Ancient DNA chronology within sediment deposits: are paleobiological reconstructions possible and is DNA leaching a factor?

In recent years, several studies have reported the successful extraction of ancient DNA (aDNA) from both frozen and nonfrozen sediments (even in the absence of macrofossils) in order to obtain genetic "profiles" from past environments. One of the hazards associated with this approach, particularly in nonfrozen environments, is the potential for vertical migration of aDNA across strata. To assess the extent of this problem, we extracted aDNA from sediments up to 3300 years old at 2 cave sites in the North Island of New Zealand. These sites are ideal for this purpose as the presence or absence of DNA from nonindigenous fauna (such as sheep) in sediments deposited prior to European settlement can serve as an indicator of DNA movement. Additionally, these strata are well defined and dated. DNA from sheep was found in strata that also contained moa DNA, indicating that genetic material had migrated downwards. Quantitative polymerase chain reaction analyses demonstrated that the amount of sheep DNA decreased as the age of sediments increased. Our results suggest that sedimentary aDNA is unlikely to be deposited from wind-borne DNA and that physical remains of organisms or their ejecta need to have been incorporated in the sediments for their DNA to be detected. Our study indicates that DNA from sediments can still offer a rich source of information on past environments, provided that the risk from vertical migration can be controlled for.