The Upper Pliocene–Quaternary Evolution of the Ioffe Calcareous Contourite Drift, Western South Atlantic.

Friday, 19 December 2014
Dmitrii Borisov, Ivar Murdmaa, Elena V. Ivanova, Oleg Levchenko, Olga Dmitrenko and Emelyan Emelyanov, Shirshov Institute of Oceanology, Moscow, Russia
The high-resolution seismic profiling during cruise 32 of the RV “Akademik Ioffe” (2010) discovered a large elongated contourite drift on the ridge of the Rio Grande fracture zone (Brazil Basin, western South Atlantic). This sedimentary body with a thickness up to 300 m, named Ioffe drift, is traced at water depth range from 3790 to 3980 m. Five seismic units are distinguished within the upper drift structure recorded to a depth of 60 mbsf. The seismic units are separated by angular discontinuities. The sediment core AI-2436 (25°51.6’S, 34°01.40’W, water depth 3800 m) retrieved near the drift top recovers about 6 m of nanno-foraminiferal ooze intercalated with foraminiferal sand interbeds. According to the planktonic foraminiferal and nannofossil stratigraphy, the sediment record embraces about 3 My of the Ioffe drift history, from the Late Pliocene to present. Absence of several foraminiferal and nannofossil zones in the core section indicates long-term hiatuses. Some zones are thicker here while others are reduced as compared to those on the nearby Rio Grande Rise. This suggests reworking of the biogenic material in the drift. The Ioffe drift was formed as a result of transport and deposition of biogenic calcareous material by the eastern branch of the Antarctic bottom water (AABW) flow. The biogenic calcareous material is mainly derived from the Rio Grande Rise where planktonic foraminiferal and nannofossil assemblages identical to those in core AI-2436 are studied at the DSDP site 516 (Barash et al., 1983). We suggest that the grain-size distribution of the calcareous sediments roughly corresponding to the foraminiferal/nannofossil ratio reflects the bottom currents velocity variations. Very high current velocities result in erosion and stratigraphic hiatuses. Angular unconformities in seismic profiles indicate episodes of intensified AABW flow velocity which led to the drift migration.

The study was supported by RFBR, research projects 14-05-31357-mol_a, 14-05-00744-a.

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