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Surface Temperature Reconstructions for the last 2,000 Years
FIGURE 5-5 Greenland temperature (Dahl-Jensen et al. 1998), Bermuda Rise SST (Keigwin 1996), and West African SST for the last 2,500 years. SOURCE: deMenocal et al. (2000). Reprinted with permission from AAAS; copyright 2000.
rafted lithic grain abundances in several North Atlantic cores were paced by variations in solar irradiance (Bond et al. 1997, 2001) (Figure 5-6).
In addition, sedimentation rates are fast enough in some estuarine and coastal settings to allow climate reconstruction for the last 2,000 years. Cronin et al. (2003) used Mg/Ca paleothermometry on microfossil shells to show temperature shifts of 2–4°C in the Chesapeake Bay, including cold excursions during the Little Ice Age and warmer periods during medieval times (about A.D. 800 to 1300). Because of estuarine pollution linked to land clearance from the mid-19th century, the most recent part of this record may reflect factors other than water temperature.
LAKE AND PEAT SEDIMENTS
In many lakes, sediments contain distinct seasonal layers, or varves, that are either biogenic (e.g., carbonate, diatom silica, and organic matter) or minerogenic (alternating coarse and fine-grained particles) in origin. Both sediment types potentially allow annual dating for sequences that span many millennia, although typically with chronological errors of a few percent (Zolitschka 2003). Lake water biology and chemistry are often sensitive to temperature, but they are also influenced by other factors such as precipitation, watershed land use, and atmospheric pollution. To obtain unambiguous climate signals from lake records, researchers often choose sites in remote locations, such as the High Arctic. The summer ice-free period in Arctic Canadian lakes is highly sensitive to temperature, for example, and this in turn has been recorded in the thickness and nature of seasonal varves (Lamoureux and Bradley 1996).
Lakes in tropical and dryland regions are usually more sensitive to water balance than they are to temperature per se. Consequently, lake sediment records provide one of the key natural archives for reconstructing histories of drought and flood in regions such as the U.S. Midwest (Laird et al. 1998), as well as long-term changes in ENSO activity (Rodbell et al. 1999). Stable isotope analysis of a varved lake sequence by