July 2009 Geology and GSA Today Media Highlights

Facies model for fluvial systems in the seasonal tropics and subtropics
Christopher R. Fielding et al., Dept. of Geosciences, 214 Bessey Hall, University of Nebraska, Lincoln, Nebraska 68588-0340, USA. Pages 623-626.

Geologists routinely recognize the deposits of ancient river systems in the rock record. The models that geologists utilize to help interpret these rocks are biased toward the deposits of perennial rivers that characterize temperate and ever-wet climate regimes, and to a lesser extent those of desert systems. Fielding et al. propose that a third, intermediate type of fluvial system, characteristic of the subhumid to semiarid (seasonal) tropics, can be readily recognized in the rock record. This distinctive "fluvial style" describes rivers that experience strongly seasonal flow, and elevated levels of evaporation over precipitation such that trees preferentially colonize the riverbed as a specialized ecological niche. Major flow events are short-lived and intense, leading to abrupt rises and falls of water level. The resulting deposits of these rivers preserve abundant, in situ tree fossils, complex lateral transitions from coarse-grained to fine-grained sediments, and an abundance of sedimentary structures formed under fast-flowing water. Recognition of this fluvial style may aid those researchers concerned with interpreting climate from ancient rocks, and those exploring for natural resources in ancient fluvial strata.

Biogenic origin for Earth's oldest putative microfossils
Bradley T. De Gregorio et al., School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA. Pages 631-634.

Microbes and bacteria were the first living organisms on Earth, and they can be preserved in Archean silica-rich rocks. One such outcrop from western Australia, dated to 3.5 billion years ago, may hold the oldest "microfossils." However, their authenticity has been called into question, and it is possible the microbe-like features may have been formed by non-biological processes inside an ancient hydrothermal vent. De Gregorio et al. have compared carbonaceous matter in this rock with that from the 1.9-billion-year-old Gunflint Formation, which contains textbook examples of microfossil features. They found striking similarities between the carbon bonding and nitrogen, sulfur, and phosphorus composition of both ancient carbonaceous materials. This similarity suggests that biological processes may be responsible for the formation of the 3.5-billion-year-old carbonaceous matter, and that hydrothermal microorganisms existed at that time on Earth.

How long was Meridiani Planum wet? Applying a jarosite stopwatch to determine the duration of aqueous diagenesis Megan E. Elwood Madden et al., School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd St., Suite 810, Norman, Oklahoma 73019, USA. Pages 635-638.