Clay minerals in Cenozoic sediments off Cape Roberts (McMurdo Sound, Antarctica) reveal the palaeoclimatic history

The clay mineral assemblages of the ca. 1600 m thick Cenozoic sedimentary succession recovered at the CRP-1, CRP-2/2A
and CRP-3 drill sites off Cape Roberts on the McMurdo Sound shelf, Antarctica, were analysed in order to reconstruct the
palaeoclimate and the glacial history of this part of Antarctica. The sequence can be subdivided into seven clay mineral units
that reflect the transition from humid to subpolar and polar conditions. Unit I (35–33.6 Ma) is characterised by an almost
monomineralic assemblage consisting of well crystalline, authigenic smectite, and therefore does not allow a palaeoclimatic
reconstruction. Unit II (33.6–33.1 Ma) has also a monomineralic clay mineral composition. However, the assemblage consists
of variably crystallized smectite that, at least in part, is of detrital origin and indicates chemical weathering under a humid
climate. The main source area for the clays was in the Transantarctic Mountains. Minor amounts of illite and chlorite appear for
the first time in Unit III (33.1–31 Ma) and suggest subordinate physical weathering. The sediments of Unit IV (31–30.5 Ma)
have strongly variable smectite and illite concentrations indicating an alternation of chemical weathering periods and physical
weathering periods. Unit V (30.5–24.2 Ma) shows a further shift towards physical weathering. Unit VI (24.2–18.5 Ma)
indicates strong physical weathering under a cold climate with persistent and intense illite formation. Unit VII (18.5 Ma to
present) documents an additional input of smectite derived from the McMurdo Volcanic Group in the south.