Alaskan Palaeogene Climates as Inferred from the Clamp Database

Abstract

CLAMP (Climate-Leaf Analysis Multivariate Program) uses Correspondence Analysis of 29 rigorously defined character states of foliar physiognomy to order leaf samples. The database of modern vegetation comprises 106 samples, each composed of >19 species of woody dicotyledons collected proximal to a meteorological station. The two principal axes in Correspondence Analysis account for ~70 % of physiognomic variation. Ordination of the samples on Axis I reflects various temperature parameters and on Axis II various water-stress parameters. Regression analysis of relative placement of the samples indicates that mean annual temperature (MAT) and cold-month mean temperature (CMMT) can be estimated with a standard error of <1° and <2 °C, respectively. Analyses of water-stress factors tend to produce only general estimates of, for example, seasonal drought and growing season precipitation; certain critical levels are emphasized by these analyses. Any fossil leaf sample of woody dicotyledons can be analysed in the CLAMP database and various palaeoclimatic parameters estimated.

The sequence of leaf assemblages on the Yakutat block, which is now at lat. 60° N at the head of the Gulf of Alaska, was, during the Eocene, located as the coastal fringe of southeastern Alaska. During the Eocene, however, the Yakutat assemblages represent vegetation at 65–70 °N, because of the subsequent counterclockwise rotation of North America. The composite Kulthieth assemblages represents the Early Eocene thermal maximum; CLAMP analyses indicate a MAT of 19.4°C and a CMMT >12 °C. Marginal Paratropical Rain forest thus extended along the coast of western North America to ~70 °N. CLAMP analyses of the Middle Eocene (~45 Ma) and latest Eocene (~34 Ma) Yakutat assemblages produce MAT estimates of ~17 °C and 16 °C, respectively, suggesting overall cooling during the Eocene. At the same time, CMMT also decreased, and climates became more extreme, although not as extreme as after ~33 Ma.

Terranes of the main part of southern Alaska were accreted to cratonic Alaska by the end of the Mesozoic. The Chickaloon assemblage has latest Palaeocene radiometric ages, and palaeomagnetic analysis of volcanic flows elsewhere on the same terrane indicates that the Chickaloon represents vegetation at ~75 °N. CLAMP analyses indicate that the Chickaloon represents microthermal mixed broad-leaved evergreen and coniferous forest. This modern vegetation type has many arcto-tertiary clades, but the Chickaloon flora is dominated by deciduous Taxodiaceae and extinct lineages of clades such as Trochodendrales and Hamamelidales. Significant numbers of extant arcto-tertiary lineages appear in Alaska ~5 Ma after their first appearance at high altitudes in middle latitudes.