LAND-SEA LINKAGES AND THE MARINE FOSSIL RECORD

The fossil record of phytoplankton hints that plankton populations and the pelagic rain of dead organic carbon available as food to the benthos increased through time in response to nutrient runoff (Martin, 1996, Palaios 11: 209-219). Phosphorus is a limiting nutrient on geologic time scales, and its supply to the oceans is limited by weathering and runoff. The phosphorus and carbon content of ancient sediments is also affected by post-burial alteration (Anderson et al., 2001, Glob. Biogeochem. Cycles 15:65-79). The hypothesis of a secular increase in carbon (C) production relative to nutrient (P) availability was therefore tested using C:P ratios (calculated from %C and %P in Jackson and Moore, 1976, Chem. Geol. 18:107-136).

A robust trend of increasing C:P ratios began at ~1.5-1.3 Ga, but by the Phanerozoic the rise in marine C:P ratios leveled off. These results suggest that P became limiting during the Vendian-Cambrian, when the marine biosphere was rapidly expanding and sequestering nutrients in living and dead biomass. Thus, by the Phanerozoic the marine biosphere may have come to depend on bioturbation to remineralize organic matter and recycle nutrients, causing C:P ratios to level off. The expansion of terrestrial forests appears to have initially enhanced continental weathering and nutrient runoff (as suggested by Sr isotopes), but land floras later began to sequester nutrients (Tappan, 1986, J. Paleontol. 60: 545-554); this may have also promoted nutrient recycling and lowered C:P ratios in marine sediments.

Our findings are in basic agreement with changes in: 1) rates of carbon burial (Hayes et al., Chem. Geol., 1999, 161: 103-125); 2) limiting nutrients through time (Anbar and Knoll, 2002, Science 297:1137-1142); and 3) the marine macrofossil record, which indicates a secular increase in "energetics" (biomass+ metabolism+food requirements; Bambach, 1999, GeoBios 32:131-144). Also, the strong rise in Sr isotopes during the Cenozoic parallels a reported rise in biodiversity (e.g., Jablonski et al., 2003, Science 300: 1133-1135). Possibly, increasing food availability supported larger populations of marine benthos, increasing their "reproductive power" (i.e., the rate at which resources can be used for reproduction; Brown, 1995, Macroecology) and potentially their evolutionary diversification. Nevertheless, the fossil record may be confounded with erosion and sedimentation.