Tuesday, June 30, 2020

How Old Is Plate Tectonics? Signals From The Mantle

A recent study has used the geochemistry of Archean basalts and komatites to suggest that plate tectonics began by 3.2 billion years ago. The chemical composition of these two volcanic rocks points to active transfer of elements from the crust into the deep mantle by this time. The authors argue that this mass transfer from surface to the interior can most efficiently be achieved only via subduction of plates.

Early in its history the earth differentiated into three chemically distinct shells, the crust, the mantle and the core. There is another type of layering on earth, one that is governed by differences in  rheology or the capacity of materials to flow and deform. The outermost rigid shell of the earth is called the lithosphere. It is made up of the crust and the uppermost part of the mantle. With the exception of earth, all other silicate or rocky planets of our solar system have an unbroken lithosphere shell riding atop a hotter ductile mantle or asthenosphere.

On earth, the lithosphere is broken into independent mobile fragments. New lithosphere is created at mid-oceanic ridges by magmatism and is destroyed at subduction zones where it sinks into the asthenosphere. This is plate tectonics. At places, the lithosphere breaks up and the two pieces drift apart to form rifts and eventually oceanic basins. At other locations, lithospheric slabs sink or collide to form orogenic mountains. The pull exerted by the sinking lithosphere slab is thought to be the major force sustaining plate motion.  This continued motion of plates makes earth a geologically dynamic place.

When did plate tectonics begin on earth? A range of dates have been proposed. Some geologists think that plate tectonics began in the Archean, as early as 3.5 billion years ago. Others insist that while some fragmentation of lithosphere into 'plates' might have occurred in the Archean, it would have been ephemeral. These proponent of late plates say that earth in its early history would have been like other rocky planets with a tectonic style dominated by a 'single lid' or an unbroken lithosphere shell enveloping a hot mantle. During this 'single lid' phase the lithosphere could sink into the mantle by delamination of a denser lower layer or by 'drips' where a dense lithosphere blob detached itself.  However, these processes were localized. A global regime of  a network of mobile plates (plate tectonics) began relatively recently in earth history, about 1 billion years ago.

The figure below summarizes the various styles of tectonics prevalent on the rocky planets of the solar system.



The evidence presented comes mostly from the composition and deformation style of certain types of crustal rocks.  In the early stages of subduction of oceanic lithosphere, rocks are subjected to high pressures but low to moderate temperatures. This results in basaltic rocks that make up the subducting oceanic crust getting transformed into a rather distinctive rock suite known as blue schists, named after the blue tinted mineral glaucophane. Blueschists first appear in the rock record around 1 billion years ago. Their earlier absence has been taken to mean the absence of subduction.

Another distinctive rock type becomes much more common around 1.2 billion years ago or so. These are volcanic rocks known as kimberlites. They are famous for being the main source of diamonds. Kimberlites solidify from explosive magmas rich in carbon dioxide and water. These magmas originate more than 200 kilometers deep in the mantle. The mantle at these depths is generally depleted in water and carbon dioxide and other volatiles. Subducting lithosphere can deliver carbon dioxide and water from the surface to great depths, enriching the mantle in these volatiles. Again, the common appearance of kimberlites after 1.2 billion years ago is seen as a sign of the beginning of subduction and plate tectonics by that date.

Early plate tectonic proponents come up with their own list of indicators. They point out to the presence of 2.2 billion year old eclogites from an ancient terrain in Congo. Eclogites are rocks containing the distinctive minerals omphacite and garnet. They form when a cold oceanic plate subducts and ocean crust basalt is subjected to high pressures and moderate temperatures.

There are hints from Indian terrains too of plate tectonics in operation from before 1 billion years ago. In the Nellore schist belt, between the Eastern Ghat and Dharwar terrains in South India, there are rock types known as ophiolites. They date to between 2.5 billion and 1.8 billion years ago. Ophiolites are slices of oceanic crust that have been scraped off from a subducting plate and thrust up into mountain ranges.

In Central India, south of the Vindhyans and north of the Tapi river lies the Central Indian Tectonic Zone (CITZ). This broad area of crust represents the zone of collision and suturing of the Dharwar continental block with the Bundelkhand continental block. This suturing is thought to have taken place around 1.6 billion years ago based on the age of peak temperature and pressure conditions recorded in metamorphic rocks (Sausar Group) of this region. There is a distinct pattern to this metamorphism. In the northern areas, rocks of the Sausar Group were metamorphosed under high pressure and high temperature conditions. In southerly regions there are signs of only high temperature and moderate pressure metamorphic conditions. Such 'paired metamorphic belts' are taken to indicate a zone where plates converge and pressure and temperature conditions vary systematically across the terrain.

Geophysical surveys in the CITZ also help in elucidating the deep lithosphere structure. They hint at the presence of dense lithosphere slabs inclined both in a southerly and in the northerly direction. This seems to indicate the remnants of ancient plates which subducted both southwards and northwards as two terrains converged and collided over the long time period from late Archean to the Mesoproterozoic (~2.5 billion to 1.6 billion years ago).

Instead of looking at distinctive rocks, the recent work by Hamed Gamal El Dien and colleagues uses a geochemical approach to time the initiation of global plate tectonics.

The early chemical differentiation of the earth depleted the mantle in elements such as barium, uranium, lead and strontium. These fall under the class known as large ion lithophile elements (LILE). Their large ionic radii means that they fit only uneasily in the atomic frame of the silicate minerals that make up the mantle. During widespread melting of the mantle in Hadean and early Archean (4.5 to 4 billion years ago) they preferentially escaped into the liquid phase resulting in a  chemically stratified earth. The growing continental crust became enriched in these elements. The mantle on the other hand became depleted in these elements and remained so as there was no mechanism to recycle these elements from the surface into the deep interior.

Hamed Gamal El Dien and colleague surveyed the chemistry of Archean age basalts and komatites from a global dataset. Both these rocks are derived from magmas sourced from fairly deep in the mantle. They found that rocks younger that 3.2 billion years show elevated values of LILE. This seems to imply a re- enrichment or a refertilization of the mantle in these elements beginning around 3.2 billion years ago. Since their data set encompassed all the continents, a global change in elemental exchange between the crust and the mantle seems to taken place around this time. This, the scientists argue, was most likely pointing to the start of subduction and global plate tectonics. The ocean floor is blanketed with sediment derived from the erosion of continental crust. As this oceanic crust sinks and dives deeper in a subduction zone, the accompanying sediment heats up and releases volatile elements into the surrounding mantle fertilizing it with a crustal chemical signature.

Another geochemical indicator that is used to distinguish continental crust versus mantle sources is the ratio of Neodymium143/Neodymium144.  Nd143 is derived from radioactive decay of Samarium147, while Nd144 is the stable isotope. During early differentiation Sm147 was preferentially enriched in the mantle. Over time, the mantle has evolved a higher Nd143/Nd144 ratio relative to the crust. Like the temporal trend in LILE, basalts and komatites younger than 3.2 billion years show a much lower Nd143/N144 ratio, suggesting an influx of crustal material into the deep mantle.

There are other mechanisms by which the mantle derived basalts and komatite rocks could inherit a crustal elemental pattern. One way is for the magma to react and get contaminated by continental crust as it ascends. The researchers rule this out by using another chemical discriminant, the ratio of Thorium/Ytterbium which is higher in continental crust than in a depleted mantle. The analysed rocks  have Th/Yb ratios inconsistent to have been derived from magmas mixing with continental crust.  Another pathway for the exchange of materials from the crust to the mantle is the sinking of lower crustal material by 'drip' or by delamination.  However, these processes generally affect only the upper levels of the mantle and would not have altered the composition of the lower mantle which is a source of the komatite magmas. Based on these observations the researchers are fairly confident that only subduction can explain the enriched mantle signals.

The late plate tectonic view has received more direct pushbacks. For example, one counter argument for the absence of older blueschists draws attention to the changing composition of oceanic crust . Archean and Early Proterozoic oceanic crust was richer in magnesium oxide and at the geothermal gradients that prevail in subduction zones this MgO rich crust would have altered to a different type of metamorphic rock known as greenschists, so named after minerals like chlorite and actinolite.

The kimberlite argument too has been countered. There are kimberlites older than 1.2 billion years,  nor do they occur with a uniform frequency through time. Rather, there are pulses centered around 2 billion years ago, 1.2 billion years ago and strikingly between 250 million years and 50 million years. Many geologists see a connection between supercontinent break up and processes in the deep mantle, wherein an influx of carbon dioxide and water initiates melting in cooler mantle beneath thick continents and deep continental fractures provide pathways for the volatile magma to rise quickly to the surface.

These debates will certainly continue for many years to come.

Robert. J Stern, in a review, outlines three conditions that had to be met before global plate tectonics could begin. First, large tracts of lithosphere slightly more dense than the asthenosphere had to form. Second, this lithosphere had to be strong enough to remain intact in a subduction zone and pull along the entire plate without it disintegrating.  And thirdly, zones of lithosphere weakness at least 1000 km long had to develop along which new plate boundaries could form.

At some point between 3 billion and 1 billion years ago, likely as a start and stop process, the earth's tectonic style began diverging from other rocky planets in our solar system, nudged by chemical differentiation and heat flow thresholds which govern the strength,  thickness and buoyancy (or density) of the lithosphere. At a critical combination of these three variables, the outer shell would have begun to regularly break along a global network of fracture zones initiating nascent plate boundaries along which dense lithosphere could sink into the mantle. The presence of copious water would also have been important. Water reacts with the olivine rich rocks of the oceanic lithosphere, transforming them into weaker serpentine rich domains, converting strong lithosphere into long zones of weaknesses that could break and bend more easily. Water released from oceanic sediment also would have acted as a lubricant at subduction interfaces, facilitating the sliding of one plate underneath another.

Plate tectonics has made earth into a restless planet. The great churn within keeps influencing climatic regimes, oscillations in ocean chemistry, and metal enrichment episodes. And it has played a major role in the maintenance of life too. As continents jostle, break apart and collide, new and varied habitats keep forming, profoundly shaping unique trajectories of biological evolution and biodiversity.

Monday, June 29, 2020

Infographic: Reefs Through Geologic History

Academia.edu is always sending me links to papers that I can download. It is a lure to get me to upgrade my membership. I am grateful for the riches that I have discovered on their site.

Last week they sent me a gem-  Palaeoecology: Past, Present and Future by David J. Bottjer. It is a full length book on the field of paleoecology summarizing its scope, its methods, and giving an overview of different ecological domains such as the shallow marine, the pelagic (deep marine), and the terrestrial environments and how they have changed through time. Dr. Bottjer is Professor of Earth Sciences, Biological Sciences and Environmental Studies with the University of Southern California Dornsife, Los Angeles.

One striking aspect of this book is the really informative diagrams that Prof. Bottjer has collated from various papers and books. They really add a lot of value, especially if you want to avoid reading every line of the text!

Here is one which shows the changing composition of reefs through time.


Source: James, N.P. and Wood, R. 2010. Reefs. In James, N.P.,  Dalrymple, R.W., Facies Models 4. Geological Association of Canada, pp. 421–447

They were not always built by corals.

I am going to learn a lot from this book.

Friday, June 12, 2020

Punctuated Equilibrium Is About Small Subtle Changes

The theory of punctuated equilibrium (PE) put forth by paleontologists Niles Eldredge and Stephen Jay Gould proposes that morphological evolution speeds up during lineage splitting events when new species form. In practice, a paleontologist sampling fossil species in a sedimentary section will find that a species over its lifetime does not show any trend in morphological changes. That species eventually goes extinct at a particular interval. Fossils of the inferred descendant species appear abruptly in the sedimentary bed above. A fossil population showing a mix of traits diagnostic of both species is not generally seen. 

Eldredge and Gould argued that this pattern of change observed in fossil species shows that the history of a species is really characterized by long periods of no change, eventually interrupted by rapid evolution to a new species.

A reassessment of one textbook example in bryozoans suggests no evidence of such a punctuated mode. 

A write up in Phys.org describes the new work. 

Revisiting a Landmark Study System: No Evidence for a Punctuated Mode of Evolution in Metrarabdotos - Kjetil Lysne Voje, Emanuela Di Martino, and Arthur Porto

I am not going into the work itself. There are well documented examples of the punctuated equilibrium mode of evolution while others as in this case may see revision from time to time. 

Instead, I wanted to comment on one of my pet peeves about this topic which is the common use of the phrase "major evolutionary change" to describe the actual evolutionary changes.

This idea which never seems to go way that PE involves big or major changes has caused a lot of misunderstanding about the theory.  As the authors of PE took pains to point out, the amount of morphological difference between ancestor and descendant species is subtle, often taking excruciating examination to recognize. 

Here is Niles Eldredge describing his work on Devonian trilobites.

I measured some 50 different lengths and widths- length of the head, distance between the eyes, height of the eyes, length of the tail and so on -on hundreds of specimens. This was tedious. Each specimen had to be cleaned to at least well enough to make all the anatomical landmarks visible. Each had to be mounted on a block of wood, stuck to a blob of plastilene, with the tops of the eyes (a flat surface) in a horizontal plane, perpendicular to my line of sight down the barrel of the microscope. There was a little scale inside the right eyepiece reticle from which to read off the various measurements. After a day of measuring, and until I got used to the microscope, I would see double from the bus window on my way home. 

 from Time Frames: The Evolution of Punctuated Equilibria.

Trilobites have compound eyes, made up of columns of lenses. All this detailed work led to the recognition that a new species had 17 columns of lenses, instead of 18 in the ancestral species found in slightly older strata!

This point about small changes was missed by many observers. Creationists conflated PE with a theory of macro-mutation or saltation, a sudden origin of big morphological change as for example seen in the transition from reptiles to mammals. They claimed that Darwin with his insistence on gradual accumulation of small changes had been wrong all along! 

Not so. The morphological differences observed involved small changes between the ancestor and descendant species.  It takes an expert to recognize this shift in form. 

The real significance of PE is not about the amount of change, but the pattern. PE proposes that very little morphological divergence takes place during the lifetime of a species. It is only when a sub-population of that species gets isolated that it may experience relatively rapid evolution. This genetic isolation results in the formation of a new species. The ancestral species may persist in the main parts of its range, while one of its peripheral populations has evolved into a new species. Biologists term this process of branching or lineage splitting as cladogenesis. 

PE is about explaining these rhythms in the history of lineages. Long periods of stasis (little or no change in form), punctuated by relatively rapid transition to a new form. This transition, though rapid in geologic time,  may take place over hundred of generations and thousand of years.

Why then isn't this transition to a new form preserved in the rock record as a fossil population with a mix of characters? The answer Eldredge and Gould prefer is that reproductive isolation takes place in outlier regions of a species range. Sediments deposited in these isolated basins will be archiving the incremental evolution of a new form, just as Darwin envisaged,  via a mixed or transitional population. However, these outliers containing transitional populations have less chance of getting preserved in the rock record.  The sedimentary sections that paleontologists examine are generally from the main part of the basin. This is not where the change to a new species has taken place.  The sudden appearance of an inferred descendant species in this strata is really recording the migration of the descendant from an isolated part of the species range where it evolved, into the central range of its extinct ancestor. 

Biologists have named this process of formation of a new species through reproductive isolation in a geographically distant area as allopatric speciation. Eldridge and Gould's theory of punctuated equilibrium invokes allopatric speciation and migration to explain the abrupt appearance of new species in the fossil record. In terms of the mechanisms of change, it is not an alternative to Darwinism as is often portrayed. Both the authors accept that even during the period of 'rapid' evolution, changes accumulate through natural selection or genetic drift incrementally across generations. But the vagaries of preservation means that sediments which record this transition are rarely available for study.