Search for: All records

While high latitude amplification is seen in modern observations, paleoclimate records, and climate modeling, better constraints on the magnitude and pattern of amplification would provide insights into the mechanisms that drive it, which remain actively debated. Here we present multi-proxy multi-site paleotemperature records over the last 10 million years from the Western Pacific Warm Pool (WPWP) – the warmest endmember of the global ocean that is uniquely important in the global radiative feedback change. These sea surface temperature records, based on lipid biomarkers and seawater Mg/Ca-adjusted foraminiferal Mg/Ca, unequivocally show warmer WPWP in the past, and a secular cooling over the last 10 million years. Compiling these data with existing records reveals a persistent, nearly stationary, extratropical response pattern in the Pacific in which high latitude (~50°N) temperatures increase by ~2.4° for each degree of WPWP warming. This relative warming pattern is also evident in model outputs of millennium-long climate simulations with quadrupling atmospheric CO₂, therefore providing a strong constraint on the future equilibrium response of the Earth System.

 

Archaeal membrane lipids are widely used for paleotemperature reconstructions, yet these molecular fossils also bear rich information about ecology and evolution of marine ammonia-oxidizing archaea (AOA). Here we identified thermal and nonthermal behaviors of archaeal glycerol dialkyl glycerol tetraethers (GDGTs) by comparing the GDGT-based temperature index (TEX 86 ) to the ratio of GDGTs with two and three cyclopentane rings (GDGT-2/GDGT-3). Thermal-dependent biosynthesis should increase TEX 86 and decrease GDGT-2/GDGT-3 when the ambient temperature increases. This presumed temperature-dependent (PTD) trend is observed in GDGTs derived from cultures of thermophilic and mesophilic AOA. The distribution of GDGTs in suspended particulate matter (SPM) and sediments collected from above the pycnocline—shallow water samples—also follows the PTD trend. These similar GDGT distributions between AOA cultures and shallow water environmental samples reflect shallow ecotypes of marine AOA. While there are currently no cultures of deep AOA clades, GDGTs derived from deep water SPM and marine sediment samples exhibit nonthermal behavior deviating from the PTD trend. The presence of deep AOA increases the GDGT-2/GDGT-3 ratio and distorts the temperature-controlled correlation between GDGT-2/GDGT-3 and TEX 86 . We then used Gaussian mixture models to statistically characterize these diagnostic patterns of modern AOA ecology from paleo-GDGT records to infer the evolution of marine AOA from the Mid-Mesozoic to the present. Long-term GDGT-2/GDGT-3 trends suggest a suppression of today’s deep water marine AOA during the Mesozoic–early Cenozoic greenhouse climates. Our analysis provides invaluable insights into the evolutionary timeline and the expansion of AOA niches associated with major oceanographic and climate changes. 

Throughout Earth's history, CO 2 is thought to have exerted a fundamental control on environmental change. Here we review and revise CO 2 reconstructions from boron isotopes in carbonates and carbon isotopes in organic matter over the Cenozoic—the past 66 million years. We find close coupling between CO 2 and climate throughout the Cenozoic, with peak CO 2 levels of ∼1,500 ppm in the Eocene greenhouse, decreasing to ∼500 ppm in the Miocene, and falling further into the ice age world of the Plio–Pleistocene. Around two-thirds of Cenozoic CO 2 drawdown is explained by an increase in the ratio of ocean alkalinity to dissolved inorganic carbon, likely linked to a change in the balance of weathering to outgassing, with the remaining one-third due to changing ocean temperature and major ion composition. Earth system climate sensitivity is explored and may vary between different time intervals. The Cenozoic CO 2 record highlights the truly geological scale of anthropogenic CO 2 change: Current CO 2 levels were last seen around 3 million years ago, and major cuts in emissions are required to prevent a return to the CO 2 levels of the Miocene or Eocene in the coming century. ▪  CO 2 reconstructions over the past 66 Myr from boron isotopes and alkenones are reviewed and re-evaluated. ▪  CO 2 estimates from the different proxies show close agreement, yielding a consistent picture of the evolution of the ocean-atmosphere CO 2 system over the Cenozoic. ▪  CO 2 and climate are coupled throughout the past 66 Myr, providing broad constraints on Earth system climate sensitivity. ▪  Twenty-first-century carbon emissions have the potential to return CO 2 to levels not seen since the much warmer climates of Earth's distant past. 

The geological record encodes the relationship between climate and atmospheric carbon dioxide (CO₂) over long and short timescales, as well as potential drivers of evolutionary transitions. However, reconstructing CO₂beyond 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 CO₂record 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 CO₂thresholds in biological and cryosphere evolution.

 

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. 

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.