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In this study, carbonate system properties were measured in the western Ross Sea (Antarctica) over the 2005–2006 and 2011–2012 austral summers with the aim of analysing their sensitivity to physical and biogeochemical drivers. Daily Advanced Microwave Scanning Radiometer 2 (AMSR2) sea ice concentration maps, obtained prior to and during the samplings, were used to analyse the sea ice evolution throughout the experiment periods. Monthly means and 8-day composite chlorophyll concentration maps from the Moderate-resolution Imaging Spectroradiometer (MODIS) Aqua satellite at 4-km resolution were used to investigate inter-annual and basin scale biological variability. Chlorophyll-a concentrations in surface waters estimated by MODIS satellite data contribute to descriptions of the variability of carbonate system properties in surface waters. Mean values of carbonate system properties were comparable across both investigated years; however, the 2012 data displayed larger variability. Sea ice melting also had a pivotal role in controlling the carbonate system chemistry of the mixed layer both directly through dilution processes and indirectly by favouring the development of phytoplankton blooms. This resulted in high pH and ΩAr, and in low CT, particularly in those areas where high chlorophyll concentration was shown by satellite maps.

The change in oceanic total dissolved inorganic carbon (DIC) concentrations relative to pre-industrial values, due to human mobilization of carbon over the past 250 years, is defined as "excess" or "anthropogenic" carbon dioxide (Excess CO 2 or C ant ; D. Wallace, 2001). This C ant represents only a small fraction of the total dissolved CO 2 , and therefore difficulties are encountered in distinguishing the anthropogenic perturbation from the predominant natural signal. This is further complicated by spatial and temporal variability of the oceanic sink for anthropogenic CO 2

<p>Over the last thirty years the Bedford Institute of Oceanography (BIO) has been maintaining the Atlantic Zone Off-Shore Monitoring Program (AZOMP), which includes annual occupation of several sections and stations in the Northwest Atlantic Ocean. Among these, the AR7W line across the Labrador Sea has one of the longest time-series where both transient tracers and dissolved inorganic carbon (DIC) have been collected since the early 1990s.</p><p>Among multiple transient tracers that have been measured along this transect (CFC-11, CFC-113, CCl<sub>4</sub> and SF<sub>6</sub>), only measurement of CFC-12 extends over the full time-series from 1992 to 2018, overlapping with DIC observations. Measurements of CFC-12 were also available for a previous cruise in 1986, extending the time-series to three decades.</p><p>In this work we present the temporal variability of CFC-12 (1986-2016) and DIC (1992-2016) concentrations as well as their distribution in the major water masses of the region.</p><p>The CFC-12 data are used to reconstruct the time-history of the tracer’s saturation at the time of convection based on multiple regression with the atmospheric input function of CFC-12 and the annual maximum mixed layer depth. The so-modelled time-varying saturation is employed to relax the constant saturation assumption of the Transit Time Distribution (TTD) method, allowing for a better estimate of anthropogenic carbon (C<sub>ant</sub>) in the region.</p><p>We present the column inventories and storage rate of C<sub>ant</sub> in central Labrador Sea between 1986 and 2016 obtained using the TTD method with time-varying saturation. We compare these estimates with a classical TTD approach that assumes constant saturation, and we highlight the differences in trends and magnitudes obtained with the two approaches.    </p><p>Finally, our work shows the multi-decadal dataset of DIC in the Labrador Sea which enables a comparison between the TTD-based C<sub>ant</sub> estimates and the measured DIC trends, providing insights into temporal variability of natural carbon in the region.</p>

<p>The Labrador Sea plays a central role in the oceanic storage of carbon. In particular, several studies have shown that this region has amongst the highest integrated column inventories of anthropogenic carbon (C<sub>ant</sub>) in the world’s ocean. The rate at which C<sub>ant</sub> is stored in this region appears to be connected to changes in ocean circulation and can therefore vary over time. Nevertheless, it is still unclear whether the temporal variability of the total Dissolved Inorganic Carbon (DIC) inventory is solely due to the changes in C<sub>ant</sub> concentrations or whether there is a contribution of the natural component of DIC to this signal.</p><p>The Bedford Institute of Oceanography has been maintaining the Atlantic Zone Off-Shore Monitoring Program (AZOMP) in the Labrador Sea since the early 1990s. The AZOMP involves annual occupations of the AR7W line that crosses the Labrador Sea and includes sampling of DIC, as well as multiple transient tracers such as CFC-12 and SF<sub>6</sub>.  </p><p>By using observations of DIC along the AR7W line, as well as previous estimates of C<sub>ant</sub> obtained with transient tracers (using a refined version of the Transit Time Distribution method; TTD) and new estimates of C<sub>ant</sub> based on the extended Multiple Linear Regression (eMLR) method, we provide a first insight on the role that the natural component of DIC plays in the temporal variability of inorganic carbon in the central Labrador Sea between 1993 and 2016.</p><p>We show that different methods to estimate C<sub>ant</sub> can lead to different conclusions on the role of the natural variability of DIC and that these discrepancies could be related to the assumptions implied in the C<sub>ant</sub> estimation methods. In particular an analysis of C<sub>ant</sub> estimates obtained with our refined version of the TTD method in different water masses, highlighted that further refinement of the tracers’ saturation assumption could be necessary in this region. This refinement could reconcile the C<sub>ant</sub> estimates from the two methods and therefore lead to an unambiguous role of the natural DIC in this region.</p>