Abstract
A ‘Neoproterozoic oxygenation event’ is widely invoked as a causal factor in animal evolution, and often attributed to abiotic causes such as post-glacial pulses of phosphorus weathering. However, recent evidence suggests a series of transient ocean oxygenation events ∼660–520 Ma, which do not fit the simple model of a monotonic rise in atmospheric oxygen (pO2). Hence, we consider mechanisms by which the evolution of marine eukaryotes, coupled with biogeochemical and ecological feedbacks, potentially between alternate stable states, could have caused changes in ocean carbon cycling and redox state, phosphorus cycling and atmospheric pO2. We argue that the late Tonian ocean ∼750 Ma was dominated by rapid microbial cycling of dissolved organic matter (DOM) with elevated nutrient (P) levels due to inefficient removal of organic matter to sediments. We suggest the abrupt onset of the eukaryotic algal biomarker record ∼660–640 Ma was linked to an escalation of protozoan predation, which created a ‘biological pump’ of sinking particulate organic matter (POM). The resultant transfer of organic carbon (Corg) and phosphorus to sediments was strengthened by subsequent eukaryotic innovations, including the advent of sessile benthic animals and mobile burrowing animals. Thus, each phase of eukaryote evolution tended to lower P levels and oxygenate the ocean on ∼104 year timescales, but by decreasing Corg/P burial ratios, tended to lower atmospheric pO2 and deoxygenate the ocean again on ∼106 year timescales. This can help explain the transient nature and ∼106 year duration of oceanic oxygenation events through the Cryogenian–Ediacaran–Cambrian.
Highlights
Marine eukaryotes are important ecosystem engineers and their evolution over ∼850–500 Ma (Figure 1a,b) surely had an impact on carbon, phosphorus and oxygen cycling [1]
The late Tonian ocean ∼750 Ma was dominated by rapid microbial cycling of dissolved organic matter (DOM) with elevated nutrient (P) levels due to inefficient organic matter removal to sediments
We hypothesise that abrupt onset of the eukaryotic algal biomarker record in the Cryogenian ∼660–640 Ma was linked to an escalation of protozoan predation
Summary
Marine eukaryotes are important ecosystem engineers and their evolution over ∼850–500 Ma (Figure 1a,b) surely had an impact on carbon, phosphorus and oxygen cycling [1]. (2) A ∼104 year timescale [16] of changes in P burial efficiency altering the ocean P inventory, which in turn controls the amount of organic matter produced and corresponding oxygen demand (Figure 2b) This modern P timescale could have differed in the Neoproterozoic. By creating an efficient particulate P removal flux to shelf sediments, it would have lowered ocean P content and corresponding O2 demand over ∼104 years, tending to oxygenate deeper waters of shelf seas (Figure 4b) and the open ocean [6] This prediction is supported by redox proxy evidence of ocean oxygenation after the Sturtian [10,11,62]. The Cambrian evolution of large zooplankton would have increased the efficiency of the biological pump [36], transferring organic matter to sediments, lowering the ocean P inventory and tending to oxygenate the ocean [1,107]
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