Obliquity Influence on Low-Latitude Coastal Precipitation in Eastern Brazil During the Past ∼850 kyr

Paleoclimate records from tropical South America typically show precession-paced variability in rainfall, caused by insolation-driven changes in the South American Monsoon System, however this mechanism may not be responsible for hydroclimate change outside of the core monsoon domain, such as in the coastal zone of tropical eastern Brazil. Our findings are based on a similar to 850 kyr-long multiproxy record from a marine sediment core collected from the eastern Brazilian margin that represents the longest continuous record of South American hydroclimate to date. Utilizing the ln (K/Al) chemical weathering proxy from the core, we determine that past hydroclimate change in the coastal zone was primarily modulated by obliquity forcing. We demonstrate that high obliquity is associated with an increase in the boreal summer interhemispheric insolation contrast which decreases the zonality of the southern trade winds and reduces moisture advection to the coastal zone. Based on the long-term coherence between the ln (K/Al) record and benthic delta C-13 records from the Atlantic during Marine Isotope Stages 16-13, we infer that an increase in the strength of the overturning circulation, caused by Northern Hemisphere high-latitude forcing, may have produced surface cooling in the western tropical South Atlantic which led to reduced moisture advection to the coastal zone. We suggest that this mechanism may have also caused the amplification in millennial-scale variability in the coastal hydroclimate system, which began after the decoupling of the coastal hydroclimate system from obliquity forcing.

Automated summary

This study found that hydroclimate change in the coastal zone is primarily modulated by obliquity forcing rather than the South American Monsoon System. High-latitude forcing in the Northern Hemisphere may have increased the overturning circulation and led to reduced moisture advection to the coastal zone.