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TitleTesting the limits in a greenhouse ocean: did low nitrogen availability limit marine productivity during the end-Triassic mass extinction?
AuthorSchoepfer, S D; Algeo, T J; Ward, P D; Williford, K H; Haggart, J W
SourceEarth and Planetary Science Letters vol. 451, 2016 p. 138-148, Open Access logo Open Access
Alt SeriesEarth Sciences Sector, Contribution Series 20160084
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf
Subjectstectonics; paleontology; marine geology; extinctions, biotic; oceanography; nitrogen; isotopes; paleoenvironment; paleogeography; Wrangellia; Panthalassic Ocean; Panthalassa; Mesozoic; Jurassic; Triassic
Illustrationslocation maps; stratigraphic columns; plots; models
ProgramGSC Pacific Division
AbstractThe end-Triassic mass extinction has been characterized as a 'greenhouse extinction', related to rapid atmospheric warming and associated changes in ocean circulation and oxygenation. The response of the marine nitrogen cycle to these oceanographic changes, and the extent to which mass extinction intervals represent a deviation in nitrogen cycling from other ice-free 'greenhouse' periods of Earth history, remain poorly understood. The well-studied Kennecott Point section in Haida Gwaii, British Columbia, Canada, was deposited in the open Panthalassic Ocean, and is used here as a test case to better understand changes in the nitrogen cycle and marine productivity from the pre-crisis greenhouse of the Rhaetian to the latest-Rhaetian crisis interval. We estimated marine productivity from the late Norian to the early Hettangian using TOC-and P-based paleoproductivity transform equations, and then compared these estimates to records of sedimentary nitrogen isotopes, redox-sensitive trace elements, and biomarker data. Major negative excursions in d15N (to -<0perthousand) correspond to periods of depressed marine productivity. During these episodes, the development of a stable pycnocline below the base of the photic zone suppressed vertical mixing and limited N availability in surface waters, leading to low productivity and increased nitrogen fixation, as well as ecological stresses in the photic zone. The subsequent shoaling of euxinic waters into the ocean surface layer was fatal for most Triassic marine fauna, although the introduction of regenerated NH+4into the photic zone may have allowed phytoplankton productivity to recover. These results indicate that the open-ocean nitrogen cycle was influenced by climatic changes during the latest Triassic, despite having existed in a greenhouse state for over 50 million years previously, and that low N availability limited marine productivity for hundreds of thousands of years during the end-Triassic crisis.
Summary(Plain Language Summary, not published)
The authors have studied ecological processes that may have contributed to the mass extinction of life at the end of the Triassic Period of Earth history, about 200 million years ago. To do this, they studied geochemical signatures reflecting the composition of the oceans and atmosphere during Triassic time, preserved in rocks deposited at that time and preserved on the archipelago of Haida Gwaii in western Canada. Haida Gwaii is so far the only place in the world where rocks of appropriate age and of sufficient quality to be studied in this manner are known to exist. Based on their analyses, the authors conclude that low levels of oxygen and high levels of nitrogen in the world's oceans in latest Triassic time changed the biological composition of the oceans significantly, and were likely strong contributing factors of the mass-extinction event recognized at the end of the Triassic Period.

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