Reconstructing Climate Variability in the North Atlantic during the Late Holocene: an Integrated Biomarker Approach

Abstract

The North Atlantic and Arctic Oceans play an important role in modulating European climate variability. Oceanic records of sea surface temperature (SST) variability covering the last 2000 years at high resolution in this region are scarce. Obtaining new high resolution reconstructions in this area is necessary to increase the level of understanding on how the climate system has varied during the last millennia, to put the recent warming into a wide context and to investigate if the natural modes of climate variability observed from instrumental records can be projected to the past. This PhD thesis focuses on generating new records of multidecadal climate variability in the North Atlantic Ocean, analyzing organic proxies (biomarkers) in marine sedimentary cores that cover the last 2000 – 4400 years. We generated new climate reconstructions in the two main paths where the northward flowing North Atlantic current meet the Arctic Ocean - the eastern Fram Strait and the Barents Sea - and also in a highly sensitive area to changes in the internal modes of climate variability and the interaction between the North Atlantic Ocean and European climate - the Skagerrak -. We used well established organic proxies (i.e. alkenones) but also recently developed ones (IP25, GDGT based proxies) to produce reconstructions based on SST but also on additional climate parameters such as sea ice cover variability, mean air temperature and export productivity. In addition, we tested the applicability of the relatively new proxies based on GDGTs in a marine sedimentary core from the Skagerrak and in soils from southern Norway and the Pyrenees. The TEX86 gave SST estimates that were similar to regional annual means in the Skagerrak but the MBT/CBT air temperature estimates were closer to summer temperatures. This bias towards a specific season was not due to changes in the synthesis of GDGTs in soils through an entire annual cycle or to an enhanced transport of the lipids to the Skagerrak with the Glomma River during this period. We suggest that the GDGTs in the Skagerrak might be a result of a mixture between lipids transported from southernmost locations and in situ production. The origin of these lipids in the Skagerrak should be addressed before applying these proxies in this area with reliability. From the records located in the eastern Fram Strait and the Barents Sea, positioned in the two main paths where the northward flowing North Atlantic Current meet the Arctic, we interpret there was a progressive increase in temperature probably due to a warming in or an enhancement of the North Atlantic Current. This trend started at ~1000 – 1300 AD but was accentuated from ~1500 - 1600 AD until the most recent part of the records. In the Skagerrak, an area influenced by 90% North Atlantic water, a long term cooling that lasted until AD 1300 was reversed by a progressive warming since AD 1600 until present even though insolation continued to decrease. We suggest that the warming trends observed in the three records during the last ~400 years were caused by changes in internal modes of climate variability such as more frequent predominantly positive phases of the Arctic Multidecadal Variability, the North Atlantic Oscillation or the Atlantic Multidecadal Oscillation. The impact of human activity on climate might be superimposed on a longer term warming trend caused by changes in natural modes of climate variability that progressively affected the study areas during the last ~400 years.

Date of Award	24 May 2013
Original language	Undefined/Unknown
Supervisor	Antoni Rosell Melé (Director)