Alkalinity distribution in the western North Atlantic Ocean margins

Abstract

Total alkalinity (TA) distribution and its relationship with salinity (S) along the western North Atlantic Ocean (wNAO) margins from the Labrador Sea to tropical areas are examined in this study. Based on the observed TA‐S patterns, the mixing processes that control alkalinity distribution in these areas can be categorized into a spectrum of patterns that are bracketed by two extreme mixing types, i.e., alongshore current‐dominated and river‐dominated. Alongshore current‐dominated mixing processes exhibit a segmented mixing line with a shared mid‐salinity end‐member. In such cases (i.e., Labrador Sea, Gulf of Maine, etc.), the y‐intercept of the high salinity segment of the mixing line is generally higher than the local river alkalinity values, and it reflects the mixing history of the alongshore current. In contrast, in river‐dominated mixing (Amazon River, Caribbean Sea, etc.), good linear relationships between alkalinity and salinity are generally observed, and the zero salinity intercepts of the TA‐S regressions roughly match those of the regional river alkalinity values. TA‐S mixing lines can be complicated by rapid changes in the river end‐member value and by another river nearby with a different TA value (e.g., Mississippi‐Atchafalaya/Gulf of Mexico). In the wNAO margins, regression intercepts and river end‐members have a clear latitudinal distribution pattern, increasing from a low of ∼300 μmol kg−1 in the Amazon River plume to a high value between ∼500–1100 μmol kg−1 in the middle and high latitude margins. The highest value of ∼2400 μmol kg−1 is observed in the Mississippi River influenced areas. In addition to mixing control, biological processes such as calcification and benthic alkalinity production may also affect ocean margin alkalinity distribution. Therefore, deriving inorganic carbon system information in coastal oceans using alkalinity‐salinity relationships, in particular, those of generic nature, may lead to significant errors.