Coupling of the cryosphere and ocean during intervals of rapid climate change in the palaeo record: a multi-proxy study of the Heinrich events of the last glacial from the Northeast Atlantic

Abstract

Determining the response of the global thermohaline circulation to freshwater perturbations is of vital importance for future climate modelling efforts. The Heinrich events of the last glacial provide classic case studies, with major episodic inputs of freshwater associated with large numbers of icebergs flooding the North Atlantic Ocean. Climate modelling experiments and proxy reconstructions have both indicated a significant decrease in the strength of the meridional overturning circulation in response to this fresh water input to the ocean during each Heinrich event. Here, I present high resolution, multi-proxy reconstructions of cryospheric and surface and deep ocean behaviour over the last 40,000 years from Ocean Drilling Project (ODP) Site 980 in the northeast Atlantic, incorporating Heinrich events 1 to 4. Oxygen, carbon and neodymium isotope reconstructions of bottom water chemistry show a unique signature at this site for every Heinrich event, indicating the influence of a different water mass during each event. Bulk sediment leachate neodymium isotope values are strongly offset towards more radiogenic values than both planktonic foraminifera and fish debris throughout the Holocene, however, the agreement between the substrates is much closer under glacial conditions. This observed offset is attributed to modification of the leachate signal by fine material transported by strengthened bottom current activity in the Holocene, suggesting that bulk sediment leachates may not always record bottom water chemistry faithfully at sediment drift sites. Rare earth element profiles suggest that foraminifera without their ferromanganese coatings removed do not undergo significant diagenetic modification in the sediment, making these a better choice for reconstructions of bottom water neodymium isotope signatures. Each Heinrich event shows a different sequence of changes in the lithologies of ice-rafted debris, which argues against a simple repeating pattern of ice sheet destabilisation at each Heinrich event. The high degree of spatial variability in IRD patterns between sites in close proximity, however, suggests that surface ocean properties and circulation likely exerted a strong control over the IRD flux records, and hence the phasing of the circum-Atlantic ice sheets cannot be simply deduced from any single sedimentary record. Evidence of perturbation in bottom water properties can be seen prior to the deposition of the main ice-rafted debris layer during some of the Heinrich events at Site 980, suggesting that circulation changes may have played a role in the destabilisation of ice sheets, though the nature of these precursor changes differs between events. These findings show that Heinrich events are not simple, repeating events. Instead, differences in fresh water input and in surface ocean properties and circulation between Heinrich events likely give rise to different patterns of middepth North Atlantic circulation. The observed contrasts in bottom water chemistry at Site 980 between different Heinrich events highlights the sensitivity of the overturning circulation to fresh water inputs and argues against a simplistic model of thermohaline circulation cessation at each Heinrich event.

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ORCID for Anya Jane Crocker: orcid.org/0000-0001-9561-5750

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