GLOBAL WARMING OR ICE AGE? - THE OLDEST RECORD OF OCEAN ACIDIFICATION ON SKELETAL REEFS PROVIDES AN ANSWER

The Late Ordovician is well known for hosting one of the most significant first order mass extinctions during the Phanerozoic. Marine biota at that time faced drastic climate changes which resulted in a loss of approximately 26% of all families. This catastrophic biotic crisis coincided with the Hirnantian ice age which caused a rapid decline in biodiversity during this interval. The Late Ordovician also witnessed the rise of reefs constructed by carbonate skeletons of various organisms to become significant ecosystems in earth’s history. Modern coral reefs react very sensitively to environmental changes and are, therefore, good indicators of current global warming. Correspondingly, we analysed the distribution pattern of Early Palaeozoic reefs from Sandbian to middle Aeronian times, spanning an interval of about 24 million years. Here we show that reef ecosystems were already in decline prior to the onset of the Hirnantian ice age. During the critical interval, the number of reef areas globally declined for more than two-thirds from the late Katian to the Hirnantian. Distribution patterns demarcate a loss of reefs in low latitudes between 12°N - 15°S persisting for more than 2 million years. Our study shows that the Late Ordovician reef crisis did not, however, exclusively result from rapidly cooling oceans. The major cause for global decline of reefs and their absence in the tropical belt during the Hirnantian can be attributed to ocean acidification during preceding greenhouse conditions.