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Branched glycerol dialkyl glycerol tetraethers (brGDGTs) with lower (sparsely-branched; SB-) and higher (overly-branched; OB-) numbers of methylated branches relative to the “regular” brGDGTs (B-GDGTs) are abundant in anoxic waters in the Black Sea. Observed changes in abundances and numbers of methylated branches of the entire series OB-GDGTs, B-GDGTs, and SB-GDGTs relative to dissolved oxygen (DO) levels in anoxic waters suggest that these compounds can potentially track changes in oceanic DO levels through time. To explore this, we determine the entire brGDGT series in surface or near-surface sediments from sites with different DO distributions in marine waters and sediments, extending the limited core-top collection of these lipids. We propose a modified methylation index based on only OB-GDGTs, called MOB, to avoid the potential impacts of terrestrial-derived B-GDGTs. Interestingly, MOB values in our extended core-top collection are strongly related to changes in bottom-water DO concentrations rather than the site-specific minimum DO values, i.e. usually within mid-depth oxygen minimum zones (OMZs). This suggests that sedimentary lipids are likely derived from heterotrophic bacteria living at the sediment-water boundary in sediments while lipids produced within mid-depth OMZs are not effectively exported to deep oceans. Analysis of MOB values in ancient sediments in the East Equatorial Pacific shows a gradual decline in bottom water DO, correlating with the progressive increase in global export productivity, organic carbon burial, and elevated level of deep-water nutrient contents since the middle Miocene. These findings highlight the potential of MOB as a tool for reconstructing past oceanic (de)oxygenation events.

The geological record encodes the relationship between climate and atmospheric carbon dioxide (CO 2 ) over long and short timescales, as well as potential drivers of evolutionary transitions. However, reconstructing CO 2 beyond direct measurements requires the use of paleoproxies and herein lies the challenge, as proxies differ in their assumptions, degree of understanding, and even reconstructed values. In this study, we critically evaluated, categorized, and integrated available proxies to create a high-fidelity and transparently constructed atmospheric CO 2 record spanning the past 66 million years. This newly constructed record provides clearer evidence for higher Earth system sensitivity in the past and for the role of CO 2 thresholds in biological and cryosphere evolution. Editor’s summary The concentration of atmospheric carbon dioxide is a fundamental driver of climate, but its value is difficult to determine for times older than the roughly 800,000 years for which ice core records are available. The Cenozoic Carbon dioxide Proxy Integration Project (CenCO2PIP) Consortium assessed a comprehensive collection of proxy determinations to define the atmospheric carbon dioxide record for the past 66 million years. This synthesis provides the most complete record yet available and will help to better establish the role of carbon dioxide in climate, biological, and cryosphere evolution. — H. Jesse Smith

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Hofig, D., Zhang, Y. G., Giosan, L., Leng, Q., Liang, J., Wu, M., Miller, B., & Yang, H. Annually resolved sediments in the classic Clarkia lacustrine deposits (Idaho, USA) during the middle Miocene Climate Optimum. Geology, 49(8), (2021): 916–920, https://doi.org/10.1130/G48901.1., The world-renowned Miocene Clarkia paleolake in northern Idaho (USA) is closely associated with Columbia River Basalt Group volcanism. The flood basalt dammed a local drainage system to form the paleolake, which preserved a plant fossil Lagerstätte in its deposits. However, the precise age and temporal duration of the lake remain unsettled. We present the first unequivocal U-Pb zircon ages from interbedded volcanic ashes at the P-33 type location, constraining the deposition to 15.78 ± 0.039 Ma. Using micro–X-ray fluorescence and petrographic and spectral analyses, we establish the annual characteristics of laminations throughout the stratigraphic profile using the distribution of elemental ratios, mineral assemblages, and grain-size structures, as well as organic and fossil contents. Consequently, the ~7.5-m-thick varved deposit at the type location P-33 represents ~840 yr of deposition, coincident with the end of the main phase of Columbia River Basalt Group eruptions during the Miocene Climate Optimum. The timing and temporal resolution of the deposit offer a unique opportunity to study climate change in unprecedented detail during global warming associated with carbon-cycle perturbations., This research was supported by U.S. National Science Foundation (NSF) grants EAR-1806015 to Zhang and EAR-1804511 to Yang and Leng. Zircon U-Pb analyses were made possible by the NSF-Major Research Instrumentation grant OCE-1626244 and the Texas A&M Research Development Fund award to Miller. We are grateful to the Kienbaum family for providing access to their private property to sample the P-33 site, and to Kunguang Yang from China University of Geosciences at Wuhan for the LA-ICP-MS analysis.

As the world warms, there is a profound need to improve projections of climate change. Although the latest Earth system models offer an unprecedented number of features, fundamental uncertainties continue to cloud our view of the future. Past climates provide the only opportunity to observe how the Earth system responds to high carbon dioxide, underlining a fundamental role for paleoclimatology in constraining future climate change. Here, we review the relevancy of paleoclimate information for climate prediction and discuss the prospects for emerging methodologies to further insights gained from past climates. Advances in proxy methods and interpretations pave the way for the use of past climates for model evaluation—a practice that we argue should be widely adopted.