Enigmatic isotope signals in the Amazon
A brief summary of Kukla et al., 2023
Oxygen isotope shifts across tropical South America differ in their magnitude and timing in the last 21 thousand years, but it’s not clear why. We argue that the confusing features in the isotope records are best explained by east-west shifts in the location of peak rainout. Rainout could shift east-to-west due to shifts in vegetation that modify how much sunlight the land surface reflects, in turn affecting the heating of the atmosphere and the circulation patterns that balance it.
The east west (“zonal”) dipole in precipitation
Proxy data show east tropical South America was drier at the Last Glacial Maximum, and wetter in the mid-Holocene. The west followed the opposite trend.
This shift corresponds with some of the largest speleothem oxygen isotope signals ever recorded in these “east” and “central” records, with much less variation in the west.
If these isotope signals track the same dynamics, why does their magnitude and timing differ?
Spatial shifts in rainout could explain the spatially distinct signals.
Changes in the precipitation isotope composition over space track “rainout”–the degree to which precipitation exceeds evaporation + transpiration.
We argue that more negative values on the top panel, or a larger y-axis span of color on the bottom panel, indicate more moisture was rained out between the two bordering isotope records.
In this framework, the amount of total rainout doesn’t change very much over time. Only the location of rainout does. It sits near western Amazonia about 20 thousand years ago, then over the Atlantic Ocean around 7 thousand years ago, and finally somewhere in between today.
Oxygen isotope profiles track where it’s rainy
When there’s more rainout between two sites, there’s also a larger decrease in the oxygen isotope composition.
The right panels illustrate our hypothesis – where we think the peak of rainout occurred at different times, and how that relates to the isotope gradient.
For example, we think rainout shifted to the west during the Last Glacial Maximum (bottom panels) because the largest decrease in the isotope composition happens between the central and western sites.
What causes rainout to shift from east to west?
We think it could be related to shifts in the atmospheric energy budget that differ from east to west.
Seasonally, the tropical rain belt moves north-to-south approximately following the latitude of maximum energy from the sun (learn more here).
Boos & Korty, 2016 showed that, in the rainiest places, the same physics can push and pull wet regions east and west (in addition to north and south). While incoming sunlight doesn’t change from east-to-west, the reflectivity of land does. If land becomes less reflective (say, as vegetation gets denser), it will heat the atmosphere more. This heat has to go somewhere (it can’t build up forever) so atmospheric circulation changes to balance it.
When we use the Boos & Korty model to study vegetation shifts consistent with the Last Glacial Maximum and Mid-Holocene, we find that the center of rainout (the circles in the figure panels) shifts east-to-west in a way that matches the proxy data.
Two points to keep in mind:
The Boos & Korty model is idealized. It lacks a lot of fancy physics that might matter. More sophisticated models usually simulate small or negligible east-west shifts in rainout, but they also tend to under-predict the magnitude of the isotope signal by a factor of 2 or 3 (Cruz et al., 2009; Liu & Battisti, 2015; Kukla et al., 2023). It remains to be seen whether the speleothem signals can be replicated by more sophisticated models.
Our interpretation assumes a hydrologic connection between sites. We argue that the east, central, and western sites (the stars in the previous map) are hydrologically connected, and we use a moisture-tracking model to show it (see our paper, Figure 2). Still, the degree of hydrologic connection and its robustness through time is up for debate.
Our interpretation is certainly not the only way to explain the isotope data. But we think it does a good job explaining its most enigmatic features, including the differences in the size and timing of the signals over space.