NO 'PHYTOPLANKTON BLACKOUT' IN THE LATE PALEOZOIC

The stratigraphical interval between the Carboniferous and early Triassic, i.e., the Late Palaeozoic – Early Mesozoic, is a time period with a very low diversity of organic-walled phytoplankton, and this period was therefore considered by some authors as corresponding to a ‘Phytoplankton Blackout.’ It has been argued that the marine food web during this interval was related to the invasion of land by plants, suggesting that a reduced nutrient input to the ocean by runoff decreased the number of acritarchs and primary production in the marine realm. Subsequently, it has also been suggested that the Phanerozoic phytoplankton record can be correlated with models of pCO₂ with high acritarch diversities corresponding to periods of high pCO₂. Conversely, the spread of terrestrial forests may have increased weathering rates via deeper rooting, releasing increased amounts of nutrients to the oceans while also increasing atmospheric pCO₂.

The present paper critically reviews the proposed scenarios and discusses the possible relations between terrestrial and marine ecosystems, in particular the possible impact of the terrestrialisation process on the marine phytoplankton. The known Palaeozoic fossil record of the phytoplankton is incomplete to a high degree. It consists almost entirely on the organic-walled fraction, because calcareous and siliceous phytoplankton remain almost unrecorded. Taking into consideration that only few modern phytoplankton taxa produce cysts, the absence of cysts in the fossil record does not necessarily imply the absence of phytoplankton. In contrast, the presence of planktotrophic larvae of marine invertebrate organisms indicates that phytoplankton must have been present in the Late Palaeozoic oceans, and the marine trophic web did indeed not collapse in the Late Devonian. The presence and abundance of filter feeding and suspension feeding benthic organisms such as brachiopods, crinoids, sponges and corals also suggest sufficient primary production in the Late Palaeozoic seas.

It can be concluded that, athough the phytoplankton is largely absent from the fossil record, a ‘phytoplankton blackout’ is unrealistic.