SOUTHERN OCEAN DIATOM DIVERSITY AND NEOGENE CLIMATE CHANGE

Neogene expansion of first Southern and then Northern Hemisphere continental ice volume resulted in important changes in ocean temperature, oceanographic fronts, and Southern Ocean current strength. Marine planktonic biota would respond to environmental forcing of this nature, with evidence recorded in the fossil record. Comprehensive datasets of sufficiently high resolution and coverage to constrain changes in microfossil species composition and diversity on a regional scale are difficult to assemble and analyze.

We present a 16 m.y. record of latest Neogene Southern Ocean diatom diversity and turnover drawn from a database of local ranges of 191 diatom taxa, from 33 Antarctic to Subantarctic drillcores. We employ a recently developed quantitative biostratigraphic technique – Constrained Optimization (CONOP) – to synthesize the incomplete and often contradictory fossil successions preserved locally and to account for the effects of shifting biogeographic ranges and failures of preservation and sampling. The resulting composite sequence of first and last appearance events is calibrated using selected paleomagnetic reversals and dated ash layers, and once normalized for uneven sampling, provides a precise record of the relative changes in regional diversity of marine diatoms. The most dramatic Late Neogene feature is a drop in diversity that begins at 3 Ma and accelerates at 2.5 Ma, roughly coincident with the acceleration of Northern Hemisphere glaciation and the onset of associated climatic changes globally. This diatom event can be resolved into two components: a prolonged decrease in the pace of origination events following a peak in the early Pliocene (during which cold-water taxa such as Fragillariopsis curta first appeared, and a conspicuous pulse of extinction events between 2.5 and 2 Ma. Previous workers have suggested that that marine planktic diatom taxa were lost somewhat catastrophically around this time and not replaced, which they infer was related to climatic cooling. Unfortunately, no other comparable records of marine plankton diversity or turnover for this region are available. However, qualitative observations of Southern Ocean diatom successions and more detailed, quantitative work on the diversity of New Zealand shelf mollusk species and genera are in agreement with these results, now better constrained by age.