Premium
A 30 ‐Year Time Series of Transient Tracer‐Based Estimates of Anthropogenic Carbon in the Central Labrador Sea
Author(s) -
Raimondi L.,
Tanhua T.,
AzetsuScott K.,
Yashayaev I.,
Wallace D.W.R.
Publication year - 2021
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1029/2020jc017092
Subject(s) - saturation (graph theory) , tracer , water column , environmental science , dissolved organic carbon , atmospheric sciences , oceanography , chemistry , geology , mathematics , physics , nuclear physics , combinatorics
We use a 30‐year time series (1986–2016) of dichlorodifluoromethane (CFC‐12) concentrations with a refined transit time distribution (TTD) method, to estimate the temporal variation of anthropogenic carbon (C ant ) in the Central Labrador Sea. We determined that the saturation of CFC‐12 and sulfur hexafluroide (SF 6 ) in newly‐formed Labrador Sea Water had departed significantly from 100% and varied systematically with time. Multiple linear regression of the time‐varying saturation, with the tracer's atmospheric growth rate and the wintertime mixed layer depth as independent variables, allowed reconstruction of the saturation history of CFC‐12 and SF 6 in wintertime surface waters, which was implemented in the TTD method. Use of the time‐varying saturation for CFC‐12 gave C ant concentrations ∼7 μmol kg −1 larger than estimates obtained assuming a constant saturation of 100%. The resulting C ant column inventories were ∼20% larger and displayed lower interannual variability compared to conventional TTD‐based estimates. The column inventory of C ant increased at an average rate of 1.8 mol m −2 y −1 over the 30‐year period. However, the accumulation rate of C ant was higher than this average in the early 1990s and since 2013, whereas inventories remained almost unchanged between 2003 and 2012. The variation in the C ant accumulation rate is shown to be linked to temporal variability in the relative layer thickness of the annually ventilated Labrador Sea Water and the underlying Deep Intermediate Water. The non‐steady C ant accumulation highlights the importance of sampling frequency, especially in regions of variable deep mixing and high carbon inventories, and potential misinterpretation of C ant dynamics.