FIGURE 5-3 Composite from Cobb et al. (2003) showing Northern Hemisphere reconstructed temperature anomalies (Mann et al. 1999), δ18O values derived from fossil corals in Palmyra, solar radiance anomalies (Lean et al. 1995, Bard et al. 2000), and one estimate of radiative forcing anomaly (Crowley 2000). SOURCE: Cobb et al. (2003). Reprinted with permission from Macmillan Publishers Ltd.; copyright 2003.

MARINE SEDIMENTS

The utility of marine sediments in recording climate change during the Holocene depends either on sufficiently rapid sediment accumulation to overcome the mixing effects of bioturbation2 (usually up to 8 centimeters) or deposition under anoxic or suboxic oceanic conditions to retain annual layers. Both types of depositional environments have been exploited in attempts to infer oceanic and atmospheric conditions affecting the record in the sediments.

Several studies of marine sediments have provided insight into past climate on a regional basis. The Cariaco Basin off Venezuela is an anoxic basin with annual layers reflecting changes in atmospheric conditions that accompany shifts in the location of the Intertropical Convergence Zone (ITCZ), in particular the location and intensity of precipitation. Haug et al. (2003) have studied the titanium concentration changes in the Cariaco Basin annual sediment layers to infer the variations in the ITCZ, which impacted rainfall on the Yucatan Peninsula. The record is somewhat ambiguous in defining the Little Ice Age or a warm period during medieval times, but the authors believe it indicates several epochs of severe drought at the beginning of medieval times

2

Bioturbation is the mixing of sediments by bottom-dwelling organisms (see, e.g., Turekian et al. 1978).



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