DID GRASSLAND EXPANSION TRIGGER BURSTS IN LACUSTRINE DIATOMITE ACCUMULATION?

Two Miocene bursts in grassland evolution are expected to have increased the accumulation of lacustrine diatomite. The predominance of diatomite mining in Miocene and younger deposits generally supports a Miocene increase, but presently available data cannot be resolved into two pulses that match the grassland radiations. Volcanic silica may have been locally important, but grassland expansion provides a more effective mechanism for widespread delivery of silica to lakes.

Grass uptake of silica for opaline phytoliths probably mobilized more silica from soils than earlier biogeochemical weathering processes. Gradual release of this relatively soluble silica along with other nutrients into rivers and lakes via runoff, wind, and groundwater might have led to a general increase in lacustrine diatom productivity. However, if grass-mobilized silica was stored in the soil/grassland recycling loop, its release may have been episodic when drops in sea level triggered fluvial incision, drainage network expansion, and soil erosion. Such pulses of silica release would produce spikes in diatomite abundance.

If Early Miocene sod-forming grasses were more effective ground cover than pre-existing non-forest plants, lacustrine diatomite abundance could have increased via decreased dilution by siliciclastic runoff. If runoff reduction was the dominant factor, diatomite abundance would not increase with the Late Miocene radiation of the C4 grasses. However, if silica supply was the critical variable, diatomite volume should have risen with the expansion of silica rich C4 grasses, but only in the tropics where those grasses thrive.

Both the Early Miocene expansion of sod-forming short grasslands and the Late Miocene C4 grass radiation were followed by diatom diversifications. Although the increased availability of silica may have contributed to these diversifications, those evolutionary expansions may have occurred independently of changes in dissolved silica supply.