Paleoclimate reconstruction of the Tatacoa region, Colombia, during the Middle Miocene Climatic Transition

The paleosols of the La Tatacoa region in Colombia provide a detailed continental record of environmental change during the Middle Miocene Climatic Transition, a time of global cooling and Antarctic ice-sheet expansion. To reconstruct these paleoenvironmental conditions, we combined stable isotopes of soil organic matter ( δ 13 C OM ) and pedogenic carbonates ( δ 13 C carb , δ 18 O carb ) with geochemical proxies to estimate mean annual precipitation (MAP), mean annual temperature (MAT), and meteoric water composition ( δ 18 O w ). Using MAT values of 17.5–26 °C derived from climatic proxies, we calculated δ 18 O w from pedogenic calcite. In the La Victoria Formation, δ 18 O w values range from –7.8‰ to –5.6‰ (26 °C), reflecting isotopically depleted rainfall of a convective wetland system during interglacial conditions. In contrast, the Villavieja Formation exhibits systematically heavier meteoric-water values ( δ 18 O w ≈ –6.9‰ to –4.6‰), indicating a fundamental hydrological reorganization during the Mi-4 glacial interval. Mean annual precipitation estimates indicate that both formations developed under broadly subhumid conditions, but Villavieja exhibits slightly higher mean MAP values (~850–1000 mm yr -1 ) than La Victoria (~540–900 mm yr -1 ) and, more importantly, an expanded maximum precipitation range, with upper-bound averages reaching ~1520–1840 mm yr -1 and an individual peaks exceeding 3000 mm yr -1 . This pattern reflects enhanced hydrological variability, expressed as a broader precipitation range and stronger wet–dry contrasts, rather than a simple uniform increase in total rainfall. We interpret this shift as a transition from the relatively stable unimodal monsoon regime that characterized La Victoria to a more seasonal bimodal precipitation regime in Villavieja. This shift occurred during glacial conditions may explain the unexpected inversion between higher MAP and heavier δ 18 O carb values. Carbon-isotope systematics further constrain an environmental change. Carbonate δ 13 C carb values range from –16.0‰ to –10.7‰ in La Victoria, reflecting wetland soils dominated by soil-respired CO₂ and fluctuating redox conditions, whereas Villavieja carbonates cluster near –13‰ to –12‰, consistent with better-drained paleosols. Two pronounced positive excursions of ~+6‰ in δ 13 C OM of soil occur in the URB unit and are interpreted as shifts in organic-matter cycling and localized carbon burial, plausibly linked to the CMX carbon maximum associated with the Mi-4 glacial stage. These excursions coincide with previously reported positive δ 18 O carb shifts and align with marine Mi-4/CMX records. The collapse of the Pebas megawetland, enrichment of tropical rainfall isotopes during glaciation, and emergence of bimodal precipitation regimes together illustrate how subtle regional boundary conditions can strongly modulate the continental expression of global climate transitions. • Provides the first terrestrial record of the coupled Mi-4/CMX isotopic excursions in northern South America. • Documents a major mid-Miocene shift from a unimodal to a bimodal rainfall regime in the tropical Andes. • Reconstructs increased mean annual precipitation in the Andean tropics during the Mi-4 glacial interval, contrary to expectations. • Identifies the collapse of the Pebas megawetland as a key driver of tropical hydrological reorganization. • Links hydrological change to interplay between global climate forcing and early Eastern Cordillera uplift. • Quantifies low silicate weathering fluxes, indicating limited carbonic acid weathering in seasonal paleosols (Ustic soil moisture regime).