Abstract
The dynamics of a large stock of organic matter contained in deep sediments of marginal seas plays pivotal role in global carbon cycle, yet it is poorly constrained. Here, dissolved organic matter (DOM) in sediments was investigated for core sediment up to ~240 meters deep in the East/Japan Sea. The upper downcore profile (≤118 mbsf, or meters below seafloor) at a non-chimney site (U1) featured the exponential production of dissolved organic carbon (DOC) and optically active DOM with time in the pore water above sulfate-methane-transition-zone (SMTZ), concurrent with the increases of nutrients and alkalinity, and the reduction of sulfate. Such depth profiles signify a biological pathway of the DOM production during the early diagenesis of particulate organic matter presumably dominated by sulfate reduction. Below the SMTZ, an insolation-paced oscillation of DOM in a ~405-Kyr cycle of orbital eccentricity was observed at site U1, implying astronomically paced paleoproductivity stimulated by light availability. Furthermore, DOM dynamics of the deep sediments were likely governed by intensive humification as revealed by the less pronounced protein-like fluorescence and the lower H/C and O/C ratios below SMTZ among 15,281 formulas identified. Our findings here provide novel insights into organic matter dynamics in deep sediments.
Highlights
As the archives of physical and biogeochemical history occurring in aquatic ecosystems, sediments in marginal seas are considered very useful in evaluating and reconstructing the paleoenvironments of both land and ocean[1,2,3]
It was reported that most (80–98%) particulate organic matter (POM) is degraded through particulate organic carbon sulfate reduction (POCSR), and only a small fraction is affected by methanogenesis above the sulfate-methane-transition-zone (SMTZ) in non-chimney sites as opposed to the chimney sites studied in a marginal sea[10]
Since we found that chimney sites were heavily affected by the dilution effects, we will focus below on the non-chimney site U1, which hopefully can mirror the paleoenvironmental changes more accurately
Summary
As the archives of physical and biogeochemical history occurring in aquatic ecosystems, sediments in marginal seas are considered very useful in evaluating and reconstructing the paleoenvironments of both land and ocean[1,2,3]. Throughout the Earth’s history, some global warming events and glacial-interglacial cycles have been linked to the orbital forcing of axial precession, axial obliquity, and orbital eccentricity[11,12,13] Pore water salinity and δ18O depth profiles illustrated the effects of Pleistocene glaciations[14], and it was estimated that global average Δδ18O constraining the glacial-interglacial cycles is 1.0 ± 0.1‰15, 16. These previous findings inspired us to explore the potential effects of these paleocycles on the quantity and quality of sedimentary DOM. The signatures of the paleo-changes in deep sediments could be hampered by clay dehydration during illitization (smectite-to-illite transformation) followed by water production (>800 mbsf) as well as by upward transport of basal fluids[18]
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