Geology topics

Masanobu Yamamoto

Publications and source records attributed to Masanobu Yamamoto.

4 recordsLinked to original sources

USGS2026

Microfocus X-ray computed tomography to evaluate ostracode shell density and other metrics for monitoring ocean acidification in the Arctic Ocean

Ocean acidification (OA) poses a significant threat to the ability of marine calcifying organisms—such as mollusks, corals, crustaceans, and plankton—to form and maintain their calcium carbonate-based shells. While some species show greater sensitivity to OA than others, the impacts on benthic marine calcifiers remain inadequately understood. This study investigates the shell density, weight, and thickness of the ostracode species Paracyprideis pseudopunctillata (subphylum Crustacea), which inhabits the continental shelves of the Arctic Ocean. Using microfocus X-ray computed tomography (microXCT) scans, we generated detailed morphometric measurements of specimens collected from both surface and down-core sediment samples to assess variations in shell properties over time. To our knowledge, this is the first study to examine the natural variability of ostracode shell characteristics in a specific region over an extended time period. For each specimen, we calculated a CT number, an index of relative calcium carbonate density, using a previously published equation. Our findings show that most living specimens of P. pseudopunctillata in the Beaufort Sea showed no overt signs of OA-related shell compromise or degradation. Shell density remained consistent between 1969 and 2018 CE, with consistent CT numbers suggesting that corrosive bottom waters are not a persistent feature in this area of the Beaufort Sea continental shelf. This is supported by a timeseries of discrete pH and carbonate saturation measurements. To help quantify diagenetic impacts on ostracode shells, we compared this calibration dataset with measurements of well-preserved adult specimens from downcore intervals of a box core (MR22, MT1, 1-29 cm) and a multicore (HLY1302 MC29, 25-35 cm), both collected from the same location, representing 2018 to 1955 CE and 1953 to 1925 CE, respectively. The CT numbers from these specimens were also consistent, which confirmed the preservation state of most specimens. However, several specimens had low CT numbers, and these shells show surficial evidence of post-mortem diagenesis. We aimed to quantify the extent of diagenesis, which has important implications for paleoenvironmental reconstructions. These findings provide important baseline data for understanding the variability of modern ostracode shell density of fossil and alive-collected specimens.

Deep Sea Research Part II: Topical Studies in Ocea↗

USGS2025

Rapid Holocene deposition in the Mackenzie Trough and Barrow Canyon areas in the western Arctic Ocean

The Arctic Ocean and terrestrial environment have recently been reported to be changing drastically, but it is unclear whether these changes are similar to natural variations in the past or how sudden and large the changes are compared to natural variations. This premise served as motivation to collect sediment cores during the summer of 2022 at four sites on the Canadian continental shelf and Alaskan upper continental slope to reconstruct changes in the marine and terrestrial environments to provide a comprehensive picture of the ocean environment during the preindustrial period before anthropogenic influences. We dated the sediments based on the 137 Cs radioactivity of bulk sediments and the 14 C concentrations of mollusk shells. The 137 Cs radioactivity shows a distinct onset corresponding to 1950 Common Era (CE) and the most prominent peak corresponding to 1963 CE. Multiple peaks appeared above the most prominent one, coinciding with nuclear power plant accidents in 1986 and 2011. Inventories of excess 210 Pb in all cores exceed the estimated supply of excess 210 Pb from atmospheric deposition, likely due to the scavenging supply of excess 210 Pb. By comparing 137 Cs and radiocarbon conventional ages, we estimated the local radiocarbon reservoir age value of each site. Using these local radiocarbon reservoir age and the conventional ages of mollusk shell samples, we established the age-depth models by the Bayesian method. The optimal ΔR values were 598, 511, 65, and –60 years at the MT1, MT2, BC2, and BC2-2 sites, respectively. The cores consist of clayey silts continuously deposited with uniquely high sedimentation rates of 0.17 to 0.74 cm y −1 . Variation in the Ca/Ti ratio indicates ~ 20, ~ 30, 50–60, 100–125, and 300-year cycles, likely attributed to the variation in the Aleutian Low that controls the Bering Strait inflow of Pacific waters influencing our core sites. These sediments will be used for further high-resolution, multi-proxy studies with forthcoming results.

western Arctic Ocean↗

USGS2021

Remote and local drivers of Pleistocene South Asian summer monsoon precipitation: A test for future predictions

South Asian precipitation amount and extreme variability are predicted to increase due to thermodynamic effects of increased 21st-century greenhouse gases, accompanied by an increased supply of moisture from the southern hemisphere Indian Ocean. We reconstructed South Asian summer monsoon precipitation and runoff into the Bay of Bengal to assess the extent to which these factors also operated in the Pleistocene, a time of large-scale natural changes in carbon dioxide and ice volume. South Asian precipitation and runoff are strongly coherent with, and lag, atmospheric carbon dioxide changes at Earth’s orbital eccentricity, obliquity, and precession bands and are closely tied to cross-equatorial wind strength at the precession band. We find that the projected monsoon response to ongoing, rapid high-latitude ice melt and rising carbon dioxide levels is fully consistent with dynamics of the past 0.9 million years.

Science Advances↗

USGS2012

Out of the tropics: the Pacific, Great Basin lakes, and late Pleistocene water cycle in the western United States

The water cycle in the western U.S. changed dramatically over glacial cycles. In the last 20,000 years, higher precipitation caused desert lakes to form which have since dried out. Higher glacial precipitation is hypothesized to result from a southward shift of Pacific winter storm tracks. We compared Pacific Ocean data to lake levels from the interior west and found that Great Basin lake high stands are older than coastal wet periods at the same latitude. Westerly storms were not the source of high precipitation. Instead, air masses from the tropical Pacific were transported northward, bringing more precipitation into the Great Basin when coastal California was still dry. The changing climate during the deglaciation altered precipitation source regions and strongly affected the regional water cycle.

Science↗