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

The eastern Mediterranean Sea (EMS) is a nutrient-poor ocean with unusually high nitrate-phosphate ratios (up to 28:1) and relatively depleted N-15/N-14 ratios (expressed as delta N-15) in deepwater nitrate (delta(NO3-)-N-15 of 2.5 parts per thousand) and sediments (<3.5 parts per thousand) that have been attributed to extensive N-2 fixation. To quantify the role of atmospheric NO3- deposition in the N cycle in the EMS, we analyzed NO3- and its delta N-15 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 delta N-15 compared to air N-2, implying a strongly depleted atmospheric source calculated to be (weighted annual estimate) -3.1 parts per thousand. The low delta N-15 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 NO3- 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 delta N-15-NO3- of 0.5 parts per thousand to 1.5 parts per thousand and that particle flux to and mineralization in the deepwater pool over the last 40-50 years can account for the unusually low delta N-15 ratios found in deepwater NO3- without the need of any significant N-2 fixation.