Testing the ancient marine redox record from oxygenic photosynthesis to photic zone euxina

Abstract

Tracing the evolution of Earth's redox history is one of the great challenges of geobiology and geochemistry. The accumulation of photosynthetically derived oxygen transformed the redox state of Earth's surface environments, setting the stage for the subsequent evolution of complex life. However, the timing of the advent of oxygenic photosynthesis relative to the Great Oxidation Event (GOE; ~2.4 Ga) is poorly constrained. After the deep ocean became oxygenated in the early Phanerozoic, hydrogen sulfide, which is toxic to most aerobes, may have transiently accumulated in the marine photic zone (i.e. photic zone euxinia; PZE) during mass extinctions and oceanic anoxic events. Here, the molecular fossil evidence for oxygenic photosynthesis and eukaryotes is reevaluated, where the results imply that currently existing lipid biomarkers are contaminants. Next, the stratigraphic distribution of green and purple sulfur bacteria biomarkers through geologic time is evaluated to test whether these compounds reflect a water column sulfide signal, which is implicit in their utility as PZE paleoredox proxies. Results from a modern case study underscore the need to consider allochthonous and microbial mat sources and the role of basin restriction as alternative explanations for these biomarkers in the geologic record, in addition to an autochthonous planktonic source.

Carotenoid biomarkers in Namibian shelf sediments: Anoxygenic photosynthesis during sulfide eruptions in the Benguela Upwelling System

“…Lipid Extraction and Fractionation. In 2014, a total of 14 samples from the regularly monitored stations were lyophilized and extracted under low-light conditions for pigment analysis by HPLC-MS (SI Appendix, Table S2) (58). Freeze-dried sediment was weighed into a clean centrifuge tube and extracted by sonication with acetone in an ice bath while simultaneously protecting the extract from light.…”

“…Instrument Measurements and Compound Identification. HPLC-MS analyses, conducted using published methods with minor modifications (58,(78)(79)(80), were performed on an Agilent 1200 series HPLC coupled to an Agilent 6520 Q-TOF MS, which was operated in auto MS/MS mode where a maximum of three precursors were selected for MS/MS fragmentation per cycle. The HPLC was equipped with an Agilent Poroshell 120 EC-C18 column (2.1 × 150 mm, 2.7 AM) column with a precolumn (EC-C18 2.1 × 5 mm; 2.7 Vm) using a solvent flow rate of 0.4 mL/min and gradient of 20% solvent B (0 min) to 100% solvent B (28.5 min), hold 100% solvent B (48.5 min), return to 20% B (50 min), finally reconditioning at 20% B until a final time of 60 min where A was 80:20 (v:v) methanol:0.05 M ammonium acetate and B was 80:10:10 (v:v:v) methanol:acetonitrile:ethyl acetate.…”

“…Sediment samples were collected in March 2014 during a cruise of the research vessel Mirabilis belonging to the Namibian Ministry of Fisheries and Marine Resources as part of the 2014 Regional Graduate Network for Oceanography Discovery Camp (58). Samples were collected from regularly monitored stations between Walvis Bay and Luderitz at the 23°, 24°, 25°, 26°, and 27°S latitudinal transects in water depths ranging from 30 to 260 m. Some previously published data from these sites include information on water depths and depth of the photic zone (59) (Fig.…”

“…The "no modern analog" problem, which refers to the absence of modern, unrestricted marine environments that host GSB and PSB and their associated pigments in the water column, contributes uncertainty to the interpretation of these aromatic carotenoids in ancient marine sediments (4,58). Because hydrogen sulfide rarely accumulates in the photic zone of contemporary, openocean water columns, the interpretation of these fossilized pigments in marine sedimentary archives is largely based on modern lacustrine systems and enclosed euxinic basins, such as the Black Sea (33,87,88).…”