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Isotopic composition of nitrate in wet and dry atmospheric deposition on Crete in the eastern Mediterranean Sea
Author(s) -
Mara Paraskevi,
Mihalopoulos Nikolaos,
Gogou Alexandra,
Daehnke Kirstin,
Schlarbaum Tim,
Emeis KayChristian,
Krom Michael
Publication year - 2009
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/2008gb003395
Subject(s) - nitrate , sea salt , deposition (geology) , mediterranean sea , flux (metallurgy) , environmental science , mediterranean climate , ammonium , environmental chemistry , aerosol , sulfate , particulates , atmospheric sciences , oceanography , chemistry , geology , geomorphology , ecology , organic chemistry , sediment , biology
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 N ratios (expressed as δ 15 N) in deepwater nitrate ( δ 15 NO 3 − of 2.5‰) and sediments (<3.5‰) that have been attributed to extensive N 2 fixation. To quantify the role of atmospheric NO 3 − deposition in the N cycle in the EMS, we analyzed NO 3 − and its δ 15 N 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 δ 15 N 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 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 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.