Seasonal calcium carbonate undersaturation in shelf waters of the Western Arctic Ocean; how biological processes exacerbate the impact of ocean acidiﬁcation

The Arctic Ocean accounts for only 4 % of the global ocean area but it contributes signiﬁcantly to the global carbon cycle. Recent observations of seawater carbonate chemistry in shelf waters of the Western Arctic from 2009 to 2011 indicate that extensive areas of the benthos are exposed to bottom waters that are seasonally undersatu- 5 rated with respect to calcium carbonate (CaCO 3 ) minerals, particularly aragonite. Our observations indicate seasonal reduction of saturation states ( Ω ) for calcite ( Ω calcite ) and aragonite ( Ω aragonite ) in the subsurface in the Western Arctic by as much as 0.9 and 0.6, respectively. Such data indicates that bottom waters of the Western Arctic shelves are already potentially corrosive for biogenic and sedimentary CaCO 3 for sev- 10 eral months each year. Seasonal changes in Ω are imparted by a variety of factors such as phytoplankton photosynthesis, respiration/remineralization of organic matter and air-sea gas exchange of CO 2 – combined these processes either increase or enhance Ω in surface and subsurface waters, respectively. These seasonal physical and biological processes also act to mitigate or enhance the impact of Anthropocene ocean 15 acidiﬁcation (OA) on Ω in surface and subsurface waters, respectively. Future monitoring of the Western Arctic shelves is warranted to assess the present and future impact on Ω values from ocean acidiﬁcation and seasonal biological/physical processes on Arctic marine ecosystems.

In this study, we assess the present state of Ω calcite and Ω aragonite conditions within surface and subsurface waters across the Western Arctic including the Eastern East Siberian Sea, Chukchi Sea, and Western Beaufort Sea shelves between 2009 and 2011. We use data collected as part of the recent RUSALCA (Russian-American Long Term Census of the Arctic) and ICESCAPE (Impacts of Climate on the Eco-Systems 5 and Chemistry of the Arctic Pacific Environment) expeditions to the Western Arctic Ocean. These data are used to assess if shelf bottom waters of the Western Arctic are potentially corrosive to CaCO 3 minerals. In the early 2000s, repeat seasonal observations during the Shelf-Basin Interactions (SBI) project showed that pH values were lower than 8.0 and values for Ω aragonite were < 1 in subsurface waters on the northern 10 slope of the Chukchi Sea but not across the shelf . We examine whether bottom waters with low pH and Ω are present across much of the shelf areas of the Western Arctic Ocean, thereby providing an assessment of the present-day seasonal exposure of the seafloor benthos to bottom waters that are corrosive for CaCO 3 shells and skeletons. 15 Furthermore, we examine the interactions and feedbacks between seawater carbonate chemistry and physical, biological and chemical processes in the Western Arctic. Such physico-biogeochemical processes significantly impact seawater carbonate chemistry (e.g. pCO 2 , pH and Ω) and include the following: marine ecosystem photosynthesis and respiration (or net community production that includes primary produc-20 tion and remineralization of organic carbon); air-sea CO 2 gas exchange; warming and sea-ice loss, and; changes in the inputs of terrestrial freshwater and organic carbon. When integrated, these processes act to either mitigate or enhance the impact of anthropogenic ocean acidification -resulting from uptake of anthropogenic CO 2 from the atmosphere -in surface and subsurface waters, respectively, of the Arctic Ocean.
individual years of data. In the paper we show surface data (typically 0-10 m depth and represents the uppermost Niskin sampler tripped at each CTD-Hydrocast station). The bottom water data represents samples typically within 5 m of the seafloor (deepest Niskin sampler tripped at each CTD-Hydrocast station) at shelf and slope stations with water depths less than 200 m. Data from the 2002-2004 Shelf-Basin Interactions 10 project in the Chukchi Sea (e.g. Bates et al., 2005a, b;Bates, 2006) are also examined for comparison.

Results
The physical and biochemical conditions of the Western Arctic Ocean are strongly influenced by northward transport of Pacific Ocean through Bering Strait (Woodgate 15 and Aagaard, 2005a; Cooper et al., 1997) and freshwater runoff from the rivers into the Arctic (Anderson et al., 2011). The inflow of Pacific Ocean water (∼ 1 Sv during summer; 1 Sv = 10 6 m 3 s −1 ) into the Chukchi Sea is comprised of Alaskan Coastal Current (ACC), Bering Sea shelf and Anadyr Current waters, with subsequent outflow through submarine canyons such as Herald Valley and Barrow Canyon, partly into the 20 Canada Basin and partly eastward along the narrow Beaufort Sea shelf. There is also an episodic inflow into the Chukchi Sea from the East Siberian Sea through Long Strait via the Siberian Coastal Current (SCC; Weingartner, 1999

Seawater carbonate chemistry variability across the Western Arctic
Seawater carbonate chemistry variability across the Western Arctic. Seawater carbonate chemistry was also highly variable across this region. Seawater TA and DIC concentrations from the three expeditions generally varied between 1400-2300 µmol kg −1 ( Fig. 2b; DIC only shown in the paper). Surface water values typically ranged from 5 1850-2250 µmol kg −1 for TA and 1800-2200 µmol kg −1 for DIC with higher values in the Chukchi Sea compared to the East Siberian Sea, Beaufort Sea and offshore in the Canada Basin (Fig. 2a). The lowest TA and DIC concentrations were generally observed in Long Strait close to the Russian coastline. These waters originate in the East Siberian Sea and are carried eastward into the Chukchi Sea with the Siberian Sea Current (SSC). The low TA and DIC contents of the SSC waters result from considerable freshwater input from river runoff into the ESS and sea-ice melt (Semiletov et al., 2004). Very low TA and DIC concentrations were also observed on the Western Beaufort Sea shelf, and likely reflect waters with seawater carbonate chemistry diluted by very low TA and DIC in Mackenzie River freshwater outflow onto the shelf and into 15 the Canada Basin (Macdonald et al., 2002). Subsurface values on the shelves and in the halocline of the Canada Basin ranged between 2150-2300 µmol kg −1 for TA and 2050-2250 µmol kg −1 for DIC (Fig. 3a).

Surface pCO 2 and pH variability
Seawater pCO 2 and pH conditions across the Western Arctic were highly variable rang-20 ing from < 100 to > 900 µatm and 7 to 8. 5,respectively (Figs. 2c,d,3b,4 2010(Arrigo et al., 2012Fig. 4c, e). These values represent some of the lowest seawater pCO 2 values observed anywhere in the global ocean. Seawater pH ranged from 8.2 to 8.4 in most surface waters (Fig. 2d). The exceptions were very high pCO 2 (> 600 µatm) and low pH (7 to 7.5) observed in the East Siberian Sea north of Wrangel Island and in the Siberian Sea Current outflowing from the East Siberian 5 Sea into the Chukchi Sea through Long Strait in 2009 (Figs. 2c, d, 3b). High pCO 2 has been observed in the Siberian Sea Current previously ). This finding is not surprising given the very high seawater pCO 2 values (∼ 500 to ∼ 1500 µatm) in the near-shore bays and estuaries of the East Siberian Sea (e.g. Tiksi Bay) and Kolyma River outflow (Semiletov et al., 1999(Semiletov et al., , 2007Pipko et al., 2008).

Subsurface pCO 2 and pH variability
Subsurface seawater pCO 2 was much higher (∼ 350-> 900) and pH much lower (∼ 7.3-7.8) when compared to surface waters (Figs. 2c,d,3b,4b,d,f). Seawater carbonate chemistry differences between surface and bottom waters were as much as 600-800 µatm for seawater pCO 2 and 1 to 1.5 for pH, respectively. This reflects strong 15 physical stratification between the surface mixed layer and subsurface waters. High seawater pCO 2 values greater than 400 µatm were observed in bottom waters close to the seafloor across the Southern Chukchi Sea and East Siberian Sea in 2009 (Fig. 4b), and in the Northern Chukchi Sea in 2010 and 2011 (Fig. 4d, f). Interestingly relatively low subsurface pCO 2 (∼ 250-400 µatm) was observed at Bering Strait and Southern 20 Chukchi Sea, presumably reflecting inflow of low pCO 2 water from the Bering Sea shelf .

Saturation states of Ω calcite and Ω aragonite
The saturation states of Ω calcite and Ω aragonite are useful indicators of the status of ocean acidification on the Western Arctic shelves.
The Ω values were highly variable with Introduction f, 3c, d). In surface waters across the Western Arctic, Ω calcite and Ω aragonite values were typically greater than 1 (Fig. 5a, c). Across much of the Chukchi Sea, Ω calcite and Ω aragonite values ranged from 3.5-5 and 2 to 3 ( Fig. 5) with surface waters highly oversaturated with respect to CaCO 3 minerals. Elsewhere, Ω calcite and Ω aragonite values were lower in the Westernmost Chukchi Sea and East Siberian Sea ranging from 2-3 5 and 1.5 to 2, respectively ( Fig. 5), with lower values in Long Strait. The low Ω calcite and Ω aragonite values in the East Siberian Sea are not unexpected (Anderson et al., 2010(Anderson et al., , 2011 given that surface waters have high seawater pCO 2 (and low pH) values that have been attributed primarily to the remineralization of organic matter introduced to the ESS from Siberian Rivers (e.g. Anderson et al., 1990;Cauwet and Sidorov, 1996;Kattner et al., 1999;Anderson et al., 2009). In contrast to surface waters of the Western Arctic shelves, surface/mixed layer waters in the Canada Basin were close to 1 to 1.5 for Ω calcite and even below 1 for Ω aragonite . Such low saturation states, especially for aragonite, have been observed in polar mixed layer waters of the Canada Basin previously (Jütterstrom and Anderson, 2010) and reflect contributions of high pCO 2 , 15 low pH/Ω sea-ice melt waters and freshwaters from the Mackenzie River. The saturation states of calcite and aragonite in subsurface waters were much lower than surface water values. Across much of the Chukchi Sea, Ω calcite and Ω aragonite values ranged from 1 to 1.5 and < 1 to 2, respectively (Figs. 5b,d,6). In many regions of the Chukchi Sea and East Siberian Sea, the near seafloor Ω calcite were close to 1 20 while Ω aragonite was often < 1. This was particularly evident in the East Siberian Sea with remineralization of organic matter back to CO 2 in subsurface waters the likely primary cause for reducing Ω values. In the Northern Chukchi Sea, the low Ω values are co-located in areas with high rates of euphotic zone marine phytoplankton primary production and thus likely reflect subsurface remineralization of vertically exported or- 25 ganic matter. Elsewhere in the Southern Chukchi Sea, the near seafloor Ω values were higher (Fig. 6), and presumably reflect input of relatively lower pCO 2 and higher pH/Ω waters inflowing from the Bering Sea shelf. Introduction

Surface enhancement and subsurface suppression of CaCO 3 saturation states
The 2009-2011 RUSALCA and ICESCAPE expeditions into the Western Arctic revealed that the saturation states for Ω calcite and Ω aragonite of bottom waters were sea-5 sonally close to or lower than 1 during the sea-ice free summertime in the Western Arctic. As with previous observations from the early 2000's during the SBI project , surface water enhancement and subsurface water suppression of Ω was observed, respectively. The causes for the seasonal divergence in Ω values for surface and subsurface waters of the Western Arctic have been previously ascribed 10 to feedbacks between biological processes and seawater carbonate chemistry and as evidence for a Phytoplankton-Carbonate Saturation State ("PhyCaSS") interaction . Similar seasonal interactions between biological processes and seawater carbonate chemistry have subsequently been observed on the Bering Sea shelf . Interactions between seawater carbonate chemistry and bi- 15 ological processes have also been observed in other environments, including the open ocean Pacific (Feely et al., 1988), in coral reefs  and anthropogenically perturbed estuaries .
In the PhyCaSS interaction, the seasonal uptake of CO 2 by phytoplankton photosynthesis in surface waters -in those areas with very high rates of marine phyto-20 plankton primary production measured during the sea-ice free period (Hill and Cota, 2005) -substantially reduce seawater DIC and pCO 2 . Photosynthesis or phytoplankton primary production thus acts to increase the pH and Ω of surface waters, thereby seasonally mitigating the long-term reduction of Ω due to gradual ocean acidification ). In the Western Arctic, summertime high rates of phytoplankton 25 primary production also result in large vertical and horizontal export of organic carbon on polar shelves such as the Chukchi Sea (Moran et al., 2005;Bates et al., 2005). Remineralization of organic matter back to CO 2 was invoked as explanation for the 14265 Introduction

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | seasonal subsurface reduction of pH and Ω in subsurface waters, with this biological process adding to the reduction of Ω due to gradual ocean acidification . In this study, as explanation for observations collected during the RUSALCA and ICESCAPE projects, we expand on the simple PhyCaSS treatment to include other physico-biogeochemical processes that can impact seawater carbonate chemistry, pH 5 and Ω, including: air-sea CO 2 gas exchange, and sea-ice melt and freshening. Potential changes in physico-biogeochemical conditions are discussed later in light of the apparent expansion of the low Ω bottom water in the Western Arctic during the last decade. Seawater carbonate chemistry, pH and saturation states for CaCO 3 can be influ-10 enced by several physical, chemical, and biological factors (Fig. 7a). Those processes that act to reduce DIC and pCO 2 , and increase pH and Ω include the following: (a) photosynthesis or primary production (i.e. net ecosystem production, NEP), and; (b) loss of CO 2 from surface waters due to air-sea gas exchange of CO 2 in those areas that are sources of CO 2 to the atmosphere (see Fig. 7a). Those processes that act to 15 increase DIC and pCO 2 , and decrease pH and Ω include the following: (c) respiration or remineralization of organic carbon back to CO 2 , and; (d) gain of CO 2 in surface waters due to air-sea gas exchange of CO 2 in those areas that are oceanic sinks of CO 2 from the atmosphere (Fig. 7a). Other processes having relatively minor impacts on pH and Ω include: (e) calcification and dissolution of CaCO 3 ; (f) warming or cooling (for 20 example, a 10 • C temperature change imparts a 0.1 change in Ω), and; (g) freshening of the mixed layer due to sea-ice melt (Fig. 7a).
Here, we examine changes in Ω for surface and subsurface waters that are imparted by ocean acidification due to anthropogenic CO 2 uptake (δΩ OA ). Superimposed on gradual changes in Ω caused by ocean acidification are seasonal changes influenced 25 by the following; (1) biological processes in the water column during the sea-ice free growing season (i.e. δΩ BIOL ) which includes photosynthesis/respiration, expressed as rate terms of primary production or net community production; (2)  of CO 2 (δΩ GASEX ), and; (3) warming/cooling of waters during the sea-ice free summertime and sea-ice covered period (δΩ TEMP ). In surface waters, the potential changes due to long-term and seasonal forcings of Ω values (∆Ω SURFACE ) can be expressed by the following: In Fig. 8, δΩ TEMP is shown as δΩ WARM and δΩ COOL to reflect the impact of warming and cooling on surface waters. Other factors, thought to have minor impacts on surface Ω are discussed later. In subsurface waters of the Western Arctic, air-sea CO 2 gas exchange and temperature changes are less important and changes in Ω (i.e. ∆Ω SUBSURFACE ) expressed as: We thus compare and contrast changes in Ω in surface and subsurface waters to determine if potentially corrosive subsurface waters are a signal of the Anthropocene (i.e. ocean acidification) that is compounded by natural physico-biogeochemical processes (Figs. 7a, 8a; albeit that these processes are likely anthropogenically perturbed). 15 The contribution of ocean acidification (δΩ OA ) to Ω changes in the Western Arctic. We estimate the present-day contribution of ocean acidification due to uptake of anthropogenic CO 2 (i.e. δΩ OA ) as follows. Recent estimates of the global ocean inventory of anthropogenic CO 2 range from 120 to 160 Pg C (Sabine et al., 2004;Khatiwala et al., 2009). In the Arctic Ocean, the inventory of anthropogenic CO 2 is approximately 2.5 20 to 3.3 Pg C or about 2 % of the global ocean inventory (Tanhua et al., 2009). Previous studies have indicated that the inventory of anthropogenic CO 2 in the upper ocean of the Arctic is approximately 35-40 µmol kg −1 of DIC. Stated another way, the mean DIC content of polar surface water is estimated to be 35-40 µmol kg −1 higher at present compared to pre-industrial times with the consequence of reducing pH and Ω. Given typical salinity, temperature, DIC and TA values for surface and subsurface waters observed during summertime of 2009 to 2011, we estimate that ocean acidification due 14267 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | to anthropogenic CO 2 uptake has reduced Ω values by ∼ 0.4 to 0.6 for Ω calcite and ∼ 0.3 to 0.4 for Ω aragonite since pre-industrial times. A schematic seasonal time series for Ω aragonite is shown with δΩ OA denoted by the dashed blue line for both surface and subsurface waters (Fig. 8b, c; a similar time series for Ω calcite is not shown but would be scaled by approximately 150 %). Superimposed on the long-term changes due to ocean acidification are changes in Ω imparted by seasonal processes (Fig. 8).
Potential physical and biological factors influencing seasonal changes in Ω in surface waters. In surface waters, marine phytoplankton primary production or net community production during the sea-ice free growing season (i.e. δΩ BIOL ) increase Ω values (Fig. 8a, b). In the Chukchi Sea, very high rates of marine phytoplankton primary 10 production or net community production (NCP; 0.8-1.2 g C m −2 d −1 and greater than 4 g C m  There are, however, counteracting processes reducing Ω in surface waters, such as ocean uptake of CO 2 from the atmosphere during the sea-ice free period (i.e. δΩ GASEX ). Previous estimates of air-sea CO 2 flux range from 10 to 40 mmol CO 2 m −2 d −1 (Bates and Mathis, 2009, references therein). This uptake is equivalent to an increase in DIC of surface waters (i.e. 0-30 m deep) of ∼ 30-25 130 µmol kg −1 with the average equivalent to a reduction in Ω aragonite of ∼ 0.7. A minor contributor is seasonal warming of surface waters (δΩ TEMP or denoted as δΩ WARM in Fig. 8b) during sea-ice retreat and melt which is estimated to increase Ω aragonite by ∼ 0.01 per • C. With surface waters exhibiting warming by ∼ 1 • C to ∼ 8 • C, the maximum increase in Ω aragonite due to warming is likely less than 0.1 (Fig. 8b).
Other potential physical and biological factors have negligible or undetermined impact on Ω. Calcification or dissolution of CaCO 3 within the benthic community has minor impact on Ω (Fig. 7a) and there is little evidence for water-column phytoplank-5 ton calcification. Vertical mixing or diffusion is likely to reduce the Ω of surface waters through entrainment of high pCO 2 , low pH/Ω subsurface waters is assumed here to have minor impact on Ω. Strong physical stratification of the water-column is likely to suppress vertical mixing and diffusion during the sea-ice free period while strong mixing and homogenization during sea-ice advance in September and October will "reset" 10 seasonally imparted changes in pH and Ω to "winter" conditions (Fig. 8a). The contribution of sea-ice melt to surface waters also appears to have minor impact on Ω. In Fig. 5, the Ω values of surface waters does not change substantially despite considerable freshening. Sea-ice melt water TA and DIC values observed in the Chukchi Sea tend to be low (∼ 80 to 500 µmol kg −1 ) with very low Ω values of < 0.1 (N. R. Bates,15 unpublished data). This suggests that sea-ice melt should reduce surface water Ω (Fig. 7a), but the relative contribution of sea-ice melt water TA and DIC compared to the TA and DIC content of surface seawater is quite small (< few %).
Summing these potential forcings, it appears that there is a net increase in Ω aragonite of ∼ +0. . On seasonal timescales, it seems that while ocean acidification brings subsurface water Ω values close to a value of one, it is biologically mediated suppression of Ω values that tips the balance and reduces Ω values below a value of one. Thus, it is likely that the presence of corrosive waters on the Western Arctic shelves is a recent phenomenon, a signal of the Anthropocene, and one that is 5 exacerbated by natural biological processes. Such seasonal transitions in Ω that are compounded by natural biological processes have also been shown to occur in the Bering Sea  and in the East Siberian Sea (Anderson et al., 2010). This analysis also suggests that there is an imbalance of the δΩ BIOL term for surface . This imbalance is likely driven by unquantified terms such as the slow release of CO 2 from shelf sediments (due to remineralization of sedimentary organic carbon and benthic ecosystem respiration; e.g. Grebmeier et al., 2008) and offshelf horizontal export of organic carbon. The former process is likely to reduce Ω 15 in bottom waters over the sea-ice covered period when the water column on the shelf is well mixed. For the latter process, previous studies have shown large plumes of suspended particulate organic matter being exported off the Chukchi Sea shelf into the halocline of the Canada Basin (e.g. Bates et al., 2005). The implication of these observations is that the Ω values of halocline waters in the Canada Basin are reduced 20 from remineralization of shelf derived, allochthonous organic carbon.
Is there any change in the distribution of low Ω bottom water in the Western Arctic? The 2009 to 2011 RUSALCA/ICESCAPE data revealed large areas of Ω aragonite undersaturation across the Chukchi Sea (Fig. 5). The summer Ω aragonite values decreased to levels lower than observed in previous years in the early 2000's during the 25 SBI project ). In the early 2000's, undersaturated bottom waters were only observed on the northern slope of the Chukchi Sea rather than across the shelf at present. It appears that low Ω bottom water has expanded across much of the Western Introduction Arctic but given the paucity of survey and time-series data, it is difficult to assess if this is a recent phenomena driven by environmental change in the Arctic. In the early 2000s, the decrease in DIC contents in surface waters due to phytoplankton primary production increased Ω aragonite by a mean value of ∼ 0.3 ). In the late 2000s, the average increase of Ω aragonite in surface water was 5 approximately ∼ 0.6 (although not shown, the net impact on Ω calcite is about 0.9), presumably reflecting increases in NCP (Arrigo et al., 2012) and reduction in ocean uptake of CO 2 (Cai et al., 2010(Cai et al., , 2012 and thus the balance of δΩ BIOL and δΩ GASEX . Arrigo et al., 2008 have shown significant increases in rates of summertime phytoplankton primary production (and a longer growing season) in surface waters across the Arctic 10 over the last two decades. Model studies also suggest that coastal shelf phytoplankton primary production has increased over the last few decades (e.g. Manizza et al., 2011).

15
< 100 µatm) have been measured in this area (e.g. Bates et al., 2011). The implications of these studies are that if rates of primary production are higher in the Western Arctic, then surface water enhancement of Ω is likely as well as concomitant reduction of Ω values -presumably linked to increased vertical export of organic carbon and subsequent benthic respiration and remineralization -in bottom waters present on the 20 shelf. Potentially counteracting this phenomena is the likelihood that a longer growing season, lower surface seawater pCO 2 and expanded areas of seasonal ice free areas may have enhanced the uptake of CO 2 from the atmosphere through gas exchange. In the central basin of the Arctic, increases in surface seawater pCO 2 -and presumably reduction in Ω -due to enhanced air-sea CO 2 gas exchange since the major sea-ice 25 loss event in 2007 appears to have reduced the sink of CO 2 into surface waters over the last few years (Cai et al., 2010(Cai et al., , 2012. It seems clear that the geographic distribution and seasonal timings of seawater carbonate chemistry, pH and Ω variability is rapidly changing in a transitioning Arctic Ocean. Introduction

Potential impact on marine ecosystems
Considering the fact that Ω aragonite and Ω calcite are generally lower in the high-latitude regions , gradual ocean acidification in the future may have an earlier impact on polar and subpolar benthic calcifying organisms compared to temperate and tropical species (e.g. Bates et al., 2009;Mathis et al., 2011a, b). 5 The seasonal presence of potentially corrosive bottom waters for CaCO 3 may have a variety of direct impacts on juvenile to adult benthic calcifying organisms, and their associated ecosystems. For example, CaCO 3 undersaturation can be harmful especially in early development stages of calcifying organisms (Lischka et al., 2011). Rapid changes in seawater chemistry of the Arctic Ocean might cause a decrease in diversity of calcifying organisms (both benthic and planktonic), with consequences for trophic flow/ecosystem structure (Walther, 2010). The most recent study on mineralogy of skeletons of bimineralic calcifiers exposed to seawater high-CO 2 conditions showed increases of calcite to aragonite ratio (Ries, 2011), favoring calcitic organisms. Experimental calcification studies on Arctic Ocean pteropods (which secrete aragonite 15 shells) showed a 28 % decrease in calcification projected for 2100 (Comeau et al., 2009). Higher solubility of aragonite compared to calcite makes this form of CaCO 3 mineral more vulnerable to dissolution in higher pH/lower Ω conditions. Given that the seawater chemistry of the Arctic Ocean is controlled by complex physical and biological processes that are vulnerable to change (Kaltin and Anderson, 2005), quantifying 20 the responses of calcifying organisms to ocean acidification and seasonally changing Ω aragonite has great importance in the future (Orr et al., 2005;Walther et al., 2009;Comeau et al., 2009;Smith, 2009;Lischa et al., 2011).

Conclusions
Previous models predict that undersaturation with respect to aragonite will occur in Introduction  (Steinacher et al., 2009). Observations have shown previously that the polar mixed layer (Yamamoto-Kawai et al., 2009) and small areas of the Chukchi Sea shelf ) already exhibit Ω values less than one. In this study, we find that large areas of the shelf benthos in the Western Arctic are seasonally exposed to bottom waters that are undersaturated with respect to CaCO 3 minerals and thus 5 potentially corrosive for biogenic or sedimentary CaCO 3 . These results suggest that the area of undersaturation for aragonite and even calcite has expanded to most of the Western Arctic shelf. The potential causes for these changes in seawater chemistry in the Western Arctic relate to ocean acidification and seasonal biological/physical processes in the region.
Ocean acidification due to anthropogenic CO 2 uptake appears to have reduced Ω values by ∼ 0.4 to 0.6 for Ω calcite and ∼ 0.3 to 0.4 for Ω aragonite since pre-industrial times. However, superimposed on the ocean acidification influence on Ω, are seasonal biological and physical processes that drive Ω values in different directions for surface and subsurface waters. As such, these processes mitigate and enhance the impact of 15 ocean acidification seasonally in surface and subsurface waters, respectively. Future changes in Ω values on the Western Arctic shelves appear unpredictable due to uncertainties about the extent of sea-ice loss, warming, changes in marine phytoplankton production and air-sea CO 2 gas exchange in the region. Expansion of the sea-ice free region, increased surface water warming and increased growing season may facilitate 20 future increases in marine phytoplankton primary production/NCP that in turn could exacerbate subsurface suppression of Ω values due to increased vertical export of organic carbon. Counteracting this may be increased air-sea CO 2 gas exchange in the region (due to longer sea-ice free periods and increased marine phytoplankton primary production/NCP) that would act to increase Ω values. Such predictions are difficult to 25 make given that air-sea CO 2 gas exchange rates appear to be rapidly changing in the Arctic Ocean  only ∼ 4 % of global ocean area (Pipko et al., 2011). In the last couple of decades, the summertime retreat of summer sea-ice from the Arctic polar shelves due to enhanced melting has exposed surface waters to the atmosphere and allowed the shallow coastal seas of the Arctic Ocean such as the Chukchi and Barents Sea (and Bering Sea) to become significant sinks for atmospheric CO 2 (Murata and Takizawa, 2003;Bates, 5 2006; Chen and Borges, 2009). Such changes may have temporarily increased the ocean uptake of CO 2 into the Arctic (Bates et al., 2006) such that the Arctic Ocean now contributes approximately 5-14 % of the annual global CO 2 uptake ) and as a consequence suppressed Ω values on the shelves. However, since the major sea-ice loss event in 2007, observations indicate that the Arctic may 10 have transitioned back to a smaller sink for CO 2 (Cai et al., 2010(Cai et al., , 2012 with potentially less impact on Ω, illustrating the dynamic and rapidly changing nature of the Arctic environment. In this study, we have not addressed any observed impact on benthic organisms or ecosystems, but, it is clear that the combination of ocean acidification and seasonal 15 biological/physical processes combine to seasonally produce conditions that are potentially unfavorable for calcifying organisms and merit future monitoring of the Western Arctic shelves.         Fig. 7. Schematic of Ω changes due to physical and biological processes superimposed onto typical Western Arctic TA and DIC variability (A) and with net seasonal changes (B). The arrows denote direction of Ω change due to individual/combined processes but arrow length does not denote magnitude of Ω change. Photosynthesis, primary production and loss of CO 2 from surface waters due to gas exchange increase Ω. Calcification/dissolution of CaCO 3 , warming and cooling, and freshening of the mixed layer have minor impact on Ω. Respiration, remineralization of organic carbon to CO 2 and gain of CO 2 by surface waters due to gas exchange decrease Ω. The purple, blue, dark green, light green and yellow lines denote Ω aragonite values of 0.5, 1, 2, 3, and 4, respectively.   In all three panels, bottom axis J to D denotes January to December. (A) annual schematic of changes in Ω aragonite on shelf waters of the Western Arctic Ocean. The sea-ice free period is approximately 3-4 months. Shelf waters are well mixed during the sea-ice covered period with mixed layer developing in late spring due to warming and sea-ice melt, and erosion of the mixed layer in September-October due to convective mixing and homogenization of shelf waters during fall cooling and sea-ice advance. The δΩ OA , δΩ BIOL , δΩ GASEX , and δΩ TEMP terms are shown in the figure with general direction of forcing of Ω. (B) annual schematic changes in Ω aragonite of surface waters. The red line denotes total change in Ω (i.e. ∆Ω SURFACE ) while other terms are denoted as follows: δΩ OA (cyan), δΩ BIOL (green), δΩ GASEX (orange), and δΩ TEMP (purple) terms. (C) annual schematic changes in Ω aragonite of subsurface waters. The red line denotes total change in Ω (i.e. ∆Ω SUBSURFACE ) while other terms are denoted as follows: δΩ OA (cyan), δΩ BIOL (green).