INSTRUMENTAL RECORDS OF MODERN FLOODING EVENTS AT CAVE KNI-51, AUSTRALIAN TROPICS

Oxygen isotopes in tropical stalagmites are commonly used to reconstruct paleomonsoon variability. However, layers of detritus contained within stalagmites can record cave flooding events, and the analysis of this material represents an important complement to isotope-based rainfall reconstructions. Each cave is unique, and therefore must be characterized by its individual hydrological characteristics and boundary conditions.

We report the first two years of a four-year study of cave flooding in cave KNI-51, tropical Western Australia. Data loggers recorded barometric pressure/water level within a single room located 500 m from the entrance and adjacent to the area from which stalagmites containing numerous flood layers were collected. Monitors were installed at three different elevations, each separated vertically from the next by approximately two meters. The lowest logger is located in a depression marking the lowest point in the room, while the middle logger is mounted against a flowstone ledge halfway to the ceiling and the highest logger is located just below the ceiling of the cave.

The last two years have been a study in contrasts of monsoon activity. The monsoon rains of 2015-16 were some of the weakest on record while 2016-17 was one of the wettest monsoon seasons since the year 2014. In 2015-16, water levels rose to the point of submerging the bottom most logger only twice; no other water level increases were detected. However, 2016-17 recorded 17 floods, eight of which filled the cave completely. The rain required to completely flood the room is a function of antecedent moisture and rainfall intensity, although 75-100 mm of rainfall over 4-7 days represents the minimum intensity required for flooding. However, flooding appears tied to total rainfall occurring across the previous five days. On average, once water levels begin to rise, the room can fill to the ceiling within an hour and drain in less than three hours.