Fingerprints of tectonics and climate change in sandy turbidites of the deep-sea Bengal Fan based on DZ U–Pb Analyses of young Tibetan magmatic arc grains

In this presentation we use detrital zircon (DZ) U–Pb analyses of sandy Brahmaputra fluvial deposits to fingerprint magmatic arc grains that serve as a tracer for sediment routing through the river system to the Bengal Fan. These young magmatic arc grains also define provenance changes over time in the deep-sea fan.

IODP Expedition 354 collected 7 cores from the Bengal Fan ~1400 km south of the Bay of Bengal shelf canyon. In addition to fingerprinting the magmatic arc grains in fluvial deposits, we analyzed U–Pb signatures of ~49 turbidites collected from deep sea cores, with age estimates from late Miocene to late Pleistocene. Samples with a Brahmaputra origin have signals that show they are from the Gangdese magmatic arc and range in age from ~ca 40–80 Ma. Young grains also come from granitic rocks that crop out in Bome in the eastern syntaxis, with zircon U–Pb ages of ca. 100–140 Ma. The U–Pb age groups from Gangdese and Bome were present in the Miocene deep-sea fan, but grow from close to zero in the Miocene to 25–30% of all grains in the middle to late Pleistocene. These changes represent tectonic drivers in the syntaxis, and both tectonic and climate drivers in the Yarlung River valley.

We also use young magmatic arc grains to infer climate change in the Plio–Pleistocene. For example, accumulation of sand-rich turbidites is biased towards periods of climate-forced low sea level when rivers were connected to slope canyons. Sand-rich turbidites display different proportions of U–Pb age groups relative to modern interglacial river sands, especially during the late Miocene and after the Plio–Pleistocene transition when glaciation began in earnest. The Gangdese and Bome age groups increase their contributions to the U–Pb signature significantly during the Plio–Pleistocene, which we suggest is a regional signal of climate change.