Cyclic and secular variation in microfossil biomineralization: clues to the biogeochemical evolution of Phanerozoic oceans
The stratigraphic occurrence and mineralogy of major protistan microfossil taxa tend to reflect evolutionary innovation in response to ocean chemistry and fertility. In foraminefera, the characteristic test composition—and, in some cases, ultrastructure—of each suborder is indicative of the degree of surface ocean CaCO 3 saturation, which varied in a cyclic manner through the Phanerozoic, at the time of origin of the suborder. High dissolved phosphate and low CaCO 3 saturation in late Precambrian-Early Cambrian surface waters may have prevented calcification in primitive non-calcareous (organic, agglutinated) foraminiferal stocks. Scattered reports of coccolithophorid-like microfossils from the Paleozoic are indicative of a secular trend in rising nutrient levels and marine productivity that controlled the initiation of calcareous oozes. Based on acritarch, carbon isotope, and phosphorite records, extremely low nutrient levels ("superligotrophic" conditions) in Cambrian-to-Devonian seas typically limited population densities of calcareous nannoplankton and prevented the formation of calcareous oozes. The overall "superoligotrophic" surface conditions of the Paleozoic were punctuated, though, by episodes of "catastrophic" eutrophication in the Late Ordovician, Late Devonia, and Late Carboniferous (Worsley et al., 1986). Following each episode, CaCO 3 rain rates were presumably enhanced because Marine C:P (MCP) burial ratios increased permanently above previous levels (Worsley et al., 1986). Nevertheless, it was not until the Carboniferous that the CCD had deepened sufficiently (via erosion of cratonic limestones) to allow pelagic calcareous oozes to begin to accumulate. Prior to this time, surface waters appear to have been sufficiently corrosive (high atmospheric pCO 2 and low CaCO 3 saturation), and the CCD sufficiently shallow, to dissolve virtually all incipient calcareous nannofossils. Following Late Permian extinctions, plankton re-expanded in response to both eustatic sea level rise (increased habitat availability) and increased nutrient levels ("mesotrophic" conditions). As organic matter (C org) and CaCO 3 rain rates increased, bioturbation rates also increased, thereby recycling nutrients back to the surface and accentuating productivity and calcareous ooze formation. MCP episodes further accelerated nutrient cycling and productivity in the Neogene, as indicated by the expansion of diatoms, which prefer nutrient-rich ("eutrophic") conditions. Ironically, while permanently increasing C:P burial ratios and productivity through the Phanerozoic, catastrophic fluctuations in nutrient levels may have also exacerbated mass extinctions via shortening of pelagic food chains. Nevertheless, re-expansion of the marine biosphere following each extinction episode resulted in a secular trend of increasing biomass and biotic diversity that may have contributed to the decline in background extinction rates through the Phanerozoic.