A 30 ‐Year Time Series of Transient Tracer‐Based Estimates of Anthropogenic Carbon in the Central Labrador Sea

• Published in: Journal of geophysical research. Oceans Vol. 126; no. 5
• Main Authors: Raimondi, L., Tanhua, T., Azetsu‐Scott, K., Yashayaev, I., Wallace, D.W.R.
• Format: Journal Article
• Language: English
• Published: 01.05.2021
• Subjects: anthropogenic carbon; CFC‐12; Labrador Sea; SF6; transit time distribution method
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2020JC017092

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 (Cant) in the Central Labrador Sea. We determined that the saturation of CFC‐12 and sulfur hexafluroide (SF6) 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 SF6 in wintertime surface waters, which was implemented in the TTD method. Use of the time‐varying saturation for CFC‐12 gave Cant concentrations ∼7 μmol kg−1 larger than estimates obtained assuming a constant saturation of 100%. The resulting Cant column inventories were ∼20% larger and displayed lower interannual variability compared to conventional TTD‐based estimates. The column inventory of Cant increased at an average rate of 1.8 mol m−2 y−1 over the 30‐year period. However, the accumulation rate of Cant 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 Cant 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 Cant accumulation highlights the importance of sampling frequency, especially in regions of variable deep mixing and high carbon inventories, and potential misinterpretation of Cant dynamics. Plain Language Summary Since the industrial revolution, humankind has emitted large amounts of carbon dioxide (CO2) to the atmosphere as a result of fossil fuel combustion and cement production. About 40% of this Anthropogenic CO2 (Cant) has been sequestered by the oceans, primarily in polar and subpolar regions. The Labrador Sea has been identified as one of the regions with the highest inventory of Cant. Here, as strong winds blow on the ocean's surface in wintertime, heat is lost to the atmosphere and water density increases. This process, defined as deep water convection, determines the mixing of surface water into the ocean interior and the transport of gases, like Cant, from surface to depth. In this work we estimate the concentrations, column inventories and storage rate of Cant in Labrador Sea between 1986 and 2016 by using gases that mimic Cant behavior. We observed that despite the overall increase in Cant that occurred between 1986 and 2016, the pace at which Cant increased has not been constant over time in the Labrador Sea. In fact, we observed periods with both fast and slow increases of Cant, which were influenced by the persistence of deep or shallow convection, respectively. Key Points In regions of ventilation, assuming a constant tracers' saturation in the transit time distribution method results in significant negative bias of anthropogenic carbon (Cant) estimates Annual estimates of Cant column inventory between 1986 and 2016 reveal a non‐steady accumulation rate of Cant in Labrador Sea The temporal variability of the Cant storage in the Central Labrador Sea appears to be linked to the strength of convection