Evolution of (bio-)geochemical processes in deep subseafloor sediments from the Nankai Trough along the tectonic migration of ocean floor and related changes in depositional and thermal conditions

Abstract

(Bio-)geochemical processes in deep subseafloor sediments can notably change over geological timescales due to variations in oceanographic, climatic or depositional conditions. These changes, in turn, can have significant impacts on global element cycles of carbon, sulfur and iron and the carbon sequestration in marine sediments. An ideal setting to study (bio-)geochemical processes under the influence of strongly changing environmental and depositional conditions is the Nankai Trough subduction zone offshore Japan in the northwestern Pacific Ocean. The sediment cores that were investigated in the framework of this cumulative doctoral thesis were taken from a 1,180 m deep hole (Site C0023) in the Nankai Trough off Cape Muroto during International Ocean Discovery Program Expedition 370, which aimed at exploring the temperature limit of microbial life in the deep subseafloor biosphere. At present, the temperature increases from 2°C at the seafloor up to 120°C at the sediment-basement interface. Over the past 15 million years, the sediments at Site C0023 have moved several hundreds of kilometers relative to its present-day position due to tectonic plate motion. During this tectonically induced migration, the sediments at Site C0023 have experienced significant changes in depositional, geochemical and thermal conditions. By combining comprehensive geochemical and rock magnetic analyses, the potential succession of (bio-)geochemical processes, in particular the cycling of iron, in the deep subseafloor sediments at Site C0023 was reconstructed along the tectonic migration of ocean floor. In a next step, the resulting evolution of (bio-)geochemical processes was quantitatively tested by reactive transport modeling. In addition, stable iron isotope analyses were performed on dissolved and sequentially extracted iron to disentangle microbially mediated and abiotic drivers of the iron cycling in deep and hot subseafloor sediments at Site C0023. The presented study contributes to an improved understanding of long-term variations in (bio-)geochemical processes in the deep subseafloor biosphere and provides important aspects for the interpretation of stable iron isotope data.