Southern Ocean seabird population shifts over the Holocene revealed by peat sequestration of mercury from guano

Monitored seabird populations have declined by up to 70% worldwide since the 1950s, yet little is known about population sizes or long-term trends prior to anthropogenic impacts. This is particularly true for the Southern Ocean, where seabirds and many other species are currently facing multiple environmental threats. Here, we identify shifts in seabird populations over the last 8100 years by applying a mercury (Hg) flux and isotope signature technique as a novel tracer of population size via guano input into peatlands on sub-Antarctic Bird Island (South Georgia). Our results show that seabirds have colonized Bird Island between 6800 and 6100 years ago, which pre-dates evidence for colonization of other sub-Antarctic islands by more than 1000 years. There are five periods when seabird populations were at maxima during the mid- to late-Holocene, coeval with periods of lower Southern Hemisphere westerly wind intensity. Our study provides unprecedented insights into cyclicity in seabird population sizes as a consequence of regional to large-scale environmental change.