Paleochemistry of manganese in corals from the Galapagos Islands

Abstract

Approximately 550 measurements of Mn/Ca ratios in three corals from the western Galapagos Islands have been performed to reconstruct a 380-year history of surface ocean variability with respect to this trace element. The time period studied encompasses 1600 A.D. to 1978. Manganese is inferred to be lattice-bound in coralline aragonite at 10–50% of its seawater proportion to calcium; uncertainty about the distribution coefficient stems from inherent variability of oceanic Mn in nearshore settings. Interannual variations at Urvina Bay, Isabela Island are generally small, with the exception of a few decades during the nineteenth century. A large positive Mn/Ca anomaly found between 1821–1830 is hypothesized to have resulted from a major volcanic eruption on nearby Fernandina Island in 1825. On intrannual timescales a pronounced cycle occurs in response to seasonal upwelling. Quarterly changes in Mn/Ca are six months out-of-phase with Cd/Ca variations-a reflection of the opposite distributions of these metals in the upper waters of the eastern Pacific. High frequency reconstructions over brief time intervals from the 17th, 18th, and 20th century reveal that the seasonal onset of warm and cool phases near Galapagos has persisted for at least 340 years. A quantitative assessment of historical changes in upwelling intensity is complicated by offsets in background Mn levels recorded by different corals. One apparent longterm feature is an overall decline in skeletal Mn concentrations from 1600–1978 which results in a net decrease of 20–30%. Several possible explanations exist for this trend, ranging from accumulation of a persistent diagenetic Mn phase in fossil aragonite to a temporal shift in oceanic/atmospheric Mn fluxes reaching the surface waters of the Galapagos Islands.