OC 210 TOPIC 2: CIRCULATION IN THE DEEP SEA
Readings: Ocean Circulation Chap 6: sections 6.3-6.7.
Intro to Physical Oceanography (on-line): Chap 13.
Part 2
H. ROLE OF THE ANTARCTIC CIRCUMPOLAR CURRENT (ACC)
1. The ACC plays a crucial role in the circulation of the deep water of the world ocean
 it is the only current which is located in all three major ocean basins (Fig. 1)
 thus it is the only direct pathway of deep water exchange between the three ocean basins
2. In the Atlantic, the formation of deep water as NADW and AABW means that the Atlantic
must be a net exporter of deep water to the deep Indian and Pacific ocean basins. This export
occurs via the ACC and brings a supply of “new” (relatively recently formed) deep water to the
deep Indian Ocean and deep Pacific Ocean (Fig. 2). Thus the ACC is the link in the deep
circulation portion of the conveyor belt between the Atlantic, Indian and Pacific basins.
3. The ACC is a wide, slow moving current (4-15 cm/s) but transports a lot of water because it
extends to the sea floor at 4000m (mean transport of ~140 Sv)
 the ACC flows eastward between ~65ºS and ~50°S (Figs. 1, 2)
 it is a geostrophic current, that is, it is produced by horizontal pressure gradients caused
by westerly winds in the southern hemisphere piling up water in the subtropical latitudes
(equatorward of 50°S).
 During the eastward flow of the ACC, vertical mixing in this region of low stability
tends to homogenize inputs from AABW, NADW and the upwelling Circumpolar Deep
Water (Fig. 2) from the deep Atlantic, Indian and Pacific Oceans.
I. TRACERS OF DEEP CIRCULATION PATHWAYS
1. Since speeds of deep water currents are so slow, we often determine the pathways of the deep
circulation indirectly by tracing the distribution of properties which are distinct for certain deep
water masses, thus oceanographers often refer to these properties as tracers.
2. What are some of the tracers of the primary deep water sources (NADW, AABW, AAIW)?
Naturally occurring tracers:
 Salinity and Temperature, which are conservative
 Nutrients (NO3, PO4, SiO2) and O2, which are non-conservative
 Remember: Conservative means that this property of seawater is neither produced nor
consumed in-situ in the ocean and non-conservative means that this property of seawater
is either produced or consumed in-situ in the ocean (like by respiration or photosynthesis
or radioactive decay)
3. Nutrients (NO3, PO4) are produced in the Deep Sea by respiration, that is, the oxidation of
organic tissue of dead plankton and fecal material for energy by bacteria and oxygen is consumed
via respiration
-the respiration reaction can be written as follows:
(CH2O)106(NH4)16(H3PO4) + 138 O2  H3PO4 + 16HNO3 + 106CO2 + 122H2O
 this ‘compound’ represents ‘organic matter’
4. The longer that the deep water parcels have been isolated from their formation regions, where
they were at the surface of the ocean, the more time respiration has had to occur and the higher
the concentrations of CO2, NO3, PO4 and lower the concentrations of O2
5. T, S, O2 and nutrients are called tracers of deep water circulation pathways because NADW,
AABW and AAIW have distinct T, S, O2 and nutrient properties when they sink in their
respective formation regions which can be used to trace the path of their flow.
Tracer distributions (T, S, nutrients, O2) in the Atlantic Ocean
a. Potential Temperature () (Fig. 3)
 cross section of  in the Atlantic doesn’t show too much structure
 main observation is the cold bottom water input via the AABW from the south
b. Salinity (Fig. 3)
 lots of vertical structure and thus is more informative than temperature
 NADW shows up as a tongue of high salinity water penetrating southward in the
Atlantic centered at a depth of ~2500m which reaches to 40-50°S
 AABW is a tongue of low salinity water at the bottom >4000m which seems to penetrate
to 40°N
 AAIW also shows as a tongue of low salinity water centered around 1000m with surface
origins at about 50°S and reaches to about 20°N
c. Oxygen (Fig. 3)
Remember: the source of dissolved O2 gas in seawater is from the O2 in the atmosphere
-thus surface waters generally have higher dissolved O2 levels than deep waters
 the longer a water parcel has been isolated from the surface ocean, the lower its O2
concentration because bacterial respiration of organic matter consumes O2
 in the Atlantic, the highest O2 levels are associated with the NADW, next highest with
AAIW and lowest with the AABW
 the low AABW O2 concentrations results because much (~50%) of the AABW formed
off Antarctica is composed of deep water that has been entrained during sinking
-this entrained deep water is old (from deep Pac and Ind) and thus depleted in O2
 the low O2 near the equator at ~1000m is due to horizontal mixing of water from the east
(Fig. 4)
-tropical thermocline water off the west coast of Africa has unusually low O2
concentrations and this region is known as an “Oxygen Minimum Zone”
 NADW has higher O2 concentrations than AABW (Fig. 4) because it is formed by
sinking surface water that has been exposed to the atmospheric O2. In contrast, AABW
is formed from upwelled deep water that is depleted in O2.
d. Nutrients (Fig. 3)
 Remember: nutrients are produced during respiration and consumed during
photosynthesis
 the meridional depth section of nutrients nitrate (NO3) and phosphate (PO4) show the
opposite trend to dissolved oxygen
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


the NADW has the lowest NO3 (and PO4) concentration because this water was formed
at the surface where photosynthesis occurred
AABW and AAIW have high NO3 (and PO4) concentrations
-because AABW has entrained nutrient rich deep water during its formation
-because surface waters in the formation region of AAIW (50-60°S) have high
concentrations of NO3
Note: the high NO3 at ~500-1000m in the tropical Atlantic (near equator) is due transport
of high NO3 water from the eastern tropical Atlantic (same process that causes low O2
level)
a. Interpreting a cross-section of a tracer
-The NO3 and O2 features at 1000m in the tropical Atlantic, referred to above, illustrates
an important point:
-when looking at a north-south cross section (Fig. 3) you should remember that
east-west currents and mixing can influence the distribution of properties (Fig.
4).
- Example: Does a salinity tongue in a north-south cross section have to imply southward
or northward flow?
-no, a salinity tongue could be due to zonal (east-west) flows, for example, the
westward flow of Mediterranean Sea water at ~1000m at ~35-40°N causes a local
maximum in Salinity at this depth
- thus one should be careful when interpreting the processes that is causing a feature in a
cross-section of a water property (T, S, NO3, O2, etc)
Question: Can you explain the vertical structure of the vertical profiles of T and S measured in
the S. Atlantic (Fig. 9 from Topic 1A) based on the meridional (N-S) cross sections of T and S
displayed in Fig. 3?
Tracer distributions (T, S, nutrients, O2) in the Pacific Ocean
Deep and Intermediate Waters present in the Pacific Ocean
 There is essentially no deep water formed in the Pacific Ocean, in distinct contrast to the
Atlantic Ocean.
 the source of deep water flow into the Pacific basin is northward flow along the bottom
that originates in the Antarctic Circumpolar Current (ACC)
 at mid-depths (2000-3000m) there is southward return flow of deep water to the ACC
that in part is the source of upwelled deep water in the S. Ocean (see Fig. 2)
 AAIW forms in the S. Pacific around 60ºS and flows northward at ~1000m
 There is a small amount of intermediate water formed in the northwest corner of the N
Pacific (Sea of Ohokst) called N. Pacific Intermediate Water
a. Temperature (Fig. 5)
 Coldest deep water found at bottom in the south (~60ºS) suggests northward inflow of
bottom water from the ACC. This deep water flow is often called Pacific Deep Water.
 The cross-section of Potential Temperature shows decreasing temperatures with depth,
as expected, except south of 60ºS where the ACC is encountered and vertical gradients in
temperature are small and as a consequence there is low stability and high vertical
mixing rates.
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b. Salinity (Fig. 5)
 the slight salinity maximum at the bottom in the south occurs in the same region as the
temperature minimum, described above, and results from northward flow of Pacific Deep
Water.
 the tongue of low salinity water at ~1000m in the S. Pacific is a result of formation and
northward flow of AAIW (like in the S Atlantic)
 the tongue of low salinity water at ~500m in the N. Pacific is a result of formation and
southward flow of N. Pacific Intermediate Water
 overall, the salinity is lower and varies less (34.5 to 34.73) in the deep Pacific Ocean
compared to the deep Atlantic Ocean (34.65 to 35.0)
 the lower salinity and northward salinity decrease in the deep Pacific is a result of
mixing with lower salinity waters associated with intermediate waters (both AAIW and
N. Pacific Intermediate Water). (This contrasts the situation in the deep Atlantic Ocean
where there is a northward increase in salinity as a result of input of NADW)
c. Oxygen (Fig. 5)
 the highest O2 in the deep Pacific is found associated with the bottom inflow of Pacific
Deep Water waters (this contrasts the situation in the Atlantic Ocean where lower O2
levels were found in AABW).
 lower O2 at mid-depth (~1500-3000m) in southward flowing water mass (this deep water
has been isolated from the surface for the longest time)
 O2 concentrations decrease northward in the deep Pacific as a result of mixing with O2
depleted Intermediate and thermocline waters in the N Pacific. (This contrasts the
situation in the Atlantic Ocean where O2 levels increase northward because NADW has
higher O2 concentrations than AABW)
 high O2 concentrations are found in AAIW and results from atmospheric input during
formation of AAIW
 The lowest O2 concentrations at found in the thermocline (500-1500m) in the N Pacific.
In this region the high rates of respiration draws down the O2 concentrations.
d. Nutrients (Fig. 5)
 the meridional cross-section of a nutrient (phosphate or PO4) distribution is essentially
the inverse of O2 (just as it was for the Atlantic)
 low PO4 (and NO3) concentrations are found in the Pacific Deep Water inflow along the
bottom in the S Pacific.
 PO4 increases northward in the deep Pacific as these waters mix with intermediate and
thermocline waters with high PO4.
 highest PO4 (and NO3) concentrations in the mid-depth waters flowing southward
because these waters have been isolated from the atmosphere for the longest time and
bacterial respiration has increased the nutrient concentrations during the waters transit
time through the basin (the same reasons as for the low O2)
e. A zonal (E-W) section of T, S and O2 in the South Pacific (Australia to S. America) shows that
the northward flowing bottom water input from the ACC (Pacific Deep Water) enters the S.
Pacific along the western edge of the basin (Fig. 6)
-the bottom water input from the ACC is relatively cold and salty compared to deep
water
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-this bottom water inflow has higher O2 (and lower nutrient) concentration compared to
deep water and is a useful tracer of this water mass
Tracer distributions (T, S, nutrients, O2) in the Indian Ocean (Fig. 7)
Key Points:
 there is no deep water formation in the Indian Ocean
 essentially, the deep water circulation pathway in the Indian Ocean is similar to that in
the Pacific Ocean (although Indian Ocean only extends to 20°N)
o thus bottom water from the ACC flows northward into the deep Indian basin
o southward flowing mid-depth deep water (1500-3000m) returning water to ACC
 the T, S, O2 and nutrient distributions in the deep Indian have similar (but not identical)
trends as the deep Pacific
a. Temperature
 northward flowing cold bottom water inflows from the south originating in the ACC
b. Salinity
 formation and northward flow of AAIW yields low salinity tongue in S. Indian at ~
1000m
 input of intermediate waters which originated in the Red Sea where there is very high
salinity (up to 40 ‰) shows up as a tongue of high salinity in the N Indian ocean in the
thermocline (500-1500m)
c. Oxygen
 relatively high O2 in northward flowing bottom water from ACC
 high O2 associated with the AAIW input at 50°S (surface origin)
 lower O2 at mid-depth (~1500-3000m) in southward flowing water mass (this deep water
has been isolated from the surface for the longest time)
 very low O2 concentrations in the northern portion of the deep basin are due to mixing
with shallow waters (<1000m) that have very low O2 concentrations which are a result of
high respiration rates caused by high rates of sinking organic matter (plankton derived)
formed in the very productive (high rates of photosynthesis) Arabian Sea surface waters
d. Nutrients
 relatively low nutrients in northward flowing bottom water from ACC
 higher NO3 (and PO4) occur at mid-depths (1500-3000m) where water is flowing
southward and has been away from the surface for the longest time
 very high nutrient concentrations in the northern portion of the deep basin as a result of
high respiration rates
Comparison of , S, O2 and nutrient distributions in deep waters of Atlantic and Pacific
oceans
a. The difference in T and S properties of deep waters in the Atlantic and Pacific oceans are a
result of the primary deep waters (NADW and AABW) being formed in the Atlantic Ocean and
the amount of mixing of these two deep water and AAIW input that occurs during the transit of
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these deep waters from their source regions in the N. and S. Atlantic to the Pacific (and Indian)
Ocean.
-generally, the longer a water parcel has been away from a deep or intermediate water
formation region, the more time mixing has to change the T and S properties of the water parcel
and the less likely the parcel has the original T and S characteristics of the originally formed
deep and intermediate waters (e.g., AAIW, NADW, AABW)
-thus the T and S depth profiles in the deep Atlantic have more curvature (“S” shape)
than depth profiles in the deep Pacific, because deep water parcels in this ocean basin are located
closer to the deep water formation regions
b. Temperature (Fig. 8)
 at mid-depth (2000-3000m) the N. Atlantic has the warmest water, next warmest is the S.
Atlantic, then the S. Pacific and finally the N. Pacific, which has the coldest deep waters
on this plot
 deeper than 3000m, the S. Atlantic has the coldest temperatures.
 This potential temperature trend reflect the strongest contribution of the relatively warm
NADW in the N. Atlantic and the increasing contribution via mixing of cold bottom
water from the AABW in the S. Atlantic and then mixing of these two deep water
sources within the ACC as deep waters move from the Atlantic to the Indian and Pacific
basins
 in the deep Pacific (>3000m),  is intermediate between values in the N. and S. Atlantic
Oceans
 downward mixing of heat from the thermocline causes deep water to warm during its
transit from the Atlantic to the Indian and Pacific basins
c. Salinity (Fig. 8)
 the N. Atlantic has the most saline deep water, then the S. Atlantic, then the S. Indian
and then the S. Pacific with the lowest salinity
 this trend reflects the strongest contribution of the relatively saline NADW in the N.
Atlantic and the increasing contribution via mixing of less saline bottom water from the
AABW as deep waters move from the Atlantic to the Indian and Pacific basins
 the low salinity of the AAIW (at ~1000m) shows up strongest in the S. Atlantic and S.
Pacific, where that water mass is formed The AAIW salinity signal is much weaker in
N. Atlantic and N. Pacific.
 There is evidence for input of the high salinity Med Water in the N Atlantic at ~1000m.
 There are low salinity surface and intermediate waters in the N Pacific where P>E.
Downward mixing of this low salinity water into the Deep Sea causes a decrease in
salinity in the N Pacific compared to S Pacific.
e. Nutrients and oxygen (Figs. 9 and 10)
- In these two figures, from left to right represents the latitudinal trend of deep water
flow going southward in the Atlantic, then following the ACC as it moves eastward in
the Southern Ocean (south of 60ºS) and then moving northward in the Pacific Ocean
- Nutrient concentrations increase from the N. to S. Atlantic and then from the S. and N.
Pacific.
- In contrast, but expected, O2 concentrations show the opposite trend with a decrease
from N. to S. Atlantic and from S. to N. Pacific
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- These trends are caused by the accumulating effect of respiration (O2 consumption and
nutrient production) as the deep water travels from the Atlantic to the Indian and Pacific
ocean basins.
- Generally, nutrient and oxygen concentrations in the deep Indian Ocean are
intermediate between those observed in the Southern Ocean and Pacific Ocean
Anthropogenically (human) Produced Tracers of Circulation in the Deep and Intermediate
Ocean
Anthropogenic circulation tracers are typically chemicals (often gases) made by humans and
been around a fairly short time (last few decades). They are sometimes called Transient Tracers
to denote their non steady-state distributions in the ocean, that is, their concentrations are
changing with time. (This is in contrast to circulation tracers in the Deep Sea like T, S, O2 and
nutrients that are at steady-state and whose distributions are not changing over time.)
a. Chlorofluorcarbons (CFCs or Freons)
 it is a gas that was used as refrigerant, for aerosol spray can and foams, three major
species F-11, F-12 and F-113
 these industrial gases have only been around during the last 40 years or so (Fig. 11)
 the Montreal Protocol (in 2000) is a treaty that banned their production because of their
role in atmospheric (stratospheric) ozone depletion
 CFCs are useful in oceanography because they act like a dye spread across the ocean
surface… the source of this dye is the atmosphere
-highest CFC levels in the surface ocean which is in contact with the atmosphere
where the CFC gas dissolves in seawater
 we can measure the CFC concentrations at very low levels (picomolar) and thus
accurately determine the CFC distribution in the deep sea
 we use the CFC distribution to estimate the pathway and rate of flow in the Deep Sea
 throughout most of the Deep Sea (>1500m) there are no measurable amount of CFCs
o Why?
o in which deep ocean basin are we most likely to find CFCs?
b. When a deep water parcel has a measurable amount of CFCs, then one can calculate the time
at which the water parcel was last at the surface of the ocean
 The higher the CFC concentration, the more recently that water parcel was at the
surface because the CFC concentration in the atmosphere has been increasing
(Fig. 11)
 In the simplest case, the measured CFC concentration in the water parcel can be
compared to the atmospheric CFC time history of CFC concentration to
determine the year when the water parcel was last at the ocean’s surface
 However, mixing will dilute the CFC concentration so this complicates
determining the “age” (time since it was last at the surface) of the water parcel
 Often the ratio of different CFC compounds (CFC-11/CFC-12) can be used to
reduce the impact of mixing on the age determination (Fig. 11)
c. The measured meridional CFC distribution in Atlantic Ocean clearly shows the penetration of
NADW in the north and a hint of AABW in the south (Fig. 12)
 This implies that these water parcels were at the surface less than 50 years ago
 For most of the deep Atlantic (30ºS to 30ºN) there is no CFC present
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d. The measured zonal CFC distribution in the Atlantic Ocean clearly shows the deep western
boundary current flowing southward along the western boundary near the equator (Fig. 12)
 this CFC cross section identifies two “bullets” of high CFC concentrations that
correspond to two layers of NADW moving southward from the formation
region (upper and lower NADW)
 the upper NADW is primarily Labrador Sea Water and the lower NADW is
primarily Greenland Sea Water
 the southwards flowing intensified western boundary current in the Atlantic
Ocean is expected based on Stommel’s theory of deep water flow (Fig. 13)
e. The water ages at the depth of NADW input in the deep Atlantic (~3000m) calculated from
CFC concentrations range from ~15 years near the input site (Labrador and Greenland Seas) to
~50 years in the S. Atlantic (Fig. 14)
f. In most regions of the deep Pacific and Indian oceans (>1500m), there are no measurable
amount of CFCs in the deep waters because these water parcels were last at the surface ocean
long before CFCs were added to the atmosphere by human activities. (Fig. 15)
Class Problem: Calculate the average speed of the southward moving NADW along the western
edge of the Atlantic basin to reach the equator based on the water parcel ages estimated from the
CFC measurements shown in Fig. 14.
J. TIME SCALE OF DEEP CIRCULATION
1. The estimated rate of formation of deep water in the source regions give us an estimate of the
deep water residence time, which in turn, gives us an estimate of how long (on average) it takes
to replace the water in the Deep Sea with water that has been at the surface of the ocean
2. In the N. Atlantic, the NADW formation rate has been estimated at 15±2 Sv.
 this formation rate represents an average over many years (decades)
 NADW formation rates likely vary year to year because surface water cooling rates
depend on air temperatures in the region, which vary, and surface salinity varies do to
precipitation, evaporation and sea ice formation
3. In the Weddell Sea, the AABW formation rate has been estimated at ~20± 5 Sv.
 about half of this is sinking of surface waters and half is entrainment of deep water
 this formation rate represents an average over many years (decades)
4. Calculate the residence time of water in the Deep Sea based on NADW and AABW formation
rates
 Assume a total deep water formation rate of 35±7 Sv
 Remember: residence time equals the volume divided by the inflow (or outflow) rate
 Mean depth of ocean (3800m) and area is 360x1012 m2
 Deep sea volume (>1500m) = (3800-1500m)*360x1012 = 8.3 x1017 m3
 Residence time = 8.3x1017 m3 / 35x106 m3/sec or 2.37x1010 sec = ~730 years
 This means that the average water parcel spends 730 years in the deep sea before it
returns to the surface ocean (via the conveyor belt circulation scheme)
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-some water parcels might only spend 100 yrs (if they upwell in the Atlantic
Ocean) and others might spend 1500 years (if they reach the far NE corner of the
N Pacific Ocean) in the Deep Sea
5. We can independently estimate the deep water residence time using the oceanic distribution of
a naturally occurring radioactive tracer, the radioactive isotope of carbon called radiocarbon or
14
C
a. What is a radioisotope?
 An isotope is the same element with a different atomic weight (same number of protons
and different number of neutrons)
 Carbon is predominantly 12C6 (99%), a little 13C6 (1%) and tiny amount of 14C6 (10-10 %)
 the extra 2 neutrons in the nucleus make 14C unstable, so it spontaneously decays to 14N
 radioisotopes decay at fixed rates, called a half-life, which means the amount of time it
takes for half of the isotope to disappear
 the half-life of 14C is 5700 years, i.e. if you start out with 1000 atoms of 14C, then after
5700 years you will have 500 atoms left (the other 500 14C atoms have decayed to 14N)
 for example, if a person died in 3700 BC and had 106 atoms of 14C in their leg bone and
you excavated that leg bone in 2000 AD, there would only be 500,000 atoms of 14C left.
 how do we estimate the 14C content of atmospheric CO2 back through time?
-measure the 14C content of cellulose in tree rings of a known age
6. Oceanographers make use of the radioactivity of 14C as a "clock" to age the deep water as it
travels around the oceans
 the basis of the approach is that the source of 14C into the oceans is the adsorption of
carbon dioxide in air that contains small amounts of 14C (as 14CO2)
- 14CO2 molecules are naturally produced in the atmosphere by cosmic rays
 once the water leaves the surface ocean, with its dissolved 14CO2 gas, there is no more
atmospheric 14CO2 input and the dissolved 14CO2 content of the water parcel decreases as
a result of radioactive decay
o thus, older parcels of water will have a lower 14C content compared to younger
parcels
 the radioactive decay rate is fixed and isn't affected by temperature, pressure or anything
 because the 14C decays at a fixed rate, the difference between the 14C you measure in the
deep water parcel and the 14C it had when it was at the surface in its formation region
depends on how long the water has been away from the surface ocean (i.e., isolated from
the 14C source)
7. The equation describing radioactive decay is as follows: (Fig. 16)

Nt = No * e(- *t)
 where Nt is the amount of the radioisotope at time t, No is the amount of the radioisotope
initially,  is the decay constant (λ=0.69 / half-life) and t is time.
 -the half-life for 14C is 5700 years.
 -for example, after 5700 years what fraction of the initial 14C will you have?
Nt/No = e (-0.69 / 5700yrs * 5700yrs)
Nt/No = e(-0.69)
-taking the natural log (ln) of both sides of this expression we get:
ln Nt/No = -0.69 (e.g., ln X = log X /0.43)
-thus Nt /No = 0.50, since the ln (0.50) = -0.69
● there is half the number of 14C atoms after 5700 years (Nt) than there were initially (No)
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8. Rearranging the radioactive decay equation gives you an expression for the amount of time (t)
that has elapsed for a given ratio of measured 14C (Nt) to initial 14C (No)
 Time = -5700 years/0.69 * ln (Nt/No)
 if Nt = 0.8*No, then Time = -5700 years/0.69*ln(0.8) = -5700/0.69*(-0.22) = 1843 years
 Thus, if you measured the amount of 14C in a parcel of deep water (Nt) that was 80% of
the 14C content it had when it was at the surface (No), e.g., the 14C of newly formed
NADW and AABW, then the age of the water parcel would by 1843 years.
9. A simpler, but less accurate, way to use the radioactive decay of 14C is a “clock” is to say that,
on average, 1% of the 14C decays every 80 years
● in the above example, the measured 14C was 80% of initial 14C and thus 20% was lost to
decay which yields a rough estimate of 1600 years for the age
● however, this rough estimate is significantly younger than the age of 1843 yrs calculated
by the radioactive decay equation.
10. By comparing the measured 14C content of water parcels in the Deep Sea to the 14C content of
the surface waters in the regions of deep water formation (off Greenland and Antarctica), we can
estimate the average time the deep water has been isolated from the surface
11. The 14C distribution in Deep Sea is at steady-state, so the overall supply rate of 14C to the
Deep Sea via the input of deep waters like NADW and AABW balances the total 14C radioactive
decay in the Deep Sea.
-Likewise, at steady-state, the 14C supplied to any specific water parcel in the ocean, by
currents and mixing, must be balanced by 14C loss via decay and upwelling of deep water back
into the surface ocean
-thus the 14C content of the Deep Sea does not decrease over time despite constant
radioactive decay of 14C. Can you picture a balanced 14C budget?
-Thus if you measured the 14C content of any deep water (say at 4000m in the equatorial
Pacific) in the years 1900, 2000 and 2100, it would be essentially the same.
- In reality, the 14C content of water parcels in the Deep Sea will change over time if the
deep water formation or circulation rates or atmosphere 14C levels change, which they do over
long time scales (say during the last Ice Age).
12. The 14C activities are expressed in parts per thousand using per mil (‰) notation relative to
the atmospheric 14C value. To convert per mil to percent of the atmospheric value, you divide ‰
by 10 and add 100, for example, -70 ‰ becomes -70/10+100 = 93% or 0.93 (expressed as a
fraction).
-The atmospheric 14C level is 0 ‰ which equals 100%.
13. The 14C content of water parcels in the Deep Sea decrease (get more negative) as one
proceeds from the Atlantic to Indian and Pacific ocean basins (Figs. 17 and 18).
-this aging trend is a result of the time it takes for water parcels in the Deep Sea to move
from the Atlantic, to the Indian and then Pacific ocean basins
-(Note that the 14C levels exceed 0 ‰ (or 100%) in the surface ocean and this is a result
of the input of 14CO2 to the atmosphere during nuclear weapons testing that began in he 1940s)
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14. When we calculate the 14C-based residence time for the deep waters in the Pacific and Indian
ocean basins, we compare the 14C activities of the average deep water to the 14C of deep water
entering the basins from the south (ACC) because it is the only deep water input to each basin.
 Using this approach, the 14C age for the deep Indian Ocean is ~300 years and the deep
Pacific Ocean is ~600 years. See if you can get a similar answer.
 Why is not surprising that the residence time of the deep water in the Pacific Ocean is a
lot longer than in the Indian Ocean?
14. Remember the average 14C age means some deep water is younger and some older
 in which basin is the youngest deep water found? Oldest?
 Why would you expect the 14C age of newly formed AABW to be older than for newly
formed NADW? Do the 14C data in Fig 18 show this?
15. Class Problem: How long, on average, does the deep water reside in the world ocean before it
returns to the surface in the N. Atlantic based on 14C measurements, if the average 14C in the deep
ocean is measured at –180 ‰? (Refer to the 14C trends shown in Fig. 18)
Part A
 Show that the answer is ~1000 years
 How does this residence time compare to the estimate derived from the estimated
NADW+AABW production rate estimate of 35±7 Sv?
Part B
In Part A, we did not consider the impact that the 14C of the newly formed AABW has on
the Deep Sea 14C age calculation. The 14C content of newly formed AABW is measured at ~85%
of atmosphere (-150 ‰ ) (see Figs 17 and 18) and is significantly lower than the 14C content of
newly formed NADW (at –70 ‰ or 93% of atmospheric 14C level).
● Since a significant portion of the deep water input to the Deep Sea is via AABW, with
lower 14C levels than NADW, will the input of AABW reduce or increase the calculated 14C age
of the Deep Sea relative to that calculated with only NADW input?
● Recalculating the deep sea residence time assuming that half the deep water input occurs
via NADW input and half via AABW input (based on their salinities) yields an average age of
670 years. See if you can obtain this result.
-thus AABW input significantly affects the calculated 14C age of deep water in the ocean.
●
K. Effect of Deep Sea circulation on ocean uptake of anthropogenic CO2
a. The long residence time for water parcels in the Deep Sea is the reason why most of the Deep
Sea has not yet accumulated any anthropogenic CO2 (Fig. 19)
-anthropogenic CO2 has only been around for ~100 years or so and most deep water
parcels were last in contact with the atmosphere ~700 years ago
b. An exception exists in the deep N. Atlantic where NADW forms. Here deep parcels of water
that were in contact with the atmosphere within the last 50 years or so (based on CFC ages,
see Fig. 12) are found and they have anthropogenic CO2 in them.
 Why would you expect to see less anthropogenic CO2 where AABW forms?
c. There is no anthropogenic CO2 in the deep Indian or Pacific oceans because these deep
waters have been away from contact with the atmosphere for 300-600 years.
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d. How will the pattern of anthropogenic CO2 distribution in the Deep Sea likely change over
the next 100 years?
K. KEY POINTS
1. Surface waters in the high latitude N. Atlantic and off Antarctica get dense (cold) enough,
primarily via heat loss to the atmosphere, to sink to great depths.
2. The deep water formed in the N. Atlantic is called the North Atlantic Deep Water (NADW)
and sinks to about 2000-3000m and spreads southward. Its production rate is ~15Sv.
3. The densest deep water forms off Antarctica where is sinks to the bottom (>4000m) and is
called Antarctic Bottom Water (AABW). Its production rate is estimated to be ~20Sv.
4. Another water mass formed at about 50-60°S in all three basins sinks to intermediate depth of
~1000m and spreads northward and is called Antarctic Intermediate Water (AAIW).
5. The temperature and salinity characteristics of water sources (AAIW, AABW, NADW) are
distinct and often allow oceanographers to trace their flow paths in the Deep Sea and calculate
the fraction that each contributes to any deep water parcel (>1500m).
6. The general circulation pathway of the deep water in the ocean is southward from the far N.
Atlantic to Antarctica via the NADW, where additional deep water is added via AABW
formation, then eastward via the ACC into the S. Indian and S. Pacific oceans and then
northward into the N. Indian and N. Pacific basins. There is return deep water flow back to the
ACC at mid-depth (~3000m) in the south Atlantic, Pacific and Indian oceans. Ultimately deep
waters rise (upwells) through the water column, as heat mixes downward to decrease the water
density, and return to the deep water formation regions via surface currents. The average current
speed in the Deep Sea is very slow at ~ 0.1 cm/s.
7. At steady-state, upwelling of ~35 Sv of deep water is needed to balance deep and bottom water
formation rates of NADW and AABW. However, we don’t know well where the upwelling
occurs nor the exact return route for surface flow back to the deep water formation regions in the
N. Atlantic and S. Atlantic.
8. The trends in deep water temperature, salinity, nutrients and oxygen between the Atlantic,
Indian and Pacific basins reflect the proximity to the deep and intermediate water formation
regions, the pathway of deep water flow and the amount of mixing that occurs along the flow
path.
9. Anthropogenically produced “transient” chemical tracers (like CFCs) can be used to determine
the flow rates of deep ocean currents in the regions near the deep water sources like the N.
Atlantic.
10. A naturally occurring radioactive tracer called radiocarbon (14C) yields an average deep
water (>1500m) residence time of about 700 years which agrees with the residence times one
would predict from estimates of the deep water formation rates of about 35 Sv.
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L. QUESTIONS/PROBLEMS
1. Where in the Deep Sea would you look for the water that has been isolated from the
atmosphere for the longest time?
2. What process causes the salinity to decrease southward in the core of the NADW?
3. Explain why there is a salinity maximum at the bottom near New Zealand and an O2 minimum
at mid-depth (~2000m) in the S Pacific? (see Fig. 6)
4. Why is there less curvature in the T vs S trends as one goes from the Atlantic to Indian to
Pacific Ocean basins? (see Fig. 8)
5. Why is using  (Pot. Temp.) rather than in-situ temperature important when making a T-S
plot?
6. If the T and S distributions in the Deep Sea are unchanging (at steady-state), what process
maintains the shape of the T vs S distribution while mixing tries to reduce the curvature?
7. Why can’t the approach of using T and S for determining the fraction of end-member water
mass sources be used for the upper ocean (<1500m)? (Hint: Are these shallow water parcels
isolated from atmospheric influences on T and S?)
8. If you measured a depth profile of NO3 concentration at the equator in the deep Pacific
(>1500m) at what depth would you expect to find the highest and lowest concentrations?
9. Explain why the 14C content of water parcels in the Deep Sea does not change over time
despite the constant decay of 14C.
10. Give an example of where zonal (E-W) mixing affects the meridional (N-S) distribution of a
seawater characteristic?
11. Why is it input to include both the 14C composition of NADW and AABW when using 14C to
calculate the age of deep water?
12. Why is the presence of anthropogenic CO2 and CFCs limited to depths <1500m in most of
the ocean? Where is the exception?
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