Holocene trends in precipitation seasonality in the northern tropical Americas

During the Holocene, the progressive decrease in Northern Hemisphere summer insolation resulted in a southward displacement of the Intertropical Convergence Zone. Accordingly, millennial-scale climatic variability in the northern tropical Americas has been marked by a progressive trend towards drier summers. However, the evolution of precipitation seasonality, which determines the annual water balance, remains poorly constrained. Here we present a 9500-year multiproxy record from Lake Ocotalito, Mexico, to evaluate long-term changes in precipitation seasonality. While geochemical indicators offer insights into summer rainfall, fossil cladoceran assemblages are used to infer lake level fluctuations linked to the annual moisture balance. Our results show that the Early Holocene was characterized by high summer precipitation, yet lake levels remained generally low, indicating a persistent annual moisture deficit consistent with relatively dry winters. From ∼8000 to 5000 cal yr BP, decreasing summer rainfall coincided with further lake shallowing, but a progressive increase in winter insolation reduced moisture deficits, setting the stage for a long-term lake deepening. After ∼5000 cal yr BP, dominance of planktonic cladocerans reflects persistent high lake levels, implying reduced seasonal moisture deficits. Since the Mid Holocene, increased El Niño frequency and a positive Pacific-North American teleconnection state have probably contributed to more frequent cold surges, resulting in higher frontal winter precipitation. Overall, our record suggests that Holocene hydroclimate variability in the region has been shaped by decreasing summer rainfall, but also by progressively wetter winters. This dual forcing has resulted in a long-term reduction of precipitation seasonality in the northern tropical Americas.