Oligocene–Holocene calcareous nannofossil biostratigraphy and diagenetic etch patterns on quaternary placoliths at ODP Site 1139 on Skiff Bank, northern Kerguelen Plateau

The distribution of calcareous nannofossils at Ocean Drilling Program Site 1139 on Skiff Bank off the western edge of the Northern Kerguelen Plateau has been used to delineate Quaternary, middle Miocene– middle Oligocene, and lower Oligocene sediments in an expanded 385- m rotary-cored section. Sedimentation rates through most of the Tertiary are high (average = 23 m/m.y.) as a result of clastic input and dilution by high siliceous productivity; chert, however, was encountered in only a single sample. These characteristics, plus the presence of multiple, reasonably well preserved siliceous and calcareous microfossil groups deposited well above the calcite compensation depth, make this site a potential candidate for more detailed coring for paleoceanographic objectives in the future. A scanning electron microscope study revealed that diagenetic etching, apparently of gephyrocapsids, caused some of these placoliths to superficially resemble Emiliania huxleyi at levels in the Quaternary well below the evolutionary first occurrence of this taxon. Other larger specimens of etched gephyrocapsids and/or reticulofenestrids resembled Pseudoemiliania lacunosa. These observations suggest that caution should be exercised when age-dating Quaternary sediments not necessarily rich in carbonate. An unusual Braarudosphaera bloom recorded in upper Oligocene sediments on Skiff Bank is similar to that reported previously at Site 737 on the Northern Kerguelen Plateau and may correlate with other such occurrences of this age in the Atlantic and Indian Oceans. The base of the pelagic section is dated as earliest Oligocene by nannofossils and foraminifers (32.8–34.3 Ma; Subchron CP16a/b and basal AP13).