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Abstract Glycerol dialkyl glycerol tetraethers (GDGTs), both archaeal isoprenoid GDGTs (isoGDGTs) and bacterial branched GDGTs (brGDGTs), have been used in paleoclimate studies to reconstruct environmental conditions. Since GDGTs are produced in many types of environments, their relative abundances also depend on the depositional setting. This suggests that the distribution of GDGTs also preserves useful information that can be used more broadly to infer these depositional environments in the geological past. Here, we combined existing iso‐ and brGDGT relative abundance data with newly analyzed samples to generate a database of 1,153 samples from several modern sedimentary settings. We observed a robust relationship between the depositional environment and the relative abundances of GDGTs in our samples. This data set was used to train and test theBranched andisoGDGT Machine learningClassification (BIGMaC) algorithm, which identifies the environment a sample comes from based on the distribution of GDGTs with high precision and recall (F1 = 0.95). We tested the model on the sedimentary record from the Giraffe kimberlite pipe, an Eocene maar in subantarctic Canada, and found that the BIGMaC reconstruction agrees with independent stratigraphic and palynological information, provides new information about the paleoenvironment of this site, and helps improve its paleotemperature reconstruction. In contrast, we also include an example from the PETM‐aged Cobham lignite as a cautionary example that illustrates the limitations of the algorithm. We propose that in cases where paleoenvironments are unknown or are changing, BIGMaC can be applied in concert with other proxies to generate more refined paleoclimate records. 

Abstract We evaluate the efficacy of the stable isotope composition of precipitation and plant waxes as proxies for paleoaltimetry and paleohydrology in the northern tropical Andes. We report monthly hydrogen (δ²H_p) and oxygen (δ¹⁸O_p) isotope values of precipitation for an annual cycle, and hydrogen isotope values of plant waxes (δ²H_wax) obtained from modern soils along the eastern and western flanks of the Eastern Cordillera of Colombia. δ²H_p, δ¹⁸O_p, as well as the unweighted mean δ²H_waxvalues ofn‐C₂₉,n‐C₃₁, andn‐C₃₃n‐alkanes in the eastern flank show a dependence on elevation (R² = 0.90, 0.82, and 0.65, respectively). In stark contrast, the stable isotope compositions of neither precipitation nor plant waxes from the western flank correlate with elevation (R² < 0.23), on top of a negligible (p‐value >0.05) correlation between δ²H_waxand δ²H_p. In general, δ²H_waxvalues along the eastern flank of the Eastern Cordillera seem to follow the trend of a simple Rayleigh distillation process that is consistent with studies elsewhere on the eastern side of the Andes in South America. Neither δ²H_pnor δ¹⁸O_p, and therefore δ²H_wax, offer reliable estimates of past elevations in the western flank, due perhaps to water vapor source mixing, evaporation overprint, contrasting plant communities, and/or differences in evapotranspiration. Thus, δ²H_waxis only reliable for paleohydrology and paleoaltimetry reconstructions on the eastern flank of the Andes, whereas interpretations based on δ²H_pand/or δ¹⁸O_pwest of the highest point of the Eastern Cordillera need to consider mixing of moisture sources in addition to precipitation amount. 

Abstract Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial cell membrane lipids that, when preserved in sedimentary archives, can be used to infer continental paleotemperatures. Although commonly used global calibrations capture a relationship between the distribution of brGDGTs and temperature, they underestimate temperatures for tropical regions as much as ~16°C. Furthermore, some global calibrations reach saturation at around 24–25°C, and, in general, they have root‐mean‐squared errors (RMSEs ≈ ~4°C) that are too large for them to resolve small variations in paleoclimate variability in tropical regions. We present an in situ regional calibration of soil brGDGTs along altitudinal transects on both flanks of the Eastern Cordillera of Colombia in the northern tropical Andes that spans ~3,200 m in elevation and 17°C and 19°C in mean annual soil and air temperatures, respectively. These new soil and air regional calibrations yield RMSEs of 1.5°C and 1.9°C, respectively. When combined with existing data from elsewhere in the tropics, the integrated data (n = 175) not only fit a linear calibration with a RMSE of 2.7°C but also fit a nonlinear calibration with a RMSE of 2.2°C. These calibrations allow for a more precise and reliable reconstruction of past temperatures in the tropics than global calibrations. 

The cross-equatorial southwesterly winds from the eastern equatorial Pacific direct moisture toward the Pacific coast of northwestern South America, where subsequent orographic lifting creates the wettest regions in the world. The Choco low-level jet is emblematic of broader westerly winds in this region and is projected to weaken by the end of the 21st century, but climate models show considerable disagreement about the extent of weakening. Using contemporary observations, we demonstrate that the configuration of westerly winds in the eastern equatorial Pacific is reflected by hydrogen isotopes in precipitation (δDp) in western Ecuador. As westerly winds strengthen, δDp increases from greater transport of δDvapor enriched in deuterium from the Eastern Pacific Warm Pool. We apply this framework to a new record of reconstructed δDp using leaf waxes in ocean sediments off the coast of Ecuador (ODP1239, 0◦40.32′ S, 82◦4.86′ W) that span the Plio-Pleistocene. Low δDp in the early Pliocene indicates weak westerly water vapor transport in a warmer climate state, which is attributed to a low sea surface temperature gradient between the cold tongue and off-equatorial regions in the eastern equatorial Pacific. Near 3 Ma, westerly water vapor transport weakens, possibly as a result of shifts in the Intertropical Convergence Zone forced by high latitude Northern Hemisphere cooling. In complementary isotope-enabled climate simulations, a weak Choco jet and westerly water vapor transport in the early Pliocene are matched by a decrease in δDp and hydroclimate changes in western Ecuador. Precipitation from the Choco jet can cause deadly landslides and weakened westerly winds in the early Pliocene implies a southward shift of these hazards along the Pacific coast of northwestern South America in the future. 

Abstract Among quantities of interest in paleoclimate, the mean annual air temperature, Ta, directly over the surface looms prominently. Most geologic estimates of past temperatures from continental regions, however, quantify temperatures of the soil or other material below the surface, Ts, and in general Ta < Ts. Both theory and data from the FLUXNET2015 data set of surface energy balance indicate systematic dependences of temperature differences ΔT = Ts − Ta and also of Bowen ratios—ratios of sensible to latent heat fluxes from surface to the atmosphere—on the nature of the land-surface cover. In cold regions, with mean annual temperatures ≲5 °C, latent heat flux tends to be small, and values of ΔT can be large, 3–5 °C or larger. Over wet surfaces, latent heat fluxes dominate sensible heat fluxes, and values of both ΔT and Bowen ratios commonly are small. By contrast, over arid surfaces that provide only limited moisture to the overlying atmosphere, the opposite holds. Both theory and observation suggest the following, albeit approximate, mean annual values of ΔT: for wetlands, 1 °C; forests, 1 ± 1 °C; shrublands, 3–4 °C; savannas, 3.5 °C < ΔT < 5.5 °C; grasslands, 1 °C where wet to 3 °C where arid; and deserts, 4–6 °C. As geological tools for inferring past land-surface conditions improve, these approximate values of ΔT will allow geologic estimates of past mean annual surface temperatures, Ts, to be translated into estimates of past mean annual air temperatures, Ta. 

Abstract We attempted to make an objective assessment of whether fossil pollen assemblages from the Sabana de Bogotá require surface uplift of ~2000 m since 6–3 Ma, as has been argued. We relied on recently published elevation ranges of plants for which fossil pollen has been found in sites 2000–2500 m high in the Sabana de Bogotá. The elevation ranges of fossil plants do not overlap, suggesting that those ranges may be too narrow. By weighting these elevation ranges by percentages of corresponding fossil pollen and summing them, we estimated probability density functions for past elevations. These probability distributions of past elevations overlap present-day elevations and therefore do not require surface uplift since deposition of the pollen. Fossil pollen assemblages include pollen from some plant taxa for which we do not know present-day elevation ranges, and therefore, with a more complete knowledge of elevation distributions, tighter constraints on elevations should be obtainable. The elevation of the oldest assemblage, from Tequendama, which lies at the southern edge of the Sabana de Bogotá and is thought to date from 16 to 6 Ma, is least well constrained. Although our analysis permits no change in elevation since the pollen was deposited, we consider 1000–2000 m of elevation gain since 15 Ma to be likely and consistent with an outward growth of the Eastern Cordillera.