Saturday, March 21, 2009

Land Plants And Expression Of Disconformities in Limestone Sequences

Geological Processes and Evolution - 2

A paper in the January issue of Geology looks at the influence of land plants on the detrital sedimentary cycle.

Evidence from Middle Ordovician paleosols for the predominance of alkaline groundwater at the dawn of land plant radiation -

Atmospheric CO2 has decreased by about 500 times from Archean times to recent. So rain water composition has become less acidic over time. But groundwater does not show such a temporal decrease in acidity. In fact groundwater may be more acidic on average in younger geological periods compared with the Archean. This is because of the evolution of land plants by Silurian times which started contributing organic acids to groundwater. The amount of organic acid from the plant system easily offset any decrease in rain water carbonic acid from decreasing atmospheric CO2.

The authors identify a geologic time window - Mid-Late Proterozoic to Late Ordovician when groundwaters were more alkaline than the time period before and after. This is because a) a spurt of continental crust formation by mid-late Proterozoic started consuming CO2 in weathering reactions and b) this stable continental crust provided a substrate for accumulation of carbonate sediments, another important CO2 sink. By early Paleozoic, atmospheric CO2 had dropped compared to the Archean (it was still about 10 times more than the recent ~ 300 ppm). And land plants had not yet evolved.

The paper in Geology demonstrates using the geochemistry of mid-Ordovician paleosols that the groundwater was alkaline in composition. There is some interesting geochemistry for those into such things like element mobility. Other studies in the past have focussed on the detrital composition of sediments and have come to the same conclusions. For example there is an abundance of detrital illite and K feldspar in the mud and sand fractions of early Paleozoic sediments as compared with younger sediments from similar provenance. In acidic groundwater K is mobile and is taken up by chelation into land plants or is transported away in solution. So diagenetic illite is more common in post-Paleozoic sediments.

I have often run into the influence of land plants on sediments from a different perspective, one that involves identification of exposure surfaces in carbonate sequences. Post Silurian during sea-level drops the exposed sea bed will get colonized by land plants. The release of organic acids leads to acidic groundwater and rapid dissolution of the carbonate sediment. Add to that is the physical disruption of sediment by the action of roots and burrowing fauna. All this leads to the development of a highly irregular, pitted surfaces with collapse structures, caverns and infill breccias and a deep chalky weathering profile often with Fe hydroxide stains and crusts. The image below is of a Pleistocene skeletal limestone from a mid shelf facies setting exposed in south Florida for the past several tens of thousands of years.

Notice the karst like profile already developing and penetrating the sequence. If this surface is overlain by sediments in the future the disconformity would be easy to spot in the field.

No such luck in the mid-late Ordovician. Look at this mid-late Ordovician disconformity in the image below. This is the top of a depositional cycle, a parasequence boundary in stratigraphic jargon. The facies setting is similar to the Pleistocene example above.

The climate too has been interpreted to be warm and seasonally moist. The contact just above the coin is an exposure surface. It is a planar surface with a few small pits but overall there is no evidence of the kind of disruptive features you see underneath younger disconformities.

This cycle is part of a cyclic sequence. I could never get a handle on the individual cycle frequency but using regional stratigraphic markers and conodont biostratigraphy a rough estimate is that surface was exposed for tens of thousands of years during Richmondian sea level falls. There is plenty of microscopic evidence of exposure in the form of preserved vadose diagenetic fabrics, but in the field it looks just like a bedding plane. In the late Ordovician there was a land plant cover but it was mostly of the bryophyte grade i.e. mosses and liverworts. These plants don't have disruptive root systems and don't release the massive quantities of organic acids for penetrative weathering profiles to develop.

There are other controls in carbonate systems that can form karstic surfaces in the absence of land plants. One is the facies setting. Supratidal and sabkha facies with dolomite and evaporite minerals dissolve quickly in carbonate under saturated groundwater and these facies often develop the kind of disruptive weathering profiles you associated with a land plant cover.

And the other control is time. Very long lived exposure of limestone, surfaces that are exposed to meteoric condition for tens of millions of years will eventually succumb and form distinctive karst. The image below is of the Knox unconformity between lower Ordovician passive margin platform carbonates and a mid Ordovician foreland basin sequence.

The Knox surface developed during a major sea-level fall that lasted at least 10 million years. A mature karst terrain developed across facies tracts. Sinkhole and caverns fills and intraformational breccias can be seen to depths of 250 m beneath the unconformity.

There have been a couple of recent studies that make connections between evolution and geological processes. One dealt with an increase in diversity in mineral species after the evolution of photosynthesis. The other pointed out a connection between sea water chemistry, evolution of animals and skeletal mineralogy.

Geology may influence evolution. The paper points out that the early radiation of land plants likely took place in alkaline groundwaters. That may explain the affinity for alkali shown by many land plant groups. Subsequently as plant life took hold over terra firma, evolution influenced geology through the transition to acidic groundwater, detrital and diagenetic sediment composition, element mobility and soil composition and characteristics of disconformities.

See: Geological Processes and Evolution