Three ODP sites located on the Marion Plateau, Northeast Australian
margin, were investigated for clay mineral and bulk mineralogy changes
through the early to middle Miocene. Kaolinite to smectite (K/S) ratios,
as well as mass accumulation rates of clays, point to a marked decrease
in accumulation of smectite associated with an increase in accumulation
of kaolinite starting at similar to 15.6 Ma, followed by a, second
increase in accumulation of kaolinite at similar to 13.2 Ma. Both of
these increases are correlative to an increase in the calcite to
detritus ratio. Comparison of our record with published precipitation
proxies from continental Queensland indicates that increases in
kaolinite did not correspond to more intense tropical-humid conditions,
but instead to periods of greater aridity. Three mechanisms are explored
to explain the temporal trends in clad, on the Marion Plateau: sea-level
changes, changes in oceanic currents, and denudation of the Australian
continent followed by reworking and eolian transport of clays. Though
low mass accumulation rates of kaolinite are compatible with a possible
contribution of eolian material after 14 Ma, when Australia became more
arid, the lateral distribution of kaolinite along slope indicates mainly
fluvial input for all clays and thus rules out this mechanism as well as
oceanic current transport as the main controls behind clay accumulation
on the plateau. We propose a model explaining the good correlation
between long-term sea-level fall, decrease in smectite accumulation,
increase in kaolinite accumulation and increase in carbonate input to
the distal slope locations. We hypothesize that during low sea level and
thus periods of drier continental climate in Queensland, early Miocene
kaolinite-rich lacustrine deposits were being reworked, and that the
progradation of the heterozoan carbonate platforms towards the basin
center favored input of carbonate to the distal slope sites. The major
find of our study is that increase kaolinite fluxes on the Queensland
margin during the early and middle Miocene did not reflect the
establishment of a tropical climate, and this stresses that care must be
taken when reconstructing Australian climate based on deep-sea clay
records alone. (c) 2005 Elsevier B.V. All rights reserved.