STUDIES ON THE DEVONIAN/CARBONIFEROUS PHYTOPLANKTON DECLINE

We have begun to investigate the characteristics of the acritarch decline that began in the Late Devonian seemingly coincident with the mass extinctions. Taxon distribution data from the PalynoData dataset were progressively filtered to produce a curve that is more robust in terms of well characterized acritarch species and their temporal distribution. Filtering involves: 1) removal of poorly constrained genera, 2) combining synonymous taxa (using Fensome et al., 1990), 3) eliminating taxa not identified to species level, and 4) rejecting records with poor temporal (e.g. system-level) resolution. Filtering removes almost 25% of the database records. Stage level binning reveals a smooth decline of standing acritarch taxon diversity beginning in the Famennian and extending through the Mississippian. The curve drops severely at the Dev/Miss with the loss of 24% (genera). The decline represents a loss of approximately 50% of genera beginning in the Famennian (116) and continuing into the middle Bashkirian of the lower Pennsylvanian (57).

Acritarchs are largely the cysts of planktonic marine algae and as such track the evolutionary activity of marine photosynthesis in the surface oceans. They are more sensitive to changing ocean and atmospheric chemistry than benthic invertebrates which depend upon buffered trophic systems for survival. The acritarch decline now appears to postdate the bulk of the Devonian extinctions, so declining phytoplankton populations do not appear causal in this regard. But declining CO₂ levels and shifting ocean chemistry may have played a fundamental role in causing the decline in green (a+b chlorophyll) phytoplankton - a group which ultimately conquered land, but never again dominated the marine realm. There is consideration from modern algae that show differences in the efficiencies of the enzyme Rubisco in various algal and cyanobacterial groups and these differences may account for the acritarch response to the CO₂ drop.