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# Seasonal Carbon Cycling in the Sargasso Sea Near Bermuda

Nicolas Gruber
Charles D. Keeling
Edition: 1
Pages: 104
https://www.jstor.org/stable/10.1525/j.ctt1pnct7

1. Front Matter
(pp. i-iv)
(pp. v-vi)
3. ABSTRACT
(pp. vii-vii)
4. ACKNOWLEDGMENTS
(pp. viii-viii)
5. 1. INTRODUCTION
(pp. 1-2)

Physical and biogeochemical processes act together in controlling the carbon balance in the upper ocean. The international oceanic sciences community recently made remarkable progress toward establishing details of this marine carbon cycle based on results from the ongoing Joint Global Ocean Flux Study time series stations in the subtropical North Atlantic (Bermuda Atlantic Time-series Study, near Bermuda) and the subtropical North Pacific (Hawaii Ocean Time-Series, near Hawaii) (Michaels et al. 1994a, Michaels et al. 1994b, Winn et al. 1994, Karl and Lucas 1996, Michaels and Knap 1996). However, underlying problems, such as the extent of biological production and of nutrient...

6. 2. PROCESSES CONTROLLING THE CARBON BALANCE IN THE UPPER OCEAN
(pp. 3-8)

We begin by discussing the processes of both biological and physicochemical origin that influence the balance of DIC in the upper ocean with special emphasis on their effects on the stable isotopic ratio of DIC, expressed by the reduced ratio:

${13_{\rm{\delta }}}{\rm{ = }}\frac{{r - {r_s}}}{r}$, (1)

where r denotes the13C/12C ratio of the sample andrsthe13C/12C ratio of the belemnite carbonate standard, PDB. Because the values of 13δ are small, they are expressed in per mil (%o). Here and subsequently, DIC denotes the analytical sum of all inorganic dissolved carbon species, i.e. [CO2]aq+ [H2C03] + [HCO3-] + [C032-] and all...

7. 3. CONSTRAINING CARBON BUDGETS BY CONCURRENT MEASUREMENTS OF DIC AND 13δ
(pp. 9-10)

To demonstrate how concurrent measurements of DIC and its isotopic ratio 13δ constrain carbon budgets we now develop the following budget equations to be used in our later analysis.

Let △DICand △13δ denote the changes in DIC and 13δ, respectively, in a constant volume of water over a time-interval △t, which begins att-1 and ends att. Expressing these changes as sums of changes, △DICiand △13δi, owing tojseparately identified processesi=l...j:

DICDICt-DICt-1$= \sum\limits_{i = 1}^j {\vartriangle {\text{DI}}{{\text{C}}_i}}$, (3)

13δ ≡ 13δt- 13δt-1. (4)

To connect the overall isotopic change, △13δ with the individual isotopic...

8. 4. SEASONAL OBSERVATIONS
(pp. 11-12)

The data utilized in this seasonal study are derived from water samples collected at Station S near the northern edge of the Sargasso Sea. Station S is located at 32° 10’ N, 64° 30’ W, about 21 krn southeast of Bermuda within the recirculation region of the North Atlantic anticyclonic subtropical gyre (Worthington 1976, 93). It has been occupied about every two to four weeks over the last 40 years (Jenkins and Goldman 1985, 467). Surface water normally flows into the area from the northeast (Worthington 1976). Horizontal velocities are small, however, according to observations (Michaels and Knap 1996) and...

9. 5. HARMONIC FITING
(pp. 13-14)

A harmonic function,

$H = \sum\limits_{k = 1}^m {[{a_k}} \sin (2\pi kt) + {b_k}\cos (2\pi kt)] + {H_o}$(7)

was fitted by the method of least squares through the composited data of temperature,Toc, sDIC, 13δoc, pCO2oc, and mixed-layer depth, MLD, as a function of time of the year. In equation (7), a,, b, and Ho represent constants that differ for each parameter while t denotes the time expressed in years. Harmonics with periods of 12 months and 6 months (m = 2) were used, except for MLD for which a 4 month harmonic (m = 3) was also included in the fit. The obtained parameters of all fits, including the coefficient of variation, R2,...

10. 6. DESCRIPTION OF THE SEASONAL MODEL
(pp. 15-22)

We have chosen a three-box model, presented schematically in Figure 6, to represent the seasonal cycle of carbon in the ocean near Bermuda. The middle box represents the mixed layer (ml) which lies in direct contact with the air-sea interface. It is overlain by an atmospheric box (atm) and directly underlain by a box representing the water in the “submixed layer” (Ib, for “layer below”). The upper-and lower-most boxes are of indeterminate size. They are involved in the model only to establish boundary conditions for the mixed layer. The boundary between the two oceanic boxes moves up and down as...

11. 7. RESULTS OF THE SEASONAL MODEL
(pp. 23-26)

The rates of change in the salinity-normalized DIC concentration calculated by the model over the course of the annual cycle, are shown in Figure 9 individually for gas exchange, vertical turbulent diffusive transport, vertical entrainment, and net biological exchange.

Substantial variations in rates are exhibited by gas exchange and biological processes during most of the year. Somewhat smaller variations are shown by vertical diffusion, whereas entrainment causes only minor variations.

The variations in gas-exchange rate mainly reflect the C02pressure gradient at the airsea boundary. During the cooler part of the year, when the C02partial pressure in the water...

12. 8. DISCUSSION
(pp. 27-34)

The seasonally varying fluxes of carbon predicted for the waters near Bermuda by our model are all influenced to some degree by uncertainties in the relationships used for the calculations. After identifying uncertainties in estimating each flux, we challenge the plausibilities of these relationships by means of sensitivity tests. The results of these tests are summarized in Table 6 with respect to the computed net community production in the mixed layer, expressed in our model by the net biological exchange flux,Fbio, and with respect to the computed seasonal variations of salinity-normalized DIC,sDICcalc, in Figure 14. Also shown in...

13. 9. SUMMARY AND CONCLUSIONS
(pp. 35-38)

The seasonal cycle of DIC in the surface waters near Bermuda is only partially a result of biological processes. To determine the rate at which DIC is converted to organic carbon via photosynthesis and is regenerated by oxidation of organic matter, observed changes in DIC must first be corrected to account for transport processes that alter the DIC concentration, including air-sea exchange, vertical diffusion, and entrainment.

We have estimated in situ biological exchange (defined as positive when DIC increases) in two ways; first from changes in the 13C/l2Cratio of DIC that are caused by a large difference in isotopic...

14. REFERENCES
(pp. 39-48)
15. APPENDIX A: FORMULAS FOR THE SEASONAL MODEL
(pp. 49-56)
16. APPENDIX B: THREE-DIMENSIONAL GLOBAL OCEAN TRACER TRANSPORT MODEL OF BACASTOW AND MAIER-REIMER (1991)
(pp. 57-58)
17. APPENDIX C: SENSITIVITY TESTS
(pp. 59-62)
18. TABLES
(pp. 63-79)
19. FIGURES
(pp. 80-96)
20. Back Matter
(pp. 97-97)