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The 2‐methylhopanes (2‐MeHops) are molecular fossils of 2‐methylbacteriohopanepolyols (2‐MeBHPs) and among the oldest biomarkers on Earth. However, these biomarkers' specific sources are currently unexplained, including whether they reflect an expansion of marine cyanobacteria. Here, we study the occurrence of 2‐MeBHPs and the genes involved in their synthesis in modern bacteria and explore the occurrence of 2‐MeHops in the geological record. We find that the gene responsible for 2‐MeBHP synthesis (hpnP) is widespread in cyano‐ and ⍺‐proteobacteria, but absent or very limited in other classes/phyla of bacteria. This result is consistent with the dominance of 2‐MeBHP in cyano‐ and ⍺‐proteobacterial cultures. The review of their geological occurrence indicates that 2‐MeHops are found from the Paleoproterozoic onwards, although some Precambrian samples might be biased by drilling contamination. During the Phanerozoic, high 2‐MeHops' relative abundances (index >15%) are associated with climatic and biogeochemical perturbations such as the Permo/Triassic boundary and the Oceanic Anoxic Events. We analyzed the modern habitat of all hpnP‐containing bacteria and find that the only one species coming from an undisputed open marine habitat is an ⍺‐proteobacterium acting upon the marine nitrogen cycle. Although organisms can change their habitat in response to environmental stress and evolutionary pressure, we speculate that the high sedimentary 2‐MeHops' occurrence observed during the Phanerozoic reflect ⍺‐proteobacteria expansion and marine N‐cycle perturbations in response to climatic and environmental change. [ABSTRACT FROM AUTHOR]

Atmospheric CO2 possibly doubled during Oceanic Anoxic Event (OAE) 1a, likely in response to submarine volcanic outgassing. Despite being important for our understanding of the consequences of carbon cycle perturbations, the response of the climate system to this increase in greenhouse forcing is poorly constrained. Here we provide a new sea-surface temperature (SST) record from the mid-latitude proto-North Atlantic based on the organic geochemical TEX86 paleothermometer. Using different calibrations, including the newly developed Bayesian Spatially-varying Regression (BAYSPAR) deep-time analogue approach, we demonstrate that SSTs increased by ∼2-4 °C during OAE 1a and decreased by ∼4-6 °C at its end, both simultaneous with changes in δ13Corg, which we argue reflects changes in pCO2. We demonstrate that a clear latitudinal SST gradient prevailed during OAE 1a, contrary to the generally accepted view that a nearly flat SST gradient existed during OAE 1a and the Early Cretaceous. These results are more consistent with climate model simulations of the Cretaceous that have failed to produce flat SST gradients. [ABSTRACT FROM AUTHOR]

Variations in atmospheric circulation across the last deglaciation in the northernmost monsoon-influenced regions of Asia are not well constrained, highlighting a fundamental gap in our understanding of Asian climate. Here we reconstruct continental air temperatures for northeast China across the last deglaciation (past 16 k.y.), based on the distribution of bacterial branched glycerol dialkyl glycerol tetraethers in a sequence of the Hani peat (Jilin Province, northeast China). Our results indicate large (as much as 10 °C) oscillations in temperature in northeast China across the deglaciation, oscillations significantly larger than observed in other temperature records from low-latitude or same-latitude East Asia, but consistent with climate model simulations. This enhanced magnitude, as well as the timing of temperature variations, provides evidence for atmospheric teleconnections with high latitudes; in particular, we suggest that highlatitude cooling associated with Arctic ice expansion and changes in Atlantic Meridional Overturning Circulation enhanced the intensity and lowered the temperature of Eurasian mid-latitude westerlies and northwesterly winds over East Asia during the last glacial, delivering cold air masses to northeast China. During the deglaciation the westerlies and therefore delivery of cold air masses weakened, amplifying the deglacial warming in this region. We conclude that changes in North Atlantic climate had a particularly strong impact on the northernmost parts of the East Asian monsoon-influenced area. [ABSTRACT FROM AUTHOR]