A δ30Si isotopic view on the climatic role of diatoms: driver or feedback?

Ros Rickaby, Kate Hendry, Bastian Georg, Katie Egan, Alex Halliday, Melanie Leng (NIGL), John Anderson (Rice University, Texas)

The evolutionary radiation of siliceous diatoms during the Cenozoic took place at the expense of the coccolithophores and forced the ocean from a high silica concentration during the Mesozoic towards the modern silica depleted ocean. At the same time, Earth underwent a major transition, from a greenhouse with high atmospheric carbon dioxide (pCO2) to an icehouse world with low pCO2. This long-term drawdown of carbon from the atmosphere may be the result of increased marine productivity associated with the expansion of the diatoms. However, it might also reflect enhanced continental weathering of silicate minerals, perhaps driven in part by the evolution of Si-bearing grasses and phytoliths. Today diatoms represent 40% of marine primary productivity, are efficient conveyors of carbon to the deep ocean and may play a key role in short- as well as long-term climate change.The aim of our project, funded by the Natural Environment Research Council (NERC) is to investigate the nature of this fundamental part of the Earth’s biogeochemical cycles by studying silicon isotopes (δ30Si) preserved in diatom opal.

We are carrying out two sets of studies.  First, we are constructing a Cenozoic history of diatom δ30Si to provide insight into the dynamics of diatom production relative to the major climatic transitions. With combined δ18OSi measurements we will investigate the factors responsible for the increasing Cenozoic success of the diatoms.  Although phytoplankton evolution appears intimately linked with climate, it is unclear whether they drive or respond to climate change.

Second, we will establish the role of diatoms in mitigating the rise of atmospheric pCO2 on a centennial timescale by reconstructing the degree of silica utilisation in the highly productive coastal zone of the Antarctica Peninsula (AP), currently responsible for ~50% of Southern Ocean productivity. We shall construct subdecadal-resolution records of silica utilisation for the last five centuries in rapidly accumulating sediments from the AP to substantiate whether our preliminary finding that diatoms are blooming in an unprecedented response to anthropogenic warming, can be extrapolated across the AP and represent a feedback to global warming.