September Geology and GSA Today highlights

August 22, 2000

Following are highlights from the September issue of Geology and a summary of the science article from the September issue of GSA Today, published by the Geological Society of America. Stories written regarding these articles are embargoed until September 1. We ask that you discuss articles of interest with the authors before publishing stories on their work, and that reference be made to Geology or GSA Today in stories published. Please contact Ann Cairns at GSA to request advance copies of articles and for additional information or assistance.

Geology

Annually dated late Weichselian continental paleoclimate record from the Eifel, Germany. Bernd Zolitschka, et al.
Sediments from Lake Holzmaar, Germany, contain annual laminations comparable to tree rings. These annual bands provide an accurate calendar year chronology for any detected environmental change. Here the authors apply this chronology to the carbonate content of the Holzmaar record that reflects eolian loess-like deposition. Assuming that intensified eolian deposition in the Eifel area is related to intensified Northern Hemispheric dust deposition, a correlation with the Greenland ice cores becomes feasible and is in good agreement with the GISP2 record. The authors correlate intensified dust accumulation with colder climatic conditions, and thus can date hitherto undated terminal moraines in northeastern Germany.

Missing molluscs as evidence of large-scale, early skeletal aragonite dissolution in a Silurian sea. Lesley Cherns, V. Paul Wright.
Scientists have long known that the fossils found in rocks only represent part of the great diversity of past life. Almost all fossils represent the remains of the hard parts of animals such as shells, and animals with only soft tissues do not make fossils. Until recently geologists thought that the fossils of animals with preservable hard parts were at least representative of the original shelly communities found in ancient sea deposits. The authors' discoveries suggest that even these shelly fossil communities provide highly inaccurate pictures of the diversity of ancient communities. Their work on 400 million year old reef deposits from Sweden suggest that many molluscs with hard parts have also disappeared from the fossil record because they were dissolved soon after death by natural acidity created by microbes in the seafloor sediment. The findings suggest that a complete reappraisal of the fossil record of marine life may be required.

Multiple taxon-multiple locality approach to providing oxygen isotope evidence for warm-blooded theropod dinosaurs. Henry C. Fricke and Raymond Rogers.
This study helps answer the long-standing question of whether dinosaurs were cold blooded or warm blooded. It is based on the idea that (1) body temperatures of cold blooded animals should decrease from the equator to the poles, while body temperatures of warm blooded animals should be the same everywhere; and that (2) these different latitudinal trends should be recorded by the chemical composition (i.e., oxygen isotope ratio) of tooth enamel. Focusing on several ~75 million year old localities, oxygen isotope ratios of tooth enamel from crocodiles and theropod dinosaurs reveal a latitudinal trend for crocodiles that is consistent with their being cold blooded animals with varying body temperatures. What is exciting is that theropods have a different latitudinal trend in isotopic ratios that indicates they were warm blooded with relatively uniform body temperatures everywhere. This isotopic approach to studying temperature regulation can be applied to a wide range of dinosaurs and other animals.

Friction marks on blocks from pyroclastic flows at the Soufriere Hills volcano, Montserrat: Implications for flow mechanisms. U. Grunewald, et al.
Pyroclastic flows are one of the deadliest of volcanic phenomena and are responsible for over half the casualties of global volcanic eruptions in the past 100 years. There is a great interest in understanding their flow mechanics so that improvements can be made in understanding how far from the volcano they travel. The authors report new observations of surface marks on boulders within pyroclastic flows from the eruption of the Soufriere Hills volcano, Montserrat. The marks show that the boulders tumbled and repeatedly collided in the flow and support a model of pyroclastic flows as flows of granular material. This observation thus rules out some other models of pyroclastic flows and provides a basis for developing physically realistic models.

Quantifying the oldest tidal record: The 3.2 Ga Moodies Group, Barberton Greenstone Belt, South Africa. Kenneth A. Eriksson and Edward L. Simpson.
The 3200 million year old Moodies Group in the Barberton Greenstone Belt, South Africa, contains the oldest preserved record of tides. Cross-beds set in the lower Moodies Group characteristically consist of alternating bundles of sandstone foresets separated by mudstone drapes. Analysis of foreset bundle thicknesses permits identification of diurnal, fortnightly, and monthly tidal periodicities. Tidal cyclicity recognized in the Moodies Group is comparable to that recorded in modern tidal settings and is most compatible with a lunar orbital shape similar to that existing today. The data are inconsistent with a markedly elliptical orbit 3200 million years ago as predicted by the lunar capture model, but rather indicate a slightly elliptical orbit that is more consistent with the impact model for the origin of the Moon.

Evolutionary paleoecology of the earliest echinoderms: Helicoplacoids and the Cambrian substrate revolution. Stephen Q. Dornbos and David J. Bottjer.
The general ecological settings present on seafloors from about 600 to 500 million years ago (late Neoproterozoic to early Phanerozoic) strongly impacted the subsequent evolution and development of animals, which were originating during this time in the event known as the Cambrian explosion, when almost all known animal groups, or Phyla, rapidly evolved. During this time of early animal evolution, animals burrowed deeper into seafloor sediments, perhaps in response to increased predation or increased seafloor oxygenation levels. This deeper burrowing softened the shallow marine muds and sands on which many of the earliest animals originally evolved and lived. The impact of this softening of sediments on early seafloor animals has been termed the Cambrian substrate revolution. One group of early seafloor animals, the helicoplacoid echinoderms (modern echinoderms include starfish and sea lilies) apparently could not adapt to the softening of the sediment on which they lived, and most likely went extinct because of it. Other early echinoderm groups, however, adapted to this increase in burrowing depth by evolving the ability to cement themselves to hard surfaces on the seafloor, such as shell fragments. The Cambrian substrate revolution, therefore, had a strong impact on early animal evolution and may have played a role in the seemingly high diversity of animal forms present during the Cambrian explosion because animals adapted to both relatively firm and relatively soft seafloor sediments that co-existed during this ecological transition.

On the history of humans as geomorphic agents. Roger LeB. Hooke.
The human population has been increasing exponentially. Simultaneously, our ability and motivation to modify the landscape by moving earth in construction and mining activities have also increased dramatically, as digging sticks and antlers have given way to wooden plows, iron spades, steam shovels, and finally today's huge excavators. As a consequence, humans have now become, arguably, the premier geomorphic agent sculpting the landscape, and the rate at which we are moving earth is increasing exponentially. As hunter-gatherer cultures were replaced by agrarian societies to feed this expanding population, erosion from agricultural fields also, until recently, increased steadily. This constitutes an unintended additional human impact on the landscape. The author explores quantitatively the evolution and impact of human earth moving.

GSA Today

The Cambrian Substrate Revolution. David J. Bottjer, et al.
The period from 600 to 500 million years ago (the Neoproterozoic-Phanerozoic transition) was an interval of rapid evolution of complex animals that exerted strong and lasting influence on ocean bottom­dwelling life. The causes of the evolutionary spurt have been difficult to pin down. David Bottjer, James Hagadorn, and Stephen Dornbos suggest that it was caused by changes in the consistency of the bottom of the ocean floor. Prior to this time, the seafloor was fairly cohesive because the sediment was covered by sticky algal and/or bacterial mats. However, during the Neoproterozoic-Phanerozoic transition, animals that grazed the algae became abundant, and the sediment previously stabilized by algal mats became soupy. The authors suggest that the new substrate consistency stimulated evolution of new adaptations and new organisms. The question of why so many types of animals evolved so rapidly during this period is an important one, and this paper provides a novel and plausible explanation.
-end-
*To view the complete table of contents of Geology, as well as that of the Geological Society of America Bulletin, see http://www.geosociety.org/pubs/cattract.htm.

*Full text Geology articles and the science article from GSA Today are available on the first of each month on the GSA Web site, http://www.geosociety.org/pubs/journals.htm.

August 23, 2000
GSA Release No. 00-20

Geological Society of America

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