It was a reason, but the main reason was slavery. Says so, right in the articles of secession put forward by the Confederacy.
Actually, the main argument for slavery can be found in the speeches and public letters of deep southerners who were appointed as commissioners to try and convince the upper south to secede by appealing to the audiences' feelings of white supremacy and the preservation of slavery. A good source for these can be found in Apostles of Disunion: Southern Secession Commissioners and the Causes of the Civil War by Charles Dew.
There's many other arguments to be found that it wasn't the main reason, though. If you can, I suggest you read "Rethinking the secession of the lower south: The Clash of Two groups" by Marc Egnal. He argues that economic differences in the north and south caused it.
The tariff issue is harder to argue, but I managed to do it once. It hinges on the fact that the south and north were splitting from 1920 on and tariffs contributed to this split. Required some obscure sources, but its definitely doable. I'll list my sources for anyone wanting to read more about it.
Beard, Charles. History of the United States. New York: The MacMillan Company, 1921.
Mark Thornton and Robert B. Ekelund, Jr. Tariffs, Blockades, and Inflation - The Economics of the Civil War. SR (Scholarly Resources) Books, 2004.
Remini, Robert V. Andrew Jackson (V.3). Twayne Publishers, 1966.
—. Henry Clay: Statesman for the Union. New York: W. W. Norton & Company, 1993.
Stampp, Kenneth M. America in 1857: A Nation on the Brink. New York: Oxford University Press, 1990.
Stanwood, Edward. American Tariff Controversies in the Ninteenth Century. Boston: The Riverside Press, 1904.
Taussig, Frank William. The Tariff History of the United States: A Series of Essays. New York: The Knickerbocker Press, 1888.
Taylor, George Rogers. The Great Tariff Debate, 1820-1830. Boston: D C Heath & Co, 1953.
Taussig's 1888 essays being the most important.
