Eric J. Barron

A warm, equable Cretaceous: The nature of the problem

Abstract The nature of the problem of warm equable paleoclimates is defined by investigating the mid-Cretaceous period. The problem consists of three components: (1) defining precisely the climatic state during any specified interval of geologic time; (2) specifying the external climatic forcing factors which may have been important; and (3) understanding the climatic response to any specific modifying influence. These components are characterized by limitations of critical importance in understanding paleoclimates. The nature of the problem of a warm Cretaceous is defined both qualitatively and quantitatively within these limits.

A “simulation” of Mid‐Cretaceous climate

A series of general circulation model experiments utilizing GENESIS have been completed for the mid-Cretaceous based on geography, variable atmospheric carbon dioxide concentrations (2 to 6 times present-day concentrations), and variable poleward oceanic heat flux (.6 to 1.2 × 1015 W increased from present day). By combining all three major variables (CO2, geography, and oceanic heat flux), the distribution of mid-Cretaceous temperatures can be achieved. In the simulations, increased CO2 is required to promote global warmth, and increased oceanic heat flux is required to prevent the tropics from overheating with higher levels of CO2. Four times present-day CO2 with 1.2 × 1015 W provided the best match to the distribution of mid-Cretaceous data. The best match to the Cretaceous observations was achieved with a globally averaged surface temperature increase of 6.2°C, at the lower end of past estimates of mid-Cretaceous warmth. This value may be a better estimate of mid-Cretaceous global warming. Finally, the model experiments can be used to provide a “paleocalibration” of the global warming expected for a doubling of atmospheric carbon dioxide. The best estimates for the mid-Cretaceous appear to be a 2.5 to 4.0°C sensitivity, in the mid to upper range of the sensitivity of current climate models used to assess future global change.

Cretaceous climate: A comparison of atmospheric simulations with the geologic record

Abstract Atmospheric simulations using realistic Cretaceous geography (100 million years ago) suggest that paleography is an important factor governing the nature of the circulation. The simulated Cretaceous atmospheric circulations are markedly different both from the present-day atmospheric circulation and from the classical hypotheses of a weaker circulation and poleward displacement of circulation features (e.g. the subtropical high). Therefore, they pose specific questions which can be tested against the geologic record. Aspects of the Cretaceous simulations are verified in comparisons with the geologic record. The simulations serve to guide areas of future paleoclimatic research and to formulate better the nature of the problem of warm, equable climates.

Response of the Mid-Cretaceous global oceanic circulation to tectonic and CO2 forcings

The mid-Cretaceous was a period of unusually active tectonism that drove enhanced volcanic outgassing and high seafloor spreading rates. This intense tectonic activity is coincident with dramatic events in the marine environment, including oceanic anoxic events 1 (Aptian-Early Albian) and 2 (Cenomanian/Turonian boundary), high biological turnover rates, and a thermal maximum. In this study, a series of mid-Cretaceous ocean general circulation model experiments were completed using the Parallel Ocean Climate Model. These experiments demonstrate the effect of enhanced atmospheric CO2 concentrations and paleogeographic change on the global oceanic circulation. The experiments reveal that paleogeography, specifically the presence/absence of a marine connection between the North Atlantic and South Atlantic basins, may have governed the nature of the mid-Cretaceous global oceanic circulation. In the absence of this connection, an Albian simulation is characterized by extremely warm, saline conditions throughout the North Atlantic and northern South Atlantic Oceans. With a gateway in a Turonian simulation, Antarctic Bottom Water ventilates the Atlantic basins. In both Albian and Turonian simulations the Pacific-Indian basins are dominated by thermohaline circulation with deep water sources in the Southern Ocean. While atmospheric CO2 concentrations influence the global temperature and salinity, an increase from present-day to 4 times present-day levels alters the global circulation very little. Differences between the Albian and Turonian numerical simulations agree well with aspects of the marine record, supporting speculation that the climatic and oceanographic changes surrounding the Cenomanian-Turonian boundary were driven by the initiation of a connection between the Atlantic Oceans.

Cretaceous rhythmic bedding sequences: a plausible link between orbital variations and climate

Abstract Simple oscillation cycles have been recognized in Cretaceous pelagic sequences as a marked periodicity in carbonate and/or organic carbon content. The estimated periodicities of these cycles resemble Milankovitch-type variations in orbital elements. However, during warm geologic periods the nature of the link between orbital variations and the sedimentary record has been problematic. Climate model studies show that the intensity of continental margin precipitation in specific low latitude regions is a function of land-sea thermal contrast which is sensitive to orbital variations. Cretaceous climate model simulations are characterized by regions of intense precipitation, the locations of which are controlled by geographic configuration. In particular, in the model simulation, the northern margin of the Tethys Ocean has all the climatic characteristics necessary to exhibit a sensitivity to orbital variations. Detailed studies of the biota, sedimentology and geochemistry of the bedding rhythms in Greenhorn Cyclothem in the Western Interior Seaway of North America are best interpreted in terms of variations in precipitation, runoff and stable stratification of the seaway. Combined the geologic and model studies support a link between orbital periodicities, variations of intense precipitation associated with specific paleogeographic situations and Cretaceous bedding patterns. This hypothesis has important implications for the sensitivity of the atmosphere-hydrosphere-lithosphere system to external forcing.

Estuarine circulation in the Turonian Western Interior seaway of North America

To understand the patterns of lithofacies, marine faunas, organic-carbon enrichment, isotopes, and trace elements deposited in the early Turonian Western Interior seaway, we conducted circulation experiments using a three-dimensional, turbulent flow, coastal ocean model driven by GENESIS, a climate model developed at the National Center for Atmospheric Research (NCAR). Circulation and chemical evolution of the seaway waters are computed under the following initial and boundary conditions: (1) paleobathymetry according to a new interpretation of the lithostratigraphy and biostratigraphy; (2) temperatures and salinities of the Boreal and Tethys oceans and adjacent drainage basins based on isotopic data, atmospheric temperatures, and precipitation-evaporation magnitudes computed by GENESIS; and (3) mean annual wind stresses over the seaway computed by GENESIS. Results show that the seaway exported freshened water much like Hudson Bay today. Runoff from eastern drainages exited the seaway as a northern coastal jet; runoff from western drainages exited as a southern coastal jet. Both jets simultaneously drew in surface Tethyan and Boreal waters, creating a strong counterclockwise gyre occupying the entire north-south extent of the seaway. The curious stratal and faunal variations of the early Turonian deposits arise from this gyre and its associated water masses.

The Cenozoic ocean circulation based on ocean General Circulation Model results

Abstract A series of ocean General Circulation Model (GCM) experiments for Paleocene, Eocene, Miocene and present-day continental geography with atmospheric forcing prescribed from atmospheric GCM experiments is utilized to investigate the changes in surface and deep-water ocean circulation through the Cenozoic. The experiments illustrate a number of similarities with previous observation-based reconstructions, although the timing of the development of particular gyre systems and the nature of the high southern latitude circulation are generally different than previous interpretations. In addition, the model results suggest substantial changes in the sites of deep water formation, including a middle Eocene subtropical source. Because of a number of experimental limitations, the experiments should be considered largely as a sensitivity analysis of the role of changing geography in modifying the ocean circulation.

Simulating the river-basin response to atmospheric forcing by linking a mesoscale meteorological model and hydrologic model system

Abstract The purpose of this article is to test the ability of a distributed meteorological/hydrologic model to simulate the hydrologic response to three single-storm events passing over the Upper West Branch of the Susquehanna River Basin. The high-resolution precipitation fields for three storms are provided by observations and by the Penn State–NCAR Mesoscale Meteorological Model (MM5) with three nested domains. The MM5 simulation successfully captures the storm patterns over the study area, although some temporal and spatial discrepancies exist between observed and simulated precipitation fields. Observed and simulated precipitation data for those storms are used to drive the Hydrologic Model System (HMS). The output from HMS is compared to the measured hydrographic streamflow at the outlet of the Upper West Branch. The Curve Number and Green-Ampt methods of rainfall-runoff partitioning are used in HMS and evaluated for streamflow simulation. The results of the hydrologic simulation compare well with observed data when using the Curve Number partitioning, but underestimate observed data when using the Green-Ampt. The likely cause is the lack of heterogeneity in hydraulic parameters. The simulated streamflow with the MM5-simulated precipitation is lower than the simulated streamflow with observed precipitation. The experiments suggest that the subgrid-scale spatial variability in precipitation and hydraulic parameters should be included in future model development

A comparison of Eocene climate model results to quantified paleoclimatic interpretations

Abstract The integration of climate model results and geologic information offers considerable potential for deriving greater insight into the geologic record. In this study, climate model results and quantified climatic interpretations derived from proxy data are compared, to assess model capabilities and to examine proxy data interpretations. Atmospheric general circulation model experiments were used to produce a range of “possible” representative Eocene climate states, based on current knowledge of the Eocene record. The climate model experiments incorporate two idealized endmembers of Eocene ocean-surface temperature distributions characterized by very different latitudinal gradients. Model results are compared to quantified interpretations of the climate of early Eocene North America in an attempt to identify one of the sea-surface temperature distributions as more likely to have existed during the Eocene. The comparisons do not produce a conclusive match between inferred paleoclimatic information and any single case of model results, but some interesting insights become apparent. Model predictions of mean annual temperature and mean annual precipitation compare favorably to interpretations from geologic evidence, but there are large differences between model results and interpreted paleoclimatic parameters of minimum surface temperature and mean annual temperature range. Several possible causes for these differences are discussed.

Middle Cretaceous reef collapse linked to ocean heat transport

Biologically defined fluctuations in Cretaceous tropical reef boundaries of the Caribbean region record a dynamic rather than stable environmental history. These fluctuations may be related to major thermal changes resulting from ocean heat transport. With simultaneous poleward movement of surface and subsurface waters on sea-level highstands, the superheated middle Cretaceous tropics cooled, the reef line contracted, diversity decreased, and reef ecosystems collapsed, leading to mass extinction. Geologic data qualitatively test and support the hypothesis of enhanced Cretaceous ocean heat transport formulated from general circulation models. In these models, four times the present-day atmospheric concentration of CO 2 and twice the present-day model value of ocean heat transport cooled the superheated tropics and provided the best match to the distribution of inferred middle Cretaceous temperature data. These dynamic changes suggest an important role for large-scale disturbance in the evolution of tropical ecosystems.