For some reason, almost no accounts of the exploration and expansion into Greenland and Vinland consider the importance of walrus ivory at the time. The trade from the Norwegian expansion areas in stockfish was brisk but the international 'gold standard' in trading was ivory. Any account of the exports of the Norse includes walrus ivory, but there is little information on where it was being harvested from. It could be traded to anyone in Europe for any goods and the value went up the further you went. Narwal ivory was equally, if not more valuable because of it's resemblance to a unicorn's horn, but walrus was the real prey and driving force. (The word 'narwhal' comes from the Old Norse náhvalr meaning 'corpse whale' since its white colour resembles that of a floating corpse!)
By the end of the 900s, Walrus stocks around Iceland were already seriously depleted, and hunters had to range futher and further asea for their prey.(What, they already depleted the stocks you say? Nah! Well, they also managed to raze the island almost bare in a hundred years too. It was forested when they arrived.) Gunnbjorn Olfsson was the one who told Eric the Red of some islands that lay west of Iceland. Why was he there? Looking for better hunting grounds.
Traditional Inuit accounts suggest that the Norse in Greenland may have been as eager to trade as to fight, and given the economic realities of mediaeval Greenland, this would seem likely. Metal was in short supply for the Greenlandic Norse as it had to be imported from Europe, as did their requirements of grain, timber and luxury goods. Payments for these imports were made in skins of walrus, polar bear and other animals, but primarily in walrus and narwhal ivory. In order to acquire the vast amounts of walrus ivory that records show were traded for European imports, the Norse had to travel 500 kilometres north of their settlements to the hunting grounds where walrus were numerous.
For more on this angle of history, you should check out Farley Mowat's "The Farfarers". Biased though he is to his own theories, it's a great book and well researched and written.

websites of interest I came across while trying to dig up more on period walrus hunting-
http://www.regia.org/bonework.htm
http://www.canadahistory.com/sections/eras/Firstcontact/graenlendingasaga.htm