Palaeontological and isotope evidence for warm saline deep waters in Ordovician oceans

TODAY'S oceans are characterized by cold bottom waters, produced at high latitudes. Here we present morphological and oxygen isotope data for brachiopods from the Caradocian (Upper Ordovician) Trenton Group, in New York, which indicate that both temperature and salinity increased with depth in Ordovician oceans. Production of warm, saline deep waters in low- to mid-latitude evaporative seas, and reduced production of cold deep water, may have been favoured by high sea level, warm global climate and fortuitous positioning of continents 1–5 . The resulting ocean circulation system was markedly different from that of today. Lower oxygen contents of warm, saline deep waters may have contributed to the deposition of organic-rich sediments, which are abundant in Caradocian strata 6,7 but rare in modern ocean sediments.