Sand dunes and other active sands generally have a low content of fine grains and, therefore, are not considered to be major dust sources in climate models. However, recent remote sensing studies have indicated that a surprisingly large fraction of dust storms are generated from regions covered by sand dunes, leading these studies to propose that sand dunes might be globally-relevant sources of dust. To help understand the dust emission potential of sand dunes and other active sands, we present in situ field measurements of dust emission under natural saltation from a coastal sand sheet at Oceano Dunes in California. We find that saltation drives substantial dust emissions from this setting. Laboratory analyses of sand samples suggest that these emissions are produced by aeolian abrasion of feldspars and removal of coatings of clay minerals on sand grains. We further find that this emitted dust is substantially finer than dust emitted from non-sandy soils and dust observed in situ over North Africa. As such, dust emitted from the sand sheet, and potentially from other active sands affected by similar dust emission processes, could have potent impacts on climate, the hydrological cycle, and human health. These measurements thus support the hypothesis that considerable emissions of fine dust can be generated by the reactivation of inactive dunes with accumulated clay minerals. This might occur due to future land-use changes and desertification, and is not currently represented in most climate models.