Oceanic suspended matter (SPM) consists of a variety of components, some of which have external sources (e.g. riverine and/or atmospheric input), and some of which are produced internally (e.g. biological activity or non-biogenic precipitation). Because of its relatively long oceanic residence time and large surface area-to-volume ratio, the SPM plays an important role in regulating the chemical composition of seawater via the sorption and desorption of trace elements and other substances. The physical and chemical properties of SPM reflect the characteristics of the water mass, including the interaction with biota and/or atmospheric input, and are able to be a clue for elucidation of the ocean biogeochemical processes.

The North Pacific is the characteristic ocean of seasonal and spatial variation of atmospheric mineral dust inputs. Mineral dust aerosols are transported over the North Pacific especially in spring season by the westerlies and deposit onto the ocean surface. It was also reported that the primary production of the subarctic North Pacific are limited by iron, which can be derived by atmospheric mineral dust.

However, there is still only a little knowledge of the SPM, as well as ultimate fate of atmospheric mineral dust particles in the North Pacific. In order to understand biogeochemical processes in the North Pacific, the impact of the sporadic event, such as Asian dust storm, on the SPM in the surface North Pacific is need to be evaluated. In this study, using both bulk and single particle chemical analysis techniques, size and elemental compositions of the SPM collected in the North Pacific area is characterized. The objectives of this study are

1) To obtain general knowledge of spatial variation in number, volume and chemical properties of the SPM collected in the North Pacific during summer, when the influence of atmospheric Asian dust transport was minimal.

2) To characterize size and chemical composition of individual mineral particles in the SPM.

3) To understand the impact of atmospheric mineral dust aerosols on the composition of SPM.

4) To understand how the phytoplankton bloom initiated by the addition of iron from sporadic atmospheric dust event affects the SPM.

SPM were obtained from the North Pacific seawater during the several cruises carried out in summer of 2004 to 2007 on board R/Vs Hakuho-maru (KH-04-3/SEEDSII iron fertilization experiment, KH-05-2 and Leg 1 of KH-06-2) and Mirai (MR07-04 and MR07-05), which belong to the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). In the R/V Tansei-maru (JAMSTEC) KT-07-7 cruise, simultaneous observations in the surface ocean and atmospheric boundary layer over the semi-pelagic western North Pacific were carried out in spring season.

The bulk elemental composition of SPM for biogenic (Si and Ca) and crustal (Al, Ti and Fe) was determined by an X-ray fluorescence spectrometry. Individual particles were also analyzed using an electron probe X-ray micro analyzer and characterized by size and elemental composition (Mg, Al, Si, P, S, K, Ca, Ti, Mn, Fe and Ba). The single particle method for studying SPM provide information unobtainable by those used only based on bulk analysis, for example, the methods make it possible to calculate the number and volume concentrations and size distributions. The elemental data obtained based on single particle analysis shows that the particles are decidedly heterogeneous, and is valuable for understanding the composition of small particles because the results based on the bulk chemical analyses are mainly influenced by the small number of larger particles.

The spatial variation in number, volume and chemical composition of the SPM collected during summer in the North Pacific are significantly affected by primary productivity. The number and volume of SPM are approximately 3 and 5 times higher in subarctic high productive regions than subtropical low productive regions. Organic types particles were most dominant in SPM number, and particles generated by biogenic skeletal material, such as opal or carbonate shell, contributed to the SPM volume in all oceanic areas. The mineral particles are, however, distributed throughout the oceanic regions in the North Pacific, and their relative abundances in the total SPM was 5-15% by number and about 2-7% by volume. The size distribution of the suspended mineral particles in the SPM was similar to that of the atmospheric mineral aerosols, indicating that wherein background level of mineral dust occurs in the marine boundary layer even in summer. Furthermore, sulfur and phosphorus, mainly of marine biogenic origin, were associated with the mineral particles in the SPM compared to mineral dust aerosols. The single particle analysis shows that bio-related elements (sulfur and phosphorus) were detected more often in the organic type SPM than other trace elements. These results suggest that chemical composition of suspended mineral particles is significantly modified by adsorption or aggregation of organic particles.

The simultaneous observations in the surface ocean and atmospheric boundary layer over the semi-pelagic western subarctic North Pacific were carried out in the spring season. During the observation, mineral dust aerosols transported from the Asian arid area, Kosa, was observed in the marine atmospheric boundary layer. The atmospheric observation suggests that mineral dust aerosols were scavenged by sea fog that appeared over the observation area, and deposited onto the ocean surface. At the ocean surface, both Al and Fe concentration in SPM were significantly increased by deposition of the atmospheric dust aerosols. The average concentrations of Al and Fe of SPM in the surface water were approximately four to five times higher than that during the non-Kosa period. Furthermore, the Al and Fe concentrations during spring were higher than those collected in the same region during the summer by more than 2.5 and 0.7-2.7 times, respectively. The atmospheric deposition of mineral dust particles onto ocean surface from one Kosa-event was directly calculated to be 270 mg m(-2) event(-1).

Average diameter of the suspended mineral particles in the mixed layer was 2.1 ± 1.6 μm, similar to that collected during the summer. Average settling velocity of suspended mineral particles was estimated to be 0.35 m day(-1), and residence time in the mixed layer (15-40 m) was estimated as 40 -110 days. However, in the summer season, number and volume concentration of suspended mineral particles in the mixed layer was less than 20% of that of the spring season. It is considered that strong seasonal bloom of phytoplankton, especially diatoms are predominant in the spring bloom in the western subarctic North Pacific, may lead to formation of the aggregates, and the suspended mineral particles are incorporated in the aggregates and scavenged rapidly into deep water.

The SEEDS II iron fertilized experiment conducted in the western subarctic North Pacific, one of the HNLC (High Nutrient low Chlorophyll) areas, shows that SPM increased significantly after the iron fertilization in response to the increase in primary production. Throughout the study, Si-rich, Ca-rich and Organic particles were dominant and their number increased inside the fertilized patch; these particles accounted for 21%, 13% and 58% of the particles examined, respectively. There was consistently higher percentage of Ca-rich particles and lower percentage of Si-rich particles inside the patch than outside of it in number, but both types of these particles apparently occupied a larger volume inside the patch than outside of it. Organic particles, that showed having peaks in smaller diameter particles, increased apparently inside the patch with time after iron fertilization. These results suggest that the increase in suspended particles following the iron enrichment was due to a combination of detrital material and live phytoplankton. The experimental studies also provide better understanding of the process and how different particle types respond to the changing conditions during a plankton bloom.

The atmospheric deposition flux of the dissolve iron flux at the Kosa-event was estimated at 130 - 230μg m(-2) event(-1), and comparable to that of SEEDS II iron fertilization experiment. The deposition flux is found to have a potential to supply enough amount of iron to lead phytoplankton bloom in HNLC (High Nutrients Low Chlorophyll) waters. This result shows that a sporadic supply of dissolved iron leading to phytoplankton bloom can naturally occur in the western subarctic North Pacific.

This thesis study shows that understanding of the physical and chemical properties in SPM are useful approach to elucidate oceanic biogeochemical cycles, including atmospheric material transport and ecosystem change by sporadic atmospheric event, quantitatively. Further investigations are needed to characterize the morphology of SPM as well as the physical and chemical interactions, such as adsorption and/or dissolution of SPM and associated trace elements. Aggregation/disaggregation processes are also important in understanding the scavenging processes of SPM from the surface into deep waters, in order to establish a relationship between SPM and vertical settling of particles.

海洋表層には、海洋生物起源の有機物・オパール・炭酸カルシウム粒子・陸起源の鉱物粒子等で構成される、懸濁態の粒子が存在する。懸濁粒子は、海水中の溶存微量元素との吸・脱着や粒子自体の生成・分解により、海水の化学成分濃度を変化させる。また、陸起源鉱物粒子の海洋への輸送は、海洋生物の栄養塩となる鉄などの供給源の一つとして重要であり、海洋生態系に影響を与える可能性がある。このため、海洋表層の懸濁粒子の分布や物理・化学的特徴を明らかにすることは、海洋における生物地球化学的物質循環を理解する上で極めて重要である。

本論文は、北太平洋で採取された表層海水中の懸濁粒子の化学組成や粒径を、バルク元素分析と個別粒子分析を用いて定量的に計測し、知見のほとんどなかった北太平洋表層における懸濁粒子濃度、化学組成、分布を明らかにした。また、黄砂による鉱物粒子の海洋への供給や鉄散布実験による人為的鉄供給が、懸濁粒子組成にどのような影響を与えているかを船舶観測によって追跡し、考察した。

本論文は全6章からなる。第1章は序章であり、懸濁粒子研究の重要性と本研究の位置づけ・目的が述べられている。第2章には船舶観測の詳細と分析手法が記述されている。懸濁粒子の化学分析では、蛍光X線分析法を用い、従来の手法では測定が困難であったケイ素を含む懸濁粒子の主要成分を、非破壊で同時に分析する方法の確立について記述されている。また、個々の粒子の粒径、化学組成を明らかにする個別粒子分析法について示されている。

第3章では、夏季の北太平洋における懸濁粒子の化学組成とその濃度分布が記述されている。北太平洋表面水中の懸濁粒子の粒子数・体積は、生物生産の高い亜寒帯海域で大きく、生物生産の低い亜熱帯海域で小さい、という傾向が示された。いずれの海域においても、粒子数には有機物粒子、粒子体積には生物起源のケイ素・カルシウムを主成分とする粒子の占める割合が大きいことが明らかにされた。これらの北太平洋における懸濁粒子の化学組成に関する広範囲のデータは1970年代以降得られておらず、詳細な分布を示すデータとして貴重である。大気からのアジア大陸起源物質の供給量の小さい夏季の北太平洋においても、懸濁粒子全粒子数・体積に占める割合は小さいが、陸起源鉱物粒子がバックグラウンドレベルで存在することが示された。鉱物粒子の化学組成が海洋大気エアロゾル中の鉱物粒子とは異なることが明らかにされ、海水中での鉱物粒子と他の粒子または溶存成分の吸着や粒子の溶解が原因として挙げられた。これらの結果は、個別粒子分析によって初めて得られたものであり、今後海水中の溶存成分と粒子成分の相互作用を明らかにする上での重要な知見を与えた。

第4章では、春季黄砂時の大気海洋同時観測から、黄砂が海洋表層の懸濁粒子組成に与える影響について記述されている。大気を通じて輸送された黄砂が海洋表層に沈着することにより、懸濁粒子中の陸起源鉱物粒子濃度が4-5倍増加したことが示された。鉱物粒子の沈降速度を基に推定された表層混合層での平均滞留時間からは、夏季に低濃度となることを説明できず、鉱物粒子は移流や拡散または粒子同士の凝集による大粒子化によって、速やかに混合層内から除去されることが示唆された。これらの結果は、大気のイベントが海洋表層の懸濁粒子組成を変化させることを直接観測によって初めて明らかにしたものであり、大変意義深い成果である。

第5章では、鉄散布実験において、生物生産が増加する水塊での懸濁粒子の変化について記述されている。鉄散布による生物生産の増加に対し、懸濁粒子の個数、体積濃度にも増加が見られ、懸濁粒子の新たな生成も観測された。春季に実測された黄砂現象一回の単位表面積あたりの海洋への溶存鉄供給は、鉄散布実験に匹敵するものであった。黄砂現象は、より広大な面積に溶存鉄を供給することから、自然現象として大規模な植物プランクトンブルームを引き起す溶存鉄の大気を通じた供給が存在することが示された。

第6章では第3から第5章までの結果をまとめ、同研究分野の新たな展開の方向性が示されている。

本論文では、過去にない手法で広範囲にわたる懸濁粒子のデータを得たという点で高く評価できる。また、懸濁粒子の化学組成の変化から、大気からの物質供給や、それに応答した生態系の変化などの物質循環が、定量的に把握できることを示した。これらの懸濁粒子の物理・化学データは、将来全海洋における懸濁粒子、沈降粒子、エアロゾル粒子を介した物質循環研究を進める上で不可欠なものである。

なお、本論文の第2章における観測、第3、第4の各章は植松光夫教授、第5章は植松光夫教授、津田敦准教授、成田祥博士との共同研究であるが、論文提出者が主体となって研究を行ったもので、その寄与が十分であると判断できる。したがって、審査員一同は博士(理学)の学位を授与できると認める。