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Abstract A giant mass-transport complex was recently discovered in the eastern Arabian Sea, exceeding in volume all but one other known complex on passive margins worldwide. The complex, named the Nataraja Slide, was drilled by International Ocean Discovery Program (IODP) Expedition 355 in two locations where it is ∼300 m (Site U1456) and ∼200 m thick (Site U1457). The top of this mass-transport complex is defined by the presence of both reworked microfossil assemblages and deformation structures, such as folding and faulting. The deposit consists of two main phases of mass wasting, each consisting of smaller pulses, with generally fining-upward cycles, all emplaced just prior to 10.8 Ma based on biostratigraphy. The base of the deposit at each site is composed largely of matrix-supported carbonate breccia that is interpreted as the product of debris-flows. In the first phase, these breccias alternate with well-sorted calcarenites deposited from a high-energy current, coherent limestone blocks that are derived directly from the Indian continental margin, and a few clastic mudstone beds. In the second phase, at the top of the deposit, muddy turbidites dominate and become increasingly more siliciclastic. At Site U1456, where both phases are seen, a 20-m section of hemipelagic mudstone is present, overlain by a ∼40-m-thick section of calcarenite and slumped interbedded mud and siltstone. Bulk sediment geochemistry, heavy-mineral analysis, clay mineralogy, isotope geochemistry, and detrital zircon U-Pb ages constrain the provenance of the clastic, muddy material to being reworked, Indus-derived sediment, with input from western Indian rivers (e.g., Narmada and Tapti rivers), and some material from the Deccan Traps. The carbonate blocks found within the breccias are shallow-water limestones from the outer western Indian continental shelf, which was oversteepened from enhanced clastic sediment delivery during the mid-Miocene. The final emplacement of the material was likely related to seismicity as there are modern intraplate earthquakes close to the source of the slide. Although we hypothesize that this area is at low risk for future mass wasting events, it should be noted that other oversteepened continental margins around the world could be at risk for mass failure as large as the Nataraja Slide.

The spatial and temporal resolutions of contemporary microwave-based sea surface salinity (SSS) measurements are insufficient. Thus, we developed a gap-free gridded daily SSS product with higher spatial and temporal resolutions, which can provide information on short-term variability in the East China Sea (ECS), such as the front changes by Changjiang diluted water (CDW). Specifically, we conducted gap-filling for daily SSS products based on the Geostationary Ocean Color Imager (GOCI) with a spatial resolution of 1 km (0.01°), using a machine learning approach during the summer seasons from 2015 to 2019. The comparison of the Soil Moisture Active Passive (SMAP), Copernicus Marine Environment Monitoring Service (CMEMS), and Hybrid Coordinate Ocean Model (HYCOM) SSS products with the GOCI-derived SSS over the entire SSS range showed that the SMAP SSS was highly consistent, whereas the HYCOM SSS was the least consistent. In the < 31 psu range, the SMAP SSS was still the most consistent with the GOCI-derived SSS (R2=0.46 ; root mean squared error: RMSE = 2.41 psu); in the > 31 psu range, the CMEMS and HYCOM SSS products showed similar levels of agreement with that of the SMAP SSS. We trained and tested three machine learning models – the fine trees, boosted trees, and bagged trees models – using the daily GOCI-derived SSS as output, including the three SSS products, environmental variables, and geographical data. We combined the three SSS products to construct input datasets for machine learning. Using the test dataset, the bagged trees model showed the best results (mean R2=0.98 and RMSE = 1.31 psu), and the models that used the SMAP SSS as input had the highest level. For the dataset in the > 31 psu range, all the models exhibited similarly reasonable performances (RMSE = 1.25–1.35 psu). The comparison with in situ SSS data, time series analysis, and the spatial SSS distribution derived from models showed that all the models had proper CDW distributions with reasonable RMSE levels (0.91–1.56 psu). In addition, the CDW front derived from the model gap-free daily SSS product clearly demonstrated the daily oceanic mechanism during the summer season in the ECS at a detailed spatial scale. Notably, the CDW front in the zonal direction, as captured by the Ieodo Ocean Research Station (I-ORS), moved approximately 3.04 km d -1 in 2016, which is very fast compared with the cases in other years. Our model yielded a gap-free gridded daily SSS product with reasonable accuracy and enabled the successful recognition of daily SSS fronts at the 1 km level, which was previously not possible with ocean color data. Such successful application of machine learning models can further provide useful information on the long-term variation of daily SSS in the ECS. The gridded gap-free SSS dataset at 0.01°×0.01° spatial resolution is freely available at 10.22808/DATA-2023-2 (Shin et al., 2023). [ABSTRACT FROM AUTHOR]

Glaciomarine laminated muds around the Antarctic continental margin are important in the marine geological record related to ice sheet dynamics. Microscopic observation and backscattered electron imagery of Pleistocene laminated muds in the Central Basin (Ross Sea) reveal that the light laminae comprise terrigenous angular to subangular silt‐sized particles, scattered diatom fragments, and eroded sand‐sized lumps of fossil‐bearing mud. In contrast, the dark laminae are clayey and biogenic with very tiny pieces of fossils. These laminated muds are interpreted to have been deposited by subglacial meltwater plumes underneath the advancing glaciers that torn off the earlier‐deposited and semi‐consolidated diatom‐rich sediments. Thus, most biogenic components of these laminated muds were recycled from older deposits, indicating that they are not related to enhanced biological production during the mud deposition. Our study suggests that the recycling of biogenic particles should be considered when interpreting the palaeoclimatic and palaeoceanographic implications of Antarctic environmental system. [ABSTRACT FROM AUTHOR]