SIDERITE AS A MORE ROBUST PALEOELEVATION PROXY: OIYUG BASIN, SOUTHERN TIBETAN PLATEAU

Calcitic carbonates from the Ramaqu section of the Oiyug Basin in southern Tibet demonstrate a consistent pattern of very depleted isotopic composition (~-19 ‰VPDB). Recent stable isotope analyses of two siderite samples from near the top of the Mio-Pliocene section along the southern flank (621F) and from the Spicer et al. (2003) floral locality along the northwestern flank (618A) of the Oiyug basin are in accord with these data (-12 and -19‰, respectively). Using Rowley et al.’s (2001) model relationship between Δ(δ ¹⁸O_p) and elevation based on atmospheric thermodynamics and Rayleigh distillation of orographically forced ascent of air masses, the siderites pin our paleoelevation estimate of the plateau at ~5300 meters by 15 Mya and persisting through the upper Miocene-Pliocene. Siderites provide a more robust measurement of δ¹⁸O because, unlike other carbonate minerals, siderites do not undergo isotopic re-equilibration during burial diagenesis (Curtis, Pearson & Somogyi, 1975; Rezaee et al., 1997) due to a higher equilibration temperature. Our results based on δ¹⁸O of the two siderites can be compared with δ¹⁸O_c, lipid δD, and previous elevation estimates based on paleoenthalpy derived from floral physiognomy (Spicer et al., 2003). We will also present a new independent estimate of water isotopic composition and temperature using Δ47 clumped isotope paleothermometry on carbonates in the Oiyug Basin. These paleoelevation proxies agree that the southern Tibetan plateau has remained high (5300-4600 meters) for at least the past 15 million years.