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Warm deep-water ocean conveyor during Cretaceous time Bernd J. Haupt Dan Seidov Earth and Mineral Sciences Environment Institute, Pennsylvania State University, University Park, Pennsylvania 16802-6813, USA ABSTRACT Because the deep water is associated with its high-latitude sources, a warm deep ocean during Mesozoic-Cenozoic time is a challenge; there is no feasible physical mechanism that could maintain warm subpolar surface oceans in both hemispheres. The goal of this study is to explore a hypothesis that a warm deep ocean can coexist with relatively cool subpolar (high latitude) sea surface in one hemisphere and a warmer subpolar sea surface in the other. A series of numerical ocean circulation experiments confirms that the ocean meridional circulation can keep the abyssal ocean warm despite the northern subpolar surface water staying relatively cool. Keywords: Cretaceous, Mesozoic, Cenozoic, paleo-oceanography, thermohaline circulation, numerical model. INTRODUCTION Observations and ocean models suggest that the ocean circulation during warm Mesozoic-Cenozoic climates was dramatically different from its present-day pattern (Barron and Peterson, 1989; Bice et al., 1997; Gordon, 1973; Hay et al., 1981; Kennett, 1977; Kutzbach and Ziegler, 1993; Poulsen et al., 1998; Seidov, 1986; Shackleton and Kennett, 1975; see also Barrera and Johnson, 1999). The warm, ice-free Cretaceous Period (65–135 Ma) presents perhaps the most challenging problem, because no consensus exists on what climatic mechanisms could maintain a warm polar climate with very small meridional and vertical thermal gradients in the world ocean. Although meridional oceanic heat transport can be called upon to explain a warm sea surface in the high latitudes (e.g., Schmidt and Mysak, 1996), keeping the high-latitude sea surface at ;10–15 8C in both hemispheres would necessitate substantial equatorially symmetric oceanic poleward heat transport. The assumption of equatorially symmetric high-latitude sea surface temperatures (SSTs) is often used in atmospheric modeling (Sloan and Barron, 1992) and implicitly in data interpretation. However, there are indications that the southern subpolar ocean was warmer than the northern oceans (e.g., Huber and Sloan, 2000). Sloan et al. (1995) argued that increased poleward heat transport is difficult to accomplish in the case of reduced oceanic thermal contrasts. They suggested that atmospheric feedbacks, in conjunction with increased greenhouse gases, might be responsible for warming the poles. A higher CO2 level during the Cretaceous is often chosen to explain the warm equable Cretaceous-Eocene climate (Allen, 1997; Barron et al., 1995; Sloan and Rea, 1995; Sloan et al., 1995; Thomas et al., 2000). Although substantial increase of CO2 is the signature of warm climates (e.g., Barron and Washington, 1985), this increase could not be too strong without overheating the tropics, as the CO2-induced warming occurs everywhere (e.g., Sloan and Rea, 1995). The equatorial regions, however, either were as warm as today or even cooler (Crowley and Zachos, 2000; see also a review by Valdes, 2000). Even an eightfold CO2 increase appears to be insufficient to raise the Earth temperature to the Cretaceous mean values (e.g., Valdes, 2000). The warm deep water is usually associated with high-latitude deep-water sources (see review in Valdes, 2000). However, because the seawater density depends on both temperature and salinity, it may be questioned whether the deep-ocean water temperature (direct geologic evidence) reflects the warm polar surfaceocean regions (a deduced supposition). Dedensification of surface waters shuts off convection and reduces meridional overturning. Reducing the overturning leads to reducing poleward heat transport and vice versa; cooling and/or increase of salinity initiates or strengthens convection and leads to stronger overturning and stronger poleward heat transport. Thus, a compromise can be found in a scenario with a relatively cool high-latitude surface at least in one hemisphere that would allow for a reasonable poleward heat transport and warm enough deep water being produced in the opposite hemisphere. The present-day deep-ocean circulation is driven by two deep-water sources—North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW). Both water masses are characterized by their own distinct temperature and salinity. The NADW, which is slightly warmer, flows southward above the northward-flowing, cooler AABW, forming the well-known layered structure of the abyss. The structure of water masses during warm climates is less well known. However, it may be assumed that the deep-ocean currents are driven similarly to their present-day analogue, as required by the Stommel and Arons (1960) theory. Therefore, it may also be assumed that past ocean circulation was sensitive to highlatitude salinity distribution (e.g., Bryan, 1986; Manabe and Stouffer, 1988; Stocker et al., 1992; see extended references in Seidov and Haupt, 1999). Asynchrony of some of the glacial cycles of the Pleistocene implies that both the southern and northern deep-water sources could be affected by asynchronous meltwater events (Antarctica and North Atlantic show mixed record indicating that either synchrony, or asynchrony could occur, e.g., Blunier et al., 1998; Bond et al., 1997). In some instances, the meridional overturning might have behaved as a bipolar seesaw with a periodicity of hundreds to a thousand years or more (e.g., Broecker, 1998; Seidov and Maslin, 2001; Stocker, 1998). In cold climates, low sea-surface salinity (SSS) in the high latitudes is due mainly to melting of sea ice or icebergs; poleward water vapor transport from the tropics is an important but secondary factor. A high-salinity signal is due to freezing seawater that leads to salt brine rejection (e.g., Gill, 1982). In warm, ice-free climates, poleward water vapor transport or river runoff can be the only causes of a low-salinity signal, whereas increased evaporation (unlikely in high latitudes) might be a cause of increased high-latitude surface salinity. However, in our approach, we can model an impact of temperature increase by changing salinity, because our concern is sea-surface density rather than temperature or salinity. A q 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400. Geology; April 2001; v. 29; no. 4; p. 295–298; 4 figures. 295 Figure 1. A: Reconstruction of land-sea distribution for mid-Cretaceous time. Crosshachured area around Antarctica is circumglobal band of surface ocean with salinity anomalies added in sensitivity tests (see text). Solid line shows position of sections in Figure 4. B: Zonally averaged sea-surface temperature (SST) representing two surface climates: intermediate (solid line) and warm (dashed line) Cretaceous climate scenarios. useful rule of thumb is that the same density increase can be achieved by either increase of salinity by ;1 psu (practical salinity unit) or by decrease of temperature by about 25 8C (Pond and Pickard, 1983). NUMERICAL EXPERIMENTS The Modular Ocean Circulation Model, version 2.2 (e.g., Cox, 1984; Pacanowski, 1996), is used here with a grid resolution of 48 latitude 3 48 longitude and 16 unevenly spaced vertical layers with annual mean SST, SSS, and wind stress. All runs are 2000 model years long, with five-fold acceleration in the deep layers (this means that the deep ocean is effectively run for 10 k.y.). A complete steady state is reached in all numerical experiments. The thermohaline ocean circulation is determined by density distribution and thus is controlled by surface density, which depends on both temperature and salinity. In the Cretaceous scenarios we specify the sea-surface thermal conditions. This requires that we make specific assumptions about the equatorto-pole SST gradients that cannot be changed in sensitivity tests. Here, in order to vary seasurface density, we keep SST unchanged and vary SSS. To test the role of hemispheric asymmetry of the Cretaceous climate, we explore two different climatic scenarios described by Poulsen et al. (1998) and Poulsen (1999). Two different types of Cretaceous surface climates (comprising SST, SSS, and wind stress) were computed in ocean model numerical experiments driven by the GENESIS atmospheric model (Thompson and Pollard, 1997) in Poulsen (1999); model output was provided to us by Poulsen (1999, personal commun.). The landsea distribution (Fig. 1A) and bottom topography are from Poulsen et al. (1998). Although it is called an intermediate Cretaceous scenario, the scenario with a relatively cool subpolar ocean is still a very warm climate, if compared to today’s. The principal 296 difference in this scenario from the ‘‘warm Cretaceous’’ scenario, with temperatures to 20 8C in the subpolar regions, is that the intermediate scenario bears a noticeable southnorth SST asymmetry, the northern subpolar ocean SST being only 6 8C, whereas the southern ocean subpolar SST is 12 8C. The equatorial SST is ;28 8C in the intermediate and 31 8C in the warm scenario (Fig. 1B shows zonally averaged SSTs, and therefore depicts a slightly lower maximum value). To test sensitivity to the density changes in the high latitudes that are known to be the strongest regulator of the ocean global thermohaline conveyor, salinity anomalies are added to the SSS field as circumglobal bands between Antarctica and 608S (crosshachured area in Fig. 1A), the amplitude being varied in different runs from 21 to 11 psu. The perturbed salinity was smoothed in 88 latitudinal bands to the north of the anomaly edge by use of a low-pass filter (Shapiro, 1971) to blend the modified SSS to the unchanged field. The wind stress and SSTs (different in the two scenarios) were held the same as in the control runs in the corresponding scenarios in order to modify only one variable at a time. Note, however, that we do not specify what kind of high-latitude impact could change salinity. Our only goal is to test whether the Cretaceous ocean circulation is sensitive to perturbations of the sea-surface density, i.e., to examine how robust the deep-ocean circulation patterns are, and whether they favor a warm or cold abyssal ocean if one of the poles is relatively cool. Possibly an equivalent temperature change would produce the same changes of the sea-surface density. As mentioned here, we keep SSTs unchanged. Thus, we choose SSS to be the variable controlling density in our idealized perturbation experiments. RESULTS The oceanic heat transport (Fig. 2), if added to other impacts, such as increased CO2 and increased water vapor (e.g., Sloan et al., 1995), seems to be consistent with this moderately warm subpolar sea surface. This is because even moderately cooler high-latitude ocean surface in the intermediate scenario leads to substantial changes of the thermohaline conveyor operation, if compared to the warm case, with strongly intensified southern overturning (see following). Figure 2 shows noticeable cross-equatorial heat transport in the intermediate scenario, whereas there is almost no cross-equatorial heat transport in the warm scenario. It is important that in this scenario the southern and northern deep-water sources trade places in driving the conveyor. The warmer Southern Ocean is saltier and therefore denser than the northern subpolar oceans, and the Drake Passage is nearly closed (200 m deep in this geometry; Poulsen, 1999). At the same time, the northern basins are colder and fresher, and the Tethys passageway prevents formation of an analogue to the present-day Gulf Stream in the Northern Hemisphere (compare the land-sea distribution in Fig. 1A with the present-day ocean geometry). Meridional temperature sections (Fig. 3) along 1208W in the Pacific Ocean (along the meridional arc in Fig. 1A) show the thermal structure in the two scenarios. The intermediate scenario, although less sensitive to additional density changes in the subpolar surface oceans, provides a realistic combination of relatively warm bottom water and warm southern but cold northern subpolar surface ocean. The warm Cretaceous scenario provides even warmer bottom water, but at the expense of unrealistically warm surface ocean in the high latitudes. Thus, the intermediate scenario is consistent with the Southern Hemisphere being warmed by southward crossequatorial oceanic heat transport, and with the warm deep ocean coexisting with a reasonable oceanic heat transport amount. In other words, we argue that a combination of warm deep GEOLOGY, April 2001 Figure 2. Global northward heat flux in PW (1 PW 5 1015 W): intermediate Cretaceous scenario (control run and experiment with low-salinity anomaly [21 psu] in Southern Hemisphere); warm Cretaceous scenario (control run and experiment with low-salinity anomaly [21 psu] in Southern Hemisphere). Figure 4. Sketch of bipolarity of deep-ocean dynamics. A: Present-day ocean; NADW— North Atlantic Deep Water, AABW—Antarctic Bottom Water. B: Intermediate Cretaceous ocean; NHW—Northern Hemisphere water, SHW—Southern Hemisphere water. C: Warm Cretaceous ocean. Note that NA present-day overturning is shown, whereas Cretaceous (B, C) global overturning can be only estimated. Arrowed lines show schemes of global meridional overturning. Figure 3. Meridional temperature sections in Pacific Ocean at 1208W (see Fig. 1). A: Intermediate Cretaceous scenario. B: Warm Cretaceous scenario. GEOLOGY, April 2001 ocean and cool high-latitude ocean surface in one of the hemispheres is possible without sacrificing the main assumption that the Cretaceous ocean was warm in the abyss. Although in the warm scenario the ocean is warm in the abyss (18–21 8C bottom water, Fig. 3B), this scenario presents the major problem for explaining the warm sea surface in the polar regions (surface temperature varies from ;30 8C at the equator to 20 8C at the poles; Fig 1B). Because the equator-topole and top-to-bottom thermal gradients are both very small, the meridional overturning (schematically shown by arrowed lines in Fig. 4), although not that small, produces poleward oceanic heat transport that is too low, insufficient to maintain polar oceans that warm, were the subpolar SST allowed to drift from the specified values (e.g., in an ocean-atmosphere model). The intermediate scenario has a robust circulation pattern undisturbed by the perturbations of the surface salinity, the heat transport being virtually unchanged (Fig. 2), whereas the circulation in the warm scenario indicates a substantial change of the meridional overturning and shows a northward cross-equato297 rial heat transport. This would have produced an asymmetry in surface climatology, where the model would allow SST to drift away from the specified values (e.g., in a coupled oceanatmosphere model). DISCUSSION AND CONCLUSIONS The Cretaceous ocean thermohaline conveyor, although different from its present-day analogue, might have operated similarly to the present-day mode, the only difference being that the amplitude might have been lower than today (because of smaller equator-to-pole density gradients), and the driving deep-water sources may have switched roles, the AABW taking the NADW role in driving the abyssal ocean currents. However, it could also be that geometry-induced north-south asymmetry was a permanent feature responsible for warming the abyssal ocean (Fig. 4). We emphasize that our experiments do not target the details of the Mesozoic-Cenozoic ocean circulations; they are not designed to explain how such warm climates could exist. This far more complex problem can be addressed only by using an advanced coupled ocean-atmosphere model. There must be some value of SSS perturbations that would cause bifurcation and reversing of the conveyor that would lead to cooling of the deep ocean. However, within a reasonable interval of SSS variances, such rebounds were not achieved in the experiments shown here, implying that SST variations must be added for the conveyor to be bifurcated and for warm deep water to be replaced by colder water. That moderately warm deep ocean can easily coexist with moderately warm, or even relatively cool, subpolar oceans removes the restriction on poleward heat transport and allows for the warm deep ocean. Thus, it removes the major paradox of too-warm high latitudes having no plausible physical mechanisms that could maintain such excessive high-latitude warmth. ACKNOWLEDGMENTS We thank Chris Poulsen for providing us with the Cretaceous surface boundary conditions and paleobathymetric data; Lisa Sloan for discussions and her help in writing the manuscript; and Eric Barron for support and valuable comments. The editor, B.A. van der Pluijm, and two reviewers provided very useful comments that helped to substantially improve the manuscript. This study is supported by the National Science Foundation (project 9975107). REFERENCES CITED Allen, P.A., 1997, Earth surface processes: London, Blackwell Science, 404 p. Barrera, E., and Johnson, C.C., editors, 1999, Evolution of the Cretaceous ocean-climate system: Geological Society of America Special Paper 332, 445 p. Barron, E.J., and Peterson, W.H., 1989, Model simulations of the Cretaceous ocean circulation: Science, v. 244, p. 684–686. Barron, E.J., and Washington, W.M., 1985, Warm Cretaceous climates: High atmospheric CO2 as 298 a plausible mechanism, in Sundquist, E.T., and Broecker, W.S., eds., The carbon cycle and atmospheric CO2: Natural variations Archean to present: American Geophysical Union Geophysical Monograph 32, p. 546–553. Barron, E.J., Fawcett, P.J., Peterson, W.H., Pollard, D., and Thompson, S.L., 1995, A ‘‘simulation’’ of mid-Cretaceous climate: Paleoceanography, v. 10, p. 953–962. Bice, K.L., Barron, E.J., and Peterson, W.H., 1997, Continental runoff and early Cenozoic bottomwater sources: Geology, v. 25, p. 951–954. Blunier, T., Chappellaz, J., Schwander, J., Daellenbach, A., Stauffer, B., Stocker, T.F., Raynaud, D., Jouzel, J., Clausen, H.B., Hammer, C.U., and Johnsen, S.J., 1998, Asynchrony of Antarctic and Greenland climate change during the last glacial period: Nature, v. 394, p. 739–743. Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I., and Bonani, G., 1997, A pervasive millennialscale cycle in North Atlantic and glacial climates: Science, v. 278, p. 1257–1366. Broecker, W.S., 1998, Paleocean circulation during the last deglaciation: A bipolar seesaw?: Paleoceanography, v. 13, p. 119–121. Bryan, F., 1986, High-latitude salinity effects and interhemispheric thermohaline circulations: Science, v. 323, p. 301–304. Cox, M., 1984, A primitive equation, 3-dimensional model of the ocean: Princeton University Ocean Group, Geophysical Fluid Dynamics Laboratory Technical Report 1, 250 p. Crowley, T.J., and Zachos, J.C., 2000, Comparison of zonal temperature profiles for past warm time periods, in Huber, B.T., et al., eds., Warm climates in Earth history: Cambridge, UK, Cambridge University Press, p. 50–76. Gill, E.G., 1982, Atmosphere-ocean dynamics: San Diego, Academic Press, 666 p. Gordon, W.A., 1973, Marine life and ocean surface currents in the Cretaceous: Journal of Geology, v. 81, p. 671–684. Hay, W.W., Barron, E.J., Sloan, J.L., and Southam, J.R., 1981, Continental drift and the global pattern of sedimentation: Geologische Rundschau, v. 70, p. 302–315. Huber, M.A., and Sloan, L.C., 2000, Climatic responses to tropical sea surface temperature changes on a ‘‘greenhouse’’ Earth: Paleoceanography, v. 15, p. 443–450. Kennett, J.P., 1977, Cenozoic evolution of Antarctic glaciation, the circum-Antarctic Ocean, and their impact on global paleoceanography: Journal of Geophysical Research, v. 82, p. 3843–3860. Kutzbach, J.E., and Ziegler, A.M., 1993, Simulation of Late Permian climate and biomes with an atmosphere-ocean model: Comparisons with observations: Royal Society of London Philosophical Transactions, ser. B, v. 341, p. 327–340. Manabe, S., and Stouffer, R.J., 1988, Two stable equilibria of a coupled ocean-atmosphere model: Journal of Climate, v. 1, p. 841–866. Pacanowski, R.C., editor, 1996, MOM-2, Documentation, user’s guide and reference manual: Princeton, New Jersey, Geophysical Fluid Dynamics Laboratory–National Oceanographic and Atmospheric Administration, 329 p. Pond, S., and Pickard, G.L., 1983, Introductory dynamical oceanography: Oxford, UK, Pergamon Press, 329 p. Poulsen, C.J., 1999, The mid-Cretaceous ocean circulation and its impact on Greenhouse climate dynamics: University Park, Pennsylvania, Earth System Science Center–College of Earth and Mineral Sciences, 219 p. Poulsen, C., Seidov, D., Barron, E., and Peterson, W., 1998, The impact of paleogeographic evolution on the surface oceanic circulation and the marine environment within the midCretaceous Tethys: Paleoceanography, v. 13, p. 546–559. Schmidt, G.A., and Mysak, L.A., 1996, Can increased poleward oceanic heat flux explain the warm Cretaceous climate?: Paleoceanography, v. 11, p. 579–593. Seidov, D.G., 1986, Numerical modeling of the ocean circulation and paleocirculation, in Hsu, K., ed., Mesozoic and Cenozoic oceans: American Geophysical Union Geodynamics Series, v. 15, p. 11–26. Seidov, D., and Haupt, B.J., 1999, Last glacial and meltwater interbasin water exchanges and sedimentation in the world ocean: Paleoceanography, v. 14, p. 760–769. Seidov, D., and Maslin, M., 2001, Atlantic Ocean heat piracy and the bi-polar climate sea-saw during Heinrich and Dansgaard-Oeschger events: Journal of Quaternary Science, v. 16 (in press). Shackleton, N.J., and Kennett, J.P., 1975, Paleotemperature history of the Cenozoic and the initiation of Antarctic glaciation: Oxygen and carbon isotope analysis in DSDP Sites 277, 279 and 28, in Kennett, J.P., et al., Initial Reports of the Deep Sea Drilling Project, Volume 29: Washington, D.C., U.S. Government Printing Office, p. 743–756. Shapiro, R., 1971, The use of linear filtering as a parameterization of atmospheric diffusion: Journal of Atmospheric Sciences, v. 28, p. 523–531. Sloan, L.C., and Barron, E.J., 1992, A comparison of Eocene climate model results to quantified paleoclimatic interpretations: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 93, p. 183–202. Sloan, L.C., and Rea, D.K., 1995, Atmospheric carbon dioxide and early Eocene climate: A general circulation modeling sensitivity study: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 119, p. 275–295. Sloan, L.C., Walker, J.C.G., and Moore, T.C., Jr., 1995, Possible role of oceanic heat transport in early Eocene climate: Paleoceanography, v. 10, p. 347–356. Stocker, T.F., 1998, The seesaw effect: Science, v. 282, p. 61–62. Stocker, T.F., Wright, D.G., and Broecker, W.S., 1992, The influence of high-latitude surface forcing on the global thermohaline circulation: Paleoceanography, v. 7, p. 529–541. Stommel, H., and Arons, A.B., 1960, On the abyssal circulation of the world ocean, I, Stationary planetary flow patterns on a sphere: Deep-Sea Research, v. 6, p. 140–154. Thomas, E., Zachos, J.C., and Bralower, T.J., 2000, Deep-sea environments on a warm Earth: Latest Paleocene–early Eocene, in Huber, B.T., et al., eds., Warm climates in Earth history: Cambridge, UK, Cambridge University Press, p. 132–160. Thompson, S.L., and Pollard, D., 1997, Greenland and Antarctic mass balances for present and doubled atmospheric CO2 from the GENESIS version-2 global climate model: Journal of Climate, v. 10, p. 871–900. Valdes, P., 2000, Warm climate forcing mechanisms, in Huber, B.T., et al., eds., Warm climates in Earth history: Cambridge, UK, Cambridge University Press, p. 3–20. Manuscript received August 10, 2000 Revised manuscript received December 11, 2000 Manuscript accepted December 21, 2000 Printed in USA GEOLOGY, April 2001