Paleosalinity of the Nanhua Basin (South China) during the Cambrian Explosion

Abstract

Similar to marine oxygen level and nutrient supply, seawater salinity has also been suggested as an important control on marine biological evolution. However, the potential links between the seawater salinity and Cambrian Explosion remain unclear. Here, we conducted high-resolution analyses of B/Ga and B/K ratios on four typical sections from different water-depth environments to constrain the paleosalinity in the Nanhua Basin (South China) from late Age 2 to Age 3 (~526–515 Ma). Our new data revealed common hypersaline conditions at shallow-water Meishucun (B/Ga = 13.8 ± 2.3 and B/K = 73.5 ± 15.7) and Wangjiaping (B/Ga = 20.6 ± 2.7 and B/K = 101.1 ± 17.1) locales during most of the period from late Age 2 to Age 3. In contrast, combined with the deep-water data in a recent study, middle- to deep-water environments were dominated by brackish to normal seawater salinity, as documented at Zhijin (B/Ga = 8.0 ± 1.1 and B/K = 48.2 ± 4.5), Daotuo (B/Ga = 6.4 ± 1.6 and B/K = 57.6 ± 12.3), Yuanjia (B/Ga = 4.3 ± 2.7), and Zhalagou (B/Ga = 6.1 ± 1.2) locales. Notably, two transient low-salinity intervals (LS1 and LS2) developed in shallow-water regions at ~526 Ma and ~ 518 Ma, respectively, because of increased freshwater inputs, during which the watermass salinity may be inhabitable for early faunas. Combined with previous redox studies, however, transient shallow-water anoxia at ~525 Ma allowed only the development of hypoxia-tolerant sponge faunas during LS1, indicating redox constrains on the early biological evolution. Additionally, although studies indicated rapid oceanic oxygenation within the hypersaline interval between LS1 and LS2, no important biotas occurred at this hypersaline interval, indicating hypersalinity restriction on metazoan diversification. Instead, when normal seawater salinity and oxic watermass were jointly formed during LS2 (~518 Ma), the Chengjiang and Qingjiang biotas experienced notable explosion. Therefore, the influences of the seawater salinity on early life evolution should be given more attention in the future.