The Plio-Pleistocene encompasses large-scale changes in climate and may provide an analog for the function of Earth's ecosystems in a high carbon dioxide (CO 2) world. Pliocene climate was characterized by atmospheric CO 2 levels similar to today, reduced sea surface temperature (SST) gradients, and events such as the restriction of the Central American and Indonesian Seaways (Cane & Molnar, 2001; Fedorov et al., 2013). Despite its relative warmth and stability, the Pliocene is defined by long-term cooling that is potentially related to tectonic plate movement and consequent changes in ocean currents and pCO 2 drawdown, especially in the Indo-Pacific region (Cane & Molnar, 2001). The Plio-Pleistocene transition around 2.7 Ma is marked by the breakdown of Pliocene climate conditions, as strong SST gradients between the high-and low-latitudes and across the Pacific Ocean basin were established, and Northern Hemisphere glaciation intensified (Fedorov et al., 2013). The development of stronger zonal SST gradients in the Pacific Ocean was likely related to gradual thermocline shoaling in the Pliocene and an increase in tropical upwelling that led to strengthened Walker circulation (Ravelo et al., 2004). The interaction between these ocean-atmosphere feedbacks and changes in insolation potentially magnified the climate's sensitivity to obliquity, leading to the 41 kyr pacing of higher amplitude glacial cycles in the early Pleistocene (Ravelo et al., 2004).

Paleolimnological evidence indicates the ancient Maya transformed terrestrial ecosystems by felling forest vegetation to construct large civic-ceremonial centers and to expand agriculture. Human settlements influenced lacustrine environments but the effects of Maya activities on aquatic ecosystems remain poorly studied. Here we analyzed a sediment core from Lake Petén Itzá, Guatemala, to infer paleoenvironmental changes resulting from Maya occupation of the archaeological site of Nixtun-Ch’ich’. Increases in charcoal and fecal stanol concentrations indicate Maya occupation of the Candelaria Peninsula by the late Early Preclassic period. Geochemical proxies reveal a period of lake ecosystem alteration during construction and expansion of the city’s urban grid in the Middle and Late Preclassic periods. Depopulation of the city in the Terminal Preclassic resulted in a decline in lake trophic state. Whereas previous studies of Petén waterbodies have indicated depressed lacustrine primary production, the core collected near Nixtun-Ch’ich’ shows evidence of ancient Maya lake ecosystem deterioration.

At Praia Rei CortiS co (PRC), coastal Portugal, we analyzed compound-specific isotopes of plant wax-derived n-alkanes in combination with molecular distribution proxies and C/N ratios to reconstruct hydrologic and environmental change in a mid-Holocene wetland. During this relatively brief segment of the Holocene (6.6-5.4k cal a BP), substantial shifts in the stable hydrogen isotope composition of terrestrially sourced C 29 n-alkanes (dD n-C29 ) record significant multi-decadal changes in precipitation origin and storm trajectory. Wetland formation at PRC occurred during a humid interval (6.6-6.5k cal a BP) with a dominantly tropical precipitation source, likely the result of a persistent negative phase of the North Atlantic Oscillation, which permitted the establishment and dominance of Sphagnum moss. A subsequent decrease in precipitation significantly reduced Sphagnum abundance at PRC, which is evidenced by decreased sedimentary C/N ratios and reduced mid-chain n-alkane abundances. From 5.9 to 5.4k cal a BP, relatively low dD n-C29 values coincide with sand invasion along the Iberian coast and cooling in the North Atlantic. Strong correspondence between dD n-C29 values and the occurrence of ice-rafted debris recorded in deep sea sediment cores during this period illustrate a marked North Atlantic control on the trajectory of mid-latitude storms and precipitation regimes in Iberia.

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