Isotopic composition of nitrate in wet and dry atmospheric deposition on Crete in the eastern Mediterranean Sea

摘要

The eastern Mediterranean Sea (EMS) is a nutrient-poor ocean with unusually high nitrate-phosphate ratios (up to 28:1) and relatively depleted ~( 15)N/~(14)Nratios (expresed as δ~(15)N) in deepwater nitrate (δ~(15)NO_3~- of 2.5%o) and sediments (<3.5%a) that have been attributed to extensive N_2 fixation. To quantify the role of tmospheric NO_3~- deposition in the N cycle in the EMS, we analyzed NO_3~- and its 615N in samples of dry and wet deposition on biweekly and per event basis from April 2006 to September 2007 on the island of Crete. Both dry and wet deposition samples have consistently negative 815N compared to air N_2, implying a strongly depleted atmospheric source calculated to be (weighted annual estimate) -3.1‰ , The low δ~(15)N of wet deposition is in agreement with data from other environments, but the consistently depleted nature of dry deposition is unusual and supports the view df an Agin from association of atmospheric NO_3~-mainly with dust and sea salt. This situation is due to high levels of sulphate compared to ammonium, sea salt, and lithogenic material. Mass balance and isotope-mixing calculations show that the present-day inputs of nitrate from external sources into the surface water layer of the eastern Mediterranean Sea have a weighted mean δ~(15)N- NO_3~- of 0.5‰ to 1.5‰ and that particle flux to and mineralization in the deepwater pool over the last 40-50 years can account for the unusually low δ~(15)N ratios found in deepwater NO_3~-without the need of any significant N_2 fixation.