Stephen R. Jacobson

Sedimentary 12-n-Propylcholestanes, Molecular Fossils Diagnostic of Marine Algae

Certain C30-steranes have been used for identifying sedimentary rocks and crude oils derived from organic matter deposited in marine environments. Analysis of a C30-sterane from Prudhoe Bay oil indicates that these C30-steranes are 24-n-propylcholestanes that apparently are derived from precursor sterols 24-n-propylidene-cholesterols and 24-n-propylcholesterol. These widely occurring sterols are biochemically synthesized in modern oceans by members of an order (Sarcinochrysidales) of chrysophyte algae. These data thus imply that C30-sterane biomarkers in sedimentary rocks and crude oils have a marine origin. Screening of a few organic-rich sedimentary rocks and oils from throughout the Phanerozoic suggests that these C30-steranes first appeared and, therefore, their source algae evolved between Early Ordovician and Devonian.

Paleoclimate of the Kimmeridgian/Tithonian (Late Jurassic) world: I. Results using a general circulation model

Abstract The Kimmeridgian/Tithonian (154.7−145.6 Ma) (middle and late Late Jurassic) was a time of expanded continental rifting, increased sea-floor spreading, and a relatively high eustatic sea level stand. These processes collectively caused the fragmentation and flooding of the megacontinent Pangea as well as the alteration of the global paleoclimate. Using a version of the Community Climate Model (CCM) from the National Center for Atmospheric Research, we report two Kimmeridgian/Tithonian paleoclimate seasonal simulations, with geologically inferred paleotopography: one using a CO 2 concentration of 280 ppm (pre-industrial level) and the other 1120 ppm. Increasing the CO 2 four-fold warms virtually the entire planet. The greatest warming occurs over the higher latitude oceans and the least over the equatorial and subtropical regions. Simulation of a warmer planet with an elevated greenhouse effect fits the distribution of paleoclimatically sensitive faunas, floras, and sedimentary rocks. Model results indicate that sea ice was restricted to the high latitudes of the Boreal and Austral seas, making landfall only in restricted areas. The trade winds bring heavy rainfall in December/January/February to eastern Gondwana and in June/July/August to the Tethys Sea margins. A strong June/July/August monsoon occurs over southeast Asia. The distribution of coals correlates to precipitation sufficient to maintain gymnosperm forests and coastal areas where water saturated sediments are a result of eustatic high stands of sea level. Evaporites are localized to areas of negative precipitation-evaporation. Runoff is restricted to regions of intense precipitation. Overall, the 1120 ppm CO 2 simulation provides a reasonable paleoclimate for the Kimmeridgian/Tithonian and provides a standard until a CCM with oceanic heat transport, a coupled atmospheric/oceanic model, or one with a finer grid cell configuration is available. The results need further scrutiny in areas with more detailed geologic information.

Palynostratigraphic framework for the cretaceous (albian‐maestrichtian) of the overthrust belt of Utah and Wyoming

Abstract The ranges of twenty selected species of palynomorphs define the palynostratigraphic framework of Cretaceous strata of the central overthrust belt of the Rocky Mountains. The palynostratigraphy is based on local ranges of dinocysts, spores, and pollen determined from more than 300 samples from independently dated outcrop reference sections in Fossil Basin, southwestern Wyoming. The stratigraphic succession in Fossil Basin is essentially continuous from middle Albian to upper Maestrichtian, except for a hiatus in the Campanian, and includes about 4500 meters of marine and nonmarine rocks. The most reliable biostratigraphic zonation for the region is one based on local ranges of palynomorphs. Use of both marine and nonmarine palynomorphs not only permits correlation between facies, but also can refine zonation of the total interval. Ratios of marine to nonmarine palynomorphs in the interval studied show a pattern that represents transgression and regression of the Western Interior Cretaceous seaway...

Acritarch biostratigraphy of the dicellograptus complanatus graptolite zone from the Vaureal formation (Ashgillian), Anticosti Island, Quebec, Canada

Abstract Forty‐two species of organic‐walled microphytoplankton (ac‐ritarchs) and a single specimen of a spore‐like microfossil are documented from the Dicellograptus complanatus graptolite Zone in the Vaureal Formation on Anticosti Island, Quebec. Our samples are from cored intervals in the New Associated Consolidated Paper #1 well. These cores contain biostratigraphically useful chitinozoans and graptolites which precisely correlate these intervals with lower to middle Ashgillian. Conodonts, ostracodes, and megafossils from outcrops of the Vaureal Formation on Anticosti Island support this lower to middle Ashgillian correlation for our sampled interval. The acritarch assemblages obtained from the sampled interval contain abundant well‐preserved specimens. Of these, some forms considered diagnostic of the Dicellograptus complanatus (graptolite) Zone are Multiplicisphaeridium bifurcatum, M. irregulare, Orthosphaeridium insculptum, Poikilofusa spinata, Sylvanidium paucibrachium, and Veryhachium hamii. Four...

A paleoclimate simulation of the Wenlockian (Late Early Silurian) world using a general circulation model with implications for early land plant paleoecology

Abstract The Silurian Period (439–409 Ma) is known for its extensive organic-rich, graptolitic, black shales and graptolitic, shelf carbonates. Physical conditions drive paleoclimate and control the zonal deposition of lithotopes. Moreover, the paleoclimate created a paleoceanic environment favorable for widespread generation, deposition, and preservation of plankton. The relationship between the paleogeographic framework, including paleotopography, and the resultant bio- and lithostratigraphy are suitable for study with a general circulation model (GCM). For this study we chose the Wenlockian Stage (430–424 Ma), the late Early Silurian. The Wenlockian northern hemispheric surface was dominated by a large ocean, the southern hemisphere by the giant Gondwanan continent. Much of Gondwanas extensive margin was in the mid-latitudes. Laurentia and Baltica occupied a tropical position, and Siberia and Kazakh laid in warm temperate latitudes to the north. Silurian stratigraphy fits a paleoatmosphere with elevated greenhouse conditions. Estimated Silurian atmospheric CO 2 values vary between wide limits. We used 1120 ppm CO 2 4x that of the pre-industrial level and at the lower end of the range of estimates. The paleoclimate was forced by the paleogeography of each hemispheres circulation. The northern hemisphere Silurian simulation is dominated by strong zonal circulation in all seasons. In contrast, the continental southern hemisphere reacts to the summer heating and winter cooling of Gondwana. This simulation furnishes paleoclimatic conditions that help explain the distribution of early land plants. This paleoclimate simulation supports a humid coastal paleoenvironment for early Silurian land plants. Furthermore, these results imply that relative humidity was more important than precipitation rates, and that intercontinental spore dispersal between Laurentia/Europe and Gondwana was not wind-aided. The GCM provides a good match with Silurian lithological and paleobiological data.

Late Jurassic Paleoclimate of Pangea Based on Results from a General Circulation Model

The progressive disintegration of Pangea occurred during the Jurassic period. The Late Jurassic Epoch represented a time when rift systems divided Gondwana in two, and separated northern Gondwana from North America. Rising eustatic sea level throughout the Jurassic flooded large parts of the continents particularly in the Northern Hemisphere. These conditions created a series of zonally oriented continents and contributed significantly to climate amelioration in the mid-Mesozoic. This chapter focuses on the changed paleoclimate caused by Pangea’s disintegration.