The "G" feature? How about this...
Possibly a 'spill' from someone carrying a bunch of collected white rubble. If someone was carrying a stash of white coral rubble (for whatever reason), then if the device they were using (bag, box, shirt, etc) to carry it ripped, broke open, lost balance, then the contents would 'spill' out creating the unusual pattern you see here. If the person rotated, spun-around trying to avoid and catch the spill, then the pattern would take on a more spiral orbit such as seen with the "G". The shape could very well have been any other elliptically based spiral orbit such an "S", an "O", a "C" or any other letter loosley resembling the shape of something spilling as you were turning to try and catch it. Just so happens the "G" was what karma had for that particular spill. The larger 'spot' within the shape of the "G" could simply be where the spill began or where it ended - causing a larger concentration of the contents to be there.
I doubt the "G" feature is anything done intentionally, but rather an accident (of sorts) from some other type of activity related to white rubble usage.
Which makes me want to think about the white arrowhead...
If in fact the arrowhead was a feature of very thin layer of white rubble, used to mark for aerial identification, then the thin color layer of 'white' would soon disappear to the sun and other elements, making it nearly un-identifiable years later. Since the original layer of white rubble would have to be 'harvested' from another location such as the lagoon and/or beach, then it would also have to be carried from that location. Thus, perhaps an explanation for the white "G" feature, from a spill while trying to create the arrowhead. Aren't the 'trails' seen in some aerial photos and maps located in that general vicinity?
Crazy thinking... possible explanation... thoughts?