Particle flux in the oceans: Challenging the steady state assumption. Issue 1 (28th January 2017)

• Record Type: Journal Article
• Authors: Giering, Sarah L. C.; Sanders, Richard; Martin, Adrian P.; Henson, Stephanie A.; Riley, Jennifer S.; Marsay, Chris M.; Johns, David G.
• Is Part Of: Global biogeochemical cycles. Volume 31:Issue 1(2017:Jan.)
• Page Start: 159
• Page End: 171
• Publication Date: 2017-01-28
• Subjects: nonsteady state -- slow‐sinking particles -- carbon export -- biogenic silica -- particle flux -- sediment trap; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1002/2016GB005424
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: Atmospheric carbon dioxide levels are strongly controlled by the depth at which the organic matter that sinks out of the surface ocean is remineralized. This depth is generally estimated from particle flux profiles measured using sediment traps. Inherent in this analysis is a steady state assumption that export from the surface does not significantly change in the time it takes material to reach the deepest trap. However, recent observations suggest that a significant fraction of material in the mesopelagic zone sinks slowly enough to bring this into doubt. We use data from a study in the North Atlantic during July/August 2009 to challenge the steady state assumption. An increase in biogenic silica flux with depth was observed which we interpret, based on vertical profiles of diatom taxonomy, as representing the remnants of the spring diatom bloom sinking slowly (<40 m d −1 ). We were able to reproduce this behavior using a simple model using satellite‐derived export rates and literature‐derived remineralization rates. We further provide a simple equation to estimate "additional" (or "excess") particulate organic carbon supply to the dark ocean during nonsteady state conditions, which is not captured by traditional sediment trap deployments. In seasonal systems, mesopelagic net organic carbon supply could be wrong by as much as 25% when assuming steady state. We conclude that the steady state assumption leads to misinterpretation of particle flux profiles when input fluxes from the upper ocean vary on the order of weeks, such as in temperate and polar regions with strong seasonal cycles in export. Key Points: Increased biogenic silica fluxes with depth imply nonsteady state conditions in the NE Atlantic in August 2009 Taxonomic flux analysis and simple modeling support the nonsteady state hypothesis By assuming steady state, net organic carbon supply to the dark ocean may be wrong by ≤ 25%