Abstract
Particulate organic carbon (POC) in the ocean often exhibits more depleted radiocarbon contents (lower Δ 14C values) than expected if its sole source were POC recently synthesized by primary production and export from the overlying surface waters. An examination of available Δ14C data sets for sinking POC show that this phenomenon is both common and globally widespread. Also, a strong correlation is found to exist between Δ14C values of organic carbon and aluminum content in sinking particles that is consistent over a range of oceanic settings. Together, these findings imply that aged organic carbon associated with lithogenic material from sediment resuspension is responsible for the observed low Δ 14C values as opposed to other processes such as incorporation of dissolved inorganic carbon or dissolved organic carbon into POC at depth. An estimate based on POC flux‐weighted Δ14C values shows that about 35% of sinking POC at the locations studied is derived from resuspended sediment. Our results suggest that resuspension of sediment and its subsequent lateral transport is an important component of the oceanic carbon cycle and should be considered in models of oceanic carbon export and burial.
Highlights
[2] The photosynthetic fixation of carbon by phytoplankton in the surface ocean, and the subsequent export and burial of a fraction of this carbon in deep waters and underlying sediments is a fundamental component of the global carbon cycle, and drives many oceanic biogeochemical processes [Hedges, 1992]
[3] One interesting geochemical characteristic of particulate organic carbon (POC) sinking from the surface ocean is that its radiocarbon content is generally lower than that expected if primary production in the overlying water column was the sole source of POC [Druffel and Williams, 1990; Druffel et al, 1992]
The near‐zero y intercept for the mixing lines implies that aged organic carbon associated with lithogenic material from sediment resuspension is responsible for the observed low normalized D14C values
Summary
[2] The photosynthetic fixation of carbon by phytoplankton in the surface ocean, and the subsequent export and burial of a fraction of this carbon in deep waters and underlying sediments is a fundamental component of the global carbon cycle, and drives many oceanic biogeochemical processes [Hedges, 1992]. Resuspension of sediment and subsequent lateral transport is evident from several observations, including material fluxes to sediment traps [Honjo et al, 1982; Biscaye et al, 1988; Freudenthal et al, 2001; Smith et al, 2001], bottom and intermediate nepheloid layers [McCave, 1983; McCave et al, 2001; Karakaş et al, 2006; Lam and Bishop, 2008], carbon supply and oxygen consumption in sediments [Jahnke, 1996], 14C ages of suspended and sinking POC [Druffel et al, 1998a; Honda et al, 2000; Hwang et al, 2004, 2008, 2009a, 2009b], aluminum contents of suspended particles [Sherrell et al, 1998], the isotopic and molecular composition of organic matter in deep sea sediments [Benthien and Muller, 2000; Ohkouchi et al, 2002; Mollenhauer et al, 2006], and regional ocean modeling [Karakaş et al, 2006]. The average normalized D14C value for all sites where measured data have been reported is −87 ± 63‰ (the uncertainty reflects the standard deviation of each average value representing each location and depth), and the average value weighted by POC flux is −95‰ (it is not doable to estimate the uncertainty of this value, but we believe that it is similar to or slightly larger than ± 63‰ as determined above), corresponding to a conventional 14C age of 800 years
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