LATE QUATERNARY GLACIAL CYCLES IN THE BAHAMIAN DEEP BASINS AND IN THE ADJACENT ATLANTIC OCEAN (ARAGONITE, DISSOLUTION).

摘要

Late Quaternary sediments in the deep Bahamian basins and in the adjacent Atlantic Ocean, display well defined synchronous cyclic patterns in the aragonite, carbonate, quartz content and the composition of clays and feldspars. The aragonite cyclic variations closely match the oxygen isotope curve of the planktonic foraminifer Globigerinoides rubra, a worldwide record of recent ice ages. This correlation ties the cycles to the glacial/interglacial rhythm of Earth's climate. The aragonite cycles are best defined in the deep Bahamian basins; towards the open Atlantic, they are gradually replaced by cycles in carbonate content. The cyclicity in the non-carbonate fraction is identical in the Bahamian basins and the adjacent Atlantic Ocean and is related to change in glacial/interglacial oceanic water circulation and sediment sources.; We have rejected the widely accepted hypothesis that the aragonite cycles reflect primarily input variations caused by flooding and exposure of the banks during Quaternary sea level cycles. This rejection is based on the three following observations: (1) the newly discovered close match of aragonite cycles and oxygen isotope record, (2) the time lag of several thousand years between the increase of aragonite and bank flooding during the last deglaciation and (3) the high correlation between the aragonite record and the well established sequence of deglaciation events. Instead, we propose aragonite dissolution cycles, as the prime control of the cycles. They are shallow-water analogues of the calcite dissolution cycles in the deep Atlantic that are known to be tied to the glacial/interglacial changes in the water structure and circulation. In the Bahamas, these cycles were recorded as variations in aragonite content because of the intermediate depth of the Bahamian basins and the proximity of carbonate banks as aragonite sources. Sedimentation rates, calculated from the stratigraphy based on the aragonite cycles, show that, in the Bahamian basins interglacial sedimentation rates are relatively high and regionally variable, whereas glacial sedimentation rates are relatively low and regionally uniform. This pattern is opposite to that of siliclastic continental margins, where sedimentation rates are high and variable during glacial intervals.