GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 182-3
Presentation Time: 9:00 AM-6:30 PM


KLUG, Paige P.1, MARTIN, Chloe N.1, DENNISTON, Rhawn F.1, UMMENHOFER, Caroline C.2, VILLARINI, Gabriele3, ASMEROM, Yemane4, POLYAK, Victor J.4, CUGLEY, John5, WOODS, David6, HUMPHREYS, William F.7 and GONZALES, Angelique N.1, (1)Department of Geology, Cornell College, Mount Vernon, IA 52314, (2)Woods Hole Oceanographic Institution, Falmouth, MA 02543, (3)IIHR--Hydroscience & Engineering, University of Iowa, Iowa City, IA 52242, (4)Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, (5)Western Australia Speleological Group, Perth, Australia, (6)Department of Parks and Wildlife, Broome, 6725, Australia, (7)Western Australia Museum, Welshpool, Australia

High-resolution reconstructions of extreme rainfall events prior to the instrumental era are important but rare. A composite stalagmite record from cave KNI-51, located in the northwest Australian tropics, provides a nearly continuous time series of cave flooding caused by extreme rainfall for the last 4,100 years. Because flood layer identification in stalagmites is to some degree subjective, each stalagmite was inspected by either two or three researchers, and the resulting data were averaged. Flood layers were assigned an age using growth models developed from high precision (2 s.d. errors generally <±30 yr) U/Th dates and assigned a value of either 0 or 1 to represent layer presence or absence. These bimodal data were then bundled into discrete years (high growth rates in these stalagmites allow for identification of multiple flood events per year in some cases).

The work presented here expands on a previously published flood event time series from the same cave (Denniston et al., 2015, PNAS, 112, 4576) and reveals similar overall trends: decreased occurrence rates of flooding mark the Little Ice Age (AD 1400-1800) and increased flooding activity in the Medieval Climate Anomaly (MCA: AD 800-1200). However, this expanded stalagmite time series provides additional replication, a more nuanced structure of the last two millennia, and reveals that the MCA contained the highest occurrence rates of cave flooding (and thus extreme rainfall events) of any interval of the last four millennia.

The rainfall driving cave flooding is derived from the Australian monsoon and tropical cyclones, with modern regional climatology revealing that the latter is more important in the highest events for 1, 2, and 3-day rainfall totals. Because La Niña (El Niño) events are associated with enhanced (reduced) cyclogenesis in the Australian region (but only modest changes in monsoon rainfall), we propose that the MCA may have been characterized by more (less) frequent La Niña (El Niño) activity than at any other time in the past 4,100 years.