Abstract
The high abundances and cyclic distribution of remains of the freshwater fern Azolla in early middle Eocene sediments from the Arctic Ocean have previously been related to episodic surface water freshening, which was speculated to be orbitally modulated. Our integrated palynological and cyclostratigraphical analysis of the recovered ‘Azolla interval’ in
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Integrated Ocean Drilling Program (IODP) Core 302-M0004A-11X resulted in the recognition of two clear periodicities: a dominant ~1.2 m cyclicity, which we relate to changes in obliquity (~40 ka) and a weaker ~0.7 m cyclicity, which we link to precession (~21 ka). Cycles in the abundances of Azolla, cysts of freshwater-tolerant dinoflagellates, and swamp-vegetation pollen show co-variability in the obliquity domain. This strong correlation suggests periods of enhanced rainfall and runoff during Azolla blooms, presumably linked to increased local summer temperatures during obliquity maxima. Larix and bisaccate conifer pollen co-vary at the precession frequency, with peak occurrences corresponding to precession minima, possibly as a result of enhance continental runoff from a more remote source area and a stronger seasonal contrast. Coeval Azolla fossils in neighboring Nordic seas were thought to have been sourced from the Arctic. However our palaeobotanical and palynological investigations reveal that five different species of Azolla had coeval blooms and spread across different areas, both on and around the Arctic and Nordic Seas. Warm climates, high precipitation and related runoff, combined with the semi-closed palaeogeographic setting of the Arctic Ocean and Norwegian-Greenland Sea, probably led, at least episodically, to in situ Azolla blooms on extraordinarily low-salinity ocean surfaces. Climatic conditions also enabled Azolla to bloom in extensive wetlands on adjacent continents from where its remains were transported to oceanic settings. The eventual demise of the Azolla may be related to a crucial decrease in mid to high-latitude precipitation, possibly caused by decreasing global temperatures in the middle Eocene. A decrease in precipitation would likely have resulted in reducing the extent of continental wetlands and a reduction in freshwater discharge into the Arctic and Nordic seas. A slight increase in surface-water salinities may have crossed the critical threshold of salinity tolerance of Azolla. In addition, a possible increase in the exchange of water masses between the North Atlantic, the Greenland-Norwegian Sea basins, and the Arctic Ocean, caused by subsidence of the ridges or/and sea-level rise, could have resulted in a higher salinity of Arctic and Norwegian-Greenland Sea surface Ocean waters
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