Eocene-Oligocene large-scale circulation of the East Asian summer monsoon recorded in neritic carbonates of the proto-South China Sea

Abstract

When the modern features of large-scale Asian monsoonal circulation were set-up is poorly constrained and pre-Neogene monsoonal archives are rare. This study investigates the temporal evolution of neritic carbonates in the proto-South China Sea to reconstruct East Asian monsoonal currents and winds during the middle to late Paleogene. Analysis of three-dimensional seismic and well core data from the North-West Palawan block reveals the occurrence of a set of thick (>125 m), aggrading Halimeda bioherms, early Oligocene in age. By analogy with Holocene green algal buildup counterparts, they are interpreted to form in nutrient-rich areas subject to upwelling currents and reflect the early presence of a strong coastal jet in the Proto-South China Sea, as seen today during the summer monsoonal season. The analysis of the underlying mixed carbonate–siliciclastic ramp shows that mesotrophic conditions already prevailed during the late Eocene, thus suggesting that modern-like summer East Asian Monsoon large-scale circulation was possibly active as early as the late Eocene in the Proto-South China Sea. Evidence for upwelling currents vanishes in late early Oligocene carbonate-producing biota, that are characterized by the significant development of euphotic hermatypic coral communities reflecting lower nutrient concentrations. This turnover coincides with the opening of the modern South China Sea and is interpreted to result from the southward drift of the North-West Palawan block which placed the area away from the Chinese margin and the coastal jet. Our results highlight therefore that many of the features of the summer East Asian Monsoon large-scale circulation are rooted in the middle Paleogene.