Date of Award

6-2022

Document Type

Union College Only

Degree Name

Bachelor of Science

Department

Geosciences

First Advisor

Donald T Rodbell

Second Advisor

David P Gillikin

Keywords

paleoclimate, peru, speleothem, hydrology, monsoon, MIS 6, caves, hydroclimate, andes, tropics

Abstract

Peruvian speleothems have been shown to be excellent recorders of paleoclimate by tracking changes in rainfall amount through oxygen isotopes in calcite. Measured changes in rainfall amount have been attributed to changes in the intensity of the South American Seasonal Monsoon (SASM). High insolation in the Southern Hemisphere and low insolation in the Northern Hemisphere is believed to increase the meridional SST gradient, leading to a southerly migration of the mean position of the Inter-Tropical Convergence Zone (ITCZ) and strengthening of the SASM intensity. Conversely, a decrease in Southern Hemisphere insolation and an increase in Northern Hemisphere insolation is believed to have led the ITCZ to a more northerly mean position and weakening of SASM intensity. I present a record from speleothem 19-11, which grew in Pacupahuain Cave between 167,068 yrs B.P. and 135,165 yrs B.P., also known as the penultimate glacial period or Marine Isotope Stage 6 (MIS 6). The δ18O record from 19-11 shows exceptional agreement with overlapping speleothem records from nearby Huagapo Cave (HC). All of these records are anti-phased with 11°S January insolation, supporting the idea that shifts in the mean position of the ITCZ modulate SASM intensity. From 165 ka to 150 ka an increase in δ18O occurs contemporaneously with a decrease in 11°S Jan insolation. This period of time reflects a broad decrease in SASM intensity, a hypothesis corroborated by decreased sediment input into regional lake records. δ18O values are lower at the beginning and end of the 19-11 record, coinciding with 11°S Jan insolation highs. Within long-term insolation driven changes in δ18O, the 19-11 record has rapid oscillations that are also seen in the HC records. Previous researchers have attributed these rapid fluctuations to high-latitude Northern Hemisphere forcing. Increased temperature in the Northern Atlantic and ice sheet melt coincides with apparent weakening of the SASM. This relationship is explained by a weakening of SST gradient and northern migration of the ITCZ. The true cause of this ice melt and increase in Northern Hemisphere temperatures is not fully understood, however, they show remarkable similarity to events recorded in other proxies during MIS 2. When compared to Chinese and Mediterranean speleothem records it is clear that some of these short-term fluctuations, namely the 161-159 ka event, have profound global significance. The cause and regional effects of short-term oscillations is an important topic for future work.

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