Introduction

The existence of an early to mid-Holocene thermal optimum is frequently referred to in the paleoclimate science literature. (Early Holocene is here loosely defined as 11-8 ka, and mid-Holocene is loosely defined as 8-4 ka, both in terms of calendar years BP.) The thermal optimum has been documented by a variety of paleocli-matic evidence, spanning many Northern Hemisphere high-latitude paleoclimatic archives, including ice cores, marine and limnic sediments, glacier records, and speleothems. A comprehensive overview is given in the 4th Assessment Report of the Intergovernmental Panel on Climate Change Working Group 1 (chapter 6, figure 6.9; Jansen et al. 2007). Based on alkenone data, Kim et al. (2004) found that the thermal optimum is most pronounced at high northern latitudes in the oceans, and that in low-latitude areas it is less pronounced or nonexistent. In some low-latitude areas, such as the Indian Ocean and parts of the tropical Atlantic, the period of the thermal optimum (8-6 ka) as recorded in the high-latitude North Atlantic is characterized by colder temperatures than in the late Holocene. Some of the sites providing evidence for a colder early to mid-Holocene are from upwelling regions; hence some of the cold anomalies may be related to enhanced wind-driven upwelling. There appears to be a poleward amplification of the thermal anomaly, as well as a clear tendency that the warmest temperature interval started earlier and lasted for a shorter period towards the Arctic than further south, where it often appeared as a warm phase between about 8 to 6 ka (Ko^ et al. 1994; Sarnthein et al. 2003; Duplessy et al. 2005; Hald et al. personal communication).

Another feature is a delayed thermal maximum in the Labrador-Irminger Seas south of Iceland, where peak Holocene temperatures occurred at about 6 ka. This feature is probably related to the late demise of the Laurentide Ice Sheet (Andersen et al. 2004; Hall et al. 2004; Berner 2006). Many authors have attributed the thermal optimum to the combined effects of the demise of the continental ice sheets, removing their influence on temperature distribution, and higher summer solar irradiance than today due to orbital factors, especially the tilt of the Earth's axis, which provided a positive summertime radiative forcing anomaly in high-latitude regions in both hemispheres.

Figure 5.1 shows selected localities where examples of the thermal optimum have been demonstrated (Figure 5.2). Figure 5.3 summarizes the marine 6 ka thermal anomaly in the Mediterranean and the north-east Atlantic-Nordic Seas by using a data-set from alkenones only, thereby reducing errors when combining different proxy methods. This figure also shows the northward amplification of the temperature anomaly.

Despite these findings, however, there are a number of North Atlantic and Nordic Seas records where the thermal optimum is not apparent, and in some there is instead a tendency for a warming throughout the Holocene

In this chapter we analyze these contrasting records in the high-latitude ocean with the aim of attributing the contrasts to the forcing and/or to dynamics of the ocean-atmosphere system, and we evaluate the interplay between the long-term responses due to orbital forcing and shorter century to millennial scale variability.

Figure 5.2 Examples of records displaying a mid-Holocene thermal maximum. (a) UK37 data from MD95-2011 (Calvo etal. 2002). (b) Pollen-based annual mean temperature reconstruction from Lake Flarken (Seppa et al. 2005). (c) Pollen-based reconstruction of mean July temperature from Vestre 0ykjamyrtj0rn (Folgefonna peninsula; Bjune et al. 2005).

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MD95-2011

MD95-2011

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Lake Flarken

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Lake Flarken

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Vestre 0ykjamyrtj0rn

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Vestre 0ykjamyrtj0rn

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