Technical Comments

Response to Comment on "Rapid Uplift of the Altiplano Revealed Through 13C-18O Bonds in Paleosol Carbonates"

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Science  03 Nov 2006:
Vol. 314, Issue 5800, pp. 760
DOI: 10.1126/science.1133131

Abstract

Clumped-isotope thermometry measurements of carbonate samples deposited in the Bolivian Altiplano as early as 28.5 million years ago and buried up to ∼5000 meters deep exhibit no relationship between burial depth and apparent temperature, and largely yield temperatures within error of plausible Earth-surface conditions. These results counter the predictions of Sempere et al. and support our previous conclusions regarding the uplift of the Altiplano.

Sempere et al. (1) suggest that the temperatures recorded by carbonate clumped-isotope thermometry in 11.4 to 10.3 million-year-old soil nodules from the northern Altiplano (2) reflect partial resetting during burial rather than deposition at low altitude. Their arguments include a testable prediction: If the soil carbonates in question underwent partial resetting during burial, then more deeply buried samples from the same or related sections should be even more strongly reset, yielding apparent temperatures above any plausible depositional temperature.

Figure 1 presents the results of carbonate clumped-isotope thermometry analyses for 32 soil and lacustrine carbonates from the northern Altiplano. These data include those in (2) as well as new measurements that are part of a broader ongoing study of modern and ancient carbonates [generated using the same analytical methods described in (2)]. This expanded suite includes soil carbonates deposited between 28.5 and 0 million years ago (Ma) and buried between 0 and ∼5000 m deep, as well as lake carbonates of similar age and burial depth. Age estimates for the new measurements are based on recently published magnetostratigraphy (3) and previously published 40Ar/39Ar dates (4) of tuffs within the Corque and Tambo Tambillo sections, and on magnetostratigraphy (5) within the Salla section. We estimated maximum burial depths for each sample based on our own measured sections near Callapa (3) and estimated section thicknesses in the Tambo Tambillo and Salla areas (4, 6).

Fig. 1.

Apparent growth temperatures for various Altiplano carbonates based on clumped-isotope thermometry plotted as a function of estimated maximum burial depth. Symbols discriminate among soil carbonates from sections near Callapa, Corque, and Salla and lacustrine carbonates from near Tambo Tambillo, as indicated by the legend. The heavy solid line indicates an estimated burial geotherm, assuming a surface temperature of 20°C and a gradient of 30°C per km. The dashed lines define a ±10°C offset from this trend, which we consider a reasonable estimate of its uncertainty. Carbonates deposited on or near the surface of the Altiplano within the past 28.5 million years and buried to 5000 m or less exhibit no systematic relationship between apparent temperature and burial depth and show no evidence for pervasive resetting of deeply buried samples.

The data presented in Fig. 1 exhibit no systematic relationship between apparent growth temperature and burial depth, are generally within analytical uncertainty of earth-surface temperatures [the only noteworthy exception was reported and discussed in (2)], and include relatively low temperatures in samples far older and more deeply buried than those reported by (2)—i.e., the temperatures of 16.9°C and 21.5°C found in 23.6- to 23.7-Ma soil carbonates that were buried to ∼5000 m. Moreover, we observe no systematic difference between surface-deposited carbonates of different types (i.e., soil versus lacustrine). Variations in temperature within this suite stem from a variety of factors, including primary differences in paleoaltitude and paleoclimate [discussed for many of the Callapa samples in (2)], unusual diagenetic resetting [e.g., the one high-temperature Callapa sample discussed in (2)], and analytical uncertainties. It is beyond the scope of this reply to discuss all of these issues in detail. Nevertheless, these data contradict the predictions of Sempere et al. and, more generally, lend no support to the suggestion that burial metamorphism has systematically reset the growth temperatures of Altiplano soil carbonates. For this reason, we maintain that the difference in average apparent temperature between 11.4 and 10.3 Ma and post-6.7-Ma soil carbonate suites reported in (2) reflects a difference in their temperatures of deposition and thus constrains paleoaltitudes using methods (and with uncertainties) that have already been discussed (2).

Sempere et al.'s geomorphic and stratigraphic arguments against a late Miocene date for uplift of the northern Altiplano are relevant but contain no quantitative paleoaltitude determinations and say nothing specific about the mid- to late-Miocene paleoaltitude of the Altiplano. Sempere et al. recognize that the Western Cordillera could have extended to higher altitudes than the Altiplano (as they do today); we suggest it is also possible that mid-Miocene altitudes in any or all of these regions might have been higher or lower than Oligocene and early Miocene altitudes. It will be difficult to know how to evaluate these issues until there is a quantitative database documenting temporal and spatial variations of paleoaltitudes across the Andean orogen.

Sempere et al.'s critique of paleoaltimetry based on fossil leaf assemblages has no direct bearing on the Ghosh et al. study (2). Although we noted that clumped-isotope thermometry results broadly agree with paleobotanical altimetry, our central arguments do not depend on this issue.

We do not agree with Sempere et al. (1) that removal of mantle lithosphere requires previous crustal thickening beneath the Altiplano. The Eastern Cordillera preserves the largest documented shortening in the Andes (7, 8) and is the most plausible candidate for the locus of development of an unstable lower-crustal and/or lithospheric-mantle root. Gravitational removal of this material could have led to simultaneous surface uplift of the eastern Altiplano and Eastern Cordillera and lower crustal flow from the Eastern Cordillera to the Altiplano, thickening the crust beneath the Altiplano. This scenario is only one of several that cannot be discounted using existing constraints. Nevertheless, it is an example of a process that is consistent with both the paleoaltitude reconstructions of (2) and the physics that govern convective removal of lithosphere, crustal thickness, and isostasy.

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