The research team, in this collaborative proposal, aims to compare speleothem records, instrumental observations of the last ~100 years, and climate models to study Caribbean climate spanning the last millennium. The research team will address two foundational questions with their project, as follows: (1) What do the high-resolution Caribbean speleothem delta oxygen-18 isotope records, together with Cariaco Basin sediments, indicate about the low-frequency, Caribbean-wide hydroclimate variations during the Late Holocene? and (2) What can be learned about the large scale Atlantic-Pacific climate interactions during the late Holocene by comparing the detailed regional Caribbean hydroclimate history with other proxy records outside the region?
The broader impacts involve the education and training of undergraduate and graduate students, most notable at the University of Puerto Rico. The research results on tropical hydrology could have widespread utility in the study of climate and water resources.
Climate model projections that are forced with increases in anthropogenic greenhouse gas suggest that throughout the 21st century the Caribbean Islands and Central America will experience a marked reduction in precipitation, reaching 10% or more of the average present rainfall amounts. While this will occur gradually as the global concentrations of greenhouse gas increase, the region will experience natural climate fluctuations that may either worsen or ease the situation. We do not have enough information on these natural variations, particularly those that persist for a long time, on the order of a decade or more. This is because actual rainfall measurements, based on raingauges and other instruments, are of relatively short extent. Thus we have to rely on climate proxies to supplement the instrumental record and help us learn about the range of natural climate variations in various areas. In this project we have been using speleothems – that is cave deposits of carbonate that are also referred to as stalactites and stalagmites – to track down variations in precipitation in the pre-instrumental past. These deposits form as rainwater trickles through the ground, dissolves the minerals in the soil and deposits them drip-by-drip inside the cave. In the tropics, such deposits carry the signal of rainfall amounts that fall throughout the rainy season. One of these ways is the imprint of rainfall amount in the ratio of different oxygen isotopes in the carbonate. Drilling through a speleothem column, we expose layers of seasonal deposits, date them, and measure the ratio between the heavy oxygen isotopes and the lighter ones and trace their change with time. The smaller the proportional amount of the heavy oxygen isotope in the carbonate layer is, the larger the amount of rainfall that leads to the formation of the layer. Plotting the variation of oxygen isotope ratio with time tells us about variations of rainfall in the past. In this project we studied a speleothem record from a cave on the border between the countries of Belize and Guatemala, in the Maya Mountains. In that area, we were able to track with annual resolution, the inferred variations in rainfall amount since the beginning of the 18th century. We found that rainfall amounts varied very little from year-to-year during the 18th century but dropped rather abruptly at the beginning of the 19th century and continued to drop throughout the 19th and 20th centuries. In particular, the vicinity of the cave experienced a persistent drop in annual rainfall between 1810 and 1840, followed by a partial recovery and then by another decline in rainfall between 1880 and 1920. A final, less dramatic decline occurred after about 1965. This pattern of rainfall decline roughly resembled the evolution of other regional indicators but those are not so finely resolved in time to allow a precise fit. What did match this evolution is the record of major tropical volcanic eruptions. The most prominent eruptions were the 1809 eruption of an unknown volcano followed by the eruption of Mount Tambora in 1815. In 1883, Krakatoa erupted and in 1963 Agung followed by Pinatubo in 1991. All these were explosive eruptions. Moreover, the 19th century eruptions were followed by more minor eruptions forming clusters that arguably explain the persistent change recorded by the Beliez-Guatemala border speleothem. Importantly, the eruption cluster came after a century where only weak sporadic volcanic eruptions occurred. Based on these results we propose that the Caribbean-Central American region is sensitive to tropical volcanic eruptions and that such eruptions in the future will further intensify the anthropogenic influence expected based on climate model integrations, particularly if such eruptions are clustered over several consecutive years.
