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A c c e p t e d M a n u s c r i p t 3 ABSTRACTThe first appearance of skeletal metazoans in the late Ediacaran (~550 million years ago; Ma) has been linked to the widespread development of oxygenated oceanic conditions, but a precise spatial and temporal reconstruction of their evolution has not been resolved. Here we consider the evolution of ocean chemistry from ~550 to ~541 Ma across shelf-to-basin transects in the Zaris and Witputs Sub-Basins of the Nama Group, Namibia. New carbon isotope data capture the final stages of the Shuram/Wonoka deep negative C-isotope excursion, and these are complemented with a reconstruction of water column redox dynamics utilizing Fe-S-C systematics and the distribution of skeletal and soft-bodied metazoans. Combined, these inter-basinal datasets provide insight into the potential role of ocean redox chemistry during this pivotal interval of major biological innovation.The strongly negative  13 C values in the lower parts of the sections reflect both a secular, global change in the C-isotopic composition of Ediacaran seawater, as well as the influence of 'local' basinal effects as shown by the most negative  13 C values occurring in the transition from distal to proximal ramp settings. Critical, though, is that the transition to positive  13 C values postdates the appearance of calcified metazoans, indicating that the onset of biomineralization did not occur under post-excursion conditions. Significantly, we find that anoxic and ferruginous deeper water column conditions were prevalent during and after the transition to positive  13 C that marks the end of the Shuram/Wonoka excursion. Thus, if the C isotope trend reflects the transition to global-scale oxygenation in the aftermath of the oxidation of a large-scale, isotopically light organic carbon pool, it was not sufficient to fully oxygenate the deep ocean. Page 4 of 74A c c e p t e d M a n u s c r i p t 4 Both sub-basins reveal highly dynamic redox structures, where shallow, inner ramp settings experienced transient oxygenation. Anoxic conditions were caused either by episodic upwelling of deeper anoxic waters or higher rates of productivity. These settings supported short-lived and monospecific skeletal metazoan communities. By contrast, microbial (thrombolite) reefs, found in deeper inner-and mid-ramp settings, supported more biodiverse communities with complex ecologies and large skeletal metazoans. These long-lived reef communities, as well as Ediacaran soft-bodied biotas, are found particularly within transgressive systems, where oxygenation was persistent. We suggest that a mid-ramp position enabled physical ventilation mechanisms for shallow water column oxygenation to operate during flooding and transgressive sea-level rise. Our data support a prominent role for oxygen, and for stable oxygenated conditions in particular, in controlling both the distribution and ecology of Ediacaran skeletal metazoan communities.Keywords: Oxygenation; Neoproterozoic; Biomineralisation; Metazoans; Ediacaran; Ecosystems Introductio...

Ocean acidification triggered by Siberian Trap volcanism has been implicated as a kill18 mechanism for the Permo-Triassic mass extinction, but evidence for an acidification event 19 remains inconclusive. To address this, we present a high resolution seawater pH record across 20 this interval, utilizing boron isotope data combined with a quantitative modeling approach. In the 21 latest Permian, the alkalinity of the ocean increased, priming the Earth system with a low level of 22 atmospheric CO 2 and a high ocean buffering capacity. The first phase of extinction was 23 2 coincident with a slow injection of isotopically light carbon into the atmosphere-ocean, but the 24 ocean was well-buffered such that ocean pH remained stable. During the second extinction pulse, 25 however, a rapid and large injection of carbon overwhelmed the buffering capacity of the ocean, 26causing an abrupt and short-lived acidification event that drove the preferential loss of heavily 27 calcified marine biota. kyrs (2) and can be resolved into two distinct marine extinction pulses, with the respective kill 37 mechanisms appearing to be ecologically selective (3). The first occurred in the latest Permian 38 (Extinction Pulse 1; EP1) and was followed by an interval of temporary recovery before the 39 second pulse (EP2) which occurred in the earliest Triassic. The direct cause of the mass 40 extinction is widely debated with a diverse range of overlapping mechanisms proposed, 41 including widespread water column anoxia (4), euxinia (5), global warming (6) and ocean 42 acidification (7). 43Models of PTB ocean acidification suggest that a massive, and rapid, release of CO 2 from 44 Siberian Trap volcanism, acidified the ocean (7). Indirect evidence for acidification comes from 45 the interpretation of faunal turnover records (3, 8), potential dissolution surfaces (9) and Ca 46 3 isotope data (7). A rapid input of carbon is also potentially recorded in the negative carbon 47 isotope excursion (CIE) that characterizes the PTB (10, 11) . The interpretation of these records 48 is, however, debated (12), and of great importance to understanding the current threat of 49 anthropogenically-driven ocean acidification (11). 50Here, we test the ocean acidification hypothesis by presenting a novel proxy record of 51 ocean pH across the PTB, using the boron isotope composition of marine carbonates ( 11 additional counterbalancing alkalinity flux. This is consistent with independent proxy data (6). 129The alkalinity source may have been further increased through soil loss (26) carbon to the atmosphere, yet remarkably, the acidification event occurs after the decline in  13 C, 139when  13 C has rebounded somewhat and is essentially stable (Fig. 2). 140Unlike the first carbon injection, the lack of change in  13 C at this time rules out very 141 13 C-depleted carbon sources, because no counterbalancing strongly 13 C-enriched source exists.

Oceanic Anoxic Event 2 (OAE 2), occurring ∼94 million years ago, was one of the most extreme carbon cycle and climatic perturbations of the Phanerozoic Eon. It was typified by a rapid rise in atmospheric CO, global warming, and marine anoxia, leading to the widespread devastation of marine ecosystems. However, the precise timing and extent to which oceanic anoxic conditions expanded during OAE 2 remains unresolved. We present a record of global ocean redox changes during OAE 2 using a combined geochemical and carbon cycle modeling approach. We utilize a continuous, high-resolution record of uranium isotopes in pelagic and platform carbonate sediments to quantify the global extent of seafloor anoxia during OAE 2. This dataset is then compared with a dynamic model of the coupled global carbon, phosphorus, and uranium cycles to test hypotheses for OAE 2 initiation. This unique approach highlights an intra-OAE complexity that has previously been underconstrained, characterized by two expansions of anoxia separated by an episode of globally significant reoxygenation coincident with the "Plenus Cold Event." Each anoxic expansion event was likely driven by rapid atmospheric CO injections from multiphase Large Igneous Province activity.

The oceans at the start of the Neoproterozoic Era (1,000–541 million years ago, Ma) were dominantly anoxic, but may have become progressively oxygenated, coincident with the rise of animal life. However, the control that oxygen exerted on the development of early animal ecosystems remains unclear, as previous research has focussed on the identification of fully anoxic or oxic conditions, rather than intermediate redox levels. Here we report anomalous cerium enrichments preserved in carbonate rocks across bathymetric basin transects from nine localities of the Nama Group, Namibia (∼550–541 Ma). In combination with Fe-based redox proxies, these data suggest that low-oxygen conditions occurred in a narrow zone between well-oxygenated surface waters and fully anoxic deep waters. Although abundant in well-oxygenated environments, early skeletal animals did not occupy oxygen impoverished regions of the shelf, demonstrating that oxygen availability (probably >10 μM) was a key requirement for the development of early animal-based ecosystems.