Hisashi Narita

Enhancement of coccolithophorid blooms in the Bering Sea by recent environmental changes

[1]xa0Since 1997, ocean color satellite images have revealed large-scale blooms of the coccolithophorid Emiliania huxleyi in the eastern Bering Sea. The blooms are often sustained over several months and have caused ecosystem changes in the Arctic Ocean, as well as in the Bering Sea. We examined continental shelf sediment profiles of alkenone, a biomarker for E. huxleyi, covering the past ∼70 years. The alkenone records suggest that large E. huxleyi blooms are a novel feature in the Bering Sea as they have occurred only since the late 1970s. Recent changes in alkenone content were closely related to the 1976–77 climatic regime shift in the North Pacific, implying that warming and freshening of Bering Sea waters promoted E. huxleyi blooms. The production rate of diatoms (total valves in sediment samples), the dominant primary producers in the Bering Sea, also increased during the past several decades. However, the ratio of alkenone content to total diatom valves in the sediments increased as E. huxleyi production increased, suggesting that the increase in the E. huxleyi production rate frequently exceeded the increase in the diatom production rate. Overall, our results indicate a possible subarctic region ecosystem shift driven by climate change.

Time variations of δ15N of organic nitrogen in deep western subarctic Pacific sediment over the last 145 ka

We present δ15N data of organic nitrogen (δ15NON) from a deep western subarctic Pacific (WSAP) sediment core over the last 145 ka. To remove the effect of inorganic nitrogen (IN) on bulk nitrogen (BN), we used the contents of IN and BN, and the isotopic ratios of IN (δ15NIN) and BN (δ15NBN). The contribution of IN to BN was largest 59% and δ15NIN had lower values than δ15NBN. However, the large contributions of IN to BN and the distinct isotopic values did not largely change any variability associated with δ15NON in the δ15NBN. Thus, it is possible to use δ15NBN in carrying out paleoceanographic research with δ15N at least in the open WSAP. The δ15NON values have been affected by long-term diagenesis making the values lower with depth in the sediment, but the values during less productive colder periods were generally lower compared to those during neighboring productive warmer periods. The results might indicate that δ15NON during the less productive colder periods (i.e., glacial periods) were lower than that during the productive warmer periods (i.e., interglacial periods). Previous studies have reported that the biological production in WSAP, where it is limited by iron at present, during glacial periods was at more reduced level than during interglacials despite there being more eolian dust supply during glacial periods. This indicates that the increase of biological production with enhanced iron supply derived from eolian dust during the glacial periods did not occur. If this is the case and alternatively the iron input from the subsurface layer to the surface-mixed layer by ventilation process is more important as suggested by recent works, the variations in δ15NON would be explained by the variations in the Fe/NO3− ratio in the upwelled water. Although δ15NON has been affected by long-term diagenesis in the sediment, the usage of this isotopic ratio provides new implication regarding the past ocean circulation in the open WSAP.

Spatial assessment of radiocaesium in the largest lagoon in Fukushima after the TEPCO Fukushima Dai-ichi Nuclear Power Station accident

Radionuclides deposited on land by global fallouts and nuclear power station accidents spread over coastal environments through estuarine areas connecting land to ocean. In this study, we monitored activity concentration of radiocaesium in surface sediment and re-suspended particles in Matsukawa-ura lagoon, the largest lagoon in Fukushima, after the TEPCO Fukushima Dai-ichi Nuclear Power Station accident. Radiocaesium distribution in surface sediment varied spatiotemporally and irregularly due to the effect of tidal waves. The effective half-life was significantly shorter than physical half-life, suggesting some system of radiocaesium discharge in the lagoon. Sediment trap observation revealed re-suspended particles from sediment were transported to the ocean. For these reasons, it is suggested that re-suspension of particles in the lagoon and their transportation to the ocean by the seawater exchange process are important processes of radiocaesium discharge. Moreover, our results show that seawater exchange process contributes to the dispersion of radiocaesium in the ocean.