Millennial-scale varnish microlamination dating of late Pleistocene geomorphic features in the drylands of western USA

Abstract

Varnish microlamination (VML) dating is a climate-based correlative age determination technique used to correlate and date various geomorphic features in deserts. In this study, we establish a generalized late Pleistocene (18-74 ka) millennial-scale microlamination sequence in fine-grained, fast-accumulating rock varnish for the drylands of western USA, radiometrically calibrate the sequence and correlate it with the δ 18 O record in the GISP2 Greenland ice core. We then use this climate-correlated varnish microstratigraphy to estimate surface exposure ages for radiometrically dated late Pleistocene geomorphic features in the study region. The VML dating of debris flow deposits on the Sehoo recessional shorelines of Lake Lahontan at the Jessup embayment of central Nevada yields a minimum-limiting age of 14.95-15.95 ka, in good agreement with a calibrated 14 C age of 15.22± 0.12 ka for the timing of the lake recession. The VML dating of a giant ejecta block on the rim of Meteor Crater in northern Arizona yields a minimum-limiting age of 49.15 ka, closely matching a thermoluminescence (TL) age of 49± 3 ka and slightly younger than a recently updated cosmogenic 36 Cl age of 56.0± 2.4 ka for the meteor impact event. The VML dating of distal Q2c fan surfaces on Hanaupah Canyon alluvial fan in Death Valley, California, yields a minimum-limiting age of 73.55 ka, in accord with cosmogenic 36 Cl depth-profile ages of 66+22/−14 ka and 72+ 24/−20 ka for the same fan deposits. The close agreement between the VML age estimates and the independently derived radiometric ages for these geomorphic features attests to the validity and reliability of millennial-scale VML dating. To further assess its potential in desert geomorphological research, we use the VML method to study alluvial-fan responses to millennial-scale climatic changes. The VML dating of a small tributary fan in Death Valley reveals two episodes of fan aggradation, one ceasing at 73.55-86.75 ka during the dry period of the last interglacial (MIS 5a) and the other finishing at 66.15 ka during the wet period of the last glacial (MIS 4). The VML and 36 Cl dating of the distal Q2c fan surfaces on Hanaupah Canyon fan reveal two episodes of large-scale fan aggradation ended at 72 + 24/− 20 ka and 73.55 ka during the wet period of MIS 4. Fanhead incision and associated within-channel or fantoe aggradation are found to take place during the relatively dry period of the glacial-to-interglacial climatic transition (12-24 ka) and the Holocene interglacial dry period (0-12 ka). These data indicate that, on the millennial to sub-Milankovitch timescale (~10 3-10 4 years), fan aggradation is a discrete sedimentational process under various climatic conditions. Because fan aggradation is ultimately controlled by the intensity and frequency of precipitation eventswhich in turn are modulated by major climatic oscillations such as Heinrich events, Dansgaard/Oeschger (DO) events, and glacial/interglacial shiftsthese major climatic changes could be the pacemaker of regionally contempor...

Nanoscale Observations Support the Importance of Chemical Processes in Rock Decay and Rock Coating Development in Cold Climates

“…The layering typical of clay minerals in HRTEM views of varnish [67] is being broken apart by the dissolution and then the reprecipitation of the Mn-rich granules into the clays-to the point where the layered texture is mostly lost (Figure 9c). These nanoscale movements do not create instability in the varnish laminations seen at the micron scale that are stable for tens of millennia [131][132][133]. Scale matters and these snanoscale movements would be analogous to cars shuffling in a parking structure composed of micron-scale laminations.…”

Rock varnish evidence for a Younger Dryas wet period in the Dead Sea basin

“…The timing for the formation of basal layers in the varnish can be determined by the age of sampled shorelines and fan delta surfaces [cf. Liu and Broecker , ]. Previous studies [ Stein et al ., ] suggested that Lake Lisan dropped from its high stand above 260 m bmsl to its B‐A lowstand below 465 m bmsl between 14.6 and 13.2 ka.…”

“…Previous studies [ Liu et al ., ; Liu , ] showed that wet events represented by the glacial age dark layers in the western United States varnish correlate in time with cold episodes of the YD and H1–H6 in the North Atlantic region. Recent studies [ Liu and Broecker , , , ] further demonstrated that fine‐grained, fast‐accumulating varnish recorded late Pleistocene and Holocene millennial‐scale wet events that were synchronous with millennial‐scale cooling events in the North Atlantic and Greenland [ Bond et al ., ; Stuiver and Grootes , ]. Hence, rock varnish can be used as a unique wetness recorder to study past climate changes, especially wetness variations of the world's deserts [ Broecker and Liu , ].…”

Co‐development of alluvial fan surfaces and arid botanical communities, Stonewall Flat, Nevada, USA

“…VML age estimates are established for five fan surfaces in Stonewall Flat. Figure shows the radiometric calibration for the generalized late Pleistocene millennial‐scale varnish microstratigraphy for the western US deserts and its correlation with the δ 18 O record in the GISP2 Greenland ice core (Liu and Broecker, ). The climate correlation‐based age assignments for the millennial‐scale VMLs shown in Figure provide the most accurate age scale upon which the VML age determinations of the fans surfaces in Stonewall Flat are made.…”

“…It is worthwhile to point out that our VML data are consistent with the hypothesis that fan aggradation is an ongoing discrete sedimentational process occurring under various climatic conditions (Liu and Broecker, ). For instance, varnish sample from site STF‐5 contains a basal layer of WP0c with a VML age estimate of 13·15 ka, favoring fan aggradation during a time of late glacial climatic transition from wet to dry period (cf.…”