Sedimentation and Stratigraphy Geology 5142 Dr. Thieme

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Sedimentation and StratigraphyGeology 5142Dr.
Thieme

• Lecture 29 Stable Isotope Records of Changing
  Ocean Chemistry, Climate, and Sea Level

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Aragonite Seas are mostly known from "icehouse"
conditions of the Neogene, late Paleozoic, and
late Proterozoic.
Calcite Seas occurred under "greenhouse"
conditions of the early and middle Paleozoic, and
the Jurassic through Eocene.
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Phanerozoic Carbonates

• Sandberg attributed the ocean chemistry cycle to
  global climate change
• high pCO2 drives down carbonate saturation in the
  oceans
• "greenhouse" oceans were still capable of calcite
  precipitation but not aragonite
• Stanley and Hardie (1998, 1999) attribute the
  ocean chemistry instead to seafloor spreading
  rates

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Aragonite Sea
Calcite Sea
High rates of seafloor spreading result in
Calcite Seas because Mg is stored in basalts and
brines are Mg-depleted. Reduced consumption of Mg
at mid-ocean ridges raises the Mg content in
Aragonite Seas.
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Phanerozoic Carbonates

• the chemical composition of carbonates is also
  controlled by local as opposed to global changes
  in ocean chemistry
• carbonate composition changes as pore water
  chemistry changes during burial and diagenesis
• carbonate composition changes if rocks are
  uplifted and exposed to subaerial weathering

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Locally, much of the Mg found in carbonate rocks
is due to dolomitization.
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Stable Isotope Geochemistry

• relative abundance of the stable isotopes of O,
  C, S, and Sr
• isotope abundance in seawater has experienced
  large fluctuations in the geologic past
• fluctuations are globally contemporaneous within
  the "mixing time" of the oceans (1000 yr)
• seawater composition is recorded indirectly by
  marine shells and sedimentary minerals

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• Carbon and Oxygen are abundant in carbonate
  rocks
• delta 13C is the abundance of the two most
  common isotopes of carbon (13C,12C)
• delta 18O is the abundance of the two most
  common isotopes of oxygen (18O,16O)
• both ratios vary in the oceans through time

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delta 13C

• The standard used for carbon isotopes is CO2 gas
  obtained by reacting belemnites of the Pee Dee
  formation with 100 percent phosphoric acid.

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delta 18O

• Standards used for oxygen isotopes are
• Peedee Belemnite (PDB), which is also the most
  commonly used standard for carbon isotopes
• Standard Mean Ocean Water (SMOW)

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Quaternary Period d18O fluctuations Water
carrying 16O is preferentially evaporated 16O-dep
leted water (more negative d18O) is stored in ice
sheets during glacial intervals Rapid isotopic
excursions occur during deglaciation
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Quaternary Period d18O fluctuations Measurements
of d18O in the shells of calcareous foraminifera
have been used to establish a series of oxygen
isotope stages for the Quaternary period. Tephra
can also be usesd to correlate some of the same
marine sediments.
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Quaternary Period d18O timescale correlations
between deep sea drilling program (DSDP) cores in
the North Atlantic and the eastern and Western
Pacific Oceans. Brunhes/Matuyama boundary is
also shown for two cores.
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d13C fluctuations in carbonate can be used to
"wiggle match" although the values are quite
different from seawater values and vary between
cores.
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• d34S fluctuations
• Rapid excursions due to the mixing of brines
  enriched in 34S with surface ocean waters.
• Anoxic conditions beneath the brines encourage
  the reduction of sulfates to form pyrite.
• late Tertiary
• late Triassic
• late Devonian
• late Cambrian?

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Strontium Isotope Curve

• 87Sr/86Sr is constant throughout the ocean at any
  given time
• variations through time are due to mixing of
  three distinct sources that supply Sr to the
  oceans
• Ocean Basalt leached by hydrothermal circulation
  at midocean ridges
• Continental Crust weathering supplies Sr to
  streams which carry it into ocean basins
• Carbonate sediment recrystallizes during burial
  diagenesis and releases some Sr back to the ocean

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• Phanerozoic Sr Isotope Curve for Marine
  Carbonates
• General decrease in 87Sr/86Sr from Precambrian
  until the Jurassic period.
• Excursions due to rapid influx of Sr depleted in
  86Sr (Ocean Basalt) during -
• Jurassic
• Permian
• Mississippian
• late Ordovician

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87Sr/86Sr has gradually increased during the late
Cenozoic, beginning 35 Ma.
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• Cenozoic Sea Level
• The "eustatic" sea level curve of Haq is based
  upon the thickness of marine sedimentary rocks
  and shows fallling sea level since the Eocene
  with brief highstands during
• early Oligocene
• early and middle Miocene
• early Pliocene
• Pleistocene interglacials