An 8000-yr Record of Vegetation and Sedimentation Change from Kaau Crater, Hawaii Shows Mid-Holocene Climate Variability in the Pacific

Abstract:

Kaau Crater is located on Oahu, Hawaii, and was formed about 1 million years ago. The crater is a wetland about 450 m in diameter, which has been accumulating sediment since at least 25,866 cal yr BP. Sediment accumulation environments suitable for stratigraphic paleoscience studies in tropical mountaintop locations, relatively unaffected by humans, are scarce. This research aims to provide a comprehensive multi-millennial record of environmental change over time, including a reconstruction of precipitation and temperature.

A continuous 450 cm organic sediment core was raised from Kaau Crater and analyzed for bulk density, organic matter (OM), stable isotopes of carbon and nitrogen, and sediment accumulation rate as well as fossil pollen. Thirteen radiocarbon measurements show that the profile represents 14,087 years of sedimentation, and continuous sedimentation since ~8000 cal yr BP. Bulk density was typically about 0.17 g cm^-3, but is particularly high (0.56 g cm^-3) in sediments deposited between 5800 and 4400 cal yr BP. High variability during this time period is also seen in OM, stable isotopes of carbon and nitrogen, and sediment accumulation rate. The OM content varies greatly throughout the core, from 12 to 94%. The ¹³C and ¹⁵N range -28.3 to -26.1‰ and 0.7 to 5.7‰. Maximum sedimentation and organic matter accumulation is between 6810-3942 cal yr BP, during which time, the organic C accumulation rate is 46.1 g m^-2 yr^-1. The C:N ratio is most variable during the mid-Holocene and the maximum is at 6326 cal yr BP. Thirty-eight pollen types were identified representing a mixture of dry- to wet-tolerant taxa. A reconstruction of annual precipitation suggests substantial variation on multi-centennial and shorter scales, particularly during the mid-Holocene (-643.4-5654 mm anomaly), but a lack of Holocene-scale wetting or drying. A reconstruction of mean annual temperature shows an overall multi-millennial cooling trend at the crater more pronounced after 5400 cal yr BP, along with diminishing dry-season insolation and a strengthening ENSO and increasing frequency of El Niño events. These results indicate the sensitivity of tropical mountaintop environments to climatic change on centennial and shorter timescales, and the differences in responses on windward versus leeward sides of tropical high islands.