letters to nature
NATURE | VOL 408 | 23 NOVEMBER 2000 | www.nature.com 453
of 3D periodic mesostructured materials without assuming any
structural models. The resolution for the structure is primarily
limited by the quality of the HREM images, which depends on the
long-range mesoscale ordering. Therefore, although further pro-
gress may give better resolution, we expect no future change to the
present conclusions about the structures of SBA-1, SBA-6 and SBA-
16, because the validity of the solutions does not depend on the
resolution. This is a characteristic of our method that makes it
different from other approaches. We also suggest that the results
presented here provide a quantitative topological description of
ordered mesostructured composites, and that such descriptions are
essential in understanding the properties and possible applications
of the composites. The resolution of periodically ordered, 3D
arrangements of bimodal (meso-micro) pores in SBA-1 and SBA-
6 makes it possible to consider the detailed characterization of the
range of complicated porous phases that are now synthetically
achievable. M
Methods
Synthesis of SBA-6
3.75 g of tetraethoxysilane (TEOS) was added with magnetic stirring to a clear solution
containing 0.5 g of the gemini surfactant 18B
4-3±1
(N,N,N,N9N9-pentamethyl-N9-
[4-(4-octadecyloxyphenoxy)-butyl]-propane-1,3-diammonium dibromide,
C
18
H
37
OC
6
H
4
OC
4
H
8
N(CH
3
)
2
C
3
H
6
N(CH
3
)
3
Br
2
), 45.4 g of doubly distilled water, and
3.69 g of benzyltrimethylammonium hydroxide at room temperature. Stirring was
continued for 20 h after the addition of TEOS at room temperature. The reaction gel
mixture was heated for 2 d at 80 8C without stirring. The precipitate was ®ltered and dried
in air at room temperature.
Determination of properties
Ar adsorption and desorption isotherms were measured at 87 K. Pore volumes (cm
3
g
-1
)
for SBA-1, SBA-6 and SBA-16 are 0.6, 0.86 and 0.45, respectively, and the ratios of the pore
volume to unit cell are respectively 0.57, 0.65 and 0.47. The surface-area/pore-volume ratio
(2.26 ´ 10
9
m
-1
) for SBA-1 is nearly three times that of SBA-6 (7.93 ´ 10
8
m
-1
). The silica
wall densities determined with an AccPyc 1300 helium pycnometer are also substantially
different for SBA-1 (2.00 g cm
-3
) and SBA-6 (2.20 g cm
-3
).
Received 23 May; accepted 6 October 2000.
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Ê
ngstrom
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Supplementary Information is available on Nature's World-Wide Web site
(http://www.nature.com) or as paper copy from the London editorial of®ce of Nature.
Acknowledgements
This work was supported in part by CREST, Japan Science and Technology Corporation
(O.T.), by the National Research Laboratory Program of Korea (R.R.), and by the National
Science Foundation (G.D.S.) and the Army Research Of®ce (G.D.S.). O.T. thanks
S. Andersson for encouragement and support. Y.S. thanks the Japan Society for the
Promotion of Science.
Correspondence and requests for materials should be addressed to O.M.
(e-mail: terasaki@msp.phys.tohoku.ac.jp) or R.R. (e-mail: r.ryoo@mail.kaist.ac.kr).
.................................................................
Improved estimates of global ocean
circulation,heat transportandmixing
from hydrographic data
Alexandre Ganachaud* & Carl Wunsch
MIT 54-1517, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
..............................................................................................................................................
Through its ability to transport large amounts of heat, fresh water
and nutrients, the ocean is an essential regulator of climate
1,2
. The
pathways and mechanisms of this transport and its stability are
critical issues in understanding the present state of climate and
the possibilities of future changes. Recently, global high-quality
hydrographic data have been gathered in the World Ocean
Circulation Experiment (WOCE), to obtain an accurate picture
of the present circulation. Here we combine the new data from
high-resolution trans-oceanic sections and current meters with
climatological wind ®elds, biogeochemical balances and
improved a priori error estimates in an inverse model, to improve
estimates of the global circulation and heat ¯uxes. Our solution
resolves globally vertical mixing across surfaces of equal density,
with coef®cients in the range …3±12† 3 10
2 4
m
2
s
2 1
. Net deep-
water production rates amount to …15 6 12† 3 10
6
m
3
s
2 1
in the
North Atlantic Ocean and …21 6 6† 3 10
6
m
3
s
2 1
in the Southern
Ocean. Our estimates provide a new reference state for future
climate studies with rigorous estimates of the uncertainties.
Obtaining a consistent picture of the oceanic circulation requires
adjusting thousands of parameters consistently with a priori error
estimates. We present here our best estimate from selected hydro-
graphic data (Fig. 1), which will improve with the appearance of
new data. Mass ¯ux is the most basic element of the circulation and
Fig. 2 shows the best-estimate coast-to-coast integrated water mass
transports for selected density classes. A volume of 15 6 2 Sv
(1 sverdrup ˆ 1 3 10
6
m
3
s
2 1
) of North Atlantic Deep Water
(NADW) is produced in the northern North Atlantic Ocean and
moves southward, entraining Antarctic Bottom Water (AABW)
from below, and Antarctic Intermediate Water (AAIW) from
above. As a result, the NADW is increased to 23 6 3 Sv as it exits
the South Atlantic at 308 S. In the Southern Ocean, a total of
21 6 6 Sv of bottom water is formed from lower Circumpolar
Deep Water (CDW)Ðwhich corresponds approximately to the
lower NADW density range. Bottom water in¯ows
(NADW ‡ AABW mixture) to the Atlantic, Indian and Paci®c
oceans are 6 6 1:3 Sv, 11 6 4 Sv and 7 6 2 Sv, respectively. In the
Indian and Paci®c oceans, most of this water returns southward at
deep and intermediate levels. These net values are the sums of large,
strongly spatially varying, ¯ows of opposing sign, and thus over-
simplify the actual circulation; a detailed description of the circula-
tion within each ocean basin will be published elsewhere
3,4
. Our
standard model estimate of the in¯ow in the South Paci®c Ocean is
in the lower range of previously published values, but it depends
directly upon the weight given to the `` PO'' phosphate±oxygen
combination (see Methods
4,5
) conservation constraints relative to
mass conservation
3
. The deep in¯ow to the North Paci®c Ocean is
also weaker than previously found
5
, as a consequence of our
consideration of heat and salt conservation in the northern parts
of those basins.
No de®nition of bottom-water formation can be completely
unambiguous because of the entrainment of ambient ¯uid during
the sinking process. In our Southern Ocean de®nition, the bottom-
* Pressent address: Laboratoire de Physique des Oce
Â
ans, IFREMER, 29280 Plouzane
Â
, France.
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