A HOLOCENE GLACIOLACUSTRINE RECORD OF THE LYMAN GLACIER AND IMPLICATIONS FOR GLACIER FLUCTUATIONS IN THE NORTH CASCADES, WA

The Holocene glacial history of the North Cascades is poorly understood, in part because most existing records rely on moraine remnants and are therefore discontinuous. To develop a more complete record of Holocene fluctuations of North Cascades glaciers, we collected and analyzed glaciolacustrine sediments (i.e., rock flour) deposited over the past ~7800 years in Lyman Lake by the upstream Lyman Glacier. We combined these results with equilibrium-line altitude reconstructions and glacier-climate modeling to quantify the climatic conditions that drove these fluctuations. Finally, we compared the Lyman Glacier’s continuous fluctuation record to existing glacier and climate records of the North Cascades. The Lyman Glacier's fluctuation history follows:

Time Period (cal yr BP)	Glacier Extent
7800 – 4900	No glacier
4900 – 4200	Intermediate
4200 – 3800	Minimum
3760 – 2900	Unsure
2900 – 2600	None
2600 – 2500	Min.
2500 – 2250	Int.
2250 – 2100	Unsure
2100 – 2000	None
2000 – 1800	Unsure
1800 – 1300	Int.
1300 – 1200	Min.
1200 – 1100	Unsure
1100 – 900	Int.
900 – 800	Minimum
800 – 600	Int.
600 – 500	Maximum
500 – 350	Int.
350 – 150	None
150 – 50	Max.
50 – Present	Min.

Our analysis indicates that: 1) the earliest Neoglacial advances in the region (starting ~6 ka) occurred asynchronously, with higher latitude and more maritime sites experiencing earlier advances; 2) Neoglacial advances remained small, infrequent and asynchronous until the last millennium; 3) Beginning at ~1.0 ka, glaciers throughout the North Cascades advanced synchronously, signaling the onset of the Little Ice Age (LIA); 4) North Cascades glaciers reached their maximum Holocene extents during the 15th and early 16th centuries (~0.55 – 0.45 ka), followed by apparent regional retreat and a final smaller 19th century (~0.15 – 0.05 ka) re-advance. The asynchronous early-to-mid Neoglacial fluctuations followed by synchronous LIA behavior suggests that local climate factors drove glacier fluctuations until the regional climate signal became strong enough to induce synchrony ca. 1.0 ka. Although the inferred regional retreat remains uncertain, the disappearance of the Lyman Glacier in the mid-LIA (~0.45 – 0.15 ka) is consistent with the precipitation record at Castor Lake (~100 km to the east), which indicates unusually dry winter conditions between ~1450 – 1850 CE (~0.5 – 0.1 ka).