physics

Steptoean positive carbon isotope excursion

Steptoean positive carbon isotope excursion is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Steptoean positive carbon isotope excursion rather than just read about it. In short: The Steptoean positive carbon isotope excursion (SPICE) is a global chemostratigraphic event which occurred during the upper Cambrian period between 497 and 494 million years ago. This event corresponds with the ICS Guzhangian-Paibian Stage boundary and the Marjuman-Steptoean stage boundary in North America.

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Reference excerpt

The Steptoean positive carbon isotope excursion (SPICE) is a global chemostratigraphic event which occurred during the upper Cambrian period between 497 and 494 million years ago. This event corresponds with the ICS Guzhangian-Paibian Stage boundary and the Marjuman-Steptoean stage boundary in North America. The general signature of the SPICE event is a positive δ13C excursion, characterized by a 4 to 6 ‰ (per mille) shift in δ13C values within carbonate successions around the world. SPICE was first described in 1993, and then named later in 1998. In both these studies, the SPICE excursion was identified and trends were observed within Cambrian formations of the Great Basin of the western United States.

Age The age of the SPICE is dated to between 497 and 494 MA, where it has primarily been identified through the use of relative dating and biostratigraphy. The onset of SPICE is generally accepted to correspond with the second wave of the End-Marjuman Biomere Extinction, and its termination corresponds to the End-Steptoean Biomere Extinction. Using trilobite and brachiopod index fossils linked to these extinctions, the upper and lower boundaries of the event can be defined. The beginning of the SPICE is identified by the extinction of shallow water polymerid trilobites, later replaced by deep water olenimorph trilobites following the observed peak δ13C value of the SPICE event. The age of SPICE can also be determined based on its correlation with the well-known Sauk II- Sauk III Sequence boundary in North America. Furthermore, in addition to biostratigraphic markers the 3 million year time frame of the SPICE event has also been determined using calculated deposition rates and the length of some of the more extensively studied SPICE sequences.

Localities, geology and δ13C characteristics

Localities

The SPICE event is expressed globally with known formations in 11 countries: United States, China, Australia, South Korea, Argentina, Canada, France, Kazakhstan, Scotland and Sweden (ordered by greatest to least number of localities). These locations span 4 modern continents (North America, Asia, Australia, Europe and South America), and represent 5 upper Cambrian paleocontinents: Laurentia, Gondwana, Kazakhstania, Siberia, and Baultica. All formations containing SPICE intervals formed between the paleolatitudes of 30°N and 60°S. For a full list of SPICE localities and formation see the following maps and table.

Geology Formations containing SPICE excursions are highly variable with geologic characteristics varying greatly amongst localities. Stratigraphic thickness in particular has very large ranges between locations, with the smallest being the Wangliangyu section of China which is less than 3m. This thickness is in contrast to the Kulyumbe section of Siberia which is greater than 800m. This variability of stratigraphic thickness suggests that the regional deposition rates during the 497 Ma to 494 Ma SPICE period were not globally uniform and more regionally dependent. Furthermore, formations containing the SPICE excursion represent a wide variety of lithologies, facies and water depths. In terms of lithology, all SPICE intervals are contained within carbonate units within carbonate and silicate sequences. The most common lithology for SPICE intervals are micritic limestones, or carbonate shales, generally interbedded with thin layers of calcareous mudstone. SPICE intervals have also been observed in dolostone units; however, these are not as common as the carbonate rocks. SPICE intervals are also highly variable when it comes to facies, with examples for shallow, intermediate and deep water settings (see map in the localities section). Considering the two most prominent areas of study, Laurentian formations (USA) tend to have stronger representation from shallow and intermediate facies (shallow/ near shore, shelf, intrashelf basin), while Gondwanan sections (China & Australia) have better representation of deep water facies (slope and basin), along with shallow and intermediate facies.

Stages of SPICE

Defining standard δ13C values of the SPICE interval, it can be noted the magnitude is highly variable from location to location, with maximum excursion values ranging from 0.64 ‰ to 8.03 ‰. Regardless of values though, the SPICE interval can be identified based on a similar pattern observed in each sequence. This pattern is identified based on 6 distinct stages: pre-SPICE, early SPICE, rising SPICE, plateau, falling SPICE, and post SPICE (see figure for visual representation of each stage).

Stage 1: Pre-SPICE All areas of the section prior to the onset of the SPICE interval. δ13C values remain near 0 ‰, similar to modern marine dissolved inorganic carbon.

Stage 2: Early SPICE Onset of SPICE, characterized by a slow increase in δ13C from 0 to approximately 1 ‰, suggesting a gradual increase in organic carbon burial and decrease in oceanic 12C.

Stage 3: Rising SPICE Rapid increase in δ13C from the early SPICE value to the max value. This shift in value is generally between 3 ‰ and 6 ‰, suggesting a rapid increase in organic carbon burial. The onset of the rising SPICE also generally corresponds to fossil indicators for the 2nd stage of the end-Marjuman biomere extinction.

Stage 4: Plateau δ13C values fluctuate but remain near the maximum value for a period of time. This stage is not observed in all SPICE intervals. After reaching the maximum value, most intervals proceed immediately into stage 5, the falling SPICE.

Stage 5: Falling SPICE Rapid decrease from the maximum δ13C value to near the standard ocean water value (0 ‰). The rate of decrease in the falling SPICE is generally more rapid than the rate of increase in the Rising SPICE. Generally interpreted as ocean water returning to standard δ13C levels.

Stage 6: Post SPICE All areas of the section immediately following the termination of SPICE.

Factors affecting the magnitude of the δ13C anomaly Despite being a global event, the magnitude of δ13C values observed within a SPICE interval appear to be highly affected by a variety of local conditions. A few common trends that have been determined are as follows:

Higher paleolatitude formations (greater than 30°S) tend to have lower δ13C values throughout the sequence. Shallower facies have lower values than deeper facies. Limestone tends to have marginally higher δ13C than dolostone.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Steptoean positive carbon isotope excursion

Start with the simplest possible case. Write down what Steptoean positive carbon isotope excursion claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Steptoean positive carbon isotope excursion before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Steptoean positive carbon isotope excursion ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Steptoean positive carbon isotope excursion

In research: Steptoean positive carbon isotope excursion appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Steptoean positive carbon isotope excursion in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Steptoean positive carbon isotope excursion is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cambrian, Events that forced the climate, Isotope excursions, so understanding it makes those chapters shorter.
In everyday life: Look for Steptoean positive carbon isotope excursion outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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Frequently asked questions

What is Steptoean positive carbon isotope excursion in simple terms?

Why does Steptoean positive carbon isotope excursion matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Steptoean positive carbon isotope excursion?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Steptoean positive carbon isotope excursion.