Abstract:
Incremental heating analysis of cryptomelane permits separating atmospheric gases released by surface and intercrystalline sites from radiogenic (40)Ar and neutron induced (39)Ar gases extracted from intracrystalline sites. The effective separation of atmospheric from radiogenic and neutron induced gases enables the analysis of Quaternary supergene cryptomelane by the (40)Ar/(39)Ar method. Analysis of 521 grains from 123 samples from three weathering profiles in Mary Valley, SE Queensland, Australia, record weathering events ranging from 6.2+ or -0.5 to 0.08+ or -0.06 Ma. The reproducibility of results for several grains from a single sample, for several samples from the same site, and for samples from different sites suggest that (40)Ar/(39)Ar analyses of cryptomelane yield reliable ages. Here we will focus on the Quaternary results. Ten out of thirteen individual grains extracted from two distinct samples from one weathering profile yield concordant spectra (291+ or -14, 313+ or -14, 320+ or -13, 330+ or -30, 330+ or -30, 290+ or -20, 345+ or -15, 300+ or -20, 290+ or -30, 330+ or -50 ka). One of the discordant grains yields a slightly older result (372+ or -14 ka), one grain does not yield a plateau, while another yields a plateau-like spectrum (260+ or -40 ka). Two grains from a hand-specimen collected from a weathering profile 25 km away yield similar plateaus (300+ or -100 and 340+ or -90 ka). Three out of four grains from one sample from a third weathering profile (4. 2 km away) yield reproducible plateau ages (300+ or -40, 370+ or -30, 340+ or -30, 280+ or -40 ka) concordant with the results obtained for the other two profiles. The age clusters suggest that SE Queensland was relatively humid at ca. 340-290 ka. This period corresponds to an interglacial interval at 330-300 ka. Oxygen isotope data for ODP site 820 (NE Australia) record a 4 degrees C increase in sea-surface temperature between 400 and 250 ka. Araucaria, Agathis and rainforest Gymnosperms predominate in ODP site 820 (NE Queensland) at 400-275 ka, and rainforest palynomorphs predominate at Fraser Island, SE Queensland, between 350-300 ka, also suggesting humid climates. The internally consistent geochronology results and the agreement between our results and independent paleoclimatic indicators suggest that (40)Ar/(39)Ar analysis of pedogenic cryptomelane provides a useful geochronometer to the study of Quaternary surficial processes.