Abstract
Atlantic Meridional Overturning Circulation (AMOC) is a key regulator of global climate. Recent coupled climate model studies and palaeoceanographic proxy data hint that AMOC may have declined in strength in recent decades as a result of anthropogenic warming. However, a lack of annual to decadal resolution paleo-records from intermediate water depths currently clouds our understanding of baseline changes in the highly variable AMOC system over the last millennium.In this thesis, I use two types of deep-sea corals, bamboo corals and Enallopsammia rostrata, to reconstruct tropical North Atlantic intermediate water radiocarbon content over the last century and temperature variation over the last ~600 years. Band-counting within the organic node is used to produce a radiocarbon independent age model for bamboo corals. However, for the E. rostrata, radiocarbon dating is used for age model development.
I find that the organic node of bamboo corals reflects the radiocarbon of the deep chlorophyll maximum depth rather than the surface mixed layer. In the locations where the chlorophyll production is mostly limited in the mixed layer, the organic nodes reflect the radiocarbon of the mixed layer. Bamboo corals calcitic radiocarbon records suggest tropical Atlantic intermediate water radiocarbon was enriched during the ~1960s and ~1980s, potentially related to a shallower North Atlantic Deep Water and/or a retreat of Antarctic Intermediate Water (AAIW) due to weakened AMOC. Calcitic skeleton radiocarbon records and seawater radiocarbon suggest the transport of bomb radiocarbon to the ~1500 m central tropical Atlantic is ~50 years.
Further back in time, my Li/Mg based temperature record from the E. rostrata shows a marked warming at the end of the Little Ice Age, coinciding with the reduction of AMOC. This warming is likely related to a deepening and warming tropical Atlantic thermocline and/or a retreat AAIW related to AMOC slowdown.
Collectively, these coral-based palaeoceanographic records provide new insights into changes in thermal structure and circulation of the Atlantic Ocean on human-relevant timescales, as well as setting the stage for use of these corals in future studies of the last millennium.
| Date of Award | 24 Jan 2023 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Laura F Robinson (Supervisor), Erica Hendy (Supervisor) & Joseph Stewart (Supervisor) |