Sediment Stratigraphy in the Hudson River Estuary

1
Sediment Stratigraphy in the Hudson River Estuary

• By Jonathan Bent

2
Background

• The Hudson River Estuary is frequently dredged by
  the Army Corps of Engineers to allow deep-hulled
  boats to travel and dock.
• Hudson River cores have many anomalous layer
  changes that might be explained by dredging, even
  in areas said not to have been dredged.

3
The Core at Issue LWB1-13
Layer 1
Layer 2
Layer 3
Layer 4

• Issue The radical change at 45 cm seems to
  suggest a cataclysmic event or anthropogenesis.
• Feature of Interest The black intrusions from
  45-60 cm, and the brown intrusions from 0-10 cm
  seem to suggest a correlation of some sort
  between the top of the core and the layer
  boundary at 45 between layers 2 and 3.
• Approach We must use pertinent proxies to
  determine the cause (and approximate age if
  possible) of this change

4
Thesis

• That there is a very strange and immediate color
  boundary at 45 cm in LWB1-13. That we can
  understand whether it is a natural boundary, or
  the result of dredging by conducting several
  proxies and tests.
• Goal To prove that a) this boundary is the
  result of dredging or b) that it is a natural
  color transition that occurs when sediments age.

5
A Very Brief (Sorry!) Overview of Tests and
Proxies Conducted
6
Logging for Density

• Density seems to change at 40 cm.
• The slightly early jump may be due to machine
  calibration error.

7
Magnetic Susceptibility Testing

• Magnetic Susceptibility usually indicates the
  presence of brick dust and coal
  slaganthropogenic elementsin the core.
• Concentrations of these particles indicate
  post-industrial sediments
• Extremely high concentrations (over 50 units)
  tend to indicate dredging boundaries, where brick
  dust and slag have concentrated, perhaps after
  precipitating preferentially from the disturbed
  and then suspended sediments.

8
Cs-137 Testing

• The Cs-137 radionuclide is anthropogenic, the
  result of nuclear fallout, and thus only exist in
  sediment deposited since 1950s large-scale
  nuclear testing.
• Error bars taken into account, there is almost
  certainly no Cs-137 at or below 60 cm. There is
  plenty at 30 cm, so sediment from after 1950 must
  start between 30 and 60 cm.

9
Loss-on-Ignition Testing

• Proxy to determine the amount by mass of organic
  content in a sample
• Entails
• Sampling a 2 cc portion of a core
• Weighing the wet sample
• Heating the sample to 100º C to boil off all
  water
• Reweighing the now dry sample
• Cooking the sample at 500-600º C for an hour to
  burn off all organic matter
• Reweighing the sample to determine mass lost upon
  the ignition of the sample.
• Organic content tends to decrease linearly in
  cores, as bacteria and other microorganisms
  consume and off-gas organic matter. If our
  boundary is a dredging boundary, we might expect
  to see a different, more dramatic, sort of change.

10
Loss-on-Ignition for LWB1-13
11
Bivalve Intrusions

• Bivalves burrow into the sediment and excrete
  younger sediment into older, deeper sediment.
• When material accretes at a standard rate and
  little in the content of the sediment changes,
  these burrow holes are almost invisible, as they
  are compressed and similar in color.
• Only at drastic or unnatural boundaries can these
  holes can be seen because of the different nature
  of the two juxtaposed sediments.
• It is important to understand 2 transitions in
  this animation a) that the very top olive-brown
  layer always transitions to the black layer
  beneath it quickly (gt5 years probably), and that
  the black layer almost certainly changes to the
  lowest gray/orange layer gradually over a long
  period of time (gt200 years).

12
Results

• The dredging boundary in the density measurements
  is very slightly off of the observed boundary.
• The cessation of Cs-137 radionuclide emissions
  could very well correlate with the dredging
  boundary, but at least two more samples must be
  taken to determine this without a doubt.
• The change in the Magnetic Susceptibility and
  Loss-On-Ignition graphs corresponds exactly with
  the observed boundary.

13
Conclusions

• That because of leaps in density, organic
  content, magnetizability and Cs-137 content, the
  boundary that exists at 45 cm in LWB1-13 is
  almost certainly unnatural and the result of
  dredging.
• That the similarity in length and size of the
  bivalve intrusions likely show that the Layer 3
  sediment as it is nownot as it was before
  compaction and organic content losswas exposed
  to benthic organisms.
• That LOI testing is a good indicator of sediment
  age and of the cause of layer boundaries.
• That Army Corps of Engineers dredging records are
  incomplete for this region.

14
Acknowledgements

• To my thesis mentor Bill Ryan
• To my thesis advisor Chris Scholz
• To Martin Stute
• To Dorothy Peteet and Tim Kenna for usage of
  their equipment and labs.
• Friends and Co-Workers Angela Slagle, Kristen
  Fountain, Robin Bell, George Lozefski, Nichole
  Anest, Chris Bertinato, Frank Nitsche