Subpolar North Atlantic paleoceanography over the last millennium and Holocene using cold-water coral geochemistry

Abstract

There is growing concern that the Atlantic Meridional Overturning Circulation might
have undergone a notable reduction over the past 150 years, potentially reaching its
weakest state over the last millennium. However, the limited availability of
instrumental data and the inconsistencies between existing proxy records – which
reflect related processes from different perspectives rather than directly measuring
ocean circulation – highlight the need for more independent, well-dated
paleoceanographic evidence to better investigate this hypothesis.
In this thesis, I analysed cold-water coral Li/Mg and Ba/Ca proxy data, coupled with
isotope dilution U-Th absolute dating, to reconstruct intermediate-depth seawater
temperature and dissolved barium concentration ([Ba]sw). Samples were collected from
the central and northeastern subpolar North Atlantic, targeting the last millennium and
the Holocene. Chapter 2 reveals a pronounced warming, accompanied by changes in
[Ba]sw at intermediate depths over the Reykjanes Ridge, after the end of the Little Ice
Age until the late 20th century, when the record ends. Together, these proxy data suggest
changes in water mixing and/or properties, both of which imply weakened source water
formations in the subpolar North Atlantic during this period. Building on this, Chapter
3 presents a long-term decrease in [Ba]sw at intermediate depths in the central subpolar
North Atlantic during the Holocene. This change could be explained by alterations in
the conservative mixing between the Northwest and Northeast Atlantic deep waters.
Alternatively, the early Holocene barium excess may reflect non-conservative
processes such as the gradual retreat of the Laurentide Ice Sheet, and/or the
accumulation of regenerated barium during sluggish deep ocean circulation. In
comparison, Chapter 4 examines the last millennium Rockall Trough and further
explores the use of paired cold-water coral-derived temperature and [Ba]sw data to trace
paleoceanographic changes in the northeast subpolar North Atlantic.
Overall, this thesis explored the co-variation of Li/Mg and Ba/Ca proxies in parameter
space to trace conservative and non-conservative processes affecting the ocean barium
cycle, and investigated their implications for ocean circulation variability. Furthermore,
the relative timing and mechanisms between North Atlantic ocean circulation
variability and climate change during the last millennium and Holocene were
investigated.

Date of Award	18 Mar 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Laura F Robinson (Supervisor), Katharine Hendry (Supervisor) & Joseph Stewart (Supervisor)