Dynamics of soil carbonate growth in the tropical domain
Résumé
Pedogenic carbonate is widespread at mid latitudes where combined warm and dry
conditions favor soil carbonate growth in summer. The mechanisms and tempo of
pedogenic carbonate formation are less certain in the tropics. There, longer periods of
soil water saturation and higher soil respiration enhance calcite dissolution,
particularly during summer. However, pedogenic carbonate growth during summer
seasons remains the assumption of most studies using stable and clumped isotope
proxies on paleosols.
To test this assumption, we investigated stable and clumped isotope values from
Quaternary and Miocene pedogenic carbonates in the tropical monsoonal domain of
Myanmar along an annual rainfall gradient. We show that carbonate growth in
Myanmar occurs in the coldest months of the year due to sustained rainfall from mid
spring to late fall. Carbonate growth timing locally varies from early winter to early
spring; this trend is partly influenced by local rainfall amount, with growth at the
wettest sites delayed to spring.
The winter bias in carbonate growth temperatures found in both Quaternary and
Miocene Burmese soils, which is so far unique amongst pedogenic carbonates,
constitutes a potential signature for past tropical monsoonal (warm summer-wet)
climates in paleosols. 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
facilitated by high winter temperatures that allow carbonate precipitation to occur
outside of the warmest months of the year. This high seasonal sensitivity is expected to
be more prominent in the geological record during times with higher temperatures and
greater expansion of the tropical realm.