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CGC-13 Fjords: climate and environmental change

Climate variability during the last millennium: Records from the fjordic coastal environments of the NW European shelf

William Austin, University of St Andrews (United Kingdom)
Alix Cage, University of St Andrews (United Kingdom)
Philip Gillibrand, Hydrodynamics Group, NIWA (New Zealand)
 

A key question in climate science is whether the warming observed during the 20^th Century is unusual in its rate and magnitude? To address this issue, high-resolution palaeoclimatic records from the past 1,000 years and covering the period of natural and anthropogenically-forced climate variability are being generated from European archives as part of the EU Framework 6 project ?Millennium'. Ocean dynamics play an important part in the regulation of climate in NW Europe and sedimentary archives from marginal marine environments, such as fjordic (or sea-loch) environments, with their high sediment accumulation rates provide suitably expanded archives spanning the past 1,000 years against which the 20^th Century changes can be compared. Additionally, since fjords bridge the land-ocean interface, palaeo-environmental records from fjordic environments provide a unique opportunity to study the link between marine and terrestrial climate.
Loch Sunart, a fjord on the NW coast of Scotland lies in a climatically sensitive region of NW Europe, experiences significant inter-annual variability and has a circulation known to respond to atmospheric forcing, e.g. precipitation and wind strength/direction. (Gillibrand et al., 2005). The 22.5m long sediment core, MD04-2832, captured from the main basin of Loch Sunart spans the last 8000 years, with the last 1000 years represented by ∼ 3 m of sediment, yielding a sampling resolution between 1-4 years for the past millennium.
The stable carbon isotope record (from benthic foraminifera) shows a δ¹³C Suess effect similar to tree-rings and which can be corrected for using a modified version of the McCarroll & Loader (2004) correction factor (scaled to account for impeded mixing across the air-ocean interface and mixing of the surface ocean reservoir with the deep ocean reservoir). Once corrected, the carbon isotope record of Loch Sunart appears to primarily reflect the varying influences of coastal and fjordic water masses (i.e. coastal water ventilation) at the core site, whilst the oxygen isotope record reflects large decadal variability in bottom water temperatures (BWTs; ∼ 1-2 °C), overlying longer-term centennial changes.
A strong and rapid warming trend is observed in the δ¹⁸O/BWT record since AD 1950 years with the warmest BWTs of the last millennium occurring at the core top in 2004/2005. However, this trend does not appear to be that unusual when we look at the BWTs of the past millennium. The BWTs between AD 1575 - 1645 are warmer than most of the 20^th Century and appears to be warmest period of the last millennium. The ∼2 °C shift to these higher BWTs occurs quite rapidly, within about 10-15 years, which is similar to the rate of change between cold to warm BWT shifts in the 20^th Century. This suggests that the 20^th Century warming seen in the Loch Sunart record may not be particularly unusual in its rate or magnitude.