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
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.