COMPARING REPLICATE STALAGMITE RECORDS FROM CAVE ACTUN TUNICHIL MUKNAL, BELIZE: A TEST OF THE HURRICANE MASKING HYPOTHESIS

Stalagmites are useful paleoclimate proxies that can record the timing of past hurricanes and droughts at high resolution in fluctuations of carbon and oxygen stable isotopes (δ¹³C and δ¹⁸O values) in calcite. Stalagmites ATM-7 and ATM-9 were collected in 2001 from the cave Actun Tunichil Muknal, in Belize. Both stalagmites were dated using layer counting and ²¹⁰Pb and ¹³⁷Cs radiometric evidence. Both stalagmites were micromilled continuously at 20 μm increments to capture weekly to monthly temporal resolution. Calcite powders were analyzed using a stable isotope mass spectrometer to measure δ¹³C and δ¹⁸O values. The δ¹⁸O values record extreme seasonal precipitation patterns, such as hurricanes, due to the anomalously low δ¹⁸O signature in tropical cyclone rainwater. The δ¹³C values record El Niño climate variability.

ATM-7 was analyzed and studied previously, but the new results for ATM-9 enable us to make a direct comparison of the two stable isotopic proxy records of past hurricanes and El Niño events. ATM-7 has an extensive paleoclimate record which we used to complete the replication test, which is a comparison of similar isotopic profiles between two or more speleothems. With the new ATM-9 data, we can test the hurricane masking hypothesis (Frappier, 2013), which suggests that the isotopically low δ¹⁸O value of hurricane rainwater can hide, or “mask”, the stalagmite record of El Niño events which produce a cave dripwater signature with higher δ¹⁸O values. The ground water from a tropical cyclone may linger for months to years before reaching the cave, concealing signals of other extreme events, such as droughts for some time after the storm event. Conversely, cave δ¹³C value records are not affected by the tropical cyclone activity, and thus preserve information about past El Niño events. Comparing the δ¹⁸O and δ¹³C records from two stalagmites allow us to see the hurricane masking effect in ATM-9, which has been previously shown for the ATM-7 record. We will present initial results of the replication test to compare these two stalagmite records, and quantify the importance of dripwater pathway hydrology in the hurricane masking isotope effect. This method will support efforts to ensure the validity of δ¹⁸O and δ¹³C value-based proxy records of past climate variability and tropical cyclone events.