Selectivity period consist of the Great Ordovician Biodiversification Event

Selectivity
patterns of sea level that offer an understanding of the effect of Ordovician events.

The evidence used, comes from carbon isotope and neodymium isotope, that record
the Great Ordovician Biodiversification Event and the end-Ordovician extinction
event, respectively. The study includes several interpretations of the sea
level fluctuations, however less study found in Britain. The isotopes correlate
conodonts and graptolites biofacies to the patterns of the sea level fluctuations.

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The carbon isotope determined the 12C/13C and Neodymium determined 143Nd/144Nd.

To be able to deduce the implications of using the isotopes and whether the
isotopes can provide evidence such as glaciation that may disrupt the marine
environment during Ordovician. Two glacial events, brief deepening and post
glacial flooding found in the end Ordovician extinction event, that confirm the
ecological destabilization of marine Ordovician environment.

 

Introduction

Two
subsequent major biotic events that are highlighted in the Ordovician period consist
of the Great Ordovician Biodiversification Event and the end-Ordovician
extinction. The Great Ordovician Biodiversification Event began around 470Ma,
lasted more than 25Ma, generally known for the large expansion in diversity in
the marine environment of Phanerozoic (Servais, 2010). The end-Ordovician
extinction, dated about 455Ma ago, distinctly recognized as the first animal
extinction during the Phanerozoic Eon and the second largest of the five mass
extinctions in the geologic history (Finnegan, 2012). These events are analysed
to contribute factors and evidence of the effect on the ecological destabilization
of marine Ordovician environment.

 

In this study, these events have
been recorded by carbon isotopes and neodymium isotopes, respectively. The carbon
isotope is stable isotope, and that differ from neodymium that is not stable
known as radiogenic isotope. Isotope measured used in carbon isotope is 12/13C.

This indicates that if the delta (?) value is positive, the sample is more enriched
in heavier isotope, which means 13C is more enriched than 12C. In contrast, if
the delta value is negative, 13C is less abundant than 12C. Thus, delta
measurement is defined as:

                             ?13C = (13C/12C) sample – 1  x 1000

                              (13C/12C) std                 

Unit
used is per mil. The same for delta measurement obtained by Neodymium, that used
143Nd/144Nd. The alpha (?) measured the product over reactant, for example, C
isotope is defined as: (13C/12C) product / (13C/12C) reactant, same formula
used for the Nd alpha measurement. The Epsilon (?) indicates the fractionate factor that depend entirely
on the size of fractionate, but stable isotope such as carbon isotope suggests
that the mass fractionation does not change over time. However, Epsilon in Nd
can be expressed as:

         ?Nd = (143Nd/144Nd) sample – (143Nd/144Nd)
VPDB x 10000

(143Nd/144Nd) VPDB

Moreover, the lower the sea level, the lower the
weathering due to the colder temperature that indicates less negative value. When
the sea level is higher, the weathering is higher and so, the temperature
increases, and that gives out more negative value.

 

The
GOBE has extensive studies of carbon isotope changes due to the causes and consequences of C isotopic fractionation still
remain poorly understoodAB1 . Nevertheless, climate cooling
during the Early Ordovician may have been an important factor in the GOBE. The
warm climate and high CO2 levels would have increased the diversification but
due to the complexity of the photosynthetic plankton that would have absorbed
CO2 from the atmosphere and release large amount of oxygen. This would have
affected the diversity of marine organisms in the higher food chain (Servais,
2010).

 

The
end-Ordovician is widely linked to the hirnantian glaciation and was
characterized as the climatic oscillation because there was a severe drop in
sea level due to the cold temperatures that caused major glaciation but latter
in the cycle, the temperature increased, the glaciers melted and sea level
stabilize once more (Finnegan, 2012). This disrupt the biotic life and caused
the second largest mass extinction in geologic history and has been recorded
from the changes of the isotope ratios in the Nd isotopeAB2 .

 

The stratigraphical framework comprises
three Global Series (Lower, Middle, Upper) and seven Global Stages
(successively: Tremadocian, Floian, Dapingian, Darriwilian, Sandbian, Katian,
Hirnantian). The Hirnantian is the stage that partly covers Britain (Servais,
2010, p.101). The Hirnantian is nearly equal to the regional Hirnantian stage,
which starts slightly, probably 100,000 years earlier. The regional Hirnantian
constitutes roughly the upper 20% of the Ashgill epoch (Servais, 2010, p.101).

The rest of the stages are contributed globally therefore, there is less
correlation and less recognition for Britain Ordovician indicates in this
report.

 

Although the Ashgill Series may be of
relatively short interval, the series includes one event of worldwide
importance, the late Ordovician glaciation and an accompanying major extinction
event. The Ashgill Series is typically developed in northern England. The first
base for the Ashgill is primarily recognized from first appearances of species
of trilobites, and brachiopods, but these are a relatively deep-water
biofacies. The problem with the definition of the Ashgill Series based on the
type area is that this section has not so far yielded conodonts, and newly
discovered graptolites are still under description (Servais, 2010, p.101).

 

 

Methods

Carbon
isotope record the carbonate and shale samples by washing and cutting the
samples off, followed by crushing the samples to even the powder using a device
called ‘automated agate mortar’ (Melchin, 2006). The powdered samples were dissolved
with few drops of HCl for organic carbon analysis and were left for about 24 hours
to remove the carbonate minerals. The samples were washed again to filter the
residue until the samples dry out (Melchin, 2006). Another device called
elemental analyser is used to determine the organic carbon contents, but to
determine organic carbon isotopic compositions, use both mass spectrometer and
elemental analyser for continuous flow (Zhang, 2010). The organic carbon
concentration is analysed when the samples were combusted in the elemental
analyser and then the pure CO2 gas collected by the mass spectrometer for
13C/12C determination. The 13C/12C determined the carbonate samples by
acid-release method. The anhydrous H3PO4 were then added to the samples of
powder at constant temperature of room temperature within 24 hours to release
CO2, and purified CO2 was sealed for carbon isotope analysis. The carbon
isotopic ratio was analysed on the mass spectrometer (Zhang, 2010).

 

The
Neodymium isotope also record carbonates and shales. The powdered sample taken
from the sedimentary rock were put into the centrifuge tubes while shaking the
mixture to diffuse adhering sediment. The calcite was dissolved with few drops
of HCl until the gas is released from the solution. Then sealed each tube with
lid overnight to ensure there is sufficient contact between the residue and the
acid. The next morning, the residue is subsequently filtered through syringe to
remove finer residue. There is small amount of Nd escaped from the
siliciclastic minerals after the residue being analysed (Holmden, 2013).

 

Nd
isotopic patterns are correlated to the conodonts biofacies and have been
determined for each sample. Conodonts were separated from carbonate with acetic
acid, then concentrated by straining and heavy liquid separation and brushed in
pure water to remove remaining material. Conodonts were weighed, projected with
a Sm-Nd tracer solution, and dissolved by drop-wise addition of HNO3 (Fanton,
2002).

 

The
measurements taken from mass spectrometer were changed for tracer isotope and
mass fractionation by a continuous process. The 143Nd/144Nd ratios were
normalized to 146Nd/144Nd ratio of 0.7219. Sm isotope ratios were normalized to
148Sm/154Sm of 0.49419 (Holmden, 2013). Isotope concentration determined the Sm
and Nd concentrations, and 147Sm/144Nd ratios. The uncertainty is ± 0.000012 (2?) (±
0.23?) for 143Nd/144Nd is based on the analyses of Nd standard. The 143Nd/144Nd
ratio of the standard measured was 0.511851 ± 0.000012 (2?) (Fanton, 2002). The samples of 143Nd/144Nd
ratios were changed for the development of 143Nd on account of deposition of
the Hirnantian glaciation and reported as ?Nd. This suggests a small error of
about 0.03 ? for the oldest samples. The uncertainty in ?Nd is about ? ± 0.4
(2?), which is based on analysis of the values of closely packed samples, and
the stratigraphic difference is relatively small ((Fanton, 2002).

 

 

Results

Carbon
Isotope (chemostratigraphy)

The
organic C-isotope chemostratigraphy shows significant fluctuations from the
Floian to the Katian. C-isotopic profiles have been compared with the biozones
represented by the graptolites. Thus, when describing the C-isotope
chemostratigraphy, graptolite biozones would follow the trend. The first gross
development of organic C-isotope occurred in the Floian stage. Figure 1.1 suggests
that the ?13Corg values increase from ? 29.4‰ of the Acrograptus filiformis
biozone to ? 21.1‰ of the Didymograptus eobifidus biozone. The ?13Corg values
then decreases towards ? 27.2‰ of the Azygograptus suecicus biozone of the
Dapingian stage. The ?13Corg clearly shows an increase of about 8‰ in the
Floian stage. The Middle and Upper Ordovician also indicates some changes in
?13Corg values. The Dapingian and Katian stages indicates certain gross
fluctuation of about 4‰, followed by a change in average ?13Corg values of
about 26.5‰ in the Dapingian to about 29‰ in the Katian.

 

The
carbonate C-isotope chemostratigraphy also shows several fluctuations given South
China as an example. Due to the presence of similar profiles and highly
correlated of the region to the global carbon isotope excursion in the GOBE. According
to figure 1.2, ?13Ccarb values increase from ? 1.1 ‰ to + 1.5‰, and shows about
3‰ increase from the top part of carbonate of the Meitan Formation to the bottom
part of the Shihtzupu Formation. Followed by the ?13Ccarb decrease of about
1.5‰ towards the upper part of the Shihtzupu Formation. The second gross
?13Ccarb rise from + 0.3‰ to + 2.7‰ shown in figure 1.2, and is recognized from
the upper part of the Shihtzupu Formation to the lower part of Pagoda Formation
in the Sandbian stage. The Pagoda Formation shows small stratigraphic change
and the ?13Ccarb values fluctuate between + 1‰ and + 2‰. Nonetheless, the
Pagoda carbonates show that the ?13Ccarb values fall from + 2.7‰ to + 0.8‰ in
the Katian stage.

 

 

Figure
1.1 C-isotopic chemostratigraphy: (A) ?13Ccarb; (B) ?13Corg; and (C) correlation
to ?13Ccarb profile (Zhang, 2010, p.109).

 

Figure
1.2 Detailed C-isotopic chemostratigraphy of Middle to Upper Ordovician in
South China interval: (A) ?13Ccarb; and (B) correlation to ?13Ccarb profile (Zhang,
2010, p.110).

 

Neodymium Isotope

Figure 2 is the
indicator of sea level change and suggests that when the ?Nd AB3 values increase, the
sea level also increases. Therefore, when the highest ?Nd values, the sea level
was at the highest. This indicates that ocean influences greatest along the
western margin of Laurentia by inputs of Nd from island arc weathering. In
contrast, the lowest ?Nd values, at which point the sea level was at the lowest
and therefore continental influences greatest along the western margin of
Laurentia inputs of Nd from weathering. In other words, when the sea level is
lower, the weathering is also lower from the colder temperature that indicates
less negative value. When the sea level is higher, the weathering is higher and
so, the temperature increases that gives out more negative value.

 

Figure 2 shows both
similarities and differences between the ?Nd record of Late Ordovician sea
level change sea level curves. The I-1, I-2, and I-3 isotopic excursions
exhibit transgressive-regressive events such as T-R cycles; 4B, 5A, and 5B, and
short ?Nd excursions between I-2 and I-3 correlate to T-R subcycles of the 5A
T-R event. The positive isotopic shifts represent transgressions characterized
by decreases in clastic content, increase of carbonate facies, and decrease of
siliclastic source terranes. The negative isotopic shifts represent regressions
characterized by shale progradation or increase in the clastic content of the
carbonates. A transgressive event should correlate to the I-4 ?Nd shift, but positive
increase of the T-R cycle 5C indicates a regressive event found at the peak of the
maximum regression.

 AB1Check
the structure of the sentence (grammar). Not clear what you mean.

 AB2Again,
how? tell the reader how these isotope systems (i.e., the changes of the
isotope ratios in the rock record tell the story, and more importantly – what
story they tell.

 AB3Define
this symbol. And define delta symbol for C isotopes.