BIOGEOCHEMISTRY AND NITROGEN ISOTOPE STRATIGRAPHY DURING THE EDIACARAN AND EARLY CAMBRIAN OF CENTRAL ASIA

The radiation of metazoans during the Ediacaran to early Cambrian interval is closely linked to fundamental oceanic and atmospheric compositional changes reflected by pronounced isotope anomalies. Recent nitrogen isotope studies from the South China Yangtze platform revealed a negative nitrogen isotope excursion across the Precambrian-Cambrian boundary. Causes triggering the isotope anomaly are suggested to be involved in evolutionary events; however, their significance on local and global scale remains elusive.

We present the first nitrogen isotope correlation of sedimentary sections spanning the Ediacaran to early Cambrian interval on two microcontinents: South China (Yangtze platform) and Kazakhstan (Malyi Karatau area). The large-scale conformity of δ¹⁵N trends from the Ediacaran until the lower Cambrian implies that obtained δ¹⁵N data represent global perturbations in the past nitrogen cycle. Furthermore, nitrogen isotope data reveal changes in paleo-environmental conditions, ocean redox structure and past seawater chemistry and provide information about the nutrient utilization of microorganisms in surface waters. During the Ediacaran δ¹⁵N data >3‰ indicate that the Ediacaran ocean contained periodically enough dissolved oxygen to stabilize nitrate similar to modern ocean conditions where nitrification-denitrification interactions and nitrate-assimilation by primary producers dominate biological nitrogen transformations. In contrast, a pronounced δ¹⁵N negative interval (δ¹⁵N <0‰) in the early Cambrian manifests the scenario of a global “oceanic anoxic event” that was involved in the course of the Cambrian radiation. Negative δ¹⁵N values argue for a significant contribution of anoxygenic phototrophs to the organic matter pool under widespread photic zone anoxia in a redox-stratified environment on at least two Precambrian-Cambrian microcontinents.