Paired Nitrogen Isotope of Chlorophyll-a and Bulk Sediment in Characterizing Coastal Nitrogen Cycling- a Comparative Study in Yellow Sea of China and Benguela Upwelling System

Yu Xin1, Maojun Yan2, Xiaosong Zhong1 and Shuhang Dong1, (1)Ocean University of China, Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Qingdao, China, (2)Shanghai Jiao Tong University, Qingdao, China
The δ15N of chlorophyll pigment is a potential molecule-specific nitrogen isotope proxy to characterize nitrogen cycling in marginal seas. Both the δ15N of chlorophyll-a15NChl-a) and bulk sediment (δ15Nsed) originate from phytoplankton that assimilates nitrogen nutrients. Thus, both the δ15NChl-a and δ15Nsed mirror the isotope of nitrogen source unless they are deviated by other nitrogen cycling processes, i.e. diagenesis, nitrogen loss or exogenous nitrogen mixing before or after the burial into sediment. A difference between the δ15Nsed and δ15NChl-a (△δ15Nsed-Chla) reflects the nitrogen cycling process that results in such a difference.

We present a comparative study on how paired δ15NChl-a and δ15Nsed clarify coastal nitrogen cycling in Yellow Sea of China and in Benguela Upwelling System. In Yellow Sea of China, the δ15NChl-a ranges from -1.00 ‰ to 3.40‰ and δ15Nsed from 3.00‰ to 5.50‰, which respectively reflects the nitrogen source assimilated by phytoplankton and the diagenesis of hereafter. The △15Nsed-Chla (1.60 ~ 6.50‰) shows a “V-shape” variation pattern with the bottom shear stress (N/m2) from inshore to offshore, suggesting that strong bottom re-suspension plays a critical role in modifying δ15Nsed at inshore while weak bottom disturbance in offshore area favors sedimentation of particulate nitrogen in smaller particle size. The northern Benguela Upwelling System is well-known of its annual existence of oxygen minimum zone where denitrification is severe. Distribution patterns of δ15NChl-a (-4.50 - 4.50‰) and δ15Nsed (3.46-8.89‰) in 27 surface sediment samples are similar. An offshore increase in both δ15Nsed and δ15NChl-a in the northern sector is consistent with 15N enrichment during progressive nitrate assimilation by phytoplankton. Higher δ15Nsed and δ15NChl-a values in the southern sector reflect assimilation of nitrate enriched in 15N by water column denitrification. The △15Nsed-Chla that the intense diagenesis of bulk N in sediments shifts the δ15Nsed, whereas the sedimentary δ15NChl-a captures the integrated N-removal in the water column.

We conclude that δ15NChl-a is an very informative and unbiased isotope proxy that is capable of capturing the major N-cycling process in the water column and sediment, and could be widely applied in nitrogen study in global marginal seas.