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The Modern Carbonate Contourite System of the Little Bahama Bank During the Last Million Years

Abstract

The Bahama Banks are a modern carbonate system with very limited terrigenous clastic input which has provided many modern analogues for ancient carbonated series. We focus on the major sedimentary body of the northwestern part of the Little Bahama Bank (LBB) slope. Our objectives are to study the export processes from the bank to the northern slope of the LBB (windward side) and sediment deposition related to the regional surface Antilles Current. This work is focused on two marine cores located at the west side of the northern slope of the LBB and collected during the CARAMBAR cruise (2010). CARKS-21 penetrates the wedge of this major sedimentary body over 12 m and CARKS-20 penetrates in its thickest part over 13.25 m. Analyses of these two marine cores allow to determine the sediment sources and to build a high-resolution stratigraphic model. The stratigraphical methods include planktonic foraminifera and coccoliths analyses, radiometric dating, XRF analyses and isotopic stratigraphy. The sedimentary analyses include spectrophotocolorimetry and grain-size measurements, and indurated thin sections. CARKS-21 extends from present to the marine isotope stage (MIS) 11 (∼ 424 kyr, from top to 9.3 m), and highlight perturbation in the sedimentation before MIS 11. CARKS-20 extends from MIS 1 to MIS 11. Glacial periods and sea-level lowstands corresponding to exposure of the bank are related to low production and exportation of sediment. Those reduced deposits show a coarsening-up unit followed by a finning-up unit forming a bigradational sequence, and are associated with allochtonous particles constituted by phosphatized planktonic foraminiferal tests and pteropod shells. Three clay-rich beds have been observed and the two most recent one are well dated and correspond to a short period of MIS 7 and MIS 11. Off-bank processes are the major sedimentary processes during sea-level highstands, resulting in high accumulation rate deposits composed mainly by carbonate foraminiferal ooze. Clay-rich deposits may be related to a slowdown of currents, allowing clay-size particle settling on the contouritic system, and an increase of clay production and export from continental areas. Current-related deposits dominate during sea-level lowstands in the northern Bahamas slope over the last 424 kyr. Phosphatized particles result from a phase of early diagenesis concomitant with transport in the basin by the Antilles Current.