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Distribution of the partial pressure of (a) CFC-12 and (b) SF 6 along the zonal section in the Fram Strait.
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The storage of anthropogenic carbon in the ocean's interior is an important process which modulates the increasing carbon dioxide concentrations in the atmosphere. The polar regions are expected to be net sinks for anthropogenic carbon. Transport estimates of dissolved inorganic carbon and the anthropogenic offset can thus provide information about...
Citations
... The Δ/Γ ratio can be constrained by two tracers with different atmospheric time histories (e.g., Waugh et al., 2003Waugh et al., , 2004. A common pair is CFC-12 and SF 6 (e.g., Stöven et al., 2016;Tanhua et al., 2008). However, SF 6 is problematic in the Nordic Seas due to the deliberate release and subsequent spreading of large amounts (320 kg) of this tracer in the Greenland Sea in 1996 (Jeansson et al., 2009;Marnela et al., 2008;Messias et al., 2008;Tanhua, Bulsiewicz, & Rhein, 2005;Watson et al., 1999). ...
... However, SF 6 is problematic in the Nordic Seas due to the deliberate release and subsequent spreading of large amounts (320 kg) of this tracer in the Greenland Sea in 1996 (Jeansson et al., 2009;Marnela et al., 2008;Messias et al., 2008;Tanhua, Bulsiewicz, & Rhein, 2005;Watson et al., 1999). This excess SF 6 may have affected the transient signal even as late as 2012, in the Fram Strait (Stöven et al., 2016). Therefore, we used CFC-12 for most years in the present study, except for 2016 in the Greenland Sea (where SF 6 was used for the upper 1,800 m). ...
... The only plausible explanation is an increased fraction of older water, which must come from the Arctic Ocean (or an error in the assumption of the shape of the TTD, i.e., the Δ/Γ ratio, or/and that the ratio is time-invariant). These waters enter the Nordic Seas via the Fram Strait, where the ages of the Arctic Ocean deep waters are in the range 170-250 years (Stöven et al., 2016). The evolution of the GSBW in the 2010s is different from that of the GSDW; the former continued the trend of aging, while the latter showed some signs of ventilation, as discussed above. ...
We evaluate the decadal evolution of ventilation and anthropogenic carbon (Cant) in the Nordic Seas between 1982 and the 2010s. Ventilation changes on decadal timescale are identified by evaluating decadal changes in mean ages and apparent oxygen utilization in each of the four main basins of the Nordic Seas (the Greenland and Iceland Seas, and the Norwegian and Lofoten Basins). The ages are derived from the transient time distribution approach, based on the transient tracers chlorofluorocarbon‐12 (CFC‐12) and sulfur hexafluoride (SF6). The different decades show different phases in ventilation, with the 2000s being overall better ventilated than the 1990s in all basins. For the Greenland Sea, we also show that the 2010s are better ventilated than the 2000s, with a clear shift in hydrographic properties. The evolution of concentrations and inventory of Cant is linked to the ventilation state. The deep waters get progressively older over the analyzed period, which is connected to the increased fraction of deep water from the Arctic Ocean.
... Formulations of gas exchange that neglect separate bubble terms (e.g., parameterizations with a single diffusive term) could lead to large errors if extrapolated between less soluble and more soluble gases in windy regions with large bubble fluxes. Additionally, this study provides evidence that bubble-induced supersaturation is an important component to quantify when using SF 6 as a transient tracer of ocean circulation and water mass age (10,47). For more soluble transient tracers such as chlorofluorocarbons (CFCs; SI Appendix, Table S2), these findings indicate that cooling-induced undersaturation should dominate bubble-induced supersaturation, in line with a recent study in the Labrador Sea (10). ...
Gas exchange between the atmosphere and ocean interior profoundly impacts global climate and biogeochemistry. However, our understanding of the relevant physical processes remains limited by a scarcity of direct observations. Dissolved noble gases in the deep ocean are powerful tracers of physical air-sea interaction due to their chemical and biological inertness, yet their isotope ratios have remained underexplored. Here, we present high-precision noble gas isotope and elemental ratios from the deep North Atlantic (~32°N, 64°W) to evaluate gas exchange parameterizations using an ocean circulation model. The unprecedented precision of these data reveal deep-ocean undersaturation of heavy noble gases and isotopes resulting from cooling-driven air-to-sea gas transport associated with deep convection in the northern high latitudes. Our data also imply an underappreciated and large role for bubble-mediated gas exchange in the global air-sea transfer of sparingly soluble gases, including O2, N2, and SF6. Using noble gases to validate the physical representation of air-sea gas exchange in a model also provides a unique opportunity to distinguish physical from biogeochemical signals. As a case study, we compare dissolved N2/Ar measurements in the deep North Atlantic to physics-only model predictions, revealing excess N2 from benthic denitrification in older deep waters (below 2.9 km). These data indicate that the rate of fixed N removal in the deep Northeastern Atlantic is at least three times higher than the global deep-ocean mean, suggesting tight coupling with organic carbon export and raising potential future implications for the marine N cycle.
... To estimate PAHs mass transport through the Fram Strait, the mean PAH concentrations of different water masses were multiplied by the average water volume from boundary currents, which were defined based on prior work (Karpouzoglou et al., 2022;Stöven et al., 2016;Wang et al., 2020). The mean PAH concentrations of different water masses were derived based on the trapezoidal integration of the PAH concentrations through the water column. ...
... It is noticeable that PAHs generally displayed medium level in the Arctic Intermediate Water (AIW) at ∼1,000 m depth, due to sinking of AW and photo-/bio-degradation during this process. Moreover, PAH concentrations were markedly lower in deep waters (>2,000 m) either for eastern or western Fram Strait, which was generally representative of old Eurasian Basin Deep Water and Greenland Sea Deep Water (EBDW/GSDW) with a mean age of 250 ± 30 years (Beszczynska-Möller et al., 2012;Stöven et al., 2016). ...
Plain Language Summary
Polycyclic aromatic hydrocarbons (PAHs) are continuously released from multiple sources, mainly from combustion processes situated in populated and industrial regions. They are capable of long‐distance transport to reach remote and deep oceans. To better understand PAH transport and fate, polyethylene passive samplers were deployed in deep waters of the Fram Strait, surface seawater of the Canadian Archipelago, as well as in air and surface water of the mid‐latitude Great Lakes. Concentrations of dissolved PAHs in the lower Great Lakes were significantly higher than those in the high Arctic, indicating the presence of emission sources in the Great Lakes. The vertical profiles of dissolved PAHs generally exhibited a “surface enrichment and depth depletion” pattern in the Fram Strait, which was potentially affected by hydrological and biogeochemical processes. Pyrogenic sources were the dominant origin of PAHs in both the Arctic and Great Lakes, with some biomass burning sources possibly from wild fires in the sub‐Arctic boreal forest regions. PAHs were exported from the Arctic Ocean to the North Atlantic, with small fluctuations of mass transport through the Fram Strait due to the continuous release of contaminants in mid‐low latitude regions, as well as those primary and secondary sources within the Arctic.
... We also find that 92% of the residual overflow transport (1.7 Sv), requiring more than 60 years to return to OSNAP East, enters the Arctic Ocean through either the Fram Strait or the Barents Sea. Our inability to recover these residual overflow particles within the duration of our Lagrangian experiment is hence unsurprising, given that recent chemical tracer studies estimate that Atlantic Waters can spend between 30-50 years circulating within the Arctic Ocean before returning to the Fram Strait (Stöven et al. 2016;Wefing et al. 2021). F . 10. Lagrangian overturning, transports and timescales for particles flowing northward across the Greenland-Scotland Ridge. ...
The strength of the Atlantic Meridional Overturning Circulation (AMOC) at subpolar latitudes is dominated by water mass transformation in the eastern Subpolar North Atlantic (SPNA). However, the distribution of this overturning across the individual circulation pathways of both the Subpolar Gyre (SPG) and the Nordic seas overflows remains poorly understood. Here, we introduce a novel Lagrangian measure of the density-space overturning to quantify the principal pathways of the time-mean overturning circulation within an eddy-permitting ocean model hindcast. By tracing the trajectories of water parcels initialised from the northward inflows across the OSNAP East section, we show that water mass transformation along the pathways of the eastern SPG accounts for 55% of the mean strength of the eastern subpolar AMOC. Water parcels following the dominant SPG pathway, sourced from the Sub-Arctic Front, form upper North Atlantic Deep Water by circulating horizontally across sloping isopycnals in less than 2 years. A slower SPG route, entrained by overflow waters south of the Iceland-Faroes Ridge, is a crucial conduit for subtropical-origin water masses to penetrate the deep ocean on subdecadal timescales. On reproducing our findings using time-averaged velocity and hydrographic fields, we further show that the Nordic seas overflow pathways integrate multiple decades of water mass transformation before returning across the Greenland-Scotland Ridge. We propose that the strong disparity between the overturning timescales of the SPG (interannual) and the Nordic seas overflows (multi-decadal) has important implications for the propagation of density anomalies within the eastern SPNA and hence the sources of AMOC variability.
... Since the history of atmospheric levels of SF 6 and CFCs is relatively well known (Bullister, 2015;Walker et al., 2000), the history of surface water concentrations can in principle be calculated as a function of salinity and temperature following the gas solubility equations determined empirically for CFC-11 (Warner and Weiss, 1985) and SF 6 (Bullister et al., 2002). However, constraints on air-sea gas exchange, surface mixed layer water renewal rates and other physico-chemical processes can influence the atmosphere-ocean equilibrium for gases and gas saturation levels must either be measured experimentally or determined through a modeling approach (Shao et al., 2013;Stoeven et al., 2016). Saturation levels for CFCs measured in the Arctic Ocean have generally been in the range of 85%-100% (Frank et al., 1998;Smethie et al., 2000;Tanhua et al., 2009) although some model results for CFC-12 and especially, SF 6 in arctic regions have indicated much lower saturations when gas exchange fails to respond sufficiently quickly to changing equilibrium conditions (Shao et al., 2013;Stoeven et al., 2016). ...
... However, constraints on air-sea gas exchange, surface mixed layer water renewal rates and other physico-chemical processes can influence the atmosphere-ocean equilibrium for gases and gas saturation levels must either be measured experimentally or determined through a modeling approach (Shao et al., 2013;Stoeven et al., 2016). Saturation levels for CFCs measured in the Arctic Ocean have generally been in the range of 85%-100% (Frank et al., 1998;Smethie et al., 2000;Tanhua et al., 2009) although some model results for CFC-12 and especially, SF 6 in arctic regions have indicated much lower saturations when gas exchange fails to respond sufficiently quickly to changing equilibrium conditions (Shao et al., 2013;Stoeven et al., 2016). SF 6 and CFC-11 input functions are illustrated in Figure 1 (inset) for a saturation level of 90% and values for T and S of 1°C and 34.9, respectively that characterize the warm core of FSBW in much of the central Arctic. ...
... The gas saturation can be measured directly in the surface mixed layer, but calculating it in deeper water requires knowledge of the water mass age and mixing history, its source region and the history of gas saturation in that source region. The SF 6 saturation has frequently been assumed to be lower than that of CFC-11, because atmosphere-ocean gas equilibration can fail to keep pace with the modern, rapidly increasing atmospheric input function of SF 6 , especially in deeper, more poorly mixed regions of source water formation (Shao et al., 2013;Stoeven et al., 2016). This is a less severe constraint for CFC-11 whose input function has been relatively flattened and has declined in recent years so that entrainment of older water during atmospheric ventilation does not alter the tracer composition of the source waters as much as it may for SF 6 . ...
Measurements of the tracers, ¹²⁹I, CFC‐11, and SF6 on water samples collected in the Arctic Ocean in 2015 have been used to calculate mean ages, Γ and mixing, Δ parameters using transit time distributions (TTDs) to constrain water circulation and mixing time scales. Values of Γ and Δ determined separately using the two tracer pairs, SF6‐CFC‐11, and ¹²⁹I‐CFC‐11 are in good agreement for gas solubilities estimated for saturation levels of 0.90, but agreement decreases for other gas saturation levels. Both Γ and Δ increase rapidly with increasing depth below the base of the intermediate water layer (ca. 1,000 m), but maintaining a value of Δ/Γ ≅ 1 supporting the use of this proportionality in applications of TTDs to deep ocean transport of substances such as anthropogenic carbon. Isolines of Γ = 20 years deepening to depths below 1,000 m over the flank of the Mendeleyev Ridge near the North Pole outline the bathymetrically steered, return flow of recently ventilated Atlantic Water toward Fram Strait. Basin interior waters are significantly older with the Γ = 25 years mean age isoline shallowing upward to depths above 500 m in the Makarov, Canada, and Eurasian Basins. Values of Δ remain relatively constant in the 6–10 years range in upper intermediate water across all three basins indicating that flow is principally advective and that the mixing specified by Δ likely occurs upstream of the central basins in regions proximal to the outflow from the Santa Anna Trough.
... As the only deep connection between the Arctic Ocean and the world ocean, Fram Strait is a confluence of several water masses (e.g., Stöven et al., 2016) The net freshwater flux of Fram Strait is projected to continue the rising trend of the historical simulation until around 2060, after which the intensity of the freshwater transport will stay relatively stable in both scenarios. Before 2060, both the volume transport increase (Figure 7g) and salinity decrease (Figure 7h) contribute to the rising freshwater outflow. ...
In this study we assessed the representation of the sea surface salinity (SSS) and liquid freshwater content (LFWC) of the Arctic Ocean in the historical simulation of 31 CMIP6 models with comparison to 39 Coupled Model Intercomparison Project phase 5 (CMIP5) models, and investigated the projected changes in Arctic liquid and solid freshwater content and freshwater budget in scenarios with two different shared socioeconomic pathways (SSP2‐4.5 and SSP5‐8.5). No significant improvement was found in the SSS and LFWC simulation from CMIP5 to CMIP6, given the large model spreads in both CMIP phases. The overestimation of LFWC continues to be a common bias in CMIP6. In the historical simulation, the multi‐model mean river runoff, net precipitation, Bering Strait and Barents Sea Opening (BSO) freshwater transports are 2,928 ± 1,068, 1,839 ± 3,424, 2,538 ± 1,009, and −636 ± 553 km³/year, respectively. In the last decade of the 21st century, CMIP6 MMM projects these budget terms to rise to 4,346 ± 1,484 km³/year (3,678 ± 1,255 km³/year), 3,866 ± 2,935 km³/year (3,145 ± 2,651 km³/year), 2,631 ± 1,119 km³/year (2,649 ± 1,141 km³/year) and 1,033 ± 1,496 km³/year (449 ± 1,222 km³/year) under SSP5‐8.5 (SSP2‐4.5). Arctic sea ice is expected to continue declining in the future, and sea ice meltwater flux is likely to decrease to about zero in the mid‐21st century under both SSP2‐4.5 and SSP5‐8.5 scenarios. Liquid freshwater exiting Fram and Davis straits will be higher in the future, and the Fram Strait export will remain larger. The Arctic Ocean is projected to hold a total of 160,300 ± 62,330 km³ (141,590 ± 50,310 km³) liquid freshwater under SSP5‐8.5 (SSP2‐4.5) by 2100, about 60% (40%) more than its historical climatology.
... where we used the water volume fluxes based on long-term observation (Stöven et al., 2016). The annual average values of water volume fluxes are 4.4 ± 3.2 and −1.4 ± 0.8 Sv for the Norwegian Atlantic Current and East Greenland Current, respectively. ...
... Recirculating Atlantic Water and Arctic Atlantic Water, whose volume fluxes are about −3.5 ± 1.9 Sv, were not discussed due to being indistinguishable in this study. Positive fluxes describe the northward fluxes into the Arctic Ocean, and negative values describe the southward fluxes from the Arctic Ocean (Stöven et al., 2016). The depth-averaged concentration ( ) was calculated using the trapezoidal integral Equation 2: ...
In the Arctic Ocean, it is still unclear what role oceanic transport plays in the fate of semivolatile organic compounds. The strong‐stratified Arctic Ocean undergoes complex inputs and outputs of polycyclic aromatic hydrocarbons (PAHs) from the neighboring oceans and continents. To better understand PAHs’ transport processes and their contribution to high‐latitude oceans, surface seawater, and water column, samples were collected from the North Atlantic Ocean and the Arctic Ocean in 2012. The spatial distribution of dissolved PAHs (∑9PAH) in surface seawater showed an “Arctic Shelf > Atlantic Ocean > Arctic Basin” pattern, with a range of 0.3–10.2 ng L⁻¹. Positive matrix factorization modeling results suggested that vehicle emissions and biomass combustion were the major PAHs sources in the surface seawater. According to principal component analysis, PAHs in different water masses showed unique profiles indicating their different origins. Carried by the Norwegian Atlantic Current (0–800 m) and East Greenland Current (0–300 m), PAH individuals’ net transport mass fluxes ranged from −4.4 ± 1.7 to 53 ± 39 tons year⁻¹ to the Arctic Ocean. We suggested the limited contribution of ocean currents on PAHs’ delivery to the Arctic Ocean, but their role in modulating PAHs’ air–sea interactions and other biogeochemical processes needs further studies.
... As mentioned in the previous section, CFC-12 was measured by both the Medusa-Aqua system and a purge-and-trap GC-ECD instrument (the syringe PT-GC-ECD system) used on board cruise MSM72. The latter is a mature system to measure CFC-12, SF 6 , and SF 5 CF 3 (Stöven, 2011;Stöven and Tanhua, 2014;Stöven et al., 2016;Bullister and Wisegarver, 2008). For comparison, information on the performance of a similar purge-and-trap system setup (Cracker PT-GC-ECD) to measure flame-sealed ampoules for CFC-12 and SF 6 is added to a comparison of the three instrument setups (Table 2). ...
This study evaluates the potential usefulness of the halogenated compounds
HCFC-22, HCFC-141b, HCFC-142b, HFC-134a, HFC-125, HFC-23, PFC-14, and PFC-116
as oceanographic transient tracers to better constrain ocean ventilation
processes. We do this mainly in terms of four aspects of the characteristics of the
potential tracers: input function (including atmospheric history and
historical surface saturation), seawater solubility, feasibility of
measurement, and stability in seawater; of these, atmospheric history and
seawater solubility have been investigated in previous work. For the latter
two aspects, we collected seawater samples and modified an established
analytical technique for the Medusa–Aqua system to simultaneously measure
these compounds. HCFC-22, HCFC-141b, HCFC-142b, HFC-134a, and HFC-125 have
been measured in depth profiles in the Mediterranean Sea for the first time
and for reproducibility in the Baltic Sea. We found that the historical surface
saturation of halogenated transient tracers in the Mediterranean Sea is
estimated to have been nearly constant at 94 % based on historical data.
Of the investigated compounds, HCFC-142b, HCFC-141b, and HFC-134a are found
to currently be the most promising transient tracers in the ocean. The
compounds that have the greatest potential as future tracers are PFC-14 and
PFC-116, mainly hampered by the low solubility in seawater that creates
challenging analytical conditions, i.e., low concentrations. HCFC-22 is
found to be likely unstable in warm seawater, which compromises the
potential as an oceanic transient tracer, although it is possibly useful in
colder water. For the compounds HFC-125 and HFC-23, we were not able to fully
evaluate their potential as tracers due to inconclusive results,
especially regarding their solubility and stability in seawater, but also with
regard to potential analytical challenges. On the other hand, HFC-125,
HFC-23, and HCFC-22 might not need to be considered because there are
alternative tracers with similar input histories that are better suited as
transient tracers.
... The former has been used with anthropogenic radionuclides in earlier studies (e.g., Smith et al., 1998Smith et al., , 2005Smith et al., , 2011Christl et al., 2015;Wefing et al., 2019), where it has also been referred to as a "tracer age model" or "dual-tracer approach". The TTD model has been applied widely in the context of ocean interior ventilation studies (e.g., Haine and Hall, 2002;Waugh et al., 2003;Tanhua et al., 2009;Smith et al., 2011;Stöven et al., 2015) and to determine the oceanic uptake of anthropogenic CO 2 (e.g., Hall et al., 2002;Waugh et al., 2006;Tanhua et al., 2008;Khatiwala et al., 2009;Olsen et al., 2010;Khatiwala et al., 2013;Stöven and Tanhua, 2014;Stöven et al., 2016;He et al., 2018). In this study the binary mixing model will be applied to samples from the surface layer, whereas the TTD model will be used for the mid-depth Atlantic layer due to the following model characteristics. ...
... For comparison to Smith et al. (2011) tracer ages and dilution factors were normalized to the entrance of the Arctic Ocean (dilution factors have been divided by 3 to account for dilution from 60 • N, where the input function from Smith et al. (2011) was defined, to the Barents Sea). For Tanhua et al. (2009) and Stöven et al. (2016), the time t = 0 is set by the isolation of waters from the atmosphere, which was here approximated as the Barents Sea opening. AAW: Arctic Atlantic Water; Init. ...
... Two recent ventilation studies in the Arctic Ocean using the TTD model with atmospherically introduced transient tracers assumed a fixed / = 1 (Tanhua et al., 2009;Stöven et al., 2016). Mean ages for the Nansen and Amundsen basins and the Fram Strait determined from these studies are in general agreement with the 129 I-236 U TTD model results (Table 2). ...
The inflow of Atlantic Water to the Arctic Ocean is a crucial determinant for the future trajectory of this ocean basin with regard to warming, loss
of sea ice, and ocean acidification. Yet many details of the fate and circulation of these waters within the Arctic remain unclear. Here, we use the
two long-lived anthropogenic radionuclides 129I and 236U together with two age models to constrain the pathways and circulation
times of Atlantic Water in the surface (10–35 m depth) and in the mid-depth Atlantic layer (250–800 m depth). We thereby benefit
from the unique time-dependent tagging of Atlantic Water by these two isotopes. In the surface layer, a binary mixing model yields tracer ages of
Atlantic Water between 9–16 years in the Amundsen Basin, 12–17 years in the Fram Strait (East Greenland Current), and up to 20 years in the Canada
Basin, reflecting the pathways of Atlantic Water through the Arctic and their exiting through the Fram Strait. In the mid-depth Atlantic layer
(250–800 m), the transit time distribution (TTD) model yields mean ages in the central Arctic ranging between 15 and 55 years, while the
mode ages representing the most probable ages of the TTD range between 3 and 30 years. The estimated mean ages are overall in good agreement with
previous studies using artificial radionuclides or ventilation tracers. Although we find the overall flow to be dominated by advection, the shift in
the mode age towards a younger age compared to the mean age also reflects the presence of a substantial amount of lateral mixing. For applications
interested in how fast signals are transported into the Arctic's interior, the mode age appears to be a suitable measure. The short mode ages
obtained in this study suggest that changes in the properties of Atlantic Water will quickly spread through the Arctic Ocean and can lead to
relatively rapid changes throughout the upper water column in future years.
... 260or more than 200 years(Jutterström and Jeansson, 2008;Stöven et al., 2016), they have been isolated from the anthropogenic CO 2 increase. Trends of aragonite and calcite saturation states are shown in Figs. 6 and S5, respectively. ...
Being windows to the deep ocean, the Nordic Seas play an important role in transferring anthropogenic carbon, and thus ocean acidification, to the abyss. Due to its location in high latitudes, it is further more sensitive to acidification compared with many other oceanic regions. Here we make a detailed investigation of the acidification of the Nordic Seas, and its drivers, since pre-Industrial to 2100 by using in situ measurements, gridded climatological data, and simulations from one Earth System Model (ESM). In the last 40 years, pH has decreased by 0.11 units in the Nordic Seas surface waters, a change that is twice as large as that between 1850–1980. We find that present trends are larger than expected from the increase in atmospheric CO2 alone, which is related to a faster increase in the seawater pCO2 compared with that of the atmosphere, i.e. a weakening of the pCO2 undersaturation of the Nordic Seas. The pH drop, mainly driven by an uptake of anthropogenic CO2, is significant all over the Nordic Seas, except for in the Barents Sea Opening, where it is counteracted by a significant increase in alkalinity. We also find that the acidification signal penetrates relatively deep, in some regions down to 2000 m. This has resulted in a significant decrease in the aragonite saturation state, which approaches undersaturation at 1000–2000 m in the modern ocean. Future scenarios suggest an additional drop of 0.1–0.4 units, depending on the emission scenario, in surface pH until 2100. In the worst case scenario, RCP8.5, the entire water column will be undersaturated with respect to aragonite by the end of the century, threatening Nordic Seas cold-water corals and their ecosystems. The model simulations suggest that aragonite undersaturation can be avoided at depths where the majority of the cold-water corals live in the RCP2.6 and RCP4.5 scenarios. As these results are based on one model only, we request additional observational and model studies to better quantify the transfer of anthropogenic CO2 to deep waters and its effect on future pH in the Nordic Seas.
