Aeolian n-alkane isotopic evidence from North Pacific for a Late Miocene decline of C4 plant in the arid Asian interior

Abstract

Aeolian deposition in the central North Pacific has been well recognized to originate from arid Asian interior. While there is no doubt about the transport of organic matters along with the mineral dust from the source region, little is known about the nature and changes of the terrestrial organic compounds preserved in the deep sea sediments. In this study, higher plant leaf wax n-alkanes from ODP Site 1208 and Site 886 in the North Pacific since the middle Miocene were analyzed to explore long-term changes in vegetation and climate in the source region. Accumulation rates of leaf wax n-alkanes show an increasing trend, consistent with the documented climatic drying of the Asian interior since the late Miocene. The records of carbon isotopic enrichment factors of C29n-alkane relative to atmospheric CO2 (εC29–CO2) show a prominent decrease from ~12 to ~8Ma. The average εC29–CO2 value prior to ~8Ma is 0.8‰ heavier than after ~8Ma. Although almost all values of εC29–CO2 (−25.3 to −21.3‰) are well within the range of C3 plants, adjustment of isotope discrimination by C3 plants is not considered as the main cause of the observed variations. Instead, changes in relative abundance of C3 vs. C4 plants are invoked to interpret the εC29–CO2 records. Higher C4 contribution (17.7±5.3%) to the local vegetation is inferred for the period prior to ~8Ma, implying a slightly warmer climate in the source region. A marked decline in C4 plants from ~12 to ~8Ma, interpreted as a result of regional temperature drop, coincides with the prominent growth of northern Tibetan Plateau around 8Ma, along with the global cooling climate. Our results therefore point to apparently close links among plateau uplift, development of drying and cooling climates, and vegetation changes in the Asian interior.