Abstract
The Xiamaling Formation is an exceptionally well-preserved sedimentary succession deposited on a marine passive margin about 1400 million years ago. We used a multi-proxy approach, including iron speciation, trace metal dynamics, and organic geochemistry, to explore the evolution of ocean chemistry through most of the Xiamaling Formation. This evolution is put in the context of the paleogeography and the sedimentological evolution of the Xiamaling depositional system.
Overall, the Xiamaling Formation is informally divided into six units based on both sedimentological and geochemical criteria. Of the six units, we fully explored four of them. Unit 4, the lowest unit we studied, is comprised of deep-water red muds, periodically interrupted by green-colored silt and sandy turbidites. Iron extraction results show that the red muds are enriched in highly reactive iron, indicating a water-column source for the iron. However, the low organic carbon contents, low hydrogen index (HI) values, and the oxidized nature of the reactive iron pool indicate deposition in oxygenated bottom waters. We interpret unit 4 to represent a low-productivity ferruginous oxygen-minimum zone (OMZ) environment, underlain by oxygenated bottom waters. The transition to unit 3 reflects an increase in primary productivity, and the development of a more biologically active OMZ, that supported anoxygenic phototrophic bacteria. Still, in this unit, the bottom waters remained oxygenated. The overlying unit 2 represents the transition to deep-water deoxygenation and anoxic waters at the sediment surface. These waters were ferruginous in the bottom part of the unit and sulfidic (euxinic) towards the top. In the uppermost unit 1, euxinic conditions continued, punctuated by more frequent water-column oxygenation towards the upper part of the unit.
We place the evolution of these chemical dynamics in the context of climate and climate change, and in particular, the placement of the Xiamaling Formation in relation to the Intertropical Convergence Zone (ITCZ) and the resulting Hadley Cell dynamics. Also, while our results demonstrate the persistence of anoxic water-column conditions high in the water column during the deposition of the Xiamaling Formation, they also demonstrate bottom water oxygenation near the seafloor during the deposition of three of the four units, and over a time interval extending to 10's of millions of years.
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