Diatom-inferred centennial-millennial postglacial climate change in the Pacific Northwest of North America

A diatom record from Moss Lake, Washington, USA spans the last 14,500 cal yr and revealed Holocene climate change in the Pacific Northwest (PNW), including evidence for periodicities related to atmospheric-ocean teleconnections and/or variations in solar output. Three main climate phases were identified: (i) Late Pleistocene to early Greenlandian (until 10,800 cal yr BP, spanning GI-1, GS-1), with a cold climate and low diatom abundance; (ii) early Greenlandian to Northgrippian (10,800–7500 cal yr BP), shifting to a warmer climate; and (iii) late Northgrippian and Meghalayan from 7500 cal yr BP onwards, with a cooler, moist climate. These climate shifts are in good agreement with the pollen record from the same core and other regional studies. Fluctuations in Discostella pseudostelligera and Aulacoseira taxa suggest climate cycles of different frequency and amplitude throughout the record. Spectral and wavelet analyses revealed periodicities of approximately 1400 and 400-500 years. We interpret the ~1400-yr and ~400-500-yr cycles to reflect alternating periods of enhanced (and reduced) convective mixing in the water column, associated with increased (and decreased) storms, resulting from ocean-atmosphere teleconnections in the wider Pacific region. The ~1400-yr periodicity is evident throughout the Late Pleistocene and late Northgrippian/Meghalayan, reflecting high-amplitude millennial shifts from periods of stable thermal stratification of the water column (weak wind intensity) to periods of convective mixing (high wind intensity). The millennial cycle diminishes during the Greenlandian, in association with the boreal summer insolation maximum, consistent with suppression of ENSO-like dynamics by enhanced trade winds. Ocean-atmosphere teleconnection suppression is recorded throughout the PNW, but there is a time discrepancy with other records, some that reveal suppression during the Greenlandian and others during the Northgrippian, suggesting endogenic processes may also modulate the Moss Lake diatom record. The large amplitude of millennial variability indicated by the lake data suggests that regional climate in the PNW was characterised over the longer-term by shifting influences of ocean-atmosphere dynamics and that an improved understanding of the external forcing is necessary for understanding past and future climate conditions in western North America.