Soil n-alkane δD and glycerol dialkyl glycerol tetraether (GDGT) distributions along an altitudinal transect from southwest China: Evaluating organic molecular proxies for paleoclimate and paleoelevation

Organic molecular proxies provide a record of the hydrogen isotopic composition of ambient precipitation and paleoenvironmental temperature. In terrestrial settings, two classes of organic biomarkers are commonly utilized to reconstruct these past environmental conditions: straight-chained normal alkanes (n-alkanes) and branched glycerol dialkyl glycerol tetraethers (brGDGTs). We measured the δDn-alkane and the ratio of cyclisation and methylation indexes of branched tetraethers (CBT/MBT) from surface soils along a ∼1000 km transect along the southeast margin of the Tibetan Plateau to assess how these proxies record changes in climate parameters that are related to elevation, despite variations in ecosystem. Abundance-weighted δDnC27-31 data record the δD of precipitation across the orogen and indicate minimal change in apparent fractionation (εn-alkane/water = −133‰) with elevation. n-Alkane δD mirrors the pattern of Rayleigh distillation of an air mass undergoing rainout as moisture moves from the Bay of Bengal into the SE margin of the Tibetan plateau. Soil tetraether temperatures provide a reliable measure of modern temperature with an MBT/CBT calculated temperature lapse rate (5.8 °C/km) close to the moist adiabat. These data reflect “ground-level” conditions that are independent of the shape of the orogen or the source of moisture. In combination with a Rayleigh distillation model that incorporates temperature and precipitation isotope data, paired tetraether temperature and leaf wax isotope data produce a reliable record of changes in environmental conditions associated with modern elevation change across the largest orogen. However, some tetraether calibrations may impart systematic temperature bias, most notably in cold or dry climates. These calibration uncertainties can result in underestimates of paleoelevations by up to 1.5-2 km. Paired isotopic and temperature paleoelevation reconstructions can minimize errors associated with an individual proxy method, however we recommend that application of tetraether temperature paleoelevation reconstruction take potential calibration biases into account in paleoelevation estimates.