The early to mid-Holocene thermal optimum is a well-known feature in a wide variety of paleoclimate archives from the Northern Hemisphere. Reconstructed summer temperature anomalies from across northern Europe show a clear maximum around 6000 years before present. For the marine realm, Holocene trends in sea-surface temperature reconstructions for the North Atlantic and Norwegian Sea do not exhibit a consistent pattern of early to mid- Holocene warmth (Figure 1). A new data-model comparison study focusing on the mid-Holocene helps to explain why.
Reconstructed temperatures depend on plankton type used
Sea-surface temperature records based on phytoplankton (alkenones and diatoms) generally show the existence of a warm early to mid-Holocene optimum. In contrast, several zooplankton (foraminifer and radiolarian) based temperature records from the North Atlantic and Norwegian Sea show a cool mid- Holocene anomaly and a trend towards warmer temperatures in the late Holocene.
Warm and cold watermasses
Model results indicate that while the seasonal summer warming of the sea surface was stronger during the mid-Holocene, sub-surface depths experienced a cooling. These results also show that the strong summer warming appears to be restricted to the upper 30–75 m. This hydrographic setting (Figure 2) can explain the discrepancies between the Holocene trends exhibited by phytoplankton and zooplankton based temperature proxy records. The relatively cool mid-Holocene zooplankton-based (foraminifer) temperatures can be explained by the presence of cool sub-surface waters during summer (July, August and September).
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| Figure 2. Modelled pre-industrial and Mid-Holocene (6000 years ago) temperature depth profiles for marine sites in the North Atlantic and eastern Norwegian Sea. |
Reference:
Andersson, C., Pausata, F., Jansen, E., Risebrobakken, R., and Telford, R.J., 2010. Holocene trends in the foraminifer record from the Norwegian Sea and the North Atlantic Ocean. Climate of the Past, 6: 179-193.