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The Mesoproterozoic Eon [1,600-1,000 million years ago (Ma)] is emerging as a key interval in Earth history, with a unique geochemical history that might have influenced the course of biological evolution on Earth. Indeed, although this time interval is rather poorly understood, recent chromium isotope results suggest that atmospheric oxygen levels were <0.1% of present levels, sufficiently lowto have inhibited the evolution of animal life. In contrast, using a different approach, we explore the distribution and enrichments of redox-sensitive trace metals in the 1,400 Ma sediments of Unit 3 of the Xiamaling Formation, North China Block. Patterns of trace metal enrichments reveal oxygenated bottom waters during deposition of the sediments, and biomarker results demonstrate the presence of green sulfur bacteria in the water column. Thus, we document an ancient oxygen minimum zone. We develop a simple, yet comprehensive, model of marine carbon-oxygen cycle dynamics to show that our geochemical results are consistent with atmospheric oxygen levels >4% of present-day levels. Therefore, in contrast to previous suggestions, we show that there was sufficient oxygen to fuel animal respiration long before the evolution of animals themselves. [ABSTRACT FROM AUTHOR]

Fluctuating climate is a hallmark of Earth. As one transcends deep into Earth time, however, both the evidence for and the causes of climate change become difficult to establish. We report geochemical and sedimentological evidence for repeated, short-term climate fluctuations from the exceptionally well-preserved ∼1.4- billion-year-old Xiamaling Formation of the North China Craton. We observe two patterns of climate fluctuations: On long time scales, over what amounts to tens of millions of years, sediments of the Xiamaling Formation record changes in geochemistry consistent with long-term changes in the location of the Xiamaling relative to the position of the Intertropical Convergence Zone. On shorter time scales, and within a precisely calibrated stratigraphic framework, cyclicity in sediment geochemical dynamics is consistent with orbital control. In particular, sediment geochemical fluctuations reflect what appear to be orbitally forced changes in wind patterns and ocean circulation as they influenced rates of organic carbon flux, trace metal accumulation, and the source of detrital particles to the sediment. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to the article "Sufficient Oxygen for Animal Respiration 1,400 Million Years Ago" in the 2016 issue.

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 lowproductivity 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. [ABSTRACT FROM AUTHOR]

Integrated Ocean Drilling Program Expedition 313 recovered Miocene sequences at Holes M0027A and M0029A on the New Jersey shallow shelf that contain a characteristic acid-resistant organic component. The palynofacies within each sequence reflects variations in terrigenous versus authigenic flux through the Miocene that are associated with sea-level change. Very high ratios of terrigenous versus marine palynomorphs and of oxidation-resistant versus susceptible dinocysts are associated with seismic sequence boundaries, consistent with their interpretation as sequence-bounding unconformities generated at times of low sea level. Comparison of palynological distance from shoreline estimates with paleodepth estimates derived from foraminiferal data allows relative sea level to be reconstructed at both sites. Ages assigned using dinocyst biostratigraphy are consistent with other chronostratigraphic indicators allowing sequence boundaries to be correlated with Miocene oxygen isotope (Mi) events. Paleoclimatic evidence from the pollen record supports the global climate changes seen in the oxygen isotope data. Although chronological control is relatively crude, Milankovitch-scale periodicity is suggested for parasequences visible in thick sequences deposited in relatively deep water where substantial accommodation existed, such as during the early Langhian at Site 29 (Middle Miocene Climatic Optimum). Palyno logical analysis thus supports the long-held hypothesis that glacioeustasy is a dominant process controlling the architecture of continental margins. [ABSTRACT FROM AUTHOR]

The appearance and subsequent evolution of land plants is among the most important events in the earth system. Plant resulted in a change of earth surface albedo and the hydrological cycle, as well as increased rock weatherability thereby causing a persistent change in atmospheric CO 2 and O 2 . Land plants are, however, themselves dependent on O 2 for respiration and long-term survival, something not considered in current geochemical models. In this perspective, we highlight two aspects of land plants' dependency on O 2 relevant for the geobiological community: (a) fossil root systems can be used as a proxy for minimum levels of past atmospheric O 2 consistent with a given fossil root depth; and (b) by identifying a positive feedback mechanism involving atmospheric O 2 , root intensity, terrestrial primary production and organic carbon burial. As an example, we consider archaeopterid fossil root systems, resembling those of modern mature conifers. Our soil-plant model suggest that atmospheric O 2 with 1 SD probably reached pressures of 18.2 ± 1.9 kPa and 16.8 ± 2.1 kPa by the Middle and Late Devonian, respectively, that is 86 ± 9% and 79 ± 10% of the present-day 21.2 kPa., (© 2021 The Authors. Geobiology published by John Wiley & Sons Ltd.)