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

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-a (δ¹⁵N_Chl-a) and bulk sediment (δ¹⁵N_sed) originate from phytoplankton that assimilates nitrogen nutrients. Thus, both the δ¹⁵N_Chl-a and δ¹⁵N_sed 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 δ¹⁵N_sed and δ¹⁵N_Chl-a (△δ¹⁵N_sed-Chla) reflects the nitrogen cycling process that results in such a difference.

We present a comparative study on how paired δ¹⁵N_Chl-a and δ¹⁵N_sed clarify coastal nitrogen cycling in Yellow Sea of China and in Benguela Upwelling System. In Yellow Sea of China, the δ¹⁵N_Chl-a ranges from -1.00 ‰ to 3.40‰ and δ¹⁵N_sed from 3.00‰ to 5.50‰, which respectively reflects the nitrogen source assimilated by phytoplankton and the diagenesis of hereafter. The △¹⁵N_sed-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 δ¹⁵N_sed 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 δ¹⁵N_Chl-a (-4.50 - 4.50‰) and δ¹⁵N_sed (3.46-8.89‰) in 27 surface sediment samples are similar. An offshore increase in both δ¹⁵N_sed and δ¹⁵N_Chl-a in the northern sector is consistent with ¹⁵N enrichment during progressive nitrate assimilation by phytoplankton. Higher δ¹⁵N_sed and δ¹⁵N_Chl-a values in the southern sector reflect assimilation of nitrate enriched in ¹⁵N by water column denitrification. The △¹⁵N_sed-Chla that the intense diagenesis of bulk N in sediments shifts the δ¹⁵N_sed, whereas the sedimentary δ¹⁵N_Chl-a captures the integrated N-removal in the water column.

We conclude that δ¹⁵N_Chl-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.