The impact of the ‘terrestrialisation process’ in the late Palaeozoic: pCO2, pO2, and the ‘phytoplankton blackout’

With few exceptions, the impact of the evolution of the terrestrial biosphere on the evolution of the marine sphere has been largely ignored. So too has the resulting complementary role of marine photosynthesis and primary productivity in the evolution of atmospheric and oceanic pCO2 and pO2 in response to land–sea interactions. The Early-Middle Palaeozoic invasion of the continents by plants is considered to be responsible for major changes in the carbon cycle and changing values of pO2 and pCO2 in the atmosphere. Some authors have also related the ‘terrestrialisation process’ to the rapid Late Devonian decline of organic-walled phytoplankton (acritarch) diversity. 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 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. In addition, the fossil record solely provides information about the diversity of cysts, but not necessarily precise data of the number and quality of the cyst-producing phytoplanktonic organisms. 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, although the phytoplankton is largely absent from the fossil record, a ‘phytoplankton blackout’ is unrealistic. A major remaining question is to understand why the cyst production decreased after the Late Devonian and why this might be correlated to changes of pCO2.