토픽 개요

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  • Course Contents

    • 　In Chapter 1, we discussed the importance of studying the distribution of water temperature and salinity (T-S distribution) in order to understand the movement of seawater, and in the supplementary section, we mentioned that the movement of fish is also roughly consistent with the T-S distribution. In this chapter, we will discuss a much more microscopic movement than that of fish, the slow but dynamic transport of materials throughout the oceans.

      　For oceanographic observations, the CTD-Niskin water sampling device is lowered from the vessel to make continuous sensor measurements of water temperature, salinity, and chlorophyll fluorescence. Water samples from the target depth are collected by the Niskin sampler and used for chemical analysis and biological measurements (see image below). The information obtained from a single depth sampling is fragmentary, but if such vertical observations are made horizontally and closely, a vertical cross section of ocean parameters can be obtained, and if monthly or seasonal observations are made, a time series distribution can be obtained.

      Figure 1


    • Matter movement - it's important to think about size ~

      　The chemical parameters measured in oceanographic observations are elements related to biological activity. Elements involved in or close to living organisms are called bioelements or biophilic elements. The oceanic distribution of pro-living elements is also broadly classified according to water mass classification. This is because water masses that sink at a certain location travel through the middle and deep layers of the ocean over a long period, and the matter contained in the water masses continues to travel with them, changing form as they do so.

      　However, only substances that are dissolved in seawater (i.e., dissolved matter) or have the same density as seawater (i.e., floating or suspended particles) can continue their journey with the water mass. If a matter becomes larger than the density of seawater, it will gravitationally fall and disappear from the water mass. So what is the matter that gravitationally falls in seawater (i.e., sedimentation particles)?

      　The first thing that comes to mind would be animal carcasses and feces. These particles are produced primarily in the surface layers of the ocean and settle at a relatively fast rate. Below is a pictorial representation of the factors that determine the vertical distribution of chemical constituents in seawater. This chapter will help you to get a concrete image of the factors.



      　Most of the particles in surface seawater are thought to be phytoplankton. They change their shape and sediment, decomposing in the process. Other particles include mineral particles that fall from the sky (yellow sand particles in the North Pacific), which are small (a few µm) and do not have sufficient sedimentation rate on their own. They are thought to sediment in the ocean by attaching to other particles (e.g., phytoplankton). To begin with, phytoplankton lives suspended in water, so it has a density almost equal to the density of seawater and is difficult to sediment. After the phytoplankton die and the various organic particles come together to form an aggregate, they may gradually begin to sediment.

      　Organic matter in suspended and sedimentary particles is used as food for microorganisms, leading to the decomposition and mineralization of organic matter. When the size of the organic matter is reduced to the limit, it is gasified and released into the atmosphere.

      　Considering the above, you can imagine that the biophilic elements (carbon, oxygen, nitrogen, phosphorus, iron, etc.) show quite complex behavior in seawater. The previous statement that "the distribution of chemical parameters is also broadly classified by water mass category" only means that "the distribution of chemical parameters changes abruptly at the boundary of a certain water mass". To consider these factors, one must have a three-dimensional and temporal picture of the movement and mixing of water masses, the suspension and sedimentation of particles, and the formation and decomposition of dissolved and particulate matter.


      　Since organic matter is responsible for mass transport in seawater, Chapter 2.1 deals with "Organic Matter in the Ocean".


    • Organic matter classification keywords and size pages to characterize  


      　The keywords in the chapter "Organic Matter in the Ocean" are "dissolved", "particulate", "gravity fall", " aggregation", "gasification", and the "size" that characterizes them all. When considering the movement of substances in seawater, the size of the substance is decisive. The largest factor that changes the size of matter in seawater is the action of living organisms. If a substance in seawater is assimilated by an organism, it becomes a particle, and if it is consumed by a higher predator in the food chain, it becomes large. If microorganisms decompose carcasses and fecal particles, they will eventually return to the dissolved state. Mass transport in seawater is easier to understand if we focus on organic matter. Organic matter is defined as "carbon compounds derived from living organisms". (However, carbon monolayers, carbonate minerals, and carbon dioxide are excluded.) This includes things that are alive now, excreta and carcasses, and things that have been decomposed or transformed by them. First, let's divide the types of organic matter into living and nonliving matter, and then classify them by size.


      Summary

      • The size of a substance determines its mode of existence, i.e., "dissolved", "particulate", "gravity fall", "agglomeration", or "gasification", which in turn determines mass transfer.
      • Most of the substances in the ocean are affected by biological activities. Most elements are found in organic matter.
      • Understand material transport and material cycles in the ocean from the movement of organic matter.

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