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doi:10.2204/iodp.proc.346.101.2015

Preliminary scientific assessment

Even the early results from the 6 weeks of core recovery during the 2 month expedition have more than met the original scientific objectives. The high quality of materials recovered and the complete documentation of their geological, geochemical, and geophysical context will lead to an unparalleled series of future studies by the expedition scientific party as well as many other scientists over the coming decades.

Furthermore, because of recent and novel advances in drilling technology and newly developed analytical tools, we were able to collect and examine sediment records that were impossible to acquire even a few years ago. The newly engineered “half piston core” system (called the half advanced piston corer [APC]) enabled us to recover the deepest piston core in DSDP/ODP/IODP history (490.4 m in IODP Hole U1427A). That achievement was also the deepest continuously recovered piston cored sequence, initiated at the mudline and penetrating ~500 m solely by piston coring. These technological advances delivered a series of “new surprises” (e.g., pristine dark–light laminae from ~12 Ma sediment recovered by piston coring from 410 m core depth below seafloor, Method A (CSF-A) (see the “Methods” chapter [Tada et al., 2015]) at IODP Site U1425 and from ~220 to 235 m CSF-A at IODP Site U1430 (Fig. F3) that will stimulate new scientific inquiry into climate dynamics during a time frame and with a high fidelity that could have only been imagined by scientists even a short time ago. To study the fate of organic carbon in the marine system and constrain rates of microbial reactions in the deep biosphere with a novel fluid extraction technique, we performed high-resolution geochemistry studies targeting the anaerobic oxidation of methane (AOM) and the relationships between metal chemistry and the degradation of organic carbon (Fig. F4). We took advantage of the impressive array of scientific equipment (e.g., color spectrometry) to develop finely tuned comparisons between disparate geographic locations and demonstrated synchroneity in the regional response to internal and external climate-related forcing (Figs. F5, F6).

We began our voyage from Valdez, Alaska (USA), with four main targeted objectives, as discussed below. After the 2 week transit, we began our studies in the Japan, Yamato, and Ulleung Basins, the Yamato Rise, and the East China Sea (Fig. F1). Ultimately, we successfully addressed and met or exceeded each of these objectives.

1. Address the timing of onset of orbital- and millennial-scale variability of the EASM and EAWM and their relation with variability of Westerly Jet circulation.

Dark and light layers of the hemipelagic sediment in the marginal sea represent changes in the intensity of EASM precipitation in South China. We recovered these dark and light layers at six sites (IODP Sites U1422–U1426 and U1430; Fig. F7). We showed that it is possible to correlate centimeter- to meter-scale (and often millimeter-scale) dark and light layers between these geographically widely spaced sites, suggesting that the marginal sea responded as a single system to climatic and/or oceanographic perturbations. We confirmed that alternations of the dark and light layers started at ~2.6 Ma and became more frequent and distinct from ~1.2 Ma to the present and have provided enhanced recovery of these important climate signals.

The presence of ice-rafted debris (IRD) and the occurrence of deepwater ventilation have previously been shown to be related to the intensity of the EAWM. Color reflectance (L*) of the sediment seems to reflect bottom water ventilation. Expedition 346 showed that deposition of IRD started at 3.0~3.2 Ma at Site U1422 and ~2.7 Ma at Sites U1423 and U1424, whereas L* increased significantly from ~2 to ~1.5 Ma at all deeper water sites. These findings obtained during the expedition provide tantalizing glimpses of meridional and temporal changes in IRD sedimentation that can be related to the evolution of EAWM climate behavior. Finally, hemipelagic sediment recovered from the marginal sea sites have high potential for studies of eolian dust, which bears directly upon important aspects of the objective relating to the Westerly Jet.

As described above, sedimentary color reflectance (L*) seems to give an indication of deepwater ventilation as well as surface productivity. We retrieved continuous hemipelagic sedimentary records to ~4 Ma at Site U1422; to ~5 Ma at Sites U1423, U1424, and U1426; and to ~12 Ma at Sites U1425 and U1430. The sediment shows orbital- to millennial-scale color cycles associated with various degrees of bioturbation and lamination. Recovery, in particular of the older laminated records, benefited tremendously from the new half APC system, which was able to recover virtually unblemished material from great depths and ages (Fig. F3).

In general, darker (lower L* values) layers of Pleistocene age are poorly bioturbated and/or finely laminated, suggesting oxygen-poor conditions, whereas the lighter layers are more bioturbated, suggesting more oxic conditions. Some of the dark layers are brownish and rich in microfossils such as diatoms, nannofossils, radiolarians, and foraminifers, suggesting high surface biological productivity. Orbital-scale dark–light color cycles appeared at ~2.6 Ma, and millennial-scale dark–light cycles became distinct at ~1.2 Ma. Orbital-scale dark–light color cycles also appeared in the Miocene interval (~12 to ~8 Ma) at Sites U1425 and U1430.

Because these cycles also appear at Yamato Basin Sites U1426 and U1427, where calcium carbonate is present, we will attempt to establish high-quality δ¹⁸O age models to apply throughout the entire marginal sea. These sites are each likely to serve as a “Rosetta stone” that will provide key age controls for the entire region. Furthermore, at Site U1425 we will apply the technique of optically stimulated luminescence (OSL) of detrital quartz grains to provide an independent control of age for the last 0.5 m.y. (Fig. F8); one that is unaffected by changes in salinity or the isotopic composition of the local seawater.

The initial cruise objective referred to the time frame “during at least the last 5 m.y.” We exceeded that oldest age by a factor of two. For example, millimeter-scale (decadal-scale) lamination is pervasive in the middle Miocene intervals at Site U1430 that record centennial- to millennial-scale color cycles in ~12 Ma sediment (Fig. F3). This laminated interval is highly diatomaceous and rich in organic carbon, suggesting high biological productivity in the surface water. The fidelity of core recovery by the half APC system is remarkable; at Site U1430 we are able to document that in these ~12 Ma laminated intervals, lighter laminae contain a startling enrichment in the relative proportion of centric versus pennate diatoms, suggesting significantly elevated nutrient inputs to the surface water and indicative of an ecosystem responding sensitively to climate dynamics.

3. Reconstruct the history of the Yangtze River discharge using cores from the northern end of the East China Sea, as it reflects variation and evolution in the EASM and exerts an impact on the paleoceanography of the marginal sea.

IODP Sites U1428 and U1429 in the northern East China Sea were selected to accomplish this objective over long timescales (e.g., ideally reaching the middle Miocene), and we were approved to drill to 800 m CSF-A. Unfortunately, we were forced to terminate these two sites at shallow depths of ~210 and ~185 m CSF-A, respectively, because of the unexpected occurrence of thick and unconsolidated sand. However, we were also fortuitously delivered the gift of higher than anticipated sedimentation rates (~42 and ~50 cm/k.y., respectively) and were able to successfully recover a continuous sequence of biocalcareous mud that was ~137 and ~179 m thick, respectively, and that covered the last ~0.35 m.y. Between the two sites, we were able to drill five separate holes with excellent stratigraphic correlations between the holes and, indeed, the sites themselves. With such high recovery and high sedimentation rates, we will be able to reconstruct very high resolution changes in sea-surface salinity and temperature. Because sea-surface salinity at these sites traces discharge of the Yangtze River, we will be able to reconstruct changes in the intensity of the EASM on orbital and millennial timescales and compare the results with paleoceanographic changes in the marginal sea. This will enable us to link climatic hydrology on the Asian continent (traced through reconstruction of Yangtze River discharge) to oceanography of the sea (traced through the surface and deepwater circulations).

4. Examine the interrelationship among the EASM, EAWM, nature and intensity of the influx through the Tsushima Strait, intensity of winter cooling, surface productivity, ventilation, and bottom water oxygenation in the region’s marginal sea and their changes during the last 5 m.y.

This single objective ties together a variety of multifaceted atmospheric and oceanographic processes. We successfully addressed and met this objective by collected study of the recovered material at all our sites, which were strategically placed to cover two meridional transects and a depth transect and to sample deposition beneath different ocean currents (Fig. F9). For example, Site U1427 in the Yamato Basin (Fig. F1) has a high sedimentation rate of ~36 cm/k.y. and covers the last 1.4 m.y. (Fig. F10). Our deepest penetration there (~550 m CSF-A) will allow for a long yet high-resolution reconstruction of the changes in the Tsushima Warm Current (TWC). We will be able to assess the different timescales of changes of the TWC and perhaps tie the younger portion of the record at Site U1427 to the very high resolution record of Yangtze River discharge at Sites U1428 and U1429. Thus, the combination of Sites U1427, U1428, and U1429 will allow us to address important questions relating the internal dynamics of the marginal sea and potential responses to external forcing.

Additionally, the depth and meridional transects described by Sites U1422 (currently at 3429 meters below sea level [mbsl]), U1424 (2808 mbsl), U1425 and U1423 (1909 and 1785 mbsl), U1426 (903 mbsl), and U1427 (~330 mbsl) (Fig. F6) will provide a heretofore unrealized opportunity to study the dynamics of ventilation and oxygenation history, which responds to changes in the EASM, EAWM, and TWC flow. The exquisite contrasts in color spectra between the dark and light cycles, which are traceable across the marginal sea, are most pronounced in the deeper sites and become progressively less so at the shallower sites, which speaks to the potential for future studies to generate a precise history of oxygenation and, perhaps, changes in the input of newly oxygenated waters during different climate phases. Moreover, the stratigraphic consistency between the evolution of key lithologic units (e.g., Subunit IB; 1.2–1.3 m.y.) from location to location regardless of depth is a marvelous example of this marginal sea’s ability to serve as a sensitive recorder of large-amplitude sea level changes associated with the climatic Mid-Pleistocene Transition as Earth’s climate evolved from being dominated by 41 k.y. cyclicity and into “the 100 k.y. world” in which we live today (Fig. F11).

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