MIOCENE-PLIOCENE CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY OF IODP SITE U1457, ARABIAN SEA

International Ocean Discovery Program (IODP) Expedition 355 drilled two sites (U1456 and U1457) in the Laxmi Basin, located in the eastern Arabian Sea. The primary objective of Expedition 355 is to better understand the impact that the Indian (southwest) summer monsoon has on weathering and erosion of the Himalayas and how this in turn affects mountain building. Laxmi Basin is located within the Indus Fan, the second largest submarine fan in the world, and has primarily been fed by the Indus River and its associated tributaries since the collision of India and Eurasia in the Paleogene. Thus, the drill sites are ideally situated to record changes in erosion from the Himalayas through time, which requires a robust chronostratigraphic framework.

Preliminary analysis of calcareous nannofossils from Site U1457 was carried out onboard the JOIDES Resolution and indicates that the section can be assigned to nannofossil zones NN9–NN21 (late Miocene to present). In conjunction with magnetostratigraphy and planktonic foraminifer biostratigraphy, the shipboard age model indicates intervals of high sedimentation rates, as well as the presence of three unconformities through the section at Site U1457. At around 8 Ma, a hiatus of ~0.5 million years disrupted consistent ~17 cm/ky sedimentation rates in the early late Miocene. After the hiatus, sedimentation rates decreased to ~10 cm/ky in the late Miocene. An ~2 million year hiatus encompasses the Miocene/Pliocene boundary and early Pliocene. When sedimentation resumed in the late Pliocene to early Pleistocene, the rate was lower (~4 cm/ky). Sedimentation was again interrupted for ~0.45 million year in the early Pleistocene, which was followed by very high sedimentation rates (~58 cm/ky) and significant sand input into the area. From the late early Pleistocene to present, sedimentation rates slowed significantly to ~7 cm/ky. In order to improve the shipboard age model, we are conducting a high-resolution study over the 2–8 Ma section at Site U1457 to refine the nannofossil biostratigraphy and define the hiatuses more precisely. These new constraints on the nannofossil biostratigraphy, when combined with other dating techniques, will produce a robust chronostratigraphic framework for examination of climate evolution and the Himalayas in the late Miocene through Pliocene.