What Fraction of the Pacific and Indian Oceans ’ Deep Water 6 is formed in the North Atlantic ?

In this contribution we explore constraints on the fractions of deep water present in Indian and Pacific Oceans which originated in the northern Atlantic and in the Southern Ocean. Based on PO 4 * we show that if ventilated Antarctic shelf waters characterize the Southern contribution, then the proportions are close to 50–50. If instead a Southern Ocean bottom water value is used, the Southern contribution is increased to 75 %. While this larger estimate may characterize the volume of water entering the Indo-Pacific from the Southern Ocean, it contains a significant portion of entrained northern water. We also note that ventilation may be highly tracer dependent: for instance Southern Ocean waters may contribute only 35 % of the deep radiocarbon budget, even if their volumetric contribution is 75 %. In our estimation, the most promising approaches involve using CFC-11 to constrain the amount of deep water formed in the Southern Ocean.

Remembering Ernst (W.B.) In 1987, Klaus Hasselmann was invited to Lamont-Doherty to present three lectures on climate.The first two dealt with what he referred to as PIPS and POPS.They didn't ring my bell.
But the third one hit home.In it Klaus laid out the distribution of properties generated by Ernst Maier-Reimer's ocean circulation model (Maier-Reimer & Hasselmann, 1987).I was particularly interested in its ability to reproduce the distribution of natural radiocarbon in the ocean.But the plots he showed were at first look incomprehensible.It turned out, that rather than presenting differences from the 14 C to C ratio in atmospheric CO 2, they were referenced to that in mean ocean water.After the lecture, I offered to come to Hamburg to help Maier-Reimer switch to a mode of presentation understandable to those conversant with the 14 C measurements.And so it was I spent three weeks with Ernst probing not only the 14 C to C distribution produced by his model, but also that of O 2 and SiO 2 .For me it was a fantastic learning experience.Not only did Ernst have an amazing mind but he had a knack of teaching by tweaking his model.Thus began a lasting collaboration and friendship.

PO 4 *
This led to an interest in determining the contributions of NADW and AABW to the ventilation of the deep Pacific and Indian Oceans.As the ratio of O 2 utilization to PO 4 release during respiration appears to be nearly constant throughout the ocean's interior (Takahashi et al. 1985;Anderson & Sarmiento, 1994), Broecker and colleagues (Broecker et al., 1985, Broecker et al., 1998)  As only differences between PO 4 * values are of importance, the choice of the constant 1.95 is arbitrary.Hence zero would have been more convenient.Other choices for the O 2 consumption to PO 4 remineralisation ratio are also possible (Hupe & Karstensen, 2000), but have little impact Based on the GLODAPv2 dataset (Key et al., 2015;Olsen et al., 2016) we have reexamined deep ocean PO 4 * distributions.The mean PO 4 * value for deep (>2000 m) Indo-Pacific waters (Figure 2) is 1.42 ±0.04 (1 SD).We select waters below 2000 m as all determinations (Johnson, 2008;Gebbie and Huybers, 2010;Khatiwala et al., 2012) Johnson (2008), Gebbie and Huybers (2010) and Khatiwala et al. (2012).
However if we use the well-ventilated shelf water value of 1.95, the north-south balance is closer to 50:50 (Broecker et al. 1998).This highlights that while the volume flux of what are typically Johnson ( 2008) uses bottom water end member values for AABW, so it is unsurprising that our estimates using a Southern Ocean bottom water value are similar to his.Gebbie & Huybers (2008) and Khatiwala et al. (2010) use surface mixed layer conditions south of the ACC (Orsi et al., 1995), taken from gridded climatologies (WOA, Conkright et al., 1994;WOCE, Gouretski & Koltermann, 2004).As discussed by Gebbie & Huybers (2010), gridded data struggles to capture shelf features and dense overflow waters, and thus excludes the end member values most characteristic of the ventilated Southern Ocean interior (Warren 1981).High adiabatic upwelling rates (Toggweiler & Samuels, 1995;Marshall & Speer, 2012) and deep mixed layers (Gordon & Huber 1990;Dong et al., 2008) may also lead to inclusion of upwelled northern waters in these Southern end members, despite little property modification in the Southern Ocean surface.
These issues may explain why the southern proportions of Gebbie & Huybers (2008) and Khatiwala et al. (2010) are larger than those using the ventilated PO 4 * end member (as in Broecker et al., 1998) and lie close to our estimates using bottom water values.the Southern Ocean's role in climate (Stocker & Johnsen 2003;Marinov et al., 2006;Barker et al., 2009;Sigman et al., 2010;Ferrari et al. 2014).

Ventilation Timescales and the Radiocarbon Budget
The difference between Southern Ocean water mass volume and tracer ventilation is particularly pronounced in the deep radiocarbon budget.Of the 220 moles per year of 14 C undergoing radiodecay in the deep sea, about 20 moles/yr are resupplied by particle rain.As 16 Sverdrups of NADW supply about 130 moles 14 C/yr, this leaves about 70 to be supplied from the Southern Ocean (see Table 2).Ventilation of radiocarbon is thus dominated by the North Atlantic, even if the Southern Ocean contributes greater volume.This is due to 14 C's long equilibration time and the limited exchange time between Southern Ocean surface waters and the atmosphere.Waters upwelled into the surface thus do not reach equilibrium for 14 C and radiocarbon gradients between surface and deep waters are very small (Broecker et al. 1985).
This, along with the presence of 14 C produced by H-bomb testing, also introduces large uncertainty into any attempt to use radiocarbon to quantify the contribution of Southern Ocean waters to the deep Indo-Pacific.The importance of northern versus southern ventilation may thus depend on the tracer and process of interest.

Constraints Based on CFCs
Further insights into SO ventilation may be obtained using CFC data.As with 14 C, the degree of surface water saturation (Schlosser et al., 1991) must be carefully considered if the input flux of CFC tracer is to be converted to a ventilation flux for southern ocean water volume (England et al., 1994).However CFCs have the advantages over 14 C of a much larger surface to deep gradient and faster and less complicated equilibration.CFC-based estimates of the flux of ventilated Southern Ocean water give values of ~15 Sv (Orsi et al., 2002;Schlitzer 2007).This is similar to values for net production of NADW (Broecker et al., 1998;Ganachaud & Wunsch, 2000;Smethie & Fine, 2001), so appears to support roughly equal ventilation of the deep Indo- Pacific between the northern Atlantic and the Southern Ocean (Broecker et al., 1998;Peacock et al., 2000;Orsi et al., 2001).However this does not rule out a much higher water flux from the south (Sloyan & Rintoul, 2001;Lumpkin & Speer, 2007;Talley 2013) -just not full equilibrium with Southern Ocean surface conditions.We also note that if diffusion down isopycnals in the open Southern Ocean is an important contributor to regional ventilation (Abernathy & Ferreira, 2015), this may not be as easily picked up as the CFC signal in shelf waters (Figure 5).The    * was originally obtained by extrapolating the observed PO 4 *temperature trend to sea water's freezing point (Table 1).As shown in Figures 3 and 4 (Warren 1981).This is also picked out by selected depth profiles along these sections (left hand panel), with the black dots showing a profile further out from the shelf edge.Entrainment of low PO 4 * waters in the subsurface reduces southern deep water PO 4 *, from 1.95 on the shelf to ~1.65 at depth.This can also be seen in the histograms in the right hand panel (encompassing larger areas than those shown in the maps and sections), which show two distinct PO 4 * populations in the top 1000 m, which mix to give the more homogenous values at depth.Note that Weddell Sea waters have higher PO 4 * than Ross Sea waters, likely due to less influence of low-PO 4 * NADW and higher local deep water formation rates, elevating PO 4 * throughout this more enclosed basin.Data are from GLODAPv2 (Key et al., 2015;Olsen et al., 2016) with profiles, maps, and sections plotted in ODV (Schlitzer 2015), with sections contoured using isopycnic gridding.

Weddell Sea
Ross Sea  * and CFC-11 through the southern portion of the Southern Ocean.The locations of the profiles in the left hand panel are illustrated with symbols and shown in the inset map: the red circles are from the Weddell Sea, the purple diamonds from the Antarctic margin in the Indian sector, and the black stars from the northern margin of the Ross Sea.In the CFC section the black dashed line indicates CFC-11 concentrations >0.5 pmol/kg and the white dotted line indicates neutral densities >28.3 kg/m 3 ; these criteria, along with depth >1500 m, are used to define the alternative deep Southern Ocean PO 4 * end member.Data are from GLODAPv2 (Key et al., 2015;Olsen et al., 2016) with profiles, maps, and sections plotted in ODV (Schlitzer 2015), with sections contoured using isopycnic gridding.(Key et al., 2015;Olsen et al., 2016) with profiles, maps, and sections plotted in ODV (Schlitzer 2015), with sections contoured using isopycnic gridding.* from the North and high PO 4 * from the South takes place along shared isopycnals, and also diapycnally in the Southern Ocean mixed layer and over rough bottom topography.Data are from GLODAPv2 (Key et al., 2015;Olsen et al., 2016) with profiles, maps, and sections plotted in ODV (Schlitzer 2015).Cross plots show all the data in this section with neutral density greater than 27.2 kg/m 3 ; the colours of the dots refer to the scale shown to the right of the cross plots.Data are from GLODAPv2 (Key et al., 2015;Olsen et al., 2016) with profiles, maps, and sections plotted in ODV (Schlitzer 2015), with sections contoured using isopycnic gridding.

Figure A2:
Indian Ocean hydrographic section for potential temperature, salinity, PO 4 *, and silicate.Cross plots show all the data in this section with neutral density greater than 27.2 kg/m 3 ; the colours of the dots refer to the scale shown to the right of the cross plots.Data are from GLODAPv2 (Key et al., 2015;Olsen et al., 2016) with profiles, maps, and sections plotted in ODV (Schlitzer 2015), with sections contoured using isopycnic gridding.
Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-8Manuscript under review for journal Biogeosciences Discussion started: 11 January 2018 c Author(s) 2018.CC BY 4.0 License.on our global-scale calculations, so we stick with the formulation of Broecker et al. (1998) above.The attraction of PO 4 * as a water mass tracer is that although the deep waters formed in the northern Atlantic range widely in temperature, all the contributors have PO 4 * values close to 0.7 (Figure 1).Further, the deep waters (i.e., >2000 m) in the deep Pacific and Indian Oceans have PO 4 * values close to 1.4.Hence were the PO 4 * for deep waters formed in the Southern Ocean known, the relative amounts of deep water produced in the two source regions could be established.Based on PO 4 * , Broecker et al. (1998) concluded that the deep Pacific and Indian Oceans received about half of their water from the northern Atlantic and half from the Southern Ocean.However,Johnson (2008),Gebbie and Huybers (2010), andKhatiwala et al. (2012), using more complex inversions of multiple tracers, concluded that only about one quarter of this water came from the northern Atlantic.If the ~16 Sverdrups of NADW(Broecker et al. 1998; Ganachaud &     Wunch, 2000;Smethie and Fine, 2001) account for only one quarter of the water ventilating the Indian and Pacific Ocean, then the Southern Ocean must supply about 48 Sverdrups.On the other hand, if half the deep water ventilating the deep sea were produced in the northern Atlantic, then the required Southern Ocean ventilation flux would be reduced to about 16 Sverdrups.
Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-8Manuscript under review for journal Biogeosciences Discussion started: 11 January 2018 c Author(s) 2018.CC BY 4.0 License.considered southern deep waters into the Indo-Pacific may substantially outweigh that of NADW, much of this water is entrained in the subsurface and does not reflect full Southern Ocean ventilation.Differences in the extent to which the Southern Ocean end member is locally ventilated may thus explain much of the difference between the north-south balance obtained by Broecker et al. (1998) versus Johnson (2008), Gebbie & Huybers (2010), and Khatiwala et al. (2010).
At the heart of this discussion lies the issue of what "counts" as ventilated Southern water.Implicit in theGebbie & Huybers (2008)  andKhatiwala et al. (2010)  studies is that any waters reaching the Southern Ocean mixed layer may be considered Southern Ocean waters.However these waters may experience little equilibration with Antarctic surface conditions, including cooling, gas exchange, and nutrient use, depending on their transit time through the Southern Ocean surface and the relaxation time of the tracer of interest.Therefore while they may count in volume fluxes from the Southern Ocean (Talley 2013; Marshall & Speer 2012; Lumpkin & Speer 2007), they may only partially reflect the exchanges of heat and CO 2 key to Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-8Manuscript under review for journal Biogeosciences Discussion started: 11 January 2018 c Author(s) 2018.CC BY 4.0 License.
reason is that low CFC-11 concentrations in a large volume may match high CFC-11 concentrations in a small volume.PO 4 * and the overturning circulation PO 4 * sections, surfaces, and tracer-tracer plots (Figures 7-9 and Supplementary Figures1-6) also highlight patterns of circulation and mixing in the deep ocean.As can be seen, low PO 4 waters themselves are less readily identified by PO 4 * , forming in frontal regions with large nutrient gradients(Talley, 1993;Talley, 1996;Sarmiento et al., 2004), and are better traced by salinity (Figures 9, A1-3).Pacific deep waters returning through the Drake Passage are also hard to identify using PO 4 * , falling in the middle of a PO 4 * mixing gradient between northern and southern waters (Figures 8), and are better identified by their low oxygen and high silicate (Figures A3, A5, A6). Conclusions The use of PO 4 * to constrain the northern and southern contributions to the waters in the deep Indian and Pacific Oceans is highly dependent on the Southern Ocean end member value.Using end members characterizing ventilated Antarctic shelf waters versus Southern Ocean deep waters brackets the NADW contribution to between 50 and 25 % respectively.There is value to both of these estimates: 75:25 may best characterize the ratio of deep Southern Ocean to North Atlantic water volume, while 50:50 better represents the fluxes of well-ventilated waters, as supported by CFC input models.In other words a large volume of the ocean's water experiences some degree of exposure to the Southern Ocean surface, but the volume of those taking on a more completely ventilated Southern Ocean signal is much smaller.Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-8Manuscript under review for journal Biogeosciences Discussion started: 11 January 2018 c Author(s) 2018.CC BY 4.0 License.

Figure 1 .
Figure 1.Plots of PO 4 * versus potential temperature for water formed in the northern Atlantic and in the Southern Ocean (based on measurements made as part of the GEOSECS expeditions).Note that all contributors of NADW have PO 4 * values within the measurement error of 0.75 µmol/kg.The Southern Ocean PO 4* was originally obtained by extrapolating the observed PO 4 *temperature trend to sea water's freezing point (Table1).As shown in Figures3 and 4, this extrapolated value is consistent with values observed close to the Antarctic margin in the Weddell Sea.
Figure 1.Plots of PO 4 * versus potential temperature for water formed in the northern Atlantic and in the Southern Ocean (based on measurements made as part of the GEOSECS expeditions).Note that all contributors of NADW have PO 4 * values within the measurement error of 0.75 µmol/kg.The Southern Ocean PO 4* was originally obtained by extrapolating the observed PO 4 *temperature trend to sea water's freezing point (Table1).As shown in Figures3 and 4, this extrapolated value is consistent with values observed close to the Antarctic margin in the Weddell Sea.

Figure 2 :
Figure 2: End member PO 4 * values for deep North Atlantic waters (blue) and deep Southern Ocean waters (red), along with deep Indo-Pacific waters (yellow).Data are from GLODAPv2 (Key et al., 2015; Olsen et al., 2016) and taken from the regions shown in the inset map.North Atlantic data are >1500 m and have CFC11>0.5 pmol/kg; Southern Ocean data are >1500 m, have CFC11>0.5 pmol/kg, and neutral density >28.3 kg/m 3 (see Figure 4); Indo-Pacific data are >2000 m.Normalised histograms of PO 4 * are shown for each region in the left hand panel, and the corresponding O 2 and PO 4 concentrations on the right, contoured with PO 4 * .

Figure 3 .
Figure 3. PO 4* sections extending out from the Antarctic continent for the Weddell and Ross Seas.As can be seen, water with a value close to 1.95 is descending in a narrow margin-hugging plume.

Figure 4 :
Figure 4: PO 4 * data in the Weddell and Ross Seas from the GLODAPv2 database.Sections (central panel) show high PO 4 * values on the shelves, that descend the continental margin in a narrow plume(Warren 1981).This is also picked out by selected depth profiles along these sections (left hand panel), with the black dots showing a profile further out from the shelf edge.Entrainment of low PO 4 * waters in the subsurface reduces southern deep water PO 4 *, from 1.95 on the shelf to ~1.65 at depth.This can also be seen in the histograms in the right hand panel (encompassing larger areas than those shown in the maps and sections), which show two distinct PO 4 * populations in the top 1000 m, which mix to give the more homogenous values at depth.Note that Weddell Sea waters have higher PO 4 * than Ross Sea waters, likely due to less influence of low-PO 4 * NADW and higher local deep water formation rates, elevating PO 4 * throughout this more enclosed basin.Data are from GLODAPv2(Key et al., 2015;Olsen et al., 2016) with profiles, maps, and sections plotted in ODV(Schlitzer 2015), with sections contoured using isopycnic gridding.

Figure 5 .
Figure 5. Circum Antarctic sections of PO 4* and CFC-11 through the southern portion of the Southern Ocean.The locations of the profiles in the left hand panel are illustrated with symbols and shown in the inset map: the red circles are from the Weddell Sea, the purple diamonds from the Antarctic margin in the Indian sector, and the black stars from the northern margin of the Ross Sea.In the CFC section the black dashed line indicates CFC-11 concentrations >0.5 pmol/kg and the white dotted line indicates neutral densities >28.3 kg/m 3 ; these criteria, along with depth >1500 m, are used to define the alternative deep Southern Ocean PO 4 * end member.Data are from GLODAPv2(Key et al., 2015;Olsen et al., 2016) with profiles, maps, and sections plotted in ODV(Schlitzer 2015), with sections contoured using isopycnic gridding.

Figure 6 .
Figure 6.Rate of deep water formation in the deep Southern Ocean as a function of its concentration of PO 4 * .Also shown is the corresponding fraction of NADW in the water ventilating the deep Pacific and Indian Oceans.As 1.64 and 1.95 µmol/kg represent reasonable limits for the PO 4 * value for deep waters formed in the Southern Ocean, the fraction of Pacific and Indian deep water supplied from the northern Atlantic could be anywhere from 23 to 54 percent.

Figure 7 .
Figure 7. PO 4 * sections for the western Atlantic and for a series of quadrants of the Southern Ocean.Low PO 4 * waters entering the Southern Ocean from the Atlantic and the high PO 4 * waters generated in the Southern Ocean are blended in the Antarctic Circumpolar Current, forming circumpolar deep water.However a PO 4* high at the seafloor and low at ~2000 m continue to trace the influence of AABW and NADW respectively.Data are from GLODAPv2(Key et al., 2015;Olsen et al., 2016) with profiles, maps, and sections plotted in ODV(Schlitzer 2015), with sections contoured using isopycnic gridding.

Figure 8 .
Figure 8. PO 4 * on a section through the Atlantic, Southern, and Pacific Oceans and on the 27.6, 28.0, and 28.11 isopycnal horizons.The depths of these horizons are shown in the section.Mixing of low PO 4* from the North and high PO 4 * from the South takes place along shared isopycnals, and also diapycnally in the Southern Ocean mixed layer and over rough bottom topography.Data are from GLODAPv2(Key et al., 2015;Olsen et al., 2016) with profiles, maps, and sections plotted in ODV(Schlitzer 2015).

Figure 9 :
Figure 9: A global hydrographic section for potential temperature, salinity, PO 4 *, and silicate.Cross plots show all the data in this section with neutral density greater than 27.2 kg/m 3 ; the colours of the dots refer to the scale shown to the right of the cross plots.Data are from GLODAPv2(Key et al., 2015;Olsen et al., 2016) with profiles, maps, and sections plotted in ODV(Schlitzer 2015), with sections contoured using isopycnic gridding.

Figure A4 :
Figure A4: Potential temperature, salinity, PO 4 *, and silicate on the 27.6 isopycnal horizon.The depth of this horizon is shown in Figure8and averages ~1000 m in the basins and is in the mixed layer in the Southern Ocean.Data are from GLODAPv2(Key et al., 2015;Olsen et al., 2016) with profiles, maps, and sections plotted in ODV(Schlitzer 2015).

Figure A5 :
Figure A5: Potential temperature, salinity, PO 4 *, and silicate on the 28.0 isopycnal horizon.The depth of this horizon is shown in Figure 8 and averages ~2500 m in the basins and ~250 m in the mixed layer in the Southern Ocean.Data are from GLODAPv2(Key et al., 2015;Olsen et al., 2016) with profiles, maps, and sections plotted in ODV(Schlitzer 2015).

Figure A6 :
Figure A6: Potential temperature, salinity, PO 4 *, and silicate on the 28.3 isopycnal horizon.The depth of this horizon is shown in Figure8and averages ~4000 m in the basins and ~400 m in the mixed layer in the Southern Ocean.Data are from GLODAPv2(Key et al., 2015;Olsen et al., 2016) with profiles, maps, and sections plotted in ODV(Schlitzer 2015).