Paleoclimatologic Analysis of a Late Jurassic Petrified Forest, Southeastern Mongolia

Abstract

The Late Jurassic Suihent petrified forest is located in the Gobi desert of southeastern Mongolia (43?34'54 N, 108006'12E). The forest is represented by well-preserved silicified trees, including in situ stumps and preferentially oriented logs, all encased in a pyroclastic surge deposit. More than 72 upright stumps and 49 horizontal logs crop out in a belt approximately 100 m wide and 720 m long. The largest stump is 1.75 m in diameter and the longest log is 13 m in length. The orientation of the logs reflects that the pyroclastic flow originated northeast of the forest. Sanidine in the ash-flow tuff has been dated at 156 ? 0.76 Ma by 40Ar/39Ar geochronology. Forty-two samples of silicified wood were collected from the site for detailed paleoclimatologic tree-ring analysis. Cell structure is excellently preserved in seventeen wood samples. Several features of the growth rings are notable and climatologically significant. These include narrow latewood, paucity of false rings, and low mean and annual sensitivity values. These characteristics indicate that growth was steady during the growing season, but ceased abruptly at the onset of unfavorable conditions, and that interannual growth was fairly consistent. Water supply was likely the limiting factor for growth. Thus, the Suihent petrified forest, which grew at a paleolatitude of 460N, provides strong evidence of a paleoclimate that was highly seasonal with a rapid cessation of the growing season. These results are consistent with the presence of a monsoonal climate in this region of central Asia during the Late Jurassic.