Heat flow measurements in the Scotia Sea collected during Atlantis I ...

Summary of data from a series of flume experiments Size range Sediment component

(0

Clay and fine silt Fine silt Clay and fine silt Coarse silt

7.00-10.00 6.25-7.00 6.25-10.00 4.25-6.00

Fine sand

3.50-4.00 3.00-3.50 2.00-3.00

Medium sand

1.00-2.00 -.50-1.00

Coarse sand

-1.00--.50

Minimum velocity for entrainment (cm/sec)

Mode of transport

1.5 7.5 1.5 2.0

suspension suspension i. suspension mt. suspension

5.0 9.0 13.0 11.0 16.0 13.0 18.0 13.0 > 20.0 16.0

mt. suspension traction Int. suspension traction mt. suspension traction mt. suspension traction mt. suspension traction

a intermiftent suspension.

Dearborn, J . H. 1977. Foods and feeding characteristics of antarctic asteroids and ophiuroids. In C. A. Llano (Ed.), Adaptations within antarctic ecosystems. Houston, Tex.: Gulf Publishing. El-Sayed, S. Z. 1971. Dynamics of trophic relations in the southern ocean. In L. 0. Quam (Ed.), Research in the Antarctic. Baltimore, Md.: King Printing. Huang, T. C., Ledbetter, M. T., and Watkins, N. D. 1982. Contrasts in AABW velocity between the Brunhes and Matuyama Epochs in the southern Pacific. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Ledbetter, M. 11979. Fluctuations of AABW velocity in the Verna Channel during the last 160,000 years. Marine Geology, 33, 71-89. Ledbetter, M. T., and Ellwood, B. B. 1982. Variations in particle align-

ment and size in sediments of the Verna Channel record AABW velocity changes during last 400,000 years. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Nowell, A. R. M., Jumars, P. A., and Eckman, J . E. 1981. Effects of biological activity on the entrainment of marine sediment. Marine Geology, 42, 133-153. Rhoads, D. C., and Young, D. K. 1970. The influence of deposit-feeding organisms on sediment stability and community trophic structure. Journal of Marine Research, 28(2), 150-178. Yingst, J . Y., and Rhoads, D. C. 1978. Seafloor stability in central Long Island Sound: P2, Biological interactions and their potential importance for seafloor erodibility. In M. Wiley (Ed.), Estuarine interactions. New York: Academic Press.

Heat flow measurements in the Scotia Sea collected during Atlantis I! cruise 107-6

Cruise 107, leg 6, of the 'v Atlantis II left Punta Arenas, Chile, on 4 March 1980 and arrived at Cape Town, South Africa, on 7 April 1980. Principal shipboard programs included rock dredging under the direction of H. J . B. Dick, chief scientist, sediment coring under the direction of B. Corliss (Woods Hole Oceanographic Institution), heat flow measurements in the Scotia Sea under the direction of V. Zlotnicki and W. Loy, and underway geophysical data collection (12- and 3.5-kilohertz echo-sounding, air-gun profiling, magnetics, and gravity). The east Scotia Sea is an actively spreading marginal sea that lies behind the active South Sandwich Islands arc (Barker 1972). Oceanic crust of the South American plate is being subducted in a westward direction beneath the South Sandwich arc and the east Scotia Sea at the South Sandwich Trench. Barker (1972) identified magnetic anomalies generated by seafloor spreading in the east Scotia Sea, thus allowing estimation of the age of the ocean floor. Heat flow measurements can be used to understand the thermal regime of the oceanic crust and the influence of the cool subducted slab on active seafloor spreading in a marginal basin only if the age of the underlying oceanic crust is known. The heat flow program was aimed at correlating heat flow and age in this marginal basin and comparing these relationships with those found for normal oceanic crust. On the Atlantis 11107-6 cruise, a 300-kilometer survey was run prior to the heat flow investigation. This survey produced

VICTOR ZLOTNICKI and WALTER Loy Joint Program in Oceanography Massachusetts Institute of Technology

and Woods Hole Oceanographic Institution Woods Hole, Massachusetts 02543

LAWRENCE A. LAWyER and JOHN G. SCLATER Department of Earth and Planetary Sciences Massachusetts Institute of Technology Cambridge, Massachusetts 02139

HENRY J .

B. DICK and

RICHARD P. VON HERzEN

Department of Geology and Geophysics Woods Hole Oceanographic Institution Woods Hole, Massachusetts 02543

1982 REVIEW

119



Heat flow values in the South Atlantic Station number 1 2 3 4 5 6 7 8 9

Position

Depth (meters)

Heat flow Conductivity Gradient (mcal/°C cm)a (m0C/cm)' (.i.cal/cm2 sec)c

56013.20'S 32021.3'W 56021.5'S 32°44.7W 5618.92'S 32©46.88W 56016.81'S 32°45.05'W 56°17.62'S 3238.86'W

3,262

1.79

N.D.d

N.D.

3,174

1.69

N. D.

N.D.

3,124

1.56

N.D.

N. D.

3,116

1.69

N.D.

N. D.

3,223

1.68

1.07

1.80

56°17.38'S 32041.29'W 5616.94'S 32038.41'W 55016.5'S 2209.7'W 54050.5S 17058.5'W

3,273

2.03

0.69

1.40

3,260

1.66

0.15

0.25

4,050

1.71

1.07

1.83

4,140

1.59

0.94

1.49

a mcal/C cm = microcalories per ocentrigrade-centimeter. b mC/cm = milli-centigrade per centimeter. c cal/cm2 sec = microcalories per centimeter squared-second. d N . D . = no data.

excellent magnetic anomaly lineations, alined nearly northsouth, as expected, from the westward subduction of oceanic crust beneath the Sandwich arc. These anomalies can be traced on seven crossings over a distance of 40 kilometers, and they are readily matched to those identified by Barker (1972). The inferred spreading rate in the east Scotia Sea varies between 5 to 6 centimeters per year 4 million years ago and 7 to 9 centimeters per year at present. The survey had been planned for an area believed to have an adequate sediment cover, a necessary condition for reliable heat flow measurements. However, ice conditions forced us to work north of the desired area. All the heat flow stations had to be taken in an area of isolated sediment ponds surrounded by unsedimented basement highs. This situation usually leads to low heat flow readings, both because of refraction and, more importantly, because the thin sediment cover does not prevent convective heat flux through the young oceanic crust (Williams et al. 1974). For normal oceanic crust of an age of 3-4 million years, the conductive heat flow is expected to be 5.6-6.8 heat flow units (HFu). The values measured at the three stations in the east Scotia Sea where measurements were obtained were much lower, between 0.25 and 1.8 HFU, due to this environmental effect (see table). Four other stations were unsuccessful due to instrument failure. Two additional heat flow values were taken on this cruise outside the east Scotia Sea. Station 8 was located just east of the South Sandwich Trench on the South American Plate. A value of 1.83 HFIJ was found, which is slightly high if the crust was formed at the mid-Atlantic ridge approximately 65 million years

120

ago. The last station produced a value of 1.49 HFU, which is within 10 percent of what would be expected on crust 50 million years old. The results of this cruise proved that heat flow measurements can be made in marginal basins on crust of a known age. The eastern side of the active spreading center had a much more uniform sediment cover on crust of approximately the same age. A subsequent cruise (Lawyer et al. 1981) took measurements there and found substantially higher heat flow values on crust of the same age. This proved that the low heat flow values measured on the Atlantis 11107-6 cruise in the east Scotia Sea are a result of extensive hydrothermal circulation in inadequately sedimented crust and are not due to any processes peculiar to marginal basins. This cruise was supported by National Science Foundation grant DPP 78-19279 to the Massachusetts Institute of Technology.

References Barker, P. F. 1972. A spreading centre in the east Scotia Sea. Earth and Planetary Science Letters, 15, 123-132.

Lawyer, L. A., Loy, W., Sciater, J . G., and Von Herzen, R. 1981. Heat flow in the east Scotia Sea. Antarctic Journal of the U.S., 16(5), 106-107. Williams, D. L., Von Herzen, R. P., Sclater, J . G., and Anderson, R. N. 1974. The Galapagos spreading center: Lithospheric cooling and hydrothermal circulation. Geophysical Journal of the Royal Astronomical Society, 38, 587-608.

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