Climate Variability of the Holocene Period

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Climate Variability of the Late Holocene
Period Rachel Harris Investigating coastal
climate changes using Paleo-Indian shell middens

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Background

• Dr. Gerard Bond came out with a study in 1997
  revealing that the climate has varied during the
  past 12,000 yearsthe Holocene period.
• This information has led other scientists to
  continue to study Holocene climate variability
  for other parts of the world.
• The tropical are of particular interest because
  they are the heat engines driving global climate.

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Approach

• Reconstruct past coastal Florida SST changes
  using modern and fossil clam shells.
• Calibrate shell d18O and Sr/Ca sensitivity to SST
  variations with modern clams.
• Use these calibrations to reconstruct past (2000
  yrs BP) changes in annual mean SST and seasonal
  SST range.
• Fossil shells of Mercenaria campechiensis
  obtained from Seahorse Key Island located off the
  west coast of Florida (600-2000 yrs BP)

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Seahorse Key
North
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• Seahorse Key, FL
• Shell Middens
•  Weeden Island culture
• (500-2000 yrs BP).
• Known for large, ceremonial shell mounds in
  coastal SE US.
• Animal-effigy ceramics.
• Shell Middens Refuse mounds containing
  artifacts, fossils, and shells.

1m
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Mercenaria Campechiensis
translucent
opaque
Middle shell layer

• Mercenaria spp. One of the most abundant clams
  found along eastern North America.
• Clam shell layers secreted daily. They record
  seasonal SST changes and typically live for 10-20
  years.
• Microsampling of middle shell layer aragonite -
  monthly resolution

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Methods

• Shells were microsampled using a Merchantek
  Micromill device.
• High-precision milling of 50 micron by 5000
  micron troughs following growth bands (monthly
  resolution).
• Typically 5-10 years sampled per shell
• About 12 shells analyzed for three intervals
  Modern, 600-900 yrs BP, and 2000 yrs BP.
• Sr/Ca ratio measured by ICP-AES (Inductively
  Coupled Plasma Atomic Emission Spectrom.).
• 1 RSD replicate precision
• Modern shells Calibration
• Ancient shells Application

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Microsampling
One year
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SSTs the Sr/Ca Ratio

• Strontium (Sr) is a trace element incorporated
  into the mineral (aragonite) lattice of the
  shell. The degree of Sr incorporation can be
  related to temperature kinetic effect.
• The shell Sr/Ca increases as temperature
  increases.
• Sr and Ca have long oceanic residence times -
  stable over many thousands of years.
• Sr/Ca is a useful tool for reconstructing past
  sea surface temperatures in corals, perhaps
  clams.
• Shell d18O measurements also useful for
  estimating SSTs but affected by salinity.

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Calibration

• Modern shell collected near SST-monitoring
  station in Cedar Key, FL. Modern SST range
  10-30C. Average SST 22C
• Sr/Ca values range from 3.0-5.0 mM/M, clear
  seasonal cycles that match shell growth bands.
  High Sr/Ca High SST.

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Calibration (cont)
Sr/Ca (mM/M)
Standard Error 3C
SST (C)
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Fossil (710 yr BP) clam Sr/Ca and d18O - 2-5C
warmer...
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Changes in mean annual SSTs
1
LIA
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Conclusions

• Sr/Ca composition of M. campechiensis appears to
  correlate well with SST (r20.67).
• Application of this calibration equation to
  fossil clam shells suggests that coastal Florida
  SSTs were 2-5C warmer than present (during the
  Medieval and Roman Warm Periods).
• Habitation of these archeological sites occurred
  during warmer periods (?).