What is the weight of a southern pine log with an average diameter of 12 inches and a length of 40 feet?

The weight depends a lot on moisture content, there are equations to determine the weight but can only be general in content.

The standard measurement unit for wood used for paper and fuel is the cord . This is a stack of wood 4 ft. x 4 ft. x 8 ft. containing approximately 128 cubic feet of bark, wood and air space. Air space can actually be as high as 40 percent but usually averages 25 percent. You can see where weight can be advantageous here. Pulpwood purchases by weight are very common and weight per cord varies widely with species and geography. A hardwood pulpwood cord generally weighs between 5,400 pounds and 6,075 pounds. A pine pulpwood cord weighs between 4,700 pounds and 5,550 pounds. You really need to determine your local average weight by species when measuring cordwood. * http://forestry.about.com/cs/forestvaluation/a/wood_vol.htm