Sequence Stratigraphy and Biodiversity: an example from Quaternay successions of the Po Plain Italy

The relation between sequence stratigraphic (sea-level driven) processes and paleoecological and macroevolutionary patterns is receiving increasing attention. Here, we evaluate stratigraphic anatomy of diversity patterns across transgressive-regressive (glacial-interglacial) cycles (4th-order sequences) from the upper Quaternary deposits of the Po Plain (Italy). The rich mollusk-dominated fossil assemblages, dominated by extant forms offer a testing ground for exploring how climate-driven sea level changes affect diversity patterns, including sample-level diversity, diversity turnover, and higher-order diversity patterns within and across systems tracts and sequences. The studied depositional sequences were densely sampled from three cores and the resulting data (150 species, 22776 specimens, and 48 Holocene and Pleistocene samples) were analyzed using single-sample and multi-sample rarefaction techniques. For both cycles, sample-level diversity decreased upward in all three cores. The late Transgressive Systems Tract (TST) samples displayed the highest richness and equitability and the Highstand Systems Tract (HST) samples displayed the lowest richness and equitability (the trend reflects primarily the increase in the dominance of most common taxa in HST samples). Most likely, this pattern resulted from a combination of environmental, ecological, and taphonomic processes. For both depositional cycles and all three cores, multi-sample rarefaction simulations suggest that species turnover is more limited in transgressive phases of the cycles. The observed trend may reflect increasing environmental heterogeneity of proximal habitats (averaged within shallowing-upward successions) and/or decreasing time averaging linked to increasing sedimentation rates expected for HST phases of the cycles. Diversity levels observed within individual late-TST systems tracts exceed the sequence and multi-sequence levels, indicating that species turnover was minimal both within and across the last two glacial-interglacial cycles. In summary, equitability and richness of the most common mollusk species track tightly the sequence stratigraphic architecture of the upper Quaternary sedimentary successions of the Po Plain.