10Be exposure age dating of Late Quaternary relative sea level changes and deglaciation of W Jura and NE Islay, Scottish Inner Hebrides

New Be-10 exposure age dating and geomorphological mapping of emerged shoreline features in W Jura and NE Islay throw new light on the regional pattern of ice sheet deglaciation and late-glacial relative sea level change. We conclude that the oldest and highest emerged shorelines in this area were produced similar to 15.7-16.3 ka, shortly after ice sheet deglaciation similar to 16.5 ka. It is envisaged that the first incursion of marine waters into coastal areas took place close to a former ice sheet margin that oscillated in position across this part of the Scottish Inner Hebrides. The first evidence of late-glacial marine sedimentation following deglaciation consists of emerged marine terrace fragments and unvegetated gravel beach ridges, the former represented by a prominent glacio-isostatically tilted shoreline that declines in altitude NE to SW, from similar to 40 m above ordnance datum (OD) in NW Jura to similar to 19 m OD in central Islay. In W Jura, north of Loch Tarbert, spectacular staircases of up to 55 unvegetated gravel beach ridges were formed shortly after regional deglaciation, possibly within 1 ka. A preliminary estimate of the average rate of relative sea level lowering across W Jura between deglaciation and the Younger Dryas is in the order of similar to 7 mmyr(-1). Geomorphological evidence from Shian Bay, W Jura, indicates a truncation of the late-glacial beach ridge staircases by a large 480-m-long beach ridge (the Colonsay Ridge) at similar to 14.9 ka, when former relative sea level was at similar to 18 m OD. This ridge may represent the product of either a stillstand in the progressive lowering of relative sea level during the late-glacial or a reversal. This raises the intriguing possibility of an association between ridge formation and the timing of the well-established global meltwater pulse 1A between similar to 14.65 and similar to 14.8 ka.

Automated summary

New Be-10 exposure age dating and geomorphological mapping provide insights into the ice sheet deglaciation and late-glacial relative sea level change in the Inner Hebrides of Scotland. The study reveals the emergence of the oldest and highest shorelines shortly after ice sheet deglaciation, and the incursion of marine waters into coastal areas close to a fluctuating ice sheet margin. The research also highlights the formation of gravel beach ridges and marine terraces during the late-glacial period, suggesting changes in relative sea level.