Cd/Ca, ¹³C, and GLACIAL NADW

Cadmium/Calcium

The rate of formation of North Atlantic Deep Water (NADW) is a key variable in theories of climate change. Boyle and Keigwin (31) were among the first to suggest that the cadmium content and the carbon isotopic composition of benthic forams are powerful tracers of deep water formation. In what way?

Cd is a trace metal in seawater, present at nM levels and below. Typical profiles (Figure NA-1) show a nutrient-like distribution and a strong correspondence to phosphate.

Figure NA-1, Typical profile of dissolved Cd from the central north Pacific, (31)

The slope of the element-element plot defines a Redfield ratio:

Eq NA-1:

A record of Cd concentration is seawater can be found in forams -- Cd is incorporated in solid CaCO₃ via solid solution. Consider the chemical reaction:

Eq NA-2:

Eq NA-3:

If these two solids form a solid solution within a single phase, then the activity of the solid scales as some measure of the concentration, e.g. the mole fraction:

Eq NA-4:

The activity of this mixture is not necessarily ideal. The energetics of this non-ideality are expressed with an activity coeffcient,:

Eq NA-5:

Thus:

Eq NA-6:

Separating the measureable quantities from the other variables:

Eq NA-7:

where D is called the distribution coefficient. To a reasonable approximation the activity coefficients of the two divalent cations are the same. Also since CaCO₃ is a major phase its activity coefficient will be approximately one. Finally, the observed value of D is about 2, thus:

Eq NA-8:

Since K is about 2x10^-5, the activity coefficient is about 10^-5. In energetic terms the free energy change would be RTln or about 14 kcal/mol to substitute the much smaller (0.17 nm) Cd ion into the Ca (radius 0.23 nm) lattice position.

Given a distribution coefficient of 2, we would expect to find a Cd/Ca ratio in the CaCO₃ of a benthic foram deposited in equilibrium with deep water of concentration 1 nM Cd (typical of the Pacific) of:

Eq NA-9:

In today's ocean, Cd is highly correlated with PO₄^-3 (figure 28.2. The reason for such a tight correlation is unknown, but appears to be robust over a wide range of oceanic conditions.

Figure NA-2. Relation of Cd to PO₄^-3 in waters throughout the world ocean.

In addition, the Cd/Ca ratio in benthic forams correlated well with the estimated ratio in the overlying bottom water (Figure NA-3), yielding an operational distribution coefficient of 2.8.

Figure NA-3. Correlation between benthic foram (four species) and bottom water Cd/Ca values.

Thus, based on its behaviour in today's ocean, Cd, as recorded in foram Cd/Ca ratios, is a powerful tool to record ocean nutrient concentrations.

Carbon-13

In an earlier lecture, it was shown that non-preformed ¹³C and PO₄^-3 covary according to the relation:

Eq NA-10:

Eq NA-11:

Thus, phosphate and ¹³C profiles should be mirror images, as shown in Figure NA-4.

Figure NA-4. PO₄^-3 and ¹³C profiles from the northwestern Pacific Ocean.

Thus ¹³C is a second potential proxy for nutrient concentrations that is well preserved in the geologic record.

Present Atlantic-Pacific contrasts

Before assessing core records of earlier distributions, it is worth reviewing the current vertical distributions of nutrients and nutrient-like elements in the Atlantic and Pacific Oceans (Figure NA-5).

Figure NA-5. Vertical profiles of P, Si, and Ba in the modern North Atlantic and North Pacific Oceans.

Because the deep waters of the North Atlantic are predominantly NADW created only recently from nutrient-depleted surface waters, nutrients are depleted throughout the water column. Deep waters in the Pacific, in contrast, are about half "aged" NADW, and about half Antarctic water. These waters enter as bottom waters, but eventually upwell to the surface where primary producers strip them of their nutrients. The "oldest" waters (in terms of time since last at the surface) are those just below the mixed layer. The nutrient distributions show this age effect, as well as the relative regeneration rates of the three elements from sinking C_org.

Ice age patterns

Profiles of Cd/Ca in benthic forams down cores from the North Atlantic show a striking change across the boundary between the last glacial and the current interglacial (Holocene; Figure NA-6). Atlantic values increase by more than 50%, whereas Pacific values decrease slightly (although barely above the scatter in the data)

Figure NA-6. Profiles of Cd/Ca in benthic forams from the North Atlantic and eastern equatorial Pacific deposited during the past 25 ky.

Similar data from a number of North Atlantic cores from depths between 1.5 and 4.5 km (Figure NA-7) show that the increase is limited to deep waters. The inferred nutrient content of glacial intermediate waters is the same as or even lower than today's value.

Figure NA-7. Glacial (green) and interglacial (red) Cd/Ca ratios in Atlantic benthic forams as a function of water depth.

¹³C profiles from the two oceans are broadly consistent with the Cd/Ca findings. Boyle and Keigwin analyzed a number of cores. Their results (assuming the global ocean is made up of 3 parts Pacific water and 1 part Atlantic) are shown in the following table.

Summary of Boyle and Keigwin Results

	Atlantic		Pacific		Global Average
	13C	Cd/Ca	13C	Cd/Ca	13C	Cd/Ca
Interglacial	1.01	0.69	0.16	.174	.37	.148
Glacial	0.25	.110	-0.25	.200	-.13	.178

This summary leads to several conclusions:

• in terms of the overall oceanic inventory of phosphate the Cd/Ca ratio suggests that the glacial ocean contained 20% more phosphate (0.178/0.148=1.20); if the present mean is 2.25 then this is a change in the mean concentration of 0.45 µM which would correspond to a change in carbon isotopic composition of -0.39--the observed shift is -0.50
• the Redfield connection between phosphate and inorganic carbon would suggest that the glacial ocean also contained 20% more inorganic carbon
• the contrast between the Atlantic and Pacific was much smaller in glacial times; a ratio of 2.5 vs 1.8 based on Cd/Ca, a ocean to ocean difference of 0.98 µM vs 0.58 µM based on the carbon isotopes
• corrected for the overall increase in inventory, the Pacific is about the same in phosphate (~-5% based on Cd/Ca, ~-1% based on ¹³C), while the Atlantic is much higher (33%,53%)

The internal distribution at present represents a mixture of NADW (low in Cd, high ¹³C) and AABW (high Cd, lower ¹³C). How can the sharp increase of phosphate in deep water of Atlantic be rationalized? The southward moving mass of NADW is influenced by two sources, a northern source produced in northern seas with low phosphate content and two southern sources, AABW and AAIW, with very high phosphate contents. Mixtures of these components satisfy the mass balance:

Eq NA-10: F_sC_s+F_nC_n=F_dC_d

If Cn is zero, then:

Eq NA-11: C_d=(F_s/F_d) * C_s

i.e., the increase in the phosphate content in glacial times can be rationalized as a decrease in the intensity of the northern source relative to the southern source or as an increase in the phosphate content of the southern source.

The present day salinity and temperature characteristics of Pacific deep waters suggest about equal proportions of southern (F_s) and northern (F_n) source waters now. Taking the Boyle and Keigwin Atlantic core as being representative of this mixing then the observed phosphate content of 1.19 would imply a southern source with 2.38, slightly above the observed 2.25.

Suppose that C_s were unchanged in glacial times. This would imply that F_s/F_d would be 0.70 (or the sources were then in the proportion 2.3:1). Alternatively, suppose that F_s/F_d were unchanged. This would imply a southern source of 3.33 which is not reasonable as the central Pacific value in the glacial is only 2.8. Taking that 2.8 as a reasonable upper limit Fs/Fd was at least 0.60 or the sources were in the ratio 1.5:1, still a substantial change from the present day.

In fact, direct measurements in the southern ocean suggests little glacial-interglacial change in the Cd/Ca ratio for this region. Thus, the PO₄^-3 proxy data support a glacial F_s/F_d ratio of about 0.7 - a substantial change from the present day.

There seem to be, a problem in the southern ocean, however. Whereas the Cd/Ca (PO₄^-3) data show no change in the deep waters, the ¹³C record (Figure NA-8) suggests that deep waters were substantially lower in ¹³C (lower than anywhere else in the oceans), with somewhat more fractionation between surface and deep waters than is observed today, implying significantly higher nutrients and higher productivity in this area.

Because such a finding has been used to explain, at least in part, the glacial decline in atmospheric CO₂, it is important to understand the discrepancy between the Cd/Ca and ¹³C records.

Figure NA-8. Downcore variations in the ¹³C content of calcite benthic (green - Cibicides sp.) and planktonic (red - N. pachyderma) foram tests from the Antarctic.

Kohfeld et al. (Paleoceanography, v. 15, p. 53; 2000) have recently evaluated the factors that affect the reconstruction of dissolved ¹³C from values measured in foram calcite. The principal factors are:

• Inorganic equilibrium: ¹³C_aq = ¹³C_T + 1.0
• Anthropogenic effects:

Eq NA-12: ¹³C = 0.0^o/oo - (0.012 ^o/oo µmol^-1kg^-1) C_anth for C < 35µmol.kg^-1 and

Eq NA-13: ¹³C = 0.3^o/oo - (0.020 ^o/oo µmol^-1kg^-1) C_anth for C > 35µmol.kg^-1

• Disequilibrium effects:

Eq NA-14: ¹³C_shell = ¹³C_eq + _disequ

Eq NA-15: Where _disequ = _diet + _carb + _T + _size

Eq NA-16: = (MX)_diet + (MX)_carb + (MX)_T + (MX)_size + B_total

and where M_x is the change in ¹³C per unit change of disequilibrium variable, x. The values for M compiled by Kohfeld et al. are:

Diet effect	0.084 o/oo per 1 o/oo change in Corg
Carbonate effect	-0.013 o/oo per 1 µmol/kg CO3-2
Temperature effect	-0.013 o/oo per 1ºC increase
Size effect	Ignored - use uniform sized tests
B (total for all effects)	2.8 (empirical)

Application of Eq NA-14 to core top data from the South Atlantic brings the foram ¹³C data into good agreement with analyses of dissolved C_T (Figure NA-9).

Figure NA-9. Effect on uncorrected ¹³C values from N. pachyderma (purple) of applying corrections for carbonate ion (blue), carbonate ion plus diet (green) and carbonate ion plus diet plus temperature (red). The final values agree well with modern measurements for the upper ocean (< 200m) corrected for anthropogenic input.

Uncorrected glacial values are lower than surface values (Figure NA-10A) as discussed earlier. However, when corrections based on the best estimates of glacial diet, carbonate, and temperature are applied, the corrected glacial values are higher than the modern values(Figure NA-10B), in agreement with the Cd/Ca data. Because the corrections are 4 to 8 times the glacial-interglacial differences, PO₄^-3 values derived from ¹³C data are more uncertain than those based on Cd/Ca ratios.

Figure NA-10. A. ¹³C data for core top (orange) and last glacial maximum (blue) foram calcite (N. pachyderma) compared to predicted preindustrial and modern upper water column (< 200m) values for the South Atlantic. B. Last glacial maximum foram ¹³C values before (blue) and after (green and yellow) corrections for diet, temperature, and dissolved carbonate changes (two models), compared to upper water column values for the South Atlantic. 

The apparent discrepancy between Cd/Ca and ¹³C estimates of Southern Ocean glacial nutrient values led to a number of imaginative explanations involving changes in air-sea exchange and deep water circulation. While these are no longer necessary, there is additional evidence of changes in deep circulation during glacial times (e.g. Figure NA-11). Because of the lack of biogenic carbonate (and, hence, Cd/Ca or ¹³C data) in the deep Pacific, hypotheses about the changes that involve the formation of Pacific deep or bottom water are currently untestable.

Figure NA-11. Estimated bottom water PO₄^-3 values between 2,400 and 3,800 m for the modern and glacial oceans. Red - greater than 1.9 µmol/kg; green - less than 1.5 µmol/kg. Note the sparse sample coverage for most of the Pacific.