Judith Barke
Geosciences building
Budapestlaan 4
3584 CD Utrecht
The Netherlands
Office: W 316 |
Phone: +31 30 253 2631
Fax: +31 30 253 5096
E-mail: J.Barke@uu.nl
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Institute of Environmental Biology, Faculty of Science, Utrecht University
Prof. Dr. Andy Lotter, Prof. Dr. Henk Brinkhuis
Palaeoecology & -biogeography of the fresh water fern Azolla in the Eocene northern hemisphere
INTRODUCTION
The discovery of high abundances of Azolla remains in Eocene marine sediments during the Arctic Coring Expedition (ACEX), suggest that the Arctic Ocean must have been a big pond about 48.5 Myr ago (Brinkhuis et al., 2006; Backman et al., 2006). In 2004 unique central Arctic drill cores have been recovered during the IODP-expedition 302, better known as ACEX, revealing huge numbers of spores of the fresh-water fern Azolla in mid-Eocene sediments. Azolla is a heterosporous fern, which produces two kinds of spores, micro- and megaspores, whereby microspores are clustered together to so-called microspore massulae (Fig.1), which have appendages -glochidia- that get attached to the megaspore to facilitate fertilization. Megaspores are embedded in so-called megaspore apparatuses together with numerous floats (Fig.2).
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Fig.1 Microspore massulae |
Fig.2 Megaspore apparatus. The section (right) shows megaspore & floats
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Mature megaspores with and without attached massulae and clusters of massulae have been found in the Arctic sediments in the order of 25x104 per gram of dry sediment, showing a very good state of preservation. These findings make it very likely that Azolla grew in situ in the Arctic Ocean, rather than being transported by rivers from adjacent continents. Azolla is nowadays to be found in ponds and lakes in tropical, subtropical and warm temperate regions, where it usually covers big parts of the water surface (Fig.3).
Fig. 3 Azolla filiculoides growing in a pond in the Netherlands
In the Eocene Arctic Ocean the necessary fresh surface-water conditions for Azolla are thought to have been created through increased precipitation exceeding the evaporation at higher latitudes (Barron et al., 1989) and enhanced runoff from bordering continents. A cyclic pattern in the Azolla record suggests that freshening of the surface waters occurred in major episodes of estimated 100 kyrs that gave rise to several extensive Azolla blooms (Brinkhuis et al., 2006).
Beside Azolla remains also high abundances of chrysophyte cysts have been found that are also indicative for fresh-water conditions. However, other siliceous microfossil as dinocysts, diatoms, silicoflagellates and ebridians found in the same sediments, are more indicative for brackish to marine conditions. This seemed at first sight contradictory, but detailed analysis of the sediment showed the presence of separate layers, in which Azolla alternates with siliceous microfossils (Fig.4). This lamination has been shown to be annual or seasonal and points to changes in the environment that favored at one stage the growth of Azolla and at the other the growth of siliceous microplankton. It is suggested that salinity might plays an important role in this interplay of species communities (Brinkhuis et al., 2006).
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Fig. 4 SEM showing organic-rich sediment layers (pale grey) dominated by Azolla, alternating with biosilica-rich layers (dark grey) containing siliceous microfossils (in this case dominantly chrysophyte cysts) (Brinkhuis et al., 2006). |
Azolla remains have not only been found in the Eocene Arctic Ocean, but as well in numerous settings in adjacent Nordic seas (Fig.5), where it was recognized by commercial oil and gas industries. Due to confidential reasons, however, it has never been published (Brinkhuis
et al., 2006). In our research study we like to investigate the dimension of this phenomenon and its palaeoecological and oceanographical implications.
Fig.5 The geographical distribution of the Azolla pulse in the early Eocene Arctic basin and in adjacent Nordic seas. The stars indicate the locations where the Azolla pulse was reported (Brinkhuis et al., 2006).
AIM OF THE STUDY
One main objective of our study is to establish a detailed geographical distribution map of the Eocene Azolla occurrences. Besides gathering already available occurrence data, we seek to collect sediment material from a broad geographical range including outcrop samples e.g. from Denmark and possibly Canada and sediments from numerous Nordic Sea locations. These sediments will then be analyzed for Azolla remains and other palynomorphs. So far it is believed that these Eocene Azolla pulses at the different locations can all be ascribed to the one big pulse in the Arctic. From there huge numbers of Azolla spores are thought to have been transported into adjacent geographical regions through episodic fresh-water spills (Brinkhuis et al., 2006). The palynological analysis of these sediments and the taxonomical identification of the Azolla remains (SEM and TEM of microspore massulae and megaspore apparatuses) will show whether this assumption is correct.
High-resolution studies of the ACEX-sediments will hopefully give us more insights into the environmental conditions in the Eocene Arctic Ocean that gave rise to the sudden onset and termination of the Azolla pulse and its cyclic occurrence on an annual/seasonal scale, as well as on a 100 kyrs scale.
LITERATURE
Backman et al., 2006. Proceedings of the Integrated Ocean Drilling Program, Volume 302: Edinburgh (Integrated Ocean Drilling Program Management International, Inc.).
Barron et al., 1989. The hydrologic cycle: A major variable during Earth history. Global and Planetary Change 1, 157-174.
Brinkhuis et al., 2006. Episodic fresh surface waters in the Eocene Arctic Ocean. Nature 441: 606-609.