Submerged carbonate reefs are preserved as a series of submarine terraces between Molokai and Hawaii along a 200‐km span of the southeastern Hawaiian Ridge. Limestones from two of the terraces have been sampled from submersibles and dated radiometrically at 13 and 120 ka. Recognition that the terraces are tilted permits assignment of about a dozen terraces from 150 to 1300 m depth to eight general reef platforms. These reefs were drowned by the combined effects of island subsidence and sea level rise at the end of successive glacial stages from 13 to 647 ka. The platforms are tilted 5 m/km southeast toward the locus of volcanic loading centered on the island of Hawaii. The end of the active period of volcanic shield building is represented on each nonactive volcano by a submerged change in slope. Dating of this slope change by reference to the dated reefs provides independent evidence that shield building ended at about the same time that the volcano changed from eruption of tholeiitic to alkalic basalt. The variation in depth of the dated tilted reefs along the ridge indicates that the end of subsidence follows the end of shield building by about one half million years. The present northwest margin of major subsidence probably crosses southeast Maui. During the last half million years the rate of southeast migration of the beginning of both subsidence and of tholeiitic volcanism does not appear to have increased. However, the rate of southeast migration of the end of both subsidence and of tholeiitic volcanism has apparently increased. This shortening of the life history of the younger volcanoes implies an increase in eruption rates.

With nearly 1 million observations of column-mean carbon dioxide concentration (X CO 2 ) per day, the Orbiting Carbon Observatory 2 (OCO-2) presents exciting possibilities for monitoring the global carbon cycle, including the detection of subcontinental column CO 2 variations. While the OCO-2 data set has been shown to achieve target precision and accuracy on a single-sounding level, the validation of X CO 2 spatial gradients on subcontinental scales remains challenging. In this work, we investigate the use of an integrated path differential absorption (IPDA) lidar for evaluation of OCO-2 observations via NASA's Atmospheric Carbon and Transport (ACT)-America project. The project has completed eight clear-sky underflights of OCO-2 with the Multifunctional Fiber Laser Lidar (MFLL)-along with a suite of in situ instruments-giving a precisely colocated, high-resolution validation data set spanning nearly 3,800 km across four seasons. We explore the challenges and opportunities involved in comparing the MFLL and OCO-2 X CO 2 data sets and evaluate their agreement on synoptic and local scales. We find that OCO-2 synoptic-scale gradients generally agree with those derived from the lidar, typically to ±0.1 ppm per degree latitude for gradients ranging in strength from 0 to 1 ppm per degree latitude. CO 2 reanalysis products also typically agree to ±0.25 ppm per degree when compared with an in situ-informed CO 2 "curtain." Real X CO 2 features at local scales, however, remain challenging to observe and validate from space, with correlation coefficients typically below 0.35 between OCO-2 and the MFLL. Even so, ACT-America data have helped investigate interesting local X CO 2 patterns and identify systematic spurious cloud-related features in the OCO-2 data set.

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We present an evaluation of airborne Intensity-Modulated Continuous-Wave (IM-CW) lidar measurements of atmospheric column CO2 mole fractions during the ACT-America project. This lidar system transmits online and offline wavelengths simultaneously on the 1.57111-µm CO2 absorption line, with each modulated wavelength using orthogonal swept frequency waveforms. After the spectral characteristics of this system were calibrated through short-path measurements, we used the HITRAN spectroscopic database to calculate the average-column CO2 mole fraction (XCO2) from the lidar measured optical depths. Using in situ measurements of meteorological parameters and CO2 concentrations for calibration data, we demonstrate that our lidar CO2 measurements were consistent from season to season and had an absolute calibration error (standard deviation) of 0.80 ppm when compared to XCO2 values calculated from in situ measurements. By using a 10-second or longer moving average, a precision of 1 ppm or better was obtained. The estimated CO2 measurement precision for 0.1-s, 1-s, 10-s, and 60-s averages were determined to be 3.4 ppm 1.2 ppm, 0.43 ppm, and 0.26 ppm, respectively. These correspond to measurement signal-to-noise ratios of 120, 330, 950, and 1600, respectively. The drift in XCO2 over one-hour of flight time was found to be below 0.1 ppm. These analyses demonstrate that the measurement stability, precision and accuracy are all well below the thresholds needed to study synoptic-scale variations in atmospheric XCO2. This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as

 ACT-America provides a unique, weather-oriented collection of high-quality measurements of atmospheric CO2, CH4, trace gases and meteorological properties. ACT-America data are free and open to the public from the ORNL DAAC. ACT-America data are uniquely suited to improve the accuracy and precision of regional inverse GHG flux estimates.