# Southern Ocean CO2 outgassing and nutrient load reduced by a well-ventilated glacial North Pacific

Madison Shankle, Graeme Macgilchrist, William Robert Gray, Casimir de Lavergne, Laurie Menviel, Andrea Burke, James Rae

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Madison Shankle, Graeme Macgilchrist, William Robert Gray, Casimir de Lavergne, Laurie Menviel, et al.. Southern Ocean CO2 outgassing and nutrient load reduced by a well-ventilated glacial North Pacific. 2025. hal-05059775

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# Southern Ocean CO2 outgassing and nutrient load reduced by a well-ventilated glacial North Pacific

Madison Shankle

mgs23@st-andrews.ac.uk

University of St Andrews https://orcid.org/0000-0002-8052-9136

Graeme MacGilchrist

Princeton University

William Gray

Laboratoire des Sciences du Climat et de l'Environnement (LSCE/IPSL), Université Paris-Saclay
https://orcid.org/0000-0001-5608-7836

Casimir de Lavergne

Sorbonne Université-CNRS-IRD-MNHN https://orcid.org/0000-0001-9267-7390

Laurie Menviel

The University of New South Wales https://orcid.org/0000-0002-5068-1591

Andrea Burke

University of St Andrews https://orcid.org/0000-0002-3754-1498

James Rae

University of St Andrews https://orcid.org/0000-0003-3904-2526

## Article

### Keywords:

**Posted Date:** January 28th, 2025

**DOI:** https://doi.org/10.21203/rs.3.rs-5631992/v1

**License:** © This work is licensed under a Creative Commons Attribution 4.0 International License.

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**Additional Declarations:** There is **NO** Competing Interest.
**Title**

Southern Ocean CO₂ outgassing and nutrient load reduced by a well-ventilated glacial North Pacific

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**Co-authors**

Madison G. Shankle<fnref n="1" />*; Graeme A. MacGilchrist<fnref n="1" />; William R. Gray<fnref n="2" />; Casimir de Lavergne<fnref n="3" />; Laurie C. Menviel<fnref n="4" />; Andrea Burke<fnref n="1" />; James W. B. Rae<fnref n="1" />

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<footnote n="1">¹ School of Earth and Environmental Sciences, University of St Andrews, St Andrews, KY16 9TS, United Kingdom</footnote>

<footnote n="2">² Laboratoire des Sciences du Climat et de l'Environnement (LSCE/IPSL), CEA-CNRS-UVSQ, Université Paris-Saclay, Gif-sur-Yvette, 91191, France</footnote>

<footnote n="3">³ LOCEAN Laboratory, Sorbonne Université-CNRS-IRD-MNHN, Paris, 75005, France</footnote>

<footnote n="4">⁴ Climate Change Research Centre, The Australian Centre for Excellence in Antarctic Science, University of New South Wales, Sydney, NSW 2052, Australia</footnote>

<footnote>*Corresponding Author: Madison G. Shankle.</footnote>

<footnote>Email: mgs23@st-andrews.ac.uk</footnote>

<footnote>Address: School of Earth and Environmental Sciences, University of St Andrews, Bute Building, Queen's Terrace, St Andrews, KY16 9TS, Scotland, United Kingdom</footnote>

<footnote>Phone number: +44 (0)1334 46 3940</footnote>
## ABSTRACT

The biogeochemistry of Southern Ocean surface waters has global impacts on nutrient distributions, the carbon cycle, and climate, motivating study of its underlying controls and changes under altered climate states. Today, poorly-ventilated North Pacific waters supply the majority of carbon and nutrients upwelling in the Southern Ocean, outpacing the rate of biological carbon assimilation and fueling CO₂ outgassing. A means of reducing this carbon/nutrient supply is central to explaining the improved biological pump efficiency evidenced in paleo-proxy reconstructions and invoked in theories for glacial CO₂ change. Past studies predominantly attribute reduced glacial carbon/nutrient load of Southern Ocean surface waters to local physical processes (e.g., reduced wind-driven upwelling or enhanced stratification). Here we propose a complementary mechanism in which the carbon/nutrient load of the source waters feeding the Southern Ocean surface is reduced remotely, prior to being upwelled. Comparing glacial North Pacific and Southern Ocean proxy records, alongside results from 3 different Earth System Models, we show that improved ventilation of the glacial North Pacific directly reduces carbon/nutrient-delivery to and CO₂ outgassing in the Southern Ocean surface. This highlights a novel interhemispheric influence on the biogeochemical state of the Southern Ocean, which may play a key role in glacial CO₂ change.

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## MAIN TEXT

### INTRODUCTION

The Southern Ocean is an important nexus for biogeochemical cycling, connecting the ocean basins as well as deep and surface layers of the ocean. The biogeochemistry of Southern Ocean surface waters supports thriving local ecosystems¹, influences global ocean nutrient distribution²,³, and regulates ocean-atmosphere carbon exchange⁴⁻⁶. Together these processes can influence the global carbon cycle through their impact on carbon dioxide (CO₂) outgassing in this region.

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Today, wind-driven upwelling supplies an over-abundance of major nutrients to Southern Ocean surface waters, where biological production – limited by availability of light⁷ and iron⁸ (a minor nutrient) – is insufficient to entirely extract the accompanying upwelled carbon and thus counteract the evasion of carbon dioxide (CO₂) to the atmosphere. This CO₂ evasion⁹,¹⁰, along with the loss of unused “preformed” nutrients back into the abyss via Antarctic Bottom Water, is why the Southern Ocean is described as having a “leaky” and relatively inefficient biological pump. Increasing the efficiency of this pump could stem this leak of CO₂ and has been invoked in many theories for atmospheric CO₂ drawdown during glacial periods⁵,¹¹⁻¹⁴. The local biological pump efficiency could be enhanced by any mechanism that reduced the supply of carbon and nutrients to the Southern Ocean surface, with productivity being better able to keep up with a reduced supply of carbon (reducing outgassing) and a reduced nutrient load of surface waters leading to less preformed nutrients being lost to the abyss.

Thus far, proposed mechanisms of reducing the carbon and nutrient load of Southern Ocean surface waters during glacial times have primarily focused on reducing the physical transport of water to the surface (i.e., reduced upwelling and/or enhanced stratification reducing the mixing of nutrient- and carbon-rich deep waters up from below). However, changes in the carbon and nutrient content of the upwelled water itself may also have played a critical role. Today the Southern Ocean receives most of its carbon and nutrients from mid-depths (1-3 km) of the Pacific basin⁹, where poor ventilation allows for their accumulation¹⁵. This is illustrated by modern profiles¹⁶ of CO₂ outgassing potential (potential pCO₂, or “PCO₂”, see Methods and Chen et al. (2022)⁹), nitrate, and phosphate spanning the Atlantic and Pacific basins (Fig. 1), which show nutrient- and carbon-rich North Pacific mid-depth waters contrasting with relatively nutrient- and carbon-poor North Atlantic Deep Water (NADW). Figure 1 also illustrates the inclined isopycnals (tilted by near-freezing waters and wind-driven Ekman divergence at the surface) that link these mid-depths in both basins to the Southern Ocean surface (e.g., on neutral density surfaces ~27.5-28 kg m⁻³ in Fig. 1). The supply of these carbon
rich, high PCO₂ Pacific waters to the Southern Ocean has been described as the deep ocean's carbon exhaust⁹. Reducing the carbon and nutrient content of the sub-surface North Pacific waters feeding the Southern Ocean surface thus provides an alternative means of reducing the glacial Southern Ocean carbon and nutrient supply that could operate independently of, or alongside, reduced upwelling or mixing. Here we explore this idea and the ability of North Pacific changes to induce important downstream effects on Southern Ocean biogeochemistry.

We first present biogeochemical proxy records from the North Pacific and Southern Ocean documenting a shared pattern of decreased surface nutrient levels during glacial periods¹⁷,¹⁸. We then show in a range of Earth System Models that reduced North Pacific surface nutrients is consistent with better ventilation (as evidenced by glacial proxy data¹⁹,²⁰) and associated depletion of sub-surface nutrients. Finally, we show that these low-carbon low-nutrient sub-surface North Pacific waters make their way south, where they directly translate into reduced nutrient load and outgassing rates in the Southern Ocean surface. These findings demonstrate an important connection between North Pacific and Southern Ocean biogeochemistry and suggest a novel mechanism for reducing the glacial carbon and nutrient load of Southern Ocean surface waters by far remote processes.

# RESULTS

## Shared biogeochemical proxy records in the North Pacific and Southern Ocean

A reduced glacial supply of nutrients and carbon to Southern Ocean surface waters is recorded in proxy data over the last glacial cycle (Fig. 2). Reduced glacial biological productivity across the Antarctic Zone of the Southern Ocean is evidenced by proxies reflecting decreased export production (Fig. 2d). Such proxies include reduced fluxes and concentrations of biogenic barium and opal in marine sediments; various elemental ratios in sediments (e.g., reduced Ba/Fe and Ca/Fe reflecting reduced fluxes of biogenic barite and calcite to the seabed); reduced ²³¹Pa/²³⁰Th (reflecting reduced particle flux to depth); and reduced calcite fluxes (reflecting
100 reduced export production of calcite-producing organisms)²¹⁻³⁰.

Alongside reduced productivity, the glacial Southern Ocean saw a more complete degree of nitrate utilization, with a greater proportion of the available nitrate pool being consumed by production. This is documented by elevated δ¹⁵N values in diatom-, coral- and foraminifera-bound organic matter³¹⁻³⁴ (Fig. 2e). Biological productivity preferentially takes up nitrate with the lighter ¹⁴N isotope, enriching the residual seawater nitrate pool in heavy ¹⁵N. As nutrient utilization becomes more complete – consuming a greater and greater proportion of the available nitrate pool – the residual heavy signal becomes increasingly incorporated into organic matter. Heavier δ¹⁵N values under glacial conditions thus indicate enhanced nitrate utilization in Southern Ocean surface waters during glacial periods³¹⁻³⁴. Critically, the glacial signal of more complete nitrate consumption alongside lower overall productivity in the Antarctic Zone is only possible if the gross overall nitrate/nutrient supply to the Southern Ocean surface was reduced.

Remarkably similar glacial trends are also recorded in the far-flung North Pacific. Reduced productivity and export production is recorded throughout the subpolar gyre by reduced barium and opal fluxes³⁵⁻³⁷ (Fig. 2a), as is enhanced nitrate consumption by heavier δ¹⁵N values of foram- and diatom-bound organic matter³⁷⁻⁴⁰ (Fig. 2b). This shared “Polar Twins” pattern¹⁷ in Southern Ocean and North Pacific proxies has prompted considerable discussion of what processes could simultaneously drive the same proxy patterns in two such far-flung regions. Previous explanations have focused on enhanced isolation of surface waters from nutrient-rich deep waters below, invoking the same physical processes (reduced upwelling or enhanced stratification) operating in both regions during glacial times¹⁷,¹⁸. Little consideration, by contrast, has been given to linkages connecting these two regions and the potential importance of reduced sub-surface nutrient and carbon content. In the North Pacific, reduced nutrient and carbon content in better ventilated sub-surface waters has recently been demonstrated as key means of reducing nutrient and carbon delivery to the surface during the Last Glacial Maximum
(LGM)¹⁹. Motivated by contemporary linkages between the North Pacific and Southern Ocean⁹,¹⁰,
we here explore the potential of North Pacific ventilation to simultaneously influence carbon
and nutrient supply to the Southern Ocean.

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A reduced sub-surface North Pacific carbon and nutrient supply

Proxy records show North Pacific surface waters exhibiting lower nutrients, higher salinity, and warmer temperatures in the LGM than today, which modeling results show to be consistent with better ventilation of and reduced nutrient content in sub-surface waters¹⁹. While reduced upwelling or mixing could also explain reduced surface nutrients, wind-driven upwelling (a dominant mode of nutrient delivery in the modern) was in fact likely increased in the North Pacific during glacial times, with Paleoclimate Modeling Intercomparison Project (PMIP) models consistently indicating substantially (~60%) greater wind stress curl over the North Pacific subpolar gyre under LGM conditions than in the pre-industrial⁴¹,⁴². Similarly, evidence of enhanced regional intermediate water formation suggests increased convective mixing at this time¹⁹,⁴³, while increases in benthic carbon isotopes (Fig. 2c) reflect enhanced ventilation and influence by heavy-δ¹³C surface waters⁴⁴,⁴⁵. Communication between the sub-surface and surface in the North Pacific therefore appears to have been increased under glacial conditions, thus requiring a reduced nutrient content of these sub-surface waters to explain reduced nutrient supply at the surface.

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Earth System Model experiments show that such a reduction in sub-surface nutrient
content can be achieved by an active meridional overturning circulation operating in the Pacific,
as demonstrated in cGENIE¹⁹ as well as higher-resolution models such as LOVELIM and
UVic⁴⁶,⁴⁷. Across these models, enhanced North Pacific Intermediate Water (NPIW) formation
under glacial-like conditions consistently drives an equilibrium state in which well-ventilated
sub-surface waters penetrate down to ~2000 m depth (hereafter referred to as "mid-depth"
waters), replacing the previously poorly-ventilated sub-surface waters with relatively carbon-

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and nutrient-poor surface waters from the low latitudes (Fig. 3; Supplementary Fig. 2). This also
reduces local surface carbon and nutrient content, as it is these sub-surface waters which
supply carbon and nutrients to the surface above.

The onset of greater convective mixing in the North Pacific may seem at odds with reduced surface nutrients and carbon, but it is important to distinguish between the transient and equilibrated responses to such ventilation. The initial onset of overturning is expected to generate a short-lived transient peak in surface nutrients and carbon as convection initially taps into nutrient- and carbon-rich sub-surface waters below⁴⁸. Over time, continued ventilation depletes this sub-surface reservoir, eventually leading to persistently low surface concentrations¹⁹. Thus, the long-term equilibrated response to ventilation of the North Pacific is one of persistently low carbon and nutrient concentrations in surface waters, even in the face of sustained convective mixing. The difference between the transient and equilibrated response is also well illustrated in ventilation experiments performed in cGENIE, which show the transient peak in surface nutrients subsiding within the first several hundred years of simulation (see their Supplementary Figure S9)¹⁹. Analogously in our own simulations, despite the initial mixing up of deep carbon, ventilation ultimately reduces outgassing rates in the North Pacific after 1000 years (Supplementary Fig. 3). (For more discussion on this, see “Discussion”.)

More active NPIW formation and ventilation thus provides a means of reducing surface
carbon and nutrients in the glacial North Pacific by reducing the carbon and nutrient load of the
sub-surface waters that supply them. We propose that these North Pacific changes could also
account for the reduced carbon and nutrient load in the glacial Southern Ocean surface. If the
well-ventilated low-carbon/nutrient sub-surface waters of the glacial North Pacific reached the
Southern Ocean, this would provide an alternative means of reducing the glacial Southern
Ocean surface carbon and nutrient load (that could act independently of or in conjunction with
physical changes like reduced upwelling or mixing), while simultaneously explaining the shared
“Polar Twins” proxy pattern between the North Pacific and Southern Ocean. Indeed, this low-
carbon low-nutrient signal is seen to propagate southward from the North Pacific in all models, notably along isopycnals outcropping in the Southern Ocean (red and yellow contours in Fig. 3 and Supplementary Fig. 2). This is a consistent pattern persisting across a range of Earth System Models spanning differing configurations, boundary conditions, and ventilation strengths, and thus is an effective demonstration of the ability of a well-ventilated North Pacific to reduce the carbon and nutrient content of waters feeding the Southern Ocean.

## Reduced carbon and nutrient delivery to the Southern Ocean

We now quantify the impact of a well-ventilated North Pacific on Southern Ocean carbon and nutrient content in the intermediate-complexity University of Victoria Earth System Climate Model (UVic ESCM) v2.9⁴⁹. The simulation analyzed here was first presented by Menviel et al. (2014), who performed North Atlantic meltwater experiments in this and the LOVECLIM v1.1 ESM to investigate marine carbon cycle responses to Atlantic meridional overturning circulation (AMOC) shutdown⁴⁶. In the North Atlantic meltwater simulation, stronger North Pacific Intermediate Water (NPIW) formation (arising from a weaker North Pacific halocline resulting from oceanic and atmospheric teleconnections) resulted in stronger ventilation of North Pacific waters down to about 3000 m depth⁴⁶.

We do not take the meltwater simulation (given its reduced AMOC strength) as a precise analog of LGM conditions, but rather capitalize on its representation of North Pacific ventilation and use it to assess the impacts of a well-ventilated North Pacific on Southern Ocean biogeochemistry. We do note, however, that similar results to those presented here are seen in more LGM-like simulations with less extreme AMOC reduction and NPIW formation (Fig. 3d and Rae et al. (2020)¹⁹).

Here we compare Southern Ocean carbon and nutrient content in two simulations, both run under identical LGM-like boundary conditions. One simulation features strong North Pacific ventilation driven by North Atlantic meltwater addition and associated enhanced NPIW
formation (the perturbed simulation, hereafter referred to as “UVic-NP”), while the other serves
as a control simulation, lacking meltwater addition and associated North Pacific ventilation
(“UVic-ctrl”). In these simulations we quantify carbon content by potential pCO₂ (PCO₂), which
being a function of both alkalinity and dissolved inorganic carbon (DIC) is an effective measure
of a water parcel’s outgassing potential⁹. Nutrient content is traced through phosphate
concentrations ([PO₄³⁻]). We also quantify differences in Southern Ocean outgassing rates
between the two simulations.

In the UVic-NP simulation, stronger NPIW formation and transport ventilates and reduces
the carbon and nutrient content of sub-surface North Pacific waters – an anomaly which
extends southwards through the basin along mid-depths (Fig. 3a, Supplementary Fig. 2a). We
see strong reductions in PCO₂ in the North Pacific from the near surface down to ~3000 m
depth, with a mean PCO₂ anomaly of -199 µatm in the sub-surface North Pacific (30-60°N, 140-
240°E, 500-2000 m) (Fig. 4a). Phosphate concentration is similarly reduced (Supplementary Fig.
3a, Fig. 4a), with a mean anomaly of -1.3 µmol kg⁻¹ in the sub-surface North Pacific. The low-
PCO₂ signal extends through mid-depths (~1000-2000 m) to southwards of 30°S, exhibiting a
concentrated core (-78 µatm on average) in the western sector of the basin (140-200°E, 1000-
2500 m), while more diffuse anomalies of -47 µatm on average pervade the basin (140-280°E,
1000-2500 m) (Fig. 4ac). After ~50°S, this low-PCO₂ signal is translated upwards along isopycnal
surfaces (σ₂ = 36.0-36.8 kg m⁻³), outcropping in the Southern Ocean surface (red and yellow
contours in Fig. 3a). Again, similar patterns are seen in phosphate concentrations (Fig. 4d),
where at 30°S phosphate is reduced by -0.3 µmol kg⁻¹ on average across the basin and by -0.5
µmol kg⁻¹ in the western sector of the basin. This negative phosphate anomaly similarly reaches
the Southern Ocean surface along outcropping isopycnals (Supplementary Fig. 3a).

Low PCO₂ drives reduced Southern Ocean outgassing
This low-PCO₂ low-nutrient signal propagated from the North Pacific translates into quantifiable negative PCO₂ and phosphate anomalies in the Southern Ocean surface (Fig. 5a, b). We find the strongest anomalies in the Pacific-sector (150-280°E) Subantarctic Zone (SAZ, ~40-60°S) where mid-depth Pacific isopycnals bounding this low-PCO₂ water (σ₂ = 36.0-36.8 kg m⁻³, red contours in Fig. 5) outcrop. We observe a mean PCO₂ anomaly of ~-8 µatm across the Southern Ocean (south of 40°S) and of ~-12 µatm across the Indian-Pacific sector (25-280°E), with maximum anomalies surpassing ~-25 µatm (Fig. 5b). Phosphate is reduced by -0.13 µmol kg⁻¹ on average across the Southern Ocean and by -0.17 µmol kg⁻¹ in the Indian-Pacific sector, with maximum anomalies surpassing -0.44 µmol kg⁻¹ (Fig. 5a).

These results likely underestimate carbon anomalies due to a low-carbon bias in UVic-ctrl in the North Pacific, where sub-surface PCO₂ values (~350-450 µatm) fall below observed pre-industrial levels (>1000 µatm¹⁶, Fig. 1). Starting from a more carbon-rich baseline in UVic-ctrl could have enabled an even greater drawdown of North Pacific carbon and nutrients in the perturbed simulation, with downstream effects on the Southern Ocean. Our results therefore perhaps represent a conservative estimate of the effect of North Pacific ventilation on Southern Ocean biogeochemistry.

Even so, the negative PCO₂ anomalies in the Southern Ocean surface drive reduced outgassing in the region (Fig. 5c). South of 40°S, outgassing is reduced by 0.18 mol m⁻² yr⁻¹ on average (representing a ~51% reduction relative to control outgassing levels). In the Indian-Pacific sector (25-280°E, south of 40°S), the reduction reaches 0.26 mol m⁻² yr⁻¹ or ~79% on average. This corresponds to net outgassing reductions of ~14 Tmol yr⁻¹ across the Southern Ocean. Furthermore, reduced outgassing is demonstrably the result of the altered biogeochemistry of the upwelling water. Changes in alkalinity and DIC dominate the outgassing anomaly signal (Fig. 5d) whereas temperature and salinity changes play a relatively minor role (see Supplementary Fig. 4). The density surfaces bounding the core low-PCO₂ water mass from the North Pacific (σ₂ = 36.0-36.8 kg m⁻³, red contours in Fig. 5) outcrop at the Southern Ocean
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surface and delineate the negative air-sea CO₂ flux anomaly, further supporting our proposed
North Pacific-based mechanism.

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With such major circulation changes as a weakened AMOC and well-ventilated North
Pacific in this simulation, changes in Southern Ocean circulation dynamics may also be
expected. Thus, in order to link the reduced outgassing signal seen in the Southern Ocean
chiefly to the reduced carbon content of the upwelled waters, two other potential processes
must be ruled out: reduced upwelling and/or enhanced stratification (reducing mixing). As
UVic's atmosphere is simulated using a non-dynamic energy-moisture balance model (in which
the equations for momentum conservation are replaced with climatological wind data and only
minor dynamical feedbacks alter this climatology), virtually no change (<0.5%) in the mean wind
stress curl over the high-latitude Southern Ocean is found between the control and perturbed
simulations. Reduced wind-driven upwelling is therefore unlikely to explain the reduced
Southern Ocean outgassing. Stratification (quantified as the density gradient between 500 m
and the surface) similarly varies by only ~1.5% on average across the Southern Ocean (south of
40°S) between the simulations, and so the reduced outgassing is also unlikely to be the result of
reduced mixing isolating surface waters from carbon-rich deep waters below.

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An additional factor potentially impacting outgassing is biological productivity. If
productivity were significantly enhanced in UVic-NP, this could increase carbon export from
surface waters, potentially contributing to CO₂ undersaturation and reduced outgassing in our
output. Instead, we find net primary production is slightly reduced in our experiment (by ~19% in
the Pacific sector of the Southern Ocean and by ~3% on average across the whole Southern
Ocean south of 40°S), consistent with the observed reduction in surface nutrients (Fig. 5a).
Reduced biological productivity would, in isolation, act to enhance outgassing by reducing the
uptake of carbon into organic matter. However, the simulated reduction in outgassing alongside
reduced levels of productivity highlights the significant role the water's biogeochemical
composition and carbon content plays in dictating its outgassing potential.

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Finally, other factors besides ocean carbon content influencing outgassing fluxes include wind speed, sea ice cover, and atmospheric pCO₂. Ocean-to-atmosphere flux of CO₂ [mol m⁻² sec⁻¹] is calculated in the model⁴⁹,⁵⁰ as

$$f = k_w * [C_{sw} - C_{atm}] \qquad (1)$$

where k<sub>w</sub> is the gas transfer velocity [m sec⁻¹] and C<sub>sw</sub> and C<sub>atm</sub> represent aqueous CO₂ concentration [mol m⁻³] (see Methods for details). Changes in atmospheric pCO₂ (impacting C<sub>atm</sub>) as well as wind speed and fractional sea ice cover (impacting k<sub>w</sub>) must therefore also be accounted for to attribute the reduced outgassing to the reduced carbon content of the water. Winds and sea ice cover are virtually unchanged between the control and perturbed simulations, resulting in negligible contributions to the reduced outgassing signal (Supplementary Fig. 4e, f). Atmospheric pCO₂ drops slightly from 191 µatm in UVic-ctrl to 185 µatm in UVic-NP. In isolation, this reduced atmospheric pCO₂ would drive greater outgassing (Supplementary Fig. 4d). The influence of this atmospheric pCO₂ change thus slightly dampens the negative outgassing anomaly in the Southern Ocean obtained in the perturbed simulation (Supplementary Fig. 4b). It is therefore changes in the water's biogeochemistry, resulting from a better ventilated North Pacific in UVic-NP, that account for the reduced Southern Ocean outgassing.

## DISCUSSION

Poorly-ventilated mid-depth waters from the North Pacific today represent a major source of carbon and nutrients to the Southern Ocean surface today⁹,¹⁰. Southern Ocean outgassing is concentrated in a narrow ring between the wintertime sea ice edge and Subantarctic Front that coincides with deep mixed layers intermittently reaching the carbon-rich waters of Indian-Pacific origin straddling the 27.8 kg m⁻³ neutral density surface – a pathway referred to as the “deep ocean’s carbon exhaust”⁹. Reducing the carbon content of these mid-depth Pacific waters – via better ventilation by enhanced NPIW formation, as emerges in our simulations –
thus provides an effective way of cutting off this deep carbon exhaust and reducing Southern Ocean outgassing. This result holds significant implications for theories of glacial CO₂ change based on enhanced biological pump efficiency, demonstrating that a central feature of these theories and of proxy data – a reduced glacial carbon and nutrient load in the Southern Ocean surface – may have been strongly influenced by improved ventilation of the North Pacific and associated changes to the biogeochemistry of the waters feeding the Southern Ocean.

The dilution of the North Pacific's mid-depth carbon reservoir is not inconsistent with a scenario of glacial CO₂ drawdown. While we expect some of the carbon lost from these mid-depths to have initially been outgassed to the atmosphere (both locally and at lower near-equatorial latitudes)⁵¹, this represents only the transient response to North Pacific ventilation¹⁹,⁴⁸. This short-lived response has been demonstrated in previous studies designed to simulate transient events (e.g., Heinrich events), which show a transient increase in atmospheric CO₂ following the onset of North Pacific ventilation (e.g., Menviel et al., 2012, 2014; Rae et al. 2014)⁴⁶,⁴⁸,⁵¹. By contrast, we probe the present simulations for their implications for the long-term equilibration response to ventilation, relevant to sustained glacial conditions. Despite an initial release of carbon to North Pacific surface waters following ventilation (as described by previous studies), we find that North Pacific outgassing ultimately (after 1000 years) decreases in our simulations (Supplementary Fig. 3). Our results suggest this is achieved by shifting the carbon and nutrient supply away from regions of poor biological pump efficiency today (e.g., the North Pacific and Southern Ocean) and towards regions of greater efficiency (e.g., the low latitudes), where biological productivity is better able to keep pace with and counteract CO₂ evasion (Supplementary Fig. 3). The same dynamics lead to a reduction of preformed nutrient load of the ocean interior and ultimately drive a global improvement in biological pump efficiency. While we cannot precisely quantify the ocean's long-term carbon uptake response to North Pacific ventilation in the present simulation (due to the confounding effects of a weakened AMOC), our results point to a significant shift of carbon out of mid-depths
and into the deep ocean, akin to the “nutrient-deepening” hypothesis of Boyle (1988)⁵². This finding highlights the need for further investigation of the North Pacific’s influence on Southern Ocean biogeochemistry, and future work should focus on quantifying changes to the ocean carbon budget in response to North Pacific ventilation.

Alongside its implications for biological pump efficiency and glacial CO₂ change, our results offer a novel explanation for the shared “Polar Twins” pattern between the North Pacific and Southern Ocean of a reduced glacial nutrient supply. Previous theories have attributed this pattern to the simultaneous action of certain physical processes in each region, such as enhanced stratification limiting the mixing of deep carbon and nutrients to the surface¹⁷. While a reduced nutrient supply is well-supported in both regions in proxy data²⁶,³³,³⁵⁻⁴⁰,⁴⁴,⁴⁵ (Fig. 2), this pattern is not exclusive to enhanced stratification, and proxy data in fact supports enhanced ventilation and convection in the North Pacific at the LGM¹⁹. Glacial model simulations also do not typically show enhanced stratification. Some PMIP2 models run under LGM conditions, for instance, exhibit increased convection and overturning in the Southern Ocean in response to enhanced sea ice formation and brine rejection⁵³,⁵⁴. These processes are also observed across multiple glacial cycles in long-running simulations of intermediate-complexity models⁵⁵. The Polar Twins proxy pattern is therefore unlikely to result from enhanced stratification, with certain glacial conditions such as increased sea ice formation and brine rejection even being expected to oppose it. In such a scenario, a reduction of the carbon and nutrient content of supplying sub-surface waters would then become essential to reducing the carbon and nutrient supply to the Southern Ocean surface.

A reduction in wind-driven upwelling has also been suggested to isolate glacial Southern Ocean and North Pacific surface waters from carbon- and nutrient-rich deep waters below¹⁸. However, PMIP3 models suggest that wind-driven upwelling was actually enhanced in the glacial North Pacific⁴¹. Winds over the glacial Southern Ocean, by contrast, may have been weaker⁵⁶. A recent analysis combining sediment proxies and climate model output indicates a
~25% weakening and 5° equatorward shift of the southern westerlies at the LGM⁵⁶. This weakening likely reduced the upwelling of carbon-rich deep waters, stemming surface outgassing. However, the equatorward shift would also have positioned the winds to preferentially pull up lighter isopycnals – including the mid-depth Pacific waters highlighted in this study that are today rich in carbon (Fig. 1). Thus, while weakened Southern Ocean upwelling may have reduced the surface carbon supply and contributed to CO₂ drawdown, a reduction in the carbon content of these mid-depth Pacific waters could have provided an additional crucial contribution. Our results bring a fresh perspective to the Polar Twins pattern, demonstrating how the carbon and nutrient content of both the Southern Ocean and North Pacific surface could have been altered by North Pacific ventilation.

## CONCLUSION

We have explored how a better-ventilated glacial North Pacific influences Southern Ocean biogeochemistry. We have demonstrated that stronger ventilation of the North Pacific reduces the carbon and nutrient load of mid-depth waters feeding into the Southern Ocean, leading directly to reduced surface carbon and nutrient load and reduced outgassing rates there. This result provides a credible and novel explanation for the shared “Polar Twins” proxy pattern between the glacial North Pacific and Southern Ocean. While not ruling out possible changes to upwelling or mixing, our findings demonstrate the ability of reduced carbon- and nutrient-content of water – outside of any reduction in physical delivery to the surface – to significantly alter Southern Ocean surface biogeochemistry.

A reduced carbon and nutrient load of the Southern Ocean surface, given its impact on biological pump efficiency, is notably a potentially important mechanism in explaining glacial CO₂ change. In this context our results highlight the importance of source-water biogeochemistry alongside local physical pathways and processes in theories of glacial CO₂ change invoking improved biological pump efficiency. In this context they also underscore the
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importance of north-south connectivity along Pacific mid-depths⁹,¹⁵ and associated inter-basin ventilation dynamics. This will be an important consideration for future studies as we continue to unravel mechanisms of ocean carbon uptake across glacial, contemporary, and future time scales.

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# METHODS

## UVic Earth System Model Simulations

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The UVic ESM uses the Modular Ocean Model (MOM) version 2, a three-dimensional ocean general circulation model with a 3.6° longitude x 1.8° latitude horizontal resolution and 19 depth layers⁴⁹,⁵⁷. The UVic ESM also includes a vertically integrated two-dimensional energy-moisture balance model of the atmosphere, a dynamic-thermodynamic sea ice model, a land surface scheme, a dynamic global vegetation model, a marine carbon cycle model, and a sediment model⁴⁹.

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To investigate the impact of a well-ventilated North Pacific on carbon delivery to the surface Southern Ocean, we compare a perturbed simulation in which strong ventilation occurs due to enhanced NPIW formation (hereafter referred to as “UVic-NP”) with a control simulation run under identical conditions but lacking the North Atlantic meltwater addition and associated enhancement of NPIW formation (“UVic-ctrl”). The perturbed UVic-NP simulation was forced by a freshwater hosing of 0.1 Sv imposed on the North Atlantic for 1000 years. Both the control and perturbed simulations were otherwise run under constant LGM-like (~21 ka BP) boundary conditions and were compared after 1000 years of model run time⁴⁶.

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## PCO₂ Calculation

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Following the example of Chen et al. (2022) and Prend et al. (2022), we trace carbon dynamics in the Pacific basin and Southern Ocean using potential pCO₂ (PCO₂), that is, the partial pressure of CO₂ (pCO₂) a water parcel would have if brought to the surface adiabatically
(thus correcting for the effect of pressure on both temperature and partial pressure)⁹,¹⁰. This property, being a function of both dissolved inorganic carbon (DIC) and alkalinity, is of more direct relevance to a water parcel's outgassing or in-gassing potential than DIC alone; sub-surface waters of greater PCO₂ than atmospheric pCO₂ would outgas if brought to the surface. It is possible for waters of high DIC content to actually have low outgassing potential (low PCO₂) if buffered by high alkalinity or if exhibiting high CO₂ solubility (e.g. by way of lower water temperatures). PCO₂ thus represents a more accurate measure of outgassing potential than DIC alone.

PCO₂ is derived by calculating pCO₂ referenced to 0 dbar from sub-surface alkalinity, DIC, potential temperature, and salinity from the model output using PyCO2SYS v1.8.1⁵⁸. The calculation uses the dissociation constants of carbonic acid from Dickson & Millero (1987)⁵⁹, of bisulphate from Dickson (1990)⁶⁰, and of hydrogen fluoride from Dickson & Riley (1979)⁶¹, and the boron to salinity ratio of Lee et al. (2010)⁶².

## CO₂ Flux Calculation

Flux of CO₂ [mol m⁻² sec⁻¹] from the ocean to the atmosphere is calculated in UVic⁴⁹,⁵⁰ as

$$f = k_w * [C_{sw} - C_{atm}] \quad (1)$$

where $k_w$ is the gas transfer velocity [m sec⁻¹] and $C_{sw}$ and $C_{atm}$ represent aqueous CO₂ concentration [mol m⁻³].

$C_{sw}$ [mol m⁻³] is calculated from dissolved inorganic carbon, alkalinity, phosphate, temperature, and salinity in the surface depth level of UVic using PyCO2SYS v1.8.1⁵⁸. The same dissociation constants are used as described in the “PCO₂ Calculation” section.

$C_{atm}$ [mol m⁻³] is calculated from atmospheric pCO₂ from the model output and CO₂ solubility, calculated as a function of temperature and salinity⁵⁰. Solubility (ff, in mol kg⁻¹ atm⁻¹) is calculated as

$$ff = \left( A_1 + A_2 \frac{100}{T} + A_3 \ln\left(\frac{T}{100}\right) + A_4 \left(\frac{T}{100}\right)^2 + S\left(B_1 + B_2 \frac{T}{100} + B_3 \left(\frac{T}{100}\right)^2\right) \right) \quad (2)$$
where $A_1 = -162.8301$, $A_2 = 218.2968$, $A_3 = 90.9241$, $A_4 = -1.47696$, $B_1 = 0.025695$, $B_2 = -0.025225$, $B_3 = 0.0049867$, $S$ = salinity in g kg⁻¹ or per mille, and $T$ = temperature in Kelvin. $ff$ is converted into mol m⁻³ atm⁻¹ using the density of seawater, and from this $C_{atm}$ [mol m⁻³] is then calculated as $C_{atm} = ff * pCO_{2,atm}$ where atmospheric $pCO_2$ is 185 µatm in the perturbed simulation (“UVic-NP”) and 191 µatm in the control simulation (“UVic-ctrl”).

The gas transfer velocity (or piston velocity) $k_w$ (in cm s⁻¹) is calculated as

$$k_w = (1 - \gamma_{seaice}) * 0.377 * (V_s^2) * (\frac{Sc}{660})^{-0.5} \quad (3)$$

where $\gamma_{seaice}$ is fractional sea ice cover (0-1), $V_s$ is the average surface wind speeds (in m s⁻¹, described below), and $Sc$ is the Schmidt number (described below). This formulation gives $k_w$ in cm s⁻¹ and so it is first converted into m s⁻¹ before being used in equation (1). The 0.377 is a proportionality constant relating the gas transfer velocity to the average wind speed squared $(V_s^2)^{50}$. $V_s$ is average wind speed from the model output subjected to a 0.8 contraction factor and 20° rotation as described in the manuscript describing UVic⁴⁹. The Schmidt number represents a further dependency of the gas transfer velocity on temperature and salinity and is calculated as described in the appendix of Wanninkhof (1992)⁵⁰.
FIGURES AND FIGURE LEGENDS

<figure><img src="image_2.png" /><figcaption>Figure 1. Modern Pacific and Atlantic basins show differing carbon and nutrient content.</figcaption></figure>

Modern transects of carbon and nutrient content of the Pacific (left) and Atlantic (right) basins from GLODAPv1¹⁶. (a) Potential pCO₂ (PCO₂) (µatm), which quantifies a water parcel's tendency to outgas CO₂ when brought to the surface, calculated from in-situ alkalinity, dissolved inorganic carbon (DIC), and potential temperature, referenced to 0 dbar (see Methods for details). Anthropogenic carbon has been removed from DIC to represent pre-industrial-like carbon content. Transects of modern (b) phosphate (PO₄³⁻) and (c) nitrate (NO₃⁻) concentrations (µmol/kg) are also plotted. White contours on all panels show neutral density surfaces [kg m⁻³].

Figure made using Ocean Data View⁶³.
<figure><img src="image_3.png" /><figcaption>Figure 2. Productivity and nutrient proxy records from the North Pacific and Southern</figcaption></figure>

**Ocean share similar patterns over the Last Glacial Period.** Various proxies showing similar patterns in (a, d) primary productivity (sedimentary barium and opal concentrations, fluxes, and mass accumulation rates (MAR); purple and pink lines), (b, e) nutrient utilization (δ¹⁵N; green lines), and (c, f) ventilation (benthic foraminiferal δ¹³C; yellow lines) between the (a-c) North Pacific and (d-f) Southern Ocean. Gray shading represents glacial conditions by demarcating marine isotope stages (MIS) 6 and 2-4. Data compiled from the following references (see Supplementary Figure 1 for a map of the sites): ODP 882 (3255 m)³⁵,³⁶; PC13 (2393 m)³⁷; MD2416 (2317 m)³⁸; JPC17 (2209 m)³⁹; SO202-07-06 (2345 m)⁴⁰ (note, “foram-bound δ¹⁵N” refers to Neogloboquadrina pachyderma); GGC27 (995 m)⁴⁴ (Uvigerina spp.); PS75/072-4 (3099 m)³³;
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ODP 1094 (2850 m)²⁶; PC493 (2077 m)⁴⁵ (Cibicidoides wuellerstorfi). The δ¹³C records depict a 3- and 5-point moving average of the cited data in the North Pacific and Southern Ocean records, respectively, and are corrected for whole-ocean δ¹³C change using the sea level record of Lea et al. (2002)⁶⁴ and a deglacial (Last Glacial Maximum to Holocene) whole-ocean δ¹³C change of 0.34‰⁶⁵.
<figure><img src="image_4.png" /><figcaption>Figure 3. Earth System Models simulating a well-ventilated glacial North Pacific show negative anomalies in potential pCO₂ (PCO₂) (μatm) throughout the Pacific basin. PCO₂ (see Methods) anomalies along 160°W resulting from a well-ventilated glacial North Pacific in the (a) UVic⁴⁶, (b) cGENIE¹⁹, (c, d) and LOVECLIM⁴⁶,⁴⁷ Earth System Models. “LOVECLIM-LGM” refers to a simulation meant to provide context of a more realistic glacial scenario with a slightly less extreme freshwater forcing in the Atlantic and resulting ventilation in the Pacific⁴⁷. Anomalies in each case refer to the perturbed (North Pacific-ventilated) simulation minus the control simulation, which was run under identical glacial-like conditions only lacking the freshwater forcing and associated North Pacific Intermediate Water formation and North Pacific ventilation. Red and yellow contours show isolines of σ₂ (potential density referenced to 2000 dbar) from the perturbed simulation in each case, bounding and characterizing the core of the low-PCO₂ water. These σ₂ isolines are meant to approximate the neutral surfaces along which these waters are expected to flow. Following convention, we subtract 1000 kg m⁻³ from all density values provided in this study. Note that we use σ₂ as a qualitative water mass marker, acknowledging that it does not delineate an exact neutral pathway of the water.</figcaption></figure>
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<figure><img src="image_5.png" /></figure>

Figure 4. Negative potential pCO₂ (PCO₂) [µatm] and phosphate (PO₄³⁻) (µmol kg⁻¹) anomalies resulting from North Pacific ventilation pervade across the Pacific basin. PCO₂ (see Methods) anomaly at (a) 45°N and (c) 30°S across the Pacific basin and the same for (b, d) phosphate anomaly. Anomalies refer to the perturbed simulation minus the control simulation (UVic-NP - UVic-ctrl). Red and yellow contours show isolines of σ₂ (potential density referenced to 2000 dbar) from the perturbed simulation (UVic-NP), meant to approximate the neutral surfaces along which these waters are expected to flow.

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<figure><img src="image_6.png" /><figcaption>**Figure 5.** The Southern Ocean surface exhibits negative anomalies in phosphate (PO₄³⁻) (μmol kg⁻¹), potential pCO₂ (PCO₂) (μatm), and air-sea CO₂ flux (mol m⁻² yr⁻¹) as a result of **North Pacific ventilation**. Anomalies resulting from a well-ventilated North Pacific (i.e., UVic-NP - UVic-ctrl) in Southern Ocean surface (a) phosphate (μmol kg⁻¹), (b) CO₂ (see Methods) (μatm), (c) air-sea CO₂ flux (mol m⁻² yr⁻¹), and (d) air-sea CO₂ flux (mol m⁻² yr⁻¹) due only to changes in the water's alkalinity and dissolved inorganic carbon (DIC) between the simulations.

Negative anomalies in air-sea flux indicate reduced CO₂ outgassing or enhanced CO₂ uptake. In</figcaption></figure>
(d), air-sea flux is first calculated in the UVic-NP simulation, and flux anomaly is then calculated as the difference between using control (UVic-ctrl) alkalinity and DIC (with all other variables held at UVic-NP values) and perturbed-simulation (UVic-NP) alkalinity and DIC, thus isolating the flux anomaly signal due only to seawater chemistry changes. Red and yellow contours show isolines of σ₂ (potential density referenced to 2000 dbar) from the perturbed simulation (UVic-NP). σ₂ is plotted at 500m depth here to avoid influence of seasonal mixed layer processes.

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## DATA AVAILABILITY STATEMENT

The Earth System Model output that supports the findings of this study are available from the
corresponding author upon reasonable request.

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## CODE AVAILABILITY STATEMENT

Scripts used to analyze the Earth System Model output for this study are available on
https://github.com/Maddie-Sh.

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## ACKNOWLEDGEMENTS

M.G.S. was supported by a St Leonard's World-Leading Doctoral Scholarship from the University of St Andrews. G.A.M. acknowledges support from UKRI (grant reference MR/W013835/1). W.R.G. and C.dL. acknowledge support from the French national LEFE program through the ROOF project. L.C.M. acknowledges support from the Australian Research Council grant SR200100008.

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J.W.B.R. and A.B. acknowledge funding from NERC (NE/N011716/1).

## AUTHOR CONTRIBUTIONS

JR, AB, GM, and MS designed the study. LM ran the Earth System Model simulations which MS analyzed. MS also compiled all proxy data and wrote the manuscript. All authors contributed to the interpretation of the results and commented on the writing of the manuscript.

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## COMPETING INTERESTS STATEMENT

The authors state that they have no competing interests in the publication of this work.
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