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The carbon dioxide system on the M ississippi R iver‐dominated continental shelf in the northern G ulf of M exico: 1. Distribution and air‐sea CO 2 flux
Author(s) -
Huang WeiJen,
Cai WeiJun,
Wang Yongchen,
Lohrenz Steven E.,
Murrell Michael C.
Publication year - 2015
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1002/2014jc010498
Subject(s) - salinity , sink (geography) , carbon dioxide , oceanography , continental shelf , environmental science , flux (metallurgy) , nitrogen , atmosphere (unit) , seasonality , sea surface temperature , seawater , carbon sink , atmospheric sciences , geology , chemistry , climate change , meteorology , geography , ecology , biology , cartography , organic chemistry
River‐dominated continental shelf environments are active sites of air‐sea CO 2 exchange. We conducted 13 cruises in the northern Gulf of Mexico, a region strongly influenced by fresh water and nutrients delivered from the Mississippi and Atchafalaya River system. The sea surface partial pressure of carbon dioxide ( p CO 2 ) was measured, and the air‐sea CO 2 flux was calculated. Results show that CO 2 exchange exhibited a distinct seasonality: the study area was a net sink of atmospheric CO 2 during spring and early summer, and it was neutral or a weak source of CO 2 to the atmosphere during midsummer, fall, and winter. Along the salinity gradient, across the shelf, the sea surface shifted from a source of CO 2 in low‐salinity zones (0≤S<17) to a strong CO 2 sink in the middle‐to‐high‐salinity zones (17≤S<33), and finally was a near‐neutral state in the high‐salinity areas (33≤S<35) and in the open gulf (S≥35). High p CO 2 values were only observed in narrow regions near freshwater sources, and the distribution of undersaturated p CO 2 generally reflected the influence of freshwater inputs along the shelf. Systematic analyses of p CO 2 variation demonstrated the importance of riverine nitrogen export; that is, riverine nitrogen‐enhanced biological removal, along with mixing processes, dominated p CO 2 variation along the salinity gradient. In addition, extreme or unusual weather events were observed to alter the alongshore p CO 2 distribution and to affect regional air‐sea CO 2 flux estimates. Overall, the study region acted as a net CO 2 sink of 0.96 ± 3.7 mol m −2 yr −1 (1.15 ± 4.4 Tg C yr −1 ).