GEOPHYSICAL CYCLES IN A DEEP ARGENTINE MARGIN SEDIMENT DRIFT

The interaction between deep ocean currents and the seafloor in regions of high sediment supply produce an array of depositional features, the largest of which can be imaged seismically. Sediment deposits produced by bottom currents are valuable physical indicators of deep circulation modes and allow changes in the deep water regime to be detected by seismically-inferred sediment architecture. Expedition TN-372 cored and seismically imaged the Valentin Feilberg drift is sculpted by Lower Circumpolar Dee Water.flowing along the southern Argentine margin, revealing three scales of cyclicity. Our high-resolution multi-channel seismic reflection profiles show pervasive 25 to 30 m cycles that extend approximately 600 m below seafloor. The 3.5 kHz subbottom profiler records reveal cycles on the order of 10 m. Core logger magnetic susceptibility data show cycles on the order of 1.5 m. If orbital variations produced these cycles through climate induced bottom water variation, then the nested cycles are consistent with the 400- and 100-kyr eccentricity cycles, and the 20-kyr precessional cycle.

We constructed a chronology for the deep drift changes using the MCS record, assuming the 400- kyr is the most likely forcing. Given this, the Valentin Feilberg drift began to accumulate around 14.5 Ma with vigorous circulation commencing at 13 Ma. The drift architecture indicates only one deep-water mass affected the drift until 8.5 Ma. From this time forward, deposition shifted to the east and deeper with multiple deep-water masses influencing sedimentation. This same period has been interpreted by other authors as the entry of NADW into the South Atlantic basins. Two other significant changes occurred in the drift construction at 5.5 Ma and 2.5 Ma, which correspond in time to the Mio-Pliocene boundary and onset of large-scale Northern Hemisphere glaciation, respectively. Early Pliocene drift sedimentation indicates a stong but steady deep current system. In contrast, the Valentin Feilberg drift records more variability over the past 2.5 Myr.