What's the default value for a std::atomic?

I find that in practice, with a variety of C++11/C++14 compilers, a std::atomic has an undefined initial value just as it would if it were a "raw" type. That is, we expect that for the expression

int a;

a may have any value. It also turns out to be true that for the expression

std::atomic< int > b;

b may also have any value. To say it another way,

std::atomic< int > b;         // b is undefined

is not equivalent to

std::atomic< int > b{ 0 };    // b == 0

or to

std::atomic< int > b{};       // b == 0

because in the latter two cases b is initialized to a known value.

My question is simple: where in the C++11 or C++14 spec is this behavior documented?