Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency

作者：Mollier Vogel Elfi^*; Ryabenko Evgenia; Martinez Philippe; Wallace Douglas; Altabet Mark A; Schneider Ralph

来源：Deep Sea Research Part I: Oceanographic Research Papers , 2012, 70: 14-25.

DOI：10.1016/j.dsr.2012.06.003

• Water column
• Surface sediments
• Nitrogen isotopes
• Denitrification
• OMZ
• South America
• Eastem tropical pacific

We present new nitrogen isotope data from the water column and surface sediments for paleo-proxy validation collected along the Peruvian and Ecuadorian margins between 1 degrees N and 18 degrees S. Productivity proxies in the bulk sediment (organic carbon, total nitrogen, biogenic opal, C-37 alkenone concentrations) and N-15/N-14 ratios were measured at more than 80 locations within and outside the present-day Peruvian oxygen minimum zone (OMZ). Microbial N-loss to N-2 in subsurface waters under O-2 deficient conditions leaves a characteristic N-15-enriched signal in underlying sediments. We find that phytoplankton nutrient uptake in surface waters within the high nutrient, low chlorophyll (HNLC) regions of the Peruvian upwelling system influences the sedimentary signal as well. How the delta N-15(sed) signal is linked to these processes is studied by comparing core-top values to the N-15/N-14 of nitrate and nitrite (delta N-15(NOx)) in the upper 200 m of the water column. Between 1 degrees N and 10 degrees S, subsurface O-2 is still high enough to suppress N-loss keeping delta N-15(NOx), values relatively low in the subsurface waters. However delta N-15(NOx) values increase toward the surface due to partial nitrate utilization in the photic zone in this HNLC portion of the system. delta N-15(sed) is consistently lower than the isotopic signature of upwelled NO3-, likely due to the corresponding production of N-15 depleted organic matter. Between 10 degrees S and 15 degrees S, the current position of perennial upwelling cells, HNLC conditions are relaxed and biological production and near-surface phytoplankton uptake of upwelled NO3- are most intense. In addition, subsurface O-2 concentration decreases to levels sufficient for N-loss by denitrification and/or anammox, resulting in elevated subsurface delta N-15(NOx) values in the source waters for coastal upwelling. Increasingly higher production southward is reflected by various productivity proxies in the sediments, while the north-south gradient towards stronger surface NO3- utilization and subsurface N-loss is reflected in the surface sediment N-15/N-14 ratios. South of 10 degrees S, delta N-15(sed) is lower than maximum water column delta N-15(NOx) values most likely because only a portion of the upwelled water originates from the depths where highest delta N-15(NOx) values prevail. Though the enrichment of delta N-15(NOx), in the subsurface waters is unambiguously reflected in delta N-15(sed) values, the magnitude of delta N-15(sed) enrichment depends on both the depth of upwelled waters and high subsurface delta N-15(NOx), values produce by N-loss. Overall, the degree of N-loss influencing subsurface delta N-15(NOx) values, the depth origin of upwelled waters, and the degree of near-surface nitrate utilization under HNLC conditions should be considered for the interpretation of paleo delta N-15(sed) records from the Peruvian oxygen minimum zone.

• 出版日期2012-12