The role of equatorial juvenile arcs in the Middle Triassic seawater 87Sr/86Sr decline during the Pangean “quiet” period

The Middle Triassic (246−235 Ma) seawater 87Sr/86Sr record exhibits an excursion following the recovery from Siberian large igneous province volcanism. Two primary mechanisms have been proposed to explain the isotopic shift: (1) a change in the 87Sr/86Sr ratio of the continental weathering flux, or (2) a change in the flux magnitude itself. We demonstrate that the excursion is best explained by the former: an influx of low-87Sr/86Sr material derived from equatorial juvenile arc terranes along the Sundaland margin of Pangea. These terranes, characterized with 87Sr/86Sr = 0.7040−0.7078, have been largely overlooked in previous models of Triassic seawater Sr cycling. Using a quantitative flux-ratio model constrained by steady-state Sr fluxes during a period of Pangean equilibrium, we show that the observed excursion requires an ∼4 × 10−4 decrease in the isotopic ratio of the global continental weathering flux. Attributing this shift solely to the Sundaland margin implies that the region contributed ∼10% of the global continental Sr flux—a value consistent with independent paleoclimate simulations. Our results show that intense tropical weathering of rapidly exhumed, juvenile volcanic arcs (rather than an unlikely alternative, given independent proxy constraints on global fluxes) drove the 87Sr/86Sr decline. This highlights the role of climatically amplified, geochemically distinct terranes in modulating global seawater isotopic signatures during periods of relative tectonic quiescence, challenging the long-held view of Pangea as a uniformly radiogenic Sr source.