DOC concentrations that are higher than background refractory DOC (~34 – 45 µmol C kg^-1; region dependent) in the interior ocean can exist as a residue of modern carbon export (Noji et al., 1999; Hansell et al., 2012). Though it is commonly thought that these signatures are fleeting and do not extend to the bathypelagic (Hansell and Carlson, 2015), our recent work reveals that significant DOC enhancements can span the entirety of the water column as a result of major export events (Lopez et al., 2020; Lopez and Hansell, 2021). In the aforementioned studies, we utilized these anomalous DOC signatures to pinpoint locations in which uniquely deep export occurred, then employed ancillary sensor, bottle, and satellite data to assess the underlying mechanisms driving that export. Here, we expand that work to the Southern Ocean, where DOC similarly serves as a useful qualitative proxy for bathypelagic export.

The Southern Ocean is a high-nutrient-low-chlorophyll region, where nitrate is plentiful but primary production is controlled by the concentration of bioavailable iron (Timmermans et al., 1998). Iron is a mandatory co-factor for various biological enzymes, such as the nitrogen-fixation enzyme nitrogenase as well as nitrate reductase (Timmermans et al., 1994; Morel and Price, 2003). An iron-starved autotrophic community quickly consumes the nutrient in the euphotic zone and as such, concentrations are consistently depleted. Previous research in the Southern Ocean has demonstrated the strong response of autotrophs to episodic inputs of iron via marked increases of phytoplankton biomass and carbon export efficiency (Boyd et al., 2000; Arrieta et al., 2004; Blain et al., 2007; Boyd et al., 2007). Though the subarctic Pacific is similarly iron limited, this micronutrient is supplied via iron-rich Asian dust (Duce and Tindale, 1991) whereas iron is largely supplied to the Southern Ocean through vertical upwelling of deep water, melting of iron-rich Antarctic ice, and continental sediment exchange (de Baar et al., 1995; Tagliabue et al., 2014; Laufkötter et al., 2018; Sergi et al., 2020). However, these processes are fluctuating as global climate and environmental conditions change (Deppeler and Davidson, 2017), thus a solid understanding of their impact on the biological pump is advantageous for future development of mitigation strategies.

Antarctic ice retains iron that is embedded through exchange with sediment and surface dust accumulation and is thus a major intermediary for iron supply to the Southern Ocean. Melt from both sea ice and continental ice impact the region, though the latter is largely confined to coastal zones and transported beneath the mixed layer and thus must be vertically upwelled to support autotrophs (Person et al., 2021). Both forms of ice melt can supply a considerable amount of bioavailable iron to surrounding waters, enhancing autotrophic carbon production and its subsequent export (Lannuzel et al., 2011; Lannuzel et al., 2016; Laufkötter et al., 2018). Discrete measurements have revealed high concentrations of bioavailable iron within ice, reaching levels up to 17.4 nM (Wang et al., 2014; Lannuzel et al., 2016), which is substantially higher than concentrations of only ~0.1 to 1 nmol kg^-1 reported in the subsurface Southern Ocean (de Baar et al., 1995; Sieber et al., 2021). The flux of iron sourced from ice melt into surrounding surface waters is estimated to be as high as 200 nmol m^-2 d^-1 in some regions (Lannuzel et al., 2016) and that ice-associated iron influx can locally increase carbon production and export up to 30 and 42%, respectively (Person et al., 2019). Production and export enhancements are likely even higher during extreme ice melt events that are becoming more common in the last decade (Fetterer et al., 2017). As such, temporal variability of ice melt dynamics is certainly impacting local biological community structure, carbon production, and export dynamics (Peck et al., 2010; Freeman et al., 2018; Moore et al., 2018), thus these impacts should be better quantified to predict future changes.

Fronts are another key source of iron supply for autotrophs because they locally enhance upward nutrient flux into the euphotic zone, thereby stimulating carbon production and deep particle export (Guidi et al., 2012; Ohman et al., 2012; Lopez and Hansell, 2021). We previously linked enhancements of bathypelagic DOC concentrations to overlying fronts in the North Pacific (30-45°N), concluding deep export to be stimulated by nitrate injection into the euphotic zone (Lopez and Hansell, 2021). The Southern Ocean similarly hosts strong fronts within the turbulent Antarctic Circumpolar Current (ACC) that are known to stimulate vertical nutrient mixing (Sokolov and Rintoul, 2007), therefore we evaluate here the impact of these fronts on export during 2008 and 2017, again using DOC as a proxy.

Though much is being learned of the Southern Ocean through major projects such as SOCCOM (Southern Ocean Carbon and Climate Observations and Modeling) and GEOTRACES, here we demonstrate the value of using DOC signatures to shed light on processes that stimulate deep export. We explored data along a transect at 103°W in 2008 and 2017, of which the DOC distributions differed substantially. Given such dichotomy, we centered our efforts on exploring the following questions:

By studying these controls on deep export, we can better understand the role of fronts within the ACC and large-scale ice melting in modulating carbon sequestration within the deep waters of the Pacific sector of the Southern Ocean.

Our analysis targeted the waters along transect P18 (30-62°S, 103°W) of the U.S. Global Ocean Ship-Based Hydrographic Investigations Program (GO-SHIP; Figure 1 ) in austral summer of both 2008 and 2017 using a variety of open-source shipboard, sensor, and satellite data (See Data Availability).

Discrete DOC samples for depths >500 m for each of the P18 cruises were analyzed for statistical significance. The distribution of DOC concentrations in the bathypelagic were then compared to locations of distinct fronts within the ACC ( Figure 2 and Table 1 ). The location of the fronts in each year was determined by abrupt changes in salinity from Argo float salinity profiles (Argo, 2021; Figures 2A, B ) and with reference to previous characterizations of the area (Sokolov and Rintoul, 2002). Float profiles were present within 500 km of the transect latitude and within 30 days of the occupation. A total of 265 and 665 Argo float profiles in 2008 and 2017 were analyzed, respectively.

Mixing dynamics within the Polar Front (PF) were assessed by comparing monthly mixed layer depths between January 2007 to January 2008 and January 2016 to January 2017. Temperature and salinity data from Argo float profiles present within 500 km of the transect longitude (103°W) and between 50-60°S were used to calculate mixed layer depth, defined as the first depth at which potential density exceeded surface density by 0.125 kg m^-3 (Monterey and Levitus, 1997).

Discrete upper-ocean iron measurements from the GEOTRACES GSci01 Antarctic Circumnavigation Expedition (Walton and Thomas, 2018), collected nearly contemporaneously with the P18 occupation in January 2017, were used in conjunction with salinity profiles to determine associations between iron concentrations and freshwater input via ice melt.

Ice area data from the National Snow and Ice Data Center (NSIDC; Fetterer et al., 2017) were then analyzed to investigate ice melt conditions during each year. Anomalies were identified by the difference in month-by-month sea ice area from the climatological mean in the 1978 – 2021 record.

Finally, bottle silicic acid concentrations normalized to salinity 35 between the two P18 cruises were analyzed to determine differences in upper-ocean nutrient uptake between 2008 and 2017 occupations.

The area of interest stretches from the subtropical gyre of the South Pacific to the high latitudes of the Southern Ocean ( Figure 1 ), with the latter notoriously hosting the most turbulent waters in the global ocean (Smith et al., 2013). Turbulence arises from extreme westerly winds between 40-70°S, aptly referred to as the “Roaring 40’s”, “Furious 50’s”, and “Screaming 60’s”. These winds give rise to the strongest oceanic current; the Antarctic Circumpolar Current (ACC; Smith et al., 2013). Several fronts are embedded within the ACC that differ in strength, location, and mixing depth (Gille, 1994; Sokolov and Rintoul, 2009). Our focus was on the dynamics of the north, middle, and south branches of the Polar Front (PF; ~50-55°N) and the Subantarctic Front (SAF, ~38-45°), as well as the northern branch of the Southern Antarctic Circumpolar Current Front (SACCf-N). The naming conventions and approximate latitudes for each frontal branch are listed in Table 1 . The northern boundary of the Polar Front (PF-N) is the location of the Antarctic Convergence (Verlencar et al., 1990), where water begins to subduct to form Antarctic Intermediate Water (AAIW). Overturning water circulation of the major water masses in this region is depicted in Figure S1 in Supplementary Material.

We were initially intrigued by this region due to the differences in the deep DOC concentrations between the 2008 and 2017 occupations of P18 ( Figures 2C, D ). A Mann-Whitney U test for DOC concentrations at depths >500 meters indicated a statistical significance between the two distributions (p = 2.2 x 10^-16). We report two main observations from these distributions; first, that the 2008 distribution contained narrow, vertically aligned features of elevated DOC concentrations (enhanced by 2-4 µmol C kg^-1 from neighboring stations at the same depths) in the mesopelagic/upper bathypelagic (<1500 m) at seven latitudes (~37°, 42°, 45°, 50°, 54°, 58°, and 61°S; Figure 2C ). Secondly, DOC concentrations in 2017 were commonly 2-6 µmol C kg^-1 higher than background throughout the deep bathypelagic at latitudes >50°S ( Figure 2D ). The enhancements were higher than uncertainty of DOC measurement, which is ±1.5 µmol C kg^-1.

The Southern Ocean is estimated to contribute up to 40% of atmospheric CO₂ uptake, demonstrating its great importance for long term carbon sequestration (Khatiwala et al., 2009; DeVries, 2014). As such, understanding the Southern Ocean’s response to anomalous conditions is critical for predicting coming changes in the biological pump. Unusual climate conditions and events are already regularly occurring, as extreme heatwaves, ice melt events, and anomalous westerly winds are gaining attention (Turner et al., 2017; Meehl et al., 2019; Parkinson, 2019). For instance, record high Antarctic temperatures were reported in summer of 2019/2020, succeeded by another heatwave in March 2022 that reached temperatures > 40°C higher than usual in some regions (Robinson et al., 2020; Colucci, 2022). Unfortunately, these anomalies may become less “anomalous” in coming years.

From this work, we reveal evidence that export to depths <1500 m in 2007/2008 was stimulated by enhanced upward mixing of iron within turbulent fronts when iron influx from ice melt was low. In contrast, when iron input from ice melt was higher-than-average in 2016/2017, deep export occurred at latitudes south of the PF-N. Post-2016, the region continued a period of decreasing summer ice cover (Meehl et al., 2019) and increased frequency of anomalously low ice area. In the ice record ( Figure 8B ), we found that such events occurred in 2018 and 2021 and perhaps exhibited above average export as well. However, this study is limited to one longitude. Other sectors of the Southern Ocean are subject to other iron sources and have different ice melt dynamics (Adusumilli et al., 2020), therefore the conclusions from this study may not extrapolate to other regions. Regardless, it is plausible that increased frequency of such events will further increase deep export and become conduits for the deep sequestration of carbon by sedimentation, microbial respiration into inorganics, and addition to the refractory DOC pool. Though this process ultimately results in atmospheric carbon sequestration, dramatic sea ice melt events can cause a ripple effect that negatively impacts factors such as ocean temperature regulation and the lifecycles of local biological community (Thomas and Dieckmann, 2010). Given that there is a need to constrain the uncertainty of the impact that climate change has on the biological pump (Henson et al., 2022), this work provides some insight on the dramatic response within the Pacific sector of the Southern Ocean.

In this work, we furthered our understanding of the Southern Ocean’s response to two prominent mechanisms of iron fertilization: deep frontal mixing and sea ice melt. We utilized signatures of enhanced DOC concentrations at bathypelagic depths to identify latitudes at which deep export occurred along the same transect in the Pacific sector occupied a decade apart (2008 vs. 2017). Antarctic ice area happened to be anomalous in both years, with high levels of ice melt present in 2017 and a low degree of melt in 2008, providing a unique opportunity to understand the impact of ice-associated iron on carbon export dynamics. We conclude that the primary influence on upper-bathypelagic export in 2008, when anomalously low ice melt was present, was upwelling/cycling of iron focused within well-known fronts in the area. In contrast, widespread export throughout the water column at latitudes >50°S in 2017, when ice melt was high, resulted from anomalously high influx of iron sourced from the melt. These conclusions present us with a dynamic view of these waters, revealing the significant and widespread influence that ice-sourced iron has on deep carbon export, as well as the role that ever-present fronts play when the influx of ice melt is low. Further, this work demonstrates the utility of bathypelagic DOC signatures as indications of modern carbon export, effectively allowing us to identify important locations and further explore the mechanisms driving that export. Given that anomalous events such as those discussed here are occurring more often, understanding the ocean’s response to a changing Antarctic climate is key to predicting future changes in the biological pump.

DOC, beam attenuation, and silicic acid data as well as sample collection methods are available from the Carbon Hydrographic Data Office (CCHDO) at https://cchdo.ucsd.edu for cruises 33RO20071215 and 33RO20161119. (Baringer and Johnson, 2021a, 2021b; Hansell, 2021a, 2021b; Mordy and Zhang, 2021a, 2021b; Gardner, 2021a, 2021b). Argo float data are available at https://argo.ucsd.edu/data (Argo, 2021). ACE GEOTRACES data are available from the GEOTRACES Intermediate Data Product (2021) at https://www.geotraces.org/geotraces-intermediate-data-product-2021/. Sea ice data presented are the final Goddard Space Flight Center (GSFC) product obtained from Version 3 of the Sea Ice Index, distributed by the National Snow and Ice Data Center (NSIDC) from platform/instruments from the Defense Meteorological Satellite Program (DMSP) -F13 SSM/I and -F17 SMMIS (Fetterer et al., 2017). Data are available at https://nsidc.org/data/explore-data.

CL conceived the idea, designed the project, completed data analysis, and wrote the manuscript. DH assisted with writing and project development. All authors contributed to the article and approved the submitted version.