Progress in geochemistry and paleoceanography of cobalt-rich crusts

Most of the paleo-oceanographic studies in the past were sediment cores, but because of their fast deposition rate (usually considered to be a thousand times faster than the growth rate of crusts), the time scale of research is often shorter; and the time-scale sediments are obtained. The core of the material has high technical requirements and sampling costs. Cobalt-rich crusts in comparison with sediment cores, convenience sampling, sampling low cost. In addition, geochemical studies have shown that cobalt-rich crusts are less disturbed by the outside world, and the exchange of many elements with seawater is negligible, basically maintaining a closed system after growth. Therefore, in recent years, oceanographers have increased their efforts in the study of cobalt-rich crusts and paleoceanography, and have achieved remarkable results, which has become a very active hot spot in the field of paleoceanography.

I. Overview of geochemical research on cobalt-rich crusts

The cobalt-rich crust grows slowly on the seabed bedrock at a rate of a few mm∕ Ma, and records the long-term history of the ore-forming environment, the marine environment and global changes, such as the source of crust growth, the paleo-oceanic circulation, and the paleo-oceanic redox Potential, marine paleoproductivity, changes in paleoclimate, etc. This information is mostly stored in the composition and geochemical characteristics of elements and isotopes. Therefore, in order to conduct dating and paleoceanographic studies of cobalt-rich crusts, it is first necessary to obtain the composition and geochemical characteristics of elements and isotopes in cobalt-rich crusts. In addition, the geochemical behavior of elements and isotopes in the crust determines whether the information stored is complete and accurate. Therefore, the important premise and basis of the study of cobalt-rich crust paleo-oceanology is to study the geochemistry of cobalt-rich crusts and find suitable tracers.

In recent years, geochemical studies of cobalt-rich crusts have involved studies on the origin, composition, and distribution of various elements in cobalt-rich crusts, as well as studies of various factors that contribute to these geochemical characteristics; whether the surrounding seawater, the bedrock material exchange occurs in the cobalt-rich crust grown, the issue of whether the element diffusion occurs and the impact of events on crusts phosphate and the like in the crust. These studies have laid a theoretical foundation for the application of various stable isotopes, radioisotopes and elements in the study of cobalt-rich crusts in paleoceanography.

(1) Diffusion of elements in cobalt-rich crusts and exchange with surrounding seawater

Henderson and Burton calculated the effective diffusion rates of elements such as U, Th, Li, Os, Sr, Be, Nd, Pb, and Hf in cobalt-rich crusts (Table 1). It can be seen from Table 1 that Li, Os and Sr diffuse rapidly in the cobalt-rich crust, so the composition of the ancient seawater cannot be accurately preserved. The chemical leaching of cobalt-rich crusts by VonderHaar et al. also confirmed that Sr in the crust was exchanged with Sr in seawater. The diffusion of U is also faster, resulting in a growth rate of ²³⁴ U∕ ²³⁸ U depth distribution faster than that obtained by ¹⁰ Be, ²³⁰ Thex, etc. This is consistent with the difference in growth rate of Neff et al. The exchange coefficient of U in the cobalt-rich crust with the dissolved state U in the surrounding seawater is 5 × 10 ^{-6 a -13} ; and Th, Nd, Pb and Be in the cobalt-rich crust Highly stable, which guarantees reliability in dating and paleoceanographic applications of cobalt-rich crusts. The effective diffusion coefficient of Hf is somewhere in between, that is, there is a certain diffusion; but David et al. think that it uses the wrong seawater Hf concentration value to calculate, thus giving a much higher effective diffusion rate; After the calculation of the seawater Hf concentration value, the effective diffusion rate is within an order of magnitude of Be, so Hf should be very stable in the crust. It can be seen from the above discussion that many elements, especially the granular active elements, are stable in the cobalt-rich crust, the diffusion in the cobalt-rich crust is negligible, and there is no material exchange with the seawater around the cobalt-rich crust growth period or The exchange of substances that occurs is negligible.

Table 1 Effective diffusion coefficients of some elements in cobalt-rich crusts

(II) Influence of bedrock on the composition of cobalt-rich crust

In addition to cobalt-rich crust in contact with the surrounding seawater, but also in contact with the bedrock in the growth period, which is a multi-metal coated with different contact tuberculosis sediments. It is known that sediment interstitial water has a great influence on the composition of matter in polymetallic nodules. However, since the detailed study of cobalt-rich crusts began in the early 1980s, it is assumed that the bedrocks are crust-forming during the growth of cobalt-rich crusts. The composition has no effect. However, the reliability of this assumption has not been verified for many years. To this end, Hein and Morgan concluded that the bedrock does not affect the crust composition based on the basic composition and other statistical analysis of the chemical and mineralogy of the cobalt-rich crust and underlying bedrock in the Central Pacific Ocean. This is the only direct and detailed evaluation of the interaction between bedrock and cobalt-rich crusts to date.

(III) Effect of phosphating on the composition of cobalt-rich crusts

When the climate is stable and the ocean circulation is weak, the dissolved phosphorus produced by strong chemical weathering accumulates in the deep sea. When the Antarctic ice sheet expands and the ocean circulation increases, the phosphorus-rich deep water rises into the middle water body due to the influence of the seamount terrain. Temporarily stored in the oxygen minimum zone (OMZ). When the OMZ is strengthened and expanded, the anoxic and phosphorus-rich water reaches the sea slope covering the crust, inhibiting the further growth of the crust, and the fluoroapatite carbonate (CFA) invades the crust to phosphate the crust. Two large and possibly three minor similar events occurred from the late Eocene to the Miocene, resulting in the formation of aquarium in the equatorial Pacific seamounts. According to the age of crust growth, one large one from 21 to 27 Ma The event and a small event of 15 Ma from time to time lead to phosphating of the crust.

The phosphating event has a significant impact on the mineralogical and chemical composition of cobalt-rich crusts, including macro and trace elements, as well as rare earth elements, which has been confirmed by many researchers. Under the influence of the phosphating event, CFA invades the cobalt-rich crust and causes diagenesis to reactivate and reorganize the cobalt-rich crust. However, there are different views on whether the phosphating event changes the composition of stable isotopes in cobalt-rich crusts. Frank et al. believe that the phosphating event has no effect on the stable isotopic composition of Pb and Nd in cobalt-rich crusts. Christensen et al. also believe that the phosphating event did not alter the stable isotopic composition of Pb in the cobalt-rich crust. Ling et al. believe that the phosphating event may affect the stable isotopic composition of Pb in cobalt-rich crusts, but has no effect on the stable isotope composition of Nd. Lee et al. believe that phosphating and diagenesis are unlikely to alter the long-term recording of Hf isotopes of cobalt-rich crusts. Therefore, when geochemical characteristics of various chemical elements in the phosphating portion of cobalt-rich crusts are used in paleoceanographic studies, care must be taken to ensure the reliability of the results.

2. Research progress on paleoceanography of cobalt-rich crusts

According to the geochemical characteristics of the crust, the chemical composition of the seawater surrounding the growth period of the crust can be directly obtained. A series of studies have given isotope distribution images or temporal evolution images of Pb, Nd, Os, Hf, Be, U, Th, etc. in the Pacific Ocean, Indian Ocean, and Atlantic Ocean. In addition, studies have also given the redox state of the seawater environment and sedimentary environment. The chemical composition and characteristics of the seawater around the crust growth period can reflect the material source of the crust and the surrounding water mass movement, and further explain the changes of the crust mineralization environment, the changes of the paleo-ocean environment, the changes of the world ocean circulation pattern and Its interactions and interrelationships with paleogeographic changes and paleoclimatic changes link the growth of crusts to paleo-oceanic environments and even global changes.

(1) Application of stable isotope in paleoceanographic study of cobalt-rich crusts

Frank et al. believe that the distribution of Pb and Nd isotopes in the past 60 Ma deep sea is controlled by two main factors: (a) the oceanic mixed mode associated with major paleogeographic changes and the changes in the water mass path with unique isotope signals. (b) Changes in the source and supply rate of the input oceanic debris material associated with climate change (ice period) or major structural uplift. Here, the main paleogeographic changes are the closure of the Isthmus of Panama from 3 to 5 Ma, and the climate change is the beginning of the Northern Hemisphere Ice Age (NHG). The main tectonic uplift refers to the uplift of the Himalayas. Therefore, information on these events can be obtained from the study of the Pb and Nd isotopes of the crust. Studies have shown that the closure of the Isthmus of Panama and the beginning of the northern hemisphere glacial period have led to the establishment of a modern ocean circulation model. Changes in ocean circulation patterns have further led to significant changes in the Pb and Nd isotopic compositions of deep waters in the Atlantic and Pacific Oceans, as well as changes in material sources. Studies of crusts in the Southwest and Central Indian Oceans have shown that the Himalayan erosion products have not significantly contributed to the deep waters of the Indian Ocean in the past 20 Ma. However, in the North Indian Ocean crust, the Himalayan erosion input in the Indian Ocean can be identified, perhaps the input is limited to a smaller range. Other related events, such as the exposure of the Greenland-Scottish Ridge, may also affect Pb and Nd entering the ocean.

Hf isotopes can also be used for the study of material sources, paleoclimate, paleocirculation and paleogeographic changes. Godfrey et al. believe that the Hf isotope ratio in the crust can be used to determine the source of Hf and may also trace the source of Fe and Mn. Piotrowski et al. believe that the drift of Hf isotopic composition in the North Atlantic crust may be caused by changes in erosion intensity during the Northern Hemisphere glacial period. The change in Hf isotopic composition in the Indian Ocean crust reflects the short-term enhancement of Himalayan erosion and the deep ocean waters of the North Atlantic (NADW). The intensity of the water entering the Indian Ocean basin by the deep (Southern) deep water (CDW) or the flow of Pacific water from the Indonesian waterway into the Indian Ocean is reduced. Lee et al. found that the Hf isotopic composition of the deep waters in the Central Pacific Ocean is closely related to paleogeographic changes and changes in the ancient circulation. David et al. confirmed that the Hf isotope can be used as a source of elements and as a tracer for water masses.

(II) Application of radioisotopes in the study of ancient oceanography of cobalt-rich crusts

In terms of radioisotopes, the ¹⁰ Be∕ ⁹ Be value is a good tracer for hydrothermal motion and changes in ancient fluxes. This value is used by von Blanckenburg and O'Nions to trace the change in NADW intensity and the source of Nd and Pb stable isotopes. Chabaux et al. believe that the Th∕ U value and the initial Th isotope activity ratio in the crust are related to changes in the past 150ka ocean circulation. Based on the depth profile study of Th in the crust, Eisenhauer et al. found that the Late Quaternary climate change interacted with the growth of the crust through the input of elements in the water column. Huh and Ku studied the changes in aeolian dust associated with paleoclimate periods such as the northern hemisphere glacial period based on the distribution of Th in nodules and crusts.

(III) Application of Elements in the Study of Ancient Oceanography of Cobalt-rich Crusts

The compositional changes of the chemical elements of the crust also reflect the changes in the paleoclimate and paleo-ocean environment during the growth period of the crust. Segl et al. found that changes in chemical composition in the crust occurred simultaneously with paleoclimatic events in the Quaternary and Late Tertiary. Halbach and Putae-nus studies have found that changes in the chemical composition of the Central Pacific crust reflect changes in the paleo-ocean environment, including carbonate dissolution rates, underlying flows, and changes in biological productivity. Banakar and Hein's study on the chemical composition of deep-water crusts in the Indian Ocean seamounts shows that ancient carbonate compensation depth (CCD), paleo-deep water, early Eocene productivity, bedrock instability, paleo-circulation, and Indian Ocean oxidative deep Changes in water conditions, etc. affect the growth of the crust. Hein et al. studied the crusts of the Central Pacific Ocean, and found that the chemical composition of the crust is related to the main changes of the ancient circulation and the growth and decline of the polar ice cap. In addition, the climate change caused by the Earth's movement will also lead to changes in the chemical composition of the crust.

(IV) Research progress on paleo-oceanography of domestic cobalt-rich crusts

The study of ancient oceanography of cobalt-rich crusts has also been carried out in China. Xu Dongyu briefly described the paleoceanographic environment of crust formation in the Central Pacific Ocean. Liang Hongfeng and others believe that the formation of seamount crusts in the South China Sea is controlled by the unique paleoceanographic environment of the South China Sea. The hydrothermal action of submarine volcanoes may also be one of the influencing factors. Liang Hongfeng et al. conducted a comparative study on the elemental geochemical behaviors of the Central Pacific Ocean, the Philippine basin, and the South China Sea crust. It is believed that crust growth is controlled by the supply of materials and paleoceanographic conditions. Pan Jiahua and Liu Shuqin discussed the geochemical characteristics of the cobalt-rich crusts in the western Pacific and their relationship with the paleo-ocean environment. Xu Dongxuan's research results show that paleo-oceanic events and environments such as water, upwelling, biological productivity and sedimentary discontinuity in the Antarctic are the main factors controlling and affecting the formation and distribution of deep-sea polymetallic nodules, cobalt-rich crusts and seamounts. . However, how these findings provide a comprehensive and accurate explanation for the ore-forming environment in the crust, and thus the discovery of the crust-forming mineralization mechanism and the establishment of the metallogenic model, still require more in-depth and effective research.

Third, the conclusion

This paper reviews the geochemical studies of cobalt-rich crusts and highlights whether the cobalt-rich crusts have undergone material exchange with the surrounding seawater bedrock, whether the elements diffuse within the crusts, and the phosphating events on the crusts. Influence and other issues. At the same time, the research progress of paleoceanography of cobalt-rich crusts (including the application of isotopes and elements in the study of crust and paleoceanology) is introduced. At present, the geochemical behavior of many elements and isotopes in cobalt-rich crusts is not fully understood, which limits the application of these elements or isotopes in paleoceanography and the development of paleoceanography. With the improvement, perfection and enrichment of analytical testing methods, we can understand the geochemical behavior and distribution characteristics of more elements or isotopes in cobalt-rich crusts. This will lay a solid theoretical foundation for the in-depth study of cobalt-rich crust paleo-oceanology and promote the further development of paleoceanographic research.

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