Palynomorphs Recovered from the ANDRILL SMS Sediment Cores Provide First Proximal Environmental Characterization of the Mid Miocene Climatic Optimum

The 2007 ANDRILL Program (AND2A), drilled from a sea-ice platform in southern McMurdo Sound, penetrated 1138.54 m sub-seafloor (mbsf) with 98% core recovery of glacially-influenced marine sediments. Biostratigraphy, magnetostratigraphy, and isotopic dating suggest that an Early and Middle Miocene section (223-1138.54 mbsf) is disconformably overlain by a Pliocene to Recent interval.

This younger interval (223 to 0 mbsf) contains sparse to moderate assemblages of penecontemporaneous and reworked marine palynomorphs (leiospheres, Micrhystridium species, and rare reworked dinoflagellate cysts) and terrestrial palynomorphs that are likely mainly reworked.

The middle Miocene section (458 to 223 mbsf) yielded rare to extremely abundant marine palynomorph assemblages. Of particular interest is the evidence for a significant algal bloom in species of Operculodinium, Pyxidinopsis, and leiospheres during part of the mid Miocene Climatic Optimum. Freshwater algae are also notably more common in some of the middle Miocene samples, especially colonial chlorococcales (Pediastrum, Scenedesmus-type) and Zygnemaceae, consistent with increases in rainfall/meltwater, standing water, and run-off. This section also yielded abundant spores and pollen, including the bryophyte spores Coptospora spp., a varied angiosperm flora and some samples have notably high counts of Nothofagidites and Podocarpidites pollen. The core contains the first accepted records of fossil grass pollen (Poaceae) from the Ross Sea region. For most of this time interval, a general climate cooler than the modern austral polar-alpine tree limit is suggested, although at times this limit (10° C January mean) may have been reached.

The Early Miocene interval (below 458 mbsf) yielded low to moderate diversity assemblages with a steady down-hole increase in the numbers of leiospheres present, possibly representing cooler to colder conditions, with some floating ice and a significant fresh water input. The terrestrial component is similar to the Middle Miocene, but characterised by more common recycled specimens.