CRETACEOUS CLIMATE-OCEAN DYNAMICS: FUTURE DIRECTIONS FOR IODP

 

A JOI/USSSP AND NSF SPONSORED WORKSHOP

 

 

CONVENED BY

 

Karen L. Bice, Timothy J. Bralower, Robert A. Duncan, Brian T. Huber,

R. Mark Leckie, and Bradley B. Sageman

 

 

The Nature Place, Florissant, Colorado

July 14-17, 2002

 

 

 

 

 

SESSION CONVENORS

 

1. Cretaceous Climate Record as Revealed by Stable Isotopes - Paul Wilson, Ken MacLeod

2. Biotic Records of Global Change during the Cretaceous - Mark Leckie, Brad Sageman, and Jochen Erbacher

3. Oceanic Anoxic Events: Causes and Consequences - Hugh Jenkyns, Lisa Pratt, Wolfgang Kuhnt

4. Sea Level Record and Mechanisms for Global Eustatic Change - Brian Huber, Mark Leckie, Lonnie Leithold

5. Atmosphere and Ocean Circulation in a Greenhouse World - Karen Bice, Chris Poulsen

6. Environmental and Biotic Consequences of Large Igneous Provinces - Bob Duncan, Tim Bralower

 

INTRODUCTION

 

In recent years, a surge in the amount and quality of paleoclimatic data has renewed interest in Cretaceous climate and ocean dynamics.  New data have provided a more precise picture of paleotemperatures and climate variations, and research on Cretaceous climate has entered an exciting, multidisciplinary phase in which geological, geochemical, geophysical and paleontological data can be integrated between marine and terrestrial realms and into modeling studies, with the goal of better constraining the controls on climate change during intervals of overall warmth. In July, 2002, the JOI-USSSP/NSF-sponsored Workshop on Cretaceous Climate and Ocean Dynamics brought together a mulitnational group of 90 scientists with diverse research interests and expertise (Appendix 1). The conference objective was to summarize the current state-of-the-art in our understanding of Cretaceous paleoclimate and to discuss future priorities. Ocean drilling has been crucial in our advances to date and is critical for the future of research in this field.

 

Social Relevance of Cretaceous Climate Research

Ultimately, our interest in Cretaceous climate stems from the current concern over modern global warming.  Extreme warmth in the middle part of the Cretaceous represents one of the best examples of  "greenhouse" climate conditions in the geological record (Barron, 1983).  Substantial evidence for this warmth includes upper ocean isotopic paleotemperatures of 22-28°C at southern high latitude sites (Huber et al., 1995), bathyal temperatures reaching 20°C in the subtropical North Atlantic (Norris and Wilson, 1998; Fassell and Bralower, 1999; Huber et al., 1999), and a large champsosaurid reptile that was intolerant of freezing conditions discovered at 78°N (Tarduno et al., 1998).  These data suggest that globally averaged surface temperatures in the mid Cretaceous were more than 10ºC higher than today.

Some of the most important earth science questions of our time relate to understanding how human activities may be modifying current and future climates. Will Earth enter another warm climate state due to rising atmospheric greenhouse gas concentrations? If so, what kinds of biota are likely to adapt/evolve and which face extinction? Will a future warm Earth system exhibit climatic and biotic stability or abrupt change and extreme states? Will Earth enter a "permanent" El Niño or will the system exhibit variability that allows for periods of increased/decreased marine productivity? From the standpoint of marine productivity, is upwelling an effective process during a warm climate interval? If not, what is? How much above modern values can tropical sea surface temperatures rise? How accurately do climate models predict the effects of increasing greenhouse gas concentrations? Cretaceous climate studies may be the best key we have to answering these questions.

One of the most critical questions involves how the biosphere will (or will not) adapt to global warming. Because sea level was high, there is a rich terrestrial record for the Cretaceous. This paleontological record serves as a "natural laboratory," an archive of biotic responses to climate change in terms of migration, extinction, adaptation and diversification. Cretaceous research allows us to assess the biotic responses to factors such as warming/cooling, humidity/aridity, and sea level change. The Western Interior Seaway is a particularly useful tableau in which these changes played out because continental impacts are so evident, thereby providing an accessible, high-resolution record with a large signal to uncertainty ratio. The Cretaceous Western Interior Seaway and other epicontinental seas also serve as important archives because the geography of these regions approximates the coastal plain-maritime interface that is today so densely populated and exhibits the fastest rate of population increase.

Of all the past warm climate periods, the Cretaceous may be the one most richly described with respect to its terrestrial fossil and marine chemical records.  These sediments therefore present a special opportunity to investigate how terrestrial and marine environments are coupled on a warmer Earth. Because Cretaceous climate was far from stable, with evident precessional and sub-precessional cycles in both terrestrial and marine records, we have the opportunity to examine climate change within a warm world on several time scales. Cretaceous climate data can therefore help to inform the public about both near- and long-term possible effects of anthropogenic climate changes. This perspective is simply not available in modern and historical records. Although the Cretaceous can not serve as a direct analog for a future greenhouse Earth (because of the very different land-sea configuration), it is clear that Cretaceous sediments may hold the best record with which to improve our understanding of climate variability and biotic responses to change on an overall warm Earth.

 

Challenges in Cretaceous Climate Research

One of the major challenges facing researchers is that most coupled and uncoupled ocean-atmosphere models underestimate the polar warmth that is indicated by mid Cretaceous fossil and geochemical data (e.g., DeConto et al., 2000), especially for Cenomanian-Turonian time when the latitudinal thermal gradient may have been at its lowest.  This failure suggests deficiencies in our ability to properly interpret the warm climate data record, our understanding of greenhouse climate dynamics, and/or deficiencies in the models. In cases where the latter is true, the immediate lesson relevant to future climate change is that these models--the same ones used in much modern climate research--are dramatically underestimating potential future polar warming. Where different models exhibit different greenhouse gas sensitivities, it is only by comparing these models to paleoclimate data that we can begin to say which extreme in CO2 sensitivity may be more reasonable than another. Model-data discrepancies have also highlighted the importance of accuracy in regional modeling and helped us to improve our ability to model the effects of elevation and vegetation on local and regional climates. While the paleoclimate community is challenged by model-data discrepancies, it is also uniquely positioned to evaluate the reliability of models used in future climate research when atmospheric conditions differ from the modern.

While the mid Cretaceous had an extreme warm climate, other times during the Cretaceous, such as the Aptian and Maastrichtian, were characterized by cooler deep water temperatures and possible ice-sheets in East Antarctica.  Evidence for marine regression suggests glacio-eustatic control of sea level during the mid-Maastrichtian (Miller et al., 1999).  Geochemical evidence has also been cited for continental glaciation during parts of the Early and Late Cretaceous (Stoll and Schrag, 1996, 2000). The recognition that the Cretaceous was not a long interval of stable, "equable," warm climate allows us to investigate climate variability on a variety of timescales using Cretaceous records. The long term overall warming trend from the Aptian to the mid Cretaceous extreme warmth, followed by an overall cooling trend to the Maastrichtian is the longest timescale variation to be explained. On shorter time scales, considerable effort is devoted to understanding what drove periods of dysoxic and anoxic marine conditions or oceanic anoxic events (OAEs)--stagnant intervals that corresponded to pulses of extinction and evolution of marine nekton and plankton (e.g., Fischer and Arthur, 1977; Bralower and Thierstein, 1984; Elder, 1991; Erba, 1994; Bralower et al., 1994). On even shorter timescales, we need to explain dramatic variations in subtropical oceans at the precessional scale (Wilson and Norris, 2001).

The Cretaceous was a time of unusually high rates of production of oceanic crust both at spreading centers and through the eruption of Large Igneous Provinces (LIPs) (e.g., Larson 1991; Tarduno et al., 1991; Coffin and Eldholm, 1994) such as Ontong Java and Kerguelen plateaus.  LIPs represent exceedingly large (> 105 km3) outpourings of predominately basaltic magma. The Ontong Java Plateau, for example, represents more than 50 million km3 of basaltic magma extruded onto the seafloor to form a 30 km thick plateau encompassing an area equal to one third of the contiguous United States.  Events of this magnitude are unknown to human experience, but the consequences must have been dramatic.  The release of gases (e.g., CO2, SO2, Cl, F, H2O) from Earth’s interior accompanying such great eruptions likely had significant consequences for the composition of the ocean and atmosphere, affected the evolution and extinction of terrestrial and marine biota (Larson, 1991; Larson and Erba, 1999; Tarduno et al., 1998; Kerr, 1998), and may have played a primary role in oceanic anoxic events.  The mechanisms linking volcanic activity, extinction/evolution and OAEs are, however, yet to be determined.  Recent and future drilling efforts will lead to a firmer understanding of the age and evolution of several LIPs, and a clearer picture of the emplacement mechanism and potential environmental perturbations.

The most significant remaining questions regarding the dynamics and forcing of Cretaceous climate and oceans include: (1) obtaining an accurate reconstruction of Cretaceous climate from proxy records, (2) understanding the mechanics of turnover in biotic communities, (3) determining mechanisms for abrupt and gradual changes in oceanic circulation and triggers for oceanic anoxic events, (4) understanding the controls on eustatic sea level change, and (5) understanding the mechanisms by which oceanic magmatism, especially LIP events, affect ocean and atmospheric chemistry, and thus life history. 

 

Workshop Structure

The challenges and questions outlined above formed the basis of six plenary sessions and keynote addresses (Appendix 2) that served as an overview of our current understanding of Cretaceous climate. On the third day of the meeting, the group went into the field to view evidence of Cenomanian-Campanian tectonic-eustatic marine cycles exposed in the Rock Canyon Anticline near Pueblo, Colorado. The final day of the meeting included break-out group discussions designed to identify the critical questions in four areas and future research directions needed to address these questions. The following report is organized by thematic plenary sessions with a summary of the presentations followed by an overview of remaining questions and avenues for future research.  Finally, we provide a list of drilling targets that will help achieve these goals. Where sources are cited with no publication year given, the reference is to a workshop abstract (2002, Available on the World Wide Web: http://cis.whoi.edu/science/GG/ccod/searchAbstracts.cfm). Another outcome of the meeting is a community web site, which continues to be updated in order to facilitate communication regarding meetings and future drilling. The original workshop web page for the Florissant meeting can be accessed through this site.

 

SCIENTIFIC OBJECTIVES FOR POST-2003 DRILLING

 

Cretaceous Climate Record as Revealed by Stable Isotopes

 

Stable Isotopes Figures with Captions                                 Return to top of Report

 

Overview

Estimating paleotemperatures continues to be a fundamental aspect of Cretaceous isotopic studies. Analyses of exquisitely preserved foraminifera (those that exhibit "glassy" preservation) have invigorated these efforts (Norris et al., Figure 1-1). Glassy Turonian foraminifera from Demerara Rise in the western tropical Atlantic yield the warmest (up to 36°C) sea-surface temperatures (SSTs) yet reported for the entire Cretaceous-Cenozoic (Wilson et al., 2002). These findings support the hypothesised "Cretaceous greenhouse" and strengthen the case for a Turonian Cretaceous thermal maximum (KTM), thereby highlighting a 20–40 m.y. mismatch between peak Cretaceous-Cenozoic global warmth (and sea level) and peak inferred tectonic CO2 production. This mismatch most likely represents either an artefact of an as yet unidentified Turonian pulse in global ocean-crust cycling, or the influence of other factors, such as CaCO3 subduction (Wilson et al., 2002) or ocean gateway change (Poulsen et al., 2003). These results from the tropics have prompted Bice et al. (in review) to re-examine material used to estimate contemporaneous high-latitude SSTs (e.g. DSDP Site 511, Falkland Plateau). In the absence of pronounced regional perturbation from mean Cretaceous sea water d18O (assumed –1‰ SMOW), d18O data from planktonic foraminiferal calcite at this site are indicative of extreme high latitude warmth during the Turonian (up to 32°C, Figure 1-2). Such extreme high latitude warmth seems difficult to explain in the absence of extreme greenhouse forcing (pCO2, 4500 to 7500 ppmv; Bice and Norris, 2003). Yet textural observations, interspecific isotopic offsets, pore water chemistry, and burial history all argue against a diagenetic explanation.  Salinites as low as 27 psu are required to explain the data as a result of freshwater input, but such brackish conditions are inconsistent with the microfossil assemblages.

MacLeod and Huber show that the concept of globally similar climate responses is not borne out by a d18O time series from three sites on Blake Nose in the western North Atlantic. Although the 5-6 million year long Maastrichtian stage is generally thought to have been a time of widespread cooling, there was 4-6°C of apparent warming at Blake Nose (Figure 1-3). Theoretical models and empirical data show that these estimates are conservative because diagenesis was acting in the opposite direction to the trend observed.  Further, low-resolution data from other sites suggests that Maastrichtian warming can be seen throughout the North Atlantic and perhaps extended into Tethys (but not into the Pacific). Thus, ocean basin scale factors that influence regional heat distribution must have played an important role during the Cretaceous. Such regional controls are clearly important today but have yet to be included in Cretaceous studies. If we are going to address these complexities, though, we need data with better temporal and spatial resolution, as well as an improved ability to separate temperature, seawater d18O (dw), and diagenetic effects on carbonate d18O. Clarke and Schouten et al. discuss efforts to separate temperature and dw effects on d18O values using preliminary Mg/Ca data and a new organic-based paleothermometer, respectively.

Another area of Cretaceous isotope studies that has undergone recent re-birth is the quest to improve our understanding of the nature and cause of oceanic anoxic events. Jenkyns and Tsilkos showed that both d15N of bulk samples and total organic carbon exhibit positive shifts in Cenomanian-Turonian black shales. A similar correlation is observed in Quaternary upwelling areas and in Toarcian (Jurassic) black shales and point to elevated productivity and denitrification in the water column over the oceanic anoxic event.  Further, examination of Cenomanian/Turonian boundary sections globally indicates that the original concept of oceanic anoxic events (OAEs), involving global stratigraphically synchronous black shales, breaks down at high-resolution (Figure 1-4). Therefore, we need to move from a lithostratigraphic definition of OAEs to a chemostratigraphic one. d13C records continue to both provide the lynch-pin in this regard and to be interpreted in terms of global partitioning of carbon between reduced and oxidized reservoirs. Pronounced d13C excursions appear to provide reliable markers even though various factors (e.g. palaeoecology, diagenesis and local carbon cycling) can tend to obscure such chemostratigraphic signals. Examining the Cenomanian-Turonian OAE2 in platform carbonates of southern Mexico, Elrick and Molina showed an abrupt -3 ‰ excursion in d13C that immediately preceded a more gradual 3-4‰ positive that is consistently expressed in multiple sections (Figure 1-5). The higher sediment accumulation rates of platform carbonates permits previously unattainable high-resolution data on the relationship between geochemical and physical processes occurring in pelagic and/or hemi-pelagic systems (anoxia, black shale deposition) and those occurring in onshore, benthic-dominated systems. Ludvigson et al. show that OAEs are also associated with environmental changes on land. They used d13C chemostratigraphy to correlate lacustrine carbonates with the marine Aptian-Albian sections. Systematic differences in d13C (~1.5 ‰) between the sections studied are attribute to floral response to moisture stress in the site immediately leeward of the Sevier mountains. d18O curves for the two study sites diverge for the time interval corresponding to OAE1b suggesting a local intensification of aridity associated with oceanic anoxia.

A series of presentations focused on isotopic constraints on the hydrological and carbon cycles in the terrestrial realm. Fricke measured phosphate d18O of dinosaur tooth enamel to infer the d18O of water ingested by the animal and concluded that, relative to the present, the Late Cretaceous of North America was typified by higher temperatures and increased vapour transport to higher latitudes. Parallel d13C measurements on trace carbonate within the tooth apatite suggest herbivorous dinosaur taxa partitioned available food resources.  Both Gonzales et al. and White et al. reported analyses of d18O of pedogenic siderite that confirm enhanced Cretaceous hydrologic cycling and provided quantitative estimates of the increase relative the present and model predictions.  Pratt et al. discussed how d13C studies of lacustrine organic matter could provide better estimates of pCO2 during times when temporal trends in d13C or carbonate and organic matter are de-coupled. One important recent finding is that we have no convincing evidence of C4 plant adaptation by Cretaceous time, even under extreme water stress conditions. A second important recent discovery is the existence of strata with significant concentrations of biomarkers indicative of forest fires. Mass balance considerations indicate that biomass burning is an under-explored explanation for pronounced negative excursions in Cretaceous d13C records. Finally, Finkelstein et al. examined the abundance of fusinite and polycyclic aromatic hydrocarbons (PAHs) in  lacustrine samples from the Late Cretaceous Fort Crittenden Formation. Both are indicative of fire and multiple abundance peaks through the section suggests repeated wildfires or peat fires in the watershed.  Assuming the hydrologic cycle was more vigorous, a seasonally wet paleoclimate could have resulted in biomass build-up and hotter fires, enhanced landscape denudation, and accelerated erosion in the watershed post fires, thus, explaining the correlation between turbidites and indicators of fire.

 

Future Directions

Worshop participants identified the following as critical needs in the areas of d18O paleothermometry and d13C records:

 

d18O Paleothermometry:

 

1) To fully evaluate the roles played by taphonomy, palaeoecology and non-equilibrium behaviour in Cretaceous palaeothermometry.

 

2) To increase stratigraphic and geographic resolution of our records and thereby assess the issue of the stability of warm climates

 

3) To determine the full magnitude of high-latitude warmth during  peak greenhouse forcing

 

4) To integrate d18O data with additional palaeo-temperature proxies  (e.g. Mg/Ca)

 

5) To further examine the issue of d18Osw in the context of sensitivity to short-term changes in continental ice volume and long-term changes in hydrothermal weathering regimes.

 

6) To improve our understanding of the paleohydrological cycle.

 

d13C Records:

 

1) To generate more multi-substrate records (e.g. benthic-planktic foraminiferal pairs, CO3-Corg) and thereby assess competing hypotheses for carbon cycle perturbations

 

2) To improve the stratigraphic resolution and reduce errors associated with pCO2 estimates

 

3) To improve our understanding and records of new isotope proxies for pCO2 and weathering (e.g. d11B; 187Os/188Os).

 

Because of the need to access undisturbed, expanded sections containing well-preserved microfossils, IODP is the key platform for advancing our understanding in these areas. One priority for future drilling is the need for a high-latitude counterpart for forthcoming ODP Leg 207. The depth-transect approach (cf. sea level section) will continue to be an important strategy.

 

 

Biotic Records of Global Change during the Cretaceous

 

Biotic Records Figures with Captions                  Return to top of Report

 

Overview

      The patterns of how life responds to the Earth's changing environments can provide important insights into the processes and rates of global change, and biotic records provide an independent and complementary proxy to geochemical tools. Tectonic forcing of climate and ocean fertility had a profound impact on terrestrial and marine ecosystems and the evolution of life during the Cretaceous. At times, greater emissions of CO2 led to increased weathering of the continents and greater delivery and availability of nutrients in the marine realm. In addition, there were possible linkages between increased hydrothermal activity on the seafloor and enhanced productivity in the surface waters (e.g., late Valanginian and mid-Cretaceous Oceanic Anoxic Events). Climate fluctuations due to greenhouse warming or reverse greenhouse cooling as a feedback to excessive burial of organic carbon, were further modulated by eustatic sea level fluctuations and albedo effects. Changing Cretaceous paleogeography created new opportunities for many groups of organisms. However, other ecosystems were stressed with the opening and closing oceanic gateways, redistribution of heat by the ocean-climate system, and the partitioning of nutrients between land and sea, and surface and deep ocean.  A number of themes emerged from the diverse array of abstracts submitted; these can be grouped by major ecosystems and by key intervals or events during the Cretaceous.

 

Pelagic Ecosystems: Since the Late Triassic, when the nannoplankton began to produce calcareous platelets thereby causing a major shift in carbonate deposition from the shelves into the deep oceans, this diverse and abundant group of unicellular, planktonic marine algae have been intimately linked to global change. In modern oceans, it has been demonstrated that the distributions of certain sensitive nannoplankton taxa mirror discrete water-masses, the existence of which is a function of ocean circulation and climate. Thus, global climate exerts, and must have exerted in the past, a major influence over calcareous nannoplankton and their distributions (Lees).

Lees delimited nannofossil palaeobiogeographic zones (PBZs) for the Late Cretaceous Indian and Pacific Oceans. These latitudinally-distributed PBZs are interpreted as indicating discrete water-masses, possessing differing temperature and nutrient properties. Movements of the fronts separating these PBZs through time can be used as proxies to primarily indicate warming or cooling trends (Figure 2-1). Comparison of available oxygen isotope sea-surface temperatures (SSTs) with the PBZ-derived warming and cooling trends shows a good correlation between the two proxies, underlining the utility of nannofossils as proxies for Mesozoic climate change.

Bown analyzed coccolithophorid and other calcareous nannoplankton evolution and diversity patterns through the Cretaceous. The data reveal no clear relationship between long-term diversity trends and previously recognized major global environmental change events in the Cretaceous. Oceanic Anoxic Events 1a (early Aptian) and 2 (latest Cenomanian) occurred well within nannoplankton diversity minima and were followed in both cases by periods of protracted diversity increase. However, the most rapid increases in diversity broadly correlate with cooler climate intervals. In particular, Bown suggests that enhanced species diversity in the Campanian was related to the onset of late Mesozoic climatic cooling with much of the diversification the result of greater palaeobiogeographic differentiation including the evolution of high-latitude-restricted phytoplankton.

The first radiation of planktonic foraminifera was thought to begin close to the base of the Aptian. However, Premoli Silva demonstrates that this group started to diversify much earlier. The first diversification occurred in the early Valanginian with the appearance of the first hedbergellids. This small fauna in both species number and size seems to persist without apparent changes through most of the Hauterivian, the end of which is characterized by an abrupt short flourishing of the gorbachikellids. The earliest Barremian coincided with a remarkable increase both in abundance and number of planktonic species and genera that continued through the Barremian accompanied by an overall increase in size (Figure 2-2). This trend continued through the early and mid-Aptian with a further increase in number of species and overall abundance and size of specimens after the early Aptian Oceanic Anoxic Event 1a (OAE1a). This increase was accompanied by the progressive appearance of more ornamented morphotypes. Premoli Silva found no turnover in planktonic foraminifera coinciding with OAE1a. The OAE1a represented only a temporary paleoenvironmental perturbation characterized in the upper water column by alternating low oxygen levels (proliferation of leupoldinids), to slightly richer oxygen levels (clavate morphotypes) or to better oxygenated waters (round-chambered taxa) (Figure 2-3). The effects of the perturbation related to the OAE1a appear to have terminated only about one million years after the event.

 

Tropical Ecosystems: Changes in the environment have modified the ecological role of reef builders through geologic time, and major extinctions have reset evolutionary patterns. In Cretaceous reefs, the relative ecologic role of scleractinian corals and rudist bivalves shifted during the mid-Cretaceous, and rudist bivalves took over as the dominant skeletal organisms for the remainder of the period (Johnson). The  role of bivalves as proxies for reefal conditions remains questionable. Johnson statistically analyzed the paleobiogeographic distributions of scleractinian corals of the Caribbean and circum-Caribbean region for all stages of the Cretaceous (Figure 2-4). Dispersion patterns for corals and rudists infer that the geographic extent of the Cretaceous tropics was greater than that of our present-day interglacial period. Knowledge of the paleolatitudinal extent of the ancient tropics is essential for analyzing the role of the tropics in ocean and atmospheric heat transport.

 

Terrestrial Ecosystems: The statistical relationship between leaf physiognomy and climate for modern plants is widely used by paleobotanists to reconstruct Late Cretaceous climate. Methods such as simple linear regression (SLR) and multiple regression (MR) are used to derive leaf/climate relationships for modern floras, which then are used to estimate temperature and precipitation from paleofloras. When applied to modern test sites, physiognomic methods provide good "ballpark estimates" of mean annual temperature (MAT) and mean annual precipitation (MAP) but can show significant discrepancies between estimated and measured climate. The magnitude of the discrepancy (up to 5°C for MAT) is determined by the choice of equation used to quantify the leaf/climate relationship, referred to here as the physiognomic transfer function (Scherer).

To determine the extent to which estimates of Cretaceous climate are biased by the choice of physiognomic transfer function, Scherer applied a variety of published physiognomic transfer functions to a well-preserved leaf megaflora from the latest Cretaceous (Maastrichtian) Jose Creek Member of the McRae Formation, southern New Mexico. For the Jose Creek assemblage, calculated MAT is 16-25°C and calculated MAP is 600-1200 mm/yr, a range of over 8°C for MAT and 600 mm/year for MAP. Variation in calculated temperature and precipitation, relative to the mean of all estimates, can exceed + 35%. However, the range of values is congruent with more qualitative estimates of climate derived from paleosols and plant life form, which indicate a warm subhumid subtropical climate for the Jose Creek Member. This implies that physiognomic transfer functions are probably accurate to the level of climate zone.

Terrestrial climates near the time of the end-Cretaceous mass extinctions are poorly understood. Wilf et al. estimated and correlated paleotemperatures for the terminal Cretaceous  (~66.7 to 65.5 Ma), using megafloral data from North Dakota and foraminiferal data from four middle- and high-latitude sites. Both plants and foraminifera indicate warming near 66.0 Ma, a warming peak from ~65.8 to 65.6 Ma, and cooling near 65.6 Ma to pre-warming temperatures, which lasted into the early Paleocene (Figure 2-5). At similar temperatures, Cretaceous floras from North Dakota were rich, but Paleocene floras were impoverished. Climatic and facies changes are insufficient explanations for plant extinctions in this area, which we attribute to the effects of bolide impact (Wilf et al.).

 

High Arctic: The Cretaceous of the Canadian High Arctic is represented by sedimentary and volcanic rocks of the Sverdrup Basin that are exceptionally well exposed on Axel Heiberg and Ellesmere Islands. These sequences contain evidence for short-term episodes of extreme warmth and cooling during the Late and Early Cretaceous, respectively (Tarduno et al.). A Turonian to Coniacian (~92 to 86 Ma) vertebrate assemblage from a site with a paleolatitude of approximately 71˚N implies that polar climates were warm (mean annual temperature exceeding 14˚C; Tarduno et al., 1998). This episode may correlate with evidence for extreme high-latitude warmth from oxygen isotope paleotemperature estimates from the Southern Ocean (Huber, 1998). The Arctic assemblage includes large (>2.4 m) champsosaurs, which are extinct crocodile-like reptiles. The vertebrate fossils overlie subaerially erupted flood basalts of the Cretaceous Strand Fiord Formation. These lavas are part of a large magmatic pulse (or large igneous province) that may include large parts of Ellesmere Island and the Arctic Ocean basin. This magmatism, coupled with coeval volcanism at six other large igneous provinces, suggests that volcanic carbon dioxide emissions may have helped cause the peak in global warmth. In contrast to the extreme warmth of the Late Cretaceous, Arctic sedimentary rocks of mid- and Early Cretaceous age show evidence of cool conditions in the form of glendonite horizons. Glendonites (pseudomorphs after the hydrated calcium-carbonate mineral ikaite), are associated with glacial marine sediments of Permian to recent age (Figure 2-6). Preliminary biostratigraphic, magnetostratigraphic and geochemistry data of Tarduno et al. suggest that Valanginian age glendonite horizons represent relatively short (<100-400 kyr) episodes at a time of low eustatic sea level.

 

Epicontinental Seas: Fisher applied an innovative technique to reconstruct the temperature and circulation of ancient surface water masses from the Cretaceous Western Interior Sea. Foraminifera construct more porous shells in warmer waters and less saline waters. Modern planktic foraminifera are less diverse at lower salinities, so shell porosity of diverse ancient planktic foraminiferal assemblages must be primarily in response to paleotemperature. Both properties are largely responsible for water density, and therefore porosity may be a proxy for relative water mass density, which is useful in interpreting paleo-ocean circulation. Specimens of the widely distributed mid-Cretaceous species Hedbergella delrioensis have been temporally studied through several sections and spatially studied along time-lines from many geographic localities across the Cenomanian-Turonian Greenhorn Sea. Stratigraphic studies show that porosity increases from the upper Cenomanian into the lower Turonian. The porosity increase is coincident with previously proposed sea level rise. The porosity increase is interpreted as an increase in the influence of warm water that entered from the Tethyan Ocean (Figure 2-7). Geographic studies along time-lines reveal high porosities in the central seaway, with porosity decreasing shoreward and northward. The distributions are interpreted as a stratified seaway, consisting of a warm central near-surface watermass underlain by deeper waters that surfaced shoreward and northward. As sea level rose the warmer central core expanded north and shoreward (Figure 2-8). If porosities are used as relative density proxy data, contour maps of porosity can be used to reconstruct geostrophic flow.

 

Late Valanginian Productivity Event: Using quantitative analyses of calcareous nannofloras and geochemistry, Tremolada et al. showed that the calcareous nannoplankton group nannoconids experienced a sharp decline across a globally documented positive carbon isotope excursion in the Berrisian-Hauterivian in northern Italy. The nannofossil assemblages of the late Valanginian are dominated by W. barnesae, with relative increase of Diazomatolithus spp., probably indicating higher fertility conditions as further supported by the decline in the oligotrophic nannoconids. Rising Sr/Ca ratios in the upper Valanginian carbonates supports the interpretation of enhanced productivity. This increase slightly leads the carbon isotope excursion, and is coeval with the onset of the nannoconid decline. According to Tremolada et al., the nannoconid decline and productivity increase probably were induced by changes in nutrient content caused by the Early Cretaceous volcanic activity connected with the emplacement of the Paranà Plateau and the "pulse" in the seafloor production. These volcanic and tectonic events provoked excessive CO2 levels in the atmosphere favoring warm and humid conditions that induced an accelerated transfer of nutrients from the continents to the oceans increasing the fertility of surface waters. Also, a nutrification event might have been directly caused by hydrothermal processes connected with the igneous activity (Figure 2-9).

 

Biotic Response to Oceanic Anoxic Events: Leckie et al. (2002) documented the evolutionary patterns of calcareous plankton (planktic foraminifera and calcareous nannoplankton) during the mid-Cretaceous (Barremian-Turonian stages, ~124-90 Ma). When combined with the radiolarian record through the same interval (Erbacher et al., 1996), there is compelling evidence that the highest rates of evolutionary turnover, i.e., speciation plus extinction, occur at or near the major Oceanic Anoxic Events. These episodes of widespread organic carbon burial occur in the early Aptian (~120.5 Ma; OAE1a), across the Aptian/Albian boundary (~113-109 Ma; OAE1b), in the latest Albian (~99.5 Ma; OAE1d), and across the Cenomanian/Turonian boundary (~93.5 Ma; OAE2). Strontium isotopic evidence suggests a possible link to times of rapid oceanic plateau formation and/or increased rates of ridge crest volcanism (Bralower et al., 1997; Jones and Jenkyns, 2001; Leckie et al., 2002). The association of plankton turnover and carbon isotopic excursions with each of the major OAEs suggests widespread changes in the ocean-climate system (Figure 2-10). The episodes of accelerated evolutionary activity in the plankton have been attributed to changing upper water column structure, increased delivery of nutrients including dissolved iron, greater oceanic productivity, and carbonate dissolution (Erba, 1994; Erbacher et al., 1996; Leckie et al., 2002). Leckie et al. (2002) concluded that plankton evolution was tectonically-forced, particularly by increased submarine volcanism and hydrothermal activity, due to tectonic influences on global climate, ocean chemistry, nutrient availability, ocean circulation, and water column structure.

According to Erba, OAE1a (late Early Aptian) and OAE2 (latest Cenomanian) correlate with the onset and end of the mid-Cretaceous greenhouse climate, interrupted by some brief cooling episodes in the Aptian to Cenomanian interval alternating with extremely warm conditions leading to black shale deposition during OAEs. High-resolution stratigraphy indicates that biotic events correlate with major igneous/tectonic events. Both OAE1a and OAE2 are interpreted as high productivity episodes probably induced by high concentrations of dissolved and particulate biolimiting metals in the oceans and increased CO2 in the atmosphere during submarine volcanism (Ontong Java and Manihiki Plateaus, and Caribbean plate, respectively). Mid-Cretaceous OAEs and cooling episodes seem correlatable with subaerial volcanism related to the formation of Kerguelen Plateau and Broken Ridge (Figure 2-11).

Changes in the nature of oceanic surface water masses during Oceanic Anoxic Events (OAE) were significant drivers of phytoplankton evolution. As an example, Watkins analyzed well-preserved nannofossil assemblages in upper Albian and lower Cenomanian hemipelagic sections from Ocean Drilling Program (ODP) Leg 171b, which record the early history of the establishment and adaptive radiation of the genus Eiffellithus. Seven distinct taxa evolved during the relatively short interval from 101.5 to 100.0 Ma. Newly evolved species tended to remain at low abundance levels until a significant disruption in the pelagic realm resulted in the precipitous decline of the dominant species. These major disruptions correspond to significant changes or shifts in the sedimentological and carbon isotopic records associated with late Albian OAE1d. The close correspondence of species originations, changes in dominance, and species extinctions to changes in sediment TOC and significant carbon isotope shifts indicates that the variability in the surface water mass during OAE1d was the principle environmental forcing mechanism behind the adaptive radiation of this genus during the late Albian (Watkins).

Holbourn and Kuhnt examined records of well preserved benthic foraminiferal assemblages across Cretaceous OAE1a, OAE1b, OAE1c and OAE2 from North Atlantic bathyal and abyssal DSDP/ODP Sites and from bathyal to neritic onshore sections in Morocco, Spain, southeast France and northern Germany (Figure 2-12). Diversity, taxonomic composition and preservation potential of benthic foraminiferal assemblages at shelf localities are strongly influenced by changes in paleo-water depth, particularly at times of major sea level change. Thus, most of the observed faunal changes at shelf sites express environmental change and taphonomic bias, rather than true extinction and radiation events. They found no evidence for a major benthic foraminiferal turnover during OAE1 and OAE2 at middle and upper bathyal sites (Figure 2-13). Most taxa recorded at these locations have stratigraphic ranges extending across oceanic anoxic events. The changes in biofacies recorded across the OAEs coincide with changes in hydrography and sedimentological facies. By contrast, abyssal benthic foraminifers underwent a marked radiation after the latest Cenomanian OAE2 in the Atlantic Ocean and the Mediterranean Tethys, which may be triggered by a general change towards better oxygenated deep-water.

With ammonite/inoceramid species loss of up to 85% in the Western Interior basin, the Cenomanian-Turonian boundary event represents a significant biotic crisis (Meyers and Sageman). Hypotheses proposed for the driving mechanism of species turnover include mainly changes in benthic and water column oxygen levels associated with OAE2, and changes in substrate consistency associated with limestone-marlstone alternation. Meyers and Sageman examined the main hypotheses for biotic turnover in light of new sediment and geochemical accumulation rates, as well as recalculated evolutionary rates across the stage boundary. These new rates are based on a high-resolution time scale developed through cyclostratigraphic analysis of the Cenomanian-Turonian Bridge Creek Limestone Member. The new high-resolution time scale facilitates an independent quantitative assessment of the rates of accumulation of environmentally sensitive geochemical proxies, and calculation of rates of evolutionary change in the C-T boundary interval. A series of major modal switches in sedimentation are identified. These modal switches may represent fundamental changes in the ocean-climate-sediment transport system. Interestingly, the highest rates of extinction do not correspond to intervals with the greatest indication of sulfidic conditions (Figure 2-14). Further comparisons of paleoenvironmental data with molluscan evolutionary rate allow evaluation of alternate biotic controls, such as substrate consistency, turbidity, and nature/frequency of environmental disturbance (Meyers and Sageman).

 

Future Directions

Based on data obtained from continental records and ocean drilling, considerable progress has been made in the last two decades toward describing and explaining Cretaceous biosphere changes. However, important unsolved problems remain in our understanding of the ocean-climate system and its impact on the biosphere. How is widespread marine biological productivity sustained for 104-105 years? How important was 400 kyr and 100 kyr cyclicity in controlling sedimentary and biotic patterns? Did metals or other trace elements play a major role in the Oceanic Anoxic Events of the Early Cretaceous and mid-Cretaceous? What was the relative role of greenhouse warming vs. reverse greenhouse cooling in controlling the availability of nutrients, burial of organic matter, water mass production, and biotic evolution during the mid-Cretaceous? What is the relative importance of upwelling vs. continental runoff as a nutrient supply mechanism and how might this have varied through different climate events? What were the rates of climate change during the OAEs? What was the frequency and extent of wildfires, and what was their impact on the carbon cycle?

 The rich Cretaceous terrestrial record, increasingly well-characterized marine records, and an increasing number of proxies and model approaches will allow us to better understand terrestrial and marine responses to internal and external climate change. Workshop participants emphsized the need to better integrate terrestrial and marine records using transects across the land-sea transitions. Lacustrine deposits were identified as a possibly under-utilized source of information. The increasing use of organic biomarkers for molecular "fingerprinting" should yield important information about community makeup and environments. Given the vast amount of hydrocarbons sourced from Cretaceous sediments, the portion of our community applying biogeochemical approaches might especially benefit from greater collaboration with the petroleum industry.

 

 

Oceanic Anoxic Events: Causes and Consequences

 

OAE Figures with Captions                                             Return to top of Report

 

Overview

 The discovery of black carbon-rich shales in deep-sea drilling sites from the Atlantic, Indian and Pacific Oceans (from DSDP Leg 1 onwards) led in the mid-1970s to the concept of Oceanic Anoxic Events.  Such events, whatever their exact nature and cause, were hypothesized to foster deposition of coeval carbon-rich sediments across environments ranging from deep oceans to shelf seas.  The original concept was primarily stratigraphic in nature, being based on the implicit assumption that the world ocean underwent a fundamental chemical and/or biological change during such events: enhanced productivity of organic-walled microfossils and bacteria and/or enhanced preservation of organic matter were both suggested as likely causes.  Among the plethora of black-shale horizons identified both on land and in the oceans, two major organic-rich horizons, one dated as early Aptian (Selli Event: OAE1a), the other at the Cenomanian-Turonian boundary (Bonarelli Event: OAE2), have proven to be of global distribution.  Both of these, for example, are recorded from submarine plateaus in the Pacific Super-Ocean, most recently the black shale of the Selli Event drilled on Shatsky Rise during ODP Leg 198. This extremely carbon-rich horizon (~35% TOC) contains biomarkers for cyanobacteria that could have utilized atmospheric elemental dinitrogen - as happens in some extreme upwelling environments today - if nitrate levels in the photic zone became vanishingly low because of utilization by plankton (Brassell, Figure 3-1).  This discovery points to upwelling and high productivity as a major forcing function behind the causality of Oceanic Anoxic Events.

Nitrogen-isotope data from Cenomanian/Turonian black shales from Italy and Morocco simi­larly are consistent with sedimentation below a water column that had undergone denitrification (Jenkyns and Tsikos), as is the case today in zones of vigorous upwelling, intense oxygen minima and high fluxes of planktonic carbon to the sea floor. Other events, particularly the Paquier Event (OAE1b) are more local, being confined - as far as we know at present - to the Atlantic-Tethyan domain.

Phil Meyers showed that low d15N values in black shales, values that falsely mimic land plant compositions, may reflect a combination of processes leading to elevated production and improved preservation of marine organic matter. Marine productivity is usually limited by availability of dissolved nitrate, but if a mid-water anoxic zone expands upward and enhances recycling of phosphorus into the photic zone, then nitrogen-fixing cyanobacteria can flourish. These organisms photosynthetically produce organic matter having a d15N value close to atmospheric nitrogen (0‰).  So, while the d15N values of most Mesozoic and Cenozoic marine sediments usually range between 4 and 8 ‰, Rau et al. (1987) reported d15N values of –2 to 3 ‰  in black shales from DSDP Sites 367, 530, and 603 (Figure 3-2).

From a study of isotopes and palynology of this Oceanic Anoxic Event from three sec­tions in the Central Atlantic and western Tethys, Erbacher and Herrle conclude: 1) that the aver­age duration of OAE1b was 45 ky; 2) a strong monsoonal circulation resulted in a dominance of the precessional signal (fertility); 3) fertility changes were superimposed on longer-term temperature cycles corresponding to the eccentricity signal; and 4) that OAE1b falls into an extremely warm and humid phase of an eccentricity cycle (Figure 3-3).  This event was probably a factor of 10 times shorter in duration than was OAE2. OAE3 (Coniacian-Santonian) again seems a more parochial affair (Wagner et al., Figure 3-4) with a record dominantly deriving from the Atlantic.  The question must be asked: is it actually useful to consider these organic-rich strata as being produced by an Oceanic Anoxic Event?  Is some minimum geographic extent of a black-shale unit required in order to qualify as the product of an OAE?

The advent of carbon-isotope stratigraphy in the nineteen eighties, with the recognition of a characteristic positive excursions related to excess global carbon burial, offered a proxy for the partitioning of the global carbon pool between oxidized and reduced reservoirs.  Recent carbon- isotope studies from England, Japan, Italy, Morocco and elsewhere now demonstrate that the both the Aptian and the Cenomanian/Turonian black shales are associated with such a positive carbon- isotope excursion in marine pelagic and shallow-water carbonate, marine organic matter and ter­restrial higher-plant material (Jahren, Hasegawa, Heimhofer et al.; Figures 3-5, 3-6, 3-7).  The Aptian event, paradoxically, is also associated with a pronounced negative carbon-isotope excursion, recently interpreted as due to dissociation of methane hydrates, a theme explored by Hope Jahren in her presentation. Because these characteristic carbon-isotope excursions, both positive and negative, can be found in deep-marine, shallow-marine and non-marine facies, they offer a novel means of correlation between sediments deposited in the oceans and on the continents. Palaeoclimatic data from conti­nental interiors may now be integrated with the wealth of data from deep-sea cores to produce a potentially global view of climate change during critical events in earth history.  The Aptian nega­tive isotope excursion is not unique in the Cretaceous: a similar phenomenon has been found in Atlantic and Tethyan sections predating the Paquier Event (Gröcke).  The question then has to asked: what is the exact relationship, if any, between OAEs and negative carbon-isotope excur­sions, when excess global carbon burial must produce a positive excursion? Perhaps both phe­nomena - OAEs and methane-dissociation events - are responding to increases in CO2-forced global temperature, the former being related to fundamental changes in the hydrological cycle, nutrient inputs, oceanic recycling, the latter to changes in the stability field of methane hydrate dependent on bottom-water temperatures.  CO2 could have been added to the atmosphere from volcanogenic sources as well as from oxidation of methane: and in one sense OAEs can be seen as the means by which the ocean-atmosphere returns to a pre-existing equilibrium. Massive seques­tration of organic carbon during the OAE would draw down CO2, return the atmosphere to pre-OAE conditions and produce global cooling.

In detail the exact relationship between the isotope curve and the total-organic-carbon stratig­raphy of any one section is complex.  The black shale or, more particularly, its most carbon-rich portions, is not fixed in exactly the same position with respect to carbon-isotope curve: in some localities the onset of the positive excursion coincides with the beginning of black-shale deposition, in others it does not.  This mismatch is seen, for example with the Cenomanian/Turonian in the Western Interior (Sageman and Meyers; Figure 3-8) where organic-carbon accumulation rates actually seem to have increased after the OAE - the OAE being defined by the duration of the carbon-iso­tope excursion.  In Italy, however, the carbon-isotope excursion begins abruptly at the base of the Bonarelli Level itself.  Hence, at these high levels of resolution, the "stratigraphic" concept of the Oceanic Anoxic Event breaks down.  But more importantly this is telling us that the most significant carbon sinks during the Cenomanian/Turonian OAE have yet to be identified, although there is some evidence that a (maybe the) major locus of organic-matter deposition was the proto-South Atlantic (Forster et al., Figure 3-9).

The Late Cenomanian oceanic anoxic event (OAE2) is the most extensive of the Mid-Creta­ceous OAEs. The duration of OAE2 was originally estimated based on biostratigraphic evidence and interpolation of geological timescale data points between 0.5 - 0.8 m.y. (Arthur et al., 1988) and 0.4 m.y. (Caron et al., 1999). Recently, orbital cyclicity has been used to estimate the duration of the event. These estimates range from 720 k.y. in Colorado (Meyers et al., 2001), approx. 400 k.y. (Kuhnt et al., 1997) in the Tarfaya Basin (Morocco) to 320 K.y. in western Canada (Prokoph et al., 2001).  The OAE2 is characterised by a large positive global carbon-isotope excursion in both carbonate and organic matter, caused by a major perturbation of the global carbon budget, probably due to the extensive burial of organic matter in black shales (Schlanger & Jenkyns 1976; Arthur & Schlanger 1979; Jenkyns 1980; Scholle & Arthur 1980; Herbin et al. 1986; Arthur & Schlanger 1987; Arthur et al. 1988; Weissert & Lini 1991; Jenkyns et al. 1994). This period of extensive carbon burial was coeval to a significant extinction event in the marine plankton, i.e. the extinction of the first clade of keeled planktonic foraminifers, the genus Rotalipora.

The OAE2 represents a major turn in earth climate history (Jenkyns et al. 1994; Norris & Wilson 1998). Decreased ocean-volume due to enhanced spreading rates and larger mid-ocean ridge volume had led to a eustatic sea level rise of 130-350 m since the early Aptian (Kominz 1984; Larson 1991a,b) and the worldwide formation of shallow, warm, epicontinental seas (Brass et al. 1982). Atmospheric CO2-partial pressure was 3-12 times higher than today due to volcanic outgassing (Larson 1991b; Berner 1991) and many terrestrial and marine paleoclimate-proxies suggest that the mid-Cretaceous was the warmest period in the last 200 m.y. (Frakes et al. 1992; Barron et al. 1995; Huber et al. 1995). The high surface temperatures resulted in enhanced evaporation within shallow, warm epicontinental seas and an intensification of the water-cycle and increased nutrient fluxes  (Föllmi et al. 1994). Recent evi­dence indicates that OAE2 was accompanied by a 40-80% reduction in CO2 levels (Kuypers et al., 1999). This was suggested to be due to a rapid increase in accumulation rates of organic mat­ter in the subtropical Atlantic Ocean and Tethyan marginal basins (Arthur et al. 1988; Kuhnt et al., 1990; Jenkyns et al. 1994; Kuypers et al. 1999). However, due to generally low sedimentation rates or even hiatuses in the late Cenomanian, a precise succession of events (carbon burial, reduction of pCO2, carbon isotope excursion, biotic extinctions and sea surface temperature changes) has never been established, leaving wide space for speculation about causal relation­ships.

 

Future Directions

Future research on OAEs must continue to characterize the environmental conditions that led to the deposition of organic-rich sediments.  Proxies that can provide direct information on nutrients in surface waters, atmospheric CO2 levels, and the original composition of biotic populations from microbes to  invertebrates, are desperately needed.  Multidisiplinary investigations must focus on events that perturbed the global ocean such as the OAEs in the early Aptian (OAE1) and the late Cenomanian (OAE2), as well as those that appear to be more regional in scale  such as OAEs 1b, 1c, and 1d.  There are a number of important questions that can best be addressed through ocean drilling:

·        What is the geographic and oceanographic distribution of these events?

·        What controls their spatial distribution and the rate of changes (chemical, physical, biological) during the event?

·        What preconditioning (tectonic, climatic, orbital) determined the time scales of the different OAEs and sustained them?

·        What are the triggering mechanisms and their manifestation, e.g. is there a role for methane?

·        What is the relationship between OAEs, ocean temperature changes and links to sea level?

·        What happened to the water column (physically, chemically, biologically) and the upper sedimentary column?

·        How did the organic matter (sources, preservation, productivity), nutrients and ocean structure change? How was that affected/stimulated by oxygen minima, euxinia, dysoxia?

·        What is the impact on the carbon cycle and its budgets, reservoirs and fluxes through the various OAEs?

·        What is the expression of these events on the continents (e.g. in lakes) versus shelf areas and the deep ocean?

 

Drilling Priorities

To address many of these questions in a rigorous fashion will require sections with a broader geographic distribution than those currently available. Dedicated drilling legs are required to obtain multiple core records of these critical intervals. Moreover, there must be a concerted effort to identify records of OAE in expanded, immature sections that will allow high-resolution studies at a millennial scale. Drilling priorities include the high latitudes (especially Arctic) transects from continent to the deep sea, and progressively "downstream" from Large Igneous Province eruptions.

 

 

Sea Level Record and Mechanisms for Global Eustatic Change

 

Sea Level Figures with Captions            Return to top of Report