Middle Cambrian
From Palaeos
| Cambrian period 542-488 | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Early Cambrian 542-521 | Middle Cambrian 521-501 | Furongian 501-488 | |||||||
| Series 2 521-510 | Series 3 510-501 | ||||||||
| Fortunian 542-535 | Stage 2 535-521 | Stage 3 521-517 | Stage 4 517-510 | Stage 5 510-506 | Drumian 506-503 | Guzhangian 503-501 | Paibian 501-496 | Stage 9 496-492 | Stage 10 492-488 |
| Cambrian topics: Stratigraphy | Geography | Climate | Life | Important Fossil Sites | References | Links | |||||||||
513.0 to 501.0 million years ago
| Top: Geological Timescale |
| Up: Cambrian Period |
| [[Previous:Early Cambrian Next: Late Cambrian]] |
| Middle Cambrian |
| Scientific History | Geography | Climate | Life | Important Fossil Sites | References | Links |
Something seems to have happened to the oceans at the end of the Early Cambrian. One indicator is that the magnesium content of seawater seems to have dropped. We know this because, in the Early Cambrian, calcium carbonate precipitated out of seawater as aragonite, a mineral form containing relatively high concentrations of magnesium as an impurity. When magnesium concentrations in seawater are lower, calcium precipitates as calcite. For reasons we don’t need to get into, both high water and low magnesium are correlated with a higher rate of sea floor spreading -- the creation of new sea bottom from magma which seeps out along mid-ocean fault zones. The Early Cambrian marked a dramatic increase in the rate of sea floor spreading. So, as we have mentioned, the seas rose and magnesium concentrations dropped.
