Too Much Salt!

Human activities impact the water quality of streams. Some of these activities include the addition of salts via road salt application before winter storms, irrigation runoff, agriculture runoff, and leaky sewage pipes. The addition of salts into streams is considered salinization; therefore, the increased salinization of streams may change the chemistry and composition of stream water, impacting the organisms occupying them.

The increased salinization of streams negatively affects the tiny organisms that occupy them. Scientists can analyze how these organisms are impacted by measuring the biochemical oxygen demand. Many freshwater organisms perform a process known as respiration to gain energy so they can grow and stay alive. This process uses oxygen and organic matter (i.e., leaves and branches) to produce this energy. The tiny organisms that live in these streams are not visible to the naked eye; therefore, they consume oxygen and organic matter that has dissolved in the stream water. Biochemical oxygen demand (BOD) measures the amount of dissolved oxygen organisms consume. Hence, BOD gives insight into the productivity of these organisms within a stream.

Washington D.C. and Maryland's four streams of concern are Hickey Run, Paint Branch, Rock Creek, and Scotts Level Branch. These urban streams are monitored by the United States Geological Survey and the University of Maryland and are known to have high salt inputs from human activities. Stream water and sediment will be collected from each study site to implement a 5-day BOD experiment. The experiment will consist of multiple treatments (i.e., low salinity, moderate salinity, and high salinity) to investigate how salinity may impact the productivity of the tiny organisms within the stream water. The dissolved oxygen will be measured before the onset of the experiment and again at the end of the experiment. The difference between these two measurements will reveal the biochemical oxygen demand. A low value indicates that less oxygen is being removed from the stream water, while a high value indicates more oxygen is being removed. The data from these experiments will be plotted to reveal any trends or potential salt thresholds for these organisms. Additionally, literature reviews will help identify the potential reasons for these effects on these organisms.

Journey to the Almost-Center of the Earth: The Outer Core and the Induction Equation

I’d like to take you on a journey to the outer core: home to an important process which scientists refer to as the Geodynamo. This process is responsible for the generation of magnetic fields in the liquid iron and nickel layer between the inner core and lower mantle. 

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. But the main idea that it was in fact the liquid metal, not the solid iron of the inner core, that generated the magnetic field, gave rise to an important equation we refer to as the Induction Equation.

A snapshot of the simulated geomagnetic field produced by Glatzmaier and Roberts (1995). The lines follow the paths of magnetic fields generated from fluid motions.

This equation was brought on by scientific discoveries in the 40s and 50s, which were mainly motivated by the studies of magnetized plasma and conductive fluids. Using the building blocks of Maxwell’s equations for electric and magnetic fields, and combining this with the Navier-Stokes equation for a fluid, we arrive at the induction equation:

 

The analogy to the dynamics of a fluid is rather profound here. The first term is a common diffusion term, showing that energy is lost by the electrical resistance of the material, as a form of heat dissipating through friction. The second term is analogous to vortex stretching: imagine you are holding a string vertically on both ends, and in the middle a ball is attached. As you swing the ball around, you sweep a circle roughly parallel with the ground. But pull the two ends together and the ball gets faster. The general idea is reflected as vortices, or swirls in the fluid, naturally elongate, or stretch, as a property of turbulence. To not overcomplicate things, the general idea is that as these vortices stretch the preexisting magnetic field, thereby strengthening it.

Source: http://www.cds.caltech.edu/~marsden/wiki/uploads/d2hfest/home/Doering.pdf

To carry this important term back to the dynamics of the outer core, one can see that essentially that what the existing velocity field is has immense implications for whether or not the total magnetic field will grow or decay. Without a velocity field that can stretch the magnetic fields, the magnetic field will naturally decay. 

This equation is inherently complex and nonlinear, and so solving it analytically is difficult. Computational simulations use advanced numerical methods and algorithms that break down the equation into manageable steps and approximate the solutions over small time intervals. Over the past 20 years, simulations based on the induction equation have been successful in replicating a dynamo, but it is still unclear as to what the exact dynamics are, specifically on earth, that have allowed this magnetic amplification.

References

1. R. Beck, Magnetic fields in spiral galaxies, The Astronomy and Astrophysics Review, 24, 4 (2015)

2. Bondi, Hermann Sir and Thomas Gold. “On the Generation of Magnetism by Fluid Motion.” Monthly Notices of the Royal Astronomical Society 110 (1950): 607-611.

3. Glatzmaiers, G., Roberts, P. A three-dimensional self-consistent computer simulation of a geomagnetic field reversal. Nature 377, 203–209 (1995). https://doi.org/10.1038/377203a0

The Mysterious Disappearance of the Ediacaran Biota

 What were the Ediacaran biota?

Almost six hundred million years before humans walked the Earth, the ocean floor was dominated by some of the earliest complex multicellular organisms, known as the “Ediacaran biota”.  They first arose 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, with a wide variety of forms, ranging from simple disc-shaped organisms to more intricate frond-like structures. 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, Ediacaran biotas are often found in the form of impressions or molds in fine-grained sedimentary rocks. It is still a mystery as to what kind of life style these organisms maintained, but current research suggests that they could have been filter feeders, photosynthesizes, or possibly engaged in symbiotic relationships.  

Illustration of Ediacaran animals and fossils (Source: Australia Post / Artist: Peter Trusler)

What happened to the Ediacaran biota?

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 organisms. 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 biotas are found. Carbonate rocks that contain Ediacaran biotas, preserved as nodules and compressions, have been studied to reconstruct the amount of oxygen present during the time that these organisms lived. A recent study that did this has found that the Ediacaran biotas may have survived under low oxygen conditions. If they maintained a lifestyle that did not require oxygen, then it is possible that an increase of oxygen could be responsible for their death. Carbonate rocks during this time record a negative carbon isotope shift, which is consistent with this idea. However, recent work reconstructing the redox 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 negative carbon isotope shift occurring at the Ediacaran-Cambrian boundary is preserved in carbonate rocks that represent 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 are relatively simple organisms, their bodies were made up mostly of water, so the falling of sea level would 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 biota. As the Ediacaran Period came to an end, new forms of life began to emerge. These new organisms may have engineered the ecosystem to better suit themselves, as the cost of the preexisting organisms. The Ediacaran biota lived and fed on microbial mats, essentially operating as grazers. The new organisms burrowed into the ground, disturbing microbial mats, destroying the food of the Ediacaran organisms. The inability to complete with bioturbation associated with new Cambrian organisms may have led to the extinction of the Ediacaran biota.

Diagram of the before (left) and after (right) the rise of bioturbation 

It is possible that environmental changes, biologic changes, or a combination of both caused the extinction of the Ediacaran biota. 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 the Ediacaran biota to disappear.

References 

Cherry, L.B., Gilleaudeau, G.J., Grazhdankin, D.V., Romaniello, S.J., Martin, A.J., and Kaufman, A.J., 2022, A diverse Ediacara assemblage survived under low-oxygen conditions: Nature Communications, v. 13, p. 7306, doi:10.1038/s41467-022-35012-y.

Darroch, S.A.F., Smith, E.F., Nelson, L.L., Craffey, M., Schiffbauer, J.D., and Laflamme, M., 2023, Causes and consequences of end-Ediacaran extinction – an update: Cambridge Prisms: Extinction, p. 1–30, doi:10.1017/ext.2023.12.

An enigmatic origin for early animal biomineralization?

What is biomineralization and why is it important?

   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, protect against predation, and maintain a healthy chemical balance.  However, when animals first acquired the ability to biomineralize remains shrouded in mystery.  Discoveries of Cloudinamorphs, an early worm-like animal with a carbonate shell, in the Mara Member of the Nama Group in southern Namibia, led to a prediction that Cloudina would be found in equivalent successions across the world.  A field excursion based on this prediction led to the Siberian Craton, where Cloudina were not found, but a groundbreaking discovery was made.  In a reef complex in the Siberian Chencha 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 574 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, starting with the carbon isotopic signature of the rock units.  Aside from being the building block of all life forms, researchers use carbon isotopes, or atoms with the same number of protons but different number of neutrons, 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 Shuram Formation in Oman.  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 carbonate deposits makes this a controversial topic.  Recent geochemical data has been used to constrain the chemical state of the Shuram oceans to better understand the conditions in which early animals may have acquired the capability to biomineralize.  Weathering proxy data reveals enhanced weathering of the continents during this time period, likely due to 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 before the Shuram Excursion contained very little oxygen, or were anoxic, while during and after the event, 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.

Geochemical plot using carbon and uranium isotope data from several continents
that highlights the unique geochemical fingerprint that defines
the Shuram Excursion  (Li et al., 2020)

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

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

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• Rachel A. Wood et al.
 

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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  

Pressure-temperature estimates for amphibolite melange block

Pressure-temperature diagram for sample A15-21A showing the zirconium-in-rutile isopleth in red and the quartz-in-garnet isomeke in blue. From Harvey et al. (2020).

 

Specfem2D output for air-filled void space inside isotropic subsurface medium

 

 

Reading stress strain curves