Abstract. The element silicon (Si) is required for the growth of silicified organisms in marine environments, such as diatoms. These organisms consume vast amounts of Si together with N, P, and C, connecting the biogeochemical cycles of these elements. Thus, understanding the Si cycle in the ocean is critical for understanding wider issues such as carbon sequestration by the ocean's biological pump. In this review, we show that recent advances in process studies indicate that total Si inputs and outputs, to and from the world ocean, are 57 % and 37 % higher, respectively, than previous estimates. We also update the total ocean silicic acid inventory value, which is about 24 % higher than previously estimated. These changes are significant, modifying factors such as the geochemical residence time of Si, which is now about 8000 years, 2 times faster than previously assumed. In addition, we present an updated value of the global annual pelagic biogenic silica production (255 Tmol Si yr−1) based on new data from 49 field studies and 18 model outputs, and we provide a first estimate of the global annual benthic biogenic silica production due to sponges (6 Tmol Si yr−1). Given these important modifications, we hypothesize that the modern ocean Si cycle is at approximately steady state with inputs =14.8(±2.6) Tmol Si yr−1 and outputs =15.6(±2.4) Tmol Si yr−1. Potential impacts of global change on the marine Si cycle are discussed.

Silicon (Si) is a pivotal element in the biogeochemical and ecological functioning of the ocean. The marine Si cycle is thought to be in internal equilibrium, but the recent discovery of Si entries through groundwater and glacial melting increased the known Si inputs relative to the outputs in the global oceans. Known outputs are due to the burying of diatom skeletons or their conversion into authigenic clay by reverse weathering. Here we show that non-phototrophic organisms, such as sponges and radiolarians, also facilitate significant Si burial through their siliceous skeletons. Microscopic examination and digestion of sediments revealed that most burial occurs through sponge skeletons, which, being unusually resistant to dissolution, had passed unnoticed in the biogeochemical inventories of sediments. The preservation of sponge spicules in sediments was 45.2 ± 27.4%, but only 6.8 ± 10.1% for radiolarian testa and 8% for diatom frustules. Sponges lead to a global burial flux of 1.71 ± 1.61 TmolSi yr −1 and only 0.09 ± 0.05 TmolSi yr −1 occurs through radiolarians. Collectively, these two non-phototrophically produced silicas increase the Si output of the ocean to 12.8 TmolSi yr −1 , which accounts for a previously ignored sink that is necessary to adequately assess the global balance of the marine Si cycle.

Abstract. The element silicon (Si) is required for the growth of silicified organisms in marine environments, such as diatoms, which consume vast amounts of Si together with N, P, and C, connecting the biogeochemical cycles of these elements. Thus, understanding the Si cycle in the ocean is critical for understanding issues such as carbon sequestration by the ocean's biological pump. In this review, we show that recent advances in process studies indicate that total Si inputs and outputs, to and from the world ocean, are 57 % and 18 % higher, respectively, than previous estimates. We also update the total ocean silicic acid inventory value, which is about 24 % higher than previously estimated. These changes are significant, modifying factors such as the geochemical residence time of Si, which is now about 8000 years and two times faster than previously assumed. In addition, we present an updated value of the global annual pelagic biogenic silica production (255 Tmol-Si yr−1) based on new data from 49 field studies and 18 model outputs, and provide a first estimate of the global annual benthic biogenic silica production due to sponges (6 Tmol-Si yr−1). Given these important modifications, we address the steady state hypothesis of the Si cycle for past and modern oceans, and propose a possible steady state scenario for the global ocean (inputs = outputs = 14.8 Tmol-Si yr−1) and boundary exchange zone. Case studies for future programs are highlighted, and potential impacts of global change on the marine Si cycle discussed.

Biogeochemical cycling of silicon (Si), largely affected by biological drivers, is pivotal to the ecological functioning of the ocean. Most knowledge regarding biological utilization of Si derives from research on phototrophic organisms circumscribed to the photic ocean (i.e., diatoms). Utilization of Si in the aphotic ocean, where heterotrophic silicifiers become relevant Si users, remains poorly investigated. Here we quantify the flux rates and stocks characterizing Si cycling across dense aggregations of the hexactinellid sponge Vazella pourtalesii established in the aphotic zone of the central Scotian Shelf, Nova Scotia, Canada. Although individual rates of silicic acid consumption were low compared to other sponge species and diatoms, the large abundance of individuals (6.5 million) over the extension of these sponge grounds (2105 km 2) leads to massive annual silicic acid consumption, invested in producing their siliceous skeletons of biogenic silica. This sponge activity accumulates large biogenic silica stocks both in the living population and in the sediments. Skeletal pieces in sediment revealed that a good portion of biogenic silica deposited to the bottom after sponge death recycles as silicic acid before being permanently buried. This biogenic silica-silicic acid turnover, facilitated by an unconventional silicification pattern that favors delamination and dissolution of V. pourtalesii spicules, causes silicic acid enrichment at oceanographic dimensions in the bottom water of the central Scotian Shelf. Silicic acid efflux from the bottom sustains a feedback mechanism that fulfills sponge needs for silicic acid and facilitates the persistence of sponge aggregations in the long term. Silicon (Si), in the form of soluble silicic acid, is a key inorganic nutrient in the ocean. Its availability modulates ocean primary productivity (Nelson et al. 1995; Tréguer et al. 1995) and the CO 2 exchange with the atmosphere (Mackenzie and Garrels 1966; Tréguer and Pondaven 2000). Regional patterns of silicic acid availability largely result from