The Mesoproterozoic deep ocean is thought to have been largely anoxic, dominated by ferruginous conditions. Phosphate was scavenged from the water column by iron oxyhydroxides, and dissolved nitrate was rapidly reduced along the redoxcline. Primary productivity was therefore generally low, and eukaryotic life was restricted to oxic marine margins. This model is supported by new nitrogen isotope data from the Lower Riphean (ca. 1.55 Ga) strata of the South Ural Mountains, where we find evidence for strong redox stratification, coupled with low organic carbon content. In contrast, unconformably overlying siliciclastic sedimentary rocks of Middle Riphean age (ca. 1.33 Ga) preserve a rare occurrence of apatite concretions, associated with relatively high concentrations of organic carbon (up to 4 wt%). The Pb-Pb isochron age for the apatite concretions of the Zigazino-Komarovo Formation is 1330 ± 20 Ma (MSWD = 3.7). We measured nitrogen isotope ratios across this interval and found δ¹⁵N values up to + 7.6‰, which are similar to those in modern upwelling zones and indicative of a significant nitrate reservoir. These observations are most parsimoniously explained by upwelling of phosphate- and nitrate-bearing waters, suggesting that the deep ocean was at least regionally and temporarily oxygenated. The lack of redox-sensitive trace element enrichment in these strata and the general scarcity of apatite accumulations in the Mesoproterozoic sedimentary record suggest that oxic deep-waters were not a global and persistent phenomenon, but our results confirm that oxic, nutrient-rich refugia existed in Mesoproterozoic oceans and were perhaps important for the radiation of early eukaryotes.

中生代深海被认为主要是缺氧的，主要受铁质条件的影响。羟基氧化铁从水柱中清除了磷酸盐，溶解的硝酸盐沿氧化还原环迅速还原。因此，初级生产力普遍较低，并且真核生物的生命仅限于有氧海洋边缘。该模型得到了来自南乌拉尔山脉下里菲安（约1.55 Ga）地层的新氮同位素数据的支持，在该数据中我们发现了强烈的氧化还原层化证据以及低有机碳含量的证据。相反，中里菲亚时代（约1.33 Ga）的不整合的上覆硅质碎屑沉积岩保留了罕见的磷灰石成矿现象，并伴有相对较高的有机碳浓度（最高4 wt％）。Zigazino-Komarovo组磷灰石结核的Pb-Pb等时年龄为1330±20 Ma（MSWD = 3.7）。我们测量了该时间间隔内的氮同位素比，发现δ¹⁵ N值最高为+ 7.6‰，与现代上升流区的N值相似，表明存在大量的硝酸盐储集层。这些观测结果最简洁地由含磷酸盐和硝酸盐的水上升流解释，表明深海至少在区域性和暂时性地被氧化。这些地层缺乏对氧化还原敏感的微量元素富集以及中元古代沉积记录中磷灰石的普遍缺乏，表明含氧深水不是一种全球性的持久性现象，但我们的结果证实存在含氧，营养丰富的避难所在中元古代海洋中，也许对早期真核生物的辐射很重要。