Maurício Santos-Andr

Rodrigo Kerr
email: rodrigokerr@furg.br

Iole B M Orselli

Thiago Monteiro

Mauricio M Mata
email: mauriciosa@furg.br

Catherine Goyet

Maurício Santos-Andrade

Maurício Santos- Andrade

Drivers of Marine CO 2 -Carbonate Chemistry in the Northern Antarctic Peninsula

unstable (aragonite) and stable (calcite) biogenic calcium carbonate minerals formed by different marine organisms. Moreover, many other climate-driven impacts have been observed around the Antarctic continent, including (a) an increase in atmospheric and ocean surface temperatures (Cook et al.,

Introduction

In the next few decades, the Southern Ocean is expected to become the next oceanic region, after the Arctic Ocean, to experience a subsaturated aragonite condition throughout the water column [START_REF] Henley | Changing biogeochemistry of the Southern Ocean and its ecosystem implications[END_REF][START_REF] Orr | Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms[END_REF]. There is an indication that calcium carbonate dissolution will prevail over precipitation, affecting et al., 2019;[START_REF] Moffat | Shelf-ocean exchange and hydrography west of the Antarctic Peninsula: A review[END_REF]) and pH reduction over different time scales [START_REF] Mcneil | Southern Ocean acidification: A tipping point at 450-ppm atmospheric CO 2[END_REF][START_REF] Midorikawa | Decreasing pH trend estimated from 35-year time series of carbonate parameters in the Pacific sector of the Southern Ocean in summer[END_REF][START_REF] Roden | Annual cycle of carbonate chemistry and decadal change in coastal[END_REF][START_REF] Sabine | Decadal changes in Pacific carbon[END_REF]. These changes are already impacting and changing the behavior of Southern Ocean ecosystems, which impact ocean health for the development of the sensitive Antarctic biota with different environmental roles, such as biogeochemical recycling and vertical energy fluxes [START_REF] Figuerola | Predicting potential impacts of ocean acidification on marine calcifiers from the Southern Ocean[END_REF][START_REF] Figuerola | A review and meta-analysis of potential impacts of ocean acidification on marine calcifiers from the Southern Ocean[END_REF][START_REF] Henley | Changing biogeochemistry of the Southern Ocean and its ecosystem implications[END_REF][START_REF] Mendes | Cryptophytes: An emerging algal group in the rapidly changing Antarctic Peninsula marine environments[END_REF].

In this sense, Southern Ocean coastal zones, such as the northern Antarctic Peninsula (NAP), are more sensitive to the effects of climate change [START_REF] Kerr | Northern Antarctic Peninsula: A marine climate hotspot of rapid changes on ecosystems and ocean dynamics[END_REF]. The surrounding waters of the NAP have experienced (a) an increase in partial pressure of carbon dioxide (pCO 2 ) in deep waters, as also observed in the Weddell Sea [START_REF] Van Heuven | Rapid invasion of anthropogenic CO 2 into the deep circulation of the Weddell Gyre[END_REF]; (b) glacier retraction and changes in sea ice thickness over recent decades, such as for the Larsen ice shelves [START_REF] Cook | Retreating glacier fronts on the Antarctic Peninsula over the past half-century[END_REF][START_REF] Cook | Ocean forcing of glacier retreat in the western Antarctic Peninsula[END_REF][START_REF] Dinniman | A model study of circumpolar deep water on the west Antarctic Peninsula and Ross Sea continental shelves[END_REF][START_REF] Dinniman | Sensitivity of circumpolar deep water transport and ice shelf basal melt along the west Antarctic Peninsula to changes in the winds[END_REF]; and (c) impacts of modes of climate variability altering hydrographic patterns in the region, such as the frequency and intensity of water masses coming from open ocean regions [START_REF] Ruiz Barlett | On the temporal variability of intermediate and deep waters in the Western Basin of the Bransfield Strait[END_REF]. Other biogeochemical impacts have been highlighted around the NAP, such as the increased anthropogenic carbon observed in recently ventilated waters [START_REF] Kerr | Carbonate system properties in the Gerlache Strait, northern Antarctic Peninsula (February 2015): II. Anthropogenic CO 2 and seawater acidification[END_REF] and its effect on decreasing pH along the NAP [START_REF] Lencina-Avila | Past and future evolution of the marine carbonate system in a coastal zone of the Northern Antarctic Peninsula[END_REF], in addition to increasing carbon dioxide (CO 2 ) uptake during summers since 2012 in coastal zones [START_REF] Monteiro | Towards an intensified summer CO 2 sink behaviour in the Southern Ocean coastal regions[END_REF].

However, our understanding of carbonate chemistry in the Bransfield Strait is minimal. Current knowledge is limited to regional (e.g., the FRUELA cruise in 1996 in the western basin of the Bransfield Strait) and temporal (either only 1 year of summer data or short-time series) assessments (e.g., [START_REF] Anadón | The FRUELA cruises. A carbon flux study in productive areas of the Antarctic Peninsula (December 1995-February 1996)[END_REF][START_REF] Ito | Sea-air CO 2 fluxes and pCO 2 variability in the Northern Antarctic Peninsula during three summer periods (2008-2010)[END_REF]. Nevertheless, many efforts to comprehend the carbonate system along the NAP over recent decades (Orselli et al., 2022 and references therein) have focused on southern regions of the western Antarctic Peninsula [START_REF] Kerr | Carbonate system properties in the Gerlache Strait, northern Antarctic Peninsula (February 2015): II. Anthropogenic CO 2 and seawater acidification[END_REF]Kerr, Orselli, et al., 2018;[START_REF] Legge | The seasonal cycle of carbonate system processes in Ryder Bay, West Antarctic Peninsula[END_REF][START_REF] Lencina-Avila | Past and future evolution of the marine carbonate system in a coastal zone of the Northern Antarctic Peninsula[END_REF][START_REF] Monteiro | Seasonal variability of net sea-air CO 2 fluxes in a coastal region of the northern Antarctic Peninsula[END_REF][START_REF] Monteiro | Towards an intensified summer CO 2 sink behaviour in the Southern Ocean coastal regions[END_REF][START_REF] Rivaro | Distribution of total alkalinity and pH in the Ross Sea (Antarctica) waters during austral summer 2008[END_REF][START_REF] Sandrini | Anthropogenic carbon distribution in the Ross sea, Antarctica[END_REF]. Some characteristics of the Bransfield Strait pinpoint particular interest in unraveling the variability of carbonate chemistry. Besides acting as a transitional environment between polar and subpolar domains, the NAP region interconnects the warm and old intermediate waters derived from the Bellingshausen Sea with the cold and young waters sourced in the Weddell Sea continental shelf [START_REF] Ruiz Barlett | On the temporal variability of intermediate and deep waters in the Western Basin of the Bransfield Strait[END_REF][START_REF] Van Caspel | On the ventilation of Bransfield Strait deep basins[END_REF]. Furthermore, the Bransfield Strait is considered a sentinel for preserving and tracking both hydrographic and biogeochemical processes around the NAP, as its deep layers maintain seawater properties of the shelf waters sourced in the Weddell Sea [START_REF] Azaneu | Trends in the deep Southern Ocean (1958-2010): Implications for Antarctic bottom water properties and volume export[END_REF][START_REF] Dotto | Multidecadal freshening and lightening in the deep waters of the Bransfield Strait, Antarctica[END_REF][START_REF] Gordon | Deep and bottom water of the Bransfield Strait eastern and central basins[END_REF][START_REF] Hofmann | Water mass distribution and circulation west of the Antarctic Peninsula and including Bransfield Strait[END_REF].

Considering the key role of the Bransfield Strait in connecting environments along the NAP, we evaluated for the first time the spatial and temporal variability of carbonate system parameters (i.e., total alkalinity [TA], total inorganic carbon [CT], pCO 2 , pH, and the saturation state of aragonite and calcite carbonate minerals-Ω ar and Ω ca , respectively) in a period spanning 30 years during 1990-2019. We used hydrographic and biogeochemical data available in historical (World Ocean Database 2018-WOD18, 1996, 2006, and 2010) and recent (Brazilian High Latitude Oceanography Group-GOAL, 2015-2019) databases to estimate equations able to reconstruct carbonate system parameters in the three basins of the Bransfield Strait over the last three decades (i.e., 1990s, 2000s, and 2010s). The results presented here will lead to a better understanding of the main drivers of interannual variability, as well as decadal changes, of carbonate chemistry in the study area.

Oceanographic Features of the Bransfield Strait

Geomorphological Setting of the Bransfield Strait

The Bransfield Strait (Figure 1a) is a coastal area of the NAP consisting of three deep basins formed by a rifting system that splits the South Shetland Islands from the Antarctic Peninsula, leading to a heritage of some volcanoes and fault systems in the region [START_REF] Fisk | Algorithms for computation of fundamental properties of seawater[END_REF]. The release of hydrothermal fluids and magma in deep and shallow waters has recently been identified, as is the case for the Humpback volcano [START_REF] Almendros | BRAVOSEIS: Geophysical investigation of rifting and volcanism in the Bransfield strait, Antarctica[END_REF] and Deception Island [START_REF] Álvarez-Valero | δD and δ 18 O variations of the magmatic system beneath Deception Island volcano (Antarctica): Implications for magma ascent and eruption forecasting[END_REF], respectively.

The depth of the Bransfield Strait increases from the southwest toward the northeast, from ∼1,200 m in the western basin, reaching ∼2,000 m in the central basin and then a maximum of ∼2,500 m in the eastern basin. All deep basins are bound by deep canyons and sills shallower than 800 m [START_REF] Clowes | Hydrology of the Bransfield Strait[END_REF][START_REF] Lopez | Hydrographic and hydrodynamic characteristics a of the eastern basin of the Bransfiled Strait[END_REF][START_REF] Masqué | Sediment accumulation rates and carbon fluxes to bottom sediments at the western Bransfield Strait (Antarctica)[END_REF]. While the western basin has an open connection to the Bellingshausen Sea, the others are semiclosed systems with much more influence from the continental shelf of the Weddell Sea.

Hydrographic Dynamics of the Bransfield Strait

The main water mass contribution influencing the intermediate levels of the Bransfield Strait is advected from the Bellingshausen Sea and the Drake Passage, with relatively warm temperatures, hereinafter referred to as modified-CDW (Figure 1b). It is derived from a mixture of surface waters and CDW intrusions over the continental shelf of the western Antarctic Peninsula [START_REF] Huneke | Water masses in the Bransfield Strait and adjacent seas, austral summer 2013[END_REF][START_REF] Moffat | Shelf-ocean exchange and hydrography west of the Antarctic Peninsula: A review[END_REF][START_REF] Sangrà | The Bransfield current system[END_REF]. Intense modified-CDW intrusions are usually related to stronger westerlies during periods of Southern Annular Mode (SAM) in positive phases (SAM+), El-Niño Southern Oscillation (ENSO) in negative phases (ENSO-) or their coupled events [START_REF] Loeb | ENSO and variability of the Antarctic Peninsula pelagic marine ecosystem[END_REF][START_REF] Ruiz Barlett | On the temporal variability of intermediate and deep waters in the Western Basin of the Bransfield Strait[END_REF]. For instance, Ruiz [START_REF] Ruiz Barlett | On the temporal variability of intermediate and deep waters in the Western Basin of the Bransfield Strait[END_REF] pointed out that the early 2000s, characterized by joint events of SAM+ and ENSO-, presented a hydrographic setting favored by modified-CDW intrusions into the western basin of the Bransfield Strait between 500 and 800 m in depth, corresponding to 80% of the water mass structure.

In addition, the deep basins of the Bransfield Strait are filled with Dense Shelf Water (DSW) advected from the north-western and southern continental shelf of the Weddell Sea and characterized by high density and low temperatures close to the freezing point of seawater (Figure 1b, [START_REF] Frölicher | Dominance of the Southern Ocean in anthropogenic carbon and heat uptake in CMIP5 models[END_REF][START_REF] Hellmer | The fate of the Southern Weddell Sea continental shelf in a warming climate[END_REF][START_REF] Sangrà | The Bransfield current system[END_REF][START_REF] Sangrà | The Bransfield gravity current[END_REF]. Thus, the DSW considered here does not account for the differentiation between High Salinity Shelf Water and Low Salinity Shelf Water sourced by ocean-atmosphere-sea ice interactions in the north-western continental shelves (e.g., [START_REF] Damini | Long-term changes on the Bransfield Strait deep water masses: Variability, drivers and connections with the northwestern Weddell Sea[END_REF][START_REF] Dotto | Multidecadal freshening and lightening in the deep waters of the Bransfield Strait, Antarctica[END_REF] and Ice Shelf Water derived from ocean-ice shelf processes in the southern continental shelves [START_REF] Newton | Global Ocean acidification observing network: Requirements and governance plan. Global ocean acidification observing network, 5-28[END_REF]. The influence of Ice Shelf Water on the composition of DSW in the deep layers of the Bransfield Strait is still a challenge (e.g., [START_REF] Damini | Long-term changes on the Bransfield Strait deep water masses: Variability, drivers and connections with the northwestern Weddell Sea[END_REF][START_REF] Van Caspel | On the ventilation of Bransfield Strait deep basins[END_REF][START_REF] Wang | Variability and dynamics of along-shore exchange on the west Antarctic Peninsula (WAP) continental shelf[END_REF], but an overflow is expected in the vicinity of the Powell Basin into the eastern basin [START_REF] Gordon | Deep and bottom water of the Bransfield Strait eastern and central basins[END_REF][START_REF] Gyldenfeldt | Flow variability at the tip of the Antarctic Peninsula[END_REF]. A larger contribution of DSW in the Bransfield Strait occurs during weaker westerlies, events of SAM-and ENSO+ or joint events of both phases [START_REF] Ruiz Barlett | On the temporal variability of intermediate and deep waters in the Western Basin of the Bransfield Strait[END_REF], which intensify the Weddell Gyre. An intensification of DSW was identified during the 2010s, representing at least 60% of the water mass structure in the deep layer of the central basin [START_REF] Damini | Long-term changes on the Bransfield Strait deep water masses: Variability, drivers and connections with the northwestern Weddell Sea[END_REF].

The averaged water mass fractions of contribution in the western, central and eastern basins of Bransfield Strait, excluding the surface level up to ∼100 m, are 80%, 40%, and 60% of CDW plus 20%, 60%, and 40% of DSW, respectively (Figure S1 in Supporting Information S1). However, a high degree of interannual variability in the water mass fractions is expected due to the rapid replenishment of DSW and CDW intrusions in the region [START_REF] Damini | Long-term changes on the Bransfield Strait deep water masses: Variability, drivers and connections with the northwestern Weddell Sea[END_REF][START_REF] Dotto | Multidecadal freshening and lightening in the deep waters of the Bransfield Strait, Antarctica[END_REF].

The boundary between the modified-CDW and DSW is associated with the surface Peninsula Front and the subsurface Bransfield Front (Figure 1c, [START_REF] Sangrà | The Bransfield current system[END_REF][START_REF] Sangrà | The Bransfield gravity current[END_REF]. The circulation pattern of both water masses in the strait characterizes a cyclonic gyre, the Bransfield Gyre [START_REF] Moffat | Shelf-ocean exchange and hydrography west of the Antarctic Peninsula: A review[END_REF], with intense baroclinic jets to the northeast associated with the Bransfield Current in the Bransfield Front [START_REF] Sangrà | The Bransfield current system[END_REF][START_REF] Sangrà | The Bransfield gravity current[END_REF]. This current plays a crucial role in toting metal, nutrients, and organisms along the Bransfield Strait [START_REF] Zhou | The western boundary current in the Bransfield Strait, Antarctica[END_REF]. Furthermore, another reason for the complex hydrography is related to glacier and sea ice melting around Antarctica [START_REF] Castro | Nutrient utilisation and chlorophyll distribution in the Atlantic sector of the Southern Ocean during Austral summer 1995-96[END_REF], including the western Weddell Sea with the collapse of Larsen ice shelves [START_REF] Massom | Antarctic ice shelf disintegration triggered by sea ice loss and ocean swell[END_REF][START_REF] Scambos | The link between climate warming and break-up of ice shelves in the Antarctic Peninsula[END_REF] and coastal ice coverage of the western Antarctic Peninsula, which is influenced by the heat transport of CDW [START_REF] Meredith | Variability in the freshwater balance of northern Marguerite Bay, Antarctic Peninsula: Results from δ 18 O. Deep-Sea Research Part II[END_REF][START_REF] Meredith | Dense waters of the Weddell and Scotia seas: Recent changes in properties and circulation[END_REF][START_REF] Shepherd | Warm ocean is eroding west Antarctic Ice Sheet[END_REF][START_REF] Shepherd | Mass balance of the Antarctic Ice Sheet from 1992 to 2017[END_REF]. For a broad review of the hydrography of the Bransfield Strait, the reader should refer to [START_REF] Clowes | Hydrology of the Bransfield Strait[END_REF], [START_REF] Damini | Long-term changes on the Bransfield Strait deep water masses: Variability, drivers and connections with the northwestern Weddell Sea[END_REF], [START_REF] Dotto | Multidecadal freshening and lightening in the deep waters of the Bransfield Strait, Antarctica[END_REF], [START_REF] García | Water masses and distribution of physico-chemical properties in the Western Bransfield Strait and Gerlache Strait during austral summer 1995/96[END_REF], [START_REF] Sangrà | The Bransfield current system[END_REF], 2017[START_REF] Ruiz Barlett | On the temporal variability of intermediate and deep waters in the Western Basin of the Bransfield Strait[END_REF].

Biogeochemical Aspects of the Bransfield Strait

The coastal zones along the NAP are areas of high biological productivity during summers, holding high stocks and concentrations of phytoplankton [START_REF] Karl | Seasonal coupling of organic matter production and particle flux in the western Bransfield Strait, Antarctica[END_REF][START_REF] Mendes | Dynamics of phytoplankton communities during late summer around the tip of the Antarctic Peninsula[END_REF], with diatoms being the most abundant phytoplanktonic group found in the north-western Weddell Sea [START_REF] Mendes | Dynamics of phytoplankton communities during late summer around the tip of the Antarctic Peninsula[END_REF]. This, therefore, implies high organic matter sedimentation around the entire peninsula [START_REF] Fischer | Stable carbon isotope ratios of plankton carbon and sinking organic matter from the Atlantic sector of the Southern Ocean[END_REF][START_REF] Masqué | Sediment accumulation rates and carbon fluxes to bottom sediments at the western Bransfield Strait (Antarctica)[END_REF]. Also, intense primary production in the north-western Weddell Sea continental shelf [START_REF] Detoni | Environmental conditions during phytoplankton blooms in the vicinity of James Ross Island, east of the Antarctic Peninsula[END_REF][START_REF] Mendes | Dynamics of phytoplankton communities during late summer around the tip of the Antarctic Peninsula[END_REF] likely promotes high dissolved organic carbon concentrations in the deep basins of Bransfield Strait [START_REF] Avelina | Contrasting dissolved organic carbon concentrations in the Bransfield Strait, northern Antarctic Peninsula: Insights into ENSO and SAM effects[END_REF]. Furthermore, regions related to islands and coastal zones in the strait are also associated with high organic carbon concentrations, such as Deception Island and Admiralty Bay [START_REF] Costa | Chemotaxonomic characterization of the key genera of diatoms in the Northern Antarctic Peninsula[END_REF][START_REF] Doval | Dissolved organic carbon distributions in the Bransfield and Gerlache Straits, Antarctica[END_REF].

The inorganic carbon cycle of the Bransfield Strait experiences large interannual variability, swinging between periods of net CO 2 source to the atmosphere (2009) and net CO 2 sink toward the ocean [START_REF] Gille | Decadal-scale temperature trends in the Southern Hemisphere ocean[END_REF]( and 2010[START_REF] Ito | Sea-air CO 2 fluxes and pCO 2 variability in the Northern Antarctic Peninsula during three summer periods (2008-2010)[END_REF]. This behavior has been recently pointed to be influenced by the strength of mesoscale structures, such as anticyclonic eddies (e.g., [START_REF] Damini | Antarctica Slope Front bifurcation eddy: A stationary feature influencing CO 2 dynamics in the northern Antarctic Peninsula[END_REF]. A long-term assessment of CO 2 fluxes in the vicinity of the Bransfield Strait has shown that coastal zones have acted as strong CO 2 sink zones since 2012 [START_REF] Monteiro | Towards an intensified summer CO 2 sink behaviour in the Southern Ocean coastal regions[END_REF], which is also observed southwards [START_REF] Brown | Enhanced oceanic CO 2 uptake along the rapidly changing West Antarctic Peninsula[END_REF]. The region has also stored ∼20-50 μmol kg -1 of anthropogenic carbon around the NAP [START_REF] Anderson | The transport of anthropogenic carbon dioxide into the Weddell Sea[END_REF][START_REF] Lencina-Avila | Past and future evolution of the marine carbonate system in a coastal zone of the Northern Antarctic Peninsula[END_REF][START_REF] Pardo | Anthropogenic CO 2 estimates in the Southern Ocean: Storage partitioning in the different water masses[END_REF], which likely comes from recently ventilated DSW [START_REF] Kerr | Carbonate system properties in the Gerlache Strait, northern Antarctic Peninsula (February 2015): II. Anthropogenic CO 2 and seawater acidification[END_REF]. Moreover, some recent studies have also shown the impacts of ENSO or SAM on physical, biological, and biogeochemical parameters in the strait [START_REF] Avelina | Contrasting dissolved organic carbon concentrations in the Bransfield Strait, northern Antarctic Peninsula: Insights into ENSO and SAM effects[END_REF][START_REF] Costa | Dynamics of an intense diatom bloom in the northern Antarctic Peninsula, February 2016[END_REF][START_REF] Damini | Long-term changes on the Bransfield Strait deep water masses: Variability, drivers and connections with the northwestern Weddell Sea[END_REF][START_REF] Dotto | Multidecadal freshening and lightening in the deep waters of the Bransfield Strait, Antarctica[END_REF]. A recent overview of state-of-the-art of carbonate chemistry and the challenges facing the NAP environments can be viewed in [START_REF] Orselli | The marine carbonate system along the northern Antarctic Peninsula: Current knowledge and future perspectives[END_REF] and references therein.

Methods

Data Collection

The following seawater hydrographic and biogeochemical variables were accessed from two data sets with broad coverage for austral summer (January, February, and March) in the Bransfield Strait: temperature (°C), salinity (unitless), dissolved oxygen as O 2 (μmol kg -1 ), TA (μmol kg -1 ), and CT (μmol kg -1 ). The compiled database used here is described below:

1. WOD18: A data set created by the National Centers for Environmental Information/National Oceanic and Atmospheric Administration with an associated high level of quality control. It includes hydrographic and biogeochemical data for different ocean regions spanning a long period of time [START_REF] Boyer | NOAA atlas NESDIS 87[END_REF]. The hydrographic variables were selected from ocean station data and high-resolution CDT/XCTD (https://www.nodc. noaa.gov/OC5/WOD/wod18-notes.html, [START_REF] Boyer | NOAA atlas NESDIS 87[END_REF] during 1990-2011. The precision for temperature and practical salinity measurements are 0.001°C-0.005°C and 0.003°C-0.02°C, respectively [START_REF] Boyer | NOAA atlas NESDIS 87[END_REF]. In situ TA and CT measurements were available [START_REF] Hofmann | Water mass distribution and circulation west of the Antarctic Peninsula and including Bransfield Strait[END_REF], 2006, and 2010. For the FRUELA cruise (17 January-5 February 1996[START_REF] Anadón | The FRUELA cruises. A carbon flux study in productive areas of the Antarctic Peninsula (December 1995-February 1996)[END_REF], TA and CT were determined through automatic potentiometric titration with hydrochloric acid and thermodynamic equations of the carbonate system estimated from pH at 15°C and TA [START_REF] Alvarez | Spatio-temporal variability of air-sea fluxes of carbon dioxide and oxygen in the Bransfield and Gerlache Straits during Austral summer 1995-96[END_REF]. Neither quality control nor precision of in situ measurements was found for 2006 and 2010 data. The oceanographic stations containing TA and CT data available in WOD18 are presented in Figure S2a in Supporting Information S1. 2. GOAL: The GOAL data set included cruises in the NAP region, almost every year, comprising data of Bransfield and Gerlache straits, Drake Passage, and Bellingshausen and Weddell seas (Dotto, Mata, et al., 2021;[START_REF] Mata | 15 years sailing with the Brazilian high latitude oceanography group (GOAL)[END_REF]. The hydrographic data set is available at PANGAEA (Mata & Garcia, 2016a, 2016b, 2016c, 2016d, 2016e, 2016f;Mata & Kerr, 2016a, 2016b, 2016c) and NAPv.1. climatology at https://doi.org/10.5281/ zenodo.4420006 (Dotto, Kerr, et al., 2021), spanning from 2003 to 2019. During austral summers, the GOAL data set includes 58028 measurements for temperature and salinity against 28725 measurements for O 2 . The precision for temperature and practical salinity measurements are ∼0.001°C and ∼0.003°C, respectively (Dotto, Kerr, et al., 2021). The TA and CT measurements spanned 2015-2019 [START_REF] Orselli | The marine carbonate system along the northern Antarctic Peninsula: Current knowledge and future perspectives[END_REF] and were measured simultaneously by potentiometric titration in a closed cell with an automated titrator (see details at Kerr, Orselli et al. (2018); Figure S2b in Supporting Information S1). Quality control of TA and CT data was performed through analyses of Certified Reference Material from the Scripps Institution of Oceanography [START_REF] Dickson | Reference materials for oceanic CO 2 analysis: A method for the certification of total alkalinity[END_REF]. The average precisions for TA and CT were 3.2 ± 1.3 and 4.0 ± 0.8 μmol kg -1 , respectively (Table S1 in Supporting Information S1). The precision was evaluated daily throughout the analysis period with replicate analysis of a single sample.

Since the hydrographic data from WOD18 and GOAL had already undergone serious quality control, only data marked with good quality by the postprocessing procedure in the data set (flag = 0 or 1) were used in this study [START_REF] Boyer | NOAA atlas NESDIS 87[END_REF]. The WOD18 and GOAL biogeochemical data sets (i.e., TA and CT) were also checked to identify potential spurious data since no quality control was previously described for either. Spurious data were excluded when greater than two standard deviations from the mean property value of each vertical profile. This represents only 3% and 8% of data for both TA and CT in WOD18 and GOAL, respectively. Validation of biogeochemical data was performed through comparisons with recently published climatologies [START_REF] Broullón | A global monthly climatology of total alkalinity: A neural network approach[END_REF][START_REF] Broullón | A global monthly climatology of oceanic total dissolved inorganic carbon: A neural network approach[END_REF]. Finally, the database used here (WOD18 + GOAL) covers 30 years (1990-2019) for hydrographic and 23 years (1996-2019) for biogeochemical variables. The TA and CT variables available in WOD18 and GOAL were used to reconstruct the carbonate chemistry in the study area for the whole period through the hydrographic data (see Section 3.3). The spatial distribution and the number of occupations of each station with hydrographic data over the last three decades are presented in Figure S2c in Supporting Information S1.

Hydrographic-Derived Parameters

Neutral density (γ n , kg m -3 ) and potential temperature (θ, °C) were estimated according to EOS-80 (Fofonoff & Millard, 1983). The γ n values were used to split the vertical layer structure in each basin (marked by their boundaries with sills shallower than 800 m, Figure 1b, [START_REF] Clowes | Hydrology of the Bransfield Strait[END_REF]Gàrcia et al., 2002), defining the surface (γ n ≤ 28.00 kg m -3 ), intermediate (28.00 < γ n < 28.27 kg m -3 ), and deep (γ n ≥ 28.27 kg m -3 ) layers. The surface layer is occupied by Antarctic Surface Water that is highly modified locally by sea-air interactions, sea ice melting and continental meltwater inputs [START_REF] Hofmann | Water mass distribution and circulation west of the Antarctic Peninsula and including Bransfield Strait[END_REF][START_REF] Wang | Variability and dynamics of along-shore exchange on the west Antarctic Peninsula (WAP) continental shelf[END_REF], whereas the intermediate layer is mainly composed of a mixture of modified-CDW and DSW [START_REF] Ruiz Barlett | On the temporal variability of intermediate and deep waters in the Western Basin of the Bransfield Strait[END_REF][START_REF] Wang | Variability and dynamics of along-shore exchange on the west Antarctic Peninsula (WAP) continental shelf[END_REF].

The deep layer is occupied by DSW that flows down the hills and canyons in the region (e.g., [START_REF] Dotto | Multidecadal freshening and lightening in the deep waters of the Bransfield Strait, Antarctica[END_REF][START_REF] Huneke | Water masses in the Bransfield Strait and adjacent seas, austral summer 2013[END_REF][START_REF] Ruiz Barlett | On the temporal variability of intermediate and deep waters in the Western Basin of the Bransfield Strait[END_REF][START_REF] Van Caspel | On the ventilation of Bransfield Strait deep basins[END_REF].

Reconstruction of Marine Carbonate Chemistry

Since measured TA and CT data were not available for the entire period of study, we applied multivariate linear equations to reconstruct TA and CT over time in vertical profiles of the Bransfield Strait. This method allows the characterization of the variability of carbonate chemistry with respect to water masses and their hydrographic signatures since (a) TA is controlled by salinity changes through freshwater input or removal, which also impacts CT; (b) blooms of primary producers and their decomposition follow temperature changes (higher temperatures in the surface layer and lower throughout the depth), contribution to CT changes and a minor impact to TA changes; and (c) CO 2 ingassing is associated with the surface layer and its lower temperatures, which promotes CT changes and no impact on TA [START_REF] Lee | Global relationships of total alkalinity with salinity and temperature in surface waters of the world's oceans[END_REF][START_REF] Millero | Distribution of alkalinity in the surface waters of the major oceans[END_REF]. Dissolved oxygen (O 2 ) fits as a good proxy for biological processes and since they exhibit much more pronounced impacts on CT than TA, is added to CT reconstructions [START_REF] Goyet | Estimation of total CO 2 concentration throughout the water column[END_REF].

This approach has been widely applied in different ocean regions, from global [START_REF] Bittig | An alternative to static climatologies: Robust estimation of open ocean CO 2 variables and nutrient concentrations from T, S, and O 2 data using Bayesian neural networks[END_REF][START_REF] Carter | Updated methods for global locally interpolated estimation of alkalinity, pH, and nitrate[END_REF] to regional [START_REF] Arrigo | Air-sea flux of CO 2 in the Arctic Ocean, 1998-2003[END_REF][START_REF] Orselli | Progress in oceanography the sea-air CO 2 net fluxes in the South Atlantic Ocean and the role played by Agulhas eddies[END_REF] scales, including along the NAP [START_REF] Hauri | Two decades of inorganic carbon dynamics along the West Antarctic Peninsula[END_REF][START_REF] Lencina-Avila | Past and future evolution of the marine carbonate system in a coastal zone of the Northern Antarctic Peninsula[END_REF][START_REF] Monteiro | Seasonal variability of net sea-air CO 2 fluxes in a coastal region of the northern Antarctic Peninsula[END_REF][START_REF] Monteiro | Towards an intensified summer CO 2 sink behaviour in the Southern Ocean coastal regions[END_REF]. For this, the following relationship (Equation 1) was applied:

TA (𝜃𝜃𝜃 S); CT (𝜃𝜃𝜃 S𝜃 O2)𝜃 (1)
where TA is total alkalinity and CT is total inorganic carbon; θ is potential temperature, S is practical salinity, and O 2 is dissolved oxygen. Briefly, measured TA and CT and the respective hydrographic parameters were tested in two groups (i.e., reconstruction and assessment) for some sets to find the best equation suitable for the Bransfield Strait. Overall, we tested three subsets:

1. Set01: a specific set of equations, considering a pair of TA and CT equations for each layer of each basin, resulting in eight pairs of equations for the whole strait. This set did not provide a pair of equations for the deep layer of the western basin because of limited Data Availability. 2. Set02: a general set of equations to increase the input data for reconstruction. A pair of TA and CT equations was tested for each layer of the whole strait, resulting in three pairs of equations for the NAP. The input data were tested through the arrangement of specific years and their different combinations. 3. Set03: a general set of equations as described for Set02 but tested via random combinations.

Different combinations of input data were tested to find the best training data, considering at least 1,000 combinations per set. The equations were evaluated using best correlation, lowest root-mean-square error (RMSE), and higher significance (95% confidence interval; p < 0.05) and significant F (high values indicate good representation through a linear regression). Comparisons of different sets for each layer (Figure S3 in Supporting Information S1) and other equations developed for broad ocean areas [START_REF] Lee | Global relationships of total alkalinity with salinity and temperature in surface waters of the world's oceans[END_REF][START_REF] Millero | Distribution of alkalinity in the surface waters of the major oceans[END_REF][START_REF] Takahashi | Climatological distributions of pH, pCO 2 , total CO 2 , alkalinity, and CaCO 3 saturation in the global surface ocean, and temporal changes at selected locations[END_REF], including the Arctic Ocean [START_REF] Arrigo | Air-sea flux of CO 2 in the Arctic Ocean, 1998-2003[END_REF], and regional areas of the Southern Ocean [START_REF] Hauri | Two decades of inorganic carbon dynamics along the West Antarctic Peninsula[END_REF][START_REF] Lencina-Avila | Past and future evolution of the marine carbonate system in a coastal zone of the Northern Antarctic Peninsula[END_REF][START_REF] Monteiro | Seasonal variability of net sea-air CO 2 fluxes in a coastal region of the northern Antarctic Peninsula[END_REF][START_REF] Monteiro | Towards an intensified summer CO 2 sink behaviour in the Southern Ocean coastal regions[END_REF] for TA (Figure S4 in Supporting Information S1) and CT (Figure S5 in Supporting Information S1) were tested. Only equations with the same input data used here were considered in this intercomparison. The best-fitted equations for the Bransfield Strait (Set03) are presented in Table 1. The TA estimates were then validated in surface (0.99; p < 0.01; n = 22), intermediate (0.99; p < 0.01; n = 19), and deep (0.81; p < 0.01; n = 12) layers, as well as CT in surface (0.99; p < 0.01, n = 26), intermediate (0.96; p < 0.01, n = 17), and deep (0.98; p < 0.01, n = 9) layers. This set of equations also presents an uncertainty range related with observation and may be classified as a weather error (i.e., an uncertainty of 10% in the estimation of the dissolved carbonate ion, [START_REF] Newton | Global Ocean acidification observing network: Requirements and governance plan. Global ocean acidification observing network, 5-28[END_REF] and similar residuals (i.e., difference between measured and reconstructed TA and CT) to other studies performing the same methodology (between -60 and 60 μmol kg -1 , Figure S6 in Supporting Information S1, [START_REF] Hauck | Data-based estimation of anthropogenic carbon and acidification in the Weddell Sea on a decadal timescale[END_REF][START_REF] Lee | Global relationships of total alkalinity with salinity and temperature in surface waters of the world's oceans[END_REF][START_REF] Takahashi | Climatological distributions of pH, pCO 2 , total CO 2 , alkalinity, and CaCO 3 saturation in the global surface ocean, and temporal changes at selected locations[END_REF]. Since Set03 represents the natural variability of the carbonate chemistry in the Bransfield Strait, TA and CT were reconstructed over 30 years . Hereafter, only the reconstructed TA and CT are used.

Reconstructed TA and CT data were used as input for CO2Sys v2.1 [START_REF] Lewis | Program developed for CO 2 system calculations[END_REF] to estimate the other carbonate system parameters (i.e., pH in seawater scale, pH sws ; pCO 2 in μatm; Revelle Factor; Ω ar and Ω ca ). Here, pH sws was used, which is 0.01 units higher than pH in the total scale (pH T , [START_REF] Millero | The marine inorganic carbon cycle[END_REF]. The dissociation constants of carbonic acid by [START_REF] Goyet | New determination of carbonic acid dissociation constants in seawater as a function of temperature and salinity[END_REF] were used, which are recognized for their good performances in high latitude environments [START_REF] Kerr | Carbonate system properties in the Gerlache Strait, northern Antarctic Peninsula (February 2015): II. Anthropogenic CO 2 and seawater acidification[END_REF][START_REF] Laika | Interannual properties of the CO 2 system in the Southern Ocean south of Australia[END_REF][START_REF] Lencina-Avila | Past and future evolution of the marine carbonate system in a coastal zone of the Northern Antarctic Peninsula[END_REF][START_REF] Wanninkhof | The optimal carbonate dissociation constants for determining surface water pCO 2 from alkalinity and total inorganic carbon[END_REF]. To comprehend the impact of different constant sets on the output data from CO2Sys, a sensitivity test was performed, the results of which are described in the Supplementary Material (Tables S2-S4 in Supporting Information S1). [START_REF] Dickson | Thermodynamics of the dissociation of boric acid in potassium chloride solutions from 273[END_REF], [START_REF] Uppström | The boron/chlorinity ratio of deep-sea water from the Pacific Ocean[END_REF], and [START_REF] Perez | Association constant of fluoride and hydrogen ions in seawater[END_REF] were used for sulfate, total boron and fluoride constants, respectively. Uncertainties related to this reconstruction were calculated following the error propagation of [START_REF] Orr | Routine uncertainty propagation for the marine carbon dioxide system[END_REF]. For pCO 2 , it was 84.8 ± 5.9 in the surface layer, 14.0 ± 0.7 in the intermediate layer, and 102.4 ± 11.7 μatm in the deep layer. Lower uncertainties for pCO 2 were associated with the intermediate layer (Table S5 in Supporting Information S1). The same pattern described in

F = 63.1 F = 39.86
Note. Statistical data related to each reconstruction are presented along with the respective equation, such as correlation (r), root mean square error (RMSE), number of samples used for each reconstruction (n), p-value (p), and the significance value (F, which expresses how many of these data are well represented via linear regressions, with high values meaning best fit via linear regressions). Note that these specific relationships present a RMSE that is about half that of those for broad oceanic areas. Acronyms are: TA-total alkalinity (μmol kg -1 ); CT-total inorganic carbon (μmol kg -1 ); θ-potential temperature (°C); S-salinity (unitless); O 2 -dissolved oxygen (μmol kg -1 ); and γ n -neutral density (kg m -3 ).

pCO 2 is observed for [H + ] (a proxy for pH uncertainty), Ω ar and Ω ca , for which uncertainties are at an order of magnitude lower in intermediate layers than in surface and deep layers (Table S5 in Supporting Information S1).

Temporal and Climate Composition Approach of Carbonate System Parameters

Interannual variability was assessed by long-term trends (±confidence bounds) based on time series anomalies for each density layer and basin of the Bransfield Strait, following [START_REF] Dotto | Multidecadal freshening and lightening in the deep waters of the Bransfield Strait, Antarctica[END_REF]. Annual anomalies were calculated as annual averages minus the 2010s average because this decade has the higher temporal coverage. Annual trends were statistically significant when the confidence bound was less than the trend value (i.e., 95% confidence level; p < 0.05).

Decadal variability was assessed through decadal averaged (±standard error) vertical profiles for the carbonate system parameters at bins of 50 m over the three decades (i.e., the 1990s during [START_REF] Dickson | Thermodynamics of the dissociation of boric acid in potassium chloride solutions from 273[END_REF]-1999, 2000s during 2000-2009, and 2010s during 2010-2019, Table S6 , Table S6 in Supporting Information S1). Each calculated average depth level in the vertical profile was assessed, point by point, through the Mann-Whitney test to identify significant differences among these three decades, considering a significance level of 95% (p < 0.05).

Vertical variability was also assessed through a composite analysis, which evaluated the role played by ENSO (https://origin.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ONI_v5.php, last access: 13 November 2022) and SAM (http://www.nerc-bas.ac.uk/icd/gjma/sam.html, last access: 13 November 2022) and their effects on carbonate chemistry within the strait. Their effects were evaluated over summers (January, February, and March) in its negative (index ≤ -0.5, SAM-/ENSO-), neutral (-0.5 < index < 0.5, SAMn/ENSOn), and positive (index ≥ 0.5, SAM+/ENSO+) phases. Differences for composite analysis were tested for significance by the Mann-Whitney test for each depth interval of 50 m in each basin. Each vertical profile shown here represents a composite profile.

Results

Western Basin of the Bransfield Strait

The surface layer of the western basin was characterized by annual average ± standard error varying from 2,315 ± 2 to 2,381 ± 1 μmol kg -1 for TA and 2,195 ± 1 to 2,291 ± 1 μmol kg S7 in Supporting Information S1).

The TA annual-average vertical distribution was lower in the intermediate layer (2,276 ± 1 μmol kg -1 ) relative to the surface layer and reached 2,392 ± 0.1 μmol kg -1 in the deep layer (Figure S7 in Supporting Information S1).

Annual averaged CT increased with depth, reaching 2,300 ± 1 μmol kg -1 in the deep layer (Figure S7 in Supporting Information S1).

In general, interannual variability showed steeper and more significant trends in the deepest layers, except for TA (Table 2, Figure S8a in Supporting Information S1). The deep layer had a sharp positive trend for CT per year, threefold higher than that in the intermediate layer (Table 2, Figure S8b in Supporting Information S1). Although data were solely restricted to the 2010s for the deep layer of the western basin, the CT trend was well-marked due to the high variability. Moreover, the annual trend for pCO 2 in the deep layer also showed a sharp positive trend of 18 ± 9 μatm yr -1 , being ninefold higher than that at the surface (Table 2). Interannual trends showed a clear reduction per year for pH sws , Ω ar , and Ω ca , which increase with depth and characterize the deep layer with trends up to three-and fivefold higher than for the intermediate and surface layers, respectively (Table 2).

Decadal-average vertical profiles of TA and CT showed no long-term variability in the surface layer (Figures 2a and2b, p > 0.05, Table S8 in Supporting Information S1). However, TA and CT decreased between the 1990s and 2000s (p < 0.01, Table S8 in Supporting Information S1) for the intermediate layer around 450-1,000 m (Figures 2a and2b). During the 2010s, TA and CT recovered to values of the 1990s (p < 0.01, Table S8 in Supporting Information S1). A progressive increase in pCO 2 values below 200 m was observed from the 1990s to the 2010s, consequently related to concurrent pH sws , and Ω ar and Ω ca reductions (Figures 2c-2f). It is noted that the vertical distribution of carbonate system parameters was mainly affected by both ENSO and SAM (Figure 3 and Figure S8 in Supporting Information S1). During SAM-(Figure 3b) and ENSO+ (Figure 3d), CT below ∼350 m increased when compared to the vertical distribution observed in years marked by a neutral phase. Furthermore, although changes were noted in TA and CT concentrations, pCO 2 , pH sws , Ω ar , and Ω ca experienced well-marked changes between 300 and 500 m in depth during SAM-and ENSO+, in which TA and CT changes were associated with pCO 2 increase, and pH sws , Ω ar and Ω ca decrease (Figure S9 in Supporting Information S1).

Central Basin of the Bransfield Strait

The surface layer of the central basin had total average concentrations for TA and CT of 2368 ± 1 and 2243 ± 2 μmol kg -1 , respectively (Figures S10a andS10b in Supporting Information S1), which are close to those of the surface waters of the western basin (Figures S7a and S7b in Supporting Information S1). The total average value of pCO 2 in the surface layer was 394.8 ± 4.0 μatm, whereas pH sws , Ω ar , and Ω ca were 8.121 ± 0.001, 1.70 ± 0.01, and 2.71 ± 0.01, respectively (Figure S11 in Supporting Information S1). The average values in the intermediate layer were marked by a minimum annual average for pH sws of 7.758 ± 0.002 and a maximum for pCO 2 of 750.2 ± 3.5 μatm relative to the other layers in the basin (Figure S10 in Supporting Information S1). Low TA and CT variabilities were observed for the whole water column, with the total average varying from 10 in the surface to 13 μmol kg -1 deep layer. The average values of Ω ar and Ω ca decreased with depth, with Ω ar reaching subsaturated condition in most of the vertical profiles and Ω ca in a few in the deep layer (Figure S10 in Supporting Information S1).

Increasing TA and CT trends were observed only in the intermediate layer (Table 2, Figure S11 in Supporting Information S1), with CT, in particular, showing higher interannual variability (Figure S11b in Supporting Information S1).

Although with no significant trends, pH sws , Ω ar , and Ω ca increased in the entire water column for the studied period (Table 2, Figure S11 in Supporting Information S1). Unfortunately, the carbonate chemistry reconstruction is limited to those years with available hydrographic data, restricting a better spatiotemporal coverage in the 1990s in this basin.

The last 30 years have been marked by a pCO 2 increase and simultaneous pH sws , Ω ar , and Ω ca decrease over the last two decades in the entire vertical profile (Figures 4c-4f). Particularly for Ω ar , depths below 200 m were below Note. If the confidence bound is higher than the trend, the trend is not statistically significant (i.e., p > 0.05; 95% confidence interval). Significant values are highlighted in bold.

saturation conditions (i.e., Ω ar < 1) in the 2010s (Figure 4e). Furthermore, TA was constant over decades and during any SAM and ENSO events (Figures 3e and3g, respectively). The pH sws , Ω ar and Ω ca were lowest in the deep layer during SAM+ and SAM-events compared to SAMn and ENSO+ (Figures S9b-S9h in Supporting Information S1).

Eastern Basin of the Bransfield Strait

The eastern basin is filled with water masses with relative homogeneity in the vertical distribution of carbonate chemistry, which implies low interannual and decadal variability (Figures S12-S14 in Supporting Information S1).

Interannual trends were small in the eastern basin compared to the other basins of the strait, and most carbonate system parameters had a negative trend although low temporal variability (Table 2, Figure S12 in Supporting Information S1). However, at surface levels, pCO 2 increased over time at a trend of 0.23 ± 0.10 μatm yr -1 , while pH sws , Ω ar , and Ω ca in the deepest layers also exhibited positive trends of 0.0003 ± 0.0001 pH sws units yr -1 , 0.0007 ± 0.0002 Ω ar units yr -1 and 0.0010 ± 0.0004 Ω ca units yr -1 , respectively (Table 2, Figure S12 in Supporting Information S1).

Discussion

What Drives Carbonate System Variability in the Bransfield Strait?

The temporal and spatial variability of carbonate chemistry in the Bransfield Strait evolves based on changes related to water mass intrusions in the region. Hydrographic and chemical changes in the water column structure includes 1996, 2004, 2011, and 2014 in the western basin; 1996, 1999, 2004, 2011, and 2014 in the central basin; and 1999, 2003, 2004, 2006, 2008, 2011, and 2014 in the eastern basin. SAM+ includes 1990SAM+ includes , 1997SAM+ includes , 2013SAM+ includes , 2015SAM+ includes , 2016SAM+ includes , 2018SAM+ includes , and 2019 in the western basin; in the western basin;[START_REF] Dickson | Thermodynamics of the dissociation of boric acid in potassium chloride solutions from 273[END_REF][START_REF] Goyet | Estimation of total CO 2 concentration throughout the water column[END_REF], 1998, 2000, 2005[START_REF] Gille | Decadal-scale temperature trends in the Southern Hemisphere ocean[END_REF], 2009, 2013, 2015, 2016, 2018, and 2019 in the central basin;and 2005, 2009, 2013, 2015, 2016, 2018, and 2019 in the eastern basin. SAM-includes 1993SAM-includes , 2010SAM-includes , and 2017 in the western basin; in the western basin;2003, 2010, and 2017 in the central basin; and 2010 and 2017 in the eastern basin. ENSOn includes 1990ENSOn includes , 1993ENSOn includes , 1997ENSOn includes , 2004ENSOn includes , 2014ENSOn includes , and 2017 in the western basin; in the western basin;[START_REF] Dickson | Thermodynamics of the dissociation of boric acid in potassium chloride solutions from 273[END_REF][START_REF] Goyet | Estimation of total CO 2 concentration throughout the water column[END_REF], 2000, 2004, 2013, 2014, and 2017 in the central basin;and 2004and , 2006and , 2013and , 2014and , and 2017and in the eastern basin. ENSO+ includes 2010and , 2015and , 2016and , and 2019 in the western basin; in the western basin;2003, 2010, 2015, 2016, and 2019 in the central basin;and 2003, 2005, 2010, 2015, 2016, and 2019 in the eastern basin. ENSOincludes 1996ENSOincludes , 2008ENSOincludes , 2011ENSOincludes , 2013ENSOincludes , and 2018 in the western basin; in the western basin;[START_REF] Hofmann | Water mass distribution and circulation west of the Antarctic Peninsula and including Bransfield Strait[END_REF], 1998, 1999[START_REF] Gille | Decadal-scale temperature trends in the Southern Hemisphere ocean[END_REF], 2009, 2011, and 2018 in the central basin;and 1999[START_REF] Gille | Decadal-scale temperature trends in the Southern Hemisphere ocean[END_REF], 2009, 2011, and 2018 in the eastern basin.

respond to the interactions of ENSO and SAM [START_REF] Avelina | Contrasting dissolved organic carbon concentrations in the Bransfield Strait, northern Antarctic Peninsula: Insights into ENSO and SAM effects[END_REF][START_REF] Loeb | ENSO and variability of the Antarctic Peninsula pelagic marine ecosystem[END_REF][START_REF] Renner | Advective pathways near the tip of the Antarctic Peninsula: Trends, variability and ecosystem implications[END_REF][START_REF] Ruiz Barlett | On the temporal variability of intermediate and deep waters in the Western Basin of the Bransfield Strait[END_REF][START_REF] Van Caspel | On the ventilation of Bransfield Strait deep basins[END_REF], which regulate the main source water mass that intrudes into the region [START_REF] Damini | Long-term changes on the Bransfield Strait deep water masses: Variability, drivers and connections with the northwestern Weddell Sea[END_REF][START_REF] Wang | Variability and dynamics of along-shore exchange on the west Antarctic Peninsula (WAP) continental shelf[END_REF]. This occurs due to the variability of modified-CDW from the western Antarctic Peninsula (i.e., a relatively old, warm, salty, less oxygenated, and TA-and CT-rich water mass) and DSW from the eastern Antarctic Peninsula (i.e., a recently ventilated, cold, salty, high oxygenated, and TA-and CT-rich water mass). However, the magnitude of the mixing processes along the water column depends on the ocean dynamics and bathymetry of each basin of the Bransfield Strait. While the western and eastern basins of the strait are more exposed to receiving a higher amount of modified-CDW intrusions at intermediate levels, the same is not true for the central basin because the Bransfield Gyre tends to constrain this water mass toward the South Shetland Islands [START_REF] Huneke | Water masses in the Bransfield Strait and adjacent seas, austral summer 2013[END_REF][START_REF] Sangrà | The Bransfield current system[END_REF]. In addition, the purest form of DSW (i.e., less mixing with ambient waters) is mainly constrained in the central basin at deep levels after sinking into the region [START_REF] Damini | Long-term changes on the Bransfield Strait deep water masses: Variability, drivers and connections with the northwestern Weddell Sea[END_REF][START_REF] Dotto | Multidecadal freshening and lightening in the deep waters of the Bransfield Strait, Antarctica[END_REF].

Interannual and decadal variabilities in the western basin of the Bransfield Strait (Figure S12 in Supporting Information S1) respond to the dynamics of the Antarctic Circumpolar Current and the modes of climate variability prevailing over time. For example, during the early 2000s, both SAM+ and ENSOn/ENSO-were characterized by increasing modified-CDW intrusions [START_REF] Loeb | ENSO and variability of the Antarctic Peninsula pelagic marine ecosystem[END_REF][START_REF] Renner | Advective pathways near the tip of the Antarctic Peninsula: Trends, variability and ecosystem implications[END_REF], mainly up to 500 m in depth, in the western Antarctic Peninsula continental shelf [START_REF] Moffat | On the characteristics of circumpolar deep water intrusions to the west Antarctic Peninsula continental shelf[END_REF][START_REF] Wang | Variability and dynamics of along-shore exchange on the west Antarctic Peninsula (WAP) continental shelf[END_REF]. Because the origin of modified-CDW implies high TA and CT concentrations, it should be associated with a concurrent rise in TA and CT, but this is not the case. The heat transported alongside by the intense modified-CDW intrusions during periods of SAM+, which reach the coastal areas of Antarctica, influences glacial retraction with consequent meltwater input into the ocean [START_REF] Abernathey | Water-mass transformation by sea ice in the upper branch of the Southern Ocean overturning[END_REF][START_REF] Meredith | Variability in the freshwater balance of northern Marguerite Bay, Antarctic Peninsula: Results from δ 18 O. Deep-Sea Research Part II[END_REF][START_REF] Meredith | Dense waters of the Weddell and Scotia seas: Recent changes in properties and circulation[END_REF][START_REF] Shepherd | Warm ocean is eroding west Antarctic Ice Sheet[END_REF][START_REF] Shepherd | Mass balance of the Antarctic Ice Sheet from 1992 to 2017[END_REF]. The freshwater input and the consequent dilution may contribute with a characteristic signature (i.e., low TA and CT relative to surrounding seawater) to the surface layer of the western basin of the Bransfield Strait, which eventually is advected from the western Antarctic Peninsula [START_REF] Moffat | Shelf-ocean exchange and hydrography west of the Antarctic Peninsula: A review[END_REF][START_REF] Shepherd | Mass balance of the Antarctic Ice Sheet from 1992 to 2017[END_REF] into the water column because of the vertical mixing [START_REF] Gille | Decadal-scale temperature trends in the Southern Hemisphere ocean[END_REF]. This dilution is a possible countereffect of the high TA and CT content associated with the modified-CDW, but with no or minimized impact on pCO 2 , pH sws , Ω ar , and Ω ca (Figure 2). Indeed, the meltwater contribution may reduce TA and CT up to 1,000 m in depth (Figures 2a and2b) as previously detected on the western Antarctic Peninsula [START_REF] Legge | The seasonal cycle of carbonate system processes in Ryder Bay, West Antarctic Peninsula[END_REF][START_REF] Lencina-Avila | Past and future evolution of the marine carbonate system in a coastal zone of the Northern Antarctic Peninsula[END_REF]. As long as SAM+ events have been more frequent over recent decades [START_REF] Marshall | The impact of a changing southern hemisphere annular mode on Antarctic Peninsula summer temperatures[END_REF], negative trends for TA and CT due to dilution, but intensification of pCO 2 , pH sws , Ω ar , and Ω ca trends (Table 2) may become marked over time if melting contributions become more intense.

The central and eastern basins showed a less intense influence of modified-CDW intrusions, which may be significant to achieve a TA:CT ratio similar to the ones for the upper layers from the western basin. The dispersion of TA and CT and the ratio between these parameters can be used to diagnose the processes influencing them (Figure 5a) because each process alters TA and CT with a certain ratio [START_REF] Zeebe | CO 2 in seawater: Equilibrium, kinetics, isotopes[END_REF]. In this case, modified-CDW intrusions might capture the meltwater signature from Antarctic Surface Water in the western Antarctic Peninsula through its mixture with CDW and promote a dilution/concentration signature for TA:CT ratios (see the inset in Figure 5a for theoretical ratios). Thus, the presence and variability of modified-CDW intrusions into the surface and intermediate layers in the Bransfield Strait contribute to the key process driving carbonate chemistry variability through dilution and concentration of salts (Figures 5a and5b).

However, DSW is the main water mass filling the water column below 500 m in the Bransfield Strait [START_REF] Damini | Long-term changes on the Bransfield Strait deep water masses: Variability, drivers and connections with the northwestern Weddell Sea[END_REF], which controls the rapid changes of carbonate system parameters in deep layers. In the western basin, the deep layer showed a positive trend for DSW contribution over the last decade [START_REF] Ruiz Barlett | On the temporal variability of intermediate and deep waters in the Western Basin of the Bransfield Strait[END_REF]. Its high carbon signature, added to the mixing with modified-CDW, may be associated with the sharp positive trend of CT (Table 2). Positive trends of carbonate system parameters associated with modified-CDW intrusions were also observed in the southern Gerlache Strait and DSW in the northern Gerlache Strait (Lencina-Avila et al., 2018), as well as in the deep layers of the Weddell Sea [START_REF] Van Heuven | Rapid invasion of anthropogenic CO 2 into the deep circulation of the Weddell Gyre[END_REF].

For the central and eastern basins, DSW contributed up to ∼60% to the mixture of source water masses filling the deep layers [START_REF] Damini | Long-term changes on the Bransfield Strait deep water masses: Variability, drivers and connections with the northwestern Weddell Sea[END_REF]. This water sinks from the shallow shelf zones directly to the seafloor as dense and cold plumes flow down the sills, a ventilation process with relatively low mixing with surrounding waters [START_REF] Dotto | Multidecadal freshening and lightening in the deep waters of the Bransfield Strait, Antarctica[END_REF][START_REF] Van Caspel | On the ventilation of Bransfield Strait deep basins[END_REF]. Thus, this process advects DSW properties from the north-western Weddell Sea continental shelf into the Bransfield Strait. In fact, the Bransfield Front also acts to constrain the contributions of modified-CDW close to the South Shetland Islands.

The presence of DSW may intensify interannual trends or decadal variability of CT and pCO 2 and decrease pH sws , Ω ar , and Ω ca in the deep layer in both western (Table 2) and central (Figure 4) basins, respectively. Beyond natural CO 2 , the DSW may transport anthropogenic carbon westward, such as previously observed by [START_REF] Kerr | Carbonate system properties in the Gerlache Strait, northern Antarctic Peninsula (February 2015): II. Anthropogenic CO 2 and seawater acidification[END_REF] in the Gerlache Strait. In addition, unpublished data by GOAL has shown that the estimated anthropogenic carbon concentration reached up to 50 μmol kg -1 in the central basin of the Bransfield Strait in the late 2010s, which are contributions expected for shelf waters around Antarctica [START_REF] Pardo | Anthropogenic CO 2 estimates in the Southern Ocean: Storage partitioning in the different water masses[END_REF]. Curiously, CT anomalies (Figure S12 in Supporting Information S1) vary fourfold more than anthropogenic carbon estimates around the NAP (∼20 μmol kg -1 , Kerr, Goyet, et al., 2018; Lencina-Avila et al., 2018), which may indicate a contribution of this carbon fraction on carbonate chemistry. Regardless of whether natural CO 2 or anthropogenic carbon dissolves into the surface of the north-western Weddell Sea, the rapid ventilation of DSW does not allow the equilibrium of CO 2 concentration between the ocean and the atmosphere to be reached. This implies that DSW ventilation advects a fraction of the whole CO 2 available to be captured before inflowing into the Bransfield Strait, promoting high CT and pCO 2 variability over time, as observed in Figures S11b andS11c in Supporting Information S1, respectively. Thus, the advection of DSW throughout the Bransfield Strait can act as an uptake signature (i.e., altering CT with no TA change; see the inset in Figure 5a for theoretical ratios). However, the magnitude of anthropogenic carbon inputs and their role over the long-term in the Bransfield Strait have not yet been assessed.

The complexity of the deep basins is intensified by organic matter cycling in the Bransfield Strait. High primary productivity takes place in the north-western Weddell Sea [START_REF] Detoni | Environmental conditions during phytoplankton blooms in the vicinity of James Ross Island, east of the Antarctic Peninsula[END_REF][START_REF] Ito | Sea-air CO 2 fluxes and pCO 2 variability in the Northern Antarctic Peninsula during three summer periods (2008-2010)[END_REF][START_REF] Mendes | Dynamics of phytoplankton communities during late summer around the tip of the Antarctic Peninsula[END_REF], transporting organic matter in DSW into the deepest layers of the Bransfield Strait, where it may be decomposed. This transport was previously indicated by [START_REF] Avelina | Contrasting dissolved organic carbon concentrations in the Bransfield Strait, northern Antarctic Peninsula: Insights into ENSO and SAM effects[END_REF] when evaluating dissolved organic carbon in the central basin of the Bransfield Strait in 2015. Organic matter decomposition is associated with increases in CT and pCO 2 and reductions in TA, pH sws , Ω ar , and Ω ca , as noticed in the central basin in the 2010s relative to the 2000s (Figure 4). Although multivariate linear equations may capture a low signal of TA changes related to organic matter decomposition [START_REF] Lee | Global relationships of total alkalinity with salinity and temperature in surface waters of the world's oceans[END_REF], all the features mentioned above may corroborate this driver of carbonate chemistry variability in the deep layer of the central basin.

Inferred Processes Controlling Carbonate Chemistry in the Bransfield Strait: A Possible Broad Avenue for Future Studies

As multivariate linear equations capture changes in θ, S, and O 2 associated with processes controlling TA and CT changes, such as freshwater input and biological processes, the results presented here point out other possible processes controlling carbonate chemistry in the Bransfield Strait. This new avenue for further investigation is highlighted by the evidence found here and described below.

First, it is well known that changes in the cryosphere system (e.g., sea ice variability and continental meltwater input) and other hydrographic dynamics can affect the carbonate system variability through changes in salinity (e.g., [START_REF] Henley | Variability and change in the west Antarctic Peninsula marine system: Research priorities and opportunities[END_REF][START_REF] Orselli | The marine carbonate system along the northern Antarctic Peninsula: Current knowledge and future perspectives[END_REF]. Freshwater input or removal can alter TA and CT. Indeed, the Antarctic Peninsula has recently been losing several ice shelves through extreme events of heat waves, atmospheric rivers and the abrupt collapse of Larsen A and B [START_REF] Massom | Antarctic ice shelf disintegration triggered by sea ice loss and ocean swell[END_REF][START_REF] Reid | Change and variability in Antarctic coastal exposure, 1979-2020[END_REF][START_REF] Scambos | The link between climate warming and break-up of ice shelves in the Antarctic Peninsula[END_REF][START_REF] Wille | Intense atmospheric rivers can weaken ice shelf stability at the Antarctic Peninsula[END_REF]. Besides, [START_REF] Dotto | Multidecadal freshening and lightening in the deep waters of the Bransfield Strait, Antarctica[END_REF] identified a change of nearly 0.06 units of salinity over the last 60 years, while our study points out a reduction of 0.02 units of salinity between the 2000s and 2010s in the deep layer of the central basin. Even if this change is considered small, it may impact TA in the long term. Thus, the carbonate chemistry in the Bransfield Strait is directly affected by this freshwater input.

Additionally, [START_REF] Gordon | Deep and bottom water of the Bransfield Strait eastern and central basins[END_REF] and von [START_REF] Gyldenfeldt | Flow variability at the tip of the Antarctic Peninsula[END_REF] indicate that the water mass contribution from the Ronne-Filchner ice shelf (southern Weddell Sea) overflows by a sill in the vicinity of the Powell Basin into the deep layer of the eastern basin of the Bransfield Strait. Due to its low temperature [START_REF] Frölicher | Dominance of the Southern Ocean in anthropogenic carbon and heat uptake in CMIP5 models[END_REF][START_REF] Hellmer | The fate of the Southern Weddell Sea continental shelf in a warming climate[END_REF] and low anthropogenic carbon content [START_REF] Pardo | Anthropogenic CO 2 estimates in the Southern Ocean: Storage partitioning in the different water masses[END_REF], this water mass has the potential to minimize the effects of anthropogenic carbon carried by DSW into the deep layer, promoting increases of pH sws values as a thermal effect. However, the eastern basin has been a challenging area for both hydrographic and biogeochemical assessments.

Second, local processes may also contribute to the variability of the carbonate system in the Bransfield Strait. For example, the presence of hydrothermal vents and volcanos at the bottom of the strait [START_REF] Almendros | BRAVOSEIS: Geophysical investigation of rifting and volcanism in the Bransfield strait, Antarctica[END_REF] could be correlated with chemosynthesis processes, which can alter TA and CT by a ratio between CO 2 ingassing and organic matter decomposition (Figure 5c). In addition, the release of CO 2 through hydrothermal vents may act as a direct process of CO 2 ingassing process into the water column with no TA change (Figure 5c). The latter can help to explain the distribution of the TA:CT distribution observed in the deep layers of the Bransfield Strait since this pattern is expected in surface waters (Figure 5c and inset in Figure 5a). Furthermore, the increase in CT may be enhanced by anthropogenic carbon intrusion through advected DSW [START_REF] Kerr | Carbonate system properties in the Gerlache Strait, northern Antarctic Peninsula (February 2015): II. Anthropogenic CO 2 and seawater acidification[END_REF], although anthropogenic carbon storage in the Bransfield Strait remains unclear. Indeed, several biological, chemical and geological processes are occurring concomitantly in the deep layers, holding a TA:CT ratio between organic matter decomposition and close to that expected at the ocean-atmosphere interface (Figure 5c). These processes acting as CO 2 inputs into the deep layer in the entire strait might enhance uncertainties for pCO 2 , as observed in the present study (Table S5 in Supporting Information S1).

Finally, evidence of nitrification in the deep layer might be another biogeochemical process with impacts on carbonate chemistry (Figure S15 in Supporting Information S1). This is characterized by changes in TA as observed in the central basin and as described by [START_REF] Wolf-Gladrow | Total alkalinity: The explicit conservative expression and its application to biogeochemical processes[END_REF]. Nitrifying organisms, such as archaea of the genus Thaumarchaeota, occur in the Southern Ocean, including the Bransfield Strait [START_REF] Signori | Microbial diversity and community structure across environmental gradients in Bransfield Strait, Western Antarctic Peninsula[END_REF]. Since this evidence is identified in SAM+, warm sea surface temperature in the Weddell Sea may favor high levels of nutrients, such as ammonium and nitrite, being advected into the deep layer of the central basin. Moreover, high O 2 concentrations in DSW [START_REF] Avelina | Contrasting dissolved organic carbon concentrations in the Bransfield Strait, northern Antarctic Peninsula: Insights into ENSO and SAM effects[END_REF] are another needful feature for nitrification. According to all of the theoretical evidence above, if nitrification processes take place in the Bransfield Strait, they could potentially influence the carbonate chemistry. However, further investigations are required to understand the magnitude of nitrification around the NAP and its impact on carbonate chemistry.

Acidification Process and Carbon Dioxide Saturation in the Bransfield Strait

Spatial and temporal variability of carbonate system parameters in the Bransfield Strait weighs changes in pH sws , Ω ar , and Ω ca , which vary in trend and magnitude among basins (Table 2, Figures S9,S12, and S13 in Supporting Information S1). The western basin may be associated with the strongest negative trends (Table 2, Figure S9d in Supporting Information S1) because of an increasing frequency of CDW intrusions, highlighting a contribution of a natural process to pH trends. For instance, the surface layer of the western basin has trends (-0.003 ± 0.0 02 pH sws units yr -1 , Table 2) similar to those of the deep layer of the northern Gerlache Strait (-0.003 ± 0.001 pH sws units yr -1 , Lencina-Avila et al., 2018), central western Antarctic Peninsula (0.002 ± 0.002 pH T units yr -1 , Hauri et al., 2015), and the surface layer of the Southern Ocean (-0.002 ± 0.0003 pH T units yr -1 , Midorikawa et al., 2012). However, these trends observed surrounding the Antarctic Peninsula are smaller than those found in other sensitive regions, exhibiting a high Revelle Factor [START_REF] Jiang | Surface ocean pH and buffer capacity: Past, present and future[END_REF][START_REF] Sabine | The oceanic sink for anthropogenic CO 2[END_REF], such as the North Sea [START_REF] Blackford | pH variability and CO 2 induced acidification in the North Sea[END_REF]. Adding up the trends of other areas of the Southern Ocean, such as the one for the south area of the Polar Front (-0.0022 ± 0.0004 pH T units yr -1 , Midorikawa et al. (2012); -0.0043 ± 0.0016 to -0.0009 ± 0.0007 pH T units yr -1 , Leseurre et al. ( 2022)), these trends in the deep layer of the western basin are one order of magnitude higher than any area in the coastal zone and surrounding ocean. Additionally, the entire Southern Ocean exhibits a pH trend of -0.0020 ± 0.0002 pH T units yr -1 [START_REF] Lauvset | Trends and drivers in global surface ocean pH over the past 3 decades[END_REF], similar to those surrounding the Antarctic Peninsula, but smaller than that estimated here in the western basin of the Bransfield Strait. Conversely, the central basin revealed interannual swinging but undefined trends (Table 2, Figure S11d in Supporting Information S1) over the 30 years investigated because of the dominance of DSW contributions. Furthermore, the intense well-mixed water column in the eastern basin may be the reason for the lower trend in the present study (Table 2, Figure S12 in Supporting Information S1).

Hence, we suggest that three possible processes may be driving pH sws trends in the western basin: (a) intensified CDW intrusions over the continental shelf of the western basin and the NAP due to the increased frequency of years marked by the SAM+ phase [START_REF] Henley | Changing biogeochemistry of the Southern Ocean and its ecosystem implications[END_REF][START_REF] Jones | Ocean acidification and calcium carbonate saturation states in the coastal zone of the West Antarctic Peninsula[END_REF]; (b) transport of meltwater signals, via vertical mixing, throughout the water column with its carbon content [START_REF] Monteiro | Towards an intensified summer CO 2 sink behaviour in the Southern Ocean coastal regions[END_REF] during more intense wind-stress in SAM+ and ENSO-events; and (c) increased mixture between modified-CDW and DSW-while the former is rich in organic matter decomposition products, the latter is also enriched with anthropogenic carbon. The latter point is associated with its increased contribution to the Bransfield Strait in the 2010s [START_REF] Damini | Long-term changes on the Bransfield Strait deep water masses: Variability, drivers and connections with the northwestern Weddell Sea[END_REF][START_REF] Dotto | Multidecadal freshening and lightening in the deep waters of the Bransfield Strait, Antarctica[END_REF][START_REF] Ruiz Barlett | On the temporal variability of intermediate and deep waters in the Western Basin of the Bransfield Strait[END_REF], which may yield the sharpest negative trends observed in the deep layer in the western basin as also noticed for the Gerlache Strait [START_REF] Lencina-Avila | Past and future evolution of the marine carbonate system in a coastal zone of the Northern Antarctic Peninsula[END_REF]. Indeed, the source area of DSW in the north-western Weddell Sea presented a reduction of 0.02 pH T units from 1992 to 2008 [START_REF] Hauck | Data-based estimation of anthropogenic carbon and acidification in the Weddell Sea on a decadal timescale[END_REF], which may be carried into the Bransfield Strait. However, other processes take place in the water column, such as organic matter decomposition in the central basin (see Figures 5b and5c), that may dampen this signature. However, since DSW is ventilated before flowing into the deepest layers of the Bransfield Strait [START_REF] Dotto | Multidecadal freshening and lightening in the deep waters of the Bransfield Strait, Antarctica[END_REF][START_REF] Van Caspel | On the ventilation of Bransfield Strait deep basins[END_REF], the signature of atmospheric CO 2 increase may be absorbed by DSW (see Figure 5c).

Trends of pH sws respond to the CO 2 increment in the water column of the Bransfield Strait. This is corroborated by the decadal rising of the Revelle Factor for the western and central basins throughout the water column (Figures 6a and6b), reaching values close to those observed in the eastern basin (Figure 6c). However, decadal changes in the central basin were more intense than those in the western basin because of the likely contribution of anthropogenic processes, such as the signature of atmospheric CO 2 increase due to the recent human footprint on the carbon cycle (i.e., anthropogenic carbon) in the Bransfield Strait. Since a high signature of modified-CDW is detected in the western basin, the decreases in pH sws in the region were related to the old and acidified water mass carrying natural products of organic matter decomposition. However, most frequent CDW intrusions respond to changes in modes of climate variability because of anthropogenic activities. Furthermore, the eastern basin has low influence of both water masses, reflecting a more mixed water mass structure in the region even with a Revelle Factor indicating that the basin has already been near CO 2 saturation since the 1990s (Figure 6c). Although this acidification process is minimal, the complexity of the eastern basin needs further investigation to clarify the reasons for its particular hydrographic and biogeochemical variability.

Overall, the Bransfield Strait demonstrated being close to reaching CO 2 saturation (i.e., reducing its capacity to store CO 2 ). This condition implies that the acidification state in the strait is associated with a decrease in pH and saturation of calcium carbonate. For example, depths below 800 m in the 2010s are characterized by Ω ar with values below 1 (Figure 4e; i.e., undersaturated condition, prevailing dissolution over precipitation). It represents an issue for much more intense negative pH sws trends with impacts on calcified structures of marine organisms (i.e., decreasing Ω values, mainly with respect to aragonite, the most unstable carbonate calcium mineral) in the water column, such as pteropods, and on the seafloor, such as echinoderms and bryozoans [START_REF] Acqua | The impact of ocean acidification on the gonads of three key Antarctic benthic macroinvertebrates[END_REF][START_REF] Figuerola | Predicting potential impacts of ocean acidification on marine calcifiers from the Southern Ocean[END_REF][START_REF] Henley | Changing biogeochemistry of the Southern Ocean and its ecosystem implications[END_REF].

Conclusions

The changes in hydrography and climate modes variability (e.g., ENSO and SAM) that regulate ocean dynamics played key roles in altering marine carbonate chemistry in the Bransfield Strait. Periods of SAM+ intensified modified-CDW intrusions into the western basin during the 2000s, which is reflected in TA and CT dilution. This is mainly caused by meltwater inputs, as a consequence of the heat transported by CDW over the continental shelf of the western Antarctic Peninsula. This impact is followed by decreases in pH, Ω ar and Ω ca as an effect of the natural low pH of both CDW and melting water. On the other hand, the 2010s were characterized by intense DSW intrusions into the deep layers of the Bransfield Strait. DSW carries a signature of CO 2 concentration from the north-western Weddell Sea, which is regulated by sea-air interactions and biological production. These processes intensified the CT positive trends over the last three decades. Also, the increased contribution of DSW into the western basin during the 2010s, for example, intensified the negative pH trends through mixing with modified-CDW intrusions. Conversely, the eastern basin exhibits a water mass structure driven by internal mixing, which minimizes any effect of water mass contribution and prevailing modes of climate variability.

It is important to highlight that biological (e.g., organic matter decomposition and nitrification) and other geochemical (e.g., hydrothermal vents as carbon inputs) processes may influence the carbonate system in the Bransfield Strait, but they require further investigation to determine their influence on TA and CT over time, and consequently on pCO 2 and pH. Another substantial contribution to carbonate chemistry might come from anthropogenic carbon, which has been little investigated in the Bransfield Strait, masking its contribution to a region that is nearing CO 2 saturation. A summary of identified and inferred hydrographic and biogeochemical processes of the 2000s and 2010s that can contribute to this variability in carbonate chemistry is provided (Figure 7).

The Bransfield Strait draws attention to the critical state of the influence of open ocean areas on coastal zones around the Antarctic Peninsula because it connects subpolar and polar systems. As part of the Southern Ocean, the Bransfield Strait draws attention to a countereffect of atmospheric CO 2 increase because the region is experiencing a significant increase in sea surface temperature and is prone to reach CO 2 saturation. The high Revelle Factor found here pinpoints the Bransfield Strait as a quasi-saturated region, which highlights a change toward a low capacity to absorb and store CO 2 .

Figure 1 .

 1 Figure 1. The Bransfield Strait and its main characteristics. (a) Distribution of the oceanographic stations used in this study. Stations in the western, central, and eastern basins of the Bransfield Strait are marked by blue dots, green triangles, and brown inverted triangles, respectively. The black dashed line shows the 1,000 m isobath, while the black dotted line shows the 200 m isobath. The numbers indicate the following: 1. Boyd Strait, 2. Deception Island, 3. King George Island, 4. James Ross Island, 5. Joinville Island, and 6. Clarence Island. (b) Ocean circulation scheme around the northern Antarctic Peninsula. Red arrows indicate the pathways of modified Circumpolar Deep Water (modified-Circumpolar Deep Water [CDW]), while the blue arrow indicates the pathway of Dense Shelf Water (DSW). The boundary between modified-CDW and DSW triggers anticyclonic eddies in the cyclonic Bransfield Gyre. The red dashed line indicates the section described in panel (c). (c) Scheme of the main hydrographic properties (seawater temperature-T and total inorganic carbon-CT) and ocean dynamics along the red dashed line depicted in panel (b). The in and out arrows represent the direction of the main current flow in the region, which is northward around the South Shetland Islands and southward near the Antarctic Peninsula. The dotted and dashed lines indicate the mean depth related to neutral density (γ n ) = 28 and γ n = 28.27 kg m -3 , respectively. The black line represents the boundary between modified-CDW and DSW.

Figure 2 .

 2 Figure 2. Vertical profiles of decadal averages of carbonate system parameters in the western basin of the Bransfield Strait during 1990-2019 for total alkalinity (TA) and during 1996-2019 for the other parameters. Plotted data are (a) total alkalinity-TA, (b) total inorganic carbon-CT, (c) partial pressure of carbon dioxide-pCO 2 , (d) pH sws (seawater scale), (e) aragonite saturation state-Ω ar , and (f) calcite saturation state-Ω ca . The averaged values are calculated for 50-m bins until the deepest sample in each decade.The total standard errors for the data were <1.9 for TA, <2.4 μmol kg -1 for CT, <5.2 μatm for pCO 2 , <0.004 pH sws units, and <0.04 for both Ω ar and Ω ca . The black horizontal dotted and dashed lines in the panels represent the average value of the neutral density (γ n ) levels of 28.00 and 28.27 kg m -3 , respectively. The vertical line in panel (e) indicates Ω ar = 1, with values below 1 referring to subsaturation of calcium carbonate. Only the 2010s presented γ n , with the value 28.27 kg m -3 , to characterize the deep layer in this basin for CT and the other CO 2 -carbonate system parameters, except for TA.

Figure 3 .

 3 Figure 3. Composited vertical profiles of (a, b, e, f, i, and j) total alkalinity-TA (μmol kg -1 ) and (c, d, g, h, k, and l) total inorganic carbon-CT (μmol kg -1 ) considering the phases of the modes of climate variability Southern Annular Mode (SAM, first and third columns) and El Niño-Southern Oscillation (ENSO, second and fourth columns). Averaged TA and CT profiles for the (first arrow: a-d) western, (second arrow: e-h) central, and (third arrow: i-l) eastern basins of Bransfield Strait. SAMnincludes 1996, 2004, 2011, and 2014 in the western basin; 1996, 1999, 2004, 2011, and 2014 in the central basin; and 1999, 2003, 2004, 2006, 2008, 2011, and 2014 in the eastern basin.SAM+ includes 1990SAM+ includes , 1997SAM+ includes , 2013SAM+ includes , 2015SAM+ includes , 2016SAM+ includes , 2018SAM+ includes , and 2019 in the western basin; in the western basin;[START_REF] Dickson | Thermodynamics of the dissociation of boric acid in potassium chloride solutions from 273[END_REF][START_REF] Goyet | Estimation of total CO 2 concentration throughout the water column[END_REF], 1998, 2000, 2005[START_REF] Gille | Decadal-scale temperature trends in the Southern Hemisphere ocean[END_REF], 2009, 2013, 2015, 2016, 2018, and 2019 in the central basin; and 2005, 2009, 2013, 2015, 2016, 2018, and 2019 in the eastern basin.SAM-includes 1993SAM-includes , 2010SAM-includes , and 2017 in the western basin; in the western basin; 2003, 2010, and 2017 in the central basin; and 2010 and and 2017 and in the eastern basin. ENSOn includes 1990[START_REF] Roy | The dissociation constants of carbonic acid in seawater at salinities 5 to 45 and temperatures 0 to 45°C[END_REF][START_REF] Goyet | Estimation of total CO 2 concentration throughout the water column[END_REF] and , 2004 and , 2014 and , and 2017 in the western basin; in the western basin;[START_REF] Dickson | Thermodynamics of the dissociation of boric acid in potassium chloride solutions from 273[END_REF][START_REF] Goyet | Estimation of total CO 2 concentration throughout the water column[END_REF], 2000, 2004, 2013, 2014, and 2017 in the central basin; and 2004 and , 2006 and , 2013 and , 2014 and , and 2017 and in the eastern basin. ENSO+ includes 2010 and , 2015 and , 2016 and , and 2019 in the western basin; in the western basin; 2003, 2010, 2015, 2016, and 2019 in the central basin; and 2003, 2005, 2010, 2015, 2016, and 2019 in the eastern basin.ENSOincludes 1996ENSOincludes , 2008ENSOincludes , 2011ENSOincludes , 2013ENSOincludes , and 2018 in the western basin; in the western basin;[START_REF] Hofmann | Water mass distribution and circulation west of the Antarctic Peninsula and including Bransfield Strait[END_REF], 1998, 1999[START_REF] Gille | Decadal-scale temperature trends in the Southern Hemisphere ocean[END_REF], 2009, 2011, and 2018 in the central basin; and 1999[START_REF] Gille | Decadal-scale temperature trends in the Southern Hemisphere ocean[END_REF], 2009, 2011, and 2018 in the eastern basin.

Figure 4 .

 4 Figure 4. Vertical profiles of decadal averages for carbonate system parameters in the central basin of Bransfield Strait during 1990-2019 for total alkalinity (TA) and during 2000-2019 for the other parameters. Data plotted are (a) total alkalinity-TA, (b) total inorganic carbon-CT, (c) partial pressure of carbon dioxide-pCO 2 , (d) pH sws (seawater scale), (e) aragonite saturation state-Ω ar , and (f) calcite saturation state-Ω ca . The averaged values are calculated for 50-m bins until the deepest sample in each decade. The total standard errors for the data were <1.2 for TA, <1.5 μmol kg -1 for CT, <5.3 μatm for pCO 2 , <0.005 for pH sws , and <0.02 for both Ω ar and Ω ca . The solid vertical line in panels (e and f) indicates Ω ar = 1 and Ω ca = 1, respectively, with values below 1 referring to subsaturation of calcium carbonate.

Figure 5 .

 5 Figure 5. Dispersal diagram of the relationship between total inorganic carbon (CT) and total alkalinity (TA) for the (a) surface, (b) intermediate, and (c) deep layers of the Bransfield Strait. The arrows in the inset (a) represent the TA:CT ratio, which characterizes the physical-biogeochemical processes that affect parameters (based on Zeebe and Wolf-Gladrow (2001)). Such processes are as follows: denitrification/nitrification (changes only in TA), concentration/ dilution due to freshwater removal/input (changing TA and CT in a 1:1 ratio), and photosynthesis/decomposition (changing TA and CT in a 0.14:1 ratio). The colors and symbols differ among basins, as indicated in panel (c). The red lines show the linear regression between CT and TA in the surface (n = 95108; TA = 0.555 (±0.002) *CT + 1,119; p < 0.001), intermediate (n = 150706; TA = 0.323 (±0.001) *CT + 1,624; p < 0.001), and deep (n = 99684; TA = -0.017 (±0.001) *CT + 2,403; p < 0.001) layers. The end-members of the TA and CT source water masses were marked by the crosses in panel (a) and were selected from Broullón et al. (2019, 2020), respectively. The source water masses are Circumpolar Deep Water in the open ocean near the western Antarctic Peninsula and Dense Shelf Water in the north-western Weddell Sea.

Figure 6 .

 6 Figure 6. Vertical distribution of the Revelle factor over three decades in the Bransfield Strait for (a) western, (b) central, and (c) eastern basins. Plotted data is the average (±standard error) at each 50 m depth intervals. The black horizontal dotted and dashed lines in the panels represent the average value of the neutral density levels of 28.00 and 28.27 kg m -3 , respectively.

Figure 7 .

 7 Figure 7. Conceptual model of the main processes affecting CO 2 -carbonate system variability over three decades in the Bransfield Strait, northern Antarctic Peninsula. Panels present the state of the area in (a) the 2000s (prevailing SAM+ and ENSO-) and (b) the 2010s (prevailing SAM-and ENSO+). The highlighted section crosses from southeastern (left) to north-eastern (right). The red-dashed lines indicate bounds by sills between basins. The acronyms are as follows: CDW-Circumpolar Deep Water; CTtotal inorganic carbon; CO 2 -ingassing carbon dioxide; POC-particulate organic carbon; CaCO 3 -calcium carbonate; Ca 2+ -calcium ion; NH 4 +ammonium ion; and NO 3 --nitrate ion.

Table 1

 1 Reconstruction Equations for Carbonate System Parameters With the Best Fit for This Study

	Layer density (kg m -3 )	TA (μmol kg -1 )	CT (μmol kg -1 )
	Surface	TA = -31.9*θ -54.5*S + 4236	CT = -29.3*θ -26.1*S -0.57*O 2 + 3311.5
	γ n ≤ 28.0	r = 0.93	r = 0.95
		RMSE = 6.35 μmol kg -1	RMSE = 8.92 μmol kg -1
		n = 72; p < 0.01	n = 49; p < 0.01
		F = 247.7	F = 139.2
	Intermediate	TA = -57.1*θ + 43.6*S -2652.3	CT = -0.73*θ + 61.2*S + 1.86*O 2 -352.1
		r = 0.93	r = 0.91
	28.0 < γ n < 28.27	RMSE = 6.35 μmol kg -1	RMSE = 8.99 μmol kg -1
		n = 41; p < 0.01	n = 37; p < 0.01
		F = 115.0	F = 53.8
	Deep	TA = 59.7*θ -126.2*S + 6805.3	CT = 135.9*θ + 797.8*S + 3.2*O 2 -26019.9
		r = 0.86	r = 0.91
	γ n ≥ 28.27	RMSE = 6.22 μmol kg -1	RMSE = 9.63 μmol kg -1
		n = 49; p < 0.01	n = 30; p < 0.01

  -1 for CT (FiguresS7a and S7bin Supporting Information S1). The surface averages of pH sws , Ω ar , and Ω ca , reaching 8.141 ± 0.001, 1.78 ± 0.01, and 2.80 ± 0.01, respectively (Figures S7d-S7f in Supporting Information S1), were higher than those in the intermediate and deep layers. The total average (i.e., average over the whole data set) of pCO 2 was higher in the intermediate layer with 542.7 ± 2.0 μatm, likely associated with modified-CDW intrusions. Furthermore, total averages of pH sws , Ω ar , and Ω ca in the intermediate layer were 7.917 ± 0.002 (minimum of 7.8), 1.03 ± 0.01 (minimum of 0.7), and 1.61 ± 0.01 (minimum of 1.1), respectively (Figure

Table 2

 2 Trends ± Confidence Bound for Carbonate System Parameters Based on a Time Series of 30 Years of Reconstructed Data During 1990-2019 in the Bransfield Strait

	Layer	Basin	Period of TA data (other parameters)	TA (μmol kg -1 yr -1 )	CT (μmol kg -1 yr -1 )	pCO 2 (μatm yr -1 )	pH sws (pH sws units yr -1 )	Ω ar (yr -1 )	Ω ca (yr -1 )
	Surface	Western	1990-2019	-0.10 ± 0.23	0.09 ± 0.51	2.11 ± 2.70	-0.003 ± 0.002	-0.010 ± 0.006	-0.015 ± 0.010
			(1996-2019)
		Central	1990-2019	-0.17 ± 0.27	-0.92 ± 0.82	-0.33 ± 0.66	0.0004 ± 0.0007	0.001 ± 0.002	0.002 ± 0.004
			(2000-2019)
		Eastern	1999-2019	-0.25 ± 0.11	-0.34 ± 0.26	0.23 ± 0.09 -0.0002 ± 0.0001 -0.0003 ± 0.0004 -0.0004 ± 0.0006
			(1999-2019)
	Intermediate Western	1990-2019	0.46 ± 0.40	2.47 ± 3.27	8.34 ± 7.86	-0.008 ± 0.002	-0.018 ± 0.009	-0.029 ± 0.014
			(1996-2019)
		Central	1990-2019	0.49 ± 0.20	1.00 ± 1.05	0.95 ± 4.88	0.0004 ± 0.0036	0.004 ± 0.007	0.005 ± 0.011
			(2000-2019)
		Eastern	1999-2019	-0.37 ± 0.43	-0.30 ± 0.37	0.36 ± 0.36	0.0002 ± 0.0001	0.0006 ± 0.0003	0.0009 ± 0.0006
			(1999-2019)
	Deep	Western	1990-2019	0.10 ± 0.24	6.77 ± 3.76	17.98 ± 8.99	-0.017 ± 0.010	-0.044 ± 0.027	-0.070 ± 0.043
			(2013-2019)
		Central	1990-2019	-0.42 ± 0.15	-0.18 ± 2.29	-0.81 ± 6.90	0.0007 ± 0.0062	0.002 ± 0.011	0.003 ± 0.018
			(2000-2019)
		Eastern	1999-2019	-0.14 ± 0.21	-0.48 ± 0.47	-0.31 ± 0.15	0.0003 ± 0.0001	0.0007 ± 0.0002	0.0010 ± 0.0004
			(1999-2019)

Acknowledgments

This study is part of the activities of the Brazilian High Latitude Oceanography Group (GOAL) within the Brazilian Antarctic Program (PROANTAR). GOAL has been funded by and/or has received logistical support from the Brazilian Ministry of the Environment (MMA); the Brazilian Ministry of Science, Technology, and Innovation (MCTI); the Council for Research and Scientific Development of Brazil (CNPq); the Brazilian Navy;

Data Availability Statement

Data from reconstructed parameters are available at https://doi.org/10.5281/zenodo.7416652 [START_REF] Santos-Andrade | Reconstructed carbonate system in the Bransfield Strait over the last 30 years[END_REF].

Conflict of Interest

The authors declare no conflicts of interest relevant to this study. the Interministerial Secretariat for Sea Resources (SECIRM); the National Institute of Science and Technology of the Cryosphere (INCT CRIOSFERA; CNPq Grant 573720/2008-8 and 465680/2014-3); and the Research Support Foundation of the State of Rio Grande do Sul (FAPERGS Grant 17/2551-000518-0). This study was conducted within the activities of GOAL projects (CNPq Grant 550370/2002-1, 520189/2006-0, 556848/2009-8, 565040/2010-3, 405869/2013-4, 407889/2013-2, 442628/2018-8, and 442637/2018-7). Financial support was also received from the Coordination for the Improvement of Higher Education Personnel (CAPES) through the project CAPES "Ciências do Mar" (Grant 23038.001421/2014-30). CAPES also provided free access to many relevant journals through the portal "Periódicos CAPES" and the activities of the Graduate Program in Oceanology. Rodrigo Kerr and Mauricio M. Mata are granted with researcher fellowships from CNPq Grant 304937/2018-5 and 306896/2015-0, respectively. Mauricio Santos-Andrade acknowledges financial support from the CNPq scholarship Grant 132983/2020-6. The authors thank the officers and crew of the polar vessels Ary Rongel and Almirante Maximiano of the Brazilian Navy, and several scientists and technicians participating in the cruises, for their valuable help during data sampling and data processing. We thank all scientists and research groups for making their data available. All data sets used and their respective websites are indicated in the manuscript. We are grateful for the constructive comments provided by Elizabeth Jones, the Editors, and the anonymous reviewer, who helped to substantially improve the manuscript.