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The Eocene Arctic Azolla Phenomenon; Cause, Duration and Impact on Organic Carbon Burial

Timme Donders1, Henk Brinkhuis2, Judith Barke2, Eveline Speelman3, Susanne Nelskamp1, and Andre F. Lotter2
1Geobiology, Geological Survey of the Netherlands, Utrecht, Netherlands.
2Laboratory of Paleobotany and Palynology, Utrecht University, Utrecht, Netherlands.
3Department of Earth Sciences - Geochemistry, Utrecht University, Utrecht, Netherlands.

In September 2004, the first-ever drilling of the Lomonosov Ridge (Arctic Coring Expedition, ACEX, IODP Expedition 302) recovered unprecedented sedimentary records of the central Arctic Ocean spanning the past ~56 Ma. With paleontological and geochemical techniques it has been possible to document the long-term development of the central Arctic for the first time. The environmental setting and paleo-climatic evolution turn out very different from that expected prior to the drilling. The ACEX data, and recent studies from the Norwegian Sea and Alpha ridge now extend the paleo-climate record further back into the Paleogene and even upper Cretaceous, revealing a warm wet greenhouse world which extended even to the high Arctic. Only the last 14 million years show the persistent influence of glacial conditions.

High organic deposition occurred at ~49 Ma with stunning concentrations of remains of the fresh water fern Azolla and freshwater tolerant dinoflagellate cysts. These suggest that, at least episodically, completely fresh surface water settings characterized the Arctic Basin, conditions that potentially allow a basin-wide occurrence of Azolla growth. Coeval Azolla fossils in neighboring Nordic seas were thought to have been sourced from the Arctic. However, new paleobotanical 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.

Integrated palynological and cyclostratigraphical analysis of the recovered ‘Azolla interval’ show 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. The eventual demise of the Azolla may be related to a crucial decrease in mid to high-latitude precipitation during global cooling in the middle Eocene, as well as a slight increase in surface-water salinities.

The organic Azolla deposits may be speculated to have been at the root of the first cooling after peak Greenhouse conditions of the early Eocene, sequestrating between 9 x 1017 and 3.5 x 1018 gC (equivalent to a 55 - 470 ppm CO2 drawdown). The large-scale organic rich deposition and high paleo temperatures described here can be an potential source for hydrocarbon generation in the Arctic ocean. A pilot study of Arctic paleo-surface temperatures and tectonic paleo-heat flow has shown that paleoclimate has a large impact on the modeled source rock maturity and on the timing of generation.


AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.