Friday, April 3, 2009

Permian Subpolar Carbonates Are Cool Too!

From the high priest of cool water carbonates Noel P. James and colleagues, a paper in the March issue of Journal of Sedimentary Research:

Carbonate Sedimentation in a Permian High-Latitude, Subpolar Depositional Realm: Queensland, Australia

High-latitude carbonate rocks that formed in subpolar Paleozoic oceans contain critical information about past climate, but they are neither well documented nor fully understood. Lower and middle Permian limestones that were deposited in such paleoenvironments at 45°S to 55°S along the southeastern margin of Gondwana are now exposed in eastern Australia.......

....All of the carbonates are distinguished by a low-diversity, high-abundance heterozoan biota with no phototrophs and express a recurring deepening-upward stratigraphic motif that is interpreted to reflect rising sea level accompanying glacial meltdown. They also record a strong poleward gradient of increasing ice-rafted debris, increasing skeleton size, decreasing invertebrate diversity, decreasing epifaunal calcareous benthic foraminifers, and reduction in crinoids;...... a recurring deepening-upward stratigraphic motif that is interpreted to reflect rising sea level accompanying glacial meltdown.

That seems to be the big difference between the large scale stratigraphic style of tropical carbonates from temperate carbonates. Tropical carbonate sequences record a shallowing up motif. This is because invariably carbonate production is so prolific in warm CO2 degassing waters that it keeps pace with or even outpaces relative sea-level rise. The sediment surface aggrades and fills up the accommodation space before new space is created either through a sinking basin floor or a rising sea-level.

Temperate water carbonate production is slower. This is because in colder water dissolved CO2 doesn't escape solution so easily. Colder sea-water is not as saturated with calcium carbonate as tropical warm sea-water. The biota too is impoverished. So a slower calcium carbonate skeleton production rate is a primary reason why sea level rise outpaces the accumulation of sediment.

There could be another reason applicable to this particular area. The basin faces southwards towards the poles. If these areas located 45°S to 55°S were exposed to severe storms there might be very significant transport of carbonate sediment into deeper and other areas of the basin from the locus of deposition. The sediment surface even if it were aggrading would be kept below storm wave base by persistent storms.

Storms affect tropical carbonates too. Large volumes of sediment is washed into the adjoining periplatform deep waters. But production is so high that the sediment surface shallows up anyway.

I'm just speculating here. It might help if I actually read the paper !

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