Links between ocean deoxygenation and phytoplankton evolution in an Early Jurassic greenhouse world: A palynological study of Lowermost Hettangian black shales from southwest England.

Abstract

The end of the Triassic is characterized by a large mass extinction event, were approximately 80% of all species became extinct. The extinction event was caused by extensive volcanism from the Central Atlantic Magmatic Province (CAMP; ~201 Ma) which lead to global warming and caused an oxygen deficiency and euxinic conditions in the oceans. The loss of oxygen also coincided with a shift from red algae towards green algae. Here, we present a composite succession of four sections (Lilstock, Kilve, East-Quantoxhead and St. Audrie’s Bay) from south-west England covering the entire Hettangian stage using a high-resolution quantative analysis of 50 samples providing information on both marine and terrestrial palynomorphs. The Hettangian was characterized by mainly green algae (acritarchs and prasinophytes) but has several episodes were red algae (dinoflagellates) became more abundant. This could mean that the entire Hettangian still experienced dysoxic conditions with several episodes with more oxic conditions. Abundances of Structureless Organic Matter (SOM) and pyrite suggest that the Hettangian also knew episodes of Photic Zone Euxinia (PZE). The biodiversity was low for terrestrial palynomorphs (pollen and spores), but it fluctuated for marine algae, which might indicate more dynamic changes in stratification and redox conditions in the oceans. These changes might be orbitally forced. Ongoing anoxic conditions in the oceans were driven by late-stage eruptive phases of CAMP volcanism, which continued for at least 600 kyr after the Triassic-Jurassic boundary but based on this results are thought to continue up into the Sinemurian.