How do We Express Isotopic Ratios?

 

The typical equation for delta notation is shown in the red box, with a hypothetical example below.

Cerium anomaly calculation

Cerium is a unique rare earth element, that unlike its neighbors, can exist in both the +3 and +4 state.  Because of its unique behavior, cerium can be used to track oxygenation in the local water column insofar as in the presence of oxygen, Ce(III) is partially oxidized to Ce(IV) on the surface of Mn oxides, where it no longer participates in solid solution exchange reactions, depleting the residual seawater in Ce relative to the other REE (Tostevin et al., 2016).  This drives what is called a negative cerium anomaly, which is pervasive in the modern, well-oxygenated ocean. Under anoxic conditions, Ce remains in the Ce(III) state and is enriched in seawater relative to its neighboring rare earth elements.   

To calculate a cerium anomaly, one must first normalize (or divide) the measured concentration by a universally accepted reference concentration, which in this case is the post-Archean Australian shale (PAAS).  Once the elemental data has been normalized, the cerium anomaly is calculated using the following equation:

             

            Where [Ce] is the shale-normalized concentration of cerium, [Pr] is the shale-normalized concentration of praseodymium, and [Nd] is the shale-normalized concentration of neodymium.  There is an alternative equation that utilizes lanthanum to calculate the anomaly.  However, due to the anomalous behavior of lanthanum, the alternative equation may drive an artificial cerium anomaly.

Journey Near the Center of the Earth: The Outer Core

Why do our compasses point north? Most people know it is because of our Earth's magnetic field, but how is it created? To answer this, I’d like to take you on a journey roughly 1,800 miles down to the outer core (like going from New York City to Denver). The outer core is home to an important process which scientists refer to as the Geodynamo. This process is responsible for the generation of magnetic fields in the earth's liquid layer, the outer core, which is made up of melted iron and nickel metal. Our Earth's magnetic field protects us and other animals from harmful solar radiation, and is therefore something we cannot live without! By studying the Geodynamo, we can get a better understanding of what happened in earth's past that allowed it to exist as well as what may happen to our field in the future.

A depiction of the Earth's magnetic protecting us from the solar rays. Credit: © ESA/ATG medialab.

To the right: the whispy white loops portray the magnetic field around the Earth, seen here as partially transparent. The outer core is highlighted in fiery red, and the black ball in the center is the inner core. The magnetic field lines gather at the North and South poles of our earth, and this is why we have the Northern and Southern Lights!

The exact mechanisms that occur to bring about this dynamo are not well known, for obvious reasons: inaccessibility to earth’s deep interior, and the complexity and chaotic behavior of magnetic field measurements. We know that the solid metal inner core is almost as hot as the surface of the Sun, so it cannot be the cause of the magnetism. This, along with the discovery of the liquid metal outer core, gave rise to the Geodynamo theory.

Scientists in the 40s and 50s were able to produce a mathematical (or analytical) model for how the Earth's field changes over time. This model, which we refer to as the Induction equation, essentially shows how the fluid stretches and twists the magnetic fields and thereby amplifies them, like water in a tornado. So how the fluid flows has immense implications for whether or not the total magnetic field will grow or decay. But the Induction Equation cannot be solved on paper to determine what type of fluid flows or magnetic fields produce the type of magnetic fields we measure on Earth.

A tornado stretching and twisting the air around it. It is visible due to the water and dust that it sucks up with it. Source: BBC America. This twisting tube is called a vortex (plural: vortices), and is an important feature in fluid flows. When the fluid is a good electrical conductor or metallic, the vortices can twist and stretch the magnetic field lines!

Although computational simulations have been successful in replicating a dynamo, it cannot reproduce the exact dynamics on Earth because of the limited range of scales that computers can capture: in the outer core, the features of the fluid flow can range from hundreds of miles all the way down to the tiny molecular level! 

To truly understand and better replicate the outer core's characteristics, we need a physical model, or a laboratory analogue. At the University of Maryland, one such experiment exists: the Three Meter.

A peek into our experiment: our former grad student stands on top of the experiment's house: a metal box. Directly below him sits the Three Meter. You are looking at the exterior, which looks like it has a tube wrapped around it. This tube is filled with hot oil that keeps the sodium inside melted!

 

As its name suggests, the experiment is 3 meters (or roughly 3 yards) large and inside has a shell that is filled with liquid metal. In our case this metal is not iron, but sodium, as it melts at regular cooking temperatures, is lightweight, and a very good conductor (the Geodynamo needs a good electrical conductor to work). We can spin the shell to bring on fluid flow in ways that may occur in the outer core, due to the spin of Earth and the natural circulation in the outer core. By taking magnetic measurements of the experiment while it is spinning, we can determine what sorts of conditions are required for there to be a growth in the magnetic field. We may be able to translate this into the conditions that Earth was under to achieve a growth in the magnetic field.

An enigmatic origin for early animal biomineralization?

What is biomineralization and why is it important?

 

A modern example of animal biomineralization is shown by
this conch shell. Mollusks use their shells to defend against
larger animals that may want a tasty snack as well as regulating
their internal chemistry (eBay).  

   Biomineralization is the process by which living organisms produce minerals, often resulting in hardened or stiffened tissues.  Animals biomineralize to create bones, teeth, and shells to provide structural support, protection from consumption, and maintain a healthy chemical balance (think about snails and their shells for example).  
However, when animals first acquired the ability to biomineralize remains shrouded in mystery.  Discoveries of an early worm-like animal with a carbonate shell, called Cloudina, in southern Namibia, led to a prediction that these organisms would be found in equivalent rock successions across the world.  A field excursion based on this prediction led to Siberia, where Cloudina were not found, but a groundbreaking discovery was made.  In a fossilized reef complex of an ancient rock formation, strange sponge-like fossils with biomineralized calcium carbonate shells were discovered.  This was a major discovery as these fossils may represent the oldest examples of sponge-like animals at approximately 600 million years old!

Top: Cloudina microfossils that were extracted from a
 microbial reefal core found in the Villarta Formation in
Spain (Alvaro et al., 2019)
Bottom:  Outcrop photo of the sponge-like bioclasts in the 
Siberian Chencha Formation reef complex (Kaufman, 2021)

So why is this important?

     Despite the significant geographical distance between these two formations, they, amongst several other formations elsewhere on the planet, host unique geochemical signatures that have left researchers perplexed for years due to the data signifying an ocean with very different chemistry than in the modern ocean.  One element that researchers use to assess ocean chemistry is carbon.  Aside from being the building block of all life forms, researchers use carbon to reconstruct shifts in the composition of seawater throughout Earth history, correlate between rock units locally and globally, and construct age models. 

 

So how does this relate back to the discovery of the organisms?

 

    The formations in which these biomineralizing organisms were found, host one of the greatest negative carbon cycle anomalies in Earth history, known as the Shuram Excursion, named after the rock formation in Oman where it was discovered.  The absence of a reliable age date for the duration of the event, the magnitude of the anomaly, the uncertainty surrounding the onset of the event, and potential for alteration in such ancient rocks makes this a controversial topic.  Recent geochemical data has been used to constrain the chemical composition of the oceans roughly 600 million years ago to better understand the conditions in which early animals may have acquired the capability to biomineralize.  Elemental data reveals enhanced weathering of the continents during this time period, likely due to large-scale tectonic rearrangement, which would have delivered nutrients, calcium ions, and sulfate to the oceans, stimulating microbial processes that would consume organic matter and sulfate to produce carbonate that would bind with the calcium ions to precipitate calcium carbonate.  The appearance of the biomineralized fossils towards the end of the Shuram suggests that the calcium levels were so high that the animals could not expel the excess calcium from their bodies at a fast enough rate so they had to get creative, and thus they started producing shells.  Building upon the weathering data, additional elemental data suggests that the oceans prior to 600 million years ago contained very little oxygen, or were anoxic, while during and after the profound change in ocean chemistry, the oceans may have reached near modern oxygen levels.  This may have been attributed to the actions of the sponge-like fossils which, during life, would have filtered massive amounts of water, clarifying the surface oceans, allowing for photosynthesizing organisms to ventilate the oceans.

 

Orientation of the continents approximately 565 million years ago which is tremendously different than the modern orientation.  The names of the continents are very different than the modern names: Am, Amazonia; Ar, Arequipa; Aus, Australia; Az, Azania; Ba, Baltica; C, Congo; I, India; K, Kalahari; Lau, Laurentia; SC, South China; Si, Siberia; T, Tarim; WAC, West African Craton Each symbol represents known Shuram Excursion intervals (modified from Busch et al., 2022).

For more information please refer to:

Álvaro, J. J., Cortijo, I., Jensen, S., Martí Mus, M., & Palacios, T. (2020). Cloudina-microbial reef resilience to substrate instability in a Cadomian retro-arc basin of the Iberian Peninsula. Precambrian Research, 336, 105479. https://doi.org/10.1016/j.precamres.2019.105479

Busch, J. F., Hodgin, E. B., Ahm, A.-S. C., Husson, J. M., Macdonald, F. A., Bergmann, K. D., Higgins, J. A., & Strauss, J. V. (2022). Global and local drivers of the Ediacaran Shuram Carbon Isotope Excursion. Earth and Planetary Science Letters, 579, 117368. https://doi.org/10.1016/j.epsl.2022.117368

Cui, H., Kaufman, A. J., Xiao, S., Zhou, C., & Liu, X.-M. (2017). Was the ediacaran shuram excursion a globally synchronized early diagenetic event? insights from methane-derived authigenic carbonates in the uppermost Doushantuo Formation, South China. Chemical Geology, 450, 59–80. https://doi.org/10.1016/j.chemgeo.2016.12.010 

Li, Z., Cao, M., Loyd, S., Algeo, T., Wang, X., & Zhao, L. (2020). Transient and stepwise ocean oxygenation during the Ediacaran Shuram Excursion. Goldschmidt Abstracts. https://doi.org/10.46427/gold2020.1541

Rothman, D. H., Hayes, J. M., & Summons, R. E. (2003). Dynamics of the Neoproterozoic carbon cycle. Proceedings of the National Academy of Sciences, 100(14), 8124–8129. https://doi.org/10.1073/pnas.0832439100

• • 
    
• Rachel A. Wood et al.
 

,

Proterozoic Modular Biomineralized Metazoan from the Nama Group, Namibia.Science296,2383-2386(2002).DOI:10.1126/science.1071599 

Uriz, M.-J. (2006). Mineral skeletogenesis in sponges. Canadian Journal of Zoology, 84(2), 322–356. https://doi.org/10.1139/z06-032  

The mysterious disappearance of the Ediacaran organisms

What were the Ediacaran organisms? 

Almost six hundred million years before humans walked the Earth, the ocean floor was dominated by some of the earliest experiments in animals, known as the “Ediacaran biota”. They roamed the ocean floor during the Ediacaran Period up until the beginning of the Cambrian Period, from ~600 million years ago to 538.8 million years ago. These creatures represent a diverse array of species, without any resemblance to life that we see today. The wide variety of forms range from simple disc-shaped organisms (Dickinsonia and Kimberella) to more intricate frond-like structures (Charnia). They are generally characterized by their soft bodies and lack of hard parts (such as shells or skeletons), making it difficult to preserve complete specimens in the rock record. Today, these animals are often found in the form of impressions or molds in fine-grained sedimentary rocks, which allows us to study them in great detail. While their forms were relatively simple, their bodies contain detailed symmetrical patterns, which may have benefitted them in life. It is still a mystery as to what kind of lifestyle these organisms maintained, but current research suggests that they could have been filter feeders, photosynthesizes, or possibly engaged in symbiotic relationships.

Illustration of the Ediacaran organisms (above) and fossils (below) (Art by Peter Trusler).

What happened to the Ediacaran organisms?

The transition between the Ediacaran and Cambrian periods is marked by the sudden disappearance of the Ediacaran biota. It is still unknown just what exactly caused the extinction of these creatures. Current hypotheses point towards environmental as well as biological causes. Clues about their lifestyle (and death) may be found by studying the rocks in which the animals are found. Carbonate rocks that contain fossils of Ediacaran organisms have been studied to reconstruct the amount of oxygen present during the time that they lived. A recent study that did this has found that the Ediacarans may have survived under low oxygen conditions. If they maintained a lifestyle that did not require much oxygen, then it is possible that an increase of oxygen could be responsible for their death. Consistent with this idea, carbonate rocks during this time record a disturbance in the carbon cycle. However, recent work reconstructing the oxidation state during this transition has found there to be no change in the oxygen level, leading the door open for other mechanisms to cause the extinction of the Ediacaran biota.

In some locations the disturbance in the carbon cycle is interpreted as the falling of sea level. Additionally, higher rates of evaporation are supported by higher oxygen isotope values during this time. Sea level drop may have had negative impacts on these organisms. As the Ediacaran biota were simple creatures, their bodies were made up mostly of water, so the falling sea level could increase the salt concentration of the oceans. This would cause the fresher water contained within their bodies to diffuse out of them, essentially dehydrating and killing them. 

Alternatively, studies have been done that point towards biological changes that may have resulted in the extinction of the Ediacaran animals. As the Ediacaran Period came to an end, and the Cambrian Period was beginning, new forms of life began to emerge. These new creatures may have engineered the ecosystem to better suit themselves. By burrowing into the ground, the new Cambrian animals disturbed the algal mats that the Ediacarans fed on, eliminating their primary food source. The inability to compete with bioturbation associated with new Cambrian organisms may have led to the extinction of the Ediacaran biota.

Illustration of the transition from the Ediacaran (left) to the Cambrian (right) Ediacaran ecosystems existed at or near the surface, while Cambrian ecosystems extended deeper into the ground. (Art by Peter Trusler).

It is possible that environmental changes, biologic changes, or a combination of both caused the extinction of the Ediacaran organisms. Scientists around the world are working to answer this question, but without a time machine it is impossible to know with complete certainty what caused these mysterious creatures to suddenly disappear. Despite these animals bearing little to no resemblance to modern organisms, understanding how they went extinct may help us better understand how the creatures of today may go extinct as well. 

(Not so) Ordinary Chondrites!: A Glance into the Early Solar System

Have you ever looked into the night sky, and caught the faint glimmer of a streak? Have you seen a movie where a giant, mysterious stone with a glowing hue crashes into the ground? Those are meteors, gifts from the great beyond! Meteors and meteorites are extraterrestrial rocks that formed from cosmic dust in outer space, then fall to Earth's surface. Although in reality, they don't usually carry aliens or grant superpowers, meteorites (or chondrites as scientists call them) can be used to study many dynamic and spectacular phenomena that occurred billions of years ago in the early Solar System.

Ordinary Chondrites (OCs) are the most common type of meteorite falling to Earth's surface today (about 80% [2]). Their main ingredients are silicate (rock) and metal grains. OCs are special because they are made of material from the solar nebula that formed the Solar System, and have remained relatively unchanged since they formed. The only major process OCs experience is "thermal metamorphism" which is when heat bakes the ingredients inside. The primitive nature of OCs make them great subjects to study early Solar System processes. They have even been used to gain insight into the mechanics of core and planetary formation [1]!

OCs contain different amounts of metallic iron - so they can be very shiny and a polished surface would glimmer in the light! There are three major categories; H, L, and LL [3]. H chondrites contain the most amount of metallic iron (15-20% by mass). Fun fact: the "H" stands for "high" iron! Geologists aren't always the best at coming up with cool names... L chondrites have low amounts of metallic iron (7-11% by mass). As you might have guessed-- the "L" stands for "low"! Finally, we have LL chondrites which have even lower amounts of metal (3-5% by mass).

You might remember that I mentioned OCs can experience thermal metamorphism (or baking). Well, scientists have ways to tell how much heat an OC has gone through! There are different levels of heat activity which are known as "metamorphic types". The temperature increases as you go from type 1 to 6. My research involves analyzing the metal grains in OCs to learn about how they are baked and changed by heat. This will help us to understand core formation (which involves a lot of thermal activity) because we will have a better idea of how metal changes when you crank the heat!

Ordinary chondrites are beautiful relics of the early Solar System which grant us an opportunity to gaze into the past, and learn more about planetary origins. To stay updated on new OC falls or to see more interesting meteorites, check out the Meteoritical Bulletin. Watch the skies--the next OC may fall in your neighborhood!

References

[1]. Bottke, W.F., DeMeo, F.E., & Michel, P. (2015). Asteroids IV. Tucson: University of Arizona Press., https://doi.org/10.1353/book.43354.

[2]. Britt, D. T. (1996). The Parent Asteroids of Ordinary Chondrite Meteorites. Lunar and Planetary Science, Volume 27, 167.

[3]. Korotev, R. Metal, Iron, & Nickel in Meteorites. meteorites.wustl.edu. Washington University in St. Louis. July 2019

(Not So) Ordinary Chondrites: A Glance into the Early Solar System

 Have you ever looked into the night sky, and caught the faint glimmer of a streak? Have you seen a movie where a giant, mysterious stone with a glowing hue crashes into the ground? Those are meteors, gifts from the great beyond! Meteors and meteorites are extraterrestrial rocks that formed from cosmic sediments in outer space, then fall to Earth's surface. Although in reality, they don't usually carry aliens or grant superpowers, meteorites can be used to study many dynamic and spectacular phenomena that occurred billions of years ago in the early Solar System.

Ordinary Chondrites (OCs) are the most abundant meteorites falling to earth's surface today (about 80% of falls [2]). Their major components are silicates and metal grains. OCs are special because they are made of precursor material from the solar nebula that formed the Solar System, and they are relatively unprocessed aside from varying degrees of thermal metamorphism. This primitive nature makes them great subjects to study early Solar System processes, and they have even been used to gain insight into the mechanics of core formation and planetary differentiation [1].

Thanks to the many samples that fall to Earth each year, scientists have designed a classification scheme to make OCs easier to study. There are three major categories; H, L, and LL [3]. H chondrites contain the most amount of iron (Fe) and tend to contain more metal (15-20% by mass). Fun fact: the "H" stands for "high" iron! Geologists aren't always the best at coming up with cool names... L chondrites are low in total iron and have less metal (7-11% by mass). And yes, you've guessed it-- the "L" stands for "low"! Finally, we have LL chondrites which are even lower in total iron and metal (3-5% by mass). In recent years, a new category named HH has emerged, and as you might guess, these samples are absolutely brimming with metallic iron [4]! However, they are quite rare, so keep your eyes peeled!

Ordinary chondrites are beautiful relics of the early Solar System which grant us an opportunity to gaze into the past, and learn more about planetary origins. To stay updated on new OC falls or to see more interesting meteorites, check out the Meteoritical Bulletin. Be sure to watch the skies--the next OC may fall in your neighborhood!

References

[1]. Bottke, W.F., DeMeo, F.E., & Michel, P. (2015). Asteroids IV. Tucson: University of Arizona Press., https://doi.org/10.1353/book.43354.

[2]. Britt, D. T. (1996). The Parent Asteroids of Ordinary Chondrite Meteorites. Lunar and Planetary Science, Volume 27, 167.

[3]. Korotev, R. Metal, Iron, & Nickel in Meteorites. meteorites.wustl.edu. Washington University in St. Louis. July 2019

[4]. Troiano, J., Rumble, D., Ricers, M., Friedrich, J. (2011) Compositions of three low FeO ordinary chondrites: Indication of a common origin with the H chondrites. Geochimica et Cosmochimica Acta Volume 75, Issue 21, 1 November 2011, Pages 6511-6519