Dynamics of Pedogenic Carbonate Growth in the Tropical Domain of Myanmar

Abstract

Pedogenic carbonate is widespread at mid latitudes where warm and dry conditions favor soil carbonate growth from spring to fall. The mechanisms and timing of pedogenic carbonate formation are more ambiguous in the tropical domain, where long periods of soil water saturation and high soil respiration enhance calcite dissolution. This paper provides stable carbon, oxygen and clumped isotope values from Quaternary and Miocene pedogenic carbonates in the tropical domain of Myanmar, in areas characterized by warm (>18°C) winters and annual rainfall up to 1,700 mm. We show that carbonate growth in Myanmar is delayed to the driest and coldest months of the year by sustained monsoonal rainfall from mid spring to late fall. The range of isotopic variability in Quaternary pedogenic carbonates can be solely explained by temporal changes of carbonate growth within the dry season, from winter to early spring. We propose that high soil moisture year-round in the tropical domain narrows carbonate growth to the driest months and makes it particularly sensitive to the seasonal distribution of rainfall. This sensitivity is also enabled 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. Clumped isotope temperatures, δ13C and δ18O values of tropical pedogenic carbonates are impacted by changes of both rainfall seasonality and surface temperatures; this sensitivity can potentially be used to track past tropical rainfall distribution.Plain Language SummarySoil carbonates are the focus of many continental paleoenvironmental studies because their isotopic composition records many features of the local environment (such as the type and density of vegetation, annual or warm season temperatures, and aridity). Soil carbonates are commonly studied in temperate and arid areas; in those environments, carbonates form during warm months when soils dry. Soil carbonates are rarer but present in the tropical domain, where their isotopic systematics and formation processes have been barely studied. This study provides stable isotopic data from soil carbonates in the tropical monsoonal domain of Myanmar, which is characterized by warm (>18°C), dry winters and abundant summer rainfall. We show that these soil carbonates grow during the coldest months of the year and follow different dynamics and isotope systematics than those of temperate and arid areas. We show that high soil wetness and warm temperatures year-round make carbonate growth particularly sensitive to the seasonal distribution of rainfall in the tropical domain. This seasonal sensitivity complicates the interpretation of soil isotopic data from past tropical ecosystems. We suggest that isotopic data from tropical paleoenvironments can be used as a proxy to reconstruct past rainfall distribution instead of average (or summer) environmental features.Key PointsTropical soil carbonates under monsoonal rainfall grow in winter and early springThe cold-season bias in carbonate growth is promoted by warm (>15°C) winter temperatures and by high soil water content in summer and fallClumped isotope temperatures in tropical paleosols are likely impacted by changes of rainfall distribution