Sources of ¦Ä¹⁵N variability in sinking particulate nitrogen in the Cariaco Basin, Venezuela

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• 作者：Enrique Montes ; Robert Thunell ; Frank E. Muller-Karger ; Laura Lorenzoni ; Eric Tappa ; Luis Troccoli ; Yrene Astor ; Ram¨®n Varela
• 关键词：Cariaco Basin ; Sinking particulate organic matter ; Nitrogen isotopes ; Nitrogen fixation ; Sediment traps ; Climate variability
• 刊名：Deep Sea Research Part II: Topical Studies in Oceanography
• 出版年：2013
• 出版时间：September, 2013
• 年：2013
• 卷：93
• 期：Complete
• 页码：96-107
• 全文大小：2742 K

文摘

Ten years of monthly observations of the ¦Ä¹⁵N of sinking particulate nitrogen (¦Ä¹⁵N-PN (in ¡ë versus atmospheric N₂)=[(¹⁵N/¹⁴N)_sample/(¹⁵N/¹⁴N)_standard)?1]1000) in the Cariaco Basin, Venezuela, confirm that the basin's bottom sediments store information about nitrogen dynamics related to seasonal and interannual variability in regional surface ocean processes. During the upwelling period of the southern Caribbean Sea (February-April), the ¦Ä¹⁵N-PN is similar to that of the thermocline nitrate (¡«3.5¡ë). This nitrate is imported into the Cariaco Basin with Subtropical Underwater (SUW), which wells up near the coast. Thus, particles generated by phytoplankton photosynthesis during this productive period bear a sub-tropical North Atlantic isotopic imprint of N₂ fixation (low compared to the global average of nitrate ¦Ä¹⁵N¡Ö5¡ë). During the non-upwelling period when surface waters are stratified (September-November), the ¦Ä¹⁵N-PN is also 3.5-4.0¡ë, and reflects a mixture of local N₂ fixation within the mixed layer, inputs of terrigenous organic matter and SUW nitrate consumption by phytoplankton below the mixed layer, which most likely exerts the strongest control on the ¦Ä¹⁵N-PN signal during this time. In the transition periods of May-July and December-January, the ¦Ä¹⁵N-PN increases to 4.5-6.5¡ë. This coincides with maxima of continental material fluxes (terrestrial PON ¦Ä¹⁵N is >6¡ë) into the Cariaco Basin. The ¦Ä¹⁵N signal in the sediments of the Cariaco Basin thus provides information about the relative strength of the local coastal upwelling, the relative input of continental material via river runoff, and local N₂ fixation. The findings contribute to interpretations of the basin's paleoclimatic nitrogen cycle variations based on observations of the sedimentary ¦Ä¹⁵N record at this location.