Proxies for Ocean Salinity, Ice
Volume, and Sea Level
Salinity
Ocean water is 3.5% salt, more than
90% of which is NaCl
Salinity is generally measured by
measuring the conductivity of the
water – Practical Salinity Units (PSU)
= the conductivity ratio of a sea water
sample to a standard KCl solution.
Average ocean salinity is 3.5% (35‰)
Varies between ~32-37 ‰.
Salinity at the top layer of the ocean
is closely linked with precipitation
and evaporation.
Salinity contributes to the density of
seawater à important for ocean
circulation
Most abundant ions
(weight %)
Cl- 1.9%
Na+ 1.1%
SO42- 0.3%
Mg2+ 0.1%
Ca2+ 0.04%
K+ 0.04%
HCO3- 0.01%
1
Salinity
<4% by weight
Sea Surface Salinity (SSS)
from Pond and Pickard, Introductory Dynamical Oceanography
Variability of SSS is controlled by the balance of evaporation and
precipitation/runoff.
2
Surface versus Deep Ocean Salinity
Variable salinity
at surface due
to evaporation,
precipitation,
runoff, etc.
Average ocean
salinity ~35‰
(or 3.5%).
Credit: NASA, Jet Propulsion Laboratory
About 90% of ocean water resides in the deep sea. The salinity of
deep ocean water is relatively uniform due to the long residence
time of salts in the ocean (most on the order of 100 million years)
compared to the mixing time of the ocean (order of 1,000 years)
Salinity-depth profile
http://earth. usc.edu/~geol150/variability/deepocean.html
Since 90% of the ocean’s water resides in the deep sea, redistribution of
SSS will create a much smaller change in the deep reservoir. The deep
ocean is much more uniform than at the surface.
3
Deep Ocean Salinity Proxy: ClConcentrations in Pore Water
Pore water is the water filling the spaces between grains of
sediment
On the order of 5% of the earth’s
hydrosphere is located in the
oceanic crust.
http://toxics.usgs .gov /definitions/pore_water.html
Pore water measurements are relevant for any chemical or
isotopic tracer that is not affected substantially by chemical
reactions with sediments over the time scale of glacialinterglacial cycles (i.e., conservative).
Extracting Pore Waters
Hydraulic press used for applying up to
40,000 lbs of pressure
Titanium, Manheim-type
squeezers
http:// iodp.tamu.edu/labs/ship/chemistry/
4
Measuring Pore Water Salinity
AgNO3
Pore water
sample +
AgNO3 (aq) + Cl-(aq) à AgCl(s) + NO3-(aq)
Potassium chromate indicator
turns orange-pink when all the
chloride ion has been
converted to AgCl
K 2CrO42-
Ag 2CrO42-
Principle of constant proportions: the ratio of major salts in
samples of seawater is constant à residence time of Cl- is on the
order of 100 million years (compared to the 1,000 year mixing
time of the ocean).
Chlorinity is a measure of the total mass of halogen ions in
seawater (includes F, Cl, Br, and I).
Salinity (‰) = 1.80655 x Chlorinity (‰) (Knudson equation)
Chlorinity is around 19.2‰, so salinity is around 34.7‰.
Pore Water Salinity Reconstruction
Simple diffusion
model:
∂c
∂c
=D
∂t
∂z
2
2
Cl- based on SPECMAP δ18O
3.7% elevated at
LGM compared
to today
Initial condition:
c(z,0) = c 0
Boundary conditions:
c(0,t) = f(t)
c(z b,t) = c b
McDuff, 1985
5
The role of advection in sediments
Simple diffusion
model:
∂c
∂c
=D
∂t
∂z
2
2
Initial condition:
c(z,0) = c 0
Existence of a signal
(curvature) in the Clprofile suggests
advection must be
occurring at rates less
than ~1.0 mm/yr.
Boundary conditions:
c(0,t) = f(t)
c(z b,t) = c b
with varying amounts
of advection
∂c
∂c
∂c
=D
−w
∂t
∂z
∂t
2
2
McDuff, 1985
Pore Water Salinity Measurements
Bermuda Rise Core
Adkins and Schrag, 2001
They use a 1-D
diffusion model
that includes
effects of
compaction
advection and
changing bottom
water
concentration
over the last
125,000 years.
The chemical composition of the bottom water over glacial
cycles provides an oscillating boundary condition on the
sediment-pore fluid system yielding a record of past ocean
composition attenuated by diffusion.
6
Pore Water Salinity Measurements
Best fit = 2.5% glacialinterglacial change in deep
seawater salinity. This is
smaller than predicted from
estimated sea-level change of
121±5 m (average modern
ocean depth = 3.8 km).
121/3800 = 3.2%
Adkins and Schrag, 2001
Difference likely due to
different bottom water sources.
Today source is NADW. In
contrast, during the LGM the
source was from the southern
ocean (evidence from benthic
foram Cd/Ca and δ13C).
Advantages/Disadvantages of Pore
Water Salinity Measurements
Advantages:
Disadvantages:
Direct measurement of
chlorinity in deep sea waters
Low resolution, but sufficient for
glacial-interglacial changes
Cl- is “conservative”
Must be measured in places
where advection is minimal (away
from ocean ridges)
Relatively easy measurement
Pore fluid cannot be directly dated,
profiles must be modeled with
assumptions about diffusion and
advection rates
7