NOAA/WDS Paleoclimatology - Cave C126 Stalagmite Stable Isotope Data for 26-15 and 11-6 ka

STUDY NOTES: Carbon and oxygen isotopic ratios of calcite stalagmites from cave C126, Cape Range Peninsula, Western Australia, for the Last Glacial Maximum through middle Holocene(excluding 15-11 ka). Chronology is based on 230Th ages obtained using a NEPTUNE MC-ICP-MS at the University of New Mexico. Stable isotopic ratios measured using a ThermoElectron Delta V Plus mass spectrometer linked to a Kiel IV automated carbonate preparation device at the University of Nevada-Las Vegas.
ABSTRACT SUPPLIED BY ORIGINATOR: Stable isotope profiles of 230Th-dated stalagmites from cave C126, Cape Range Peninsula, Western Australia, provide the first high-resolution, continental paleoclimate record spanning the Last Glacial Maximum, deglaciation, and early to middle Holocene from the Indian Ocean sector of Australia. Today, rainfall at Cape Range is sparse, highly variable, and is divided more or less equally between winter and summer rains, with winter precipitation linked to northwest cloud bands and cold fronts derived from the southern mid- to high-latitudes, and summer precipitation due primarily to tropical cyclone activity. Influences of the Indo-Australian summer monsoon at Cape Range are minimal as this region lies south of the modern monsoon margin. The interaction of these atmospheric systems helps shape the environment at Cape Range, and thus C126 stalagmite-based paleoclimatic reconstructions should reflect variability in moisture source driven by changing ocean and atmospheric conditions. The C126 record reveals slow stalagmite growth and isotopically heavy oxygen isotope values during the Last Glacial Maximum, followed by increased growth rates and decreased oxygen isotopic ratios at 19 ka, reaching a δ18O minimum from 17.5 to 16.0 ka, coincident with Heinrich Stadial 1. The origin of this oxygen isotopic shift may reflect enhanced moisture and lower oxygen isotopic ratios due to amount effect-driven changes in rainfall δ18O values from an increase in rainfall derived from tropical cyclones or changes in northwest cloud band activity, although the controls on both systems are poorly constrained for this time period. Alternatively, lower C126 stalagmite δ18O values may have been driven by more frequent or more intense frontal systems associated with southerly-derived moisture sources, possibly in relation to meridional shifts in positioning of the southern westerlies which have been linked to southern Australia megalake highstands at this time. Finally, we also consider the possibility of contributions of tropical moisture derived from the Indo-Australian summer monsoon. The Intertropical Convergence Zone and associated monsoon trough shifted southward during Heinrich events and other periods of high northern latitude cooling, and although clearly weakened during glacial periods, rainfall with low δ18O values associated with the monsoon today suggests that even small contributions from this moisture source could have accounted for some of the observed oxygen isotopic decrease. Despite a pronounced isotopic excursion coincident with Heinrich Stadial 1, no identifiable anomaly is associated with Heinrich Stadial 2. The Holocene is also characterized by overall low δ18O values and rapid growth rates, with decreasing oxygen isotopic values during the earliest Holocene and at ∼6.5 ka, roughly coincident with southern Australia megalake highstands. The origins of these stalagmite oxygen isotopic shifts do not appear to reflect increases in mean annual temperature but are tied here largely to changes in the δ18O values of precipitation and may reflect a more southerly influence of the Indo-Australian summer monsoon at this time. C126 stalagmite carbon isotopic ratios offer an important complement to the oxygen isotopic time series. Stalagmite δ13C values averaged −5‰ during the Last Glacial Maximum and early deglaciation, and reached a plateau during the oxygen isotopic minimum at 17.5 ka. However, δ13C values decreased sharply to −12‰ between 11 and 8 ka, a shift interpreted to reflect increases in plant density in response to the onset of interglacial conditions. Stalagmite δ13C values at 6 ka are lower than expected for the modern C4-dominated vegetation and thin soils of Cape Range, suggesting that a more C3-rich environment was present during elevated rainfall conditions of the early and middle Holocene. The Cape Range stalagmite time series thus reveals for the first time the millennial-scale sensitivity of the moisture source variations in northwestern Australia, a result that has implications for precipitation dynamics across much of the continent.