(2020) Foraminifera-Bound Nitrogen Isotopes: From Mechanistic Understanding to a Case Study in the Equatorial Pacific
Ren H, Chen Y-C, Fang W-N, Studer A, Yu J, Martinez-Garcia A, Haug G, Spero H & Sigman D
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14d: Plenary Hall, Tuesday 23rd June 00:30 - 00:33
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Dear Dr. Ren, Thank you very much for this informative talk. I am wondering about the interpretation regarding the differences between bulk-sediment d15N and foraminifera-bound d15N (slides 21-23 from your presentation). If the large glacial-interglacial increases in bulk-sediment d15N correspond to terrestrial nutrient input rather than denitrification, why wouldn't the d15N of the terrestrial-sourced nutrients also be recorded into the foraminifera-bound d15N?
Dear Dr. Doherty, Thank you for the question. Much of the terrestrial N input comes in the form of particulate organic matter or detritus, which is quickly incorporated into the sediments and does not influence the open ocean N cycle. For instance, many coastal sites show clear changes in C:N ratio that suggest changes in terrestrial versus marine Organic N. There are also some evidence that some inorganic forms of N could be absorbed into particles, such as minerals, and be incorporated into the sediments as well. For example, there is debate on whether sediments should be acid leached before analyzing TN in order to remove the influence from these mineral associated inorganic N. These above two would contaminate the TN signal in the sediments, but is not reflected by forams because these N source did not get into the open surface water biogeochemical cycle. Of course, some terrestrial N as dissolved nitrate, ammonium, or DON would influence coastal Biogeochemistry. This influence should be recorded by forams as well, since it is a biogeochemical N source to this environment. Since I am discussing the difference between foram-bound and TN, I think it is likely that bulk sedimentary TN are influenced by the N sources that gets into the sediments but did not influence surface ocean Biogeochemistry.
Abby, if the forams record pray how is that related to nitrate in the water column?
Dear Adina, Thank you for the great question! This is a question that needs to be addressed in the field. We find that the three dinoflagellates-bearing forams have similar d15N as nitrate source in oligotrophic regions, suggesting that their food represents the total N export in these environment. So the possibilities are (1) they are not picky on their food, so eating whatever they can get in the water column, thus if we take the average over 100 specimens accumulated over hundreds of years in the sediments, the d15N value gets very close to the total N export; or (2) they happen to consume on specific groups that represent total N export in these environment.
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