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A sulfidic driver for the end-Ordovician mass extinction
Stockholm University, Faculty of Science, Department of Geological Sciences.
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2012 (English)In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 331, 128-139 p.Article in journal (Refereed) Published
Abstract [en]

The end-Ordovician extinction consisted of two discrete pulses, both linked, in various ways, to glaciation at the South Pole. The first phase, starting just below the Normalograptus extraordinarius Zone, particularly affected nektonic and planktonic species, while the second pulse, associated with the Normalograptus persculptus Zone, was less selective. Glacially induced cooling and oxygenation are two of many suggested kill mechanisms for the end-Ordovician extinction, but a general consensus is lacking. We have used geochemical redox indicators, such as iron speciation, molybdenum concentrations, pyrite framboid size distribution and sulfur isotopes to analyze the geochemistry in three key Hirnantian sections. These indicators reveal that reducing conditions were occasionally present at all three sites before the first pulse of the end-Ordovician extinction, and that these conditions expanded towards the second pulse. Even though the N. extraordinarius Zone appears to have been a time of oxygenated deposition, pyrite is significantly enriched in 34S in our sections as well as in sections reported from South China. This suggests a widespread reduction in marine sulfate concentrations, which we attribute to an increase in pyrite burial during the early Hirnantian. The S-isotope excursion coincides with a major positive carbon isotope excursion indicating elevated rates of organic carbon burial as well. We argue that euxinic conditions prevailed and intensified in the early Hirnantian oceans, and that a concomitant global sea level lowering pushed the chemocline deeper than the depositional setting of our sites. In the N. persculptus Zone, an interval associated with a major sea level rise, our redox indicators suggests that euxinic conditions, and ferruginous in some places, encroached onto the continental shelves. In our model, the expansion of euxinic conditions during the N. extraordinarius Zone was generated by a reorganization of nutrient cycling during sea level fall, and we argue, overall, that these dynamics in ocean chemistry played an important role for the end-Ordovician mass extinction. During the first pulse of the extinction, euxinia and a steepened oxygen gradient in the water column caused habitat loss for deep-water benthic and nektonic organisms. During the second pulse, the transgression of anoxic water onto the continental shelves caused extinction in shallower habitats.

Place, publisher, year, edition, pages
2012. Vol. 331, 128-139 p.
Keyword [en]
mass extinction, Ordovician, geochemistry, sulfur
National Category
Geochemistry
Research subject
Geochemistry
Identifiers
URN: urn:nbn:se:su:diva-75463DOI: 10.1016/j.epsl.2012.02.024ISI: 000306030500012OAI: oai:DiVA.org:su-75463DiVA: diva2:516743
Available from: 2012-04-19 Created: 2012-04-19 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Ocean chemistry and the evolution of multicellularity
Open this publication in new window or tab >>Ocean chemistry and the evolution of multicellularity
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Oxygen has been assumed to be a vital trigger for the evolution of multicellular life forms on Earth, partly based on its power to promote substantial energy flux in cell respiration and partly as biosynthesis of compounds like collagen require oxygen. However, the co-evolution of large life and the Earth’s chemical environment is not well understood at present, and there is particular disagreement in the field about whether the Cambrian explosion of animal life forms was a chemical or biological event. Here, I discuss the evolution of multicellularity, divided in simple or complex forms, in light of the evolution of ocean water column chemistry in both the Proterozoic and the early Paleozoic. Even if the appearance of animals is confined to the Ediacaran, other fossil evidence of complex multicellularity can be argued to occur in the Paleo-, Meso- and Neoproterozic. These finds are, if anything, reason enough to keep searching for early experiments in complex multicellularity. In this search, we may have to expand our toolbox by looking at e.g. trace element aggregations and the isotopic composition of key elements. 

Research over the last couple of years have accentuated that much of the interval between the Ediacaran and the Devonian was dramatic with transitional ocean chemistry at the same time that large forms of animal life experienced dynamic radiation and ecological expansion. Results presented here describe some aspects of this time, including geochemistry from Chengjiang and a mechanism for preserving non-mineralized Cambrian animals that was partly dependent on specific ocean chemistry. Also, geochemical proxies using iron and molybdenum are used to infer a Paleozoic atmosphere with less than 50% of present levels of oxygen. The possibility that the subsequent rise is due to terrestrial plants and linked to the appearance of large predatory fish is discussed. Finally, the first mass extinction in the end-Ordovician is linked to low oxygen concentrations in the water column. It appears that more than oxygen was critical to allow the radiation of large life forms on Earth, but that chemistry and tectonic activity were intimately intertwined to biology, in a dance of permitting and being determined by certain aspects of ecology.

Abstract [sv]

Under lång tid har vi sett atmosfärens syrehalt som avgörande för att stora livsformer skulle börja utvecklas på jorden, delvis eftersom syre är ett energirikt bränsle men också för att det krävs vid sammansättningen av vissa ämnen som djur behöver, till exempel proteinet kollagen. Men, i själva verket, har vi inte lyckats reda ut detaljerna om hur utvecklingen av tidigt, stort liv och miljö satt samman, och om den kambriska explosionen framförallt var en biologisk eller kemiskt händelse. I den här avhandlingen diskuterar jag hur utvecklingen av flercellighet, då uppdelat i enkla och komplexa former, kan vara kopplad till hur havens kemi förändrats både i proterozoikum (2.5-0.5 miljarder år sedan) och paleozoikum (0.5-0 miljarder år sedan. Även om fossil från moderna djur dyker upp runt ediacaran och kambrium, så finns det långt äldre fossil som kan påvisa flercellighet. Dessa fossil ger, om inte annat, anledning att leta vidare efter fler spår av pre-kambrisk flercellighet och kanske kan vi utöka våra sökmetoder till att också tolka ansamlingar, eller isotopsammansättningar, av spårmetaller.

Den kambriska explosinen av djurliv (med startskott för 543 miljoner år sedan) är ett etablerat begrepp, men den senaste årens forskning har satt fokus på att en längre period, från ediacaran till devon, var en dynamisk tid med skiftande havskemi, nya djurarter och experimentella ekologiska nätverk. I den här avhandlingen presenteras några resultat som belyser just denna övergångstid, som geokemin i Chengjiang som beskriver hur havets kemi skiftar från syrefritt till sulfatfritt till syrerikt, och hur djur utan skal och ben kunde bli bevarade genom att flera unika förhållanden sammanföll. En annan studie visar hur molybden använts för att påvisa att atmosfärens syrehalt, under den här perioden, var högst hälften av vår moderna nivå. Vi hävdar att stigningen som skedde i devon, delvis tack vare växternas intåg på land, och att stigningen kan speglas i att fiskar först då hade råd att jaga och växa sig stora. Slutligen visar jag också på hur det första stora massutdöendet kan vara sammankopplat med syrefria hav, snarare än kyla och mer syre än djuren klarade av. Ett komplext samspel mellan flera kemiska ämnen, utöver syre, tektonisk aktivitet och biologi ser ut att höra samman med den dramatiska uvecklingen för stora livsformer på jorden.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University, 2012. 51 p.
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper, 350
Keyword
Geochemistry, biology, paleontology, evolution, multicellularity
National Category
Geochemistry
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-75466 (URN)978-91-7447-486-2 (ISBN)
Public defence
2012-06-11, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
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Supervisors
Available from: 2012-05-10 Created: 2012-04-19 Last updated: 2012-05-03Bibliographically approved

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