IMPROVEMENTS ON THE DEGLACIAL CHRONOLOGY AT MONO LAKE, CA FROM 14C-DATING OF PROGRESSIVE LEACHES

The dramatic expansion of the Great Basin lakes at the end of the last glacial period is well-known but poorly understood, in part because of difficulties in dating the time and duration of the expanded lakes. At Mono Lake the final highstand is marked by the sequence of Ashes 1-2-3-4, readily-identifiable in deep lake sediments as high as 50-60 m above average late-Pleistocene levels. Carbonate ¹⁴C dates and ⁴⁰Ar/³⁹Ar on rhyolitic ashes in the Wilson Creek Formation sediments both have complications, and for the high-precision and –accuracy required to correlate global changes, improved chronological constraints are required.

We have built on previous progressive leaching experiments to separate and radiocarbon date the oldest carbon in a sample, on the premise that all younger carbon is contamination added after the time of the original precipitation. After weighing carbonate into a sealed vial, the vial is evacuated, phosphoric acid is added, and the vial attached to the graphitization rig. Using variable concentrations of acid, sampling intervals, and amounts of heat, we have dated multiple steps on ostracodes, thinolite, and gastropods. The results show that surface area-to-volume ratio is the most important factor in separating multiple steps: thinolite crushed to >300 microns can be controlled easily, while ostracodes are quite thin and delicate, making the reaction much more difficult to control.

Eleven steps on a thinolite fan from above Ash 1 show an increase of more than 2000 years from first to last step, and ~400 year increase over a lightly-leached bulk sample. A weighted mean of the final ages, 10,920 +/-20 ¹⁴C yrs BP, minus a 1000-year reservoir effect, calibrates to 11,300 +/- 70 cal yr BP, close to the end of the Younger Dryas interval recorded in Hulu cave (11,470 +/- 100 ka). Several sets of leaches on one ostracode sample yielded 4000- to 7000-year differences between first and last leach steps and 3500 years between the oldest leach and a bulk sample date. Calibration of the oldest leach age corresponds to 13,900+/-170 cal yr BP (with 1 ka reservoir correction). These results suggest that both the cold Younger Dryas and the warm Bolling-Allerod were extremely wet. Future work will increase the number of reliably-dated carbonates, and clarify the relation of the Mono Lake highstand to global climate changes.