Oc 210 topic 2: circulation in the deep sea

OC 210 TOPIC 2: CIRCULATION IN THE DEEP SEA

Readings: Ocean Circulation Chap 6: sections 6.3-6.7.

Part 2

• 
  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.

• 
  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.
  

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

(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

1. 
   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)

• 
  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
  

• 
  the low O₂ near the equator at ~1000m is due to horizontal mixing of water from the east (Fig. 4)
  

• 
  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
  
• 
  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 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)
  

1. 
   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.
  

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

-this bottom water inflow has higher O₂ (and lower nutrient) concentration compared to deep water and is a useful tracer of this water mass

• 
  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)
  
  • 
    thus bottom water from the ACC flows northward into the deep Indian basin
    
  • 
    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
  

-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

- 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.)

• 
  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
  

• 
  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
  
  • 
    Why?
    
  • 
    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
  

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)

-How long has CFC been around?.....about 30 years

=(70° - 0°)*110km/°*1000m/km*100cm/m / 30 years

= 0.6 cm/sec

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)
  

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?
  

• 
  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)
  

• 
  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
  
  • 
    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)

N_t = N_o * e^(-l*t)

• 
  where N_t is the amount of the radioisotope at time t, N_o is the amount of the radioisotope initially, l 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?
  

N_t/N_o = e^(-0.69)

-taking the natural log (ln) of both sides of this expression we get:

ln N_t/N_o = -0.69 (e.g., ln X = log X /0.43)

-thus N_t /N_o = 0.50, since the ln (0.50) = -0.69

● there is half the number of ¹⁴C atoms after 5700 years (N_t) than there were initially (N_o)
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 (N_t) to initial ¹⁴C (N_o)

• 
  Time = -5700 years/0.69 * ln (N_t/N_o)
  
• 
  if N_t = 0.8*N_o, 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 (N_t) that was 80% of the ¹⁴C content it had when it was at the surface (N_o), 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.

-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%.

-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)
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)

• 
  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

● 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

2. 
   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?
     

3. 
   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.
   

4. 
   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.

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?

Directory: oc210 -> OC210 -> Lecture%20Notes
Lecture%20Notes -> Topic 3: surface ocean circulation
Lecture%20Notes -> Oc 210 topic 2: circulation in the deep sea (Fall 2009)

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