Free and kerogen‐bound biomarkers from late Tonian sedimentary rocks record abundant eukaryotes in mid‐Neoproterozoic marine communities

Abstract

Lipid biomarker assemblages preserved within the bitumen and kerogen phases of sedimentary rocks from the ca. 780-729 Ma Chuar and Visingsö Groups facilitate paleoenvironmental reconstructions and reveal fundamental aspects of emerging mid-Neoproterozoic marine communities. The Chuar and Visingsö Groups were deposited offshore of two distinct paleocontinents (Laurentia and Baltica, respectively) during the Tonian Period, and the rock samples used had not undergone excessive metamorphism. The major polycyclic alkane biomarkers detected in the rock bitumens and kerogen hydropyrolysates consist of tricyclic terpanes, hopanes, methylhopanes, and steranes. Major features of the biomarker assemblages include detectable and significant contribution from eukaryotes, encompassing the first robust occurrences of kerogen-bound regular steranes from Tonian rocks, including 21-norcholestane, 27-norcholestane, cholestane, ergostane, and cryostane, along with a novel unidentified C 30 sterane series from our least thermally mature Chuar Group samples.Appreciable values for the sterane/hopane (S/H) ratio are found for both the free and kerogen-bound biomarker pools for both the Chuar Group rocks (S/H between 0.09 and 1.26) and the Visingsö Group samples (S/H between 0.03 and 0.37). The more organic-rich rock samples generally yield higher S/H ratios than for organic-lean substrates, which suggests a marine nutrient control on eukaryotic abundance relative to bacteria. A C 27 sterane (cholestane) predominance among total C 26 -C 30 steranes is a common feature found for all samples investigated, with lower amounts of C 28 steranes (ergostane and crysotane) also present. No traces of known ancient C 30 sterane compounds; including 24-isopropylcholestanes, 24-n-propylcholestanes, or 26-methylstigmastanes, are detectable in any of these pre-Sturtian rocks. These biomarker characteristics support the view that the Tonian Period was a key interval in the history of life on our planet since it marked the transition from a bacterially dominated marine biosphere to an ocean system which became progressively enriched with eukaryotes. The eukaryotic source organisms likely encompassed photosynthetic primary producers, marking a rise in red algae, and consumers in a revamped trophic structure predating the Sturtian glaciation. K E Y W O R D S eukaryotes, HyPy, lipid biomarkers, Neoproterozoic, steranes | 327 ZUMBERGE Et al.

The global explosion of eukaryotic algae: the potential role of phosphorus?

“…Therefore, our results do not completely match with the biomarker record before about 750 Ma, where steranes are not found, suggesting that eukaryotic algae were of minimal importance in the global marine ecosystem. Indeed, steranes are absent even under a variety of depositional conditions (represented by sedimentary rocks of varying organic matter content) likely representing different degrees of nutrient availability [2,3,108–113].…”

“…Despite this, the ratio of steranes to hopanes in ancient sediments suggests the eukaryotic algae failed to make any significant biological or ecological impact until well into the Neoproterozoic Era, ca. 750 Ma [2,3]. The Neoproterozoic Era was characterised by extreme biochemical and climatic volatility, which resulted in dramatic alterations in the marine redox state and fluctuating surface-ocean oxygen concentrations [4].…”

“…These glaciations not only altered the Earth’s climate, but they resulted in extensive continental weathering that may have released large quantities of nutrients into the oceans [2,6–8]. Secondly, in the late Neoproterozoic Era, a shift in ecosystem structure and function resulted in the dramatic expansion and diversification of eukaryotic algae, the so called ‘rise of the algae’ [2,3,9]. This ecological transition resulted in irreversibly altered benthic and pelagic ecosystems and the eventual emergence of metazoan life.…”

“…During the middle Neoproterozoic Eon (800-650 Ma), fundamental shifts in the phosphorus cycle may have resulted in increased marine P concentrations [7,21,26]. Since, the apparent increase in P concentration occurred around the same time as the first appearance of algal steranes in the biomarker record (780-729 Ma) [3], the two events could be linked [2]. As the availability of phosphorus is regarded as a critical factor regulating phytoplankton and their communities [24], the response of different organisms to nutrient availability should ultimately impact overall community structure [27].…”

Nature and Habitat of pre-Cryogenian Algae

“…These novel insights into the paleo-functionalisation of fossil lipids suggest a significant ecological role of Rhodophytes in Tonian ecosystems. While red algae locally rose to ecological relevance prior to the Cryogenian glaciations, the persistent and global rise of algae was delayed to the radiation of Chlorophytes after the Cryogenian/Ediacaran transition [1,2].…”