20,000 Days In The Life Of A Clam

My Whatsapp profile description says, "what's a million years here and there".

It is a tongue in cheek acknowledgment of the vast spans of time geologists often have to contend with. If I am studying a rock that formed more than a billion years ago, a 5 to 10 million year uncertainty in nailing down its exact time of formation is acceptable. Uncertainty in estimating the time of formation may occur due to our as yet not so perfect understanding of the decay rate of various radioactive isotopes being used for dating, or due to limits of sensitivity of the instruments measuring the radioactive isotopes in the mineral. We are making great strides in measurement techniques with a 0.1 % accuracy now achievable. 

Sedimentary rocks are harder to date than igneous rocks since minerals with radioactive elements rarely form in them at the time of their deposition (some limestones and black shales are exceptions). Often, using some indirect methods we can bracket their maximum and minimum age. Take the example of the Alwar Group sedimentary rocks which occur in the northern Aravalli mountains in Rajasthan. They are estimated to have been deposited sometime between 2.1 billion years ago and 1.8 billion years ago, an uncertainty of 300 million years! We know from our understanding of sediment accumulation processes that deposition was not uniformly spread out over the 300 million years. Rather, the sequence of sediments would have been deposited in discrete pulses lasting 10 to 20 million years, separated by long phases of non-deposition. 

Amazingly, even though we don't know exactly when during the 300 million year interval these sediments came to be deposited, we can track fairly accurately what was happening then on a daily basis. Some strata of the Alwar Group are of shallow marine origin. As the daily tide flooded in and ebbed, a thin layer of sand was deposited during each of these high energy phases. During the slack phase in between, a thin layer of mud was deposited. Stacks of these tidal bundles made of a sand and mud couplet record the passage of daily tides. Observing the stacking pattern closely reveals even more details. Sets of bundles of thicker sand-mud couplets alternate with sets of thinner bundles. Each set formed during alternating spring (thick layers) and neep tide (thin layers) cycles. 

We can interpret the ancient record by comparing the patterns with those forming today in different settings. The principle "present is the key to the past" is used with some caution, but it works well in this case.

Such tidal rhythmites are fairly common in the geologic record. The pictures to the left is a section of a core from  Cretaceous age sediments laid down in an estuary. The sedimentary section is made up of sets of thicker silt layers capped by a darker mud layer, overlain by a set of thinner silt and mud couplets. Each silt layer represents deposition during the flood or ebb tide. During the slack period, stirred up organic rich mud settled down. Again, we don't know the exact age of the rock to a certainty of few hundred thousand years, but we can track daily events. Image source: G. Shanmugan; AAPG Bulletin v. 84, 2000.

Speaking of tides, the earth's rotation is slowing down due to tidal friction. Marine organisms like corals, brachiopods, and clams build a calcium carbonate skeleton to house their soft tissues. Their shell grows by a daily addition of a thin mineral layer. Geologists have studied the pattern of skeleton growth of Paleozoic corals. Besides daily growth bands, they can identify seasons too, as corals lay down thicker bands during the dry season and thinner bands during the wet phase. The number of days in the year are estimated by counting the number of daily bands in each season. It turns out that there were about 420 days in a year during the early and middle Silurian (between 443 million to 419 million years ago). By middle Devonian (roughly 370 million years ago), some 50 million years later, the number of days had reduced to about 410. Using bivalve shells, scientists estimate that there were 370 days in a year by Late Cretaceous times (about 80 to 70 million years ago). 

There are other examples closer to our own existence on earth of natural rhythms being preserved in rock. Geologists and climate experts routinely use mineral bands in cave stalagmites to understand variations in rainfall. A recent study by Gayatri Kathayat and colleagues from Uttarakhand, North India, reveals details of the course of Indian monsoons over thousands of years, mapping dry and wet phases lasting few centuries each. Despite an uncertainty of a few decades in the absolute age of each layer, it gives us a broad picture of climate change through the Holocene. 

We accept the fuzziness of our estimates of the age of an event while being able to sharply resolve the changes taking place in that cloud of uncertainty.

I could have named this post "Ode to Laminae", an appreciation of thin layers that form in tune with earth cycles and which preserve in their layering valuable information on ancient tides, earth moon dynamics, changing of seasons, and longer term climate change.  I just thought the title of the post and the paper it refers to is better click bait. 

Sometime in the Late Miocene (about 10 million years ago, what's a few tens of thousands of years here and there) a giant clam living on the western margin of the Makassar Strait (Indonesia) built a shell with daily growth increments. I will post the entire abstract of the paper below so you can get an idea of the details of ocean conditions scientists can tease out today with sophisticated instrumentation.  

 Iris Arndt et.al., 20,000 days in the life of a giant clam reveal late Miocene tropical climate variability.

Giant clams (Tridacna) are well-suited archives for studying past climates at (sub-)seasonal timescales, even in ‘deep-time’ due to their high preservation potential. They are fast growing (mm-cm/year), live several decades and build large aragonitic shells with seasonal to daily growth increments. Here we present a multi-proxy record of a late Miocene Tridacna that grew on the western margin of the Makassar Strait (Indonesia). By analysing daily elemental cycle lengths using our recently developed Python script Daydacna, we build an internal age model, which indicates that our record spans 20,916 ± 1220 days (2 SD), i.e. ∼57 ± 3 years. Our temporally resolved dataset of elemental ratios (El/Ca at sub-daily resolution) and stable oxygen and carbon isotopes (δ18O and δ13C at seasonal to weekly resolution) was complemented by dual clumped isotope measurements, which reveal that the shell grew in isotopic equilibrium with seawater. The corresponding Δ47 value yields a temperature of 27.9 ± 2.4 °C (2 SE) from which we calculate a mean oxygen isotopic composition of late Miocene tropical seawater of −0.43 ± 0.50 ‰. In our multi-decadal high temporal resolution records, we found multi-annual, seasonal and daily cycles as well as multi-day extreme weather events. We hypothesise that the multi-annual cycles (slightly above three years) might reflect global climate phenomena like ENSO, with the more clearly preserved yearly cycles indicating regional changes of water inflow into the reef, which together impact the local isotopic composition of water, temperature and nutrient availability. In addition, our chronology indicates that twice a year a rainy and cloudy season, presumably related to the passing of the ITCZ, affected light availability and primary productivity in the reef, reflected in decreased shell growth rates. Finally, we find irregularly occurring extreme weather events likely connected to heavy precipitation events that led to increased runoff, high turbidity, and possibly reduced temperatures in the reef.

Tell me geology isn't the coolest field of study.

Plastic In Sediment, Antarctica Ice Core, Alfred Wallace

A few interesting readings:

1) Sedimentation Shifted - How rivers move sediment along their course to the sea is an important aspect of sedimentology research. Grain size, shape, and density, all affect how currents move sediment, and where and in what proportions sand, silt, and mud particles come to be deposited. Now there is a new kid on the block: plastic. Catherine Russell has written a fascinating article diving deep into experimental work on how plastic impacts sediment transport. The work she describes has important implications for our understanding of plastic pollution in rivers, and the role plastic particles plays in enhancing erosion rates and sediment redistribution in riverbeds. 

2) Antarctica: 1.2-Million-Year-Old Ice- Scientists use gases trapped in old ice to measure ancient atmospheric composition and estimate past climatic conditions. A long running drilling program in Antarctica had so far recovered 800,000 year old ice. That record has been recently broken. Scientists have reached the very bedrock of the Antarctica continent. The oldest ice at the very bottom is 1.2 million years old. This is the longest continuous record of our climate that we have so far.  It hold much valuable information on climate fluctuations through the Pleistocene and Holocene. This article is a press release of the University of Bern. 

3) Beyond Evolution: Alfred Russel Wallace’s critique of the 19th century world- Alfred Russel Wallace is the co-discover of evolution through natural selection along with Charles Darwin. He was a brilliant naturalist and made foundational contributions to natural history. But he also was very sympathetic to the plight of local people suffering under colonialism and the environmental degradation the race to strip the land of resources was causing. Marshall A. summarizes nicely Wallace's observations on the impact of environmental damage, both in his native Wales and also during his travels in the far away Malay archipelago. 

Let me take this opportunity to share again this lovingly crafted documentary on the life and work of Alfred Wallace. It is made as a paper-puppet animation, produced by Flora Litchman and Sharon Shattuck and narrated by George Beccaloni of the  Natural History Museum London and Andrew Berry of Harvard University.

 

What a fine example of science outreach. 

Shear Luck Near Sunderdunga River

As I settled down for lunch by the Sunderdunga riverside during my recent Kumaon trek, I noticed a polished boulder nearby. It had a striking appearance dominated by a large crystal of feldspar set in a much finer grained material. This finer matrix had a pronounced streaky fabric, as if made up of very fine layers. Upon closer examination, these layers or foliation was due to the planar arrangement of minerals like amphiboles, mica, quartz, and feldspar. The larger eye catching feldspar grain in the center of the boulder seems a little flattened along one axis and elongated along the orthogonal, giving it a crude sigmoid shape.

I had chanced upon a rock caught up in a shear zone. These are fault zones where movement of the crust causes intense rock deformation. The type of deformation I observed typically occurs at a deeper level where high temperatures make rocks soft and ductile. Rocks caught up in fault zones at shallower levels undergo brittle deformation. They have a broken appearance, made up of sharp edged fragments set in a crushed finer matrix. The rock is fractured, and these cracks get filled with minerals like calcite and quartz. 

This typical brittle like deformation was absent in this rock. There was no sign of any fracturing and breakage of the rock. Instead, the finer grained minerals seemed to flow around the larger feldspar crystal. Grain size reduction occurs by plastic rearrangement of atomic layers and recrystallization of softer minerals during deformation. The stronger resistant minerals which remain as large crystals are called porphyroclasts. Since rocks are sliding past, there is a rotational component to deformation also. Larger grains often show signs of being rotated, while finer groundmass wraps around.

The end stage of such ductile deformation are rocks known as mylonites. These have a flinty or glassy appearance due to the extreme grain size reduction.

I suspect this particular rock has not quite reached the mylonite stage. Let us call it a protomylonite. It does show a clear contrast between the finer matrix made up of stretched and elongated minerals and a large porphyroclast.  

The asymmetry of the porphyroclast gives geologists an idea of the sense of motion along faults. The annotated photo below shows the relative sense of shear or motion.  

Hundreds of such measurements have been made in the Greater Himalaya. When measured in-situ,  the direction of relative motion is 'top to the south', indicating the general direction of movement of Himalaya thrust faults.  Deformation is not uniformly distributed throughout the Greater Himalaya but appears restricted to narrow zones. These zones of intense shearing containing deformed rocks including mylonites have allowed the recognition of  major thrust fault zones such as the Main Central Thrust which emplaces the Greater Himalaya slab on top of the Lesser Himalaya.

There are minor shear zones too. I think this rock was eroded from one such shear zone in the Sunderdunga valley. 

Coming back to the brittle versus ductile deformation regimes. Almost all the deformation you observe in the Greater Himalaya took place in the ductile regime. Here are a few examples from the Greater Himalaya of ductile deformation seen in schists and gneisess. These are my observations from various treks in the Kumaon. 

A cross section of the Himalaya is presented here to showcase the metamorphic gradients along the Greater Himalaya slab (green). For this reading, you can ignore the rest of the Himalaya orogen shown in the figure. Temperature gradient increases towards the core of the slab with kyanite (k) and sillimanite (sill) as the prime high grade metamorphism indicators. This example is from the Nepal Himalaya, but the arrangement of the different Himalaya divisions is identical in adjacent Kumaon. 

 Source: Mike Searle et.al. Tectonophysics 2017

Notice the localization of mylonites along the Main Central Thrust zone. Metamorphism of rocks above around 600 degree centigrade during the Eocene (~35 million years ago) and in the Miocene (~25-16 million years ago) has resulted in the ubiquity of ductile deformation observed in the Greater Himalaya. In hotter pockets in the core, metamorphic rocks partially melted and the resulting granitic magma was injected along penetrative weak planes, forming dikes, sills, and small plutons.

Channeled between two great fault zones, the Main Central Thrust at the base and the South Tibetan Detachment as roof, this hot mushy crustal material was then tectonically extruded to shallower levels, its ductile fabrics frozen and preserved as the rocks cooled. Subsequent tectonism has superimposed brittle deformation on the Greater Himalaya ductile structures. 

Finally, another beautiful example of a gneiss showing ductile shearing.  Fish shaped white feldspar are set in a biotite mica and quartz matrix which flows around the porphyroclasts. Can you guess the sense of relative motion?

Observing features that you have seen only in a textbook - that is the great joy of going out in the field.

Darwin- Patagonian Christmas

A friend recently returned from a trip to Patagonia, Argentina. I mentioned to her that Charles Darwin had spent some time exploring the Patagonian coastline and had made some interesting observations about the local fauna and the native people.

Being a bit vague on the details I dug into one of the most reliable source on Darwin's life and work, the Darwin Online archive.  I accessed his Beagle Diary, scrolling down to the time the Beagle docked at Port Desire on December 8, 1833. Below is his entry from December 24, 1833:

24th Took a long walk on the North side: after ascending some rocks there is a great level plain, which extends in every direction but is divided by vallies. — I thought I had seen some desart looking country near B. Blanca; but the land in this neighbourhead so far exceeds it in sterility, that this alone deserves the name of a desart. — The plain is composed of gravel with very little vegetation & not a drop of water. In the vallies there is some little, but it is very brackish. — It is remarkable that on the surface of this plain there are shells of the same sort which now exist. — & the muscles even with their usual blue colour. — It is therefore certain, that within no great number of centuries all this country has been beneath the sea. —1 Wretched looking as the country is, it supports very many Guanacoes. — By great good luck I shot one; it weighed without its entrails &c 170 pounds: so that we shall have fresh meat for all hands on Christmas day.

John van Wyhe, ed. 2002-. The Complete Work of Charles Darwin Online- Beagle Diary.

As always his keen geology eye had spotted shells which were similar to living shelly creatures, leading him to the conclusion that this region must have been under the sea in the past. All along the South American coastline he noticed oyster and shell beds a meter or two above the current sea level. He concluded that there must have been vertical movement of the crust. 

He was a good hunter too. Apart from the Guanacoe, one of this shipmates shot a rhea. The crew ate that too. Darwin realized only too late that he wanted the entire bird to compare its form to rhea elsewhere in S.America. Eating valuable zoological samples became a common occurrence all through the voyage of the Beagle. 

The Beagle then sailed south to Port Julian. On his walks there Darwin came across bits of spine and hind legs of a largish creature, later identified as a Megatherium or ground sloth. That led him to muse about what could have killed off this variety, as nothing like it existed today. The sedimentary layers entombing the fossil did not indicate any kind of flood. How could an entire species die off?  Taking inspiration from cuttings of apple trees which were clearly part of the parent, Darwin wondered if all individuals of animal species too shared some life force. If so, could a common disturbance or event kill all of them off? 

In February 1834 the Beagle sailed into the Straits of Magellan and Tierra del Fuego. He immediately formed a low opinion of the natives whom he though of as uncivilized savages, living naked and unkempt in an utterly desolate landscape: 

Their country is a broken mass of wild rocks, lofty hills & useless forests, & these are viewed through mists & endless storms. In search of food they move from spot to spot, & so steep is the coast, this must be done in wretched canoes. — They cannot know the feeling of having a home — & still less that of domestic affection; without, indeed, that of a master to an abject laborious slave can be called so. — How little can the higher powers of the mind come into play: what is there for imagination to paint, for reason to compare, for judgement to decide upon. — to knock a limpet from the rock does not even require cunning, that lowest power of the mind. Their skill, like the instinct of animals is not improved by experience; the canoe, their most ingenious work, poor as it may be, we know has remained the same for the last 300 years. Although essentially the same creature, how little must the mind of one of these beings resemble that of an educated man. What a scale of improvement is comprehended between the faculties of a Fuegian savage & a Sir Isaac Newton — Whence have these people come?

John van Wyhe, ed. 2002-. The Complete Work of Charles Darwin Online- Beagle Diary.

Darwin compared the Fuegian natives with natives he had met further north along the Patagonian coast. Those tribes appeared better dressed, with some European contact spoke a smattering of English and Spanish, and had learned to eat with a fork and knife. 

Could the savage Fuegian change their life habits too? Darwin was doubtful. He thought that hundreds of years of living in this habitat had formed an instinct that could not change, a view reinforced when Jemmy a Fuegian native who had lived with Europeans was seen to have gone back to his native state. Perhaps, Darwin thought,  differences between human populations were more deep than realized.

Much more of South America remained to be explored. At this time his interest lay primarily in geology, but questions about species extinction and fixity, and the nature of human differences in life habits and intellect, lingered on in his mind. 

If you would like to read more about Darwin I will recommend Adrian Desmond and James Moore's fine biography, Darwin: The Life Of A Tormented Evolutionist.

Landscapes: Sunderdunga Valley Kumaon Himalaya

In mid November, I explored the Sunderdunga valley in the Kumaon region of Uttarakhand. It was a good rigorous walk through some extraordinarily beautiful landscapes. This area is better known for the famous Pindari Glacier trek. Kafni Glacier is another option for trekkers. All three routes begin at village Khati. The picture taken of the high ranges from nearby Dhakori shows the three glacial valleys.

And here are some more photos of the route with a brief commentary.

The entrance to Sunderdunga valley with the vigorous Sunderdunga river flowing through.

The first day walk to Jatoli village was through golden and green forests.

Village Jatoli in the mid November sun. We stayed there overnight at the Kumaon Mandal tourist guesthouse. They provide excellent clean accommodation. 

About 4 km walk upstream from Jatoli the next day and the land cover changes abruptly. The forest is gone. There is no marked trail from here on and a walk over a rugged boulder strewn region begins. 

We navigate our way over steeply dipping metamorphic rocks and scree cones. 

Numerous rock falls make for tricky passages. You can spot my companions climbing their way up the steep slope. 

After slogging for about 8 km through this terrain we arrive at Kathaliya, situated at about 10,500 feet ASL. We have climbed about 2500 feet from Jatoli to Kathaliya. A small trekkers shed has been constructed here. We stayed there for the next couple of days. 

Next morning, ahead of Kathaliya camp, we encountered the full glory of Sunderdunga valley. Here, it is a occupied by a wide boulder strewn river bed with several small active channels. The earthy colors of rock and grass were in stunning contrast to the blue sky. A solitary shepherd's hut can been seen in the lower right. 

Another view of the valley.

Boulder bed! The surrounding Greater Himalaya are made up of high grade metamorphic rocks. You can spot quartzo- feldspathic gneiss, amphibolite gneiss, mica schist and gneiss, and mica, garnet, and kyanite bearing schist and gneiss in the river bed. Quite a treat to walk along this metamorphic treasure! 

 Crossing the wider channels on rickety wooden bridges was fun! 

Here we are near Maiktoli Top, a high vantage point. Jagdish Bisht, me, Ratan Singh Danu, Lucky, and Kapil. They made my trip safe, comfortable, and memorable.

Why I go to these places. A clear view of the bands of metamorphic rocks exposed along the spectacular cliff face of the Sunderdunga ridge!

 

Village Khati is such a pretty place.

The high bare peaks in the background speak of a worrying trend. Everyone I talked to told me that this trek would have been impossible a few years ago in mid November. The upper part of the valley and the rocky ridges would have been blanketed in a thick snow pack. This area still had not received a single snowfall when I left on 22nd November. The two or three big snowfalls of the year now occur mostly in January and February. The pastoral and agriculture economy depends on a healthy winter snow cover to rejuvenate the high meadows, and to replenish springs and streams.

My guide tells me that Sunderdunga valley is the tougher route amongst the three treks to the nearby glaciers. I am so glad I walked this valley!