February Geology and GSA Today media highlights

Rivers are a familiar feature of Earth’s landscape and have a significant impact on human existence, either as a source of water or as a hazard through flooding or triggering of landslides and debris flows. Because they play a key role in shaping Earth’s landscape, it is vital to understand how fluvial systems work. Although considerable research has focused on alluvial rivers that dominate lowland landscapes, much less attention has been paid to rivers that incise bedrock, which typify fluvial channels found in mountainous areas. Whittaker et al. focus on such channels that drain mountains in central Italy, where present-day earthquake activity is contributing to landscape instability. Current landscape models, which can be used to describe how this area might evolve as tectonic activity continues, make important assumptions about how rivers behave. Whittaker et al. challenge these assumptions by showing that rivers dynamically adjust to uplift on active faults and that traditionally used mathematical relations, which link river discharge to channel shape, do not apply in these settings. The results help to clarify the long-term erosion mechanics of bedrock rivers, explain how some rivers manage to cut across active faults, and challenge geomorphologists to improve their landscape evolution models.

Fish tooth δ[delta]18O revising Late Cretaceous meridional upper ocean water temperature gradients
Emmanuelle Pucéat, Université de Bourgogne, UFR Sciences de la Terre, Dijon 21000, France; et al. Pages 107-110.

The Cretaceous period is known as the warmest period of the past 250 Ma, with atmospheric CO2 levels that may have been as high as 5 to 10 times modern levels during the climatic optimum of the middle Cretaceous (ca. 95 Ma). This climatic optimum was followed by a long-term cooling through the end of the Cretaceous (ca. 65 Ma). Using the oxygen isotope composition of fossil fish teeth, a paleo–upper ocean temperature proxy exceptionally resistant to diagenetic alteration, Pucéat et al. show that low- to middle-latitude thermal gradients of the Middle Cretaceous climatic optimum and of the cooler latest Cretaceous are similar to modern levels, despite a cooling of 7 °C between the two periods. These new results imply that no drastic changes in meridional heat transport are required to explain the Late Cretaceous climate. Based on climate models, such a cooling without any change in the low- to middle-latitude thermal gradient supports an atmospheric CO2 decrease as the primary driver of the climate evolution recorded during the Late Cretaceous.