Oceanic Euxinia in Earth History: Causes and Consequences

Abstract

Euxinic ocean conditions accompanied significant events in Earth history, including several Phanerozoic biotic crises. By critically examining modern and ancient euxinic environments and the range of hypotheses for these sulfidic episodes, we elucidate the primary factors that influenced the generation of euxinia. We conclude that periods of global warmth promoted anoxia because of reduced solubility of oxygen, not because of ocean stagnation. Anoxia led to phosphate release from sediments, and continental configurations with expansive nutrient-trapping regions focused nutrient recycling and increased regional nutrient buildup. This great nutrient supply would have fueled high biological productivity and oxygen demand, enhancing oxygen depletion and sulfide buildup via sulfate reduction. As long as warm conditions prevailed, these positive feedbacks sustained euxinic conditions. In rare, extreme cases, euxinia led to biotic crises, a hypothesis best supported by evidence from the end-Permian mass extinction. 251

Confounding effects of oxygen and temperature on the TEX 86 signature of marine Thaumarchaeota

“…Suboxia and anoxia prevail in some modern enclosed basins and in restricted waterways present early in Earth history such as the Atlantic Ocean during the Oceanic Anoxic Events (OAE) of the Cretaceous and Jurassic periods (49). Presently, the ocean is also home to several large OMZs where subsurface waters experience varying degrees of suboxia and anoxia.…”

“…Our results suggest decreased DO concentrations of warm ancient seawater would raise reconstructed temperatures to values somewhat higher than the actual SST. During particular intervals of Earth's history, such as the many OAEs and Paleocene/Eocene thermal maximum (PETM), widespread dysoxic or even anoxic conditions impinged on the photic zone, as indicated by a sedimentary biomarker (isorenieratane) for photic zone anoxia (31,49,53,55,56). Although some increase in temperature during OAEs and the PETM is consistent with paleoclimate model simulations and planktonic foraminiferal δ 18 O-based SST reconstructions, TEX 86 values (as high as 0.95) and TEX 86 -derived temperatures (up to 43°C) observed during these low-oxygen events seem anomalously high (31,(53)(54)(55)(56)(57)(58).…”

Extinctions in ancient and modern seas

“…In historical and modern times, bottom-water anoxia driven by excessive nutrient loading and decomposition of organic matter has led to some extirpations in estuarine and coastal waters [11,33,42]. Widespread anoxia is also associated with several major extinctions in the fossil record (Table 1), triggered by elevated atmospheric CO 2 , global warming, diminished thermohaline circulation, and eutrophication [59]. Other forms of pollution (e.g., pesticides and toxic algal blooms) have also contributed to historical extinctions and current risk [11,15,33,38,42], but identifying definitive examples of these in the fossil record is challenging.…”

Nitrogen cycle feedbacks as a control on euxinia in the mid-Proterozoic ocean

“…This is responsible for the (rare) sulphide build-up that occasionally occurs during coastal upwelling events 7,8,16 . Here we explore the implications of these simple feedbacks in the context of the Proterozoic ocean, specifically in terms of the ongoing discussion 17,18 about what caused significant euxinia to occur against the backdrop of largely ferruginous conditions.…”