In geology, it has long been a common saying that "the deeper the water, the more reducing the environment, and the less dissolved oxygen there is." However, this may be incorrect! Here are three main perspectives: (1) In most cases, the open ocean is oxygen-rich, with the exception of the "Oxygen Minimum Zone" (OMZ) found at depths between 600-1200m beneath the influence of mid- to low-latitude ocean currents. (2) Apart from the upper ocean, dissolved oxygen content increases with depth. (3) The dissolved oxygen in the ocean is entirely controlled by the temperature difference between the sea surface and the seafloor, having no relation to biochemical processes.

          We express our gratitude to Acta Geologica Sinica for publishing our research, which is now accessible on CNKI in Chinese version with English Abstract.  We can search for it under the title "Control of Water Depth on Organic Matter Enrichment in Confined Environments" or directly click on the article link provided here:

https://www.geojournals.cn/dzxb/dzxb/article/abstract/2024007?st=article_issue

          "As sea levels rise, marine transgression occurs, water bodies deepen, and bottom waters become hypoxic, facilitating the preservation of organic matter and deposition of organic-rich shales. Conversely, as sea levels fall, marine regression occurs, water bodies shallow, and oxygen levels increase, making it unfavorable for the preservation of organic matter in shales." This is a common notion in geology, sedimentology, marine sedimentology, and sedimentary mineral resources. It is almost universally accepted that "deep-water shelves are the optimal environments for the enrichment of organic matter in marine shales." However, these views are erroneous.

          This conventional wisdom stems from intuition. It is imagined that oxygen from the atmosphere must travel a long and arduous journey to reach the lakebed or seafloor. Consequently, it is postulated that deeper waters are more hypoxic and reducing. The "thermohaline circulation" theory posits that in mid- to low-latitude oceans, strong stratification and weak convection lead to oxygen-rich bottom waters due to the sinking of cold polar waters carrying oxygen to these regions. This perspective likens the ocean to a basin on the scale of a washbasin (Figure 1). This scale, measured in meters, is clearly inadequate for comparison with the vastness of the ocean spanning thousands of kilometers.

Figure 1: Atlantic Oxygen Convection Model (Paul R. Pinet: Invitation to Oceanography, Second Edition, www.jgpub.com/oceanlink)

         The hypothesis of cold water sinking mechanisms was first proposed by Stommel (1958, 1960, 1961) and experimentally verified by Rossby (1965) (Figure 2). Subsequently, oceanographers extended this washbasin model globally. However, from mathematical or physical perspectives, considering the ocean's vastness (approximately 10,000 kilometers from pole to equator and an average depth of 4 kilometers), such a comparison is fundamentally flawed.

Figure 2: Rossby (1965) Experimental Diagram (Washbasin Scale, Meters in Length and Width)

         In recent years, the limitations of Stommel's hypothesis have been recognized. Munk and Wunsch, Huang Ruixin and Nilsson, among others, have proposed new perspectives: "thermohaline circulation" should be driven by mechanical energy provided by winds and tides, facilitating the meridional overturning circulation that transports heat, freshwater, and other substances. Given the ocean's extremely low efficiency in converting thermal energy into mechanical energy, it cannot be considered a heat engine. This undoubtedly challenges the traditional view of thermohaline circulation.

         Our study combines environmental science insights on dissolved oxygen distribution to conduct in-depth research on the relationship between water depth and the degree of organic matter enrichment in surface sediments. The findings are as follows:

         Apart from the upper water column (from the sea surface to the storm wave base or the extent of ocean currents), dissolved oxygen in the water column increases with depth. The seafloor is oxygen-rich; high-latitude lakes, oceans, or winter lakes are oxygen-rich from shallow to deep waters. The upper water column extends from the sea surface to depths of 300-600 meters.

         The OMZ occurs beneath the upper water column in mid- to low-latitude oceans, at depths between 600-1200 meters. Below the OMZ, dissolved oxygen content increases with depth (Figure 3). The top boundary of the OMZ near the equator can be as shallow as 300 meters. However, due to storm waves and ocean currents, the top boundary of the upper water column is generally not shallower than 200 meters. The maximum biomass is found within 0-50 meters, with extremely low biomass beyond 150 meters. Thus, the OMZ's occurrence is unrelated to biological activity.

(a)                                                             (b)

 Figure 3: Relationship Curves between Dissolved Oxygen and Depth in Different Oceans. High-latitude regions are oxygen-rich from the sea surface to the seafloor.

 (a) Dissolved Oxygen Distribution in Mid- to Low-Latitude Oceans; (b) Comparison of Dissolved Oxygen in Various Oceans and the Arctic Ocean

          Deep-water shelf environments at depths of 50-200 meters are oxygen-rich and do not possess the conditions for developing black shales. Instead, they feature fine sand and silt. Influenced by ocean currents and tidal flows, they develop numerous medium- to large-scale, and even giant, sandwave landforms with wavelengths of 1-2 kilometers and heights of 150 meters (Figure 4). Sandwave landforms can even be found in deep-sea environments at depths of 2000 meters (Figure 5).

Figure 4: Photograph of Sandwave Landforms at 2000 Meters Depth on a Seamount in the Western Pacific (Taken by ROV)

Figure 6: Distribution Map of Surface Sediment Types and TOC Content in the Northern South China Sea, with Low TOC Content and Predominantly Fine Silt.

          Open water bodies, including oceans, continental shelves, bays, or epicontinental seas, cannot enrich organic matter (as shown in Figure 6), with TOC contents ranging from approximately 0.2-0.8%. Only confined lagoon environments have the potential to enrich organic matter. Therefore, the so-called "marine shales" are actually transitional marine-continental shales. There may be no true "marine shales."

         Seasonal temperature variations induce vertical convection in marine water bodies, particularly in winter, allowing oxygen to penetrate and saturate waters at various depths through natural vertical convection. In winter, or in high-latitude oceans and lakes, waters are oxygen-rich from the surface to the seafloor (Figure 7). Oceanographers believe that the OMZ is controlled by the decomposition and oxygen consumption of floating plant and animal remains (known as particulate organic carbon, or POC) in the ocean, leading to the lowest oxygen content at depths of 600-1200 meters. However, Figures 8 and 9 show that high productivity and POC occur in high-latitude regions without OMZs, indicating no intersection between high POC and OMZs. Thus, this claim is incorrect and contradicts oceanography.

Figure 6: Dissolved Oxygen Distribution in Different Reservoirs across Seasons

 (Note: Clearly, surface water temperature determines dissolved oxygen distribution. In winter, cold surface waters and relatively warm bottom waters induce strong vertical convection, keeping waters oxygen-rich. In summer, convection weakens, and the lowest oxygen content occurs within 5-30 meters.)

         The high productivity observed in high-latitude waters (Figures 8 and 9) benefits from their oxygen-rich state. For instance, Antarctic krill production reaches 6.5-10 billion tons annually, exceeding China's total grain production of 7 billion tons. Despite this high biomass, no OMZ zone appears beneath the upper water column in high-latitude oceans, with waters remaining oxygen-rich from the surface to the seafloor (as seen in the Arctic Ocean in Figure 3b). In other words, no discernible impact of biological oxygen consumption on dissolved oxygen levels is observed.

Figure 8: Global Ocean Productivity (Chlorophyll Concentration). Highest Productivity under Cold High-Latitude Climate Conditions

 (http://kejiao.cntv.cn/20101101/100692.shtml)

Figure 9: Global Annual POC Flux (2003-2018)

 (https://www.mdpi.com/2072-4292/11/24/2941)

Abstract 

          When discussing the development of organic-rich shales, associations are often made with deeper, hypoxic environments, considering the sedimentary centers of basins and deep lakes—semi-deep lakes as reducing environments more conducive to the development of high-quality source rocks. However, the characteristics of organic matter enrichment in modern water bodies contradict this view, necessitating an in-depth analysis of their relationship. Utilizing information on dissolved oxygen distribution in environmental science and ecology, as well as organic matter content in surface sediments of modern lakes, this study examines the enrichment characteristics of organic matter at different depths in confined water bodies. The findings indicate that dissolved oxygen concentrations in water bodies increase with depth (with the exception of the upper water column). Lake bottoms and seafloors are predominantly oxygen-rich under most circumstances. Deeper waters generally exhibit lower primary productivity and are not necessarily low-energy or hypoxic environments, which is unfavorable for the preservation of organic matter. Conversely, relatively confined environments such as lagoons, bays, and lake bays serve as better gathering places for organic matter, minerals, and volcanic ash. This paper argues that in small, confined—semi-confined water bodies, shallower depths are more conducive to organic matter enrichment. The contribution of exogenous organic matter cannot be overlooked. Open water bodies, such as deep-water shelves and large deep lakes—semi-deep lakes, do not possess the conditions for developing high-quality source rocks.

 Conclusions

          Small lakes, lagoons, or semi-confined bays represent confined environments more prone to the enrichment of organic matter, minerals, and volcanic ash. Large water bodies and continental shelves constitute open environments with strong hydrodynamic forces, making organic matter enrichment difficult.

         Deeper waters are not necessarily more reducing, and dissolved oxygen content in deeper waters may not necessarily be lower. For non-steady-state water bodies, such as winter lakes or high-latitude oceans and lakes, waters are oxygen-rich from the surface to the seafloor. For steady-state water bodies, such as summer lakes or mid- to low-latitude oceans, dissolved oxygen generally increases with depth, with the upper water column being oxygen-rich. The OMZ is located at depths between approximately 600-1200 meters, with dissolved oxygen gradually increasing downward to the seafloor, which is oxygen-rich.

         Shallower waters are more conducive to organic matter enrichment and carbon sequestration. The carbon sequestration capacity of water bodies is inversely related to their area and depth (Figure 10). Only well-confined water bodies constitute low-energy environments.

         Warm and humid climates are more conducive to the enhancement of terrestrial primary productivity (Figure 11), inhibiting the growth of algae in water bodies and reducing water productivity. Cold climates induce algal blooms in water bodies, resulting in high chlorophyll levels (Figure 8) and high POC contents (Figure 9), thereby enhancing water productivity. This contradicts sedimentology and petroleum geology.

         From Figures 8 and 9, it can be observed that oceans near the equator and in tropical regions exhibit extremely low primary productivity, only slightly better than deserts! Figure 11 depicts terrestrial primary productivity, which is indeed high in tropical regions. However, the carbon storage in their soils is as low as that in oceans. For instance, the Brazilian rainforest, dominated by red soil with strong decomposition due to high temperatures, possesses the lowest carbon storage, far lower than that in Northeast China's black soil and the carbon storage in the Qinghai-Tibet Plateau's cold temperate zone.

         It is recommended that the depth and confinement of water bodies be considered as important parameters in evaluating source rocks or shale oil and gas resources.

Figure 10: Relationship between Organic Carbon Content in Lake Surface Sediments and Water Depth. TOC is inversely related to water depth.

Figure 11: Global Terrestrial Primary Productivity. Low in Tropical Regions, Increasing towards Polar Cold Climates.

(https://data.tpdc.ac.cn/zh-hans/data/d6dff40f-5dbd-4f2d-ac96-55827ab93cc5)

Mao Xiaoping,

Tel: +86 13911360200

Wechat: 13911360200 or maoxp9

 School of Energy Resources, China University of Geosciences (Beijing)

 May 12, 2025

Part of References

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