It doesn’t say what type of artillery this was, but I’d assume based on the number of rounds that it was anti-aircraft fire.
North Korea fired about 30 artillery rounds into the sea from its western coast and the South immediately responded with 100 shots from a marine base on an island near the sea border, an officer at the Joint Chiefs of Staff in Seoul said. The North said it would continue to fire rounds.
N. Korea exchanges fire with neighbor – North Korea- msnbc.com
Dysthymia 
50 more rounds in the afternoon: http://www.khaleejtimes.com/DisplayArticle08.asp?xfile=data/international/2010/January/international_January1434.xml§ion=international