LETTERS. ; Glacial greenhouse-gas fluctuations controlled by ocean circulation changes. Andreas Schmittner 1 & Eric D. Galbraith 2

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1 Vol Month 28 doi:.38/nture73 ; Glcil greenhouse-gs fluctutions controlled y ocen circultion chnges Andres Schmittner & Eric D. Glrith 2 Erth s climte nd the concentrtions of the tmospheric greenhouse gses cron dioxide (CO 2 ) nd nitrous oxide (N 2 O) vried strongly on millennil timescles during pst glcil periods. Lrge nd rpid wrming events in Greenlnd nd the North Atlntic were followed y more grdul cooling, nd re highly correlted with fluctutions of N 2 O s recorded in ice cores. Antrctic temperture vritions, on the other hnd, were smller nd more grdul, showed wrming during the Greenlnd cold phse nd cooling while the North Atlntic ws wrm, nd were highly correlted with fluctutions in CO 2. Arupt chnges in the Atlntic meridionl overturning circultion (AMOC) hve often een invoked to explin the physicl chrcteristics of these Dnsgrd Oeschger climte oscilltions 3, ut the mechnisms for the greenhouse-gs vritions nd their linkge to the AMOC hve remined uncler 4 8. Here we present simultions with coupled model of glcil climte nd iogeochemicl cycles, forced only with chnges in the AMOC. The model simultneously reproduces chrcteristic fetures of the Dnsgrd Oeschger temperture, s well s CO 2 nd N 2 O fluctutions. Despite significnt chnges in the lnd cron inventory, CO 2 vritions on millennil timescles re dominted y slow chnges in the deep ocen inventory of iologiclly sequestered cron nd re correlted with Antrctic temperture nd Southern Ocen strtifiction. In contrst, N 2 O co-vries more rpidly with Greenlnd tempertures owing to fst djustments of the thermocline oxygen udget. These results suggest tht ocen circultion chnges were the primry mechnism tht drove glcil CO 2 nd N 2 O fluctutions on millennil timescles. Our model simultes the coupled ocen tmosphere se-ice iosphere system 9 ; it includes dynmic terrestril vegettion nd cron cycle model, nd three-dimensionl ocen generl circultion model with ocen ecosystem dynmics nd cycling of nitrogen, phosphorous, oxygen nd cron. The model uses simple, energylnce tmosphere nd ws run under glcil conditions (Methods). The model is forced y vrying idelized freshwter perturtions to the North Atlntic (Fig. ), mimicking surrounding ice-sheet fluctutions. We note tht the simultions re idelized, in tht the true forcing ehind Dnsgrd Oeschger (D-O) cycles remins unknown. The model forcing is thus ritrry nd ws chosen only to trigger AMOC vritions (Sv) (Sv) (Tg N yr ) College of Ocenic & Atmospheric Sciences, Oregon Stte University, Corvllis, Oregon 9733, USA. 2 Atmospheric nd Ocen Sciences, Princeton University, Princeton, New Jersey 844, USA. Nture nture73.3d 27//8 7::3, 2, 3, AMOC SAT Greenlnd, 2, 3, Time (model yers) Freshwter forcing Atmospheric N 2 O Figure Model simultions of glcil climte nd greenhouse-gs fluctutions. The five model sensitivity runs differ in the length (yellow, 4 yers; green, 7 yers; lue,, yers; nd red,,7 yers) of the simulted stdil (cold phse in Greenlnd)., Time series of (from top) freshwter forcing, AMOC, Greenlnd surfce ir temperture (SAT), ocen (ºC) (p.p..v) 74 P pref /P tot (%) , 2, 3, Preformed nutrients SAT Antrctic Atmospheric CO 2 nomly, 2, 3, Time (model yers) N 2 O production (left scle) nd tmospheric N 2 O concentrtion (right scle)., Time series of glol frction of preformed nutrients (left scle) nd corresponding chnges in tmospheric p CO2 ccording to the simple theory 22, Antrctic SAT, nd tmospheric CO 2 simulted y the complex model. Red lines correspond to the simultion shown in Figs 2 nd (p.p.m.v) (p.p.m.v)

2 NATURE Vol Month 28 In response to the forcing, sinking of North Atlntic Deep Wter (NADW) stops nd the AMOC rpidly spins down from 3 Sv ( Sv 6 m 3 s 2 ) t model yer to lmost Sv fter yers. Five sensitivity experiments hve een conducted to ssess the influence of the durtion of the AMOC oscilltions. In four experiments, the AMOC is switched ck on fter 4, 7,, nd,7 yers, respectively. In one experiment, it remins turned off (lck lines, Fig. ). The modelled climtic response, including rpid cooling in the North Atlntic nd grdul wrming in the Southern Hemisphere, is cused y reduced northwrd het trnsport in the Atlntic, s descried in detil elsewhere,2. It is qulittively consistent with reconstructions ut quntittively underestimtes the surfce ir temperture chnges over Greenlnd nd Antrctic, presumly owing to missing tmospheric dynmics. The experiments were not designed to reproduce ny prticulr oserved event. We choose the time period from to 4 kyr efore present (BP) for the comprison in Fig. 2 ecuse the durtion of the stdil phse corresponds well to one of our experiments. After the AMOC collpse (following yer ), mrine N 2 Oproduction rpidly decreses y 4% to less thn.8 Tg N yr 2 during yer 6 (Fig. ). Figure 3 shows tht N 2 O production decreses lmost everywhere in the glol ocen except for the North Atlntic. The decrese is lrgest in the low-oxygen regions of the estern tropicl Pcific nd the northern Indin Ocen, ut it is lso noticele in the North Pcific nd elsewhere. Decresed productivity 3 nd etter ventiltion of thermocline wters led to incresing susurfce oxygen concentrtions, reducing Indo-Pcific N 2 Oproduction 4.Simulted N 2 O concentrtions co-vry strongly with Greenlnd tempertures with little time lg (, yers), consistent with the pleorecord. Modelled N 2 O mplitudes re 4 prts per illion y volume (p.p..v.), nd lrger over longer stdils. This is in excellent greement with ice-core dt (Fig. 2 inset) nd consistent with the erlier finding tht N 2 O increses re lrger for longer interstdils, which generlly follow longer stdils (see lso Supplementry Note). After the rpid contemporneous increses in simulted Greenlnd tempertures nd N 2 O concentrtions, they remin constnt, in contrst to oservtions, owing to the idelized forcing pplied (Supplementry Fig. 3) nd the lck of tmospheric nd ice-sheet vriility. Despite this difference, the simulted N 2 O concentrtions nd Greenlnd tempertures remin highly correlted, consistent with the oservtions. The mplitude of our simulted N 2 O vriility is much lrger thn tht found in previous study ( p.p..v.) tht used zonlly verged ocen model 8. As noted ove, the simulted N 2 O production chnges show strong zonl structure nd, together with the nonliner dependence of N 2 O production on oxygen concentrtions, suggest tht zonlly verged model leds to systemticlly ised underestimte. Sensitivity tests showed tht the simulted mplitude is insensitive (,%) to prmeter uncertinties within their s rnge of the empiricl N 2 O production eqution 6, nd tht it is only modertely sensitive to the glcil rtio of mrine to terrestril N 2 O production. Decresing this rtio from /3 to /4 cuses only % reduction of the N 2 O mplitude. Methne vritions on millennil timescles testify to chnges in terrestril systems during D-O events 7, which presumly lso ltered N 2 O production rtes to some degree. However, the history of N 2 O chnges recorded in ice cores is mrkedly different from tht of methne, nd the fct tht our model cn reproduce the correct mplitude of the glcil N 2 O vritions shows tht chnges in ocen N 2 O (p.p..v) Nture nture73.3d 27//8 7::4 CO2 (p.p.m.v) δ 8 O ( ) 4 Greenlnd N 2 O CO 2 3 Model NGRIP GRIP 2 Durtion (kyr) Antrctic Ice-core records Figure 2 Exmple of millennil chnges in climte nd greenhouse gses s recorded in polr ice cores from 4 to kyr BP in comprison with model simultion., Ice-core records;, model. Light lue shding indictes the stdil (cold in Greenlnd) phse following D-O interstdil 3 nd immeditely followed y rpid wrming into D-O interstdil 2. This stdil coincides with n event of wide spred ice-rfted deris in North Atlntic sediments (Heinrich event H). In, Greenlnd oservtions include d 8 O (ref. 28; temperture proxy) nd N 2 O concentrtions ; Antrctic records include d 8 O (Dronning Mud Lnd 29, grey, nd Byrd 7, Amplitude (p.p..v) δ8o ( ) δ 8 O ( ) N 2 O (p.p..v) Model Freshwter forcing Age (kyr BP) Model time (kyr) CO 2 (p.p.m.v) Wter flux (Sv) Temperture ( C) Temperture ( C) lck), nd CO 2 (Tylor Dome 3 ) on the ge scle of ref. 24, shifted from the GISP2 ge scle y.8 kyr in order to synchronize with the NGRIP ge scle (Supplementry Informtion). In, model output shows imposed North Atlntic freshwter forcing, temperture chnges over Greenlnd (u 3u W, 68u 78u N), tmospheric CO 2 nd N 2 O, nd surfce ir temperture over Antrctic (68u 78u S). Inset, mplitude of N 2 O chnges versus the durtion of corresponding stdils, from two ice-core records following ref. (lue), nd from four model simultions (ornge).

3 NATURE Vol Month 28 8º N 4º N º N 4º S 8º S 8º N 4º N º N 4º S 8º S 8º N 4º N º N 4º S 8º S c yr yr 6 Difference N 2O production (mg N m 2 yr ) º E º E º W º W sources could hve dominted glcil vriility of tmospheric N 2 O on millennil timescles, oviting the need to invoke lrge chnges in the terrestril N 2 O source,8. The simultions lso resolve puzzling feture of the N 2 O oservtions. Reference notes tht for long D-O oscilltions, N 2 O egins to increse efore the rpid wrming in Greenlnd, suggesting led nd potentilly cusl reltionship for the AMOC resumptions. The simultions show comprle led, ut our experimentl set-up precludes N 2 O effects on climte nd hence N 2 O vritions cnnot influence the model AMOC. Rther, the recovery of N 2 Ois relted to the long-term djustment of the upper-ocen nitrte nd oxygen inventories fter the AMOC collpse. After the initil decrese, upper-ocen nutrient levels nd glol productivity clim fter yer 6 (Supplementry Fig. 3), followed closely y N 2 O production. Most of this increse occurs in the North Atlntic nd Arctic ocens, s nutrient inventories in these sins slowly djust to the ltered circultion. In contrst to N 2 O, simulted CO 2 concentrtions increse slowly y out 2 p.p.m.v., on millennil timescle, fter the AMOC shut down (Fig. ), nd decrese gin only fter the AMOC hs resumed, co-vrying strongly with Antrctic temperture nd ventiltion in the Southern Ocen 9. Despite n increse in terrestril cron on millennil timescle, tmospheric CO 2 increses owing to Nture nture73.3d 27//8 7:: Figure 3 Simulted ocen production of N 2 O., During the interstdil t yer ;, during the stdil t yer 6; nd c, the difference (yer 6 minus yer ). lrger-mplitude decrese in the mrine cron inventory (Supplementry Fig. ). This occurs despite smll increse in the soluility of CO 2 in the ocens 4,9, through much lrger reduction in the glol efficiency of the ocenic iologicl pump, mechnism which cn e most clerly understood through the chnge in the ocen preformed nutrient inventory 2. When surfce wters in regions with high nutrient concentrtions principlly the Southern Ocen sink into the ocen interior, the cpcity of those unutilized (preformed) nutrients to sequester cron vi the iologicl pump goes unrelized (Supplementry Fig. 2). In contrst, when nutrient-depleted wters from the sutropicl Atlntic flow north to sink s NADW, they entrin reltively little preformed nutrients, therey encourging higher efficiency of the iologicl pump 2. A recently proposed theory quntittively links chnges in the tmospheric prtil pressure of CO 2 (p CO2, in p.p.m.v.) to chnges in the preformed frction (P pref /P tot ) of the glol nutrient inventory 22 : Dp CO2 ~32 DP pref =P tot (see Methods). The quntity P pref /P tot is closely correlted to p CO2 in our simultions (Fig. ), incresing from 62% to more thn 72% on millennil timescle. The simple theory yields p CO2 chnges of,3 p.p.m.v., somewht overestimting the chnges simulted y the complex model. The discrepncy rises mostly from simulted chnges in lnd cron, which decreses during the first 2 yers, giving rise to the rpid initil increse in tmospheric CO 2 y p.p.m.v., efore reversing sign to grdully dmp the long term p CO2 increse (Supplementry Fig. ; see Supplementry Informtion for discussion on uncertinties in lnd cron simultions). During the simulted shutdown of the AMOC, two fctors conspire to cuse the decresed efficiency of the iologicl pump, oth of which relte to the volumetric contriutions of wter mss endmemers to the ocen interior. First, diminished input of low-preformed-nutrient NADW to the deep ocen cuses the ocen interior to grdully ecome more dominted y the high-preformed-nutrient wters of the Southern Ocen. Second, wekened Southern Ocen strtifiction, cused y reduced input of slt to the deep wters vi NADW, llows more rpid production of wters with high preformed nutrients 9. Modelled p CO2 vritions (Fig. ) re,2 p.p.m.v. for long D-O oscilltions, consistent with ice-core dt 23,24, nd smller for shorter oscilltions, in greement with previous results 9. The simultion of smll-mplitude p CO2 chnges during short D-O oscilltions, which re likely to hve een more influenced y terrestril processes, cnnot currently e evluted ecuse of the corse time resolution of ville p CO2 dt, ut my e testle in the future s higher resolution dt ecome ville 23. However, the simulted decrese of p CO2 fter the rupt wrming in Greenlnd is clerly fster thn oserved (Fig. 2; see lso ref. 23). This discrepncy points to processes not cptured y the model, such s the impct of wind-shift-induced precipittion chnges on vegettion nd lnd cron storge 4 nd/or ocen sediment interctions, nd indictes tht more work needs to e done in order to fully reproduce the oserved evolution of glcil CO 2 fluctutions on millennil timescles. The simultions point to n importnt mechnistic contrst etween the ocenic control on these two greenhouse gses (CO 2 nd N 2 O), explining how contrsting tmospheric histories,23 cn e generted through unified ocenic process. The prtil pressure of CO 2 grdully increses fter the interstdil stdil trnsition owing to relese of cron from the intermedite nd deep ocen, relted to chnges in the glol effectiveness of the iologicl pump 9. The stdil decrese of N 2 O, on the other hnd, is much fster ecuse it is controlled y djustments of upper-ocen oxygen cycling. Our results emphsize the role of ocen circultion nd iogeochemicl cycling for tmospheric greenhouse-gs concentrtions. Given model projections of slowing AMOC 2, nd suggestions tht the current ocen sink for cron is lredy decresing 26 nd tht the ocen source of N 2 O might increse in the future 9, further progress in understnding ocen iogeochemicl cycles will e required 3

4 NATURE Vol Month 28 in order to refine the quntifiction of climte sensitivity to nthropogenic forcing. METHODS SUMMARY A two-dimensionl (verticlly verged) energy-moisture-lnce tmospheric model, including prescried sesonlly vrying winds, provides thermodynmiclly consistent solution for lnd ocen surfce conditions, without the computtionlly demnding requirements of complex three-dimensionl tmosphere. The model is well tested, nd the ocen iogeochemicl trcer distriutions re in good greement with oservtions when integrted under present-dy conditions 9. In order to improve comprison with the pleorecord, we simulte spects of the glcil climte y prescriing () lnd surfce conditions in the presence of Northern Hemisphere (Lurentide nd Fennoscndin) ice sheets 27 (tht is, ledo, topogrphy, no vegettion), nd (2) decresing the outgoing long wve rdition t the top of the tmosphere y 2.4 W m 22 ( simple pproximtion of reduced tmospheric greenhouse-gs concentrtions). See Supplementry Informtion for detils of the simultion of the glcil ckground climte. Mrine N 2 O production ws clculted from simulted oxygen concentrtions nd oxygen consumption rtes using n empiricl formul (eqution (8) of ref. 6) tht reflects oth nitrifiction nd denitrifiction pthwys of N 2 O production. N 2 O production increses nonlinerly with decresing oxygen concentrtions, with most production in the oxygen-depleted regions of the thermocline in the estern tropicl Pcific nd the northern Indin Ocen (Fig. 3). Terrestril N 2 O production nd the strtospheric sink were ssumed constnt. See Supplementry Informtion for more detils of the N 2 O clcultion. The reminerlized nutrient concentrtion is given y P remi AOU 3 R P:O, where AOU is the pprent oxygen utiliztion (AOU O 2 st 2 O 2 ), O 2 is the dissolved oxygen concentrtion in se wter, O 2 st is the temperture dependent oxygen sturtion concentrtion, nd R P:O is (constnt) phosphorous to oxygen rtio. The efficiency of the iologicl pump cn e expressed s the iologiclly sequestered frction of the totl nutrient inventory, P remi /P tot 2 P pref / P tot. Received 29 April; ccepted 3 Septemer 28.. Broecker, W. S., Peteet, D. R. & Rind, D. Does the ocen-tmosphere system hve more thn one stle model of opertion? Nture 3, 2 26 (98). 2. Rhmstorf, S. Ocen circultion nd climte during the pst 2, yers. Nture 49, (22). 3. Stocker, T. F. Pst nd future reorgniztions in the climte system. Qut. Sci. Rev. 9, 3 39 (2). 4. Menviel, L., Timmermnn, A., Mouchet, A. & Timm, O. Meridionl reorgniztions of mrine nd terrestril productivity during Heinrich events. Pleocenogrphy 23, PA23, doi:.29/27pa44 (28).. Kohler, P., Joos, F., Gerer, S. & Knutti, R. Simulted chnges in vegettion distriution, lnd cron storge, nd tmospheric CO 2 in response to collpse of the North Atlntic thermohline circultion. Clim. Dyn. 2, (2). 6. Mrchl, O., Stocker, T. F. & Joos, F. Impct of ocenic reorgniztions on the ocen cron cycle nd tmospheric cron dioxide content. Pleocenogrphy 3, (998). 7. Scholze, M., Knorr, W. & Heimnn, M. Modelling terrestril vegettion dynmics nd cron cycling for n rupt climtic chnge event. Holocene 3, (23). 8. Goldstein, B., Joos, F. & Stocker, T. F. A modeling study of ocenic nitrous oxide during the Younger Drys cold period. Geophys. Res. Lett. 3, 92, doi:.29/ 22gl48 (23). 9. Schmittner, A., Oschlies, A., Mtthews, H. D. & Glrith, E. D. Future chnges in climte, ocen circultion, ecosystems nd iogeochemicl cycling simulted for usiness-s-usul CO 2 emission scenrio until yer 4 AD. Glo. Biogeochem. Cycles 22, GB3, doi:.29/27gb293 (28).. Meissner, K. J., Wever, A. J., Mtthews, H. D. & Cox, P. M. The role of lnd surfce dynmics in glcil inception: study with the UVic Erth System Model. Clim. Dyn. 2, 37 (23).. Schmittner, A., Senko, O. A. & Wever, A. J. Coupling of the hemispheres in oservtions nd simultions of glcil climte chnge. Qut. Sci. Rev. 22, (23). 2. Crowley, T. J. North Atlntic deep wter cools the southern hemisphere. Pleocenogrphy 7, (992). 3. Schmittner, A. Decline of the mrine ecosystem cused y reduction in the Atlntic overturning circultion. Nture 434, (2). 4. Schmittner, A. et l. Lrge fluctutions of dissolved oxygen in the Indin nd Pcific ocens during Dnsgrd-Oeschger oscilltions cused y vritions of North Atlntic Deep Wter suduction. Pleocenogrphy 22, PA327, doi:.29/26pa384 (27).. Fluckiger, J. et l. N 2 O nd CH 4 vritions during the lst glcil epoch: Insight into glol processes. Glo. Biogeochem. Cycles 8, GB2, doi:.29/ 23GB222 (24). 6. Nevison, C., Butler, J. H. & Elkins, J. W. Glol distriution of N 2 O nd the DN 2 O- AOU yield in the susurfce ocen. Glo. Biogeochem. Cycles 7, 9 (23). 7. Blunier, T. & Brook, E. J. Timing of millennil-scle climte chnge in Antrctic nd Greenlnd during the lst glcil period. Science 29, 9 2 (2). 8. Sowers, T., Alley, R. B. & Juenville, J. Ice core records of tmospheric N 2 O covering the lst 6, yers. Science 3, (23). 9. Schmittner, A., Brook, E. J. & Ahn, J. in Ocen Circultion: Mechnisms nd Impcts Pst nd Future Chnges of Meridionl Overturning (eds Schmittner, A., Ching, J. C. H. & Hemming, S. R.) (Vol. 73, Geophysicl Monogrph Series, Americn Geophysicl Union, 27). 2. Sigmn, D. M. & Hug, G. H. in Tretise On Geochemistry Vol. 6 (eds Hollnd, H. D. & Turekin, K. K.) (Elsevier Science, 23). 2. Toggweiler, J. R. et l. Representtion of the cron cycle in ox models nd GCMs 2. Orgnic pump. Glo. Biogeochem. Cycles 7, 26, doi:.29/ 2GB4 (23). 22. Ito, T. & Follows, M. J. Preformed phosphte, soft tissue pump nd tmospheric CO 2. J. Mr. Res. 63, (2). 23. Ahn, J. & Brook, E. J. Atmospheric CO 2 nd climte on millennil time scles during the lst glcil period. Science 322, 83 8 (28) Ahn, J. & Brook, E. J. Atmospheric CO 2 nd climte from 6 to 3 k BP. Geophys. Res. Lett. 34, L73, doi:.29/27gl29 (27). 2. Schmittner, A., Ltif, M. & Schneider, B. Model projections of the North Atlntic thermohline circultion for the 2st century ssessed y oservtions. Geophys. Res. Lett. 32, L237, doi:.29/2gl24368 (2). 26. Le Quere, C. et l. Sturtion of the Southern Ocen CO 2 sink due to recent climte chnge. Science 36, (27). 27. Peltier, W. R. Glol glcil isostsy nd the surfce of the ice-ge erth: The ICE- G (VM2) model nd GRACE. Annu. Rev. Erth Plnet. Sci. 32, 49 (24). 28. Andersen, K. K. et l. High-resolution record of Northern Hemisphere climte extending into the lst interglcil period. Nture 43, 47 (24). 29. EPICA. One-to-one coupling of glcil climte vriility in Greenlnd nd Antrctic. Nture 444, 9 98 (26). 3. Indermuhle, A. et l. Atmospheric CO 2 concentrtion from 6 to 2 kyr BP from the Tylor Dome ice core, Antrctic. Geophys. Res. Lett. 27, (2). Supplementry Informtion is linked to the online version of the pper t Acknowledgements This study ws funded y the NSF Mrine Geology nd Geophysics progrmme grnt 7283-OCE, nd the NSF Pleoclimte progrmme grnt 6239-ATM. We thnk J. Ahn, E. Brook, R. Toggweiler nd V. Brovkin for discussions nd comments on the mnuscript. Author Contriutions A.S. designed the study nd performed the numericl experiments. E.D.G. performed the comprison with the proxy record. A.S. nd E.D.G. discussed the results nd contriuted eqully to the nlysis of the dt nd writing of the pper. Author Informtion Reprints nd permissions informtion is ville t Correspondence nd requests for mterils should e ddressed to A.S. (schmitt@cos.oregonstte.edu). 4 Nture nture73.3d 27//8 7::

5 NATURE Vol Month 28 Author Queries Journl: Nture Pper: nture73 Title: Glcil greenhouse-gs fluctutions controlled y ocen circultion chnges Query Reference Query AUTHOR: When you receive the PDF proofs, plese check tht the disply items re s follows (doi:.38/ nture73): Figs, 2, 3 (colour); Tles: None; Boxes: None. Plese check ll figures (nd tles, if ny) very crefully s they hve een re-lelled, re-sized nd djusted to Nture s style. Plese ensure tht ny error rs in the figures re defined in the figure legends. For Nture office use only: Lyout % Figures/Tles/Boxes % References % DOI % Error rs % Supp info (if pplicle) % Title % Colour % Acknowledgements % Authors % Text % Author contris (if pplicle) % Addresses % Methods (if pplicle) % COI % First pr % Received/Accepted % Correspondence % Disply items % AOP (if pplicle) % Author corrx % Nture nture73.3d 27//8 7::