You can easily create your own method to generate a sequence representing a column given that input:
public static IEnumerable<T> GetColumn<T>(
this IEnumerable<IList<T>> data,
int columnNumber)
{
return data.Select(row => row[columnNumber]);
}
Now you can write code like:
var firstColumnSum = veryLargeJaggedArray.GetColumn(0).Sum();
var secondColumnAverage = veryLargeJaggedArray.GetColumn(1).Average();