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| Titel |
Orbitally-forced Azolla blooms and middle Eocene Arctic hydrology; clues from palynology |
| VerfasserIn |
Judith Barke, Hemmo A. Abels, Francesca Sangiorgi, David R. Greenwood, Arthur R. Sweet, Timme Donders, André F. Lotter, Gert-Jan Reichart, Henk Brinkhuis |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042553
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| Zusammenfassung |
| The presence of high abundances of the freshwater fern Azolla in the early Middle Eocene
central Arctic Ocean sediments recovered from the Lomonosov Ridge during IODP
Expedition 302, have been related to the presence of a substantial freshwater cap. Azolla
massulae, belonging to the newly described Eocene species Azolla arctica Collinson et al.,
have been found over at least a ~4 m-thick interval. There are strong indications
that Azolla has bloomed and reproduced in situ in the Arctic Ocean for several
hundreds of thousands of years. Possible causes for the sudden demise of Azolla at
~48.1 Ma include salinity changes due to evolving oceanic connections or sea-level
change.
Distinct cyclic fluctuation in the Azolla massulae abundances have previously been related to
orbitally forced climate changes. In this study, we evaluate the possible underlying forcing
mechanisms for these freshwater cycles and for the eventual demise of Azolla in an integrated
palynological and cyclostratigraphical approach.
Our results show two clear periodicities of ~1.3 and ~0.7 m in all major aquatic and
terrestrial palynomorph associations, which we can relate to obliquity (41 ka) and precession
(~21 ka), respectively. Cycles in the abundances of Azolla, freshwater-tolerant dinoflagellate
cysts, and swamp vegetation pollen show co-variability in the obliquity domain. Their strong
correlation suggests periods of enhanced rainfall and runoff during Azolla blooms, possibly
associated with increased summer season length and insolation during obliquity maxima.
Cycles in the angiosperm pollen record are in anti-phase with the Azolla cycles. We interpret
this pattern as edaphically drier conditions on land and reduced associated runoff during
Azolla lows, possibly corresponding to obliquity minima. The precession signal is
distinctly weaker than that for obliquity, and is mainly detectable in the cold-temperate
Larix and bisaccate conifer pollen abundances, which is interpreted as a response to
stronger seasonality with colder winters and warmer summers during precession
minima.
Together with the Azolla demise at 48.1 Ma, a concurrent decline of swamp vegetation
suggests drier local conditions on land, saltwater intrusion, and possibly decreased runoff into
the Arctic Ocean, causing salinity changes which could have been fatal for Azolla
blooms.
After the Azolla demise, the cyclic distribution of freshwater tolerant dinoflagellate cysts
suggests that runoff cycles continued to influence the central Arctic although at decreased
intensity. |
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