PNNL

Abstract

Continental margin sediments play a critical role in global biogeochemical cycles, as they are generally dominated by high primary production and associated burial of marine organic matter. However, the highly heterogeneous nature of continental margins complicates our understanding of these settings as preserved in the rock record. This study investigates the complex interplay of carbon, iron, and sulfur biogeochemical processes that prevail in the bathymetrically isolated Santa Monica Basin (SMB). We explore solid-phase and pore fluid profiles along a transect from 71 to 907 meters water depth that includes oxygenated (>60 µM O2) bottom waters near the coast and oxygen-deficient waters (~4 µM O2) in the deeper basin. The geochemical data of the basinal sediments are further scrutinized by means of reactive transport modeling. We found that the basin sediments do not follow the traditional geochemical signatures of oxygen deficient settings. A lack of dissolved sulfide accumulation and sulfurized iron persists despite the sediments being deposited under reducing conditions (without bioturbation/bioirrigation), with strong organic carbon input (TOC up to 5.0 wt%), and active dissimilatory sulfate reduction. Not only did we find an exceptional enrichment in highly reactive Fe oxides in the surface sediments (~45% of total Fe), but the enrichment, including ferrihydrite, persists downcore and coexists with high levels of dissolved Fe. The enhanced preservation of FeOOH and lack of iron-sulfide precipitation is in part explained by detection via Mössbauer spectra of iron oxides bounded to organic matter (Fe[III]-OM complexes). The modeled Fe budget shows that most of the Fe oxides in the surface sediments are internallyrecycled by upward diffusion and subsequent oxidation of Fe2+ . Sulfide oxidation coupled to Fe(III) reduction effectively precludes sulfide accumulation while enhancing build-up of dissolved Fe, fueling the iron cycle and consuming most of the reactive organic carbon immediately below the sediment-water interface. The sediments deposited in the basin contain a heterogeneous mixture of organic material, indicated by the Corg/N ratios and radiocarbon data, with a significant portion of recalcitrant organic matter that limits carbon oxidation via sulfate reduction. In the absence of labile organics, other than within the uppermost layers, the organic-rich sediment profiles are dominated by iron cycling that enhanced the formation of Fe(III)-OM complexes and limits the production and preservation of reduced sulfur phases. This study highlights key local controls on Fe availability in marginal basins and describes an intricate biogeochemical carbon iron-sulfur cycling in modern and possibly ancient marine systems with important implications for Fe availability in the marine realm.