Complex sentence: He subsequently led colonization efforts on Kitadaitōjima and Okidaitōjima, which also had substantial deposits of guano, which was mined for phosphate-based fertilizer and gunpowder.
Split into simpler sentences: He subsequently led colonization efforts on Kitadaitōjima and Okidaitōjima. Those two islands had substantial deposits of guano, which was mined for phosphate-based fertilizer and gunpowder.