Variations responsible for the timing of the

Variations in the
recurring cycles of the Earth-Moon system and Earth-Sun orbital patterns create
gravitational and temperature effects that modify the environment on Earth’s
surface.  These variations affect
systems such as the twice-daily tides, day-night interchanges, tidal patterns, and
the yearly solar pattern.  The general
term for these changes is “orbital forcing,” which is the effect on climate of
slow changes in the tilt of the Earth’s axis and the shape of the orbit.  “Forcing” signifies a physical process that
affects the Earth’s climate. This mechanism is also believed to be
responsible for the timing of the ice age cycles.  Due to the observable recurring nature of
these cycles, variations in them are evident in the rock record through
different sedimentation processes, having been used paleontologically to provide
short-term age determination in the past.  These cycles have been separated into four different
groups based on frequency and the orbital cycles that they represent.  The groups include the calendar band,
solar band, Milankovitch band, and galactic band, which are then broken down
by the number of years within each of their orbital cycles.  In addition, major sea level variations
over time are also noticeable in the rock record, and can be correlated with
the aforementioned cycles, specifically the timing of ice ages.  The magnitude and timing of the changes is
highly variable; however, the cycles and sea level variations still
provide insight into the tectonic and climatic history of the Earth.

Creating timescales
using microrhythmic sequences

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Current timescales
rely on biostratigraphic scales for the Phanerozoic; however,
they are relative, not absolute.  For some eras, radiometric scales may be
used,
but for pre-Cretaceous rocks, their sparseness and unreliability make them
of limited practical use.  New timescales
can be created using microrhythmic sequences, which describe various
cycles that exert gravitational effects on Earth.  These cycles are thought to be the result of
changes in sea level as well as changes in vegetation and erosion patterns on
adjacent areas of land, which are mainly driven by climate.  One sequence in particular,
the Milankovitch band, shows the effects of eccentricity,
axial tilt, and precession; this cycle has specifically been found to
correlate with the frequency of ice ages in the past.  This sequence has an effect on the
solar energy reaching the outer atmosphere due to the changing distance between
the Earth and the Sun, or the seasonal distribution of insolation.  Groups of orbital cycles were created to
describe the extent of each cycle’s gravitational effects on Earth, or
in other words, the capability of the cycle to exert orbitally forced changes
in the temporal energy distribution reaching Earth’s outer atmosphere (Figure
1).  Figure 1 gives the range of orbitally forcing
frequencies which may help contribute to the development of the new timescales;
these cycles include the calendar band, solar band, Milankovich band,
and galactic band (House, 1995).

Calendar band

The calendar band
frequency focuses on annual and lesser orbital cycles (? 1.0
a).
 The lower order cycles may be separated
into the tidal, whose effects
result mainly from gravitational changes in the Earth-Moon system,
and the solar, which result from
daily to annual changes in the energy received from the Sun.  Cycles at frequencies of ? 1.0 a are
recognizable in the sedimentary record as rhythmites and in the fossil record
as growth banding, where the accretion of tissue in both plants and
animals reflects environmental rhythms.  Although
many cycles have been created and categorized through the data collected from
fossils, this paper focuses only on the data collected from sedimentary
records (House, 1995).

Semidiurnal tidal effects (0.00135
a)

The orbit of the
Moon around the Earth attracts a tidal wave around the Earth approximately once
a day.
 Due to the centrifugal force
resultant upon the rotation of the Earth, tides are developed at opposite
sides of the Earth at points closest to and furthest away from the Moon.  Therefore, given the rotation of the
Earth,
the typical tidal cycle is formed giving two tides a day; this pattern has
been named semidiurnal.  Usually,
the two semidiurnal tides are not equal, meaning the alteration of
levels reached by successive tides is not the same, resulting in a lower high
tide followed by a higher high tide.  This
inequality stems from the fact that the orbital plane of the Moon around the
Earth has a low and constantly changing angle with the axis of the Earth’s
rotation.  For example,
higher high water will occur when a point is closest to the moon,
and vise versa for lower high water (House, 1995).

The twice-daily
rise and fall of sea level can follow several patterns varying from one in
which there is an extreme change in the usual semidiurnal sequence to one which
is daily, but in reality reflects the period of one lunar day.  Both patterns have been recorded from
sedimentary sequences and fossil shells, which have allowed for the
obtaining of precise information on local tidal regimes, and on the timing
of astronomical controls. H.C. Sorby, the founder of petrography,
was the first to recognize and distinguish different directions of depositing
flow between the ebb and the flow sedimentary regimes.  Multiple examples of such rhythmites on the
coasts of different countries all showed similar deposition sequences (Figure
2): fine sand is deposited at times of tidal current flow and clays are
deposited from suspension during times of minimal current flow at either high
or low slack tides.  It is also
important to note that such records only occur when the environment allows
continuous deposition (House, 1995).

Annual cycle (1.0 a)

            The
Annual cycle is the frequency that has received the most study.  There are 365 solar days in the Earth year;
environmentally, it represents the complete cycle of the dominant
extremes of solar radiation reaching the outer atmosphere of the Earth.  There are, however,
lag effects in how this process affects climate at the surface of the Earth
where highest or lowest temperatures are delayed.  A classic example of this frequency is seen in
sediments in lakes near glacial regimes where spring melts and summer organic
debris comprise a couplet or two of laminae resulting in a single varve.  A varve is an annual layer of sediment or
sedimentary rock, which in this case, has led to the
establishment of a post-glacial chronology (House, 1995).  Although there are many examples of ancient
varved sediments all around the world, a local example occurs in the Lockatong
Formation of the Newark Basin, with long sequences from the Triassic (van
Houten,
1964).

 

Solar band

            The
frequencies within this band contain orbital cycles between annual and the
Milankovitch band (1.0 a – 10.0 ka), and have been referred to as the solar
frequency band because solar phenomena and atmospheric and magnetospheric
reactions to them are dominant.  Their
orbitally forced signatures in the sedimentary record may not be as distinctly
prominent as cycles in the other three groups, but several gravitational
elements continue to be important (House, 1995).

One of the more
recognizable cycles is El Niño.  Off
the coast of Peru, the cold Humboldt Current from the south usually
gives way around the end of December to tropical and warmer waters from the
north.
Changes in rainfall, sedimentation and ecology result.  Stronger changes occur about every four years,
with occasional peak frequencies up to 9.9 a.  While these effects may be initiated by
orbital factors, their processes are complex (House,
1995).

Another fairly
discernable cycle is that of the solar year (11.0 a).  Sunspots are localized vortexes on the Sun’s
surface produced by magnetic activity.  The
spots reduce luminosity of the Sun and thus, the solar energy received
by the outer atmosphere of the Earth.  In
1843,
S.H.
Schwale discovered that there is a periodicity when spots reach a maximum every
11 a,
and this is what is termed the solar year.  These, of course, are not controlled by
orbital factors, but they do have some time relevance that aids in
creating an additional time scale (House, 1995).

Milankovitch band

Milankovitch
cycles are cyclical movements related to Earth’s orbit around the Sun.  The orbital effects associated with these
movements operate by changing the seasonal distribution of insolation and the
distance between the Earth and the Sun from time to time.  Such changes alter the amount of solar energy, or
insolation, reaching the outer atmosphere of the Earth.  The three cycles include eccentricity,
axial tilt, and precession.  Milankovitch’s
theory states that these three cycles combined affect the extent of solar heat
that reaches the Earth’s surface and thus, influences climatic
patterns.  Eccentricity describes
how the path of Earth’s orbit around the Sun changes from an almost perfect
circle to an oval shape on a 100,000-year cycle.  The axis is also not upright; it
tilts, with the angle of the tilt varying between 22 and 24 degrees every 41,000
years.
 At the maximum angle,
regions in the extreme upper and lower hemispheres will experience the hottest
summers and coldest winters, and it can be concluded that the
angle of tilt varies the strength of the seasons.  Additionally, precession describes how
the Earth moves around on its axis as it spins.  The axis staggers toward and away from the Sun
due to tidal forces from the Sun and moon over the span of 19,000
to 23,000
years,
which varies the timing of the seasons.  The combination of these small changes in
Earth-Sun geometry change the amount of sunlight each hemisphere receives
annually, where in the orbit the seasons occur, and how intense the
seasonal changes are (Riebeek, 2006).

            Furthermore,
Milankovitch’s theory helps to explain the timing of the ice ages, which took
place
when orbital variations caused the Northern Hemisphere around Northern
Europe to receive less sunlight in the summer.  Short, cool summers did not melt all of the snow
from the previous winter, so the snow accumulated year after year, creating a
white surface of increasing area.  The growing
white surface reflected more sunlight back into space, and temperatures continued
to drop even further, until eventually, an ice age would be at its
maximum.  Based on Milankovitch’s
calculations, he predicted that the ice ages would peak every 100,000
and 41,000
years,
with additional occurrences every 19,000 to 23,000 years.  Paleoclimate records show peaks at those exact
intervals.  On land,
there are about three or four recorded ice ages as evidenced by misplaced
boulders and glacial loess deposits; however, ocean core data has
revealed ten ice age events in the last million years, and approximately
100 in the last 2.5 million years (Riebeek, 2006).

Determining an ice
age in the rock record requires analyzing oxygen isotopes that are trapped in
ocean sediments.  Evidence
supporting Milankovitch’s theory of the timing of ice ages came from a series
of fossilized coral reefs in the South Pacific that formed in a shallow marine
environment during warm interglacial periods.  As ice ages progressed, more water froze
into ice caps and lowered the ocean level—this left the reed exposed.  When the ice caps melted,
the ocean levels rose again and warmed, creating another reef.  During these instances, the peninsula on
which the reefs formed was being simultaneously uplifted by tectonic processes.  Today, these reefs, whose age was easily determined
due to the coral’s decaying uranium, form a series of steps along the shore of
Papua New Guinea, and show the millennia between ice ages in
addition to the defined the maximum length of each.  The intervals shown by the corals fell at the
exact intervals that Milankovitch predicted they would (Riebeek,
2006).

Galactic band

The frequencies
within this band contain long period orbital cycles (> 1.0
Ma),
and are linked to periodic extinctions as well as major glaciation periods.  In terms of the periodic extinctions, there
was an increase in proposals of many different hypotheses that arose after the
publication of a claim that the extinctions of fossil groups occurred at
regular intervals during the post-Paleozoic.  The debates focused on what cyclical event
caused the extinctions, such as Earth passing through regions of cosmic dust
clouds,
and whether the timing of the extinctions in the original claim had accurate
statistical evidence to support it.  After
much debate and skepticism, the proposal was rejected based on
inconsistencies and missing extinctions in their timeline.  However, were a regular orbital
effect proven, it would have been of great advantage in calibrating
those time scales using the Milankovitch band (House, 1995).

The major
glaciation periods were suggested to line up with the cosmic year, ranging
between 220-250 Ma, which was also the period of time it was estimated
for the solar system to move around the Milky Way Galaxy.  The Pleistocene (1.6 Ma),
Permian (250 Ma), Ordovician (440 Ma), and Vendian (600 Ma),
being the periods of major glaciations, do not appear to be separated by equal
amounts on current radiometric data, so this suggestion is highly speculative
(House,
1995).

Sea level changes in
the rock record

Sea level changes
have occurred throughout Earth’s history, with the causes of the changes varying.  Locally, sea level may change if
tectonic forces cause the land to shift up or down.  However, in terms of global changes
in sea level, the variations must be due to either changes in the volume of
water in the oceans or changes in the volume of the ocean basins (Kominz,
2001). 

Changes in volume of water in the oceans

            The
two largest reservoirs of Earth’s water are its oceans (97%), and its glaciers
(2.7%).  Throughout the last three
billion years, the main variable influencing these percentages was the amount
of water that was being held in glaciers on the continents.  When ice ages occur, and glaciers remove a
large amount of water from the oceans, continental shelves can be exposed.  Inversely, as these glaciers recede, melt,
and release their previously landlocked water back into the oceans, sea level
will rise (Kominz, 2001).

Several different
methods have been used to determine the magnitude and timing of historical sea
level change such as dredging of continental shelves, studying ancient corals,
and carbon dating.  These methods have
shown that human activity used to occur near the present shelf-slop boundary, and
that fossils of wildlife only found in shallow water are now covered by over
100 meters of water.  The conclusions
made based on the aforementioned methods explain that in the near distant past,
sea level was substantially lower than it is currently (Kominz, 2001). 

When looking
further back into the record of sea level change, the stable isotope,
Oxygen-18, is a valuable tool in estimating actual sea level changes.  Water in the atmosphere typically has a lower
Oxygen-18 to Oxygen-16 ratio because evaporation of the lighter isotope
requires less energy.  This is
significant because it means that the snow that accumulates in glaciers is low
in Oxygen-18, and also that the ocean is proportionally enriched with the
heavier isotope.  Since marine organisms
make their shells out of the calcium, carbon, and oxygen that are present in
the sea water at that specific time, the ratios of Oxygen-18 to Oxygen-16 found
in the organisms’ shells differ based on changing oceanic conditions.  When a marine organism dies, it sinks to the
bottom of the seafloor where it decays and leaves behind its shell to become
part of the sedimentary record.  By using
this varying isotopic ratio and applying it to oceanic sediments and fossils,
scientists can estimate when sea level was rising or falling due to
glaciation.  Unfortunately, the amount of
Oxygen-18 present in an organism’s shell is not solely due to the amount of
Oxygen-18 present at the time of shell formation, but can also be attributed to
the temperature and salinity of the environment at that time.  This means that during periods of glaciation,
the shells are even further enriched with Oxygen-18, and oxygen isotope records
can reveal a combined history of changing local oceanic temperatures and
salinity in addition to the record of global glaciation (Kominz, 2001).

Moving back
through the Cenozoic (0-65 Ma), continuous sedimentation on the ocean floor
helped guarantee that palaeoceanographic data remained clear.  Oxygen-18 levels in fossil shells show a
general period of cooling for the past 50 million years, and two rapid
increases in the isotope 12.5 Ma and 28 Ma. 
These increases were largely due to the formation of the Greenland Ice
Sheet and the Antarctic Ice Sheet. 
Although enormous continental glaciers and ice sheets were not common in
the history of the Earth, they are known to have been present throughout a few
extended periods.  This is verified by
the amount of evidence found in the continental sedimentary record, which
indicates that ice sheets were present throughout history.  Specifically, there is sedimentary evidence
that shows glaciation in Ordovician to Silurian rocks (420-450 Ma), Devonian
rocks (380-390 Ma), and in Carboniferous to Permian rocks (350-270 Ma).  Since the sedimentary record matches the
previously discovered concept of predictable, high frequency, periodic growth
and retreat of glaciers during the Carboniferous to Permian glaciation, it can
be assumed that there were very large-scale sea level changes that occurred
during this time period.  It is fair to
conclude that large-scale (10-100 m), high frequency (20,000-400,000 years)
variations in sea level occurred during intervals of time when continental
glaciers were present on Earth (Kominz, 2001).

Changes in volume of ocean basins

            Tectonics
is thought to be the driving force of long-term (50 million+ years) sea level
change due to the fact that plate tectonics changes the shapes and areal
extents of the ocean basins.  While the
amount of water present on Earth has remained constant for the last 4 billion
years, plate tectonics has been steadily reshaping the surface features of the
planet, thus changing the total area taken up by oceans over time.  When a supercontinent forms, sea level fall
occurs because one continent is dipping under another and decreasing the
overall landmass on Earth while simultaneously increasing its oceanic
area.  Continental breakup has the
opposite effect.  During continental
breakup, the continents are stretched, which increases the amount of land mass
and decreases the amount of oceanic area, resulting in sea level rise (Kominz,
2001).

            Some
bathymetric features within the Earth’s oceans are large enough to greatly
influence sea level as they change size and shape.  The largest physiographic feature on Earth is
the mid-ocean ridge system, which spans over 60,000 km in length at widths of
between 500 and 2,000 km.  The ridge
occurs at a location on the ocean floor where two tectonic plates are spreading
apart, creating new ocean crusts and lithosphere along the ridge.  The width of these ridges is directly related
to the rate the tectonic plates are moving apart, so fast spreading ridges are
broad and slow spreading ridges are narrow. 
If the average spreading rates for all of the ocean’s ridges were to
decrease, the average volume taken up by ocean ridges would also decrease,
resulting in an increase in the volume of ocean basin available for water and
an overall fall in sea level (Kominz, 2001).

            Changes
in sea level due to the changing in volume of oceanic ridges has been
quantitatively estimated by scientists before. 
Since oceanic ridge volume is directly dependent upon the age of the
ocean floor, scientists can estimate ridge volume when the age of the ocean
floor is known.  The method of
quantifying sea level change based on oceanic ridges is not perfect though, as
the oldest oceanic crust that has not yet been subducted is only 200 million
years old.  This time constraint means
that uncertainty in sea level change estimates increases when researching closer
to 90 million years ago, and that it is not possible to make an accurate
quantitative estimate on sea level change past this point in time.  In the past 80 million years, it is estimated
that sea level has fallen 230 m (+/- 120 m) due to changes in oceanic ridge
volumes (Kominz, 2001).

Sea level change estimated from observations on the continents

            Estimates
of sea level change can also be made based on the sedimentary strata deposited
on the continents.  The continents are a
great place to obtain observations of sea level change because past sea levels
have been substantially higher than they are now, and also because continents
experience uplift in many places.  Uplift
occurs when land that was previously below sea level is moved vertically until
it is well above sea level, allowing scientists to estimate the magnitude of
sea level rise and fall in that area for a given time.  The fact that the continents are not
stationary, and move vertically in response to tectonic driving forces can also
be problematic when investigating sea level change.  Since the continents are always moving, all
indicators of sea level change found in continental strata are assumed to be
relative.  Although an enormous amount of
information about sea level change can be extracted from continental
sedimentary deposits, periods of non-deposition and the constant movement of
the Earth’s tectonic plates make obtaining a global signature from these
observations extremely problematic (Kominz, 2001).

            Aside
from long-term changes in sea level, there is sedimentary evidence of sea level
fluctuations that are substantially shorter than the 50-100 million-year
variations, but longer than ones caused by orbital variations (