FIGURE 3.7 Variation in the ratio of Mg²⁺ to Ca²⁺ in the ocean over the past 550 million years. Red bars represent values estimated from measurements of fluid inclusions in halite crystals from salt deposits. The gray line is a model. Shown at the top are summaries of geological evidence consistent with the model and measurements. When Mg/Ca > 2, aragonite rather than calcite tends to precipitate from the oceans as the primary nonbiogenic carbonate mineral, and MgSO₄, rather than KCl, is the first mineral to precipitate when seawater evaporates to form salt deposits. SOURCE: Loewenstein et al. (2001). Reprinted with permission of AAAS.

are needed to determine how closely they are balanced and how much their changing rates influence the climate system. Volcanism rates are commonly estimated from seafloor generation rates, which themselves must be estimated since most of the ocean floor has been subducted. Seafloor generation rates are calculated from plate tectonics reconstructions and ridge or trench lengths or from global sea level determined from shoreline markers. However, uncertainties are large and results vary. For example, scientists disagree on whether the global rate of seafloor generation has changed over the past 100 million years (Rowley, 2002, versus Engebretson et al., 1992). Interpreting sea-level records is complicated by uncertainties about whether the volume of ocean water has remained constant over the past 500 million years. Figure 3.8 shows deduced sea-level variations for the past 500 million years. The double-humped curve (second column of the chart) has become a backbone of Phanerozoic climate studies and is often regarded as a proxy for the CO₂ supply side of

FIGURE 3.8 Amplitude of sea-level change, in meters relative to modern, extracted from the stratigraphic record. The “backstripped” values account for the effects of sediment compaction, loading, and variations in water depth on basin subsidence. SOURCE: Miller et al. (2005). Reprinted with permission from AAAS.