Arctic bottom water temperatures
A benthic foraminiferal MG/Ca-temperature field calibration and application

27th October 2016
14h00–17h00, DeGeer salen

Main supervisor: Helen Coxall
Co-supervisor: Martin Jakobsson

Examiners:
Per Andersson (Naturhistoriska riksmuseet)
Helena Filipsson (Lund University)

Abstract
The Arctic is more sensitive to modern global temperature rise than the rest of the planet, a phenomenon known as polar amplification. It is predicted that warming of Arctic bottom waters by only 1–2°C could mobilize vast stocks of fossil carbon on subsea shelves and margins. This could exacerbate global warming through release of greenhouse gases. To date, very little is known about the importance of these potential ocean-carbon system climate feedbacks, therefore records of past Arctic bottom water temperatures (BWT) and climate variability are of paramount importance for future climate modeling. This provides the motivation for this Ph.D. project/licentiate thesis as a component of the 2014 SWERUS-C3 program (Swedish- Russian-US Arctic Ocean investigation of Climate-Cryosphere-Carbon interactions), with the overarching aim to reconstruct Arctic Ocean bottom temperatures through glacial-interglacial cycles.

Reconstructing Arctic BWT in the Quaternary using palaeoceanographic proxies has proved challenging due to the unique physical and chemical environment of the Arctic Ocean. Mg/Ca palaeothermometry in calcitic microfossils has shown promise but a large amount of groundwork is still required to develop reliable Mg/Ca temperature calibrations. In this licentiate thesis, I present the results of two studies that investigate the applicability of Mg/Ca paleothermometry using Arctic benthic foraminifera. The first study explores the influence of temperature, salinity and carbonate ion concentration on Mg/Ca in six benthic species (Elphidium clavatum, Nonionellina labradorica, Cassidulina neoteretis, Quinqueloculina arctica, Oridorsalis tener and Cibicidoides wuellerstorfi) using material recovered from Arctic marine surface sediments and their complementary field oceanographic measurements. The applicability of Mg/Ca paleothermometry in Arctic shelves compared to the central Arctic is also explored. In the second study, I use benthic foraminifera Mg/Ca and stable isotope analyses in E. clavatum to infer late Holocene bottom water variability from the Herald Canyon (72 m water depth) in the Chukchi Sea. This area is characterized by exceptionally high sedimentation rates (~200 cm/kyr), which, with strong radiocarbon age control, provide unprecedented detail of Holocene Pacific-Arctic Ocean history. The obtained Mg/Ca values reveal bottom water variations over millennial time scales moving towards a cooling trend at ~1200 cal. yr BP. This signal is similarly recorded in Mount Logan (Yukon, Canada) δ18O ice core record suggesting that Herald Canyon benthic Mg/Ca is recording BWT variability that is linked to Pacific atmosphere-climate oscillations. I conclude that benthic foraminiferal Mg/Ca proxy can be successfully applied in the Arctic Ocean, although using specific foraminifera taxa and preferentially in depositional environments associated with high sedimentation rates.

 

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