Found it, here you go.
"Brave New World also brings in trade caravans, special units that set up trade routes with Civs and city states. These dont supplant trade routes; caravans also a second way to bring in cash. And as other Civs grow, the amount of gold that comes in builds up. In the modern era, I was getting significant amounts. I was able to establish new trade caravans with city states when the gosh-darned
Zulus passed the trade embargo, which shut down my trade caravans with them (
oddly enough, I could still trade luxury and strategic resources through the Diplomacy screen), and establish a new revenue stream."
SOURCE:
http://bit.ly/17H0X5G