(2020) A Seasonally Resolved Coral Nitrogen Isotope Record from the Florida Keys: Implications for the Impact of Anthropogenic Nitrogen on the Gulf of Mexico
Wang Y, Wang XT, Flannery JA, Toth LT, Richey JN, Chen Y-C, Rao ZC, Ren HA & Sigman DM
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12c: Plenary Hall, Wednesday 24th June 22:15 - 22:18
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If all the excess nitrogen is denitrified as you suggest, then there must be some signal of that would show that. Does your study suggest that the total amount of denitrification is limited by the supply of nitrogen? Is there any suggestion of a limitation on this process that could decouple the rate of denitrification from the nitrogen supply in the future if nitrogen loads continue to increase?
This is a great question. It appears to be the case for our study period. Our coral nitrogen isotope record suggests that the impact of anthropogenic nitrogen on the open water in the GoM was weak or minimal. This implies that the coastal nitrogen removal pathway would have been efficient in removing riverine nitrogen input in the early 21st century. However, if the nitrogen load continues to increase in the future, it is quite likely that the rate of sedimentary denitrification cannot keep up and some of the riverine nitrogen would reach the open ocean and affect the ecosystem there.
Hi Yuwei, I really liked the timeseries of d15N from the coral, this looks like a valuable indicator of environmental changes. At some point you mention that the seasonal cycle of d15N is inconsistent with the seasonality of riverine nitrogen inputs. However, if your site isn't directly adjacent to the rivers you are referring to, then could the "mismatch" be due to a lag between the "peak riverine input" and the advection of nitrogen to your field site?
This is a great question. We are glad that you like our coral nitrogen isotope record. The riverine nitrogen input with high d15N is the highest during the late spring or summer. However, in our coral record the highest d15N was mostly observed in the winter. If the seasonal d15N variation in coral was caused by riverine nitrogen input, it would mean that it takes 6 months to transport riverine nitrogen to the open ocean only several hundred kilometers away. This timescale seems too long for the circulation of the GoM. For example, the loop current in the GoM is one of the fastest currents in the Atlantic, traveling at the speed of 0.8m/s or over 2000km/month.
Thanks for an interesting talk. For several years of your record it appears that Sr/Ca and 15-N are positively correlated, but for other years there is no correlation or even an inverse correlation between these properties. Do you have any hypotheses that could explain these trends?
Thanks for the feedback. This is a great question. Sr/Ca is a proxy for temp. while d15N is a proxy for the N cycle. For most of the record, high Sr/Ca was correlated with high d15N. But during a couple of years, the was not the case, indicating the decoupling of biogeochemistry from the temp record. This might be caused by the natural N cycle processes on the reef. For example, low d15N during the summer might be caused by N2 fixation and ammonium recycling. If the rates of N2 fixation and ammonium recycling were not high during those year, it could be responsible for the decoupling of biogeochemistry from the temp. This is a working hypothesis. To better understand the seasonality, we plan to do more field work in the near future.
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