The paleobotanical record of Myanmar is poorly documented despite its importance for understanding the evolution of Asian monsoonal ecosystems through time. Here, we describe seventeen species of fossil wood from silicified specimens collected in the upper lower to lowermost middle Miocene Natma Formation, central Myanmar. These species share affinities with modern Fabaceae, Dipterocarpaceae, Burseraceae, Moraceae and Cupressaceae. Five new species are described, belonging to genera Koompassioxylon Kramer (K. kalewensis n. sp.), Pahudioxylon Chowdhury, Ghosh & Kazmi (P. adenantheroides n. sp.), Dipterocarpoxylon (Holden) Den Berger (D. fugax n. sp.), Shoreoxylon Den Berger (S. glomeratum n. sp.), Artocarpoxylon Prakash & Lalitha (A. informe n. sp.). They include the first record of genus Dryobalanoxylon Den Berger in Myanmar, as well as a great variety of fossil dipterocarps (7 species) as found in today’s Southeast Asian rainforests. The nearest living relatives of this assemblage reflect different ecotones of seasonal forests with coastal, mixed to dry deciduous, and wet evergreen species. This reconstruction implies a wet, warm, and monsoonal climate in Myanmar during the late early Miocene. The presence of fossil dipterocarp species typical of wet evergreen forests contrasts with Burmese Eocene dry dipterocarp assemblages and indicates wetter conditions during the Miocene. Our reconstructions support a long-term change from seasonal to everwet ecosystems for dipterocarp trees.

Pedogenic carbonate is widespread at mid latitudes where combined warm and dry conditions favor soil carbonate growth from spring to fall. The mechanisms and tempo of pedogenic carbonate formation are more ambiguous in the tropics, where longer periods of soil water saturation and higher soil respiration enhance calcite dissolution. This paper provides bulk and clumped isotope values from Quaternary and Miocene pedogenic carbonates in the tropical monsoonal domain of Myanmar where annual rainfall reaches up to 1700 mm. We show that carbonate growth in Myanmar is delayed to the coldest months of the year by sustained rainfall from mid spring to late fall. We propose that high soil moisture year-round in the tropical domain makes carbonate growth more episodic than in temperate ecosystems, and particularly sensitive to the seasonal distribution of rainfall. This sensitivity is also enhanced by high winter temperatures, allowing carbonate growth to occur outside the warmest months of the year. This high sensitivity is expected to be more prominent in the geological record during times with higher temperatures and greater expansion of the tropical realm. The winter bias in TD47 values found in Burmese soils, unique for pedogenic carbonates, constitute a potential signature for past tropical monsoonal (warm summer-wet) climates in paleosols, and are also found in our Miocene samples.