Studies on geochemical characteristics and biomineralization of Cambrian phosphorites, Zhijin, Guizhou Province, China

Abstract

Phosphate rocks, an important ore resource in Guizhou Province, China, are mainly hosted within the Sinian Doushantuo Formation and the Cambrian Meishucun Formation. In addition, the phosphate rocks of the Cambrian Meishucun Formation are rich in biological fossils. Although numerous studies investigating the genesis of phosphate deposits have been performed, the relationship between biological activity and the formation of phosphate deposits in the lower Cambrian Meishucun Formation has not been convincingly explained. This study focuses on the biological fossil assemblage, the characteristics of phosphorus, and the relationship between biological and phosphorus enrichment of the lower Cambrian phosphorites. The primary objectives of our study are to analyze the role of organisms in the formation of phosphorites, restore the phosphorus-formation environment of the Cambrian Meishucun Formation, and construct a sedimentary model of the phosphorites in the Meishucun Formation. The results indicate that there is a significant positive correlation between biological activity and the deposition of phosphorites, that is, the higher the degree of biological enrichment and differentiation, the stronger the deposition. The geochemical analysis of several profiles in the Zhijin phosphorite block shows that the phosphorite block was deposited in an oxygen-rich environment and was affected by a high-temperature hydrothermal fluid. Upwelling ocean currents supplied abundant phosphorus and other nutrients, which provided the conditions for small shells and algae to flourish. Biochemical activity was a crucial factor in the deposition of the phosphorite.

The Paleoecological Environment during the Ediacaran–Cambrian Transition in Central Guizhou Province, China: Evidence from Zn Isotopes

“…Geotectonically, the central region of Guizhou is located at the southwestern margin of the Yangtze Craton of the South China Block [53,54] (Figure 1a). The Taozichong Formation in Qingzhen, Guizhou Province of China, is located in the southern margin of the "Central Guizhou Uplift" region in the Yangtze Platform [7,10,52], and a large "ancient anticlinal" structure was developed there [55][56][57]. The tectonic movement in the region is complex, mainly dominated by NE-trending folds, and there are also NW-, NNE-, and NE-trending faults (Figure 1b).…”

“…The sedimentary facies of the lower Cambrian in South China from east to southwest include continental slope deep-water basin, deep-water slope, and littoral tidal flat-shallow water slope, and the phosphorous rock series in this period are the products of upwelling ocean current and transgression process [59,60] (Figure 2). Previous studies have shown that the paleogeographic position of the study area during the early Cambrian Meishucun period was a submerged platform containing phosphorus carbonate on the side of a shallow sea extending toward the ocean, which was dominated by a subtidal closed marine environment [7,10,51,[59][60][61] (Figure 2). The strata in the study area mainly developed from the bottom up into the Dengying Formation of the upper Ediacaran, the Taozichong Formation (Cambrian Terreneuvian Fortunian), and the Niutitang Formation (Cambrian Terreneuvian Stage 2) of the lower Cambrian (Figures 1c and 3a) [10,[48][49][50][51][52], which is a set of marine strata dominated by carbonate, phosphorus carbonate and clasolite (Figure 3a) [10,51,52,[60][61][62].…”

“…The "deep time" Earth evolution, paleoenvironmental conditions, coupled changes in the geosphere, and the relationship between biological and environmental evolution are the most important issues in current Earth science research [1][2][3][4][5][6][7]. Many unique rock associations and major geological events simultaneously occurred during the Cambrian period [8][9][10], highlighting the significance of the relationship between sedimentary records and paleogeography, paleoocean, paleoclimate, and paleoecology.…”

“…When an important phosphorus-forming event occurred in the early Cambrian period, a large area of phosphorus-rich sequence was formed on the continental shelf [30]. Extensive phosphorus enrichment in the ocean is believed to have resulted from intense continental weathering [31], surface ocean cyanobacterial proliferation [32], and ocean upwelling [6][7][8], which shed light on the biotic-abiotic and continent-ocean interactions during the critical time period in the dawn of the Cambrian explosion. Major events of phosphorite accumulation are associated with disruptions in the biogeochemical cycle of phosphorous over time, which leads to changes in paleogeography, paleoceanography, and biological evolution [33].…”

“…As an important ore resource in Guizhou Province, China, phosphate rocks mainly occur in the Sinian Doushantuo Formation and the Cambrian Meishucun Formation [6][7][8]. The Ediacaran-Cambrian continuous stratigraphic sequence of Qingzhen in central Guizhou Province is an ideal area for studying the sedimentary evolution, as well as the paleoclimate and paleoenvironmental conditions of the early Cambrian period [10,[48][49][50][51][52].…”