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The million-year variability of the marine nitrogen cycle is poorly understood. Before 57 million years (Ma) ago, the 15 N/ 14 N ratio (δ 15 N) of foraminifera shell-bound organic matter from three sediment cores was high, indicating expanded water column suboxia and denitrification. Between 57 and 50 Ma ago, δ 15 N declined by 13 to 16 per mil in the North Pacific and by 3 to 8 per mil in the Atlantic. The decline preceded global cooling and appears to have coincided with the early stages of the Asia-India collision. Warm, salty intermediate-depth water forming along the Tethys Sea margins may have caused the expanded suboxia, ending with the collision. From 50 to 35 Ma ago, δ 15 N was lower than modern values, suggesting widespread sedimentary denitrification on broad continental shelves. δ 15 N rose at 35 Ma ago, as ice sheets grew, sea level fell, and continental shelves narrowed., (Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

The history of atmospheric O 2 partial pressures (Po 2 ) is inextricably linked to the coevolution of life and Earth's biogeochemical cycles. Reconstructions of past Po 2 rely on models and proxies but often markedly disagree. We present a record of Po 2 reconstructed using O 2 /N 2 ratios from ancient air trapped in ice. This record indicates that Po 2 declined by 7 per mil (0.7%) over the past 800,000 years, requiring that O 2 sinks were ~2% larger than sources. This decline is consistent with changes in burial and weathering fluxes of organic carbon and pyrite driven by either Neogene cooling or increasing Pleistocene erosion rates. The 800,000-year record of steady average carbon dioxide partial pressures (Pco 2 ) but declining Po 2 provides distinctive evidence that a silicate weathering feedback stabilizes Pco 2 on million-year time scales., (Copyright © 2016, American Association for the Advancement of Science.)

Methane is an important greenhouse gas and energy resource generated dominantly by methanogens at low temperatures and through the breakdown of organic molecules at high temperatures. However, methane-formation temperatures in nature are often poorly constrained. We measured formation temperatures of thermogenic and biogenic methane using a "clumped isotope" technique. Thermogenic gases yield formation temperatures between 157° and 221°C, within the nominal gas window, and biogenic gases yield formation temperatures consistent with their comparatively lower-temperature formational environments (<50°C). In systems where gases have migrated and other proxies for gas-generation temperature yield ambiguous results, methane clumped-isotope temperatures distinguish among and allow for independent tests of possible gas-formation models., (Copyright © 2014, American Association for the Advancement of Science.)