Global Biogeochemical Cycles

A time series study of silica production and flux in an eastern boundary region: Santa Barbara Basin, California

Authors

  • Rebecca F. Shipe,

  • Mark A. Brzezinski


Abstract

A high-resolution data set describing the annual silica cycle between January 1994 and August 1999 in the Santa Barbara Basin (SBB) captures processes and events occurring in both surface and deep basin waters. Increases in biogenic silica (bSi) concentrations in the surface waters indicated strong bloom events in the early spring of each year and sporadic blooms during the summer and fall. Lithogenic silica (lSi) was present at elevated concentrations in surface waters in the winter and spring. At times, the siliceous biomass in the SBB is comparable to that in major global upwelling regions. However, anomalously low annual depth-integrated bSi concentrations were observed during the E1 Niño conditions of 1997–1998. A 1.5-year time series of biogenic silica production rate measurements was used to calculate the highest resolution estimate of annual silica production to date, an estimated 5.5 mol Si m −2 yr−1. This silica production rate approaches that estimated for the major global upwelling zones (8.3 mol Si m−2 yr−1). Eastem boundary regions outside of the major coastal upwelling zones would account for 30–40% of global silica production if the Santa Barbara Basin is representative of these coastal regions. Since the sediments of the SBB contain a high-resolution climate record, an understanding of the coupling between surface processes and deep basin sedimentation events is of value. Fluxes of bSi and lSi to 470 m were not strongly coupled to the seasonal cycle of particulate concentrations or production rates in surface waters. Continuous sediment trap collections of sedimenting particles indicate that both lSi and bSi tend to be exported at the highest rates during the spring and summer. As particles sink, lSi becomes enriched relative to bSi and consistently dominates the mass flux of silica at 470 m. Brief increases in the percent contribution of bSi to Si flux may create microlaminae within larger-scale laminations which are thought to be annual varves. A comparison of annual bSi sedimentation rates with annual silica production estimates reveal that ∼30% of the annual silica production is exported to a depth of 470 m. This export rate is the highest observed to date and suggests the potential for large quantities of silica burial in coastal sediments.

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