Climate proxies are any biological or geological record that preserves temperature or precipitation fluctuations prior to human records. This project’s climate proxy is stalagmites, which are cave formations created from drip water on a cave floor. In particular, this study focuses on carbon isotopes, which record vegetation and moisture changes, spanning parts of the last 500,000 years from Cape Range, Western Australia. Cape Range is located approximately halfway up the western coast of Australia and sits on the boundary between tropical monsoon rains coming from the northern tropics and middle latitude rainfall moving up from the south. The majority of summer rainfall comes from tropical cyclones while winter precipitation comes from southern rainfall. The percentage of annual rainfall derived from these weather systems varies substantially from year to year. Little is understood of rainfall variability at Cape Range during the ice ages. A recent study by Cheng et al. (2016) produced a continuous 640,000-year record of monsoonal variability using oxygen isotopes in stalagmites from southern China. Although the Cape Range stalagmite record is highly discontinuous, comparison of the Chinese record with carbon isotopes in Cape Range stalagmites suggests an interconnectedness of precipitation over the past 500,000 years. In-phase changes are seen between the two systems (simultaneous wetter or drier intervals), a finding that is surprising given their location in different hemispheres. It is still unclear why this relationship exists, but a possible explanation is an expansion and contraction of monsoon rains over past millennia. Analysis of additional stalagmites is needed to develop a more continuous record from Cape Range and better test this hypothesis.
James Garrett, ’17