods to directly date moraine deposition by measuring isotope concentrations in quartz bearing boulders on moraine crests.
The Telken Moraines
Samples from five boulders are processed from this moraine to constrain boulder erosion rates in this area. The relatively close bracketing of moraine ages from the stratigraphic relationships with the basalt flows, i.e. 760 to 1016 ka, allows the calculation of an expected isotope concentration if one assumes that the boulder has been exposed for its entire history. Any difference between the expected concentration and the measured concentration is attributed to boulder erosion. These analyses indicate that boulder erosion rates in this area are 1.4 ± 0.7 mm/kyr (Kaplan et al., in press, 2005). Actual erosion rates will be lower if the boulder was exhumed after moraine deposition.
The Deseado Moraines
Analysis of 10Be in eleven Deseado samples (and 26Al in two of these) suggests that these moraines are likely older than ~300 ka (Douglass, in preparation). Zero-erosion ages for the oldest samples from Deseado I and II are ~300 and ~350 ka, respectively. Ages based on an erosion rate of 1.4 mm/kyr increase these ages close to, or above the upper stratigraphic age limit for these moraine (~ 760 ka), suggesting that our erosion rate estimates may be overestimates. The age of the Deseado moraines are likely 250-450 ka, but potentially as old as 350-750 ka. I am currently waiting for results of 36Cl analyses from nine of these same samples. It may be possible to improve these erosion rates estimates and moraine ages by using paired 10Be and 36Cl analyses (e.g. Phillips et al., 1997).
The Moreno Moraines
Exposure ages based on pairs of in-situ cosmogenic 10Be and 26Al in quartz from surfaces of thirteen large erratic boulders of granitic or metamorphic rock indicate that the Moreno I and II moraines were deposited ~ 145 ka (Kaplan et al., in press, 2005). Results from seven boulders from the Moreno III moraine are more varied, but clearly indicate that this moraine segment is significantly older than the rest of the Moreno system, probably 200-300 ka, potentially as old as 300-650 ka.
These ages indicate that a major MIS 6 glaciation occurred in Patagonia. After stage 6, the next five youngest (Fenix) moraines correspond to MIS 2. There is no preserved record of a local glacial advance during MIS 4, despite the observation that peaks in Patagonia-derived eolian dust concentration occur in the Vostok, Antarctica ice core at 150 ka (MIS 6), 65 ka (MIS 4) and 22 ka (MIS 2) (Petit et al., 1999). Evidently, MIS 4 moraines were overrun by the Fenix advances. Thus, the Patagonia glacial record has revealed for the first time, unequivocally, that during the last two glacial cycles both the timing and the relative magnitude of glacial maxima in the southern Andes has been in phase with the ~100,000 yr growth and decay of Northern Hemisphere ice sheet volume.
The Fenix Moraines
Exposure ages were determined by measuring in-situ cosmogenic 10Be, or pairs of 10Be and 26Al, from quartz in the surfaces of 46 large erratic boulders atop five moraines just east of Lago Buenos Aires. The results indicate that these moraines were deposited sequentially from east to west between 22.8 ± 0.8 and 15.7 ± 0.7 ka (Kaplan et al., 2004; Douglass, in preparation).
The surface exposure chronology of the Fenix moraines is consistent with that determined via 14C dating in the Chilean Lake district (Lowell et al., 1995; Denton et al., 1999a), and is similar to cosmogenic nuclide-based glacial chronologies from western North America (Gosse et al., 1995; Phillips et al., 1996, 1997; Licciardi et al., 2001, 2004). In fact, the structure of the last mid-latitude South American ice age-specifically, the overall timing, a maximum ice extent prior to 22 ka and deglaciation after 16 ka-is indistinguishable from that of the last major glaciation in the Northern Hemisphere, despite a peak in Southern Hemisphere insolation during this time period. The similar mid-latitude glacial history in both polar hemispheres implies that a global climate forcing mechanism, such as atmospheric cooling due to advection of water, rather than slower oceanic redistribution of heat, synchronizes the ice age climate on sub-orbital time scales (Lowell et al., 1995; Denton et al., 1999b; Kaplan et al., 2004).
The age of the youngest moraine in the Fenix system, the Menucos moraine, is 13.4 ± 0.75 ka and is more consistent with deposition during the Antarctic Cold Reversal (ACR; ~14.8-12.7 ka) and older than the European Younger Dryas Chronozone (YD; ~12.7-11.5 ka), given currently accepted production rates and scaling factors (Stone, 2000).
The Fachinal Moraines
At Fachinal, Chile, ninety km west of the Lago Buenos Aires moraines, a pair of moraines sits atop a delta at the mouth of the Aviles River, which flows northward into Lago General Carrera (the Chilean name for Lago Buenos Aires). Suspecting, based on results highlighted above, that one or both of these sharp-crested moraines corresponds in age to either the ACR or YD, we undertook surface exposure dating by measuring in situ cosmogenic 10Be and 36Cl in 16 erratic boulders. The exposure ages of individual boulders range from 15 to 5 ka, but four relatively old ages are interpreted to contain inheritance due to prior exposure. Surprisingly, the mean exposure ages for the outer and inner moraines at Fachinal are 8.5 ± 0.7 and 6.2 ± 0.8 ka, respectively, documenting for the first time in this region two distinct early Holocene glacier advances (Douglass et al., in press).
I infer that the ice advances recorded at ~8.5 and ~6.2 ka at Fachinal are a result of a northward migration of the Southern Westerlies causing an increase in precipitation and/or a decrease in temperature at 46°S latitude. The older advance precedes the currently accepted initiation of Holocene glacial activity in southern South America by about 3000 years (Porter, 2000; Glasser et al., 2004). Both of these advances are temporally consistent with Holocene climate oscillations that occurred in Greenland and the rest of the world (e.g., Alley et al., 1997; Mayewski et al., 2004). If there are causal links between these events, then rapid climate changes appear to be either externally forced (e.g. solar variability), or are rapidly propagated around the globe (e.g., atmospheric processes).
A chronosequence of 34 soils on the 4 moraine groups near Lago Buenos Aires provides an excellent opportunity to study the long-term mechanisms and rates of soil development (Douglass and Bockheim, in prep). This exceptionally stable landscape-moraines older then 760 ka are subdued, but identifiable landforms-is ideally suited to study soil development over such long time intervals. The soils are classified primarily as coarse-loamy, mesic Typic Calcixerolls or Calcic Haploxerolls and occur under short grass-shrub steppe in a semi-arid climate on moraines comprised of a variety of rocks from the Andean Cordillera. The dominant soil-forming processes are the accumulation of organic matter, carbonate and clay-sized particles.
A-horizon quantities of organic carbon range between about 1 and 4 kg m-2, and are higher for the Deseado and Telken soils. On average, the Deseado soils have the thicker A horizons and Telken soils have higher organic carbon concentrations. These data can be described as a highly significant (p = 0.001) step function increase between the Moreno and Deseado soils, with similar horizon quantities for the Fenix and Moreno as well as the Deseado and Telken soils. These soils can also be fit with a log regression (r2 = 0.45; p < 0.001), which would require that soil organic matter is exceedingly stable in this environment. However, most studies suggest that soil organic carbon equilibrates in less than 5 to 20 kyr, even in arid environments (Birkeland, 1999). Based on this prior work, our preferred interpretation is that the greater accumulation of organic carbon in the Deseado and Telken soils is not related to soil age, but to slight differences in soil environment such as microclimate effects, moisture retention, or grazing practices.
Profile accumulations of pedogenic carbonate and clay-sized particles also have significant positive correlations with time (Figure 5a and b). Both of these constituents are best fit with a compartment model with a linear input function and a loss function that is dependent on the concentration of the constituent. The time integral of this model is: Q = I/k x (1- e-kt), where Q is the profile quantity, I is the constant influx rate, k is the loss rate constant, and t is time. The r2 values for these regressions are 0.51 and 0.38, respectively for carbonate and clay (both p < 0.001). These data can also be fit with log-linear regressions, but these regressions have lower correlation coefficients (r2 = 0.46 and 0.18 for carbonate and clay, respectively). These plateaus in carbonate and clay accumulation are most likely the result of either decreased influx in the earliest part of the record or attainment of equilibrium between influx and loss (e.g. leaching of carbonates or slow soil erosion).
There are no appreciable changes in soil redness, and preservation of easily weatherable minerals in even the oldest soils suggests that there is little chemical weathering in this environment. Measured dust input (27 g m-2 yr-1) can explain the accumulation of both clay-sized particles and carbonate.
Calibration of rates and mechanisms of soil formation creates a powerful correlation
tool that can be used to correlate other glacial deposits in Argentina to the
well-dated moraines at Lago Buenos Aires.
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Last updated 12/3/2004 by Daniel Douglass