In the Cox Bay chronosequence, Si in the “unweathered” clay fraction (BC horizon; δ30Si = −0.52 ± 0.16‰, 2SD, n = 3) is isotopically lighter compared to the primary lithogenic minerals in the parent beach sand material (C material; δ30Si = −0.27 ± 0.10‰, 2SD, n = 3) (Fig. 4A).