地球科学最新文献

The Atlantic Meridional Overturning Circulation is the main driver of northward heat transport in the Atlantic Ocean today, setting global climate patterns. Whether global warming has affected the strength of this overturning circulation over the past century is still debated: observational studies suggest that there has been persistent weakening since the mid-twentieth century, whereas climate models systematically simulate a stable circulation. Here, using Earth system and eddy-permitting coupled ocean–sea-ice models, we show that a freshening of the subarctic Atlantic Ocean and weakening of the overturning circulation increase the temperature and salinity of the South Atlantic on a decadal timescale through the propagation of Kelvin and Rossby waves. We also show that accounting for upper-end meltwater input in historical simulations significantly improves the data–model agreement on past changes in the Atlantic Meridional Overturning Circulation, yielding a slowdown of 0.46 sverdrups per decade since 1950. Including estimates of subarctic meltwater input for the coming century suggests that this circulation could be 33% weaker than its anthropogenically unperturbed state under 2 °C of global warming, which could be reached over the coming decade. Such a weakening of the overturning circulation would substantially affect the climate and ecosystems.

The solar cycle includes multi-scale variations in the near-Earth space regions including plasmasphere, inner radiation belt (IRB), ionosphere, mesosphere and lower thermosphere (MLT). We present a thorough analysis of the extent of solar cycle effect on those four regions by using mesospheric and thermospheric geopotential height and temperature from SABER on TIMED, ionospheric hmF2 from Chinese Meridian Project, high-energy protons in IRB and electron density in plasmasphere from Van Allen Probes within 2013–2018 intervals. By analyzing evolutions of these quantities, we find that entire IRB, ionosphere and MLT region shrink at solar minimum and stretch at solar maximum by ∼10³, 50–10², and 1 km scales, respectively, while plasmapause shows an opposite trend. Fourier spectra of these quantities have been investigated by Lomb–Scargle periodogram. The mid-term periodic oscillations (13.5-day, 45-day, and 52-day) have been observed in MLT region, matching well with plasmapause locations and geomagnetic indices, which have not been observed in solar EUV radiation and IRB. This may indicate that those oscillations facilitate energy exchange and mass transportation between MLT region and plasmasphere due to magnetic storms and substorms. The oscillation periods of higher energy (102.6 MeV) in IRB have not been observed in MLT region except for annual variations. The impact of higher energy protons on MLT regions may not be significant, although they could penetrate deeper into MLT region. Our results reveal relationships between some quantities and solar cycle multi-scale modulation, which may provide assistance and monitors for mass transportation in the near-Earth space regions.

The influence of upper ocean dynamics on the acoustic field in the South Eastern Arabian Sea (SEAS) is studied using in situ oceanographic/acoustic measurements from a moored buoy, along with satellite-derived and climatological data sets. Upper-ocean variability at the site is quantified using Mixed Layer Depth (MLD), Isothermal Layer Depth (ILD), Barrier Layer Thickness (BLT), Maximum Spice Depth (MSD), and Sonic Layer Depth (SLD), along with surface variability factors such as Sea Surface Temperature, Sea Surface Salinity, Spice, and Sea Level Anomaly. The mixed layer acoustic duct (MLAD) varies from 2 to 100 m, with BLT varying from 5 to 99 m, and a mean SLD of 43 m. A thick transition layer connects the mixed layer with the thermocline during winter. The observations reveal that maximum SLD, MSD, and BLT occurred during January–March. Unlike other seasons when SLD follows MLD, winter SLD is influenced by BLT, suggesting strong salinity stratification due to low-salinity water intrusion from the Bay of Bengal by East India Coastal Current. During these months, the SLD varies from 80 to 100 m, with the corresponding minimum cut-off frequency varying from 300 to 200 Hz. Results are correlated with estimated Sound Pressure Level (SPL) from Ambient Noise Measurements during November 2018 to November 2019. SPL variation follows SLD for low and mid-frequencies, with the highest SPL noted during January-February. Acoustic propagation simulations at 250 and 1,000 Hz revealed features like acoustic duct leakage and channeling, indicating energy transfers between the surface acoustic duct and deeper layers.

The wide distribution of tuff layers, locally named the “green bean rocks” (GBRs) in the Yangtze Block straddling the Early Middle Triassic marine sequence indicates intense volcanic eruption(s). Sr, Nd, and S isotope compositions and trace elements of marine sediments were analyzed spanning the tuff layers to elucidate their responses to the volcanic eruptions and related environmental changes. The Sr isotope compositions of marine sediments are comparable to those of open seawater during the time interval of ca. 245–248 Ma. Sr and Nd isotope compositions of the samples show synchronous increases in the ⁸⁷Sr/⁸⁶Sr ratios and εNd(t) values during the deposition of GBRs. The elevated ⁸⁷Sr/⁸⁶Sr ratios and εNd(t) values are proposed to be caused by the input of volcanic tephra and increased influx of weathering product of mafic rocks (most likely the Emeishan flood basalts). The S isotope compositions of sulfates exhibit a negative shift in the GBRs, which could possibly be attributed to greater input of lighter ³²S from weathering products and volcanic eruptions. The variation of Th/U ratios indicate that the GBRs formed in an anoxic environment, resulting from high marine productivity as a consequence of more nutrients from weathering and volcanic materials. The responses of Sr, Nd, and S isotopes to volcanic eruptions during the Early Middle Triassic indicate this event resulted in adverse effects, namely enhanced eutrophication and low O₂ levels, acidic precipitation, toxic components, etc., that could cause ecological destruction both on land and in the sea.