24. PALYNOLOGY OF THE PLEISTOCENE GLACIAL/INTERGLACIAL CYCLES OF THE AMAZON FAN (HOLES 940A, 944A, AND 946A)1Carina Hoorn2 |
ABSTRACTThe composition and distribution of organic matter in sediments of the Amazon Fan during the Pleistocene glacial/interglacial sequences has been studied using samples from Holes 940A, 944A, and 946A (Ocean Drilling Program Leg 155). The results show that glacial lowstand deposits are characterized by large concentrations of a palynological assemblage composed of Andean and tropical lowland taxa, minor amounts of reworked Paleozoic, Cretaceous, and Tertiary taxa, and large volumes of organic debris (e.g., wood, vessels, epidermis, and cuticles). In general, these intervals do not contain many taxa diagnostic for the lowland savannah areas. In all the studied sites, there is an increase of ferns, fern allies, reworked and corroded palynomorphs, and fungal spores at the time of the last glacial. Interglacial highstand deposits are instead characterized by low concentrations of the terrigenous palynological assemblage and a predominance of marine microfossils. Although no marked differences exist in the composition of the organic matter along the fan, there is a decrease in the concentration of palynomorphs from proximal (Sites 940 and 944) to distal (Site 946) sites. There is no compositional difference between the levees and the debris flow. The palynological sequences in the Amazon deep sea fan allow us to characterize the composition of the glacial/interglacial intervals and to correlate the last glacial between the different sites. Moreover, they give an indication of the distribution and concentration of palynomorphs within the fan. These palynological data give no indication of major vegetational changes in the drainage basin. If extensive savannah vegetation existed in Amazonia during the Pleistocene glacial intervals, as claimed in the refugia theory, evidence of it was either obliterated or went unrecorded in the sediments of the Amazon Fan. |
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