Christopher R. Scotese

Jurassic and cretaceous plate tectonic reconstructions

Abstract Five plate tectonic reconstructions are presented illustrating the breakup of Pangea and the evolution of the worlds ocean basins during the Early Jurassic (Pliensbachian), Late Jurassic (Volgian), Early Cretaceous (Aptian), Late Cretaceous (Cenomanian), and latest Cretaceous (Maestrichtian). Indicated on the maps are the location of the spreading centers, intracontinental rifts, subduction zones, and transform faults which were active during each interval.

Quantifying the role of geographic change in Cenozoic ocean heat transport using uncoupled atmosphere and ocean models

Abstract A series of five Cenozoic atmospheric general circulation model (AGCM) experiments has been performed using a new set of paleogeographic reconstructions for 55, 40, 33, 20 and 14xa0Ma. The five continental reconstructions incorporate the tectonic evolution of early Eocene to middle Miocene continental positions and topography. With all other model boundary conditions and forcings held constant, the series of AGCM experiments captures a

Vegetation–atmosphere interactions and their role in global warming during the latest Cretaceous

Forest vegetation has the ability to warm Recent climate by its effects on albedo and atmospheric water vapour, but the role of vegetation in warming climates of the geologic past is poorly understood. This study evaluates the role of forest vegetation in maintaining warm climates of the Late Cretaceous by (1) reconstructing global palaeovegetation for the latest Cretaceous (Maastrichtian); (2) modelling latest Cretaceous climate under unvegetated conditions and different distributions of palaeovegetation; and (3) comparing model output with a global database of palaeoclimatic indicators. Simulation of Maastrichtian climate with the land surface coded as bare soil produces high–latitude temperatures that are too cold to explain the documented palaeogeographic distribution of forest and woodland vegetation. In contrast, simulations that include forest vegetation at high latitudes show significantly warmer temperatures that are sufficient to explain the widespread geographic distribution of high–latitude deciduous forests. These warmer temperatures result from decreased albedo and feedbacks between the land surface and adjacent oceans. Prescribing a realistic distribution of palaeovegetation in model simulations produces the best agreement between simulated climate and the geologic record of palaeoclimatic indicators. Positive feedbacks between high–latitude forests, the atmosphere, and ocean contributed significantly to high–latitude warming during the latest Cretaceous, and imply that high–latitude forest vegetation was an important source of polar warmth during other warm periods of geologic history.

Plate tectonic reconstructions of the Juan Fernandez microplate: Transformation from internal shear to rigid rotation

Side-scan sonar, swath bathymetry and magnetic anomaly date define a detailed, three-phase history of the Juan Fernandez microplate. The ∼6 m.y. history is presented in a series of discrete time steps to document the growth and reorganization of propagating spreading centers and structural feature, and microplate kinematic evolution. Prior to the microplate, the East Pacific Rise at the Pacific-Antarctic-Nazca triple junction was offset by a long transform fault zone, likely the fastest slipping transform on Earth at anomaly 3A time. The microplate originated from an intratransform setting between anomaly 3A (5.95 Ma) and anomaly 3 (5.24 Ma) time. Its early development resembled a large propagating rift system, and microplate core structures suggest the entire offset zone may have experienced deformation. Fast propagation of the East Ridge dominated microplate growth until ∼2.6–1.9 Ma when seafloor spreading became the dominant growth process. The microplate rotation rate increased threefold (from 9 to 29° m.y.−1 average) from phase 1 (4.2–2.6 Ma) to phase 2 (2.6–1.1 Ma) of the microplates history, then reduced fourfold (29 to 7° m.y. −1 average; phase 3, 1.1 Ma to Present). Phases 2 and 3 of the microplates rotational history support the edge-driven model for microplate kinematics of Schouten and others to a good approximation. The Pacific-Nazca shear couple drove microplate rotation during phase 2, but development of the southeastern boundary enabled a transfer to the Nazca-Antarctic plate pair (phase 3). West Ridge propagation and reorganization of the south-western boundary may have decoupled the Pacific plate from the microplate, thus facilitating the shear couple transfer. The recent continued deceleration in microplate rotation rate and westward migration of the Pacific-Antarctic ridge axis relative to the microplate may indicate that the process of microplate “death” has begun. We speculate that the Juan Fernandez microplate will accrete to the Antarctic plate, perhaps within the next million years, like the extinct Friday microplate has done, thereby accomplishing another northward migration of the Pacific-Antarctic-Nazca triple junction. Our reconstructions illustrate that the Easter and Juan Fernandez microplates are more similar than previously thought in terms of their origin, growth, rift propagation, ridge segmentation and overall tectonic evolution.

Paleozoic plate dynamics

Current plate motions can be accounted for by a balance of active forces, slab pull, ridge push, and, for continental plates, trench suction, with drag beneath the plate as a resistive force. If we assume that the same forces have acted through time, we can reconstruct plate motions from the geometry of past plate boundaries. Paleozoic reconstructions are made with paleomagnetic, tectonic, climatic, and biogeographic data, as no ocean floor remains. PALEOMAP reconstructions are used to estimate past plate speeds and to test simple dynami- cal models in order to determine which ranges of forces best accounts for the observations. Over the last 600 m.y., plate speeds averaged over 40- to 100-m.y. intervals show considerable variation; Gondwanas speed oscillates from 20 to 60 km/m.y. over a long timescale (200-400 m.y.) with considerable noise superposed. Over the Paleozoic Era motions for large continental regions average 28 krn/m.y.; force balance models based on present-day observations suggest that continental regions without a large attached slab would move 30 mrn/yr. The opening and closing of the ocean between Laurentia and Gondwana 560-400 Ma is used to test dynamical models and the parameter values assumed. In the late Precambrian, Laurentia rifted away from Gondwana. In the earliest Cambrian it was near 40oS; by Late Cambrian and Ordovician it had moved to the equator. During the Silurian and Devonian, Laurentia reversed direction and later collided with Gondwana at 40oS. In a model of the forces acting on the plates, slab pull, ridge push, and trench suction are assumed to balance plate drag. Only certain ranges of ridge-push and trench parameters can model both the opening and subsequent closing of the ocean. The dynamic models, with parameter values inferred from present rates, bracket the rates required by the reconstructions.

New Internet software aids paleomagnetic analysis and plate tectonic reconstructions

Internet server-side applications are becoming an invaluable source of information for many scientists. This is particularly evident in the case of molecular biology, where raw information and computation tools related to various genome projects are easily and quickly accessible through several Web servers. As illustrated by the pioneering efforts of the Ocean Drilling Stratigraphic Network (ODSN) service for making online plate reconstructions, the Cornell Interactive Mapping System, the Harvard Centroid-Moment Tensor Project, and the DRAGON project, geophysics has just begun to systematically use comparable network programming technology Within the next few years, many local resources that are currently accessible only at the scale of university departments will be rendered available to a wide public of researchers and students.

Latitudinal temperature gradients and high-latitude temperatures during the latest Cretaceous: Congruence of geologic data and climate models

A major challenge in paleoclimatology is disagreement between data and models for periods of warm climate. Data generally indicate equable conditions and reduced latitudinal temperature gradients, while models generally produce colder conditions and steeper latitudinal gradients except when using very high CO2. Here we show congruence between temperature indicators and climate model output for the cool greenhouse interval of the latest Cretaceous (Maastrichtian) using a global database of terrestrial and marine indicators and fully coupled simulations with the Community Climate System Model version 3. In these simulations we explore potential roles of greenhouse gases and properties of pre-anthropogenic liquid clouds in creating warm conditions. Our model simulations successfully reproduce warm polar temperatures and the latitudinal temperature gradient without overheating the tropics. Best fits for mean annual temperature are simulations that use 6× preindustrial levels of atmospheric CO2, or 2× preindustrial levels of atmospheric CO2 and liquid cloud properties that may reflect pre-anthropogenic levels of cloud condensation nuclei. The Siberian interior is problematic, but this may relate to reconstructed elevation and the presence of lakes. Data and models together indicate tropical sea-surface temperatures ∼5 °C above modern, an equator-to-pole temperature difference of 25–30 °C, and a mid-latitudinal temperature gradient of ∼0.4 °C per 1° latitude, similar to the Eocene. Modified liquid cloud properties allow successful simulation of Maastrichtian climate at the relatively low levels of atmospheric CO2 indicated by proxies and carbon cycle modeling. This supports the suggestion that altered properties of liquid clouds may be an important mechanism of warming during past greenhouse intervals.

Siberian glaciation as a constraint on Permian–Carboniferous CO2 levels

Reconstructions of Phanerozoic CO2 levels have generally relied on geochemical modeling or proxy data. Because the uncertainty inherent in such reconstructions is large enough to be climatically significant, inverse climate modeling may help to constrain paleo-CO2 estimates. In particular, we test the plausibility of this technique by focusing on the climate from 360 to 260 Ma, a time in which the Siberian landmass was in middle to high latitudes, yet had little or no permanent land ice. Our climate model simulations predict a lower limit for CO2—the value beneath which Siberia acquires “excess” ice. Simulations provide little new information for the period in which Siberia was at a relatively low paleoaltitude (360–340 Ma), but model results imply that paleo-CO2 levels had to be greater than 2–4× modern values to be consistent with an apparently ice-free Siberia in the late Permian. These results for the later period in general agree with soil CO2 proxies and the timing of Gondwanan deglaciation, thus providing support for a significant CO2 increase before the end-Permian boundary event. Our technique may be applicable to other time intervals of unipolar glaciation.

Paleozoic stromatactis and zebra carbonate mud-mounds: Global abundance and paleogeographic distribution

Carbonate mud-mounds with zebra and stromatactis structures are present in every Paleozoic system and series, but are more common in Devonian and Carboniferous deposits, reaching their acme in Mississippian System (lower Carboniferous) rocks. Global distributions illustrate that mud-mounds spanned the planet ranging from tropical to polar circles. Such a wide latitudinal span signifies that they not only grew in and occupied warm depositional environments, but also in settings where oceanic waters were cold and seasonally light limited. Moreover, their proliferation during the Devonian and Carboniferous was at a time when planet-wide climatic ice-house conditions are thought to have prevailed. Mud-mounds, therefore, may also be products of cool and cold-water carbonate sedimentation.

Plate motions, Gondwana Dinosaurs, Noah's Arks, Beached Viking Funeral Ships, Ghost Ships, and Landspans

Gondwana landmasses have served as large-scale biogeographic Noahs Arks and Beached Viking Funeral Ships, as defined by McKenna. The latitudinal trajectories of selected Gondwana dinosaur localities were traced through time in order to evaluate their movement through climate zones relative to those in which they originally formed. The dispersal of fauna during the breakup of Gondwana may have been facilitated by the presence of offshelf islands forming landspans (sensu Iturralde-Vinent and MacPhee) in the Equatorial Atlantic Gateway and elsewhere.