Isotopic composition of nitrate in wet and dry atmospheric deposition on Crete in the eastern Mediterranean Sea GB4002

The eastern Mediterranean Sea (EMS) is a nutrient-poor ocean with unusually high nitrate-phosphate ratios (up to 28:1) and relatively depleted ¹⁵N/¹⁴N ratios (expressed as 6δ¹⁵N) in deepwater nitrate (δ¹⁵NO₃^- of 2.5%o) and sediments (<3.5%o) that have been attributed to extensive N₂ fixation. To quantify the role of atmospheric NO₃^- deposition in the N cycle in the EMS, we analyzed NO₃^- and its δ¹⁵N 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 (δ¹⁵N compared to air N₂, implying a strongly depleted atmospheric source calculated to be (weighted annual estimate) -3.1%o. The low δ ¹⁵N 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 of an origin 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 (δ¹⁵N-NO ₃^- of 0.5%o to 1.5%o and that particle flux to and mineralization in the deepwater pool over the last 40-50 years can account for the unusually low δ¹⁵N ratios found in deepwater NO ₃^- without the need of any significant N₂ fixation.