The Biological Pump in the Past

While the early focus of hypotheses for glacial/interglacial CO₂ change was on the biological pump (Broecker, 1982a,b), a number of alternative mechanisms, mostly involving the calcium carbonate budget, attracted increasing attention during the late 1980s and the 1990s (Archer and Maier-Reimer, 1994; Boyle, 1988b; Broecker and Peng, 1987; Opdyke and Walker, 1992). As problems have been recognized with these mechanisms as the sole driver of glacial/interglacial CO₂ change (Archer and Maier-Raimer, 1994; Sigman et al., 1998), the biological pump is receiving attention once again. Hypotheses exist for both low- and high-latitude changes in the biological pump. Their strengths, weaknesses, and central questions are now fairly clear. Currently, the most popular low-latitude hypotheses depend largely on whether the nitrogen cycle alone can drive a large-scale change in low-latitude export production. As of the early 2000s, while this question has not yet been directly addressed, no new observations have arisen that would overturn the traditional view that the nitrogen cycle could have had only a limited effect without cooperation from the phosphorus cycle and that the phosphorus cycle could not have been adequately dynamic over glacial/interglacial cycles (e.g., Froelich et al., 1982; Haug et al., 1998; Redfield, 1942, 1958; Redfield et al., 1963; Ruttenberg, 1993; Sanudo-Wilhelmy et al., 2001). Thus, while the low-latitude biological pump has by no means been eliminated as the driver of glacial/interglacial cycles, our attention is focused on the polar ocean, the Antarctic in particular. Figure 11 A 6 Myr biogenic opal record from Ocean Drilling Program Site.