High Be-10 contents in tektites reported in literature are taken as evidence of a source material, melted at the impact site, enriched in atmospheric Be-10; i.e., a soil or sediment. In 0.8 Ma Australasian tektites, Be-10 content increases with distance from the putative impact location in Indochina, with geographic averages from 69 x 10(6) atoms/g (Indochina) to 136 x 10(6) atoms/g (Australia). Here we report, for the first time, Be-10 contents in microtektites collected from Antarctica and the South China Sea. We show that microtektites are similar to 30 x 10(6) atoms/g richer in Be-10 than tektites from the same geographic areas. Antarctic microtektites, with an average Be-10 content of 184 x 10(6) atoms/g after correction for in situ production, are the richest impact glass ever measured. The simplest explanation for such systematic size and geographic trends is that the source depth of the melt within the target surface decreases with ejection velocity. Indeed, higher initial kinetic energy implies higher launch distances and higher fragmentation of the ejecta. Antarctic microtektite source depth may tentatively be restricted to the upper tens of centimeters at the impact site. Alternative models invoking a marine or loessic sediment source, or a secondary enrichment in the microtektite (either by atmospheric scavenging, selective fractionation by volatilization, or post-depositional contamination) fail to reproduce the observed relationships.