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The Role of the Sun in Holocene Climate Change
Reference
Blaauw, M., van Geel, B. and van der Plicht, J. 2004. Solar forcing of climatic change during the mid-Holocene: indications from raised bogs in The Netherlands. The Holocene 14: 35-44.

Background
"Raised bogs," in the words of the authors, "are dependent on precipitation alone for water and nutrients." In addition, the various species of plants that are found in them each have their own requirements with respect to depth of water table. As a result, the vertical distribution of macro- and micro-fossils in raised bogs reveals much about past changes in local moisture conditions, especially, as the authors note, "about changes in effective precipitation (precipitation minus evapotranspiration)."

At the same time, changes in the carbon-14 content of bog deposits reveal something about solar activity, because, as Blaauw et al. describe the connection, "a decreased solar activity leads to less solar wind, reduced shielding against cosmic rays, and thus to increased production of cosmogenic isotopes [such as 14C]." Consequently, it is possible to compare the histories of the two records (effective precipitation and 14C) and see if inferred changes in climate bear any relationship to inferred changes in solar activity.

What was done
Two cores of mid-Holocene raised-bog deposits in the Netherlands were 14C "wiggle-match" dated at high precision, as per the technique described by Kilian et al. (1995, 2000) and Blaauw et al. (2003), while changes in local moisture conditions were inferred from the changing species composition of consecutive series of macrofossil samples.

What was learned
The authors report that nine out of eleven major mid-Holocene 14C increases (for which they provide evidence that the rises were "probably caused by declines in solar activity") were coeval with major wet-shifts (for which they provide evidence that the shifts were "probably caused by climate getting cooler and/or wetter"). In the case of the significant wet-shift at the major 14C rise in the vicinity of 850 BC, they additionally note that this prominent climatic cooling has been independently documented in many parts of the world, including "the North Atlantic Ocean (Bond et al., 2001), the Norwegian Sea (Calvo et al., 2002) [see also Andersson et al. (2003)], Northern Norway (Vorren, 2001), England (Waller et al., 1999), the Czech Republic (Speranza et al., 2000, 2002), central southern Europe (Magny, 2004), Chile (van Geel et al., 2000), New Mexico (Armour et al., 2002) and across the continent of North America (Viau et al., 2002)."

What it means
The findings of their most recent study, according to Blaauw et al., "add to the accumulating evidence that solar variability has played an important role in forcing climatic change during the Holocene." The broad geographic extent of the observations of the 850 BC phenomenon also adds to the accumulating evidence that many of the climate changes that have occurred in the vicinity of the North Atlantic Ocean have been truly global in scope, providing even more evidence for their having been forced by variations in solar activity, which would be expected to have considerably more than a regional climatic impact.

References
Andersson, C., Risebrobakken, B., Jansen, E. and Dahl, S.O. 2003. Late Holocene surface ocean conditions of the Norwegian Sea (Voring Plateau). Paleoceanography 18: 10.1029/2001PA000654.

Armour, J., Fawcett, P.J. and Geissman, J.W. 2002. 15 k.y. palaeoclimatic and glacial record from northern New Mexico. Geology 30: 723-726.

Blaauw, M., Heuvelink, G.B.M., Mauquoy, D., van der Plicht, J. and van Geel, B. 2003. A numerical approach to 14C wiggle-match dating of organic deposits: best fits and confidence intervals. Quaternary Science Reviews 22: 1485-1500.

Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.

Calvo, E., Grimalt, J. and Jansen, E. 2002. High resolution U37K sea temperature reconstruction in the Norwegian Sea during the Holocene. Quaternary Science Reviews 21: 1385-1394.

Kilian, M.R., van Geel, B. and van der Plicht, J. 2000. 14C AMS wiggle matching of raised bog deposits and models of peat accumulation. Quaternary Science Reviews 19: 1011-1033.

Kilian, M.R., van der Plicht, J. and van Geel, B. 1995. Dating raised bogs: new aspects of AMS 14C wiggle matching, a reservoir effect and climatic change. Quaternary Science Reviews 14: 959-966.

Magny, M. 2004. Holocene climate variability as reflected by mid-European lake-level fluctuations, and its probable impact on prehistoric human settlements. Quaternary International 113: 65-79.

Speranza, A.O.M., van der Plicht, J. and van Geel, B. 2000. Improving the time control of the Subboreal/Subatlantic transition in a Czech peat sequence by 14C wiggle-matching. Quaternary Science Reviews 19: 1589-1604.

Speranza, A.O.M., van Geel, B. and van der Plicht, J. 2002. Evidence for solar forcing of climate change at ca. 850 cal. BC from a Czech peat sequence. Global and Planetary Change 35: 51-65.

Van Geel, B., Heusser, C.J., Renssen, H. and Schuurmans, C.J.E. 2000. Climatic change in Chile at around 2700 BP and global evidence for solar forcing: a hypothesis. The Holocene 10: 659-664.

Viau, A.E., Gajewski, K., Fines, P., Atkinson, D.E. and Sawada, M.C. 2002. Widespread evidence of 1500 yr climatic variability in North America during the past 14000 yr. Geology 30: 455-458.

Vorren, K.-D. 2001. Development of bogs in a coast-inland transect in northern Norway. Acta Palaeobotanica 41: 43-67.

Waller, M.P., Long, A.J., Long, D. and Innes, J.B. 1999. Patterns and processes in the development of coastal mire vegetation multi-site investigations from Walland Marsh, Southeast England. Quaternary Science Reviews 18: 1419-1444.


Reviewed 25 February 2004