The researchers attribute the presence of Synedropsis fossils in these sediments to the presence of sea ice, and silica-enriched waters that favour their preservation. They propose that, like Synedropsis species found in polar regions today, the ACEX species were also sea-ice specialists uniquely adapted for surviving the lengthy polar darkness and freezing temperatures. "These diatoms provide the most compelling evidence for ancient sea ice, as they rely on this medium for their survival," said Catherine Stickley. Moreover, their analysis of quartz grain textural characteristics further supports sea ice as the dominant transporter of ice-rafted debris at this time.
"It is likely that sea ice formed in autumn and winter and melted in spring and summer, as seasonal sea ice does today," they say. Synedropsis species probably over-wintered within the sea ice and then bloomed there in the spring when there was enough sunlight. They would have been released into stratified surface waters as the ice melted, rapidly sinking to the sea bottom as aggregates, leaving other diatom species to dominate summer production. And, indeed, these seasonal changes can be discerned in the sediment cores.
The researchers conclude from their analysis, which cover a two-million year period, that episodic sea ice formation in marginal shelf areas of the Arctic started around 47.5 million years ago, about a million years earlier than previous estimates based on ice-raft debris evidence only. This appears to have been followed hal
|Contact: Dr. Rory Howlett|
National Oceanography Centre, Southampton (UK)