Radiometric Age Calibration of Varnish Microstratigraphy, Western USA

Figure Caption    LU = Layering Unit, WH = Wet event in Holocene, WP = Wet event in Pleistocene, YD = Younger Dryas, H = Heinrich event, MIS = Marine Isotope Stage. Please see Liu (2003) and Liu and Broecker (2007) for more discussion on radiometric age calibration of varnish microstratigraphy.

Figure Caption    A generalized varnish layering sequence presumed to represent wetness variations during the last glaciation (12–74 ka) in the drylands of western USA (left panel) and their temporal correlation with the climatic changes in the Greenland ice core record (GISP2 data subjected to a 200-year smoothing) (right panel). Here LU-1 to LU-5 represent five layering units and WP0–WP7 denote eight Mn- and Ba-rich major black layers in the generalized varnish sequence. Letters a–e mark minor black layers within each of the major black layers. Radiometric ages for calibration derive from independently dated geomorphic features on which varnish samples were collected, providing maximum-limiting ages for the formation of basal layers in the varnish. Age constraints for minor black layers in WP0 and WP1 are adapted from our previously published results (Liu and Broecker, 2008). For the purpose of comparison with the available radiometric age control, ages for all of the minor black layers in the generalized layering sequence are assigned on the basis of the depicted tentative correlation between short-lived wet events represented by those minor black layers and millennial-scale cooling events (also marked by letters a-e) in the ice core record. Numbers in green represent both the interstadial phases of DO events 1–22 in the ice core record and related dry phases signified by orange/yellow layers in the varnish record (cf. Liu and Broecker, 2013). Dash lines represent a series of sawtooth-shaped progressive cooling cycles (i.e., Bond cycles) (Bond et al., 1993). H=Heinrich event, YD=Younger Dryas, DO=Dansgaard/Oeschger event, WP=wet event in Pleistocene, MIS=Marine Isotope Stage.

Figure Caption    LU = Layering Unit, WP = Wet event in Pleistocene, MIS = Marine Isotpe Stage. Note that the age assignments for LU-1 to LU-4 are based on climatic correlation of the varnish record with the GISP2 record (Grootes et al., 1993; Bender et al., 1994; Meese et al., 1994), and those for LU-5 to LU-8 are based on similar correlation between the varnish record and the SPECMAP record (Imbrie et al., 1984; Martinson et al., 1987). Please see Liu (2003) and Liu and Broecker (2007) for more discussion on radiometric age calibration of varnish microstratigraphy.