Earth science: Volcanic cause of catastrophe

From the timing, it looks as if an episode of marked oceanic oxygen deficiency during the Cretaceous was the result of undersea volcanism. Studies of such events are relevant to the warming world of today. At times in the past, ocean waters have been depleted in dissolved oxygen and large amounts of organic matter, probably produced by enhanced primary production or preservation, were buried on the ocean floor. These oceanic anoxic events (OAEs) often coincided with elevated temperatures, high atmospheric CO2 and mass extinctions. The event known as OAE2 occurred at the Cenomanian–Turonian boundary about 93.5 million years ago, coincident with an extinction event mainly affecting marine invertebrates. The trigger for OAE2 has not been identified, though large-scale magmatic activity is suspected. Steven Turgeon and Robert Creaser now establish a clear link between the onset of OAE2 and magmatism through a direct proxy, the seawater osmium isotope ratios recorded in organic-rich black shales.