Pai-Sen Yu

Sedimentary records of event deposits are crucial for regional natural disaster risk assessments and hazard history reconstructions. After Super Typhoon Haiyan passed through the South China Sea in 2013, five gravity cores were collected along the typhoon path in the southern South China Sea basin (>3800 mbsl). The results showed that Super Typhoon Haiyan deposits with clear graded bedding are preserved at the top of all cores. The thickness of the typhoon layers ranges from 20 to 240 cm and is related to changes in typhoon intensity. The lack of river-connected submarine canyon systems limited the transportation of terrestrial sediments from land to sea. Super Typhoon Haiyan-induced large surface waves played an important role in carrying suspended sediment from the Philippines. The Mn-rich layers at the bottom of the typhoon layers may be related to the soil and rock composition of the Palawan region, which experienced tsunami-like storm surges caused by Super Typhoon Haiyan. These Mn-rich layers may serve as a proxy for sediment export from large-scale extreme terrigenous events. This study provides the first sedimentary record of extreme typhoon events in the deep ocean, which may shed light on reconstructing regional hazard history.

Despite technological advances, some core disturbances due to piston coring process still occur and can be extremely difficult to identify characteristics of sedimentary structures during qualitative visual core description and core lithologies. Few quantitative methods have offered plausible ways to define the exact magnitude of the distorted sedimentary structures in imperfect sedimentary records in the context of overly limited sedimentary records. In this study, an investigation was conducted on the Kaoping slope offshore southwestern Taiwan, a region known for its high sedimentation rate (> 0.9 g/cm²/year), using both the LGD-GPC (giant piston corer) and giant gravity corer techniques. To examine how GPC procedures have shaped the retrieval of sedimentary sequences and affected core quality, we apply well-developed, non-destructive analyses, including physical property measurements, visible color reflectance and digital core images with quantitative curvature index (CI), to recovered sediments. Cores at 22° 23′N, 120° 13′E were as the basis for non-destructive measurement comparison. Evidence from digital core imaging and the CI calculation further supports the mechanism of piston suction. We suggest that shear deformation of sediment against the core barrel would be the prominent factor that govern the observed core disturbances in LGD-GPC cores. Meanwhile, excessive pulling forcing from a modified TORI piston and a slowly lift speed during LGD-GPC coring process may further aggravate core disturbances at the revisited site. The sedimentary length of core LGD-T43-B-PC is overestimated by more than 200%. We further demonstrate that an integration of the CI and color reflectance is an effective way to evaluate the transformation and distorted sedimentary sequences.

The quality of marine sediment cores is significant for investigations on marine geohazards, paleoceanography, paleoclimatology, and marine environmental changes. The identification of genuine or artificial sedimentary structures is essential to the development of long piston coring procedures. In this study, we highlight an overlooked primary process of a standard procedure and its effect on the deformation of a sedimentary sequence. The LGD-T32/LGD-T43 cruises of the R/V Legend were designed to test the capability of 8-m-long piston coring offshore southwestern Taiwan. To examine how giant piston coring procedures have shaped the retrieval of sedimentary sequences and affected core quality, we apply well-developed, non-destructive analyses, including physical property measurements, visible color reflectance and the curvature index, to recovered sediments. The continuous sequences of four giant sediment cores at the revisited site are therefore used to provide the basis for comparison with non-destructive results. Rather than dewatering or lithifying the units themselves, we suggest that during the coring procedure, an abnormal pulling force could destroy the balance among the sediments and thus cause the observed deformations of sedimentary features with different magnitudes. We demonstrate that an integration of the curvature index and color reflectance is a better way to evaluate the transformation and distorted sedimentary sequences. Furthermore, using a Taiwan Ocean Research Institute (TORI)-revised piston could potentially produce an abnormal pulling force inside a tube and causing oversampling and distorted sedimentary features, as evidenced by integrated results.

We investigated an alkenone-based sea surface temperature (SST) and the hydrographic change records of the subarctic Northwestern (NW) Pacific from the last glacial to interglacial. The core we investigated is a piston core (LV 63-41-2, 52.56° N, 160.00° E; water depth 1924 m) retrieved from the southern offshore east coast of the Kamchatka Peninsula, which is a location of high sedimentation rate, with highly dynamic interactions with the cold/warm water masses of the Bering Sea/the NW Pacific. Based on our alkenone analysis with a previously well-established chronology of the core, we found high glacial C_37:4 contents suggesting larger freshwater influences prior to the last deglacial in approximately 27–16 ka BP. The most significant features of what we found are alkenone indicative of “warming” intervals with minimum alkenone productions that occurred prior to the stadial Heinrich event 1 and the Younger Dryas. In contrast, for the interval corresponding to the Bølling–Allerød period, our alkenone analysis shows relatively “colder” but maximum alkenone productions. We conclude that this particular subarctic alkenone SST proxy record is mainly masked by non-thermal environmental influences, such as strong shifts of timing and duration of the sea ice retreat and/or salinity changes in surface water at this site, which could cause changes in water stratification that affect nutrient supplies of the upper ocean that modulate growth durations of phytoplankton/coccolithophore productions. Our studies suggest that this subarctic alkenone “SST” proxy record is indicative of the changes of seasonality that control the timing and duration of the blooming seasons of coccolithophores. The alkenone “SST” proxy is also dominantly driven by water stratification effects that, instead of SSTs, reflect most likely a combination of the following local to regional climate and ocean current patterns: (1) the amount of meltwater inputs from high mountain glaciers at Kamchatka; (2) less saline, nutrient-rich Alaskan Stream waters from the Cordilleran Ice Sheet in the Gulf of Alaska; (3) downwelling waters associated with the interactions between the southward Eastern Kamchatka Current and the spinning-up of the North Pacific Subarctic Gyre; and (4) the strength of the Kuroshio Current since the last glacial.

The nature of the Eocene stratigraphy of the South China Sea (SCS) has been a mystery for decades. In the southern SCS, the Eocene system with marine deposits is often considered as representing part of the ‘Proto-South China Sea’ [ 1]. In the northern SCS, however, the Eocene is dominated by non-marine sequences of South China origin [ 2], probably also related to the Proto-SCS [ 3]. Then the question is what kind of SCS northern margin was in the Eocene: part of the South China continent or part of Proto-SCS or even an eastern extension of the shrinking Tethys Ocean? Furthermore, the significance of the Eocene issue is enhanced by the recent debates on the opening age of the modern SCS, whether in the Eocene or in the early Oligocene [ 4]. The existence of the marine Eocene in the northern SCS was implied by reworked Eocene nannofossils found in offshore oil exploration wells over 20 years ago [ 5] by Eocene foraminifers and nannofossils found in cutting samples from deep-water oil wells [ 6] and by Eocene marine deposits reported recently from Taiwan [ 7]. Until now, however, solid direct evidence of marine Eocene depositions in the northern SCS has been absent. Here, we report new discoveries of deep- and shallow-water marine Eocene sediments based on the results of the International Ocean Discovery Program (IODP) drilling in the northern margin of the SCS.

Radiolarians are a very diverse microzooplanktonic group, often distributed in regionally restricted assemblages and responding to specific environmental factors. These properties of radiolarian assemblages make the group more conducive for the development and application of basin‐wide ecological models. Here we use a new surface sediment data set from the western Pacific to demonstrate that ecological patterns derived from basin‐wide open‐ocean data sets cannot be transferred on semirestricted marginal seas. The data set consists of 160 surface sediment samples from three tropical‐subtropical regions (East China Sea, South China Sea, and western Pacific), combining 54 new assemblage counts with taxonomically harmonized data from previous studies. Multivariate statistical analyses indicate that winter sea surface temperature at 10 m depth (SSTw) was the most significant environmental variable affecting the composition of radiolarian assemblages, allowing the development of an optimal calibration model (Locally Weighted‐Weighted Averaging regression inverse deshrinking,R²_cv = 0.88, root‐mean‐square error of prediction = 1.6°C). The dominant effect of SSTw on radiolarian assemblage composition in the western Pacific is attributed to the East Asian Winter Monsoon (EAWM), which is particularly strong in the marginal seas. To test the applicability of the calibration model on fossil radiolarian assemblages from the marginal seas, the calibration model was applied to two downcore records from the Okinawa Trough, covering the last 18 ka. We observe that these assemblages find most appropriate analogs among modern samples from the marginal basins (East China Sea and South China Sea). Downcore temperature reconstructions at both sites show similarities to known regional SST reconstructions, providing proof of concept for the new radiolarian‐based SSTw calibration model.