怎么样使用CSS和bootstrap图标制作上下跳动的指示箭头动画效果？

<a href="#" class="scroll-down">
 <span> <i class="glyphicon glyphicon-chevron-down"></i> </span>
</a>

/*向下滑动的动画效果*/
@-webkit-keyframes drop {
 0%  { top:0px; opacity: 0;}
 30% { top:10px; opacity: 1;}
 100% { top:25px; opacity: 0;}
}
@keyframes drop {
 0%  { top:0px; opacity: 0;}
 30% { top:10px; opacity: 1;}
 100% { top:25px; opacity: 0;}
}
/*应用动画，添加按钮效果*/
.scroll-down {
  border: 2px solid #bbb;
  border-radius: 50%;
  margin: 0 auto;
  height: 50px;
  width: 50px;
  display: block;
  text-align: center;
  z-index: 10;
  -webkit-transition: all 0.125s ease-in-out 0s;
  -moz-transition: all 0.125s ease-in-out 0s;
  -ms-transition: all 0.125s ease-in-out 0s;
  -o-transition: all 0.125s ease-in-out 0s;
  transition: all 0.125s ease-in-out 0s;
}
.scroll-down span {
  position: relative;
  color: #bbb;
  font-size: 24px;
  -webkit-animation-name: drop;
  -webkit-animation-duration: 1s;
  -webkit-animation-timing-function: linear;
  -webkit-animation-delay: 0s;
  -webkit-animation-iteration-count: infinite;
  -webkit-animation-play-state: running;
  animation-name: drop;
  animation-duration: 1s;
  animation-timing-function: linear;
  animation-delay: 0s;
  animation-iteration-count: infinite;
  animation-play-state: running;
}