Coastal vegetation shifts and climate change inferred from ancient sedimentary plant DNA
Microscopic identification and enumeration of fossil pollen is the most commonly used approach to reconstruct shifts in the paleovegetation and to reconstruct climate change. However, this approach is time consuming and can suffer from low taxonomic resolution, especially for dry vegetation C4 grasses. An alternative approach is molecular community analyses from the vegetation paleome. Terrestrial plant material can enter water bodies in the form of pollen or in the form of plant litter (e.g. via river runoff). 18S rDNA from terrestrial plants has been recovered from Pleistocene Mediterranean deep-sea sediments(Boere et al., 2011) and represented up to 10% of the pyrotags in marine and lacustrine Black Sea sediments (Coolen et al., in review at PNAS), indicating that marine sediments harbor a hitherto unexplored vegetation paleome. A more suitable barcoding gene for plants is the short (~100 bp) hypervariable chloroplast-located trnL-P6 loop. TrnL-P6 barcoding enables the identification of most plants at the genus level (>75%) and one-third of all plants at the species-level. We recently explored the Ion Torrent NGS platform (low-cost sequencing of short reads) to sequence the fossil trnL-P6 pool to reconstruct late-Pleistocene to mid-Holocene vegetation and climate changes in the Black Sea. For example, this approach revealed a high relative abundance of the fern Dryopteris during the first half of the Younger Dryas (~12.8-11.6 ka cal BP) indicative of open grassland and/or secondary forest during this cold stadial climate. Dry conditions during the early Younger Dryas was confirmed from a predominance of DNA from C4 grasses and trees like Citrus and Prunus. Interestingly, a shift to a predominance of C3 grasses, and therefore higher precipitation, occurred during the second half of the Younger Dryas. A predominance of different ferns, notably Athyrium, was indicative of closed dense forest vegetation with moist and shadow-rich ground cover during the following Preboreal until 10.4 ka cal BP. Similar conditions returned abruptly at the peak of the HCO, which apparently started in this region at 8 ka cal BP. A high humidity and stable mild temperatures year-round must have occurred in the last millennia of deglaciation as indicated by a predominance of mountainous laurel-type evergreen and moist riperian mixed forest trees between 10 and 9 ka cal BP (Coolen et al., in prep.). These vegetation types were not identified through our parallel pollen survey (Marinova-Filipova et al., 2012), and neither pollen nor the aquatic plankton paleome revealed the major continental climate shift from dry to wet conditions at 8 ka cal BP.