ABSENCE OF PRESERVED MANGANIFEROUS RHIZOLITHS WITHIN THE PETM INTERVAL AND ITS CLIMATIC IMPLICATIONS

A measured section of the Willwood Formation at Polecat Bench in the Bighorn Basin, Wyoming shows that the kind of fossil root systems varies across the Paleocene-Eocene Thermal Maximum (PETM) interval. In particular, manganiferous rhizoliths (Mn rhizoliths) are abundant in strata directly below and above the PETM interval. Mn rhizoliths are relatively sparse during the initial decline in δ¹³C values and are absent from the remainder of the PETM.

Where present, Mn rhizoliths are found in discrete horizons and weather as black and red-brown nodular masses spaced from 10 cm to several meters apart. The masses are typically a few centimeters to ~50 cm in diameter and extend vertically 5 cm to > 50 cm, but most have vertical dimensions between 10 cm and 30 cm. In a few locations the Mn rhizoliths were seen to branch horizontally and could be traced for several meters. The color and preservation are similar to other nodules previously reported from the Paleogene of the Bighorn Basin as casts of tree trunks and primary root systems preserved by calcite and a mixture of goethite and manganese oxides. Locally nodular masses similar to those described above, but irregular in shape and only centimeters in longest dimension are seen up to 75 cm below the level of more typical Mn rhizoliths, and are interpreted as locally preserved extensions of the root systems.

The Mn rhizoliths are present only in weak paleosols developed on crevasse-splay and avulsion deposits. Weak pedogenesis is indicated by the lack of extensive mottling, rooting, or burrowing and common preservation of relict bedding. Crevasse-splay and avulsion deposits are present throughout the study area, including the PETM interval, indicating that Mn rhizolith distribution is not dependent on the presence of those facies. Previous workers have attributed similar manganese oxide and carbonate concentrations to the decay of plant material in wet soils with alternating reducing and oxidizing conditions. A similar mechanism may be responsible for the preservation of Willwood Mn rhizoliths. The lack of Mn rhizoliths within the PETM interval is interpreted as the result of better-drained soil conditions and lower water tables, which is consistent with lower paleoprecipitation estimates of previous studies.