Will Earth Become Venus?

I came across an article written by economist Sanjeev Sabhlok on the long term climate future of the earth titled - Limestone proves the impossibility of a runaway greenhouse effect on Earth.  Mr Sabhlok has been reading some geology and has found out that the earth can naturally regulate the earth's carbon dioxide levels over geologic time.

The process operates like so: During times of increased volcanism, CO2 levels in the atmosphere increase to a point where the earth starts warming. This in turn enhances rock weathering reactions which pull back CO2 and washes it down into the ocean where it is sequestered as a bicarbonate or carbonate molecule. A fraction of this carbonate gets locked up in limestone precipitating on the sea floor. 

Besides this mechanism, photosynthesis also pulls out CO2 from the atmosphere. This CO2 goes into building organic molecules. Some of  that organic matter sinks to the ocean floor and is buried, creating another long term carbon sink.

All these natural adjustments to atmospheric CO2 means that a runaway increase where CO2 levels keep rising thousand fold unabated is unlikely to occur. Earth will not turn into a Venus. Mr Sabhlok says that most climate scientists ignore this natural regulator in their panic over a runaway greenhouse effect.

Mr Sabhlok has written quite a nice summary of the geological evolution of the earth's atmosphere. But he entirely misses the point about why scientists ignore geologic sequestration of CO2 in their climate change projections. They do so because it works too slowly to matter to us. Our concern is not a distant future where surface temperatures may or may not reach a Venus like 450 deg C, but one where there is a spike of 3-4 deg C in the next few decades to centuries which nevertheless will result in extreme damage to human society and the ecosystems we depend on.

The geologic thermostat that Mr Sabhlok describes can't prevent these smaller shorter time scale perturbations in atmospheric conditions. Some numbers he shares demonstrates the inadequacy of weathering to neutralize CO2 at short time scales. He quotes from a video put up by a Dr. Johnson Haas; " Typically on an annual basis … about 0.03 gigatonnes of carbon is extracted from the atmosphere and goes into limestone which goes into long-term geologic storage. … [E]ven at that slow rate the drawdown of CO2 from our atmosphere by shell building organisms … would completely exhaust the atmosphere of CO2 in less than a million years”. 

What he doesn't add is the impact of human emissions. Our activity is emitting an eye popping 40 billion tons of CO2 to the atmosphere every year. This is 2 orders of magnitude more than what limestone can suck in. About half of this CO2 gets absorbed by the ocean, the vast majority getting locked as a stable bicarbonate molecule (HCO3). The rest remains in the atmosphere, cumulatively increasing its CO2 levels. Over the past 250 odd years, human activity has increased the amount of CO2 in the atmosphere by about 1.5 trillion tons.

When emissions eventually go to zero, absorption by oceans will quickly start reducing atmospheric CO2, putting the brakes on warming. And in the long run, several hundred to a few thousand years after we achieve a net-zero emission scenario,  CO2 levels will come down to pre-Industrial amounts. But as long as emissions continue, the earth will keep warming and become a very unpleasant place. The geologic past informs us of the havoc wrecked by increased CO2 levels and a warmer earth. The Late Devonian (372 million years ago), the Late Permian (252 million years ago), and the late Triassic (201 million years ago) mass extinctions were all triggered by increased CO2 levels and warming from sustained volcanism.

The Inter Governmental Panel on Climate Change Synthesis Report outlines many scenarios that might unfold towards the year 2100. No contributing climate scientist on that report is panicking about a runaway greenhouse effect. Instead, they highlight that incremental increases in temperature over the next few decades will place a debilitating burden on our society through myriad impacts on our health, water security, agriculture, and biodiversity. While fixating on an implausible runaway effect, Mr. Sabhlok stays silent on the real impending danger that we are facing.

His sanguine advice that "We should sleep soundly, knowing that no matter how much CO2 mankind emits by burning fossil fuels, our amazing living planet will never go the way of Venus" is utterly irresponsible. 

Earth may never go the way of Venus, but if we don't stop burning fossil fuels our amazing planet will turn into a living hell for us and our immediate descendants. 

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Fun Facts: I didn't want to quibble about some of the specifics in my post, but I want to share this with you. 

1) Biocalcification (limestone formation) results in the emission of CO2! Since most of the carbonate in the ocean is in the form of HCO3, we can write the precipitation equation as- 

Ca + 2HCO3 -----> CaCO3 + H2O + CO2 --------- Eq.1. 

For every molecule of CO2 that gets locked up in limestone, one molecule is released in the ocean and eventually into the atmosphere.  Limestones over time do constitute a CO2 sink, but precipitation of carbonate sediment is not that effective an offset of atmospheric CO2 in the here and now. 

2)  On the other hand, dissolution of CaCO3 in the deep ocean adds alkalinity,  neutralizing the increase in ocean acidity due to CO2 released by the oxidation of organic matter. It is Eq.1. in reverse.

CaCO3 + H2O + CO2 ------> Ca + 2 HCO3 ----- Eq.2. 

Carbonate equilibria can be counterintuitive and complex! 

3) Mr Sabhlok says that "we are currently close to the lowest levels of CO2 in the Earth’s history". It is true that CO2 levels have steadily decreased over geologic time. But they have sharply increased in the past 150 years from 280 ppm in the late 1800's to more than 400 ppm today and will continue to increase as long as we keep burning fossil fuels. The last time earth saw such CO2 levels was 14 million years ago.

Easterly Tilt Of The Deccan Plateau - Update

I first wrote about this topic in 2011 in response to a question by a reader. I thought I would update my post with some new maps and explanations. Why is there such a pronounced pattern of easterly flow of the rivers in the Indian Peninsula.  I keep getting asked this question.  It was time for an update on this interesting topic on geology and landscapes. 

The region south of the Tapi river covering the Deccan basalts and the southern Indian peninsula exhibits an easterly drainage with the rivers flowing into the Bay of Bengal. The map below shows the Indian peninsular region with easterly drainage. The Deccan Plateau is mostly but not entirely covered by the Deccan basalts. South of this region is the Karnataka Plateau with a Precambrian geology. Along the east coast there are Permian-Triassic and Cretaceous basins.

Source: Hetu C. Sheth: Deccan Beyond the Plume Hypothesis

The question posed to me was - What is the relationship between the Deccan volcanics and the easterly tilt of the Indian plateau (i.e. the plateau covering the Deccan volcanics and the southern Indian peninsular region)?

The easier more intuitive answer would have been that the western ghats provide the topography and Deccan volcanism created a lava pile that is thicker to the west and which thins to the east, thus generating an east sloping surface. Rivers follow the slope to the Bay of Bengal. 

There are some geologic age inconsistency in this answer and this also does also not fully explain why the region south of the Deccan Volcanics too has an easterly drainage. Clearly, something more is going on. 

To understand the evolution of the Peninsular drainage patterns let us look back to the time when the Peninsula didn't exist. In early Mesozoic, India was part of Gondwanaland and was joined to Antarctica and Australia to the east, and Africa to the west. The triangular shape of south India with characteristic eastern and western coastlines had not formed yet. 

How can we find out the direction rivers were flowing back then? Geologists look to clues in the sedimentary basins of that age. The composition of sand in sandstone is matched to the most likely source terrain. And current directions can be inferred from studying ripples preserved on the surface of ancient sand. 

The paleo geographic maps below shows Gondwanaland and the location of the Pranhita Godavari basin in the Mesozoic. 

 

Source: Sankar Kumar Nahak and Coworkers 2024.

West North West flowing rivers originating in the highlands of the future Antarctica and in the Eastern Ghats were funneling sediment to the basin. Much of the interior of the region that would become the southern Peninsular India was a peneplain. There wasn't much topography towards the west for an easterly drainage network to develop. 

India broke away from Antarctica beginning about 140 million years ago. A distinct eastern continental margin formed. Several NE- SW oriented basins developed along the edge of the Indian continent. Since by this time an expanding Indian Ocean lay to the east, the orientation of a natural drainage system would have been from the west towards the east. 

We can say with some confidence that by 90 to 80 million years ago, east flowing rivers originating in the interior of the Indian continent were depositing sediment along the eastern Indian margin. See this map of sediment distribution along the Indian east coast. 

 Source: K.S. Krishna and Coworkers 2016.

It shows the thickness of  Mid- Late Cretaceous sediment, ranging in age from about 100 million years ago to 65 million years ago. The sediment lobes coincide with the mouths of the Godavari, Krishna rivers and other southern rivers, indicating that the paleo Godavari and the paleo Krishna system had begun building deltas from that time. Since there were no Western Ghats then, these rivers may have been shorter, with their source somewhere in the Archean and Proterozoic terrain of Peninsular India. 

Further to the south, geologists find a similar story with the ancient Cauvery. The Cauvery basin formed when Sri Lanka detached from the Indian continent. Its delta and marine deposits too contains sediment from the Late Cretaceous. 

The easterly drainage pattern of Peninsular India developed before Deccan Volcanism and the formation of the Western Ghats. 

India broke away from Madagascar about 88 million years ago  and subsequently from the Seychelles about 66-64 million year ago. The latter separation coincided with Deccan Volcanism and the eventual formation of the western Indian continental margin. Block faulting that accompanies continental breakup would have created a north south oriented high area, which would eventually evolve into the present day Western Ghats. The thinning of the lava pile to the east also would have created an easterly slope. Rivers originating in the western highland now would flow across the length of the Peninsula. 

New streams would have incised the fresh volcanic surface as lava buried the older etched landscape. But the regional  pattern of easterly flow persisted.

Some geologists maintain that there has been some fairly recent Cenozoic age (past 15-20 million years) uplift of the Western Ghats which has accentuated relief and produced the youthful looking topography of scarps, waterfalls, and deep canyons. These earth movements would have certainly given new energy to the drainage system, but there is some geologic evidence to suggest that the streams originating in the western ghat region are antecedent to the uplift of the ranges. 

For example, in the Mahabaleshwar area easterly drainage cuts across the axis of a north south oriented gentle anticlinal structure, implying that the drainage predates the uplift and warping of lava flows. Evidence from sedimentation patterns of the eastern river deltas also show that the easterly drainage originated much earlier than the formation of the Western Ghats. 

What then created that initial slope to the east that imprinted the drainage network that continues today? 

One reason is that the eastern margin formed first. Basin formation along the eastern edge of the continent would have created a relief difference between the western interior and the eastern depressions, resulting in stream networks flowing eastwards.  Secondly, the new oceanic crust made of lava that formed when India and Antarctica separated in the Late Jurassic and Early Cretaceous would have cooled by Late Cretaceous times. Becoming colder and denser it has been sinking and dragging the Peninsular region with it. 

Earlier eastern basin formation and a tug from the floor of the Bay of Bengal may have been enough to impress an east flowing drainage. Later, the east sloping lava surface and the rise of the Western Ghats reinforced this distinction between the west and the east perpetuating the direction of river flow initiated since Cretaceous times. 

How Old Is Himalaya Topography?

 Is this true?

The answer is no. To be fair, aside from the click bait, the article itself does not make any such claim. It covers a new study on some igneous rocks from Arunachal Pradesh that formed during an earlier pre-Himalaya stage of the India Asia plate convergence. 

By mid late Cretaceous times, between 100 to 66 million years ago, the dense ocean lithosphere that was the front edge of the Indian plate was sinking underneath the Ladakh terrain, a splinter of Asian continental plate disconnected from the mainland. The subduction of the Indian plate triggered extensive melting in the deep subsurface, building over time a magmatic arc. The schematic below shows the plate tectonic scenario. 

Source: Sankar Chatterjee and coworkers: Gondwana Research.

This arc has been well studied in the Kohistan and Ladakh areas. The new work found that a body of granitic rock known as the Lohit pluton is also an eastern extension of the Kohistan Ladakh arc complex. 

Eventually, the oceanic crust of the Indian plate was consumed and the Indian continental crust collided with the Ladakh terrain, which by this time had sutured with the Asian mainland. Himalayan mountain building begins from this point on, roughly sometime after 50 million years ago. 

How do geologists know when topography began to form in the Himalayan region? Various dating methods tell us when a particular rock or mineral crystallized or cooled below a particular temperature. But how do we date the formation of topography? Sedimentary geology, my field of specialization, has played a big role in giving us insights into the tectonic and topographic evolution of the Himalaya. I'll summarize how the story of the formation of the different Himalaya ranges came to be written by these geologists. 

Due to inherited geologic history and subsequent conditions during continental collision, the entire region between Ladakh and the Himalaya front can be subdivided into six geologic terrains running along the length of the mountain arc. These are, from north to south,  the Gangdese magmatic arc, the Indus-Tsangpo suture zone, the Tethys Himalaya, the Greater Himalaya, the Lesser Himalaya, and the sub Himalaya (Siwalik). Each is made up of distinctive rock associations. 

The earliest topography began to from in the north, at the zone of contact between the two continental plates. Over time, there was a step wise progression of southwards topography formation. As each of the geologic terrains rose up, newly developed stream networks started eroding the rocks and delivered distinctive sediment mixtures to two types of sedimentary basins which had developed adjacent to the mountains. The Indus and Bengal basins at the two extremities of the Himalaya received sediments from the Indus and the Yarlung-Tsangpo/Brahmaputra. A second type of basin, known as a foreland basin formed to the south of the orogen. This moat like depression which runs parallel to the Himalaya, and is fed by streams running transverse to the ranges, also became an archive of material removed from the mountains. 

By carefully identifying the sedimentary grain types deposited in these basins and how their proportions change through the oldest to the youngest layers, geologists have been able to piece together the evolution of new topography and geologic provenance through time. 

I am presenting this reconstruction through a series of time slices which show topography formation in each of the geologic terrains and the resulting stream networks transporting the derived sediment from source to sink. The representation only shows the central foreland basin from Himachal Pradesh to Nepal. I have not shown the sedimentary history of the western and eastern basins. The trends at the Himalaya extremities are similar with some difference due to variations in the dominant geology of the contributing catchments. 

I have also not covered a short phase of basin formation in the Indus Tsangpo suture zone between 30 to 20 million years ago. This basin received sediments from both the Gangdese Arc and the Indian plate and was then uplifted to form the Indus Group ranges which include the famous Kailash mountain. For more details of this episode of Himalaya mountain building do refer to my post - Is Mount Kailash the Oldest Mountain in the Himalaya?

In the diagrams, the legend "foreland sandstone diagnostic grains" refers to the arrival of a suite of distinct grain types in the foreland basin. This signals the uplift and erosion of new rock types in the growing mountain range. Earlier formed ranges will in most cases continue to contribute sand, but it is the first appearance of a new grain type in successive strata that is the indicator of tectonic and topographic changes. 

The inspiration for these diagrams is from P.G. DeCelles and coworkers paper on the timing of the India Asia collision, published in the May 2014 issue of Tectonics.

A) Initial Continental Collision- 58-54 million years ago: What was the timing of the collision?

As long as Indian oceanic lithosphere was subducting under Asia, a deep trench and forearc basin separated the two continents. Sediments eroded from Asia were trapped in these depressions and could not travel on to the Indian continent. This situation changed on continental impact. By then the intervening basins were shallower, the Tethyan Ocean had started retreating and rivers originating on the Asian tectonic plate could now flow across the zone of collision and deposit sediments on the Indian continent. These Asia derived delta sediments rich in volcanic rock fragments eroded from the Gangdese Arc began to be deposited on the Indian continental shelf between 58-54  million years ago. TH, GH, and LH are the future Tethys Himalaya, Greater Himalaya, and Lesser Himalaya.

B) Proto Himalaya Stage- 45-30 million years ago. 

Preceding and during collision, slices of the Indian oceanic plate were thrust up and sandwiched between the two continental plates. Igneous rocks of this "suture zone" are made up of magnesium rich silicate minerals (olivine, pyroxene) and magnesium aluminum oxides (spinel). Only a remnant arm of the Tethys persists, but rivers can now flow across the Indian continent and deliver sediments to the foreland basin. This basin is located roughly over what is now the southern Lesser Himalaya and the Siwalik ranges. Gangdese Arc and suture zone derived sediments first appear in the foreland by 45 million years ago.

C) Early Himalaya Stage- 35-20 million years ago. 

A pulse of low grade metamorphic rock fragments start appearing in the early Oligocene to Miocene age foreland basin sediments. These are derived from the newly forming Tethyan fold and thrust belt. Volcanic rock fragments are now absent in the central foreland, suggesting that the growing Tethyan ranges are a barrier to rivers originating in the Gangdese Arc. The Indus and the Yarlung Tsangpo, initiated in the furrow of the suture zone and flowing parallel to the northern ranges before cutting across the rising mountain chain, continue to transport arc derived sediment to the basins at the western and eastern extremities. 

D) Main Himalaya Stage- From 20 million years ago.

Three pulses of mountain building are recognized during the Main Himalaya Stage. The early phase from 20 -11 million years ago records the uplift of the Greater Himalaya which are made up of high grade metamorphic rocks containing minerals like mica, feldspar, and garnet. Deformation continued southwards between 11-5 million years ago resulting in the formation of the Lesser Himalaya ranges. These shed low grade metamorphic and dolostone (magnesium calcium carbonate) detritus. 

Eventually the foreland basin got caught up in the progressing orogeny. New faults propagated southwards. Slices of the oldest foreland basin deposits were broken and accreted to the mountain front. These freshly exhumed rocks became the major source of sediments feeding the youngest depositional phase in the foreland. Pliocene-Pleistocene  (5 -0.5 million years ago) layers of the Siwalik hills are made up of rock fragments cannibalized from Eocene and Oligocene foreland strata. 

How is the age of the foreland basin sediments determined? The entire exercise of unraveling the topographic history of the Himalaya depends on that! 

These sediments have been dated using a variety of methods. Fossils provide age ranges for packages of sediment. Magnetic signals preserved in iron rich mineral grains are measured and pegged to an absolute date by comparing the magnetic pattern to a global magnetic chronology. Techniques such as fission track dating of zirconium silicate sand (zircon) tells geologists when that zircon in its source was being uplifted and cooled thus giving a fair idea of the time of its erosion, transport, and deposition as a sedimentary particle. 

Gathering all this age information on the sediments, the timing of the arrival of distinctive rock fragments and minerals in the foreland agrees well with the exhumation history of their provenance as deduced from bedrock geochronology. 

The growth of mountain chains is a long and complex process. The word "collision" may invoke ideas of near instant crustal response, but deformation and surface uplift moves rather slowly across strong and rigid plates. Surface and deep crustal process are linked. For example, the formation of the Tethyan fold and thrust belt resulted in crustal thickening and the deep burial and metamorphism of rocks that eventually became the Greater Himalaya. There was a long time lag between the initiation of collision and the main Himalaya pulse of uplift. The rise of  the Greater Himalaya took place a good 35 million years after the India Asia impact. 

Foreland basins give us valuable insights into the relationship between sedimentation and tectonics, but they too need careful evaluation. They are not static entities. Compare the proto Himalaya stage with the main Himalaya stage and you will notice that as mountain building moved southwards the location of the foreland shifted too in response to the migrating load of the thickened crust. This process continues to this day. If the region of the Siwalik hills was the foreland a few million years ago, today it is the Ganga alluvial plains. 

Orogeny and drainage impact the sedimentation patterns in the foreland. Different rivers breaking through along the mountain front may not be bringing uniform sediment mixtures at the same time. Hinterland differences in geology of the catchment results in variable sand composition along the length of the foreland. Take the example of river Ravi. It drains mostly the Lesser Himalaya and hence its brings with it low grade metamorphic sand grains. On the other hand, the Sutlej flows through a significant portion of the Greater Himalaya. High grade metamorphic grains make up a large proportion of its sand. 

Further, chemical reactions taking place in the subsurface may dissolve some types of minerals. This may create an apparent trend in sand composition through time, which may not accurately reflect the actual history of the provenance. 

Researchers need to be cognizant of these issues when they construct their answers. 

Understanding the timing of Himalaya uplift provides valuable insights into the geodynamic forces at play during continental collisions. But the interest in this question goes beyond geology. Climate scientists want to know more about the linkage between Himalaya evolution and the advent and shifts in the Asian monsoon. And the value of studying the Himalaya spills into many other areas. Orogeny exposed enormous volumes of fresh rock to chemical weathering, mobilizing nutrients and organic carbon which would then be sequestered in fluvial and marine environments. These elemental cycling and budgets are keenly studied by surface systems specialists. 

Visit the picturesque Kangra region in Himachal Pradesh. Climb the thick sandstone layers leading up to the historic Kangra fort. Go to nearby Jwalamukhi, where natural gas emanating from the deep keeps alight an eternal flame. Ascend the hills towards the famous town of Dharamshala. You will be traveling through the Miocene foreland basin. Sedimentary petrologists for decades have worked on these and other sites along the Himalaya frontal ranges amassing data on sandstone composition. Even as million dollar instrumentation keep revealing new facets of the earth, their main tool has remained the humble petrologic microscope mounted with a grain counting stage. From this labor of love has emerged the story of how and when the Himalaya came to be.

Joshimath Landslide, Human Evolution, Early Animals

Sharing some of my readings over the past couple of weeks- 

1) Movement of Joshimath Landslide in India: The town of Joshimath in Gharwal Himalaya is built on an ancient landslide. Geological reports going as far back as the 1970’s had warned that excessive modification of the slope due to urbanization may result in slope movement and eventual failure. These warnings proved correct as slope movement since 2018 has caused major damage to houses and livelihoods. Landslide expert Dave Petley reports on a new study of the region that uses radar technology to track earth movements. 

It concludes that removal of vegetation, mismanaged groundwater seepage and blocked drainage paths contributed to accelerated movements of the Joshimath landslide system. There is unplanned and unregulated construction happening in other Himalayan towns. Authorities must take geological advice seriously and plan their growth accordingly.

2) The Olduvai Effect- New questions about meat eating in human origins: How about some food for thought?… I mean literally. Was meat eating connected to the evolution of larger brains in our human ancestors beginning around 2 million years ago? Paleoanthropologist John Hawks writes about recent work on East African sites across the period from 2.6 to 1.2 million years ago that throws doubt on the “meat made us human” hypothesis. 

The study he discusses finds no evidence for a systematic increase in meat eating across the studied period. The evidence also points to different hominin groups having flexible strategies for obtaining meat. But can we tie increased meat eating to one particular branch of the hominin tree? This is a very interesting article on how anthropologists retrieve and analyze evidence from sites and how the geography and time depth of sampling influence the conclusions that are drawn. 

3) Complex animals living millions of years before the Cambrian Explosion revealed by seabed tracks: What do we know about early animal life before the evolution of shells made their preservation more likely starting around 530 million years ago? That animals were present much before they acquired shells is inferred from molecular data that places their origin and diversification a good 50-100 million years before the Cambrian. But there is another way to understand animal evolution before body fossils appeared. It is through studying their movement on the sea bed. Tracks and burrows made by mobile animals start appearing in the rock record by 550 million years ago. 

James Ashworth describes some recent fascinating work that has decoded the morphology of these fossil trails and compared their shape with those made by some common modern sea floor animals. The researchers then propose that the changing shapes of trails across a 10 million year period is indicative of increasing complexity in animal locomotion. Early trails were made by simpler animals with short round bodies and limited sensory capabilities. Later in time, sinuous tracks made by worm like animals characterized by a slender anterior-posterior body profiles appear. A very clever way of understanding morphological changes during early animal evolution.

Fellow- Geological Society of India

I'm happy to announce that earlier this month I was elected a Fellow of the Geological Society of India in recognition of my efforts to introduce and popularize geology among the general public. 

 

As I came to know, some senior office bearers have been reading my blog and recommended my induction into the Society. I feel honored to be a part of this distinguished body. 

I want to thank you readers for your support and encouragement over the years. It has kept me seeking new topics to learn and write about. I started writing many years ago because I felt that there was a lack of popular style writings on the beauty of geology as a science and its relevance to society. Dramatic events such as earthquakes made news. But the field with its many sub specializations, and as a mode of inquiry into the history of the earth remained invisible to the lay audience. I aimed my writings to fill this lacunae. 

Interestingly, biologists were the first to start interacting with me. They were researchers interested in how landscapes impacted biodiversity and evolution. I have had many a fruitful exchange with them. Since then, my readership has expanded and I hear from people from diverse backgrounds. There is a sense of satisfaction that my collection of writings is being used as a resource by many science enthusiasts. A 17 year archive of my posts on varied geoscience topics is available on this blog for your perusal.

Let  me share an email I recently received from a student.

These are the moments when you think it has all been worth it.

Be sure to hum "He's A Jolly Good Fellow" when you are reading my posts. And get your friends to subscribe to this blog. Pronto!