Evolution of (Bio‐)Geochemical Processes and Diagenetic Alteration of Sediments Along the Tectonic Migration of Ocean Floor in the Shikoku Basin off Japan

Abstract

 The tectonic migration of ocean floor led to a transition from an organic carbon-starved to an elevated organic carbon burial environment  Diagenetic transformation of iron oxides into pyrite within the carbon-lean sediments occurred several millions of years after deposition Accepted ArticleThis article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

Influence of Early Low‐Temperature and Later High‐Temperature Diagenesis on Magnetic Mineral Assemblages in Marine Sediments From the Nankai Trough

“…Diagenetic processes at Site C0023 have been reconstructed by Köster et al (2021a) based on sedimentary Fe pool analysis. In the following, we discuss the rock magnetism of Site C0023 along with the findings of Köster et al (2021a) and propose a burial diagenetic magnetic model for this site (Figures 7 and 8). Figure 8 gathers key information on the timing of key diagenetic events for the different formations, and stability fields for key iron minerals.…”

“…This was accompanied by burial temperature and resulting (bio-)geochemical process changes (Horsfield et al, 2006;Köster et al, 2021a;Tsang et al, 2020). The Lower Shikoku Basin facies (Lithologic Unit IV ∼637-1,112 mbsf; 13.53-2.53 Ma) was deposited in an organic carbon-starved, low sedimentation abyssal environment (Köster et al, 2021a). Aerobic conditions prevented reductive dissolution and favored detrital magnetic mineral preservation, mainly iron (oxyhydr)oxides (Figures 7 and 8).…”

“…Site C0023 experienced depositional environment changes during its tectonic displacement from the Shikoku Basin to the Nankai Trough. This was accompanied by burial temperature and resulting (bio-)geochemical process changes (Horsfield et al, 2006;Köster et al, 2021a;Tsang et al, 2020). The Lower Shikoku Basin facies (Lithologic Unit IV ∼637-1,112 mbsf; 13.53-2.53 Ma) was deposited in an organic carbon-starved, low sedimentation abyssal environment (Köster et al, 2021a).…”

“…Aerobic conditions prevented reductive dissolution and favored detrital magnetic mineral preservation, mainly iron (oxyhydr)oxides (Figures 7 and 8). As Site C0023 moved toward the Japanese island arc, sedimentation rates and organic carbon supply increased, leading to organoclastic iron and sulfate reduction as well as biogenic methanogenesis, and, thus, to anoxic depositional conditions for the Upper Shikoku Basin facies (Unit III at ∼494-637 mbsf; 2.43-0.43 Ma, Köster et al, 2021a) (Figures 7 and 8). Following the onset of methanogenesis, sulfate diffusing down from the overlying seawater reacted with biogenic methane during AOM and a shallow SMT formed (Köster et al, 2021a).…”

“…T max is the RockEval parameter and T max ∼ 435°C indicates the onset of catagenesis. The two sulfate-methane transitions (SMTs; Köster et al, 2021a) discussed in the text are reported. Time is indicated on a log scale to illustrate recent events and consequent processes (<0.5 Ma) that control diagenetic processes.…”