Driftwood as a tool for investigation of Holocene pan-Arctic sea ice dynamics
Keywords: climate
dendrochronology
proxy reconstruction
proxy
ocean currents
oceanography
palaeoclimate
glaciation
Arctic
Holocene
driftwood
Arctic Ocean
sea ice
geochemistry

Arctic sea ice is rapidly declining in area and volume, with impacts on local and global climatic and ecological conditions. Without knowledge of past changes, this modern trend cannot be placed within a broader context that would aid future predictions for Arctic sea ice. Driftwood forms an under-utilised proxy for reconstructing Arctic sea ice extent over the Holocene (~12,000 yrs ago to present): its transport and deposition is determined by sea ice and surface current dynamics; making it a robust proxy for sea ice reconstructions. Driftwood in the Arctic results from the falling of trees into the large rivers that drain the circum-Arctic, which upon flowing into the Arctic Ocean can then become locked up in forming sea ice. This enables the driftwood to travel across the Arctic Ocean without sinking, making it an invaluable proxy for sea ice extent by recording variations in Arctic Ocean surface currents (and therefore sea ice drift) and ice cover. Given it takes on average multiple years for driftwood to travel from origin to destination in a high Arctic beach, multi-year sea ice is required for its transport. Driftwood is abundant on many raised Arctic beaches, as it is captured on shorelines that have risen out of the sea due to the retreat of the weight of glaciers at the end of the last ice age. Analysis of the driftwood’s age and provenance can be used to form a reconstruction of the driftwood transport routes through time, and therefore Arctic sea ice extent and dynamics. I present a first pan-Arctic collation of Holocene driftwood data providing a reconstruction of higher spatial and temporal resolution than other proxy-based methods. The collation of 913 driftwood samples from across the western Arctic with spatiotemporal distribution and available provenance data enabled the production of a proxy-based reconstruction of Holocene Arctic Ocean surface current and sea ice dynamics. This revealed that Holocene sea ice extent and drift is characterised by a gradual progression from millennial to centennial shifts in the relative position of the Transpolar Drift and Beaufort Gyre, which is consistent with the dynamics of the Arctic Oscillation. Building on this I present the development of novel methodological approaches which successfully employ driftwood as a proxy for Arctic Ocean surface current and sea ice dynamics. These reveal a 500-year history of driftwood incursion to northern Svalbard, directly reflecting regional sea ice conditions and Arctic Ocean circulation. This record indicates centennial- to decadal-scale shifts in source regions for driftwood incursion to Svalbard, aligning with Late Holocene high variability and high frequency shifts in the Transpolar Drift and Beaufort Gyre strengths and associated ﬂuctuating climate conditions. Driftwood occurrence and provenance also tracks the northward seasonal ice formation shift and migration of seasonal sea ice to the peripheral Arctic seas in the past century. A distinct decrease in driftwood incursion during the last 30 years matches the observed decline in pan-Arctic sea ice extent in recent decades. Lastly, I present the development of novel techniques to refine the provenance of driftwood through radiogenic isotopic analysis (87Sr/86Sr). The use of geochemical provenance techniques can potentially address some of the limitations of current methods, warranting further development. The results show the utilisation of 87Sr/86Sr ratios to establish provenance for Arctic driftwood has potential, with current confounding factors in contamination issues of driftwood during transport and in the scale, spatial heterogeneity and temporal variations of pan-Arctic source regions. Combination of all techniques can further define the role of atmospheric and oceanic circulation in sea ice and climatic changes throughout the Holocene. The components of the thesis describe the successful employment and further development of driftwood as a proxy for Arctic Ocean surface current and sea ice dynamics.