Late Cretaceous and modern diatom ecology: implications for our changing oceans

Photomicrographs of diatom resting spores. Scale bars =10 mm (From Davies and Kemp, 2016)

Diatoms are unicellular algae with golden-brown photosynthetic pigments with a fossil record that extends back to Early Jurassic. They live in aquatic environments, soils, ice, attached to trees or anywhere with humidity, and their remains accumulate forming diatomite, a type of soft sedimentary rock. The most distinctive feature of diatoms is their siliceous skeleton known as frustule that comprise two valves. The formation of this opaline frustule is linked  in modern oceans with the biogeochemical cycles of silicon and carbon.

Past fluctuations in global temperatures are crucial to understand Earth’s climatic evolution. During the Late Cretaceous the global climate change has been associated with episodes of outgassing from major volcanic events, orbital cyclicity and tectonism before ending with the cataclysm caused by a large bolide impact at Chicxulub, on the Yucatán Peninsula, Mexico. Following a major diatom radiation after the Cenomanian-Turonian anoxic event, the development of the first extensive diatomites provides the earliest widespread geological evidence for the rise to prominence of diatoms in ocean biogeochemistry. Studies of the greenhouse Cretaceous climates are especially topical since such warm, high CO2 periods of the past are often invoked as potential analogues for present warming trends (Davies and Kemp, 2016).

A. Chain of Stephanopyxis turri (From Davies and Kemp, 2016)

Because their abundance and sensitivity to different parameters,  diatoms play a key role in Paleoceanography, particularly for evidence of climatic cooling and changing sedimentation rates in the Arctic and Antarctic oceans and to estimate sea surface temperature. Like Stephanopyxis, a common planktonic genus in the present oceans distinguished by its long stratigraphic range from the Albian to modern. Stephanopyxis can be found in tropical or warm water regions and evidence suggests a similar ecological adaptation during the Cretaceous. Meanwhile, resting spore development is generally associated with the onset of unfavourable environmental conditions and sporulation generally occurs in response to a sudden change in one or more environmental factors.

Since the start of the Industrial Revolution the anthropogenic release of CO2 into the Earth’s atmosphere has increased a 40%. In this context, warming of the present surface ocean is  leading to increased stratification in both hemispheres. Based on traditional views of diatom ecology, ocean stratification would  lead to decreased diatom production and a reduced effectiveness of the marine biological carbon pump. But recent ocean surveys, and records of the stratified seas of the Late Cretaceous, suggest that increased stratification may lead to increased rather than decreased diatom production and export. This would then result in a negative-rather than positive feedback to global warming (Davies and Kemp, 2016).

 

References:

A. Davies, A.E.S. Kemp, Late Cretaceous seasonal palaeoclimatology and diatom palaeoecology from laminated sediments, Cretaceous Research 65 (2016) 82-111

Martin, R. E. and Quigg, A. 2012 Evolving Phytoplankton Stoichiometry Fueled Diversification of the Marine Biosphere. Geosciences. Special Issue on Paleontology and Geo/Biological Evolution. 2:130-146.

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