New thoughts about the Cretaceous climate and

Earth-Science Reviews 115 (2012) 262–272

Contents lists available at SciVerse ScienceDirect

Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev

New thoughts about the Cretaceous climate and oceans William W. Hay a,⁎, Sascha Floegel b a b

,1

Department of Geological Sciences, University of Colorado at Boulder, 2045 Windcliff Dr. Estes Park, CO 80517, USA GEOMAR, Helmholtz-Zentrum für Ozeanforschung Kiel, Gebäude Ostufer, Wischhofstr. 1–3, D-24148 Kiel, Germany

a r t i c l e

i n f o

Article history: Received 3 January 2012 Accepted 17 September 2012 Available online 6 October 2012 Keywords: Cretaceous Paleoclimate Paleotemperature Atmospheric circulation Hadley cells

a b s t r a c t Several new discoveries suggest that the climate of the Cretaceous may have been more different from that of today than has been previously supposed. Detailed maps of climate-sensitive fossils and sediments compiled by Nicolai Chumakov and his colleagues in Russia indicate widespread aridity in the equatorial region during the Early Cretaceous. The very warm ocean temperatures postulated for the Mid-Cretaceous by some authors would likely have resulted in unacceptable heat stress for land plants at those latitudes, however, and may be flawed. Seasonal reversals of the atmospheric pressure systems in the Polar Regions are an oversimplification. However, the seasonal pressure differences between 30° and 60° latitude became quite pronounced, being more than 25 hPa in winter and less than 10 hPa in summer. This resulted in inconstant winds, affecting the development of the gyre-limiting frontal systems that control modern ocean circulation. The idea of Hasegawa et al. (2012) who suggest a drastic reduction in the size of the Hadley cells during the warm Cretaceous greenhouse is supported by several numerical climate simulations. Rapid contraction of the Hadley cell such that its sinking dry air occurs at 15° N latitude rather than 30° N is proposed to occur at a threshold of 1000 ppmv CO2 in the atmosphere. This change will probably be reached in the next century. © 2012 Elsevier B.V. All rights reserved.

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2. The Chumakov paleoclimate data and maps . . . . . . . . . 3. The Supercontinent Effect . . . . . . . . . . . . . . . . . 4. The Dead Zone Effect . . . . . . . . . . . . . . . . . . . . 5. Seasonal reversals of high latitude atmospheric pressure systems 6. The location of the descending limbs of the Hadley Cells . . . 7. Possible ‘hothouse’ conditions in the Cretaceous . . . . . . . 8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction

2. The Chumakov paleoclimate data and maps

In 2008 Hay published a summary of the evolution of ideas concerning Cretaceous climate and its causes. Since that paper was prepared, several new discoveries have come to light that suggest that the climate of the Cretaceous may have been, at least at times, more different from that of today than has been previously supposed.

During the late 1980s and early 1990s Nicolai M. Chumakov and colleagues in Russia worked on a ‘Warm Earth Project’ to assemble an extensive database on climate-sensitive sediments and fossils. In 1995 Chumakov described the project, and in 1995 and 2004 the group published a series of six maps showing their interpretation of climate zones for the Berriasian, Aptian, Albian, Cenomanian, Santonian, and Maastrichtian. These maps present significant new information and update the maps of Chris Scotese (www.Scotese.com) and Ronov et al. (1989). The maps have been reproduced in color in Skelton et al. (2003), and are reprinted here with their legend as Figs. 1–7.

⁎ Corresponding author. Tel.:+1 970 586 8698. E-mail addresses: [email protected] (W.W. Hay), sfl[email protected] (S. Floegel). 1 Tel.: +49 431 600 2317. 0012-8252/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.earscirev.2012.09.008

W.W. Hay, S. Floegel / Earth-Science Reviews 115 (2012) 262–272

With one exception the zones recognized by Chumakov et al. (1995) were analogous to those of today: northern and southern high-latitude temperate humid belts corresponding to today's polar regions, northern and southern mid-latitude warm humid belts, northern and southern hot arid belts, corresponding to today's desert regions, and an equatorial humid belt, corresponding to the region of the Intertropical Convergence Zone (ITCZ). All of these appear on the maps of the Albian, Cenomanian, Santonian, and Maastrichtian. The exception is a tropical–equatorial hot arid belt, which replaces the equatorial humid belt and northern and southern hot arid belts on the maps of the Berriasian and Aptian. There appears to be a conspicuous absence of fossil evidence for the wet region under the ITCZ and there is evidence of widespread arid conditions throughout the low latitudes. The idea that the ITCZ could shut down and that little or no rain would fall in the equatorial region on a global basis is untenable. Air must rise over the Earth's thermal equator even if it does not exactly coincide with the geographic equator. Today, the thermal equator is a few degrees north of the geographic equator, reflecting the unequal distribution of land and sea in the northern and southern hemispheres. To replace the rising air, there must be a surficial equatorward flow of warm air from the mid-latitudes picking up moisture from the sea surface. Today, the ITCZ migrates northward and southward with the seasons, but its annual average position is slightly north of the equator. Hay (2008) suggested that without polar ice, there should be a seasonal reversal of atmospheric pressure systems in the Polar Regions. This would lead to an alternation between two and three cell circulation in each hemisphere. Because the northern pole is water, and southern pole land, both hemispheres would have two cell circulation during the northern hemisphere summer and three cell circulation during the northern hemisphere winter. Note that in Fig. 10 in Hay (2008) the months are mis-labeled: ‘January’ should read ‘July,’ and ‘July’ should read ‘January.’ In reality the complexity of the polar geography, particularly in the northern hemisphere, makes a simple evaluation of the reversals difficult. The legend for the Chumakov maps is shown in Fig. 1. The Chumakov maps are shown below in Figs. 2–7. There are two possible explanations for the apparent lack of an equatorial humid zone over land in the Early Cretaceous Berriasian and Aptian maps: (1) the Supercontinent Effect — the continental

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area is so broad that moisture from the oceans cannot penetrate into the interior; or (2) the Dead Zone Effect — that the temperatures are so warm that plant life cannot carry out photosynthesis and is excluded even though moisture is available. Although the maps do not indicate findings of plant fossils, they do show dinosaur occurrences, and the dinosaurs must have had some vegetation to feed on. 3. The Supercontinent Effect The Supercontinent Effect has long been known from studies of the climate of Pangaea (Crowley et al., 1987, 1989; Kutzbach and Gallimore, 1989; Otto-Bliesner, 1999; Fluteau et al., 2001). In the Early Cretaceous, Gondwana was largely still intact, and the distance across the continent in the equatorial region was between 11,000 and 12,000 km, more than four times the width of North America at 40°N today. The maps of the results of general circulation models for the Late Jurassic presented by Sellwood and Valdes (2006) suggest that the equatorial–tropical regions would be only seasonally wet, with year-round rainfall restricted to small areas on the western margin of the continent. What the Chumakov maps show is that much of the Earth was in a supercontinent state from the Carboniferous to the mid-Cretaceous, much longer than most of us have supposed. 4. The Dead Zone Effect The Dead Zone Effect has been suggested as a regional life-limiting mechanism. Extremes of cold and warm temperatures inhibit different life forms. While it is obvious that prolonged freezing temperatures are inimical to both plants and animals, less is known about their ability to withstand higher temperatures. The habitats of mammals are limited by their ability to maintain temperature control over heat stress in hot climates (Sherwood and Huber, 2010). Plants also have problems carrying out photosynthesis under heat stress. As John Ellis commented in Nature (Ellis, 2010): Rubisco is the most important enzyme on the planet — virtually all the organic carbon in the biosphere derives ultimately from the carbon dioxide that this enzyme fixes from the atmosphere. But Rubisco is also one of the most inefficient enzymes on the planet. It evolved when the atmospheric composition was different from

Fig 1. Legend for the Chumakov maps.

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temperature, so that at temperatures between 35 °C and 42 °C this partially offsets the deactivation of the Rubisco itself. Above 42 °C the activity of Rubisco activase declines and the photosynthetic ability of the plant rapidly decreases. This suggests that plant life may not be able to survive in regions where the daytime temperatures are above 42 °C and would be inhibited to a greater or lesser extent at temperatures above 35 °C. It has been suggested that tropical and equatorial temperatures were within this range at least during the warmest part of the mid-Cretaceous. Bice and Norris (2002) and Bice et al. (2006) have suggested that in the late Turonian the temperature of the Atlantic

that of today, and its failure to adapt significantly to the modern atmosphere limits agricultural productivity. Studies of cotton and tobacco leaves by Crafts-Brandner and Salvucci (2000) show that the effectiveness of the critical enzyme for photosynthesis, Ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco) declines when temperatures exceed 35° to 40 °C. However, they found that the overall activity of Rubisco is controlled by another compound, termed Rubisco activase. Under heat stress and without the influence of Rubisco activase, Rubisco itself gradually deactivates and photosynthesis slows. However, the supply of Rubisco activase increases with

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equatorial region (Demerara Rise) may have been as warm as 42 °C. The implication is that temperatures on land would have been even higher. However, temperatures would have been lower in elevated regions, such as the site of the rift system that would later become the South Atlantic. With a sea level temperature of 42 °C and a lapse rate of 6 °C the temperature on 2 km high rift shoulders would have been a more hospitable 30 °C. Fig. 8 shows the general pattern of temperatures during the Cretaceous, with suggested lower and upper limits, along with a widely

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5. Seasonal reversals of high latitude atmospheric pressure systems

Fig. 10 shows our preferred interpretation of the mean annual and seasonal meridional temperature gradients for the warm Earth of the Turonian. Although there might have been some areas on land where heat stress might have affected plant growth, it is more likely that vegetation would have been limited by the availability of moisture.

Simulations of the paleoclimate of the early Turonian, carried out by Sascha Flögel and Robert DeConto using the GENESIS 2.0 general circulation model, show significant changes in the high-latitude atmospheric

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large circum-Arctic drainage area, the small Arctic ‘Ocean’ would have a relatively low salinity, similar to today's Black Sea. The lack of calcareous foraminifera and nannofossils in the Cretaceous sediments of the northern part of the Western Interior Seaway indicates that Arctic ‘Ocean’ salinities were below 33‰ and/or temperatures below 7 °C (Fisher et al. 1994). If the salinity was below 24.7‰ the Arctic ‘Ocean’ would have behaved as a lake, with the temperature of maximum density above the freezing point. This would result in the formation of a relatively thin low-salinity layer, and it is unlikely that this would not freeze during the polar night.

pressure regimes, but not the simple seasonal reversals proposed by Hay (2008, 2009). Detailed maps of seasonal sea level pressures are presented in Flögel (2001). Two factors complicate the high-latitude atmospheric circulation: (1) During the Cretaceous the Arctic ‘Ocean’ was much smaller and fresher than that of today. The Arctic Basin was almost completely surrounded by land, and connected to the world ocean through long shallow meridional seaways (see the paleotectonic–paleogeographic maps of Kazmin and Napatov (1998). Lacking broad and deep connections to the world ocean while receiving fresh-water inputs from a

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SIN LATITUDE Fig 9. Different ideas of meridional temperature gradients and global average temperatures in the Cretaceous. The solid lines (A) represent the present day surface temperature at sea level, corrected for the elevation of Antarctica; note that the modern thermal equator is north of the Equator in response to the hemispheric differences in land and sea. The four major groups of opinions concerning Cretaceous temperatures are: (1) tropical sea surface temperatures were only slightly warmer than today, but polar temperatures were much warmer (5 to 8 °C) — indicated by the fine dotted line (B), except when ice was present (0 to −5 °C) — indicated by the short-dashed line (C); (2) tropical sea surface temperatures were 32 to 34 °C, with very warm polar regions 10 to 18 °C — indicated by the long dashed lines (D); (3) the Bice et al. (2006) proposal that tropical sea surface temperatures were about 42 °C and polar temperatures >18 °C — indicated by the broadly dashed lines (E). The increasing shades of grey at the top of the figure mark the temperature range over which photosynthesis becomes increasingly inhibited.

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SIN LATITUDE Fig 10. Our preferred meridional temperature gradients for the Turonian warm episode, (upper set, D) of Fig. 9, compared with those of the present (lower set, A). Mean annual temperatures are solid lines, JJA averages are dashed lines, and DJF averages are dotted lines. The reddish area at the top of the diagram represents temperatures at which photosynthesis would become increasingly inhibited.

(2) Through much of the Cretaceous the southern hemisphere paleogeography was very different from today. Antarctica was still connected to India and Australia/New Guinea. Except for the Pacific sector, the high-latitude circum-Antarctic Seaway was a relatively narrow passage between South America and southern Africa. These paleogeographies produce complicated seasonal changes in the polar regions, resulting in both latitudinal and longitudinal shifts of the highs and lows with the seasons. However, the major effect at mid-latitudes is the same as might be expected from complete seasonal reversals — major changes in the strength and location of the westerly winds. This is shown in Fig. 11A and B. In Fig. 12 the only complete seasonal reversal is at the very high latitudes (80°–90°) in the northern hemisphere. The southern hemisphere shows large seasonal pressure differences between the land and ocean areas. One consequence of the seasonal reversals of high-latitude pressure systems is the perturbation of the westerly wind systems which drive the ocean circulation into the large, warm-water tropical– subtropical gyres seen today. The westerlies are driven by the pressure difference between 60° and 30° latitude. Fig. 12 shows that for the Late Cenomanian–Early Turonian (ca. 93.5 Ma) simulations these differences are maximal (> 25 hPa) in the winter and minimal (b10 hPa) in the summer. It is the latitudinal changes in the speed of the westerlies that force the ocean waters to sink along and between two bounding frontal systems, the polar front and the subtropical front (see discussion in Hay, 2009). Hay (2009) postulated that the westerly winds become slower and ultimately unstable in response to the shifting locations and strength of the high-latitude atmospheric pressure systems in summer. This effect is apparent in the simulations shown in Fig. 11A and B. Weak, unstable winds would cause breakdown of the well-organized ocean circulation system seen today and its replacement by less well-organized mesoscale eddies.

Recent studies of Cretaceous ocean circulation by Alexandre et al. (2010) confirm the idea of large ocean eddies dominating the circulation. Although their ocean model has a resolution of only 2.5 × 2.5° and is not ‘eddy resolving’ it does show eddy-like features. Their Fig. 5 is a long-term average for the global ocean, showing two general circulation cells with fine structure produced by eddies. For these eddies to show up on a time-averaged plot they must occupy specific sites much of the time. Analogs in the modern ocean would include the ‘Great Whirl’ and ‘Socotra Gyre’ in the Arabian Sea off northeastern Africa first described over a century and a half ago (Findlay, 1886). A detailed analysis of this perennially recurring circulation feature has been presented by Fischer et al. (1996). The Agulhas current eddies passing around the southern tip of Africa are another example of repetitive eddies in a narrow region of the ocean (Gordon and Haxby, 1990). The classic eddy-resolving model of ocean circulation by Semtner and Chervin (1988) shows a number of areas where turbulent flow produces trains of eddies that characterize specific regions in the modern ocean. A recent simulation of the Equatorial Current system in the Pacific, reproduced here in Fig. 13, shows this phenomenon.

6. The location of the descending limbs of the Hadley Cells The recent paper by Hasegawa et al. (2012) suggests shrinkage of the Hadley circulation during the mid-Cretaceous super greenhouse. They were able to locate the site of the descending limb of the Hadley Cells by determining divergent paleowind directions in Cretaceous desert regions of eastern Asia. Their conclusion was that the shift from about 30°N to 15°N occurs as a sudden jump which occurs at a pCO2 threshold level of about 1000 ppmv. This level could be reached in the next century. A jump in the site of the descending limb of the Hadley circulation would result in a period of climate chaos much more severe than anything we have experienced thus far or expected in the near future.


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Fig 11. Average sea level pressure and winds for Late Cenomanian–Early Turonian (ca. 93.5 Ma) simulated using GENESIS 2.0. (A). June–July August (JJA); (B) December–January–February (DJF).

This prompted us to make an inspection of the recent literature on modeling of Cretaceous climates to see if this phenomenon has appeared in other studies. The easiest way to find the location of the

descending limb of the Hadley circulation is from meridional plots of the precipitation. The descending limb corresponds to a precipitation minimum. Several modeling exercises show the minimum near 15°N

Fig 12. Seasonal variations in average sea level pressure for Late Cenomanian–Early Turonian (ca.93.5 Ma) simulated using GENESIS 2.0. Red line is JJA; blue line is DJF. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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and S rather than 30° (Price et al., 1998; Flögel, 2001; Sellwood and Valdes, 2006). A location of the descending limb of the Hadley circulation much closer to the equator than it is today is not only supported by climate modeling, it seems quite reasonable. The surface energy transport by the Hadley circulation is equatorward, not poleward. Much of the energy is carried as latent heat, released in the Intertropical Convergence as the air rises. As Earth warms and the Hadley circulation expands, it must reach a point at which the poleward energy transport becomes so inefficient that the climate switches to a new state, with much narrower Hadley circulation. It would be very important to determine the CO2 levels and temperature gradients involved in this transition. 7. Possible ‘hothouse’ conditions in the Cretaceous The idea that the warm equable climate of the Cretaceous could have been the result of a reversal of the deep ocean circulation so that it would carry warm saline waters formed in the tropics poleward rather than cold saline polar waters equatorward as it does today goes back to Thomas Chrowder Chamberlin (1906). After initial interest the idea received little attention. Then in the 1980s it was revived by Brass et al. (1982a, 1982b) at the Rosenstiel School of Marine and Atmospheric Science in Miami. Again, the idea remained just a suggestion. In the last few years it has been revived, but initially in a different context — to explain the end-Permian extinction (Kidder and Worsley, 2004, 2010). Fischer (1982) had used the terms ‘greenhouse’ and ‘icehouse’ to characterize the two major climatic states of the Earth during the Phanerozoic. Kidder and Worsley proposed that there are actually three basic conditions for Earth's climate: Icehouse, Greenhouse, and Hothouse, and that the Greenhouse can be subdivided into Cool and Warm states. (1) The ‘Icehouse’ has polar ice and alternating glacial–interglacial episodes in response to Milankovitch orbital cycles. The present (interglacial) global average temperature is about 15 °C; during the glacials it is about 6 °C lower. Icehouse climates like that of the Neogene and Quaternary have a strong latitudinal temperature gradient (50–60 °C) inducing high wind velocities, wind shear, and wind erosion. The permanent atmospheric low pressure over the polar ice stabilizes the wind systems and the surface ocean currents they drive. Icehouse climates follow continental collisions which expose large areas of silicate rocks to weathering, enabling reaction with atmospheric CO2 to draw down its concentrations to below 300 ppmv.

The major nutrients required by ocean phytoplankton are phosphate and some fixed form of nitrogen, usually nitrate. Most of the phosphate used by organisms in the photic zone is upwelled from deeper waters. However, the ultimate source of phosphate is from the weathering of silicate rocks on land. Diatomic nitrogen is readily available as it makes up most of the atmosphere. However, to be used by organisms it must be fixed into another form. Most ocean phytoplankton utilize it in the form of nitrate (NO3−). In the ocean the fixation is done by specialized bacteria through an enzyme that requires iron. However, iron is insoluble in alkaline solutions, and seawater is alkaline. Iron is introduced into the ocean as iron oxide coatings on airborne dust and fine-grained clastics. Glacial action and physical weathering under icehouse conditions provide an enhanced supply of dust, and the strong winds carry it out over the ocean. Under these conditions, the phosphate supplied by rivers then becomes the ultimate limiting nutrient in the ocean. The major deep circulation of the ocean is initiated through oxygen-rich, cold, saline water sinking in the Polar Regions. Most of it returns to the surface through broad diffuse upwelling in the tropics, but some works its way upward through intermediate layers to contribute to the upwelling along the eastern margins of the ocean basins in the mid-latitudes. The end result is that the ocean depths are highly oxic and organic productivity in the surface waters is high. Low oxygen levels occur only at mid-depths beneath the areas of very high organic productivity, such as the mid-latitude eastern ocean margin upwelling regions. The region of hypoxic conditions is known as the oxygen minimum layer. Kidder and Worsley (2012) recognize two versions of the greenhouse climate, cool and warm. Both are restricted to times dominated by relative orogenic quiescence, when little silicate rock is available to consume CO2 through weathering. Because of the lower consumption rate, atmospheric CO2 levels are much higher than during the icehouse. (2) The ‘Cool Greenhouse” has some polar ice and alpine glaciers, but no ice streams calving icebergs into the ocean. Global average temperatures are thought to range between 21° and 24 °C. Atmospheric CO2 levels are thought to have been between 2 and 4 times those of today (600–1200 ppmv) (Flögel et al., 2011a). Seasonal changes of polar ice allow for some instability in the high-latitude atmospheric pressure systems. The lesser high to low latitude temperature gradient (ca. 40 °C) means that wind velocities, wind shear, and wind erosive power are all less than in the icehouse. Ocean circulation is more sluggish

Fig 13. The equatorial ocean circulation in the Pacific simulated by the Geophysical Fluids Dynamics (GFDL) model CM 2.6, showing persistent turbulent eddies.


and less well defined The warmer ocean absorbs less oxygen from the atmosphere. The thermohaline circulation slows, but in most areas the supply of oxygen to the deep sea still exceeds its consumption by the rain of organic matter. The oxygen minima can expand, but deep-sea anoxia does not develop. With less dust available and slower winds, the supply of the iron required by nitrogen-fixing bacteria is diminished. This means that the limiting nutrient switches from being phosphorous to nitrate. The cool greenhouse condition would be that prevailing though much of the Early and latest Cretaceous. (3) The ‘Warm Greenhouse’ lacks polar ice. Global average temperatures may have ranged from 24° to 30 °C. Kidder and Worsley (2012) believe that atmospheric CO2 levels ranged between 4 and 16 times present (1200–4800 ppmv). Seasonal reversals of the atmospheric pressure systems at the poles should occur. The lower latitudinal temperature gradients (b 34 °C) mean reduced winds and wind shear, and little wind erosion. The warmer ocean would absorb even less oxygen. Isolated basins could become anoxic. (4) The “Hothouse” condition requires very special conditions and is relatively short-lived (b1 to ca. 3 my). Kidder and Worsley (2010, 2012) believe that this is the result of anomalously large inputs of CO2 into the atmosphere during the formation of Large Igneous Provinces (LIPs) or, in the case of the Paleocene–Eocene Thermal Maximum (PETM), from the massive release of methane from gas hydrates. Atmospheric CO2 concentrations rise above 16 times present (4800 ppmv). Once the oversupply of CO2 ends, weathering rapidly draws atmospheric concentrations back down (Flögel et al., 2011b). They term the condition resulting from these unusual hothouse conditions ‘HEATT episodes:’ (HEATT = haline euxinic acidic thermal transgression). The major difference from the warm greenhouse condition is that a reversal of the ocean thermohaline circulation takes place. In the tropics shallow seas become warm and saline, becoming sources for large amounts of dense ocean bottom water. It should be noted here that this condition is not readily reproduced by present ocean models because processes in marginal seas are poorly represented or neglected. This warm haline water returns to the surface both in the equatorial and in the Polar Regions where it results in anomalous high-latitude warmth. Oxygen in the ocean becomes highly depleted causing widespread euxinic conditions. Euxinic conditions differ from anoxic conditions in the degree of consumption of oxygen in decomposing organic matter. Initially it is the dissolved O2 that is consumed, first producing hypoxic conditions detrimental to aerobic life forms, then anoxic conditions in which essentially all of the free oxygen has been consumed. Oxidation of remaining organic matter then proceeds by using the oxygen in the sulfate ion, resulting in ‘euxinic’ conditions (‘euxinic’ is derived from the Latin name for the Black Sea, Pontus euxinis). This process produces H2S as a byproduct— a gas very poisonous to aerobic life. Furthermore, H2S scavenges iron from the ocean, dropping it to the sea floor in the form of pyrite framboids. This would effectively shut down the nutrient system in the ocean. The very high atmospheric CO2 levels would promote acidification of the ocean, contributing to the demise of many forms of marine life. Finally, as the ocean warms, sea level rises and expands the areas of marginal seas resulting in a powerful positive feedback. HEATT episodes are times of major extinctions. In their 2012 paper Kidder and Worsley cite three possible hothouse episodes in the Cretaceous: the Early Aptian OAE 1a (120 Ma), the Cenomanian Turonian OAE 2 (93 Ma), and the end Cretaceous event (65 Ma). The corresponding LIP emplacements are two phases of Ontong-Java — phase I between 119 and 125 Ma and phase II between 86 and 94 Ma — and the emplacement of the Deccan traps (66 Ma).

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Much depends on what atmospheric CO2 levels actually were during the Cretaceous. Geochemical modeling efforts show possible long-term trends, but not the shorter term variations the Kidder and Worsley hypothesis requires. Data based on samples of fossil plants, ancient soils and other proxies vary greatly. A useful summary of the current state of knowledge is presented in Fig. 6 of Quan et al. (2009), along with a helpful set of references. We find the Kidder and Worsley hothouse scenario attractive because it provides an explanation for the ocean-wide OAEs of the Aptian and Cenomanian–Turonian. 8. Conclusions The detailed maps of climate sensitive fossils and sediments compiled by Nicolai Chumakov and his colleagues in Russia make a convincing argument for widespread aridity even in the equatorial region during the Early Cretaceous. The arguments for very warm ocean temperatures in the MidCretaceous may be flawed. Such warm temperatures would likely have resulted in unacceptable heat stress for land plants at those latitudes. The idea that there should be seasonal reversals of the atmospheric pressure systems in the Polar Regions is an oversimplification. The complex paleogeographies of both Polar Regions in the Cretaceous result in much local variability and instability. However, the seasonal ranges of pressure difference between 30° and 60° latitude become quite pronounced, being more than 25 hPa in winter and less than 10 hPa in summer. This results in inconstant winds and would affect the development of the gyre-limiting frontal systems that control modern ocean circulation. The idea of Hasegawa et al. (2012) that the descending limb of the Hadley circulation may jump from near 30°N to 15°N is supported by several numerical climate simulations. The proposed threshold for the jump is suggested to be around 1000 ppmv CO2 and will probably be reached in the next century. The suggestion by Kidder and Worsley (2010, 2012) that there may be an extreme climate state, the ‘hothouse’ may provide an answer to the enigma of ocean-wide anoxia in the Aptian, Cenomanian–Turonian and end Cretaceous events. Acknowledgments The authors benefitted greatly from discussions with numerous colleagues, including Robert Spicer who called our attention to the RUBISCO problem, Michael Schulz, Poppe de Boer, João Trabucho Alexandre, and Wolf Christian Dullo. Special thanks is due to Nikolai Chumakov and Robert Spicer who granted us permission to reproduce the maps from Skelton et al. (2003) and Chumakov (2004). This work was supported by the German Science Foundation (SFB 754 sub-project A7). References Alexandre, J.T., Tuenter, E., Henstra, G.A., van der Zwan, K.J., van de Wal, R.S.W., Dijkstra, H.A., de Boer, P., 2010. The mid-Cretaceous North Atlantic nutrient trap: black shales and OAEs. Paleoceanography 25, 4201 http://dx.doi.org/10.1029/ 2010PA001925. Bice, K.L., Norris, R.D., 2002. Possible atmospheric CO2 extremes of the warm midCretaceous (late Albian–Turonian). Paleoceanography 17, 1070 http://dx.doi.org/ 10.1029/2002PA000778. Bice, K.L., Birgel, D., Meyers, P.A., Dahl, K.A., Hinrichs, K.-U., Norris, R.D., 2006. A multiple proxy and model study of Cretaceous upper ocean temperatures and atmospheric CO 2 concentrations. Paleoceanography 21 http://dx.doi.org/ 10.1029/2005PA001203. Brass, G.W., Saltzman, E., Sloan, J.L.I.I., Southam, J., Hay, W., Holser, W., Peterson, W., 1982a. Ocean circulation, plate tectonics, and climate. In: Berger, W.H., Crowell, J.C. (Eds.), Climate in Earth History. National Academy Press, Washington, D.C, pp. 83–89. Brass, G.W., Southam, J.R., Peterson, W.H., 1982b. Warm saline bottom waters in the ancient ocean. Nature 296, 620–623. Chamberlin, T.C., 1906. On a possible reversal of deep-sea circulation and its influence on geologic climates. Journal of Geology 14, 363–373.

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Chumakov, N.M., 1995. The problem of the warm biosphere. Stratigraphy and Geological Correlation 3, 205–215. Chumakov, N.M., 2004. Climatic zones and climate of the Cretaceous period. In: Semikhatov, M.A., Chumakov, N.M. (Eds.), Climate in the epochs of major biospheric transformations. Transactions of the Geological Institute of the Russian Academy of Sciences (550), 105–123 (Moscow, Nauka [in Russian]). Chumakov, N.M., Zharkov, M.A., Herman, A.B., Doludenko, M.P., Kalandadze, N.N., Lebedev, E.A., Ponomarenko, A.G., Rautian, A.S., 1995. Climate belts of the MidCretaceous time. Stratigraphy and Geological Correlation 3, 241–260. Crafts-Brandner, S.J., Salvucci, M.E., 2000. Rubisco activase constrains the photosynthetic potential of leaves at high temperature and CO2. Proceedings of the National Academy of Sciences 97, 13430–13435. Crowley, T.J., Mengel, J.G., Short, D.A., 1987. Gondwanaland's seasonal cycle. Nature 329, 803–807. Crowley, T.J., Hyde, W.T., Short, D.A., 1989. Seasonal cycle variations on the supercontinent of Pangaea. Geology 17, 457–460. Ellis, R.J., 2010. Tackling unintelligent design. Nature 463, 164–165. Findlay, A.G., 1886. A Directory for the Navigation of the Indian Ocean. Richard Holmes Laurie, London . (1062 pp.). Fischer, A.G., 1982. Long-term climatic oscillations recorded in stratigraphy. In: Berger, W.H., Crowell, J.C. (Eds.), Climate in Earth History. National Academy Press, Washington D.C., pp. 97–104. Fischer, J., Schott, F., Stramma, L., 1996. Currents and transports of the Great Whirl–Socotra Gyre system during the summer monsoon, August 1993. Journal of Geophysical Research 101 (C2), 3573–3587. Fisher, C.G., Hay, W.W., Eicher, D.L., 1994. An oceanic front in the Greenhorn Sea (Late middle through late Cenomanian). Paleoceanography 9, 879–892. Flögel, S., 2001. On the influence of precessional Milankovitch cycles on the Late Cretaceous climate system: comparison of GCM-results, geochemical, and sedimentary proxies for the Western Interior Seaway of North America: Electronic Doctoral Dissertation, University of Kiel, Germany (http://e-diss.uni-kiel.de/diss_552/) 236 pp. Flögel, S., Wallmann, K., Poulsen, C.J., Zhou, J., Oschlies, A., Voigt, S., Kuhnt, W., 2011a. Simulating the biogeochemical effects of volcanic CO2 degassing on the oxygenstate of the deep ocean during the Cenomanian/Turonian Anoxic Event (OAE2). Earth and Planetary Science Letters 305, 371–384 http://dx.doi.org/10.1016/ j.epsl.2011.03.018. Flögel, S., Wallmann, K., Kuhnt, W., 2011b. Cool episodes in the Cretaceous — exploring the effects of physical forcing on Antarctic snow accumulation. Earth and Planetary Science Letters 307, 279–288. Fluteau, F., Besse, J., Broutin, J., Ramstein, G., 2001. The Late Permian climate. What can be inferred from climate modeling concerning Pangaea scenarios and Hercynian range altitude? Palaeogeography, Palaeoclimatology, Palaeoecology 167, 39–71. Frakes, L.A., 1999. Estimating the global thermal state from Cretaceous sea surface and continental temperature data. In: Barrera, E., Johnson, C.C. (Eds.), Evolution of the Cretaceous Ocean-Climate System. Geological Society of America, Special Paper 332, 49–57. Frakes, L.A., Probst, J.-L., Ludwig, W., 1994. Latitudinal distribution of paleotemperature on land and sea from early Cretaceous to middle Miocene. Comptes Rendus de l'Académie des Sciences, Paris 318 (2), 1209–1218.

Gordon, A.L., Haxby, W.F., 1990. Agulhas eddies invade the South Atlantic: evidence form Geosat altimeter and shipboard conductivity–temperature–depth survey. Journal of Geophysical Research 95 (C3), 3117–3125. Hasegawa, H., Tada, R., Jiang, X., Suganuma, Y., Imsamut, S., Charusiri, P., Ichinnorov, N., Khand, Y., 2012. Drastic shrinking of the Hadley circulation during the midCretaceous supergreenhouse. Climate of the Past 8, 1323–1337 (www.clim-past. net/8/1323/2012/doi:10.5194/cp-8-1323-2012). Hay, W.W., 2008. Evolving ideas about the Cretaceous climate and ocean circulation. Cretaceous Research 29, 725–753. Hay, W.W., 2009. Cretaceous oceans and ocean modeling. In: Hu, X., Wang, C., Scott, R.W., Wagreich, M., Jansa, L. (Eds.), Cretaceous Oceanic Red Beds: Stratigraphy, Composition, Origins and Paleoceanographic and Paleoclimatic Significance. SEPM Special Publication, 91. SEPM (Society for Sedimentary Geology), pp. 243–271 (ISBN 978-1-56576-135-3). Kazmin, V.G., Napatov, L.M. (Eds.), 1998. The Paleogeographic Atlas of Northern Eurasia: Institute of Tectonics of the Lithospheric Plates. Russian Academy of Natural Sciences, Moscow, Russia. Kidder, D.L., Worsley, T.R., 2004. Causes and consequences of extreme Permo-Triassic warming to globally equable climate and relation to the Permian-triassic extinction and recovery. Palaeogeography, Palaeoclimatology, Palaeoecology 203, 207–237 http://dx.doi.org/10.1016/S0031-0182(03)00667-9. Kidder, D.L., Worsley, T.R., 2010. Phanerozoic large igneous provinces (LIPs), HEATT (Haline Euxinic Acidic Thermal Transgression) episodes and mass extinctions. Palaeogeography, Palaeoclimatology, Palaeoecology 295, 162–191 http://dx.doi.org/ 10.1016/j.palaeo.2010.05.036. Kidder, D.L., Worsley, T.R., 2012. Human-induced hothouse climate? GSA Today 22, 4–11. Kutzbach, J.E., Gallimore, R.G., 1989. Pangaean climates: megamonsoons of the megacontinent. Journal of Geophysical Research 94, 3341–3357. Otto-Bliesner, B., 1999. Effects of tropical mountain elevation on the climate of the Late Carboniferous: climate model simulations. In: Crowley, T.J., Burke, K.C. (Eds.), Tectonic Boundary Conditions for Climate Reconstructions. Oxford Monographs on Geology and Geophysics, 39, pp. 100–115. Price, G.D., Valdes, P.J., Sellwood, B.W., 1998. A comparison of GCM simulated Cretaceous ‘greenhouse and ‘icehouse climates: implications for the sedimentary record. Palaeogeography, Palaeoclimatology, Palaeoecology 142, 123–138. Quan, C., Sun, C., Sun, Y., Sun, G., 2009. High resolution estimates of paleo-CO2 levels through the Campanian (Late Cretaceous) based in Ginkgo cuticles. Cretaceous Research 30, 424–428. Ronov, A.B., Khain, V.E., Balukhovsky, A.N., 1989. Atlas of Lithological–Paleogeographical Maps of the World: Mesozoic and Cenozoic of Continents and Oceans. In: Barsukhov, V.L., Laviorov, N.P. (Eds.), Editorial Publishing Group, Moscow, pp. 1–79. Sellwood, B.W., Valdes, P.J., 2006. Mesozoic climates: general circulation models and the rock record. Sedimentary Geology 190, 269–287. Semtner Jr., A.J., Chervin, R.M., 1988. A simulation of the global ocean circulation with resolved eddies. Journal of Geophysical Research 93 (C12) 15, 502–15, 522. Sherwood, S.C., Huber, M., 2010. An adaptability limit to climate change due to heat stress. Proceedings of the National Academy of Sciences 107, 9552–9555. Skelton, P.W., Spicer, R.A., Kelley, S.P., Gilmour, I., 2003. The Cretaceous World. Cambridge University Press, Cambridge, UK. 360 pp.