The influence of water mass mixing on the dissolved Si isotope composition in the Eastern Equatorial Pacific

Abstract

Silicon isotopes are a powerful tool to investigate the cycling of dissolved silicon (Si). In this study the distribution of the Si isotope composition of dissolved silicic acid (δ30Si(OH)4) was analyzed in the water column of the Eastern Equatorial Pacific (EEP) where one of the globally largest Oxygen Minimum Zones (OMZs) is located. Samples were collected at 7 stations along two meridional transects from the equator to 14°S at 85°50′W and 82°00′W off the Ecuadorian and Peruvian coast. Surface waters show a large range in isotope compositions δ30Si(OH)4 (+2.2‰ to +4.4‰) with the highest values found at the southernmost station at 14°S. This station also revealed the most depleted silicic acid concentrations (0.2 μmol/kg), which is a function of the high degree of Si utilization by diatoms and admixture with waters from highly productive areas. Samples within the upper water column and the OMZ at oxygen concentrations below 10 μmol/kg are characterized by a large range in δ30Si(OH)4, which mainly reflects advection and mixing of different water masses, even though the highly dynamic hydrographic system of the upwelling area off Peru does not allow the identification of clear Si isotope signals for distinct water masses. Therefore we cannot rule out that also dissolution processes have an influence on the δ30Si(OH)4 signature in the subsurface water column. Deep water masses (>2000 m) in the study area show a mean δ30Si(OH)4 of +1.2±0.2‰, which is in agreement with previous studies from the eastern and central Pacific. Comparison of the new deep water data of this study and previously published data from the central Pacific and Southern Ocean reveal substantially higher δ30Si(OH)4 values than deep water signatures from the North Pacific. As there is no clear correlation between δ30Si(OH)4 and silicic acid concentrations in the entire data set the distribution of δ30Si(OH)4 signatures in deep waters of the Pacific is considered to be mainly a consequence of the mixing of several end member water masses with distinct Si isotope signatures including Lower Circumpolar Deep Water (LCDW) and North Pacific Deep Water (NPDW).