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.

Ganga Earthquake, Nile, Deep Sea Habitats

Some readings over the past few weeks:

1) An Earthquake Changed the Course of the Ganges. Could It Happen Again?  Sediment load carried by big rivers like the Ganga often choke up channels and force the river to cut another path. Sudden channel shifts can also occur due to tectonic movements. A recent survey of the Ganga about 100 km south of Dhaka, Bangladesh , identified an old channel of the Ganga. Exploring this area, researchers came across veins of sand cutting across the sediment layers. These veins or sand dikes were  injected into the surrounding sediment. They are a sign of major ground trembling triggered by a large earthquake. Ground motion may pressurize buried sand and inject it  upwards. Further studies showed that this event may have been a 7-8 magnitude earthquake that occurred 2500 years ago. Kevin Krajick writes about this discovery and the larger geologic context.

2)  Lessons from the Nile about rivers and society. The Nile river has sustained agriculture, human habitation, and royal dynasties for millennia. A Nature Geoscience editorial summarizes a collection of sedimentological and geomorphologic studies that track the evolution of the Nile through the Holocene. Phases of the river cutting a deep valley changed to phases of the river flooding laterally and building fertile floodplains. This geomorphological evolution driven by climate change and more recent dam building have influenced agriculture and social systems in the past as well as the present. 

3) Ocean Exploration: Meet The Deep. I highly recommend this site. National Ocean and Atmospheric Administration, U.S.A., has compiled some stunning photographs and videos of deep sea habitats and the diverse forms of life that inhabit this little understood world. Corals, sponges, brittle stars, molluscs, worm tubes- attached to the bottom, living around energy sources such as hydrothermal vents and cold seeps. It is an absolutely fascinating exploration of the biodiversity of the deep sea. 

Octocoral and Brittle Star: Source NOAA Ocean Exploration

As a bonus, these images make great wallpaper for your mobile phone!

Deep Sea Mining, Indian Ocean, Infectious Diseases

Some readings for you:

1) Mining the bottom of the sea: The deep sea bed is considered the last frontier on earth for mining. Large patches of the sea bed are littered with metallic lumps or nodules rich in manganese, cobalt, zinc, and nickel. These elements are considered vital for powering the world's green economy. Nauru, a tiny Pacific Ocean island nation situated northeast of Papau New Guinea, along with a Canadian mining company, wants to start mining a region of the Pacific between Hawaii and Mexico known as the Clarion-Clipperton Zone. Scientists warn that a hurried push to mine the deep ocean bed will result in an irreversible loss to biodiversity, ecologic functioning, and ocean health. Elizabeth Kolbert writes about the complex legal and regulatory issues and conflicts of interest related to international deep sea mining.

As things stand in June 2024, a deep sea mining code is still being decided by the International Sea Bed Authority. Rohini Krishnamurthy of Down to Earth has the latest news on the progress made on this issue. Negotiations are hampered by a lack of basic science and divergence of views between member states.

2) Indian Ocean headed for a near-permanent state of marine heat wave:  Rapid fossil fuel emissions over the past century or so has changed the earth's energy balance. More energy is now coming in than is being radiated out to space. More than 90% of this excess energy is ending up in the ocean as heat. As a result, the world's oceans are warming up. The Indian Ocean is warming rapidly too. Recent studies have found that it may be heading towards a scary sounding situation known as 'permanent heatwave state' where the sea surface temperatures exceed a threshold value for 220-250 days a year.

Environment and climate journalist Nidhi Jamwal summarizes the findings of this research and a new book titled The Indian Ocean and its Role in the Global Climate System. The consequences are far reaching, impacting tropical cyclones, biodiversity, and fisher folk livelihood.

3) Probing the pathogens that afflicted ancient humanity: Pathogens and humans have been co-evolving for millennia. Paleoanthropologist John Hawks charts out the history of some of the common infectious diseases afflicting humanity. Infection patterns are not random. Rather, they follow networks of transmission shaped by ecology and culture. Very illuminating essay!

Deep Pacific Upside Down Waterfall

This passage from Helen Czerski's Blue Machine: How The Ocean Shapes Our World gives us a glimpse of the wondrous undersea universe we are just beginning to explore.

"We see upside-down waterfalls, she says. I don't understand what she means at first, and it takes me a few seconds to process the video as Deb keep talking. In those vertical chimneys, the walls crack and hydrothermal fluids come leaking out and  you get something that looks like half a toadstool growing out of a tree in an old growth forest. And suddenly I see it. This is a gigantic hydrothermal chimney looming out of the darkness, and hot water is indeed leaking out of its side. But because hot water is less dense than cold water, the hot water keeps flowing rapidly upwards. When it first hits cold water, its clearly dumped some minerals and made a ledge that sticks out- that's the toadstool shape that Deb is referring to. The water flowing upwards has had to flow outwards underneath the ledge before it can carry on upwards. But the ledge has developed a hollow on its underside like an upside-down bowl, so there is a pool of hot water there, held in the hollow as if it were filling up the inside of an umbrella. The boundary between hot and cold water shimmers like a mirror. And then the hot water is spilling out of its hollow and continuing upwards into the gloom. It really is an upside down waterfall".

Helen Czerski is watching this footage captured by a remotely operated vehicle exploring the area around the Juan de  Fuca Ridge, an undersea mountain chain a few hundred kilometers west of Seattle. Here, the Pacific and the Juan de Fuca tectonic plates diverge. Scientists are closely monitoring this ridge for seismic and volcanic activity, using a network of sensors  called the Regional Cabled Array. Deb Kelly is the Director of this project. Hydrothermal chimneys are sulfide and carbonate mineral deposits that form when hot mineral saturated sea water emerges through cracks in the ocean crust. The are common near mid oceanic ridges where the interaction of sea water and rock heated up by magma generates vigorous hydrothermal systems.

I'm only a quarter into this book and am enjoying every page of it. Highly recommended!

Books: Transformer , Blue Machine

 These two books came highly recommended via friends and social media.

I had really enjoyed his earlier book, The Vital Question. It was an exploration of energy constraints on the evolution of complex life on earth and perhaps elsewhere in the universe. Transformers takes a deeper look at the chemical processes underlying cellular function and their commonalities and differences across the spectrum of life.

Growing up as a hard rock geologist, I received very little training in how the ocean functioned. And to be honest, I have not updated myself on this immensely important topic in earth sciences. Blue Machine is my chance to learn more about how oceans shape our world. Looking forward to this one!