SPECIAL ISSUE FEATURE

WjREENHOUSE AND THE ORLD OCEAN

BY REISHI TAKASHIMA, HIROSHI NISHI, BRIAN T. HUBER, AMD R. MARK LECKIE

Earth's climate has alternated between hurricanes, and enhanced amounts of icant achievements of DSDP and OOP greenhouse (warm) and icehouse (cool) precipitation. Understanding the ocean- research and discusses future prospects modes throughout the Phanerozoic (Fig- climate system during past greenhouse for Integrated Ocean Drilling Program ure 1A). At present, Earth is in the midst climate modes is essential for more accu- (10DP) investigations in the field of Me- of an icehouse climate. Nevertheless, the rately predicting future climate and envi- sozoic paleoceanography. rise of industrialization in the last two ronmental changes in a warming Earth. centuries has led to a dramatic increase The Mesozoic-early Cenozoic is NEW INSIGHTS in atmospheric CO^ from the burning known as a typical greenhouse pe- Determination of Mesozoic Ocean of fossil fuels, which, in turn, has led to riod caused largely by increased CO, Temperature History significant global warming (e.g., Rud- from elevated global igneous activity An important DSDP and ODP achieve- diman, 2000). Global warming could (Figure 1A-C). The mid-Cretaceous is ment was the reconstruction of the his- profoundly impact human life as a result marked by a major warming peak (Fig- tory of Mesozoic ocean temperature of consequent global sea-level rise, more ure 1D); it is characterized by globally changes based on geochemical methods numerous and increasingly powerful averaged surface temperatures more such as oxygen isotopes, TEX^, and al- than 14°C higher than those of today kenone analyses. Oxygen-isotope data Reishi Takashima ([email protected]. (Tarduno et al., 1998), a lack of perma- have provided the greatest source of hokudai.ac.jp) is Research Fellow, Depart- nent sheets (Frakes et al., 1992), and paleotemperature reconstructions from ment of Earth and Planetary Sciences, Hok- ~ 100-200-m-higher than that ancient oceans. However, the increasing kaido University, Sapporo,)apan. Hiroshi of today (Haq et al., 1987; Miller et al., prevalence of diagenctic alteration in Nishi is Associate Professor, Department 2005a) (Figure IE). Studies using Deep older or more deeply buried rocks limits of Earth and Planetary Sciences, Hokkaido Sea Drilling Project (DSDP) and Ocean or prevents reliable isotopic data from University, Sapporo, Japan. Brian T. Huber Drilling Program (ODP) cores have being gleaned from biogenic calcite pre- is Curator, Smithsonian Institution, National advanced understanding of Mesozoic served in terrestrial outcrops. Compared Museum of Natural History, Washington, oceanography and climate, demonstrat- to many land-based sections, calcareous D.C, USA. R. Mark Leckie is Professor, ing that Mesozoic ocean circulation and microfossils of Cretaceous age recovered Department of Ceosciences, University of marine ecosystems differed greatly from from samples drilled at DSDP and ODP Massachusetts, Amberst, MA, USA. those of today. This paper reviews signif- sites are often better preserved, and usu-

82 Oceanography I Vol. 19, No. 4, Dec. 2006 Figure 1. Compilation (G) Percent of world's showing the changes original in climate, and geologi- reserves generated by cal and paleonrologi- source rocks (Klemme cal events through the andlllminshek,1991) Phanerozoic.

(F) Percentage extinction (%) of marine genera (Raup and Sepkoski, 1986) and major Oceanic Anoxic Events 20

(E) Sea level changes and continental glaciation (Ridgwell, 2005)

(D) Temperature (Frakesetal.,1992)

(C) Carbon dioxide Ratio of the mass of atmospheric COj at a past time to that at present (Berner, in press)

(B) Production rate of (Stanley,1999)

(A) Climate mode (Frakes et al, 1992)

i i i i I M i i | i r i i | i i i I I I | I I'I I 0{Ma) 100 200 300 400 500 600

Oceanography I Vol. 19, No. 4, Dec. 2006 83 ally have not been as seriously affected 2003; fenkynsetal., 2004). where the difference of temperature be- by complex tectonic and/or weather- According to isotopic records of sur- tween the mid-Cretaceous and the pres- ing processes. Exquisitely preserved face-dwelling planktic foraminifera, sea ent oceans is nearly 30°C (Figure 2). Bice foraminifera from the low- surface temperatures reached a maxi- and Norris (2002) estimate that at least Demerara Rise (ODP Sites 1258-1261; mum of 42°C at the Demerara Rise (Bice 4500 ppm CO, would be required to 4-15°N), mid-latitude Blake Nose (ODP et al., 2006), 33%: at the Blake Nose (Hu- match the above-mentioned maximum Sites 1049,1050,1052; 30°N), and ber et al., 2002), and 31°C at the Falkland temperatures, which is 11 times greater high-latitude Falkland Plateau (DSDP Plateau (Huber et al., 2002; Bice et al., than the modern atmospheric concen- Site 511; 60°S) in the Atlantic Ocean 2003) during the Turanian (~ 93-89 Ma tration. Using a more recent climate have been especially useful for recon- [million years ago]) (Figures 2 and 3). model, Bice et al. (2006) conclude that structing vertical and latitudinal tem- At comparable in the modern 3500 ppm or greater atmospheric C02 perature gradients of the mid- through ocean, August surface water tempera- concentration is required to reproduce Late Cretaceous ocean (Figures 2 and 3). tures are 25-28°C at 0-20°N, 20-28°C at the estimated maximum sea surface tem-

The TEXS() method is especially useful 20^0°N, and 0-5°C at 60°S (Thurman peratures of the Mesozoic tropical ocean. for organic carbon-rich and and Trujillo, 1999). Consequently, these Because the Mesozoic paleo-tem- has provided excellent paleo-tempera- data suggest that Cretaceous warming perature estimates based on geochemical ture determinations (e.g., Schouten et al., was most prominent at high latitudes proxies are still insufficient in sediments older than Albian (> 112 Ma) and in ar- eas outside of the Atlantic Ocean, further investigations are needed to reconstruct a reliable spatial and temporal tempera- Mid-Cretaceous ture history during the greenhouse cli- 35- sea-su face mate of the Mesozoic. ^ temperature - ? gradient 30 -0' o o Oceanic Anoxic Events Defining the concept of Oceanic Anoxic 25- 8 Events (OAEs) was one of the most im- portant achievements of the early DSDP. 20 o V o m 2L Cretaceous marine sediments in Europe E * V cr are mainly comprised of white lime- i „ 15- * / Present stone and chalk; however, distinct black, " / sea-surface ' temperature laminated organic-rich layers, termed 10 <% ' gradient ,260 390/1049 1257 392 \ "black shales," are occasionally interca- 144 1050 \ fi-533 MS 2 58 511 / 52S356 463,7 '25E 6271052 lated within these sequences (Figure 4). W /' YY W YY \. Y 5 - Because organic carbon is preferentially 1— —I— —I I 1 1 T 1 1 80*S GO'S 40"S 20°S 0 20°N 40° N 60"N 80"N preserved under anoxic conditions, ear- Latitude lier workers suggested that these black

y ODP/DSDP sites Y other core sites shales had accumulated locally in a

Paleo sea surface temperature weakly ventilated, restricted basin under ^Maastrichtian OTuronian DCenomanian Alate Albian regional anoxic conditions. In the mid- 1970s, however, the discovery of black Figure 2. Latitudinal variations of surface ocean paleo temperature derived from oxy- gen isotopes of planktonic foraminifera and TEX^. Modified from Huber etal. (2002), shales at many DSDP drill sites from Bice et a I. (2003), and Jenkyns et al. (2004). the Atlantic, Indian, and Pacific Oceans

84 Oceanography I Vol.19, No. 4, Dec, 2006 led to recognition of widespread anoxic Burial of organic carbon, which pref- trial rocks such as dark gray- to black- conditions in the global ocean spanning erentially sequesters iso topically light colored mudstones, carbon isotope limited stratigraphlc horizons (Figure 5). carbon during OAEs, resulted in a posi- excursions are a useful marker for rec- Schlanger and Jenkyns (1976) termed tive 513C (13C/12C) excursion of 2-3%o ognizing OAEs (e.g., Groke et ah, 1999; these widespread deposit tonal black in the geologic record (Figure 3). Even Takashima et ah, 2004). Recent advances shale intervals "Oceanic Anoxic Events." if black shales are not visible in terres- in biostratigraphy and correlation us-

Bulk Carbon Isotope Ocean Crust Production Paleo-temperature (Huberetal.,2002) [green line} Sea Level and Carbonate Platform (Stanley and Hardie 1998) Blake Plateau S. high latitude Oceanic Anoxic Cm) 2 Drowning [rEd arrow) (km /year) [Site 1049) (Site 511 and 690) Events 0 Oarnonarp 10 14 IB 22 26 30 <*C) 6 10 14 18 22 26 CO J I I I I I J I I I I I l_

• • - % V>

A%

Caribbean Plateau 1 00°0 tfi O O Of

Kerguelen Plateau

I P,aj ma rial Trap ~* Ontong • Benthk foraminifera Q Plankricforaminifera

Parana H Flood

Karoo Flood Basalt

Circum Atlantic L Magmatic Province I I I 4 6 (x10skms) Large Igneous Provinces {Jones and Jenkyns, 2001)

Figure 3. Compilation showing -Cretaceous changes in sea level, oceanic-crusc production, pa I eo-temperature, bulk carbon isotopes, carbonate- platform drowning events and OAEs. Large Igneous Province data are from Jones and Jenkyns (2001), Bulk carbon isotopes are from (1) Van de Schoot- brugge et al. (2005); (2) Hesselbo et ai. (2000); (3) Morettini etal. (2002); (4) Dromart et al. (2003); (S) Weissercet at (1998); (6) Erbacher et al, (1996); (7) Jenkyns et al. (1994); (8) Jarviset ai, (2002); and (9) Abramovich et al. (2003). Carbonate-platform drowning data are from Simo et al. (1993) and Weis- sert and Mohr (1996),

Oceanography I Vol. 19, No. 4, Dec, 2006 85 Figure 4. Cretaceous black shales intercalated in pelagic limestone sequence, central Italy. Figure courtesy of R. Cocci on i.

ing carbon isotopes have revealed thai modern examples for this model. have further consumed dissolved oxygen OAEs occurred at least eight times in the These two models predict differ- in the water column, while simultane- Cretaceous and at least one to four times ent vertical thermal gradient profiles of ously releasing CO, to the atmosphere in the Jurassic (Figure 3). The Toarcian the water column that can be inferred (Gale, 2000; Jahren, 2002). However, be- OAF, Weissert OAE, OAE la, and OAE 2 from the oxygen isotopes of planktic cause there really is no modern analog are global-scale anoxic events associated and benthic foraminifera. For example, for global ocean anoxia, these models with prominent positive excursions of the OAE lb in the earliest Albian (about suffer accordingly. 5UC and worldwide distribution of black 112 Ma) is characterized by a sudden OAEs have had a significant influ- shales (Figure 3). increase in surface water temperatures ence on the evolution and diversity of Two models, that of a stagnant ocean and strengthening of the vertical strati- ancient marine communities through and expansion of the oxygen-minimum fication of the water column (Erbacher the Phanerozoic. Numerous records layer, have been proposed to explain et ah, 2001), suggesting similarity to the demonstrate a high turnover rate of mi- black shale in the OAEs (e.g., Pedersen STO model (Figure 7A). On the other cro fossils at or near OAE intervals (Jarvis and Calvert, 1990). The stagnant ocean hand, the OAE 2 (about 93.5 Ma) shows et al., 1988; Erbacher et al, 1996; Pre- model (STO model) attributes OAEs to sudden warming of deep water and col- moli Silva and Sliter, 1999; Leckie et al., depletion of bottom water oxygen as a lapse of vertical stratification (Huber 2002; Erba, 2004). During the Cenoma- result of dense vertical ocean stratifica- et al„ 1999), which probably induced nian-Turonian (C/T) boundary OAE 2, tion (Figure 6A). A modern analog is enhanced upwelling and productivity for example, anoxic environments ex- seen in stratified silled basins such as the similar to the expanded OMZ model panded from the photic zone (Damste Black Sea. The expanded oxygen-mini- (Figure 78). Deep-water warming may and Koster, 1998; Pan cost et al., 2004) to mum layer model (OMZ model) pro- have contributed to a decrease in oxygen greater than 3500-m depth in the Atlan- poses that increased surface ocean pro- solubility in the deep ocean and may tic Ocean (Thurow et al., 1992), resulting ductivity caused expansion of the oxy- have triggered the disassociation of large in about 20 percent of marine organisms gen-minimum layer in the water column volumes of methane hydrate buried in becoming extinct in various habitats (Figure 6B). Upwelling sites such as the sediments of the continental margins. within an interval of less than one mil- Moroccan and Peruvian margins provide Oxidation of the released methane could lion years (Figure IF). Black shales in the

86 Oceanography Vol. 19, No. 4, Dec. 2006 O TOC Increase in <&&& Mountain Ranges J Shallow Ocean • Black Shale and/or Organic Rich Sediment J LIPs (Pre-Cenomanian) I | Epi-Continental Sea — Mid-Ocean Ridges | LIPs (Cenomanian-Turonian) • Land •*• Subduction Zones • Deep Ocean

Figure 5. Distribution of black shales and/or increased organic carbon sediments at OAE 2. Data are from Schlanger et al. (1987); Arthur etal. (1987, 1988); Jenkyns, (1991); Thurowetal. (1992); Kassab and Obaidalla (2001); Wang et al. (2001); Lebedeva and Zverev (2003); Yurtsever et al. (2003); Coccioni and Lucrani (2005); Fisher et al. (2005); and Takashima and Nishi {unpublished data).

(A) Stagnant ocean model (8) Expanded oxygen minimum layer model

Figure 6. Representative models for black shale deposition: (A) the stagnant ocean model, and (B) the oxygen-mini mum-layer model.

Oceanography I Vol. 19, No. 4, Dec. 2006 87 (A) OAE 1 b (Strengthened water column stratification) (B)OAE 2 {Collapse of water column stratification) Oxygen Isotope

CJ Planktonk Records Foraminifera 5,SOforaminifera(%o)

i i 'i i Hedbergelta ptanispira

m- .

—|— —r 10 14 22 tea shallower habitant Figure 7. Vertical ocean temperature structure, reconstructed 11 from oxygen isotopes, during (A) OAE lb (Erbacher et al., 2001) deeper "5. Z habitant and (B) OAE 2 (Huber et al„ 1999) intervals at the Blake Nose, western North Atlantic. bent hie Fora mini Fere

Black shale

Marl

1 1 1 Limestone — Slump

OAEs, especially OAE la and OAE 2, fre- the black shales provides strong support ing OAEs may have drawn down CO, quently yield no calcareous nan no fossils, for this hypothesis. These proxies further from the ocean-atmosphere by burying planktic foraminifera, or radio la rians, indicate that anoxic conditions occa- organic carbon in black shales, thereby suggesting that anoxic conditions had sionally occurred at very shallow water punctuating long-term global warmth expanded to within the euphotic zone of depths during the C/T OAE. (e.g., Arthur et al., 1988; McElwain et al., the surface water column (e.g., Hart and OAEs also served as an effective ther- 2005). The Late anoxic event Leary, 1991; Coccioni and Luciani, 2005). mostat for the greenhouse Earth, Be- could be an extreme example where Discovery of abundant cyanobacteria cause the change in organic burial in the widespread anoxia caused not only sig- biomarkers (e.g., Kuypers et al., 2004), OAE pelagic sections was two to three nificant bio tic extinction (about 40 per- nonthermophilic archaca (e.g., Kuypers orders of magnitude greater than the cent), but also induced glaciation after et al., 2001), and green sulfur bacteria mean conditions at other time intervals, deposition of black shales (Caplan and (e.g., Dameste and Roster, 1998) within burial of massive organic carbon dur- Bustin, 2001).

88 Oceanography I Vol. 19, No. 4, Dec. 2006 OAEs have benefited human life be- and intra-continental seaways (e.g., Hays an ice-free interval, the mechanisms for cause they are a major cause of the large and Pitmann III, 1973) (Figure 5^ the large and rapid observed sea-level volumes of oil and gas that we consume The most widely cited reconstructions changes during the Cretaceous have long today. These hydrocarbons were derived of past sea-level changes were established been debated (e.g., Skelton et al., 2003). from organic-rich sediments that formed by Exxon Production Research Company Miller and his colleagues demonstrated under anoxic conditions. Indeed, many (EPR) (Haq et al., 1987), which have that several rapid sea-level falls recorded petroleum source rocks were formed since been refined (e.g., Hardenbol et on the New Jersey margin could be ex- during greenhouse warming peaks be- al., 1998). These sea-level plots consist plained only by glacio-eustacy (Miller tween the Middle Jurassic and mid-Cre- of short- (105-106year) and long-term et al., 1999; 2005a). Through integra- taceous (Figure 1G). (10'-10s year) curves that are correlated tion of data on occurrences of ice-rafted with detailed chrono-, bio-, and magne- and/or glacial deposits around polar re- Mesozoic Sea-Level Changes and tostratigraphies for last 250 million years gions, positive oxygen isotope values of the Existence of Ice Sheets (Figure 3). According to the EPR curves, foraminifera, and intervals of rapid sea- Rising sea level attributed to global Late Cretaceous sea level rose as much as level fall, it is quite possible that warming is one of the most serious 260 m above the present level. The EPR waxed and waned during the greenhouse and imminent problems for mankind curves, however, have been criticized climate of the Mesozoic. Although un- because of the concentration of hu- because: (I) the supporting data are certainty remains in age and ice volume, man populations in coastal areas. Fluc- proprietary, (2) the sequence boundar- several geologically short-term glacial tuations in global sea level result from ies do not translate into custatic (glob- events during the Cretaceous have been changes in the volume of the ocean or al) sea-level changes, and (3) inferred proposed (e.g., middle Cenomanian the volume of ocean basins. The former amplitudes of sea-level fluctuations [96 Ma], middleTuronian [91-90 Ma], depends mainly on the growth and de- appear to be conjecture (e.g., Christie- middle Campanian [the Campanian is cay of continental ice sheets over short Blkk et al., 1990). OOP drilling on the from 83.5-70.8 Ma], and earliest and (10"-]0s year) timescales, while the lat- New jersey passive continental margin late Maastrichtian [70.6 and 66.1 Ma]). ter fluctuates on longer (106-107year) (ODP 174AX) provided new insights These results imply that greenhouse pe- timescales resulting from tectonic effects into the amplitudes of, and mechanisms riods can exhibit significant short-term such as variations in seafloor-spreading for, sea-level changes for last 100 million climatic variability, in contrast to previ- rates, ocean-ridge lengths, and collision/ years. The area around the drilling sites ously proposed models suggesting long- break-up of continents (e.g., Ruddiman, is an excellent location for sea-level stud- term stable and equable climates. 2000; Miller et al., 2005a, b). Because ies because of quiescent tectonics and the Mesozoic era exhibited the break-up well-constructed biostratigraphic and Biocakification Crises During the of Gondwana, predominantly ke-free Sr isotopic age control (Sugarman et al., Mesozoic Ocean climates, high rates of seafloor spread- 1995). The proposed sea-level curve de- The Mesozoic is marked by the poleward ing, as well as the emplacement of large veloped using data collected during New expansion of shallow-water carbonate igneous plateaus on the ocean floor Jersey drilling is well correlated with platforms as well as several occurrences (see Coffin et al., this issue), the Meso- those of Russian and EPR curves, but of their global "drowning" or "collapse" zoic ocean was characterized by much the estimated maximum global sea-level events (e.g., Johnson et al., 1996; Simo et higher sea level than at present. Sea amplitude is - 100 m during Late Cre- al., 1993). These drowning events were level peaked in mid- to Late Cretaceous taceous (Miller et al., 2005a) (Figure 3), not due to sea-level rise because shal- (~ 100-75 Ma) when the total land area in contrast to the much greater sea-level low-water carbonate platforms usually flooded was more than 40 percent great- amplitude estimated by EPR. grow upwards much faster than sea level er than at present, resulting in the expan- Because the Mesozoic greenhouse pe- rises. Although eutrophication of surface sion of continental shelf environments riod is generally assumed to have been oceans associated OAEs were considered

Oceanography I Vol.19, No. 4, Dec, 2006 89 to be the cause of these drowning events, al. (2006) estimated that Cretaceous CO; drilling targets. Submerged continental ODP Legs 143 and 144 revealed that atmospheric concentration ranged be- rift sites such as the Somali Basin should some shallow-water carbonate platforms tween 600 and 2400 ppm, 1.5 to 6 times also be targeted as they record a continu- survived during OAE la in the central the present concentration. If the current ous paleoceanographic history from the

Pacific (Wilson et al., 1998). Weissert rate of CO; increase continues, Creta- Early Cretaceous or older. We expect that and Erba (2004) pointed out that the ceous values may be attained within new IODP research from these Creta- coincidence between drowning events 1500-6000 years, but current trends are ceous sites could provide new insights of shallow-water carbonate platforms already having clear affects on both the to the process of abrupt global warming and the crisis of heavily calcified plank- ocean-climate system and the biosphere. and its impact on Earth's biosphere. ton groups, and termed these events In fact, a recent ocean-climate model "biocalcification crises." Although the predicts that rapid atmospheric release ACKNOWLEDGEMENTS mechanism responsible for biocalcifica- of CO will produce changes in ocean We express sincere gratitude to Dr. R. tion crises remains poorly constrained, chemistry that could affect marine eco- Coccioni for providing photographs of recent hypotheses point to elevated systems significantly, even under future black shales in Italy. Extremely helpful pCO^-induced lowered surface ocean pathways in which most of the remain- reviews by two anonymous reviewers pH, which affected carbonate-secreting ing fossil fuel C02 is never released (Cal- and the editorial assistance of Drs. R. organisms (e.g., Lcckie et al., 2002; Weis- deira and Wickett, 2005). Burger and K. Fujioka helped to improve. sert and Erba, 2004). Improved understanding of the Me- the quality of the manuscript, Su\ sozoic ocean-climate system and forma- FUTURE PROSPECTS OF tion of OAEs is important to better pre- REFERENCES THE IODP FOR MESOZOIC dict environmental and biotic changes Abramovich, S., G. Keller, D. Stiiben and Z. Berner. 2003. Characterization (if late Campanian and OCEANOGRAPHY in a future greenhouse world. However, Maastrichtian planktonic foraminiferal depth The greenhouse climate of the mid-Cre- Cretaceous DSDP and ODP cores with habitats and vital activities based on stable iso- taceous was likely related to major global continuous recovery and abundant well- topes. Palaeogeography, Palaeoclimatology, Pal- aeoecology 202:1-29. volcanism and associated outgassing preserved fossils suitable for isotopic Arthur, M.A., S.O. Scblanger, and H.C. Jenkyns. of COr OAEs may be recognized as a study are very limited. To better under- 1987. The Cenomanian-Turonian oceanic an- oxic event, II. Palaeoceanographic controls tin negative feedback in response to sudden stand the ocean-climate dynamics of the organic-matter production and preservation. warming episodes by preventing further Mesozoic greenhouse Earth, a denser Pp. 401-422 in Marine Petroleum Source Rocks, acceleration of warming through remov- global array of deep-sea cores is needed I. Brooks and A. J. Fleet, eds. Geological Soci- ety Special Publication 26. Black-well, Oxford, al of organic carbon from the ocean-at- to provide more detailed reconstructions United Kingdom. mosphere (C02) reservoir to sediment of global climate changes and OCM no- Arthur, M.A..W.E. Dean, and L.M. Pratt. 1988. reservoirs. This process resulted in the graphic conditions. Though far from Geochemical and climatic effects of increased marine organic carbon burial at the Cenoma- emplacement of a large volume of or- complete, the Mesozoic paleoceano- nian/Turonian boundary. Nature 335:714—717. ganic matter during the mid-Cretaceous, graphic record is much better studied in Berner, R,A. In press. GEOCARBSULF: A com- which now serves as a major source of areas of the Atlantic Ocean and Tethys bined model for Phanerozoic atmospheric O, and CO,. Geochimica et Cosmochimica Acta. fossil fuels (Larson, 1991). However, Sea than in the Indo-Pacific Oceans be- Bice, K.L., and R.D. Norris. 2002. Possible atmo- present human activities are rapidly con- cause most seafloor formed there in the spheric CO extremes of the middle Cretaceous suming these fuels, returning the carbon Mesozoic has already been subducted (late Albian-Turonian). Paleoceanography 17:1029/2002PA000778. to the ocean-climate system. Prc-indus- (Bralower et al., 1993). Mesozoic marine Bice, K.L., B.T. Huber, and R.D, Norris, 2003. Ex- trial CO; levels of about 280 ppm have sequences deposited at middle to high treme polar warmth during the Cretaceous increased over the past 200 years to the latitudes of the Pacific Ocean, such as the greenhouse?: Paradox of the Late Turonian W record at DSDP Site 511. Paleoceanography current levels exceeding 380 ppm, main- continental margin of eastern Asia and 18:1029/2002PA000848. ly as a result of human activities. Bice et the Bering Sea, are appropriate future Bice, K.L., D. Birgel, P.A. Meyers, K.A. Dahl,

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