The nitrogen isotope composition of sediments from the proto‐North Atlantic during Oceanic Anoxic Event 2. (23rd July 2015)

• Record Type: Journal Article
• Authors: Ruvalcaba Baroni, I.; van Helmond, N. A. G. M.; Tsandev, I.; Middelburg, J. J.; Slomp, C. P.
• Is Part Of: Paleoceanography. Volume 30:Number 7(2015:Sep.)
• Page Start: 923
• Page End: 937
• Publication Date: 2015-07-23
• Subjects: Paleoceanography -- Periodicals; 551.46
• DOI: 10.1002/2014PA002744
• Languages: English
• ISSNs: 0883-8305

Abstract

<abstract abstract-type="main" id="palo20219-abs-0001"> <title>Abstract</title> <p id="palo20219-para-0001">Sediment records of the stable isotopic composition of N (<italic>δ</italic><sup>15</sup>N) show light <italic>δ</italic><sup>15</sup>N values at several sites in the proto‐North Atlantic during Oceanic Anoxic Event 2 (OAE 2) at the Cenomanian‐Turonian transition (∼94 Ma). The low <italic>δ</italic><sup>15</sup>N during the event is generally attributed to an increase in N<sub>2</sub> fixation and incomplete uptake of <inline-formula><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pgj2f6znvj6" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math display="inline" altimg="urn:x-wiley:palo:media:palo20219:palo20219-math-0001" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow /><mml:mrow><mml:mn>4</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:math></alternatives></inline-formula> for phytoplankton growth. A compilation of all reliable data for the proto‐North Atlantic during OAE 2 demonstrates that the most pronounced negative shift in <italic>δ</italic><sup>15</sup>N from pre‐OAE 2 to OAE 2 occurs in the open ocean but with <italic>δ</italic><sup>15</sup>N never lower than −3‰. Using a box model of N cycling for the proto‐North Atlantic during OAE 2, we show that N<sub>2</sub> fixation is a major contributor to the <italic>δ</italic><sup>15</sup>N signal, especially in the open ocean. Incomplete uptake of <inline-formula><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pgj2f6znvm9" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math display="inline" altimg="urn:x-wiley:palo:media:palo20219:palo20219-math-0002" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow /><mml:mrow><mml:mn>4</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:math></alternatives></inline-formula> for phytoplankton growth is important in regions dominated by downwelling, with lateral transport of <inline-formula><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pgj2f6znvkr" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math display="inline" altimg="urn:x-wiley:palo:media:palo20219:palo20219-math-0003" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow /><mml:mrow><mml:mn>4</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:math></alternatives></inline-formula> acting as a major source. In the southern proto‐North Atlantic, where bottom waters were euxinic, the light <italic>δ</italic><sup>15</sup>N signature is largely explained by upwelling of <inline-formula><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pgj2f6znvbw" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math display="inline" altimg="urn:x-wiley:palo:media:palo20219:palo20219-math-0004" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow /><mml:mrow><mml:mn>4</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:math></alternatives></inline-formula>. Our study provides an overview of regional differences in <italic>δ</italic><sup>15</sup>N in the proto‐North Atlantic and highlights the role of lateral exchange of water and nutrients, in addition to local biogeochemical processes, in determining <italic>δ</italic><sup>15</sup>N values of OAE 2 sediments.</p> </abstract>