Award: OCE-1241247

Palau?s marine lakes and the thick sediment sequences they contain provide a treasure trove of climatic and evolutionary history. Over 150 meters of sediment cores were collected from 11 lakes during month-long field campaigns in 2013 and 2016. Radiocarbon dating has shown that several of these lakes contain sediments from lake inception as long ago as 11,000 years, the beginning of the Holocene period. The analysis of molecular fossils and their hydrogen isotopic composition in many of these sediments have revealed a dynamic climate and environmental history. While Palau, at 7.5 degrees north latitude, has a very wet climate today, receiving over ten feet of rain per year, it appears to have been much drier during the period from 6,000 to 2,000 years ago. Curiously, other researchers have found that conditions closer to the equator in the western tropical North Pacific were wetter at that time. For instance, rainfall reconstructions from cave deposits in central and northern Borneo indicate the wettest conditions of the last 10,000 years occurring just when Palau was driest. Because both Palau and Borneo experience the same rainfall anomaly in response to El Niño, when they suffer drought, and La Niña, when they both become even wetter, changes in the El Niño system cannot explain the differing climates during the mid-Holocene period. Another clue to the cause of this mid-Holocene climate condition comes from reconstructions of the strength of monsoons in Asia, India, Africa, and South America, which all show weakening during the same time. The most parsimonious explanation is that the tropical rain band--known as the Intertropical Convergence Zone, or ITCZ--was driven south by a decline in solar radiation in the northern tropics caused by changes in Earth?s orbital geometry. Analyses of the hydrogen isotopic composition of molecular fossils from four of Palau?s marine lakes have also demonstrated that the period known as the Little Ice Age, that lasted from 1400 to 1850 A.D. was characterized by dry conditions relative to the preceding centuries the modern climate. Climate reconstructions from other islands in the Pacific lend support to our interpretation of the Little Ice Age also being a time when the ITCZ was located further south than it is today. Research with computer models of the climate system show that a combination of increased volcanic activity and diminished solar activity were likely responsible for a tropical rain band located closer to the equator during the Little Ice Age. Last Modified: 04/02/2018 Submitted by: Julian P Sachs