The processes determining the concentration of oxygen, phosphate and other organic derivatives within the depths of the Atlantic Ocean

Abstract

The great oceans of the world all contain at intermediate depths less oxygen and more nitrate and phosphate than is found at either lesser or greater depths. This is one of the most marked physical features of the sea which must be attributed to the action of biological agencies. Those who have discussed this condition recently are agreed that it originates through the oxidation of organic matter derived primarily from the surface layers of the ocean, where alone the original synthesis of organic matter can occur. The condition obtaining at any depth is considered to depend upon the balance between the rate at which oxygen is removed from the water by respiratory and other metabolic processes and the renewal of oxygen in the layers in question by movements of the water. One group of investigators has emphasized the latter factor as the dominant one in determining the observed distribution of oxygen (Jacobsen, 1916; Dietrich, 1937; Wüst, 1935; Wattenberg, 1929, 1938). Oxygen content is reduced to the greatest extent at those depths in which the water is in minimal motion and hence the renewal of oxygen is least. Seiwell (1937) and Sverdrup (1938) have pointed out that this condition is not a necessity and is indeed in certain situations contrary to the apparent facts. They have shown that the observed distribution of oxygen may be accounted for by assuming various suitable relations between the rates at which oxidation occurs as a function of the depth and the rates of renewal by the circulation of water. These discussions appear to consider the state of the water to depend upon factors operative more or less locally and in situ. Specifically, oxidation is assumed to follow the sinking of organic matter from the surface to the depth in question in the discussions of Wattenberg (1937) and Seiwell (1937). The renewal of oxygen is assumed to depend on the horizontal circulation. Several considerations appear to have been given insuffcient weight in discussions of this subject. It is not at all clear why the depth of the oxygen minimum layer varies so greatly from place to place or what its relation is to the particular nutritive conditions in the sea's surface. Observations made in the relatively shallow water of the Gulf of Maine indicate that organic decomposition and oxidation take place for the most part not far from the sea surface. It seems not unreasonable to assume that the properties of the water which depend upon organic decomposition may have been determined primarily at a time when the water was relatively near the sea surface and that the water has subsequently moved into its observed position. The recent evidence, reviewed by Montgomery (1940), that mixing processes along surfaces of constant potential density may occur with great ease, even in the absence of directional flow, provides a convenient mechanism for establishing a distribution of oxygen and the products of organic activity at great depths which is dependent in large part on processes taking place much nearer the sea surface in remote regions. These considerations have suggested that the wellmarked evidence of decomposition which is observed at great depths in the central Atlantic Ocean may be due to the flow of water along surfaces of equal potential density from regions near the sea surface in high northern and southern latitudes rather than to the decay of organic matter derived directly from the overlying surface waters. This view requires that the characteristics of the water show a marked continuity in their distribution along layers of constant potential density, and that these layers emerge at or near to the sea surface in places suitable to produce the peculiar character of the layers in question. To test this possibility, data secured by the "Meteor," the "Discovery," and the "Atlantis" have been examined. The most illuminating information was secured from two north-south sections which together extend from Greenland to Antarctica. The South Atlantic was traversed by a section, made by the "Discovery" in April-May, 1931, extending along the thirtieth meridian from 57°36'S to 14°27'N ("Discovery" Reports, 1932). This is Section 2 of Clowes (1938). A section of the North Atlantic was constructed from the data secured by the "Atlantis" extending from 1°N to 34°N west of the fortieth meridian in March, 1932 (Stations 1158-1179), and from 39°N to 49°N near the fortieth meridian in September, 1935 (Stations 2485-2491), and by the "Meteor" (Stations 120-125) extending from 50°N to 58°N near the forty-fourth meridian, occupied in March, 1935 (Bull. Hydrographique, 1933, 1936). The data have been converted into suitable common units. Phosphate has been expressed as milligram-a toms phosphorus per cubic meter (γ-atoms P per liter) uncorrected for salt error. The function of the oxygen content which is of importance is the quantity which has disappeared from the water owing to metabolic processes. This has been approximated by assuming the water to have been saturated with air at the time it acquired its temperature and salinity at the sea surface and subtracting the recorded oxygen content from the value calculated on this assumption. The "apparent oxygen utilization" so obtained has been expressed as cubic centimeters per liter. In plotting the data for the sections, a rectangular grid on which latitude is represented horizon tally and sigma-τ is represented vertically has been chosen. Mon tgomery (1938) has shown that surfaces of equal sigma-τ are approximately of constant potential density. Consequently, on such a diagram the path of free movement by lateral mixing or flow is along horizontal lines. Any correlation of the distribution of a component of the sea water with potential density becomes at once apparent, if present. The surface of the sea and surfaces of any particular depth are represented by curved lines on the diagram. A grid of this type has been used by Spilhaus (1941) to distinguish different water types in the complex situation which exists at the margin of the Gulf Stream.