THE SERIOUS CHALLENGE OF OBTAINING PRIMARY RECORDS OF PALEOALTITUDE IN TIBET (Invited Presentation)

There is intense interest in understanding the uplift history of Tibet. Many sedimentary and volcanic deposits spanning most of the Cenozoic are potentially available for isotopic reconstruction of this uplift history. In this talk we focus on sedimentary carbonates from paleosols and paleolakes, and their carbon, oxygen, and clumped isotope compositions.

In our view, primary clumped isotope compositions reflecting original soil and lake temperatures are rare in Tibet and appear to be confined to a few Mio-Pliocene deposits that have experienced modest (2-3 km) of burial. Clearly reset clumped isotope compositions are more the norm in Tibet, where time and deep burial have reset most T(47)>50°C in both soils and lake carbonates. A few records yield mixed T(47) results; these cannot be definitively rejected and also should not be accepted until replicated in other basins.

Converting oxygen isotope values to paleoaltitude estimates is also a challenge, starting with the observation that most early Cenozoic marine carbonates do not preserve primary marine oxygen isotope compositions. Low (<-15‰) values are commonplace in soil and lake carbonates, and we suspect, based on burial and thermochronologic histories, and petrographic evidence, that these low values are not primary but the result of isotopic exchange or replacement. We regard most pre-Miocene oxygen isotope results from Tibet with great caution.

In marked contrast to the challenges of the oxygen isotope record, the carbon isotope values from paleosols in Tibet is largely undisturbed and markedly lower (-15 to -5 per mil) than those in modern soils (-5 to +10 per mil), reflecting a gradual increase vegetation cover and presumably warmer, wetter conditions in the suture zone of southern Tibet prior to the mid-Miocene.