Friday, June 25, 2021

Articles: Trace Fossils, Supercontinents, Harappan Hydrology

 Some interesting geology rich readings from the past few weeks:

1) Ichnology is a branch of palaeontology that studies the traces made by organisms in soft sediment. These could be tracks and trails as animals move around on a substrate, or burrows constructed as escape structures or as dwellings, or bite marks on shells and bones. All these are indicative of behavior, which otherwise would be hard to discern from just the fossilized remains of body parts. Science writer Jeanne Timmons has written this lovely article on Ichnofossils and what they tell us about past ecology and animal behavior.

Trace fossils, the most inconspicuous bite-sized window into ancient worlds.

2) The earth has seen over its long geological history episodes of continents coming together to form a supercontinent, then breaking up and drifting apart forwhat seems an eternity, but eventually coalescing to form another giant landmass. When did this supercontinent cycle begin on earth. What are the forces that initiated and subsequently has maintained this mode of surface reconfiguration, and what are its consequences on tectonics, and the physical and chemical evolution of earth. A great review article by Ross N. Mitchell and colleagues.

The Supercontinent Cycle.

3) The rivers that sustained the Bronze Age Harappan Civilization have been the subject of lively research in recent years. Ajit Singh and colleagues have worked on the Markanda river catchment in the Sub-Himalaya dun region. Markanda joins the Ghaggar-Hakra river flowing through present day Harayana, Punjab and Rajasthan. They find that during the Mature Harappan Period (2600 B.C. to 1900 B.C.), large floods in the Himalaya foothill rivers sustained flow in downstream reaches, making  agricultural viable, even as northwestern parts of India experienced a reduction in summer monsoon strength.

Larger floods of Himalayan foothill rivers sustained flows in the Ghaggar–Hakra channel during Harappan age (behind paywall).

Tuesday, June 15, 2021

Lessons From A Hot Past

This short editorial published recently in Nature Geoscience is worth reading and thinking about. It summarizes our findings of past climate change and how earth systems such as sea level, glaciers, and the biosphere responded to these climate swings. 

Reconstructing temperatures going back to the Eocene (~50 million years ago) and later in the Miocene (~ 15 million years ago) reveal a very different world. These finding do come with a caveat. The rates of change are averaged over thousands of years, while we today stare at an unfolding catastrophe in our lifetimes. 

There is data though from more recent times that can tell us in finer temporal detail how climates fluctuated. Carbon dioxide trapped in Antarctica ice sheets points to changing atmospheric composition on a centennial scale and tree ring data informs us about seasonal changes in rainfall.

The current level of CO2 in the atmosphere of about 415 ppm (parts per million) are the highest since the Pliocene, more than 3 million year ago. We are also pumping CO2 at rates which are unprecedented in geologic memory, a shift from about 280 ppm to our present levels in just about 150 years The past may not provide a perfect analogue for the rapid changes we are experiencing, but it does send us a sobering warning that civilization's envelopes of comfort will be breached not so far in the future.

Nature Geoscience Editorial: Lessons From A Hot Past.

Monday, June 7, 2021

Liesegang Banding In Proterozoic Badami Sandstone

The Chalukya era (6th-8th CE) rock cut caves and sculptures at Badami in Karnataka are an archeological wonder. But there is plenty of geology there to admire. In January 2020, I spent some time wandering through Badami. The sandstone layers are 900 million years old river deposits. I wrote a long post about them, explaining the primary sedimentary structures that one can observe in these rocks, and what they tell us about the water depths and currents during deposition of the sediment. 

But these primary structures, i.e. sedimentary layer orientations that form during deposition, are not the only interesting features of these rocks. Chemical reactions in these sediments after their burial has overprinted an intriguing fabric on to the rock.

In the picture a very distinct dark and light banding is seen in one of the Badami rock surfaces. This is Liesegang banding. 

The dark bands are rich in iron oxide. The lighter bands have little or no iron oxide. Such banding forms by the mobilization of ions from one location in the sediment and their precipitation at another. Ions diffuse along a concentration gradient in the water filled pore spaces. Robert A. Berner's book, Early Diagenesis: A Theoretical Approach, has a good explanation for the formation of Liesegang banding. I am reproducing that below.

"Mobilization of different components of a substance can occur at two or more different locations. The best example of this is the formation of Liesegang banding.In Liesegang banding we have the interdiffusion of two dissolved ions which cab react with one another to form a relatively insoluble solid. The two ions can come from different sources and when their concentrations at a given site build  up, via diffusion, to sufficiently high values, precipitation of the insoluble solid occurs. This precipitation suddenly lowers concentration in the neighborhood of the solid, and as a result the diffusion profiles become altered. Continued interdiffusion results in a new build-up in concentration and precipitation at another site. Depending on the geometry of the situation, this process may result in Liesegang rings (3-dimensional), tubes (2-dimensional), or layers (1-dimensional). A common example of Liesegang phenomenoa are rhythmic bands of iron oxides often found in sandstones. In this case precipitation is most likely brought about by the interdiffusion of dissolved Fe++ (from an anoxic) source) and dissolved O2 (from an oxic source). Where the Fe++ and O2 meet, Liesegang banding occurs".

The iron (Fe++) would already have been present in the sediment perhaps in discrete grains of pyrite (FeS2), or trapped in carbonaceous plant debris.   Rainfall fed groundwater is the common source of oxygen.  As pyrite gets oxidized it releases Fe++ and sulphur ions. The ferrous ions get oxidized to ferric ions (Fe+++). These then nucleate to form iron oxide or hydroxides. Rapid diffusion of ions towards a growing crystal will eventually lower the concentration of ferric ions in the region surrounding the grain to below the nucleation threshold, at which point crystal growth stops. This threshold is reached at a different location where pyrite oxidation is releasing a fresh supply of Fe++. At this new location the concentration of ferric ions build up again to levels where they start nucleating into iron oxide. This migration of zones of dissolution (of pyrite) , diffusion, and nucleation results in the distinct banding. I've summarized this explanation from a paper by P. Ortoleva and colleagues on redox (reduction-oxidation) front propagation and formation of mineral banding.

Formation of redox fronts during the burial of a sedimentary rock can be economically important. For example, a certain type of sandstone hosted uranium deposit known as 'roll-front' occur where oxidizing fluids containing dissolved uranium meet reduced components such as pyrite or organic matter. 

Here is another close up of these Liesegang bands. They have a ring or a tube like geometry. The cross bedding indicated by the arrow is a primary structure formed by the movement of sand sculpted into ripples or waves on the river bed. The Liesegang bands have been imprinted over the cross beds subsequently. 

The chemical reactions that occur in sediment after their deposition are of great interest to geologists.  They play a large role in the reorganization of porosity and permeability through the dissolution and re-precipitation of minerals.Throughout the history of a sedimentary basin, fluids move through these pore networks mobilizing elements, and under favorable conditions, enriching them at particular locations. Geologists prospecting for metal and hydrocarbon deposits want to understand this process.

Thursday, June 3, 2021

Permian Seafloor Gardens Of Glass

In Metazoa:Animal Minds and the Birth of Consciousness, author Peter Godfrey-Smith describes the Hexactinellida, a group of sponges that construct hard parts made of silicon dioxide as a support for its soft tissue. In an earlier post I had written briefly about amorphous varieties of silica. The Hexactinellidae's skeleton is made up of opal, denoted by the chemical formula SiO2.nH2O. Sponges put together their skeleton using a variety termed opal-A , the A indicating amorphous. Over geologic time the amorphous opal-A often transforms by expelling water and re configuring the geometry arrangement of silicon and oxygen atoms to opal-CT and chalcedony, both silicon dioxide varieties showing the first glimmer of a crystalline structure.

Hexactinellida are popularly called the glass sponges because of their transparent silica frame. The basic elements of this skeleton are tiny rods or spicules which are joined to form dagger, star or snowflake like shapes. These then group together to form a hard mesh that supports the soft tissue. Upon death, the silica skeleton disintegrates, leaving a carpet of spicules on the sea floor. 

The sketches below are from Godfrey-Smith's book. They are drawings by Rebecca Gelernter of  sponges collected on the Challenger expedition of the 1870's.

One fascinating function of these glass elements could be as collectors of light. Sponges often have colonies of photosynthetic organisms like diatoms living inside them. The speculation is that the glass channels light energy into the interior of the sponge body, which the diatoms use as a power source for photosynthesis.

Glass gardens on the sea floor is an evocative way to describe these sponge communities. And occasionally in geologic history these gardens have proliferated on a scale that is simply hard to imagine. Some time back I read a very interesting paper by Edward J. Matheson and Tracy D. Frank on Late Permian age (~260 million years old) sedimentary rocks deposited on the northwestern shores of the supercontinent Pangea. Different sedimentary rock types were deposited in this long lived basin. One distinct layer, termed the Tosi Chert, contains significant amounts of chalcedony and chert. A closer examination revealed that these two silicon dioxide minerals were derived from a siliceous sponge precursor.

Scattered through these Permian rocks are 'ghosts' of spicules. The Tosi Chert was once a glass sponge garden colonizing a gently sloping sea floor.  It was staggering in scale. These sponge meadows extended over 75,000 sq km. To the east of these sponge habitats lay an arid Laurentian desert, Laurentia being the northern continent which had joined the southerly placed Gondwana to form the supercontinent Pangea. To the west was the subtropical epicontinental Phosphoria Sea. An epicontinental sea is a shallow sea that floods the interiors of continents during times of a global sea level high. Since siliceous sponges were the dominant benthos these depositional systems are called glass ramps, the latter term indicating a uniformly sloping sea bed. The paleogeographic map below shows the position and range of the  'spicule belt' (in orange) on the northwestern edge of Pangea.  The pale pink area is the desert.

Source: An epeiric glass ramp: Permian low-latitude neritic siliceous sponge colonization and its novel preservation (Phosphoria Rock Complex) Edward J. Matheson and Tracy D. Frank

The Tosi sponge communities lived during a time of sea level rise. The sedimentary variation within the Tosi Chert indicates that sponges occupied environments  ranging from subtidal settings to near shore tidal flats. In the open ocean subtidal regions the sediment was mostly sponge debris. Nearer to the shore the environments were more variable. Calcium carbonate mineralizing organsims such as molluscs lived in patchy zones. Abiogenic ooids formed in some areas. In other regions, currents transported quartz detritus from adjacent areas.  Wind blown silt size mica and iron oxide particles sourced from the eastern deserts mixed with the biogenic sediment. Landward, in shallow ponds and depressions, layers of gypsum precipitated from saline waters. 

These environments of deposition of the Tosi Member are depicted in the block graphic below. 

Source: An epeiric glass ramp: Permian low-latitude neritic siliceous sponge colonization and its novel preservation (Phosphoria Rock Complex) Edward J. Matheson and Tracy D. Frank

These conditions persisted for hundreds of thousands of  years. Eventually, sea level began to fall and the sponge communities began to die out. Calcareous biota replaced the silica sponges. The glass gardens were buried under layers of lime sediment.

Like an artist dismantling a patiently constructed exhibit of installation art, nature relentlessly ground up the delicate glass sponges and transformed them into rock. But this change took its own interesting route. 

As sea level dropped, a mosaic of tidal flats and lagoons developed. In the arid climate, high rates of evaporation resulted in the development of hypersaline magnesium rich brines. These denser pools of water percolated downwards through the shallow buried silica rich sediment. The magnesium calcium carbonate mineral dolomite started precipitating within the sponge rich sediment. Along with dolomite, the calcium sulphate mineral gypsum formed at places. 

The dolomite rich sediment then underwent another transformation. The opal skeletons of the sponges started dissolving. The released silica however did not diffuse away in to the open sea. Rather, the high amounts of released silica created zones of silica supersaturation within the pore spaces of the sediment resulting in the precipitation of chalcedony and chert. Silica got redistributed within the Tosi sediment package, first dissolving and then reprecipitating a few millimeters away. The new silica minerals were not spread evenly but formed compact masses giving the evolving rock a nodular appearance.    Here and there the original shapes of the sponge spicules were preserved, although they were no longer made up of opal, having being replaced by chalecdony and chert. 

The photomicrographs show examples of dolomite and silica nodule replacement of the original sponge skeletal debris. The pale area in the image to the left is a chert nodule with a diffuse boundary that gives way to a darker dolomite matrix. The image to the right shows a bioturbated dolomite rock with some chert replacement. Tiny lath shaped particles are ghosts of sponge spicules.

 Source: An epeiric glass ramp: Permian low-latitude neritic siliceous sponge colonization and its novel preservation (Phosphoria Rock Complex) Edward J. Matheson and Tracy D. Frank

Today the Tosi Chert is not that attractive or spectacular rock to look at. It is a few meters thick, has a grey to red to purple color and is made up mainly of  silica nodules and dolomite with minor amounts of quartz, anhydrite and gypsum. Layers of limestone, lithified from patchy molluscan and ooid sediment, interfinger with silica rich strata.

Calcium carbonate secreting organisms have been the most prolific biogenic sediment producers in Phanerozoic shallow marine settings. Siliceous sponges more commonly occur in deeper water and high latitude settings.  Occasionally though,  siliceous sponges did take over the shallow marine domain. The extensive Mid-Late Permian Pangean sponge belt is an example of such ecological opportunism, where silica rich sea water and nutrient availability resulted in prolific growth and persistence of sponge communities over vast areas of the northwestern Laurentian margin. Those majestic glass gardens, perhaps harboring photosynthetic symbionts are now gone, transformed to dull looking rock, but look closely and the ghosts of those long dead sponges are waiting to tell you their story.